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Amorphous carbon films prepared by photo-CVD from acetylene
Volume
4, number
A~O~HOUS M. DANNO Department Received
MATERIALS
5,6,7
July 1986
LETTERS
CARBON FILMS PREPARED
BY PHOTO-CVD
FROM ACE~LEN~
and M. HANABUSA
of Electrical Engineering,
Toyohashi University of Technology,
Toyohashr 440, Japan
1 Aprif 1986
An attempt has been made to deposit carbon films by photo-CVD, utilizing 185 nm light from a low-pressure mercury lamp to photolyze acetylene. The produced films are electrically resistive and optically transparent in the infrared, but they are soft. Comparing the present results with those obtained by plasma CVD, it becomes clear that the presence and acceleration of carbon ions play important roles in determining the physical properties, such as refractive index and hardness. Possibilities of producing hard amorphous or diamond-like carbon films using photoinduced effects are discussed.
1. Introduction In recent years there has been a growing interest in the production and applications of the carbon thin films. In particular, diamond-like carbon films which are amorphous with a certain degree of crystallinity are attracting a great deal of attention. Also hydrogenated amorphous carbon films (a-C:H) prepared at low substrate temperatures are of some interest. All of these films are highly electrically insulating, hard and optically transparent in the infrared, and should be distinguished from graphite carbon, which is electrically conductive and optically opaque. They have been prepared by various methods, including ion beam methods [l-3], ionized deposition [4], plasma deposition [5-IO], sputtering [ 111, CVD [12] and electronassisted CVD (EA CVD)
[ 141, Even though many kinds of photoinduced effects are available as the processes leading to deposition of thin films, a technique based on photolysis of material gases induced by UV light is in particular popular. Photo-CVD has been used to produce many kinds of thin films, which include silicon, metals and dielectrics. However, to our knowledge, this method has never 0 167-577x/86/$03.50 0 Elsevier Science Publishers B.V. (Noah-Hoard Physics Pub~shing Division)
been applied to the carbon films of great interest today. The objective of the present work is to examine what kind of carbon films can be produced by photo CVD. In the present work we photolyzed acetylene gas by 185 nm W light generated by a low-pressure mer cury lamp and produced carbon films on various substrates. The substrate was kept at relatively low temperatures, because otherwise a pyrolytic reaction exceeded the photoinduced reaction and graphite-like carbon films could easily be produced. The choice of low substrate temperatures was supported also by the fact that high-quality a-C:H films have been produced by plasma CVD with negative self-bias at low substrate temperatures [8- 101. In the following we show that by the present photoinduced method hydrogenated amorphous carbon films can be produced as by plasma CVD, but their quality is inferior because they are soft. It has been asserted that the quality of the a-C:H films, including hardness, is closely related with the presence of energetic ions in plasma [9]. Since ions are absent in the photolysis of acetylene by 185 nm photons, the present result seems to endorse their importance in producing good a-C:H films. Also we show by a simple experiment that energetic electrons are effective only to the same degree as photons in producing hard a-C:H films. On the basis of these observation we discuss possible solutions for improvement of quality of the carbon films produced by photoinduced effects. 261
Volume 4, number 5,6,7
MATERIALS LETTERS
2. Experimental The apparatus used in the present photoinduced experiment is shown in fig. 1. It consists of a lamp house, which contains a 40 W, N-shaped low-pressure mercury lamp (Sen Tokushu Kogen Co.) and is filled with nitrogen gas, and a cell, which is made of stainless steel and 20 cm in diameter and 20 cm high. The lamp and the substrate were separated by 40 mm through a quartz window, and the total UV power measured at the location of the substrate was about 4 mW/cm2, while the 185 nm component was about 20% of the total power. The UV light is effectively absorbed by acetylene with a rate of 10 atm-l cm-l [ 151. In the cell a substrate was placed on a tungsten heater, and the acetylene gas, which was 99.5% pure, was blown toward the substrate through a nozzle. The flow rate was 5.3 ml/min at a cell pressure of 3 Torr. A variety of substrates were tried, but mainly tungsten, quartz, and silicon were used in the subsequent experiment; they were cleaned ultrasonically in alcohol. To protect the window from carbon deposition, argon gas was blown to it. This scheme helped to reduce unwanted coverage of the window, but at the same time deposition on the substrate was slowed down considerably. Therefore, we often did not blow the inert gas, and instead, replaced the window when it became dirty. For comparison we deposited carbon films by electronic decomposition of acetylene. A tungsten wire, placed about 10 mm above the substrate, was heated to generate electrons. The electrons were drawn toward the substrate by a bias voltage (150 V)
I
I
II
I
I
‘vrheimocouple Fig. 1. Experimental arrangement used for photo-CVD of aC:H films from acetylene.
July 1986
applied to the substrate holder. The arrangement is identical to the one used previously for EA CVD to prepare diamond-like films, where the substrate temperature was raised to 8OO’C by an independent heater and also hydrogen gas was added to hydrocarbon gas [ 131. In the present experiment we intended to compare the effects of electrons with those of photons under conditions as much identical as possible, and therefore no hydrogen was added and the substrate was kept at about 2OO’C. The wire was heated to a temperature between 1300 and 19OO’C. The current flowing between the wire and the substrate holder was 5 mA at a wire temperature of 1900°C. The flow rate was 5.3 ml/min, while the gas pressure was kept at 3 Torr. The substrate was either silicon or quartz plate.
3. Results When we began the present photo-CVD experiment, it became immediately clear that transparent, highly resistive carbon films could be deposited. The films were deposited first at room temperature. They passed a scotch tape test easily, but were polished off with a Q dip after many strokes. Therefore, we raised the substrate temperature to 150°C; then the films became more adhesive and could not be scraped by the Q dip. However, it could be scratched easily by a pointed object; in this sense, we call them soft. The temperature was raised further in an attempt to improve film property; however, the deposition rate decreased substantially with increasing temperature, even though the adhesive force was not improved; it was about 500 A/h at 15O’C and 200 A/h at 300°C. At the higher temperature of 500°C, the film started to loose its transparency. The deposited films were amorphous, as confirmed by the absence of diffraction pattern in X-ray and electron analysis. The sample prepared at 5OO”C, however, was different from those prepared at lower temperatures, and a diffraction pattern characteristic of graphite was observed. The presence of graphite-like elements indicates the initiation of pyrolysis of acetylene. Hydrogen was detected from an.optical absorption loss spectrum around 3.4 pm due to a C-H stretch vibration. The result for a film deposited at 150°C is shown in fig. 2. Judging from the location and linewidth of the spectral peak [8,16], a monohydride
July 1986
MATERIALS LETTERS
Volume 4, number 5 $6,7
1 OOk
1
3500 WAVE
3000 NUMBER
2500 (cm-‘)
Fig. 2. IR absorption loss spectrum observed for a C-H stretch vibration.
bonded to sp3 - type carbon dominates among the possible C-H bonds. The optical absorption coefficient measured between 200 nm and 10 pm for a photoinduced film at 15O’C is shown in fig. 3. The optical property agrees with that of the films prepared by plasma CVD with negative self-bias [8]. The results show that the films are transparent in the IR range and loose transparency in the UV range. From the absorption measurements an optical gap of 1.9 eV was determined. A relatively large value for the optical gap indicates a high concentration of bonded hydrogen in the films. The electrical resistivity measured at room temperature was of the order of lo8 fi cm. This value is fairly high, but less than those measured for amorphous carbon films prepared by plasma CVD, which were as high as 1012 R cm [g]. The close analogy observed for optical transparency between the films prepared by photo-CVD and plasma CVD does not hold for the refractive index n; the value of y2was only 1.64 for the films deposited at 15O”C, while for the films prepared by plasma CVD its value was higher than 1.80, depending on deposition conditions, in particular self-bias voltage between the electrodes and gas pressure [8,9]. Related with the refractive index is the hardness of the films; the films with a low refractive index are known to be soft, which is true for the present photoinduced films.
WAVELENGTH
( n m)
Fig. 3. Optical absorption coefficient measured for a photoinduced carbon film.
The properties of the carbon films obtained by electronic excitation were almost identical to those of the photoinduced films. The deposition rates depended on the wire temperature; 60, 1900, and 9600 A/h at 1300, 1500, and 1900°C, respectively, for silicon substrates with a bias of 150 V. Without bias they were considerably lower; at a wire temperature of 19OO”C, the rate was 6000 A/h with the bias, while 1200 A/h without it, for quartz substrates. These results show that even though the gas is thermally decomposed by the heated wire, energetic electrons are more effective to deposit films. The optical and electrical properties were almost identical to those of the photoinduced films; the films were soft too. It is concluded that both W photons and electrons produce almost the same kind of a-C films.
263
Volume 4, number 5,6,7
MATERIALS LETTERS
4. Discussion and conclusions The fact that carbon films prepared by the present photo-CVD method were not graphite-like can be explained by the presence of energetic photons. Graphitelike fdms are known to be produced by low-energy processes such as pyrolysis. Acetylene molecules are excited by the 6.7 eV photons to decompose into C,H and H or C, and HZ [ 171. Even though the details of the process leading to deposition are unknown, the excitation and decomposition of acetylene by UV photons are good enough to produce electrically resistive and optically transparent carbon films. We can explain also why the films produced by photo-CVD were soft. It was pointed out that growth rates, refractive index, and density increased with VB/ P”.s, which represented the average energy of the ions, where VB is the self-bias voltage across electrodes and P the gas pressure [9]. Therefore, it was asserted that the presence of energetic ions was critically important for the production of hard films. In the present arrangement with the low-pressure mercury lamp as a light source, it is impossible to generate ions, because for this purpose light with wavelengths shorter than 110 nm is required. Therefore, the assertion made previously for a close connection between ions and film properties is well supported by the present experiment. Also it becomes clear that one obvious method to improve film quality in photo-CVD is to use a light source with short wavelengths to generate ions and then accelerate them by a proper bias. There are rare gas resonance lamps available for this purpose. We face a different problem for production of diamond-like films. Here, crystallinity becomes important, and one obvious solution to improve it is to raise the substrate temperature. As shown in this experiment, at higher substrate temperatures pyrolytic effects become dominant and there is a tendency that graphite-like films are produced. In thermal CVD hydrogen gas was introduced to overcome this problem, because atomic hydrogen plays a critical role [ 121; the same method could be employed for photoCVD. However, we believe that a much more powerful light source than the low-pressure mercury lamp used in the present experiment is required to obtain a reasonable deposition rate, because it is expected to decrease considerably at such high temperatures. Alternatively, a light source with short wavelengths may be employed because acetylene absorbs light much 264
July 1986
more strongly below 1.50 nm [ 15 1. In a different scheme we may employ some kind of hybrid methods, where photoinduced effects are utilized together with other excitation techniques such as plasma or ion processes. However, it is clear from the present experiment that we cannot simply replace photons with electrons as the excitation source, or just use a simple combination of both. In conclusion, by photolysis of acetylene by 185 nm light, optically transparent and electrically resistive amorphous carbon films could be deposited at low substrate temperatures. Formation of graphite films was avoided because of the presence of energetic photons. The deposited films were soft, however, because of the absence of energetic ions. Thermal electrons accelerated by a positive bias were also used, but produced the same kind of films. Some suggestions were made on how to improve film hardness or grow diamond-like films by photoinduced effects. References
[II S. Aisenberg and R. Chabot, J. Appl. Phys. 42 (1971) 2953.
PI E.G. Spencer, P.H. Schmidt, D.C. Joy and F.J. Sansalone, Appl. Phys. Letters 29 (1976) 118.
131 C. Weissmantal, K. Bewilogua, D. Dietrich, H.J. Erler, H.J. Hinneberg, S. Klose, W. Nowik and G. Reisse, Thin Solid Films 72 (1980) 19. 141 T. Mori and Y. Namba, J. Appl. Phys. 55 (1984) 3276. 151 L. Holland and S.M. Ojha, Thin Solid Films 38 (1976) L17. 161 H. Voraand T.J. Moravec, J. Appl.Phys. 52 (1981) 6151. 171 T.J. Moravec and J.C. Lee, J. Vacuum Sci. Technol. 20 (1982) 338. Ial B. Dischler, A. Bubenzer and P. Koidl, Appl. Phys. Letters 42 (1983) 636. PI A. Bubenzer, B. Dischler, G. Brandt and P. Koidl, J. Appl. Phys. 54 (1983) 4590. IlO1 R.E. Sah, B. Dischler, A. Bubenzer and P. Koidl, Appl. Phys. Letters 46 (1985) 739. IllI T. Miyasato, Y. Kawakami, T. Kawano and A. Hiraki, Japan. J. Appl. Phys. 23 (1984) L234. 1121 S. Matsumoto, Y. Sato, M. Kamo and N. Setaka, Japan. J. Appl. Phys. 21 (1982) L183. 1131 A. Sawabe and T. Inuzuka, Appl. Phys. Letters 46 (1985) 146. 1141 D. Bauerle, ed., Laser processing and diagnostics (Springer, Berlin, 1984). I151 H. Okabe, Photochemistry of small molecules (Wiley, New York, 1978). 1161 B. Dischler, A. Bubenzer and P. Koidl, Solid State Commun.48 (1983) 105. II71 H. Okabe, J. Chem. Phys. 78 (1983) 1312.
4, number
A~O~HOUS M. DANNO Department Received
MATERIALS
5,6,7
July 1986
LETTERS
CARBON FILMS PREPARED
BY PHOTO-CVD
FROM ACE~LEN~
and M. HANABUSA
of Electrical Engineering,
Toyohashi University of Technology,
Toyohashr 440, Japan
1 Aprif 1986
An attempt has been made to deposit carbon films by photo-CVD, utilizing 185 nm light from a low-pressure mercury lamp to photolyze acetylene. The produced films are electrically resistive and optically transparent in the infrared, but they are soft. Comparing the present results with those obtained by plasma CVD, it becomes clear that the presence and acceleration of carbon ions play important roles in determining the physical properties, such as refractive index and hardness. Possibilities of producing hard amorphous or diamond-like carbon films using photoinduced effects are discussed.
1. Introduction In recent years there has been a growing interest in the production and applications of the carbon thin films. In particular, diamond-like carbon films which are amorphous with a certain degree of crystallinity are attracting a great deal of attention. Also hydrogenated amorphous carbon films (a-C:H) prepared at low substrate temperatures are of some interest. All of these films are highly electrically insulating, hard and optically transparent in the infrared, and should be distinguished from graphite carbon, which is electrically conductive and optically opaque. They have been prepared by various methods, including ion beam methods [l-3], ionized deposition [4], plasma deposition [5-IO], sputtering [ 111, CVD [12] and electronassisted CVD (EA CVD)
[ 141, Even though many kinds of photoinduced effects are available as the processes leading to deposition of thin films, a technique based on photolysis of material gases induced by UV light is in particular popular. Photo-CVD has been used to produce many kinds of thin films, which include silicon, metals and dielectrics. However, to our knowledge, this method has never 0 167-577x/86/$03.50 0 Elsevier Science Publishers B.V. (Noah-Hoard Physics Pub~shing Division)
been applied to the carbon films of great interest today. The objective of the present work is to examine what kind of carbon films can be produced by photo CVD. In the present work we photolyzed acetylene gas by 185 nm W light generated by a low-pressure mer cury lamp and produced carbon films on various substrates. The substrate was kept at relatively low temperatures, because otherwise a pyrolytic reaction exceeded the photoinduced reaction and graphite-like carbon films could easily be produced. The choice of low substrate temperatures was supported also by the fact that high-quality a-C:H films have been produced by plasma CVD with negative self-bias at low substrate temperatures [8- 101. In the following we show that by the present photoinduced method hydrogenated amorphous carbon films can be produced as by plasma CVD, but their quality is inferior because they are soft. It has been asserted that the quality of the a-C:H films, including hardness, is closely related with the presence of energetic ions in plasma [9]. Since ions are absent in the photolysis of acetylene by 185 nm photons, the present result seems to endorse their importance in producing good a-C:H films. Also we show by a simple experiment that energetic electrons are effective only to the same degree as photons in producing hard a-C:H films. On the basis of these observation we discuss possible solutions for improvement of quality of the carbon films produced by photoinduced effects. 261
Volume 4, number 5,6,7
MATERIALS LETTERS
2. Experimental The apparatus used in the present photoinduced experiment is shown in fig. 1. It consists of a lamp house, which contains a 40 W, N-shaped low-pressure mercury lamp (Sen Tokushu Kogen Co.) and is filled with nitrogen gas, and a cell, which is made of stainless steel and 20 cm in diameter and 20 cm high. The lamp and the substrate were separated by 40 mm through a quartz window, and the total UV power measured at the location of the substrate was about 4 mW/cm2, while the 185 nm component was about 20% of the total power. The UV light is effectively absorbed by acetylene with a rate of 10 atm-l cm-l [ 151. In the cell a substrate was placed on a tungsten heater, and the acetylene gas, which was 99.5% pure, was blown toward the substrate through a nozzle. The flow rate was 5.3 ml/min at a cell pressure of 3 Torr. A variety of substrates were tried, but mainly tungsten, quartz, and silicon were used in the subsequent experiment; they were cleaned ultrasonically in alcohol. To protect the window from carbon deposition, argon gas was blown to it. This scheme helped to reduce unwanted coverage of the window, but at the same time deposition on the substrate was slowed down considerably. Therefore, we often did not blow the inert gas, and instead, replaced the window when it became dirty. For comparison we deposited carbon films by electronic decomposition of acetylene. A tungsten wire, placed about 10 mm above the substrate, was heated to generate electrons. The electrons were drawn toward the substrate by a bias voltage (150 V)
I
I
II
I
I
‘vrheimocouple Fig. 1. Experimental arrangement used for photo-CVD of aC:H films from acetylene.
July 1986
applied to the substrate holder. The arrangement is identical to the one used previously for EA CVD to prepare diamond-like films, where the substrate temperature was raised to 8OO’C by an independent heater and also hydrogen gas was added to hydrocarbon gas [ 131. In the present experiment we intended to compare the effects of electrons with those of photons under conditions as much identical as possible, and therefore no hydrogen was added and the substrate was kept at about 2OO’C. The wire was heated to a temperature between 1300 and 19OO’C. The current flowing between the wire and the substrate holder was 5 mA at a wire temperature of 1900°C. The flow rate was 5.3 ml/min, while the gas pressure was kept at 3 Torr. The substrate was either silicon or quartz plate.
3. Results When we began the present photo-CVD experiment, it became immediately clear that transparent, highly resistive carbon films could be deposited. The films were deposited first at room temperature. They passed a scotch tape test easily, but were polished off with a Q dip after many strokes. Therefore, we raised the substrate temperature to 150°C; then the films became more adhesive and could not be scraped by the Q dip. However, it could be scratched easily by a pointed object; in this sense, we call them soft. The temperature was raised further in an attempt to improve film property; however, the deposition rate decreased substantially with increasing temperature, even though the adhesive force was not improved; it was about 500 A/h at 15O’C and 200 A/h at 300°C. At the higher temperature of 500°C, the film started to loose its transparency. The deposited films were amorphous, as confirmed by the absence of diffraction pattern in X-ray and electron analysis. The sample prepared at 5OO”C, however, was different from those prepared at lower temperatures, and a diffraction pattern characteristic of graphite was observed. The presence of graphite-like elements indicates the initiation of pyrolysis of acetylene. Hydrogen was detected from an.optical absorption loss spectrum around 3.4 pm due to a C-H stretch vibration. The result for a film deposited at 150°C is shown in fig. 2. Judging from the location and linewidth of the spectral peak [8,16], a monohydride
July 1986
MATERIALS LETTERS
Volume 4, number 5 $6,7
1 OOk
1
3500 WAVE
3000 NUMBER
2500 (cm-‘)
Fig. 2. IR absorption loss spectrum observed for a C-H stretch vibration.
bonded to sp3 - type carbon dominates among the possible C-H bonds. The optical absorption coefficient measured between 200 nm and 10 pm for a photoinduced film at 15O’C is shown in fig. 3. The optical property agrees with that of the films prepared by plasma CVD with negative self-bias [8]. The results show that the films are transparent in the IR range and loose transparency in the UV range. From the absorption measurements an optical gap of 1.9 eV was determined. A relatively large value for the optical gap indicates a high concentration of bonded hydrogen in the films. The electrical resistivity measured at room temperature was of the order of lo8 fi cm. This value is fairly high, but less than those measured for amorphous carbon films prepared by plasma CVD, which were as high as 1012 R cm [g]. The close analogy observed for optical transparency between the films prepared by photo-CVD and plasma CVD does not hold for the refractive index n; the value of y2was only 1.64 for the films deposited at 15O”C, while for the films prepared by plasma CVD its value was higher than 1.80, depending on deposition conditions, in particular self-bias voltage between the electrodes and gas pressure [8,9]. Related with the refractive index is the hardness of the films; the films with a low refractive index are known to be soft, which is true for the present photoinduced films.
WAVELENGTH
( n m)
Fig. 3. Optical absorption coefficient measured for a photoinduced carbon film.
The properties of the carbon films obtained by electronic excitation were almost identical to those of the photoinduced films. The deposition rates depended on the wire temperature; 60, 1900, and 9600 A/h at 1300, 1500, and 1900°C, respectively, for silicon substrates with a bias of 150 V. Without bias they were considerably lower; at a wire temperature of 19OO”C, the rate was 6000 A/h with the bias, while 1200 A/h without it, for quartz substrates. These results show that even though the gas is thermally decomposed by the heated wire, energetic electrons are more effective to deposit films. The optical and electrical properties were almost identical to those of the photoinduced films; the films were soft too. It is concluded that both W photons and electrons produce almost the same kind of a-C films.
263
Volume 4, number 5,6,7
MATERIALS LETTERS
4. Discussion and conclusions The fact that carbon films prepared by the present photo-CVD method were not graphite-like can be explained by the presence of energetic photons. Graphitelike fdms are known to be produced by low-energy processes such as pyrolysis. Acetylene molecules are excited by the 6.7 eV photons to decompose into C,H and H or C, and HZ [ 171. Even though the details of the process leading to deposition are unknown, the excitation and decomposition of acetylene by UV photons are good enough to produce electrically resistive and optically transparent carbon films. We can explain also why the films produced by photo-CVD were soft. It was pointed out that growth rates, refractive index, and density increased with VB/ P”.s, which represented the average energy of the ions, where VB is the self-bias voltage across electrodes and P the gas pressure [9]. Therefore, it was asserted that the presence of energetic ions was critically important for the production of hard films. In the present arrangement with the low-pressure mercury lamp as a light source, it is impossible to generate ions, because for this purpose light with wavelengths shorter than 110 nm is required. Therefore, the assertion made previously for a close connection between ions and film properties is well supported by the present experiment. Also it becomes clear that one obvious method to improve film quality in photo-CVD is to use a light source with short wavelengths to generate ions and then accelerate them by a proper bias. There are rare gas resonance lamps available for this purpose. We face a different problem for production of diamond-like films. Here, crystallinity becomes important, and one obvious solution to improve it is to raise the substrate temperature. As shown in this experiment, at higher substrate temperatures pyrolytic effects become dominant and there is a tendency that graphite-like films are produced. In thermal CVD hydrogen gas was introduced to overcome this problem, because atomic hydrogen plays a critical role [ 121; the same method could be employed for photoCVD. However, we believe that a much more powerful light source than the low-pressure mercury lamp used in the present experiment is required to obtain a reasonable deposition rate, because it is expected to decrease considerably at such high temperatures. Alternatively, a light source with short wavelengths may be employed because acetylene absorbs light much 264
July 1986
more strongly below 1.50 nm [ 15 1. In a different scheme we may employ some kind of hybrid methods, where photoinduced effects are utilized together with other excitation techniques such as plasma or ion processes. However, it is clear from the present experiment that we cannot simply replace photons with electrons as the excitation source, or just use a simple combination of both. In conclusion, by photolysis of acetylene by 185 nm light, optically transparent and electrically resistive amorphous carbon films could be deposited at low substrate temperatures. Formation of graphite films was avoided because of the presence of energetic photons. The deposited films were soft, however, because of the absence of energetic ions. Thermal electrons accelerated by a positive bias were also used, but produced the same kind of films. Some suggestions were made on how to improve film hardness or grow diamond-like films by photoinduced effects. References
[II S. Aisenberg and R. Chabot, J. Appl. Phys. 42 (1971) 2953.
PI E.G. Spencer, P.H. Schmidt, D.C. Joy and F.J. Sansalone, Appl. Phys. Letters 29 (1976) 118.
131 C. Weissmantal, K. Bewilogua, D. Dietrich, H.J. Erler, H.J. Hinneberg, S. Klose, W. Nowik and G. Reisse, Thin Solid Films 72 (1980) 19. 141 T. Mori and Y. Namba, J. Appl. Phys. 55 (1984) 3276. 151 L. Holland and S.M. Ojha, Thin Solid Films 38 (1976) L17. 161 H. Voraand T.J. Moravec, J. Appl.Phys. 52 (1981) 6151. 171 T.J. Moravec and J.C. Lee, J. Vacuum Sci. Technol. 20 (1982) 338. Ial B. Dischler, A. Bubenzer and P. Koidl, Appl. Phys. Letters 42 (1983) 636. PI A. Bubenzer, B. Dischler, G. Brandt and P. Koidl, J. Appl. Phys. 54 (1983) 4590. IlO1 R.E. Sah, B. Dischler, A. Bubenzer and P. Koidl, Appl. Phys. Letters 46 (1985) 739. IllI T. Miyasato, Y. Kawakami, T. Kawano and A. Hiraki, Japan. J. Appl. Phys. 23 (1984) L234. 1121 S. Matsumoto, Y. Sato, M. Kamo and N. Setaka, Japan. J. Appl. Phys. 21 (1982) L183. 1131 A. Sawabe and T. Inuzuka, Appl. Phys. Letters 46 (1985) 146. 1141 D. Bauerle, ed., Laser processing and diagnostics (Springer, Berlin, 1984). I151 H. Okabe, Photochemistry of small molecules (Wiley, New York, 1978). 1161 B. Dischler, A. Bubenzer and P. Koidl, Solid State Commun.48 (1983) 105. II71 H. Okabe, J. Chem. Phys. 78 (1983) 1312.