Fabrication of optical components with

molds and stamps

INTRODUCTION

Advantages over conventional photolithography:

• tolerates a wide range of materials needed for photonics,chemistry, and biology (polymers, sol-gels, coloids,suspensions),

• is compatible with a wide range of substrates including

Replica molding Micro-transfer molding

Replication using flexible or hard molds is a simple lithography technique that can

be used to generate micro and nanostructures in polymers with a resolution less

than 100 nm.

• is compatible with a wide range of substrates includingglass, plastics, ceramics, or carbon,

• optically functional surfaces can be achieved,

• is suitable for mass production of cheap devices.

Soft Lithographic Techniques

Soft lithography is based on pattern transfer using a mold that is deforming or

patterning the substrate material. Most of the soft lithographic techniques

use mold formed of a master template.

• The mold replicates the inverse of master features, and it can be used in

turn to deform or pattern the next material.

Soft mold- fabrication steps

Soft Lithographic Techniques

transfer features from master to replica by curing a liquid

transfer of features

from master toreplica by pressing

transfer of material

on master toreplica by stamping

Replica Molding

Molding is the transfer of a topographic pattern from one material to another by curing or solidifying a liquid precursoragainst the original patterned substrate.

The replication steps include:

• fabricating the desired topographic patterned master;

• transferring this pattern to PDMS by curing a PDMS prepolymer in contact with the master and releasing the PDMS from the master;

• solidifying a liquid precursor against the PDMS mold and releasing the solidified structure to isolate a replica of the master.

Byron D. Gates. Materials Today 8, 44-49, 2005

Replica Molding

Scanning electron micrograph of polyurethane with patterned microstructures on its surface that was formed by replica molding against a stretched PDMS mold

Y. Xia and G. M. Whitesides, “Soft Y. Xia and G. M. Whitesides, “Soft lithography”, Angew. Chem. Int. Ed. 37 (1998) pp.

550-575

Replica molding with PDMS can transfer a pattern of periodic features with vertical deflections <2 nm into a UV cured Polyurethane

The ultimate resolution of replication with a PDMS transfer element is currentlyunknown and may be limited by capillary and van der Waals interactions.

Microtransfer MoldingReplication of the recesses of a topographically

patterned mold and transfer of these features to

another substrate:

• A liquid prepolymer, such as polyurethane (PU) or

thermally curable epoxy fills the recessed regions of

the PDMS master.

• The excess prepolymer is removed;

• The prepolymer is placed in contact with a rigid

substrate and cured.

• After curing, the PDMS master is removed.

•The film of polymer that remains between the features

on top of the substrate is called scum and can be

removed using reactive ion etching (RIE), if the

features need to be separated.

Microtransfer Molding

Two-layered structure of

isolated microstructures of

epoxy on lines of epoxy epoxy on lines of epoxy supported on a glass slide

G. M. Whitesides

Micromolding in Capillaries

• The PDMS master is placed onto a substrate, having

the channels faced down.

Micromolding in capillaries (MIMIC) resembles microtransfer molding, but in

this technique the soft PDMS master has a microchannel structure.

•The liquid prepolymer is placed next to the open ends

of the channels, and the capillary forces pull the liquid

to fill the channels.

•The liquid is crosslinked, and the PDMS master

removed.

In this technique no scum is created, the structures are formed in a single step.

The structures fabricated with this technique are obviously limited to channels. Since the rate of

filling of the capillary is proportional to the cross section of the channel, the slow filling of small

capillaries might limit the feature size.

Micromolding in Capillaries

Polymeric microstructures

of polyurethane fabricated

by MIMIC

• It is, however, still a challenge to fill <100 nm wide channels with the desired solutions. The resistance to liquid flow through the channels increases with decreasing size of the channel. It may be slow to fill the channel with the liquid, but applying a vacuum to the channel or heating the liquid can assist the filling process.

by MIMIC

UV-Molding

• Ultraviolet-molding (UV-molding) uses a rigid master,

• Curing- using UV radiation: a UV-curable prepolymer that is crosslinked by exposing it to UV radiation.

• The prepolymer is molded onto a master and a substrate is placed on top of it.

Step and Flash Imprint Lithography™

Step and flash imprint lithography™ (SFIL™) is a special version of UV-molding. It is a registered trademark of the Willson group.

•SFIL™ replicates topography of a rigid

transparent master using a photocurable

prepolymer solution, such as photocurable

polyurethane (PU) or organosilicon as the

material to be molded.

•A low-viscosit y liquid fills the spaces of

the transparent master, usually made of

quartz.

Douglas J. Resnick, S.V. Sreenivasan, C. Grant Willson

Materials Today 8 (2005) p.34-42

quartz.

•Exposing the prepolymer to UV light

causes the crosslinking and the master is

removed.

•Post-processing: etch of the residual layer

of the monomer, followed by a selective

etch into transfer layer.

S-FIL tools

• Imprint lithography relies on the parallel orientation of template and substrate.

Inaccurate orientation may yield a layer of cured etch barrier that is nonuniform

across the imprint field. Thus, it is necessary to develop a mechanical system where

template and substrate are brought into coparallelism during etch barrier exposure.

• This was originally achieved in S-FIL by way of a two-step orientation scheme. In step

one, the template stage and wafer chuck are brought into coarse parallelism via

micrometer actuation. The second step uses a passive flexure-based mechanism that

takes over during actual imprin The first step-and-repeat system was built at The

University of Texas at Austin by modifying a 248 nm Ultratech stepper that was

donated by IBM.

Step and Flash Imprint Lithography

Imprinted images obtained with the S-FIL process: 50 nm dense lines,

20 nm semidense lines, 60 nm posts, and a three-tiered structure.

Step and Flash Imprint Lithography

• SFIL does not suffer the thermal mismatch induced by thermal expansion.

• The pressure is also very low, so pressure induced deformations are also minimal, allowing the use of brittle substrates

• High aspect ratios are also possible.

• It is a very fast technique compared with the other soft lithographic techniques, only 5 minutes per cycle 1. techniques, only 5 minutes per cycle 1.

• The transparent master allows the features to be optically aligned to the underlying layer.

• The fabrication on non-planar substrates is however difficult with SFIL.

• Air bubbles can be trapped between the master and the polymer.

Embossing (Imprinting)

• Embossing is the process of imprinting a pattern into an initially flat solid surface by pressing a mold and softening the surface. The difference between replica molding and embossing techniques is the material to be molded: in embossing it is a solid film, in replicamolding a liquid precursor.

• The polymer is embossed thermally, with pressure, or with solvent assistance. Embossing can use a rigid or a soft mold.

• Embossing techniques:

– nanoimprint lithography, which uses a rigid master and solvent assisted

– micromolding, which uses a soft mold as a master.

Nanoimprint Lithography

Nanoimprint lithography (NIL), developed by Chou et

al. also called hot embossing, uses usually a hard

rigid master to physically deform a solid polymer

film that is on a rigid substrate surface.

The polymer on a substrate surface is heated above

its glass transition temperature (Tg), and the

master is pressed against it.

The polymer is deformed filling the voids in the

master, then it is cooled below Tg, master, then it is cooled below Tg,

The master is removed revealing a pattern that is the

inverse of the master.

Solvent Assisted Micromolding• solvent-assisted micromolding (SAMIM) uses a solvent to restructure a polymer film

by swelling or dissolving the polymer

• room-temperature processing

• avoids thermal cycling of the substrate which can be time intensive and lead to oxidation of the substrates. Instead of heat, an organic solvent softens the polymer film

After coating the surface of a PDMS mold or polymer film with the solvent, the mold is brought into contact with the polymer film. The softened polymer film adapts to the topography of the PDMSmold mold The PDMS is gas permeable, which preventstrapping of gas pockets at the mold-polymer interface. The solvent also passes through the PDMS mold and evaporates.During solvent evaporation, the restructured polymer film hardens with a topographic pattern complementary to that of the PDMS mold. The mild processing conditions of SAMIM areconducive to patterning fragile optical components, such as organic light-emitting diodes (OLEDs) and organic-based distributed feedback lasers.SAMIM can pattern nanoscale structures : SAMIM can pattern linewidths of at least 60 nm, such as the lines in the Novolac-based photoresist

Printing- Microcontact printing (µCP

• Printing using a topographically patterned stamp is a straightforward method of projecting a pattern onto a surface.

• Microcontact printing (µCP) transfers an ‘ink’ onto a surface in a pattern defined by the raised regions of a stamp

• An elastomeric stamp coated with the ink is brought into conformal contact with the surface of

• interest. The ink transfers from the stamp to this surface by a process of chemisorption or physisorption. Releasing the PDMS stamp from the surface reveals the printed pattern of ink.

• The first application of this technique30 was to pattern alkanethiols on Au.

Application of monolayers on

gold by : microcontact printing:

Schmidt-Wenkel, P.; Stutz, R.; Wolf, H. IBM J. Res. Dev. 2001, 45, 697

Nanotransfer Printing• Nanotransfer printing (nTP) avoids the need to expose a substrate to harsh

solvents (e.g. acids and bases).

• The thin film transfers to the printed surface by covalent binding to reactive SAMs (e.g. alkanedithiols), cold welding between two metal layers, or condensation between silanol or titanol groups.

• nTP simplifies nanofabrication by eliminating the need for developers and etchants during pattern transfer, and can transfer complex patterns in a single step

steps: steps: •coating the stamp with a thin film (e.g. 20

nm thick

•Au); •bringing the stamp into contact with a

surface to be

•patterned;

•releasing the thin film as a continuousstructure or as isolated features by removal of the stamp.

Materials for Soft Lithography

Properties of PDMS- after curing

• Stability temperatures: –50°C to +200°C

• Tensile strength : 7.1 MPa

• Elongation at break: 1.4

• Young’s modulus (tensile modulus) in the range of 1 – 5 MPa

• Surface energy: low, approximately 21.6 mJ/ m2

Poly(dimethylsiloxane) PDMS

• Surface energy: low, approximately 21.6 mJ/ m2

• Linear coefficient of thermal expansion: 310 ppm/°C

• Hydrophobicity: hydrophobic, water contact angle 90 – 120

• Permeability to gases: passes gases easily

• Optical properties: optically transparent down to ~300 nm

• Viscosity (prior to curing): medium viscosity, at 23ºC immediately after mixing: 4000 mPa×s

• Elastic modulus: 1.8 MPa

• Compressibility high, 2 N/mm2

Experiments in IMT

Steps

• Master fabrication

• Mold fabrication

• Replication

� Lithographic mask

� Resit or Silicon

� Polymer (PDMS, PMMA, epoxy resin)

In case the master can be easily fabricated (or only a small quantity of

structures have to be replicated) and the replica must be the negative of this

master, the original master can be used as a mold to directly fabricate the

structures.

MASTER FABRICATION

Resist master

Photoresist (AZ, SU8, etc.) Electronoresist (PMMA, SU8)

Photo-EBL – feature size < 1µµµµm

silicon substrate

SiO2 (optional)

resist

Photo-lithography

EBL – feature size < 1µµµµm

MASTER FABRICATION

Resist masterPhotoresist (AZ, SU8, etc.)

Period 3 µm

16 µm

MOLD FABRICATION

Resist masterElectronoresist (PMMA, SU8)

Dots - φ ~150 nm Lines – 100nm

MOLD FABRICATION

Metallic masterMetal (PMMA- double layer + metal deposition + lift-off)

Metallic master for replication of high aspect ratio gratings

Metalic nanostructures for plasmonics and for nanoelectrodes

Metallic master for photonic crystals (φ << 100 nm)

MASTER FABRICATION

Silicon master

resist

SiO2Si Lithography

SiO etching

isotropic etching

SiO2 etching

Oxide removal

Si etching (isotropic)HNO3: HF – 50:1

HNO3 : HF: CH3COOH – 25:1:10

MASTER FABRICATION

Silicon master (2)

Lithography

SiO2 etching

anisotropic etching

SiO2 etching

Si <100> oriented

etching

(anisotropic- KOH)

Oxide

removal

REPLICATION

Casting with a prepolymer + removing excess

•• Process flow Process flow --MTMMTM

TransferTransfer

Curing + Mold Removal

Micro-lenses Antireflection coating Gratings, waveguides,

REPLICATION

Polymers

PDMS• mixing of the base and the curing agent Sylgard 184 (Dow-Corning) in weight ratio 10:1• degassing by placing the polymer mixture in a dessicator connected to a vacuum pump for 30 min• curing:60 oC, 3h;

Epoxy resin:• mixing Dinox 010S resin with triethylenetetraamine in ratio 10:1, • degasing to eliminate the gaseous inclusions

Materials

• degasing to eliminate the gaseous inclusions• curing at 80 oC, 30 min ,

PMMA 7 % PMMA solution in monomer mixed with an initiator (e.g. methyl-ethyl ketone peroxide) • degasing to eliminate the gaseous inclusions• curing 100oC, 1h

Release agents

• 230 Fluid Dow-Corning• chlor-alkylsilane and alkylsiloxane derivatives• polyvinyl alcohol solution

REPLICATION

The refractive index can be increase by the addition of titanium, zirconium, and other

metal oxide nanoparticles with high refractive index

Doped polymers

Materials

Polymers doped metal oxide nanoparticles have been prepared by� sol-gel process,

� polymerization of monomer containing nanoparticles and � dispersing nanoparticles into a polymeric matrix

MATERIALS

MaterialsExample: doped PMMA

Refractive index increased by the addition of or ZrO2 or TiO2

� 2.5% PMMA (Aldrich, 120000 MW dissolved in methyl-methacrylate -MMA) doped (100:1) with

zirconium (IV) propoxide mixed with isopropanol and chelated with acetylacetone in mole ratio 1: 10: 2 in nitrogen environment

� 2.5% PMMA ( in MMA ) doped (20:1) with Titanium isopropoxide (Aldrich) chelated with acetyl acetone in mole ratio 1: 2 in nitrogen environment

Refractive index decreased by the addition of SiO2

PMMA transmittance

0

20

40

60

80

100

300 400 500 600 700 800

Wavelength (nm)

Tra

nsm

itta

nce

(%)

7% PMMA

PMMA + ZrO2

PMMA + SiO2

Refractive index decreased by the addition of SiO2

� PMMA + chelated Si-trimethoxypropyl (10:1)

PMMA, PMMA + SiO2, PMMA+ZrO2

1,46

1,48

1,5

1,52

1,54

1,56

1,58

1,6

1,62

1,64

400 500 600 700 800

Wavelength (nm)

Refr

acti

ve

ind

ex

PMMA PMMA+SiO2 PMMA+ZrO2

RESULTS

Diffraction Gratings

Master (photoresist) Replica in PDMS

RESULTS

Diffraction GratingsProfilometry -WYKO DEKTAK 8 instrument- The tactile probe geometry has some influence on the measurement: 0.71 um corner radius probe and 45º- In the X direction the probing has been done at a constant rate and a constant speed

RESULTS

MasterPyramids

Replica in PDMS

RESULTS

PyramidsSide 32 µm

Profilometry

Mold

Replica

RESULTS

Master

Pyramids

plateau

30 µm

Replica in PDMS

Profilometry

Replica

RESULTS

Pyramids

Master

RESULTS

Pyramids in PMMA

RESULTS

Microlenses

Si master

Replica

RESULTS

PMMA master

DOEs with feature size < 1 µµµµm

Mold in PDMS (negative copy)

Replica in Epoxy Resin (positive )

RESULTSStructures with feature size < 1 µµµµm

Polimeric waveguides

width < 200nm

Master for microchannels-width < 250 nm)

PDMS channels

RESULTSDOEs with feature size < 1 µµµµm

PMMA Master

a) b)Fig. 1. Diffractive structures in epoxy resin obtained by replica molding with a master obtained by

EBL in a) a thin layer of PMMA -950K layer; b) double PMMA layer (φ~150 nm, h ~200-300 nm).

RESULTS

2D and 3D crossed grating structure (for biosensors) in PDMS obtained using a PMMA master (a) monolayer –PMMA 950K and (b) bi-layer: PMMA 450k/PMMA 950k) and PDMS mold.

Line width: ~ 150 nm, pitch: 400 nm

Master

DOEs with feature size < 1 µµµµm

Replica

Applications

DOEs

Applications

Antireflection coatings for solar cells and photodetectors

60

70

80

Re

fle

cta

nc

e [

%]

0

10

20

30

40

50

200 400 600 800 1000

Wavelength [nm]

Re

fle

cta

nc

e [

%]

structured

not-structured

Processe/results from literature

Mold/master fabrication- Si etching

Soft lithography replication of polymeric microringoptical resonators

The master

devices are precisely fabricated using

directelectron beam

The substrates are silicon wafers

covered with a 5 µm thermally grown amorphous silica film (refractive index 1.445). A thin film (2µm) of negative, epoxy-type electron beam resist SU-8 (refractive index

1.565) is spin coatedon the substrates and exposed by electron beam lithography to form the master device. Afterdeveloping in SU-8 developer, the structure is covered by PDMS and

baked at 80◦C for 1 h.

Yanyi Huang, George T. Paloczi, Jacob Scheuer, and

Amnon Yariv6 October 2003 / Vol. 11, No. 20 / OPTICS

EXPRESS 2452

electron beam lithography.

The replicas

are produced by the molding

technique and

subsequent

ultraviolet curing.

baked at 80◦C for 1 h.Once cured, the PDMS mold is peeled off. To mold the replicas, a drop of SU-8 is placed ontoa new silica substrate and stamped with PDMS mold. The replicated device is cured under UVlight until solidified. Both the master and replicated devices are cleaved to expose the waveguideend-facets for optical measurement.

Microring resonators

fabrication of the

soft PDMS mold

fabrication of the PS microring

resonatorsfabrication of the SU-8 microring resonators

PDMS waveguides

David A. Chang-Yen, Richard K. Eich, Bruce K. Gale, JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 23, NO. 6, JUNE 2005, p.2088

PDMS curing at 150 C for 60 min ���� refractive index 1.47PDMS curing at room temp. ���� refractive index 1.45

Lenses

• Replication process: UV curable epoxy is dispensed into the lenscavities of the stamp ~a!.

• A lens backing plate is placed on the stamp to cover the epoxy ~b!

• and irradiated in UV light ~c!.

• After curing the stamp is peeled off giving the replicated lens array ~d!.array ~d!.

Kunnavakkam et al., Appl. Phys. Lett., Vol. 82, 2003, 1152

Lenses

a)Replicated lens array on the

quartz backingplate;

b) enlarged view of the

lenses. The white regions in the lenses The white regions in the lenses are reflections from the

llumination source. A 12

312 lens master with a pitch of

1.25 mm was used to prepare the stamp.

The central 10310 array in the

replica is usable. The lens is ;1 mm in

diameter ~lateral dimension!.

Lenses

Comparison of surface profiles of the master and replicated

lens.The profile was obtained using a Wyko scanning optical

profilometer. The

central regions of the master central regions of the master and replica are similar in

shape. The left and

right portions of the lens are not leveled leading to an

apparently large

shrinkage. The height

measurement error is about 0.5% according to instrumentspecifications.

Lens arrays- REM

Teng-Kai Shin, Jeng-Rong Ho, and J.-W. John Cheng, IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 16, NO. 9, SEPTEMBER

2004, p.2078

a)The footprint with designed shape and size for the microlens is defined on a metallic mask through which the excimer laser writes the footprint directly onto a polycarbonate (PC) plate. By monitoring the power intensity and number of laser shots, depth of the holes drilled by the excimer laser can be well controlled. b) Athin liquid polymethylmethacrylate(PMMA) film is then coated on the PC-based pedestal through spin coating. As the liquidPMMAis rapidly spreading out, due to its own weight and viscosity, a film, suspended on the pedestals and with special curvature, is formed on the pedestal. The effect of the liquid surface tension results in good surface uniformity, providing that the liquid PMMA is at reasonable thickness. After baking at 60 C for 5 min, the liquid film is solidified and stuck fixedly on the pedestal.c) A soft PDMS mold with microlens patterns is formed by the replica molding method. Specifically, a liquid PDMS mixture (Sylgard 184, Dow Corning, silicone elastomer: curing agent ) was cast onto the PMMA film obtained in Step 2. After baking at 60 C for 20 min, the solidified PDMS mold can be easily stripped from the PMMA film. d) A concave soft mold can be fabricated by a second replica molding using the convex PDMS mold obtained in Step 3. To make sure both the concave and convex PDMS molds can be separated after baking, the weight ratio of the silicone elastomer and the

curing agent for the concave mold should be different from that for the convex mold.

e) both concave and convex polymer microlens arrays can be fabricated by another replica molding by casting the polymeric

liquid, PMMA in this study, in the corresponding convex and concave molds, respectively.

Lens - REM

PMMA lens