Polymer Microfabrication (Part I)

Prof. Tianhong Cui, Mechanical Engineering

ME 8254

Polymers for Microfabrication

• Examples diverse– PDMS– PMMA– Polyurethane– Polyimide– Polystyrene

• Disadvantages– Low thermal stability– Low thermal and electrical conductivity– Techniques for fabrication on

microscale not as well developed

• Advantages over silicon– Inexpensive– Flexible– Transparent to visible/ UV– Easily molded– Surface properties easily

modified– Improved biocompatibility or

bioactivity

PDMS

• Polydimethylsiloxane• Advantages

– Deforms reversibly– Can be molded with high fidelity– Optically transparent down to ~300 nm– Durable and chemically inert– Non-toxic– Inexpensive

Soft Lithography

• Developed by Whitesides, et. al. at Harvard• Microcontact printing

– Elastomeric stamp– Patterns of self-assembled monolayers (SAMs) and proteins– SAMs allow a variety of surface modifications

• Thickness variation by changing tail length• Modification of tail group changes surface properties• Variety available for different substrate materials

– Other SAM advantages• Self healing and defect rejecting• Ultrathin resists and seed layers• Do not require clean room facilities• Low cost

– Fabricated using a PDMS mold of “photoresist” structure

About soft lithography

Conception of soft lithography:

A collection of techniques based on printing, molding and embossingwith an elastomeric stamp.

Character of soft lithography :

Low costExperimentally convenientThree-dimensional and curved structures Tolerates a wide variety of materialsGenerates well-defined and controllable surface chemistriesGenerally compatible with biological applications

Soft lithography VS. photolithography

Limitations of photolithography application related to biological systems:

Intrinsically expensive process (electronics-qualified clean-room facility )

Photolithography is carried out by projecting a pattern on a photomask ontoa photoresist film ( barrier in rapid, inexpensive prototyping of test patterns anddevices

Provides little or no control over surface chemistry, and it is not applicable to curvedor non-planar substrates

Photolithography will continue as the dominant technology in fabrication ofsemiconductor devices and systemsAt the current stage of development, soft lithography still relies on the use ofphotolithography to generate the master

Some patterning techniques

Microcontact printing (μCP)

Replica molding (REM)

Solvent-assisted micromolding (SAMIM)

Microtransfer molding

Micromolding in capillary

Phase-shifting edge lithography

Nanotransfer printing

Decal transfer lithography

Nanoskiving

Based on printing, molding andEmbossing with an elastomericstamp

Schematic illustration of the four major stepsinvolved in soft lithography

Outlines the four major steps:

(i) Pattern design

(ii) Fabrication of the mask and then the master,

(iii) Fabrication of the PDMS stamp

(iv) Fabrication of micro- and nanostructureswith the stamp by printing, molding and embossing

The term soft lithography ‘refers to the fabrication ofpatterned copies using the PDMS stamp

Microcontact printing (μCP)

a) PDMS was used to fabricate a stampfrom a master template

b) The stamp was removed from the masterby peeling away the cured polymer

c) The stamp was exposed to the alkanethiolink

d) After inking, the stamp was brought intocontact with the Au substrate

e) The patterned substrate was then etchedin an aqueous, basic solution of cyanideion and dissolved oxygen to produce thedesired features

SEM images of test patterns that werefabricated using μCP

SEM images of test patterns of silver (a–c, 50 nmthick; d, 200 nm thick), gold (e, 20 nm thick), andcopper ( f, 50 nm thick) that were fabricatedusing μCP with HDT, followed by wet chemicaletching.

(a) and (b): Patterns were printed withrolling stamps

(c–f ): Patterns were printed with planar stampsThe bright regions are metals; the dark regionsare Si/SiO2 exposed where the etchant hasremoved the unprotected metals.

(g-h): SEM images of silicon structures fabricatedby anisotropic etching of Si(100), with patternedstructures of silver or gold as resists

Replica molding (REM)

(i) Creating a topographically patterned master

(ii) Transferring the pattern on the master into PDMSby REM

(iii) Transferring the pattern on the PDMS back intoa replica of the original master by solidifying a liquidprepolymer (UV-curable polyurethane (PU) or epoxy)

Advantages of REM:(i)Can produce many copies ( > 50) of molds

(ii) Can work with a wide range of polymers otherthan e-beam-sensitive materials

(iii) Allows patterning over large areas rapidly, not limitedto a serial process defined by EBL

(iv) Can be applied to transfer patterns to non-planar surfaces

SEM images of test patterns that werefabricated using REM

(a-b): AFM images of Cr structureson a master, and a PU replica preparedfrom a PDMS mold cast from this master

(c-d ): AFM images of Au structures onanother master, and a PU replica producedfrom a PDMS mold cast from this master.

Solvent-assisted micromolding (SAMIM)

(a) An elastomeric mold is placed on a polymerFilm (containing the solvent) that is spin coated

onto a substrate

(b) After releasing the pressure, the wholestructure is left undisturbed for a period of timefor solidification

(c) The mold is then removed.

Substrate was dipped intothe polymer solution

SEM images of test patterns that werefabricated using SAMIM

a–c) :SEM images of quasi-three-dimensional structures in photoresist (Microposit1805, Shipley; ~1.6 um thick) spin-coated on Si/SiO2, polystyrene (PS, 2.0 um thick),and ABS (0.85 um thick), respectively

d) : An AFM image of nanostructures in a thin (~0.4 mm thick) film of Microposit 1805spin-coated on Si/SiO2.

The solvent we used was ethanol for the photoresist and acetone for PS and ABS

Microtransfer molding (μTM)

a) A thin layer of liquid prepolymer is applied to thepatterned surface of a PDMS mold

b) the excess liquid is removed by scraping with aflat PDMS block or by blowing off with a streamof nitrogen

c) This mold, filled with the prepolymer, is thenplaced in contact with the surface of a substrate,and the prepolymer is cured to a solid byilluminating the mold with UV light or by heatingit.

d) When the mold is peeled away carefully, apatterned microstructure is left on the surface ofthe substrate.

Polymeric microstructures fabricated using μTM

a) A photograph of arrays of 3-cm longwaveguides of PU fabricated on Si/SiO2.

b) An SEM image of the ends of thewaveguides.

c) An SEM image of an array of isolatedmicrocylinders of epoxy on 5-um linesof epoxy, supported on a glass slide

d ) An SEM image of a three-layerstructure on a glass slide made from a

thermally curable epoxy.

e-f ) SEM images of microstructures ofglasses fabricated by molding with sol-gel materials, followed by thermalconsolidation.

Micromolding in capillary (MIMIC)

a) Fabricate elastomeric master by casting poly

b) Cut the ends of master to open and allow the liquidprepolymer to enter

c) Place a drop of liquid prepolymer at one open end ofthis network

d) Liquid spontaneously fills the channel by capillary action

e) Remove the PDMS stamp and get patterned polymericlayer

SEM images of microstructures of variousmaterials fabricated using MIMIC

a) An SEM image of quasi-three-dimensional structures of PUformed on Si/SiO2.

b–d ) SEM images of patternedmicrostructures of polyanilineemeraldine HCl salt, zirconia , andpolystyrene beads, respectively,that were fabricated from theirsolutions or suspensions usingMIMIC.

e- f ) SEM images of free-standingmicrostructured membranes ofpolyurethane.

Nanotransfer printing

A schematic illustration of procedures for using nanotransferPrinting to fabricate 3D nanostructure:

i)Coating a high-resolution stamp with a thin layer of AuPrepare the stamp for printing

ii) Contact with a substrate that presents a surface chemistrythat bond to the Au

iii) Transfer the Au from the stamp to the sunstrate

Au-Au cold welding bonds these layers

SEM of 3D nanostructures formed bynanotransfer printing

a) Arrays of nanochannels and integrated micro/Nanochannel systems

b) A ten layer stack of crossed nanochannels

c-d) Low and high-resolution images of a squarearray of free stand capsules

e-f) Low and high-resolution images of an array offree standing “L”-shaped structures formed by printingWith a stamp that is coated with Au at a steep angle

Nanoskiving

Fabrication of metal nanowires by nanoskiving

a) the replication of a flat or topographicallypatterned PDMS surface in epoxy,

b) the deposition of a thin film of metal on theepoxy by e-beam evaporation or sputtering.

c) After embedding the thin metallic filmin more epoxy and curing, sectioning with anultramicrotome produces sections with a thickness(z) as small as 50 nm using astandard 45° diamond knife.

d) After sectioning, transfer of the epoxysection to a silicon substrate and removal of theepoxy with an oxygen plasma leaves free-standingmetal nanostructures.

SEM of nanostructures formed by Nanoskiving

(A) SEM image of 100-nm wide, 1-μm high step-shaped nanostructures fabricated bynanoskiving a metal-coated epoxy substrateprepatterned with 1-μm wide lines with 1-μmspacing.

(B) SEM image of parallel gold nanowires with20-nm spacing, by nanoskiving a flat epoxysubstrate coated with a multilayer, compositeAu/SiO2 film, followed by complete etching ofthe SiO2 spacing layers using reactive ionetching.

(C) Dark-field optical microscopy image of “L-shaped” nanostructures patterned over a ∼3-mm2 area by nanoskiving in a directionparallel to the patterned substrate.

(D) SEM image of double loop-shaped goldnanostructures.

SEM of nanostructures formed by Nanoskiving

E) SEM image of loop-shaped SiO2nanostructures on a SiO2/Si(100)substrate by using gold loop-shapednanostructures as a physical mask duringreactive ion etching with CF4.

(F) SEM image of an array of “U-shaped”gold nanostructures positioned on thecurved surface of a glass rod.

(G) SEM image of parallel gold nanowiresdraped over 20-μm wide “truncated-V”-shaped trenches, etched in a Si(100)surface.

(H) Cross-bar nanostructures fabricated by the orthogonal stacking of two epoxyslabs containing arrays of gold nanowires on top of each other.