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Low pressure chemical vapor deposition of silicon carbide thin films from hexamethyldisilane
Thin Solid Films 252 (1994) 13-18
Low pressure chemical vapor deposition of silicon carbide thin films from hexamethyldisilane Hsin-Tien Chiu*, Jen-Shiou Hsu Department of Applied Chemistry, National Chiao Tung University, Hsinchu 30050, Taiwan
Received 28 October 1993; accepted 2 June 1994
Abstract Silicon carbide thin films were grown by low pressure chemical vapor deposition using hexamethyldisilane Me,SiSiMe, as the single-source precursor. Deposition of uniform thin films on Si( 1 1 1) substrates was carried out at temperatures 1123- 1323 K in a hot-wall reactor. The growth rates were 0.9-55 nm min-’ depending on the conditions employed. Estimated energy of activation is 110 kJ mol- ‘. Bulk elemental composition of the thin films, studied by an electron probe X-ray microanalyzer, is best described as Sic, (x = 0.8-2.3). The Si/C ratio increased with increasing temperature of deposition. The films were cubic polycrystals, a = 0.435-0.438 nm, as indicated by scanning transmission electron microscopy, electron diffraction and X-ray diffraction. The lattice parameter decreased with increasing temperature of deposition. Elemental distribution within the films, studied by Auger depth profiling and X-ray photoelectron spectroscopy, was uniform. Gas phase products H,, CH,, C2H4, Me,SiH and Me,Si were identified and possible reaction pathways were proposed. Keywords:
Chemical vapour deposition;
Silicon carbide
1. Introduction
Silicon carbide is a potentially useful material for many modern electronic devices [ 11. In order to grow silicon carbide thin films with homogeneous elemental distributions at low temperatures by chemical vapor deposition (CVD), there are many interests to employ organosilanes as the single-source precursors. For example, in addition to MeSiCl,, which has been used frequently, Me,Si [2], Me,iH, [3], Et,SiH, [4], and 1,3-disilacyclobutane ( SiH,CH,)2 [ 51 have been employed to deposit silicon carbide thin films. Additionshown that volatile air-stable ally, we have organopolysilanes of small molar mass, such as (Me,Si),SiMe,, (Me,Si),SiMe, (Me$i)$i [6], and (Me,Si), [7,8], can be used to grow silicon carbide thin films on silicon substrates by low pressure chemical vapor deposition (LPCVD). The simplest member of the permethylpolysilane family, hexamethyldisilane (HMDS), Me,SiSiMe,, has been utilized to grow silicon carbide thin films by conventional CVD [9] and to
*Corresponding author. 0040-6090/94/.$7.000 1994 - Eisevier Science S.A. All rights reserved SSDf 0040-6090(94)06219-B
deposit organosilicon polymers by plasma-enhanced CVD [lo]. In this study, we report the growth of silicon carbide thin films from HMDS by LPCVD at relatively low temperatures 1123- 1323 K.
2. Experimental
details
The precursor, Me$iSiMe,, was purchased from Merck and transferred under argon into a reservoir prior to the experiments without further purification. Si( 1 1 1) wafers, cut into squares 1 cm x 1 cm, were cleaned using standard procedures to remove silicon dioxide and grease on the surfaces. The CVD experiments were carried out in a remote plasma-enhanced hot-wall reactor described previously [8]. It consists of a quartz tube of 30 mm diameter heated externally by a tube furnace and evacuated by a mechanical pump. The hydrogen plasma can be ignited by applying radio-frequency (RF, 13.56 MHz) power to
the external electrodes preceding the deposition U-trap was installed between the quartz tube pump when collection of gas phase products was desired. For each experiment, a base
zone. A and the at 77 K pressure
14
H.-T. Chiu, J.-S. Hnr I Thin Solid Films, 252 (1994) 13- 18
10-l Pa was obtained before the substrates were loaded under an Ar atmosphere. The reactor was evacuated again while it was heated to the desired temperature of deposition. Then, the substrates were etched in hydrogen plasma (RF power = 40 W, H2 flow rate = 40 seem) for 10 min. After the completion of this process, the precursor was vaporized at a controlled pressure into the reactor. The deposition experiments were performed at temperatures between 1123 and 1323 K. The samples were annealed for 10 min after the depositions, then allowed to cool to room temperature. X-Ray diffraction (XRD) patterns of the films were obtained with a diffractometer MAC MXP-3 using Cu Kx radiation. Scanning electron micrographs (SEM) were obtained on a Hitachi S-570 instrument operated at an accelerator voltage of 20 kV. Scanning transmission electron micrographs (STEM) and electron diffraction patterns (ED) were taken from a JEOL JEM-2000FX microscope operated at an accelerator voltage of 160 kV. Elemental compositions were analyzed using a JEOL JCX-733 electron probe X-ray microanalyzer (EPMA). Auger and X-ray photoelectron spectra (XPS) were obtained from a Perkin-Elmer PHI-560AM SAM/1605 ESCA. Infrared spectra were measured on a Nicolet 520 instrument. Volatile products trapped at 77 K were analyzed by gas chromatography-mass spectrometry (GC-mass) on a VG Trio 3000 instrument. On-line analyses of gas phase products were performed using a residual gas analyzer (RGA) VG Quadruples Sensorlab 200D.
3. Results and discussion The deposition experiments (Table 1) were conducted in a low pressure hot-wall CVD reactor capable of performing prezetching of the surface of the Si substrates by Hz plasma. Our previous studies indicated that this process improved thin film’s adhesion and homogeneity significantly [8]. Smooth thin films with good adhesion to the substrates (Scotch Tape test) were grown at temperatures of 1123-1323 K. SEM photographs of the films deposited at 1173 and 1323 K are shown in Figs. l(a) and l(b). They indicate that the size of the microcrystallines increased with increasing temperatures of deposition. From the thicknesses of the Table 1 Typical growth conditions thin films from HMDS
used for the deposition
Deposition temperature, T, Total pressure, P,,,,, Pressure of HMDS, PHMDS Carrier gas and flow rate Substrate Plasma surface preetching
of silicon carbide
112331323 K 4- 105 Pa 3-20 Pa No carrier gas or H, 20 seem Si(l I 1) H, 20 seem, 13.56 MHz, 40 W, IO min __._~_
(a)
(b)
(4 Fig. 1. SEM photographs of thin films deposited on Si( I 1 1). (a) T, = Surface, T, = II73 K, PHMDS = 8 Pa, I = 60 min; (b) surface, rr = 1173 K, 1323 K, PHMOs = 8 Pa, I = 60 min; (c) cross-section, P HMDS = 20 Pa, I = 45 min; (d) cross-section, r, = 1173 K, P,,,, = I3 Pa, H, flow rate = 20 seem, I = 60 min.
films, estimated from the cross-sectional SEM studiessuch as those shown in Figs. l(c) and (d), the growth rates are estimated to be 0.9-55 nm min-’ depending on the growth conditions. Corresponding Arrhenius plots are shown in Fig. 2. Experiments carried out under H, flow show little effects on the plot. At temperatures between 1123 and 1223 K, straight lines showing apparent activation energies 110 and 113 kJ mol- ’ are obtained. These values indicate that the reactions were surface-controlled processes since the C-H, C-Si and Si-Si bond dissociation energies of HMDS, 435, 311 and 337 kJ mol - 1, respectively [ 11,121, exceed the apparent activation energies significantly even after the mass transport contributions are considered. At temperatures exceeding 1273 K, the growth rates do not increase significantly with increasing temperatures of deposition, suggesting that the reactions were controlled by diffusion of the precursor molecules to the surfaces. Employing H, in the reactions shows little influence on the growth behavior also. The films grown at 1173 and 1323 K, removed by etching the Si substrates from the backside with a solution of HF and HNO,, were further studied by STEM and ED (Fig. 3). The bright-field STEM photograph (Fig. 3(a)) and the ED pattern (Fig. 3(b)) indicate that the film grown at 1173 K shows clusters of
H.-T.
Chiu, J.-S.
Hsu / Thin Solid Films, 252 (1994) 13-18
T (K) 1123
1223
1323
T-
t ._ E E C
20.;5
0.90 l/T
(x 10-3 K-l)
carrier gas carrier gas
O-
PHMDS
= 13 Pa, no
l n -
PHMDS
= 20 Pa, no
PHMDS
= 13 Pa, Flow Rate (H2) = 20 seem
Fig. 2. Arrhenius
plot of thin films deposited
(4
at
I 123- 1323 K.
15
very fine f.c.c. (face-centered cubic) crystals distributed evenly. On the other hand, the polycrystals grown at 1323 K have larger sizes and their distribution within the film is not even. As shown in Fig. 3(c), there are areas of high transmission and low transmission. In addition to ring patterns of randomly oriented fine crystals, the ED pattern in Fig. 3(d) shows slightly diffused dot patterns. These are characteristic of f.c.c. crystals with preferred growth orientation in the [l 1 1] direction. From the ED studies, we estimate that the lattice parameter, a, is 0.437 and 0.436 nm for the films grown at 1173 and 1323 K, respectively, close to the value of /I-Sic (0.4358 nm). The ED pattern of a film deposited at 1123 K shows diffused rings only, suggesting that the film is amorphous. XRD patterns of the films deposited without and with H, flow are shown in Figs. 4 and 5, respectively. There are little differences between the patterns shown in Figs. 4 and 5. Major Cu Kx peaks of P-Sic (1 1 l), (2 0 0), (2 2 0) and (3 1 1) reflections are observed. As shown in Fig. 6, the lattice parameter, a, calculated from the XRD data, although close to the value of B-Sic, shrinks with increasing temperatures of deposi-
(b)
Cd) Fig. 3. STEM studies of thin films deposited and (d) ED, T, = 1323 K.
at P HMDS = 4 Pa. (a) Bright-field
micrograph
and (b) ED, T, = II73 K; (c) bright-field
micrograph
16
Chiu, J.-S. Hsu I Thin Solid Films, 252 (1994) 13-18
H.-T.
d 75
Fig. 4. XRD patterns of thin films deposited 1323 K, (b) 1273 K, (c) 1223 K, (d) 1173 K.
0.6
at PHMDs = 8 Pa. (a)
1123
(4
ALL
1223
1323 T (K)
b.
A
c d
25
60 28
75 ldegreel
0.6 -
Fig. 5. XRD patterns of thin films deposited at P,,,, = 20 Pa, H, flow rate = 20 seem. (a) 1323 K, (b) 1273 K, (c) 1223 K, (d) 1173 K. 0.6-
P = 0.436 5
O*‘:
“E i
1;3
lb)
0.436
T (IO
Fig. 7. Effect of temperature of deposition carrier gas, (b) H, flow rate = 20 seem.
$ ‘E z J 0.434
(a) No
i12’23 T (K)
1123
Ol -
on Si/C ratio.
1323
PHMDS
=4 Pa, no carrier gas
PHMDS
=4 Pa, Flow Rate (Hp) = 20 seem
Fig. 6. Effect of temperature
of deposition
on lattice parameter.
tion, from 0.438 nm at 1173 K to 0.435 nm at 1323 K, an indication of variable elemental ,compositions. Bulk elemental composition of the thin films, determined by EPMA, is Sic, (x = 0.8-2.3). In Figs. 7(a) and (b), Si/C ratios of the thin films, derived from the EPMA studies, are plotted against the temperature of deposition. It shows that the Si/C ratio increased with increasing temperature of deposition. Employing H, into the reactions shows little effect again. This result and the XRD data can be correlated and rationalized as fol-
lows. Stoichiometric P-Sic has a f.c.c. sublattice of Si atoms with half of the tetrahedral sites occupied by C atoms to form a zinc blende type of structure. For thin films with low Si/C ratio, which is C rich, more than half of the tetrahedral sites could be occupied by carbon atoms, causing the lattice parameter to enlarge. On the other hand, for thin films with high Si/C ratios, the lattice parameter could shrink because less than half of the tetrahedral sites are occupied. This explains why athin s,,,, is larger than aS+o when the growths were performed at low temperatures of deposition but smaller at high temperatures. Auger depth profiles of the thin films, such as that shown in Fig. 8, indicate uniform elemental distributions. High resolution XPS of two thin films, deposited without and with H2 carrier gas and characterized as received, are shown in Fig. 9.
H.-T. Chiu, J.-S. Hsu I Thin Solid Films, 252 (1994) 13-18
4
8 Etch Time (min)
Fig. 8. Auger depth profile of a thin film. T, = 1223 K, P,,,, 13 Pa.
=
a
I
1
104
I
I
100
98
Binding Energy MI)
288
284
280
Binding Energy (eV)
Fig. 9. XPS of thin films characterized as received. Si,, region, (a) T, = 1223 K, P,,,, = 4 Pa, and (b) T, = 1173K, P,,,s = 3 Pa, H, flow rate = 20 seem; C,, region, (c) T, = 1223 K, P,,,s = 4 Pa, and (d) T, = 1173 K, PHMDS= 3 Pa, H, flow rate = 20 seem.
Again, little differences are observed for these films. The SiZp region shows major signals at 100.6 and 100.4 eV and the C,, region shows major signals at 283.8 and 284.0 eV, close to the values expected for silicon carbide. Since none of the studies mentioned above, ED, XRD and XPS, show signals assignable to pure silicon or carbon, we conclude that formation of clusters of free carbon and free silicon phases in the films is unlikely. Clearly, this is the consequence of using single-source
17
precursor. FT-IR studies showed major absorptions assignable to Si-C stretchings at 790-800 cm-’ for all of the films. For thin films deposited at 1123 K, additional weak absorption signals due to residual C-H stretchings were observed at 2900-3100 cm-‘. In order to understand the reaction pathways, we analyzed the gas phase products formed in the CVD processes by a residual gas analyzer. At PHMDsof 40 Pa and 1073 K, only 15% HMDS decomposed to generate Hz, CH,, C2H, and Me,SiH. When the temperatures were increased above 1223 K, the precursor decomposed completely to generate, in addition to the gases mentioned above, Me,Si. With increasing temperature of deposition, the concentration of Me,SiH decreased while the concentration of Me,Si increased. At 1323 K, Me,Si was the sole silicon containing gas product. Below 1223 K, Arrhenius plots of the ions originated from H,, CH, and C2H, were drawn and the slops were calculated to be 114, 51 and 79 kJ mol- ’ respectively. Although the value 114 kJ mol-’ is very close to the apparent energy of activation estimated from the growth rates mentioned above, it is not easy to correlate the data and derive definite mechanistic conclusions. Nonetheless, formation of the hydrogen molecules by combining gaseous H atoms is a less favored route because the C-H bonds of HMDS are strong and energies higher than the apparent activation energies are needed to dissociate gaseous HMDS molecules into H atoms and Me,SiSiMe,CH,. fragments. It is more likely that the HZ molecules were formed by combining surface hydrogens. Formation of CH, and C2H, by combining surface hydrocarbon species is also speculated. In general, removal of surface species as gaseous products to increase nucleation sites is an important step in thin film growth. To account for our observations, a generalized reaction route, although speculative, is proposed in Scheme 1. In route a, HMDS is adsorbed on the surface first, then eliminates Me,SiH to form “Me,Si=CH,” attached to the surface. After a methane molecule is removed in step 2, the adsorbed species “MeSiCH” may undergo several different decomposition steps further, such as 3, 4 and 5, before residual atoms and fragments “SiCI”, “Sic”, and “Si” are generated and incorporated into the lattice. In route b, decomposition of HMDS generates a SiMe, molecule and a “SiMe,” fragment adsorbed on the surface. After the rearrangement in step 7, this species undergoes further elimination of CH, and H, via step 8 to form a “Sic” unit and elimination of CH, and C,H, via step 9 to leave an adsorbed Si atom. Gas phase reactions analogous to steps 1, 6 and 7, involving the formation of transient species Me,Si=CH,, SiMez and MeHSi=CH,, have been reported in the literature [ 11,131. To account for the low Si/C ratios of the films grown at low temperatures, we postulate that the reaction via route a is the major pathway because Me,SiH
18
H.-T.
Chiu, J.-S. Hsu I Thin Solid Films, 25.2 (1994) 13- 18
77 Scheme
9 97
were surface-catalyzed. The lattice parameter of the cubic polycrystalline thin films, a = 0.435-0.438 nm, decreased with increasing temperature of deposition. Depending on the deposition conditions, bulk elemental composition of the thin films is determined to be Sic, (x = 0.8-2.3). In general, the Si/C ratio increased with increasing temperature of deposition. Elemental distribution within the films, studied by Auger depth profiling, was uniform. Employing hydrogen in the reaction shows little influence on the growth behavior. Based on the gas phase products detected, a possible reaction pathway involving the formation of transient species Me2Si=CH2, SiMe, and MeHSi=CH, adsorbed on the surface by eliminating Me,SiH and SiMe, from the precursor molecule is proposed.
1.
Acknowledgments
was the major Si-containing product detected and the reaction through step 4 is the only way in the proposed scheme to obtain a composition of low Si/C. At high temperatures, route b becomes the favored pathway because the concentration of SiMe, increased in the experiment and the reaction through step 9 can be held responsible for the high Si/C ratios. In contrast to the routes mentioned above, we speculate that the activation process in the APCVD of silicon carbide from HMDS [9] involves either Si-Si or Si-C bond homolysis in the gas phase because the apparent energy of activation of the process, 312 kJ mol-‘, is much larger than the value found in this study and very close to the Si-Si and the Si-C bond dissociation energies reported before, 337 and 311 kJ mol- ’ respectively [ 11,121.
4. Conclusions We have shown that silicon carbide thin films can be grown by LPCVD employing air-stable hexamethyldisilane Me,SiSiMe, as the single-source precursor at relatively low temperatures 1123- 1323 K. Pretreatment of the surfaces of the Si( 1 1 1) substrates by ‘hydrogen plasma ensures the deposition of uniform thin films with good adhesion. The growth rates were 0.9-55 nm min-’ and the apparent energy of activation is estimated to be 110 kJ mol- ‘, indicating that the reactions
We thank the National Science Council of the Republic of China (NSC-Sl-0208-M009-508) for support, and the Instrument Center of the NSC for sample analyses.
References
ill
For example see: E. Emin, T.L. Aselage and C. Wood (eds.), Novel Refractory Semiconductors, Mater. Res. Sot. Symp. Proc. 97, Materials Research Society, Pittsburgh, PA, 1992. 121A. Figueas, S. Garelike, C. Combescure and C. Dupuy, J. Cryst. Growth, 110 (1991) 528. [31 I. Golecki, F. Reidinger and J. Marti, Appl. Phys. Lett., 60 (1992) 14. [41 J.M. Grow, R.A. Levy, Y.T. Shi and R.L. Pfeffer, J. Electrochem. Sot., 140 (1993) 851. [51 D.J. Larkin and L.V. Interrante, Chem. Muter., 4 (1992) 22. [61 H.-T. Chiu and P.-F. Wu, J. Chin. Chem. Sot., 38 (1991) 231. 1323. [71 H.-T. Chiu and S.-F. Lee, J. Mater. Sci. Lett., lO(l991) PI H.-T. Chiu and S.-H. Huang, J. Muter. Sci. Left., 12( 1993) 537. S. Nishino and J. Saraie, J. Electrochem. Sot., [91 K. Takahashi, 139 (1992) 3565. D.C. Apperley and J.P.S. Badyal, Chem. [lOI J.L.C. Fonseca, Mater., 4 (1992) 1271. [Ill I.M.T. Davidson and A.B. Howard, J. Chem. Sot., Faraday Trans., I (1975) 69. and A. Salzer, Organometallics, VCH, Wein1121 C. Elschenbroich heim, 1989, p. 94. T.J. Barton and N. Till[I31 I.M.T. Davidson, S. Ijadi-Maghsoodi, man, J. Chem. Sot., Chem. Commun., ( 1984) 478.
Low pressure chemical vapor deposition of silicon carbide thin films from hexamethyldisilane Hsin-Tien Chiu*, Jen-Shiou Hsu Department of Applied Chemistry, National Chiao Tung University, Hsinchu 30050, Taiwan
Received 28 October 1993; accepted 2 June 1994
Abstract Silicon carbide thin films were grown by low pressure chemical vapor deposition using hexamethyldisilane Me,SiSiMe, as the single-source precursor. Deposition of uniform thin films on Si( 1 1 1) substrates was carried out at temperatures 1123- 1323 K in a hot-wall reactor. The growth rates were 0.9-55 nm min-’ depending on the conditions employed. Estimated energy of activation is 110 kJ mol- ‘. Bulk elemental composition of the thin films, studied by an electron probe X-ray microanalyzer, is best described as Sic, (x = 0.8-2.3). The Si/C ratio increased with increasing temperature of deposition. The films were cubic polycrystals, a = 0.435-0.438 nm, as indicated by scanning transmission electron microscopy, electron diffraction and X-ray diffraction. The lattice parameter decreased with increasing temperature of deposition. Elemental distribution within the films, studied by Auger depth profiling and X-ray photoelectron spectroscopy, was uniform. Gas phase products H,, CH,, C2H4, Me,SiH and Me,Si were identified and possible reaction pathways were proposed. Keywords:
Chemical vapour deposition;
Silicon carbide
1. Introduction
Silicon carbide is a potentially useful material for many modern electronic devices [ 11. In order to grow silicon carbide thin films with homogeneous elemental distributions at low temperatures by chemical vapor deposition (CVD), there are many interests to employ organosilanes as the single-source precursors. For example, in addition to MeSiCl,, which has been used frequently, Me,Si [2], Me,iH, [3], Et,SiH, [4], and 1,3-disilacyclobutane ( SiH,CH,)2 [ 51 have been employed to deposit silicon carbide thin films. Additionshown that volatile air-stable ally, we have organopolysilanes of small molar mass, such as (Me,Si),SiMe,, (Me,Si),SiMe, (Me$i)$i [6], and (Me,Si), [7,8], can be used to grow silicon carbide thin films on silicon substrates by low pressure chemical vapor deposition (LPCVD). The simplest member of the permethylpolysilane family, hexamethyldisilane (HMDS), Me,SiSiMe,, has been utilized to grow silicon carbide thin films by conventional CVD [9] and to
*Corresponding author. 0040-6090/94/.$7.000 1994 - Eisevier Science S.A. All rights reserved SSDf 0040-6090(94)06219-B
deposit organosilicon polymers by plasma-enhanced CVD [lo]. In this study, we report the growth of silicon carbide thin films from HMDS by LPCVD at relatively low temperatures 1123- 1323 K.
2. Experimental
details
The precursor, Me$iSiMe,, was purchased from Merck and transferred under argon into a reservoir prior to the experiments without further purification. Si( 1 1 1) wafers, cut into squares 1 cm x 1 cm, were cleaned using standard procedures to remove silicon dioxide and grease on the surfaces. The CVD experiments were carried out in a remote plasma-enhanced hot-wall reactor described previously [8]. It consists of a quartz tube of 30 mm diameter heated externally by a tube furnace and evacuated by a mechanical pump. The hydrogen plasma can be ignited by applying radio-frequency (RF, 13.56 MHz) power to
the external electrodes preceding the deposition U-trap was installed between the quartz tube pump when collection of gas phase products was desired. For each experiment, a base
zone. A and the at 77 K pressure
14
H.-T. Chiu, J.-S. Hnr I Thin Solid Films, 252 (1994) 13- 18
10-l Pa was obtained before the substrates were loaded under an Ar atmosphere. The reactor was evacuated again while it was heated to the desired temperature of deposition. Then, the substrates were etched in hydrogen plasma (RF power = 40 W, H2 flow rate = 40 seem) for 10 min. After the completion of this process, the precursor was vaporized at a controlled pressure into the reactor. The deposition experiments were performed at temperatures between 1123 and 1323 K. The samples were annealed for 10 min after the depositions, then allowed to cool to room temperature. X-Ray diffraction (XRD) patterns of the films were obtained with a diffractometer MAC MXP-3 using Cu Kx radiation. Scanning electron micrographs (SEM) were obtained on a Hitachi S-570 instrument operated at an accelerator voltage of 20 kV. Scanning transmission electron micrographs (STEM) and electron diffraction patterns (ED) were taken from a JEOL JEM-2000FX microscope operated at an accelerator voltage of 160 kV. Elemental compositions were analyzed using a JEOL JCX-733 electron probe X-ray microanalyzer (EPMA). Auger and X-ray photoelectron spectra (XPS) were obtained from a Perkin-Elmer PHI-560AM SAM/1605 ESCA. Infrared spectra were measured on a Nicolet 520 instrument. Volatile products trapped at 77 K were analyzed by gas chromatography-mass spectrometry (GC-mass) on a VG Trio 3000 instrument. On-line analyses of gas phase products were performed using a residual gas analyzer (RGA) VG Quadruples Sensorlab 200D.
3. Results and discussion The deposition experiments (Table 1) were conducted in a low pressure hot-wall CVD reactor capable of performing prezetching of the surface of the Si substrates by Hz plasma. Our previous studies indicated that this process improved thin film’s adhesion and homogeneity significantly [8]. Smooth thin films with good adhesion to the substrates (Scotch Tape test) were grown at temperatures of 1123-1323 K. SEM photographs of the films deposited at 1173 and 1323 K are shown in Figs. l(a) and l(b). They indicate that the size of the microcrystallines increased with increasing temperatures of deposition. From the thicknesses of the Table 1 Typical growth conditions thin films from HMDS
used for the deposition
Deposition temperature, T, Total pressure, P,,,,, Pressure of HMDS, PHMDS Carrier gas and flow rate Substrate Plasma surface preetching
of silicon carbide
112331323 K 4- 105 Pa 3-20 Pa No carrier gas or H, 20 seem Si(l I 1) H, 20 seem, 13.56 MHz, 40 W, IO min __._~_
(a)
(b)
(4 Fig. 1. SEM photographs of thin films deposited on Si( I 1 1). (a) T, = Surface, T, = II73 K, PHMDS = 8 Pa, I = 60 min; (b) surface, rr = 1173 K, 1323 K, PHMOs = 8 Pa, I = 60 min; (c) cross-section, P HMDS = 20 Pa, I = 45 min; (d) cross-section, r, = 1173 K, P,,,, = I3 Pa, H, flow rate = 20 seem, I = 60 min.
films, estimated from the cross-sectional SEM studiessuch as those shown in Figs. l(c) and (d), the growth rates are estimated to be 0.9-55 nm min-’ depending on the growth conditions. Corresponding Arrhenius plots are shown in Fig. 2. Experiments carried out under H, flow show little effects on the plot. At temperatures between 1123 and 1223 K, straight lines showing apparent activation energies 110 and 113 kJ mol- ’ are obtained. These values indicate that the reactions were surface-controlled processes since the C-H, C-Si and Si-Si bond dissociation energies of HMDS, 435, 311 and 337 kJ mol - 1, respectively [ 11,121, exceed the apparent activation energies significantly even after the mass transport contributions are considered. At temperatures exceeding 1273 K, the growth rates do not increase significantly with increasing temperatures of deposition, suggesting that the reactions were controlled by diffusion of the precursor molecules to the surfaces. Employing H, in the reactions shows little influence on the growth behavior also. The films grown at 1173 and 1323 K, removed by etching the Si substrates from the backside with a solution of HF and HNO,, were further studied by STEM and ED (Fig. 3). The bright-field STEM photograph (Fig. 3(a)) and the ED pattern (Fig. 3(b)) indicate that the film grown at 1173 K shows clusters of
H.-T.
Chiu, J.-S.
Hsu / Thin Solid Films, 252 (1994) 13-18
T (K) 1123
1223
1323
T-
t ._ E E C
20.;5
0.90 l/T
(x 10-3 K-l)
carrier gas carrier gas
O-
PHMDS
= 13 Pa, no
l n -
PHMDS
= 20 Pa, no
PHMDS
= 13 Pa, Flow Rate (H2) = 20 seem
Fig. 2. Arrhenius
plot of thin films deposited
(4
at
I 123- 1323 K.
15
very fine f.c.c. (face-centered cubic) crystals distributed evenly. On the other hand, the polycrystals grown at 1323 K have larger sizes and their distribution within the film is not even. As shown in Fig. 3(c), there are areas of high transmission and low transmission. In addition to ring patterns of randomly oriented fine crystals, the ED pattern in Fig. 3(d) shows slightly diffused dot patterns. These are characteristic of f.c.c. crystals with preferred growth orientation in the [l 1 1] direction. From the ED studies, we estimate that the lattice parameter, a, is 0.437 and 0.436 nm for the films grown at 1173 and 1323 K, respectively, close to the value of /I-Sic (0.4358 nm). The ED pattern of a film deposited at 1123 K shows diffused rings only, suggesting that the film is amorphous. XRD patterns of the films deposited without and with H, flow are shown in Figs. 4 and 5, respectively. There are little differences between the patterns shown in Figs. 4 and 5. Major Cu Kx peaks of P-Sic (1 1 l), (2 0 0), (2 2 0) and (3 1 1) reflections are observed. As shown in Fig. 6, the lattice parameter, a, calculated from the XRD data, although close to the value of B-Sic, shrinks with increasing temperatures of deposi-
(b)
Cd) Fig. 3. STEM studies of thin films deposited and (d) ED, T, = 1323 K.
at P HMDS = 4 Pa. (a) Bright-field
micrograph
and (b) ED, T, = II73 K; (c) bright-field
micrograph
16
Chiu, J.-S. Hsu I Thin Solid Films, 252 (1994) 13-18
H.-T.
d 75
Fig. 4. XRD patterns of thin films deposited 1323 K, (b) 1273 K, (c) 1223 K, (d) 1173 K.
0.6
at PHMDs = 8 Pa. (a)
1123
(4
ALL
1223
1323 T (K)
b.
A
c d
25
60 28
75 ldegreel
0.6 -
Fig. 5. XRD patterns of thin films deposited at P,,,, = 20 Pa, H, flow rate = 20 seem. (a) 1323 K, (b) 1273 K, (c) 1223 K, (d) 1173 K. 0.6-
P = 0.436 5
O*‘:
“E i
1;3
lb)
0.436
T (IO
Fig. 7. Effect of temperature of deposition carrier gas, (b) H, flow rate = 20 seem.
$ ‘E z J 0.434
(a) No
i12’23 T (K)
1123
Ol -
on Si/C ratio.
1323
PHMDS
=4 Pa, no carrier gas
PHMDS
=4 Pa, Flow Rate (Hp) = 20 seem
Fig. 6. Effect of temperature
of deposition
on lattice parameter.
tion, from 0.438 nm at 1173 K to 0.435 nm at 1323 K, an indication of variable elemental ,compositions. Bulk elemental composition of the thin films, determined by EPMA, is Sic, (x = 0.8-2.3). In Figs. 7(a) and (b), Si/C ratios of the thin films, derived from the EPMA studies, are plotted against the temperature of deposition. It shows that the Si/C ratio increased with increasing temperature of deposition. Employing H, into the reactions shows little effect again. This result and the XRD data can be correlated and rationalized as fol-
lows. Stoichiometric P-Sic has a f.c.c. sublattice of Si atoms with half of the tetrahedral sites occupied by C atoms to form a zinc blende type of structure. For thin films with low Si/C ratio, which is C rich, more than half of the tetrahedral sites could be occupied by carbon atoms, causing the lattice parameter to enlarge. On the other hand, for thin films with high Si/C ratios, the lattice parameter could shrink because less than half of the tetrahedral sites are occupied. This explains why athin s,,,, is larger than aS+o when the growths were performed at low temperatures of deposition but smaller at high temperatures. Auger depth profiles of the thin films, such as that shown in Fig. 8, indicate uniform elemental distributions. High resolution XPS of two thin films, deposited without and with H2 carrier gas and characterized as received, are shown in Fig. 9.
H.-T. Chiu, J.-S. Hsu I Thin Solid Films, 252 (1994) 13-18
4
8 Etch Time (min)
Fig. 8. Auger depth profile of a thin film. T, = 1223 K, P,,,, 13 Pa.
=
a
I
1
104
I
I
100
98
Binding Energy MI)
288
284
280
Binding Energy (eV)
Fig. 9. XPS of thin films characterized as received. Si,, region, (a) T, = 1223 K, P,,,, = 4 Pa, and (b) T, = 1173K, P,,,s = 3 Pa, H, flow rate = 20 seem; C,, region, (c) T, = 1223 K, P,,,s = 4 Pa, and (d) T, = 1173 K, PHMDS= 3 Pa, H, flow rate = 20 seem.
Again, little differences are observed for these films. The SiZp region shows major signals at 100.6 and 100.4 eV and the C,, region shows major signals at 283.8 and 284.0 eV, close to the values expected for silicon carbide. Since none of the studies mentioned above, ED, XRD and XPS, show signals assignable to pure silicon or carbon, we conclude that formation of clusters of free carbon and free silicon phases in the films is unlikely. Clearly, this is the consequence of using single-source
17
precursor. FT-IR studies showed major absorptions assignable to Si-C stretchings at 790-800 cm-’ for all of the films. For thin films deposited at 1123 K, additional weak absorption signals due to residual C-H stretchings were observed at 2900-3100 cm-‘. In order to understand the reaction pathways, we analyzed the gas phase products formed in the CVD processes by a residual gas analyzer. At PHMDsof 40 Pa and 1073 K, only 15% HMDS decomposed to generate Hz, CH,, C2H, and Me,SiH. When the temperatures were increased above 1223 K, the precursor decomposed completely to generate, in addition to the gases mentioned above, Me,Si. With increasing temperature of deposition, the concentration of Me,SiH decreased while the concentration of Me,Si increased. At 1323 K, Me,Si was the sole silicon containing gas product. Below 1223 K, Arrhenius plots of the ions originated from H,, CH, and C2H, were drawn and the slops were calculated to be 114, 51 and 79 kJ mol- ’ respectively. Although the value 114 kJ mol-’ is very close to the apparent energy of activation estimated from the growth rates mentioned above, it is not easy to correlate the data and derive definite mechanistic conclusions. Nonetheless, formation of the hydrogen molecules by combining gaseous H atoms is a less favored route because the C-H bonds of HMDS are strong and energies higher than the apparent activation energies are needed to dissociate gaseous HMDS molecules into H atoms and Me,SiSiMe,CH,. fragments. It is more likely that the HZ molecules were formed by combining surface hydrogens. Formation of CH, and C2H, by combining surface hydrocarbon species is also speculated. In general, removal of surface species as gaseous products to increase nucleation sites is an important step in thin film growth. To account for our observations, a generalized reaction route, although speculative, is proposed in Scheme 1. In route a, HMDS is adsorbed on the surface first, then eliminates Me,SiH to form “Me,Si=CH,” attached to the surface. After a methane molecule is removed in step 2, the adsorbed species “MeSiCH” may undergo several different decomposition steps further, such as 3, 4 and 5, before residual atoms and fragments “SiCI”, “Sic”, and “Si” are generated and incorporated into the lattice. In route b, decomposition of HMDS generates a SiMe, molecule and a “SiMe,” fragment adsorbed on the surface. After the rearrangement in step 7, this species undergoes further elimination of CH, and H, via step 8 to form a “Sic” unit and elimination of CH, and C,H, via step 9 to leave an adsorbed Si atom. Gas phase reactions analogous to steps 1, 6 and 7, involving the formation of transient species Me,Si=CH,, SiMez and MeHSi=CH,, have been reported in the literature [ 11,131. To account for the low Si/C ratios of the films grown at low temperatures, we postulate that the reaction via route a is the major pathway because Me,SiH
18
H.-T.
Chiu, J.-S. Hsu I Thin Solid Films, 25.2 (1994) 13- 18
77 Scheme
9 97
were surface-catalyzed. The lattice parameter of the cubic polycrystalline thin films, a = 0.435-0.438 nm, decreased with increasing temperature of deposition. Depending on the deposition conditions, bulk elemental composition of the thin films is determined to be Sic, (x = 0.8-2.3). In general, the Si/C ratio increased with increasing temperature of deposition. Elemental distribution within the films, studied by Auger depth profiling, was uniform. Employing hydrogen in the reaction shows little influence on the growth behavior. Based on the gas phase products detected, a possible reaction pathway involving the formation of transient species Me2Si=CH2, SiMe, and MeHSi=CH, adsorbed on the surface by eliminating Me,SiH and SiMe, from the precursor molecule is proposed.
1.
Acknowledgments
was the major Si-containing product detected and the reaction through step 4 is the only way in the proposed scheme to obtain a composition of low Si/C. At high temperatures, route b becomes the favored pathway because the concentration of SiMe, increased in the experiment and the reaction through step 9 can be held responsible for the high Si/C ratios. In contrast to the routes mentioned above, we speculate that the activation process in the APCVD of silicon carbide from HMDS [9] involves either Si-Si or Si-C bond homolysis in the gas phase because the apparent energy of activation of the process, 312 kJ mol-‘, is much larger than the value found in this study and very close to the Si-Si and the Si-C bond dissociation energies reported before, 337 and 311 kJ mol- ’ respectively [ 11,121.
4. Conclusions We have shown that silicon carbide thin films can be grown by LPCVD employing air-stable hexamethyldisilane Me,SiSiMe, as the single-source precursor at relatively low temperatures 1123- 1323 K. Pretreatment of the surfaces of the Si( 1 1 1) substrates by ‘hydrogen plasma ensures the deposition of uniform thin films with good adhesion. The growth rates were 0.9-55 nm min-’ and the apparent energy of activation is estimated to be 110 kJ mol- ‘, indicating that the reactions
We thank the National Science Council of the Republic of China (NSC-Sl-0208-M009-508) for support, and the Instrument Center of the NSC for sample analyses.
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