Generic Substrate for the Surface Forces Apparatus: Deposition and

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Generic Substrate for the Surface Forces Apparatus: Deposition and Characterization of Silicon Nitride Surfaces Yuval Golan*,† Department of Materials Engineering, Ben-Gurion University of the Negev, Beer Sheva 84105, Israel

Norma A. Alcantar, Tonya L. Kuhl, and Jacob Israelachvili*,‡ Department of Chemical Engineering, and Materials Department, University of California, Santa Barbara, California 93106 Received January 28, 2000. In Final Form: June 1, 2000 We introduce a novel substrate for direct normal and lateral force measurements in the surface forces apparatus (SFA). While most SFA experiments are typically carried out using molecularly smooth mica surfaces, the new layered structure allows the use of just about any material which can be deposited as a thin film using evaporation or chemical vapor deposition. In this work, we demonstrate the use of this method for preparing plasma enhanced chemical vapor deposited silicon nitride substrates. Chemical, structural, optical and mechanical characterization of the substrates was carried out using secondary ion mass spectrometry, atomic force microscopy, transmission electron microscopy, ellipsometry, multiple beam interferometry and SFA force measurements. The latter showed an extremely low adhesion energy (γ < 1 µJ/m2) between silicon nitride surfaces prepared using this method, which is attributed to the surface roughness (ca. 2.2 nm rms).

Introduction The surface forces apparatus (SFA) has emerged as an important technique for measuring normal1 and lateral2 (friction) forces in air and in liquids. The technique traditionally employs mica substrates, which can be readily cleaved to produce molecularly smooth surfaces. Recently, there has been an increased effort to expand the SFA technique to materials other than mica. This is due to the interest in measuring a wider range of interactions across surfaces of greater technological interest such as metals, ceramics and polymers, namely, “real” or “engineering” surfaces rather than the model surfaces studied so far. In addition, the preparation and mounting of mica substrates for the SFA requires considerable effort and skill compared to, e.g., tips for the atomic force microscope (AFM), and the large-scale production of readyto-use substrates could significantly increase the accessibility of the SFA technique. Several SFA studies were previously carried out with surfaces other than mica, including noble metals,3-6 polymers,7 silica,8,9,10 alumina11,12 and iron oxide.13 In many of these studies, the films were deposited on top of * Corresponding authors. † E-mail: [email protected]. Fax: 972-7-6472944. Telephone: 972-7-6461474. ‡ E-mail: [email protected]. Fax: 1-805-8937870. Telephone: 1-805-8938407. (1) Israelachvili, J. N.; Adams, G. E. J. Chem. Soc., Faraday Trans. 1 1978, 74, 975. (2) Homola, A. M.; Israelachvili, J.; Gee, M. L.; McGuiggan, P. M. J. Tribology 1989, 111, 675. (3) Smith, C. P.; Maeda, M.; Atanasoska, L.; White, H. S.; McClure, D. J. J. Phys. Chem. 1988, 92, 199. (4) Levins, J. M.; Vanderlick, T. K. J. Phys. Chem. 1995, 99, 5067. (5) Levins, J. M.; Vanderlick, T. K. J. Colloid Interface Sci. 1997, 185, 449. (6) Knarr, R. F.; Quon, R. A.; Vanderlick, T. K. Langmuir 1998, 14, 6414.

conventionally prepared mica surfaces which obviously complicated rather than simplified the substrate preparation process. Moreover, the use and interpretation of multiple beam interferometry (MBI) becomes much more difficult when multilayered substrates are used. In this work, we present a layered structure for generic substrates prepared from almost any material which can be deposited as a thin film using evaporation or chemical vapor deposition, such as ceramics (alumina, silica, silicon nitride), semiconductors (CdS, PbTe, ZnSe), and polymers (via e.g. spin-coating). We demonstrate the use of this process for preparing amorphous silicon nitride (Si3N4) substrates, a hard material which has a wide range of applications such as antireflection coatings for solar cells,14 electronic devices,15,16 transistor dielectrics,17,18 protective layers17 and tribological coatings.19-21 (7) Luengo, G.; Pan, J.-M.; Heuberger, M.; Israelachvili J. Langmuir 1998, 14, 3873. (8) Horn, R. G.; Smith, D. T.; Haller, W. Chem. Phys. Lett. 1989, 162, 404. (9) Vigil, G.; Xu, Z.; Steinberg, S.; Israelachvili, J. N. J. Colloid Interface Sci. 1994, 165, 367. (10) Drake, J. M. Personal communication. (11) Horn, R. G.; Clarke, D. R.; Clarkson, M. T. J. Mater. Res. 1988, 3, 413. (12) Berman, A.; Steinberg, S.; Campbell, S.; Ulman, A.; Israelachvili, J. N. Tribology Lett. 1998, 4, 43. (13) Campbell, S.; Pan, J.-M.; Steinberg, S.; Israelachvili, J. N. To be published. (14) Kishore, R.; Singh, S. N.; Das, B. K. Sol. Energy Mater. Sol. Cells 1992, 26, 27. (15) Bencher, C.; Ngai, C.; Roman, B.; Lian, S.; Vuong, T. Solid State Technol. 1997, 3, 109. (16) Doshi, P.; Jellison, G. E., Jr.; Rohatgi, A. Applied Optics 1997, 36, 7826. (17) Lu, Z.; He, S. S.; Ma, Y.; Lucovsky, G. J. Non-Crystalline Solids 1995, 187, 340. (18) Quinn, L. J.; Mitchell, S. J. N.; Armstrong, B. M.; Gamble, H. S. J. Non-Crystalline Solids 1995, 187, 347. (19) Cerny, F.; Suchanek, J.; Hnatowicz, V. Thin Solid Films 1998, 317, 490.

10.1021/la000125h CCC: $19.00 © 2000 American Chemical Society Published on Web 07/22/2000

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Experimental Section Electron Beam Evaporation. Curved silica disks were solvent cleaned with chloroform and ethanol, dried, and exposed to a short UV-ozone treatment prior to mounting in an evaporation chamber which was pumped to a base pressure of ca. 10-6 Torr. A thin, 1.7 nm thick Cr precoat layer was first evaporated onto the silica disk, after which, the standard 50 nm thick reflective silver layer was deposited (deposition rate: 0.15 nm/ s). Another 1.7 nm thick Cr layer was then deposited to aid the adhesion between the subsequent layer and the silver. It is important to note that both Cr adhesion layers were critical for ensuring good mechanical stability of the layered substrates. Plasma Enhanced Chemical Vapor Deposition (PECVD). PECVD of Si3N4 was carried out in a VII 790 series vertical showerhead plasma reactor (Plasma-Therm LR Inc.). This technique was selected over other deposition methods as it allows relatively low deposition temperatures, and produces high purity films with better stoichiometric control.22,23 Following a standard cleaning cycle, the substrates were mounted on the lower plate and heated to 100 °C. A continuous, 0.25 µm thick Si3N4 film was then deposited using 160 sccm of SiH4, 450 sccm of N2 and 1.45 sccm of NH3. The temperature was raised to 350 °C at 1 °C/min and a subsequent 1.75 µm thick layer of Si3N4 was deposited using similar gas flows (silane flow rate was decreased to 150 sccm). The base pressure and RF power were 900 mTorr and 20 W, respectively, and the deposition rates were ca. 10 to 11.5 nm/min for the low and high deposition temperatures, respectively. Atomic Force Microscopy (AFM). AFM imaging was carried out in air with a Digital Instruments Dimension-3000 AFM operating in tapping mode. Images were obtained in the height and amplitude modes simultaneously using single cantilever Si tips and a scan rate of 1 Hz. Transmission Electron Microscopy (TEM). TEM and electron diffraction were carried out with a JEOL 2010 TEM operating at 200 kV. Samples were prepared by mounting freshly cleaved, thin (≈80 nm) mica sheets onto 400 mesh Cu TEM grids. The grids were placed in the PECVD chamber and 40 nm of Si3N4 were subsequently deposited at 350 °C as described above. Secondary Ion Mass Spectrometry (SIMS). SIMS was performed using a Physical Electronics 6650 Quadrupole DSIMS. A 70 µm Cs+ primary ion beamspot (3 keV) was rastered at 60° off-normal to form 0.5 mm × 0.5 mm craters. Both positive and negative secondary ions were monitored, and data were accepted from the center 5% of the crater area. Charge neutralization was accomplished using a static, defocused, 700 eV electron beam. Surface Forces Apparatus (SFA). Friction and swelling experiments were carried out using an SFA-3 surface forces apparatus modified for friction experiments.2 The upper and lower surfaces were mounted on a friction-sensing device equipped with semiconductor strain gauges, and a piezoelectric bimorph slider, respectively. Compression experiments were performed using an SFA MK-2. The refractive index of the silicon nitride films was independently measured using ellipsometry. Using these values, distance calculations were carried out with the equations normally used in the SFA.24 Since the fringes were somewhat more diffuse compared to conventional mica surfaces, the resolution of the wavelength measurement is approximately 0.3 nm, resulting in a corresponding reduction in distance measurement resolution. The effect of the ultrathin Cr adhesion layers was neglected due to their extremely low effective absorbance and reflectance. Materials. Tetradecane (Aldrich, 99+%) was filtered using pre-rinsed Acrodisc 0.2 µm PTFE filters (Gelman Sciences, MI). Analytical grade chloroform (Fischer) and 200 proof, dehydrated ethanol (Quantum Chemical Co., Tuscola, IL) were used without further purification. (20) Tomizawa, H.; Fischer, T. E. ASLE Transactions 1986, 30, 41. (21) Gates, R. S.; Hsu, S. M. Special publication #876; National Institute of Standards and Technology: Gaithersburg, MD, 1995. (22) Smith, D. L.; Alimonda, A. S.; Chen, C.-C.; Ready, S. E.; Wacker, B. J. Electrochem. Soc. 1990, 137, 614. (23) Smith, D. L J. Vac. Sci. Technol. A 1993, 11, 1843. (24) Israelachvili, J. N. J. Colloid Interface Sci. 1973, 44, 259.

Figure 1. (a) Cross section of a pair of layered substrates for the SFA. The curved silica disks are coated with a reflective Ag film, followed by a thin, continuous silica or silicon nitride layer deposited at low temperature in order to protect the Ag mirrors. A high-quality Si3N4 layer (or any other material deposited by evaporation or chemical vapor deposition) is then deposited at higher temperature to form the substrate surface. Note that in this case, the Ag mirror is deposited on a stiff, nondeforming silica substrate. (b) Cross section of a mica substrate traditionally used in the SFA. In this case, the Ag mirror is mounted on a relatively soft, conforming glue layer.

Results and Discussion Deposition of Si3N4 Substrates for the SFA. One of the main problems that hamper the chemical vapor deposition of high-quality substrates (suitable for SFA studies) directly on Ag-coated surfaces is that deposition at high-temperature (HT) is required for high-quality films; however, elevated temperatures are detrimental for the optically reflecting Ag mirror layers beneath. To overcome this problem, curved silica disks were coated with a reflective Ag film followed by a thin, continuous “protective” layer of silica or Si3N4 deposited at low temperature (LT). A high-quality Si3N4 layer may then be deposited at HT to form the substrate surface, as shown in Figure 1a. In these substrates, the Ag mirror is deposited on a stiff, nondeforming silica substrate that is different from the conventional mica substrates, where the Ag mirror is mounted on a soft, conforming glue layer (Figure 1b). As a result, the optical technique of the SFA, which is highly sensitive to topographic features when conventional mica surfaces are used,25,26 is much less sensitive (25) For a detailed analysis of the effect of surface roughness in the optical technique of the SFA, see: Heuberger, M.; Luengo, G.; Israelachvili, J. Langmuir 1997, 13, 3839.

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Figure 2. Schematic showing the relationship between the morphology of the contact and the subsequent shape profile of the fringes of equal chromatic order (FECO) seen in the SFA. (a) Layered Si3N4 substrates. The “stiff” mirror is much less sensitive to topographic features. (b) Clean mica surfaces in flattened contact. The Ag mirrors are mounted on a conforming glue layer so that surface features and deformations are readily seen in the fringes, as shown in part c.

Figure 3. Fringes of equal chromatic order (FECO) as observed in-situ during experiments with two Si3N4 surfaces separated by a thin film of tetradecane.

to topography in the case of the more rigid Si3N4 layered substrates. Figure 2a shows a cross-section of two Si3N4 surfaces in contact (bottom), and the typical shape of the resulting fringes of equal chromatic order (FECO) obtained using the optical technique of the SFA. Despite the considerable roughness of the surfaces (see below), there is no apparent deformation in the fringe shape besides a somewhat more diffuse fringe profile. Conversely, the FECO obtained with conventional mica surfaces are much less diffuse (Figure 2b) and readily deform when, e.g., a particle is present between the surfaces, as shown in Figure 2c. The actual FECO obtained in-situ during SFA experiments with Si3N4 surfaces separated by a thin film of tetradecane are shown in Figure 3. Upon addition of a water droplet, the surfaces were immediately wetted (the contact angle was very small), ruling out notable contamination of the surfaces. Normal force profiles of these surfaces in both pure, dry nitrogen and tetradecane were monotonically repulsive from a separation distance of ca. 150 nm, indicative of their nonadhesive nature within the experimental normal force detection limit, which was better than 10 µN/m using a normal force-measuring spring of stiffness k ) 3 × 105 mN/m. This corresponds to an adhesion energy of γ ) F/4πR e 1 µJ/m2. Comparing (26) Golan, Y.; Drummond, C.; Tenne, R.; Israelachvili, J. Wear, in press.

Figure 4. AFM images of PECVD Si3N4 surfaces imaged while still mounted on the curved SFA disks: (a) 0.5 µm scan, z-scale ) 24 nm; (b) 6.0 µm scan, z-scale ) 36 nm.

this value to a typical van der Waals energy of, e.g., 50 mJ/m2 indicates a dramatic decrease in adhesion by 4 orders of magnitude, which is attributed to the surface roughness described below.27,28 Morphology. The Si3N4 films were essentially featureless in TEM imaging, with no apparent grains or domains seen in the micrographs (not shown). Transmission electron diffraction confirmed the amorphous nature of the films, and showed a diffuse, broad “amorphous” ring with a d spacing centered at ca. 0.135 nm, which was calibrated according to crystalline mica. The surface topography was characterized using tapping mode AFM. Images of unlubricated Si3N4 surfaces are shown in Figure 4a and 4b, with scan sizes of 0.5 and 6.0 µm, respectively. A characteristic mound-shaped topography is observed, with an rms roughness, Rrms, of 2.2 nm (Figure 4a) and 4.4 nm (Figure 4b).25,29,30 Substrates with varying roughness open new possibilities for the study of surfaces of (27) The absence of a measurable adhesion force suggests an adhesion parameter of at least 1.6, as defined in Fuller, K. N. G.; Tabor, D. Proc. R. Soc. London A 1975, 345, 327. (28) For an extension of the JKR theory to the layered structure used in the SFA, see: Sridhar, I.; Johnson, K. L.; Fleck, N. A., J. Phys. D: Appl. Phys. 1997, 30, 1710.

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Figure 6. SIMS depth profile obtained with positive Cs ions for (a) layered PECVD Si3N4 and (b) a high-purity Si3N4 standard. Figure 5. Secondary ion mass spectrum (SIMS) obtained with negative Cs ions for (a) layered PECVD Si3N4 and (b) a highpurity Si3N4 standard.

concrete technological interest; to the best of our knowledge, there have been very few adhesion studies4,5 and essentially no systematic friction studies of rough surfaces using the SFA. Chemistry. Secondary ion mass spectrometry (SIMS) was used to compare the chemical composition of the PECVD layers with a high purity, polycrystalline Si3N4 standard sample. The mass spectra obtained from the PECVD and the reference Si3N4 samples are shown in Figure 5a and b, respectively. While the peaks at m/q ) 28 and m/q ) 42 (SiN) are of similar magnitude for both samples, the PECVD sample showed lower oxygen and carbon levels and was higher in hydrogen. Since at least part of the carbon is present in the form of adsorbed hydrocarbons, the level of hydrogen seen in the PECVD sample is probably a lower limit of the hydrogen content in this sample. The relatively high levels of hydrogen in PECVD Si3N4 originate from the silane and ammonia precursors which decompose in a sequence of reactions to release atomic hydrogen which is subsequently incorporated into the Si3N4 matrix.22,23,31 It is important to mention (29) The root-mean-square roughness is defined as Rrms ) {[∑(Zi Zave)2]/N}1/2, where Zave is the average z value in the image, Zi is the z value for the ith pixel, and N is the total number of pixels in the image. Note that Rrms depends on N and may vary for different image sizes obtained from the same surface. (30) PECVD-deposited films with rougher surfaces can be obtained primarily by lowering the deposition temperature, as well as by varying other deposition parameters. Nevertheless, it should be kept in mind that other material properties can be affected at the same time (e.g., porosity, stoichiometry). (31) Morello, G. J. Non-Cryst. Solids 1995, 187, 308.

that Ag impurities were not seen within the detection limit of our measurements. In addition, it is well-known that under ambient conditions, the surface of Si3N4 is covered with a thin oxide, or oxynitride layer.32 Although we have not been successful in quantitatively determining its thickness using SIMS, the presence of the native oxide layer in our samples was clearly evident. SIMS analysis of Si3N4 is complicated by the difficulty to discriminate between the Si+ (m/q ) 27.9769) and N2+ (m/q ) 28.0056) ions. To determine the stoichiometry of the PECVD substrates, we have used the complex ions formed upon interaction with the Cs+ ion beam, CsN+ and CsSi+, assuming that the degree of formation of the complex ion CsN2+ is negligible. The steady-state SIMS depth profiles are shown in Figure 6a and 6b for PECVD Si3N4 and the Si3N4 standard, respectively. By comparing the CsSi+/CsN+ ratio in both samples, the PECVD material appears to be somewhat Si rich, with a stoichiometry of Si3N3.86. Mechanical Properties. One of the unique capabilities of the SFA is the accurate and straightforward measurement of the compression modulus of ultrathin films.33,34 We have used an SFA-Mk2 in order to evaluate the compression modulus of the Si3N4 layered substrates. The onset of interaction in the normal force profile was determined as the zero distance point in the compression curves shown in Figure 7, and the Si3N4 layered structures were compressed with respect to this point using the coarse motor control of the SFA.2 The response was monitored by the optical technique of the SFA.24 The applied force (32) Zhmud, B. V.; Sonnefeld, J.; Bergstrom, L. Colloids Surf., A 1999, 158, 327 (and references therein). (33) Chen, Y. L.; Helm, C. A.; Israelachvili, J. N. Langmuir 1991, 7, 2694. (34) Hu, H.-W.; Granick, S. Science 1992, 258, 1339.

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Figure 8. Apparent increase in thickness due to swelling, as determined using multiple beam interferometry in the SFA, versus time elapsed from injection of tetradecane between the layered Si3N4 substrates. The slope of the linear region shown (up to approximately 40 h) corresponds to an apparent swelling rate of ca. 2 nm/h.

Figure 7. Force versus distance plots obtained in the SFA upon compression of layered Si3N4 surfaces in tetradecane. (a) The complete range tested (0 to -500 nm), showing mechanical failure at F/R ≈ 3.8 N/m. Inset: Mechanical damage, as seen in the fringes of equal chromatic order (FECO). (b) The region between 0 and -100 nm. Showing a slope corresponding to a compression modulus of 6.6 GPa for the LT Si3N4. (c) The region between -100 and -350 nm, showing a slope corresponding to a compression modulus of 13.3 GPa for the HT Si3N4.

is plotted as a function of distance in Figure 7a; note that negative distances represent compression of the substrates. At a distance of -390 nm (corresponding to a force of ca. 3.9 N/m), clear evidence of mechanical failure is seen, which is also evident in the FECO obtained insitu as shown in the inset. An important feature in Figure 7a is the different slope obtained in the region from 0 to -100 nm, followed by a considerably higher slope, which continues to a point where mechanical damage is evident. It is reasonable to assign the lower slope to the compression of the softer, more porous LT layer, and the higher slope to the compression of the denser HT layer. Parts b and c of Figure 7 show the linear fits to the two regions, which correspond to compression moduli of 6.6 GPa (LT Si3N4) and 13.3 GPa (HT Si3N4), respectively. We note that Young’s modulus for polycrystalline silicon nitride is

substantially higher, 240-330 GPa.35,36 These values compare to the range of compression moduli of 4-40 GPa for the mica/glue substrate, and ca. 12 GPa for vapor deposited silica substrates.10 When dealing with porous substrates for the SFA, it is important to evaluate the physical and optical consequences of solvent penetration into the medium. Moreover, both swelling and change in refractive index are expected to occur simultaneously, which notably complicates the evaluation. We have followed this process by injecting a droplet of tetradecane between two Si3N4 surfaces and then bringing them into contact. Using the optical technique of the SFA, the position of the FECO fringes was monitored as a function of time as shown in Figure 8. The slope of the linear region shown (up to approximately 40 h) corresponds to an apparent volumeswelling rate of ca. 2 nm/h. However, since the refractive index of the film continuously changes due to penetration of tetradecane, it was necessary to use an independent technique for measuring the refractive index of Si3N4 substrates before and after soaking them in tetradecane. Using variable angle spectroscopic ellipsometry, the refractive indices at λ ) 550 nm were 2.256 for the HT dry Si3N4 substrates and 2.085 for the substrates which were soaked in tetradecane for 72 h. For the case of two identical substrates in contact, we have the simple case of a onelayer interferometer of thickness 2Y, which is described by the equation24

4µY ) nλ, n ) 1, 2, 3, ...

(1)

where λ is the fringe wavelength, n is the fringe order and µ is the mean refractive index of the substrate material. Since µ was decreasing with increasing tetradecane content, it is evident that the curve shown in Figure 8 is in fact a lower limit for the amount of swelling. However, soaking the substrates in the liquid for several days prior to use is a good way to overcome the swelling complication, and ensures optical and physical stability throughout the measurements. (35) Komeya, K.; Matusi, M. In Materials Science and Technology; Cahn, R. W., Haasen, P., Kramer, E. J., Eds.; VCH: 1994; Vol. 11, Chapter 10, p 534. (36) Due to the sphere-on-a-flat geometry used for measuring the compression modulus in the SFA, a small correction factor is necessary in order to accurately compare our results with the literature values for Young’s modulus. In this case, this correction can be neglected due to the very large difference in the elastic moduli.

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microscope. Although the surfaces appeared stable while separated, it was observed that the substrates immediately shattered as soon as the surfaces were brought into contact, thus ruling out the application of these substrates in aqueous conditions.

Figure 9. Friction traces obtained using an SFA-3 modified for friction measurements with Si3N4 surfaces lubricated with tetradecane. (a) Triangular wave voltage supplied to the bimorph slider, which corresponds to the displacement of the driver for laterally moving the lower surface in the SFA. (b) The corresponding friction trace. Note the fine structure in the traces which is repeatedly reproduced from cycle to cycle.

Friction. The layered substrates were first used to study the microtribology of Si3N4 surfaces. Although the complete study will be published elsewhere,37 it is noteworthy to point out the sensitivity of the friction sensing device to the topography of the surface. Friction traces obtained with Si3N4 surfaces lubricated with tetradecane are shown in Figure 9. Figure 9a shows the triangular wave voltage supplied to the bimorph slider, which corresponds to the displacement of the driver for laterally moving the lower surface in the SFA, and the corresponding friction trace is shown to the right in Figure 9b. Interestingly, the fine structure in the traces is repeatedly reproduced from cycle to cycle, and represents a convoluted pattern of the tribological topography of the two surfaces. A final note on the incompatibility of the Si3N4 substrates in aqueous conditions: Due to the smaller size of the water molecule, larger amount of water can penetrate into the amorphous matrix and hence increase the strain in the layered structure. In controlled experiments, a water droplet was injected between two dry Si3N4 surfaces, which were carefully adjoined under observation using the SFA (37) Golan, Y.; et al. In preparation.

Summary and Conclusions We have introduced an alternative generic substrate which can replace mica and expand the number of surfaces that can be studied in the SFA to a large variety of materials. A layered structure was designed in order to protect the Ag mirrors during the deposition. This was achieved by depositing a continuous protective film at LT directly onto the mirrors, followed by a high quality HT layer. We have demonstrate the use of this structure for constructing PECVD-deposited Si3N4 substrates. TEM confirmed the amorphous nature of the surfaces. SIMS showed that the PECVD Si3N4 was rich in hydrogen, and low in oxygen and carbon compared to a high-purity stoichiometric standard, with a Si to N ratio of 3:3.86. AFM showed a mound-like topography, with an rms roughness ca. 3 nm. The compression modulus was 6.6 and 13.3 GPa for the LT and HT layers, respectively. The surfaces were shown to damage at about 3.9 N/m, as well as immediately upon contact in water. Nonaqueous liquid penetration into the porous matrix can cause physical and optical instabilities, which were successfully overcome by soaking the substrates in the liquid prior to use. In addition to the wider range of interactions available for study using this technique, the new substrates will allow us to use the SFA for the study of materials properties such as compressibility, porosity, swelling, and the effect of surface roughness. Acknowledgment. We thank Dr. Tom Mates for expert help with the SIMS measurements, and Dr. Carlos Drummond for helpful discussions. We also thank Professor David Clarke for providing the Si3N4 standard and for useful discussions. Y.G. thanks INTEL Israel for a research award (Supervisor: Dr. Andy Sharon, INTEL Lachish). This work was supported by the MRSEC Program of the National Science Foundation under award DMR-9123048. LA000125H