Langmuir 1994,10, 3116-3121
3116
Surface Reactions during the Growth of Si02 Thin Films on Si(100) Using Tetraethoxysilane J. B.Danner and J. M. Vohs* Department of Chemical Engineering, University of Pennsylvania, Philadelphia, Pennsylvania 19104 Received January 21, 1994. In Final Form: June 10, 1994@ The reactions of tetraethoxysilane (TEOS) on oxidized Si(100)surfaces were studied using temperature programmed desorption (TPD),X-ray photoelectron spectroscopy(XPS), and high-resolutionelectron energyloss spectroscopy (HREELS).The surface reaction pathways followed by TEOS were found to depend on the thickness of the silica layer. Changes in the observed reactivity with thickness were correlated to the relative populations of the various oxidation states of silicon present in the surface layer. The structure of the transition region between a Si(100) substrate and a Si02 film deposited using TEOS was found to be similar to that produced by thermal oxidation using oxygen.
Introduction The growth of dielectric layers of Si02 is a n important step in the production of silicon-based microelectronic devices. In order to optimize processing conditions and device performance, a n ideal Si02 deposition procedure would produce films of uniform thickness and high purity with minimal defects at the lowest possible temperature. The traditional methods for the deposition of Si02 on silicon are direct thermal oxidation with oxygen or chemical vapor deposition (CVD)using silane and oxygen. Although these processes are applied commercially, they often produce films with nonuniform thicknesses and require relatively high processing temperatures. A newer alternative to these traditional methods is the CVD of Si02 using tetraethoxysilane (TEOS). It has been reported that this process produces films of superior quality at lower The work presented here focuses on elucidating the surface chemistry involved in the growth of Si02 layers using this technique. A common feature to all methods for the deposition of Si02 thin films on Si(100) is the formation of a SiO, (0 .e x I 2) transition layer at the interface between the substrate and the Si02 film. It is generally thought that the thickness ofthe transition region is three to four atomic layers and that it contains all of the possible formal oxidation states of silicon, i.e. Si+,Si2+,Si3+,and Si4+.3,4 The concentration of oxygen and the relative populations of the higher silicon oxidation states in the transition layer increase with distance from the substrate until the stoichiometry of the layer reaches that of SiO,. Thus, as the SiO, layer grows, significant changes occur in its composition and structure. It is therefore likely that the surface reactions of TEOS which occur during the growth of the transition layer depend on film thickness. Although several reports concerning the reaction of TEOS on Si(100)526and on amorphous SO2' can be found in the literature, studies of the surface reactions of TEOS as a function of the thickness of a silica layer have yet to
* Author to whom correspondence should be addressed.
Abstract published inAdvanceACSAbstracts, August 1,1994. (1)Becker, R. S.;Pawlik, D.; Anzinger, H.; Spitzer,A. J . Vac.Sci. Technol. B 1987,5,1555. (2) Raupp, G . B.; Shemansky, F. A.; Cale, T. S. J . Vac.Sci. Technol. E 1992,10,2442. (3) Braun, W.; Kuhlenbeck, H. Surf. Sci. 1987,180, 279. (4)Hattori, T.J . Vac.Sci. Technol. B 1993,11, 1528. (5)Danner, J. B.;Vohs, J. M. Langmuir 1993,9,455. (6)Crowell, J. E.; Tedder, L. L.; Cho, H.; Cascarano, F. M.; Logan, M. A. J . Vac.Sci. Technol. A 1990,8, 1864. (7) Tedder, L. L.; Lu, G . ;Crowell, J. E. J.App1. Phys. 1991,69,7037. @
be reported. In the work presented here, we expand on our previous study of the reaction of TEOS on to include characterization of the reaction of TEOS on oxidized silicon surfaces. In particular, temperature programmed desorption (TPD), X-ray photoelectron spectroscopy (XPS),and high-resolution electron energy loss spectroscopy (HREELS)were used to examine the reactions of TEOS on SiO, layers with thicknesses varying between 0 and 12 A and on a stoichiometric Si02 thin film.
Experimental Section Experiments were performed in two separate ultrahigh vacuum (UHV) surface analysis systems. One system was equipped with a mass spectrometer (UTI), a retarding field electron energy analyzer (Omicron) for performing both lowenergy electron diffraction and Auger electron spectroscopy, and a high-resolution electron energy loss spectrometer(McAllister). The second chamber was equipped to perform photoelectron spectroscopy and contained an Al(Ka) X-ray source, a hemispherical electron energy analyzer (VG Scientific),and a mass spectrometer (UTI). The base pressure in each chamber was typically 2 x 1O-IO Torr. Substrates consisted of 5 mm x 5 mm x 0.5 mm pieces of boron-doped Si wafers (15 Q cm) oriented in the (100)direction. Prior to being placed under vacuum, the silicon substrates were subjected to a wet etchingloxidation procedure similar to that described by Henderson.8 This procedure cleaned the surface and produced a thin protective oxidelayerwhich could be removed by heating in vacuum. After etching and oxidizing, the silicon wafer was clipped into a tantalum foil sample holder which was spot-welded to two pins of an electrical feedthrough on a UHV sample manipulator. A chromel-alumel thermocouple was attached to the back surface of the sample using a ceramic adhesive (Aremco 516). The sample could be heated to 1100 K and cooled to 90 K via conduction from the tantalum holder. Once in vacuum, the protective oxide layer was desorbed by heating to 1100 K. This produced a clean, ordered 2 x 1 reconstructed surface as determined by XPS and LEED. Electronic grade TEOS(Aldrich)was contained in glass sample vials and purified by freeze-pump-thaw cycles and vacuum distillationprior to use. The TEOS was admitted into the vacuum chamber via a variable leak valve equipped with a 0.625 cm diameter dosingneedle. TPD experiments were performed with a heating rate of 5 Ws. HREEL spectra were collected using a 3.5-eV electron beam directed 60" from the surface normal with detection in the specular direction. The sample temperature was held at 90 K during HREELS analysis. To characterizesurfaceintermediates formed at higher temperatures, the sample was heated to the ( 8 ) Henderson, R. C. J . Electrochem. SOC. 1972,119, 772.
0743-7463/94/2410-3116$04.50/00 1994 American Chemical Society
TEOS Reactions on Si Surfaces temperature of interest and then rapidly quenched to 90 K, at which point an HREEL spectrum was collected. The thickness and composition of deposited silica layers were estimated using quantitative analyses of Si(2p) photoelectron spectra. The 2p electrons in elemental silicon have a binding energy of 99.1eV. Bonding of silicon to one, two, three, and four oxygen atoms results in shifts ofthe 2p binding energiest o 100.8, 101.7, 102.6, and 103.4eV, re~pectively.~ Therefore, the Si(2p) photoelectron spectrumof an oxidized silicon substrate can consist of up to five peaks, one from the substrate and four which correspondto the silicon atoms coordinated to differing numbers of oxygen atoms. Although the Si-0 bond is roughly 33%ionic, it is convenient to refer to silicon atoms bonded to one,two, three, or four oxygen atoms as having formal oxidation states of $1, +2, f 3 , and +4, respectively. For convenience,this nomenclature is used below. The relative populations ofthe Si oxidation states in the silica layer were estimated by fitting the Si(2p) photoelectron spectrum with Gaussian shaped peaks. In fitting the data the full-widthat half-maximumof all peak intensities were used as adjustable parameters. The oxide film thickness was estimated using a method similar to that described by Feldman and M a ~ e r . ~ In several cases it was useful to have silica films thicker than those that could be conveniently grown inside the UHV analysis system. These films were grown ex situ by oxidizing a freshly etched Si(100)sample in a 1:1:3solution of HCl, H202, and H2O. Once in vacuum, films produced in this manner were annealed for 15min at 900 K. This procedure resultedin silica films which were free of surface hydroxyl groups and had low levels of carbon contamination as determined by AES and HREELS.
Results In order to investigate the influence of the thickness of a silica film on the adsorption and reaction of TEOS, a series of TPD and XPS experiments were performed. In addition to determining the gaseous products of the surface reactions, each TPD experiment also served to deposit a small amount of SiO, on the surface. Information on the effect of the build-up of the SiO, transition layer on the TEOS decompositionpathways was obtained by observing changes in the TPD spectra collected during successive experiments. The thickness and composition of the SiO, layer were determined after each TPD experiment using XPS. This allowed changes in the desorption spectra to be compared to trends in the relative populations of the various silicon oxidation states in the deposited layer. Temperature Programmed Desorption. Figure 1 displays TPD spectra for mass-to-charge (mle)ratios of 27 and 2 collected during 12 consecutiveexperiments in which a n initially clean Si(l00)-2x 1surface was exposed to 0.6 langmuirs of TEOS at 90 K. In addition to the mle ratios displayed in Figure 1,mle 25,26, and 28 were also detected and had peak shapes and temperatures similar to that of mle 27. Based on the relative intensities of the mle 25, 26,27, and 28 signals, the peaks in the mle 27 spectra can be assigned to desorption of ethylene. The peaks in the mle 2 spectra correspond to desorption of molecular hydrogen. Other than a small amount of molecular TEOS which desorbed at 195 K, ethylene and hydrogen where the only gaseous species detected. A maximum temperature of 950 K was used in all of the TPD experiments to ensure that the deposited SiO, layer did not desorb. Following the adsorption of TEOS on the clean Si(100)2 x 1 surface (experiment l),ethylene desorbed in three overlapping peaks centered a t 520, 630, and 720 K and hydrogen desorbed in a single peak centered a t 780 K. The relative intensities of the three ethylene desorption peaks were 0.3:1.0:0.9. This result is nearly identical to that obtained in previous TPD studies of the reaction of TEOS on Si(100)-2xl . 5 s 6 As will be discussed below, XPS (9) Feldman, L. C.; Mayer, J. W. Fundamentals of Surface and Thin Film Analysis; Elsevier Science Publishers, Inc.: New York, 1986.
Langmuir, Vol. 10, No. 9, 1994 3117 TEOZ
iiOx
: d 11* 10
Hydrogen
600
400
800
1000 600 Temperature (K)
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1000
Figure 1. Series of (a) mle 27 and (b) mle 2 TPD spectra for 12 consecutive 0.6 langmuir doses of TEOS. 6 -
-
h
4u)' 5 $
4
-
Y 0
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c
-i€i
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Figure 2. Thickness of the SiO, layer as a function of total TEOS exposure. analysis indicated that SiO, had been deposited on the surface during the TPD experiment. Several trends can be observed in the TPD spectra from experiments 2 through 12. The ethylene peaks initially centered a t 520,630, and 720 K as well as the hydrogen peak a t 780 Kattenuated with each successiveexperiment and by experiment 12 were no longer detectable. Of the three ethylene peaks, the highest temperature feature decreased in intensity less rapidly than the other two. An additional peak in the mle 27 spectra centered a t 830 K was observed in experiments 5 through 12. Comparison to the relative intensities ofthe signals for mle 25,26, and 28 again allows this new peak to be assigned to desorption of ethylene. In contrast to the lower temperature desorption features, the ethylene peak at 830 K increased in intensity with each successive TPD run for experiments 5 through 12. Additional TPD experiments resulted in mle 27 and 2 spectra which were indistinguishable from those obtained during experiment 12. XPS analysis indicated that after the first 12 TPD cycles the silica layer was approximately 6 A in thickness. Attempts to grow thicker silica layers using TPD of TEOS proved to be unsuccessful. Figure 2 shows the relationship between total TEOS exposure and the thickness of the deposited layer. The data suggest that after 6 A of silica had been deposited, the reactivity of the surface toward TEOS decreased significantly. In order to continue to examine the dependence of the reactivity of adsorbed
Danner and Vohs
3118 Langmuir, Vol. 10, No. 9, 1994
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TEOS I SiOx Thickness
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(e)
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-/\200
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Figure 3. TPD spectra for mle 27 following exposure of silica films(a)8A,(b)12A, and(c)1OOO~inthicknessto0.6langmuirs of TEOS.
TEOS on the SiO, layer thickness, it was necessary to alter the experimental procedures. At this point, the 6-A film was heated to 900 K and then exposed to 2 x Torr of TEOS for 15 min. This produced a silica film approximately 8 A in thickness. The TPD spectrum obtained for mle 27 following exposure of the 8-A silica film to 0.6 langmuir of TEOS is displayed in curve a of Figure 3. The spectrum contains two ethylene desorption peaks centered a t 450 and 830 K. Hydrogen was not observed as a gaseous product in this experiment. Given the constraints of the UHV surface analysis systems used in this study, it was difficult to grow a film thicker than 8 in situ. In order to examine the surface reactions of TEOS on a thicker silica layer, a 12-A film (as determined by X P S ) was prepared ex situ using the procedure described above. The mle 27 TPD spectra obtained following exposure of the 12-A silica film to 0.6 langmuir of TEOS is displayed in curve b of Figure 3. The mle 27 spectrum contains a broad feature due to ethylene desorption between 400 and 600 K, similar to that in the TPD spectra obtained following exposure of the 8-A film to TEOS. However, in contrast to the data from the 8-A film, ethylene desorption at 830 K was not observed. TPD experiments were also performed using a Si02 film which was 1000 A in thickness (as determined by ellipsometry). Curve c of Figure 3 displays the TPD spectrum for mle 27 followingexposure of the 1000-Afilm to 0.6 langmuir of TEOS. The spectrum exhibits a broad ethylene peak between 400 and 600 K. The spectrum is nearly identical to that obtained from the 12-Afilm. Again, hydrogen desorption was not detected. X-ray Photoelectron Spectroscopy. Prior to each of the TPD experiments described above, XPS was used to characterize the thickness and composition of the SiO, layer. Figure 4 displays a subset of the Si(2p) spectra collected in these experiments as a function of the SiO, layer thickness. Also displayed in this figure are curve fits which were used to determine the intensity corresponding to the individual oxidation states of silicon. Panel a in Figure 4 displays the Si(2p) XPS spectrum of the clean silicon substrate. This spectrum contains a single peak centered a t 99.1 eV which corresponds to elemental silicon. The Si(2p)spectrum obtained after the first TEOS TPD experiment is displayed in panel b of Figure 4. In this spectrum there is intensity on the high binding energy side of the substrate peak originating from silicon atoms in higher oxidation states. Analysis of the data showed that the silica layer was less than one monolayer in thickness (Le. it did not completely cover
A
Bindlng Energy (eV)
Figure 4. Si(2p)photoelectron spectra of (a)Si 100) and silica films (b) 2 A, (c) 4 A, (d) 6 A, (e) 8 A, and (0 12 in thickness. Also displayed in the figure are the curve fits which were used to estimate the relative populations of the various oxidation states of silicon in the deposited layer.
6
the surface) and that the distribution of oxidation states in the silica film was approximately 55% Si+ and 45% Si2+. Intensity attributable to Si3+or Si4+was not detected. The XPS spectrum of a 4-A silica layer collected after TPD experiment 5 is displayed in panel c of Figure 2. The data indicate that the oxide layer contained 42%Si+,52% Si2+,and 6% Si3+.This was the first point in this series of experiments in which the SiO, layer was found to contain Si3+. Comparison to the TPD results shows that the emergence of silicon in a +3 oxidation state coincides with the appearance of ethylene desorption a t 830 K. The XPS spectrum collected after TPD experiment 12 is displayed in panel d of Figure 4. An analysis of this spectrum indicates that the oxide layer was 6 in thickness and that the distribution of the silicon oxidation states was 36% Si+, 42% Si2+,and 22% Si3+.As shown in panel e of Figure 4, the 8-fi film was the first to include silicon in a n oxidation state of $4. The composition of this silica layer was calculated to be 29% Si+, 36% Si2+, 24% Si3+,and 12% Si4+. The XPS spectrum of the film prepared by ex situ wet-chemical oxidation is displayed in panel f of Figure 4. This silica layer was estimated to be 12 A in thickness and to have a silicon oxidation state distribution of 7% Si+, 9% Si2+,20% Si3+,and 64% Si4+. The XPS data contained in Figure 4 are summarized in Figure 5 which displays the relative populations of the oxidation states of silicon in the deposited layer as a function of thickness. Also included in this figure are data for a 10-fi film which was prepared in the same manner a s the 12-A film. For films less than 3 A in thickness, only Si+ and Si2+were detected in the oxide. The relative population of +1 and +2 oxidation states decreased with increasing film thickness. The relative population of Si3+is negligible for films less than 3 A in thickness, increases for films 3-8 A in thickness where it reaches a maximum, and then decreases slightly for thicker films. Silicon atoms in a +4 oxidation state were only detected for films greater than 6 in thickness and their relative population increased for thicker films. At a thickness of 12 A the relative population of Si4+exceeds 60%, which suggests that the stoichiometry of the top few atomic layers is SiO2.
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A
Langmuir, Vol. 10, No. 9, 1994 3119
TEOS Reactions on Si Surfaces 60
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motions associated with these modes are depicted below.
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m
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s! na, 20
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a i 0 1 2
Thickness (A) Figure 6. Relative populations of silicon atoms in oxidation states of il,$2, f 3 , and +4 in the oxide layer as a function of thickness.
TEOS / 4 A SiOx Film SiOx
~
(a) ,
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1000
,
2000
,
,
3000
,
,
4000
Energy Loss (l/cm)
Figure 6. HREEL spectra of (a) 4 A thick silica film and following exposure of this film t o 0.6 langmuir of TEOS at 90 K and subsequentheating t o (b) 400 K, (c) 750 K, and (d) 900 K.
High-ResolutionElectron Energy Loss Spectroscopy. The dependence of the reaction of TEOS on silica film thickness was also investigated using HREELS. In these experiments, a silica film was deposited in a stagewise fashion similar to that used in the TPD experiments. An initially clean Si(100) sample was exposed to 0.6 langmuir of TEOS. An HREEL spectrum was then collected. HREEL spectra were also collected after briefly annealing the dosed-surface to 400,750, and 900 K. This adsorption-heating cycle was then repeated several times, thus allowing the surface species to be characterized both as a function of temperature and silica film thickness. As a n example of one of these cycles, Figure 6 displays HREEL spectra of a 4-A silica film before and after dosing with 0.6 langmuir of TEOS. The spectrum of the undosed surface contains peaks centered a t 340, 790,1025,2250, and 2880 cm-l. The three lowest energy peaks correspond t o the normal modes of siloxane bonds.lOJ1 The atomic (10)Thiry, P. A.; Liehr, M.; Pireaux, J. J.;Sporken, R.; Caudano, R.; Vigneron, J. P. Lucas, A. A. J . Vac.Sci. Technol. B 1985,3, 1118. (11)Schaefer, J. A.; Gopel, W. Surf. Sci. 1985,155, 535.
The peak a t 2250 cm-l is a t a n energy characteristic of a Si-H stretch in which the silicon atom also has three oxygens in its coordination sphere (i.e. 03Si3+-H).12 The presence of this peak demonstrates that hydride ligands remained on the surface after the previous reaction cycle. The remaining peak in the spectrum a t 2880 cm-I can be assigned to a v(C-H) mode of CH or CH2 species which remain on the surface after the previous reaction cycle. The HREEL spectrum obtained following exposure of the 4-A silica film to 0.6 langmuir of TEOS at 90 K and subsequent annealing to 400 K is displayed in curve b of Figure 6. In addition to the three Si-0 modes, losses are evident at 1410,2250,2905, and 3615 cm-'. The mode a t 1410 cm-' is in the frequency range characteristic of d(CH,) modes (x = 2 or 3). Unfortunately, the identification of the exact hydrocarbon species which gives rise to this peak is prevented by the domination of the lower frequency region of the spectrum by the Si-0 modes. It is clear, however, that hydrocarbon fragments remain on the surface after heating to 400 K. The peaks a t 2905 and 3615 cm-l can be assigned to C-H and 0-H stretching modes, respectively. The OH stretch most likely is due either to a small amount of water adsorbed from the background during data collection or to the formation of a few surface hydroxylgroups during TEOS decomposition. The peak a t 2250 cm-I can again be assigned to 1 4 0 3 SP-H). Further heating of the surface to 750 K (curve c Figure 6) resulted in several changes in the HREEL spectrum. A shoulder centered a t 2077 cm-l appeared on the low energy side of the v(03Si3+-H) peak. This frequency is characteristic of a v(Si-H) mode in which the silicon atom is fully reduced, i.e. v(Si,--H).l2 Two inferences can be drawn from the presence of this peak: silicon atoms which are not coordinated to oxygen are still present a t the interface, and the surface hydrocarbon species have undergone dehydrogenation,producing additional surface hydrides. Further changes in the spectrum upon heating included a decrease in the intensities of the d(CH,) and v(C-H) modes and a slight decrease in the frequency of the d(CH,) mode. The attenuation of these modes is consistent with the TPD results which show that ethylene desorbs between 400 and 750 K. However,the persistence of the d(CH,) mode after heating to 750 K indicates that some hydrocarbon fragments, most likely -CHzCH3 species, remain on the surface. These fragments react a t 830 K to liberate ethylene. Curve d of Figure 6 displays the HREEL spectrum after annealing to 900 K. Changes in the HREEL spectrum as a result of heating include the disappearance of the 6(CH,) mode, the attenuation of the v(C-H) mode and a shift in its frequency t o 2880 cm-l. Additionally, the intensity of the v(Si0-H) peak decreased. The disappearance of the d(CH,) mode and attenuation of the v(C-H) mode are consistent with the loss of hydrocarbon species from the surface resulting from the desorption of ethylene at 830 K. The v(C-H) mode a t 2880 cm-l again indicates the presence of adsorbed CH or CH2 species a t (12)Lucovsky, G. J . Vac.Sci. Technol. 1979,16, 1225.
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3120 Langmuir, Vol. 10,No.9,1994
sor A TEOS I SiOx
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Energy Loss (llcm)
Figure 7. HREEL spectra ofTEOS-grownsilica films ranging in thickness from 2 t o 6 A.
the end of the thermal cycle. The decrease in intensity ofthe v(Si,-,-H) peak is also consistent with the TPD results which show that hydrogen desorbs from the surface between 750 and 900 K. The v(Si3+-H) peak a t 2250 cm-l has not decreased in intensity which suggests that these hydride species do not participate in the reactions which liberate hydrogen. Sets of HREEL spectra such as the one described above were collected for a series of five film thicknesses varying from 2 to 6 A. The spectra collected following annealing to 900 K from each of these five experiments are displayed in Figure 7. Several trends can be identified from these data. The three Si-0 stretching modes increase in intensity and energy with increasing film thickness. This dependence of the vibrational modes of thin silica films on thickness has been reported previously.lOJ1 The increase in the intensity of the modes results from a n increase in the coverage of the SiO, film, while the shift t o higher frequencies can be associated with an increase in the average Si-0-Si bond angle in the oxide layer.” A second trend apparent in the data is an increase in the intensity of the v(03Si3+-H) mode with thickness. This demonstrates that for the thicker films, hydride species remained on the surface even after heating to 900 K.
Discussion A good starting point for discussing the reactions involved in the nucleation of a silica film on Si(100)-2x 1 is to examine the structure of the substrate prior to film deposition. The atomic structure of Si(100)-2x 1has been studied extensively11J3and a schematic side view of the surface is shown in Figure 8a. The surface contains dimer rows of silicon atoms in which each atom in the dimer contains a single dangling bond. The high level of reactivity of this surface has been attributed to the dangling bonds of the dimer atoms.14 Silicon atoms in the second and third atomic layers are also accessible to adsorbed species. Indeed, it has been suggested that during thermal oxidation with 0 2 , oxygen insertion (13) Hamers, R. J.; Tromp, R. M.; Demuth, J. E. Phys. Rev. B 1986, 34, 5343. (14) Yoshinobu, J.; Tanaka, S.; Nishijima, M. Jpn. J . A p p l . Phys. 1993,32,1171.
.
Sltco” Atom
0 Ow3”
\
Dmnglmmg Scnd
Figure 8. Schematic diagrams of surface structures. Descriptions of each structure are given in the text.
initially occurs between the first and second and second and third atomic layers prior to oxidation of the dimer b o n d ~ . ~ lKnowledge J~ of these details is useful in the interpretation of the surface reaction of adsorbed TEOS. The TPD spectra from the TEOS-dosed Si(100)-2x1 surface contained three separate ethylene desorption peaks. The origin of these peaks has been described previously in detai15J6and can be summarized a s follows. TEOS dissociates on the Si(100)-2x 1surface via cleavage of C-0 bonds to form surface ethyl groups and SiO, species. Upon heating to 520 K the adsorbed ethyl groups undergo P-hydride elimination to produce gaseous and adsorbed ethylene. The ethylene bonds in a di-a configuration across a surface silicon dimer (see Figure 8b). This reaction also produces adsorbed hydrides which react a t 780 K to produce gaseous Hz. A portion of the di-a bound ethylene desorbs at 630 K giving rise to the second ethylene peak in the TPD experiment. The origin of the third ethylene TPD peak is less clear; however, it may result from the desorption of di-a ethylene species which are stabilized by the presence of oxygen atoms in the coordination sphere of one or both of the surface Si dimer atoms. As will be discussed below, further support for the stabilization of adsorbed ethylene by surface oxygen is provided by trends in the ethylene TPD peak intensities. A schematic of the proposed intermediates in these reactions is shown in Figure 8b. The growth of the SiO, transition layer results in the attenuation and eventual disappearance of the hydrogen and the three ethylene TPD peaks which were observed in the TPD results from the Si(100)-2x 1 surface. Since the ethylene TPD peaks a t 630 and 720 K result from the desorption of di-a ethylene species bound to surface silicon dimers, these peaks are indicative ofthe presence of silicon dimers on the surface. The TPD and XPS results suggest (15)D’Evelyn, M. P.; Nelson, M. M.; Engel, T. Surf. Sci. 1987,186, 75. (16) Rueter, M. A.; Vohs, J. M. Surf. Sci. 1992,262,42.
TEOS Reactions on Si Surfaces
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that such sites are present for films less than 6 in thickness. As can be seen in Figure 2, this is also the thickness range during which the reactivity of the surface toward TEOS is the greatest. As noted above, previous XPS and HREELS studies have shown that the oxidation of Si(100)-2xl with 0 2 initially proceeds via oxygen insertion into subsurface Si-Si bonds.11J5 The Si-Si bonds between the first and second and second and third layers undergo oxidation prior to the surface Si-Si dimer bonds. This oxidation mechanism accounts for coexistence of both silicon atoms in higher oxidation states (i.e. +1 and $2) and intact silicon dimers a t the interface. This is shown schematically in Figure 8c. It is interesting that the XPS spectra of silica films less than 4 in thickness produced both by reaction of TEOS and oxidation with 0 2 are similar; they both contain primarily silicon in +1 and +2 oxidation state^.^ This is somewhat surprising since the silicon atoms in the TEOS reactant are fully oxidized (i.e. Si4+). This suggests that for the reaction conditions used in this study, the structure of the transition region is governed primarily by thermodynamics. It is therefore likely that local structures such as those depicted in Figure 8c are also produced during the initial stages of film growth using TEOS. This model of the surface is useful in assigning the ethylene peaks a t 630 and 720 K in the TPD spectra. As noted earlier, the ethylene desorption feature a t 630 K can be assigned to di-aethylene coordinated to unoxidized surface silicon dimers. This assignment is consistent with the TPD of ethylene from clean Si(100)-2x1in which di-a bound ethylene desorbs a t a similar t e m p e r a t ~ r e . ' ~ Electron withdrawing substituents, such as oxygen, in the coordination sphere of the surface silicon dimer atoms would be expected to increase the Si-C bond energy in the adsorbed ethylene complex.l* Thus, the ethylene peak a t 720 K is most likely due to the desorption of ethylene coordinated to partially oxidized surface silicon dimers. These two types of adsorbed ethylene species are shown in Figure 8b. This model also accounts for the trend observed in the relative intensities of the two ethylene peaks. As the surface becomes increasingly oxidized the feature associated with unoxidized silicon dimers attenuates more rapidly than that associated with the partially oxidized silicon dimers. As shown in Figure 5, for films less than 4A in thickness the silicon atoms are primarily in +1 and +2 oxidation states. However, for thicker films there is a rapid increase in the relative population of silicon in higher oxidation states. This is also accompanied by the appearance of a high-temperature ethylene desorption feature (830 K) in the TPD spectra. Comparison oftheXPS and TPD results shows that the intensity of the high-temperature ethylene peak correlates with the relative population of Si3+in the surface region. Therefore, the surface species which reacts to liberate ethylene a t 830 K appears to be coordinated to surface silicon atoms in a +3 oxidation state. Although the hydrocarbon species which gives rise to the high temperature ethylene peak could not be definitively identified from the HREEL spectra, they are most likely adsorbed ethyl groups. As discussed above, dissociative adsorption ofTEOS occursvia c-0 bond scission upon interaction with surface silicon atoms in oxidation states of 0, +1, or +2 to produce adsorbed ethyl species at temperatures below 450 K. Upon heating to 520 K these surface ethyl species react to form both gaseous and adsorbed ethylene. By analogy, it is reasonable to assume
A
(17) Yoshinobu, J.; Tsuda, H.; Onchi, M.; Nishijima, M. J. Chem. Phys. 1987,87, 772. (18) Patai, S.; Rappoport, Z. The Chemistry of Organic Silicon Compounds; John Wiley & Sons: New York, 1989.
that upon interaction with Si3+,dissociation of TEOS also occurs via C-0 bond scission to form surface ethyl species. However, since surface Si3+atoms do not have any silicon nearest neighbors, it is unlikely that dehydrogenation of the adsorbed ethyl groups would lead t o the formation di-a ethylene complexes. It appears that in this case, dehydrogenation results in the formation of only gaseous ethylene. The HREELS results indicate that the hydrogen liberated in this reaction remains on the surface and is bound to Si3+. The TEOS TPD results obtained for films greater than 8 in thickness contained a single, broad ethylene desorption feature centered between 400 and 600 K. Comparison of the XPS and TPD results reveals that the presence of this feature correlates with the population of Si4+in the oxide film. Thus, the broad ethylene desorption feature between 400 and 600 K is characteristic of the reaction of TEOS on stoichiometric Si02 films. Others have shown that alkoxysilanes react with strained siloxane bonds on Si02 surfaces to produce adsorbed alkoxy and trialkoxysilane ~ p e c i e s . The ~ ~ *ethylene ~~ detected in the TPD spectra results from decomposition of the ethoxy ligands in these surface complexes.
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Summary The reactions ofTEOS on a silica-covered silicon surface were found to depend strongly on the silica film thickness. For silica films less than 12A in thickness, changes in the surface reaction pathways were found to correlate with the relative population of silicon oxidation states in the surface region. As long as silicon dimers are present on the surface, TEOS dissociatively adsorbs via cleavage of C-0 bonds in the ethoxy ligands to form surface ethyl groups. At temperatures near 520 K, these ethyl groups undergo ,%hydride elimination to form gaseous and adsorbed ethylene. The adsorbed ethylene bonds in a di-a configuration across a surface silicon dimer. Ethylene species coordinated to dimer sites, in which both silicon atoms are in a n oxidation state of zero, desorb a t 630 K. On partially oxidized surfaces, a portion ofthe dimer sites are composed of Si atoms which have one or two oxygens in their coordination sphere (Le. Si+ and Si2+). Ethylene coordinated to these dimer sites desorbs at 720 K. For silica films 5-8 in thickness the surface Si atoms are primarily in a +3 oxidation state. TEOS dissociates on these sites to form adsorbed ethyl groups. These species undergo &hydride elimination a t 830 K to form gaseous ethylene. For films thicker than 12 the stoichiometry of the topmost atomic layer approaches that of SiOz. The surface is, therefore, composed primarily of Si4+. TEOS reacts with strained siloxane bonds on this surface to form adsorbed ethoxy and ethoxysilane species. These species decompose between 400 and 600 K producing gaseous ethylene. The structure of the transition region between the Si(100) substrate and the deposited Si02 film was found to be similar to that produced by thermal oxidation. This result suggests that for the conditions used in this study the structure of the transition region is controlled primarily by thermodynamics. Acknowledgment. This work was supported by the National Science Foundation through the Presidential Young Investigators Program. (Grant No. CTS89-57056) and the Shell Oil Co. Foundation. We also acknowledge the University of Pennsylvania's Laboratory for Research on the Structure of Matter (LRSM) for the use of their facilities. The LRSM is supported by the National Science Foundation MRL Program (Grant No. DMR91-20668).
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(19) DuBois, L. H.; Zegarski, B. R. J . A m . Chem. Soc. 1993, 115, 1190. (20)Tedder, L. L.; Crowell, J.E . J .Vac.Sei. 2'echnol.A 1981,9,1002.
