hydrogen silsesquioxane for direct electron-beam...

Hydrogen silsesquioxane for direct electron-beam patterning of stepand flash imprint lithography templates

D. P. Mancini,a) K. A. Gehoski, E. Ainley, K. J. Nordquist, and D. J. ResnickMotorola Labs, Physical Sciences Research Laboratories, Tempe, Arizona 85284

T. C. Bailey, S. V. Sreenivasan, J. G. Ekerdt, and C. G. WillsonTexas Materials Institute, University of Texas at Austin, Austin, Texas 78712

~Received 28 May 2002; accepted 29 July 2002!

The feasibility of using hydrogen silsesquioxane~HSQ! to directly pattern the relief layer of stepand flash imprint lithography~SFIL! templates has been successfully demonstrated. HSQ is aspin-coatable oxide, which is capable of high resolution electron-beam lithography. Negative actingand nonchemically amplified, HSQ has moderate electron-beam sensitivity and excellent processinglatitude. In this novel approach, 636 3 0.25 in.3 quartz photomask substrates are coated with a 60nm indium tin oxide~ITO! charge dissipation layer and directly electron-beam written using a 100nm film of HSQ. Direct patterning of an oxide relief layer eliminates the problems of criticaldimension control associated with both chromium and oxide etches, both required processes ofprevious template fabrication schemes. Resolution of isolated and semidense lines of 30 nm hasbeen demonstrated on imprinted wafers using this type of template. During this evaluation, a failureof the release layer to provide a durable nonstick surface on ITO was discovered and investigated.This problem was successfully remedied by depositing a 5 nmoxide layer over the patternedITO/HSQ template. ©2002 American Vacuum Society.@DOI: 10.1116/1.1515311#

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I. INTRODUCTION

Step and flash imprint lithography~SFIL! is being devel-oped as a possible low cost alternative next generationthography ~NGL!.1,2 Unlike conventional optical methodthat use a mask to project an aerial image onto a waferstrate, SFIL replicates patterns by using a template havinrelief image etched into its surface. To form a pattern,organosilicon, photocurable liquid is placed onto a waand the template relief surface is pressed into contact wthe substrate using minimal pressure. A broadband illumtion source is used to flood expose the imprint area throthe back of the transparent template. This cross-linksphotocurable material and forms a polymer replica oftemplate relief. It is largely the absence of any image projtion optics and the simplicity of the irradiating source thmake SFIL a low-cost process. In spite of its relative siplicity and low cost, SFIL has demonstrated resolution bel30 nm.3

Templates have typically been fabricated from chromiucoated photomask blanks using standard phase shift etchcessing techniques,1 and more recently with very thin chromium to decrease etch bias and improve resolution.3,4 Bothprocesses result in a transparent, all-quartz template witelectron-beam~e-beam! patterned relief image etched into isurface.

In still another approach, a 6025 quartz mask blankcoated with a thin transparent conducting oxide~TCO! filmsuch as indium tin oxide~ITO!, followed by coating with athin layer of SiO2 deposited using chemical vapor depositi~CVD!.5 The substrate is then coated with resist and e-be

a!Electronic mail: [email protected]

2896 J. Vac. Sci. Technol. B 20 „6…, Nov ÕDec 2002 1071-1023Õ200

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patterned. Finally, using the resist as a mask, the oxidetched to provide the template’s relief surface. This methology offers several advantages over the chromium/qufabrication scheme. The significant critical dimension~CD!bias associated with etching chromium is eliminated. Incporation of a transparent ITO layer as a permanent eleminto the template provides charge dissipation for both e-bewriting and subsequent scanning electron microsco~SEM!-based inspections.6 The ITO layer also adds contrasto an all-quartz template necessary for optical inspectioFinally, the ITO layer acts as an etch stop during the oxetch, mitigating loading effects which would otherwise cauan etch depth bias between small and large features. Uthis methodology, Dauksheret al.6 have demonstrated imprinted features as small as 20 nm.

The current work involves using hydrogen silsesquioxa~HSQ; FoX-12, Dow Corning! as a spin-coatable organisilicon oxide which is directly e-beam written. While thprimary use of HSQ is as a low-k dielectric, several investi-gators have demonstrated its usefulness as a high-resole-beam resist. Resolution of sub-20 nm features has bshown.7–9 In its cured state, HSQ becomes a durable oxmaking it a very convenient material for direct patterningSFIL template relief structures. Processing of HSQ ase-beam resist is less complicated since it is not chemicamplified and can be developed in the standard tetrameammoniumhydroxide~TMAH ! based developers used commonly for conventional resists. In addition, HSQ is espcially well-suited for patterning on thick and insulating sustrates such as 6025 plates. A wide process latituderelative invariance to changes in softbake conditions mfor robust processing and consistent results. Direct patternof SFIL relief structures would also provide improved C

28962Õ20„6…Õ2896Õ6Õ$19.00 ©2002 American Vacuum Society

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2897 Mancini et al. : Hydrogen silsesquioxane 2897

control through elimination of the oxide and/or chromiuetches associated with previous fabrication methods, wsaving several processing steps.

In this study, characterization and analysis of the quaand feasibility of SFIL template relief structures made wHSQ is presented. This analysis includes atomic forcecroscopy~AFM! of template surfaces for both ITO field anexposed HSQ regions. The linearity, exposure latitude,contrast of HSQ used as a direct-write material on ITOalso examined. Finally, SEM images of features from btemplates and imprinted wafers are shown. Additional desconsiderations such as the compatibility of the fluoroalkylane release layer with ITO and HSQ are also discussed

II. EXPERIMENT

Templates were fabricated on 63 6 3 0.25 in.3 ~6025!quartz mask blanks supplied by Ulcoat. ITO films were dposited in a customized rf sputter system operating apower of 100 W and an Ar/O2 pressure of 6 mTorr. The filmswere then annealed at 350 °C for 10 min on a hotplatefurther increase conductivity and optical transmission at 3nm. Plasma enhanced chemical vapor deposition~PECVD!oxide was deposited in a Plasma Therm 790 systemtemperature of 250 °C.

A Leica VB6 electron beam exposure system operatin100 keV with a thermal field emitter source was used forexposures. Resolution patterns were exposed using asize of 5 nm, with a beam current of 1 nA and a spot size7 nm. HSQ was supplied by Dow Corning as FoX-12FoX-15, and was diluted using methyl ethyl ketone~MEK!as needed to achieve the desired film thickness of 80–nm. Coating of 6025 plates was done on a Laurell spinsystem with softbaking performed on a Cee® model 1hotplate. Development of exposed substrates was done uShipley LDD26W developer.

AFM images were collected in ambient using a DigiInstruments Dimension 3000-1 system in tapping fomode. Film thickness measurements were performed oNanometrics model 210XP and a Tencor Instruments AlStep 300 profilometer. SEM and critical dimension resuwere obtained using Hitachi 7800 and Hitachi S4500 fiemission CD SEMs.

The x-ray photoelectron spectroscopy~XPS! data wereobtained using a Physical Electronics PHI5700 ESCA sysequipped with an Al monochromatic source~Al Ka radiationat 1486.6 eV!. Wide range~survey! scans were obtained wita step size of 1 eV and pass energy of 93.9 eV; high restion scans were taken with a step size of 0.1 eV and penergy of 11.75 eV. The Ag 3d5/2 XPS peak at 368.3 eV froma sputter-cleaned Ag foil was used to calibrate the syste

Following pattern fabrication, a 13 1 in.2 square imprint-ing template containing the relief image was cut from tcenter region of the 6025 plate using a precision diamwheel saw. The template was cleaned using a JelUVO-42 UV-ozone cleaner, and then vapor primed withlow surface energy release agent monomer~Gelesttridecafluoro-1,1,2,2-tetrahydrooctyl trichlorosilane! and an-

JVST B - Microelectronics and Nanometer Structures

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nealed. The template was mounted in the SFIL imprint stper located at the Texas Materials Institute, UniversityTexas at Austin, where it was used to print 200 mm wafePrior to printing, wafers were coated with a 60 nm translayer consisting of Shipley AR2 600 DUV antireflectivcoating. The photocurable material used to replicate thelief pattern on the wafer consisted of 4 wt. % Darocur 11~Ciba! photoinitiator in 1,3-bis~methacryloxypropyl!tetra-methyldisiloxane.

III. RESULTS AND DISCUSSION

AFM analysis of an exposed region of HSQ after devoping is shown in Fig. 1~a!. The peak to valley range over15 mm per side square area was 16.2 nm; the corresponrms roughness is approximately 0.26 nm. These valuesgest that while the surface is generally very smooth, sospikes or undulations are present which affect surface ctour. For comparison, AFM analysis@Fig. 1~b!# of an ITO

FIG. 1. AFM of an e-beam exposed region of a HSQ surface~a!, and AFMof surface of a CVD SiO2 on ITO ~b!.

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FIG. 2. Exposure latitude of HSQ for semidense lines expressed as line~nm!/space size~nm! ~a!, and exposure latitude of HSQ for isolated line~b!.

J. Vac. Sci. Technol. B, Vol. 20, No. 6, Nov ÕDec 2002

surface covered with 15 nm of PECVD oxide taken over amm square area shows a peak to valley range of 17.6 nma rms roughness value of 0.51 nm.

Critical dimension measurements of resolution pattfeatures were recorded for a dose array ranging from 1002200mC/cm2. The results of these measurements are shoin Fig. 2~a! for semidense lines and in Fig. 2~b! for isolatedlines. HSQ shows good CD linearity of lines from 20 to 10nm in size as well as excellent exposure latitude overdose range. The operating dose that could be expecteproduce semidense lines to size was approximately 1mC/cm2, while that for isolated lines was approximate1900mC/cm2.

Analysis of HSQ contrast on an ITO-coated quartz wawas done by measuring retained film thickness of expo100 mm square regions over a dose range of 100–1mC/cm2. From this data the contrast,g, was calculated to be1.63, a value that is in close agreement with previouslyported results.8 A material with this moderate level of contrast would be expected to produce scumming and bridgof unexposed regions between dense lines. This artifactgenerally observed to a varying degree for dense linesposed at higher doses and/or where spacing was equalless than the HSQ thickness of 90 nm. While scummingconventional resists is objectionable since it often caupoor etch pattern transfer, SFIL processing may be moregiving as long as the thickness of scumming is not signcant.

Figure 3 depicts top down SEM images of the finishtemplate for a variety of line sizes and pitches at e-bedoses of 1150 and 1900mC/cm2. Line edge roughness i

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FIG. 3. Top down SEM micrographs oa HSQ/ITO template expressed as dsigned line CD/designed space CDe-beam dose; actual measurement:~a!80/125, 1150mC/cm2; 78 nm,~b! 60/125, 1150mC/cm2; 52.8 nm, ~c! 40nm iso, 1150mC/cm2; 25.2 nm, ~d!60/125, 1900mC/cm2; 61.2 nm, ~e!30/100, 1900mC/cm2; 38.1 nm, and~f! 20/100, 1900mC/cm2; 25.5 nm.

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minimal and resolution is excellent with no scummingbridging evident. Figure 3~e! depicts 30 nm/100 nm semdense lines with a trace level of scumming evident.

The first imprinting tests done using an ITO/HSQ teplate resulted in complete delamination of the cured ebarrier from the wafer, with the etch barrier remaining aherent to the template surface. Since the printing surfacthe template is comprised of over 95% ITO, the failure of trelease layer was especially catastrophic. Use of the flualkylsilane release agent, while successful for quatemplates,10 had never been studied for use with ITO sufaces. Therefore the failure of this test, though disappointwas not entirely unexpected. Dauksheret al.6 used tempera-ture programmed desorption of methanol to compare thefaces of SiO2 and ITO. Their results indicate the surfachydroxyl groups on ITO bond methanol less strongly ththose on SiO2 , which may indicate a difference in hydroxygroup reactivity. In order to further investigate this phenoenon, samples with SiO2 and ITO surfaces were treated withe release agent, annealed, and subjected to various

FIG. 4. F 1s XPS spectra of a quartz/ITO/oxide sample with SiO2 surface~a!and quartz/ITO sample with ITO surface~b!, illustrating the lack of dura-bility of the fluorocarbon release layer on the ITO surface.


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ments. The samples were rinsed in acetone and isopralcohol ~IPA!, and then buffed with a cleanroom wipe satrated with IPA to simulate the mechanical effects possiseen during imprinting. XPS analysis of the F 1s signal fromthe surfaces indicates that the fluorocarbon layer bondethe SiO2 surface remains intact after ultrasonic bath trements in the solvents, and also remains after the light bing. The film on the ITO, however, is present after the svent bath, but is almost completely removed during tbuffing step. These data are shown in Fig. 4.

In an attempt to remedy this, the next template wcapped with a 5 nmlayer of CVD SiO2, coating both theITO and patterned HSQ surfaces. Since the silane baselease layer has been shown to adhere strongly to quthrough reactions with silanol functional groups,9 surfacemodification of ITO by addition of an oxide layer would bexpected to provide a more compatible surface chemistr

Figure 5 shows top down SEM micrographs of an imprinted wafer taken from three different die~imprint Nos. 2,6, and 15! corresponding to the same template site, featur60 nm/125 nm and 30 nm/100 nm semidense lines. Restion of these images is again excellent, in replication oftemplate. Repeating defects are evident, however, in b60/125 and 30/100 line sets, with individual defects froboth sets observed to be progressively worsening. Prevwork has shown that SFIL templates pretreated with a flroalkylsilane release layer exhibit a self-cleaning mechan

FIG. 5. SEM micrographs of top down die-to-die comparison of 60 nm/1nm and 30 nm/100 nm semidense lines for three different die on the swafer.


FIG. 6. Cross-sectional SEMs of imprinted wafer~a! 60 nm 1:1 pitch,~b! 60 nm 1:5 pitch,~c! 30 nm 1:2 pitch, and~d! 30 nm 1:3 pitch.

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whereby defects deposited on the template after pretreatmare removed during the printing process via contact withetch barrier.10 As a result of this tendency, a template canitself of defects by printing more die. The nature and caof the template defects that led to the printed defects shin the progression of Fig. 5 is not known. However, it a

J. Vac. Sci. Technol. B, Vol. 20, No. 6, Nov ÕDec 2002

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pears from this series of micrographs that once a tempdefect is established, a nucleus for continued growth mialso be created.

Angled and cross sectional SEM photos depicting 30 a60 nm dense and semidense lines of an imprinted wafershown in Fig. 6. While these images show a high level

FIG. 7. SEM cross-section micrographs of an imprinted wafer illustrating 50 nm lines and the effect of HSQ scumming of dense features~a! 1:1 pitch and~b!1:3 pitch.

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resolution, excessive roughness and rounding is noted atops of features. This condition is worse for lines havingnarrower pitch. This result is not surprising given the hidegree of scumming between lines noted on the tempFigure 7~a! further illustrates this phenomena showinrounded and shallow imprinting of 50 nm lines having a 1pitch; the result of a highly scummed dense line region.contrast, Fig. 7~b! shows imprinted 50 nm lines having a 1pitch with well-defined sidewalls and a full aspect ratio.

IV. CONCLUSIONS

Direct e-beam patterning of HSQ has been used succfully on 6025 quartz/ITO substrates to fabricate the relayer of SFIL templates. Resolution of 20 nm on templaand 30 nm on imprinted wafers was achieved. AFM analyshowed that surface roughness of ITO and HSQ is veryand well within limits useful for SFIL. e-beam patterningHSQ on 6025 plates demonstrated that HSQ offers very glatitude and linearity, but possesses only moderate conthat results in scumming between dense features especiahigh doses. The fluoroalkylsilane release layer, used prously for quartz SFIL templates, failed on bare ITO surfacallowing the photocurable etch layer to stick to the templaThis problem was overcome by depositing a 5 nm oxidelayer over the patterned ITO/HSQ template.

ACKNOWLEDGMENTS

The authors gratefully acknowledge Anne DinsmoLester Casoose, and Kathy Palmer for providing SEM anasis. The authors also thank Jeff Baker, Eric Newlin, and Alpho Rios for providing processing assistance. The work


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Diana Convey in providing AFM characterization is also aknowledged. The authors express gratitude to Laura Siragfor her support in this work. The authors thank Byung JChoi, Matthew Colburn, and the rest of the UT SFIL teafor their efforts in SFIL process and tool development, aJose Lozano and Allison Carroll for their help in XPS anasis. Finally, the authors wish to acknowledge the financsupport of DARPA~BAA 01-08/01-8964 and MDA972-97-1-0010! and SRC~96-LC-460!.

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