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Fabrication With Molds and Stamps In Integrated Photonics
Citation preview
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
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