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Contact print technology hasbeen widely used since the1960s to pattern integrated cir-cuits. In addition to having veryhigh throughput, the equip-ment is relatively inexpensiveand far less complex than mod-

ern projection printers and scanners. Be-cause of its many advantages, contactprinting remained a significant lithogra-phy technology in the IC industry well in-to the 1980s.

Despite these capabilities, contactprinting has become obsolete for mostcritical applications due in part to the veryhigh level of defects that result from theprinting process.1 Modern projection

printers have equaled the throughput,and far surpassed the resolution and over-lay accuracy of the most advanced contactprint systems. However, it is the high de-fect level associated with vacuum contactprinting that has kept this technology

from being used for applications where itsresolution and overlay capabilities are ac-ceptable.

Using phase-shift masks (PSMs), it hasalready been demonstrated that 193 nmphotolithography can produce sub-100nm features.2 A combination of improvedoptics, a reduced wavelength to 157 nm,and the introduction of more complexprocessing will surely enable further re-ductions in feature size.

Along this path, such improvementscome with an ever-increasing cost forphotolithographic tools. As conventionalprojection lithography reaches its limits,next-generation lithography (NGL) toolsmay provide a means to further pattern

Release Layers for

Contact andImprint Lithography

SEMICONDUCTOR I N T E R N A T I O N A L 71JUNE 2002www.semiconductor.net

If contact and imprint lithographytechniques are going to supplantmore established optical meth-ods, defect levels must be con-trolled. Low surface energy re-

lease layers have shown effectiveresults, dramatically improving

mask life and cleaning frequency.

At a Glance

Douglas J. Resnick andDavid P. Mancini,

Motorola Labs, Tempe, Ariz.S.V. Sreenivasan and C.

Grant Willson,University of Texas,

Austin, Texas

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six0206con.qxd 05/15/2002 10:15 AM Page 71

SEMICONDUCTOR I N T E R N A T I O N A L72JUNE 2002 www.semiconductor.net

shrinks, but are expected to come at aprice tag that is prohibitive for manycompanies.

A significant part of the cost of a pro-jection printing tool is tied up in the op-tics and the light source. Looking ahead,this trend is not expected to change astool manufacturers first address the chal-lenges of building lenses from calciumfluoride (CaF2) for 157 nm tools, and lat-er deal with the issues of building multi-layer reflective optics for extreme ultravi-olet (EUV, 13 nm) systems.

As a result, several researchers haveconsidered imprint lithography toachieve high resolution while minimiz-ing tool cost. Investigations by this groupand others looking into the sub-100 nmregime indicate that final imprinted reso-lution is limited only by template resolu-tion. Routine use of these techniques forsemiconductor manufacture will only beconsidered, however, if defect levels canbe controlled.

For either contact lithography or im-print lithography, defects mainly resultfrom damage imparted to the resist layercaused by direct contact between a maskand a resist-coated substrate. Conven-tional chromium-coated quartz pho-tomasks have high surface energies andthus have a high potential to adhere tophotoresist. When intimately contacted,resist tends to be pulled from the waferand remain on the mask.

Thus, not only is the resist coating ofthe immediate wafer damaged, but sub-sequent layers are likely to be affected, es-pecially if the defect falls into a clearfieldarea of the mask. The defect will also cre-ate a gap between the wafer and mask,potentially degrading resolution in near-

by areas. As a result, defects adhering to amask have a cumulative effect, propagat-ing to subsequent layers until the mask iscleaned.

A potential solution to these problemsis to apply a low surface energy releaselayer directly onto the mask. If a robustrelease layer can be developed, the im-

pact to lithography could be significant.At the very least, a good release layer willminimize the number of mask cleansnecessary in the contact print process.Optimistically, it could help to enable animprinting process extendable to the 10nm regime.

Surface treatment historyAttempts have been made to reduce de-fect levels by employing a measured gapbetween the substrate and mask. Thisvery commonly used technique, knownas proximity printing, is effective at re-ducing defect levels. However, resolutionis rapidly lost as the gap increases, creat-ing a compromise between defect densityand maximum resolution.

Surface treatments for masks have alsobeen used to improve their scratch resist-ance.3 However, such techniques do notlower surface energy and thus do not de-crease the tendency for particulates to ad-

here. Coatings have also been applied tomasks to keep their surfaces clean.4 Suchstrippable lacquer coatings are applied inrelatively thick films, then peeled fromthe mask surface just prior to use. Thesecoatings were never intended for in situuse on masks during the contact printingprocess for several reasons: They are too

thick to permit the level of contact need-ed for high-resolution printing; they arenot anti-sticking, and may create or at-tract more defects when contacted withresist; and, depending on their chemistryand thickness, they may absorb signifi-cant amounts of the exposing light in-tended for the photoresist.

Conversion coatings using fluorinatedsilane-based monomers have also beenapplied to masks to alter their surface en-ergy.5 This process, known as SURCAS(surface conversion for anti-sticking),successfully lowers the surface free ener-gy of a coated mask, but lacks adequatedurability in consecutive runs requiredby a production contact print operation.

Attempts have also been made to applylow surface energy release coatings di-rectly to resist-coated wafers.6 A formula-tion of poly vinyl alcohol mixed with asurfactant and a lubricative monomerwas applied to a wafer and baked follow-

RELEASE LAYERS FOR CONTACT AND IMPRINT LITHOGRAPHY

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At the very least, a good release layer willminimize the number of mask cleans necessary

in the contact print process. Optimistically, itcould help to enable an imprinting process

extendable to the 10 nm regime.



ing a normal resist application process.Significant improvements in device yieldand mask durability resulted when usedin hard contact compared with trialsdone with uncoated wafers. However,this process suffers from the obviousdrawback that each wafer must be takenthrough an additional coating and bakingstep.

Teflon AFAmorphous fluoropolymers look espe-cially promising for release-layer technol-ogy.7 Teflon AF is a family of amorphousfluoropolymers made by DuPont Fluoro-products (Wilmington, Del.). These ma-terials are made by the copolymerizationof 2,2-bistrifluoromethyl-4,5-difluoro-1,3-dioxole (PDD) with other fluorine-con-taining monomers.

As with other fluoropolymer resins,Teflon AF has good thermal stability andchemical resistance along with a very lowsurface energy. However, unlike otherfluoropolymers such as polytetrafluo-roethylene (PTFE), Teflon AF is amor-phous. This characteristic offers manyadditional properties that make it particu-larly useful as a mask coating. Because ithas none of the crystallites found in semi-crystalline materials such as PTFE, coat-ings (


many consecutive exposures. Todemonstrate this capability, a resolu-tion mask was coated with 100 nm ofCytop CTL-107M and used to expose250 consecutive wafers in vacuumcontact. An identical uncoated maskwas also used for a series of 10 consec-utive vacuum exposures. Figure 3shows photographs of both masks af-ter the test, clearly demonstrating theeffectiveness of using a release coat-ing.

Imprint lithographyImprint lithography techniques areessentially micromolding processes inwhich the topography of a templatedefines the patterns created on thesubstrate. As in contact lithography, arelease layer is necessary to avoid thetransfer of resist from the substrate tothe template. Because this technolo-gy may be suitable for dimensions assmall as 10 nm, it is not possible tospin-coat materials such as Teflon AFand Cytop onto the template surface.Good conformality cannot be obtainedand the release layer must be extremelythin so that the template feature size isnot impacted.

Whitesides et al have formed a tem-plate by applying a liquid precursor topolydimethylsiloxane over a master maskproduced using either electron-beam oroptical lithography.11 The liquid iscured, and the PDMS solid is peeledaway from the original mask. The PDMStemplate can then be coated with a thiolsolution, which is subsequently trans-ferred to a substrate and coated with athin layer of gold.

The process of curing the PDMSagainst the master can result in unwant-ed adhesion of the PDMS to the exposedregions of the silicon wafer. The PDMSis sealed irreversibly to the silicon mastersurface by a cross-linking reaction. Toprevent this adhesion, the master surfaceis passivated by the gas phase depositionof a long-chain, fluorinated alkylchlorosi-lane (CF3(CF2)6(CH2)2SiCl3).

The fluorosilane reacts with the freesilanol groups on the surface of the mas-ter to form a Teflon-like surface with alow interfacial free energy. The passivat-ed surface acts as a release layer that fa-cilitates the removal of the PDMS

stamp from the master.Because the PDMS is easily de-

formable, the technology is not well suit-ed for devices requiring precise patternplacement. Nanoimprint lithography,developed by Chou et al, uses a solidmold, such as silicon or nickel.12

The imprint process is accomplishedby heating a resist above its glass transi-tion temperature and imparting a rela-tively large force to transfer the image in-to the heated resist. To minimize possibleadhesion between the resist and themold, a fluorinated material is typicallyadded to the resist. Features as small as10 nm have been imaged using this ap-proach.

Step and flash imprintlithographyDevices that require several lithogra-phy steps and precise overlay requirean imprinting process capable of ad-dressing registration issues. Step andFlash Imprint Lithography (S-FIL), atechnique developed by Willson et al,solves the problem of overlay by usinga transparent quartz template.13

The steps required for patterningare schematically depicted in Figure4. The process employs a tem-plate/substrate alignment scheme toset the template parallel to the sub-strate. A low-viscosity liquid etch bar-rier material is then injected betweenthe template and substrate. The gap isclosed and ultraviolet light is illumi-nated through the template, therebycuring the etch barrier. The templateis withdrawn, leaving a preciselyreplicated inverse of the pattern onthe template. The viscosity of theetch barrier is sufficiently low, so thatminimal pressure (~2-4 psi) and no

additional heating is necessary to movethe liquid into the stencil. Finally, be-cause the template is transparent, con-ventional overlay schemes can be used toalign patterns.

The UV-curable etch barrier formula-tion in S-FIL is a solution of organicmonomer, silylated monomer, anddimethyl siloxane oligomer (DMS).14Each component plays an important rolein the imaging process. The free radicalgenerator initiates polymerization uponexposure to actinic illumination. The or-ganic monomer ensures adequate solu-bility of the free radical generator and ad-hesion to the underlying organic transferlayer. The silylated monomers and the


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DMS provide the silicon re-quired to give a high-oxygenetch resistance. Both mono-mer types help maintain thelow viscosity required duringimprinting. The silylatedmonomer and DMS derivativealso lower the surface energy ofthe etch barrier, thereby en-hancing the separation pro-cess.

The template release layermust have a low enough sur-face energy to enable tem-plate/substrate separation, butmust also be durably bonded tothe template surface to remainfunctional after many imprints.An approach similar to that ofWhitesides was pursued.14Alkyltrichlorosilanes form strong cova-lent bonds with the surface of fused sili-ca, or SiO2. In the presence of surfacewater they react to form silanol interme-diates, which undergo a condensation re-action with surface hydroxyl groups andadjacent silanols to form a networkedsiloxane monolayer.

When this functional group is synthet-ically attached to a long fluorinatedaliphatic chain, a bifunctional moleculesuitable as a template release film is cre-ated. The silane-terminated end bonds it-self to a template’s surface, providing the

durability necessary for repeated im-prints. The fluorinated chain, with itstendency to orient itself away from thesurface, forms a tightly packed comb-likestructure and provides a low-energy re-lease surface. Annealing further en-hances the condensation, creating ahighly networked, durable, low surfaceenergy coating. The release layer forma-

tion process is depicted in Figure 5.Similar to the amorphous fluoropoly-

mers used in contact lithography, themonolayer acts effectively as a self-clean-ing agent. This attribute is depicted inFigure 6. A dirty template was used to im-print several die on a silicon wafer. Theprogression of pictures indicates that de-fects that start on the template embedthemselves in the etch barrier, and by theseventh imprint, there are no detectableparticles. It is also interesting to note thatthere does not appear to be any degrada-tion of the release layer over time. Con-

tact angle measurements show nochange after more than two months.14

The effectiveness of the monolayer isalso observed in the printing of very high-aspect-ratio sub-100 nm lines as depictedin Figure 7. A quartz template wasformed by patterning a high-resolution e-beam resist. The pattern was then trans-ferred, first into a thin chrome layer and

subsequently into the quartzplate. The resist and chromi-um were then stripped fromthe template. Etch depth intothe quartz was ~120 nm. Linesmeasuring 40 and 30 nm (withaspect ratios of 3:1 and 4:1, re-spectively) were clearly re-solved with no evidence of ad-hesion loss.15

ConclusionAs new nanolithography tech-nologies continue to be devel-oped, their potential as cost-ef-fective alternatives to optical li-thography methods continuesto grow. Surface acoustic wave(SAW) devices, photonic crys-tals, waveguides and high-den-

sity memories are just a few of the prod-ucts that can be produced provided de-fect levels can be reliably controlled.

Low surface energy release layers com-prised of amorphous fluoropolymershave already been proven to be effectivefor improving defect levels in contactprint lithography. This patented technol-ogy is used routinely within Motorola’sPhysical Sciences Research Labs, andhas been shown to provide dramatic im-provements in both mask life and maskcleaning frequency.

In addition, release layers have beensuccessfully applied to the surfaces of im-print templates with similar results. It isunclear at this early stage of developmentwhether nano imprint lithographies willsupplant more established opticalmethodologies. However, it is clear that asuccessful release layer technology is re-quired to control defects and enable thepotential of imprint lithography to beachieved. •

AcknowledgmentsWe gratefully acknowledge Bill Dauksher,Kevin Nordquist, Steve Smith, DolphRios, Eric Newlin, Steve Young, JenniferClift, David Standfast and Todd Bailey fortheir process help. We would also like tothank Lyndi Noetzel, Kathy Palmer, The-resa Hopson, Andy Hooper and Alec Tal-in for their characterization work. We alsoappreciate the information provided byGeorge Whitesides and Stephen Chou.Finally, we thank Laura Siragusa and Jim


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Prendergast for their support. This workwas partially funded by DARPA (BAA 01-08/01-8964 and MDA972-97-1-0010) andSRC (96-LC-460).

References1. J.H. Bruning, “Optical Lithography — ThirtyYears and Three Orders of Magnitude,” Proc.SPIE, 1997, p. 14.2. K. Patterson et al, “Improving Performance of193 nm Photoresists Based on Alicyclic Poly-mers,” Proc. SPIE, 1998, p. 425.3. S. Long, G.E. McGuire, “The Boriding ofChromium Photomasks for Increased AbrasionResistance,” Thin Solid Films, Vol. 64 (1979), p.433.4. G.W.W. Stevens, “Reduction of Waste Result-ing From Mask Defects,” Solid State Technology,August 1978.5. T. Matsuzawa, H. Yanazawa, N. Hashimoto,H. Mishimagi, “Surface Conversion for Anti-sticking to Reduce Patterning Defects in Pho-tolithography,” J. Electrochem. Soc.: Solid-StateScience and Technology, January 1981.6. D.L. Flowers, “Lubrication in Lithography,” J.Electrochem. Soc., Vol. 124 (1977), p. 1608.7. U.S. Patent #6,300,042.8. J.H. Lowry, J.S. Mendlowitz, N.S. Subramani-an, “Optical Characteristics of Teflon AF Fluoro-

plastic Materials,” Optical Engineering, Septem-ber 1992.9. J. Harwood, “Teflon AF — a New Polymer forElectronics,” IEEE/ISHM 1990 IEMT sympo-sium, p. 503.10. D.P. Mancini et al, “Low Surface EnergyPolymeric Release Coating for Improved Con-tact Print Lithography,” BACUS Symposium onPhotomask Technology, 2001.11. Y. Xia, G.M. Whitesides, “Soft Lithography,”Angew. Chem. Int. 1998, 37, p. 550.12. S.Y. Chou, P.R. Krauss, P.J. Renstrom,

“Nanoimprint Lithography,” J. Vac. Sci. Technol.B, Vol. 14, No. 6, p. 4129.13. M. Colburn et al, “Step and Flash ImprintLithography: A New Approach to High-Resolu-tion Printing,” Proc. SPIE, 1999, p. 379.14. T. Bailey et al, “Step and Flash Imprint Li-thography: Template Surface Treatment and De-fect Analysis,” J. Vac. Sci. Technol. B, Vol. 18, No.6, p. 3572.15. D.J. Resnick et al, “High Resolution Tem-plates for Step and Flash Imprint Lithography,”SPIE Microlithography 2002.


For more info go to www.semiconductor.net/info and enter #47.

Doug Resnick is a section manager in the Advanced Process Characterization Laboratories (APCL) atMotorola. He is responsible for providing lithographic and plasma-processing solutions for a diversegroup of programs that are developing products in Motorola’s Physical Sciences Research Labs (PSRL).He has a Ph.D. in solid-state physics from Ohio State University.

David Mancini works in Motorola’s PSRL as the group leader of the optical lithography area. He has aB.S. in chemical engineering from Arizona State University, and an M.S. in chemical engineering from theUniversity of Connecticut.

S.V. Sreenivasan has served on the College of Engineering faculty at the University of Texas at Austinsince 1994, and specializes in developing analytical and experimental tools for understanding kinematicsand dynamics of complex mechanical systems. In late 2001, he co-founded Molecular Imprints Inc. tocommercialize semiconductor equipment technology developed at the University of Texas. He has a Ph.D.in mechanical engineering from Ohio State University.

Grant Willson joined the faculties of the Departments of Chemical Engineering and Chemistry at theUniversity of Texas at Austin in 1993. He has a B.S. and Ph.D. in organic chemistry from the Universityof California at Berkeley, and an M.S. in organic chemistry from San Diego State University.


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