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Silicon dioxide thin films prepared by photochemical vapor deposition from silicon tetraacetate
201
Thin Solid Films, 232 (1993) 201-203
Silicon dioxide thin films prepared by photochemical deposition from silicon tetraacetate Toshiro Department
Maruyama of Chemical
and Engineering,
Teruoki
Tago
Faculty
of Engineering,
Kyoto
University,
Kyoto
vapor
606 (Japan)
(Received February 18, 1993; accepted April 19, 1993)
Abstract Silicon dioxide thin films were prepared by a direct photochemical vapor deposition method. The raw material was silicon tetraacetate which is non-toxic and easy to handle. A 6 W low-pressure mercury lamp was used as a light
source. Thin films were obtained in an inert atmosphere at a substrate temperature of above 110 “C. The deposition rate was higher than for thermal chemical vapor deposition, which is only possible in the presence of oxygen gas.
1. Introduction Recently, the photochemical vapor deposition (photoCVD) method has attracted much attention because of the possibility of lowering the process temperature. In addition, photo-CVD has been reported to be effective in increasing deposition rates, obtaining spatially selective deposition, and improving the surface morphology or film quality. The photo-CVD method appears to be a promising method for low-temperature processing in VLSI technology. The photo-CVD SiO, process [ 1, 21 has depended mainly on the photolytic generation of electrically neutral atomic oxygen which subsequently oxidizes silane. The photolytic generation of atomic oxygen can be accomplished by either direct photolytic or mercury photosensitization (catalytic) techniques. Si, H, has also been reported to be susceptible to direct photolysis [3]. However, little is known about the source material which solely decomposes to produce silicon dioxide thin film in the photo-CVD method. This paper proposes silicon tetraacetate (tetraacetoxysilane, Si (CH,COO),) as a source material for obtaining SiO, films by the direct photo-CVD method in an inert atmosphere. Silicon tetraacetate is non-toxic and easy to handle in solid form at room temperature (the melting point is 110 “C). Silicon tetraacetate decomposes thermally into silicon dioxide and acetic anhydride at temperatures of 160- 170 “C [4] : Si( CH, COO), -
SiO, + 2( CH, CO), 0
(1)
Our previous paper [5] on conventional chemical vapor deposition (thermal CVD) of SiO, showed that SiOZ film was obtained from silicon tetraacetate at a reaction
0040-6090/93/$6.00
temperature above 120 “C. However, the film could only be prepared if 0, was present in the reaction environment, and no film was obtained in an inert atmosphere. In this paper, therefore, the deposition conditions of the photo-CVD method are described in comparison with the thermal CVD methods. Comparisons are also made of the structure and hardness of the deposited film.
2. Experiment A 6 W low-pressure mercury lamp was used as a light source. The main resonance lines of this lamp were 184.9 nm and 253.7 nm. Figure 1 is a schematic diagram of the experimental apparatus and reactors. In the reactor, both the substrate and the mercury lamp are
Low pressure Hg lamp
Si(CHxCOO),
Subkte
v
5’’ Furnace
Mass flow controller
N!
Fig. 1. Schematic representation reactors.
0 1993 -
of the experimental equipment and
Elsevier Sequoia. All rights reserved
T. Muruyamu,
202
T. Tago / Photo-CVD
set horizontally with a separating distance of 2 cm. The lamp irradiates the substrate vertically. The flux density at the substrate surface was 53 mW cme2. Silicon tetraacetate (Shin-Etsu Chemical Co., Ltd.) was used as the source material. It was heated at temperatures of lOO- 130 “C (the melting point is 110 “C), and the vapor was carried into the reactor with nitrogen gas as carrier. The flow rate of the carrier gas was 300 cm3 min’. Deposition was carried out in an inert (nitrogen) atmosphere at atmospheric pressure. (For comparison, both thermal CVD and photo-CVD were done in air.) Borosilicate glass plates and silicon ( 100) single-crystal wafers were used as substrates. The substrate was placed on a temperature-controlled electric heater. The substrate temperature ranged from 110 “C to 300 “C. The film thickness was determined from weight gain measurements. The composition of the film was measured by X-ray photoelectron spectroscopy. IR spectra were obtained using a Fourier transform IR spectrometer (Shimadzu FTIR-4300). The samples were prepared by depositing films onto silicon single-crystal substrates. The crystallinity of the film was checked by X-ray diffraction with Cu Ka radiation. The dynamic hardness of the film was measured with an ultramicro dynamic hardness meter (Shimadzu DUH-200). The UV absorbance spectrum of silicon tetraacetate was obtained by means of a multipurpose recording spectrophotometer on hexane solutions of 12.9 mol mP3 in the wavelength range between 200 nm and 400 nm.
3. Results and discussion Figure 2 shows the UV absorbance spectrum of silicon tetraacetate. Strong light absorption was observed in the range 200-240 nm. Thus, the effective UV component in photo-CVD is 184.9 nm light of the low-pressure mercury lamp. Transparent amorphous thin films were obtained at substrate temperatures above 110 “C. X-ray photoelectron spectroscopy showed that the films were near-stoichiometric Si02. The lower limit of the substrate temperature is equal to that used for thermal CVD. In
0
li. xi)
100
Wavelength Fig. 2. UV absorbance
4w.l [nm]
spectrum
of silicon tetraacetate.
of SiO,
(4
I/ 1.5
(b)
2.0
2.5
3.0
1000/T [K.‘]
Fig. 3. Arrhenius plots of deposition rates for photo-CVD onto borosilicate glass; (a) with the corresponding rate for silicon substrates and (b) with the corresponding rates for thermal CVD and photo-CVD in air.
thermal CVD, however, SiOZ films could only be grown in the presence of oxygen gas, while photo-CVD is possible in an inert (nitrogen) atmosphere. Therefore, the reaction mechanism of photo-CVD must be different from that of thermal CVD. Arrhenius plots of deposition rates for photo-CVD onto borosilicate glass substrates are shown in Figs. 3(a) and 3(b). The deposition rates were obtained at a source temperature of 110 “C. Also shown in Fig. 3(a) is the deposition rate onto silicon substrates. Apparently, the nature of the substrate influences the deposition rate. Figure 3(b) compares the deposition rates for thermal CVD and photo-CVD in air. The deposition rate for photo-CVD is higher than that for thermal CVD in air. However, the deposition rate for photoCVD in air is nearly equal to that for thermal CVD. This can be explained as follows. An absorption spectrum of O2 gas has a sharp absorption band (Schumann-Runge band) at wavelength 1755200nm. In photo-CVD in air, therefore, the reduction in 184.9 nm light by O2 surrounding the lamp is so great that little light reaches the vicinity of the silicon tetraacetate which flows along the substrate. As a result, the films are deposited in the same manner as for thermal CVD. Figures 4(a) and 4(b) show IR absorption spectra of films deposited at different substrate temperatures. The
T. Maruyama,
T. Tago 1 Photo-CVD
203
of SiO,
IS043
IS6nm
100
200
Temperature Fig. 5. Dynamic hardness (b) thermal CVD.
IOXnm
(b)
2000
1500
1000
Wave number [cm’
Fig. 4. IR transmission and (b) thermal CVD.
spectra
500
of films prepared
by (a) photo-CVD
and
4. Conclusions
1 of films prepared
[“cl
the substrate temperature suggests that the photochemical reaction plays an important role in the development of the Si-0-Si bond of the film. Also shown in this figure is the dynamic hardness of the film produced by thermal CVD. It is strongly dependent on the substrate temperature, and is greater than that of the film prepared by photo-CVD, except at substrate temperatures less than 120 “C where the IR absorption spectrum shows insufficient development of the Si-0-Si bond.
I IOT
4000
400
300
by (a) photo-CVD
parameters shown in these figures are the substrate temperature and the film thickness. The photo-CVD spectra (Fig. 4(a)) are similar to each other, i.e. absorption peaks at about 1080 cm-i, 800 cm-’ and 460 cm-’ represent Si-0-Si asymmetric-bond stretching vibration, network Si-0-Si symmetric-bond stretching vibration, and network Si-0-Si bond-bending vibration. No trace of the C-H bond (1273, 849 cm-i) or Si-H bond (880 cm-‘) can be identified. For thermal CVD (Fig. 4(b)), however, the spectrum at a substrate temperature of 110 “C is different. It shows strong absorption peaks at about 3330 cm-’ and 950 cm-i. The peak at about 3330 cm-’ is due to absorbed water, and the peak at 950 cm-’ is due to Si-OH bonds containing nonbridging oxygen atoms. This result is consistent with ref. 5, i.e. the thermal CVD method cannot sufficiently develop the Si-0-Si bond of the film at a temperature below 150 “C. In contrast, the photo-CVD method can develop this bond at a temperature above 110 “C. Figure 5 shows the dynamic hardness of the film as a function of substrate temperature. It is nearly independent of the substrate temperature, consistent with the results of IR absorption spectra. This independence of
Silicon dioxide thin films were prepared by a direct photochemical vapor deposition method. The raw material was silicon tetraacetate which is non-toxic and easy to handle. A 6 W low-pressure mercury lamp was used as a light source. At a substrate temperature above 110 “C, thin films were obtained in an inert atmosphere. The deposition rate was higher than for thermal chemical vapor deposition, which is only possible in the presence of oxygen gas. Acknowledgments This work was supported by the Iketani Science and Technology Foundation, Yazaki Science and Technology Foundation, Nippon Sheet Glass Foundation, and General Sekiyu Research and Development Encouragement and Assistance Foundation. References 1 J. W. Peter, Tech. Digest Int. Electron Devices Meet., 1981, IEEE, New York, 1981, p. 240. 2 R. F. Sarkozy, Tech. Digest Symp. on VLSI Technology, 1981, Jpn Sot. Appl. Phys., Tokyo, 1981, p, 68. 3 Y. Mishima, M. Hirose, Y. Osaka and Y. Ashida, J. Appl. Phys., 55 (1984) 1234. 4 T. Maruyama and K. Aburai, Jpn. J. Appl. Phys., 27 (1988) L2268. 5 T. Maruyama and J. Shionoya, Jpn. J. Appl. Phys., 28( 1989) L2253.
Thin Solid Films, 232 (1993) 201-203
Silicon dioxide thin films prepared by photochemical deposition from silicon tetraacetate Toshiro Department
Maruyama of Chemical
and Engineering,
Teruoki
Tago
Faculty
of Engineering,
Kyoto
University,
Kyoto
vapor
606 (Japan)
(Received February 18, 1993; accepted April 19, 1993)
Abstract Silicon dioxide thin films were prepared by a direct photochemical vapor deposition method. The raw material was silicon tetraacetate which is non-toxic and easy to handle. A 6 W low-pressure mercury lamp was used as a light
source. Thin films were obtained in an inert atmosphere at a substrate temperature of above 110 “C. The deposition rate was higher than for thermal chemical vapor deposition, which is only possible in the presence of oxygen gas.
1. Introduction Recently, the photochemical vapor deposition (photoCVD) method has attracted much attention because of the possibility of lowering the process temperature. In addition, photo-CVD has been reported to be effective in increasing deposition rates, obtaining spatially selective deposition, and improving the surface morphology or film quality. The photo-CVD method appears to be a promising method for low-temperature processing in VLSI technology. The photo-CVD SiO, process [ 1, 21 has depended mainly on the photolytic generation of electrically neutral atomic oxygen which subsequently oxidizes silane. The photolytic generation of atomic oxygen can be accomplished by either direct photolytic or mercury photosensitization (catalytic) techniques. Si, H, has also been reported to be susceptible to direct photolysis [3]. However, little is known about the source material which solely decomposes to produce silicon dioxide thin film in the photo-CVD method. This paper proposes silicon tetraacetate (tetraacetoxysilane, Si (CH,COO),) as a source material for obtaining SiO, films by the direct photo-CVD method in an inert atmosphere. Silicon tetraacetate is non-toxic and easy to handle in solid form at room temperature (the melting point is 110 “C). Silicon tetraacetate decomposes thermally into silicon dioxide and acetic anhydride at temperatures of 160- 170 “C [4] : Si( CH, COO), -
SiO, + 2( CH, CO), 0
(1)
Our previous paper [5] on conventional chemical vapor deposition (thermal CVD) of SiO, showed that SiOZ film was obtained from silicon tetraacetate at a reaction
0040-6090/93/$6.00
temperature above 120 “C. However, the film could only be prepared if 0, was present in the reaction environment, and no film was obtained in an inert atmosphere. In this paper, therefore, the deposition conditions of the photo-CVD method are described in comparison with the thermal CVD methods. Comparisons are also made of the structure and hardness of the deposited film.
2. Experiment A 6 W low-pressure mercury lamp was used as a light source. The main resonance lines of this lamp were 184.9 nm and 253.7 nm. Figure 1 is a schematic diagram of the experimental apparatus and reactors. In the reactor, both the substrate and the mercury lamp are
Low pressure Hg lamp
Si(CHxCOO),
Subkte
v
5’’ Furnace
Mass flow controller
N!
Fig. 1. Schematic representation reactors.
0 1993 -
of the experimental equipment and
Elsevier Sequoia. All rights reserved
T. Muruyamu,
202
T. Tago / Photo-CVD
set horizontally with a separating distance of 2 cm. The lamp irradiates the substrate vertically. The flux density at the substrate surface was 53 mW cme2. Silicon tetraacetate (Shin-Etsu Chemical Co., Ltd.) was used as the source material. It was heated at temperatures of lOO- 130 “C (the melting point is 110 “C), and the vapor was carried into the reactor with nitrogen gas as carrier. The flow rate of the carrier gas was 300 cm3 min’. Deposition was carried out in an inert (nitrogen) atmosphere at atmospheric pressure. (For comparison, both thermal CVD and photo-CVD were done in air.) Borosilicate glass plates and silicon ( 100) single-crystal wafers were used as substrates. The substrate was placed on a temperature-controlled electric heater. The substrate temperature ranged from 110 “C to 300 “C. The film thickness was determined from weight gain measurements. The composition of the film was measured by X-ray photoelectron spectroscopy. IR spectra were obtained using a Fourier transform IR spectrometer (Shimadzu FTIR-4300). The samples were prepared by depositing films onto silicon single-crystal substrates. The crystallinity of the film was checked by X-ray diffraction with Cu Ka radiation. The dynamic hardness of the film was measured with an ultramicro dynamic hardness meter (Shimadzu DUH-200). The UV absorbance spectrum of silicon tetraacetate was obtained by means of a multipurpose recording spectrophotometer on hexane solutions of 12.9 mol mP3 in the wavelength range between 200 nm and 400 nm.
3. Results and discussion Figure 2 shows the UV absorbance spectrum of silicon tetraacetate. Strong light absorption was observed in the range 200-240 nm. Thus, the effective UV component in photo-CVD is 184.9 nm light of the low-pressure mercury lamp. Transparent amorphous thin films were obtained at substrate temperatures above 110 “C. X-ray photoelectron spectroscopy showed that the films were near-stoichiometric Si02. The lower limit of the substrate temperature is equal to that used for thermal CVD. In
0
li. xi)
100
Wavelength Fig. 2. UV absorbance
4w.l [nm]
spectrum
of silicon tetraacetate.
of SiO,
(4
I/ 1.5
(b)
2.0
2.5
3.0
1000/T [K.‘]
Fig. 3. Arrhenius plots of deposition rates for photo-CVD onto borosilicate glass; (a) with the corresponding rate for silicon substrates and (b) with the corresponding rates for thermal CVD and photo-CVD in air.
thermal CVD, however, SiOZ films could only be grown in the presence of oxygen gas, while photo-CVD is possible in an inert (nitrogen) atmosphere. Therefore, the reaction mechanism of photo-CVD must be different from that of thermal CVD. Arrhenius plots of deposition rates for photo-CVD onto borosilicate glass substrates are shown in Figs. 3(a) and 3(b). The deposition rates were obtained at a source temperature of 110 “C. Also shown in Fig. 3(a) is the deposition rate onto silicon substrates. Apparently, the nature of the substrate influences the deposition rate. Figure 3(b) compares the deposition rates for thermal CVD and photo-CVD in air. The deposition rate for photo-CVD is higher than that for thermal CVD in air. However, the deposition rate for photoCVD in air is nearly equal to that for thermal CVD. This can be explained as follows. An absorption spectrum of O2 gas has a sharp absorption band (Schumann-Runge band) at wavelength 1755200nm. In photo-CVD in air, therefore, the reduction in 184.9 nm light by O2 surrounding the lamp is so great that little light reaches the vicinity of the silicon tetraacetate which flows along the substrate. As a result, the films are deposited in the same manner as for thermal CVD. Figures 4(a) and 4(b) show IR absorption spectra of films deposited at different substrate temperatures. The
T. Maruyama,
T. Tago 1 Photo-CVD
203
of SiO,
IS043
IS6nm
100
200
Temperature Fig. 5. Dynamic hardness (b) thermal CVD.
IOXnm
(b)
2000
1500
1000
Wave number [cm’
Fig. 4. IR transmission and (b) thermal CVD.
spectra
500
of films prepared
by (a) photo-CVD
and
4. Conclusions
1 of films prepared
[“cl
the substrate temperature suggests that the photochemical reaction plays an important role in the development of the Si-0-Si bond of the film. Also shown in this figure is the dynamic hardness of the film produced by thermal CVD. It is strongly dependent on the substrate temperature, and is greater than that of the film prepared by photo-CVD, except at substrate temperatures less than 120 “C where the IR absorption spectrum shows insufficient development of the Si-0-Si bond.
I IOT
4000
400
300
by (a) photo-CVD
parameters shown in these figures are the substrate temperature and the film thickness. The photo-CVD spectra (Fig. 4(a)) are similar to each other, i.e. absorption peaks at about 1080 cm-i, 800 cm-’ and 460 cm-’ represent Si-0-Si asymmetric-bond stretching vibration, network Si-0-Si symmetric-bond stretching vibration, and network Si-0-Si bond-bending vibration. No trace of the C-H bond (1273, 849 cm-i) or Si-H bond (880 cm-‘) can be identified. For thermal CVD (Fig. 4(b)), however, the spectrum at a substrate temperature of 110 “C is different. It shows strong absorption peaks at about 3330 cm-’ and 950 cm-i. The peak at about 3330 cm-’ is due to absorbed water, and the peak at 950 cm-’ is due to Si-OH bonds containing nonbridging oxygen atoms. This result is consistent with ref. 5, i.e. the thermal CVD method cannot sufficiently develop the Si-0-Si bond of the film at a temperature below 150 “C. In contrast, the photo-CVD method can develop this bond at a temperature above 110 “C. Figure 5 shows the dynamic hardness of the film as a function of substrate temperature. It is nearly independent of the substrate temperature, consistent with the results of IR absorption spectra. This independence of
Silicon dioxide thin films were prepared by a direct photochemical vapor deposition method. The raw material was silicon tetraacetate which is non-toxic and easy to handle. A 6 W low-pressure mercury lamp was used as a light source. At a substrate temperature above 110 “C, thin films were obtained in an inert atmosphere. The deposition rate was higher than for thermal chemical vapor deposition, which is only possible in the presence of oxygen gas. Acknowledgments This work was supported by the Iketani Science and Technology Foundation, Yazaki Science and Technology Foundation, Nippon Sheet Glass Foundation, and General Sekiyu Research and Development Encouragement and Assistance Foundation. References 1 J. W. Peter, Tech. Digest Int. Electron Devices Meet., 1981, IEEE, New York, 1981, p. 240. 2 R. F. Sarkozy, Tech. Digest Symp. on VLSI Technology, 1981, Jpn Sot. Appl. Phys., Tokyo, 1981, p, 68. 3 Y. Mishima, M. Hirose, Y. Osaka and Y. Ashida, J. Appl. Phys., 55 (1984) 1234. 4 T. Maruyama and K. Aburai, Jpn. J. Appl. Phys., 27 (1988) L2268. 5 T. Maruyama and J. Shionoya, Jpn. J. Appl. Phys., 28( 1989) L2253.