Langmuir 1994,10, 619-622
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Spatially Resolved Mineral Deposition on Patterned Self-Assembled Monolayers Peter C. Rieke,' Barbara J. Tarasevich,t Laurie L. Wood, Mark H. Engelhard, Donald R. Baer, and Glen E. Fryxell Pacific Northwest Laboratory, P.O. Box 999, Richland, Washington 99352
Connie M . John$ Charles Evans & Associates, 301 Chesapeake Drive, Redwood City, California 94063
David A. Laken and Milt C. Jaehnig FEI Corp., 19500 N W Gibbs Drive, Suite 100, Beaverton, Oregon 97006-6907 Received September 20, 1993. In Final Form: December 14, 1993" Electron and ion beam lithographic techniques were used to pattern self-assembled monolayers with organic functional groups. Nucleation and growth of minerals from aqueous solution were confined to the patterned regions. A vinyl-terminatedself-assembled monolayer (SAM) was selectively deposited in ion and electron beam etched regions of a methyl-terminated SAM. Sulfonation of the vinyl groups produced a surface patterned in either hydrophobic methyl groups or hydrophilic sulfonate groups. Subsequent growth of FeOOH films was confined to the sulfonated regions. Condensation images were used to image each step in the lithographic scheme. Resolution of the SAM patterning step was 1-3 pm, while resolution of the mineral deposition step was 10-15 wm. Introduction , Functionalized organic surfaces can be used to control the deposition of inorganic materials from aqueous solutions.lI2 The surface plays an active role in promoting heterogeneous nucleation of the mineral phase. This approach to film growth has many advantages for the formation of thin filmswith advantageous microstructures, deposition of films on polymer, metal, and ceramic substrates, and uniform coating of complex shapes. In addition it should be possible to pattern the mineral deposition by patterning the surface functionalization. In this work, we demonstrate that mineral thin films can be spatially confined to the micrometer scale on selfassembled monolayers patterned with mineral nucleation sites. Many minerals have useful optical, piezoelectric,
* To whom correspondence should be addressed.
+ Current address:
Department of Chemistry, Penn State University, University Park,PA 16802. t Current address: Shaman Pharmaceuticals,213 E. Grand Ave., South San Francisco, CA 94080-4812. 0 Abstract published in Advance ACS Abstracts, February 15, 1994. (1)Zhao, X.K.;Fendler,J. H. J.Phys. Chem. 1991,95,3716.Landau, E. M.; Levanon, M.; Leiserowitz, L.; Lahav, M.; Sagiv, J. Nature 1986, 318,353. Rajam, S.;et al. J. Chem. SOC.,Faraday Tram. 1991,87,727. Heywood, B. R.; Rajam, S.; Mann, S. J. Chem. SOC.,Faraday Trans. 1991,87,735.Addadi,L.;Moradian,J.;Shay,E.;Maroudas,N. G.; Weiner, S. Proc. Natl. Acad. Sci. U.S.A. 1987,84,2732.Ulman, A. Adv. Mater. 1993,5,55. (2)Rieke, P. C.; Bentjen, S.B. Chem. Mater. 1993,5,43.Campbell, A. A.; Fryxell, G. E.; Graff, G. L.; Rieke, P. C.; Tarasevich,B. J. Scanning Microsc. 1993,7,423. Bentjen, S. B.; Nelson, D. A.; Tarasevich, B. J.; Rieke, P. C. J. Appl. Polym. Sei. 1992,44,965.Rieke, P. C.; Tarasevich, B. J.; Fryxell, G. E.; Bentjen, S. B.; Campbell, A. A. In Supramolecular Architecture: Synthetic Control in Thin Films and Solids. A.C.S. Symposium Series 499;American Chemical Society: Washington, DC, 1992. Heuer, A,; et al. Science 1992,255,1098.Tarasevich, B. J.;Rieke, P. C.; McVay, G. L.; Fryxell,G. E.;Campbell, A. A. In Chemical Processing of Advanced Materials; J. Wiley & Sons: New York, 1992. Tarasevich, B. J.; Rieke, P. C. In Materials Synthesis Utilizing Biological Processes, Vol. 174;Rieke, P. C., Calvert, P. D., Alper, M., Eds.; Materials Research Society: Pittsburgh, PA, 1988. Rieke, P. C.; Bentjen, S. B.; Tarasevich, B. J.; Autrey, T. S.; Nelson, D. A. In Materials Synthesis Utilizing Biological Processes, Vol. 174; Rieke, P. C., Calvert, P. D., Alper, M., Eds.; Materials Research Society: Pittsburgh, PA, 1990.
0743-7463/94/2410-0619$04.50/0
magnetic and other sensorltransducer properties. These materials, difficult to etch by conventional chemical and ion beam techniques, are easily and rapidly patterned by this process. This technique should find application in miniaturization of sensor devices and in their integration into electronic circuits. Our process is similar in concept to the patterned metalization work described by Calvert and c o - ~ o r k e r s .The ~ important difference is that we deposit an oxide or oxyhydroxide mineral from supersaturated solution rather than a metal by electroless reduction. Electron and ion beam lithographic techniques were used to pattern self-assembled monolayers (SAMs) with functional organic groups that either inhibit or promote mineral deposition. These techniques allow rapid and precise writing of complex patterns into SAMs. Patterning of SAMs is possible by other means. For example, Lopez et al. have used a ball point pen filled with ~ i l a n e .Abbot ~ et al. have used fibers to micromachine pattern boundaries on SAMs.5 Deep ultraviolet irradiation can be used to pattern SAMs by photocleavage of terminal groups.31~ These schemes might also be adapted for patterned mineralization. Experimental Procedure Figure 1 is a schematic of the procedure for lithography. First a hydrophobic SAM is deposited over the entire substrate. The primary requirement of this layer is the inability to promote heterogeneousnucleation of the inorganicmineralto be deposited. The SAM is etched using an energetic beam to reexpose the (3)Dulcey, C. S.;et al. Science 1991,252,551. Calvert, J. M.;et al. Thin SolidFilms 1992,210,359.Calvert, J. M.;etal. J.Vac.Sci. Technol., B 1991,9,3447. Dressick, W. J.; Dulcey, C. S.; Georger, J. H.; Calvert, J. M. Chem. Mater. 1993,5,148.Calvert, J. M.; Chen, M. S.; Dulcey, C. S.; Georger, J. H.; Peckerar, M. C.; Schnur, J. M.; Schoen, P. E. J. Electrochem. SOC.1992. 139. 1677. (4)Lopez, G. P.; Biebuyck, H. A.; Frisbie, C. D.; Whitesides, G. M. Science 1993,260, 647. (5)Abbott, N. L.; Folkers, J. P.; Whitesides, G. M. Science 1992,57, 1380. (6)Roznyai, L. F.;Benson, D. R.; Fodor, S. P. A.; Schultz, P. G. Angew. Chem., Int. Ed. Engl. 1992,31,759.
0 1994 American Chemical Society
620 Langmuir, Vol. 10,No. 3,1994
Letters
Deposit an inert SAM on substrate
Etch selected regions @beam with ion or electron
Deposit a second SAM terminated with functional end group in etched regions
Mineral growth from aqueous solution occurs only on patterned region Y
Figure 1. Schematic of the lithography process. underlying silicon wafer. We have used both ion and electron beams but show in this report only the results for ion beam etching. In the etched area, a functionalizedSAM is deposited. A terminal functional group is chosen that promotes the heterogeneous nucleation of the inorganicmineral. The now-patterned surface is placed in an aqueous deposition solution, from which the mineral precipitates as a thin film on the functionalized regions of the surface. For this work, an octadecyltrichlorosilane(OTS) was initially deposited on a silicon wafer.' The electron beam used was from an PHI Auger spectrometer. The beam was rastered first in one, dimension and then in the other to create a simple cross pattern with each arm approximately 50 pm in width. The beam was operated at 2 kV at 1 pA current for a dosage of 2.4 C/cm2. Certainlyfiner and more complexpatterns could be programmed into a more versatile electron gun, but the simple cross pattern served the purpose of demonstrating the concept. The electron beam desorbed somehydrocarbon but left a graphitic-likeresidue on the surface.8 High-energy beams (>20 kV) do not damage the films, and it is possible to image the surfaces by electron microscopy without significant damage. Two ion guns were used to produce the images reported here. Both guns were produced by FEI Corporation; one was the primary ion source in a Charles Evans & Associates time-offlight secondary ion mass spectrometer (TOF-SIMS) and the other was the milling source in an FEI ion milling machine. Both instruments were easily adapted to ion milling a 20 thick SAM? The resolution of the beam is less than 1pm but the beam has much wider, low-intensity tails that may damage the relatively sensitive SAM surface. Three bars of some nominal width separated by a distance equal to the width were etched into the surface. The length of each bar was 5 times the width. The actual dimensions of the bars varied in width from approximately 50 pm to approximately 1.0 pm. The etched surface was then cleaned of residual carbon and repopulated with silanol groups using a brief electrochemical ~~
(7) Prior to SAM deposition the silicon wafer was cleaned in an air plasma and treated for 2 min in 0.1 M KOH and 5 min in 0.1 M HN03. The wafer was then placed for 1 h in a 1%by weight solution of OTS in clean, dry cyclohexane. (8) Rieke, P. C.; Baer, D. R.; Fryxell, G. E.; Engelhard, M. H.; Porter, M. S. J. Vacuum Sci. Technol.A 1993,II, 2292. Laibinis, P. E.; Graham, R. L.; Biebuyck, H. A.; Whitesides, G. M. Science 1991,254,981.
oxidation by poising the sample at 1.6 V versus the saturated calomel electrode (SCE). The unetched surface was not affected at this potential; 2.2-2.5 V versus SCE was required to damage the unetched monolayer. Vinylsilane was deposited in the etched region by immersion of the substrate in a 1% solution of (n-vinylheptadecy1)trichlorosilane in cyclohexane. A sulfonic acid group was introduced to the surface by exposure of the substrate to SO3 gas:l0J1 No apparent reaction occurredon the methyl-terminated region, although a slight decrease in contact angle of the methyl SAM region suggested some oxidation. Detailed XPS studies of this reaction have shownthat essentially100%of the vinyl groups undergo reaction and that sulfur is undetectable on the methylterminated regions.12 The final, almost anticlimatic, step in the scheme of Figure 1 is deposition of the mineral film, in this case, iron oxyhydroxide (FeOOH).13 Deposition occurred by immersion of the substrate in an aqueous solution supersaturated with respect to the desired mineral phase.14 Becausethe sulfonated portion of the substrates directs nucleation and growth, the resulting film growth is confinedto this portion. Other minerals might also be deposited; the key is in designing an appropriate functionalized organic surface.
Results and Discussion Scanning electron microscopy (SEM),TOF-SIMS, optical microscopy, and condensation imaging were used to determinethe chemicalpropertiesof the surfaceand image these properties on the micrometer scale. Parts A-F of Figure 2 present images of the surface after each step outlined in Figure 1. For the sake of brevity, we present only figures of the ion beam etched samples; very similar results were obtained with the electron beam. Shown in Figure 2A is an optical micrograph of damage induced by the ion beam. Each bar is 10 X 50 pm. Bars 1 X 5 pm were not quite resolved,suggestinga resolutionof approximately 1 pm for the ion beam etch step. The ion beam etched away the surface to a depth below the native oxide layer. Figure 2B shows an image of water condensed on the ion beam etched bars shown in Figure 2A. The image was obtained by setting the sample on top of a Millipore filter holder filled with crushed dry ice. Condensation drops formed as the sample cooled and were observed by optical microscopy. Imaging is feasible due to the difference in contact angle wetting of the two s~rfaces.~J5 Large drops are formed when the edges of two spreadingadjacentdrops meet and coalesce into one. Rapid spreading and coalescence of drops implies a low advancing contact angle. Because the wettability of the etched areas changed dramatically from step-to-step, condensation images proved particularly informative. The regions etched by the ion beam are readily apparent in Figure 2B. Each bar contained an oblong drop that (9)Milling times were typically a few seconds and were roughly determinedby the time requiredfor decay of the bright secondaryelectron yield from the approximately20 A thick silicon oxide layer on the silicon wafer. Typical dosages were less than 3 X 10-3 C/cm2. (10) Gilbert, E. E. Sulfonation and ReZated Reactions; Robert E. Krieger Publishing Co.: New York, 1977. (11) The samples were placed in a glass vacuum chamberand pumped down to rough pump pressures. SO3 gas was introduced from a valved flask containing SO3 solid. After 1.5 min of exposurethe sample chamber was flushed with dry nitrogen and the samples were removed and rinsed well with clean water. (12) Rieke, P. C.; Williford, R. E., Pacific Northwest Laboratory, unpublished data. (13) Schwertman, U.; Cornell, R. M. Iron Oxides in the Laboratory; VCH: New York, 1991. (14) The sample was placed in a 3 mM Fe(NO& solution in 10 mM HNO3 and heated in a water bath at 70 "C for 45-60 min. (15) Beysens, D.;Knobler,C. M.Phys.RevLett. 1986,57,1433. Family, F.; Meakin, P. Phys. Rev. A 1989,40,3836. Fritter, D.;Knobler, C. M.; Beysens, D. A. Phys. Rev. A 1991,43, 1991. Steyer, A.; Guenoun, P.; Beysens, D.; Knobler, C. M. Phys. Rev. A 1991,44, 8271.
Letters
Langmuir, Vol. 10, No. 3, 1994 621
Figure 2. Optical micrographs (OM) and condensationimages (CI) of a ion beam etched sample at sequential steps corresponding to Figure 1: (A) OM of the as etched methyl-terminatedSAM; (B) CI of the sample in A; (C) CI after electrochemical cleaning; (D) CI after depositionofthe vinyl monomer; (E) CI after sulfonation of the vinyl monomer;(F) OM of the brown FeOOH mineral deposit. Each bar is 10 X 50 pm.
appeared to withdraw from the ends of the etched regions as the drops coalesced into the middle. The methylterminated SAM surface contained smaller, randomly distributed, hemisphericaldrops. In comparison with the figures discussed below, neither of these surfaces was distinctly hydrophilic but were sufficiently different to discern a small difference in hydrophobicity. While the etched regions were somewhat more hydrophilic,they were not at this point suitable for deposition of a second monolayer. Etched regions smaller than those shown in Figure 2B were not readily imaged by this technique, as coalescencebetween the bars occurred rapidly and formed a large drop covering all three bars. Figure 2C shows a condensation image of the etched region after the electrochemical cleaning step. A water drop uniformly filled each bar. The bars were clearly resolved from each other and the unchanged methylterminated SAM surface. Each set of features was surrounded by a region depleted of drops. Bars of 1 X 5 pm could also be resolved, but it proved impossible to capture a condensation image of these as the drops on adjacent bars coalesced faster than it was possible to take a photograph. Previously we have found that electron beam damage leaves a graphitic residue on the surface9 that is easily removed by the electrochemical cleaning step. SIMS analysis of the ion beam etched regions shows little remaining carbon and the condensation figures show an enhanced hydrophilicitywith electrochemicalcleaning. In the case of ion beam etching, the primary purpose of electrochemicaloxidation is probably to reintroducesilanol groups to the surface of the exposed silicon wafer. Figure 2D shows a condensation image of etched samples after the vinyl-terminated SAM was deposited. On large
etched areas the images were readily apparent by the difference in the number-density of drops although the drop size was very similar. On the small features shown in the figure it is difficult to distinguish the vinylterminated region from the methyl-terminated region. Only the peculiar geometric arrangement of the drops, a slightlylarger drop size, and the contrast of the underlying etched region allowed the vinyl-terminated regions to be identified. Features smallerthan this could not be resolved by condensation imaging. The etched areas were considerably more hydrophobic than prior to vinyl monomer deposition. This was consistent with the similarity in contact angles of methylterminated SAMs, loso,and vinyl-terminated SAMs, 98'. The contact angle on the methyl-terminated region after deposition of the vinyl monomer was greater than 108" indicating that vinyl monomer was not deposited over the methyl-terminated surface. This result confirmed the successful selective deposition of the vinyl monomer in the etched regions. Figure 2E shows a condensation image of the ion beam etched sample after sulfonation of the vinyl-terminated SAM. Condensate formed very rapidly on the sulfonated regions such that little condensate formed on the hydrophobic surface. This is reflected in the figure as small sparse drops on the methyl-terminated regions with fully wetted sulfonated regions. If the image was allowed to develop until the hydrophobic portion appeared, as in previous figures, the sulfonated regions began to coalesce. The contact angle of a macroscopic sulfonate-terminated surface was about 10". The results of Figure 2E were consistent with a highly hydrophilic surface and demonstrated that selective chemical reactions can be achieved
Letters
Figure 3. Optical micrograph of patterned SAM surface after deposition of clearly resolved,brown, FeOOH film. Each bar is 50 X 250 pm.
on SAM with dimensions of a few micrometers. As above, a series of bars 1 X 5 pm could not be resolved because single water drop covered the etched areas as well as the regions between etched areas. The above images describe in detail the steps required to successfullypattern a SAM surface with electron or ion beams. A very hydrophilic sulfonated region has been patterned into a very hydrophobic methyl-terminated SAM surface. These particular surfaces were chosen for their respective ability to strongly promote and to strongly inhibit the nucleation and growth of iron oxyhydroxide films. Shown in Figures 2F and 3 are optical micrographs of FeOOH films deposited on the sulfonated regions of this surface. The brown tint of the iron film was clearly visible. The bars in Figure 3 were 50 X 250 pm (Note one of the bars was overwritten twice with a slight offset. The small dark dots in each figure were tiny water spots formed from contaminants during evaporation of previous condensation images.) and those in Figure 2F were 10 X 50 pm. The methyl-terminated regions remained hydrophobic and no optically detectable iron was present. Energy-dispersive spectra of the two regions confirmed that iron was present in easily detectable quantities on the sulfonated region but undetectable on the hydrophobic regions. The thickness of the deposit in Figure 3 is about 100 nm. TEM analysisof similar films showed highly uniform dense films. The diffraction pattern suggests the films are composed of weakly crystalline goethite.
The larger bars of Figure 3 demonstrate the ability to clearly resolve features separated by 50 pm. The edges of the features are sharp and closely resemble the etched pattern. Closer examination shows distortion near the edge to be about 2 pm wide-more or less the resolution of the ion beam etch step. The bars in Figure 2F were easily visible. However, it appeared as though a thin FeOOH film extended between the bars and that on this scale we have not quite been successfulin confining mineral deposition to the desired areas. An SEM micrograph of these features confirmed the presence of iron between the bars and indicated that the bars were not quite resolved. From the optical density of the two regions, the deposition on the in-between regions appeared less thick than in the etched regions and suggested a slower deposition rate between the bars. Comparison of the two figures suggests that resolution is more dependent on feature separation than on the resolution of the ion beam etch step. While this could be due to some damage to the methyl-terminated regions, it may also be a reflection of the relative rates of lateral and vertical growth of the mineral film. In addition the thickness,judged qualitatively from the optical density, is much greater for the large set of bars. This suggests that the kinetics of deposition depend on the total area exposed. The ability to just resolve features separated by 10 pm defines the current resolution capability of the lithographic technique. The purpose of this work is to outline the requisite steps for a new and novel lithographic technique. While the work described here involves deposition of FeOOH, many other minerals can be deposited as patterned films provided the necessary surface functionality can be patterned into the SAM substrate. Indeed, finding the necessary surface functionality to induce the nucleation and growth of other minerals is the major focus of our research.2 The resolution of the mineral deposition step was on the order of 10to 15pm and appeared to be limited by growth of the film between adjacent features. This might be a fundamental limit to resolution and will be the focus of further work. The resolution of the SAM patterning step was a few micrometers and could feasibly be much smaller than 1.0 pm using fully optimized etching conditions. Acknowledgment. The authors thank Charles Evans and Associates and FEI Corp. for the generous use of their equipment and resources. This research was supported by the U.S. Department of Energy, Office of Basic Energy Sciences and Office of Industrial Technology. Additional funding was received from the National Institutes of Health through the Small Business Innovative Research Program (Grant 1R43 GM47771-01, C. M. John). Pacific Northwest Laboratory is operated for the U.S. Department of Energy by Battelle Memorial Institute under Contract DE-AC06-76RLO 1830.
