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Plasma etching of amorphous GeSx thin films
Thin Solid Films, 136 (1986) 123 127 PREPARATION AND CHARACTERIZATION
123
P L A S M A E T C H I N G OF A M O R P H O U S GeSx T H I N FILMS z. G. IVANOVA AND E. D. ATANASSOVA Institute of Solid State Physics, Bulgarian Academy of Sciences, 1784 Sofia (Bulgaria)
A. TONEVA Central Laboratory of Solar Energy and New Energy Sources, Bulgarian Academy of Sciences, 1784 Sofia (Bulgaria)
(ReceivedMarch 25, 1985;acceptedAugust 27, 1985)
The process of CF4 plasma etching of amorphous GeS~ films (1.0 ~< x ~< 4.0) was investigated. A negative effect of photostimulated selective etching was established, the GeS1.5 composition showing optimum parameters. Some possibilities for improving selective etching using ion implantation were found.
1. INTRODUCTION Over the past several years an increased interest in chalcogenide amorphous films as inorganic resists has been observed. Extensive studies of the production of lithographic patterns in the submicron range have been developed mainly on the basis of G e - S e and As-S systems I 3. The selective solubility of amorphous GeSxbased films has also been investigated 4 6. A negative effect of photostimulated selective dissolution was established, and this was enhanced by the introduction of gallium or indium. A suitable low temperature and chemical modification improve the selective solubility parameters of the films 7. The fundamental functions of these resists are usually based on a distinct change after irradiation in the chemical solubility in alkaline solutions. However, nowadays dry plasma etching is successfully used as a suitable resist fine-pattern delineation process in fabricating electronic devices 8. A CF4 plasma was first used by Chang and Chen 9 to form a negative Ag/As2S3 resist in lithographic applications. A maximum etch rate ratio of 1.8 between the unexposed and exposed films was obtained. When the plasma etching technique was used for the Ag/Ge-Se system 1°, fine-pattern delineation of less than 1 gm linewidth was easily attainable. In this report the possibilities of selective plasma etching of amorphous GeSx films are studied. The results are compared with those obtained with conventional wet etching 4-6. These materials are preferable to the inorganic resists so far known in that they do not contain the poisonous elements arsenic or selenium. 2. EXPERIMENTALDETAILS Thin films (about 0.5 lam) from previously synthesized bulk glasses of the GeSx 0040-6090/86/$3.50
© ElsevierSequoia/Printedin The Netherlands
124
z.G.
IVANOVA, E. D. ATANASSOVA, A. TONEVA
system were prepared by vacuum thermal evaporation. They were deposited onto glass substrates at a rate of about 0.1 lam min-1 (substrate temperature T~ ~ 40 °C). Compositions were selected from both the first (x = 2.0, 3.0 and 4.0) and the second (x = 1.5) glass-forming regions 11 (monocrystalline GeS being used as the initial material for the GeS films). The electron diffraction results showed that all the films were amorphous. The photoexposure was made with a 500 W mercury lamp having an intensity of about 0.6 W cm 2. The plasma etching was carried out in an entirely stainless steel planar reactor with radial flow 12 The glow r.f. discharge at a frequency of 13.56 MHz was produced in an atmosphere of CF4. The pressure P of the etching gas was within the range 0.52.5 Torr. The slices were placed on the grounded electrode whose temperature T varied from 20 to 300 °C, depending on the glass transition temperature 13 Tg of the corresponding composition so that T ~ Tg. The r.f. power density frange used in the experiments was 0.1-0.3 W c m - z. The thickness measurements after every etch step were made with a Talystep measuring instrument (Rank Taylor Hobson). The accuracy was in the range 1~o-5~o for thicknesses of 0.5-0.01 ~tm respectively. 3.
RESULTS AND DISCUSSION
The basic parameters, determined in order to establish the etching selectivity of amorphous GeSx films in a CF 4 plasma, are the following: c~ and ct' denote the etching times of the unexposed and exposed parts of the films respectively and fl (~) is the remaining film thickness. A negative effect ofphotostimulated selective etching has been observed for all compositions, i.e. the unexposed part is etched more quickly than the exposed part (the etch time ratio ct'/ct > 1). The film structure and the bonding geometry are likely to have changed after exposure in such a way that the chemical stability of the films has increased and they etch more slowly. A similar dependence has also been established when investigating the selective solubility of GeSx-based films in alkaline solutions 4 6. The results of the CF 4 plasma etching of the studied films have been summarized in Table I. For each composition the values of ct'/ct and fl, obtained at a suitable etching regime (P = 2.2 Torr, f = 0.3 W cm-2 and T = 250-300 °C), are the maximum values. It has been seen that both the GeS~.5 from the second glassforming region and the GeS compositions have better parameters of selective
TABLE I SELECTIVE PLASMA ETCHING PARAMETERS OF GeS x FILMS x
1.0 1.5 2.0 3.0 4.0
Composition (at.~;,) Ge
S
Initial thickness (I.tm)
50 40 33 25 20
50 60 67 75 80
0.49 0.51 0.50 0.49 0.48
~ (min)
fl (~o)
2'./~
17 19 20 20 22
70 75 67 52 48
2.2 2.5 1.8 1.5 1.4
PLASMA ETCHING OF
a-GeSx
125
THIN FILMS
etching. It should be noted that the greatest shift in the absorption edge in the optical transmission spectra to shorter wavelengths (i.e. a photobleaching effect) has been observed mainly for these compositions (Fig. 1). In Fig. 2 the etching characteristics of GeSI.5 films before (curve 1) and after exposure (curve 1') are given. It has been established that one previous annealing at T~, <~ Tg does not lead to the improvement in the selective etching parameters (curves 2 and 2') which has been observed when using alkaline solutions for some GeSx-based films 4' 6. loo p
~'~,
!
I 3,5
75
~ 04
~
0,3
1\2;@?, \\\. 30
26
22
10-3 x W o v e n u m b e r {crn-1 )
18
40
,
20 30 Etching time {rain}
40
50
Fig. 1. Optical transmission spectra of GeSt.5 (curves 1 and 1') and GeS3 (curves 2 and 2') films (curves 1 and 2, films before exposure; curves 1' and 2', films after exposure). Fig. 2. Etching characteristics of GeS1.5 films: curves ! and 2, unexposed; curves 1' and 2', exposed; curves 2 and 2', films previously annealed at T~n = 300 °C.
The values of ~'/~ and fl obtained after plasma etching in CF4 have been found to be higher in comparison with those in the conventional wet technique. For the compositions in Table I the use o f N a O H 4 or K O H 5,6 has led to ~'/~ ~ 1.2-1.3 and fl ~ 45%-50%. As far as the other known systems are concerned, the values of ~'/~ can be said to be similar to those 9 in AszS3 films but they are smaller than those 1° in GezsSe7 s. It should be noted at this point, however, that the latter two are silversensitized systems. Figure 3 shows the exposure characteristics and the sensitivity of the films. The contrast 7, as estimated from the slope of the curve, is about 2.0 for the GeSL5 composition. The curve is saturated at fl .~ 75% for 30-40 min of exposure. The mechanism of CF 4 plasma etching of arsenic and germanium chalcogenides is not fully understood yet. CF4 dissociation by glow discharge produces mainly 14 CF3 +, C F 3 - , CF 3, F and F'. The atomic fluorine free radicals seem to attack the GeSx compositions, forming volatile fluorides such as GeF 4 and SF6, which are pumped out through the vacuum system. The type of curve in Fig. 2 leads us to believe that the etching does not begin spontaneously. An apparent etching process has not been observed during the first 4 or 5 min. This seems to be connected with some chemisorption of reactive etching particles on the film surface. The following stage is a real chemical reaction, leading to the etching observed. Since in our case the energy of the bombarding particles is about 40-50 eV, physical sputtering is quite unlikely. The ultimate elucidation of the etching mechanism of GeSx films in a CF plasma requires further investigation. Like As2S 3 9 and G e - S e ~0, these compositions are resistant to a n 0 2 plasma.
126
z . G . IVANOVA, E. D. ATANASSOVA, A. TONEVA
Studies have recently appeared in which ion implantation in G e - S e films is shown to be applicable in high resolution storage, holography and lithography15 17. Thus submicron lines have been produced in silver-sensitized G e - S e films irradiated with 20 keV finely focused helium, nitrogen, argon and xenon ion beams 16, t 7. We have carried out previously some experiments irradiating GeS and GeS1.5 films with different ions (N +, O +, Cr + or Ni +) with a dose of 1015 1017 ions cm - 2 a current from 0.2 to 5.0 ~tA cm - 2 and an energy of 20-50 keV. The etching characteristics of GeS films irradiated with N + ions are given in Fig. 4. On comparison of the case of UV exposure (cf. Table I) with the above example it can be concluded that for the implanted films the values of~'/~ are increased (from 2.2 to 2.6) with a negligible decrease in fl (from 70~o to 66Yo). The study of the effect of ion implantation conditions (the type of ions and the dose and energy of irradiation) could result in modified GeSx films with selective etching characteristics which are useful in practical applications, thus avoiding long UV exposure times.
@100! 7si
i
so /
2 a~
25 SI 132
103 104 Exposure {J/cm2)
\
0,1 10
N,,
~
20 30 Etching time Imir]
&C'
Fig. 3. Exposure characteristics of GeS,. 5 films at a power intensity of about 0.6 W cm 2. Fig. 4. Etching characteristics of GeS films: curve 1, unimplanted; curve 1', after implantation with 25 keV N + ions at a dose of 2 x 1016 ions cm - 2.
4. CONCLUSION To summarize, our study has shown that amorphous GeSx(1.0 ~< x ~< 4.0) films can be etched in an r.f. CF4 plasma. A negative effect of photostimulated selective etching has been established following UV exposure, compositions from the second glass-forming region giving better results (for GeS1. 5 the etch time ratio ct'/~ -- 2.5 and the remaining film thickness fl = 75~o). The sensitivity, similar to that of other non-silver types of inorganic photoresist, is at present not high enough. Some previous experiments have demonstrated that, using ion implantation, the etching selectivity can be improved without the introduction of silver in the compositions. REFERENCES 1 2 3 4
M.J. Bowden, Solid State Technol., 24 (6) (1981) 73. D.I. Blezkan, V. S. Gerasimenko, I. M. Grankin, A. V. Kolomeiko and V. P. Pogrebuyak, Ukr. Fiz. Zh. (Russ. Edn.), 26 (1) (1981) 14. Y. Mizushima and A. Yoshikawa, Amorphous Semiconductor Technology and Devices, NorthHolland, Amsterdam, 1982, p. 277. Z.G. Ivanova, E. Vateva and M. D. Babacheva, in R. Grigorovici and M. Manaila (eds.), Proc. 6th Int. Conf. on Amorphous Semiconductors, Bucharest, Romania, 1982, p. 249.
PLASMA ETCHING OF
a-GeSx THIN
FILMS
127
5 Z.G. Ivanova, in E. Vateva and A. Buroff(eds.), Proc. 7th Int. Conf. on Amorphous Semiconductors, Gabrovo, Bulgaria, 1984, p. 268. 6 Z.G. Ivanova and E. Vateva, Thin Solid Films, 120 (1984) 75. 7 E. Vateva, M. Nikoforova, Z. G. Ivanova and D. Arsova, J. Non-Cryst. Solids, 70 (1985) 29. 8 G.N. Taylor, T. M. Wolf and L. E. Stillwagon, SolidState Technol., 27 (2) (1984) 145. 9 M.S. Chang and J. T. Chen, Appl. Phys. Lett., 33 (10) (1978) 892. 10 A. Yoshikawa, O. Ochi and Y. Mizushima, Appl. Phys. Lett., 36 (1) (1980) 107. 11 Y. Kawamoto and S. Tsuchihashi, J. Am. Ceram. Soc., 52 (11) (1969) 626. 12 A.R. Reinberg, U.S. Patent 3,757,733, September 11, 1973. 13 Y. Kawamoto and S. Tsuchihashi, J. Am. Ceram. Soc., 54 (3) (1971) 131 ; 54 (10) (1971) 526. 14 H.F. Winters, J.W. Coburn and E. Kay, J. Appl. Phys., 48 (12) (1977) 4973. 15 K.L. Chopra, K. S. Harshavardham, S. Rajagopalan and L. K. Malhotra, Appl. Phys. Lett., 40 (5) (1982) 428. 16 A. Wagner, D. Barr, T. Venkatesan, W. S. Crane, V. E. Lamberti, K. L. Tai and R. G. Vadimsky, J. Vac. Sci. Technol., 19 (4) (1981) 1363. 17 T. Venkatesan, J. Vac. Sci. Technol., 19 (4) (1981) 1368.
123
P L A S M A E T C H I N G OF A M O R P H O U S GeSx T H I N FILMS z. G. IVANOVA AND E. D. ATANASSOVA Institute of Solid State Physics, Bulgarian Academy of Sciences, 1784 Sofia (Bulgaria)
A. TONEVA Central Laboratory of Solar Energy and New Energy Sources, Bulgarian Academy of Sciences, 1784 Sofia (Bulgaria)
(ReceivedMarch 25, 1985;acceptedAugust 27, 1985)
The process of CF4 plasma etching of amorphous GeS~ films (1.0 ~< x ~< 4.0) was investigated. A negative effect of photostimulated selective etching was established, the GeS1.5 composition showing optimum parameters. Some possibilities for improving selective etching using ion implantation were found.
1. INTRODUCTION Over the past several years an increased interest in chalcogenide amorphous films as inorganic resists has been observed. Extensive studies of the production of lithographic patterns in the submicron range have been developed mainly on the basis of G e - S e and As-S systems I 3. The selective solubility of amorphous GeSxbased films has also been investigated 4 6. A negative effect of photostimulated selective dissolution was established, and this was enhanced by the introduction of gallium or indium. A suitable low temperature and chemical modification improve the selective solubility parameters of the films 7. The fundamental functions of these resists are usually based on a distinct change after irradiation in the chemical solubility in alkaline solutions. However, nowadays dry plasma etching is successfully used as a suitable resist fine-pattern delineation process in fabricating electronic devices 8. A CF4 plasma was first used by Chang and Chen 9 to form a negative Ag/As2S3 resist in lithographic applications. A maximum etch rate ratio of 1.8 between the unexposed and exposed films was obtained. When the plasma etching technique was used for the Ag/Ge-Se system 1°, fine-pattern delineation of less than 1 gm linewidth was easily attainable. In this report the possibilities of selective plasma etching of amorphous GeSx films are studied. The results are compared with those obtained with conventional wet etching 4-6. These materials are preferable to the inorganic resists so far known in that they do not contain the poisonous elements arsenic or selenium. 2. EXPERIMENTALDETAILS Thin films (about 0.5 lam) from previously synthesized bulk glasses of the GeSx 0040-6090/86/$3.50
© ElsevierSequoia/Printedin The Netherlands
124
z.G.
IVANOVA, E. D. ATANASSOVA, A. TONEVA
system were prepared by vacuum thermal evaporation. They were deposited onto glass substrates at a rate of about 0.1 lam min-1 (substrate temperature T~ ~ 40 °C). Compositions were selected from both the first (x = 2.0, 3.0 and 4.0) and the second (x = 1.5) glass-forming regions 11 (monocrystalline GeS being used as the initial material for the GeS films). The electron diffraction results showed that all the films were amorphous. The photoexposure was made with a 500 W mercury lamp having an intensity of about 0.6 W cm 2. The plasma etching was carried out in an entirely stainless steel planar reactor with radial flow 12 The glow r.f. discharge at a frequency of 13.56 MHz was produced in an atmosphere of CF4. The pressure P of the etching gas was within the range 0.52.5 Torr. The slices were placed on the grounded electrode whose temperature T varied from 20 to 300 °C, depending on the glass transition temperature 13 Tg of the corresponding composition so that T ~ Tg. The r.f. power density frange used in the experiments was 0.1-0.3 W c m - z. The thickness measurements after every etch step were made with a Talystep measuring instrument (Rank Taylor Hobson). The accuracy was in the range 1~o-5~o for thicknesses of 0.5-0.01 ~tm respectively. 3.
RESULTS AND DISCUSSION
The basic parameters, determined in order to establish the etching selectivity of amorphous GeSx films in a CF 4 plasma, are the following: c~ and ct' denote the etching times of the unexposed and exposed parts of the films respectively and fl (~) is the remaining film thickness. A negative effect ofphotostimulated selective etching has been observed for all compositions, i.e. the unexposed part is etched more quickly than the exposed part (the etch time ratio ct'/ct > 1). The film structure and the bonding geometry are likely to have changed after exposure in such a way that the chemical stability of the films has increased and they etch more slowly. A similar dependence has also been established when investigating the selective solubility of GeSx-based films in alkaline solutions 4 6. The results of the CF 4 plasma etching of the studied films have been summarized in Table I. For each composition the values of ct'/ct and fl, obtained at a suitable etching regime (P = 2.2 Torr, f = 0.3 W cm-2 and T = 250-300 °C), are the maximum values. It has been seen that both the GeS~.5 from the second glassforming region and the GeS compositions have better parameters of selective
TABLE I SELECTIVE PLASMA ETCHING PARAMETERS OF GeS x FILMS x
1.0 1.5 2.0 3.0 4.0
Composition (at.~;,) Ge
S
Initial thickness (I.tm)
50 40 33 25 20
50 60 67 75 80
0.49 0.51 0.50 0.49 0.48
~ (min)
fl (~o)
2'./~
17 19 20 20 22
70 75 67 52 48
2.2 2.5 1.8 1.5 1.4
PLASMA ETCHING OF
a-GeSx
125
THIN FILMS
etching. It should be noted that the greatest shift in the absorption edge in the optical transmission spectra to shorter wavelengths (i.e. a photobleaching effect) has been observed mainly for these compositions (Fig. 1). In Fig. 2 the etching characteristics of GeSI.5 films before (curve 1) and after exposure (curve 1') are given. It has been established that one previous annealing at T~, <~ Tg does not lead to the improvement in the selective etching parameters (curves 2 and 2') which has been observed when using alkaline solutions for some GeSx-based films 4' 6. loo p
~'~,
!
I 3,5
75
~ 04
~
0,3
1\2;@?, \\\. 30
26
22
10-3 x W o v e n u m b e r {crn-1 )
18
40
,
20 30 Etching time {rain}
40
50
Fig. 1. Optical transmission spectra of GeSt.5 (curves 1 and 1') and GeS3 (curves 2 and 2') films (curves 1 and 2, films before exposure; curves 1' and 2', films after exposure). Fig. 2. Etching characteristics of GeS1.5 films: curves ! and 2, unexposed; curves 1' and 2', exposed; curves 2 and 2', films previously annealed at T~n = 300 °C.
The values of ~'/~ and fl obtained after plasma etching in CF4 have been found to be higher in comparison with those in the conventional wet technique. For the compositions in Table I the use o f N a O H 4 or K O H 5,6 has led to ~'/~ ~ 1.2-1.3 and fl ~ 45%-50%. As far as the other known systems are concerned, the values of ~'/~ can be said to be similar to those 9 in AszS3 films but they are smaller than those 1° in GezsSe7 s. It should be noted at this point, however, that the latter two are silversensitized systems. Figure 3 shows the exposure characteristics and the sensitivity of the films. The contrast 7, as estimated from the slope of the curve, is about 2.0 for the GeSL5 composition. The curve is saturated at fl .~ 75% for 30-40 min of exposure. The mechanism of CF 4 plasma etching of arsenic and germanium chalcogenides is not fully understood yet. CF4 dissociation by glow discharge produces mainly 14 CF3 +, C F 3 - , CF 3, F and F'. The atomic fluorine free radicals seem to attack the GeSx compositions, forming volatile fluorides such as GeF 4 and SF6, which are pumped out through the vacuum system. The type of curve in Fig. 2 leads us to believe that the etching does not begin spontaneously. An apparent etching process has not been observed during the first 4 or 5 min. This seems to be connected with some chemisorption of reactive etching particles on the film surface. The following stage is a real chemical reaction, leading to the etching observed. Since in our case the energy of the bombarding particles is about 40-50 eV, physical sputtering is quite unlikely. The ultimate elucidation of the etching mechanism of GeSx films in a CF plasma requires further investigation. Like As2S 3 9 and G e - S e ~0, these compositions are resistant to a n 0 2 plasma.
126
z . G . IVANOVA, E. D. ATANASSOVA, A. TONEVA
Studies have recently appeared in which ion implantation in G e - S e films is shown to be applicable in high resolution storage, holography and lithography15 17. Thus submicron lines have been produced in silver-sensitized G e - S e films irradiated with 20 keV finely focused helium, nitrogen, argon and xenon ion beams 16, t 7. We have carried out previously some experiments irradiating GeS and GeS1.5 films with different ions (N +, O +, Cr + or Ni +) with a dose of 1015 1017 ions cm - 2 a current from 0.2 to 5.0 ~tA cm - 2 and an energy of 20-50 keV. The etching characteristics of GeS films irradiated with N + ions are given in Fig. 4. On comparison of the case of UV exposure (cf. Table I) with the above example it can be concluded that for the implanted films the values of~'/~ are increased (from 2.2 to 2.6) with a negligible decrease in fl (from 70~o to 66Yo). The study of the effect of ion implantation conditions (the type of ions and the dose and energy of irradiation) could result in modified GeSx films with selective etching characteristics which are useful in practical applications, thus avoiding long UV exposure times.
@100! 7si
i
so /
2 a~
25 SI 132
103 104 Exposure {J/cm2)
\
0,1 10
N,,
~
20 30 Etching time Imir]
&C'
Fig. 3. Exposure characteristics of GeS,. 5 films at a power intensity of about 0.6 W cm 2. Fig. 4. Etching characteristics of GeS films: curve 1, unimplanted; curve 1', after implantation with 25 keV N + ions at a dose of 2 x 1016 ions cm - 2.
4. CONCLUSION To summarize, our study has shown that amorphous GeSx(1.0 ~< x ~< 4.0) films can be etched in an r.f. CF4 plasma. A negative effect of photostimulated selective etching has been established following UV exposure, compositions from the second glass-forming region giving better results (for GeS1. 5 the etch time ratio ct'/~ -- 2.5 and the remaining film thickness fl = 75~o). The sensitivity, similar to that of other non-silver types of inorganic photoresist, is at present not high enough. Some previous experiments have demonstrated that, using ion implantation, the etching selectivity can be improved without the introduction of silver in the compositions. REFERENCES 1 2 3 4
M.J. Bowden, Solid State Technol., 24 (6) (1981) 73. D.I. Blezkan, V. S. Gerasimenko, I. M. Grankin, A. V. Kolomeiko and V. P. Pogrebuyak, Ukr. Fiz. Zh. (Russ. Edn.), 26 (1) (1981) 14. Y. Mizushima and A. Yoshikawa, Amorphous Semiconductor Technology and Devices, NorthHolland, Amsterdam, 1982, p. 277. Z.G. Ivanova, E. Vateva and M. D. Babacheva, in R. Grigorovici and M. Manaila (eds.), Proc. 6th Int. Conf. on Amorphous Semiconductors, Bucharest, Romania, 1982, p. 249.
PLASMA ETCHING OF
a-GeSx THIN
FILMS
127
5 Z.G. Ivanova, in E. Vateva and A. Buroff(eds.), Proc. 7th Int. Conf. on Amorphous Semiconductors, Gabrovo, Bulgaria, 1984, p. 268. 6 Z.G. Ivanova and E. Vateva, Thin Solid Films, 120 (1984) 75. 7 E. Vateva, M. Nikoforova, Z. G. Ivanova and D. Arsova, J. Non-Cryst. Solids, 70 (1985) 29. 8 G.N. Taylor, T. M. Wolf and L. E. Stillwagon, SolidState Technol., 27 (2) (1984) 145. 9 M.S. Chang and J. T. Chen, Appl. Phys. Lett., 33 (10) (1978) 892. 10 A. Yoshikawa, O. Ochi and Y. Mizushima, Appl. Phys. Lett., 36 (1) (1980) 107. 11 Y. Kawamoto and S. Tsuchihashi, J. Am. Ceram. Soc., 52 (11) (1969) 626. 12 A.R. Reinberg, U.S. Patent 3,757,733, September 11, 1973. 13 Y. Kawamoto and S. Tsuchihashi, J. Am. Ceram. Soc., 54 (3) (1971) 131 ; 54 (10) (1971) 526. 14 H.F. Winters, J.W. Coburn and E. Kay, J. Appl. Phys., 48 (12) (1977) 4973. 15 K.L. Chopra, K. S. Harshavardham, S. Rajagopalan and L. K. Malhotra, Appl. Phys. Lett., 40 (5) (1982) 428. 16 A. Wagner, D. Barr, T. Venkatesan, W. S. Crane, V. E. Lamberti, K. L. Tai and R. G. Vadimsky, J. Vac. Sci. Technol., 19 (4) (1981) 1363. 17 T. Venkatesan, J. Vac. Sci. Technol., 19 (4) (1981) 1368.