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Growth of Ultrasmooth Octadecyltrichlorosilane Self-Assembled Monolayers on SiO2 Yuliang Wang and Marya Lieberman* Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, Indiana 46556 Received August 5, 2002 Ultrasmooth octadecyltrichlorosilane (OTS) monolayers (2.6 ( 0.2 nm thick, RMS roughness ∼1.0 Å) can be obtained reproducibly by exposing clean native SiO2 surfaces to a dry solution of OTS in Isopar-G. A clean room is not required. Atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), contact angle data, and ellipsometry show that film formation occurs through a “patch expansion” process and terminates once a single monolayer is formed, after about 2 days. These monolayers are suitable as substrates for high-resolution electron beam and AFM or STM lithography. Further observations highlight the importance of controlling water content during deposition of siloxane self-assembled monolayers. OTS covers the surface much faster when there is a little water in the OTS solution; contact angle and ellipsometry data indicate formation of a hydrophobic, 2.6 nm thick film after about 2 h. However, these OTS films have a totally different growth mechanism than films grown from dry solutions and are not really monolayers. The OTS forms platelike islands that then adsorb onto the surface; the resulting overlayers have RMS roughness of more than 3 Å. Continued exposure to the OTS solution results in continued island deposition and increased roughness.
Introduction The reaction of n-octadecyltrichlorosilane (OTS) with silicon oxide to form self-assembled monolayers (SAMs) has been widely studied by many groups in recent years.1,2 These methyl group terminated SAMs and their functionalized derivatives have many potential applications in a wide variety of fields, especially for surface modification and patterning in microfabrication.3 The characteristics of these SAMs have been investigated extensively by various methods, such as FTIR, AFM, and ellipsometry.4-16 These studies provided valuable information for our understanding of the growth behavior of OTS on (1) (a) Netzer, L.; Iscovici, R.; Sagiv, J. Thin Solid Films 1983, 100, 67. (b) Wasserman, S. R.; Tao, Y.-T.; Whitesides, G. M. Langmuir 1989, 5, 1074. (c) Tillman, N.; Ulman, A.; Schildkraut, J. S.; Penner, T. L. J. Am. Chem. Soc. 1988, 110, 6136. (d) Biebaum, K.; Grunze, M.; Baski, A. A.; Chi, L. F.; Schrepp, W.; Fuchs, H. Langmuir 1995, 11, 2143. (2) (a) Tillman, N.; Ulman, A.; Penner, T. L. Langmuir 1989, 5, 101. (b) Brzoska, J. B.; Azouz, I. B.; Rondelez, F. Langmuir 1994, 10, 4367. (c) Lambert, A. G.; Neivandt, D. J.; McAloney, R. A.; Davies, P. B. Langmuir 2000, 16, 8377. (3) (a) Brandow, S. L.; Chen, M. S.; Wang, T.; Dulcey, C. S.; Calvert, J. M.; Bohland, J. F.; Calabrese, G. S.; Dressick, W. J. J. Electrochem. Soc. 1997, 144, 3425. (b) Vossmeyer, T.; Jia, S.; Delonno, E.; Diehl, M. R.; Kim, S. H.; Peng, X.; Alivisatos, A. P.; Heath, G. R. J. Appl. Phys. 1998, 84, 3664. (c) Liu, J.; Casavant, M. J.; Cox, M.; Walters, D. A.; Boul, P.; Lu, W.; Rimberg, A. J.; Smith, K. A.; Colbert, D. T.; Smalley, R. E. Chem. Phys. Lett. 1999, 303, 125. (4) Banga, R.; Yarwood, J.; Morgan, A. M.; Kells, J. Langmuir 1995, 11, 4393. (5) Le Grange, J. D.; Markham, J. L.; Kurkjian, C. R. Langmuir 1993, 9, 1749. (6) Berquier, J.-M.; Fernandes, A.-C.; Chartier, P.; Arribart, H. SPIE Fourier Transform Spectrosc. 1989, 1145, 300. (7) Mathauer, K.; Frank, C. W. Langmuir 1993, 9, 3446. (8) Kallury, K. M. R.; Tripp, C. P.; Hair, M. L. Langmuir 1992, 8, 947. (9) Le Grange, J. D.; Markham, J. L.; Kurkjian, C. R. Langmuir 1993, 9, 1749. (10) Tripp, C. P.; Hair, M. L. Langmuir 1992, 8, 1120. (11) McGovern, M. E.; Kallury, K. M. R.; Thompson, M. Langmuir 1994, 10, 3607. (12) Offord, D. A.; Griffin, J. H. Langmuir 1993, 9, 3015. (13) Tidswell, I. M.; Rabadeau, T. A.; Pershan, P. S.; Kosowsky, S. D. J. Chem. Phys. 1991, 95, 2854. (14) Tidswell, I. M.; Ocko, B. M.; Pershan, P. S. Phys. Rev. B 1990, 41, 1111. (15) Silberzan, P.; Leger, L.; Benattar, J. J. Langmuir 1991, 7, 1647.
the SiO2 surface. From AFM studies and also X-ray diffraction results most of the investigations have found an “island aggregation” mechanism for the growth of OTS SAMs, which starts with the building of islands in solution that adhere to the substrate and grow together into a complete monolayer.17 There is a “uniform” or “continuous” model supported by Wasserman et al. based on X-ray reflectivity measurements and IR-ATR data.18 This model indicates that an incomplete monolayer consists of OTS molecules that are uniformly distributed over the substrate in a disordered manner, which later fills out and is rearranged into the final highly ordered film structure. Since X-ray reflectivity measurements and also IR average information over a large measurement area, the use of atomic force microscopy (AFM), which determines the local structure of the surface, would be appropriate to study the growth mechanism of such films at a microscopic level. Based on the properties of OTS and the possible chemical reactions involved in SAM growth, the conditions of the experiments have a dramatic influence on the formation of the SAMs. For example, the dryness of the OTS growth solution and the substrates, the soaking time, and pretreatment of the substrate are all crucial factors that affect the structure of the film. Hoffmann and Friedbacher studied the effects of water content of the OTS solution on the growth mechanism.19 They found that both a continuous growth mechanism and an island type growth mechanism are involved in the formation of OTS monolayers, and that with increasing water content or increasing age of the adsorbate solution, island type growth is strongly favored. Silberzan et al.15 along with Angst and Simmons20 have shown that a water film (one or several (16) Wei, M.; Bowman, R. S.; Wilson, J. L.; Morrow, N. R. J. Colloid Interface Sci. 1993, 157, 154. (17) (a) Cohen, S. R.; Naaman, R.; Sagiv, J. J. Phys. Chem. 1986, 90, 3054. (b) Biebaum, K.; Grunze, M.; Baski, A. A.; Chi, L. F.; Schrepp, W.; Fuchs, H. Langmuir 1995, 11, 2143. (18) Wasserman, S. R.; Whitesides, G. M.; Tidswell, I. M.; Ocko, B. M.; Pershan, P. S.; Axe, J. D. J. Am. Chem. Soc. 1989, 111, 5852. (19) Vallant, T.; Brummer, H.; Mayer, U.; Hoffmann, H.; Leitner, T.; Resch, R.; Friedbacher, G. J. Phys. Chem. B 1998, 102, 7190. (20) Angst, D. L.; Simmons, G. W. Langmuir 1991, 7, 2236.
10.1021/la020697x CCC: $25.00 © 2003 American Chemical Society Published on Web 01/17/2003
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layers thick) covering the silica substrate is necessary for the formation of a complete monolayer. Tripp and Hair used infrared spectroscopy to determine that there is no direct reaction of OTS with the surface hydroxyl groups and that OTS binds only to a small extent with the first water layer on the surface of fumed silica.21 Here we show how different growth environments (i.e., dry vs wet) affect the mechanism of OTS film growth at a microscopic level. We report a reproducible way to prepare ultrasmooth OTS SAMs on silicon oxide with a RMS roughness e 1 Å. Grunze and Fuchs had reported that an OTS film formed under clean room conditions can achieve this smoothness.1d By controlling the surface pretreatment, the water content in the growth solution and the soaking time, we can prepare such a smooth OTS monolayer in a normal lab environment. These OTS SAMs are suitable as resist materials that could be chemically processed by high-resolution electron-beam lithography or scanning probe modification and then used to direct the deposition of molecules from solution. Results The RMS roughness of a typical 500 nm thick layer of thermally grown silicon dioxide on commercial wafers is 5-10 nm. Although this roughness is just 1% of the oxide thickness, it is several times the anticipated thickness of an OTS monolayer (2.6 nm). Hence, if an ultrasmooth OTS layer is needed (e.g., as an electron-beam resist or as a substrate for nanolithography), it is necessary to use a much thinner oxide layer. The native oxide found on wafers is in the right thickness range (1-3 nm) with a RMS roughness less than 1 Å. However, we find that wafers that have sat around for a few months, even in a clean room, are often contaminated with grease or other material that hampers formation of uniform OTS overlayers. To consistently grow clean native oxide films suitable as substrates for ultra-smooth OTS, a rigorous degreasing and cleaning procedure for wafer, tweezers, and glassware is necessary, culminating in HF stripping of the existing native oxide. The action of room-temperature piranha acid on the cleaned wafer, as described in the Experimental Section, yields a silicon oxide layer between 8 and 15 Å thick with an RMS roughness of less than 0.5 Å. These clean native oxide surfaces must be used within a few hours of preparation or they too gather contaminants. We compared the growth rate and mechanism of OTS in dry solution with that in wet solution (water concentration >50mM). For dry solutions, a mixture of chloroform, carbon tetrachloride, and Isopar-G was dried by passage through activated alumina and fresh OTS was added to make a 5 mM OTS solution. Figure 1 shows AFM images of partially and fully grown OTS films formed by exposing clean native oxide to this dry solution of OTS. Image (a) was taken after 18 h immersion of the sample. At this time, the contact angle of the sample was measured as 90° and the ellipsometric thickness as 14 ( 2 Å, both values indicating partial coverage of the hydrophilic oxide surface. The light area is the growing OTS film and the dark areas are the substrate. The areal coverage of the OTS film measured from these AFM images is 60%. Image b is a line section of a. The height of the OTS film is 2.4 nm. This is very close to the expected thickness of ∼2.6 nm for an OTS SAM on SiO2. Since ellipsometry averages the film height over a large area, we predict an ellipsometric thickness of (0.6 × 24 Å), or 14.4 Å, which is what (21) (a) Offord, D. A.; Griffin, J. H. Langmuir 1993, 9, 3015 (b) Tripp, C. P.; Hair, M. L. Langmuir 1992, 8, 1120.
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we observed. Because compression from the AFM tip can have the effect of minimizing the actual height of OTS films,22 it is likely that the observed thickness at the OTS regions represents a fully formed OTS film. Image c shows the surface morphology of the completely formed OTS monolayer. This AFM image was taken after 48 h of soaking in the dry OTS solution, when the contact angle of 115 ( 3° and the ellipsometric thickness of 24 ( 2 Å both indicated complete monolayer coverage. The RMS roughness for this 10 µm × 10 µm area is 1.04 ( 0.05 Å, a value that meets the standard we set for further nanofabrication. Image d shows a line section of this image; its main feature is the smoothness of the OTS layer. Under dry conditions, the RMS roughness for the partially formed OTS film is 0.97 Å, the same as that for the complete OTS film. Once the OTS molecules form a monolayer patch on the surface, these patches are as smooth as the final fully grown OTS SAM. OTS layers prepared in wet solution are very rough at all stages of film growth, and layer growth is not selflimiting. Figure 2a is an AFM image of a clean native oxide surface taken after 30 min soaking in wet OTS solution, when the contact angle of 100 ( 3° and the ellipsometric thickness of 12 ( 2 Å both indicate partial coverage of the substrate surface. Small, irregular islands about 23 Å high and with an average surface area of 2 × 104 nm2 are observed; at this soaking time, the islands cover about 40% of the substrate surface, and give an RMS roughness of 5.5 Å. The samples were rinsed in several solvents with concomitant ultrasonication, a process which removes most physisorbed contaminants and dust particles, so we believe the islands are chemically bound to the substrate. Figure 2c shows the surface morphology after 2 h of soaking in wet OTS solution; at this time, the contact angle of 115 ( 3° and the ellipsometric thickness of 26 ( 2 Å are both consistent with full coverage of the oxide surface. However, it is clear from the topography that this “full coverage” film is very inferior in quality to the one produced under dry conditions. The film is composed of many small islands with an average surface area of 1 × 103 nm2 and height about 20-25 Å. The coverage is much denser, about 80-90% of the surface being covered with islands. The RMS roughness is still 3.09 ( 0.18 Å, a value 3 times larger than the dry condition. For a uniformly distributed surface morphology, the RMS roughness does not depend on the image size.17b X-ray photoelectron spectroscopy (XPS), AFM and ellipsometry were used to compare the effects of continued exposure of OTS SAMs to the dry and wet OTS solutions. XPS is very sensitive to the chemical composition of surfaces in the top 50-100 Å. The clean native oxide films, which consist of 8-15 Å of SiO2 on top of elemental Si, show two peaks in the Si 2p binding energy range. The peak at 103.3 eV corresponds to electrons originating from the Si(IV) in SiO2, and one at a lower binding energy of 99.3 eV corresponds to electrons originating from the underlying elemental silicon(0) atoms, whose intensity is somewhat attenuated by passing through the SiO2 overlayer. A weak signal in the C 1s binding energy range is also observed at 284.5 eV; this is due to adventitious carbon from pump oil in the XPS system and general contamination in the lab ambient. Because the OTS molecule (C18H37SiCl3) contains both carbon and silicon (IV), the formation of monolayers and multilayers can be observed by changes of the XPS peak (22) Gu, Y.; Akhremitchev, B.; Walker, G. C.; Waldeck, D. H. J. Phys. Chem. B 1999, 103, 5220.
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Figure 1. AFM images of partially and fully grown OTS monolayers on SiO2 deposited under dry conditions. (a) Partially grown OTS film after 18 h soaking. Coverage is about 60%. Light area is the OTS monolayer; dark area is the bare SiO2 substrate. The “patch expansion” growth habit can be seen in this image. The white line and the arrows are used for line section analysis as shown in b. (b) Line section of a. The height difference between the OTS layer and the substrate is 2.4 nm. (c) Fully grown OTS monolayer after 48 h. (d) Line section of c.
intensities with soaking time. If only a monolayer is formed, there should be no changes of the relative intensities of the carbon and the two silicon peaks once
full coverage is achieved. Multilayer formation, in contrast, should result in larger intensities for the peaks due to the material present in the multilayer and smaller intensities
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Figure 2. AFM images of partially and fully grown OTS monolayers on SiO2 deposited under wet conditions. (a) Partially grown OTS film after 30 min soaking. The coverage is about 40%. The light area is the OTS film. (b) A line section of a. The height difference between the OTS and the substrate is 2.3 nm. The thickness obtained from ellipsometric measurement is 1.2 nm. (c) Fully covered OTS film after 2 h. (d) Line section of c. The OTS layer height is 2.4 nm.
for the peaks from the underlying substrate, and these changes in relative peak intensities should continue to accrue during longer soaking times.
Figure 3 shows XPS data for samples soaked for long periods in dry (3a,c) and wet (3b,d) OTS solutions. Parts a and b show spectra of the C 1s binding energy region,
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Figure 3. XPS results for multilayer film formation under wet and dry conditions. The times shown in each figure tell how long samples were soaked in the OTS solutions. (a) Carbon peak intensity changes for samples grown under dry conditions. (b) Carbon peak intensity changes for samples grown under wet conditions. (c) Silicon peak intensity changes for samples grown under dry conditions. (d) Silicon peak intensity changes for samples grown under wet conditions. Table 1. Contact Angle, Ellipsometric Thickness, and RMS Roughness Data for Multilayer OTS Growth
soaking time contact angle (deg) ellipsometric OTS thickness (nm) RMS roughness (Å)
native oxide, no exposure to OTS 18 ( 2 1.0 ( 0.1 (SiO2 thickness) 0.46 ( 0.03
substrate: 3 days
dry solution 1 week
10 days
3 hours
wet solution 3 days
1 week
112 ( 4 2.4 ( 0.2
113 ( 2 2.4 ( 0.1
118 ( 5 2.6 ( 0.2
115 ( 3 2.6 ( 0.2
120 ( 5 7.2 ( 0.5
120 ( 5 9.6 ( 1.0
1.01 ( 0.07
0.97 ( 0.05
1.07 ( 0.08
3.40 ( 0.30
9.13 ( 0.45
14.51 ( 0.63
and parts c and d display the Si 2p binding energy region. The time intervals were selected to exceed the time required for monolayer formation, as judged by ellipsometry and contact angle measurements. The RMS roughness for each sample, along with ellipsometry and contact angle measurements, is given in Table 1. The ratio of the intensity of the Si(IV) peak at 103.3 eV to the Si(0) peak at 99.3 eV is 0.37 for clean native oxide, and the ratio of the carbon peak to the Si peak at 99.3 eV is 0.12. When the clean native oxide is immersed in a dry OTS solution, formation of a monolayer is complete after 2 d and is accompanied by a slight increase in the ratio of the intensity of the Si(IV) peak to the Si(0) peak (ratio
) 0.44) and an increase in the intensity of the carbon peak relative to the Si peak at 99 eV (ratio ) 0.26). The silicon peak intensity changes reflect the presence of additional Si(IV) atoms from the OTS molecules and also increased attenuation of electrons from the buried elemental silicon substrate by the overlying OTS monolayer. The carbon intensity change may include both a loss of weakly attached “adventitious” carbon and a gain from the hydrocarbon chains of the OTS molecules. These peak intensity changes are not altered by increased soaking time; the Si(IV)/Si(0) ratio for the three different soaking times is constant. Nor do the ellipsometric film thickness,
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Figure 4. AFM 3-D comparison of surface topography for dry and wet growth conditions. (a) Dry condition. (b) Wet condition.
contact angle, or RMS roughness change significantly even after more than 10 days of soaking time. When SiO2 is exposed to a wet OTS solution, the picture is quite different. Although ellipsometry and contact angle measurements indicate that monolayer formation is complete after just 2 h, longer soaking times result in continued deposition of material. After 3 h of soaking, the ellipsometric thickness is 2.6 ( 0.2 nm, consistent with monolayer formation, but after 3 days, the ellipsometric thickness increases to 7.2 ( 0.5 nm and after 7 days, to 9.6 ( 1.0 nm. Continued high contact angle values suggest that the deposited material is oriented so as to display the hydrocarbon tails of the OTS on the top of the surface. In the XPS, the ratio of the intensity of the Si(IV) peak at 103.3 eV to the Si(0) peak at 99.3 eV climbs from 0.42 at nominal monolayer coverage to 0.95 at 3 days and 1.36 at 7 days. The intensity of the Si (IV) peak is much larger than for monolayer coverage and, taken together with the ellipsometry results, indicates deposition of multiple layers of OTS. The carbon peak too increases in intensity relative to the underlying silicon(0) peak, from a value of 0.51 at nominal monolayer coverage to 2.2 after 3 days of soaking and 3.4 after 7 days of soaking. These intensity ratio changes result from both deposition of additional C and Si(IV) from OTS and from attenuation of the underlying
elemental silicon. Attenuation is expected to be quite strong for overlayers thicker than about 5 nm.23 The surface morphology for these samples was also investigated by AFM. Figure 4 contrasts the very smooth OTS monolayer produced after a 10 day exposure to dry OTS solution with the rough multilayer given by a 7 day exposure to wet OTS solution. The RMS roughness value for the rough multilayer is 14.5 Å, more than 10 times worse than the RMS roughness of the smooth monolayer produced under dry conditions. Monolayer formation under dry conditions requires 2 days, and continued exposure to the dry OTS solution does not deposit more material on the monolayer. On the multilayer samples prepared under wet conditions, AFM shows many small patches or islands of 20-25 Å height. This is the same type of surface morphology seen for partial and (nominally) full coverage samples prepared under wet conditions. Discussion Growth Mechanism. Previous papers have shown that OTS does not directly react with the silica surface OH (23) Gross, T.; Lippitz, A.; Unger, W.; Guttler, B. Surf. Interface Anal. 2000 29 (12), 891-894.
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Figure 5. Proposed reaction mechanisms for OTS growth under dry (a) and wet (b) conditions.
group and that a water film (one or several molecular layers) covering the silica is vital for the OTS to form a complete film.15,20 In our dry solution growth, primary nucleation is very slow, secondary nucleation does not compete with patch expansion, and the observed morphology of the patches of SAM is very smooth. We see no indication that preformed siloxane “islands” are involved under these conditions, as they clearly are in wet conditions. Not only is the concentration of water in the dry solution very low, but due to the hydrophobicity of the solvent Isopar-G used in this study, the thin layer of water molecules that is present on the hydrophilic SiO2 surface tends to stay there.15,25 Thus, only free molecules of OTS are available in the dry solution. During film growth, these OTS molecules prefer attachment sites next to OTS molecules that are already attached to the surface, which eventually fills in the gaps between SAM patches to form a complete monolayer, as shown in Figure 5a. The main indicators of this growth mechanism are the absence of secondary nucleation events and the fact that the thickness and RMS roughness of SAM patches (both measured by AFM) do not change during their growth from partial to complete coverage of the substrate. Bierbaum et al.17b observed very swift (minutes) nucleation of ragged SAM patches and continued secondary nucleation events from a dicyclohexane solution of OTS; since the humidity was 45%, these solutions may have contained enough “island aggregates” like the ones we observed in wet OTS solutions to account for the faster rate of nucleation events. In contrast to thiols on gold, which lie down at low coverage and stand up to form densely packed films at higher coverage, siloxanes on silicon dioxide form densely packed SAMs in which the OTS molecules are nearly upright even at low coverage.20,24 From our studies of the partially covered OTS film in the dry condition, we see the clear upright edge of the OTS patches. It is not clear how OTS molecules find their way to the edges of a SAM patch. One possibility would be that the hydrophobic OTS tail physisorbs to the hydrophobic SAM surface and skates (24) Schreiber, F. Prog. Surf. Sci. 2000, 65, 151. (25) Thiel, P. A.; Madey, E. Surf. Sci. Rep. 1987, 7, 211.
around until the polar OTS headgroup can encounter the SiO2 surface and be hydrolyzed by silanol groups in the SAM patch or on the SiO2 surface. Another possibility would be that the polar OTS headgroup is attracted to the hydrophilic SiO2 surface, is hydrolyzed by the water layer present there, and that the resulting OTS silanol slides around on the water layer until its hydrophobic tail is trapped by interactions with a nearby SAM patch. The OTS silanol could then be covalently trapped by further hydrolysis reactions with silanol groups in the SAM or on the SiO2 surface. Under wet conditions, OTS appears to hydrolyze and react with other OTS molecules within a matter of minutes to form large, flat aggregates. These aggregates adsorb on the surface of the substrate, where they may react with the surface water film or with surface silanol groups. Because the aggregates are extensively cross-linked before they hit the surface, they can be firmly attached to the surface by a few covalent bonds. This process continues until all the available surface area is covered by the OTS aggregates. The aggregates show considerable selforganization. They appear on the surface as patches approximately one SAM layer high and as large as 2 × 104 nm2. The hydrophobic tails of the OTS molecules are mostly facing up into solution, as indicated by the relatively high contact angles for partially covered and “monolayer” coverage surfaces. Because the aggregates are preformed in solution, they cover the surface quickly but are not able to form the type of smooth monolayer observed under dry conditions. Continued exposure to the wet solution does not result in “filling in” voids in the monolayer, but rather in continued film growth. Enough hydrophilic silanol groups remain exposed at the edges of the aggregates (or at defect sites on the surfaces of the aggregates) that the remaining bulky aggregates continually adsorb on the surface and are covalently bound, leading to a multilayer that steadily increases in thickness and roughness. In a dry environment where only the water layers absorbed on the substrate can react with individual OTS molecules, molecules attach on the surface one by one
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instead of by adsorption of aggregates containing hundreds or thousands of OTS molecules, as may happen in the wet condition. Once the OTS molecules are attached on to the surface, cross-links between OTS molecules form so the molecules are interconnected to form a complete film. Because the formation of the complete film is achieved by connecting each individual OTS molecule horizontally, the growth time required to have a complete monolayer is much larger than for the wet condition. As the soaking time increases there is no multilayer formation, because once all the available attachment sites are filled by OTS molecules, there is no way to put more free OTS molecules on the surface. Even if the OTS molecules can physisorb on the OTS monolayer, rinsing or sonicating can easily remove these molecules and leave the monolayer unchanged. In short, deposition from dry solution onto clean native oxide is a reproducible way to form an ultra-smooth OTS monolayer without access to a clean room. Relevance to Molecular Electronics Applications. Many molecular electronics applications require placement of individual molecules on surfaces in specific locations. There are several approaches to this difficult patterning task, but all require that the surface to which the molecules adhere be sufficiently smooth and flat that the molecule can later be distinguished from topological features of the surface. Some surfaces, like graphite, gold (111), silicon (100), and mica, can easily be prepared to display atomically flat terraces which have been very useful as viewing platforms for molecular adsorbates and SAMs.26 However, placing molecules in certain locations on these terraces requires a second processing step; either directed deposition (moving molecules by pushing them with an STM or AFM tip, dip-pen nanolithography27) or a lithography/deposition process (high-resolution electronbeam lithography or scanning probe modification of a resist film followed by deposition of molecules, constructive nanolithography, or “molecular lift off”28). In particular, siloxane SAMs can be used as resist materials that can be chemically processed by high-resolution electron-beam lithography or scanning probe modification and then used to direct the deposition of molecules from solution.29 The chemical patterning of siloxane films can already be done on a large size scale, for applications where the surface topography is not critical.30 However, for ultrahighresolution patterning, ultrasmooth siloxane SAMs, such as the ones reported here, are necessary. Conclusions Water content during OTS SAM deposition is crucial to film quality. Under wet conditions, OTS aggregates are first formed in solution due to hydrolysis. These islands then attach to the surface to form a rough overlayer, which renders the surface nonpolar but does not block deposition of additional OTS aggregates. This finding warrants caution in the use of contact angle measurements as a criterion for siloxane SAM formation. One of the char(26) (a) Fisher, A. J.; Blochl, P. E. Phys. Rev. Lett. 1993, 70, 3263. (b) Yao, X.; Ruskell, T. G.; Workman, R. K.; Sarid, D.; Chen, D. Surf. Sci. Lett. 1996, 366, 743. (c) Lu, X.; Hipps, K. W. J. Phys. Chem. B 1997, 101, 5391. (27) Piner, D.; Zhu, J.; Xu, F.; Hong, S.; Mirkin, C. A. Science 1999, 283, 661. (28) Hang, Q.; Wang, Y.; Lieberman, M.; Bernstein, G. H. Appl. Phys. Lett. 2002, 80, 4220. (29) (a) Lercel, M. J.; Tiberio, R. C.; Chapman, P. F.; Craighead, H. G.; Sheen, C. W.; Parikh, A. N.; Allara, D. L. J. Vac. Sci. Technol. B 1993, 11, 2823. (b) Yang, X. M.; Peters, R. D.; Kim, T. K.; Nealey, P. F.; Brandow, S. L.; Chen, M. S.; Shirey, L. M.; Dressick, W. J. Langmuir 2001, 17, 228. (30) Kumar, A.; Biebuyck, H. A.; Whitesides, G. M. Langmuir 1994, 10, 1498.
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acteristics of self-assembled monolayers is self-limiting growth; that is, once a monolayer of molecules has formed, it should block the deposition of further molecules on the substrate surface. The OTS films formed from wet solutions are not self-limiting and should not be thought of as true SAMs. Under dry conditions, the growth of OTS follows a patchexpansion mode to form an ultra-smooth (RMS roughness ∼1 Å) self-assembled monolayer that is suitable for use as an electron-beam resist or for other applications where angstrom differences in height matter. Experimental Section (1) Sample Preparation. Solvents, Chemicals, and Substrates. Octadecyltrichlorosilane (95%, Aldrich), Isopar-G (Exxon, a mixture of branched C9-C13 naphthenes, iso- and n-paraffins, and alkanes), CHCl3 (100%, J. T. Baker), carbon tetrachloride (99.9+%, HPLC grade, Aldrich), hexane (99.9%, Fisher), acetone (99.5%, Aldrich), and ethanol (absolute, Aaper) were commercially available and used as received. DI water, 18.2 MΩ cm milli-Q water, concentrated sulfuric acid (Fisher), H2O2 (30%, Fisher), detergent (Neutrad, Fisher Scientific), concentrated nitric acid, and activated aluminum oxide (Aldrich) were used as obtained. P-doped single-sided polished Si wafers (MEMC Electronic Materials, Inc.) of [100] orientation, 11.000-16.000 Ω‚cm resistivity, and 650 µm thickness were cut into 1 cm × 1 cm pieces with a diamond glass cutter before cleaning. All glassware and tweezers were rinsed with hexane, cleaned in a detergent bath (∼50 °C), and rinsed with DI water. The cleaned glassware was soaked in concentrated nitric acid for 24 h, rinsed with running DI water, washed again with detergent, and rinsed with milli-Q water. It was wrapped in aluminum foil and dried in an oven for 24 h. New glass vials (disposable scintillation vials, 20 mL, Kimble Glass, Inc.) and their caps were rinsed with running DI water, then cleaned together with the tweezers in boiling ethanol for at least 30 min in a 500 mL beaker. They were rinsed with DI water and blown dry with a strong Ar flow. The wafers were first cleaned 30 min in boiling ethanol in a clean beaker. Next the wafers were placed into individual clean vials and sonicated in CHCl3, acetone, and ethanol (a VWR AQUASONIC model 50T ultrasonicater was used): one vial per wafer, each solvent 2 or 3 times (fresh solvent each time), and each time around 5 to 10 min. The wafers were then rinsed in running DI water and blown dry with Ar. RCA cleaning procedures were carried out next; the wafers and vials were soaked for 15 min in RCA bath 1 (50:1:1 DI water:concentrated NH4OH:30% H2O2) and bath 2 (50:1:1 DI water:concentrated HCl:30% H2O2). The temperature was set at 70°C for both baths. Wafers were rinsed with DI water after each bath. The cleaned wafers were soaked in a 10:1 DI H2O: HF solution for 30 s to remove the original native silicon oxide. After that, the wafers and vials were blown dry with high purity N2. The vials were wrapped in aluminum foil and stored in an oven at T ) 150° for further use. A piranha solution (7:3 concentrated H2SO4:30% H2O2) was prepared in a cleaned vial and the wafers were soaked for 30 min at 90 °C to grow a fresh native oxide. They were rinsed with DI water and blown dry with Ar. Then these wafers were placed into individual clean vials and sonicated 3 × 5 min in CHCl3, acetone, and ethanol and milli-Q water, respectively. Finally the wafers were blown dry with a strong Ar flow and used as the substrate for the growth of OTS SAMs. SAM Growth. For the dry OTS solution, a chromatography column filled with activated aluminum oxide was used to dry all the solvents. (Filtration through activated alumina removes water to below 5 ppm levels. Pangborn A. B.; Giardello M. A.; Grubbs, R. H.; et al. Organometallics 1996, 15, 1518-1520.) CHCl3 was passed through the column until the column was wet. CHCl3 (1.5 mL), 1 mL CCl4, and 10 mL isopar-G were mixed, passed through the column, and collected in a cleaned vial. A disposable syringe (1 mL size) was used to add fresh OTS liquid to make the final OTS concentration 5 mM. Then the vial was tightly closed and the solvents and OTS were mixed in the sonicator for 5 s.
Growth of Ultrasmooth OTS SAMS on SiO2 The cleaned wafers were rinsed with milli-Q water and dried with Ar until no trace of water drops was visible. The wafers were placed in the OTS solution and the vial was closed. After being soaked for the desired time at room temperature, the wafers were removed and rinsed with copious CHCl3. After that the samples were sonicated in a clean vial in CHCl3, acetone, alcohol, DI water, and milli-Q water and finally dried with Ar. Samples were stored in a cleaned storage box in a desiccator for at least 12 h before they were analyzed. For the wet OTS solution, additional H2O was added into the OTS solution to make the total water concentration >50mM and mixed well by sonication. Wafers were added immediately and soaked for the desired time at RT. (2) Instrumentation. AFM Measurement. AFM was carried out using a Digital Instruments (Santa Barbara, CA) Multimode Nanoscope III instrument operating in tapping mode. The tips used were DI NanoSensors TappingMode Etched Silicon Probes, model TESP (force constant, 20-100N/m; resonant frequency, 200-400 kHz; tip radius curvature, 5-10 nm). The imaging setpoint was set for 1.5 V. Image analysis was performed offline using roughness and section commands provided in the AFM software. Contact Angle Measurement. All measurements of contact angles are advancing angles and were performed with a KRUSS G 10 contact angle measuring instrument. On each sample at
Langmuir, Vol. 19, No. 4, 2003 1167 least four different locations were measured and results were averaged. Ellipsometric Measurement. The thicknesses of OTS layers were obtained with an Rudolph AutoEl III ellipsometer. The measurement was performed using 632.8 nm He/Ne laser light incident upon the sample at 70°. Both a single-layer and doublelayer models were used to verify the validity of the thickness measurement. For each sample, the measurement was done at several places on the surface and the results were averaged. The refractive index of OTS was assumed to be 1.50 and of SiO2 was assumed to be 1.46. XPS Measurement. X-ray photoelectron spectroscopy (XPS) was done using a Kratos XSAM 800 with an Al KR X-ray source (1486.6 eV). The takeoff angle was fixed at 90°. The 1 cm × 1 cm SiO2 (or OTS on SiO2) samples were mounted on sample stubs with conductive carbon tape. All peaks were fitted with GaussLorentz peaks using the Kratos Vision II software to obtain peak area information. A linear base line was used in the fitting processes.
Acknowledgment. We are grateful for support from the DARPA Moletronics program, N00014-99-0472 LA020697X
