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High-quality amorphous hydrogenated silicon carbide coatings by remote plasma chemical vapor deposition from a single-source precursor
ELSEVIER Journal of Materials Processing Technology 53 (1995) 477482
Journalof Materials Processing Technology
High-quality a m o r p h o u s h y d r o g e n a t e d silicon carbide coatings b y r e m o t e plasma chemical v a p o r deposition f r o m a single-source p r e c u r s o r
A. M. Wr6bel a, S. Wickramanayakab, Y. Nakanishib, and Y. Hatanakab apolish Academy of Sciences, Centre of Molecular and Macromolecular Studies, Sienkiewicza 112, 90-363 L6d~, Poland bResearch Institute of Electronics, Shizuoka University, Hamamatsu 432, Japan
The amorphous hydrogenated silicon carbide (a-SiC:H) films were produced by the remote hydrogen plasma chemical vapor deposition (CVD) using tetrakis(trimethylsilyl)silane (TMSS) molecular cluster as a novel single-source precursor. The remote plasma CVD process has been examined in terms of mechanism of the activation step. The determined temperature dependence of the film deposition rate suggests that the examined remote hydrogen plasma CVD is a non-thermally activated process. The susceptibility of particular bonds in TMSS molecule to the activation step has been characterized using suitable model source compounds. The films have been characterized by ellipsometry, Auger electron spectroscopy, scanning electron microscopy, and reflection high energy electron diffraction analysis. There is reported the effect of substrate temperature (Ts) on such properties of the film as the compositional uniformity, surface morphology, and refractive index. The films exhibit an excellent morphological homogeneity, outstanding compositional uniformity and stoichiometry near pure silicon carbide at Ts=300-400°C. The refractive index alters from 1.5 to 2.4 with rising T s in the range of 30-400°C. 1. INTRODUCTION The. development of remote plasma chemical vapor deposition (CVD) technique that takes place in the recent years, is an important step towards the fabrication of defectless, highquality thin-film materials. This technique, in contrast to direct (conventional) plasma CVD, offers better control over growth conditions [1]. In particular, some damaging effects arising from direct interaction of the plasma and the growing film, namely charged-particle bombardment and vacuum-ultraviolet (VUV) irradiation [2], can be avoided. Using biatomic gas for the plasma generation, such as hydrogen, a homogeneous activation of the source compound, involving only hydrogen radicals, is achieved. The resulting chemical structure of the deposit is expected to be much more uniform than that of the materials produced by the direct plasma CVD. Taking into account the mentioned beneficial features of the remote hydrogen plasma CVD we have used this technique for the deposition of a-SiC:H films from TMSS molecular cluster, (Me3Si)4Si , as a novel single-source precursor. Due to several Elsevier Science S.A. 0924-0136(95)02004-6
SSDI
478
A.M. Wr6bel et al. / Journal of MateriaL~ Processing Technology 53 (1995) 477-482
properties important in terms of the CVD processes, namely, relatively high volatility, the presence of four reactive disilane (Si-Si) bonds in the molecular skeleton, and relatively low carbon to silicon ratio (C/Si=2.4), this compound is a good candidate for the production of silicon carbide films. The present work gives insight to the mechanism of the activation of TMSS molecules in the remote hydrogen plasma CVD process and reports such important properties of the deposited films as compositional uniformity, microstructure, and refractive index. 2. E X P E R I M E N T A L The remote plasma CVD apparatus used for the present study (Figure 1) consisted of two major parts: (1) plasma generation tube of 31 mm i.d. (made of fused silica) with a coil coupled via a matching network with a radiofrequency (13.56 MHz) power supply unit, and (2) an afterglow tube (made of Pyrex glass) expanded to 78 mm i.d. in the deposition section which contained the substrate holder equipped with a heater and a temperature control unit. A source compound injector (4 mm i.d.) was located in front of the substrate holder. To prevent VUV irradiation of the film during growth, a Wood's horn light trap was inserted into the afterglow tube before the deposition section. The distance between the plasma edge and substrate was about 25 cm. Source compound vapor
H2
Pressure gauge
~/~
°
1
o
Vacuum pump Light trap
Figure 1. Schematic diagram of the remote plasma CVD apparatus.
Films were deposited on p-type c-Si wafers ( < 111 > orientation, resistivity 90-100 flcm) under following conditions: total pressure p=0.2 Torr (27 Pa), hydrogen flow rate F(H2) =200 sccm, r.f. power input P=200 W, and the substrate temperature Ts=30-400°C. TMSS source compound was evaporated at the temperature of 80°C in argon flowing through the evaporator with the rate F(Ar)= 1 sccm and fed to the reactor with the flow rate F(TMSS) =0.5 sccm. Film thickness and refractive index were measured ellipsometrically using a Nippon Infrared Industrial Company EL-101D ellipsometer. The thickness of the film
A.M. Wrrbel et aL / Journal of Materials Processing Technology 53 (1995) 477-482
479
samples deposited in the present study ranges from about 200 to 500 rim. Auger electron spectroscopic (AES) analysis was performed using an ULVAC AQM 808 system. Compositional AES depth profiling was carded out by sputter-etching of the film sample with a 2 keV Ar + beam. The morphology of film surface was examined by scanning electron microscopy (SEM) using a Jeol JSM 6100 electron microscope. 3. RESULTS AND DISCUSSION 3.1. Mechanism of the activation step To study of the activation of the source compound in remote hydrogen plasma CVD the mass flow regime in the remote part of the apparatus has been characterized. For this purpose the Peclect number Pe=vL/D (where, v is the average flow velocity of hydrogen, L is the distance between the plasma edge and the substrate, and D-diffusivity of the source compound) has been evaluated. This number being the ratio of convection to diffusion, is useful for assessing the importance of back diffusion of the source compound molecules. For the present experimental conditions and hydrogen concentration and temperature data given below there has been calculated v = l . 9 x l 0 3 cm s-1. Assuming the mean size of TMSS molecule as 1.07 nm (the value has been computed using HyperChem v.4.0 for Windows software) we evaluated D < 103 cm 2 s-1. Taking into account these data and L=25cm we found P e > > 10. This value of Pe indicates that the diffusion of the source compound to the plasma section can be neglected [3] and thus the activation of TMSS molecules exclusively takes place in the deposition section. Moreover, the zero value of thermal activation energy found from the Arrhenius plot of the substrate temperature dependence of the deposition rate suggests that the activation step proceeds in the gas-phase and the deposition process is controlled by the diffusion of the active species from the gas-phase to the substrate. Concentration of the atomic hydrogen in the reactor determined by the NO~ titration method is [H] =5. lx1015 cm "3. Using this value and concentration of the gas molecu-les in the reactor (under the pressure of 0.2 Torr and at the estimated temperature of 306 K) as 6.4x1015 cm -3, we find the fraction of hydrogen atoms at the reaction site to be fn=0.79. On the basis of the latter value and the flow rate data of the reagents we have evaluated the approximate number of hydrogen radicals per molecule of the source compound as NH = fHXF(H2)/F(TMSS) = 3x 102 atom/molecule. In order to estimate the susceptibility of particular bonds in TMSS molecule towards the reaction with hydrogen radicals, the deposition experiments were performed using methane, tetramethylsilane (TMS), and hexamethyldisilane (HMDS) as the model source compounds which simulate the C-H, Si-C, and Si-Si bonds, respectively. To avoid an undesirable effect which might arise from the thermochemical reactions the experiments were carried out at unheated substrate. The reactivity of these compounds in the remote hydrogen plasma CVD process was analyzed by the yield of the deposition, R/FM, where R is the deposition rate, F is the volumetric flow rate of the source compound vapor, and M is the molecular weight of the source compound. The composite parameter R/FM expresses the thickness of the deposit per mass unit of the source compound fed to the reactor and was found to be very sensitive to the molecular structure of the compound [4,5]. The values of the deposition rate determined for the examined model compounds and the calculated deposition yields are listed in Table 1, which, for comparison, also contains respective data for TMSS.
480
A.M. Wr6bet et al. / Journal of Materials Processing Technology 53 (1995) 477-482
Table 1 Yield of the deposition process (R/FM) determined for the model compounds simulating particular bonds in TMSS source compound, at the following deposition conditions: p=0.2 Tort, F(H2)=200 sccm, P(r.f.)=200 W, and Ts=30-35°C. Bond
Model compound
Molecular weight, M (g/mol)
Flow rate, F (sccm)
Deposition rate, R (nm/min)
R/FM x 22.4 x 102 (nm/g)
C-H
CH 4
16
8.0
0
0
Si-C
SiMe 4
88
1.0
0.2
5
Si-Si
(Me3Si)2
146
0.4
2.0
77
(Me3Si)4Si
321
0.5
5.4
75
The zero value of the deposition rate observed for methane accounts for inactivity of C-H bonds in the investigated activation step. The very close values ef R/FM found for HMDS and TMSS (Table 1) which, on the other hand, are higher than that of TMS by more than one order of magnitude, prove that the Si-Si bonds play a predominant role in the activation of TMSS. The contributions of particular bonds in TMSS to the activation step, evaluated from the deposition yield data in Table 1, are: 0% for C-H, 7% for Si-C and 93% for Si-Si. These results account for a high selectivity of the activation of source compound in the examined remote plasma CVD.
3.2. Properties of the deposited material To examine the compositional uniformity of the films the AES depth profiling was performed for the materials produced at various substrate temperatures Ts). A typical compositional AES depth profile presenting variation of the atomic concentrations of silicon, carbon, and oxygen with the film depth is illustrated in Figure 2. It is apparent from the data in this figure that the concentrations of particular elements, except for the film surface and film/substrate interface regions, remain constant independently of the depth, thus revealing an excellent compositional uniformity of the deposit, which appears to be much better than that of a-SiC:H films produced from organosilicon precursors by the direct plasma CVD [6,7]. The constant level of oxygen throughout the film depth (6 at. %) as noted in Figure 2, suggests that this element is predominantly uptaken during the film growth and its main source is the etching of silica tube with hydrogen plasma. The oxygen content in the bulk of the deposit was found to drop to 1-2 at. % with rising T s to 300-350°C. The effect of the substrate temperature on the composition and compositional uniformity of the film is shown in Figure 3 which presents the AES atomic concentration ratio Si/C determined for the surface region (after sputter-etching of the film with a 2 keV Ar ÷ beam for 2 min) and the bulk (at the depth of 100 nm) as a function of T s. The dotted lines marked at Si/C=4.2 and 1.0 correspond to the stoichiometry of TMSS source compound and pure silicon carbide, respectively.
.4.M. Wr6bel et al. / Journal of Materials Processing Technology 53 (1995) 477-482
481
DEPTH (nm) 0
100
300
200
400
500
20
25
100
80 z
__ 60 F-I..-Z u.l Z 0
1
40
20
it
0 0
10
5
o
15
SPUTTERING TIME (rain)
Figure 2. AES compositional depth profile of the film deposited on c-Si wafer at the temperature T s = 100°C. 1.2
1
I
I
U
o
1.0
< t~
oz0.8 U~ <
~0.6 o rj
~0.4
................................... T M S S
o E-~ <
0.2
I
I
I
I
I00
200
300
400
SUBSTRATE
TEMPERATURE
(°C)
Figure 3. AES atomic concentration ratio Si/C determined for the film surface region (A) and the bulk at the depth of 100 nm (O), as a function of the substrate temperature.
As can be noted in Figure 3 the ratio Si/C for the film surface region and the bulk increases with rising substrate temperature and at T s - 3 5 0 ° C reaches value close to 1.0, which corresponds to that of pure silicon carbide. Furthermore, the data in Figure 3 prove that the
482
A.M. Wr6bel et al. /Journal of Materials Processing Technology 53 (1995) 477-482
difference in the stoichiometry of the surface region and the bulk is diminishing with increasing T s. The films produced at Ts->300°C display an outstanding compositional uniformity which is revealed by the same values of the atomic ratio Si/C found for the surface region and the bulk. The quality of films deposited at various substrate temperatures have been assessed by the SEM examination of surface morphology. The results of these examination revealed that the substrate temperature does not influence the surface morphology of the deposit. The film surface, being smooth, exhibited an excellent morphological homogeneity, irrespective of the deposition temperature. The morphology of the examined film surfaces was found to differ drastically from that of the direct plasma CVD films. An inherent feature of the latter materials, is morphological heterogeneity manifested by appearance of a two-phase structure comprising spherical or spheroidal powder particles embedded in a continuous film matrix [8]. The presence of powder particles markedly deteriorates the film's quality in terms of practical properties. The reflection high energy electron diffraction patterns obtained for the films deposited at different substrate temperatures in the examined range, revealed the lack of reflexes attributed to crystalline structure. This accounts for the amorphous structure in the produced films. The optical properties of the deposited material are characterized by the refractive index measured ellipsometrically for the film samples produced at different substrate temperature. The refractive index was found to vary from 1.5 to 2.4 with rising T s in the range of 30400 °¢. This trend is presumably due to the densification of the film resulting from thermallyinduced crosslinking process leading to the formation of carbidic network structure, as reflected by the increase of the atomic concentration ratio Si/C (Figure 3). In summary, the remote plasma CVD is apparently a substantial progress towards the production of high-quality a-SiC:H films. ACKNOWLEDGEMENTS The present work was financially supported by the Japanese Ministry of Education (Monbusho). The study is a part of the KBN research project No. 226859203. REFERENCES
1. G. Lucovsky, D. V. Tsu, R. A. Rudder, and R. J. Markunas, in Thin Film Processes II, J. L. Vossen and W. Kern (eds.), Academic, Boston, MA, 1991, Chap. 4. 2. A. M. Wr6bel and G. Czeremuszkin, Thin Solid Films, 216 (1992) 203. 3. K. F. Jensen, in Chemical Vapor Deposition. Principles and Application, M. L. Hitchman and K. F. Jensen (eds.), Academic, London, 1993, Chap. 2, pp. 53-56. 4. A. M. Wr6bel, S. Wickramanayaka, and Y. Hatanaka, J. Appl. Phys., 76 (1994) 558. 5. A. M. Wr6bel and W. Stafczyk, Chem. Mater., 6 (1994) 1766. 6. M. J. Loboda, S. Baumann, M. J. Edgell, and K. Stolt, J. Vac. Sci. Technol., A10 (1992) 3532. 7. L. Maya, J. Vac. Sci. Technol., A12 (1994) 754. 8. A. M. Wr6bel and M. R. Wertheimer, in Plasma Deposition, Treatment, and Etching of Polymers, R. d'Agostino (ed.), Academic, Boston, MA, 1990, Chap. 3, pp. 211-219.
Journalof Materials Processing Technology
High-quality a m o r p h o u s h y d r o g e n a t e d silicon carbide coatings b y r e m o t e plasma chemical v a p o r deposition f r o m a single-source p r e c u r s o r
A. M. Wr6bel a, S. Wickramanayakab, Y. Nakanishib, and Y. Hatanakab apolish Academy of Sciences, Centre of Molecular and Macromolecular Studies, Sienkiewicza 112, 90-363 L6d~, Poland bResearch Institute of Electronics, Shizuoka University, Hamamatsu 432, Japan
The amorphous hydrogenated silicon carbide (a-SiC:H) films were produced by the remote hydrogen plasma chemical vapor deposition (CVD) using tetrakis(trimethylsilyl)silane (TMSS) molecular cluster as a novel single-source precursor. The remote plasma CVD process has been examined in terms of mechanism of the activation step. The determined temperature dependence of the film deposition rate suggests that the examined remote hydrogen plasma CVD is a non-thermally activated process. The susceptibility of particular bonds in TMSS molecule to the activation step has been characterized using suitable model source compounds. The films have been characterized by ellipsometry, Auger electron spectroscopy, scanning electron microscopy, and reflection high energy electron diffraction analysis. There is reported the effect of substrate temperature (Ts) on such properties of the film as the compositional uniformity, surface morphology, and refractive index. The films exhibit an excellent morphological homogeneity, outstanding compositional uniformity and stoichiometry near pure silicon carbide at Ts=300-400°C. The refractive index alters from 1.5 to 2.4 with rising T s in the range of 30-400°C. 1. INTRODUCTION The. development of remote plasma chemical vapor deposition (CVD) technique that takes place in the recent years, is an important step towards the fabrication of defectless, highquality thin-film materials. This technique, in contrast to direct (conventional) plasma CVD, offers better control over growth conditions [1]. In particular, some damaging effects arising from direct interaction of the plasma and the growing film, namely charged-particle bombardment and vacuum-ultraviolet (VUV) irradiation [2], can be avoided. Using biatomic gas for the plasma generation, such as hydrogen, a homogeneous activation of the source compound, involving only hydrogen radicals, is achieved. The resulting chemical structure of the deposit is expected to be much more uniform than that of the materials produced by the direct plasma CVD. Taking into account the mentioned beneficial features of the remote hydrogen plasma CVD we have used this technique for the deposition of a-SiC:H films from TMSS molecular cluster, (Me3Si)4Si , as a novel single-source precursor. Due to several Elsevier Science S.A. 0924-0136(95)02004-6
SSDI
478
A.M. Wr6bel et al. / Journal of MateriaL~ Processing Technology 53 (1995) 477-482
properties important in terms of the CVD processes, namely, relatively high volatility, the presence of four reactive disilane (Si-Si) bonds in the molecular skeleton, and relatively low carbon to silicon ratio (C/Si=2.4), this compound is a good candidate for the production of silicon carbide films. The present work gives insight to the mechanism of the activation of TMSS molecules in the remote hydrogen plasma CVD process and reports such important properties of the deposited films as compositional uniformity, microstructure, and refractive index. 2. E X P E R I M E N T A L The remote plasma CVD apparatus used for the present study (Figure 1) consisted of two major parts: (1) plasma generation tube of 31 mm i.d. (made of fused silica) with a coil coupled via a matching network with a radiofrequency (13.56 MHz) power supply unit, and (2) an afterglow tube (made of Pyrex glass) expanded to 78 mm i.d. in the deposition section which contained the substrate holder equipped with a heater and a temperature control unit. A source compound injector (4 mm i.d.) was located in front of the substrate holder. To prevent VUV irradiation of the film during growth, a Wood's horn light trap was inserted into the afterglow tube before the deposition section. The distance between the plasma edge and substrate was about 25 cm. Source compound vapor
H2
Pressure gauge
~/~
°
1
o
Vacuum pump Light trap
Figure 1. Schematic diagram of the remote plasma CVD apparatus.
Films were deposited on p-type c-Si wafers ( < 111 > orientation, resistivity 90-100 flcm) under following conditions: total pressure p=0.2 Torr (27 Pa), hydrogen flow rate F(H2) =200 sccm, r.f. power input P=200 W, and the substrate temperature Ts=30-400°C. TMSS source compound was evaporated at the temperature of 80°C in argon flowing through the evaporator with the rate F(Ar)= 1 sccm and fed to the reactor with the flow rate F(TMSS) =0.5 sccm. Film thickness and refractive index were measured ellipsometrically using a Nippon Infrared Industrial Company EL-101D ellipsometer. The thickness of the film
A.M. Wrrbel et aL / Journal of Materials Processing Technology 53 (1995) 477-482
479
samples deposited in the present study ranges from about 200 to 500 rim. Auger electron spectroscopic (AES) analysis was performed using an ULVAC AQM 808 system. Compositional AES depth profiling was carded out by sputter-etching of the film sample with a 2 keV Ar + beam. The morphology of film surface was examined by scanning electron microscopy (SEM) using a Jeol JSM 6100 electron microscope. 3. RESULTS AND DISCUSSION 3.1. Mechanism of the activation step To study of the activation of the source compound in remote hydrogen plasma CVD the mass flow regime in the remote part of the apparatus has been characterized. For this purpose the Peclect number Pe=vL/D (where, v is the average flow velocity of hydrogen, L is the distance between the plasma edge and the substrate, and D-diffusivity of the source compound) has been evaluated. This number being the ratio of convection to diffusion, is useful for assessing the importance of back diffusion of the source compound molecules. For the present experimental conditions and hydrogen concentration and temperature data given below there has been calculated v = l . 9 x l 0 3 cm s-1. Assuming the mean size of TMSS molecule as 1.07 nm (the value has been computed using HyperChem v.4.0 for Windows software) we evaluated D < 103 cm 2 s-1. Taking into account these data and L=25cm we found P e > > 10. This value of Pe indicates that the diffusion of the source compound to the plasma section can be neglected [3] and thus the activation of TMSS molecules exclusively takes place in the deposition section. Moreover, the zero value of thermal activation energy found from the Arrhenius plot of the substrate temperature dependence of the deposition rate suggests that the activation step proceeds in the gas-phase and the deposition process is controlled by the diffusion of the active species from the gas-phase to the substrate. Concentration of the atomic hydrogen in the reactor determined by the NO~ titration method is [H] =5. lx1015 cm "3. Using this value and concentration of the gas molecu-les in the reactor (under the pressure of 0.2 Torr and at the estimated temperature of 306 K) as 6.4x1015 cm -3, we find the fraction of hydrogen atoms at the reaction site to be fn=0.79. On the basis of the latter value and the flow rate data of the reagents we have evaluated the approximate number of hydrogen radicals per molecule of the source compound as NH = fHXF(H2)/F(TMSS) = 3x 102 atom/molecule. In order to estimate the susceptibility of particular bonds in TMSS molecule towards the reaction with hydrogen radicals, the deposition experiments were performed using methane, tetramethylsilane (TMS), and hexamethyldisilane (HMDS) as the model source compounds which simulate the C-H, Si-C, and Si-Si bonds, respectively. To avoid an undesirable effect which might arise from the thermochemical reactions the experiments were carried out at unheated substrate. The reactivity of these compounds in the remote hydrogen plasma CVD process was analyzed by the yield of the deposition, R/FM, where R is the deposition rate, F is the volumetric flow rate of the source compound vapor, and M is the molecular weight of the source compound. The composite parameter R/FM expresses the thickness of the deposit per mass unit of the source compound fed to the reactor and was found to be very sensitive to the molecular structure of the compound [4,5]. The values of the deposition rate determined for the examined model compounds and the calculated deposition yields are listed in Table 1, which, for comparison, also contains respective data for TMSS.
480
A.M. Wr6bet et al. / Journal of Materials Processing Technology 53 (1995) 477-482
Table 1 Yield of the deposition process (R/FM) determined for the model compounds simulating particular bonds in TMSS source compound, at the following deposition conditions: p=0.2 Tort, F(H2)=200 sccm, P(r.f.)=200 W, and Ts=30-35°C. Bond
Model compound
Molecular weight, M (g/mol)
Flow rate, F (sccm)
Deposition rate, R (nm/min)
R/FM x 22.4 x 102 (nm/g)
C-H
CH 4
16
8.0
0
0
Si-C
SiMe 4
88
1.0
0.2
5
Si-Si
(Me3Si)2
146
0.4
2.0
77
(Me3Si)4Si
321
0.5
5.4
75
The zero value of the deposition rate observed for methane accounts for inactivity of C-H bonds in the investigated activation step. The very close values ef R/FM found for HMDS and TMSS (Table 1) which, on the other hand, are higher than that of TMS by more than one order of magnitude, prove that the Si-Si bonds play a predominant role in the activation of TMSS. The contributions of particular bonds in TMSS to the activation step, evaluated from the deposition yield data in Table 1, are: 0% for C-H, 7% for Si-C and 93% for Si-Si. These results account for a high selectivity of the activation of source compound in the examined remote plasma CVD.
3.2. Properties of the deposited material To examine the compositional uniformity of the films the AES depth profiling was performed for the materials produced at various substrate temperatures Ts). A typical compositional AES depth profile presenting variation of the atomic concentrations of silicon, carbon, and oxygen with the film depth is illustrated in Figure 2. It is apparent from the data in this figure that the concentrations of particular elements, except for the film surface and film/substrate interface regions, remain constant independently of the depth, thus revealing an excellent compositional uniformity of the deposit, which appears to be much better than that of a-SiC:H films produced from organosilicon precursors by the direct plasma CVD [6,7]. The constant level of oxygen throughout the film depth (6 at. %) as noted in Figure 2, suggests that this element is predominantly uptaken during the film growth and its main source is the etching of silica tube with hydrogen plasma. The oxygen content in the bulk of the deposit was found to drop to 1-2 at. % with rising T s to 300-350°C. The effect of the substrate temperature on the composition and compositional uniformity of the film is shown in Figure 3 which presents the AES atomic concentration ratio Si/C determined for the surface region (after sputter-etching of the film with a 2 keV Ar ÷ beam for 2 min) and the bulk (at the depth of 100 nm) as a function of T s. The dotted lines marked at Si/C=4.2 and 1.0 correspond to the stoichiometry of TMSS source compound and pure silicon carbide, respectively.
.4.M. Wr6bel et al. / Journal of Materials Processing Technology 53 (1995) 477-482
481
DEPTH (nm) 0
100
300
200
400
500
20
25
100
80 z
__ 60 F-I..-Z u.l Z 0
1
40
20
it
0 0
10
5
o
15
SPUTTERING TIME (rain)
Figure 2. AES compositional depth profile of the film deposited on c-Si wafer at the temperature T s = 100°C. 1.2
1
I
I
U
o
1.0
< t~
oz0.8 U~ <
~0.6 o rj
~0.4
................................... T M S S
o E-~ <
0.2
I
I
I
I
I00
200
300
400
SUBSTRATE
TEMPERATURE
(°C)
Figure 3. AES atomic concentration ratio Si/C determined for the film surface region (A) and the bulk at the depth of 100 nm (O), as a function of the substrate temperature.
As can be noted in Figure 3 the ratio Si/C for the film surface region and the bulk increases with rising substrate temperature and at T s - 3 5 0 ° C reaches value close to 1.0, which corresponds to that of pure silicon carbide. Furthermore, the data in Figure 3 prove that the
482
A.M. Wr6bel et al. /Journal of Materials Processing Technology 53 (1995) 477-482
difference in the stoichiometry of the surface region and the bulk is diminishing with increasing T s. The films produced at Ts->300°C display an outstanding compositional uniformity which is revealed by the same values of the atomic ratio Si/C found for the surface region and the bulk. The quality of films deposited at various substrate temperatures have been assessed by the SEM examination of surface morphology. The results of these examination revealed that the substrate temperature does not influence the surface morphology of the deposit. The film surface, being smooth, exhibited an excellent morphological homogeneity, irrespective of the deposition temperature. The morphology of the examined film surfaces was found to differ drastically from that of the direct plasma CVD films. An inherent feature of the latter materials, is morphological heterogeneity manifested by appearance of a two-phase structure comprising spherical or spheroidal powder particles embedded in a continuous film matrix [8]. The presence of powder particles markedly deteriorates the film's quality in terms of practical properties. The reflection high energy electron diffraction patterns obtained for the films deposited at different substrate temperatures in the examined range, revealed the lack of reflexes attributed to crystalline structure. This accounts for the amorphous structure in the produced films. The optical properties of the deposited material are characterized by the refractive index measured ellipsometrically for the film samples produced at different substrate temperature. The refractive index was found to vary from 1.5 to 2.4 with rising T s in the range of 30400 °¢. This trend is presumably due to the densification of the film resulting from thermallyinduced crosslinking process leading to the formation of carbidic network structure, as reflected by the increase of the atomic concentration ratio Si/C (Figure 3). In summary, the remote plasma CVD is apparently a substantial progress towards the production of high-quality a-SiC:H films. ACKNOWLEDGEMENTS The present work was financially supported by the Japanese Ministry of Education (Monbusho). The study is a part of the KBN research project No. 226859203. REFERENCES
1. G. Lucovsky, D. V. Tsu, R. A. Rudder, and R. J. Markunas, in Thin Film Processes II, J. L. Vossen and W. Kern (eds.), Academic, Boston, MA, 1991, Chap. 4. 2. A. M. Wr6bel and G. Czeremuszkin, Thin Solid Films, 216 (1992) 203. 3. K. F. Jensen, in Chemical Vapor Deposition. Principles and Application, M. L. Hitchman and K. F. Jensen (eds.), Academic, London, 1993, Chap. 2, pp. 53-56. 4. A. M. Wr6bel, S. Wickramanayaka, and Y. Hatanaka, J. Appl. Phys., 76 (1994) 558. 5. A. M. Wr6bel and W. Stafczyk, Chem. Mater., 6 (1994) 1766. 6. M. J. Loboda, S. Baumann, M. J. Edgell, and K. Stolt, J. Vac. Sci. Technol., A10 (1992) 3532. 7. L. Maya, J. Vac. Sci. Technol., A12 (1994) 754. 8. A. M. Wr6bel and M. R. Wertheimer, in Plasma Deposition, Treatment, and Etching of Polymers, R. d'Agostino (ed.), Academic, Boston, MA, 1990, Chap. 3, pp. 211-219.