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The Phase Behavior of Multicomponent Self-Assembled Monolayers Directs the Nanoscale Texturing of Si(100) by Wet Etching Krista R. Finnie† and Ralph G. Nuzzo*,†,‡ Department of Materials Science & Engineering, School of Chemical Sciences, and The Frederick Seitz Materials Research Laboratory, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801 Received August 19, 2000. In Final Form: November 8, 2000 We demonstrate a novel soft-lithography-based methodology for generating nanoscale structures on Si(100) substrates via wet chemical etching. The feature generation apparently results from the phase dynamics of a multicomponent resist ink which phase separates on the Si surface. The monolayers formed from inks comprised of different mole fractions of docosyltrichlorosilane and octyltrichlorosilane contact printing are annealed in air and then placed into KOH etching solutions to generate dense textures of nanoscale features. We examine the resulting etch structures via atomic force microscopy and find that the different mole fractions of the model inks used here influence the nanoscale textures of the structures obtained. The photoluminescence of the etched samples was examined, both for the samples obtained from the etching and following a subsequent treatment in a buffered HF solution.
Introduction Nanoscale silicon-based structures, architectures with potential applications ranging from high surface area supports to bio- and chemical sensors, have become an area of great current interest.1-8 As each application is intimately tied to the properties arising from the precise details of the architecture and composition of the silicon nanostructure, developing flexible methods for the generation of these materials is a subject of timely interest for research.1-5,9-11 One common method for generating nanoscale features in Si is through the wet chemical etching of a Si substrate under potentiostatic control using a dilute, buffered HF electrolyte solution. This method underlies most of the recent efforts reported in the literature focused on the generation of photoluminescent porous Si nanostructures.1,2,4,5,9,10 Since the latter properties originate from quantum confinement effects, much attention has been given to using this process to generate very fine-scale features in the etch structures (e.g., e10 nm). Larger features, while perhaps losing the photoluminescent properties of porous Si, still have many potential uses.2,3 * To whom correspondence may be addressed. Phone: 217-2440809. Fax: 217-244-2278. E-mail:
[email protected]. † School of Chemical Sciences. ‡ Department of Materials Science & Engineering. (1) Doan, V. V.; Sailor, M. J. Science 1992, 256, 1791-1792. (2) Stewart, M. P.; Buriak, J. M. Adv. Mater. 2000, 12, 859-869. (3) Velev, O. D.; Kaler, E. W. Adv. Mater. 2000, 12, 531-534. (4) Schuppler, S.; Friedman, S. L.; Marcus, M. A.; Adler, D. L.; Xie, Y.-H.; Ross, F. M.; Chabal, Y. J.; Harris, T. D.; Brus, L. E.; Brown, W. L.; Chaban, E. E.; Szajowski, P. F.; Christman, S. B.; Citrin, P. H. Phys. Rev. B 1995, 52, 4910-4925. (5) Lehmann, V.; Gosele, U. Appl. Phys. Lett. 1991, 58, 856-858. (6) Harper, J.; Sailor, M. J. Anal. Chem. 1996, 68, 3713-3717. (7) Harper, T. F.; Sailor, M. J. J. Am. Chem. Soc. 1997, 119, 69436944. (8) Dancil, K.-P. S.; Greiner, D. P.; Sailor, M. J. J. Am. Chem. Soc. 1999, 121, 7925-7930. (9) Brus, L. E.; Szajowski, P. F.; Wilson, W. L.; Harris, T. D.; Schuppler, S.; Citrin, P. H. J. Am. Chem. Soc. 1995, 117, 2915-2922. (10) Canham, L. T. Appl. Phys. Lett. 1990, 57, 1046-1048. (11) Ranjan, V.; Singh, V. A.; John, G. C. Phys. Rev. B 1998, 58, 1158-1161.
The present report describes work stimulated by a longstanding interest in the patterning of materials by soft lithography, especially as it relates to methods that control wet etching processes (and the structures that result from them).12-18 Precise control over surface properties in the latter patterning methods is often accomplished by the deposition of self-assembled monolayers, and these materials thus provide in principle one possible way of controlling the generation of features in the underlying substrate via wet etching.12,14,19-21 Monolayers of alkyltrichlorosilanes formed by both immersion of a Si substrate in an adsorbate solution or contact printing have been well characterized (while both methods give high-quality self-assembled monolayers (SAMs), printing requires much shorter process times to generate high surface coverages of adsorbate).21-32 These (12) Kumar, A.; Biebuyck, H. A.; Abbott, N. L.; Whitesides, G. M. J. Am. Chem. Soc. 1992, 114, 9188-9189. (13) Kumar, A.; Biebuyck, H. A.; Whitesides, G. M. Langmuir 1994, 10, 1498-1511. (14) Kim, E.; Kumar, A.; Whitesides, G. M. J. Electrochem. Soc. 1995, 142, 628-633. (15) Xia, Y.; Mrksich, M.; Kim, E.; Whitesides, G. M. J. Am. Chem. Soc. 1995, 117, 9576-9577. (16) Zhao, X.-M.; Wilbur, J. L.; Whitesides, G. M. Langmuir 1996, 12, 3257-3264. (17) Black, A. J.; Paul, K. E.; Aizenberg, J.; Whitesides, G. M. J. Am. Chem. Soc. 1999, 121, 8356-8365. (18) Huck, W. T. S.; Yan, L.; Stroock, A.; Haag, R.; Whitesides, G. M. Langmuir 1999, 15, 6862-6867. (19) Abbott, N. L.; Kumar, A.; Whitesides, G. M. Chem. Mater. 1994, 6, 596-602. (20) Wilbur, J. L.; Kim, E.; Xia, Y.; Whitesides, G. M. Adv. Mater. 1995, 7, 649-652. (21) Finnie, K. R.; Haasch, R.; Nuzzo, R. G. Langmuir 2001, 17, 69686976. (22) Allara, D. L.; Parikh, A. N.; Rondelez, F. Langmuir 1995, 11, 2357-2360. (23) Bierbaum, K.; Grunze, M. Langmuir 1995, 11, 2143-2150. (24) Calistri-Yeh, M.; Kramer, E. J.; Sharma, R.; Zhao, W.; Rafailovich, M. H.; Sokolov, J.; Brock, J. D. Langmuir 1996, 12, 2747-2755. (25) Hoffmann, H.; Mayer, U.; Brunner, H.; Krischanitz, A. J. Mol. Struct. 1995, 349, 305-308. (26) Hoffmann, H.; Mayer, U.; Krischanitz, A. Langmuir 1995, 11, 1304-1312. (27) Hoffmann, H.; Mayer, U.; Brunner, H.; Krischanitz, A. Vib. Spectrosc. 1995, 8, 151-157.
10.1021/la0011984 CCC: $20.00 © 2001 American Chemical Society Published on Web 01/26/2001
Nanoscale Structures on Si(100)
films have been shown to be robust enough to serve as barriers in many different processes,20,21,28,29,33-39 though only the long-chained alkyltrichlorosilanes deposited by printing have proven capable of withstanding wet chemical etching conditions for any length of time.21 Generally, though, these monolayers are poor resists and the features generated using them contain significant densities of defects.21,32 Despite the apparent failure of this class of monolayer to serve as useful etch resists, the failure mechanisms identified in the earlier studies32,40 suggested to us interesting possibilities for manipulating the structural outcome via rational molecular design. Short-chain alkyltrichlorosilane resists fail relatively quickly, whereas long-chain alkyltrichlorosilanes actually serve as resists for a reasonable (albeit still too short to be commercially useful) length of time.21 SAMs formed from mixtures of long- and short-chain amphiphiles, at least for the case of thiolate SAMs on Au, have been shown to form nonideal 2-D mixtures and thus have a disposition to phase separate.41,42 We show in this report that these differences can be exploited in such a way as to both create and control the nanoscale structures generated by the wet chemical etching of a silicon substrate. By printing monolayers comprised of both short- and long-chain alkyltrichlorosilanes, mixtures that appear to generate nanoscale domain textures in the SAM, the failure rates of local regions within the film can be influenced. These kinetic biases serve to transfer the nanoscale latent image of the resist SAM into the substrate. Although the process remains far from optimized, the current demonstration illustrates the effectiveness of the concept using 2-D phase dynamics as a template for chemical fabrication in Si and suggests opportunities to improve it by appropriate molecular design. Experimental Section Materials and Film Formation. The substrates used in these experiments were Si(100) wafers (test grade, p-type, obtained from Silicon Sense, Nashua, NH). The substrates were cut into ∼4 cm2 pieces for further modification. The substrates were cleaned by repeated rinsing with deionized water, acetone, and isopropyl alcohol and dried under a nitrogen stream. The substrates were exposed, in a home-built UV/ozone chamber (lowpressure mercury lamp, λ ) 185 and 254 nm), for 15 min prior to modification to remove hydrocarbon impurities and produce (28) Jeon, N. L.; Finnie, K.; Branshaw, K.; Nuzzo, R. G. Langmuir 1997, 13, 3382-3391. (29) Kluth, G. J.; Sung, M. M.; Maboudian, R. Langmuir 1997, 13, 3775-3780. (30) Parikh, A. N.; Allara, D. L.; Ben Azouz, I.; Rondelez, F. J. Phys. Chem. 1994, 98, 7577-7590. (31) Parikh, A. N.; Schivley, M. A.; Koo, E.; Seshadri, K.; Aurentz, D.; Mueller, K.; Allara, D. L. J. Am. Chem. Soc. 1997, 119, 3135-3143. (32) Wasserman, S. R.; Tao, Y.; Whitesides, G. M. Langmuir 1989, 5, 1074-1087. (33) Jeon, N. L.; Nuzzo, R. G.; Xia, Y.; Mrksich, M.; Whitesides, G. M. Langmuir 1995, 11, 3024-3026. (34) Jeon, N. L.; Clem, P. G.; Nuzzo, R. G.; Payne, D. A. J. Mater. Res. 1995, 10, 2996-2999. (35) Jeon, N. L.; Clem, P. G.; Payne, D. A.; Nuzzo, R. G. Langmuir 1996, 12, 5350-5355. (36) Jeon, N. L.; Clem, P.; Jung, D. Y.; Lin, W. B.; Girolami, G. S.; Payne, D. A.; Nuzzo, R. G. Adv. Mater. 1997, 9, 891-895. (37) Jeon, N. L.; Lin, W. B.; Erhardt, M. K.; Girolami, G. S.; Nuzzo, R. G. Langmuir 1997, 13, 3833-3838. (38) Xia, Y.; Whitesides, G. M. Angew. Chem., Int. Ed. Engl. 1998, 37, 550-575. (39) Xia, Y. N.; Kim, E.; Zhao, X. M.; Rogers, J. A.; Prentiss, M.; Whitesides, G. M. Science 1996, 273, 347-349. (40) Cotton, F. A.; Wilkinson, G. Advanced Inorganic Chemistry, 3rd ed.; Wiley: New York, 1972; p 321. (41) Laibinis, P. E.; Nuzzo, R. G.; Whitesides, G. M. J. Phys. Chem. 1992, 96, 5097-5105. (42) Tamada, K.; Hara, M.; Sasabe, H.; Knoll, W. Langmuir 1997, 13, 1558-1566.
Langmuir, Vol. 17, No. 4, 2001 1251 a hydrophilic surface. The mixed monolayer (docosyltrichlorosilane (DTS) and octyltrichlorosilane (OCTY)) films and pure DTS films were made by spin coating an unpatterned poly(dimethylsiloxane) (PDMS) stamp using a photoresist spinner (Headway Research, Garland, TX) with a freshly prepared 10 mM solution of either DTS or a mixture of DTS and OCTY (Gelest, Tullytown, PA) dissolved in toluene (Aldrich, Milwaukee, WI) at 3000 rpm for 30 s. The stamp was then dried under a nitrogen stream for 30 s and then brought into contact with the substrate for a predetermined length of time (typically a contact time of 5 min was used). The humidity levels present during the reaction were held at high levels (near 60%), using a controlled humidity enclosure (the enclosure was constructed by mounting a Duracraft humidifier in a decommissioned, sealable fume hood). The PDMS stamps used in these experiments were fabricated according to previously reported methods and rinsed with isopropyl alcohol prior to each use.12,13,43 After contact printing, the samples were annealed in air at 75 °C for 1 h, prior to etching in a 4 M KOH/ 15% isopropyl alcohol solution held at 40 °C for 5-20 min. Hydrogen-terminated surfaces were formed by etching the samples in a 6:1 buffered HF oxide etching solution (Ashland Chemical) for 30 s. Atomic Force Microscopy (AFM). A Digital Instruments Dimension-3000 scanning probe microscope was used to acquire AFM images of pure and mixed monolayer films, both pre- and postetching. A standard Si3N4 tip was used, and the images were acquired in the contact mode with a scan rate of 2 Hz with the contact forces reduced to the minimum necessary to maintain feedback. Photoluminescence. Photoluminescence studies were performed on samples etched for 15 min in a 4 M KOH/15% isopropyl alcohol solution held at 40 °C (pre- and postetching in buffered HF for 30 s). The samples were excited using a He-Cd (Liconix, Santa Clara, CA) laser with 5 mW incident power at 442 nm and a 50 µm spot size. The spectrometer used was a Spex 270M (Metuchen, NJ); the photoluminescence was detected using a photomultiplier tube (Hamamatsu, Bridgewater, NJ). A bandpass color filter was employed to remove the scattered laser light.
Results and Discussion Formation of Mixed Monolayer Films. Figure 1 shows AFM images of contact printed samples made from two different mole fractions of DTS and OCTY; these micrographs were recorded after annealing the SAM (see above) but prior to etching. Figure 1A is a monolayer formed from a 10 mM solution composed of 50% DTS and 50% OCTY, while Figure 1B is a monolayer formed from a 10 mM solution composed of 80% DTS and 20% OCTY. A simple comparison of the two images reveals a crucial difference in the changes seen in the domain structure between the two mole ratios of the silanes used to print these films. The sample formed from an 80% DTS solution shows well-defined light domains surrounded by darker regions. The light regions are most likely comprised of DTS-rich phases, while the darker regions can be attributed to those containing larger mole fractions of OCTY. The 50% DTS sample appears quite different, with only diffuse regions of light and dark and essentially no texture that can be easily assigned to compositionally sensitive domains. These images strongly suggest that, at higher concentrations of DTS, the interactions between the long chains are sufficiently strong so as to drive the formation of domains in the transferred SAMs. As the mole fraction of DTS is reduced, the mixing between DTS and OCTY increases, forming a more diffuse set of structures within the monolayer. These differences are further highlighted in Figure 2, which shows cross sections of each of the AFM images. For the 80% DTS sample, the difference in height between the light and dark regions is apparent (10 (43) Kumar, A.; Whitesides, G. M. Appl. Phys. Lett. 1993, 63, 20022004.
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Figure 3. After 5 min in a 4 M KOH etching solution at 40 °C, bare silicon (A) still appears smooth by AFM, whereas the 50% DTS (B) and the 80% DTS (C) show a developing grainlike structure. The 100% DTS sample (D) still appears smooth at this magnification.
Figure 1. AFM images of mixed monolayers of DTS and OCTY prior to etching: (A) 50% DTS, 50% OCTY; (B) 80% DTS, 20% OCTY. Ink concentrations were held at 10 mM.
Figure 2. Cross sections of the AFM images seen in Figure 1. The domain structure seen in the 50% DTS sample (A) is less defined than that for the 80% DTS sample (B).
Å), as are the lengths of the light domains (∼100 nm), while the 50% DTS sample is much more homogeneous, with only slight (or negligible) differences being seen in the height profiles. We should note as a point of caution here that the compositions of the SAMs do not appear to follow in a simple way from the concentrations used to prepare the inks. Several lines of evidence gathered in control experiments support this notion (but most notably the fractions evident in Figure 1B) and it also follows behavior seen in studies of mixed monolayer formation from solution immersion.44,45
Architecture of Etched Structures on Si(100). The compositional variations present in the mixed monolayer films translate into distinct textural differences with the exposure of the samples to a (nonisotropic) wet chemical etching solution comprised of 4 M KOH/15% isopropyl alcohol, at 40 °C. Figure 3 illustrates the differences seen between a blank silicon control, a full monolayer of DTS, and the two mixed monolayer resists after a short etch time (5 min). The Si control still appears essentially flat at this magnification (it actually has roughened somewhat), as does the pure film of DTS. The two mixed monolayers appear quite different, however. The 80% DTS sample is rough, with well-defined light domains surrounded by shallowly etched dark regions. The observed features are on the length scale of 0.1 µm or smaller. The 50% DTS sample, reminiscent of the sample prior to etching, has much larger, more diffuse domains, ones centered at approximately 0.25 µm or smaller; these grains are more sparsely distributed across the substrate than is the case seen in Figure 3C. The differences in the etch pits can be quite clearly seen in Figure 4, which shows representative AFM cross sections for the mixed SAM samples. The depths of the features generated at this point are approximately 1.5 nm, which corresponds to approximately 1-5% of the etch depth found on the Si blank in a separate control experiment. As etch times are increased, considerable changes in the surface architecture can be seen for all four samples (Figure 5). The etchant used in this demonstration has a very high etch rate for silicon but is inherently anisotropic. With time, it will generate (111) facets on the (100) substrate. Such features are seen in two of the samples. When examined by AFM, both the bare silicon and the pure DTS film surfaces appear quite rough, with faceted (44) Folkers, J. P.; Laibinis, P. E.; Whitesides, G. M.; Deutch, J. J. Phys. Chem. 1994, 98, 563-571. (45) Zhang, H. L.; Chen, M.; Li, H. L. J. Phys. Chem. B 2000, 104, 28-36.
Nanoscale Structures on Si(100)
Figure 4. Cross sections of the AFM images seen in Figure 3. Etching of the substrate is more pronounced in the 50% DTS sample (A) with fewer protected regions than for the 80% DTS sample (B).
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Figure 6. Cross sections of the AFM images presented in Figure 5. The bare silicon substrate appears rough, with large features apparent (A) as is also the case for the 100% DTS-modified substrate (D). The architectures of the 50% DTS (B) and the 80% DTS (C) samples are much more corrugated in appearance, with smaller, more regular features being seen.
Figure 5. After 15 min in a 40 °C, 4 M KOH etching solution, large etched structures appear in AFM images for bare silicon (A). The 50% DTS (B) and the 80% DTS (C) samples are textured on the nanoscale; the sizes of the grainlike structures appear to be more uniform. The 100% DTS (D) sample is heavily faceted due to the slower etching rate of the (111) planes.
Figure 7. AFM images of samples after a 15 min etch in 40 °C, 4 M KOH and a 30 s etch in a 6:1 buffered HF solution: (A) bare silicon; (B) 50% DTS; (C) 80% DTS; (D) 100% DTS.
features of the order of several hundred nanometers up to a micrometer in size (laterally) being evident. It is interesting to note the striking resemblance the samples bear to each other. This suggests that, while the DTS film initially protects the underlying substrate, it ultimately fails (apparently uniformly), leading the Si substrate to be etched anisotropically in much the same way as an unprotected substrate (albeit to a lesser depth overall). The surface architecture of the other two samples is considerably different from the blank silicon and the pure DTS film samples. The feature sizes observed on the 80% and 50% samples are a fraction of the lateral sizes seen in the other samples (e100-250 nm), have much higher aspect ratios, and appear to have a more uniform size and shape as well. Between these two samples, a few differences exist, as seen in Figure 6. The grain sizes seen in the 80% DTS sample appear more uniform than that of the 50% DTS sample. Additionally, the etch pits observed in the 80% DTS samples are generally deeper than those
seen in the 50% DTS sample, which can be attributed to the etch resist properties of the films. The domains observed in the 80% DTS sample are better defined, comprised of densely packed regions of DTS, features that the 50% DTS sample lacked. As dense films of DTS are known to be more resistant to etching conditions than shorter chain films, it would follow that regions of densely packed DTS would protect better than would a less coherent mixture of phase-separated long and short chains. This difference seems to generate the kinetic discrimination needed to produce the slightly finer-scale nanostructured features seen in Figure 5B. Despite these subtle differences, the samples are otherwise very similar. Photoluminescence Studies. One of the most intensively studied forms of nanostructured Si is so-called porous Si.1-8,10,11 This material, when properly prepared by electrochemically mediated wet etching, is strongly luminescent. This remarkable property has been shown to result from quantum confinement effects.1,2,4 While the
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sizes of the features generated in this study appear to be too large to generate useful optical properties via quantum confinement, we none the less undertook an exploratory study of the structures generated using the mixed resist inks. The photoluminescence (PL) of these samples was examined in the range of 450-850 nm using excitation at 442 nm. A control sample of porous Si was strongly photoluminescent under these same conditions. As the current method for generating porous silicon produces a hydrogen-terminated surface, the samples were measured as prepared and after being immersed in a dilute buffered HF solution for 30 s. AFM images (Figure 7) and X-ray photoelectron spectroscopy (XPS) data (not shown) reveal that the surface architecture is changed slightly after the HF etch, a result due largely to the stripping of oxide surface layers left behind by the etching solution. These samples are, at best, weakly luminescent as seen in representative data shown in Figure 8 for the sample derived from the 80% DTS SAM. Porous silicon is thought to have features on the nanometer length scale,5 whereas the features observed in this study are on the order of 100 nm or larger. This appears to account for the weak activity seen in the visible wavelength range here. The spectrum is most intense for the HF-etched sample (although still weak) and, notably, is blue-shifted (750-650 nm) relative to the as prepared sample. The data presented in this paper demonstrate that the phase behavior in a multicomponent SAM (here the nanoscale phase separation of long and short chains) can be used as an effective “latent image” to generate features of related scale in silicon by wet etching. Given the nature of the SAMs used here, ones derived from organosilane coupling agents, the character of the dynamics is both complex and presumably limited as well by the reactions which serve to both (hydrolytically) cross-link and anchor the adsorbates to the surface. We believe a better-defined structural outcome might be obtained using an ink system that could more effectively equilibrate the domains formed at length scales directed either thermodynamically by molecular design or dynamically via the perturbations
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Figure 8. PL spectra of 80% DTS/20% OCTY etched for 15 min in a 4 M KOH/15% IPA solution held at 40 °C (A) and after etching in a buffered HF solution for 30 s (B).
provided during the processing. We are exploring these options in current research and will report on approaches to both in future publications Acknowledgment. This work was supported by the National Science Foundation (Grant CHE 9626871) and DARPA (SPAWAR: N66001-98-1-8915). Imaging studies were carried out at the Center for Microanalysis of Materials, University of Illinois, which is supported by the Department of Energy under Contract DEFG0291ER45439. LA0011984
