- Email: [email protected]
Radiation effects of vacuum ultraviolet lasers in amorphous Si3N4 films
Nuclear Instruments and Methods in Physics Research B 91 (1994) 659-662 North-Holland
NlONil B
Beam Interactions with Materials 8 Atoms
Radiation effects of vacuum ultraviolet lasers in amorphous S&N, films Kazuo Nakamae a~*,Kou Kurosawa b, Masato Ohmukai a,1, Masahito Katto a, Masahiro Okuda a, Wataru Sasaki b, Shigenori Nozawa ’ and Tatsushi Igarashi ’ a Department of Physics and Electronics, University of Osaka Prefecture, Sakai, Osaka 593, Japan b Department of Electrical Engineering, Miyazaki Uniuersity, Miyazaki 889-21, Japan ’ USHIO Inc., Himeji, Hyogo 647, Japan
The effects of irradiation of silicon nitride films with an argon excimer laser having a capability of emitting 126 nm photons of a fluence of 1014 per cm2 per pulse have been studied. We found that laser irradiation induces crystalline silicon precipitation accompanied with nitrogen desorption within thin surface layers of silicon nitride films. We showed that laser-induced electronic excitation played a crucial role in the process and further that the amount of precipitated silicon depends on the film surface temperature.
1. Introduction
It is well known that defect formation or bond rearrangements are induced by electronic excitation in some insulators and semiconductors [l]. Self-trapped excitons have been widely approved for the processes. In materials in which excitons are self-trapped, electronic excitation at surfaces is known to also induce sputtering or ejection of constituent atoms or ions. In almost all of these cases, however, the microscopic phenomena have been investigated, because the main interests are in the strong electron-phonon coupling. Takigawa et al. have found, recently, that valence electron excitation of amorphous silicon dioxide results in oxygen desorption and moreover crystalline silicon precipitation macroscopically [2]. Such a phenomenon must become a topic of strong interest not only as a novel phenomenon from the mechanistic point of view but also as a novel fabrication method of silicon crystals in insulating silicon dioxide from the practical point of view. Amorphous silicon nitride is also an important material for electrical insulation and passivation of semiconductor devices as well as amorphous silicon dioxide. We tried to apply WV laser-induced sputtering to
* Corresponding author. Present address: Institute of Laser Engineering, Osaka University, Yamadaoka, Suita 565, Japan. Tel. +816 877 5111 (ext. 6591), fax +816 877 4799, e-mail [email protected]. 1 Present address: Department of Electrical Engineering, Akashi College of Technology, Akashi, Hyogo 647, Japan.
insulating silicon nitride. We have found recently that WV lasers induce crystalline silicon precipitation within thin surface layers of silicon nitride films 131. Now the interesting point is in its mechanisms, i.e., whether the surface temperature plays an important role or not, and whether exciton formation is crucial in silicon nitride as well as in silicon oxide. In this paper we will describe experimental results for the silicon precipitation and discuss the mechanisms.
2. Experimental Amorphous S&N, films were grown with a plasmaenhanced chemical vapor deposition method using SiH, and NH, gas. Single crystals of GaAs and Si were used as the substrate materials. Films with two different thicknesses of 51 and 94 nm were fabricated on GaAs substrates and a film with a thickness of 120 nm was fabricated on a Si substrate. Heat diffusion distances are estimated to be 437 nm in S&N,, 575 nm in GaAs, and 871 nm in Si within 5 ns pulse duration. The argon excimer laser that we have designed and constructed ourselves [5-71 has a capability to emit light pulses with 5 ns duration at 126 nm (corresponding to 9.8 eV photon energy). The penetration depth of the photon from this laser in S&N, is calculated to be 6.7 nm based on the optical constants [4]. In Fig. 1 is illustrated the cavity part of it. A film was set as the laser-cavity reflector combined with an output coupler of a MgF, single crystal plate, all of which were in an atmosphere of an argon gas which pressure is about 30 kg/cm2. The reflectivity of Si,N, films used in this
0168-583X/94/$07.00 0 1994 - Elsevier Science B.V. All rights reserved SSDZ 0168-583X(94)00066-5
IX. SURFACE AND SPUTTERING
K. Nakamae et al. /Nucl.
660
I&r.
and Meth. in Phys. Res. B 91 (1994) 659-662
study has not been measured, but it is expected to be 28% at 126 nm based on the optical constants. Laser pulses of 5 mm diameter which came through two MgFz plates (one serves as a the cavity mirror and the other as a pressure window) were introduced into a jonlemeter (ED-ZOO, Gen Tee) in vacuum for monitoring the energy. The output energy per pulse largely depended upon the reflectivity and surface finish of the cavity reflector. We obtained an output energy of 65 mJ/cm’ with the Si,N,/GaAs reflector and 25 ml/cm’ with the Si,N,/Si reflector. Taking into account the transmission coefficient (63%) of the MgFz plates, we can estimate the laser fluences on the film surface to be 170 mJ/cm’ and 63 mJ/cm’. Observations of the laser-irradiated thin films were carried out with an X-ray photoelec~on spectrometer (XPS) (ESCALAB 200X, VG Scientific) and also with a Raman spectrometer (NR-1000, JASCO). In the case of XPS spectra, photoelectrons excited with Mg-K, radiation (energy = 1253.8 eV) were detected from selected areas with 0.6 mm diameter on sample surfaces. Since silicon nitride films were insulators, the shift of the X-ray photoelectron spectrum due to charge-up is observed, the energy was calibrated with respect to the value 532.5 eV for 0-1s core level emission. Si(KL-L& Auger spectra, whose chemical shifts are about three times as large as the Si-2p XPS peaks, were also detected [7]. Raman spectra that were excited with the 514.5 nm line from an argon ion laser, were taken from a selected area with 0.2 mm diameter on the sample surfaces.
l....l.... .l.,,“.‘I”‘~~
b)
SWN4lGaAs (94nm)
[c) Si3N41Si (120nm) A
3. Results We could see beam patterns on tie surfaces of the silicon nitride films with the naked eyes after only one laser shot. The beam patterns looked as if shallow holes of 5 mm in diameter were digged, whose appear-
OUTPUT MIRROR (MgF2) ANODE
CATHODE
-7OOKV
Fig. 1. Schematic illustration of the argon excimer laser we have designed and constructed by ourselves. Argon gas filied in the anode pipe is pumped by an electron beam accelerated to - 700 kV.
100 95 105 BINDING ENERGY (eV) Fig. 2. Si-2p XPS spectra taken from a 51 nm Si,N, film on 110
GaAs (a), from a 94 nm film on GaAs (b), and from a 120 nm film on Si (c). Curves ~$1were taken from the non-irradiated surfaces and curves #2 from the laser-irradiated surfaces. The shadow parts of the curves show the signals of elemental silicon and the number with these parts mean the integrated intensities of these area.
ante was almost independent of the film thickness and the substrate material. In Figs. 2 and 3 are shown Si-2p XPS and Si(KLL) Auger spectra. In these figures curves labeled #l were taken from non-irradiated surfaces and curves labeled as #2 were taken from laser-irradiated surfaces. In the 51 nm thickness film whose spectra are illustrated in (a), curve #1 contains a peak at 101.1 eV, which indicates that the main constituent is Si,N, before the
K. Nahzmae et al. /Nucl. Instr. and Meth. in Phys. Res. B 91 (1994) 659-662
laser irradiation. Curve #2 in Fig. 2a includes two peaks at 99.4 eV and 102.4 eV, and thus the laser irradiation splits the Si-2p peak into two peaks. The Si(KLL) Auger peak at 1612.6 eV splits into three peaks at 1609, 1612, and 1616.7 eV after laser irradiation. The 99.4 eV XPS peak and the 1616.7 eV Auger peak correspond to those from silicon in silicon single crystals. The 94 nm film showed almost the same results as those of the 51 nm film, as shown in Figs. 2b and 3b. The integrated intensity of the 99.4 eV XPS
661
I”“I”“I”“I”~‘I”“I””
- SisN4 _ (d=94nm)
after
/\, L
before
A 1....,....,....,....I....I.... (a)
100
SDN4IGaAs
200
300
400
500
600
700
_ 800
RAMAN SHIFT (l/cm) Fig. 4. Raman spectra taken from a 94 nm Si,N, film on GaAs before laser irradiation (lower curve) and after laser irradiation (upper curve).
peaks for the 94 nm film is apparently larger than those for the 51 nm film. It should be remarked here that the peak positions and the shape of the N-1s peaks for these three films did not change significantly by laser irradiation, although their integrated intensities were found to decrease significantly. In Figs. 2c and 3c are reproduced Si-2p XPS and Si(KLL) Auger spectra taken from the 120 nm thickness film on Si, which apparently indicate that laser irradiation has induced precipitation of silicon together with slightly oxidized silicon. Raman spectra taken from the 94 nm film are shown in Fig. 4. The upper curve horn the irradiated surface shows a sharp peak at 520 cm-r, which is assigned to the phonon mode of the non-irradiated surface. The 520 cm-’ peak corresponds with one from silicon crystals, and thus the surface contains silicon crystals. In these curves peaks at 290 cm-’ are assigned to the phonon mode of the GaAs substrate. (c)
1600
Si3N41Si (120nm)
1604
1608
A
1612
4. Discussion
1616
1620
KINETIC ENERGY (eV) Fig. 3. S#KL,,,L,,,) Auger spectra taken from a 51 nm Si,N, film on GaAs (a), from a 94 nm film on GaAs (b>, and from a 120 nm film on Si (c). Curves #l were taken from the non-irradiated ted surfaces.
surfaces and curves #2 from the laser-irradiaThe shadow parts of the curves show the signals of elemental silicon.
In the XPS experiments, Ga3d and As-3d-peaks were measured for all samples. For the 51 nm film, these peaks were observed after laser irradiation but not before laser irradiation. The integrated intensity after laser irradiation of 51 nm film is roughly equal to that taken from an unirradiated 10 nm Si,N, film on GaAs, which implies that a substantial part of the 51 nm film was etched off by laser irradiation. A laser pulse having 170 mJ/cm2 has an ability to etch off the film on GaAs by about 40 nm at the most. It was reasonable that XPS peaks of Ga-3d and As3d were not observed at all on the 94 mn films even after laser irradiation. In the case of the film on Si, we can IX. SURFACE AND SPUTTERING
662
K. Nakamae et al. /Nucl. Instr. and Meth. in Phys. Res. B 91 (1994) 659-662 2500
/
““I””
/
I”‘_
Si 3 N4 (m.p.1: g 2 3
2000
-:
-.
;-
Si (m.p.)
1500
2 I3 1000 !2 E
500 0 0
50
200 150 100 DEPTH (nm) Fig. 5. Temperature depth profiles calculated from the laserdeposition energy under the assumption of a simple one-dimensional thermal-diffusion prosess. Curve #1 is for a 51 nm Si,N, film on G&s, curve #2 is for a 94 nm Si,N, film on GaAs and curve #3 is for a 120 nm film on Si. The melting points of Si,N,, GaAs and Si are 2153 K, 1510 K and 1685 K, respectively.
exclude the possibility that elemental silicon signals come from the substrate in view of the result that Ga and As signals are not observed from the 94 nm film on GaAs after irradiation. Combining the results of the XPS observation with the Raman spectra, we can conclude that crystalline silicon precipitates in the surface layers of relatively thick SisN, films by irradiation with the argon excimer laser. We can estimate the amount of precipitated silicon based on the integrated intensity of the respective Si-2p XPS peaks. The 94 nm film provided a larger amount of silicon than the 51 nm film. Moreover the precipitated silicon in the 94 nm film on GaAs was larger than that in the 120 nm film on Si. The interesting point is whether the surface temperature is crucial or not. The temperatures of the surfaces and interfaces are estimated from the deposition energy by assuming only a one-dimensional thermaldiffusion process. In Fig. 5 are illustrated the calculated temperature depth profiles. The temperatures of the Si,N,/GaAs interfaces were found to be about 1200 K independent of the film thickness, and were lower than the melting point of GaAs. The surface temperatures largely depend upon the film thicknesses. The temperature of the 94 nm film apparently exceeds the melting point of Si,N, but that of the 51 nm film is almost the same as the melting point. The amount of precipitated silicon for the 94 nm film was larger than that of the 51 nm film, which indicates that it increases with the surface temperature.
As shown in Fig. 5, the surface temperature for the 120 nm film on Si could not reach the melting point and is much lower than those for the films on GaAs. It can be considered that the silicon precipitation is not induced by thermal processes but by electronic processes, although the fact that the detailed processes have not been clarified yet. It should be added finally that the Kr, excimer laser (146 nm) induces the same surface reaction as the Ar, excimer laser. By considering that the fundamental band gap energy of Si,N, is around 5 eV, both Ar, and Kr, excimer laser photons have the ability to induce band-to-band excitation via an efficient one photon absorption process. It is reasonable that both of these lasers induce the same surface reaction.
5. Summary We showed argon excimer laser induced crystalline silicon precipitation in thin surface layers of Si,N, films. Even though the film surface temperature was much lower than the melting point of S&N,, the silicon was observed to be precipitated. The electronic excitation was considered to play a crucial role in the process, in spite of a fact that the temperature rose clearly. The detailed process has to be clarified yet, but self-trapped excitons are inferred as the main cause for it. We are now carring out luminesence experiments and will report elsewhere.
References [l] N. Itoh (ed.), Defect
[2]
[3] [4] [5] [6]
[7]
[8]
Procesess Induced by Electronic Excitation in Insulator (World Scientific, Singapore, 1989). Y. Takigawa, K. Kurosawa, W. Sasaki, K. Yoshida, E. Fujiwara and Y. Kato, J. Non-Crystal. Solids 116 (1990) 293. M. Ohmukai, H. Naito, M. Okuda, K. Kurosawa and W. Sasaki, Jpn. J. Appl. Phys. 32 (1993) L1062. E.D. Palik (ed.), Handbook of Optical Constants of Solids (Academic Press, Orland, FL, 1985). W. Sasaki, Y. Uehara, K. Kurosawa, E. Fujiwara, Y. Kate and M. Yamanaka, Rev. Laser Eng. 14 (1986) 370. K. Kurosawa, W. Sasaki, M. Okuda, Y. Takigawa, K. Yoshida, E. Fujiwara and Y. Kato, Rev. Sci. Instr. 61 (1990) 728. K. Kurosawa, Y. Takigawa, W. Sasaki, M. Okuda, E. Fujiwara, K. Yoshida and Y. Kate, IEEE J. Quant. Electron. 61 (1991) 71. M.S. Hegde, R. Caracciolo, KS. Hatton and J.B. Wachtman Jr., Appl. Surf. Sci. 37 (1989) 16.
NlONil B
Beam Interactions with Materials 8 Atoms
Radiation effects of vacuum ultraviolet lasers in amorphous S&N, films Kazuo Nakamae a~*,Kou Kurosawa b, Masato Ohmukai a,1, Masahito Katto a, Masahiro Okuda a, Wataru Sasaki b, Shigenori Nozawa ’ and Tatsushi Igarashi ’ a Department of Physics and Electronics, University of Osaka Prefecture, Sakai, Osaka 593, Japan b Department of Electrical Engineering, Miyazaki Uniuersity, Miyazaki 889-21, Japan ’ USHIO Inc., Himeji, Hyogo 647, Japan
The effects of irradiation of silicon nitride films with an argon excimer laser having a capability of emitting 126 nm photons of a fluence of 1014 per cm2 per pulse have been studied. We found that laser irradiation induces crystalline silicon precipitation accompanied with nitrogen desorption within thin surface layers of silicon nitride films. We showed that laser-induced electronic excitation played a crucial role in the process and further that the amount of precipitated silicon depends on the film surface temperature.
1. Introduction
It is well known that defect formation or bond rearrangements are induced by electronic excitation in some insulators and semiconductors [l]. Self-trapped excitons have been widely approved for the processes. In materials in which excitons are self-trapped, electronic excitation at surfaces is known to also induce sputtering or ejection of constituent atoms or ions. In almost all of these cases, however, the microscopic phenomena have been investigated, because the main interests are in the strong electron-phonon coupling. Takigawa et al. have found, recently, that valence electron excitation of amorphous silicon dioxide results in oxygen desorption and moreover crystalline silicon precipitation macroscopically [2]. Such a phenomenon must become a topic of strong interest not only as a novel phenomenon from the mechanistic point of view but also as a novel fabrication method of silicon crystals in insulating silicon dioxide from the practical point of view. Amorphous silicon nitride is also an important material for electrical insulation and passivation of semiconductor devices as well as amorphous silicon dioxide. We tried to apply WV laser-induced sputtering to
* Corresponding author. Present address: Institute of Laser Engineering, Osaka University, Yamadaoka, Suita 565, Japan. Tel. +816 877 5111 (ext. 6591), fax +816 877 4799, e-mail [email protected]. 1 Present address: Department of Electrical Engineering, Akashi College of Technology, Akashi, Hyogo 647, Japan.
insulating silicon nitride. We have found recently that WV lasers induce crystalline silicon precipitation within thin surface layers of silicon nitride films 131. Now the interesting point is in its mechanisms, i.e., whether the surface temperature plays an important role or not, and whether exciton formation is crucial in silicon nitride as well as in silicon oxide. In this paper we will describe experimental results for the silicon precipitation and discuss the mechanisms.
2. Experimental Amorphous S&N, films were grown with a plasmaenhanced chemical vapor deposition method using SiH, and NH, gas. Single crystals of GaAs and Si were used as the substrate materials. Films with two different thicknesses of 51 and 94 nm were fabricated on GaAs substrates and a film with a thickness of 120 nm was fabricated on a Si substrate. Heat diffusion distances are estimated to be 437 nm in S&N,, 575 nm in GaAs, and 871 nm in Si within 5 ns pulse duration. The argon excimer laser that we have designed and constructed ourselves [5-71 has a capability to emit light pulses with 5 ns duration at 126 nm (corresponding to 9.8 eV photon energy). The penetration depth of the photon from this laser in S&N, is calculated to be 6.7 nm based on the optical constants [4]. In Fig. 1 is illustrated the cavity part of it. A film was set as the laser-cavity reflector combined with an output coupler of a MgF, single crystal plate, all of which were in an atmosphere of an argon gas which pressure is about 30 kg/cm2. The reflectivity of Si,N, films used in this
0168-583X/94/$07.00 0 1994 - Elsevier Science B.V. All rights reserved SSDZ 0168-583X(94)00066-5
IX. SURFACE AND SPUTTERING
K. Nakamae et al. /Nucl.
660
I&r.
and Meth. in Phys. Res. B 91 (1994) 659-662
study has not been measured, but it is expected to be 28% at 126 nm based on the optical constants. Laser pulses of 5 mm diameter which came through two MgFz plates (one serves as a the cavity mirror and the other as a pressure window) were introduced into a jonlemeter (ED-ZOO, Gen Tee) in vacuum for monitoring the energy. The output energy per pulse largely depended upon the reflectivity and surface finish of the cavity reflector. We obtained an output energy of 65 mJ/cm’ with the Si,N,/GaAs reflector and 25 ml/cm’ with the Si,N,/Si reflector. Taking into account the transmission coefficient (63%) of the MgFz plates, we can estimate the laser fluences on the film surface to be 170 mJ/cm’ and 63 mJ/cm’. Observations of the laser-irradiated thin films were carried out with an X-ray photoelec~on spectrometer (XPS) (ESCALAB 200X, VG Scientific) and also with a Raman spectrometer (NR-1000, JASCO). In the case of XPS spectra, photoelectrons excited with Mg-K, radiation (energy = 1253.8 eV) were detected from selected areas with 0.6 mm diameter on sample surfaces. Since silicon nitride films were insulators, the shift of the X-ray photoelectron spectrum due to charge-up is observed, the energy was calibrated with respect to the value 532.5 eV for 0-1s core level emission. Si(KL-L& Auger spectra, whose chemical shifts are about three times as large as the Si-2p XPS peaks, were also detected [7]. Raman spectra that were excited with the 514.5 nm line from an argon ion laser, were taken from a selected area with 0.2 mm diameter on the sample surfaces.
l....l.... .l.,,“.‘I”‘~~
b)
SWN4lGaAs (94nm)
[c) Si3N41Si (120nm) A
3. Results We could see beam patterns on tie surfaces of the silicon nitride films with the naked eyes after only one laser shot. The beam patterns looked as if shallow holes of 5 mm in diameter were digged, whose appear-
OUTPUT MIRROR (MgF2) ANODE
CATHODE
-7OOKV
Fig. 1. Schematic illustration of the argon excimer laser we have designed and constructed by ourselves. Argon gas filied in the anode pipe is pumped by an electron beam accelerated to - 700 kV.
100 95 105 BINDING ENERGY (eV) Fig. 2. Si-2p XPS spectra taken from a 51 nm Si,N, film on 110
GaAs (a), from a 94 nm film on GaAs (b), and from a 120 nm film on Si (c). Curves ~$1were taken from the non-irradiated surfaces and curves #2 from the laser-irradiated surfaces. The shadow parts of the curves show the signals of elemental silicon and the number with these parts mean the integrated intensities of these area.
ante was almost independent of the film thickness and the substrate material. In Figs. 2 and 3 are shown Si-2p XPS and Si(KLL) Auger spectra. In these figures curves labeled #l were taken from non-irradiated surfaces and curves labeled as #2 were taken from laser-irradiated surfaces. In the 51 nm thickness film whose spectra are illustrated in (a), curve #1 contains a peak at 101.1 eV, which indicates that the main constituent is Si,N, before the
K. Nahzmae et al. /Nucl. Instr. and Meth. in Phys. Res. B 91 (1994) 659-662
laser irradiation. Curve #2 in Fig. 2a includes two peaks at 99.4 eV and 102.4 eV, and thus the laser irradiation splits the Si-2p peak into two peaks. The Si(KLL) Auger peak at 1612.6 eV splits into three peaks at 1609, 1612, and 1616.7 eV after laser irradiation. The 99.4 eV XPS peak and the 1616.7 eV Auger peak correspond to those from silicon in silicon single crystals. The 94 nm film showed almost the same results as those of the 51 nm film, as shown in Figs. 2b and 3b. The integrated intensity of the 99.4 eV XPS
661
I”“I”“I”“I”~‘I”“I””
- SisN4 _ (d=94nm)
after
/\, L
before
A 1....,....,....,....I....I.... (a)
100
SDN4IGaAs
200
300
400
500
600
700
_ 800
RAMAN SHIFT (l/cm) Fig. 4. Raman spectra taken from a 94 nm Si,N, film on GaAs before laser irradiation (lower curve) and after laser irradiation (upper curve).
peaks for the 94 nm film is apparently larger than those for the 51 nm film. It should be remarked here that the peak positions and the shape of the N-1s peaks for these three films did not change significantly by laser irradiation, although their integrated intensities were found to decrease significantly. In Figs. 2c and 3c are reproduced Si-2p XPS and Si(KLL) Auger spectra taken from the 120 nm thickness film on Si, which apparently indicate that laser irradiation has induced precipitation of silicon together with slightly oxidized silicon. Raman spectra taken from the 94 nm film are shown in Fig. 4. The upper curve horn the irradiated surface shows a sharp peak at 520 cm-r, which is assigned to the phonon mode of the non-irradiated surface. The 520 cm-’ peak corresponds with one from silicon crystals, and thus the surface contains silicon crystals. In these curves peaks at 290 cm-’ are assigned to the phonon mode of the GaAs substrate. (c)
1600
Si3N41Si (120nm)
1604
1608
A
1612
4. Discussion
1616
1620
KINETIC ENERGY (eV) Fig. 3. S#KL,,,L,,,) Auger spectra taken from a 51 nm Si,N, film on GaAs (a), from a 94 nm film on GaAs (b>, and from a 120 nm film on Si (c). Curves #l were taken from the non-irradiated ted surfaces.
surfaces and curves #2 from the laser-irradiaThe shadow parts of the curves show the signals of elemental silicon.
In the XPS experiments, Ga3d and As-3d-peaks were measured for all samples. For the 51 nm film, these peaks were observed after laser irradiation but not before laser irradiation. The integrated intensity after laser irradiation of 51 nm film is roughly equal to that taken from an unirradiated 10 nm Si,N, film on GaAs, which implies that a substantial part of the 51 nm film was etched off by laser irradiation. A laser pulse having 170 mJ/cm2 has an ability to etch off the film on GaAs by about 40 nm at the most. It was reasonable that XPS peaks of Ga-3d and As3d were not observed at all on the 94 mn films even after laser irradiation. In the case of the film on Si, we can IX. SURFACE AND SPUTTERING
662
K. Nakamae et al. /Nucl. Instr. and Meth. in Phys. Res. B 91 (1994) 659-662 2500
/
““I””
/
I”‘_
Si 3 N4 (m.p.1: g 2 3
2000
-:
-.
;-
Si (m.p.)
1500
2 I3 1000 !2 E
500 0 0
50
200 150 100 DEPTH (nm) Fig. 5. Temperature depth profiles calculated from the laserdeposition energy under the assumption of a simple one-dimensional thermal-diffusion prosess. Curve #1 is for a 51 nm Si,N, film on G&s, curve #2 is for a 94 nm Si,N, film on GaAs and curve #3 is for a 120 nm film on Si. The melting points of Si,N,, GaAs and Si are 2153 K, 1510 K and 1685 K, respectively.
exclude the possibility that elemental silicon signals come from the substrate in view of the result that Ga and As signals are not observed from the 94 nm film on GaAs after irradiation. Combining the results of the XPS observation with the Raman spectra, we can conclude that crystalline silicon precipitates in the surface layers of relatively thick SisN, films by irradiation with the argon excimer laser. We can estimate the amount of precipitated silicon based on the integrated intensity of the respective Si-2p XPS peaks. The 94 nm film provided a larger amount of silicon than the 51 nm film. Moreover the precipitated silicon in the 94 nm film on GaAs was larger than that in the 120 nm film on Si. The interesting point is whether the surface temperature is crucial or not. The temperatures of the surfaces and interfaces are estimated from the deposition energy by assuming only a one-dimensional thermaldiffusion process. In Fig. 5 are illustrated the calculated temperature depth profiles. The temperatures of the Si,N,/GaAs interfaces were found to be about 1200 K independent of the film thickness, and were lower than the melting point of GaAs. The surface temperatures largely depend upon the film thicknesses. The temperature of the 94 nm film apparently exceeds the melting point of Si,N, but that of the 51 nm film is almost the same as the melting point. The amount of precipitated silicon for the 94 nm film was larger than that of the 51 nm film, which indicates that it increases with the surface temperature.
As shown in Fig. 5, the surface temperature for the 120 nm film on Si could not reach the melting point and is much lower than those for the films on GaAs. It can be considered that the silicon precipitation is not induced by thermal processes but by electronic processes, although the fact that the detailed processes have not been clarified yet. It should be added finally that the Kr, excimer laser (146 nm) induces the same surface reaction as the Ar, excimer laser. By considering that the fundamental band gap energy of Si,N, is around 5 eV, both Ar, and Kr, excimer laser photons have the ability to induce band-to-band excitation via an efficient one photon absorption process. It is reasonable that both of these lasers induce the same surface reaction.
5. Summary We showed argon excimer laser induced crystalline silicon precipitation in thin surface layers of Si,N, films. Even though the film surface temperature was much lower than the melting point of S&N,, the silicon was observed to be precipitated. The electronic excitation was considered to play a crucial role in the process, in spite of a fact that the temperature rose clearly. The detailed process has to be clarified yet, but self-trapped excitons are inferred as the main cause for it. We are now carring out luminesence experiments and will report elsewhere.
References [l] N. Itoh (ed.), Defect
[2]
[3] [4] [5] [6]
[7]
[8]
Procesess Induced by Electronic Excitation in Insulator (World Scientific, Singapore, 1989). Y. Takigawa, K. Kurosawa, W. Sasaki, K. Yoshida, E. Fujiwara and Y. Kato, J. Non-Crystal. Solids 116 (1990) 293. M. Ohmukai, H. Naito, M. Okuda, K. Kurosawa and W. Sasaki, Jpn. J. Appl. Phys. 32 (1993) L1062. E.D. Palik (ed.), Handbook of Optical Constants of Solids (Academic Press, Orland, FL, 1985). W. Sasaki, Y. Uehara, K. Kurosawa, E. Fujiwara, Y. Kate and M. Yamanaka, Rev. Laser Eng. 14 (1986) 370. K. Kurosawa, W. Sasaki, M. Okuda, Y. Takigawa, K. Yoshida, E. Fujiwara and Y. Kato, Rev. Sci. Instr. 61 (1990) 728. K. Kurosawa, Y. Takigawa, W. Sasaki, M. Okuda, E. Fujiwara, K. Yoshida and Y. Kate, IEEE J. Quant. Electron. 61 (1991) 71. M.S. Hegde, R. Caracciolo, KS. Hatton and J.B. Wachtman Jr., Appl. Surf. Sci. 37 (1989) 16.