Copyright

by

Gregory Blachut

2016

The Dissertation Committee for Gregory Blachut Certifies that this is the approved

version of the following dissertation:

Design of Silicon-containing Block Copolymer Materials for

Applications in Lithography

Committee:

Carlton Grant Willson, Supervisor

Christopher J. Ellison, Co-Supervisor

Venkat Ganesan

Chris A. Mack

Deji Akinwande

Design of Silicon-containing Block Copolymer Materials for

Applications in Lithography

by

Gregory Blachut, B.S.

Dissertation

Presented to the Faculty of the Graduate School of

The University of Texas at Austin

in Partial Fulfillment

of the Requirements

for the Degree of

Doctor of Philosophy

The University of Texas at Austin

December 2016

Dedication

This dissertation is dedicated to my parents, for their love and support.

v

Acknowledgements

I would like to thank Professor Grant Willson for being an extraordinary advisor

in all sense of the word. I would also like to thank my other advisor as well, Professor

Christopher Ellison, for his exceptional guidance throughout the years. Perhaps the

biggest testament to the character of both of these men lies in the droves of talented

students that flock to their groups. It has been my great honor to work alongside some of

these students throughout my years at UT, and to even call some of them my close

friends. I thank Michael Maher, Yusuke Asano, William Durand, Austin Lane, and

Sunshine Zhou for our close collaborations. In addition, I am graciously beholden to the

mentorship of Christopher Bates, Julia Cushen, Josh Katzenstein, William Bell, and Ryan

Deschner. I am also very fortunate to have worked with a number of eager and forgiving

undergraduates who also deserve thanks and recognition: Ziang Li, Adam Wrobel, Gary

Cantu, Andrew Dinhobl, and Paul Ortega. The administrative assistance and friendship of

Donna Martin, Kathleen Sparks, Mindy Maloney, and Kate Baird is also very

appreciated. Last, the sense of comradery from the rest of the Willson and Ellison group

members will be sorely missed.

I was very fortunate as well to have the opportunity to collaborate directly with

Lam Research and IMEC as part of this research. Diane Hymes, Stephen Sirard, Johan

Vertommen, David Hellin, Bart Coenegrachts, Mohand Brouri, Christophe Noben, Geert

Vandenberghe, Roel Gronheid, Paulina Rincon Delgadillo, BT Chan, Nadia

Vandenbroeck, Hari Pathangi, Arjun Singh, Robert Seidel, Jan Doise, Grant Garner,

Rikimaru Sakamoto, Yasanobu Someya, Hiroyuki Wakayama and many others

vi

contributed to this monumental effort. In addition, a different collaboration with Seagate

was made possible by XiaoMin Yang and Kim Lee.

Last, I would like to thank Dr. Justin Shearer for being my under-graduate

research mentor and enabling my jump to graduate school.

vii

Design of Silicon-containing Block Copolymer Materials for

Applications in Lithography

Gregory Blachut, Ph.D.

The University of Texas at Austin, 2016

Supervisor: C. Grant Willson

Co-supervisor: Christopher J. Ellison

Continual advancement in microelectronic performance has made

microelectronics essentially ubiquitous, enriching modern life in ways unimaginable even

a few decades ago. The advancement in microelectronic devices is made possible by

advancements in the manufacturing processes used to make them. Chief among these

technologies is lithography, the process by which the individual components on the

device are patterned. At present, complex and complicated double-patterning processes

are being used to extend the resolution of the lithographic methods used in high-volume

manufacturing, but only at great cost. Future generations of microelectronic devices will

require even further use of multiple-patterning processes, at which point the economics of

manufacturing could prevent the commercialization of such devices.

This economic reality has spurred interest in alternative patterning technologies.

One of the leading potential methods is to exploit the self-assembly of block copolymers

(BCPs). BCPs are a type of polymer consisting of two or more chemically distinct blocks

that are covalently joined together. The components of a BCP can phase-separate, and the

resultant features form on the 5 to 50 nm length-scale. This size range is coincidentally

ideal for next-generation semiconductor devices. However, BCPs on their own do not

viii

immediately form device-relevant features. Processes known collectively as directed self-

assembly (DSA) are needed to properly guide BCPs.

The work in this dissertation focuses on a very specific class of BCPs, those that

contain silicon in just one of the blocks. The presence of silicon in the molecule produces

many lithographic advantages, but also requires specialized processing steps. Chapter 1

provides an overview of lithography and block copolymer self-assembly. Chapter 2

introduces the materials and techniques needed to control the behavior of silicon-

containing BCPs. Chapter 3 presents and characterizes a variety of silicon-containing

BCPs. Last, Chapters 4 and 5 describe two implementations of silicon-containing BCP

DSA, one for semiconductor patterning, and the other for hard disk drive applications.

ix

Table of Contents

List of Tables ....................................................................................................... xiii

List of Figures ...................................................................................................... xiv

List of Schemes .................................................................................................. xxxi

Chapter 1: Patterning in microelectronic applications .............................................1

1.1 History of integrated circuits and modern computing ..............................1

1.2 Photolithography .......................................................................................5

1.3 BCPs and self-assembly ..........................................................................10

1.4 BCP challenges and opportunities in lithography ...................................12

1.5 Goals of this dissertation .........................................................................15

Chapter 2: Methods and materials for BCP thin film control ...............................16

2.1 Surface neutralization .............................................................................16

2.1.1 Cross-linkable polymeric materials ............................................18

2.1.2 Graftable polymeric brushes .......................................................19

2.1.3 Polarity-switching top coats ........................................................21

2.2 Island-hole testing ...................................................................................23

2.2.1 Origin of topography...................................................................24

2.2.2 Half-height (0.5 periodicity) topography ....................................25

2.2.3 Confined island-hole ...................................................................27

2.2.4 Representative island-hole test ...................................................30

2.3 Consequences of neutral surfaces ...........................................................31

2.4 Conclusion .....................................................................................32

Chapter 3: Design of silicon-containing block copolymers for lithography ..........34

3.1 Background information .........................................................................34

3.2 Previous work in high-χ BCPs ................................................................36

3.3 UT silicon-containing high-χ BCPs ........................................................38

3.3.1 Silicon-containing BCP development .........................................38

x

3.3.2 Summary of styrene-styrene silicon-containing BCPs for lithography

.....................................................................................................40

3.3.3 BCP synthesis overview .............................................................46

3.3.4 BCP data for individual samples.................................................47

3.3.5 BCP pitch ....................................................................................49

3.3.6 Characterization procedures ........................................................50

3.3.7 Calculating χ by dynamic mechanical spectroscopy ..................52

3.3.8: Extrapolating scaling trends .......................................................58

3.4 Conclusions .............................................................................................65

3.5 Experimental ...........................................................................................66

3.5.1 BCP synthesis .............................................................................66

3.5.2 Characterization ..........................................................................66

3.5.3 Regression procedures ................................................................67

Chapter 4: Directed self-assembly for semiconductor applications .....................68

4.1 Background .............................................................................................68

4.2 Material design and characterization ......................................................72

4.2.1 Overview of materials .................................................................72

4.2.2 Surface treatments .......................................................................74

4.3 Typical process to create DSA wafers ....................................................78

4.4 Qualitative comparison of the two DSA processes.................................80

4.5 Documentation and consequences of the sidewall in DSA ....................86

4.6 Pattern transfer ........................................................................................89

4.7 Mask layout and CD-SEM measurements ..............................................95

4.8 Quantitative comparison of the DSA processes....................................103

4.8.1 Overview of the different DSA processes ................................103

4.8.2 Guide stripe pitch ......................................................................105

4.8.3 Guide stripe width .....................................................................108

4.8.4 Backfill brush composition .......................................................111

4.8.5 BCP film thickness ...................................................................115

4.8.6 Annealing time and temperature ...............................................118

xi

4.9 Conclusions and future work ................................................................123

4.10 Acknowledgements .............................................................................124

4.11 Experimental .......................................................................................125

4.11.1 Processing ...............................................................................125

4.11.2 Conditions for each DSA process ...........................................126

Chapter 5: Direct self-assembly for bit patterned media applications .................127

5.1 Background ...........................................................................................127

5.2 Process overview ..................................................................................131

5.3 DSA with the brush reflow process ......................................................133

5.4 DSA with no brush................................................................................136

5.5 A comparison of the two DSA processes .............................................137

5.6 Developing a selective deposition strategy ...........................................145

5.6.1 Water contact angle measurements ...........................................146

5.6.2 Quantification of the lack of selectivity in the current grafting

procedure...................................................................................146

5.6.3 Grafting selectivity onto chromium oxide ................................149

5.6.4 Water contact angle hysteresis on NIL resist samples ..............159

5.7 DSA on unetched NIL resist .................................................................162

5.8 Conclusion ............................................................................................165

5.9 Acknowledgements ...............................................................................166

5.10 Experimental .......................................................................................166

5.10.1 Materials .................................................................................166

5.10.2 Initial pre-patterning ...............................................................166

5.10.3 BCP and top coat processing ..................................................167

5.10.4 Thin Film analysis...................................................................167

Appendix A: Characterization of BCP samples...................................................168

A.1 SEC Data ..............................................................................................168

A.2 MALDI data .........................................................................................174

A.3 SAXS data ............................................................................................179

A.4 DMS data .............................................................................................182

xii

Appendix B: Practical considerations in anionic polymerization ........................186

B.1 Monomer and solvent purification and storage ....................................186

B.2 Reactor setup ........................................................................................195

B.3 Conducting a polymerization ...............................................................197

B.3.1 Cooling the reactor ...................................................................197

B.3.2 Dispensing the initiator ............................................................198

B.3.3 Dispensing monomer................................................................200

B.3.4 Taking an aliquot ......................................................................202

B.3.5 Terminating the reaction ..........................................................204

B.4 Safety ....................................................................................................205

References ............................................................................................................208

xiii

List of Tables

Table 1.1 Smallest dimensions of Intel’s commercial devices ...............................9

Table 2.1: List of polymers that can and cannot form confined island-hole features

...........................................................................................................28

Table 3.1: Summary of BCPs and key lithographic parameters ...........................42

Table 3.2: BCP sample characterization summary ...............................................48

Table 3.3: Summary of ODT data used to determine χ at 180 °C ........................53

Table 3.4: Parameters for domain spacing calculations ........................................59

Table 3.5: Summary of empirical model for predicting scaling limits .................63

Table 4.1: Polymer brushes available at IMEC ....................................................75

Table 4.2: Weight % composition of the brush formulations used in Figure 4.29.114

Table 4.3: Summary of processing conditions used to make Figure 4.7 ...........126

Table B.1: Purification reagents for monomers in anionic polymerization ........188

xiv

List of Figures

Figure 1.1: The disassembled ENIAC sits on display at the University of

Pennsylvania. Copyright 2016 Penn Engineering. .............................2

Figure 1.2: A tilted view SEM of Intel’s 22 nm FinFET Tri-Gate transistor. Image

courtesy of Maltiel Consulting. Copyright Intel Corporation. ............4

Figure 1.3: Cross-section (left) and overhead view (right) of SADP process.

Copyright David Abercrombie.13 ........................................................8

Figure 1.4: Theoretical phase diagram of a diblock copolymer. f = volume fraction, L

= lamellae, G = gyroid, C = cylinders, S = spheres (bcc lattice), Scp =

spheres (close-packed lattice). Reproduced with permission from

Matsen, M. W. “The Standard Gaussian Model for Block Copolymer

Melts.” J. Phys. Condens. Matter 2002, 14, R21–R47. Copyright 2002

IOP Publishing Ltd. ..........................................................................10

Figure 1.5: Top-down SEM showing a region of random fingerprint (left) next to a

region of directed self-assembly (right) of PS-b-PMMA. Reproduced

with permission from Stoykovich, et al. “Block Copolymers and

Conventional Lithography.” Mater. Today 2006, 9, 20–29. Copyright

2006 Elsevier. ...................................................................................12

Figure 1.6: Silicon-containing BCP wetting layer formation despite the neutralization

of the bottom interface. Image courtesy of Michael J. Maher and IBM

Almaden. ...........................................................................................15

Figure 2.1: Process used to neutralize a substrate for a BCP. Image courtesy of

Christopher Bates. .............................................................................17

xv

Figure 2.2: General structure of the XST materials made by free radical

polymerization. .................................................................................19

Figure 2.3: Thin film IR spectra of top coat as cast from amyl acetate (blue curve),

the corresponding TMA salt case from methanol (red curve), and the

TMA salt after heating at 190 °C for 30 s (green curve). Used with

permission from Maher, et al. “Interfacial Design for Block Copolymer

Thin Films.” Chem. Mater. 2014, 26, 1471–1479. Copyright 2014 ACS.

...........................................................................................................22

Figure 2.4: Structure of poly(styrene-alt-maleic anhydride)-r-poly(3,5-di-tert-

butylstyrene-alt-maleic anhydride) top coat. ....................................23

Figure 2.5 (A) Symmetric and (B) asymmetric BCP wetting in thin films. In (A),

islands are formed, while in (B), holes are formed. Used with permission

from Maher, et al. “Interfacial Design for Block Copolymer Thin

Films.” Chem. Mater. 2014, 26, 1471–1479. Copyright 2014 ACS.24

Figure 2.6: Three-dimensional representation of AFM micrograph showing

approximately 20 nm islands. ...........................................................25

Figure 2.7 Illustration of the origin of 0.5 L0 step heights when a BCP interfaces with

one neutral and one preferential surface. Used with permission from

Kim, et al. “Consequences of Surface Neutralization in Diblock

Copolymer Thin Films.” ACS Nano 2013, 7, 9905–9919. Copyright

2013 ACS. .........................................................................................26

xvi

Figure 2.8: Sample confined island-hole test of 18 nm PS-b-PTMSS on cross-linked

PS. The top labels are the chemistry of the top coat (number corresponds

to the mol % of the styrenic component that is 3,5-di-tert-butylstyrene.

Rest is styrene). Images are AFM micrographs at the labelled BCP film

thickness. Bottom graphs are traces of the topography. Used with

permission from Maher, et al. “Interfacial Design for Block Copolymer

Thin Films.” Chem. Mater. 2014, 26, 1471–1479. Copyright 2014 ACS.

...........................................................................................................30

Figure 2.9: Tilted SEM of 22 nm PS-b-PTMSS with the PS block fully removed

showing high-aspect ratio features that persist through the entire film.

Used with permission from Maher, et al. “Interfacial Design for Block

Copolymer Thin Films.” Chem. Mater. 2014, 26, 1471–1479. Copyright

2014 ACS. .........................................................................................32

Figure 3.1: Poly(3,4,5-trimethoxystyrene)-b-poly(4-pentamethyldisilylstyrene).45

Figure 3.2: Polymers showing signs of oxidation damage during and after rheology

experiments. ......................................................................................54

Figure 3.3: (A) Log-log plot of L0 calculated at two different χN values (10.5 and 14)

versus χ at 180 °C. Line is least squares residual fit. (B) Residual plot of

the linear regression through the points in plot (A). Lines are added to

guide the eye. Numbers in both graphs correspond to the BCPs as

follows: (1) PS-b-PTMSS (2) PS-b-PDSS (3) PMOST-b-PTMSS (4)

PMOST-b-PDSS (5) PVBD-b-PDSS (6) PTMSS-b-P2VP (7) PMVBD-

b-PDSS. .............................................................................................62

xvii

Figure 4.1: Fabrication of chemical pre-patterns used for the directed self-assembly

of PS-b-PMMA with frequency multiplication. Reproduced with

permission from Liu et al. “Chemical Patterns for Directed Self-

Assembly of Lamellae-Forming Block Copolymers with Density

Multiplication of Features.” Macromolecules 2013, 46, 1415–1424.71

Figure 4.2: Polymers introduced into IMEC: cross-linkable poly(4-methoxystyrene)

(XPMOST), 4 polymer “brushes” of hydroxyl-terminated polystyrene-r-

poly(4-tert-butylstyrene), the silicon-containing BCP poly(4-

methoxystyrene)-b-poly(4-trimethylsilylstyrene), and the polarity

switching top coat. ............................................................................73

Figure 4.3: Island-hole data of PMOST-b-PTMSS performed at IMEC on polymer

brushes. B-50 and BB-68 show holes, which implies symmetric

(PTMSS) wetting of the two surfaces. BB-85 shows islands and holes at

the two thicknesses, implying neutral “half” features are present. ...76

Figure 4.4: “Confined” island-hole test of the top coat. The progression from island

to spinodal islands to holes in this BCP film thickness range suggests the

neutrality of the top coat. Underneath the BCP is a PMOST-preferential

PS under-layer. ..................................................................................77

Figure 4.5: (A) Schematic of the hybrid process used to create the chemical pre-

pattern with shallow topography. (B) Depiction of the corresponding

pre-pattern surface produced by the chemo-epitaxy process. ...........79

Figure 4.6: Cross-section STEM micrograph of the 19 nm tall guide stripes after etch

and resist strip but before depositing the brush or the BCP. The

chromium capping layer was added to protect the features during cross-

sectioning and to prevent charging during imaging. .........................80

xviii

Figure 4.7: Representative micrographs of DSA under best conditions using (A)

chemo-epitaxy process with XPS guide stripe (B) chemo-epitaxy

process with XPMOST guide stripe (C) hybrid process with 19 nm tall

XPMOST guide stripe. Scale bar is valid for all images. .................81

Figure 4.8: Summary of the cross-sectional STEM EELS study of BCP on tall guide

stripes in the hybrid flow. Guide stripe width (measured by CD-SEM)

(A) ~0.5 L0 (B) ~1.0 L0 (C) 1.5 L0. (Top) STEM EELS mappings of

cross-sectioned DSA film stacks. The legend in the top right describes

the elemental mapping color scheme for all three EELS images. Scale

bar is valid for every image. (Bottom) Schematic of the proposed BCP

domain orientation on guide stripes corresponding to difference

quantizations of the BCPs natural periodicity, along with depictions of

the anticipated polymer chain configuration. As shown in the legend,

blue corresponds to PMOST and red to PTMSS. .............................83

Figure 4.9: Orientation of BCP chains in the process flow, depicting the oxidized

sidewalls as guiding interfaces. .........................................................87

Figure 4.10: Illustration of a possible DSA scheme using sidewall guided assembly

to relax the initial pre-pattern. ...........................................................88

xix

Figure 4.11: Top down and angled SEM of (a) 20 nm polystyrene-b-poly(4-

trimethylsilylstyrene) (PS-b-PTMSS) (b) 20 nm poly(4-

methoxystyrene)-b-poly(4-trimethylsilylstyrene) (PMOST-b-PTMSS)

(c) 20 nm polystyrene-b-poly(4-pentamethyldisilylstyrene) (PS-b-PDSS)

(d) 14 nm PMOST-b-PTMSS (e) 14 nm PS-b-PDSS. Reproduced with

permission from Durand et al. “Design of High-chi Block Copolymers

for Lithography.” J. Poly Sci. A 2015, 53. 344-352. Copyright 2015

Wiley. ................................................................................................90

Figure 4.12: Over-etch study on BCP features. The wafer was etched for 75 s total.

Features from chips at the edge of the wafer are etched more heavily

than those at the center (note: the DSA quality was not uniform across

the wafer, so only the quality of the lines, and not their vertical

alignment, was the metric for over-etching). A similar wafer etched for

60 s showed no difference from center to edge, implying that the

threshold for domain distortion is between 60 and 75 s for the features

on the outer chips. Scale bar is 200 nm and valid for all four images.91

Figure 4.13: Initial demonstration of fully developed PMOST-b-PTMSS DSA

features at IMEC. The wafer was etched to the point just before the

features were expected to start distorting. (Top right) Top-down SEM of

the region submitted for cross-section. The area was chosen for its good

DSA result. (Top left) Tilted-view SEM of the features. (Bottom) Cross-

section SEM of the features. Measurements indicate that approximately

15.5 nm (out of 33 nm initial film thickness) remain. Scale bars are each

50 nm. ...............................................................................................92

xx

Figure 4.14: Tilted-view SEM showing pattern transfer of BCP features into silicon

nitride and then silicon. Scale bars are each 100 nm. .......................93

Figure 4.15: Pattern transfer attempt for a “deep” silicon etch. (Top left) Top-down

SEM of area to be cross-sectioned. BCP was etched to a lesser degree

than in previous tests, and the white lines are noticeably thicker than in

other BCP pattern transfer images. (Top right) Top down SEM of silicon

nitride etch. The features have more waviness line edge roughness than

previous, and some bridging appears to be occurring. (Bottom left)

Tilted view SEM and (Bottom right) cross-section SEM of the silicon

etch step. The silicon was etched to the point of exhausting the silicon

nitride mask to determine the maximum etch depth possible. Analysis

shows aspect ratios between 3.5 and 5.0. The bridging is probably the

result of an insufficient BCP removal in the first step. .....................95

Figure 4.16: Schematic of the “vinaigrette” mask used to print pre-patterns for DSA

at IMEC. The different purple regions correspond to regions with

different purposes including cross-sectioning and defect contribution

analysis. The “scatterometry target” region was used for this project.97

Figure 4.17: (A) Optical micrograph of the pattern produced by the scatterometry

target fields. The pitch of the lines-space pattern increases to the right,

and the width of the printed line increases in the vertical direction. (B)

SEM micrograph of one of the scatterometry targets. Each block is

labelled with a mask critical dimension (CD) and pitch (nanometers for

both). The top dark block contains horizontal features, and the bottom

block contains vertical features, where DSA is expected to occur. The

height and width of each individual box is 60 µm for scale. ............98

xxi

Figure 4.18: CD-SEM micrographs of (A) resist structures after exposure and

development (B) and resist structures after trim etch both show the resist

as the thin gray line. The resist and mat are depicted in cross-section

after etch in (C) to show how the sidewall (also called “shoulder”)

remains exposed to form the black halo seen in (B) around the resist

features. The line pitch in (A) and (B) is 79 nm for scale. ...............99

Figure 4.19: Representative micrographs of different degrees of DSA and their

corresponding score from the image scoring program. ..................100

Figure 4.20: Representative heat map of DSA scores across one wafer with a dose

gradient applied (the right-hand side of wafer has the highest dose so

smallest guide stripe width). The two heat maps show the same data but

are printed in different color schemes. ............................................102

Figure 4.21: Schematic of the anticipated DSA schemes on the wafer, labelled with

the density multiplication (either 4x or 5x) and the number of periods the

guide stripe width corresponds to (either W=0.5 or W=1.5). .........102

Figure 4.22: Direct comparison of the chemo-epitaxy process with XPMOST and

XPS guide stripes at the four DSA schemes (A) under conditions

optimized for each process (B) under deliberately sub-optimal

conditions. .......................................................................................104

Figure 4.23: Comparison of the hybrid process to the chemo-epitaxy process. The 9

nm data correspond to the chemo-epitaxy process made with XPMOST,

while the other two thicknesses correspond to hybrid process with

XPMOST. Error bars are 3 standard deviations. ............................105

xxii

Figure 4.24: (A) Top-down SEM of fingerprint pattern of ~20 nm BCP (B) 2D fast

Fourier transform (FFT) of the image in (A). FFT software provided

courtesy of http://www.ejectamenta.com. ......................................106

Figure 4.25: DSA score versus pre-pattern pitch of a chemo-epitaxy process wafer

made with XPS. The data were taken from the region where the guide

stripe pinned approximately 0.5 L0 of BCP. The box plot represents the

five number summary of the data. ..................................................107

Figure 4.26: DSA score versus pre-pattern pitch of a hybrid process wafer. The data

were taken from the region where the guide stripe pinned approximately

0.5 L0 of BCP (A) and 1.5 L0 (B). The box plot represents the five

number summary of the data...........................................................108

Figure 4.27: CD-SEM micrograph of the resist features highlighting the approximate

resist edge and the presence of the partially etched guide stripe. ...109

Figure 4.28: DSA score versus guide stripe width for (A) XPS chemo-epitaxy

process wafer (B) hybrid process wafer for 20 nm PMOST-b-PTMSS at

79 nm guide stripe pitch (4x density multiplication). .....................110

Figure 4.29: DSA image score versus backfill brush at 4x density multiplication with

0.5 L0 guide stripes for (A) XPS guide stripe chemo-epitaxy process and

(B) XPMOST guide stripe chemo-epitaxy process. Compositions in bold

are pure polymers and compositions in italics are blends. The boxplots

represent the five number summary of the data. .............................114

Figure 4.30: DSA score versus nondimensionalized BCP film thickness for a series

of XPS chemo-epitaxy wafers. The points represent average scores and

the error bars represent maximum and minimum values for 4x density

multiplication and circa 0.5 L0 guide stripe width. .........................116

xxiii

Figure 4.31: A subset of the DSA score versus nondimensionalized BCP film

thickness for series of XPS chemo-epitaxy wafers. The points represent

average scores and the error bars represent maximum and minimum

values for the four DSA regimes identified on the wafer. ..............117

Figure 4.32: DSA score versus nondimensionalized BCP film thickness for series of

hybrid process wafers. The points represent average scores and the error

bars represent maximum and minimum values for the three identified

DSA schemes. 5x W=0.5 is omitted for clarity, but performed the worst

overall. ............................................................................................118

Figure 4.33. Optical micrograph of an XPS chemo-epitaxy wafer with a completed

DSA film stack after being annealed at 215°C for 5 minutes. The spots

are top coat dewetting from the BCP surface. ................................119

Figure 4.34: SEM micrographs of (A) XPS chemo-epitaxy process annealed at

170°C for 70 minutes and (B) the same film stack annealed at 180°C for

10 minutes. (B) is enlarged to show mark areas BCP domains are

beginning to nucleate from the horizontal features.........................121

Figure 4.35: DSA score versus temperature for the four DSA schemes on an XPS

chemo-epitaxy wafer. Error bars are 3 standard deviations ............122

Figure 4.36: (A) DSA score versus annealing time (at 205°C) for two DSA schemes

on an XPS chemo-epitaxy wafer. (B) Representative micrographs of best

DSA result from the 5x W=0.5 region demonstrating the improvement

upon annealing longer. ....................................................................123

xxiv

Figure 5.1: Fabrication of rectangular patterns from BCP lamellae. In D and E, a

second patterning step is used to chop the line space pattern into discreet

rectangles. Used with permission from Ruiz, R.; et al. ACS Nano 2011,

5, 79–84. Copyright 2011 ACS. .....................................................130

Figure 5.2: The (A) materials and (B) process steps used for producing 10 nm DSA

in the NIL process. Arrow 1 and 2 describe the process with and without

a grafted brush, respectively. ..........................................................131

Figure 5.3: Top-down SEM of 10 nm PVBD-b-PDSS on etched NIL resist with a

neutral brush grafted at 200 °C for 10 minutes. The resist lines run

vertically, while the BCP domains travel mostly horizontally. The

patchy regions are an artifact of SEM and not indicative of poor BCP

assembly. .........................................................................................133

Figure 5.4: Top-down SEM micrograph of defect-free 5 nm patterns over a large

area. The sample was made by grafting a neutral brush to the substrate

with the reflow process. The BCP film was lightly etched for image

contrast. Scale bar = 100 nm. ..........................................................134

Figure 5.5: Presumed mechanism of reflow process. The polymer brush (purple) is

drawn into the trenches of the resist lines (blue). ...........................135

Figure 5.6: Top-down SEM micrographs of reflow process control experiments. In

both images, the resist lines run vertically. A) Substrate treated with

BST 105 0.1 wt. % for 250 °C for 5 minutes (omitting the 150 °C reflow

step). B) Substrate treated with BST 105 0.33 wt. %, with both a 150 °C

10 minute bake and a 250 °C 5 minute bake. .................................136

xxv

Figure 5.7: Top-down SEM micrograph of defect-free 5 nm patterns over a large

area. The sample was made without grafting a neutral brush to the

substrate. The BCP film was lightly etched for image contrast. Scale bar

= 200 nm. ........................................................................................137

Figure 5.8: Top-down SEMs of BCP assembly on substrates patterned by NIL,

demonstrating the effect of resist height on DSA quality for A) the no

brush process and B) the reflow brush process. “h” is the average height

of the NIL resist for both images in a given row, as measured by AFM.

All BCP films were 33 nm (3.3 L0) thick and were annealed at 190°C for

2 minutes. Scale bar = 200 nm. .......................................................140

Figure 5.9: BCP self-assembly (top) and cross-sectional STEM with EELS mapping

(bottom) for three surfaces investigated in this study: A) SiO2 with

brush, B) SiO2 without brush, and C) 2 nm chromium oxide on Si. The

height of the NIL resist for the two samples made on SiO2 and the one

sample on chromium oxide were approximately 12 nm and 8 nm,

respectively. Samples made on SiO2 were etched by oxidizing plasma

before STEM analysis, while the sample made on chromium was not.

The starting BCP film thickness for all three samples was approximately

33 nm. The samples on SiO2 were annealed at 190°C for 2 min, while

the sample prepared on chromium oxide was annealed at 205°C for 10

min. Scale bar for the top-down SEM images is 100 nm. Scale bar for

the cross-section EELS images is 15 nm. .......................................142

xxvi

Figure 5.10: Enlarged STEM EELS image showing domain mismatch. Arrows

indicate domains at the top of the film that do not directly correspond to

domains lower in the film. Dashed lines are added as guides-to-the-eye

to show the direction of the domains. .............................................145

Figure 5.11: W.C.A. on substrates grafted with OH-brush (approximately 71% PS) at

250°C for 5 minutes. Results are the average of at least four separate

droplets, with the left and right droplet interface measured separately by

the software. The error bars indicate 95% confidence intervals. ....149

Figure 5.12: W.C.A on various modified substrates. The first 6 substrates were all

chromium oxide, and the last was silicon oxide. As controls, one

chromium oxide substrate was solely sonicated and rinsed with IPA

(first entry), and a different chromium oxide substrate was spin coated

with pure MIBK, annealed, and sonicated (second entry). The error bars

indicate 95% confidence intervals. Only one measurement was taken for

the last entry on silicon oxide due to its low value. The inset shows the

chemical structure of the 12 kg/mol PS made by anionic polymerization.

.........................................................................................................151

Figure 5.13: W.C.A. on etched chromium oxide (green) and etched NIL resist (blue)

after application (100 °C 60 s anneal) of various chemical treatments.

PtBuSty-OH is hydroxyl-terminated poly(4-tert-butylstyrene). The error

bars indicate 95% confidence intervals...........................................154

Figure 5.14: Demonstration of chromium oxide wetting change (by island-hole flip)

due to a change in grafting temperature. The island-holes were measured

by AFM. The grafted polymer was a hydroxyl-terminated PS-r-PtBuSty

with approximately 25 mol % PtBuSty. .........................................158

xxvii

Figure 5.15: W.C.A. on NIL resist treated with an OH-brush (250 °C for min) (blue)

and with decene (60 s at 100 °C) (green) as a function of trial number.

Each droplet was deposited at the same spot as the previous droplet,

except for the last trial in OH-brush. Lines are included to guide the eye.

.........................................................................................................161

Figure 5.16: Top-down SEM of 10 nm PVBD-b-PDSS on unetched NIL (without a

brush treatment) at the edge of the patterned region. ......................162

Figure 5.17: Top-down SEM of 10 nm PVBD-b-PDSS on unetched NIL resist

(without a brush treatment) showing a region where the features briefly

run parallel to the resist. ..................................................................164

Figure 5.18: Top-down SEM of 10 nm PVBD-b-PDSS on unetched NIL resist

(without a brush treatment) showing a region where the BCP runs

perpendicularly to the resist and patches of BCP rise above the resist

lines. ................................................................................................165

Figure A.1: SEC traces of PS (top trace) and PS-b-PTMSS (bottom trace) for (A)

disordered (B) 18 nm (C) 19 nm (D) 22 nm. ..................................168

Figure A.2: SEC traces of PMOST (top trace) and PMOST-b-PTMSS (bottom trace)

for (A) 14 nm (B) 20 nm. The SEC trace of PMOST-b-PTMSSdisordered

appears in Durand et al.40 ................................................................169

Figure A.3: SEC traces of PMOST (top trace) PMOST-b-PDSS (bottom trace) for

(A) 14.5 nm (B) 18 nm (C) 23 nm. .................................................170

Figure A.4: SEC traces of PTMSS (top trace) and PTMSS-b-P2VP (bottom trace) for

(A) 10.0 nm (B) 10.5 nm. The tailing observed in the bottom BCP trace

is due to P2VP interacting with the gel material and not an indication of

poor control of synthesis. ................................................................171

xxviii

Figure A.5: SEC traces of PVBD (top trace) and PVBD-b-PDSS (bottom trace) for

(A) 8.5 nm (B) 9.5 nm (C) 10.0 nm (D) 10.5 nm. ..........................172

Figure A.6: SEC traces of PMVBD (top trace) and PMVBD-b-PDSS (bottom trace)

for (A) 6.5 nm (B) 10.5 nm. ............................................................173

Figure A.7: MALDI spectrum for PS-b-PTMSS for (top to bottom) 18 nm, 19 nm, 22

nm. The data for the PS-b-PTMSSdisordered appears in Durand et al.40174

Figure A.8: MALDI spectrum for PMOST-b-PDSS for (top to bottom) 14.5 nm, 18

nm, 23 nm. The data for the disordered sample appears in Durand et al.40

.........................................................................................................175

Figure A.9: MALDI spectrum for PTMSS-b-P2VP for (top to bottom) 10.0 nm, 10.5

nm. (Left) homopolymer PTMSS aliquots (right) BCP..................176

Figure A.10: MALDI spectrum for PVBD-b-PDSS for (top to bottom) 8.5 nm, 9.5

nm, 10.0 nm, and 10.5 nm. .............................................................177

Figure A.11: MALDI spectrum for PMVBD-b-PDSS for (top to bottom) 6.5 nm,

10.5 nm. (Left) homopolymer PDSS aliquots (right) BCP. ............178

Figure A.12: SAXS profiles of ordered PS-b-PTMSS samples, from top to bottom:

18 nm, 19 nm, 22 nm. Data shifted vertically for clarity. ...............179

Figure A.13: SAXS profiles of ordered PMOST-b-PTMSS samples, from top to

bottom: 14 nm, 20 nm. Data shifted vertically for clarity. .............179

Figure A.14: SAXS profiles of ordered PMOST-b-PDSS samples, from top to

bottom: 14.5 nm, 18.0 nm, 23.0 nm. Data shifted vertically for clarity.

.........................................................................................................180

Figure A.15: SAXS profiles of PTMSS-b-P2VP samples, from top to bottom: 10.0

nm, 10.5 nm. Data shifted vertically for clarity. .............................180

xxix

Figure A.16: SAXS profiles of PVBD-b-PDSS samples, from top to bottom: 8.5 nm,

9.5 nm, 10.0 nm, 10.5 nm. Data shifted vertically for clarity. ........181

Figure A.17: SAXS profiles of PMVBD-b-PDSS samples, from top to bottom: 6.5

nm, 10.5 nm. Data shifted vertically for clarity. .............................181

Figure A.18: DMS plots of the storage modulus (G’) of PS-b-PTMSS taken upon a

constant 5 °C/min temperature ramp. Each sample was taken at strain =

2% and angular momentum (ω) = 1 rad/s. ......................................182

Figure A.19: DMS plots of the storage modulus (G’) of PMOST-b-PTMSS and PS-

b-PDSS taken upon a constant 5 °C/min temperature ramp. Each sample

was taken at strain = 2% and angular momentum (ω) = 1 rad/s. ....182

Figure A.20: DMS plots of the storage modulus (G’) and delta of PMOST-b-PDSS

(disordered sample) taken upon a constant 5 °C/min temperature ramp.

The sample was taken at strain = 2% and angular momentum (ω) = 1

rad/s. No indication of an ODT appears in the graph, precluding the

direct calculation of χ. .....................................................................183

Figure A.21: DMS plots of the storage modulus (G’) of PTMSS-b-P2VP taken upon

a constant 5 °C/min temperature ramp. Each sample was taken at strain

= 2% and angular momentum (ω) = 1 rad/s. ...................................183

Figure A.22: DMS plots of the storage modulus (G’) of PVBD-b-PDSS taken upon a

constant 5 °C/min temperature ramp. Each sample was taken at strain =

2% and angular momentum (ω) = 1 rad/s. ......................................184

xxx

Figure A.23: DMS plots of the storage modulus (G’) and delta of PVBD-b-PDSS

(8.5 nm disordered sample) taken upon a constant 5 °C/min temperature

ramp. The sample was taken at strain = 2% and angular momentum (ω)

= 1 rad/s. No indication of an ODT appears in the graph, precluding the

direct calculation of χ by this sample. .............................................184

Figure A.24: DMS plots of the storage modulus (G’) of PMVBD-b-PDSS taken upon

a constant 5 °C/min temperature ramp. The sample was taken at strain =

2% and angular momentum (ω) = 1 rad/s. ......................................185

Figure B.1: Top-down illustration of 5 port reactor. Ports 1 and 2 are at the back of

the reactor (clamps attach to these ports) and ports 4 and 5 are in the

front. ................................................................................................196

xxxi

List of Schemes

Scheme 2.1: Controlled radical polymerization of poly(styrene)-r-poly(4-tert-

butylstyrene). ..............................................................................21

Scheme 3.1: Synthesis of PTMSS-b-poly(oligosaccharide) by click chemistry.

Oligosaccharide chemistry: (a) N-maltoheptaosyl-3-acetamido-1-

propyne (b) N-xyloglucooligosaccharide-3-acetamido-1-propyne (c)

mono-6A-N-propargylamino-6A-deoxy-β-cyclodextrin. Adapted with

permission from Cushen, et al. “Oligosaccharide/silicon-Containing

Block Copolymers with 5 nm Features for Lithographic

Applications.” ACS Nano 2012, 6, 3424–3433. Copyright 2012 ACS.

.....................................................................................................39

Scheme 3.2: Synthesis of PTMSS-b-PLA by anionic polymerization and then ring-

opening polymerization. Used with permission from Cushen, et al.

“Thin Film Self-Assembly of Poly (trimethylsilylstyrene-B-D,l-

Lactide) with Sub-10 nm Domains.” Macromolecules 2012, 45,

8722–8728. Copyright 2012 ACS. .............................................40

Scheme 3.3: Sequential anionic polymerization of styrene-styrene BCPs. ..........47

Scheme 3.4: Sequential anionic polymerization of PTMSS-b-P2VP. ..................47

1

Chapter 1: Patterning in microelectronic applications

1.1 HISTORY OF INTEGRATED CIRCUITS AND MODERN COMPUTING

It is hard to find a better example of the transformative powers of technology than

the semiconductor and the integrated circuit (IC). In the span of less than half a century,

computers and other devices based on these technologies went from being laboratory

demonstrations to essentially ubiquitous, effectively permeating every aspect of modern-

day society.

The first electronic computer in the world was the Electronic Numerical

Integrator and Computer (ENIAC), deployed initially in 1946.1 At full capacity, the

computer filled a 30 by 50 ft. room, weighed 30 tons, and drew 150 kW of power-

necessitating its own dedicated power lines. Commissioned by the military to calculate

artillery firing tables, the “general-purpose” tool was adapted to solve other complex

problems in fields as diverse as weather predictions and nuclear reactions. As Figure 1.1

shows, the ENIAC now sits decommissioned at its original home, the University of

Pennsylvania, partially due to advances in the IC, making the vacuum-tube technology

the ENIAC was based on obsolete.

2

Figure 1.1: The disassembled ENIAC sits on display at the University of Pennsylvania.

Copyright 2016 Penn Engineering.

The IC was made possible by the invention of the transistor by William Shockley,

John Bardeen, and Walter Brattain of Bell Labs in 1947, just one year after the startup of

the ENIAC. The implication of their discovery was that the function of the power-

hungry, prone to failure vacuum tubes in the ENIAC could be instead performed by the

solid state transistor. For their work, the trio was awarded the 1956 Nobel Prize in

Physics. The transistor computers that initially emerged consisted of individually

constructed components that were later assembled together. There was much resistance to

the idea of integrating multiple transistors on one device (the core concept of ICs). The

math was simple: for a 20 component device to have a 50% chance of working, each

individual component would need to have a success rate of 96.6%.2 Such device yields

3

were unheard of at the time. Of course, manufactures now ship working devices with

billions of working transistors.

It was not until 1958, more than a decade after the invention of the transistor, that

Jack Kilby of Texas Instruments demonstrated an electronic circuit made from a single

semiconducting material. Kilby was eventually awarded the 2000 Nobel Prize in Physics

for this achievement. In 1959, Robert Noyce of Fairchild Semiconductor demonstrated

their own IC, with several practical improvements over Kilby’s result. From that point

on, manufacturers rushed to produce improved and more powerful ICs. Although

advancements in ICs partially came about as the result of improved transistor structures,

the battle for improving devices was and continues to be largely a battle of scaling.

Scaling, the phenomenon where manufacturing improvements allow for a decrease in the

size of IC features, improves devices in two ways.2 First, device function is improved by

increasing transistor speed and decreasing power consumption. In effect, a smaller

transistor requires less time and less power to switch states. Scaling also helps by

decreasing device cost. Multiple individual ICs are made simultaneously on a single

wafer by a series of unit processes, so increasing the number of ICs on a wafer reduces

the cost per device. Making the size of individual transistors smaller increases the density

of transistors, which makes each transistor cheaper. Gordon Moore, later a founder of

Intel, famously observed in 1965 that the number of transistors on an IC was doubling

every year.3 Historically, this rate of doubling has actually been approximately every two

years, but the tenacity with which “Moore’s Law” has held is remarkable nonetheless.

An elegant and contemporary example of device improvement due to the

interplay between transistor design and scaling is the advent of FinFET transistors.

FinFET transistors, first employed commercially by Intel in 2012,4 improve upon the

design of traditional planar transistors. The three-dimensional transistor design, as shown

4

in Figure 1.2, offers more control over the flow of current through the transistor. The

effect is better transistor performance at lower operating voltages compared to planar

transistors. In addition to improved transistor design, manufacturers are able to make

these transistors smaller than the planar transistors of preceding generations, bringing

upon improvements due to scaling as well.

Figure 1.2: A tilted view SEM of Intel’s 22 nm FinFET Tri-Gate transistor. Image

courtesy of Maltiel Consulting. Copyright Intel Corporation.

At the forefront of manufacturing technologies used to make microelectronics is

lithography. Lithography is the process by which IC design information is actually sent to

the level of the wafer to form device relevant features.2 The limitations of lithography

have long gated the rate at which ability of manufacturers can make smaller features, and

the subject of this dissertation is materials and processes that can potentially improve the

resolution of existing manufacturing technologies.

5

1.2 PHOTOLITHOGRAPHY

The chief form of lithography used today in mass production of ICs is

photolithography.5 In this process, light is projected onto a substrate through a mask

which contains the design information of the IC. The substrate itself is coated with a

light-sensitive polymer called a photoresist. The photoresist interacts with the photons to

either become more soluble (positive-tone) or less soluble (negative-tone) in a specific

solvent called a developer. After development, one is left with a relief image that can be

transferred into the underlying substrate to pattern the metal or dielectric layers that

constitute the transistor and the connecting wires. Very rarely does the photoresist

material actually become incorporated in the final IC product, but the crucial role of

photoresist in enabling modern computing cannot be overstated.

The chief metric of a photolithography technology is the ultimate resolution or

“critical dimension” that the technology can produce. It is photolithographic resolution

that limits the size of transistors. The light source, the optics used to manipulate the light

source, the mask, and the photoresist all affect this value. Rayleigh’s criterion2 can be

applied to quantify these contributions:

𝐶𝐷 = 𝑘1

𝜆

𝑁𝐴 (1.1)

where k1 is a process parameter constant, λ is the wavelength of the light source, and NA

is the numerical aperture of the lens. Advancements in the resolution of photolithography

have come through advancements in these variables. k1 quantifies the ability of the

photoresist to distinguish between small changes in light dose, and is theoretically limited

to 0.25 (typical values used today are 0.3 to 0.8, and it is difficult to imagine any

remaining routes to decrease k1).2 Improvements in design and processing of photoresists

have largely contributed to improvements in k1,6 although technologies such as phase-

6

shift masks have also contributed.7 Decreasing the λ of the light source has been another

traditional way to increase resolution. For example, mercury arc lamps operating at 365

nm have been replaced with krypton fluoride and argon fluoride excimer lasers operating

at 248 and 193 nm, respectively.5 However, efforts to decrease λ further to 157 nm have

been abandoned due to issues with optics at these wavelengths, and extreme ultraviolet

(EUV) lithography operating at 13.5 nm still faces challenges in terms of throughput,

photoresist sensitivity, and defectivity.8 Last, NA, the measure of the optics of the system,

also already operates near physical limits. The most recent improvement in NA was the

incorporation of water as the material between the objective lens and the substrate, a

practice known as immersion lithography.8 The increase in refractive index brought about

by replacing air with water allowed an increase in NA above 1.0, but efforts to replace

water with an even higher refractive index material have stalled. At the time of this

writing, manufactures print features with immersion lithography very near the 36 nm half

pitch theoretical limit of this technology, and EUV lithography is still not commercially

viable.

A potential alternative to photolithography is step and flash imprint lithography

(SFIL), now known as nanoimprint lithography (NIL).9 In NIL, a template is used to

“stamp” a pattern into a fluid. A blanket ultraviolet exposure hardens the fluid, and the

template is removed to leave behind a topographic pattern. NIL has resolution below that

of immersion lithography,10 and sufficient throughput to be considered viable for

manufacturing.11 However, concerns about defectivity due to contacting the wafer with

the template have hampered but not stopped NIL deployment for manufacturing.

To sidestep the current lack of improvement in photolithography resolution,

manufacturers have looked to multi-patterning solutions to pattern ever smaller

features.12 For example, various flavors of “litho-etch-litho-etch” (LELE),13 “litho-freeze-

7

litho-etch”,14 and “litho-litho-etch”15 have been shown to surpass the 36 nm half pitch

limit by using two (or more) photolithography steps to pattern photoresist on the same

layer. Another multi-patterning technique, self-aligned double patterning (SADP), is

already used in production to double resolution by using deposition and etch steps to

create spacers around an initial, photolithographically defined feature.12,13 Since every

feature has two sidewalls, creating spacers on the sidewalls effectively halves the pitch.

Both of these solutions suffer from an increase in complexity and cost for each step,

which is compounded with poor device yields. As shown in Figure 1.3, to create device

relevant features with SADP, complicated block mask patterns need to be applied to the

features. This involves additional immersion lithography steps, as well as etch and

deposition steps. LELE also requires multiple expensive immersion lithography steps and

is highly dependent on overlay precision to produce correct structures.

8

Figure 1.3: Cross-section (left) and overhead view (right) of SADP process. Copyright

David Abercrombie.13

Despite the cost drawbacks of SADP, its viability for manufacturing has been

demonstrated. However, efforts to go beyond simply double patterning and enter the

realm of self-aligned multiple patterning (SAMP) increase the cost and complexity even

further.16 At some point, an SAMP-like process will probably fail to meet the yield and

cost requirements for manufacturing. Table 1.1 shows the smallest sizes of Intel’s recent

commercial devices17 and Table 1.2 shows the International Technology Roadmap for

Semiconductors (ITRS) technology trends targets18 for upcoming nodes. These values

reinforce the fact that the industry is already operating below immersion lithography’s

~36 nm half pitch fundamental limit by using SADP and that aggressive scaling is

required to make features even smaller.

9

Table 1.1 Smallest dimensions of Intel’s commercial devices

Node Fin ½ pitch (nm) Interconnect ½ pitch (nm)

22 30 40

14 21 26

Table 1.2 2013 ITRS technology trends target

Node Logic ½ pitch

(nm)

Flash ½ pitch

(nm)

DRAM ½ pitch

(nm)

FinFET ½ pitch

(nm)

16/14 40 18 28 30

10 32 15 24 24

7 25 13 20 19

5 20 11 17 15

3.5 16 9 14 12

As an alternative method to these complicated pattern multiplication processes,

the self-assembly of block copolymers (BCPs) is being investigated for lithography.

BCPs, a class of polymers specifically containing two or more chemically distinct

polymers connected by a covalent bond, phase separate much like oil and water.

However, due to the covalent bond between the two blocks, the phase separation occurs

on the macromolecular length scale, approximately 5 to 50 nm full pitch, which

coincidentally is approximately equivalent to the target pitch sizes for upcoming

semiconductor nodes. The topic of this dissertation is a class of BCPs that contain silicon

in one of the two blocks. The silicon imparts numerous favorable lithographic

advantages, but also necessitates special processing and materials.

10

1.3 BCPS AND SELF-ASSEMBLY

BCPs can self-assemble into a variety of repeated structures based on volume

fraction (f), degree of polymerization (N), and chemical incompatibility of the

components (χ).19,20 Figure 1.4 shows a theoretical phase diagram of the simplest BCP, a

diblock copolymer.21 By changing f and the product χN, BCPs can be made to produce

spheres, cylinders, lamellae, or other, more complex morphologies. Importantly, theory

requires that χN, which quantifies segregation strength,22 be greater than at least 10.5 to

produce ordered structures.

Figure 1.4: Theoretical phase diagram of a diblock copolymer. f = volume fraction, L =

lamellae, G = gyroid, C = cylinders, S = spheres (bcc lattice), Scp = spheres

(close-packed lattice). Reproduced with permission from Matsen, M. W.

“The Standard Gaussian Model for Block Copolymer Melts.” J. Phys.

Condens. Matter 2002, 14, R21–R47. Copyright 2002 IOP Publishing Ltd.

The size of the lamellae that a symmetric diblock BCP phase separates into is

governed by thermodynamics and well described by Equation 1.2 for a symmetric

diblock:

𝐿0 = 𝛼𝜒1/6𝑁𝛿 (1.2)

11

where L0 is the equilibrium periodicity or the full pitch of a BCP, α is a constant

encompassing the statistical monomer length of the BCP, and δ is a constant that has

been calculated to be 1/2 in the weak segregation regime (χN ≈ 10) and 2/3 in the strong

segregation regime (χN >> 10).22 Regardless of segregation strength, L0 scales more

strongly with N than χ. Remembering that a threshold segregation strength is required to

form ordered features (χN > 10.5), the mathematical implication of this formula is that the

smallest features are realized at high χ and low N. This has been corroborated with many

experimental studies.23 Choosing or designing BCPs with high χ is a recurring theme in

this dissertation.

Of the morphologies predicted in Figure 1.4, the lamellar morphology is

anticipated to be most useful to make dense line-space patterns for IC fabrication, and the

BCPs featured in this work are all lamellae-forming with f ≈ 0.5.24 Although cylinders

have been demonstrated for line-space applications as well,25 concerns exist that these

structures will be unsuitable for pattern transfer.26 Cylinders have successfully been

demonstrated for via-hole shrink applications, but this area is not a focus of this

research.27

BCPs must be annealed to give mobility to the individual polymer chains and

enable the material to adopt its equilibrium morphology, both in thin films and in bulk.

Two methods of annealing polymers are thermal annealing and solvent annealing. In

thermal annealing, the polymer is heated above its glass transition temperature (Tg), and

then usually cooled rapidly to vitrify the equilibrium state. In solvent annealing, the

vapors of one or more solvents, sometimes chosen to be preferential for only one

component of the BCP, are brought into a chamber containing the BCP. The solvent

vapors are absorbed and plasticize the BCP and allow the macromolecular chains to

reorganize. Although solvent annealing has been used successfully in the laboratory,28 its

12

low rate of throughput and irreproducibility (among other reasons) make it unsuitable for

high-volume manufacturing. In contrast, thermal annealing can be very high throughput,

and can be readily implemented in a manufacturing environment since heating elements

are commonplace in modern IC fabrication.29

The spontaneous phase separation of BCPs produces features with randomly

aligned structures which lack long-range order in thin films. For a BCP to be used for

lithography, it must be aligned to a pre-pattern, a process known as directed self-

assembly (DSA). As Figure 1.5 shows,24 BCPs can be coaxed to align along a desired

trajectory, provided that the substrate is properly constructed. DSA is a blanket term for

many different approaches to align BCPs, only some of which are useful for

lithography.30 Chapters 4 and 5 discuss the DSA of some of the silicon-containing BCPs.

Figure 1.5: Top-down SEM showing a region of random fingerprint (left) next to a

region of directed self-assembly (right) of PS-b-PMMA. Reproduced with

permission from Stoykovich, et al. “Block Copolymers and Conventional

Lithography.” Mater. Today 2006, 9, 20–29. Copyright 2006 Elsevier.

1.4 BCP CHALLENGES AND OPPORTUNITIES IN LITHOGRAPHY

The implementation of BCP DSA for IC fabrication is contingent on the

resolution of a number of issues. At a high level, low defectivity is absolutely necessary

for this technology, or any other technology, to be implemented in manufacturing.

13

Currently, the best defectivity numbers for DSA are on the order of 10 cm-2.31 Although

this is an absurdly low number of defects from a macroscopic perspective given the

nanometer length-scales in question, this degree of defectivity is still orders of magnitude

too high for manufacturing. Alongside defectivity, a cost-effective implementation of

DSA into existing processes is necessary for manufacturing.32 Both of these issues must

be investigated on a commercial scale and not a laboratory scale.

An issue that can be tackled on a laboratory scale is BCP design for lithography,

especially in terms of scaling. The most ubiquitous BCP for line-space applications is

poly(styrene)-b-poly(methyl methacrylate) (PS-b-PMMA). Because of a number of

favorable thin film properties, this BCP has become the standard for DSA studies.

However, due to its relatively low χ, the BCP is limited to forming approximately 22 nm

full pitch features.33 Comparing the scaling requirements predicted in Table 1.2 with the

resolution limit of PS-b-PMMA shows that this material cannot meet all foresee-able

requirements, and necessitates the design of new materials.

For a BCP to be useful for lithography, a number of properties need to be

demonstrated. First, of course, the BCP needs to be able to form features substantially

smaller than 20 nm full pitch to compete with PS-b-PMMA. This usually means that its χ

needs to be higher than that of PS-b-PMMA. Second, there needs to exist a method to

selectively remove one of the two blocks to produce a topographic etch mask. This is not

always trivial, especially if both BCP polymers are composed entirely of organic

materials. In the case of PS-b-PMMA, the chemical dissimilarity between the styrene and

the methacrylate enables the methacrylate to be removed while leaving behind the

styrene. Last, the BCP lamellae need to be oriented perpendicular to the substrate in thin

films. Disparate surface energies at either the top or bottom interfaces cause one of the

two components of a BCP to segregate to that interface, forming a wetting layer and

14

driving the orientation of the features parallel to the substrate, resulting in ultimately

lithographically useless structures. PS-b-PMMA is unique in that its two components

have approximately the same surface tensions at elevated temperatures, so it can generate

perpendicularly oriented structures upon thermal annealing.

In the Willson and Ellison groups we have investigated incorporating silicon into

one of the two BCP components to create materials suitable for lithography. The

incorporation of silicon solves a number of issues. First, silicon-containing polymers are

known to be generally resistant to oxygen etch, permitting at least one direct route to high

aspect ratio mask features.34,35 Second, organo-silicon moieties, such as the trimethyl

silyl, tert-butyldimethylsilyl, and tert-butyldiphenylsilyl ether protecting groups in

organic chemistry, are known to be very hydrophobic. The inherent hydrophobicity of the

desired silicon groups opens the door to BCPs with high χ–values as a large enthalpic

penalty is realized when mixing hydrophobic components with more hydrophilic or polar

moieties. However, silicon-containing BCPs suffer from the drawback that their

processing in thin films is more difficult than that of PS-b-PMMA. The hydrophobicity of

organosilicon, while advantageous for χ, makes the silicon-containing block very low in

surface tension, driving the formation of wetting layers with the top interface. This is

shown in Figure 1.6. Despite the perfect “neutralization” of the bottom surface, the BCPs

interactions with the air interface forces the features at the top to lie parallel to this

interface. Although in this specific example one could imagine etching away the top

interface and only using the perpendicularly oriented features at the bottom of the film for

patterning, in practice the film thicknesses used for DSA are much thinner than in this

SEM, and the BCP interaction with the air interface dominates the BCP behavior. Much

early work in the Willson and Ellison groups focused on controlling this orientation

behavior and at present we declare success in controlling both top and bottom interface.

15

Figure 1.6: Silicon-containing BCP wetting layer formation despite the neutralization of

the bottom interface. Image courtesy of Michael J. Maher and IBM

Almaden.

1.5 GOALS OF THIS DISSERTATION

This dissertation aims to provide a summary of silicon-containing BCPs designed

specifically for lithography, as well as two different demonstrations of DSA using these

materials. Chapter 2 provides an overview of the materials and methods used to control

BCP behavior in thin films. Chapter 3 introduces a number of silicon-containing BCPs

along with a comparison of their properties. Chapters 4 and 5 describe two different

implementations of directed self-assembly (DSA) with these materials to make device-

relevant features. It is the intent of the author not only to provide details about the

specific materials and processes used in this document, but also to generalize the results

of BCP design and DSA implementation to guide future developments in this field.

16

Chapter 2: Methods and materials for BCP thin film control

This chapter surveys the techniques and tools developed in the Willson and

Ellison labs to control the orientation behavior of BCPs in thin films. Much of this work

has been already described in the dissertations of Christopher Bates,36 Michael Maher,37

and William Durand,38 and the intent of this Chapter is only to provide enough context to

understand some of the pertinent studies utilized later in the manuscript. The reader is

encouraged to seek out the previous dissertations or the published literature39–41 for more

in-depth descriptions of the topics discussed here. Portions of this Chapter have been

adapted from Maher, M. J.; Bates, C. M.; Blachut, G.; Sirard, S.; Self, J. L.; Carlson, M.

C.; Dean, L. M.; Cushen, J. D.; Durand, W. J.; Hayes, C. O.; Ellison, C. J.; Willson, C. G.

Interfacial Design for Block Copolymer Thin Films. Chem. Mater. 2014, 26 (3), 1471–

1479 (the author provided the BCP samples and characterization information).

2.1 SURFACE NEUTRALIZATION

As stated in the introduction, the key principle for generating perpendicular

lamellar BCP features in thin films is the neutralization of both the top and bottom

interfaces for each BCP component. Neutralization is more formally described as the

process by which each surface the BCP interacts with is made to be non-preferential

towards either block. The mathematical formalism of a neutral surface is expressed as

Equation 2.1:

𝛾𝐴,𝑖 = 𝛾𝐵,𝑖 (2.1)

where γ is the surface tension of block A or B with interface i. From just a surface energy

perspective, the enthalpic penalty incurred for both BCP blocks interfacing with a neutral

surface is identical to the penalty for just one block interfacing with a surface (to form a

wetting layer), not guaranteeing perpendicular orientation on its own. However, there are

17

additional enthalpic forces at play in the form of the energetics of chain stretching or

compression.42,43 Perpendicularly oriented features have fewer stretching penalties

compared to features oriented parallel to the substrate since the BCP chains are aligned

parallel to the substrate and are subjected to essentially no confining force by the thin

film. The thin film confinement forces that act on parallel BCP features (perpendicular

chains) are discussed shortly in the context of island-hole testing.

In practice, a surface is made neutral for BCPs by applying some kind of chemical

treatment to change the surface tension. Although this can be done with monolayers of

small molecules,44,45 typically for DSA applications this is done by using statistically

random copolymers consisting of two or more monomers. Importantly, the random

copolymers can be composed of the monomers that comprise the BCP46,47 or of

completely different monomers,48,49 depending on the application. As Figure 2.1 shows,

the surface neutralization treatment (SNT) needs to be rendered insoluble by some

mechanism so that the BCP can be spin-coated directly on top. The two following

sections discuss two approaches to insolubilize the SNT: cross-linking and grafting.

Figure 2.1: Process used to neutralize a substrate for a BCP. Image courtesy of

Christopher Bates.

As already explained, for silicon-containing BCPs, and most BCPs besides PS-b-

PMMA, an SNT is also needed to neutralize the top interface to generate perpendicular

features. However, there is no way to solution-deposit a conventional SNT on top of the

BCP. Any solvent (for spin-coating or in general) that dissolves a neutral SNT is also

18

inevitably able to dissolve the BCP and destroy the filmstack. To circumvent this spin-

coating limitation, polarity-switching top coats50 were developed that enable SNTs to be

coated directly on top of the BCP out of a non-destructive, polar solvent, but still produce

a non-preferential substrate for neutralization. These materials will also be discussed

shortly.

2.1.1 Cross-linkable polymeric materials

One way to render the SNT insoluble on the substrate is to cross-link the polymer

after spin-coating. SNTs made by cross-linking are typically referred to as mats.

Azide48,51 and benzocyclobutene51–53 moieties have been used for this purpose as both are

stable at room temperature but cross-link at elevated temperature ( > 200 °C). In addition,

azide cross-linking can be performed by UV exposure.51

The cross-linkable materials developed in the Willson and Ellison groups, known

specifically as XSTs, were inspired by the disparate surface energies accessible by

random copolymers of poly(methyl methacrylate) (PMMA) and poly(4-tert-butylstyrene)

(PtBuSty).54 Both of the corresponding monomers are commercially available,

facilitating the synthesis of a vast library of XSTs with vastly different surface energies.39

For cross-linking, vinylbenzylchloride was incorporated into the polymer at

approximately 3-6 mol %. In a post-polymerization modification step, the chlorine was

displaced by the nucleophilic substitution of sodium azide to install azido-functionality

into the polymer. The polymer could then be cross-linked by heating. Figure 2.2 shows

the general structure of the XSTs.

19

Figure 2.2: General structure of the XST materials made by free radical polymerization.

In a typical procedure, a cross-linkable polymer was dissolved in a suitable spin-

coating solvent and diluted to give the desired film thickness. Typically, an

approximately 0.4 wt % solution yielded a 10 nm film at ~3000 rpm. The wafer was then

baked at 250 °C for five minutes. Although in the laboratory the bake was typically done

open to air, it can also be performed under vacuum or under the purge of an inert gas. The

wafer was cooled and washed with organic solvents to rinse away uncross-linked

material, and then soft-baked. Typically, no more than 1-2 nm of film thickness is lost

during the wash step, so the final XST film thickness is largely determined by the spin-

coating step. In addition, the deposition by spin-coating produces a more-or-less

conformal coating of the XST mat on the substrate, precluding area selective deposition

of the materials.

2.1.2 Graftable polymeric brushes

The other way by which an SNT can be insolubilized is through a grafting

mechanism. Directly analogous to self-assembled monolayers of small molecules,

polymer “brushes” are functionalized to have surface-reactive groups (often just one) that

enable covalent bonding of the polymer to a substrate.46 The standard functional group

for substrate grafting is an alcohol, which grafts by some poorly defined condensation

reaction with other hydroxyl groups present on the substrate.47,55,56 However, there are a

20

number of other functional groups, including perhaps most famously silanes like HMDS,

which can perform this function as well.

There are a number of differences between brushes and mats in their practical

uses. Since brushes only graft to the substrate in regions of some defined chemistry, it is

possible to selectively deposit brushes on a wafer, for example by patterning and

etching.57 This property makes brushes critical for DSA applications. In contrast, mats

indiscriminately cover the substrate. In addition, since at maximum a monolayer of brush

grafts to the surface the final film thickness of a brush layer is on the order of 5 – 8 nm,

regardless of initial coating thickness.58 If an application requires an SNT film thicker

than approximately 8 nm, mats need to be used instead.

In the Willson and Ellison groups, the need arose to make a generic family of

brushes whose surface energies spanned a reasonably large range in order to be relevant

to a variety of BCPs. In addition, it was deemed necessary to use a polymer structure with

only one grafting group to replicate as closely as possible materials previously reported to

successfully produce DSA59 (although there is no published evidence in the literature that

a polymer with multiple grafting groups is less optimal for this task). Controlled radical

polymerization (CRP) was identified as the ideal method to produce a library of brush

materials on large scales, where the single grafting point could be installed in the

initiator. A hydroxyl-terminated initiator for nitroxide-mediated polymerization (NMP),60

a form of CRP, had been used for this purpose previously.61 However, the quality of

acrylic polymers produced by NMP with the most common reaction mediator is not great,

so replicating the same PMMA-r-PtBuSty chemistry used in the XSTs was not ideal.

Instead, as Scheme 2.1 shows, random copolymers were made by combining styrene and

4-tert-butylstyrene with a hydroxyl-terminated NMP initiator. These materials are

discussed in context in Chapter 4.

21

Scheme 2.1: Controlled radical polymerization of poly(styrene)-r-poly(4-tert-

butylstyrene).

2.1.3 Polarity-switching top coats

The last SNTs developed in the Willson and Ellison groups are polarity-switching

top coats. Again, the goal was to have a spin-coatable surface neutralization strategy for

the top interface, but the solubility of the BCP in organic spin-coating solvents precluded

the use of copolymers like those described above. Instead, a spin-coatable top coat would

have to be deposited out of a polar solvent like IPA, methanol, or water. However, any

polymer soluble in methanol or water is inherently not going to be neutral for the BCP,

rather, it will prefer the non-silicon, non-hydrophobic block. To circumvent this solubility

catch-22, a polarity-switch is required. Maleic anhydride was discovered to have suitable

chemistry.50 As the thin film IR data in Figure 2.3 shows, polymers of maleic anhydride

can be ring-opened with a base such as trimethylamine (TMA) to produce a polymeric

salt soluble in IPA, methanol, or water, and then ring-closed by heating to regenerate the

original polymer.

22

Figure 2.3: Thin film IR spectra of top coat as cast from amyl acetate (blue curve), the

corresponding TMA salt case from methanol (red curve), and the TMA salt

after heating at 190 °C for 30 s (green curve). Used with permission from

Maher, et al. “Interfacial Design for Block Copolymer Thin Films.” Chem.

Mater. 2014, 26, 1471–1479. Copyright 2014 ACS.

Among the many maleic anhydride-containing polymers surveyed, polymers

pairing maleic anhydride with styrene and styrene-derivatives emerged as the best

polymer structure. The alternating copolymerization of maleic anhydride with styrene

and styrene-derivatives creates an advantageous situation where the resulting polymer has

a predictable composition determined by the feed ratio of monomers, leading to a logical

progression in surface energies of the top coats. This is in contrast to other surveyed top

23

coats50 whose components had much different reactivity ratios, leading to polymers with

non-uniform compositions and ultimately in a lack of reproducibility in the materials.

Figure 2.4 shows the chemical structure of the most successful top coat family,

poly(styrene-alt-maleic anhydride)-r-poly(3,5-di-tert-butylstyrene-alt-maleic anhydride),

used to neutralize the top interfaces of all the BCPs that will be discussed in Chapter 3.

As already discussed, these top coats were deposited as ring-opened trimethylamine

(TMA) salts, and not the as-synthesized, ring-closed form depicted in Figure 2.4. By

changing the ratio of styrene (the high surface energy component) to 3,5-di-tert-

butylstyrene (the low surface energy components) in the feed, the surface energies of the

resulting polymer top coats could be controlled monotonically.

Figure 2.4: Structure of poly(styrene-alt-maleic anhydride)-r-poly(3,5-di-tert-

butylstyrene-alt-maleic anhydride) top coat.

2.2 ISLAND-HOLE TESTING

Determining the composition of a neutral SNT for a given BCP is a process that

needs to be performed empirically. A common misconception is that the surface tension

of a polymer with air or with a liquid can be correlated directly to its surface tension with

another polymer. Although some information about surface energy can be inferred from

indirect measurements, the complexity of surface interactions necessitates an empirical

approach. Fortunately, there exist techniques that allow for BCP interactions with

surfaces to be unambiguously determined, which enables the determination of neutral

24

surfaces. These techniques depend on the formation of BCP terraces to quantized values

of the periodicity, known as “island-hole” tests.62

2.2.1 Origin of topography

When a BCP is annealed with at least one free surface, the domains orient

themselves so that each interface is wetted by the BCP for which it has the greatest

affinity. As Figure 2.5 shows, if both interfaces are wet by the same component, the BCP

quantizes to integer multiples of L0 (“symmetric” wetting) while if each interface is wet

by different blocks, the BCP quantizes to half-integer multiples of L0 instead

(“asymmetric” wetting).62

Figure 2.5 (A) Symmetric and (B) asymmetric BCP wetting in thin films. In (A), islands

are formed, while in (B), holes are formed. Used with permission from

Maher, et al. “Interfacial Design for Block Copolymer Thin Films.” Chem.

Mater. 2014, 26, 1471–1479. Copyright 2014 ACS.

If the BCP film thickness is set by spin-coating to be incommensurate to either of

these values, the BCP still quantizes to either full or half L0 multiples, but does so at two

different terrace heights above and below the initial film thickness due to the

conservation of mass (also shown in Figure 2.5). The situation where the majority of the

film rises up to a quantized multiple, and a minority of the film lowers itself to that

multiple minus one, results in a situation where “holes” in the film structure are

observable by top-down AFM or optical microscopy. Conversely, the opposite scenario

also exists where the majority of the film lowers itself to a discreet value, and only a

25

minor portion of the film rises up, resulting in microscopic “islands”. Figure 2.6 shows a

representative AFM micrograph of islands to illustrate how such a film might appear. The

initial film thickness, plus the observation of islands or holes, allows one to infer whether

the BCP is engaging in symmetric or asymmetric wetting behavior. If the identity of the

preferential block of one interface is known, the identity of the second block can be

inferred. In this way, one can qualitatively determine if a candidate SNT prefers one

block or the other, which enables subsequent SNTs to be chosen to hone in on a neutral

surface.

Figure 2.6: Three-dimensional representation of AFM micrograph showing

approximately 20 nm islands.

2.2.2 Half-height (0.5 periodicity) topography

In addition to using the island-hole test to narrow down the range of a neutral

SNT composition, a new island-hole phenomenon was discovered that was indicative of a

neutral substrate. When a BCP is annealed with one preferential surface and one neutral

surface, the BCP quantizes to symmetric and asymmetric simultaneously in different

regions of the film.39,63 As seen in Figure 2.7, this is due to both components of parallel-

oriented lamellae interfacing with one surface. The resulting topography manifests itself

26

as features that are only 0.5 L0 tall, a height discernable from 1.0 L0 by AFM. The

presence of half-features indicates that a surface is neutral, and confining a BCP between

two surfaces independently determined to be neutral results in the perpendicular

orientation of the BCP features. The ability to unambiguously verify that a surface is

neutral removes the “guess-and-check” process used previously to screen BCPs for

desired perpendicular orientation in thin films.

Figure 2.7 Illustration of the origin of 0.5 L0 step heights when a BCP interfaces with one

neutral and one preferential surface. Used with permission from Kim, et al.

“Consequences of Surface Neutralization in Diblock Copolymer Thin

Films.” ACS Nano 2013, 7, 9905–9919. Copyright 2013 ACS.

New evidence suggests that half features may only be kinetically trapped features

and that full L0 features are the system’s lowest energy state at equilibrium. Experiments

demonstrated that BCP films on “neutral” surface treatments, originally showing half L0

features, slowly formed full L0 features as thermal annealing was continued. Often, the

resulting topographies became very hard to interpret as they displayed a superposition of

both half and full features. Half features were seen to transform into both full islands and

full holes, implying that it was possible transition to both symmetric and asymmetric

wetting. Furthermore, it was observed that surfaces “further” away from neutral

generated full features at shorter time intervals than surfaces “closer” to the anticipated

neutral composition. The implication of the kinetic nature of BCP half/full topography on

BCP thin film orientation is yet to be fully appreciated. At this point in time, the evidence

27

suggests that as long as both surfaces (top and bottom treatments) produce half features

when tested individually, then the BCP will always orient itself perpendicular to the

substrate in thin films. At this point in time, it is not clear if there is a threshold “off-

neutral” value which generates half features but does not produce defect-free orientation

of the BCP features, or conversely, if it is possible to find a “perfectly-neutral” substrate

that never devolves into full features.

2.2.3 Confined island-hole

Alongside the synthesis of a library of alternating top coats with a range of

predictable surface energies came the discovery that certain BCPs could form island-hole

structures under a top coat,39 which came to be known as the “confined” island-hole test

and further drove the development of neutral surfaces. The typical island-hole test has the

BCP on top of a surface of interest. Due to the presence of a free interface (the top

interface), a BCP can readily form the island-hole topography used as a diagnostic.

However, the ability of BCPs to form the same topography underneath a supposedly rigid

surface (the top coat) had never been reported to the best of our knowledge. The

experiment is now reversed: a cross-linkable layer of a polymer highly preferential to one

of the two components of a BCP is first coated and annealed, the BCP is then coated, and

then the TMA salt of the top coat is coated last. Annealing the wafer above 160 °C

(typically 180-200 °C) ring-closes the top coat and allows island and holes to form. The

presence of symmetric or asymmetric BCP wetting can be translated directly to top coat

wetting preference because the wetting preference of the bottom surface is fixed when

one chooses the chemistry of this surface. The confined island-hole test, along with

predictable top coat surface energy due to the synthesis method employed, allows one to

hone in on a neutral top coat.

28

An important caveat is that not every polymer in a BCP actually performs the

confined island-hole test. Table 2.1 lists specific polymers that have and have not been

observed to form island-hole features (both full and half) under a top coat. For a BCP to

form confined island-hole features, both of its blocks need to be able to form confined

features (for example, PS-b-PMMA, a BCP composed of a block that is known to form

confined island-holes (PS) and a BCP of a block that is not known to form confined

features (PMMA) will not form confined features). This list was compiled by spin-

coating top coats on various BCPs with different pairs of blocks, annealing the sample,

and checking for the formation of island-hole topography. For example, it was

determined that polystyrene-b-poly(4-trimethylsilylstyrene) (PS-b-PTMSS) readily

formed confined island-hole features, but neither poly(styrene)-b-poly(t-butyl acrylate)

nor poly(4-trimethylsilylstyrene)-b-poly(methyl methacrylate) were ever observed to. It

stands to reason that the incorporation of poly(t-butyl acrylate) and poly(methyl

methacrylate) somehow arrests the formation of confined features.

Table 2.1: List of polymers that can and cannot form confined island-hole features

29

At this point it is not clear what property governs why some polymers produce

features under a top coat and others do not. From Table 2.1, essentially every polystyrene

derivative has been seen to produce confined island-holes, but the close structural

analogue poly(2-vinylpyridine) (P2VP) has not. Most acrylates and methacrylates do not

form confined features, but there is at least one exception in the silicon-containing

methacrylate, poly(methyl trimethylsilyl methacrylate). Even stranger, the structures of

polylactide and poly(ethyl glycolide) differ only by replacing two methyl groups with

two ethyl groups, yet polylactide does not form confined features while poly(ethyl

glycolide) does. One could imagine that the viscosity of the components could play a

role- with lower viscosity components being able to form confined topography more

readily- but poly(dimethylsiloxane), which is a liquid at room temperature and

presumably extremely viscous at >160 °C annealing temperatures, does not form

confined-island holes. A crucial piece missing to the puzzle is actual behavior of the BCP

in situations where confined island-hole features do not form. Presumably, the BCP is

still rearranging itself to phase-separate and to preferentially wet both interfaces. It has

been documented that certain BCPs can change their effective L0’s when confined by

rigid substrate if the film thickness is incommensurate with the BCP L0.64 Perhaps for

some polymers the energetic penalty of stretching or compressing in thin films is less

than the penalty for forming topography, preventing the appearance of confined island-

holes. Further studies- both experimental and modelling- are needed to determine what

BCPs do if they do not form island-hole morphologies, and the factors that determine

either behavior.

30

2.2.4 Representative island-hole test

Figure 2.8 shows the results of a representative island-hole test, in this case, a

confined island-hole test. The 18 nm PS-b-PTMSS was spin-coated at either 1.19 L0 (~21

nm) or 1.36 L0 (~24 nm) onto an approximately 10 nm cross-linked PS film, known to

preferentially wet the PS block. The candidate top coats, all of the chemistry depicted in

Figure 2.4, were deposited on top of the BCP and the entire film stack was annealed (the

number in the label corresponds to the mol % of the styrenic component that is 3,5-di-

tert-butylstyrene, while the rest is styrene).

Figure 2.8: Sample confined island-hole test of 18 nm PS-b-PTMSS on cross-linked PS.

The top labels are the chemistry of the top coat (number corresponds to the

mol % of the styrenic component that is 3,5-di-tert-butylstyrene. Rest is

styrene). Images are AFM micrographs at the labelled BCP film thickness.

Bottom graphs are traces of the topography. Used with permission from

Maher, et al. “Interfacial Design for Block Copolymer Thin Films.” Chem.

Mater. 2014, 26, 1471–1479. Copyright 2014 ACS.

In the left-hand side image, the BCP interfaces with a very hydrophobic surface,

drawing the low surface energy PTMSS component to the top of the film. The bottom

31

substrate is again PS wetting, so asymmetric wetting occurs. For the BCP to accomplish

this when at 1.19 L0, the majority of the film rises to 1.5 L0, and a small amount decreases

to 0.5 L0, and holes are formed. Conversely, in the right-hand side image, PS wets both

top and bottom interface, causing the symmetric wetting scenario to perform holes. In-

between these two extremes is a top coat composition that produces half-islands and half-

holes. The AFM height trace clearly shows that these features are only approximately

half the size of the full features on either side. In addition, both island-like and hole-life

features can be seen within the same 0.5 L0 interval (1.0 L0 < BCP film thickness < 1.5

L0), which provides further evidence for neutrality.

2.3 CONSEQUENCES OF NEUTRAL SURFACES

The chief consequence of identifying and verifying the neutrality of a given

surface is the ability to make perpendicularly oriented BCP structures. As shown in

Figure 2.9, a BCP sandwiched between two neutral SNTs produce lamellar structures

that span the entire thickness of the film, which makes them viable structures for

patterning the underlying substrate.

32

Figure 2.9: Tilted SEM of 22 nm PS-b-PTMSS with the PS block fully removed

showing high-aspect ratio features that persist through the entire film. Used

with permission from Maher, et al. “Interfacial Design for Block Copolymer

Thin Films.” Chem. Mater. 2014, 26, 1471–1479. Copyright 2014 ACS.

In addition, it was discovered that neutralizing just one surface provided much

leeway in the neutrality of the other surface.65 If one surface was identified as being

perfectly neutral for a BCP, the other surface had a window of “near neutrality” where a

range of compositions and surface energies could produce perpendicularly oriented

features, depending on the film thickness and other parameters. This has favorable

implications for applications where the neutrality of a surface cannot be explicitly

determined, or perhaps the surface energy changes during processing. However, work

suggests that BCP through-film structure is not necessarily guaranteed to be perfectly

perpendicular to the substrate if the surfaces are not neutral.66 The importance of this

discovery depends on the final application.

2.4 Conclusion

In conclusion, three different types of SNTs for silicon-containing BCPs were

introduced and described. They differ in their constituent chemistries, which has

implications on how they are used in thin films. In addition, the island-hole technique

33

used to determine neutral surfaces is described. The next Chapter introduces the BCPs

that were found to be compatible with the polarity-switching top coat SNTs.

34

Chapter 3: Design of silicon-containing block copolymers for

lithography

The work in this chapter has been reproduced in part with permission from

Durand, W. J.; Blachut, G.; Maher, M. J.; Sirard, S.; Tein, S.; Carlson, M. C.; Asano, Y.;

Zhou, S. X.; Lane, A. P.; Bates, C. M.; Ellison, C. J.; Willson, C. G. “Design of High-χ

Block Copolymers for Lithography.” J. Polym. Sci. Part A Polym. Chem. 2015, 53, 344–

352. Copyright 2015 Wiley.40 For this manuscript, the author prepared the BCP samples,

provided characterization data for the BCPs, and helped design the experiments.

3.1 BACKGROUND INFORMATION

Poly(styrene)-b-poly(methyl methacrylate) (PS-b-PMMA) is the most studied

BCP for line-space applications. Numerous examples of PS-b-PMMA being used for

frequency densification have been reported for bit-patterned media67,68 and semi-

conductor69,70 applications. The key property of PS-b-PMMA that makes it so ubiquitous

is that neither component of PS-b-PMMA segregates preferentially to the top interface at

elevated temperatures, resulting in the spontaneous production of perpendicularly

oriented features. An additional beneficial property is that the difference in chemistry of

the two blocks (aryl vs. ester) produces enough contrast to enable preferential removal of

the PMMA in laboratory71 and industrial72 reactive ion etch systems. However, PS-b-

PMMA’s relatively modest χ-value73 precludes it from forming lithographically useful

features below approximately 20 nm.33

Much effort has been put towards making BCPs with χ-values higher than PS-b-

PMMA to provide access to smaller feature sizes.23 For a BCP to be useful for

lithography, however, it needs to satisfy at least the following constraints:

35

1. The constituent blocks must be sufficiently different to yield a χ and L0

adequate for the application.

2. One of the blocks needs to be removed with reactive ion etching and leave

behind the other domain as an etch mask.

3. Lamellar features need to be oriented perpendicularly in thin films.

Much has already been said about these three design constraints. For applications

that require high throughput, such as high-volume manufacturing,74 the following design

constraints are added:

4. Processing must be performed on time-scales and with methods commensurate

with wafer throughput requirements.

5. The BCP etch mask needs to be able to directly pattern the underlying layer.

These constraints reinforce the need for a process that uses only spin-coating and

thermal annealing, as these unit operations are already deployed in manufacturing

facilities. It furthermore precludes processes like sequential infiltration synthesis,75 used

to imbue organic-only BCPs with etch contrast, since these processes necessitate

additional unit operations.

At UT, it was decided to pursue silicon-containing BCPs as materials for

lithography. Silicon-containing polymers are known to be etch resistant in an oxidizing

plasma environment,34,40 providing at least one direct route to isolating domains for mask

formation. In addition, organo-silicon polymers are exceptionally hydrophobic. Pairing

the hydrophobic polymers with more polar polymers opens the door to high-χ values due

to the enthalpic penalty of mixing the two materials. However, this hydrophobicity is a

double-edged sword. As discussed in Chapter 2, the silicon-containing domains readily

segregate to the top surface due to their low surface energies, forming wetting layers and

36

driving the lamellar features parallel to the substrate. Of course, the polarity-switching

top coat strategy39,50 was successfully employed to neutralize the top interfaces of these

materials and control thin film orientation.

This chapter introduces efforts by other researchers to make high-χ BCPs and then

details the work done in the Willson and Ellison Groups. A number of silicon-containing

BCPs with various chemistries and χ-values are presented and discussed. The

characteristics of these materials are then used to speculate about the nature of BCPs that

can make features as small as 8 or 6 nm.

3.2 PREVIOUS WORK IN HIGH-Χ BCPS

Much work has been performed to synthesize BCPs with χ and ultimate resolution

better than that of PS-b-PMMA. Although there are many examples of clever and

ingenious strategies to increase the incompatibility of the components, the examples

presented here fail to satisfy one or more lithographic constraints, necessitating the

development of silicon-containing BCPs compatible with the top coat strategy.

Much work has been done on high-χ BCPs made with poly(dimethylsiloxane)

(PDMS). This hydrophobic polymer has been paired with more polar polymers like

polystyrene (PS),76 poly(methyl methacrylate) (PMMA),77 and poly(2-vinylpyridine)

(P2VP)78 to produce materials that can form features much smaller than 20 nm full pitch.

In addition, the silicon-containing PDMS is used to form a hard etch mask, just like in the

strategy employed at UT. Unfortunately, the wildly disparate surface energies of PDMS

and organic polymers cause PDMS-containing BCPs to form wetting layers. To

circumvent this, researchers have relied on cylinder-forming BCPs of PDMS instead of

lamellae-forming. The PDMS wetting layer is removed by a fluorine plasma chemistry,

and then an oxygen-containing plasma is used to develop the cylinders, which lie down in

37

the film. Concerns remain about the viability of lying down cylinders as hard masks.26

Due to hexagonal packing, only one layer of cylinders can be used as an etch mask. In

addition, since the cylinders are completely embedded in the sacrificial material, a large

degree of line edge roughness is anticipated to arise as one etches further into the film,

and the dimensions of the lines are anticipated to be highly dependent on the etch

process. Top coats have been developed to mitigate the wetting layer in PS-b-PDMS,79

but this strategy requires a solvent annealing step which introduces its own problems.

A number of researchers have paired low surface energy polymers with high

surface energy polymers to create BCPs with large enthalpic differences between the two

blocks. For example, the hydrophobic poly(4-tert-butylstyrene) (PtBuSty) has been

paired with P2VP80 and poly(hydroxystyrene) (PHOST),81,82 both very polar polymers, to

form BCPs capable of making features below 10 nm full pitch. However, perpendicularly

oriented features of these BCPs have not been demonstrated, precluding lamellae from

being used. In addition, neither of the blocks in PtBuSty-b-P2VP or PtBuSty-b-PHOST

can be preferentially removed over the other in a plasma etch, necessitating costly and

complicated sequential infiltration of organometallics to produce etch selectivity.83

In addition to just trying to increase the χ by increasing the enthalpic penalty

between the two materials, BCPs have been designed with other lithographic goals in

mind. One noteworthy example is epoxidized PS-b-poly(isoprene) (PS-b-PI) from the

Bates group.84,85 Utilizing chemistry that only epoxidized (to make more polar) the lower

surface energy PI block, at a certain degree of epoxidation they were able to equalize the

surface energies of the two components, producing perpendicularly oriented thin film

features through thermal annealing. This is only the second reported example (besides

PS-b-PMMA) where a BCP spontaneously and reproducibly forms perpendicular

features. Unfortunately, no immediate etch selectivity is possible between the two

38

organic components, again necessitating some organometallic infiltration scheme to

impart etch selectivity, but this result demonstrates a potential strategy to design other

materials for BCP lithography.

The last significant example of a high-χ BCP is poly(cyclohexylethylene)-b-

PMMA (PCHE-b-PMMA).86 This material has χ high enough to make sub-10 nm full

pitch features, but remarkably does it without an immediately obvious hydrophobic-

hydrophilic interaction. Although the saturated hydrocarbon PCHE is inarguably more

hydrophobic than the carbonyl-containing PMMA, intuitively the magnitude of the

differences are not as vast as the differences in other organic-only high-χ materials like

PtBuSty-b-PHOST. Since PCHE-b-PMMA forms features just as small as these extreme

examples, this implies that another mechanism besides the hydrophobic-hydrophilic

interaction penalty produces the high χ-value. It is anticipated that an affect like

conformational asymmetry87 between the PCHE and the PMMA contributes substantially

to χ in this material, and demonstrates that changing polar/non-polar interactions is not

the only route to small features. Demonstrations of perpendicularly oriented features and

etch contrast between PCHE and PMMA has not been reported, unfortunately.

3.3 UT SILICON-CONTAINING HIGH-Χ BCPS

3.3.1 Silicon-containing BCP development

Initial work at UT focused on producing high-χ silicon-containing BCPs without

too much focus on amenability to thin film processing. These materials have among the

highest-χ reported in the literature, but were reliant on solvent annealing which hampered

efforts to control them in thin films.

Oligosaccharides were paired with PTMSS to produce BCPs with very high χ-

values.88 As seen in Scheme 3.1, these BCPs were made by click chemistry to combine

39

together two starkly different compounds. PTMSS, the silicon-containing component,

was synthesized by controlled radical polymerization, and then modified with an azido-

terminus. Oligosaccharides, hydrophilic biomolecules readily dissolved in water, were

supplied with alkyne endgroups. These two functionalized polymers were then linked

together by copper catalyzed azide-alkyne cycloaddition. The resultant materials formed

features of less than 10 nm full pitch, a monumental achievement at the time, but suffered

from thermal instability of the oligosaccharides, precluding thermal processing of the

materials.

Scheme 3.1: Synthesis of PTMSS-b-poly(oligosaccharide) by click chemistry.

Oligosaccharide chemistry: (a) N-maltoheptaosyl-3-acetamido-1-

propyne (b) N-xyloglucooligosaccharide-3-acetamido-1-propyne (c)

mono-6A-N-propargylamino-6A-deoxy-β-cyclodextrin. Adapted with

permission from Cushen, et al. “Oligosaccharide/silicon-Containing

Block Copolymers with 5 nm Features for Lithographic Applications.”

ACS Nano 2012, 6, 3424–3433. Copyright 2012 ACS.

In a similar vein, PTMSS was combined with the hydrophilic but slightly less

exotic polymer poly(D,L-lactide) to produce PTMSS-b-PLA.89 As seen in Scheme 3.2, the

monomers contained different functional groups that required separate polymerization

40

procedures to synthesize. PTMSS was made by anionic polymerization, but terminated

with ethylene oxide to install a primary alcohol at the end of the molecule. This primary

alcohol was then used to initiate the polymerization of D,L-lactide by ring-opening

polymerization. The resultant materials displayed very high χ-values, and produced

features well below 20 nm full pitch. Although solvent annealing was typically employed

to anneal the BCPs, a thermal annealing scheme based on polarity-switching top coats

was successfully demonstrated for these materials.50 However, coating the polar top coat

on top of the hydrophilic PLA is exceedingly difficult and other materials were pursued

instead.

Scheme 3.2: Synthesis of PTMSS-b-PLA by anionic polymerization and then ring-

opening polymerization. Used with permission from Cushen, et al. “Thin

Film Self-Assembly of Poly (trimethylsilylstyrene-B-D,l-Lactide) with

Sub-10 nm Domains.” Macromolecules 2012, 45, 8722–8728. Copyright

2012 ACS.

3.3.2 Summary of styrene-styrene silicon-containing BCPs for lithography

As introduced in Chapter 2, the development of top coats with controllable and

predictable structures occurred simultaneously with investigation into styrene-styrene

BCPs. Serendipitously, it was discovered that these materials were mutually compatible,

which spurred the development40 of silicon-containing BCPs with increasingly high χ

values and correspondingly small features without sacrificing the ability to control their

orientation behavior in thin films.

41

Table 3.1 summarizes the general properties of a series of BCP structures that

were successfully synthesized and that were compatible with the polarity-switching top

coat orientation strategy. Of note is that all of the BCPs in Table 3.1 are composed of

polymers listed in Chapter 2 as forming confined island-hole features, with the sole

exception of poly(4-trimethylsilylstyrene)-b-poly(2-vinylpyridine) (PTMSS-b-P2VP).

This BCP does not form confined island-hole features due to the incorporation of the

P2VP block, but its large χ nonetheless makes it an attractive BCP for lithographic

applications.

42

Table 3.1: Summary of BCPs and key lithographic parameters

a Abbreviations: PS = polystyrene, PTMSS = poly(4-trimethylsilylstyrene), PDSS = poly(4-

pentamethyldisilylstyrene), PMOST = poly(4-methoxystyrene), PVBD = poly(5-

vinylbenzo[d][1,3]dioxole), P2VP = poly(2-vinylpyridine), PMVBD = poly(4-methoxy-6-

vinylbenzo[d][1,3]dioxole). b BCPs were made by sequential anionic polymerization. The names of the components

correspond to the order in which they were added to the polymerization, from left to right. c χ determined by extrapolating order-disorder transition from rheology, normalized to 118Å3. d Interpolated from a linear regression. Uncertainty is estimated to be approximately 5 - 10%.

43

Table 3.1 reports the calculated χ value of each BCP chemistry to quantify the

chemical incompatibility of the two components, as well as the calculated pitch of each

BCP at two χN values. The χ value was calculated at the reference temperature of 180 °C

for every sample (as will be explained later on in the manuscript) since 180 °C is the

approximate annealing temperature of the SAXS samples used to determine L0 and 180

°C is also a relevant annealing temperature in thin films. χN = 10.5 is the mean-field

condition for the order-disorder transition (ODT) of a BCP, so the L0 at that χN value is

the smallest features that the BCP could ever theoretically form. χN = 14 is a more

conservative estimate of the segregation strength needed to form usable lithographic

structures, and pitches at this χN value are included to provide better estimates of the

practical scaling limits of these BCP structures.33 The uncertainty in χ reported in Table

3.1 is propagated from the uncertainty in the constituent values used in the calculations.

The uncertainty in L0 is more crudely estimated to be between 5 and 10 % of the reported

value, and is attributed largely to inconsistencies in the use and state of the SAXS

instrument used to gather the data. It bears noting that the reported L0 values at χN are

estimations based on calculations and not a measured value from a single sample.

Subsequent sections discuss these calculations in more detail.

The first entry in Table 3.1, PS-b-PTMSS, is significant because most of the

development of the maleic-anhydride alternating top coats and the confined island-hole

test was carried out with this material. BCPs of this chemistry were the first materials to

have both top and bottom interfaces unmistakably neutralized to produce perpendicular

features.39 However, the relatively modest χ value of this material afforded no real

scaling advantage over PS-b-PMMA (producing approximately the same sized features),

and necessitated further development of materials.

44

The two entries following PS-b-PTMSS in Table 3.1, polystyrene-b-poly(4-

pentamethyldisilylstyrene) (PS-b-PDSS) and poly(4-methoxystyrene)-b-poly(4-

trimethylsilylstyrene) (PMOST-b-PTMSS), demonstrate the evolution of BCP design to

increase χ. Both molecules differ from PS-b-PTMSS by only one moiety, yet this change

dramatically increases the chemical incompatibility of the blocks. PS-b-PDSS contains an

extra organo-silicon moiety compared to PTMSS, making the hydrophobic part of the

BCP even more hydrophobic, while PMOST-b-PTMSS modifies PS by adding a

methoxy functional group to the “polar” repeat unit of the BCP. In both cases, χ

approximately doubles compared to the parent molecule PS-b-PTMSS, presumably due

to increasing the enthalpic penalty of mixing the two components together. Ultimately,

this results in molecules that can make approximately 17 nm pitch features in the

“medium”-segregation regime (χN ≈ 14), with ultimate theoretical resolution around 13-

14 nm. This compares favorably to the 18-22 nm resolution limit of PS-b-PMMA, but

nonetheless falls short of the 10 nm full pitch goal post. Because of this, poly(4-

methoxystyrene)-b-poly(4-pentamethyldisilylstyrene) (PMOST-b-PDSS) was

synthesized and analyzed. It incorporates both of the aforementioned structural

modifications, and its χ is triple that of PS-b-PTMSS, but can ultimately only make circa

11 nm pitch features, again short of the 10 nm goal.

Following the philosophy of “more is better”, poly(3,4,5-trimethoxystyrene)-b-

poly(4-pentamethyldisilylstyrene) (PTMOST-b-PDSS) (Figure 3.1) was synthesized to

make the polar component of the BCP even more polar and further increase the enthalpic

incompatibility of the two blocks. This material did indeed make sub-10 nm features (in

bulk samples), but is not included as a viable lithographic material in Table 3.1 because

the PTMOST block was fundamentally incompatible with the top coat strategy. All

PTMOST-containing BCP films washed away when a top coat was deposited on top out

45

of methanol or even out of water. Since this essentially precludes control over the thin

film orientation of the BCP, this structure was not further pursued, but does provide an

interesting comparison to the other BCPs capable of ~10 nm full pitch in Table 3.1.

Figure 3.1: Poly(3,4,5-trimethoxystyrene)-b-poly(4-pentamethyldisilylstyrene).

Despite the incompatibility of PTMOST with the top coat orientation approach,

materials structurally similar to PTMOST made BCPs with χ-values high enough to form

sub-10 nm pitches without being dissolved by methanol. Both poly(5-

vinylbenzo[d][1,3]dioxole)-b-poly(4-pentamethyldisilylstyrene) (PVBD-b-PDSS) and

poly(4-methoxy-6-vinylbenzo[d][1,3]dioxole)-b-poly(4-pentamethyldisilylstyrene)

(PMVBD-b-PDSS) from Table 3.1 are similar to the PTMOST structure in Figure 3.1 in

that all are styrene-derivatives with multiple ethers, yet the dioxole ring in PVBD-b-

PDSS and PMVBD-b-PDSS somehow creates a structure not damaged by methanol

while retaining a high degree of polarity. This led to BCPs with some of the smallest

pitches seen yet, soundly below 10 nm and approaching 8 nm, but that were still

compatible with the top coat neutralization strategy.

PTMSS-b-P2VP, as already mentioned, is different than the other BCPs in Table

3.1 in that it does not perform the confined island-hole test. However, the P2VP polymer

is extremely polar and made a very tempting target to pair with the hydrophobic silicon-

containing blocks to produce a high χ material.80 As Table 3.1 shows, this is indeed the

46

case, as PTMSS-b-P2VP has a very large χ value which translates into very small

features. The high χ value of this material has motivated progress towards synthesizing

poly(4-pentamethyldisilylstyrene)-b-poly(2-vinylpyridine) (PDSS-b-P2VP), which

should have an even larger χ value.

3.3.3 BCP synthesis overview

The lithographically relevant parameters reported in Table 3.1 (χ and pitch) were

determined from characterizing and analyzing multiple BCP samples of each chemistry

and generalizing the results. This initially involved synthesizing a number of BCP

samples at different molecular weights. All the BCPs were synthesized by controlled

living anionic polymerization90,91 where the two monomers were sequentially added to

the initiator, as seen in Scheme 3.3. Before adding the second monomer, an aliquot of the

first polymer was taken for characterization. In all of the BCPs featured in Table 3.1,

except for PTMSS-b-P2VP, the polar monomer was added first, and then the silicon-

containing monomer was added second. The polymerization needs to be performed this

way because the styrenes modified with ether functional groups are more electron rich

than silicon-containing styrenes and the corresponding carbanion at the benzylic position

is more unstable. When attempted in reverse, the more stable silicon-containing styrene

carbanion is unable to start the polymerization of the polar styrene. The general structure

shown in Scheme 3.3 and the order in which the blocks are referred to in Table 3.1 (and

throughout the text) reflects this. The synthesis of PTMSS-b-P2VP is an exception

because the P2VP carbanion is not strong enough to start the polymerization of a styrene.

As depicted in Scheme 3.4, the silicon-containing monomer is instead added first to the

initiator, followed by the 2-vinylpyridine monomer.

47

Scheme 3.3: Sequential anionic polymerization of styrene-styrene BCPs.

Scheme 3.4: Sequential anionic polymerization of PTMSS-b-P2VP.

3.3.4 BCP data for individual samples

A subset of BCPs made by anionic polymerization and their characterization

information appears in Table 3.2. Part of this data has been previously published in

Durand et al.,40 and the naming convention introduced in that paper, where each sample

is identified by its approximate L0, is followed in this manuscript. The methodologies

used to characterize the BCPs are explained in great detail since nuances in the

methodologies define the certainty to which the reported values are known.

48

Table 3.2: BCP sample characterization summary

a Periodicity from bulk SAXS measurements, annealed at approximately 180 °C. Values in

paranthesis indicate measurements of a disordered sample. b MN (in kg/mol) of aliquot and BCP by MALDI c Dispersity index by SEC. d Degree of polymerization, normalized to 118 Å3

e Dispersity index calculated from MALDI due to sample-specific SEC difficulties

49

3.3.5 BCP pitch

A key value reported in Table 3.2 is the L0 of the BCP sample as measured by

small angle x-ray scattering (SAXS). Within a given class of BCPs, L0 increased

monotonically with increasing BCP molecular weight (MN,BCP) or degree of

polymerization (N), as is expected based on Equation 1.2. When comparing two BCP

samples of different classes but that form approximately the same feature size (ie, PS-b-

PTMSS18 nm and PMOST-b-PDSS18 nm, both forming approximately 18 nm features), the

BCP class with the higher χ-value (PMOST-b-PDSS) forms these features at much

smaller MN,BCP or N. Similarly, when comparing two BCP samples of different classes but

approximately same MN,BCP or N (ie, MN,BCP = 30.7 kg/mol, 29.7 kg/mol and N = 430,

427 for PS-b-PTMSS18 nm, PS-b-PDSS20 nm, respectively), the higher χ-value material

(PS-b-PDSS) forms larger features (~18 vs. 20 nm). The trade-off, of course, is that the

higher χ-value materials stay ordered at lower MN,BCP or N, and can ultimately produce

smaller features. For example, the sample PS-b-PTMSSDis is disordered at MN,BCP = 27.7

kg/mol and the location of its SAXS peak implies the limit of the PS-b-PTMSS class of

materials is more than 17 nm, but the sample PMOST-b-PTMSS14 nm remains ordered at a

much smaller size of MN,BCP = 17.5 kg/mol with a much smaller SAXS peak at

approximately 14 nm.

There exists some discrepancy between the L0 measured by SAXS and the L0

observed in thin films. For example, PMOST-b-PTMSS20 nm has a SAXS peak at 19.6

nm, has been measured to be 19 to 21 nm on different instruments by top-down SEM

inspection, and flips island-hole behavior at multiples of 18.7 ± 0.2 nm. These values

span a range of almost 2.5 nm or approximately 12% of the average full pitch. The SAXS

L0 value is extracted from a bulk sample, so it stands to reason this value could be

intrinsically different from the BCP behavior observed in thin films. Discrepancies in

50

SEM measurements could be attributed to differences in the calibration of the

instruments. The island-hole flipping behavior is harder to explain by instrument error

since two different ellipsometers of different brands with different fitting models have

both produces the same measurement. Again, the island-hole flip is a thin film

phenomenon which could deviate substantially from bulk SAXS measurements. Another

caveat is that L0 is temperature-dependent property of the BCP, so different annealing

conditions could contribute to different results. This is anticipated to be the reason for the

slight discrepancy between the L0 values reported by Durand et al.40 and the results

reported here. Data for the temperature dependency of L0 for these ordered silicon-

containing BCPs has not been collected, but from data collected for disordered BCP

samples, the change in L0 across a 60 °C temperature range is estimated to be on the order

of half a nanometer. Also possible to explain the differences between Table 3.2 and

Durand et al. is drift in the SAXS instrument. Throughout the years, the detector and the

optical train on the SAXS instrument have been repaired and/or replaced repeatedly.

Despite calibrating the instrument before each sample run, it is possible that a systematic

error in the tool has emerged that would skew the L0 one way or the other. Because of all

these factors, the uncertainty in reported L0 values have an estimated uncertainty between

5 and 10%, and the reported and discussed L0 is always the value from SAXS. The raw

SAXS profiles are included in Appendix A.

3.3.6 Characterization procedures

The number average molecular weight (MN) of each homopolymer aliquot (MN,Ali)

and of each corresponding BCP (MN,BCP) are reported in Table 3.2 and were determined

by matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF)

spectrometry. Since MALDI-TOF is a non-destructive mass spectrometry technique, the

51

data collected are the ratio of polymer chain mass to charge (m/z), allowing for the

absolute determination of the molecular weight and molecular weight distribution of a

sample. BCP MN is needed to calculate the degree of polymerization N of the sample,

which is used in calculating χ. MN,Ali is reported to guide future synthesis of materials.

Calculating MN by MALDI is preferable to calculating it by size exclusion

chromatography (SEC) since SEC requires the calculation of the change in refractive

index of the solution (dn/dc) for each sample to properly determine the exact MN. This is

difficult because the present sample set includes polymers of many different chemistries,

which would require repeated determination of dn/dc, and in addition, some of the

sample MN’s are in the regime where dn/dc is not a constant, so dn/dc would need to be

calculated individually for each sample. MALDI does not face these drawbacks, and in

fact performs better as the MN decreases. The raw MALDI spectrum are included in

Appendix A.

The dispersity index (Đ) reported in Table 3.2 was, in contrast, determined by

SEC, and is reported to quantify the quality of each BCP samples. Products produced by

unintended side-reactions during the anionic polymerization appear as peaks in SEC.

These side-reactions would potentially not appear in a MALDI spectrum, for example,

necessitating the use of SEC to characterize the MN distribution. The Đ-values in Table

3.2 are all below 1.10, indicating that the homopolymers and the BCPs were polymerized

in a living manner without terminating side-reactions. This is partially due to deliberately

only choosing monomers that undergo anionic polymerization without side-reactions, and

also due to rigorously following the air-free/water-free procedures to ensure no events

occurred during polymerization to generate a large amount of dead homopolymer. The

PTMSS-b-P2VP samples in Table 3.2 have Đ > 1.10, but this is most likely due to the

SEC’s inability to properly measure such small samples (MN,ALI < 4 kg/mol), which no

52

doubt fall outside the instrument’s range. Inspection of the raw SEC traces (all of which

are included in Appendix A) corroborates that these homopolymer peaks have narrow

dispersity and no indication of poor synthesis control. In addition, the Đ of the two

smallest PMVBD-b-PDSS samples are instead reported from MALDI due to difficulties

in obtaining these SEC traces.

From the MN,BCP values, N was calculated, and this was used to calculate χN for

each sample. The reported N-value is normalized to 118 Å3, a common but arbitrary

reference volume for BCPs.92 Normalization of N allows direct comparisons to be made

between different BCPs. As previously discussed, χN is a measure of the segregation

strength of the BCPs. The BCPs reported here bridge the weak segregation limit and the

strong segregation limit, so it was necessary to calculate χN to quantify how close a BCP

sample was to its disordered state.

3.3.7 Calculating χ by dynamic mechanical spectroscopy

In addition to determining N of the BCP, the last piece required to calculate the χ-

values is the order-disorder transition temperature (TODT) of individual samples. Since χ is

a function of temperature as shown by Equation 3.1:

𝜒 =𝑎

𝑇+ 𝑏 (3.1)

where T is absolute temperature and a and b are constants, there exists a temperature at

which a given BCP sample goes from ordered to disordered (χN > 10.5 to χN < 10.5).

Such a transition be can monitored by dynamic mechanical spectroscopy (DMS). Upon

heating a bulk BCP sample, the transition from order to disorder is accompanied by a

precipitous drop in the elastic modulus (G’).93 However, not every BCP sample will have

a measurable TODT. For a TODT to be measurable, the value needs to be above the glass

53

transition temperature (Tg) of the BCP sample but below the degradation temperature of

the sample. For example, it was shown that a relatively high χ BCP, poly(4-tert-

butylstyrene)-b-poly(methyl methacrylate), with a relatively high dependency of χ on

temperature (a = 41 K from Equation 3.1) had accessible ODTs only for samples

between approximately 17-31 kg/mol in weight.54 In practice, this range can be even

smaller depending on the specific BCP’s properties, as well as how far away from the

degradation point one wishes to operate. Using a rheometer, the TODT of a number of the

samples in Table 3.2 were discovered and are reported in Table 3.3.

Table 3.3: Summary of ODT data used to determine χ at 180 °C

a Temperature of order-disorder transition measured by rheology b Uncertainty in the temperature, degree of polymerization, and temperature coefficient c Calculated χ value at 180 °C d Propagated uncertainty in χ value

As seen in Figure 3.2, even below the nominal degradation temperature, visible

discoloration occurs due to oxidative damage in the benzodioxole polymers (PVBD and

54

PMVBD) when they are heated to find the OTD. Compared to the corresponding ether,

poly(3,4,5-trimethoxystyrene), the benzodioxole moieties show significantly greater

oxidative damage, even at a modest temperature of 160 °C. After being raised above 250

°C, the samples are clearly an amber color. It is anticipated that the benzodioxole

transforms into a catechol, as has been reported by other researchers.94 Ironically, this is

anticipated to actually raise χ in our favor, as the phenolic moieties on the catechol are

actually more hydrophilic than the parent benzodioxole. Of course, this means that the χ

determined by this method is not the true χ of the parent molecule, but is the effective χ of

the partially modified benzodioxole BCP. Since the typical thin film annealing conditions

also subject the BCP to > 160 °C temperatures in air (presumably allowing the same kind

of oxidation to occur) no attempts were made to differentiate between the actual χ and the

effective χ.

Figure 3.2: Polymers showing signs of oxidation damage during and after rheology

experiments.

55

Of the seven classes of BCPs introduced in Table 3.1, only one class, PMOST-b-

PDSS, did not have a sample with an accessible TODT in Table 3.3. A value for a

hypothetical TODT was estimated for the sample PMOST-b-PDSSDis. An upper value was

concretely established by the DMS experiment (the sample was disordered even as low as

145 °C). A lower bound of approximately 85 °C was determined from comparing it to

other materials (the χ of PMOST-b-PDSS would have to be lower than that of PMOST-b-

PTMSS for this sample’s TODT to be less than 85 °C, which is unlikely due to scaling

arguments of L0 vs. χ and previous trends associated with changing PTMSS to PDSS).

This uncertainty in TODT for PMOST-b-PDSSDis is reflected in Table 3.3, whereas the

uncertainty in TODT is lower for the other samples and is attributed to the discrepancy

between the thermocouple’s location in the rheometer and the polymer sample’s location.

Table 3.3 again reports the N of each sample, as well as calculated uncertainty in N

arising from uncertainty in MN,BCP (depending on how one analyzed the MALDI

spectrum, the calculated MN,BCP could differ by approximately 3%).

With just N, one could use the mean field theory result χN = 10.5 to calculate the

χ of the BCPs, but this value would only be true at the TODT. Since the measured TODT’s

span a range of almost 100 °C, these χ-values could not be used to make accurate

comparisons between the BCP materials, and instead necessitates the calculation of χ at

the same temperature using Equation 3.1. To use this equation, the constants a and b

need to be fitted, however, only three of the seven BCP classes had samples with multiple

TODT’s, precluding the direct determination of a and b for each class. Since b is a constant

for a given BCP, Equation 3.1 can be combined with itself to write Equation 3.2,

removing the need to calculate b explicitly:

56

𝜒1 −𝑎

𝑇1= 𝜒2 −

𝑎

𝑇2 (3.2)

A value for a is still needed to perform the calculation. Again, this value can be

calculated from Equation 3.1 for PS-b-PTMSS, PVBD-b-PDSS, and PTMSS-b-P2VP

since multiple samples with these chemistries had measurable TODT’s, and these are the

values reported in Table 3.3. To estimate a for the other four BCP chemistries, inferences

had to be made from the samples that did not have measurable TODT’s. For example,

although PMVBD-b-PDSS10.5 nm had a TODT at 247 °C, PMVBD-b-PDSSDis had no

measurable TODT, which placed an upper limit on a for the PMVBD-b-PDSS BCP class.

In addition, a has been experimentally determined for a number of different BCPs in the

literature,86 supplying guidance for reasonable values for a. Because of the lack of solid

evidence for most of these materials, the uncertainty in a has been over-estimated in

Table 3.3, sometimes being stated as 2/3 of the actual value. It was discovered that the χ

dependency on a does not greatly affect the uncertainty over the temperature range 160

°C < T < 260 °C, due to the Arrhenius-like relationship between χ and temperature.

Solving Equation 3.2 for χ2 and replacing χ1 and T1 with the more precise χODT

and TODT yields Equation 3.3:

𝜒2 = 𝜒𝑂𝐷𝑇 + 𝑎 (1

𝑇2−

1

𝑇𝑂𝐷𝑇) (3.3)

Again, T2 is set equal to 180 °C to be in line with the SAXS data and because this

temperature is approximately the temperature at which thin film annealing is performed,

making it a relevant temperature at which to compare χ. χODT is again from χN = 10.5 at

the ODT. The calculated values of χ2 are listed in Table 3.3 for each individual sample

with a TODT (and PDSS-b-PMOSTDis). For the BCP chemistries with multiple samples

that have a TODT, it is satisfying to see that the calculated χ-values are within the margin

57

of error. In Table 3.1, the average of the two χ-values is reported for the BCPs with more

than one measurable TODT, and the reported uncertainty is the larger of the two in Table

3.3. For the other four BCP chemistries, the reported χ-value and uncertainty at 180 °C in

Table 3.1 is the result from the one sample in Table 3.3.

To calculate the final uncertainty in χ, the variance formula was used with

Equation 3.3 to yield Equation 3.4:

Δχ = √(𝜕χ

𝜕𝑎∙ Δ𝑎)

2

+ (𝜕χ

𝜕𝑇𝑂𝐷𝑇∙ Δ𝑇𝑂𝐷𝑇)

2

+ (𝜕χ

𝜕𝑁∙ Δ𝑁)

2

(3.4)

where each partial derivative is calculated as follows:

𝜕χ

𝜕𝑎= (

1

𝑇2−

1

𝑇𝑂𝐷𝑇) (3.5)

𝜕χ

𝜕𝑇𝑂𝐷𝑇= −

𝑎

𝑇𝑂𝐷𝑇2 (3.6)

𝜕χ

𝜕𝑁= −

10.5

𝑁2 (3.7)

The variance in a contributed most significantly to Δχ for PMOST-b-PDSS and PMVBD-

b-PDSS. These two polymers had the largest initial Δa, and χ had to be extrapolated 65

and 67 ° to 180 °C, respectively, among the largest extrapolation required. Both PVBD-

b-PDSS samples also had larger-than-average a variance contributions, also because of

the large temperature range of extrapolation. The variance contribution of TODT was only

very significant for PMOST-b-PDSS, again because the actual TODT was never measured

for any of these samples and had to be inferred. The variance due to N was generally the

58

lowest contribution to Δχ and increased as N decreased for the higher-χ BCPs, due to

scaling as 1/N2.

The preceding analysis is important because it allows for direct comparison of

BCP chemistries. Specifically, it shows that χ was increased substantially by polymer

modification, a very satisfying result in terms of “engineering” materials for lithographic

applications. In addition, χ is a parameter in Equation 1.2, needed to calculate L0 at a

given segregation strength. Three materials, PVBD-b-PDSS, PTMSS-b-P2VP, and

PMVBD-b-PDSS, emerged as very high-χ materials whose full pitch can be as small as

10 nm. From the data described above, more information about the full potential of these

materials can be gleaned.

3.3.8: Extrapolating scaling trends

The actual pitch a BCP can make is perhaps the most significant parameter in

lithographic applications. Although χ is often used as the benchmark to compare different

BCP materials, L0 is not determined solely by χ. Equation 1.2 links L0 to χ and N and is

reprinted here:

𝐿0 = 𝛼𝜒1/6𝑁𝛿 (1.2)

where α contains the statistical monomer length, approximately 1 nm but different for

every BCP chemistry, and δ has been calculated to be 1/2 in the weak segregation regime

and 2/3 in the strong segregation regime.19,22 The BCP materials described here span

these two theoretical extremes, which necessitates the extraction of δ from data. Taking

the logarithm of both sides of Equation 1.2 yields Equation 3.8,

log (𝐿0) = log(𝛼𝜒1/6) + 𝛿log (𝑁) (3.8)

and provides a linear relationship for L0 and N. Equation 3.8, along with the SAXS L0

and N data reported in Table 3.2 and with the χ calculated at 180 °C were used to

59

generate Table 3.4. Of note is that the relationship expressed in Equation 1.2 is perhaps

more accurately represented by the radius of gyration (RG) of the BCP rather than N, but

N is the value more readily determined in glassy polymers.

Table 3.4: Parameters for domain spacing calculations

a Parameters forL0 = α·χ1/6·Nδ extracted from linear regression of log-log plot of N and L0 and the

confidence interval on the values. α has units of nm. b Coefficient of determination for the linear regression of log-log plot of N and L0 c Range of χN values of the samples used to calculate the regression

Table 3.4 reports the calculated values for α and δ, their confidence intervals, the

coefficient of determination for the linear regression, and calculated L0’s at a number of

χN values for each BCP chemistry. Both α and δ vary substantially from sample to

sample, but the confidence interval for these values are very large and it is difficult to

draw meaningful conclusions solely from these values. The confidence intervals for δ

60

capture values reported for other BCPs (0.5 to 0.8)54 despite only fitting 2-4 BCP

samples.

In order to calculate L0 at a number of χN values for Table 3.4, the quantity χN

was defined as a constant:

𝜒𝑁 = 𝐶 (3.9)

Solving Equation 3.9 for N and substituting into Equation 1.2 yields Equation 3.10:

𝐿0 = 𝛼𝜒1/6−𝛿𝐶𝛿 (3.10)

The χN range of the BCP samples used to derive α and δ are included in Table 3.4, which

shows the range over which Equation 3.10 can be trusted to produce accurate values for

L0. Unfortunately, samples of χN > 14 were not available for the three BCPs with the

highest χ-values, and a χN > 16 sample was not available for PS-b-PTMSS, necessitating

that the calculated L0’s at these χN’s be extrapolated. It is anticipated but cannot be

guaranteed that the power-law behavior calculated at lower χN’s will be applicable at

these higher χN’s. The coefficient of determination of the log-log relationship in all cases

is very close to 1., strengthening the argument that the linear fit of the log-log data is a

good fit.

The increments for χN were chosen to represent a range of values that might

represent the lower bound of the segregation force required to make viable BCP

structures for lithography,85,95 but of course, this requires extrapolating far beyond the χN

range of the initial data for PVBD-b-PDSS, PTMSS-b-P2VP, and PMVBD-b-PDSS,

making the calculations at χN = 14 and 16 less meaningful. As seen in Table 3.4, the

confidence intervals for L0 of PTMSS-b-P2VP at χN = 14 and 16 are very large and do

not make significant predictions about the material’s performance.

Importantly, Table 3.4 predicts that the 10 nm features made by these three

highest-χ BCPs are not very highly segregated, even by the standards of the samples in

61

this study. Etch tests are underway with some of these near-10 nm samples to determine

if pattern transfer can be accomplished nonetheless.

Curiosity drove us to see if we could decouple the scaling of the BCPs from their

explicit chemistries, in other words, was there a universal scaling law for BCPs? This

would help guide efforts to make BCP materials that could make 8 nm features and

below. Plotting the calculated pitch of each BCP chemistry against the chemistry’s

calculated χ at constant χN produced a plot with a power law behavior. These data were

then plotted on a log-log plot, as shown in Figure 3.3A and quantified by the following

two empirical equations:

𝐿0 = 𝛽𝜒𝜆 (3.11)

log (𝐿0) = log(𝛽) + 𝜆log (𝜒) (3.12)

where β and λ are empirical constants. For clarity, Figure 3.3A shows only the results for

χN = 10.5 and 14, but χN = 12 and 16 follow the same general trend. In Figure 3.3A, the

points are numbered according to the BCP chemistry to which they belong, with

increasing number corresponding to increasing χ (an explicit listing also appears in the

caption). The BCPs satisfyingly fall on a line until the last three, high-χ BCPs. With these

last three samples, PVBD-b-PDSS and PMVBD-b-PDSS fall slightly below the trendline,

while PTMSS-b-P2VP falls significantly above it. This fact can also be discerned from

the residual plot in Figure 3.3B. Again, points 5 and 7 (PVBD-b-PDSS and PMVBD-b-

PDSS) are below zero, and point 6 (PTMSS-b-P2VP) is above zero. The residuals for the

rest of the data points seem to be randomly distributed with no clear trends.

62

Figure 3.3: (A) Log-log plot of L0 calculated at two different χN values (10.5 and 14)

versus χ at 180 °C. Line is least squares residual fit. (B) Residual plot of the

linear regression through the points in plot (A). Lines are added to guide the

eye. Numbers in both graphs correspond to the BCPs as follows: (1) PS-b-

PTMSS (2) PS-b-PDSS (3) PMOST-b-PTMSS (4) PMOST-b-PDSS (5)

PVBD-b-PDSS (6) PTMSS-b-P2VP (7) PMVBD-b-PDSS.

The values of β and λ were extracted from Equation 3.12 and are summarized in

Table 3.5. Remarkably, the exponent for χ, λ, is approximately constant at -0.44 for this

range of χN values. The prefactor β, however, is a function of χN and increases with

increasing χN. Unfortunately, the calculated uncertainties from Table 3.4 and Equation

3.10 for L0 and χ resulted in even larger uncertainties in β and λ. The power law

relationship between L0 and χ is very sensitive to even small degrees of uncertainty in the

constituent values used in the calculation. Nonetheless, these values were used to

evaluate Equation 3.11 for L0 = 8 and 6 nm to probe approximately what kind of

materials are needed to make features this small. As seen in Table 3.5, the predicted χ-

values ranged from 0.15 to 0.25 depending on χN for 8 nm features, and from 0.29 to

63

0.53 for 6 nm features. These values were then used to calculate the N of the hypothetical

BCP, and approximate MN (the true MN would be dependent on the density of the BCP

blocks). Again, the large uncertainties in β and λ created even larger uncertainties in these

χ-values, in this case to the point of making them near meaningless. The smallest

uncertainties were ± 50% of the calculated χ values, and the largest uncertainties were +

200% and – 80%. Due to this large range of values, the uncertainties were not reported in

Table 3.5.

Table 3.5: Summary of empirical model for predicting scaling limits

a Parameters for L0 = β·χλ extracted from linear regression of log-log plot of L0 and χ at constant

χN.

To decrease the uncertainty in β, λ, and χ at 8 and 6 nm, better values are needed

for χ of the known BCP chemistries and L0 of the individual BCP samples. The largest

contributor to the uncertainty in χ of the known BCP chemistries is the extrapolation of χ

from the TODT to the reference temperature (180 °C). If additional BCP samples were

available that contained measurable TODT’s, and the TODT’s bracketed the reference

64

temperature so that interpolation was used to calculate χ, the uncertainty in χ would

decrease substantially. The uncertainty in L0 is due to the variability of the SAXS

measurements. As discussed previously, the measurements were taken on potentially

unstable equipment over a period of many years, with many variations in the sample

preparation protocol from operator to operator. Uniformly constructing and annealing the

samples, and then taking the measurements all at once on the same equipment in a

consistent manner, would substantially decrease the variability in the output of the SAXS

experiment and the corresponding uncertainty in L0.

With the concerns about the uncertainty of the hypothetical χ-values at 8 and 6 nm

in mind, there are still meaningful inferences to be made from Table 3.5. First, to go

from materials that can make ~10 nm full pitch features (like the ones described in this

manuscript) to materials that can make ~8 nm full pitch features, at the minimum a 50%

increase in χ is required. The structural changes that transformed PS-b-PTMSS into

PMVBD-b-PDSS increased χ almost 400%, but it is not immediately clear if any

additional changes are possible to produce a material with a further incremental increase

in χ (and that is still compatible with the top coat orientation strategy). In addition, to

further increase the resolution from 8 nm to 6 nm, χ needs to double. Again, the route to a

material with such a large χ is not clear. Furthermore, the calculated N and MN of the

hypothetical 6 nm materials indicate BCPs composed of essentially oligomers; it is hard

to imagine that these materials, which are essentially liquid crystals, would follow trends

extrapolated from materials an order of magnitude larger. Deviations from the described

trend, however, could be beneficial and not detrimental. As shown in Figure 3.3A, it

appears PVBD-b-PDSS and especially PMVBD-b-PDSS produce smaller features that

those predicted by the model that describes the behavior of BCPs with lower χ values at

higher N’s. Conversely, the features made by PTMSS-b-P2VP are much larger. These

65

differences in L0 are beyond the error expected from the SAXS measurements and

indicative that a single empirical model that describes all BCP behavior is probably non-

existent.

3.4 CONCLUSIONS

In conclusion, variety of lamellae-forming BCPs were synthesized and

characterized as candidate materials for lithography. The BCPs were alike in that they all

had a styrenic (or nearly styrenic, in the case of P2VP) backbone. Each BCP contained a

silicon-containing block to be used as an etch mask, and was paired with a polar block.

The chemistry of the polar block was the single largest difference from BCP to BCP, and

choosing especially polar blocks created BCPs with high χ-values that ultimately

produced 10 nm and smaller features.

A general model was proposed to correlate L0 and χ, regardless of chemistry, and

was fit by the seven BCP examples introduced in this chapter. At different segregation

strengths, a power law model described the L0 behavior quite well. This model was used

to draw inferences about the kinds of materials needed for 8 and 6 nm pitch feature

production. Unfortunately, the model predicts that substantial increases in χ are needed to

produce features this small. However, the actual BCP data at small L0 seems to deviate

from the predicted trends, with samples making both larger and smaller L0’s than

predicted, which invites optimism about the scaling limits of BCPs.

BCPs described in this Chapter were used in directed self-assembly (DSA)

experiments to make device relevant line-space features. For example, Maher et al. used

PS-b-PTMSS22 nm at IBM to make patterns through a variety of mechanisms.96 Cushen et

al. used PMOST-b-PTMSS20 nm (as well as a 17 nm sample not introduced here) at HGST

for bit-patterned media applications.97 The last two chapters describe the work of the

66

author and coworkers with PMOST-b-PTMSS20 nm and PVBD-b-PDSS10.5 nm at IMEC

and Seagate, respectively.

3.5 EXPERIMENTAL

3.5.1 BCP synthesis

BCPs were synthesized according to the following general procedure. The

monomers were trap-to-trap distilled out of dibutyl-butyl magnesium twice and finally

collected in an empty, flame dried Schlenk flask. A 500 mL glass reactor with 5 threaded

ports was charged with a Teflon-coated magnetic stir bar and the following were attached

to the ports: a thermocouple well, a septum-capped inlet port (attached to a Schlenk line

and a pressure gauge), a 500 mL solvent flask of ~150g THF, and 2 glass blanks. The

reactor was flame dried under high vacuum and purged five times with Ar. The purified

THF was added to the reactor while maintaining a slight 3 psig overpressure. After

cooling to – 78 °C, a solution of sec-butyllithium (1.4 M in cyclohexane, Aldrich) was

added to the reactor through the septum via an air-tight syringe. The first monomer was

then added, also by syringe, and allowed to react for 1 to 3 hours. An aliquot of

approximately 3 mL was taken and injected into degassed methanol, before adding the

second monomer by syringe. The reaction continued to be stirred for 1 to 3 hours and

then terminated by addition of degassed methanol. The THF solution was concentrated

slightly (target: 5 g THF / 1 g polymer) and precipitated into methanol. The isolated

white powder was dried in vacuo.

3.5.2 Characterization

SEC was performed using a Viscotek VE 2001 triple detector gel permeation

chromatograph with an Agilent 1100 Series isopump and autosampler. THF was used as

67

an eluent at 1 mL/min and 23 °C with three I-series mixed bed high-MW columns,

calibrated against PS standards.

MALDI data were collected on an AB SCIEX Voyager-DE PRO MALDI-TOF

using trans-2-[3-(4-tert-butyl-phenyl)-2-methyl-2-propenylidene] malononitrile (DCTB)

as a matrix with silver trifluoroacetate as the counterion. The baseline of the MALDI

spectra was fitted to a quadratic function and subtracted.

SAXS measurements were collected using a Molecular Metrology instrument

equipped with a high brilliance rotating copper anode source (Kα radiation) and a two-

dimensional 120 mm gas filled multiwire detector. The beam was calibrated using silver

behenate with its primary reflection peak set at 5.838 nm. Samples were prepared by

annealing polymer inside of a Viton o-ring in between Kapton tape on a hotplate.

Rheology data were collected on a TA Instruments AR2000EX using a 25 mm

parallel plate operating in strain controlled mode. Each sample was measured at strain =

2% and angular momentum ω = 1 rad/s in the linear viscoelastic regime. Samples were

ramped at 5 °C/min under nitrogen flow while monitoring the storage modulus, the loss

modulus, and the difference between the two (delta).

3.5.3 Regression procedures

Equations 1.2 and 3.11 were fitted by least squares residual linear regression of

the logarithm of both values. Uncertainty in the slope and y-intercept was determined by

the method of maximum/minimum values for the first and last data points.

68

Chapter 4: Directed self-assembly for semiconductor applications

The work in this chapter has been reproduced in part with permission from

Blachut, G.; Sirard, S.M.; Maher, M.J.; Asano, Y.; Someya, Y.; Lane, A.P.; Durand,

W.J.; Bates, C.M.; Dinhobl, A.M.; Gronheid, R.; Hymes, D.; Ellison, C.J.; Willson, C.G.;

“A hybrid chemo-/grapho- epitaxial alignment strategy for defect reduction in sub-10 nm

directed self-assembly of silicon-containing block copolymers.” Chemistry of Materials

2016. DOI:10.1021/acs.chemmater.6b03633. The author performed the majority of the

studies in this manuscript. Parts of the chapter have also been reproduced with permission

from Cushen, J.; Wan, L.; Blachut, G.; Maher, M. J.; Albrecht, T. R.; Ellison, C. J.;

Willson, C. G.; Ruiz, R. “Double-Patterned Sidewall Directed Self-Assembly and Pattern

Transfer of Sub-10 nm PTMSS-b-PMOST.” ACS Appl. Mater. Interfaces 2015, 7, 13476-

13483. The author prepared the BCP samples and provided the BCP characterization data

for this manuscript. Copyright 2015-2016 American Chemical Society.

4.1 BACKGROUND

A number of creative methods to produce long range order in thin films of block

copolymers (BCPs) appear in the literature. Electric fields,98 heterogenous substrates,99

pressure gradients,100 and temperature gradients101 have all been used to order BCPs

along a desired axis. However, to make device-relevant features with BCPs, directed self-

assembly (DSA) techniques are required. DSA is needed to precisely place the BCP

features, especially in relationship to pre-patterned structures on the substrate. A key

practical aspect of DSA is that DSA implementations need to densify or multiply a sparse

initial pre-pattern; simply replicating a pattern 1:1 is insufficient. In addition to

“frequency multiplication” or “pattern interpolation” to increase the final feature density,

69

DSA can rectify errors in the initial lithography.102,103 This phenomenon may become

more significant as the feature size decreases.

DSA techniques have been broadly categorized as grapho-epitaxy104,105 or chemo-

epitaxy,102,106,107 depending on how the pre-pattern is constructed. In grapho-epitaxy,

topography on the substrate serves as the registration point for BCP domains. In chemo-

epitaxy, chemical contrast, often thought of in terms of surface energy differences or

gradients, serves as the anchoring force for different components of the BCP. Grapho-

epitaxy is often easier to implement since there are an assortment of ways to create the

requisite topography,108–110 and the pre-pattern’s pitch and dimensions can be hundreds of

nanometers. However, grapho-epitaxy often necessitates the sacrifice of usable areal

space due to the topography, making it less desirable for some high density applications.

Chemo-epitaxy, in comparison, typically sacrifices no areal space, and the near-planar

under-layer adds minimal complexity to the film stack, helping to facilitate subsequent

pattern transfer. Unfortunately, chemo-epitaxy is often much harder to implement. The

surface tension of the patterned regions must be precisely tuned for a BCP system,57,59

and often custom materials are required for a given BCP chemistry. The initial

lithography for chemo-epitaxy is more demanding as well: the pre-pattern pitch is often

no more than 4 or 5 times the BCP natural periodicity (L0), and the actual feature size

might be a fraction of L0. It should be noted that most DSA schemes actually implement

aspects of both grapho- and chemo-epitaxy: the surface tension of topographic features is

a significant parameter in determining the orientation of BCP domains,105 and “planar”

chemo-epitaxy often contains height differences between the different chemical

regions.111 In addition, recently “hybrid” epitaxy processes have emerged, where

chemical pre-patterns and topography are deliberately used in conjunction for

DSA.97,112,113 The difference between hybrid processes and most grapho-epitaxy

70

processes is that the topography in hybrid processes is shallower than the BCP film

thickness, as to not sacrifice any usable areal space. Hybrid processes often have the

advantage that their defectivity is lower than chemo-epitaxy processes.

Progress has been made in implementing a chemo-epitaxy process for PS-b-

PMMA on 300 mm wafer processing equipment with state-of-the-art 193 nm immersion

lithography (193i) at IMEC114–116 and other laboratories.117 The process at IMEC is based

on a modified LiNe process,57,59 depicted in Figure 4.1. To compatibilize the laboratory-

scale LiNe process (which used electron beam lithography) with 193i, a film stack was

developed to allow high resolution photolithography. Specifically, a 13 nm thick silicon

nitride layer is needed, in conjunction with the 7 to 8 nm thick polymer underlayer, to

form a suitable anti-reflective coating for 193i.114 The LiNe flow employs a trim etch to

simultaneously reduce the dimension of the lithographically-defined resist features

(“trim”) and transfer the resist feature pattern into the underlying polymer layer. A

second polymer is then “backfilled” into the region exposed by the trim etch, producing a

surface with alternating surface energy stripes of unequal width. The surface tension of

these two polymeric regions needs to be tuned precisely to the BCP chemistry and often

requires refinement for a new BCP formulation. To be “drop-in” compatible with 300

mm wafer processing, the LiNe process utilizes only spin-coating to deposit polymers,

and only baking (thermal annealing) for the various annealing steps.

71

Figure 4.1: Fabrication of chemical pre-patterns used for the directed self-assembly of

PS-b-PMMA with frequency multiplication. Reproduced with permission

from Liu et al. “Chemical Patterns for Directed Self-Assembly of Lamellae-

Forming Block Copolymers with Density Multiplication of Features.”

Macromolecules 2013, 46, 1415–1424.

This chapter describes the implementation of a 193i-compatible DSA process for

silicon-containing BCPs at IMEC. Modifying the existing chemo-epitaxy process at

IMEC to be suitable for silicon-containing BCPs was possible but required the

introduction of new polymeric materials with surface energies relevant for the BCP.

Beyond the introduction of new materials, the processing steps used to fabricate pre-

patterns and form DSA were largely unchanged, demonstrating the viability of silicon-

containing BCPs as smaller pitch replacements for PS-b-PMMA. First, the materials and

processes are introduced. In addition to a traditional chemo-epitaxy process, a hybrid

process utilizing shallow topography was implemented to investigate the advantages of

such a scheme. The DSA results are presented in summary and an in-depth study of the

mechanism of DSA in the hybrid process follows. After that, various DSA process

parameters are quantitatively analyzed. The hope is that an exhaustive description of the

procedures and parameters creates a roadmap for implementation of future materials.

72

4.2 MATERIAL DESIGN AND CHARACTERIZATION

4.2.1 Overview of materials

A silicon-containing BCP, polymer brushes, guide stripe material, and a top coat

were designed, synthesized, and analyzed at The University of Texas specifically for this

collaboration. The chemistry of the materials are depicted in Figure 4.2. Nissan

Chemical graciously demetallized the polymers through an ion-exchange process,

formulated the polymers for spin-coating, and shipped them to IMEC.

73

Figure 4.2: Polymers introduced into IMEC: cross-linkable poly(4-methoxystyrene)

(XPMOST), 4 polymer “brushes” of hydroxyl-terminated polystyrene-r-

poly(4-tert-butylstyrene), the silicon-containing BCP poly(4-

methoxystyrene)-b-poly(4-trimethylsilylstyrene), and the polarity switching

top coat.

The BCP chemistry and periodicity (L0) were chosen in tandem for lithographic

performance. A full pitch of approximately 20 nm was selected as an initial target for the

experiment; this value is 10% smaller than the ~22 nm practical limit of poly(styrene)-b-

poly(methyl methacrylate) (PS-b-PMMA)33 and 30% smaller than IMEC’s ~28 nm

process of record (POR) PS-b-PMMA,116 demonstrating the ability of silicon-containing

BCPs to form lithographically viable features smaller than PS-b-PMMA. Importantly, 20

nm is 1/4 of 80 nm, so a hypothetical 20 nm BCP multiplies the approximate resolution

limit of 193i by a factor of four.12 While frequency multiplication factors larger than 4x

are possible, sparser pre-patterns generally lead to larger defectivity as the driving force

for alignment decreases. The approximate resolution limit for the simplest silicon-

containing BCP we had success orienting, poly(styrene)-b-poly(4-trimethylsilylstyrene)

(PS-b-PTMSS), is 20 nm, but diffuse interfaces in BCPs near the order-disorder transition

(ODT) complicate pattern transfer and made this sample unattractive for a 20 nm

lithographic material.40,85,95 Poly(4-methoxystyrene)-b-poly(4-trimethylsilylstyrene)

(PMOST-b-PTMSS) emerged as an alternative BCP chemistry. The BCP monomers were

readily available, and the synthesis of PMOST-b-PTMSS could be performed in a living

and controlled manner by anionic polymerization. The resolution limit of PMOST-b-

74

PTMSS was determined to be approximately 14 nm, so a sample large enough to form 20

nm features was well within the ordered regime. Most importantly, PMOST-b-PTMSS

responded to the confined island-hole test, enabling the identification of neutral top coats

to generate perpendicular features.39 This BCP, introduced as PMOST-b-PTMSS20 nm in

Chapter 3, was made on a 50 g scale to ensure sufficient material for multiple

applications, and in fact this material was used in bit patterned media studies at Hitachi

Global Storage Technologies97 and Seagate Technology.

4.2.2 Surface treatments

Polymer brushes for surface modification for DSA were also prepared on large

scale. As mentioned in Chapter 2, random copolymers of polystyrene (PS) and poly(4-

tert-butylstyrene) (PtBuS) were made as PS wets the poly(4-methoxystyrene) (PMOST)

BCP block and PtBuS wets the poly(4-trimethylsilylstyrene) (PTMSS) block, so each

BCP component’s affinity for the brush surface could be modified by changing the

composition of the brush. Table 4.1 lists the polymer brushes introduced into IMEC,

their composition, and their wetting preference for the BCP. Island-hole118 tests were

performed to determine the wetting preference. Brushes were grafted onto blank wafers,

and BCP films of various thicknesses were coated. It was important to perform this test at

IMEC since the demetallization of the polymers by Nissan Chemical had the potential to

change the brush properties. Figure 4.3 shows the results of the island-hole test which

was used to determine wetting behavior and populate Table 4.1. Two of the tested

brushes showed “hole” features which indicate symmetric wetting when 1.5 L0 < BCP

film thickness < 2.0 L0, and the third showed neutral wetting behavior with a flip from

islands to holes within the same BCP film thickness range. (Interestingly, the data show

that the affinity of PTMSS for PtBuSty is higher than the affinity of PMOST for PS,

75

since the sample that contains the most PS, BB-85, is only neutral for the BCP, all of the

other brushes are preferential for PTMSS, and none of the samples are PMOST

preferential.) Fortunately, from these existing samples a brush composition was suitable

for DSA (discussed further on in the chapter).

Table 4.1: Polymer brushes available at IMEC

Sample Composition (mol % PS)1 Wetting preference

BB-27 27 PTMSS-wetting2

BB-50 50 PTMSS-wetting

BB-68 68 PTMSS-wetting

BB-85 85 neutral 1 as determined by 1H NMR 2 this result is inferred from the results of the other tests

76

Figure 4.3: Island-hole data of PMOST-b-PTMSS performed at IMEC on polymer

brushes. B-50 and BB-68 show holes, which implies symmetric (PTMSS)

wetting of the two surfaces. BB-85 shows islands and holes at the two

thicknesses, implying neutral “half” features are present.

A top coat consisting of maleic anhydride alternating with styrene and 3,5-di-tert-

butylstyrene, described previously in Chapter 2, was synthesized for PMOST-b-PTMSS.

Although the top coat neutrality was confirmed at the UT, it was unclear if the ion

exchange process for demetallization would leave the polymer, and its surface energy,

unchanged. Confined island-hole tests,39 where the top coat was applied on top of the

BCP, were performed at IMEC, and are shown in Figure 4.4. As the BCP film thickness

increased from 1.6 to 1.9 L0, the topography changed from dark spots which correspond

to islands, to mottled dark regions, to white spots. Just like the island-hole flip observed

77

for BB-85 in Figure 4.3, this island-hole flip implies that, despite the additional

processing, the top coat remained neutral for the BCP.

Figure 4.4: “Confined” island-hole test of the top coat. The progression from island to

spinodal islands to holes in this BCP film thickness range suggests the

neutrality of the top coat. Underneath the BCP is a PMOST-preferential PS

under-layer.

The last material introduced to IMEC from UT for DSA was a cross-linkable mat

of poly(4-methoxystyrene) (XPMOST). As will be explained shortly, this polymer was

used to form the guide stripes that anchors the BCP in DSA. The performance of this

material in DSA was compared directly to commercial cross-linkable poly(styrene)

(XPS) installed at IMEC for the DSA of PS-b-PMMA. Although the polar component of

the BCP, PMOST, wets both XPS and XPMOST, it was anticipated that the chemical

similarity between XPMOST and PMOST would increase the quality of DSA compared

to the XPS. As an IMEC rule forbade the deposition by hand of any material that enters

the 193i scanner, the XPMOST had to be coated by a track from a small volume dispense

unit (SVDU). This greatly reduced the number of experiments possible with XPMOST,

so a majority of the data was collected on the readily available XPS.

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4.3 TYPICAL PROCESS TO CREATE DSA WAFERS

Two related processes were used to create the pre-patterns for DSA, and their

relative performance was compared. A relatively standard chemo-epitaxy process,

mimicking IMEC’s establish LiNe process, was created for the silicon-containing BCP,

as was a so-called hybrid process, combining chemical guiding surfaces with topography.

Compared to pure grapho-epitaxy, the topography of a hybrid process is kept shallow to

avoid sacrificing usable areal space. Figure 4.5A shows the process used to make the

hybrid pre-patterns. XPMOST was first spin-coated at 19 nm and cross-linked by thermal

annealing. This cross-linked mat was then coated with a positive-tone resist, exposed

with a commercial 193i scanner, and the resist was developed. The wafer was then

reactive ion etched to shrink the width of the resist (“trim” step) and to remove the

XPMOST mat in the regions not covered by the resist to expose the underlying substrate

(“breakthrough” step). Stripping the resist yielded thin lines of mat that later became the

guide stripes. The polymer brush was then applied by spin-coating and grafted by a

thermal anneal to modify the surface exposed during the breakthrough etch. The final

grafting thickness of the brush was only approximately 7-8 nm, far less than the height of

the guide stripe. The height difference between the brush region and the top of the guide

stripe formed the topography in the system. Excess, ungrafted brush was then stripped

with organic solvent to yield the hybrid pre-pattern. To instead make the chemo-epitaxy

pre-pattern, the mat material, in this case either XPMOST or XPS, was coated at only

approximately 8 nm (versus 19 nm) to produce an essentially flat pre-pattern after

processing, as depicted in Figure 4.5B. Beyond the changes in guide stripe coating

thickness and chemistry, the subsequent steps of cross-linking, patterning, etching, and

depositing brush in the chemo-epitaxial process are otherwise identical to the hybrid

process. For both processes, the DSA film stack was completed by spin-coating the BCP

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on the pre-pattern, and then the top coat directly on top of the BCP. The wafer was then

heated to anneal the BCP, and then etched a second time to remove the top coat and the

sacrificial PMOST block for imaging. The resulting features were observed chiefly by

automated critical-dimension SEM (CD-SEM).

Figure 4.5: (A) Schematic of the hybrid process used to create the chemical pre-pattern

with shallow topography. (B) Depiction of the corresponding pre-pattern

surface produced by the chemo-epitaxy process.

One oversimplification shown in Figure 4.5 that ends up affecting the DSA

significantly is the incorrect depiction of the guide stripe as having edges normal to the

substrate. In reality, due to the different etch rates between the acrylic photoresist and the

styrenic guide stripe and the imperfect anisotropy of the plasma etch, the guide stripes

have a slanted sidewall. As seen in Figure 4.6, the angle of the sidewall is relatively

shallow, approximately 45 to 60° from the substrate. Since the sidewall was directly

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exposed to the plasma, it became oxidized and resulted in a very polar surface, which was

anticipated to significantly affect the DSA.

Figure 4.6: Cross-section STEM micrograph of the 19 nm tall guide stripes after etch

and resist strip but before depositing the brush or the BCP. The chromium

capping layer was added to protect the features during cross-sectioning and

to prevent charging during imaging.

4.4 QUALITATIVE COMPARISON OF THE TWO DSA PROCESSES

The DSA of PMOST-b-PTMSS was accomplished via both processes, but the

hybrid process produced higher quality DSA than the chemo-epitaxy process using either

guide stripe chemistry. The three micrographs in Figure 4.7 show the best DSA achieved

by each process, after each process was optimized for pre-pattern pitch, guide stripe

width, brush composition, and BCP film thickness (discussion of the optimization of the

process parameters appears later). As shown in Figure 4.7C, the hybrid process produced

DSA with far fewer defects than either the chemo-epitaxy process made with XPS

(Figure 4.7A) or XPMOST (Figure 4.7B). Unfortunately, the defectivity achieved with

the hybrid process on this initial DSA attempt was still much higher than IMEC’s 28 nm

PS-b-PMMA process,116 and determining the contributors of defectivity is an ongoing

area of research. Still, the ability of the hybrid process to produce better DSA than the

similar chemo-epitaxy process warranted further investigation.

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Figure 4.7: Representative micrographs of DSA under best conditions using (A) chemo-

epitaxy process with XPS guide stripe (B) chemo-epitaxy process with

XPMOST guide stripe (C) hybrid process with 19 nm tall XPMOST guide

stripe. Scale bar is valid for all images.

Cross-sectional STEM with electron energy loss spectroscopy (EELS) was used

to study the BCP orientation on the topography in the hybrid process. Regardless of the

features observed in top-down SEM, BCPs can adopt complex and non-uniform

morphologies in thin films that may render them useless or at least sub-optimal for

pattern transfer.59 Multiple guide stripe widths were analyzed to probe the influence of

this pre-pattern characteristic on the resulting through-film BCP structures. Figure 4.8

shows STEM EELS cross-sections of the pre-pattern and BCP film stack at three guide

stripe widths. Below each STEM image is a corresponding illustration of the proposed

chain conformation that maintains the pattern fidelity discernable from the EELS images,

offering insight into the key interactions that govern DSA. The green or blue vertical

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segments in each EELS image map silicon and oxygen, respectively, and correspond to

the PTMSS block (image acquisition differences caused oxygen to be measured over

silicon in Figure 4.8C, but the qualitative result remains the same). The color magenta

maps carbon and corresponds to several different carbonaceous regions in the film: the

vertical magenta domains between the green or blue domains correspond to the PMOST

block, the magenta horizontal stripe on top of the BCP domains is the top coat, the

thinner magenta horizontal line under the BCP corresponds to the brush layer, and the

magenta trapezoid corresponds to the guide stripe. The silicon substrate (green) and the

metal oxide capping layer used for sample preparation (blue) are also seen at the very

bottom and top of the images, respectively. A key difference immediately discernable

from the STEMs is that each guide stripe pins a different number of BCP domains,

demonstrating three different DSA regimes. Going from Figure 4.8A to Figure 4.8C, the

guide strip width increase from 13 to 18 to 28 nm (circa 0.5 to 1.0 to 1.5 L0), which

corresponds roughly to one, two, and three BCP domain situated on top of the guide

stripe, respectively, which is also depicted in the illustrations.

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Figure 4.8: Summary of the cross-sectional STEM EELS study of BCP on tall guide

stripes in the hybrid flow. Guide stripe width (measured by CD-SEM) (A)

~0.5 L0 (B) ~1.0 L0 (C) 1.5 L0. (Top) STEM EELS mappings of cross-

sectioned DSA film stacks. The legend in the top right describes the

elemental mapping color scheme for all three EELS images. Scale bar is

valid for every image. (Bottom) Schematic of the proposed BCP domain

orientation on guide stripes corresponding to difference quantizations of the

BCPs natural periodicity, along with depictions of the anticipated polymer

chain configuration. As shown in the legend, blue corresponds to PMOST

and red to PTMSS.

Importantly, the STEM EELS micrographs show that the BCP domains travel

vertically through the film without adopting complex 3D morphologies. However, the

tapered, wedge-shaped PTMSS domains in Figures 4.8A and 4.8C and the short PTMSS

domains sitting on top of the guide stripes in Figures 4.8B and 4.8C are both sub-optimal

features. Since the PTMSS domains become the hard mask for pattern transfer, any

degree of non-uniformity in these domains impedes this process. The wedge-shaped

PTMSS domains are a result of the domains ordering over the angled sidewall of the

guide stripe. The XPMOST guide stripe draws the PMOST block to the top of the guide

stripe, placing a PMOST to PTMSS intramolecular interface at the guide stripe edge and

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the PTMSS domain directly over the sloped sidewall. Hypothetically, modifying the DSA

scheme by changing the guide stripe to be PTMSS preferential places a PMOST block on

either sidewall and avoids PTMSS domain distortion. Wedge-shaped domains or other

distortions are tolerable for PMOST since PMOST is the sacrificial block. The other non-

uniformity in the PTMSS domains, the short PTMSS domains on top of the guide stripes

in Figures 4.8B and 4.8C, are not readily fixed. The height difference between PTMSS

domains on the guide stripes and over the backfill brush region is an inescapable

consequence of using shallow topography. While height non-uniformity complicates

subsequent pattern transfer, the uniform structures present in the top half of the film offer

a potential route for isolating viable mask features.

Wedge-shaped PTMSS domains are absent in Figure 4.8B which indicates a

fundamental difference in the conformation of the BCP in this scenario. The immediately

discernable difference is that the intermolecular boundary of two PMOST blocks- and not

the intramolecular block-block interface- is now situated at the edge of the guide stripe.

As a consequence, at the center of the guide stripe sits a full domain of PTMSS, flanked

by half-domains of PMOST on either side. At first glance, the opposite situation, where

PMOST is the component in the center of the guide stripe, flanked by two PTMSS

domains, is equally valid from an enthalpic standpoint, since in both cases a total of one

domain of PTMSS and one domain of PMOST sit on top of the guide stripe. Ostensibly,

the guide stripe sidewall, oxidized during the trim etch, is polar wetting and produces the

key pinning force to anchor the polar PMOST block and induces the BCP conformation

seen. The net effect is that since the PTMSS domains are not ordered above the sidewalls,

no wedge-shaped domains are formed. On further inspection, it appears the PTMSS

domain sitting on top of the guide stripe potentially has curvature at its base, which could

be attributed to a PMOST wetting layer forming underneath the block to minimize

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unfavorable enthalpic interactions. To minimize sub-optimal wetting situations, the guide

stripe should be made perfectly neutral for the BCP in the case of 1 L0 pre-patterns. Since

one full domain of PMOST and PTMSS sit on top of the guide stripe in this situation, a

neutral guide stripe’s interfacial contribution to the overall free energy of the system

would be lower, potentially improving the quality of DSA.

Despite the location of the PTMSS domains directly over the sidewall regions

when guide stripe width = 0.5 or 1.5 L0, it is anticipated that the sidewalls also pin the

PMOST block in this situation due to the unfavorable enthalpic interactions of the

oxidized sidewall with the hydrophobic PTMSS block. To accomplish this, the PMOST

domains must “spill-over” the guide stripe, as illustrated in Figures 4.8A and 4.8C. This

proposed topology further exacerbates the sub-optimal wedge shape of the PTMSS

domains. Unfortunately, due to the chemical similarities between the guide stripe and the

PMOST, it is not possible to tell with certainty if a layer of PMOST interfaces with the

sidewall. Efforts are underway to incorporate nitrogen into the guide stripe to serve as an

atomic label in STEM EELS to determine the identity of the BCP component interfacing

with the sidewall.

The preceding analysis of the hybrid process implicates the sidewall as having a

large role in the DSA. From tests done on blanket films, it was shown that the hydroxyl-

terminated brushes do not graft to oxidized XPMOST (or XPS) at the conditions used to

graft them to the inorganic substrate, so the polarity of the sidewall is not significantly

decreased before the BCP is applied. Although polymers oxidized by plasma have been

shown to rearrange to hide polar functional groups from the surface,119 the confined

volume of the guide stripe and its cross-linked nature probably prevents a large degree of

rearrangement from occurring. The BCP is forced to interface with this very polar

surface, which changes the nature of the DSA.

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4.5 DOCUMENTATION AND CONSEQUENCES OF THE SIDEWALL IN DSA

Since nearly all processes used to make pre-patterns for DSA involve patterning

and plasma etching a surface modification layer covered by resist, it is likely that many

processes already rely on or exploit a polar sidewall as an anchoring surface. A

discussion of some of the most significant contributions to this idea follows, and

concludes with a proposal for a novel way to perform DSA.

One of the first documented instances where an oxidized region of film served as

an anchoring surface for DSA was in work done by Yang et al.107 Plasma etching was not

used to oxidize the surfaces, rather, self-assembled monolayers of

octadecyltrichlorosilane were patterned by interference extreme ultraviolet lithography

(EUVL). The exposed regions became very hydrophilic, while the unexposed regions

remained hydrophobic.44 PS-b-PMMA was then deposited on top of the surface, and after

annealing, the BCP domains aligned themselves to the stripes of alternating surface

energy. Although in this example plasma etching was not used to create oxidized

surfaces, it does demonstrate the potential of a surface modified by oxidation to function

as a pinning line.

Work done by collaborators at HGST explicitly implicates the role of the oxidized

sidewall in DSA. Cushen et al.97 report defect-free DSA of PMOST-b-PTMSS over a

large area using solely XPS for the guide stripe and PS for the backfill brush, subject to

the constraints of shallow topography (brush height << guide stripe height), relatively

dense pre-patterns, and 1 L0 guide stripe width, much like the situation depicted in Figure

4.8B. The authors’ interpretation was that the sidewalls of the guide stripe, oxidized

during etching, provided the chemical contrast for DSA (since the chemically similar

guide stripe and brush imparted no chemical contrast), as seen in Figure 4.9. However,

the DSA was also highly dependent on the topography of the system, which strongly

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confined the BCP chains and promoted the desired alignment. Even slight deviations

from this confining geometry resulted in no DSA.

Figure 4.9: Orientation of BCP chains in the process flow, depicting the oxidized

sidewalls as guiding interfaces.

Subsequently, Nealey and coworkers reported evidence implicating “three-toned”

DSA as a guiding mechanism in chemical pre-patterns produced by the LiNe process for

PS-b-PMMA, where the top of the guide stripe, the oxidized sidewall, and the backfill

brush constitute three chemically unique surfaces for DSA.111 The authors were

investigating chemo-epitaxy but discovered that even in their planar process, a degree of

topography remained in the pre-pattern, and a “shoulder” of oxidized sidewall remained

exposed to the BCP and probably assisted in DSA. The authors concluded that the extra

guiding surface provided by the sidewall could help to decrease defectivity.

These examples show the potential role of the oxidized sidewall in DSA, but one

largely unexplored possibility is to rely on sidewall guided DSA to relax the initial

lithographic requirements for the pre-pattern. Since each lithographically defined feature

has two sidewalls, a DSA scheme using the sidewalls as anchors in effect doubles the

number of available pinning surfaces. This idea was first formalized by Cushen et al. as

“double-patterned sidewall” DSA, as a direct analogy to self-aligned double patterning

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techniques.97 We hypothesize that DSA with a large degree of density multiplication can

be achieved solely through the sidewall with careful design of the guide stripe and brush,

resulting in less demanding initial lithography. This concept becomes especially

important as BCP feature size shrink and initial lithographically-defined pre-patterns

become proportionally sparser. Figure 4.10 shows an illustration of how such a DSA

scheme might look. The sidewall is deliberately created and left unobstructed to serve as

the pinning force. The guide stripe and brushes are made to be “nearly-neutral” to the

BCP to reflect the small degree of asymmetry present on top of the guide stripe and in the

brush region (in both regions, there is one fewer blue domain than red due to blue being

pinned by the guidestripe). In this way, the enthalpy of the system is minimized while

guide surfaces remain to form DSA. There are many similarities with Figure 4.10 and the

previously described work, however, the nuanced changes in the guide stripe and brush

surface energies make this scheme potentially more optimized for DSA.

Figure 4.10: Illustration of a possible DSA scheme using sidewall guided assembly to

relax the initial pre-pattern.

One undervalued phenomenon in these studies is the inability of the brush

deposition step to modify the oxidized sidewall. As stated explicitly by Cushen et al., the

hydroxyl-terminated brushes graft to inorganic oxide (specifically SiO2) upon heating,

but fortuitously, they do not graft to the oxidized XPS (or XPMOST in our case). This

selectivity is not general and seems to be almost unique to these two polymers (XPS and

XPMOST). The hydroxyl-terminated brushes do graft to oxidized surfaces of Novolac,

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Nanoimprint lithography resist, acrylics, and many if not most other polymers. Due to

these developments, the need to carefully design the chemistry of the guide stripe is

becoming increasingly apparent.

4.6 PATTERN TRANSFER

The ultimate goal of DSA is to create a topographic pattern to serve as an etch

mask for underlying layers. To accomplish this, the sacrificial component (in this case

PMOST) needs to be removed without removing the mask component (PTMSS), and

then the mask component needs to be able to withstand the etch process used to transfer

the pattern into the underlying layer. The silicon nitride anti-reflective layer upon which

the DSA film stack is constructed is an unfortunately difficult target for silicon-

containing BCPs to etch since both materials contain silicon. However, progress towards

this goal was accomplished at IMEC.

The first step of pattern transfer was to ensure the BCP pattern could be fully

developed to produce well-defined structures without any toppling or distortion of the

domains. An etch recipe had previously been developed to etch UT silicon-containing

BCPs by Steven M. Sirard of Lam Research,35,120 and great results had been

demonstrated on fingerprint patterns. As shown in Figure 4.11, the BCPs can be etched

to fully remove the organic material in the interstitial regions without toppling, bending,

or bowing the silicon-containing domains.40 This work was done with BCPs of three

different chemistries and two pitch ranges to demonstrate that the etch process was robust

and not highly dependent on block chemistry or pitch. Notably, the 20 nm PS-b-PTMSS

result, the 14 nm PMOST-b-PTMSS result, and the 14 nm polystyrene-b-poly(4-

pentamethyldisilylstyrene) (PS-b-PDSS) result show a large degree of bridging between

the features. This is attributed to the BCP samples being close to their order-disorder

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transition temperatures85 and not an artifact of the etch, as the two 20 nm samples do not

contain bridging.

Figure 4.11: Top down and angled SEM of (a) 20 nm polystyrene-b-poly(4-

trimethylsilylstyrene) (PS-b-PTMSS) (b) 20 nm poly(4-methoxystyrene)-b-

poly(4-trimethylsilylstyrene) (PMOST-b-PTMSS) (c) 20 nm polystyrene-b-

poly(4-pentamethyldisilylstyrene) (PS-b-PDSS) (d) 14 nm PMOST-b-

PTMSS (e) 14 nm PS-b-PDSS. Reproduced with permission from Durand et

al. “Design of High-chi Block Copolymers for Lithography.” J. Poly Sci. A

2015, 53. 344-352. Copyright 2015 Wiley.

A similar BCP etch process was used at IMEC to etch the PMOST-b-PTMSS.

Typically, only a partial etch (targeting approximately 50% of a 30 nm thick BCP film)

was used for most experiments, as over-etching the features would remove the organic

surface treatments below the BCP and topple the features. Fortunately, only a brief etch

was necessary to generate sufficient contrast for top-down SEM inspection, but for

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pattern transfer, the features needed to be fully developed. Initially, existing DSA wafers

of a fixed BCP thickness were re-etched for additional time increments to determine the

point at which the features began to distort. XPS guide stripe chemo-epitaxy wafers were

used for this experiment to avoid the complications of topography present in the hybrid

process. As Figure 4.12 shows, it was discovered that the BCP etch rate across the wafer

was not uniform, and that features in the center of the wafer remained intact as features at

the edge of the wafer became heavily distorted. While not ideal from a manufacturing

standpoint, this etch rate uniformity proved beneficial since the advent of feature collapse

on just one wafer could be coupled with an estimation of the etch rate difference

(measured on blanket resist wafers) to pinpoint the amount of time the BCP could be

safely etched.

Figure 4.12: Over-etch study on BCP features. The wafer was etched for 75 s total.

Features from chips at the edge of the wafer are etched more heavily than

those at the center (note: the DSA quality was not uniform across the wafer,

so only the quality of the lines, and not their vertical alignment, was the

metric for over-etching). A similar wafer etched for 60 s showed no

difference from center to edge, implying that the threshold for domain

distortion is between 60 and 75 s for the features on the outer chips. Scale

bar is 200 nm and valid for all four images.

A DSA wafer was etched to the point before feature collapse and cross-section

micrographs were obtained to examine the BCP in greater detail. As seen in Figure 4.13,

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under these etch conditions, the features are fully developed and no indication of any

organic material (either sacrificial BCP block or underlying brush) remains.

Figure 4.13: Initial demonstration of fully developed PMOST-b-PTMSS DSA features at

IMEC. The wafer was etched to the point just before the features were

expected to start distorting. (Top right) Top-down SEM of the region

submitted for cross-section. The area was chosen for its good DSA result.

(Top left) Tilted-view SEM of the features. (Bottom) Cross-section SEM of

the features. Measurements indicate that approximately 15.5 nm (out of 33

nm initial film thickness) remain. Scale bars are each 50 nm.

The etch transfer into silicon nitride and eventually silicon was then

demonstrated. As Figure 4.14 shows, pattern transfer without much loss of pattern

fidelity or mask height was possible. The silicon nitride was first etched with a CH3F

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based chemistry known to etch silicon nitride selectively over photoresist. Fortuitously,

this gas chemistry also etched the silicon nitride without quickly etching the BCP mask.

It is possible that the residual carbon in the BCP domains acts as the component that

resists the etch in lieu of the silicon also present in the BCP mask. Further tests are

needed to explore this possibility. The silicon etch step that followed was performed with

Cl2 chemistry known to etch silicon, and was able to further transfer the pattern.

Unfortunately, these etch tests suffer from an imperfect degree of DSA in the initial

pattern, which masks the origin of the bridging and wiggling seen in the silicon nitride

and silicon etch steps. Ideally, further etch development should be performed with DSA

containing fewer defects to unambiguously elucidate the origin of any imperfections in

the resulting structures.

Figure 4.14: Tilted-view SEM showing pattern transfer of BCP features into silicon

nitride and then silicon. Scale bars are each 100 nm.

Another experiment showed the need to properly develop the BCP before pattern

transfer. The goal of the experiment was to etch the underlying silicon as much as

possible with the DSA film stack to determine the maximum etch depth possible. To aid

in this attempt, the BCP was deliberately under-etched compared to previous pattern

transfer demonstrations (Figures 4.13 and 4.14) to leave behind as much mask as

possible to serve as an etch barrier. The silicon nitride was then etched like normal, and

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then the silicon was etched until all of the silicon nitride was consumed. As seen in

Figure 4.15, aspect ratios of 3.5 to 5.0 in the silicon are possible, with etch depth of

approximately 35 to 45 nm. This comes from an initially 33 nm thick BCP film, which

means that there is a net gain in feature height despite the unfavorable silicon-containing

film stack. Unfortunately, a large amount of bridging is visible in the final silicon step,

believed to be a result of the BCP under-etch in the first step. Despite the top-down SEM

of the BCP showing clean features, the subsequent silicon-nitride etch shows some

degree of bridging, which becomes unmistakably present in the high-aspect ratio features

of the silicon etch. Evidently, small amounts of organic matter remain in the BCP trench

after the shorter etch time, which is enough to prevent the full development of silicon

nitride and silicon. This level of bridging was not observed in the experiment shown in

Figure 4.14 which contained a longer BCP etch, despite using the same combination of

etch chemistries. Evidently, the duration of the BCP etch step needs to be balanced to

avoid toppling the features due to over-etching and not removing all of the matter in the

trenches due to under-etching, but fortunately it appears that such a process window does

exist.

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Figure 4.15: Pattern transfer attempt for a “deep” silicon etch. (Top left) Top-down SEM

of area to be cross-sectioned. BCP was etched to a lesser degree than in

previous tests, and the white lines are noticeably thicker than in other BCP

pattern transfer images. (Top right) Top down SEM of silicon nitride etch.

The features have more waviness line edge roughness than previous, and

some bridging appears to be occurring. (Bottom left) Tilted view SEM and

(Bottom right) cross-section SEM of the silicon etch step. The silicon was

etched to the point of exhausting the silicon nitride mask to determine the

maximum etch depth possible. Analysis shows aspect ratios between 3.5 and

5.0. The bridging is probably the result of an insufficient BCP removal in

the first step.

4.7 MASK LAYOUT AND CD-SEM MEASUREMENTS

The DSA work done at IMEC was performed on 300 mm wafers which provided

a unique opportunity to explore multiple process variables on the same wafer, especially

the variables associated with pre-pattern geometry. The rest of the chapter describes how

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experiments were performed to quantitatively evaluate DSA performance on this

platform.

Figure 4.16 shows a schematic of the “vinaigrette” mask used to expose each

wafer. The vinaigrette reticle defined an area approximately 14 mm tall and 20 mm wide

on the substrate, and 11 columns and 19 rows of the reticle were printed on a wafer.

Many different regions of line-space patterns exist on the vinaigrette mask to support a

variety of DSA experiments, but the “scatterometry targets” were most useful for this

project. This region contained a matrix of 60 by 60 µm fields whose pitch and line width

on the mask were systematically biased, as shown in Figure 4.17A. The pitches range

from 72 to 200 nm, and the mask dimensions are biased from approximately 30% to 60%

of the pitch. Figure 4.17B shows a close up of an individual “target” and shows how

each target contained a region of horizontal and vertical stripes; due to light polarization

during the exposure, only the vertical lines printed. (Conveniently, the resist was left

intact in the top horizontal non-printing section, protecting the underlying mat and

creating a region where island-hole formation could be observed for diagnostic purposes).

As seen in Figure 4.17B, each target was labeled with a pitch and nominal critical

dimension (CD), although the final resist dimensions on the wafer was largely

determined by the exposure conditions. In addition to the variation in pre-pattern

structures available within one exposure, the commercial 193i exposure tool allowed for

a systematic bias of the exposure dose on a chip-by-chip basis to create a corresponding

variation in line widths, generating a large matrix of essentially unique chips. However,

the large amount of data present on each wafer necessitated the development of a robust

methodology to analyze each wafer.

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Figure 4.16: Schematic of the “vinaigrette” mask used to print pre-patterns for DSA at

IMEC. The different purple regions correspond to regions with different

purposes including cross-sectioning and defect contribution analysis. The

“scatterometry target” region was used for this project.

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Figure 4.17: (A) Optical micrograph of the pattern produced by the scatterometry target

fields. The pitch of the lines-space pattern increases to the right, and the

width of the printed line increases in the vertical direction. (B) SEM

micrograph of one of the scatterometry targets. Each block is labelled with a

mask critical dimension (CD) and pitch (nanometers for both). The top dark

block contains horizontal features, and the bottom block contains vertical

features, where DSA is expected to occur. The height and width of each

individual box is 60 µm for scale.

Automated CD-SEM was used to inspect the wafer after the exposure step, after

the trim etch step, and after the DSA film stack was etched. The locations of different

scatterometry target fields were programmed into the CD-SEM, and the instrument

automatically imaged a select number of chips at these locations. It was necessary to

measure the resist features to elucidate information about the guide stripe width. The CD-

SEM software could measure the width of resist features before etching very well, but

measuring the resist features after etch was more complicated. This was unfortunate since

the after-etch resist width correlates most closely to the DSA variable of guide stripe

width. As shown in Figure 4.18A, the resist features before etch had a much different

brightness level than the darker background regions, which facilitated the identification

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of the resist edge by the software and enabled trouble-free width determination. The

corresponding after-etch resist features in Figure 4.18B, however, had nearly the same

brightness as the background, which complicated the CD-SEM’s image detection

procedure. Offline assessment of the micrographs was necessary to determine the etched

resist width measurements, which slowed down the acquisition of this crucial variable. Of

note is that a black “halo” can be seen around the after-etch resist features, which is

inferred to be unremoved cross-linked mat (Figure 4.18C). This “shoulder”, partially

etched but not fully removed, becomes the oxidized sidewall that was discussed

previously.

Figure 4.18: CD-SEM micrographs of (A) resist structures after exposure and

development (B) and resist structures after trim etch both show the resist as

the thin gray line. The resist and mat are depicted in cross-section after etch

in (C) to show how the sidewall (also called “shoulder”) remains exposed to

form the black halo seen in (B) around the resist features. The line pitch in

(A) and (B) is 79 nm for scale.

DSA inspection was also performed by CD-SEM. Rather than attempt a direct

measurement of the DSA, the CD-SEM simply took a micrograph of the BCP features

present at the desired coordinates. As discussed previously, most wafers were made with

a gradient in the exposure dose chip-to-chip, so the “same” location on different chips

could potentially have much different guide stripe widths (as will be discussed later,

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guide stripe width was a major factor in the quality of DSA). This necessitated imaging at

least half the wafer to ensure that the full range of guide stripe widths were sampled

(most wafers had symmetry about the x-axis removing the need to image the whole

wafer). Although the micrographs could be and were examined by hand, the need for a

quantitative method to compare different micrographs arose due to the sheer number of

micrographs taken and the subtle differences in DSA quality that emerged. A computer

program was written to evaluate an image for DSA quality, where the output was an

estimation of the percent of pixels in a micrograph “involved” in DSA (the program’s

algorithm is proprietary to Lam Research and unfortunately cannot be described here). As

seen in Figure 4.19, the scores produced by the program satisfyingly differentiated

between different degrees of DSA and thus were reliable metrics for DSA quality. Scores

of 100% were never observed, presumably due to ever-present line-edge roughness, and

defect-free micrographs of DSA typically scored 80 to 85%.

Figure 4.19: Representative micrographs of different degrees of DSA and their

corresponding score from the image scoring program.

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The output of the image scoring program was often used to create a wafer map to

help determine trends across a wafer. A sample wafer heat map appears in Figure 4.20

and helps illustrate an important point about the results. This wafer, like the majority of

the samples, was produced with a dose gradient on the wafer (the right-hand side of the

wafer is high dose) to create a corresponding gradient of guide stripe widths. Since this

heat map represents the scores of micrographs taken at the same scatterometry target on

each chip, any variation in the scores in this heat map is almost entirely due to the guide

stripe width variation caused by the systematic exposure dose bias. Going from left to

right, two regions of “good” DSA can be clearly differentiated by their difference in color

from a region of “poor” DSA right down the middle of the wafer. Evidently, two

different regimes of guide stripe widths produce DSA. As shown in Figure 4.21, the two

regimes are anticipated to correspond to guide stripes pinning 0.5 and 1.5 L0 (10 and 30

nm) of BCP, and appear in both 4x and 5x density multiplication. The ability of the guide

stripe to pin different amounts of BCP based on its width was previously demonstrated by

the cross-section STEM experiment of the hybrid process presented in Figure 4.8, and a

further study of the guide stripe width for both the chemo-epitaxy and hybrid process

appears further on in the chapter. In the STEM experiment it was uncovered that the

hybrid process actually had a third regime of DSA (1 L0 guiding). However, this regime

performed worse than either 0.5 or 1.5 L0, and was omitted in subsequent studies. In

addition to the guide stripe width, the image scoring program was used to explore the

following process parameters: guide stripe pitch, guide stripe chemistry, brush chemistry,

BCP film thickness, anneal temperature, and anneal time. The micrographs presented in

Figure 4.7 and the comparisons made between the hybrid process and the chemo-epitaxy

process with either guide stripe material are the result of optimizing these parameters

separately for each process.

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Figure 4.20: Representative heat map of DSA scores across one wafer with a dose

gradient applied (the right-hand side of wafer has the highest dose so

smallest guide stripe width). The two heat maps show the same data but are

printed in different color schemes.

Figure 4.21: Schematic of the anticipated DSA schemes on the wafer, labelled with the

density multiplication (either 4x or 5x) and the number of periods the guide

stripe width corresponds to (either W=0.5 or W=1.5).

103

4.8 QUANTITATIVE COMPARISON OF THE DSA PROCESSES

The DSA image scoring process was used to determine the optimal conditions for

each of the DSA processes, and then the optimized products were compared against each

other. This section describes a summary of the results, and then subsequent sections

describes the individual optimizations one-by-one.

4.8.1 Overview of the different DSA processes

XPMOST chemo-epitaxy was compared directly to XPS chemo-epitaxy. The

chemical similarity between the XPMOST guide stripe and the PMOST block of the BCP

was expected to result in a greater degree of anchoring between the two materials,

manifested in better DSA. Figure 4.22 shows that this hypothesis was true. Figure 4.22A

shows the two chemo-epitaxy processes under their final optimized conditions. In all

cases, XPMOST outperforms XPS to produce better DSA, supporting the guide stripe

strength argument. This effect is even more pronounced in Figure 4.22B, where the

conditions used to create both chemo-epitaxy wafers were far from ideal. (Section 4.11.2

describes the exact conditions used to make all three wafers). In this experiment, the

ability of the XPMOST to produce better DSA over XPS is clear. However, the

improvement in DSA scores for XPMOST in the best result for both cases, 4x density

multiplication with 0.5 L0 wide guide stripes, is negligible. To this effect, XPMOST

failed to produce a practical improvement over XPS in the criteria of overall defectivity.

The ability of XPMOST to produce better DSA than XPS for PMOST-b-PTMSS is

significant from a process window standpoint, but further optimizations in the process are

required to reduce defectivity.

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Figure 4.22: Direct comparison of the chemo-epitaxy process with XPMOST and XPS

guide stripes at the four DSA schemes (A) under conditions optimized for

each process (B) under deliberately sub-optimal conditions.

One such attempt to reduce defectivity was the creation and implementation of the

hybrid process. This process largely mirrored the XPMOST chemo-epitaxy process, but

increased the thickness of the XPMOST guide stripe to produce topography. Figure 4.23

shows the results of increasing the thickness of the guide stripe. Wafers were prepared

identically except for changing the XPMOST mat coating thickness to 9, 15, and 19 nm

by changing the spin-coating speed. The 9 nm result corresponds to the chemo-epitaxy

process, and the two other thicknesses constitute the hybrid process. At 4x density

multiplication, the DSA scores increased monotonically with increasing guide stripe

thickness, and produced among the highest DSA scores recorded. The results were less

consistent at 5x density multiplication, but since this region of the wafer performed worse

than the 4x region overall, the implementation of the hybrid process was deemed an

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overall success. Going forward, all instances of the hybrid process used a 19 nm thick

guide stripe.

Figure 4.23: Comparison of the hybrid process to the chemo-epitaxy process. The 9 nm

data correspond to the chemo-epitaxy process made with XPMOST, while

the other two thicknesses correspond to hybrid process with XPMOST.

Error bars are 3 standard deviations.

4.8.2 Guide stripe pitch

The effect of the pre-patterned guide stripe pitch on DSA was investigated by

comparing DSA scores taken at different pitches in the vicinity of 100 nm on the same

wafer. The bulk periodicity of the BCP was determined to be approximately 19.7 nm by

SAXS taken at room temperature of a sample annealed at 180 °C. A fast Fourier

transform (FFT) of the fingerprint BCP pattern, made and imaged on IMEC CD-SEMs,

was also performed as seen in Figure 4.24. The peak location in the FFT was determined

to be 19.3 to 19.6 nm, corroborating the SAXS value. Therefore, pitches near and slightly

below 100 nm were anticipated to capture the 5x density multiplication regime. Figure

4.25 shows a statistical summary of the DSA scores of pitches near 100 nm taken from a

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single wafer prepared with the chemo-epitaxy process (XPS guide stripes). A clear trend

in the DSA quality emerges, with a DSA score maximum centered around a pitch of 98

nm (the scores at 97 and 99 nm are not statistically different, however). If one assumes

the best DSA occurs when the pre-pattern pitch is most commensurate with the natural

periodicity of the BCP, a value of 98 nm implies the effective thin film L0 of the BCP is

19.6 nm, which agrees well with the values determined by the two other methods.

Figure 4.24: (A) Top-down SEM of fingerprint pattern of ~20 nm BCP (B) 2D fast

Fourier transform (FFT) of the image in (A). FFT software provided

courtesy of http://www.ejectamenta.com.

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Figure 4.25: DSA score versus pre-pattern pitch of a chemo-epitaxy process wafer made

with XPS. The data were taken from the region where the guide stripe

pinned approximately 0.5 L0 of BCP. The box plot represents the five

number summary of the data.

The pre-pattern pitch was also investigated at approximately 80 nm to probe the

4x density multiplication regime. Figure 4.26 shows the results from a hybrid process

wafer. Unfortunately, the quality of the resist structures degraded markedly below 78 nm,

so only pitches of 78, 79, and 80 nm were compared. Results are included from the

region on the wafer corresponding to where the guide stripe pins 0.5 L0 (Figure 4.26A)

and 1.5 L0 (Figure 4.26B). The 1.5 L0 DSA scores are higher than the 0.5 L0 scores, and

both are higher than the sub-optimal XPS chemo-epitaxy results reported in Figure 4.25.

In Figure 4.26B it appears that 79 nm performed slightly better than 80 nm, but the

relationship is flipped in Figure 4.26A. Both pitches scored better than 78 nm, even

though 78 nm is closer to the “magic” BCP L0 value of 19.6 nm determined for the 5x

density multiplication (78 / 4 = 19.5, 79/4 = 19.75, 80/4 = 20). The DSA quality trend

due to pre-pattern pitch at 4x density multiplication is less conclusive than the same

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experiment at 5x, but nonetheless demonstrates that high quality DSA can be achieved in

this pre-pattern pitch regime.

Figure 4.26: DSA score versus pre-pattern pitch of a hybrid process wafer. The data

were taken from the region where the guide stripe pinned approximately 0.5

L0 of BCP (A) and 1.5 L0 (B). The box plot represents the five number

summary of the data.

4.8.3 Guide stripe width

The effect of guide stripe width on DSA was also studied with the aid of the DSA

scoring software. Guide stripe width was approximated from resist measurements taken

by CD-SEM after the trim and breakthrough etch step but prior to the resist strip step. As

mentioned previously, this measurement was difficult for the CD-SEM to make due to

reduced contrast between the resist features and the background regions and required

manual correction of the majority of the micrographs. In addition, the presence of the

partially etched shoulder of the guide stripe makes the “true” width of the guide stripe

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ambiguous. Figure 4.27 shows a zoomed-in CD-SEM micrograph and highlights these

points.

Figure 4.27: CD-SEM micrograph of the resist features highlighting the approximate

resist edge and the presence of the partially etched guide stripe.

After measuring the width of the guide stripes, the DSA process was completed,

and the wafer was imaged by CD-SEM. The DSA scores calculated from these

micrographs were then mapped to the previously measured guide stripe widths to create

Figure 4.28. Figure 4.28A is the correlation of DSA scores on guide stripe width for an

XPS chemo-epitaxy process wafer, and Figure 4.28B is the same relationship for a

hybrid process wafer. These results were measured at 79 nm guide stripe pitch (4x

density multiplication), and the results at 98 nm pitch (5x density multiplication) are

qualitatively similar. XPMOST chemo-epitaxy results also mirror the XPS chemo-

epitaxy results. A striking number of contrasts emerge from these graphs. First, the

relationship for the chemo-epitaxy process is approximately parabolic and has a

maximum between 16 and 20 nm. The DSA scores in the hybrid process, however, are

almost uniformly high from 7 to 16 nm, and actually dip to their lowest values at the

guide stripe widths that produce the highest scores for the chemo-epitaxy process. In

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addition, Figure 4.28B captures the start of the 1.5 L0 guide stripe DSA regime at

approximately 24 nm for the hybrid process, while in Figure 4.28A this regime is absent,

although the window is known to exist at guide stripe widths closer to 30 nm for the

chemo-epitaxy process. It appears that in addition to producing higher overall higher

scoring DSA micrographs, the hybrid process is much more tolerant of variation in the

guide stripe width than the chemo-epitaxy process.

Figure 4.28: DSA score versus guide stripe width for (A) XPS chemo-epitaxy process

wafer (B) hybrid process wafer for 20 nm PMOST-b-PTMSS at 79 nm

guide stripe pitch (4x density multiplication).

The ideal guide width is expected to be 10 nm (0.5 L0) in the chemo-epitaxy

process in this guide stripe width regime, however the measured maximum appears

between 0.8 and 1.0 L0. Some of this 6 to 10 nm deviation from 10 nm could likely be

attributed to the uncertainty between the resist and guide stripe width measurement, as

discussed previously. However, other research has also documented the preference of

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guide stripes wider than 0.5 L0 in the chemo-epitaxy DSA of PS-b-PMMA.121,122 This

explanation could account for the wider than expected guide stripes, even though the

deviation seen here is larger than that seen for PS-b-PMMA (typically reported as 0.6-0.8

L0). Also possible, but less likely, is the hypothesis that the guide stripe actually pins 1.0

L0 of BCP. This hypothesis would explain why the ideal guide stripe width is closer to 20

nm than it is to 10 nm. However, it does not explain the absence of any indication of 0.5

L0 guide stripe DSA, which would still be expected to occur even if 1.0 L0 was the

predominant DSA scheme. In addition, the only previous account of 1.0 L0 DSA by

Cushen97 describes the need for a large degree of topography for the 1.0 L0 DSA to occur,

which is absent in the chemo-epitaxy process. A cross-sectional analysis of the DSA in

the chemo-epitaxy process could potentially clear up this situation, but the experiment

would have to be performed in such a way that the guide stripe boundary was

discernable, which was not the case in the STEM EELS experiments used to probe the

hybrid process.

4.8.4 Backfill brush composition

Three candidate PS-r-PtBuS hydroxyl-terminated brushes, representing a range of

surface energies, were screened for DSA behavior in both processes to determine the

optimal brush composition (the fourth brush, introduced in Table 4.1, was determined to

be outside of the range of surface energies needed for DSA and largely ignored). The

surface energy of the backfill brush used in a DSA process must be precisely tuned to

produce the best DSA.59 The optimum brush surface energy for DSA, however, is

different from the surface energy that is non-preferential to the bulk BCP.59,63 Due to the

unequal number of BCP domains that interface with the region defined by the backfilled

brush (for n density multiplication, where one PMOST block is pinned by the guide

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stripe, the ratio of PMOST to PTMSS in the backfill region is n – 1 : n), the backfill

brush deviates from neutrality in the direction of the majority block to compensate for

asymmetry, and Nealey et al. suggest that the ideal backfill brush surface energy is

“neutral” to the weighted average of the BCP domains interfacing with the backfill

brush.59 However, complex substrate-BCP interactions,45 substrate reorganization

effects,123 and the sensitivity of surface energy to small chemical changes56 preclude any

diagnostic test of the brush outside of actual DSA implementation. To this effect, as

described previously, a series of functionalized brushes were prepared with different feed

amounts of PS and PtBuS (Table 4.1) to test DSA performance. The brush samples are

labelled as BB-nn, where nn is the mole percent of PS in the brush, measured by NMR.

Since PS has a higher surface energy than PtBuS, higher PS content results in a more

hydrophilic and higher surface energy brush. As described previously, island-hole118

experiments determined that BB-85, the sample with highest PS content, is

approximately neutral for the BCP,63 while BB-50 and BB-68, with more PtBuS content,

are PTMSS wetting. To test these brushes, XPS guide stripe chemo-epitaxy wafers were

made with each brush and inspected. DSA was observed on the wafer produced with

slightly-preferential BB-68 brush, no BCP features were seen on the wafer made with the

more preferential BB-50 brush (it is expected that the BCP features were aligned parallel

to the substrate), and BCP fingerprint pattern without DSA was observed on the wafer

produced with neutral BB-85 brush. Evidently, these three brushes bracket the range of

relevant surface energies for DSA, and the composition and surface energy of slightly-

preferential BB-68 brush is close to ideal for DSA.

To further hone in on the ideal backfill composition, the brushes were mixed to

yield blends with compositions intermediate to the parent polymers. This blending

strategy is not new; Nealey et al. reported that blending homopolymer brushes of PS and

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PMMA yielded blends with wetting properties proportional to the amount of PS and

PMMA in the blend,124 so it was reasoned that blending different PS-r-PtBuS brushes

was a valid way to produce substrate-modifying materials with surface tensions

intermediate to that of the parent brushes. The DSA scores with several brush

compositions are shown in Figure 4.29A for XPS guide stripe and Figure 4.29B for

XPMOST guide stripe. Compositions in bold indicate pure brush and refer to the

corresponding BB-nn sample, while compositions in italics indicate a blended brush,

described in Table 4.2. Comparing Figures 4.29A and 4.29B, the ideal brush

compositions are surprisingly different for the two guide stripe chemistries. Compared to

the ideal brush composition for XPS, 71% PS, the ideal brush composition for XPMOST

is more hydrophobic at approximately 68%. Apparently, to compensate for the higher

surface energy XPMOST guide stripe, a lower surface energy brush is required. This

observation suggests that there is an additional relationship between the guide stripe and

brush surface energies yet to be explicitly quantified. BCP wetting behavior, elucidated

from island-hole studies, serves as a guide for design of backfill brush but at this point,

the ideal backfill brush chemistry for DSA must be empirically determined.

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Figure 4.29: DSA image score versus backfill brush at 4x density multiplication with 0.5

L0 guide stripes for (A) XPS guide stripe chemo-epitaxy process and (B)

XPMOST guide stripe chemo-epitaxy process. Compositions in bold are

pure polymers and compositions in italics are blends. The boxplots represent

the five number summary of the data.

Table 4.2: Weight % composition of the brush formulations used in Figure 4.29.

Sample Composition % BB-50 % BB-68 % BB-85

66 10 90 0

68 0 100 0

71 0 84 16

73 0 70 30

85 0 0 100

B

0

10

20

30

40

BB3 16% BB4,84% BB3

30% BB4,70% BB3

BB4DSA

imag

e sc

ore

(%

)

Backfill brush formulation (% PS)

XPS Guidestripe

A

68 71 73 85

20

30

40

50

60

70

10% BB2, 90%BB3

BB3 16% BB4, 84%BB3

DSA

imag

e sc

ore

(%

)

Backfill brush formulation (% PS)

XPMOST Guidestripe

66 68 71

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There was no statistically significant difference between using BB-71, optimal for

XPS chemo-epitaxy, and BB-68, optimal for XPMOST chemo-epitaxy, in the hybrid

process. Since the process worked equally well with either brush composition, a thorough

optimization was not performed. For all experiments with the hybrid process, BB-68, the

optimal XPMOST chemo-epitaxy brush, was used.

4.8.5 BCP film thickness

The film thickness (t) of the BCP was determined to be a primary contributor

towards DSA quality and was studied by systematically varying t from approximately 1.0

to 2.5 L0 on wafers patterned with XPS guide stripes. As seen in Figure 4.30, varying the

t caused the average DSA scores to vary substantially from 45 to 65% in a periodic

fashion. Three local score maxima are seen at approximately t = 1.2, 1.7, and 2.4 L0, with

corresponding minima at approximately t = 1.2, 1.3, and 2.1 L0. Smaller BCP t values

resulted in better DSA for PS-b-PMMA,125,126 but this is the first documentation of a

periodic dependency of the DSA performance on t. Curiously, other BCP behaviors in

thin films have a periodic dependency on film thickness. For example, the perpendicular

orientation of BCPs in thin films can be preferred at certain film thicknesses due to the

delicate interplay between conformational entropy and surface energy effects.42,43,50,65

Such phenomenon are likely at play here due to confinement effects (both in thickness

and in-plane), and further modeling studies are needed to understand the interdependence

of BCP t and DSA.

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Figure 4.30: DSA score versus nondimensionalized BCP film thickness for a series of

XPS chemo-epitaxy wafers. The points represent average scores and the

error bars represent maximum and minimum values for 4x density

multiplication and circa 0.5 L0 guide stripe width.

An interesting phenomenon emerges when the data is examined not just for 4x

density multiplication with 0.5 L0 wide guide stripes, which is the DSA scheme that

scored the best for this process, but also for the other three schemes. Figure 4.31

summarizes a subset of the data presented in Figure 4.30, with the addition of the results

for DSA with 1.5 L0 wide guide stripes, as well as 5x density multiplication. As

explained, 4x density multiplication with 0.5 L0 wide guide stripes produced the best

results, but surprisingly, 5x density multiplication with 1.5 L0 wide guide stripes scored

nearly as high and came in a clear second. The only immediate connection between the

seemingly opposite DSA schemes is that both have the same number of BCP domains

(which amounts to the same ratio of components) sitting in the brush backfill region, as

shown in Figure 4.21. As was discussed in the polymer brush section, the brush must be

chosen specifically for the BCP components that interface with the backfill region, and

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the brush used for this experiment was the composition found to produce the best DSA in

4x density multiplication with 0.5 L0 wide guide stripes, which by our understanding

could also be the ideal brush for DSA with 5x density multiplication and 1.5 L0 wide

guide stripes, and helps to explain these results. Since the brush is necessarily sub-

optimal for the other two DSA schemes, it makes sense that the DSA in those regions

performs poorly.

Figure 4.31: A subset of the DSA score versus nondimensionalized BCP film thickness

for series of XPS chemo-epitaxy wafers. The points represent average scores

and the error bars represent maximum and minimum values for the four

DSA regimes identified on the wafer.

The same analysis was also applied for the hybrid process. Figure 4.32 shows the

DSA score versus BCP film thickness for three DSA schemes (5x density multiplication

with 0.5 L0 wide guide stripes performed the worst and was omitted for clarity). In the

range of BCP film thicknesses investigated, there was a much smaller variation in DSA

score than in the XPS chemo-epitaxy study. There is some indication that again the film

thicknesses commensurate with the BCP’s L0 (1.5 and 2.0 L0) produce the worst DSA,

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but this effect is less pronounced than for XPS chemo-epitaxy. Most surprising is that 4x

density multiplication with 1.5 L0 wide guide stripes produced the best DSA results,

instead of either of the two DSA schemes previously identified as working the best in

chemo-epitaxy. The reason for this change is not immediately obvious but could again be

potentially attributed to the brush chemistry used to make the wafer.

Figure 4.32: DSA score versus nondimensionalized BCP film thickness for series of

hybrid process wafers. The points represent average scores and the error

bars represent maximum and minimum values for the three identified DSA

schemes. 5x W=0.5 is omitted for clarity, but performed the worst overall.

4.8.6 Annealing time and temperature

The process variables of annealing time and temperature were examined with the

DSA image scoring technique. Optimizing the aforementioned process parameters

decreased but did not eliminate defects, chief among them dislocation defects where the

BCP strays from the pre-pattern for only a few periods. Defects are thermodynamically

unstable127 but annihilation of defects is a kinetic process.128,129 Studying the effect of

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anneal time and temperature on the DSA was performed to better understand the

importance of these parameters on defectivity.

Wafers with the same XPS chemo-epitaxy process were made and annealed at

170°C, 180°C, 190°C, and 200°C. Temperatures above 215°C caused the top coat film to

dewet from the underlying BCP, as seen in Figure 4.33. The top coat is inherently

unstable on top of the BCP, but when annealed only below its glass transition

temperature (Tg), the top coat lacks the mobility to dewet. Dynamic scanning calorimetry

(DSC) measured the Tg to be approximately 200 to 210°C,39 further corroborating that

annealing at or above the Tg of the film leads to dewetting.

Figure 4.33. Optical micrograph of an XPS chemo-epitaxy wafer with a completed DSA

film stack after being annealed at 215°C for 5 minutes. The spots are top

coat dewetting from the BCP surface.

The samples annealed at 170°C and 180°C produced DSA with many regions

believed to be horizontally oriented BCP lamellae (Figure 4.34). Although many

micrographs showed indication of alignment with the underlying substrate, these areas

surrounded gray regions of parallel features. Samples annealed at 190°C and 200°C did

not suffer from these patches. The 170°C sample was remade and annealed for an

additional 70 minutes to confirm the areas were not a processing mistake (Figure 4.34A).

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Even with the longer anneal, the gray regions persisted. Closer examination of these

defect regions (Figure 4.34B) showed what appears to be correctly oriented BCP

domains nucleating from the gray regions. Since the substrate consists of alternating

bands of material that preferentially wet different components of the BCP, perhaps the

mechanism by which DSA occurs involves a step where some patches of the BCP

initially orient themselves parallel to the substrate, and only gradually orient themselves

perpendicularly. Taking “snapshots” of the DSA process by annealing several wafers at a

temperature where the gray regions were previously not witnessed (>190°C) for short

time increments could potentially capture this process in action. Taking real time

micrographs of the surface using a non-destructive imaging technique like AFM could

also accomplish this goal, if a technique was discovered that could image the BCP

features through the top coat.

121

Figure 4.34: SEM micrographs of (A) XPS chemo-epitaxy process annealed at 170°C for

70 minutes and (B) the same film stack annealed at 180°C for 10 minutes.

(B) is enlarged to show mark areas BCP domains are beginning to nucleate

from the horizontal features.

The samples annealed at 190°C and 200°C for 10 minutes produced DSA without

the gray patches and the DSA was analyzed by the image scoring software. Figure 4.35

summarizes these results. In all cases, the 200°C sample produced a superior DSA result.

Besides imbuing the BCP with additional thermal energy to cure defects, it is not obvious

if there is an additional mechanism by which raising the temperature leads to lower

defectivity. Surface tension, which governs a number of important interactions in BCP

DSA, is a function of temperature, and the rate of surface tension change is different for

every polymer. It is possible that changing the temperature causes the surface tensions of

the guide stripe, the brush, and the BCP to become better matched for DSA, but it is not

clear how to isolate these effects for further study. Since annealing at 200°C worked

better than 190°C, but annealing at 215°C becomes problematic due to the Tg of the top

coat, an annealing temperature in-between these two, 205°C, was chosen for every other

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DSA experiment. No indication of top coat dewetting was seen at 205°C, even after

annealing for one hour at this temperature.

Figure 4.35: DSA score versus temperature for the four DSA schemes on an XPS

chemo-epitaxy wafer. Error bars are 3 standard deviations

To further probe the kinetics of DSA defectivity, two XPS chemo-epitaxy wafers

were prepared identically, and one was annealed for 5 minutes and the other was

annealed for 60 minutes. The summary of the DSA image score analysis appears in

Figure 4.36A. These wafers were prepared with an exposure that did not print the pre-

pattern at 4x density multiplication, so only the regions containing 5x density

multiplication were compared. The DSA score when the guide stripe width = 0.5 L0 was

much worse than when width = 1.5 L0, but this region also experienced the larger

increase in DSA quality after the longer anneal. Figure 4.36B compares micrographs of

the “best” DSA from this “worse” region at 5 and 60 min, showing how the longer anneal

resulted in fewer dislocation defects and much longer segments of uninterrupted DSA

patterns. Compared to this result, the improvement in DSA quality at the “better” region

of width = 1.5 L0 was marginal. A statistically significant difference arose between the

top scores of this region at the two anneal durations, but the micrographs from the two

123

anneal times display little in terms of a practical difference. The number of dislocation

defects are decreased but not eliminated at the longer annealing time.

Figure 4.36: (A) DSA score versus annealing time (at 205°C) for two DSA schemes on

an XPS chemo-epitaxy wafer. (B) Representative micrographs of best DSA

result from the 5x W=0.5 region demonstrating the improvement upon

annealing longer.

4.9 CONCLUSIONS AND FUTURE WORK

In conclusion, the DSA of a sub-20 nm silicon-containing BCP was implemented

at IMEC. Materials suitable for the control of the BCP were synthesized at UT and

introduced into IMEC’s 300 mm cleanroom. Two different processes were used for DSA.

A hybrid process was designed to use shallow topography as well as chemo-epitaxy as

guiding surfaces, and was compared to a more traditional chemo-epitaxy process. In

general, the hybrid process achieved higher DSA quality than the corresponding chemo-

epitaxy process, and the process window for a number of parameters was larger as well.

Cross-section STEM EELS experiments demonstrated the BCP features travel vertically

through-film, without forming any complex 3D morphologies, despite the presence of the

124

shallow topography. The sidewall of the guide stripe is anticipated to play a large role in

aligning the BCP features.

This is only the second account- after PS-b-PMMA- of the DSA of a BCP using

processes suitable for HVM. The optimizations described in previous sections should

provide a roadmap for the DSA of future materials using similar processes, regardless of

material chemistry.

Experiments are underway at IMEC to follow up on the work presented in this

chapter. A guide stripe material containing a nitrogen in the repeat unit has been

synthesized and introduced into IMEC to enable precise determination of the BCP/guide

stripe interface in STEM with EELS experiments. The integration of silicon-containing

BCPs smaller than 20 nm is also planned, which also entails a need for a double-

patterned sidewall DSA scheme as discussed previously.

4.10 ACKNOWLEDGEMENTS

The DSA described here was performed at IMEC, Leuven, Belgium. The work

was made possible by generous support from Lam Research Corporation, managed by

Diane Hymes. Stephen M. Sirard of Lam also contributed as technical consult. The Lam

office at IMEC, managed by Johan Vertommen, contributed greatly to the effort, above

and beyond their official roles. David Hellin, Mohand Brouri, Christophe Noben, and

Bart Coenegrachts specifically helped with etch recipes and logistics and deserve special

recognition for their contributions. Geert Vandenberghe generously managed the IMEC

portion of the collaboration. Roel Gronheid, Paulina Rincon Delgadillo, BT Chan, Nadia

Vandenbroeck, Hari Pathangi, Arjun Singh, Robert Seidel, Jan Doise, and Grant Garner

contributed to the DSA research from IMEC. Rikimaru Sakamoto, Yasanobu Someya,

125

and Hiroyuki Wakayama, all of Nissan Chemical, contributed to the collaboration.

Michael J. Maher, Yusuke Asano, and Andrew Dinhobl contributed to material synthesis.

4.11 EXPERIMENTAL

4.11.1 Processing

Samples were prepared on commercial 300 mm wafer handling equipment at

imec as previously described.111,116 Si wafers with 13 nm of silicon nitride as an anti-

reflective coating were coated with 8 nm of XPS mat or 8 or 19 nm of XPMOST mat and

annealed at 315 °C for 5 min under nitrogen to cross-link. Wafers were coated with

commercial AIM5484 resist and exposed by an ASML 1950 or 1970 193i scanner. After

development, resist features were trim etched in a Lam Research Kiyo® conductor etch

chamber. Resist was stripped with Orgasolv in a TEL ACT12 track. Hydroxyl-terminated

brush (1% w/v propylene glycol monomethyl ether acetate) was manually spin-coated in

the same ACT12 track. The brush was annealed under nitrogen at 250 °C and then

ungrafted brush was stripped by RER 600 organic solvent. PMOST-b-PTMSS (1% w/v

methyl isobutyl ketone) was manually spin-coated to give a film thickness of 30-35 nm

(1.6 to 1.8 L0). Top coat in the form of a ring-opened trimethylamine salt (0.5% w/v

methanol) was spin-coated directly on top of the BCP. The wafer was then annealed at

205 °C for 5 minutes. The wafer was then etched in a Lam Research Flex™ dieletric etch

chamber. Inspection was performed by Hitachi Hi-Tech CG4000 and CG5000 CD-SEMs.

Cross-sectional STEM images with EELS mapping were provided by Nanolab

Technologies.

126

4.11.2 Conditions for each DSA process

Table 4.3: Summary of processing conditions used to make Figure 4.7

Process step XPS chemo- XPMOST chemo- Hybrid

Mat deposition Commercial NLD-

128 = 8 nm

XPMOST 0.5 wt.%

PGMEA 3000 RPM

= 9 nm

XPMOST 0.5 wt.%

PGMEA 600 RPM

= 19 nm

Brush composition

(see Table 4.2)

BB-71 BB-68 BB-68

BCP deposition 1.2 wt. % MIBK

1500 RPM = 33 nm

1.2 wt. % MIBK

1500 RPM = 33 nm

1.2 wt. % MIBK

1500 RPM = 33 nm

Top coat deposition 0.5 wt. % methanol

1500 RPM = 19 nm

0.5 wt. % methanol

1500 RPM = 19 nm

0.5 wt. % methanol

1500 RPM = 19 nm

Anneal 205°C 5 mins 205°C 5 mins 205°C 5 mins

CD to image for best

feature formation

18 nm 18 nm 14 nm

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Chapter 5: Direct self-assembly for bit patterned media applications

The work in this chapter has been reproduced in part with permission from a

manuscript prepared by Lane, A.P.; Yang, X.; Maher, M.J.; Blachut, G.; Asano, Y.;

Someya, Y.; Mallavarapu, A.; Sirard, S.M.; Ellison, C.J.; Willson, C.G.; “Directed self-

assembly of 5 nm Block Copolymer Lamellae” (to be submitted). For this manuscript, the

author synthesized polymeric surface treatments, and designed and carried out

experiments.

5.1 BACKGROUND

Another pattern-densification application of the directed self-assembly (DSA) of

block copolymers (BCPs) is bit patterned media (BPM) for hard disk drives (HDD). In

direct analogy to the transistor density scaling described by Moore’s Law,3 Kryder’s Law

describes the scaling of the areal density of magnetic media in HDDs.130 Impressively,

the data stored on a HDD increased 1,000-fold from 1990 to 2005 by continual

refinements to the technology used to make the magnetic head, which addresses the data,

and the magnetic media, which stores the data.130 However, a decade later, an overhaul of

HDD technology is required to continue this aggressive scaling.

As described by the Advanced Storage Technology Consortium (ASTC)

roadmap,131 a “new paradigm” will be necessary in order to make next-generation

devices with more storage density. The need for such a technological switch has become

even more pressing as solid-state flash memory emerges as a viable alternative to HDDs

for storage media in high-tech devices.132 The limitations of current HDD technology are

largely a consequence of the limited storage density of the current magnetic media. So-

called conventional multigrain media (CMM) stores bit of data by grouping individual

magnetic grains together to make a weakly-coupled magnetic cluster. The density of data

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storage is, therefore, inversely proportional to the size of each grain times the number of

grains that constitute a bit. Decreasing the number of grains in a bit also decreases the

signal-to-noise ratio of the magnetic media,133 and decreasing the area (or volume) of

each grain allows the magnetic moment of the grain to spontaneously flip (the so-called

superparamagnetic effect) with increased likelyhood.134 State-of-the-art, 1 terabit/in2

(Tb/in2) devices operate essentially at the limit of both of these constraints.133 New

technologies are needed to circumvent these physical limitations.

The technology with the most support from the HDD industry is heat-assisted

magnetic recording (HAMR). Since the superparamagnetic effect limits the size of the

grains, using a magnetic material with a higher coercivity, or resistance to magnetic

flipping, will allow the use of smaller grains.135 However, the higher coercivity makes

each grain prohibitively difficult to rewrite at room temperature. To circumvent this

coercivity “catch-22”, HAMR uses a laser to locally heat the magnetic material, making

the grains more susceptible to magnetization.135 Implementation of HAMR is contingent

on the resolution of numerous technical difficulties, including the reliability of the laser

used to heat the media, the reliability of the lubrication layer used to protect the media,

and many other practical concerns.

Instead of making the grains more resistant to spontaneous demagnetization, BPM

physically isolates the grains of a bit from neighboring bits to increase magnetic stability

without requiring an overhaul of the materials used in the head or the storage media. In

CMM, the volume of a magnetic element is essentially the volume of an individual grain,

while in BPM each grain can be magnetically coupled to its neighbors, making the

combination of grains the effective volume.11 This greatly decreases the threshold volume

needed for stable storage, which increases the areal density of the HDD. The trade-off is

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that the magnetic media needs to be lithographically patterned to create the isolated

magnetic islands that form the bits.

A geometric analysis reveals that 1 Tb/in2 BPM patterns, which are approximately

equivalent to the storage density limit of CMM, require the center to center distance of

each bit be approximately 25 nm. To produce patterns with 10 Tb/in2 storage density,

which is the long term vision of the ASTC roadmap,131 the center to center distance needs

to be approximately 8 nm. Both of these pitches are well beyond the resolution capability

of state-of-the-art 193 immersion lithography (193i) used for the high volume

manufacturing of integrated circuits (approximately 80 nm full pitch).12 Step-and-flash

Imprint Lithography (SFIL), now commonly known as Nanoimprint Lithography (NIL),

has sufficient resolution capability10 and throughput needed for high volume

manufacturing,11 but is gated by the availability of a master template with 25 nm or 8 nm

full pitch features. Electron beam lithography (EBL) has sufficient resolution to make

features this small for a template,108 but making a full template would take at least one

month,102 at which point beam stability issues would significantly impact the quality of

the produced template.

In much the same way that BCP DSA can multiply the frequency of 193i, BCP

DSA can be used to multiply the density of an initial EBL-patterned template to produce

a final NIL template with sufficient density for BPM. Many proof-of-concept

demonstrations of BPM using DSA have been reported by Hitachi Global Storage (now

Western Digital),67,136 Toshiba,137 and Seagate.138 However, much work remains to be

done to reach the scaling goals for this technology. Although the concept of magnetically

isolated islands evokes images of cylindrical pillars, research suggests that the best shape

for BPM features would be rectangular11,68 in order to exploit the spatial anisotropy of a

magnetic head. These features can be made by aligning BCP lamellae in one direction,

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and then applying a second pattern of different pitch that is oriented perpendicular to the

original pattern, as shown in Figure 5.1.

Figure 5.1: Fabrication of rectangular patterns from BCP lamellae. In D and E, a second

patterning step is used to chop the line space pattern into discreet rectangles.

Used with permission from Ruiz, R.; et al. ACS Nano 2011, 5, 79–84.

Copyright 2011 ACS.

To facilitate the development and mass production of NIL templates, Seagate has

developed a two-step NIL and DSA process.139 A “seed” NIL template (with sparse

patterns produced by EBL) is patterned onto a substrate, and then the DSA process is

performed on this pattern. The product wafer, now with a much denser pattern, is then

used to make a second NIL template, which functions as the master template for BPM

production. This route minimizes the reliance on EBL patterns for research and

development. Although the cited reference demonstrated DSA with cylinder-forming

BCPs, the process is general enough to be compatible with a number of different DSA

materials, including lamellae.

The rest of this chapter details the collaboration between Seagate and the

University of Texas to develop a DSA process with the silicon-containing, high-χ

materials previously discussed. Some of the learning from IMEC was used to aid in the

131

process development, but the nuances of working with NIL necessitated new procedures

to solve new problems.

5.2 PROCESS OVERVIEW

Figure 5.2 shows the materials and processes used in conjunction with the

Seagate NIL patterns. The 10 nm BCP used for the project, depicted in Figure 5.2A, is

poly(5-vinyl-1,3-benzodioxole-b-4-pentamethyldisilylstyrene) (PVBD-b-PDSS). As

discussed in Chapter 3, this BCP chemistry has a χ value large enough to access 10 nm

full pitch feature, yet the polar component is not soluble in methanol, allowing the top

coat surface neutralization strategy to be employed. The hydroxyl-terminated PS-r-PtBuS

brushes developed for the IMEC project were also introduced at Seagate, and the

chemistry is depicted in Figure 5.2A.

Figure 5.2: The (A) materials and (B) process steps used for producing 10 nm DSA in

the NIL process. Arrow 1 and 2 describe the process with and without a

grafted brush, respectively.

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In the standard process, a 150 mm wafer was patterned by NIL using a

commercial acrylate-based resist and a template containing approximately 18 nm line

widths in a matrix of 40 to 50 nm full pitches. Occasionally, the wafer was coated with 2-

3 nm of sputtered chromium, a standard mask material for manufacturing NIL templates,

and the NIL step was performed on top of this layer. The NIL pattern was then etched

with an oxygen-based reactive ion etch (RIE) to clear out the approximately 8 nm-thick

residual layer of NIL resist remaining between the guide lines. Since this “breakthrough”

etch also trimmed down the height and width of the NIL guide lines, typically each

quadrant of a wafer was etched for different times to vary the extent of resist trimming.

At this point, a polymeric brush was sometimes applied, but as will be discussed, a

degree of success was achieved without this step as well. In Figure 5.2B, the arrow

labelled 1 shows the process with the brush deposition step, and the arrow labelled 2

shows the process without the brush deposition step. After the brush was applied and

annealed, the BCP was then spin coated out of methyl isobutyl ketone (MIBK), and the

trimethylamine salt of the top coat39 was spin coated out of methanol. The wafer was then

heated to anneal the BCP and to ring-close the top coat and produce a neutral interface

for the BCP. This anneal also imparts mobility into the BCP, which enables DSA,

presumably by a hybrid chemo-/grapho- epitaxy mechanism where the polarity and

topography of the resist lines drives the correct assembly of the BCP. To image the BCP,

a second O2 RIE step was then used to remove the top coat and lightly decorate the BCP

features.

The LiNe-style chemo-epitaxy process could not be replicated exactly because

was impossible to do with the NIL resist since there is no way to strip the cross-linked

NIL resist from an underlying polymer mat (which would then function as guide stripes).

Thus, the resist lines themselves had to be used as guide stripes.140 However, one

133

unanticipated issue emerged with the brush deposition step as a result. It was discovered

that, unlike for the cross-linked poly(4-methoxystyrene) (XPMOST) and poly(styrene)

(XPS) guide stripes used in Chapter 4,97 the brushes did indeed graft appreciably to the

etched NIL resist. As Figure 5.3 shows, a pre-pattern treated at 200 °C with a neutral

brush produces only poorly aligned BCP domains, and importantly, the domains seem to

align mostly perpendicular to the direction of the NIL resist lines. Apparently, the brush

was grafted more-or-less uniformly on both the exposed silicon oxide surface and the

etched resist, producing no highly preferential surfaces that would direct the assembly

along the guide stripe direction. This result prompted a new strategy for mitigating the

grafting promiscuity of the brushes.

Figure 5.3: Top-down SEM of 10 nm PVBD-b-PDSS on etched NIL resist with a neutral

brush grafted at 200 °C for 10 minutes. The resist lines run vertically, while

the BCP domains travel mostly horizontally. The patchy regions are an

artifact of SEM and not indicative of poor BCP assembly.

5.3 DSA WITH THE BRUSH REFLOW PROCESS

One successful method developed to promote the grafting selectivity of the

brushes to the substrate was a so-called “reflow” grafting process. In this process, a very

thin layer of brush was spin coated onto the etched NIL resist, and the wafer was baked at

134

a temperature only slightly higher than the Tg of the brush. Then, a second high

temperature bake was applied to graft the polymer. As Figure 5.4 shows, the DSA

produced by this method was remarkably good, especially compared to Figure 5.3. To

produce this wafer, a neutral brush was spin coated as a 0.1 wt % solution in PGMEA,

annealed at 150 °C for 10 minutes, and then 250 °C for 5 minutes, before being rinsed

with organic solvent to remove excess brush. The BCP film stack was then made as

normal.

Figure 5.4: Top-down SEM micrograph of defect-free 5 nm patterns over a large area.

The sample was made by grafting a neutral brush to the substrate with the

reflow process. The BCP film was lightly etched for image contrast. Scale

bar = 100 nm.

The mechanism by which this process forms is hypothesized to be a “reflowing”

of the neutral brush. As shown in Figure 5.5, the brush presumably flows into the

trenches due to capillary action during the modest heating step, which is done at a

temperature where significant grafting to the sidewalls of the resist does not occur.

During the second, high temperature anneal, the majority of the grafting presumably

135

occurs with the substrate and the base of the resist structures, since the brush was largely

in contact with these regions. The reflow process leaves behind a large amount of

unmodified, polar sidewall to act as a pinning surface.

Figure 5.5: Presumed mechanism of reflow process. The polymer brush (purple) is

drawn into the trenches of the resist lines (blue).

The ability of the “reflow” process to produce high quality DSA was studied in

more detail. Experiments were performed to isolate the effect of depositing a very thin

layer of brush and grafting without a reflow step, and to isolate the effect of depositing a

slightly thicker film with the “reflow” process. First, a 0.1 wt. % brush solution was spin

coated on etched resist features, and then was annealed at 250 °C without a preliminary

low temperature bake step. As Figure 5.6A shows, the DSA produced by this treatment

was heavily distorted. Although the BCP lines run the same direction as the resist lines,

the spacing between features was uneven, and regions with poor assembly were also

observed. It is not explicitly clear how the omission of the 150 °C bake step leads to these

structures, but the conclusion can be drawn that it is necessary for good DSA. In a second

experiment, the full process was performed, but this time with a slightly thicker brush

layer from a 0.33 wt. % brush solution (versus a 0.1 wt. % solution). As Figure 5.6B

shows, the DSA produced by this method is poorly aligned, and in fact appears that the

lines run perpendicularly to the resist lines, similar to what happened when the brush was

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grafted at a much greater film thickness and annealed to saturation. This result could be

interpreted as the film produced by the 0.33 wt % solution is too thick to fully reflow into

the trenches and instead has an opportunity to graft to the resist sidewall, removing the

anchoring force for DSA.

Figure 5.6: Top-down SEM micrographs of reflow process control experiments. In both

images, the resist lines run vertically. A) Substrate treated with BST 105 0.1

wt. % for 250 °C for 5 minutes (omitting the 150 °C reflow step). B)

Substrate treated with BST 105 0.33 wt. %, with both a 150 °C 10 minute

bake and a 250 °C 5 minute bake.

5.4 DSA WITH NO BRUSH

In the process of developing the brush reflow process, it was serendipitously

discovered that well-aligned DSA could be achieved without the use of a brush. Simply

building the BCP filmstack on top of the etched NIL resist lines produced features that

followed the resist lines to a large degree, as shown in Figure 5.7. This was initially a

very puzzling result as even the appearance of BCP features oriented perpendicular to the

substrate was unanticipated. As will be discussed shortly, in the unpatterned regions of

the film, the BCP features did actually orient parallel to the substrate (forming no visible

features upon top-down inspection), as is expected when the BCP is in contact with a

137

preferential substrate (the oxide on the wafer). However, it appears the that combination

of a neutral surface produced by the top coat and the confinement effects of the

topography provided enough driving force to produce perpendicularly oriented BCP

features within the area defined by the NIL patterning, with these features aligned parallel

to the guide stripes. The etched sidewall of the resist lines furthermore anchored the polar

PVBD block of the BCP, aligning the BCP along the resist lines. Further studies were

carried out to compare the DSA produced by the no-brush process with the DSA

produced by the “reflow” process.

Figure 5.7: Top-down SEM micrograph of defect-free 5 nm patterns over a large area.

The sample was made without grafting a neutral brush to the substrate. The

BCP film was lightly etched for image contrast. Scale bar = 200 nm.

5.5 A COMPARISON OF THE TWO DSA PROCESSES

The routes to produce DSA with the NIL resist- without a brush or with a thin

brush- were further analyzed. One of the chief goals was to determine which process

performed better as the height of the NIL resist was reduced. Successful DSA

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implementation depends on transferring the pattern into the underlying substrate, and it

was predicted that lower topography from the NIL resist would help make this process

less challenging.

As mentioned previously, the final height of the resist lines was controlled by

changing the duration of the trim etch used to remove the residual resist layer in-between

the resist features. The heights of the resist features trimmed in this manner were

measured by AFM, and then the brushless and the reflow brush process were

implemented on different wafers. Figure 5.8 shows the DSA results as a function of NIL

resist height for both these processes. The brushless process had an enhanced ability to

generate DSA as the height of the resist, and presumably the driving force for DSA, was

decreased. At 6 nm resist height, the DSA still appeared very good for the brushless

process, while the reflow brush process showed BCP features devolved into random

fingerprints. At even shorter resist heights of approximately 1 to 3 nm, the brushless

processes finally ceased to produce DSA, and instead produced a featureless film in lieu

of the fingerprints observed with the brush reflow process. The lack of fingerprint pattern

in the brushless process can be justified by the high degree of preferentiality of the

surface towards the polar component of the BCP, driving the features parallel to the

substrate. This result was corroborated by SEM micrographs taken in the unpatterned

area, also shown in Figure 5.8A, showing a lack of well-defined lamellar features due to

parallel BCP orientation. In contrast, the brush reflow process produced fingerprints, at 6

nm resist height as stated, and also at 1 to 3 nm height and in the unpatterned area of the

wafer. This result suggests that the sparse brush coating deposited by the 0.1 wt % spin

coating step was still sufficient to meaningfully modify the surface energy of the

substrate. The inability of 6 nm features to form good DSA in the brush reflow process

suggests that the conditions of the reflow process are inadequate to preferentially modify

139

only the substrate when the feature height decreases sufficiently. For this reason, the

brushless process appeared better suited to produce DSA that could be successfully

pattern transferred to produce a NIL template.

140

Figure 5.8: Top-down SEMs of BCP assembly on substrates patterned by NIL,

demonstrating the effect of resist height on DSA quality for A) the no brush

process and B) the reflow brush process. “h” is the average height of the

NIL resist for both images in a given row, as measured by AFM. All BCP

films were 33 nm (3.3 L0) thick and were annealed at 190°C for 2 minutes.

Scale bar = 200 nm.

141

The brushless and the brush reflow process were further analyzed by cross-

sectional STEM with EELS mapping, as was performed on the IMEC 20 nm DSA result

in Chapter 4 The goal of this study was to determine the through-film structure of the

BCP. Figure 5.9 shows the results of this study. In addition to the brushless process on

silicon oxide (Figure 5.9B), the brushless process was replicated on chromium oxide

(Figure 5.9C). The top set of images show top-down SEM micrographs of the DSA at

the approximate location where the cross-sectioning was performed. The DSA quality

was very good for all three samples, but the “reflow” process in Figure 5.9A showed a

higher degree of defectivity than the other two. The NIL resist height was 12 nm for this

experiment, and Figure 5.8B established that somewhere between 14 and 6 nm, the resist

height becomes small enough to deteriorate DSA in the reflow process, possibly

explaining the increased defectivity in the top-down DSA. The bottom set of images are

the STEM micrographs. The green domains are Si and correspond to the PDSS BCP

block and the wafer substrate. The color magenta maps carbon and corresponds to the

PVBD domains in-between the PDSS, and the slightly thicker magenta areas correspond

to the NIL resist guide stripe. Blue is Cr, used both as a capping layer to make the

sample, as well as the substrate in Figure 5.9C.

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Figure 5.9: BCP self-assembly (top) and cross-sectional STEM with EELS mapping

(bottom) for three surfaces investigated in this study: A) SiO2 with brush, B)

SiO2 without brush, and C) 2 nm chromium oxide on Si. The height of the

NIL resist for the two samples made on SiO2 and the one sample on

chromium oxide were approximately 12 nm and 8 nm, respectively.

Samples made on SiO2 were etched by oxidizing plasma before STEM

analysis, while the sample made on chromium was not. The starting BCP

film thickness for all three samples was approximately 33 nm. The samples

on SiO2 were annealed at 190°C for 2 min, while the sample prepared on

chromium oxide was annealed at 205°C for 10 min. Scale bar for the top-

down SEM images is 100 nm. Scale bar for the cross-section EELS images

is 15 nm.

The most immediate difference between the samples is the presence of a wetting

layer at the bottom of the BCP film in the two brushless samples, Figures 5.9B and 5.9C.

A large, continuous PDSS domain is clearly seen running parallel to the substrate in both

brushless samples, while the domains in the reflow process (Figure 5.9A) seem to run

continuously through-film. The lack of a wetting layer in Figure 5.9A is presumably due

to the minute amount of brush grafting by the reflow process, neutralizing the substrate

and promoting the perpendicular orientation of both domains. In the case of the two

brushless samples, the substrate is preferential towards the polar PVBD BCP block,

pinning this component and generating the wetting layers. Although difficult to discern

from the micrographs, a ~2 nm half domain of PVBD (corresponding to one monolayer

143

of BCP) is assumed to be the bottom-most layer of the wetting layer and sits below the

full ~5 nm green PDSS domain that is clearly visible in both micrographs. If the PDSS

domain was instead the bottom-most layer, the green domain would only appear half as

thick as the vertically oriented green domains, but that is clearly not the case. Notably, in

the case of the chromium sample, a wetting layer of two PDSS domains forms, this is due

to the trench width being slightly wider where this micrograph was taken. Despite the

presence of the wetting layers, the BCP domains still manage to orient themselves

vertically at the top of the film, generating the well-aligned features seen in top-down

inspection. This is an indication of the strong driving force for alignment provided by the

polar resist sidewalls and the driving force for perpendicular orientation provided by the

top coat. The presence of the wetting layer does not necessarily preclude the possibility

of successful pattern transfer. When subjected to a reactive ion etch, typically 5 to 10 nm

of a silicon-containing polymer is consumed initially before the hardmask forms.40

Pattern transfer experiments are needed to determine the viability of these complex BCP

structures for lithography. Of course in the case of Figure 5.9C, the presence of two

wetting layers adds more complexity than the situation in Figure 5.9B.

The large degree of domain tilting in Figure 5.9A, and the minor tilting possibly

seen in Figures 5.9B and 5.9C, are probably due in part to sample preparation and not an

actual representation of the domain alignment. Although it is possible that the BCP

domains could be tilted with respect to the substrate, the NIL resist also appears to have

the same degree of tilt. This situation is highly unlikely, which seems to imply that the

entire film was skewed in that direction during sample preparation. Thermal drift of the

sample during image acquisition could also explain the tilt.

Evident from the STEM micrographs is the mismatch of BCP domains with the

periodicity of the guide stripes. For example, in Figures 5.10A and 5.10B, it appears that

144

the BCP domains at the top of the film do not necessarily correspond directly to the

domains closer to the substrate and in-between the resist lines. After these cross-section

experiments were performed, a thorough study of the pre-pattern pitch and DSA was

conducted, and it was discovered that a pre-pattern pitch of approximately 42 to 44 nm

resulted in the best DSA performance. Ostensibly, the pre-pattern pitch used to generate

Figures 5.10A and 5.10B was incommensurate with the natural periodicity of the BCP,

leading to the mismatch observed in domain location. This behavior was not, however,

observed in Figure 5.9C; the features at the top of the film continue into the film without

interruptions (despite, of course, the double wetting layer). This sample was made after

the investigation on pre-pattern pitch was performed, and the cross-section was

deliberately taken at the ~43 nm pre-pattern pitch expected to be commensurate with the

BCP. Satisfyingly, changing the pitch to this “optimal” value seemed to mitigate the issue

of mismatched domains. Interestingly enough, despite the commensurability mismatch

between the pre-pattern and the BCP, high quality DSA was still observed in Figures

5.10A and 5.10B, in contrast to the significantly increased defectivity observed in the

chemo-epitaxy results in the previous chapters. It appears that the topography of the resist

structures presents a formidable barrier to forming dislocations, which might be

advantageous for DSA.

145

Figure 5.10: Enlarged STEM EELS image showing domain mismatch. Arrows indicate

domains at the top of the film that do not directly correspond to domains

lower in the film. Dashed lines are added as guides-to-the-eye to show the

direction of the domains.

5.6 DEVELOPING A SELECTIVE DEPOSITION STRATEGY

The STEM EELS results highlight the need to develop a method to render the

substrate neutral for the BCP and inhibit the formation of complex three dimensional

structures. Having BCP features that travel only vertically through-film would be

certainly advantageous for pattern transfer. However, as demonstrated by the initial

experiments in Figure 5.3, this must be done in such a way to not chemically modify the

oxidized sidewalls of the NIL resist lines, so the driving force for BCP alignment is not

removed. The options available to develop a selective grafting methodology are 1) to find

a combination of anneal time and temperature that produce selectivity, 2) to find a

functional group that only grafts to the substrate and not to the oxidized NIL resist

sidewall, and 3) to find a substrate that is much more reactive than the oxidized resist to a

given functional group. The chemical reaction of a functionalized polymer grafting to a

substrate is governed by a finite rate constant,46 so some combination of time and

temperature can potentially be identified that maximizes the grafting selectivity. Since the

grafting reaction is presumably performed only by the reactive functional group present

146

in the polymer, the identity of that functional group could be tailored to exploit a

selective grafting reaction.141 In a comparable way, the chemistry of the substrate can in

turn also be changed to promote a selective reaction.142 This section discusses the steps

taken to identify conditions and materials that together modify only the substrate and

promote perpendicular assembly of the BCP without sacrificing DSA control.

5.6.1 Water contact angle measurements

Water contact angle measurements were used as a proxy to determine the extent

of modification of a surface. The wettability of water on a substrate, quantified by

macroscopic water contact angle measurements, can be used to determine the relative

hydrophilicity of a surface. A few noteworthy caveats emerge. Although the shape of a

water droplet on a substrate is determined by the thermodynamic equilibrium between the

droplet, the surface, and the surrounding gas, in practice the true water contact angle is

obscured by surface topography and advancing-receding drop hysteresis,143,144 and care

must be taken when interpreting absolute results from water contact angles. For the

purposes of our investigation, however, the water contact angle needs to only

approximate the state of a surface after modification. This goal is relatively achievable

due to the wide range of contact angles formed by water on the surfaces of interest. Oxide

surfaces, such as freshly etched silicon oxide, typically have very small (nearly zero

degree) contact angles as the polar surface is readily wet by water. In contrast, polymers

and polymer treatments can have contact angles greater than 90°,46,97 depending on the

chemical identity of the polymer and the extent of grafting.

5.6.2 Quantification of the lack of selectivity in the current grafting procedure

In most “trim etch then backfill” processes, including the modified LiNe chemo-

epitaxy processes59 described for our silicon-containing BCPs in Chapter 4, the brush is

147

typically grafted under conditions that saturate the substrate. At saturation, the polymeric

brush has been grafted to such a density that further grafting is impossible due to the

volume excluded by the grafted polymers. Operating in the saturated grafting regime is

typically desirable because of the uniformity of the resulting surface, both spatially on a

single wafer and also from wafer to wafer. Empirically it has been established that an

annealing temperature of 250 °C produces a nearly saturated brush layer after only

approximately 5 mins,115 which is ideal for high wafer throughput. However, as

demonstrated previously in Figure 5.3, such annealing conditions were inappropriate for

the NIL resist system since it appears that polymer grafted indiscriminately to both the

substrate and the etched NIL resist, producing incorrectly aligned BCP features.

To investigate this situation as a starting point for selectivity studies, blanket films

of silicon oxide and etched NIL resist were grafted with OH-brush in a similar way as

were the DSA samples (~30-40 nm thick films, annealed at 250 °C for five minutes, and

rinsed clean with toluene and IPA). The brush composition was ~71 mol % PS (the rest

PtBuS), approximately neutral for the 10 nm PVBD-b-PDSS BCP. The water contact

angle measurement results shown in Figure 5.11 corroborate the poor DSA. Under these

grafting conditions, both substrates show nearly the same water contact angle, implying

that a significant amount of the OH-brush covers both surfaces. Compared to the water

contact angle of unmodified silicon wafer (nearly zero degrees) and of etched NIL resist

(approximately 33°), the approximately 90° water contact angles show a large degree of

grafting. The small ~4° difference between the average contact angle values shown in the

graph becomes even smaller when one takes into account contact angle hysteresis present

in the NIL resist samples (described in Section 5.6.4). When only the water contact

angles from the initial droplet from a region of NIL resist wafer are counted, the average

value rises to 90.8°, even closer to the 91.0° result on bare wafer. Ellipsometry was used

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to further study the surfaces. After grafting and rinsing, the silicon oxide wafer had a 4.9

nm thick film of brush, a thickness typical for a brush layer. The etched NIL resist film,

initially 8.0 nm thick, grew only to 9.5 nm after grafting and rinsing, implying a much

sparser covering of polymer brush. Still, clearly very little selectivity exists at these

conditions.

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Figure 5.11: W.C.A. on substrates grafted with OH-brush (approximately 71% PS) at

250°C for 5 minutes. Results are the average of at least four separate

droplets, with the left and right droplet interface measured separately by the

software. The error bars indicate 95% confidence intervals.

5.6.3 Grafting selectivity onto chromium oxide

Although the resist on silicon film stack is convenient for research, a better film

stack for NIL mask production includes a thin layer of chromium metal directly under the

resist. This layer is etched with the resist structures (either NIL resist or BCP domains) as

an etch mask, and then the chromium is used to mask the etching of silicon underneath.

The O2-based trim etch process used in the NIL DSA process does not remove the

chromium, so the regions between the NIL resist structures are coated with chromium

oxide after oxygen plasma exposure (and not silicon oxide as is the case without the

chromium layer). Serendipitously, it was discovered that the grafting reactivity of

chromium oxide is much different, and in most cases stronger. This enhanced selectivity

provided a path to the goal of achieving selective deposition.

The grafting of a number of functional groups to chromium oxides has been

reported, including a number of functional groups not typically considered as reactive

towards substrates (such as alkenes and alkynes).145 These traditionally unreactive

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unsaturated hydrocarbons graft to chromium oxide at relatively modest temperatures, for

example, reaching saturation contact angles at only 100 °C after 12 hours.146 This

property, a large degree of grafting at very modest temperatures, seemed advantageous

for our purposes.

In the course of conducting studies to determine the extent of grafting on

chromium oxide films, it was discovered that all organic functional groups tested

significantly modified the chromium oxide surface. This phenomenon is summarized by

Figure 5.12. A polymer of PS, made by a controlled anionic polymerization procedure,

was grafted to chromium oxide films at various relatively low temperatures, sonicated

and rinsed clean, and the water contact angle was measured. The sample annealed at 80

°C produced a contact angle of approximately 32°, the sample at 100 °C produced a

contact angle of approximately 36°, and the sample annealed at 140 °C produced a

contact angle of approximately 44°. For comparison, the water contact angle of freshly

etched chromium oxide was too low to be measured by our goniometer ( < ~10°). The

data show that even these modest annealing conditions were sufficient to modify the

substrate, with the extent of modification somewhat dependent on the thermal driving

force.

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Figure 5.12: W.C.A on various modified substrates. The first 6 substrates were all

chromium oxide, and the last was silicon oxide. As controls, one chromium

oxide substrate was solely sonicated and rinsed with IPA (first entry), and a

different chromium oxide substrate was spin coated with pure MIBK,

annealed, and sonicated (second entry). The error bars indicate 95%

confidence intervals. Only one measurement was taken for the last entry on

silicon oxide due to its low value. The inset shows the chemical structure of

the 12 kg/mol PS made by anionic polymerization.

This result is remarkable due to the lack of any true reactive groups in the

polymer structure. As the inset in Figure 5.12 shows, the end-groups and backbone of the

polymer are aliphatic, and the pendant group is an aryl ring. Neither of these moieties is

typically considered especially reactive for substrate modification, yet clearly some kind

of modification is occurring. It could not be ruled out that some other aspect of the

processing steps was responsible for the surface modification, or that a functionalized

contaminant was being attached to the surface. As a control, a wafer of silicon was

similarly etched, coated with the PS, and annealed at 140 °C (the highest temperature

used in this study). In contrast to the contact angles on chromium oxide, the silicon oxide

only produced a water contact angle of approximately 11°, which again is near the

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measurement limit of our instrument. Although this contact angle is greater than that of

freshly etched silicon, it is much less than the 44° contact angle measured for the

chromium film. Any modification of the silicon oxide by this process is much less than

the modification of chromium oxide, which supports the observation that chromium

oxide seems naturally more reactive than the silicon oxide.

Although at this point we were optimistic that the high reactivity of chromium

oxide films provided a potential route to grafting selectivity over etched NIL resist, were

were driven to further investigate the nature of the grafting observed in the PS

experiment. The film produced by annealing PS at 140 °C on top of the silicon oxide was

measured by ellipsometry and found to be 0.5 to 1.0 nm thick. We were unable to

measure the corresponding film thickness of the residual layer on chromium oxide due to

difficulties with finding and fitting an ellipsometry model to the chromium layer, but

assumed it was probably denser than the silicon oxide film due to the higher contact

angle. This value is nonetheless very thin for a surface modification layer, and implies

either a very sparse residue of PS or a monolayer or patchy monolayer of small

molecules. A PS residue could be adhered to the surface by van der Waals forces and not

by a covalent bond,147 assuming the identity of the residue is in fact PS.

In an attempt to elucidate the identity of the residual film on chromium oxide,

additional control experiments were performed to evaluate the effect of the processing

steps on the surface modification. For the previously described experiments, the PS was

spin coated from methyl isobutyl ketone (MIBK), and after annealing, sonicated with

toluene, rinsed with IPA, and then blown dry. Two controls were performed to

investigate the effect of 1) MIBK and 2) sonicating and rinsing. An etched chromium

oxide wafer shard was put on an 80 °C hot plate, and then was completely covered by

drops of MIBK. After 60 s, the wafer was cooled down, blown clean with a filtered air

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gun, sonicated with toluene, rinsed with IPA, and then blown dry with the same air gun to

mimic all the process steps performed on the PS. A second etched chromium oxide wafer

was simply sonicated with toluene, rinsed with IPA, and then blown dry with an air gun.

The water contact angles of these two samples also appear in Figure 5.12. Remarkably,

processing the wafers without polymer still modified the surface to an extent discernable

by water contact angles. In the case of sonicating and rinsing the chromium oxide wafer

shard, the contact angle increased to approximately 22° (again, freshly etched chromium

oxide was significantly closer to zero degrees). Even more surprising is the result of the

wafer annealed with MIBK: the sample’s contact angle, approximately 32°, is

indistinguishable from the corresponding sample annealed with PS at 80 °C. Due to

MIBK’s boiling point of approximately 117 °C, annealing a chromium oxide wafer with

MIBK at higher temperatures was not performed.

Having established the reactivity of chromium oxide towards substrate for surface

modification, a number of small molecules with different functional groups, as well as a

hydroxyl-terminated polymer brush, were grafted at relatively low annealing

temperatures and compared to similarly prepared, etched NIL resist films. The small

molecules were long chained molecules functionalized with 1) an alkene (decene), 2) an

alcohol (decanol), 3) a carboxylic acid (undecanoic acid). In addition, a hydroxyl-

terminated polymer, poly(4-tert-butylstyrene) (PtBuSty-OH), was also used. Each

compound was deposited onto etched chromium oxide and etched NIL resist films and

annealed at a modest 100 °C for 60 s. Decene and decanol, both liquids, were deposited

by dropping the chemicals out of a pipette directly onto the hot wafer. Undecanoic acid, a

waxy solid, was dissolved in IPA (1 wt %) and pipetted onto the hot wafer shards; as IPA

evaporated, more of the solution was deposited. PtBuSty-OH was spin coated out of

PGMEA (1.5 wt %) to form a film approximately 50 nm thick before annealing. After

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annealing, the wafer shards were cooled, washed with toluene, tetrahydrofuran, IPA, and

water, and blown dry. The water contact angle was measured repeatedly for each wafer.

A wafer of etched but otherwise not treated NIL resist was also measured alongside these

samples. Figure 5.13 summarizes the results of the study.

Figure 5.13: W.C.A. on etched chromium oxide (green) and etched NIL resist (blue)

after application (100 °C 60 s anneal) of various chemical treatments.

PtBuSty-OH is hydroxyl-terminated poly(4-tert-butylstyrene). The error

bars indicate 95% confidence intervals.

In all 4 trials, the water contact angle of the chromium oxide samples was much

higher than those of the corresponding etched NIL resist samples. The lowest chromium

oxide water contact angle was approximately 70° for decene, and the highest was

approximately 90° for undecanoic acid. In contrast, all of the water contact angles on NIL

resist were approximately 40 to 50°. Although these values are higher than the water

contact angle of etched, untreated NIL resist films- determined to be approximately 33°-

they are still significantly less than the values for the chromium oxide films, implying a

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much larger degree of surface modification on the chromium oxide than the NIL resist.

This is exactly the sought-after behavior.

The water contact angles for the three small molecules follow the trend

undecanoic acid > decanol > decene. Since the tail of all three molecules is essentially the

same structure (a 10 or 11 carbon chain), we presume that the differences in water contact

angles on chromium oxide for these three treatments can be attributed to the reactivity of

their functional groups towards the chromium oxide surface. This implies the reactivity

towards the chromium oxide is carboxylic acid > alcohol > alkene. The corresponding

NIL resist films showed no significant difference in water contact angles, however. At

annealing condition of 100 °C for 60 s, no significant difference in reactivity of the

carboxylic acids, alcohols, and alkenes for the etched NIL resist was observed. The three

NIL resist samples, however, did produce water contact angles greater than the etched but

untreated NIL resist film. Compared to the ~40° angles of the three films treated with

small molecules, the untreated film produced an average angle of 33°. This difference in

water contact angle might signal that some degree of grafting is actually occurring at

these modest annealing conditions after all. However, also possible is that the 100 °C 60 s

anneal could enable the etched NIL resist to reorganize and hide polar functional groups

from the surface,119 producing a higher water contact angle without actually grafting the

small molecules.

Although the ability of the small molecules to significantly modify the

chromium oxide substrate without similarly modifying the etched NIL resist was

established, for DSA reasons a modification step performed with a polymer might be

preferable. Small molecules might affect the BCP in some unforeseen way, and from a

practical standpoint, dispensing volatile, flammable materials in a manufacturing process

is discouraged. As shown in Figure 5.13, PtBuSty-OH was also tested the same way as

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the small molecules, and the results follow the same general trend. The PtBuSty-OH

treated chromium oxide film produced a high water contact angle, between that of the

decanol and the carboxylic acid. Since the chemistry of the polymer- and thus, its

response to the water contact angle measurement- is much different than that of the small

molecules, meaningful inferences about the relative reactivity of the polymer to the small

molecules cannot be drawn. Interestingly enough, the water contact angle on the NIL

resist substrate was significantly higher than was measured on the small molecule

samples, almost 10° higher. While the practical importance of this difference on DSA has

not yet been tested, the increase in water contact angle produced by the brush application

draws notice. Perhaps a different mechanism of physical attachment could be suggested

for the polymer onto the NIL resist, such as interpenetration of part of the brush polymer

into the resist, leaving behind a sparse layer of “stuck” material. If this is the case, then

the partial modification of the NIL resist by polymer treatments seems somewhat

inevitable. However, to test this hypothesis, similar studies with differently

functionalized polymers, such as alkenes or carboxylic acids, but otherwise similar

chemistries, should be performed.

Having established that grafting conditions seem to exist that preferentially

modify the chromium oxide over the etched NIL resist, one last test was performed to

determine to what extent a surface modification had to be performed to elicit a practical

change in the wetting behavior of the BCP. A PS-r-PtBuSty polymer of 25 mol %

PtBuSty, a composition slightly more hydrophobic than the neutral brush for PVBD-b-

PDSS when grafted to saturation, was coated onto etched chromium oxide wafers and

annealed at different temperatures to control the extent of grafting. Because the brush was

only slightly more hydrophobic than neutral, any switch from polar to non-polar grafting

(via island-hole118 experiments) would indicate a rather significant modification of the

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surface towards the BCP. After washing off the ungrafted brush, water contact angle

measurements were taken to quantify the extent of grafting. Then, a film of PVBD-b-

PDSS was spin coated onto the substrates, and annealed to produce island-hole features.

As seen in Figure 5.14, changing the grafting temperature allowed us to change the

extent of grafting- the sample annealed at 160 °C had a water contact angle more than a

20° higher than that of the sample annealed at 100 °C. The AFM micrographs in Figure

5.14 further show that this grafting difference is significant enough to change the BCP

wetting condition. The sample grafted at 100 °C produced holes, indicative of

asymmetric wetting in this film thickness regime, which translates to the polar PVBD

component wetting the substrate. In contrast, the sample grafted at 160 °C showed the

opposite topography, islands, which implies the other component, the low surface energy

PDSS, is now wetting the substrate. In both of these cases, the chemistry of the

modification layer is the same, yet the BCP behavior on the substrate was modified by

changing the extent of grafting.

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Figure 5.14: Demonstration of chromium oxide wetting change (by island-hole flip) due

to a change in grafting temperature. The island-holes were measured by

AFM. The grafted polymer was a hydroxyl-terminated PS-r-PtBuSty with

approximately 25 mol % PtBuSty.

The water contact angle values determined by this study should not be interpreted

as absolute measures by which to judge a specific surface modification, rather the results

should serve as proof that a route to controlled grafting exists. The contact angle

measurements do provide a rough estimate of the extent of grafting needed to produce an

optimal DSA surface. Since the role of the backfill region, mediated by the brush

modification, is to provide a non-preferential interaction to generate perpendicular

orientation of the BCP features through the entire film, a neutral or near-neutral substrate

is desired. From Figure 5.14, this surface probably has a water contact angle between 68

and 91°, but the exact extent of treatment will be empirically determined. In Figure 5.14,

temperature was used as the handle to modify the extent of treatment, but an equally valid

handle is the composition of the brush. At a given grafting extent, fixed by the time and

temperature of the anneal, a brush lower in surface energy than the one employed in

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Figure 5.14 would produce a correspondingly more hydrophobic surface, perhaps more

ideal for DSA. At the time of manuscript preparation, DSA experiments with the

hydroxyl-terminated brushes and low temperature “selective” grafting conditions are

ongoing.

5.6.4 Water contact angle hysteresis on NIL resist samples

In the process of conducting the water contact angle study, a hysteresis

phenomenon was observed in the contact angles. It was discovered that repeated water

contact angle measurements on etched or unetched NIL resist resulted in continuously

decreasing water contact angles. In a typical experiment, a series of droplets were

deposited on a substrate and the contact angle was reported as an average of the

measurements. Each droplet was deposited in the middle of the wafer shard to avoid edge

effects. However, each trial seemed to affect the subsequent trials by producing a more

polar surface, resulting in a lower water contact angle. It is anticipated that the chief form

of substrate modification was the rearrangement of functional groups present on the

surface due to the presence of water. Such reorganization behavior has been previously

documented both at polymer interfaces148–151 and polymers modified by plasma.119 It is

also possible that the polymer itself becomes swollen with water, decreasing the water

contact angle of subsequent trials.

Figure 5.15 shows the evolution of water contact angle measurements as a

function of trial number on treated surfaces from two such water droplet studies. The

behavior observed on these two surfaces is characteristic of all grafting studies performed

on NIL resist films. For both surfaces, the first trial produced the highest water contact

angle, and subsequent trials produced lower contact angles. For the experiments with the

decene treatment (green), the biggest decrease was between the third and the fourth trials,

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after which the contact angle oscillated around 45°. It is hypothesized that the surface

reorganization reached a steady-state and the oscillations were due to noise in the

measurement. The difference between the largest and smallest measurements was

approximately 15°. The experiments with the OH-brush (blue) showed a more gradual

decrease in contact angle, but without any indication that a steady-state was reached after

6 trials. To convince ourselves that the initial measurement obtained was actually much

higher than the later measurements and not a mistake, the last measurement for the OH-

brush was taken at a much different spot on the wafer. The resultant water contact angle

was much higher than the preceding measurement and almost as high as the first one. The

contact angle measurements spanned a range of 7°, both a smaller value and a smaller

percent than the decene experiment. This might be indicative that the surface

functionalized with a polymer has a lesser or slower ability to rearrange itself than the

surface modified with a small molecule (decene). Although deionized water was used for

all contact angle measurements, it is unfortunately impossible to simultaneously disprove

that a contaminant in the water was modifying the substrate.

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Figure 5.15: W.C.A. on NIL resist treated with an OH-brush (250 °C for min) (blue) and

with decene (60 s at 100 °C) (green) as a function of trial number. Each

droplet was deposited at the same spot as the previous droplet, except for the

last trial in OH-brush. Lines are included to guide the eye.

Despite the pronounced affect the hysteresis had on the water contact angle

measurements, little effort could be made to mitigate this behavior. Ideally, either

different spots on the wafer or different wafers entirely could be used to make individual

measurements to not modify the substrate, but due to material constraints, often there was

no practical way to avoid measuring the same spot on the wafer multiple times. The

wafer shards were necessarily small, and even sometimes displayed macroscopically

visible variations in NIL resist quality. Often, only an area of less than a square

centimeter was trustworthy, so multiple measurements had to be taken in the same

region. Still, valuable inferences about the NIL resist surface after modification could be

made even with the uncertainty introduced by the polymer reorganization.

Measurements performed on the other surfaces, silicon oxide or chromium oxide,

did not display a discernable degree of trial hysteresis.

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5.7 DSA ON UNETCHED NIL RESIST

While examining the ability of the brushless process to generate DSA, it was

discovered that an unetched NIL resist substrate also produced an unexpected degree of

BCP alignment. As seen in Figure 5.16, substrates not subjected to an RIE, and not

treated with a brush, generated BCP domains running perpendicularly to the resist walls.

The thickness of the BCP layer was kept lower than the height of the resist lines to form

isolated “ladder” structures. These short BCP lines are essentially rectangles with a large

in-plane aspect-ratio, a morphology that has been identified as ideal for the bits in

magnetic media.11 Currently, these features contain too much pattern-placement error to

be used for devices, but perhaps with the implementation of a template specifically

designed to guide such structures, the BCP could be aligned with more predictability.

Figure 5.16: Top-down SEM of 10 nm PVBD-b-PDSS on unetched NIL (without a

brush treatment) at the edge of the patterned region.

The domain orientation perpendicular to the resist sidewalls is itself a peculiar

phenomenon. Naively, one might anticipate that the resist sidewall, known to be

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preferential to the PVBD block even when not etched, would anchor this block and drive

the assembly of the BCP parallel to the resist sidewalls. However, when confined to a

small area, as is the case with the BCP in the relatively narrow trenches, the energetics of

BCP stretching or compressing in the small space become significant. As has been

predicted43,50 and verified experimentally,65 BCPs confined by hard surfaces have a

tendency to orient themselves perpendicularly to the confinement structures, even if the

interfaces are not perfectly neutral for the BCP. This becomes even more likely if the

space available to the BCP is not a commensurate multiple of its natural periodicity, L0,

which is almost certainly the case here. Observed very infrequently were regions where

the BCP briefly ran parallel to the substrate, as seen in Figure 5.17. It is not clear what is

locally different about the pre-pattern to allow these structures to form, but does imply

that there exists a way to modify the substrate to either promote the formation of parallel-

aligned features or promote the formation of perpendicularly aligned features. Unclear

also is the behavior of the BCP at the very bottom of the film. Again, due to the

preferential wetting of PVBD to the NIL resist, one might naively expect a wetting layer

to form, but this complicated confinement situation does not lend itself to speculation.

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Figure 5.17: Top-down SEM of 10 nm PVBD-b-PDSS on unetched NIL resist (without a

brush treatment) showing a region where the features briefly run parallel to

the resist.

In other regions of the wafer, patches of BCP material seemed to rise above the

NIL resist lines and form somewhat continuous films, as seen in Figure 5.18. The

features are still aligned more or less perpendicularly to the resist lines, but a degree of

waviness and randomness is clear. We anticipate that if the film thickness of the BCP was

increased to the point where the majority of the film thickness was higher than the height

of the resist features, there would be a continuous film of somewhat aligned BCP. In fact,

we anticipate that the result originally depicted in Figure 5.3, where a brush was grafted

to saturation on etched resist structures, is essentially this phenomenon. The BCP

domains in-between the resist lines orient themselves perpendicularly to the lines due to

confinement, and nucleate the formation of similarly aligned BCP structures on top, even

though the BCP structures on top do not interact directly with the resist trench. Although

the practical ramifications of perpendicularly aligned DSA features are uncertain, their

presence provides clues to the major forces at play in this kind of topographic DSA.

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Figure 5.18: Top-down SEM of 10 nm PVBD-b-PDSS on unetched NIL resist (without a

brush treatment) showing a region where the BCP runs perpendicularly to

the resist and patches of BCP rise above the resist lines.

5.8 CONCLUSION

In summary, a 10 nm full-pitch, silicon-containing BCP was integrated with the

NIL process used to create dense BPM templates at Seagate. Application of the polarity-

switching top coat, necessary for perpendicular orientation of BCP features, was not an

issue at all. Ironically, applying the bottom surfaces was more difficult due to the

proclivity of the polymeric brushes to graft to etched NIL resist. Several routes towards

DSA were developed, both with and without a neutralizing brush. Cross-sectional STEM

EELS, however, showed that a brush was required to generate the most lithographically

useful features. Methods to apply a brush without sacrificing the quality of BCP

alignment were investigated, and the most promising results involve exploiting the

reactivity of etched chromium towards organic functional groups.

Future work involves testing the “selective” grafting procedure with actual DSA

structures, and analyzing the output both for alignment to the pre-pattern and through-

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film structure. If these results are satisfactory, transferring the BCP pattern into

chromium is required, which we are currently collaborating with Lam Research to

accomplish. After the demonstration of DSA to produce line-space patterns, a second

patterning step is required to chop the line structures into rectangular bits. This step might

also require a pattern densification by DSA to achieve the resolution goal, which brings

with it interesting technological challenges.

5.9 ACKNOWLEDGEMENTS

Austin Lane and Michael Maher contributed to the development of the processes

and the production of the samples. Austin Lane contributed to the production of the

manuscript. Dr. XiaoMin Yang and Dr. Kim Lee of Seagate facilitated the collaboration,

contributing time, materials, and resources.

5.10 EXPERIMENTAL

5.10.1 Materials

Solvents for spin coating and rinsing were purchased from Sigma Aldrich and

used as received.

5.10.2 Initial pre-patterning

150 mm silicon wafers were used as received or coated with a 2-3 nm layer of

sputtered chromium metal (performed on an AJA International ATC 2200-V sputtering

system). A 20 nm commercial NIL resist was deposited on the wafers and patterned with

a Molecular Imprints Imprio HD 2200 Nanopatterning System. As described, the

template typically produced a pre-pattern with 18 nm structures with pitches of 40 to 50

nm in a 2.4 mm band around the center of the wafer. Trim etching was performed by

exposing the wafer with resist to an oxygen-based plasma (performed in an Oxford

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Plasmalab System 100 etcher). Samples were routinely characterized by AFM and SEM

after trim etching.

5.10.3 BCP and top coat processing

Spin coating and annealing were performed on a Brewer Science 100CB spin

coating/hot plate station. BCP films of various thicknesses were deposited out of MIBK

(0.9 wt % at 3000 RPM for 18 nm film, 1.2 wt % at 2000 RPM for 33 nm film). A ~0.5

wt % methanol solution of the trimethylamine salt of the top coat was then spin coated

directly on top to produce a ~15 nm thick film. The filmstack was annealed between 190

°C to 205 °C for 2-10 mins, depending on experiment (generally, longer anneals and

higher temperatures led to better DSA results). For inspection by SEM, the filmstack was

subjected to a second oxygen-based RIE step (also in the Oxford Plasmalab System 100)

for 90 – 105 s. This time was sufficient to remove the top coat, partially remove the

PVBD domains, and partially oxidize the PDSS domains.

5.10.4 Thin Film analysis

SEM micrographs of NIL patterns and BCP films were obtained using a

Raith150-Two system operating at an accelerating voltage of 10 keV. AFM height data

was obtained from an Asylum MFP-3D Origin system operating in tapping mode. Cross-

sections were made by focus ion beam milling by Nanolab Technologies, CA. Cross-

sectional STEM and EELS images were obtained from Nanolab Technologies, CA. Film

thickness measurements were made on a Nanometrics reflectometer (model # 1000-

01195).

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Appendix A: Characterization of BCP samples

The data summarized in Chapter 3 for different samples of BCPs is included

below. Data appearing in Durand et al.40 has been omitted for brevity except for the cases

where a discrepancy existed between my data and the published data. Very rarely were

the discrepancies practically significant, and each case is commented on separately.

A.1 SEC DATA

Figure A.1: SEC traces of PS (top trace) and PS-b-PTMSS (bottom trace) for (A)

disordered (B) 18 nm (C) 19 nm (D) 22 nm.

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Figure A.2: SEC traces of PMOST (top trace) and PMOST-b-PTMSS (bottom trace) for

(A) 14 nm (B) 20 nm. The SEC trace of PMOST-b-PTMSSdisordered appears

in Durand et al.40

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Figure A.3: SEC traces of PMOST (top trace) PMOST-b-PDSS (bottom trace) for (A)

14.5 nm (B) 18 nm (C) 23 nm.

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Figure A.4: SEC traces of PTMSS (top trace) and PTMSS-b-P2VP (bottom trace) for

(A) 10.0 nm (B) 10.5 nm. The tailing observed in the bottom BCP trace is

due to P2VP interacting with the gel material and not an indication of poor

control of synthesis.

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Figure A.5: SEC traces of PVBD (top trace) and PVBD-b-PDSS (bottom trace) for (A)

8.5 nm (B) 9.5 nm (C) 10.0 nm (D) 10.5 nm.

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Figure A.6: SEC traces of PMVBD (top trace) and PMVBD-b-PDSS (bottom trace) for

(A) 6.5 nm (B) 10.5 nm.

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A.2 MALDI DATA

Figure A.7: MALDI spectrum for PS-b-PTMSS for (top to bottom) 18 nm, 19 nm, 22

nm. The data for the PS-b-PTMSSdisordered appears in Durand et al.40

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Figure A.8: MALDI spectrum for PMOST-b-PDSS for (top to bottom) 14.5 nm, 18 nm,

23 nm. The data for the disordered sample appears in Durand et al.40

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Figure A.9: MALDI spectrum for PTMSS-b-P2VP for (top to bottom) 10.0 nm, 10.5 nm.

(Left) homopolymer PTMSS aliquots (right) BCP.

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Figure A.10: MALDI spectrum for PVBD-b-PDSS for (top to bottom) 8.5 nm, 9.5 nm,

10.0 nm, and 10.5 nm.

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Figure A.11: MALDI spectrum for PMVBD-b-PDSS for (top to bottom) 6.5 nm, 10.5

nm. (Left) homopolymer PDSS aliquots (right) BCP.

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A.3 SAXS DATA

Figure A.12: SAXS profiles of ordered PS-b-PTMSS samples, from top to bottom: 18

nm, 19 nm, 22 nm. Data shifted vertically for clarity.

Figure A.13: SAXS profiles of ordered PMOST-b-PTMSS samples, from top to bottom:

14 nm, 20 nm. Data shifted vertically for clarity.

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Figure A.14: SAXS profiles of ordered PMOST-b-PDSS samples, from top to bottom:

14.5 nm, 18.0 nm, 23.0 nm. Data shifted vertically for clarity.

Figure A.15: SAXS profiles of PTMSS-b-P2VP samples, from top to bottom: 10.0 nm,

10.5 nm. Data shifted vertically for clarity.

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Figure A.16: SAXS profiles of PVBD-b-PDSS samples, from top to bottom: 8.5 nm, 9.5

nm, 10.0 nm, 10.5 nm. Data shifted vertically for clarity.

Figure A.17: SAXS profiles of PMVBD-b-PDSS samples, from top to bottom: 6.5 nm,

10.5 nm. Data shifted vertically for clarity.

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A.4 DMS DATA

Figure A.18: DMS plots of the storage modulus (G’) of PS-b-PTMSS taken upon a

constant 5 °C/min temperature ramp. Each sample was taken at strain = 2%

and angular momentum (ω) = 1 rad/s.

Figure A.19: DMS plots of the storage modulus (G’) of PMOST-b-PTMSS and PS-b-

PDSS taken upon a constant 5 °C/min temperature ramp. Each sample was

taken at strain = 2% and angular momentum (ω) = 1 rad/s.

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Figure A.20: DMS plots of the storage modulus (G’) and delta of PMOST-b-PDSS

(disordered sample) taken upon a constant 5 °C/min temperature ramp. The

sample was taken at strain = 2% and angular momentum (ω) = 1 rad/s. No

indication of an ODT appears in the graph, precluding the direct calculation

of χ.

Figure A.21: DMS plots of the storage modulus (G’) of PTMSS-b-P2VP taken upon a

constant 5 °C/min temperature ramp. Each sample was taken at strain = 2%

and angular momentum (ω) = 1 rad/s.

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Figure A.22: DMS plots of the storage modulus (G’) of PVBD-b-PDSS taken upon a

constant 5 °C/min temperature ramp. Each sample was taken at strain = 2%

and angular momentum (ω) = 1 rad/s.

Figure A.23: DMS plots of the storage modulus (G’) and delta of PVBD-b-PDSS (8.5

nm disordered sample) taken upon a constant 5 °C/min temperature ramp.

The sample was taken at strain = 2% and angular momentum (ω) = 1 rad/s.

No indication of an ODT appears in the graph, precluding the direct

calculation of χ by this sample.

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Figure A.24: DMS plots of the storage modulus (G’) of PMVBD-b-PDSS taken upon a

constant 5 °C/min temperature ramp. The sample was taken at strain = 2%

and angular momentum (ω) = 1 rad/s.

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Appendix B: Practical considerations in anionic polymerization

Anionic polymerization is a great path to low dispersity BCPs with defined and

controlled microstructures. The BCPs introduced in this Dissertation were all synthesized

via this technique. However, anionic polymerization is not trivial to implement. The high

degree of reactivity of the carbanion propagating groups necessitates that stringent

procedures are followed to exclude any moisture or oxygen which will deleteriously

affect the polymerization, even in low amounts. In addition, there are a great number of

safety considerations when performing the polymerization that need to be addressed.

These hazards have, can, and will cause bodily harm, and knowledge about them is

necessary to mitigate danger as much as possible.

B.1 MONOMER AND SOLVENT PURIFICATION AND STORAGE

Perhaps the biggest historical hurdle to successfully conducting an anionic

polymerization was removing contaminants from the monomers. Water, oxygen, radical-

trapping inhibitors, and any other protic species will adversely affect the polymerization

by terminating reacting groups or by coupling multiple reacting groups together (while

also terminating them). The actual, hands-on procedures to perform the following

purification steps cannot be well-described in writing without necessarily omitting safety

and practical information concerning the use and manipulation of the glassware. Unless

the researcher is very experienced in air-free/water-free techniques, there is no substitute

for an apprenticeship period. However, a high-level description is provided to convey

information garnered through experience. First, generic information about handling the

purification reagents is presented, and then information about performing a successful

distillation is presented. Last, taking solvent is briefly described.

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Oxygen and other gaseous impurities are removed by repeated freeze-pump-thaw

steps with liquid nitrogen (LN2), which exploits vapor liquid equilibrium to draw out

oxygen from the monomer. Order-of-magnitude calculations suggest that just three

freeze-pump-thaw cycles can easily reduce the oxygen partial pressure to one millionth of

the original. A freeze-pump-thaw is necessarily performed before every distillation, and

most monomer purifications have procedures involving three or more distillations,

guaranteeing at least three freeze-pump-thaw cycles. In addition, typically one to two

extra freeze-pump-thaws are performed before the first distillation, bring the number to

four or five. Last, before any freeze-pump-thaw cycles are performed, it is common

practice to open the monomer flask straight to vacuum while stirring vigorously until the

monomer just begins to boil. In this way, a bulk of the gas is removed from the monomer

without necessarily removing any significant amount of material, which improves the

effectiveness of the subsequent freeze-pump-thaw cycles. The oxygen content in the

drybox is typically 3 to 6 ppm, and it has been observed that exposing the monomer to

this degree of oxygen does not noticeably detract from the quality of the polymerization

product.

Non-gaseous impurities are removed by distilling the monomers over

organometallic bases that function as purification reagents. The purification reagents for

monomer classes are as listed in Table B.1. It is worth noting that these materials are

highly pyrophoric and require utmost care when handling AND when disposing.

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Table B.1: Purification reagents for monomers in anionic polymerization

Monomer class Purification reagent Reference

styrene 2x di-n-butyl magnesium, 1 pipette / 10 g monomer 152

methacrylate 1x CaH2, 1 g / 5 g monomer

1x trioctylaluminum, titrated until yellow holds

54

2-vinylpyridine 1x CaH2, 1 g / 5 g monomer

1x triethylaluminum, titrated until yellow holds

152

diphenylethylene 2x n-butyl lithium, 1 pipette / 10 g monomer 54

ethylene oxide 2x butyl magnesium chloride, 1 pipette / 10 g

monomer

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Di-n-butyl magnesium, n-butyl lithium, and butyl magnesium chloride are alike in

that these purification reagents are dispensed into Schlenk flasks with stir bars in the

drybox via a glass pipette, and then removed from the drybox (closed to the outside

environment). The solvent from these reagents only is then removed while the flask is

hooked up to a thoroughly purged manifold. Isolated di-n-butyl magnesium is a solid,

which requires a special procedure to dry fully. First, the solution is stirred as quickly as

possible and the argon in the flask is slowly bled out into the vacuum. Once the argon is

removed and a reasonably high vacuum is reached (~ 900 mtorr), the flask is opened

entirely and the solvent removed while the stirring continues. This way, the di-n-butyl

magnesium is isolated as small flakes. If the procedure is not followed, the purification

reagent may instead turn into a gel intermixed with solvent that is hard to fully dry.

Performing this part of the procedure on only one flask at a time helps prevent the

occurrence of gelling. Once dry flakes are isolated (typically seen coating the entire wall

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of the glassware), one may open multiple flasks to the same vacuum. The flasks can be

submerged in warmed water baths (no more than 60 °C or so) to promote drying.

Typically, it may take 60 – 120 minutes to dry two such flasks. Note: n-butyl lithium and

butyl magnesium chloride are liquid when isolated, so they do not require this thorough

process.

Triethyl aluminum (TEA) and trioctyl aluminum (TOA), conversely, are not

isolated from the solution in which they are received. Instead, one distills the monomer

into an empty Schlenk flask charged with a stir bar, and then the tared flask is taken into

the glovebox where TEA or TOA are added drop-by-drop to the stirred solution until a

yellow color persists. The closed flask is then taken out of the glovebox and weighed

again. One can estimate how much of the monomer solution is now solvent (based on the

nominal concentration of the purification reagent) to accurately determine how much

monomer solution needs to be later dispensed for the reactions. The TEA- or TOA-

containing monomer is then isolated by distillation into a clean flask after approximately

one hour of stirring.

After the monomer is distilled out of the purification reagents into the final

monomer flasks, the flasks containing purification reagents should be closed while under

vacuum and disconnected from the Schlenk line. In a clear hood with no stray solvent

bottles, the neck of the flask should be filled with tetrahydrofuran (THF), and then the

plug opened just to the point that THF begins to bleed into the flask. The level of THF

should be constantly replenished while swirling the flask to make sure the sidewalls are

wetted with THF. Once one is sure the sidewalls have been wetted with THF, it is safe to

fully open the flask to air. Small amounts of isopropyl alcohol (IPA) should then be

added to the solution repeatedly and swirled. This step should be repeated with methanol

and water once one is confident that every part of the glassware has had time to react

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with the IPA (and then the methanol). Typically, addition of only a few drops of water

causes the solution to gel. It is recommended that this mixture is poured out into the

waste as soon as possible or the flask will become hard to clean. Acetone can be used to

rinse the flask repeatedly at this point as it seems to dissolve the gel reasonably well.

Typically there will still be some visible impurities in the glassware. Soaking the flask

alternatively with THF and then dilute acid (but transitioning with acetone and water

rinses in between) usually dissolves most residues in a matter of hours. Rarely, it is

necessary to put HCl solution directly into the flask and then to let the flask sit overnight.

Label the flask as containing concentrated acid to avoid any confusion in the future.

Calcium hydride (CaH2) is the only purification reagent that does not need to be

handled inside of a drybox. The powder is poured into the Schlenk flask with a funnel,

and one needs to make sure to clean off any dust on the glass that makes contact with the

plug. Typically, CaH2 is purged 5 times with argon when connected to the vacuum, but

never flame dried. The monomer is distilled over into the CaH2 flask, and then allowed to

stir. After an adequate time interval, which can be anywhere from one hour to overnight,

the monomer is distilled, typically into an empty flask for TEA or TOA treatment.

Distilling out of CaH2 can be difficult because the powder gets carried over into the next

flask. It has been recommended that glass wool be inserted into the distillation line to

block CaH2, but this has produced mixed results as very often the glass wool is blown

into the source or destination flask via argon purges. Instead, a slow distillation is

recommended, especially of low boiling monomers like methyl methacrylate and 2-

vinylpyridine. If a visible amount of CaH2 is transferred, a second distillation is

recommended. The CaH2–containing flask does not need to be cleaned as delicately as

flasks containing the other purification reagents, but care is needed since CaH2 is still

highly reactive. In addition, it can stain glassware and build deposits which can in turn

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prevent a high degree of vacuum from being reached when the deposits are on the glass

region that interfaces with the plug. CaH2 should be emptied into its own glass disposal

beaker using alcohol. IPA and methanol are both good at removing residual powder from

the flask. It has been found that acetone and THF causes CaH2 to form insoluble residues

and actually hamper the cleaning effort. Once all visible CaH2 is removed, these solvents

can be used. Acid, either dilute or concentrated, can again be used to deal with any

insoluble salts.

Conducting the distillation of high boiling point monomers is difficult since the

monomers begin to slowly polymerize in a hot water bath. Durand38 describes custom

glassware that maintains an open connection to the vacuum to help with this process. In

addition, Durand describes angled distillation glassware to help condense and isolate

monomer. Typically, monomers like styrene or methyl methacrylate can be distilled at a

30 – 40 °C water bath. 4-methoxystyrene requires 55 – 65 °C, while 4-

trimethylsilylstyrene needs approximately 70 °C. Monomers larger than 4-

trimethylsilylstyrene (FW 176 g/mol) need near boiling water for transfer.

Heat wrap can be used to expedite the distillation of high boiling point monomers.

Heat wrap is recommended even for 4-methoxystyrene (FW 134 g/mol), and definitely

for monomers larger than this. The heat wrap needs to be wrapped tightly around the

glass in order for the heat to transfer effectively and evenly. Of special importance is the

need to wrap the region where the plug sits in both the sending and receiving flask.

Monomer tends to pool here because the thick glassware and Teflon plug take longer to

heat than the rest of the apparatus. In addition, the tight-fitting plug can wick away liquid

monomer, further slowing down the transfer. Often this area of the glassware is neglected

due to the cramped geometry, but with practice becomes extremely easy to wrap. Once

wrapped tightly, the heat wrap needs only to be operating at 5 – 15% of total power.

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More power and hotter temperatures have caused the polymerization of the monomer

inside the glass tubing. Often monomer will condense in the U-shaped portion of the

Schlenk line. This portion of glassware can also be wrapped with heat wrap, or it can be

warmed with a heat gun. Often, the monomer that condenses here is ignored since the

amount it is so small and the rest of the monomer continues transferring nonetheless.

Low boiling point monomers, and especially solvents, can also pose difficulties if

they transfer too quickly. If CaH2 is used, a quick transfer again propels the material into

the next flask. Even with other purification reagents, too fast of a transfer can cause the

solution to bump due to the small tubing cross-section, transferring the purification

reagent into the distillation tubing and sometimes the next flask. Ensuring adequate head

space (flasks filled no more than half way) and vigorous stirring helps to prevent this

from occurring. In addition, using a modest temperature is recommended. For solvents,

this usually means keeping the source flask in a room temperature water bath or even an

ice bath. Last, for severe problems with bumping, the angled distillation tubing can be

used in reverse to create an “uphill” path for the liquid to follow, helping to trap the

purification reagent behind in the source flask.

A common issue is not knowing when a distillation is finished and not knowing

how to efficiently transfer to the next step. Action needs to be taken when the flask looks

visibly dry, and definitely when the stir bar stops stirring. First, the plug on the sending

flask needs to be closed, and the heat wrap turned off. The LN2 in the receiving flask

should be topped off, and one should wait for approximately 3 – 5 minutes. Then, the

LN2 should be topped off again, and the heat wrap removed to inspect the glass tubing. If

liquid monomer is found, one should attempt to heat it with the heat gun to transfer it into

the receiving flask. After no more than 1 – 2 minutes of trying to recover this monomer,

or if no liquid monomer was detected, the system should be opened to vacuum and

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allowed to achieve baseline. Any residual liquid monomer should be heated once again,

and typically within five minutes baseline is achieved. The receiving flask, which is still

open to vacuum, should be closed and thawed safely, and then the glassware should be

disassembled and reassembled as needed for the next distillation step.

When distilling into the final purification flask, the entire glass assembly,

including the U-shaped portion of the Schlenk line, needs to be flame dried with a butane

torch repeatedly. Typically, the entire assembly is flame dried three times, undergoes five

pump purges, and at least one of those pump purges is a freeze-pump-thaw of the source

flask (where the neck is also flame dried when the stop cock is opened). This is to ensure

that as much water is removed as possible. Care must be taken not to heat the metal or

rubber sections for too long. Care must also be taken to flame dry the reverse side of the

glassware as well as the front. Heat wrap, if used, should be applied during the pump

stage of the last freeze-pump-thaw cycle while both flasks are open to vacuum.

Isolated monomer should be thawed to the point where approximately half of it is

still frozen, and then the flask should be backfilled with argon and sealed at a modest

overpressure (1-3 psig). The flask of half-frozen monomer should be massed to determine

the final distillation yield and then put into the glovebox as soon as possible. Typically,

losses after three distillations amount to no more than 1 g for 4-trimethylsilylstyrene and

lighter monomers (ignoring, of course, any large issue that arise). One should anticipate

larger losses than 1 g for higher boiling-point monomers.

Once fully distilled and isolated, the monomers have a finite but unfortunately

undefined lifetime before they begin to autopolymerize and become useless for anionic

polymerization. From experience, styrene and methyl methacrylate can be stored in a

drybox freezer for at least one night before being used, but anecdotes suggest they may

have a lifetime of a week or so. Some monomers like diphenylethylene and 4-

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methoxystyrene freeze at freezer temperatures, so they have displayed longer lifetimes

(on the order of months for diphenylethylene).

Of special note are the styrenes 4-trimethylsilylstyrene and 4-

pentamethyldisilylstyrene. These have a notably short lifetime once purified, and

typically by the next day have turned into a stringy solution believed to be an indication

of autopolymerization. These monomers are typically distilled and used on the same day

for the best polymerization products.

Compared to the involved process used to purify monomer, obtaining purified

solvent is relatively trivial. The solvent delivery system (SDS) delivers suitable solvent

straight to a flask. The flask, fitted with a plug but open, should be attached to the Cajon

fitting, subjected to vacuum, and allowed to baseline. If baseline is not readily achieved

(no more than 10 mtorr or so above the baseline achieved with all the valves closed), the

flask should be taken off the vacuum and the Cajon fitting and plug should be inspected.

Very often the rubber o-ring in the Cajon fitting on the SDS can tear due to repeated use.

Once a suitable vacuum is achieved, the entire apparatus should be pump-purged with

argon at least two times, and then flame dried with pump-purge at least three times.

Again, flame dry the neck but avoid flame drying the metal fittings. The flask should be

left under vacuum and allowed to cool before dispensing solvent. Solvent should be

dispensed only to a flask under vacuum. Drawing a line on the flask with a permanent

marker parallel to the lip of the metal ledge on which the flask sits indicates a fill level

that yields approximately 130 – 140 g of THF. Sometimes, closer to 100 g is desired,

which requires practice to approximate. Once the solvent is taken, the flask should be

filled with argon and closed, and vacuum should be enabled yet again. Inevitably, a

sizeable amount of solvent remains in the neck of the flask which needs to be removed by

vacuum to ensure a good seal in the flask. The ribbed metal piece quickly becomes very

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cold under vacuum and can be warmed gently with the heat gun. Once baseline is

achieved with the neck of the closed flask, the SDS should be turned to argon and the

neck refilled. While under argon, the plug should be briefly opened to allow trapped THF

to travel into the flask. The flask should be closed a second time, and then evacuated one

more time. This time, typically only a tiny amount of residual solvent needs to be

removed and baseline is achieved quickly. The plug should be confirmed to be tightly

closed, and then the flask is removed. At this point, the weight of the flask should be

noted and compared with the tare weight to determine the amount of solvent received.

The flask should be stored way from light in a safe place where it will not be knocked

over.

B.2 REACTOR SETUP

Reactors should be set up to test as soon as possible since achieving vacuum

baseline is often problematic. Once solvent is acquired, the reactor can often be set up on

the same Schlenk line right away. The reactors used in the Willson and Ellison labs are

five port threaded reactors. Very often not all five ports are used, but there exists many

different combinations of glassware than can be used in conjunction with the reactor.

Typically, using Figure B.1 as a reference, port 1 has a glass blank, port 2 has the solvent

flask, port 3 should have the vacuum adaptor, port 4 has another blank, the burette

adaptor, or simply a burette, and port 5 has a thermocouple well. In this configuration, a

right-handed operator can access the valve on the solvent flask without too much trouble,

and a path to the septum on port 3 is still open. Putting the solvent on either of the front

two ports blocks access to the septum and also puts too much torque on reactor, held up

by clamps on ports 1 and 2. Putting the burette adaptor or a burette on port 5 often puts it

too close to the solvent flask in port 2, so port 4 is a better choice. A line going to vacuum

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and to a pressure gauge are attached to the adaptor in port 3. Be sure to place a stir bar

and any other necessities (such as LiCl salt for methyl methacrylate polymerization) in

the reactor before pulling vacuum.

Figure B.1: Top-down illustration of 5 port reactor. Ports 1 and 2 are at the back of the

reactor (clamps attach to these ports) and ports 4 and 5 are in the front.

Once all the parts are attached (inspect glassware for cracks or hairline fractures),

the reactor can be opened to vacuum. Sometimes it is prudent to incrementally test the

vacuum of the reactor by first closing the plug leading to the reactor and inspect just the

Schlenk line connector, then by opening the reactor but closing the reactor’s pressure

gauge to test the bulk of the glassware, and then finally by opening the pressure gauge

plug. Leaks, manifested as more than 10 - 100 mtorr above baselines pressure, are

common. First thing to try is to tighten all of the black plastic threaded screws. Next,

rotate the solvent flask gently without applying too much torque. Sometimes this bulky

piece of glassware needs to be reseated. If that fails, attempt to rotate the other glass

pieces very gently and slowly to see if they need to be reseated. If the vacuum does not

reach baseline at this point, it is necessary to disassemble the vacuum piece-by-piece to

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determine the culprit. Often, very short, stubby tubing, either on the blanks or on the

vacuum adaptor, causes an insufficient seal to form. The glassware should be replaced

with some with longer tubing. In addition, the white Teflon ferrule fittings sometimes

need to be replaced. Anecdotally, it seems that ferrule fittings that initially sit much

looser on their glass tubing make better connections. Fittings that are initially tight

should be replaced with looser fittings.

B.3 CONDUCTING A POLYMERIZATION

With a reactor that achieves baseline, it is possible to proceed with the actual

polymerization. The reactor should be pump-purged at least two times, and then flame

dried with pump-purging at least three times. The glassware, including the vacuum

adaptor and the Schlenk line, should be flame dried as well. Care should be taken not to

heat any plastic, metal, or rubber portion for too long.

B.3.1 Cooling the reactor

Once a satisfactory flame dry is performed, the reactor should be allowed to cool

under vacuum, and then filled with argon to approximately two psig. The plug to the

reactor should be closed, but the Schlenk line should be left open to argon. The solvent

flask should then be opened and the solvent allowed to flow into the reactor. Screw the

plug closed once the solvent has emptied into the reactor. Stirring should now commence.

Now the reactor should be cooled with LN2 while monitoring the temperature with a

thermocouple. The Dewer containing LN2 may be lifted up closer to the reactor to

submerge more of the reactor in the LN2 to speed up the cooling. While the reactor is

cooling the pressure in the reactor also needs to be monitored. The reactor should be

refilled repeatedly with argon at this point to three psig, and never allowed to reach

below one psig. One needs to be alert since there is a delay between the temperature of

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the bottom of the solvent and the temperature of the solvent in the thermocouple well.

The solvent may get too cold and freeze- the target temperature is -78 °C but the freezing

point of THF is -108 °C. Typically, it takes approximately two dewer refills of LN2 to

reach this temperature for a 500 mL reactor flask. If the solvent is kept stirring, it should

be fine even if part of it freezes. At approximately -75 to -85 °C, IPA and crushed dry ice

should be added to the LN2 flask. Any residual LN2 will quickly evaporate. If the solvent

had ceased stirring due to freezing, it should also be warmed with IPA and dry ice after

pouring out the excess LN2.

Once the reactor is submerged with IPA, the Schlenk line needs to be switched

from argon to vacuum and the pressure monitored again. The slow warming of the

reactor to -78 °C will increase the pressure. This should be countered by slowly bleeding

off pressure by slightly opening up the port to the reactor. This becomes much more

important if the solvent froze, since the temperature may change abruptly. Constantly

refill the dry ice in the IPA and stir the slurry to maintain even cooling. Once the pressure

and temperature of the reactor has equilibrated (in practice around -72 °C), the initiator

can be added.

B.3.2 Dispensing the initiator

The preferred initiator for anionic polymerization is sec-butyllithium (sBuLi) 1.4

M in cyclohexane from Sigma-Aldrich. It is worth repeating that this organometallic is

extremely pyrophoric and that a related compound was the cause of the death of a

researcher a few years back. Only dispense the compound in the glovebox with a glass,

gas-tight syringe.

Before operating the glovebox, fill two 50 mL beakers halfway with THF and set

them near the reactor. These will be used to quench and purge the sBuLi syringe.

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Deliberately decide which beaker to take from and which beaker to purge into. Clear out

space near the reactor to remove clutter. Solvent bottles should be removed as well.

In the glovebox, the sBuLi is drawn out. A good idea is to remove the sBuLi from

the glovebox freezer right before dispensing the THF into the reactor to allow the sBuLi

time to warm up. Typically, an amount of sBuLi solution is brought into the syringe and

then the plunger is moved to about 10% of the syringe’s capacity further than the desired

amount. The syringe is then held upside down, with the needle point up, while the

plunger is pushed closed against the glovebox shelf. This removes any gas in the syringe

and also wets the syringe barrel with sBuLi, cleaning it in the process. The drawback is

that impurities concentrate in the sBuLi. (Because of this, small 25 mL bottles are

recommended for use despite costing a nominal premium.) The desired amount of sBuLi

is then taken up, and the needle is immediately removed from the glovebox and taken to

the reactor(s). As a rule of thumb, for safety the syringe should not be not filled more

than halfway.

Before inserting the needle into the reactor, one droplet of sBuLi is typically

dropped onto the fume hood countertop to “clean” the needle tip of any residue. The

importance of this practice is unsubstantiated but followed nonetheless. The needle is

immediately inserted into the reactor and the syringe and needle apparatus is arranged so

that one can easily see the graduations on the barrel. The sBuLi is dispensed into the

reactor, forming a characteristic neon yellow color. After all the reactors are initiated

with sBuLi, THF from one of the beakers is drawn up into the syringe and mixed with

any residual sBuLi solution before being purged into the second beaker with the needle

below the THF level. This is repeated five times to clean the syringe and ensure no sBuLi

remains in the syringe. The reactor(s) with sBuLi is(are) allowed to stir for approximately

one hour, the time it takes for the glovebox to be prepared for monomers.

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It is prudent to take more sBuLi than anticipated since inevitably some portion of

the sBuLi will “die” to residual impurities in the THF, and additional sBuLi needs be

delivered to the reactor to compensate for this loss. There are two ways to determine how

much extra sBuLi is needed. The color of the reaction should be monitored as the sBuLi

is added drop-by-drop to the reactor. At some point, the characteristic yellow color

develops. The plunger position at this point in time should be noted. At the point that the

color begins to hold, regardless of how much liquid actually entered the reactor,

experience shows that the reactor has received an effective “dose” of 0.2 to 0.6 mL of the

1.4 M sBuLi. The remainder that is necessary should be estimated based on the goals of

the polymerization and calculated from the plunger position. The total amount of liquid

dispensed and the point at which color first held are typically noted. These values can be

used in the second procedure, known as “dead fraction calculation”. Essentially, simply

from the amount of sBuLi dispensed into the reactor, the molecular weight of the product

polymer as determined by some technique like MALDI, and the amount of monomer

used in the reaction, the number of sBuLi molecules that “sprouted” into chains can be

calculated “posthumously”, which can be used then to calculate how many milliliters of

1.4 M sBuLi “died” due to solvent. This value typically scales with amount of solvent

dispensed into the reactor, and ranges from 0.04 to 0.15 mL / 100 g THF. This value may

change from reaction to reaction, but is still tracked to inform subsequent polymerization

attempts.

B.3.3 Dispensing monomer

The historical way to dispense monomer into threaded-port reactors was to use a

graduated burette. However, this method fell out of favor while working with the high-χ,

silicon-containing BCPs for a number of reasons. First, the purification procedure is

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slower since multiple burettes need to be filled one by one if running multiple reactions.

Second, when working with the silicon-containing monomers, the relatively small

amount of the non-commercially available monomer used (~5 g of each monomer for ~10

g total) became difficult to accurately measure out from the burette into the reactor.

However, using a burette works and is recommended once one starts manipulating ~20 g

of monomer or more at a time.

The more convenient way found to dispense the monomer, especially at the scale

at which we are operating, is to use a plastic syringe. Specifically, the ubiquitous Luer

Lock Norm-Ject® plastic syringes by Henke Sass Wolf (Germany), coupled to a standard

stainless steel needle, are able to hold a sufficient seal that the monomer can be removed

from the drybox and injected into the reactor and form essentially flawless BCPs. The

graduations printed on the syringes are sufficient to withdraw and deliver monomer

accurately. The nominally 5 mL (and smaller) syringes are especially convenient since

the barrel of the syringe fits into the glass neck of a standard Schlenk flask, permitting

one to insert even a relatively short needle (~20 cm) very far into the flask. The

nominally 10 mL syringes are unfortunately too thick to fit into the neck of the flask.

These require a standard 30 cm steel needle to adequately draw up monomer. The length

of the needle becomes important because the drybox antechamber can only fit a certain

length of apparatus. A fully withdrawn 10 mL syringe will require the corresponding 30

cm needle to be bent to a large degree. This strains the Luer joint significantly and is

anticipated to be a point of failure for at least one reaction. A fully withdrawn 5 mL

syringe with a 20 cm needle or a 10 mL syringe withdrawn to approximately 8 mL with a

30 cm needle fit comfortably in the glovebox antechamber.

Before inserting into the reactor, one droplet of monomer is dropped outside the

reactor (usually directly on the wall of the reactor) and then the needle is inserted

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immediately after. Just like with the sBuLi, this practice is believed to help keep the

needle tip clean.

The needle should be positioned in such a way that the monomer flows directly

into the THF vortex. Monomer that splashes onto the side of the reactor creates pockets

of high monomer concentration and has led to the formation of gels on the glass wall.

The monomer should be added drop-by-drop, with an eye on the temperature of the

reactor. Typically, the addition of most styrenes can warm the reactor to approximately

negative 60 °C without causing evidence of chain termination to occur, but it is

anticipated that this value is too warm for the silated-styrenes and acrylic and methacrylic

monomers, and negative 65 °C is a more conservative target.

It is also possible to use glass air-tight syringes to dispense monomer. However,

they need to be cleaned thoroughly between uses because small but not insignificant

amounts of monomer adhere to their Luer lock assemblies. Multiple reactions have been

contaminated with monomer from previous reactions despite a relatively thorough degree

of cleaning occurring in-between. In this regard, the cheap, disposable plastic syringes

are preferred.

B.3.4 Taking an aliquot

After the first monomer is inserted, an aliquot needs to be taken to analyze the

first block of the BCP. Typically, the monomer is allowed to react for approximately one

hour for styrenes (but longer for diphenylethylene or methyl methacrylate). During this

time, methanol is degassed via argon from the Schlenk line in a 125 mL Erlenmeyer flask

sealed with a septum. Also during this time, needles without warped tips and without

signs of rust should be identified, cleaned, and put in the drying oven. The line running to

the reactor should be under vacuum for all reactors (but the plug still closed).

203

To take an aliquot, the methanol purge first needs to be stopped. While

maintaining the flow of argon, the purging needle is raised out of the methanol (but kept

in the flask), then the vent needle is moved, and then immediately after that the purging

needle is removed. The argon flow to that needle should now be turned off. Repeat for all

methanol Erlenmeyer flasks.

Next, one needle is removed from the drying oven and taken to a reactor. That

reactor’s line is switched from vacuum to argon. The plug to the reactor is opened, and

then the hot needle is inserted immediately through the septum but not into the THF (aim

to insert at least 10 – 15 cm of the hot needle into the reactor). Argon is allowed to flow

through the needle. Once the needle has cooled down (~30 s), a plastic 5 mL Luer

locking plastic syringe with its plunger removed is attached to the needle. The plunger is

then pressed against the barrel opening without re-inserting the plunger fully into the

barrel. The syringe and the face of the plunger are allowed to be purged with argon for

approximately one minute. Then, the plunger is inserted all the way to the bottom of the

barrel, and held in place with a thumb. The pressure in the reactor is allowed to rise to 3

psig, and then the plug to the reactor is closed. The needle is then submerged into the

THF, and an aliquot of approximately 3 mL is drawn up. After a sufficient amount of

liquid is drawn up, the needle is lifted out of the THF and argon is drawn into the syringe

to clear out the needle and prevent spillback of the solution into the reactor’s THF. The

needle and syringe, now very cold, are quickly removed from the reactor and inserted

immediately through the septum of the methanol flask, then the needle tip is inserted

below the level of the methanol, and the syringe contents are quickly pushed out. A white

precipitate forms. Some color may persist in the syringe, indicating living anion. The

syringe should be quenched with THF before being thrown out.

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The procedure is repeated with each reactor. Keep in mind that now there is argon

in each line. The pressure in each reactor should be adjusted to maintain 3 psig, and the

lines switched to vacuum after isolating the reactors from each line.

After the aliquot is taken, the second monomer is typically dispensed in the same

way as the first. If only one monomer is being used, an aliquot is typically not taken.

Care must be taken during the aliquot taking process because every aliquot

inevitably terminates some portion of the anion in the reactor. As Figure A.7 shows,

these dead homopolymer from the aliquots are visible in the MALDI of the BCP powder

(of course, MALDI is more sensitive to low molecular weight materials). Figure A.7 is

especially unique because the three PS-b-PTMSS polymers featured in this figure were

made by taking two aliquots (and building two “blocks” of PS) to accurately hit a

molecular weight target, so both aliquots are seen in the MALDI. The other MALDI

spectrum (Figure A.8, for example) show only one aliquot since only one was taken.

B.3.5 Terminating the reaction

To terminate the reactions, a new, clean Erlenmeyer flask of methanol is purged

with argon. The needle that is going to be used to inject the methanol can be used as the

vent needle for this operation (but not for the aliquots- those needles should be dried in

the oven up until use). When sufficient time has elapsed (~ 45 minutes), a new 5 mL Luer

locking plastic syringe with its plunger removed is attached to the needle, and the plunger

is pressed up against the barrel without being inserted. The argon is again allowed to

purge the syringe for one minute, before the plunger is fully inserted, the argon turned

off, the needle dropped below the level of the methanol, and approximately 3 mL of

methanol is removed. The methanol is injected into each reactor one-by-one without too

much focus on procedure. Typically only one milliliter is needed to terminate a reaction,

205

but the color should be monitored during the termination process. Sometimes, a gradient

of color develops that indicates inefficient mixing that should be rectified in future

polymerizations. After the color disappears, the reaction is allowed to stir for five more

minutes before the argon the reactor is disconnected from vacuum and the plug opened to

vent pressure.

B.4 SAFETY

Of course, these procedures should only be conducted in the safest ways possible.

The various risks associated with the procedures are discussed below.

The biggest risk with these procedures is warming the distillation flasks after they

have been cooled to LN2 temperatures. At LN2 temperatures, argon and oxygen can

condense into liquid. Once the flask is warmed, the liquid evaporates and increases the

pressure in the flask by many orders of magnitude, potentially shattering the glass.

Glassware fitted with pressure-release valves should be used at all times to help prevent

catastrophic glass failure. In addition, one needs to be conscious of whether or not a flask

is open to vacuum. A flask should only be thawed immediately after being shut off from

the vacuum. A flask that has not been subjected to vacuum should not be thawed. The

Schlenk line valve and the plug on the flask should be confirmed to be open before

assessing whether or not a sufficient vacuum has been achieved to allow for safe thawing.

In addition, monomers such as ethylene oxide or isoprene pose specific dangers

since there are mechanisms by which the pressure in the purification flasks may rise to a

dangerous level without being cooled by LN2. The temperature of the solutions of both

monomers needs to be maintained at or below ice water temperature. Ethylene oxide

boils at only 10 °C, so care must be taken to keep this monomer a liquid. In addition,

ethylene oxide may be highly contaminated, which can cause the pressure in at least the

206

first purification flask to increase substantially. It may be prudent to bleed off some of the

pressure while thawing to prevent pressure buildup. Isoprene has been ignored by the

author since it is not a material he has worked with, but bears mention here since multiple

accidents have been caused by this material. At elevated temperatures, isoprene reacts

with its purification reagent to polymerize and generates a substantial amount of heat.

Isoprene’s boiling point is only 34 °C, which may be quickly reached by a runaway

reaction, leading to a surprising buildup of pressure and broken glassware. Again, both of

these monomers, and similar compounds, need to be kept at or below ice water

temperature at all times. It is not recommended that one ever try to work with these

materials without hands-on training from an expert, regardless of their proficiency with

other anionic polymerization materials.

In the same vein, all purification reagents may produce gaseous materials as by-

products of the purification process. This is especially significant for CaH2 which may

produce H2 gas. When the monomer is first thawed and allowed to react with the

purification reagent, any large degree of bubbling should be noted since this might

indicate a release of gaseous by-products. These should be bled out into the vacuum. If

one goes to freeze-pump-thaw the purification flask before distillation and the pressure

reaches > 400 mtorr before baselining, it is recommended that a second freeze-pump-

thaw be performed to remove any additional volatile by-products. Of course, these

purification reagents should only be used in flasks with pressure release valves.

The cryogens used to cool the reactor (dry ice) and cool the vacuum trap and

purification flasks (LN2) also need to be treated with care. Use personal protective

equipment (PPE) in the form of gloves to avoid prolonged contact with these materials.

Be cautious when filling up the vacuum trap since LN2 may pool in the cuff of a lab coat

while pouring and cause burns on the wrist.

207

LN2 poses another specific threat. Oxygen may condense in the vacuum trap if

too much air is continuously pulled into the vacuum line. Liquid oxygen reacts violently

with any organic compounds, such as those that may also be condensed in the vacuum

trap. Do not run the vacuum continuously open to oxygen (for example, in a traditional

distillation) with an LN2 trap. Use a dry ice trap instead.

The butane torch used to flame dry the glassware can also pose a risk. Do not

flame dry with solvent bottles (plastic or glass) in the hood. Metal tins now exist in each

fume hood to conveniently move all the solvent bottles at once out of the hood. Glass

solvent bottles, typically used for waste, should also be removed. Metal fittings, rubber,

and plastic should not be flame dried. Use the flame-resistant lab coats provided by the

department at all times.

And last, as already mentioned, the purification reagents and initiator are highly

pyrophoric organometallics. Practices must be maintained to exclude air and water from

these materials. Sigma-Aldrich publishes an online safety training that discusses the safe

handling of pyrophoric materials.

208

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