Forming Nanostructures by the Top-Down Approach

Photolithography and Microelectronics: Limitations

Nanolithography: Electron Beam Lithography Scanning Near-Field Photolithography

Soft Lithography: Chemically Printing on Surfaces

Scanning Probe Microscopies: Writing on Surfaces

– Learning Objectives Part 3 –

Top-Down Approach

CHM4M2– Nanoscale Science –

After completing PART 3 of this course you should have an understanding of, and be able to demonstrate, the

following terms, ideas and methods.

(i) Appreciate what is meant by top-down and bottom-up wrt the fabrication of nanostructures

(ii) Understand the process of photolithography as applied to the microelectronics industry.

(iii) Understand the limits of photolithography.

(iv) Understand the process of e-beam lithography.

(v) Understand the process on scanning near field optical lithography

(vi) Understand the process of dip-pen nanolithography.

(vii) Understand the process of nanooxidation on surfaces, induced by SPMs.

(viii) Understand that SPMs can not only image, but draw and move particles on surfaces.

What is Meant by Top-Down?

We discussed in Part 2 the Bottom-Up approach to nanostructures: Whereby atoms were assembled into molecules, and molecules into nanostructures (i) by covalent bonds (dendrimers), and (ii) by noncovalent bonds (supramolecules).

The alternative approach is from the Top-Down: 6 x 1023 atoms of silicon (28 grams) can be continually divided until only two remain!

This Top-Down approach has been enormously successful, and has been the mainstay of the microelectronics industry for the last forty years…but they are far from reaching 2 atoms

Using a process called photolithography, feature sizes of less than 200 nm (about 1000 silicon atoms laid side by side) are routinely made on silicon chips. Additionally, using this technology 3 billion transistors per second are made in the US alone!

Lithography Definition: A method of printing from a metal or a stone surface on which the printing areas are not raised but made ink-receptive as opposed to ink-repellent.

QuickTime™ and aPhoto - JPEG decompressor

are needed to see this picture.

Processes 1 and 2 results in the mask - the equivalent of a photographic negative

Laser Beam

Glass Substrate

Thin Chromium

Layer

UV Light

Mask

Lens

Silicon Wafer withLayer of Photoresista

Silicon Chips

1

2

3

The exposed parts of the photoresist are removed, and the exposed silicon is etched away with a chemical reagent, allowing the pattern to be transferred to the silicon, resulting in the silicon chip.

4

A laser beam writes the circuit pattern for a microchip on a layer of light sensitive polymer that has been spun coated on a thin layer of chromium supported on a glass substrate. The irradiated polymer is selectively removed by a solvent. The unirradiated polymer film is left on the chromium.

1

The exposed chromium is then etched away, by a chemical reagent, whilst the chromium that is covered by the polymer is not etched away. When the chromium has been removed to expose the glass, the rest of the polymer is then removed by an organic solvent.

2

When a beam of UV light is directed at the mask, the light passes through the gaps in the chromium. A lens shrinks the pattern by focussing the light onto a layer of photoresist on a silicon wafer.

3 4

Photolithography: The Basis of the Microelectronics Industry

Limitations to Photolithography

The questions that need to be addressed in terms of nanoelectronics are,

(i) can photolithography be used to create structures of less than 100 nm?

and

(ii) if so what is the limit of miniaturisation?

Presently, the photolithography process uses wavelengths of UV light of <250

nm. To create structures, with dimensions less than 250 nm, using a mask

with features less than 250 nm, will lead to diffraction of the UV light which

blurs the projected image. This problem has been overcome by various

technological breakthroughs related to the design of the mask. However,

making mask structures less than half the wavelength of the light being used

results in the projected image being so diffracted that it will no longer be

viable. Thus, structures of sub 200 nm have been achieved, and with

refinements of the technology there is still some scope for miniaturisation.

An obvious answer to this problem is to use UV light of even shorter wavelengths.

Indeed, this avenue of research is being investigated, but there are at least two

problems that need to be overcome, if smaller wavelengths are used:

(i) Conventional lenses are not transparent to extreme (short) wavelength

UV.

(ii) The UV irradiation energy is inversely proportional to the wavelength

and thus the UV light damages the masks and lenses.

As you can imagine there is a great deal of research effort involving chemists to

design, synthesise and characterise new materials that can address these

problems.

What you continually have to bear in mind is that the microelectronics industry

want to keep using this photolithography process for as long as possible, as the

cost associated with building new fabrication plants using other technologies

are huge.

However, at some point the microelectronics industry will have to bite-the-bullet,

and adopt new technologies if they are going to have increased capacity and

performance.

There are several technologies that are currently under investigation. Two of

which are.

X-Ray Lithography

Electron Beam Lithography

At this point, in time these two technologies look as if they may be able to be

developed to a scaleable process for manufacturing silicon chips.

We shall discuss only electron beam lithography.

Electron Beam Lithography

Inducing Cross-linking or Cleavage of Bonds

Non-Specific Chemistry

Silicon

“Organic”

eeee

ee

eee

eeee

ee

1

2 The unirradiated “organic” is removed with an organic solvent, leaving the cross-linked insoluble network pattern.

The electron beam initiates a chemical reaction in the organic material, either

(i) leading to fragmentation to smaller molecular components, which are soluble in some solvent (positive tone resist), or

(ii) crosslinking to form an insoluble network (negative tone resist).

1

2

Serial Writing is very slow, compared to Photolithography

Negative Tone Electron Beam Lithographic Resist

Spin Coated 10 -100s nm

3

4

A chemical etchant is employed to remove the exposed silica, and in so doing also etches the irradiated organic material, result in the pattern transfer to the silicon.

3

The pattern is then doped with appropriate materials to create an active pattern, i.e. will conduct electrons

4

The Organic Material Requirements For a Negative Tone Resist

· Must interact with the electron beam

· Must cross-link to form a network

· Must have a high sensitivity to the electron beam (energy efficiency)

· The network must be insoluble

· The network must have good mechanical strength

· The network must be resistant to the etchant that is used to remove

the silicon in the pattern transfer step (aspect ratio)

Poor Negative ToneResolution Resist

Good EtchDurabilty Resist

SAL-601

Composite of Novolac Resin, Acid Generator

and Cross Linking Agent

Neither materials have good sensitivity towards the electron beam to make them crosslink efficiently, and neither can make a high resolution (thin) and tall (good etch durabilty) structures.

Good Resolution Positive Tone Resist

Poor EtchDurabilty Resist

COOMe

Me

n

PMMA

New Materials Used as Negative Tone E-Beam Resist

Me

O 5O

OMe

J. Fujita, Y. Ohnishi, Y. Ochiai, S. MatsuiAppl. Phys. Lett., 1996, 68, 1297

N

N

NN

O O

O

O

O

O

M. Yoshiiwa, H. Kageyama, Y. Shirota, F. Wakaya, K. Gamo, T. Takai

Appl. Phys. Lett., 1996, 69, 2605

T.Tada, T. KanayamaJpn. J. Appl. Phys ., 1996, 35, L63

These materials were shown to have better sensitivities toward the electron beam, but the etch ratios were still poor.

X

Y

OOO

X

Y

O

n n

ORRO

RO

RO OR

OR

PMMA

Resolution

Etch Ratio

20 nm

6

14 nm 20 nm (10 nm)

6 <1 (<1)

Next Generation Resists

Resolution equals or surpassed PMMAEtch ratio much better than SAL 601Sensitivity much better than previous medium molecular weight materials

Introduced strained cyclopropane ring

Sensitivity enhanced.Crosslinking increased

Large -surface Sensitivity enhanced

14 nm

Scanning Electron Micrographs of Resist Patterns

ORRO

RO

RO OR

OR

R = Pentyl

100 nm

35 nm

20 nm

ScanningElectron Micrographs

‘A Triphenylene Derivative as a Novel Negative/Positive Tone Resist of 10 nm Resolution A.P.G. Robinson, R.E. Palmer, T. Tada, T. Kanayama, M.T. Allen, J.A. Preece, and K.D.M. Harris, Microelectronic Engineering, 2000, 53, 425-428.

‘Multi-adduct Derivatives of C60 for Electron Beam Nano-Resists’ T. Tada, K. Uekusu, T. Kanayama, T, Nakayama, R. Chapman, W.Y. Cheung, L. Eden, I. Hussain, M. Jennings, J. Perkins, M. Philips, J.A. Preece, E.J. Shelley, Microelectronic Engineering, 2002, 61, 737-743.

Electron Beam Lithography

Inducing Chemical Transformations

Specific Chemistry

Patterning: Direct-Beam Writing

e

beam

NO2

NH2A single molecular monolayer

S

N O2

S

N O2

S

N O2

S

N O2

S

N O2

S

N O2

S

N O2

S

N H2

S

N H2

S

N O2

S

N O2

S

N O2

S

H N

S

H N

S

N O2

R1O

R1O

e-beam

A u

A u

A u

Background: Chemical Nanolithography with Electron Beams

W. Eck, V. Stadler, W. Geyer, M. Zharnikov, A. Gölzhäuser, M. Grunze,

Adv. Mater. 2000, 12, 805-808.

Excellent system as chemical reactivity between nitro and amino group is different.

AFM micrograph in frictional mode.

And furthermore…

O2N O

Si(OMe)3

Film Formation

Immerse Si/SiO2 into 5 mM/anhy. THF under Ar

(Sonication at 25°C)Reaction times: 2 hours

Sonicate twice in fresh THF for 5 minRinse intensively with CHCl3, EtOH and UHP H2ODry under Ar

Film Characterisation:

Contact Angle (surface type)

AFM (roughness)

Elipsometry (thickness)

XPS (elemental composition)

NPPTMS

Procedure from: N. Tillman, A. Ulman, J.S. Schildkraut, TL. Penner, J. Am. Chem. Soc., 1988, 110, 6136-6144.

SAM on Si/SiO2

NO2

O

SiO

OO

Si

NO2

O

SiO

O

Si

NO2

O

SiO

O

Si

NO2

O

SiO

O

Si

NO2

O

SiO

O

Si

NO2

O

SiO

O

Si

1.1 nm

(a) 3 min

(e) 447 min

(d) 273 min

(c) 163 min

(b) 97 min

NO 2 (405.6 eV)

NH 2 (399.6 eV)

Inte

nsi

ty /

arb

itra

ry u

nit

s

394399404409Binding energy / eV

XPS Chemical Modification

Secondary back scattered electrons initiate the chemistry

SAM Thickness= 1.2 0.2 nmCalculated = 1.1 nm

O

NH

Si

O

O

C

CF3

O

Si/SiO2

680685690695700Binding energy (eV)

Inte

nsi

ty (

arb

itra

ry u

nit

s)

F (1s)

• Immersion of the irradiated surface in a 10% TFAA solution in dry THF overnight

O

NH2

Si

O

O

Si/SiO2

Confirming the Chemical Transformation: NO2 to NH2

O

NO2

Si

O

O

Si/SiO2

• Immersion of the irradiated surface in a 10% TFAA solution in dry THF overnight

XPS

• E-beam

Patterning: Direct-Beam Writing

e

beam

5 m

NO2

NH2

SEM Image

primary beam energy

= 5 and 6 keV

doses between

= 25 and 300 µCcm-2

P. Mendes, S. Jacke, Y. Chen, S.D. Evans, K. Kritchley, K. Nikitin, R. E. Palmer, D. Fitzmaurice, J.A. Preece, Langmuir, 2004, 20, 3766-3768.

Shuqing Sun, Karen S. L. Chong, and Graham J. Leggett*

J. Am. Chem. Soc., 2002, 124, 2414

Scanning Near-FieldOptical Lithography

Au

SNOM

Background: Scanning Near Field Photolithography

SH SO3 SH

CO2H

Planar Surface

Nanoscale Molecular Patterns Fabricated by Using Scanning Near-Field Optical LithographyShuqing Sun, Karen S. L. Chong, and Graham J. LeggettJ. Am. Chem. Soc., 2002, 124, 2414

Generation of Nanostructures by Scanning Near-Field

Photolithography of Self-Assembled Monolayers and Wet Chemical

Etching

Shuqing Sun and Graham J. Leggett*

Nano Letters 2002, 2, 1223-1227

Conclusions

Soft Lithography

Soft Lithography: Chemically Printing on Surfaces

We are all familiar with chemistry in a round bottom flask, where reagent A and reagent B are both dissolved up in a solvent, and they then react to form product C, which still remains in solution.

But there is a fascinating area of chemistry which utilises chemistry taking place on surfaces.

This type of chemistry is a very mature area of science, because the applications of modifying surfaces are huge. For instance, surfaces can be made water repellent, corrosion resistant, non-stick and chemical resistant.

The application of surface chemistry in novel lithographic techniques is an area which is currently receiving a great deal of research, because the structures which can be created are on the nanometre scale and are literally only one molecule thick.

These novel lithographic techniques rely upon the formation of what are referred to as Self-Assembled Monolayers or SAMs.

The most popular SAMs are formed between a gold surface and alkyl thiols.

http://mrsec.wisc.edu/Edetc/SlideShow/slides/lithography/soft.html

Self-Assembled Monolayer Formation

Gold Substrate

SH

SAu

-H

The result of SAM formation is a highly ordered two dimensional solid of the organic moiety, as a result of the sulfur atoms being bonded in the three centre hollow of the gold atoms. These stable ordered structures allow SAMs to literally be written onto surfaces.

21 3Nano-Contact Printing

54

1 A monomer of PDMS is poured over a master, which has been produced by photolithography (200 nm features) or even electron beam lithography (20 nms).

3 The PDMS stamp is peeled of the master.

2 The liquid monomer is cured, to form the rubbery solid PDMS polymer.

4 The PDMS stamp is inked with a solution of the thiols, and pressed against a gold substrate.

5 The thiols form a SAM on the gold surface only where the stamp has been brought into contact with the gold.

This technique can produce structures down to 50 nm lateral dimension and only one molecule thick (about 1 nm!).

PDMS Monomer

PDMS Stamp

PDMS StampInked with Thiols

Gold SurfaceSAM of Thiol

Master

50 nm

Scanning Probe Lithography

Moving Atoms One By One to Create Nanostructures

There are a group of techniques referred to as Scanning Probe Microscopies (SPM), examples of which are the Atomic Force Microscope (AFM) and Scanning Tunnelling Microscopy (STM).

They have quite literally revolutionised the way the atomic world is viewed, and in part have been responsible for the increased research activity in nanoscale science

Indeed, the significance of these techniques was recognised with the award of the Nobel Prize in Physics to Rohrer, Binning and Gimzewski in 1986.

http://www.nobel.se/physics/laureates/1986/index.html

The SPMs allow atomic mapping of surfaces, such that individual atoms on a surface can be visuallised, or adsorbate molecules on the surface can be visuallised (see Nature 2001, 413, 619-621),

Additionally, they can induce chemical reactions on a surface.

Furthermore, molecules and atoms can be moved and positioned on a surface (see www.almaden.ibm.com/vis/stm).

We shall look in more details at SPMs in Part 4, but the following examples

illustrate the power of these techniques for creating nanostructures by,

(i) depositing molecules onto a surface (Dip Pen Lithograpghy),

(ii) Inducing chemical reactions on a surface (NanoOxidation), and

(iii) Moving individual atoms/molecules on a surface.

Additionally, the examples show how surfaces can be imaged at the nano and sub nanoscale.

Dip-Pen Nanolithography (DPN) is an new Atomic Force Microscope (AFM) based soft-lithography technique which was recently discovered in the labs of Prof Merkin.

DPN is a direct-write soft lithography technique which is used to create nanostructures on a substrate of interest by delivering collections of molecules (thiols) via capillary transport from an AFM tip to a surface (gold)

Dip-Pen Nanolithography

10 nm

http://www.chem.northwestern.edu/~mkngrp/

Scientific American

2001

Potential Applications of Dip-Pen Nanolithography

D. Piner, J. Zhu, F. Xu, and S. Hong, C. A. Mirkin, "Dip-Pen

Nanolithography", Science, 1999, 283, 661–63.

Hong, S.; Zhu, J.; Mirkin, C. A. "Multiple Ink Nanolithography: Towards a

Multiple-Pen Nanoplotter," Science, 1999, 286, 523-525.

Hong, S.; Mirkin, C. A. "A Nanoplotter for Soft Lithography with Both

Parallel and Serial Writing Capabilities" Science, 2000, 288, 1808-1811.

Further Reading on DPN

200 nm

An AFM tip in a humid

atmosphere, such that a water

condensate gathers at the tip

substrate surface, can be

utilised to create a conducting

medium when a bias is applied

between the tip and substrate.

This conduction initiates an

electrochemical oxidation of

the surface as the tip is moved

across the substrate surface,

and a line of oxide is drawn

across the surface.

Writing by Inducing NanoOxidation on a Surface

TiO2

SiO2

Carbon nanotubes have previously been used

as tips in atomic force microscopes (AFM) for

producing images. But now for the first time

nanotube tips have been used as pencils for

writing 10-nm-width structures on silicon

substrates. Ordinary graphite pencils write by

wearing themselves down, but this is not the

case with nanotube pencils developed at

Stanford. The robustness of the nanotube

tips permits a writing rate- --0.5 mm/sec---five

times faster than was possible with older AFM

tips.

NANOTUBE NANOLITHOGRAPHY.

http://www.stanford.edu/group/quate_group/index.html

The way the nanotube writes is for an electric field, issuing from the nanotube, to remove hydrogen atoms from a layer of hydrogen atop a silicon base. The exposed silicon surface oxidizes; thus the "writing" consists of narrow SiO2 tracks. The

Stanford results should help the development of nanofabrication, since tip wear problems have been an obstacle to the use of probe microscopes in lithography and data storage at the nm size scale.

Writing by Moving Individual Atoms Or Molecules!

The AFM and STM can be utilised to move atoms and molecules which have

been adsorbed to a surface, either by physically pushing the atoms/molecules

(AFM) or picking them up by electrostatic forces (STM) and positioning them at

another point on the surface.

Thus, these processes in principle allow the creation of nanostructures.

The SPM is being used as a robotic arm on the nanoscale, but is controlled from our macro world, to position individual molecules!

Is this the Top-Down or Bottom-Up?

The following pictures illustrate the power of these techniques for controlling the positioning of atoms on the nanoscale.

The Surface of Platinium (STM)

Xe on Ni (STM)

Quantum CoralFe Atom Ring on Copper

Carbon Monoxide ManCO2 on Platininum Surface

Summary: Top-Down Approach

PHOTOLITHOGRAPHY is the mainstay of the microelectronics industry for

creating patterns on a surface. However, the miniaturisation that can be achieved

will hit physical barriers in the coming years.

E-BEAM LITHOGRAPHY is one methodology that is being employed in research

labs as the possible successor to photolithography, for creating patterns on

surfaces with sub 100 nm features sizes. The main problems that need to be

confronted to make this process a viable methodology are the requirements to (i)

increase the speed of the serial process and (ii) to have materials that respond

efficiently to the e-beam. It should be pointed out that there are other techniques

that are also being investigated, such as X-ray lithography.

NANOCONTACT PRINTING is a simple methodology for creating nanostructures on

surfaces by chemically imprinting structures. This process, however, is too slow

to be used in the electronics industry as a mass production technology, but could

be used to build prototype or very specialised devices. Nanocontact printing has

other potential uses in areas such as sensing or biological evaluations.

SCANNING PROBE MICROSCOPIES are being used in several ways for creating nanostructures:

(i) A modified AFM tip with an optiocal probe is used as a nanoscale light pencil to induce chemical reactions on a surface (Scanning Near Field Optical Lithography SNP)

These approaches allow the creation of films one molecule thick, and with several nanometres lateral dimension.

(iii) An AFM tip induces a chemical oxidation at the surface, and

This approach allows nanostructures of 10 of nms to be created, but in principle, should be able to produce smaller structures. Unfortunately, the process produces oxides, which are generally not very good conductors. However, we are only limited by the imagination of the chemist to use other reactions in this process!

(iv) An STM positions individual molecules on surfaces.

This approach is the ultimate limit of fabrication. The control of matter on this length scale is already shedding new light on basic quantum physics (quantum coral). However, its use in the electronics industry for creating structures is a long way off: The process is extremely slow and generally requires extremely low temperatures in order that the adsorbates stick to the surface.

(ii) An AFM tip is used as a nanoscale pencil to either deposit a chemical that will react with the surface (Dip-Pen Lithography)