Langmuir 1990, 6, 621-627
621
Effects of Chemical Treatments on S i c Surface Composition and Subsequent MoS2 Film Growth Stephen V. Didziulis' and Paul D. Fleischauer Chemistry and Physics Laboratory, The Aerospace Corporation, El Segundo, California 90245 Received June 15,1989. I n Final Form: October 16,1989 The surface composition of hot-pressed silicon carbide (Sic) samples and the growth of lubricating molybdenum disulfide (MoS,) films on these surfaces are examined as a function of surface treatment. Sic samples polished in air exhibit significantly less surface silicon oxide (approximately one monolayer) relative to unpolished samples, as determined by X-ray photoelectron spectroscopy. The surface oxide and surface carbon contamination are both reduced by etching with aqueous HF under a nitrogen atmosphere. Annealing the sample to 350 "C under vacuum removes still more surface carbon contamination, particularly C-0 species. The crystallographic orientation of radio frequency sputter-deposited MoS, films on the Sic substrates at 220 "C is strongly dependent on the amount of surface contamination. As the contamination concentration decreases, more MoS, crystallites grow with basal planes oriented parallel to the sample surface, as detected by X-ray diffraction. These results support an active site model for the growth of MoS, films with basal planes oriented perpendicular to the surface and suggest that the active site on Sic is a chemisorbed carbon contaminant.
Introduction Silicon-based ceramics show great promise as lightweight structural materials because of their high hardnesses and high melting points. The inclusion of silicon carbide (Sic) or silicon nitride (Si,N,) components in most mechanical systems requires that the ceramic surfaces be lubricated effectively. The effects of surface chemistry on lubricant performance for ceramics have received minimal attenti~n.'-~In this paper, the effects of chemical treatments on the surface composition of hotpressed silicon carbide engineering samples are studied, and the interactions of the resulting chemically different surfaces with lubricating MoS, (molybdenum disulfide) films are examined. Previous surface chemistry studies of Sic in the form of single crystals and polycrystalline material have focused primarily on its s e m i c ~ n d u c t i n gand ~ ~tribological+l2 properties. Sic grows in cubic zinc blende (p) and hexagonal wurtzite (a)structures, with many different polytypes of the a phase existing. In all of these structures, the silicon and carbon atoms are tetrahedrally coordinated to their nearest neighbors. Tribologi~ts'~ often assume that samples undergoing no treatment before use are covered with an oxide layer thick enough to treat the material surface as silicon dioxide (SiO,). Therefore, the thermal oxidation of Sic has been the subject of some detailed research.14-16 Changes in surface composition that occur (1) Miyoshi, K.; Buckley, D. H. ASLE Trans. 1983,26,53. (2) Johanmir, S.; Fischer, T. E. STLE Trans. 1986, 31, 32. (3) Cranmer, D. C. Tribol. Trans. 1988, 31, 164. (4) Wheeler, D. R.; Pepper, S. V. Surf. Interface Anal. 1987,10, 153. ( 5 ) Kaplan, R. J. Appl. Phys. 1984, 56, 1636. (6) Dayan, M. J. Vac. Sci. Technol. A 1986, 4, 38. (7) Porte, L. J. Appl. Phys. 1986, 60, 635. (8) Bermudez, V. J. Appl. Phys. 1988,63, 4951. (9) Miyoshi, K.; Buckley, D. H. ASLE Trans. 1979,22, 245. (10) Miyoshi, K.; Buckley, D. H.; Srinivasan, M. Ceram. Bull. 1983, 62, 485. (11) Miyoshi, K.; Buckley, D. H. ASLE Trans 1979,22,146. (12) Miyoshi, K.; Buckley, D. H. Appl. Surf. Sci. 1982, 10, 357. (13)Rahaman, M. N. DeJonghe, L. C. Am. Ceram. SOC.Bull. 1987, 66, 782. (14) Costello, J. A.; Tressler, R. E. J. Am. Ceram. SOC.1981, 64, 327. (15) Lu, W.-J.; Steckl, A. J.; Chow, T. P.; Katz, W. J. Electrochem. SOC.1984, 131, 1907.
0743-7463/90/2406-0621$02.50/0
with polishing and chemical etching have not been studied; polishing and chemical etching are incorporated into this study as preparative steps before MoS, film deposition. Molybdenum disulfide (MoS), is a widely used solid lubricant that is particularly effective in inert or vacuum environments." MoS, is a layered compound. Each layer consists of a plane of molybdenum bound to two layers of sulfur atoms in a sandwich structure. Strong Mo-S bonding exists within each layered sandwich, but bonding is weak between the sulfur atoms in the basal planes of adjacent sandwiches. The structure produces highly inert (0001) basal planes and reactive edge planes containing dangling bonds. The layered structure is also responsible for the lubricating properties of MoS,, because the inert basal planes can slip easily over one another. It is theorized, therefore, that the most effective lubrication with MoS, would result from crystallites strongly adhered to the substrate surface with their basal planes oriented parallel to the surface. I t has been proposed that surface active sites on substrates interact strongly with the reactive edge planes of MoS, crystallites, forcing the radio frequency (rf)-sputtered films to grow with their basal planes perpendicular to the surface.Is An absence of active sites would allow the film to grow with its crystallite basal planes parallel to the surface, in the lowest energy configuration for the film. This theory will be tested in this work by varying the Sic surface composition and examining the orientation of the crystallites in the MoS, film. This work represents the first study of MoS, films on Sic surfaces. The Sic substrates were polished to a uniform sample roughness and subjected to a variety of chemical treatments, including methanol washes and dilute hydrofluoric acid (HF) and concentrated nitric acid ("0,) etches. The surface composition was studied with X-ray photoelectron spectroscopy (XPS) to determine the relative concentrations and chemical states of the Si, (16) Muehlhoff, L.; Bozack, M. J.; Choyke, W. J.; Yates, J. T., Jr. J. Appl. Phys. 1986,60,2558. (17) Winer. W. 0. Wear 1967. 10. 422. (18) Bertrand, P. A. J.Mater. Res: 1989,4,180. Bertrand,P. A.Langmuir 1989, 5 , 1387.
0 1990 American Chemical Society
Didziulis a n d Fleischauer
622 Langmuir, Vol. 6, No. 3, 1990
C, a n d a n y impurity atoms. MoS, films were t h e n rfsputter-deposited o n t o the S i c substrates. The crystallinity and orientation of the films were examined with X-ray diffraction (XRD), and film microstructures were
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studied with scanning electron microscopy (SEM). T h e
results of these studies a r e compared t o previous studies of MoS, films deposited o n stainless steel a n d silicon substrates. Experimental Section Polycrystalline, hot-pressed S i c coupons manufactured by ESK Engineered Ceramics were used in this study. These materials were fully hot isostatically pressed, with 0.5% A1,0, used as a sintering agent. The S i c crystallite diameter in the material ranged from approximately 1to 4 pm. The dominant polytypes of a-Sic detected by XRD were 4H and 6H, with possible contributions from the 2H and the cubic 3C structures. Unless otherwise stated, all samples were polished in air with diamond paste down to 0.25-pm particle size to ensure uniform surface roughness. After being degreased with acetone and methanol in an ultrasonic cleaner, the samples were chemically treated under dry nitrogen atmospheres in glovebags. The samples were subjected to combinations of chemical treatments, including anhydrous methanol rinses; brief (approximately 1s) etches in a buffered H F solution composed of HF, NH,F, and distilled, deionized water in a 1:10:20 ratio followed by methanol rinses; and 1-s etches in concentrated nitric acid followed by methanol rinses. The glovebags were attached directly to the load locks of the X-ray photoelectron spectrometer vacuum systems, and samples were introduced into vacuum without exposure to air after the etches. Surface analysis was performed with two different XPS spectrometers. High-resolution work was done with a Surface Science Instruments (SSI) SSX-100 spectrometer in an ionpumped ultra-high-vacuum chamber with a base pressure of Torr. The spectrometer uses monochromless than 1 X atized A1 K a X rays and a high-throughput, spherical-sector electron energy analyzer with a 30" angle of acceptance and a multichannel detector. The data reported here were obtained with a 300-pm spot size and a 50-eV analyzer pass energy. These conditions yield a gold 4f7/, peak from a gold foil sample with a full width at half-maximum (fwhm) of 0.90 eV at a binding energy of 84.0 eV, relative to the spectrometer Fermi level. All S i c binding energies are reported relative to the spectrometer Fermi level; no sample charging occurred in the semiconducting samples. Unless otherwise reported, all spectra were obtained with the sample normal a t an angle of 60" relative to the central axis of the electron energy analyzer (a takeoff angle of 30") to enhance surface sensitivity. The surface compositions of S i c samples used as substrates for MoS, film deposition were examined with a MacPherson ESCA 36 XPS spectrometer. This spectrometer (described el~ewhere'~) was used because of ease in sample-handling capability for these large (10 x 10 X 5 mm) samples relative to the SSI spectrometer. The fwhm of the gold 4f,/, peak obtained with the MacPherson spectrometer is 1.4 eV. After surface treatment under N, and analysis with the MacPherson, samples were removed from the vacuum system under N,, placed in a desiccator, and transported to the rf sputter deposition chamber for MoS, film deposition. During transfer of the samples to the deposition system, a strong nitrogen purge was maintained, although the samples almost certainly were exposed to air for brief periods. MoS films were deposited in a system described elsewhere.2'*21 It should be noted, however, that the chamber base Torr. Samples were pumped down pressure was only 1 x overnight, and the MoS, target was conditioned for 100 min before film deposition. Films were deposited at two different
BINDING ENERGY (eV)
Figure 1. Si 2p XPS peaks of (a) unpolished S i c taken with the analyzer a t 10" relative to the surface normal, (b) unpolished S i c a t 60°, (c) polished S i c at lo", and (d) polished S i c a t 60". Fits to the 60" data in b and d are also shown.
sample temperatures: at ambient temperatures (approximately 70 " C ) for the uncoupled sample stage (designated AT films) and at elevated temperatures of approximately 220 "C (designated H T films). The substrates were maintained a t 220 "C for 16 h before deposition of the H T films. Film thicknesses were initially estimated from the deposition rates obtained in previous experiments involving MoS, films deposited on steel substrates:' where 6 min was needed for 2000-8, H T films and 8 min for 2000-8, AT films. The film thicknesses were later confirmed with electron microscopy. After deposition, the films were stored in a vacuum desiccator (typically for several days) until further analyses were performed. XRD analyses were performed with a Phillips Electronics APD-3720 vertical powder diffractometer equipped for normal 8-28 scans using Cu K a X-rays. Samples were oriented so that the scattering vector was always parallel to the surface normal. This method ensures that only reflections from planes oriented parallel to the surface will appear in the data.,, SEM was performed on the thin films with a Cambridge Stereoscan S-200 scanning electron microscope. Samples were coated with a thin gold film (
