Nanostructures can be patterned with focused electron or ion beams in thin, stable, conformal films of water ice grown on silicon. We use these patterns to reliably fabricate sub-20 nm wide metal lines and exceptionally well-defined, sub-10 nanometer beam-induced chemical surface transformations. We argue more generally that solid-phase condensed gases of low sublimation energy are ideal materials for nanoscale patterning, and water, quite remarkably, may be among the most useful.Engineering and Applied SciencesMolecular and Cellular Biolog

Branton, Daniel

Golovchenko, Jene

King, Gavin M.

Schurmann, Gregor

Nano Letters

PubMed

Nanostructures can be patterned with focused electron or ion beams in thin, stable, conformal films of water ice grown on silicon. We use these patterns to reliably fabricate sub-20 nm wide metal lines and exceptionally well-defined, sub-10 nanometer beam-induced chemical surface transformations. We argue more generally that solid-phase condensed gases of low sublimation energy are ideal materials for nanoscale patterning, and water, quite remarkably, may be among the most useful.Engineering and Applied SciencesMolecular and Cellular BiologyVersion of Recor

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 Nanometer Patterning with Ice  (Article begins on next page)The Harvard community has made this article openly available.Please share how this access benefits you. Your story matters.Citation King, Gavin M., Gregor Schurmann, Daniel Branton, and Jene A.Golovchenko. 2005. Nanometer patterning with ice. Nano Letters5(6): 1157-1160.Published Version doi:10.1021/nl050405nAccessed February 17, 2015 8:52:47 PM ESTCitable Link http://nrs.harvard.edu/urn-3:HUL.InstRepos:3109370Terms of Use This article was downloaded from Harvard University's DASHrepository, and is made available under the terms and conditionsapplicable to Other Posted Material, as set forth athttp://nrs.harvard.edu/urn-3:HUL.InstRepos:dash.current.terms-of-use#LAANanometer Patterning with IceGavin M. King,*,† Gregor Schu 1rmann,‡ Daniel Branton,‡ andJene A. Golovchenko†,§Department of Physics, Department of Molecular and Cellular Biology,and DiVision of Engineering & Applied Sciences, HarVard UniVersity,Cambridge, Massachusetts 02138Received March 3, 2005; Revised Manuscript Received April 12, 2005ABSTRACTNanostructures can be patterned with focused electron or ion beams in thin, stable, conformal films of water ice grown on silicon. We usethese patterns to reliably fabricate sub-20 nm wide metal lines and exceptionally well-defined, sub-10 nanometer beam-induced chemicalsurface transformations. We argue more generally that solid-phase condensed gases of low sublimation energy are ideal materials for nanoscalepatterning, and water, quite remarkably, may be among the most useful.Electron beam (e-beam) lithography is a powerful establishedmethod used to pattern nanometer scale structures such assingle electron transistors, single molecule detectors, andnanoelectromechanical devices.1-4 E-beam lithography typi-cally involves applying, chemically transforming, and chemi-cally dissolving a polymer resist.5 The desire to decreasedevice size, to enhance the role of quantum mechanicaldevice characteristics, and to pattern increasingly complexsubstrates requires new lithographic approaches to definenanometer scale structures. Here we demonstrate a newapproach to nanoscale e-beam patterning based on thecondensation and beam stimulated sublimation of water ice.Electron and ion beam stimulated sublimation of solidcondensed gases has been studied in the context of cryo-electron microscopy,6 plasma confinement,7 and astrophys-ics,8 but not, to our knowledge, for high-resolution patterning.We have discovered that frozen water, a common substance,can be used to generate nanoscale patterns of metals andpotentially very useful chemical transformations on a sub-strate surface.We chose this approach because it is known that electronsand very high energy ions create electronic excitations,molecular dissociations, and ionization in ice, but theconsequences of these effects on nanoscale patterning havenot been explored. In insulating surface layers with lowsublimation energies, “Coulomb explosions” can lead to largelocalized agitation and ejection of surface atoms.8 In addition,dissociation products of condensed molecular gases can beejected after diffusion to the surface from deeper regionswithin the beam exposure volume.6,9 These processes, as wellas others, have been discussed,7,10,11 but we feel it fair toconclude that the fundamental physics responsible for theseobservations is still obscure.In our experiments, we deposit very thin, conformal layersof ice from water vapor onto cryogenically cooled siliconsubstrates in the chamber of a combined scanning electronmicroscope (SEM) and focused ion beam (FIB) apparatus(FEI Co., Hillsboro, Oregon). Subsequent exposure of theice surface to focused energetic electron or gallium ion beamsstimulates local removal of ice and ultimately exposes theunderlying silicon substrate in whatever patterns the beamsare programmed to produce. Afterward, using lower beamdoses, the exposed regions can be inspected nondestructivelywith the SEM or FIB operating in its standard scanningimaging mode. With the sample still cold, the exposedpatterned regions of the substrate can then be metallized ormodified by other techniques (ion implantation, reactive ionetching, sputter deposition, etc.) in the same vacuum chamberor an adjoining one. Finally, the ice can be removed eitherby in situ sublimation (eliminating liquid surface tensioneffects) or by rinsing.Typically, we deposit ice at a rate of 1 nm/s using aleak valve controlled water vapor flow that is directed ontothe cooled sample (Figure 1). Uniform ice coverage over1m m 2 is achieved with a single needle (inner diameter100 ím) water vapor source held 5 mm above the samplesurface. At operating sample temperatures of 128 K thedeposited ice is amorphous and sublimes at a rate of only0.3 monolayers/hour with a sublimation energy of 0.45eV.12,13 In practice, no appreciable ice sublimation is observedover several hours when working at 128 K, even with icefilms <10 nm thick. The ice is promptly removed bysublimation when the sample temperature is raised to180 K. We have successfully patterned ice with a focused* Corresponding author. Telephone: 303-492-7818; Fax: 303-492-5235;E-mail: gking@jilau1.colorado.edu.† Department of Physics.‡ Department of Molecular and Cellular Biology.§ Division of Engineering & Applied Sciences.NANOLETTERS2005Vol. 5, No. 61157-116010.1021/nl050405n CCC: $30.25 © 2005 American Chemical SocietyPublished on Web 04/29/2005ion beam (30 keV Ga+, 10 pA, diameter 10 nm) or afocused electron beam (1-30 keV, 30-150 pA, diameter5 nm). All data presented here were patterned with anelectron beam. After patterning, samples were transferredwhile cold to a connected chamber with a chromium sputterdeposition source.An in situ SEM image of a 75 nm thick layer of ice on asilicon substrate at 128 K immediately after 5 keV e-beampatterning is shown (Figure 2a). The e-beam dose used tocreate each of the 500 nm square patterns was increased fromleft to right. The sharp rise in contrast between the fourthand fifth squares indicates that a dose above 8.8  105 íC/cm2 is required to remove the entire ice layer and exposethe underlying silicon surface. An atomic force microscope(AFM) line scan and an SEM image of the same sample areshown (Figure 2b) after sputter deposition of 40 nm of Cr,removal from the vacuum chamber, and a rinse in isopropylalcohol14 to remove the remaining ice and its overlayer. Onlythe Cr that comes in contact with the patterned ice-freeregions of the substrate remains after complete ice removal.No aggressive methods such as ultrasonication or mechanicalscrubbing were needed to assist the liftoff. Figure 2b confirmsthat 5 keV e-beam doses greater than 8.8  105 íC/cm2 arerequired to remove the 75 nm ice layer thickness and ensuresubsequent pattern transfer to the deposited Cr. This criticaldose for water ice resist is roughly 3 orders of magnitudelarger than that required for a typical exposure of the polymerresist polymethyl methacrylate (PMMA). Assuming theamorphous ice layer has a density13 0.91 gm/cm3,w ecalculate the sputter yield (i.e., H2O molecules ejected perincident electron) S ) 0.03 for 5 keV electrons. S decreasesby over an order of magnitude as the beam energy increasesfrom 1 to 30 keV, but it does not vary significantly withtemperature between 128 and 158 K. In contrast to therelatively low electron sputter yield, we found S  12 for30 keV gallium ions in an FIB!15Figures 2c and 2d present results of varying the line doseunder the same conditions as previously described for thearea dose study, except with a thinner (20 nm) ice layer.Figure 2c is an in situ cryogenic SEM image of micron-long, single pixel lines of varying doses. Here the line doseis increased from 1.1 íC/cm (left) to 5.6 íC/cm (right). After6 nm Cr deposition, an SEM image and an AFM line scan(Figure 2d) yield the critical line dose under these conditionsof 2.2 íC/cm. We note the patterned metal dot at thebottom end of each line allows for easy identification ofunderexposed regions.The AFM scans in Figures 2b and 2d also reveal thegrowth of a layer of material  1 nm thick in the regionwhere the beam hit the ice even with doses too low to ejectall of the ice down to the underlying substrate. We attributethis thin layer to a beam-induced surface chemical transfor-mation that involves both the ice layer and the underlyingsilicon substrate because (a) the effect is observed with nometallization step; (b) the effect is specific to silicon, i.e.,we see no similar growth of material on SiO2 or Si3N4Figure 1. Schematic of the apparatus. Within the main vacuumchamber at <1  10-6 Torr, water from the vapor phase is directlycondensed onto cryogenically cooled substrates, typically at 128K. The water vapor pressure is controlled by leak valves (not shown)connected to a MgSO4â7H2O water vapor source. The coldfinger(located 5 mm above the sample surface) is held at 80 K, ensuringa sufficient thermal gradient to keep unwanted species fromcondensing on the substrate. Ice deposition rates of 1 nm/s areeasily achieved. Substrate temperatures can be controlled to (1Kand raised to sublime off unwanted ice. An Ar sputter gun allowsfor metallization in the loadlock chamber.Figure 2. Patterning a silicon surface. (a) In situ cryogenic (sampleT ) 128 K) SEM image of 500 nm  500 nm squares formed ina 75 nm thick ice layer by a 5 keV, 112 pA electron beam. Theelectron beam patterning dose was increased from 2.2 (left side,square barely visible) to 35.2  105 íC/cm2 (right). (b) Room-temperature SEM image of the same sample after metallization (40nm Cr). A topographic AFM line scan thru a region of the surface(dashed line) is shown above the SEM image. (c) In situ cryogenicSEM image of single pixel lines defined in a 20 nm thick ice layeron silicon (e-beam doses as stated). (d) Post metallization SEMand AFM line scan of the same sample.1158 Nano Lett., Vol. 5, No. 6, 2005substrates; (c) the material grows only to a self-terminatingthickness above the silicon (111) surface; and (d) the final(self-terminated) thickness of the material scales with theamount of ice initially deposited on the silicon surface. When5 nm of ice was deposited, subsequent e-beam exposureproduced a 0.5 nm high chemical transformation; a 75 nmice thickness produced a 3.0 nm high structure. Furthere-beam exposure did not increase the material’s height. Incontrast to contamination lithography,16 observations (b) and(c) imply that we can rule out deposition of unspecified (e.g.,hydrocarbon) vacuum contaminants in the chamber. Sincethe electron range in our experiments is larger than the icelayer thickness, observation (d) suggests that the stimulatedsurface chemistry required for the material growth persistsonly as long as the ice layer that is being removed still exists.Considering the possible species present (H2O, its atomicor molecular fragments, silicon and hot electrons) on or nearthe silicon-ice interface when the beam impacts, we postulatethat the thin layer of material is likely silicon oxide. This isfurther supported by the observation that the thin layers ofmaterial can be removed with hydrofluoric acid, leavingbehind a depression in the silicon surface (Figure 4f). Theelectronic properties of the thin layer of material are currentlyunder investigation.The spatial resolution that we have achieved for icepatterned Cr lines on silicon using 20 nm thick ice layersis shown in Figure 3 with beam exposures (a) at 30 keVand (b) at 20 keV. We note that this apparent difference inline width is potentially related to aperture alignment,astigmatism adjustment, and focus optimization steps neces-sary after changing the beam acceleration voltage. Thenarrowest dimensions of the material tentatively identifiedas silicon oxide (Figure 4) are, remarkably, much smallerthan those obtained for Cr (Figure 3). A study of the lateraldimensions of these putative oxide structures as a functionof beam energy illustrates the evident trend toward higherresolution with increasing electron beam energy. At 10 keV(Figure 4a), the structure’s full width at half-maximum(fwhm) measures 18 nm. The fwhm is reduced to 14 nm at25 keV and to 10 nm at 30 keV (Figures 4b and 4c). Themeasurements listed in Figure 4 are values obtained withoutsubtracting the effective diameter of the AFM probe tip.When we account for the AFM tip effective diameter of 5nm (Figure 4d), the true width of these lines are of order13, 9, and 5 nm for the 10, 25, and 30 keV exposures,respectively. Figure 4e shows a 2 ím  1.5 ím AFM imageof two lines defined by a 30 keV electron beam. Here wesee that the 3.0 íC/cm line measures 1.2 nm in heightabove the Si, while the 1.5 íC/cm line on the left is only0.8 nm high.Although the “record” line widths of e-beam exposedPMMA are in the single digit nanometer range, such resultswere achieved with specialized higher energy beams (100keV) and attentive ultrasonication during resist develop-ment.17-19 A more typical minimum line width achieved withe-beam exposed PMMA on bulk silicon substrates withcommercial e-beam lithography tools is of order 30 nm.20To reliably achieve thinner line widths, angled depositionsof metals or other materials on exposed resists are common.Our demonstrated sub-20 nm metal lines are achievable withan ice resist on bulk silicon with e-beam energies <30 keVand without directional deposition of metal. We speculatethat proximity effects21,22 are minimized with ice resistbecause the excitation of ice by backscattered electronexposure away from the point of beam incidence is not anadditive process. Instead, excited ice that is not ejected fromthe surface can relax to its initial unexposed state.Patterning with ices of any condensed gas is a straight-forward and practical process. Ice resist does not requireFigure 3. SEM analysis of sub-30 nm wide Cr lines on a bulksilicon substrate. Both structures were defined on 20 nm ice resistlayers and subsequently metallized with 8 nm Cr. (a) 30 keV line,4.5 íC/cm dose. (b) 20 keV line, 4.4 íC/cm dose.Figure 4. AFM analysis of the local electron beam inducedchemical transformation of a silicon substrate. All data except panel(d) are tapping mode AFM images with associated AFM linescans.All results in (a-c) and (e) were produced with beam doses toolow to eject all of the ice down to the silicon substrate. (a-c) Linewidth as a function of beam energy (energies as stated). (d) TEMimage of the AFM tip taken after acquiring the data in (a-c). (e)Line height as a function of dose (30 keV beam). Left line, 1.5íC/cm; right line 3.0 íC/cm. (f) Trenches are formed when raisedlines similar to that shown in (e) (right line) are treated for 15 swith 48% w/w HF.Nano Lett., Vol. 5, No. 6, 2005 1159spinning or baking. All processing and patterning steps canoccur in a single evacuated chamber and be monitored athigh resolution. The final removal of unexposed resist leavesminimal residues. Environmentally harmful solvents are notrequired and complete dry removal of the ice layer can beperformed by in situ sublimation. Although in our currentapparatus dry resist removal by sublimation after Cr metal-lization causes large loose Cr flakes to settle on the substrate,an inverted configuration may avoid this problem. Com-pletely dry resist processing will be particularly useful forpreparing delicate micro- and nanoelectromechanical deviceson a variety of substrates, especially those exhibiting complexthree-dimensional geometries.It will be important to discover the resolution limits andto minimize the critical dose requirements by testing a widerange of beam energies, beam diameters, and other condensedgases. To avoid induced interface chemistry, rare gases areclearly called for. Alternatively, to yield desired chemicaltransformations, other gases can be selected to react withthe substrate and produce precisely defined thicknesstunnel junctions and gate insulators needed for nanoscaleelectronic devices. The gate insulator needed for nanoscalefield effect transistors is an important example for whichthe material grown with water may well be suited. Weanticipate that the rich and varied chemical, electronic, andmass transport properties of energetic beam stimulated solidcondensed gases will provide many opportunities for dis-covery and innovation in connection with nanoscale pat-terning.Acknowledgment. This work was supported by awardsfrom the NSF (#DMR-0073590), DOE (#DE-FG02-01ER45922), NIH (#RO1HG02338), and Agilent Technolo-gies. We thank the Harvard Center for Imaging andMesoscale Structures for facilities support as well as DamonFarmer and Trygve Ristroph for assistance and discussions.References(1) Pashkin, Y.; Nakamura, Y.; Tsai, J. Appl. Phys. Lett. 2000, 76, 2256.(2) Liu, K.; Avouris, Ph.; Bucchignano, J.; Martel, R.; Sun, S.; Michl,J. Appl. Phys. Lett. 2002, 80, 865.(3) Craighead, H. Science 2000, 290, 1532.(4) Ito, T.; Okazaki, S. Nature 2000, 406, 1027.(5) Rai-Choudhury, P., Handbook of Microlithography, Mircomachining,and Microfabrication; SPIE Optical Engineering Press: London, UK,1997.(6) Talmon, Y.; Davis, H. T.; Scriven, L. E.; Thomas, L. E. J. Mircoscopy1979, 117, 321.(7) Erents, S. K.; McCraken, G. M. J. Appl. Phys. 1973, 44, 3139.(8) Brown, W. L.; Lanzerotti, L. J.; Poate, J. M.; Augustyniak, W. M.Phys. ReV. Lett. 1978, 40, 1027.(9) Brown, W. L.; Augustyniak, W. M.; Lanzerotti, L. J.; Johnson, R.E.; Evatt, R. Phys. ReV. Lett. 1980, 45, 1632.(10) Ellegaard, O.; Schou, J.; Sorensen, H.; Borgesen, P. Surf. Sci. 1986,167, 474.(11) Watson, C. C.; Tombrello, T. A. Radiat. Eff. 1985, 89, 263.(12) Sack, N.; Baragiola, R. Phys. ReV.B1993, 48, 9973.(13) Echlin, P. Low-Temperature Microscopy and Analysis; PlenumPress: New York, 1992.(14) Liquid water also works for this purpose. Techniques to remove theCr overlayer in situ have not been explored.(15) Conventional organic polymer resists are 10-100 times moresensitive to ion irradiation than to electron exposure (Wolf, S. andTauber, R., Silicon Processing for the VLSI Era; Lattice Press: SunsetBeach, California, 1986). Based on this measured ion sputter yieldwe estimate the effectiveness of ice resist as an ion implantationmask: 4 nm of ice is required per monolayer of implanted ions(30 keV Ga).(16) Broers, A.; Hoole, A.; Ryan, J. Microelectron. Eng. 1996, 32, 131.(17) Vieu, C.; Carcenac, F.; Pepin, A.; Chen, Y.; Mejias, M.; Lebib, A.;Manin-Ferlazzo, L.; Couraud, L.; Launois, H. Appl. Surf. Sci. 2000,164, 111.(18) Chen, W.; Ahmed, H. Appl. Phys. Lett. 1993, 62, 1499.(19) Craighead, H. G.; Howard, R. E.; Jackel, L. D.; Mankiewich, P. M.Appl. Phys. Lett. 1983, 42, 38.(20) Raith 150 Ultrahigh-resolution E-beam lithography and metrologytool, Product data sheet: minimum feature size ) 30 nm. RaithGmbH, Dortmund (Germany).(21) Chang, T. H. P. J. Vac. Sci. Technol. 1975, 12, 1271.(22) Chen, W.; Ahmed, H. AdV. Imag. Elect. Phys. 1998, 102, 87.NL050405N1160 Nano Lett., Vol. 5, No. 6, 2005

Nanometer Patterning with Ice

Gavin M. King

Gregor Schürmann

Daniel Branton

Jene A. Golovchenko

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Nanometer Patterning with Ice

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