Langmuir 1993,9, 455-459
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Pathways and Intermediates in the Reaction of Tetraethoxysilane on Si(100)-2X1 J. B. Danner, M. A. Rueter, and J. M. Vohs' Department of Chemical Engineering, University of Pennsylvania, Philadelphia, Pennsylvania 19104 Received July 13,1992. In Final Form: October 26,1992
The adsorption and reaction of tetraethoxysilane (TEOS) on Si(lOO)-2Xlsurfaces was studied wing temperature programmed desorption (TPD) and high resolution electron energy lose spectroscopy (HREELS). TEOS adsorbs dissociativelyon the Si(lOO)-2Xlsurface via cleavage of the C-0 bonds in the ethoxyligandsforminga mixture of adsorbed di- and triethoxysiloxaneand ethyl groups. The adsorbed ethoxysiloxanefragments decomposeat temperatures between 300 and 450 K to produce SiOzand ethyl groups. The surface ethyl groups undergo a complex reaction mechanism resulting in the production of ethylene at three separate temperatures during temperature programmed desorption experiments. Introduction Alkoxysilanes [RnSi(OR).+n]have been used for many years as coupling agents to enhance the adhesion between two dissimilar materials and as modifiers to control the physical and chemical properties of surfaces.'-3 More recently alkoxysilanes have been used in the microelectronics industry as gaseous precursors for the growth of Si02 dielectric layers using chemical vapor deposition (CVD). In this application the reactant of choice is generallytetraethoxysilane (TEOS).4-8The use of TEOS hae severaladvantages over the alternative method of Si02 depoeition,high temperature oxidationof silane? including being less hazardous and having the ability to produce films which have uniform thickness on stepped surfaces.6 TEOS also has the added advantage of incorporating both oxygenand silicon into a single source,thereby simplifying the operation of the CVD reactor. Given ita technological importance, it is not surprising that CVD of Si02 from TEOS has been the subject of a number of investigations.q-8 Although these studies have provided much insight into growth rates and overall kinetics of Si02 deposition, the details of the surface processes involved remain largely unknown. Crowell et al. have carried out one of the few fundamental studies of the surface chemistry of the growth of Si02 thin films using TEOS.' These researchers used temperature programmed desorption (TPD) and Fourier transform infrared Spectroscopy (FTIR)to study the adsorption and reaction of TEOS on Si(100)and amorphousSi02 surfaces. During TPD experiments withTEOS-dosed Si(100),they detectsd ethylene and hydrogen as the primary gaseous products. Ethylene was found to desorb in three distinct states centered at 500,600, and 720 K, while hydrogen desorbed in a single peak centered at 750 K. A small amount of acetylene was also found to desorb at 720 K. On the basis of theee reeults and those from FTIR studies of TEOS adsorbed on amorphous silica, Crowell et al.
* Author to whom correspondence should be addressed.
(1)Plueddeman, E. P. Sikrne Coupling Agents; Plenum Press: New York, 1991. (2)Mdn, H.Polym. Compos. 1984,5,101. (3)Finklea, H.0.; Murray, R. J. J. Phys. Chem. 1979,83,353. (4)Crowell,J. E.;Tedder, L.L.; Cho, H.; Cascarano, F. M.; Logan, M. A. J. Voc. Sci. Technol. 1990,A8, 1864. (5) Becker, F. 5.;Treichel, H.; Rahl, 5.J.Electrochem. SOC.1989,136, 3033.
(6) Becker, F. S.; Pawlik, D.; Anzinger, H.; Spitzer, A. J. Voc. Sci. Technol. 1987,86,1665. (7) Becker, F. S.; mhl, 9. J. Electrochem. SOC.1987,134,2924. (8) Jin,T.; Okuhara, T.; White, J. M.J. Chem. SOC.,Chem. Commun. 1987,1248.
0143-7463/93/2409-0455$04.OOI0
proposed that the three ethylene peaks in the TPD experiments were due to sequential decompositionof the ethoxy ligands on dissociatively adsorbed TEOS. These reactions were thought to proceed via @-hydrideelimination. In a recent TPD study performed in our laboratory, it was found that reaction of ethyl groups on Si(100) also resulted in the desorption of ethylene in three states centered between 500 and 800 K.g Indeed, our TPD spectra for the decomposition of ethyl groups on Si(100) are nearly identical to those of Crowell et al. for reaction of TEOS on this surface. This similarity suggests that surface ethyl groups are produced during the reaction of TEOS on Si(100) and therefore casts some doubt on the TEOS decomposition mechanism proposed previously. The goal of the work presented here was to develop a more thorough understanding of the mechanism for the reaction of TEOS onSi(100)and, specifically,to identifythe surface intermediates involved in this reaction. We have studied the interaction of TEOS and ethyl groups with Si(100)2x1 surfaces using TPD and high resolution electron energy loss spectroscopy (HREELS). Experimental Section, Experiments were performed in an ion pumped ultrahigh vacuum chamber with a base pressure of 2 X 1W0Torr. The chamber was equipped with a quadrupole mass spectrometer (UTI lQQC),optics for low energy electron diffractionand Auger electron spectroscopy(Omicron),a high reaolutionelectronenergy loss spectrometer (McAllister),and an ion gun (PhysicalElectronics).
Substrates consisted of 5 mm X 5 mm X 0.5 mm pieces of boron-doped Si wafers (16 0 cm) oriented in the (100) direction. Prior to being placed under vacuum, the silicon substrab were subjected to an etching/oxidation procedure similar to that described by Henderson.loJ1 This procedure produced a thin oxide layer that protected the silicon surface during subsequent preparation steps. After etching and oxidizing, the silicon wafer was clipped into a tantalum foil sample holder which was spotwelded to two pine of an electrical feedthroughat the bottom of a ultrahigh vacuum (UHV) sample manipulator. A chromeV alumel thermocouple was attached to the back surface of the sampleusing a high temperature ceramic adhesive (Aremco616). Sample temperatures up to 1200 K were obtainedby conduction from the resistively heated tantalum holder, while cooling to 90 K was made possible by exposing the atmospheric side of the electrical feedthrough to liquid nitrogen. Once in vacuum, the (9) Rueter, M. A.; Vohs, J. M.Surf. Sci. 1992,262,42.
(10)Rueter, M.A.; Vohs, J. M. J. Voc. Sci. Technol. 1991,A9,2916. (11)Henderson, R. C.J. Electrochem. SOC.1972,119,772.
0 1993 American Chemical Society
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1 L Exposure
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Figure 1. TPD spectra following exposure of the Si(100)-2Xl surface to (a) 1 langmuir of TEOS and at 90 K and (b) 4 langmuirs of DE2 at 90 K. Mass-to-charge ratios of 27 and 2 correspond to CzH4and Hz, respectively. protective oxide layer was removed by heating to 1100 K. This produced a clean, ordered 2x1 surface as determined by AES and LEED. Electronic grade tetraethoxyailane (Aldrich) and diethylzinc (Strem) were contained in glass sample vials and purified by freeze-pump-thaw cycles and vacuum distillation prior to use. The reactants were admitted into the vacuum chamber via a variable leak valve equipped with a 0.625 cm diameter dosing needle. TPD experimenta were performed using a computer multiplexing system that allowed up to eight mass signals to be monitored simultaneously. A heating rate of 5 K/s was used in all experiments. HREEL spectra were collected using a 3.5-eV electron beam directed 60° from the surfacenormal with detection in the specular direction. The sample temperature was held at 90 K during HREELS analysis. To characterize surface intermediates formed at higher temperatures, the sample was heated to the temperature of interest and then rapidly quenched to 90 K, at which point the HREEL spectrum was collected.
Results TPD of TEOS on Si(100)-2X1. In order to compare to the TPD results reported by Crowell et d.,4TPD spectra for the reaction of TEOS on Si(100)-2X1were collected. Figure l a displays the TPD spectra obtained following expoeure of the Si(100)-2Xlsurfaceto 1langmuir of TEOS at 90 K. As shown in this figure, the primary gaseous products were ethylene, which desorbed in three overlapping peaks centered at 500, 600, and 720 K, and hydrogen, which desorbed in a single peak centered at 750 K. These TPD spectra are in good agreement with those reported by Crowell et al.: with the exception that we did not detect any acetylene at 720 K. Due to overlap in the mass spectral cracking patterns of acetyleneand ethylene, however, it is possible that a small amount of acetylene may have gone undetected. As noted above, the desorption of ethylene from TEOSdosed Si(100) is similar to that obtained for the reaction of ethyl groups on this surface. This can be seen by comparing the TEOS TPD spectra in Figure l a with the TPD spectra for the reaction of diethylzinc (DEZ) on Si(100)displayed in Figure lb. It has been previously shown
that diethylzinc adsorbs dissociatively on Si(100) to form adsorbed ethyl groups and zinc atoms, upon heating the zinc desorbs in a broad peak centered at 500 K (not shown in Figure lb), while ethylene desorbs in three overlapping peaks centered at 550,650, and 770 K.g Note that, with the exception of a small offset in the peak temperatures, the ethylene and hydrogen TPD spectra in Figure 1 are qualitatively similar, suggesting that common surface intermediates were produced in each experiment. It is likely that the temperature offset is due to differences in the coverage between the two experiments. HREEL studies of the adsorption of diethylzincon Si(100)indicate that this molecule adsorbs dissociatively forming surface ethyl groups which upon heating undergo 8-hydride These results, therefore, elimination to produce eth~lene.~ suggest that dissociation of TEOS on Si(100)proceeds by cleavage of the C-O bonds in the ethoxy ligands producing adsorbed ethyl species. Identification of Surface Species with HREELS. In order to further identify the pathways and intermediatea involved in the reaction of TEOSon Si(l00), the vibrational spectra of the surface species were characterized using HREELS. Figure 2 displays the HREEL spectra obtained following exposure of the Si(100) surface to 50 langmuirs of TEOS at 90 K. TPD experimenteshow that for these conditions the surface is covered with multilayers of molecular TEOS. Fundamental losses are evident in the spectrum at 441, 792, 1086, 1435, and 2930 cm-l. By comparison to the IR spectrum of gaseous TE0Sl2 and the HREEL spectrum of ethoxy groups adsorbed on Si(111)-7X7,13these losses can be assigned to 6(Si-O), u(Si-O), u-(C-O), 6(CH3), and v(C-H) vibrations, respectively. In addition to these features, smaller peaks are also evident at 1878,2172, and 2621 cm-l, which can be assigned to u(C-0) + u(Si-O), u(C-0) + u(C-O), and u(C-0) + 6(CH3) combination modes. The C-C stretch of (12) Pouchert, C. J. The Aldrich Library ofznfrared Spectra; Aldrich Chemical Co.: Milwaukee, WI, 1981. (13) Ying, Z.; Ho, W. Surf. Sci. 1988, 198, 473.
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TEOSISl(100) 50 L Expoaure
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Figure 2. HREEL spectrum of condensed multilayers of TEOS on Si(100) following a 50-langmuir exposure at 90 K. TEOSISi(100) 5 L Exposure
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Figure 3. HREEL spectra of (a) the clean Si(100)-2xl surface, (b) following a 5-langmuir exposure of TEOS at 90 K, and following subsequent heating to (c) 450 K, (d) 600 K, (e) 750 K, and (01100 K.
the ethoxy ligands could not be resolved; however, it most likely contributed some intensity to the low energy side of the peak at 1086 cm-l. HREEL spectra were also collected as a function of sample temperature following exposure of the substrate to 5 langmuirs of TEOS at 90 K. These spectra are displayed in Figure 3. Temperatures for this series were chosen in order to characterize the surface before and after each desorption feature in the TPD spectra. Also displayed in Figure 3 is the spectrum of the clean Si(100)2x1 surface (curve a). This spectrum contains a small peak centered at 900 cm-l. This feature has been observed in previous HREELS studies of Si(100) and has been
attributed to surface carbide species which are present in levels below the AES detection limit.lOJ4-17 The HREEL spectrum of Si(100)following a 5 langmuir dose of TEOS at 90 K is displayed in curve b of Figure 3. Losses are evident in this spectrum at 460,810,1103,1435, and 2940 cm-'. By comparison to the spectrum of condensed TEOS, these modes can be assigned to &SiO),v(Si-01, v(C-O),6(CH3),and u(C-H),respectively. With the exception of the 6(CH3) mode all of these peaks occur 10-20 cm-l higher in frequency than the corresponding peaks in the spectrumof TEOS multilayers. The intensity of the u(C-0) peak relative to the other peaks in the spectrum of the surface dosed with 5 langmuirs of TEOS is also significantly less than that in the spectrum of condensed TEOS. These differences in the two spectra can be attributed to dissociative adsorption of TEOS on the Si(100) surface. The decrease in the intensity of the v(C-0) mode may suggest that dissociation occurs at the C-O bonds in the ethoxy ligands. Breaking of one of the C-O bonds in TEOS upon adsorption would result in the formation of surface tetraethoxysiloxane, i.e. OSi(OCHZCH& and anadaorbed ethyl group. Cleavageof additional C-0 bonds could result in the formation of a mixture of adsorbed tri- and diethoxysiloxanes. With the exception of the surface4 stretching mode, the vibrational modes of the adsorbed ethyl groups, namely 6(CH3), u(C-H), and 6(CHz),would be expected to occur at energies close to those of the analogous modes of the ethoxy groups in the adsorbed ethoxysiloxane. The surface-carbon stretching mode of ethyl groups adsorbed on Si(l00) occurs at 630 cm-l and has a relatively small HREELS cross section.9 This mode is not resolvable from the nearby S i 4 stretching mode in the spectrum of the TEOS-dosed surface. Heating the TEOS-dosed surface to 300 K to desorb any physisorbed species produced little change in the HREEL spectrum, other thana slight decrease in the peak intensities and a marginal improvement in the overall resolution. As shown in spectrum c of Figure 3, heating to 450 K resulted in more dramaticchanges in the HREEL spectrum, the most noticeable being the attenuation of the C-0 stretching mode at 1103 cm-l. Other changes include a shift in the Si-0 bending mode from 460 to 435 cm-l, and a shift in the C-H stretching mode from 2940 to 2930 cm-l. Several new peaks have also appeared in the spectrum centered at 964, 1070, and 2085 cm-l. It should also be notad that a peak near 3500 cm-l characteristic of an 0-H stretching mode was not observed in any of the spectra. Thus, adsorption of water from the background gas in the chamber was not significant during the collection of the HREEL spectra. The attenuation of the v(C-0) peak indicates that by 450 K many of the ethoxy ligands have decomposed. The spectrum can therefore be attributed primarily to adsorbed hydrocarbon and SiO, (e.g. Si0 or SiOz) species. Unfortunately, due to overlap of several of the hydrocarbon vibrational modes with those of the SiO, species it is somewhat difficult to unambiguously determine the identity of the adsorbed hydrocarbon fragments. Several of the peaks suggest, however, that some di-a-bonded ethylene has been formed. In particular, the peak at 2085 cm-1 can be assigned to the Si-H stretch of adsorbed (14) Lee, F.; Backman, A. L.; Lin, R.; Gow, T. R.; Maeel, R. 1. Surf. Sci. 1989,216,173. (15)Lin,R.;Gow,T.R.;Backman,A.L.;Cadwell,L.A.;Lee,F.;Maeel, R. I. J. Vac. Sci. Technol. 1989,B7,726. (16)Yoshinobu,J.;Tsuda, H.; Onchi, M.;Nishijima,M.J.ChemPhys. 1987,87,772. (17)Forater, A.;Liith, H. J. Vac. Sci. Technol. 1989,B7,720.
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458 Langmuir, Vol. 9, No.2, 1993
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Figure 4. HREEL spectrum of an Si02 thin film produced by sputter depositing oxygen on the Si(100)surface.
hydrogen?JOJ4 Thus, dehydrogenation has occurred as would be required for the formation of ethylene. On the basis of comparison to the HREEL spectrum of ethylene adsorbed on Si(lOO)9 the peak at 1070 cm-l can be tentatively assigned to the v(C-C) of adsorbed di-u ethylene, while the peak at 1430 cm-l can be assigned to CH2 bending and CHa deformation modes. The peak at 964 cm-1 is most likely associated with the C-CHa rock of surface ethyl groups.@ These results suggest that both di-a ethylene and ethyl groups are present on the surface at 450 K. This conclusion is consistent with the TPD results. The HREEL spectrum obtained after heating the TEOS-dosed surface to 600 K is displayed in curve d of Figure 2. Although this spectrum is similar to that obtained at 450 K, several changes have occurred. The C-H stretch is now centered at 2920 cm-I, the frequency expected for adsorbeddi-a ethylene, and the Si-H stretch at 2085 cm-l has increased in intensity. These results indicate that by 600 K the majority of the adsorbed ethyl groups have undergone @-hydrideelimination forming adsorbed ethylene and hydrogen. Small changes in the frequencies of the Si-0 stretching modes also occurred upon heating to 600 K. In order to further characterize the Si-0 stretching modes, an HREEL spectrum of a sputter deposited Si02 thin film was collected. As shown in Figure 4, Si02 has three characteristic vibrational modes centered at 485, 830, and 1160 The normal mode corresponding to each vibration is also displayed in this figure.'@ The frequencies of these modes are known to be a function of film thickness and occur at 50-100 cm-l lower in frequency for films less than 50 A thick.18 Thus, the peak centered near 1060 cm-l in the spectra of the TEOS-dosed surface at 450 and 600 K contains a contribution from this Si-O mode as well as the C-C stretching mode of adsorbed di-u ethylene. Heating the TEOS-dosed surface to 750 K resulted in the near disappearance of the peak at 1430cm-l due to the C-H bending mode of adsorbed di-u ethylene (see curve d Figure 3). This is consistent with the TPD data which show that the majority of the hydrocarbons have desorbed by this temperature. The vibrational modes of the surface SiO, have shifted slightly in energy and appear at 420, 790, and 1030cm-l. The peak at 910 cm-I can be assigned (18) Thiry, P. A.; Liehr, M.; Pireaux, J. J.; Sporken,R.; Caudano, R.; Vigneron, J. P.; Lucas, A. A. J. Vac. Sci. Techml. 1985, B3, 1118. (19)Schaefer, J. A.; GOpel, W. Surf. Sci. 1985,155, 535.
to the Si-C stretch of a surface carbide.lOJ*-l7 Two peaks are resolvable near the energy characteristic of an Si-H stretch. The peak at 2085 cm-l can be assigned to the monohydride species detected at lower temperatures. The additional v(Si-H) mode at 2240 cm-l suggeats the preeence of a second surface hydride species. Lucovsky has shown that the Si-H stretching mode of hydrogen adsorbed on amorphous silicon is approximately 180 cm-l lower in frequencythan that of hydrogen adsorbed on si1ica.m The v(Si-H) peak at 2240 cm-l can therefore be assigned to hydrogen adsorbed on the silicon atoms of the surface SiO,. A small C-H stretch centered at 2880 cm-l was also observed in the spectrum of the surface heated to 750 K. This peak most likely correspondsto adsorbed CH2 or CH species. Further dehydrogenationof these hydrocarbon fragments results in the deposition of carbon onto the surface. This can be seen in curve f of Figure 2 which displays the HREEL spectrum obtained after heating to 1100 K. This spectrum is similar to that obtained before dosing TEOS, except for an increase in the intensity of the S i 4 stretch at 900 cm-l. The lack of S i 4 stretching modes demonstrates that this temperature was also sufficient to desorb the deposited silica. Discussion The results of the HREELS and TPD experiments suggest that TEOS adsorbs dissociativelyon Si(100)below 300 K via cleavage of the C-O bonds in the ethoxy ligands, producing adsorbed ethoxysiloxanesand ethyl groups. The decrease in intensity of the C-O stretching mode upon heating the TEOS-dosed surface to 450 K indicates that by this temperature cleavage of the C-O bonds has started to occur. At this temperature the surface is covered with adsorbed ethyl groups, ethylene, hydrogen atoms, and SiO,. These results are consistent with the TPD results and suggest that ethoxy groups are most likely not directly involved in the reactions which produced gaseous ethylene during the TPD experiments as has been previously suggested. The correspondence in the TPD spectra obtained from the TEOS-and diethylzinc-dosed surfaces allow a similar reaction mechanism to be proposed for both systems. In the case of diethylzinc, the lowest temperature ethylene peak in the TPD spectra has been attributed to surface ethyl groups undergoing &hydride elimination.@In addition to gaseous ethylene this reaction also results in the formation of adsorbed di-u ethylene and hydrogen atoms. The second ethylene peak at 600 K is at a temperature Characteristic of the desorption of di-u ethylene from this surface.9~~~ Thus, this peak can be assigned to di-u ethylene. The HREELS results of the present investigation are also consistent with these assignments. The third ethylenedesorption state at 720 K is somewhat more difficult to explain. Unfortunately, with the exception of the C-H Stretchingmode, the vibrational modes of the adsorbed hydrocarbonsin the HREEL spectrum at 750 K overlap with those of the surface SiO, species. As a result, the HREELS results do not provide a definitive assignment of the adsorbed hydrocarbons at this temperature. In our previous study of the reaction of ethyl groups on Si(100)-2Xl,the high temperature ethylene TPD peak was assigned to recombination of adsorbed CHa species. In the present investigationthere is only indirect (20) Lucowky, G. J.
Vac. Sci. Technol. 1979,16,1226. (21)Clemen,L.; Wallace, R. C.; Taylor, P. A.; Dmear, M. J.; Choyke, W. J.; Weinberg, W. H.; Yatea, J. T. Surf.Sei. 1992,268,206.
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evidence that adsorbed CH2 groups are formed; however, a similar assignment may still be applicable. Studies of the adsorptionofethylene onSi(100)-2xl, however, further complicate the assignment of this temperature ethylene desorption feature.21*22During TPD of ethylene-dosed Si(100)-2Xl, ethylene desorbs near 575 K. For high ethylene coveragesa second desorption feature is observed at 620 K whose intensity is less than 5% of the lower temperature peak. Thus,the high temperature ethylene desorption state observed in the present study does not appear to be produced by the adsorption of ethylene. Isotopic labeling studies have also shown that ethylene adsorbed on Si(100) does not undergo C-C bond scission to any appreciable extent.= It has been demonstrated, however, that the presence of adsorbed hydrogen affects the ethylene desorption temperature. In coadsorption experiments it was found that the ethylene desorption temperature increased as the hydrogen coverage was increased.21 Thus, it is poesible that the high temperature ethylene peak from the TEOS-dosed surface may be a due to adsorbed ethylene stabilized by nearby adsorbed hydrogen atoms rather than due to recombinationof CH2 species.
Conclusions
(22) Cheng, C. C.; Choyke, W. J.; Yates, J. T.Surf. Sci. 1990,231,289.
The TPD and HREELS results suggest that tetraethoxysilane adsorbs dissociatively on the Si(100)-2Xl surface via cleavage of the C-O bonds in the ethoxy ligands, resulting in the formation of adsorbed ethyl groups and SiO,. The ethyl groupsundergo 8-hydride elimination at 500 K to produce gaseous ethylene,adsorbed di-a ethylene, and adsorbed hydrogen atoms. A portion of the di-a ethylene desorbe at 600 K. A higher temperature ethylene desorption state at 720 K results from the desorption of ethylene stabilized by nearby adsorbed hydrogen atoms or possibly from the recombinationof CH2 species formed at lower temperature. The silica species formed by the reaction of monolayer coverages of TEOS on Si(100)-2X1 is unstable and desorbs at temperatures above lo00 K.
Acknowledgment. We acknowledge the support of the National Science Foundation (Grant No. CTS89570561,and the Laboratory for Research on the Structure of Matter at the University of Pennsylvania (NSG-MRL Program, Grant No. DMR88-19885) for the use of their facilities.
