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Langmuir 1998, 14, 1328-1336
Direct Organometallic Synthesis: The Metal-Etching Reactions of Isobutyl Iodide on Al(111)1 Shrikant P. Lohokare,† Elizabeth L. Crane,† Lawrence H. Dubois,‡ and Ralph G. Nuzzo*,† Department of Materials Science and Engineering, School of Chemical Sciences and the Frederick Seitz Materials Research Laboratory, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, Defense Advanced Research Projects Agency, Arlington, Virginia 22203, and MIT Lincoln Laboratories, Lexington, Massachusetts 02173 Received July 3, 1997. In Final Form: October 2, 1997 We report a study of the thermal decomposition and reactions of isobutyl iodide on Al(111). Using temperature-programmed reaction and Auger electron spectroscopies, it was found that more than one product-forming pathway involving the alkyl moiety exists on this surface. A first-order, β-hydride elimination reaction converts surface-bound isobutyl groups derived from the dissociation of the C-I bond to gas phase isobutene and dihydrogen at temperatures above ∼420 K. Competing with this unimolecular process is a collection of complex associative reactions which effect the etching of the aluminum surface via the formation of volatile organometallic species. This includes formation and subsequent desorption of diisobutylaluminum iodide (desorption peak maximum at ∼490 K), diisobutylaluminum hydride (∼515 K), methylaluminum dihydride (∼725 K), and AlIx, x ) 1-3 (∼620 K). The kinetics of the processes yielding the various aluminum hydrides are coupled to that of the β-hydride elimination pathway (which serves as the hydrogen atom source) and are strongly coverage dependent. The formation of MeAlH2 reveals the occurrence of a kinetically competitive β-methyl elimination reaction of the surface alkyl groups.
Introduction Understanding the mechanisms of the reactions of alkyl groups on surfaces is important because of the centrality of such intermediates in a variety of technologically important processes as diverse as heterogeneous catalysis2-4 and the chemical vapor deposition (CVD) of metals from organometallic precursors.5-8 The generation of these transient species by the dissociative chemisorption of alkyl halides on metal surfaces has proven to be a useful procedure for forming these intermediates, and numerous studies employing this preparation method have been reported in recent years.9 Mechanistic studies of the thermal decomposition of alkyl groups larger than the methyl moiety on a variety of surfaces (Pt,9 Al,5,6 Cu,10 and Ni11 ) have demonstrated that the dominant thermolytic reaction pathway is β-hydride elimination, which results in the formation of the corresponding alkene and adsorbed hydrogen. The reactions occurring on the †
University of Illinois at Urbana-Champaign. Defense Advanced Research Projects Agency and MIT Lincoln Laboratories. ‡
(1) Dedicated to the memory of Professor Brian E. Bent, a friend and colleague whose kind words and gentle spirit will be long remembered and deeply missed. (2) Herman, W. A. Angew. Chem., Int. Ed. Engl. 1982, 21, 117. (3) Rooney, J. J. J. Mol. Catal. 1985, 31, 147. (4) Gardin, F.; Maire, G. Acc. Chem. Res. 1989, 22, 100. (5) Bent, B. E.; Nuzzo, R. G.; Dubois, L. H. J. Am. Chem. Soc. 1989, 111, 1634. (6) Bent, B. E.; Nuzzo, R. G.; Dubois, L. H. Mater. Res. Soc. Symp. Proc. 1989, 131, 327. (7) Creighton, J. R.; Parmeter, J. E. Crit. Rev. Solid State Mater. Sci. 1993, 18, 175. (8) Simmonds, M. G.; Gladfelter, W. L. In CVD of Metals; HampdenSmith, M. J., Kodas, T. T., Eds.; VCH: Weinheim, Germany, 1995; pp 45. (9) Zaera, F. Acc. Chem. Res. 1992, 25, 260. (10) Jenks, C. J.; Chiang, C. M.; Bent, B. E. J. Am. Chem. Soc. 1991, 113, 6308. (11) Tjandra, S.; Zaera, F. J. Vac. Sci. Technol. 1992, A10, 404.
coinage metals (Au, Ag, and Cu) promote alkyl coupling to form C-C bonds. This coupling occurs preferentially on silver surfaces12 and to a lesser extent on gold and copper.13,14 An excellent and comprehensive review of work in this area has been given by Zaera.9 The present study of the thermolytic reactions of isobutyl groups on aluminum surfaces is prompted by their intermediacy in the growth of aluminum thin films by metal-organic chemical vapor deposition (MOCVD)15 using triisobutylaluminum (TIBA) and the corresponding reverse process, the commercial synthesis of aluminum alkyls.16-19 The reaction of alkyl halides with aluminum is also an industrially important process for the bulk synthesis of aluminum sesquihalides.16 Inhibiting metaletching reactions occurring in competition with aluminum deposition in CVD metallization processes is also of crucial importance in the uniform (void free) filling of small vias in integrated circuit fabrication.20 We previously reported a detailed investigation of the thermal decomposition of alkyl halides on aluminum surfaces.21 The present study extends this work and brings to light the participation of (12) Zhou, X. L.; White, J. M. Surf. Sci. 1991, 241, 270. (13) Paul, A.; Bent, B. E. J. Catal. 1994, 147, 264. (14) Chiang, C.-M.; Wentzlaff, T. H.; Bent, B. E. J. Phys. Chem. 1992, 96, 1836. (15) Dubois, L. H.; Bent, B. E.; Nuzzo, R. G. In Surface Reactions; Madix, R. J., Ed.; Elsevier: Amsterdam, 1995; p 135. (16) Eisch, J. J. In Comprehensive Organometallic Chemistry; Wilkinson, G., Stone, F. G. A., Ebel, E. W., Eds.; Pergamon: Oxford, 1982; Vol. 1. (17) Kochi, J. K. Organometallic Mechanisms and Catalysis; Academic: New York, 1978. (18) Mole, T.; Jeffrey, E. A. Organometallic Compounds; Elsevier: New York, 1972. (19) Ziegler, K. Organometallic Compounds; Reinhold: New York, 1960. (20) Sze, S. M. Semiconductor Devices: Physics and Technology; Wiley: New York, 1985. (21) Bent, B. E.; Nuzzo, R. G.; Zegarski, B. R.; Dubois, L. H. J. Am. Chem. Soc. 1991, 113, 1137.
S0743-7463(97)00716-6 CCC: $15.00 © 1998 American Chemical Society Published on Web 01/17/1998
Direct Organometallic Synthesis
multiple decomposition pathways in the reactions occurring on the Al(111) surface. The reaction mechanisms characterized on this surface seem to partially mirror the more complex reactivity patterns seen in the formation and decomposition of discrete metal alkyls in solution.22,23 In this report we focus on the thermal decomposition reactions of a prototypical alkyl halide, isobutyl iodide, on the Al(111) surface. Temperature-programmed reaction (TPRS) and Auger electron (AES) spectroscopies are used to identify the various products formed in the decomposition processes and rank their relative energetics. A β-methyl elimination process of a surface isobutyl group and the formation of a highly stable methylalane product, methylaluminum dihydride, is also characterized directly for the first time. The methyl moiety is believed to serve as the source of the carbon contamination found in aluminum thin films deposited at high temperature using triisobutylaluminum as the MOCVD precursor.7,8 The formation of the aluminum alkyl halide and hydride etching products described here appears to have no corresponding mechanistic parallel in the reactions reported for the decomposition of alkyl iodides on transition metal surfaces.9-12 Experimental Section Experiments were performed in an ultrahigh-vacuum (UHV) chamber with typical base pressures of e2 × 10-10 Torr. The system was equipped with two quadrupole mass spectrometers (VG SXP 300), a cylindrical mirror analyzer with a coaxial electron gun (Physical Electronics) for Auger electron spectroscopy, an ion gun (Physical Electronics) for sputtering the crystal, and a Bayard-Alpert type ionization gauge (Varian). Both mass spectrometers were shrouded and differentially pumped by 30 L/s ion pumps (Varian). The chamber was pumped by a 10 in. diffusion pump (5000 L/s) with a liquid nitrogen trap. Additional pumping was provided by a titanium sublimation pump and a 540 L/s turbomolecular pump. A 6 mm diameter Al(111) single crystal was mounted on the manipulator and could be heated to >800 K with a button heater and cooled to 90 K by flowing liquid nitrogen through a differentially pumped, rotatable feed-through. The sample was firmly held on the face of the button heater by means of tantalum wire straps (0.1 mm thick) spot welded to the body of the heater and running through thin slits cut into the sides of the crystal. The temperature of the crystal was monitored by means of a chromel-alumel thermocouple inserted into a hole spark-eroded into the side of the crystal. Heating rates were controlled by a Eurotherm temperature controller/programmer (818P) in conjunction with a programmable dc power supply (HewlettPackard). The cleaning of the Al(111) crystal surface was performed by repetitive cycles of Ar+ bombardment (1 kV, 6 mA) at 300 K for 20 min and at 700 K for 10 min. The crystal was then annealed at 750 K for 5 min. The exposures of isobutyl iodide (Aldrich) were made through a precision leak valve (Duniway Stockroom) at a surface temperature of 300 K (background pressure during the dosing was held at 2 × 10-8 Torr for all exposures). The exposures are expressed in units of langmuirs (1 × 10-6 Torr s). The isobutyl iodide was stored over a clean copper wire and always kept shielded from light to avoid photochemical degradation. The sample was purified by several freeze-pump-thaw cycles prior to use. TPR spectra were collected by linearly ramping the surface temperature from 300 to 800 K at a rate of 12 K/s. Data was collected in digital form (Labtech data acquisition software in conjunction with a Metrabyte CIO-DAS08-AO PCB) with the crystal placed at a distance of 1 mm from a skimmer (Beam Dynamics) mounted on the shield cone of the mass spectrometer differential pumping region. The mass spectrometer used in this study differs in significant ways from the instrument used (22) Adkins, H.; Scanley, C. J. Am. Chem. Soc. 1951, 73, 2854. (23) Grosse, A. V.; Mavity, J. M. J. Org. Chem. 1940, 5, 106.
Langmuir, Vol. 14, No. 6, 1998 1329 in our earlier work.21 Of particular significance is the ionizer assembly, which is based on an open (pass-through) design. This change greatly facilitates the detection of organoaluminum species by TPRS. The cleanliness of the surface was checked by Auger electron spectroscopy after each TPR spectrum was acquired. With care during the dosing and frequent light baking of the vacuum chamber between blocks of experiments, it was found that the surface was atomically clean after each temperature ramp. The surface order could be restored after each run by annealing the surface at 750 K for 5 min. A molecular beam source was used in conjunction with a lineof-sight mass spectrometer to estimate the variation of the ionization and fragmentation cross sections of isobutene monitored during TPRS as a function of its internal energy. The molecular beam source is described in more detail elsewhere.24 Briefly, the molecular beam source was housed in the first stage of a differentially pumped two-stage chamber. The beam was formed by the continuous expansion of isobutene (Aldrich) through a 25 mm diameter molybdenum aperture (Ladd Optics). The aperture seat and the connected tubing were enclosed inside a cylindrical copper block bearing grooves holding tungsten wire which in turn was insulated with ceramic tubing. The beam source could be heated from room temperature to ∼850 K by a 50 V-10 A dc power supply (Hewlett-Packard). The stagnation temperature of the beam was measured at the throat of the beam nozzle by a chromel-alumel thermocouple spot welded to the nozzle edge. The entire source assembly was mounted on an X-Y-Z translation precision manipulator to facilitate the alignment of the beam source with respect to the collimating apertures and line-of-sight mass spectrometer. The beam source chamber was pumped by a 10 in. diffusion pump. The beam was skimmed by a 400 mm diameter (Model 1, Beam Dynamics) nickel skimmer mounted on the inside wall of the source chamber. The skimmernozzle distance was on the order of 1 cm and was adjusted to optimize the beam intensity.
Results and Discussion The results of our study are presented along with their interpretation in three sections. In the first, we present a brief overview of the results reported in our earlier study of the thermal decomposition of isobutyl iodide on aluminum, describe the effects of the coadsorbed iodine atoms on the surface reactions of isobutyl groups observed in the present work, and provide a discussion of the underlying kinetics involved in this more complex reaction scheme. The second section provides a detailed mechanistic description of the new reaction pathways characterized in the present work. A comprehensive discussion of the results and their significance in both aluminum deposition and etching is given in the third section. A Reexamination of the Thermal Decomposition Mechanism of Isobutyl Iodide on Al(111). The β-hydride elimination reaction is a common decomposition pathway for metal-bound alkyl groups with β hydrogen atoms.3-5,9-11 It was concluded on the basis of the results reported in our earlier study21 that most alkyl groups on aluminum surfaces also follow this decomposition pathway. The evidence supporting this conclusion stemmed from the fact that only three significant types of desorption products were detected by TPRS for a variety of alkyl iodides. The specific products found for isobutyl iodide were isobutene (monitored by m/e ) 56 (C4H8+) and 41 (C3H5+)), dihydrogen (m/e ) 2 (H2+)), and an aluminumcontaining product detected as the aluminum monoiodide ion (m/e ) 154 (AlI+)). The precise nature of the latter product was not determined due to instrumental limitations that precluded high sensitivity and high-resolution analysis of metal-containing fragments. The reaction (24) Lohokare, S. P.; Nuzzo, R. G. To be submitted.
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Figure 1. TPRS spectra as a function of isobutyl iodide exposure to an Al(111) surface at 300 K: (a) m/e ) 41 (C3H5+) and (b) m/e ) 56 (C4H8+).
pathway suggested by these data can be summarized in the following simple scheme: 1
(CH3)2CHCH2I + Al f (CH3)2CCH2 + /2 H2 + AlI (1) The β-hydride elimination reaction forms isobutene and hydrogen at temperatures >450 K. The hydrogen was detected by TPRS in the form of dihydrogen which desorbs in a peak coincident with that of the isobutene (∼500 K). The iodine remains on the surface during this process and only desorbs as aluminum iodide (AlIx, x ) 1-3) above 600 K. Auger analysis of the surface at the end of the thermal decomposition indicated that little or no residual carbon or iodine was left on the surface. Reactions occurring on the (100) surface appear to follow this relatively simple pathway almost exclusively, and on the basis of a less comprehensive examination of TPRS data for a (111) substrate, the reaction scheme was assumed to be a general one for the aluminum system. We now believe, on the basis of data reported below, that other reactions contribute to the product-forming pathways on at least the Al(111) surface. These other products also contribute to the intensity seen in the m/e ) 41 and 56 TPR spectra. Figure 1a shows TPR spectra monitored at m/e ) 41 (C3H5+) as a function of the exposure of isobutyl iodide to Al(111) at 300 K. The observed desorption profiles and the peak temperatures match those reported in the earlier study.21 As before, we find that the peak maximum shifts toward lower temperatures (from 515 to 490 K for the
Figure 2. Plot of ratios of mass spectrometer intensities for the m/e 56:41 ratio as a function of (a) the internal energy of isobutene sampled from a heated molecular beam source (O) and (b) TPR spectra for 1 (]), 2 (3), 6 (4), and 8 (0) langmuir exposures of isobutyl iodide on an Al(111) surface at 300 K. The intensities were measured at various points along the TPRS traces and plotted as a function of the temperature at which the values were taken. Note: Uncertainty exists for the ratios measured at low exposures of isobutyl iodide (e2 langmuirs) especially in the temperatures g520 K (due to the low intensities), and hence, the anomalous decrease in the profile is seen at those exposures.
spectra shown) with increasing exposures. Saturation of the overlayer was obtained at an isobutyl iodide exposure of ∼8 langmuirs. Figure 1b shows the results of a comparable TPRS study for m/e ) 56 (C4H8+), the molecular ion for isobutene. In principle, either of the masses monitored (m/e ) 56 or 41) could be used to determine the kinetics of an elementary surface reaction which yields isobutene as the predominant hydrocarbon product.21 With identical mass spectrometer settings (i.e., multiplier gain, resolution, ionization energy, etc.), the ratio of the yield of the parent ion (m/e ) 56) to that of the most abundant ionization fragment (m/e ) 41) of an authentic isobutene sample at 300 K in our apparatus was found to be 0.52. The ratios calculated from the TPRS spectral intensities from Figure 1, parts a and b, in the temperature range 480-520 K (i.e., the region of the TPRS peaks) at saturation coverage (8 langmuir exposure) are found to lie between 0.17 and 0.35. The differences between the expected and measured values are thus on the order of 33-66%. The surface temperature must contribute at least some of this difference since isobutene is evolved at 515 K in the TPRS experiment and therefore contains greater internal energy. The data shown in Figure 2 (upper curve) demonstrate the significant effect of the temperature on the electron impact ionization and fragmentation cross sections of isobutene. The measured m/e 56:41 ratio at 300 K using the beam protocol (∼0.52) is experimentally indistinguishable from that found for a background pressure of
Direct Organometallic Synthesis
isobutene using similar instrumental parameters. The data in this figure show that the measured ratio declines sharply as the nozzle temperature is raised. However, it falls to a minimum value of ∼0.26 at ∼460 K. The beam experiments suggest that this ratio begins to increase again above this temperature, rising to a characteristic value of ∼0.35 at 520 K. The data shown in the figure are similar to the effects reported by Amirav and co-workers,25 who studied the effects of internal energy on the ionization and fragmentation cross sections of propyl iodide. The lower curves in Figure 2 analyze the temperature dependence of the m/e 56:41 ratios over a broad range of surface coverages of the isobutyl and iodine fragments. The points shown were calculated using absolute intensities measured at eight common temperatures across the TPRS wave forms (between 470 and 540 K) shown in Figure 1, parts a and b. The measured TPRS ratios only converge with the expected ratios of the calibration curve for temperatures g520 K, and then only for the highest coverages. The more surprising result is the substantially lower ratios obtained for temperatures below 520 K for all coverages of isobutyl iodide. For submonolayer coverages, the ratios do not converge to the expected values. The data thus seem to suggest that the product(s) measured is (are) not isobutene with the thermal energy distribution found in the molecular beam. Hence, one asks what other factors must be considered. Part of the difference between the expected and measured m/e 56:41 ratio might be attributed to reaction mechanisms other than β-hydride elimination which yield products with different electron impact fragmentation ratios. Thus, although m/e ) 41 is the most abundant ionization fragment of isobutene, it is also likely that it would be a daughter ion of other products containing the isobutyl group as well. The fragmentation cross sections of different species containing the isobutyl group need not be equal. The data therefore must be analyzed in a way which sheds light on the nature of the competing reactions involved. Competing mechanisms, especially those involving branching between associative and dissociative pathways, are potentially analyzable via their coverage dependencies. Figure 3 shows a plot of the peak desorption temperatures (Tmax) for m/e ) 41 and 56 as a function of isobutyl iodide exposure. As can be seen, the Tmax. values measured for the two data channels are not coincident. We take this as firmly establishing that multiple pathways yielding C4 fragments must be operating. It is these latter results that initially prompted us to undertake a detailed search for other C4-containing products. The absence of any significant intensity for m/e ) 58 leads us to conclude that isobutane is not formed to any significant degree; this result is consistent with the findings of our earlier study.21 We also find no evidence suggestive of the reductive coupling of alkyl groups (to form a C8 product). This later result follows the ratestructure correlation developed in a study of alkyl groups on Ag, Au, and Cu surfaces by Bent and Paul.13 They report that alkyl coupling rates are fastest when the carbon-metal bonds are weak. The bond strength of the Al-C bond is substantial,15 and thus, on the basis of this correlation, little or no coupling is to be expected. Given the knowledge that alkyl halides are aggressive etching agents of aluminum in solution,23,24 we decided to reexamine the possible role that these reactions might play in the thermolytic surface reactions and determine (25) Danon, A.; Amirav, A.; Silberstein, J.; Salman, I.; Levine, R. D. J. Phys. Chem. 1989, 93, 49.
Langmuir, Vol. 14, No. 6, 1998 1331
Figure 3. Plot of peak maximum temperature for m/e ) 41 (C3H5+) and 56 (C4H8+) TPRS spectra as a function of isobutyl iodide exposure to an Al(111) surface at 300 K.
Figure 4. TPRS spectra of m/e ) 154 (AlI+) as a function of isobutyl iodide exposure to an Al(111) surface at 300 K.
whether organoaluminum products are being formed to any significant degree. Alkyl Halide Induced Etching Reactions on Al(111). On the basis of precedents established by condensed-phase reactions, one expects that organoaluminum-etching products should contain halogen. The major iodine-containing product detected in our previous study21 was a nonorganic aluminum iodide species, AlIx (monitored by m/e ) 154), of undetermined stoichiometry (x ) 1-3). The desorption of the AlI-containing fragment occurs at a significant rate at temperatures above ∼600 K. However, the quality of the data in the earlier study for these latter species was very poor due to the limitations imposed by the design of the spectrometer ionizer assembly. The system used here substantially improves the detectability of metal-containing products by at least 1 order of magnitude. Figure 4 shows TPR spectra measured in the present study for m/e ) 154 (AlI+) plotted as a function of increasing exposures of isobutyl iodide at
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equilibrium involving metal and the various volatile aluminum halides which is established under these conditions can be written as follows:
2Al + AlX3 S 3AlX
Figure 5. TPRS spectra of (bottom to top) m/e ) 2 (H2+), 15 (CH3+), 27 (Al+), 28 (AlH+), and 29 (AlH2+) for a saturation exposure of isobutyl iodide to an Al(111) surface at 300 K.
300 K. For low exposures (4 langmuirs). The absence of any corresponding feature in this temperature range in the m/e ) 154 desorption spectra (Figure 4) suggests that the dihydrogen product is not an aluminum iodide containing moiety. Also, from the similar lack of corresponding features in the thermal desorption spectrum measured at m/e ) 84 (Figure 7) and m/e ) 41 (Figure 1a), we can discount the possibility of an isobutylaluminum species being evolved. It will be recalled that the TPRS traces for the CH3+ cracking fragment (m/e ) 15, Figure 5) also show a high-energy desorption state above 700 K, one which is coincident with features seen in the m/e ) 2 (Figure 9) and 28 (Figure 5) data. The data thus suggest the presence of surface-bound methyl groups which decompose through a variety of pathways in this temperature range. The presence of such groups provides evidence that a β-methyl elimination reaction can occur. As mentioned above, this latter process is believed to be the source of carbon contamination in the Al-CVD process using TIBA.7,8,30 In the present case, Auger electron spectroscopic analysis carried out after the desorption experiment shows little carbon remaining on the surface. This suggests that the coverages of CH3 are low and what may exist of these are stabilized in the form of an aluminum hydride complex (possibly CH3AlH2, methylaluminum dihydride) which can desorb with modest efficiency above 700 K. The features observed around ∼515 K in the TPRS traces shown in Figure 5 for m/e ) 15, 27, 28, and 29 merit further discussion at this point. Although monitored to detect CH3+, Al+, AlH+, and AlH2+ species, respectively, these ions are also important electron impact cracking fragments of isobutene. Their assignment in the 500 K (30) For depositions carried out on Si, it was observed that the surface silicon atoms scavenged alkyl groups and hydrogen atoms in reactions which produced measurable quantities of alkylsilanes; at least one of these silane products contained a primary methyl group (ref 1). Gates, S. M; Kunz, R. R.; Greenleif, C. M. Surf. Sci. 1989, 207, 364. Kao, C.-T.; Dubois, L. H.; Nuzzo, R. G. J. Vac. Sci. Technol. 1990, 9, 228.
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Figure 10. Plot of normalized AES intensities for carbon (272 eV) and iodine (511 eV) for an Al(111) surface at 300 K saturated with isobutyl iodide as a function of surface temperature.
region of the spectra is thus complicated by the fact that products other than isobutene (e.g. diisobutylaluminum iodide or diisobutylaluminum hydride) also evolve in this temperature range. The nonideal line shapes seen are suggestive of this underlying complexity. Given our current uncertainties of how to model the complex kinetics which must be involved and the lack of suitable temperature-corrected, mass spectral standards, it is difficult to estimate the quantitative mass balances for the various reactions identified above. It is clear, however, that the associative reactions which generate aluminum-etching products are strongly coverage dependent. A qualitative analysis of the system was also made as a function of surface temperature using Auger electron spectroscopy. Auger spectra were taken of a surface saturated with isobutyl iodide at 300 K after flash annealing to the desired temperature. Two peaks were monitored: the carbon KLL peak at 272 eV and the iodine MNN peak at 511 eV. The beam current was minimized due to the adsorbate's extreme sensitivity to electronstimulated decomposition and desorption. The noise limits in the data are therefore fairly large. The results of the AES experiments are summarized in Figure 10 which shows a plot of the normalized intensities for the C and I peaks as a function of the surface temperature. It can be seen that the depletion of carbon (i.e., the isobutyl fragments) and iodine occurs nonuniformly over a wide temperature range. A more quantitative analysis was not possible due to beam-induced desorption and decomposition of the adsorbate. However, it does appear that the depletion of the carbon occurs more readily than does that of the iodine. We undertook preliminary studies by Fourier transform infrared reflection absorption spectroscopy to see if spectroscopic signatures could be found which might provide a structurally rationalized basis for the proposed etching reactions (the data is not shown). Isobutyl moieties on an Al(100) surface have been characterized by infrared reflection absorption spectroscopy, as reported previously.31 Our results show broad, weak C-H stretching modes centered at ∼2880 and ∼2940 cm-1, in good (31) Zegarski, B. R.; Dubois, L. H. Surf. Sci. Lett. 1992, 262, L129.
Direct Organometallic Synthesis
Langmuir, Vol. 14, No. 6, 1998 1335
Scheme 1
agreement with their data. Flash annealing the surface to 500 K at 5 K/s followed by cooling to 300 K leads to the disappearance of these features with no evidence for gross reorganizations of the adsorbate or the formation of any new species. The product-forming pathways thus appear to follow simple thermally activated mechanisms. Mechanistic Implications. We noted above the more significant features seen in the reaction kinetics for the thermal decomposition of isobutyl iodide on the Al(111) surface which suggested the operation of more than one reaction pathway. The data shown in Figures 1-9 allow the construction of a comprehensive reaction scheme for the fragments generated by the dissociation of isobutyl iodide on Al(111). This picture is illustrated in Scheme 1. The changing characteristics of the “isobutene” desorption spectra seen with increasing exposures (Figure 1) can be explained on the basis of strongly coverage dependent reaction mechanisms which involve the competition of both dissociative and associative pathways. We find a reasonable efficiency for the formation of diisobutylaluminum iodide and diisobutylaluminum hydride throughout the coverage range studied. Even at the lowest coverages, a measurable fraction of the surfacebound alkyl groups are lost as aluminum-etching products, the most important of which appears to be the hydride product DIBAH. The rate of this process is limited by the rate of the β-hydride elimination reaction which serves as the hydrogen atom source. With increasing exposure, the desorption of diisobutylaluminum iodide also becomes kinetically competitive. Though the formation of DIBAI is thermodynamically favored, a detailed mechanistic understanding of how the associative desorption of this product operates remains unclear. The yields of the various aluminum alkyls are not well understood at this point. To rigorously quantify them would require detailed mass spectral studies which not only establish the fragmentation patterns and ion yields of the various organometallic species but also provide quantitative insights as to how these patterns are affected by their internal energy distributions. Without this knowledge, the relative peak intensities for the hydride and the iodide species (e.g., Figure 7) cannot be correlated easily with their concentrations. The detection of small quantities of desorbing species always raises the question of the role being played by defects in the reaction mechanism. The sensitivity of the product partitioning to surface defect density was therefore qualitatively evaluated by carrying out TPRS experiments on substrates bearing sputtering-induced damage not removed by annealing. Figure 11 shows the results of such an experiment for the desorption of (C4H7)2Al+ (m/e
Figure 11. TPRS spectra for m/e ) 141 ((C4H7)2Al+) on (a) an Al(111) surface annealed at 750 K for 5 min and saturated with isobutyl iodide at 300 K (bottom spectrum), and (b) an Al(111) surface induced with defects (sputtered but not annealed) and saturated with isobutyl iodide at 300 K (top spectrum).
) 141). The lower spectrum corresponds to data obtained using a well-annealed (111) surface. The top spectrum corresponds to data obtained using an unannealed surface in which imperfections were induced by Ar+ ion sputtering (1 keV, 6 mA). Only a slight increase in the desorption intensity and a slight shift of the peak maximum toward higher surface temperature are seen in comparison to the annealed surface shown in the bottom spectrum. At most, the metal-etching reactions forming the dialkylaluminum species are enhanced only modestly by the presence of these gross surface inhomogeneities. The kinetic parameters for the thermal decomposition of isobutyl iodide on both Al(111) and Al(100) surfaces have been reported.21 It is clear, from the data reported above, that the rate analysis made in that work is complicated by the fact that multiple reaction pathways are involved in removing the isobutyl groups and iodine from the surface. Even so, the apparent activation parameters were found to be simple inasmuch as they were well-modeled by a single, first-order rate law. Any associative reaction which leads to aluminum etching should not follow a true first-order rate law, however. We know from the current data that at least two associative reactions contribute to aluminum etching. The first of these yields DIBAH, and the other DIBAI. The production of DIBAH requires surface-bound hydrogen. We believe that the rate of the DIBAH desorption is limited by the rate of hydrogen atom production via β-hydride elimination since desorption of this species occurs at ∼500 K. This would give the same first-order rate dependence characterized in our earlier work. Kinetic arguments suggest that the DIBAI yield should be low, given that its rate is not reaction limited by the β-hydride elimination pathway and that consumption of isobutyl moieties via this pathway is not reflected in a large deviation of the TPRS kinetics from apparent first-order behavior. The activation energies (Ea) and preexponential factors (A) for the β-hydride elimination kinetics demonstrate strong substrate rate-structure sensitivities. On the Al(111) surface, the activation energy measured was ∼23 kcal/mol (∼1 eV) with a corresponding preexponential
1336 Langmuir, Vol. 14, No. 6, 1998
factor of 8 × 109 s-1. The corresponding values measured on the Al(100) surface were ∼28 kcal/mol and 4 × 1011 s-1, respectively. Kinetic compensation is thus evidenced in the activation parameters, but overall, the rate of the β-hydride elimination reaction on the (111) surface at 500 K is 3-5 times faster than it is on the (100). Other significant rate-structure sensitivities have been reported for reactions occurring on the (111) and (100) surfaces of aluminum. For example, TPRS studies of the desorption of dihydrogen (m/e ) 2) from an Al(111) surface predosed with atomic hydrogen showed that the process occurs at a significant rate at temperatures near ∼370 K.32 In an important study, Nozoye and co-workers33,34 showed that this reaction generates products other than H2. Their data demonstrated that aluminum hydrides (AlH3 or Al2H6) are important desorption products on the (111) surface. On the (110) and (100) surfaces, the yields of the metal hydrides were much lower, being nearly 0 for the (110) substrate.34 This suggests that this type of gas phase etching of Al is highly anisotropic, showing a pronounced selectivity for competitive etching (versus recombination) on the (111) crystal orientation. The occurrence of the associative (i.e., metal etching) reactions observed in the present study thus finds precedent in these results. We note that crystallographic selectivities are commonly found in chemical etching processes; of par(32) Winkler, A.; Pozgainer, G.; Rendulic, K. D. Surf. Sci. 1991, 251/ 252, 886. (33) Hara, M.; Domen, K.; Onishi, T.; Nozoye, H.; Nishihara, C.; Kaise, Y.; Shindo, H. Surf. Sci. 1991, 242, 459. (34) Kondoh, H.; Hara, M.; Domen, K.; Nozoye, H. Surf. Sci. 1993, 287/288, 74. (35) Knor, K. In Catalysis Science and Technology; Springer: Berlin, 1982; Vol. 3.
Lohokare et al.
ticular importance are those used in the commercial fabrication of silicon-based, microelectronic devices.36-41 We believe it possible that a similar etching anisotropy is evidenced in the thermal decomposition of isobutyl iodide on aluminum as well. Further studies will be needed to clarify this point. We close with a brief comment on what is perhaps the most unusual finding in this study, namely, the apparent high-temperature desorption of CH3AlH2 (or perhaps the corresponding dimer (CH3AlH2)2). It is hard to rationalize the kinetic competence of this latter process given the known thermal stabilities of H atoms32 and methyl species42 on Al(111). It is also expected that the β-methyl elimination rate for surface-bound isobutyl groups is not likely to be rate limiting in this instance.15 Further work will be needed to clarify the mechanistic basis of this latter issue. Since the yields of this product are very low, it may be that substrate defects play an important role here. Acknowledgment. Financial support from the National Science Foundation (CHE-9300995 and CHE9626871) is gratefully acknowledged. LA970716G (36) Froitzheim, H.; Kohler, U.; Lammering, H. Surf. Sci. 1985, 149, 537. (37) Gallois, B. M.; Besmann, T. M.; Stott, M. W. J. Am. Ceram. Soc. 1994, 77, 2949. (38) Ghez, R.; Iyer, S. S. IBM J. Res. Dev. 1988, 32, 804. (39) Kisielowski, K. C.; Beyer, W. J. Appl. Phys. 1989, 66, 552. (40) Waltenburg, H. N.; Yates, J. T., Jr. Chem. Rev. 1995, 95, 1589. (41) Yang, S. K.; Peter, S.; Takoudis, C. G. J. Appl. Phys. 1994, 76, 4107. (42) Chen, J. G.; Beebe, J. T. P.; Crowell, J. E.; Yates, J. T., Jr. J. Am. Chem. Soc. 1987, 109, 1726.
