- Email: [email protected]
Polycrystalline silicon precipitates on SiO2 using an argon excimer laser
Applied Surface Science 137 Ž1999. 78–82
Polycrystalline silicon precipitates on SiO 2 using an argon excimer laser Masato Ohmukai a
a,)
, Yasuo Takigawa b, Kou Kurosawa
c
Department of Electrical Engineering, Akashi College of Technology, Uozumicho-Nishioka, Akashi, Hyogo, 674-8501, Japan b Department of Electronics, Osaka Electro-Communication UniÕersity, Neyagawa, Osaka, 572-8530, Japan c Department of Electrical Engineering, Miyazaki UniÕersity, Miyazaki, 889-2192, Japan Received 28 May 1998; accepted 17 July 1998
Abstract We are developing an argon excimer laser which oscillates at 126 nm Ž9.8 eV.. Since the photon energy of the laser is as high as 9.8 eV, the laser can induce bond breaking in most of materials without any reactive gas or solution. We performed irradiation of an argon excimer laser on crystal and glass SiO 2 , and then investigated the surfaces by means of X-ray photoelectron spectroscopy, Raman scattering, X-ray diffraction and reflection of high energy electron diffraction measurements. The results indicate that polycrystalline silicon precipitates on the surface with a preferential orientation. q 1999 Elsevier Science B.V. All rights reserved. PACS: 82.50.Fv Keywords: Growth; Radiation effect; Silicon
1. Introduction We are developing an argon excimer laser w1x as a high-fluence and short-wavelength laser. The wavelength of the laser is 126 nm Ž9.8 eV. which is in the vacuum ultraviolet region. The largest fluence attained up to now is 80 mJ Ž16 MW for a duration of 5 ns. w2x. Since photon energy of the argon excimer laser is no less than the binding energy of SiO 2 bonds Ž9 eV., the irradiation of argon excimer laser light can induce bond breaking and chemical changes of SiO 2 in the surface layer w3,4x.
)
Corresponding author. Tel.: q81-78-946-6124; Fax: q81-78946-6138; E-mail: [email protected]
In this article we report further results on irradiation effects of an argon excimer laser on crystal and glass SiO 2 . We investigated the surface chemical changes by means of X-ray photoelectron spectroscopy ŽXPS. and Raman scattering measurements. We evaluated the physical properties of the surface layer by means of X-ray diffraction ŽXRD. and reflection of high energy electron diffraction ŽRHEED..
2. Experimental details The argon excimer laser developed by us is illustrated in Fig. 1. The laser oscillates between a reflec-
0169-4332r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 Ž 9 8 . 0 0 3 7 1 - 7
M. Ohmukai et al.r Applied Surface Science 137 (1999) 78–82
Fig. 1. Schematic illustration of an argon excimer laser. A crystal or glass SiO 2 is fitted up as a reflector.
tor and an output mirror which are located at both ends of an anode pipe. Inside the anode pipe, a laser medium of argon gas was charged at a pressure of 35 atm. The output mirror and window are made of MgF2 plates. An SiO 2 sample was put as a reflector in an argon gas environment. During the laser oscillation, several pulses of intense laser light irradiate an SiO 2 sample. A repetition frequency is about 1 shotrh. The beam size is 5 mm in diameter. From the fact that the output energy of the laser was about 10 mJ, the energy fluence was estimated to be 125 mJrcm2 on the SiO 2 sample surface. The value was calculated from the transmittance of the MgF2 mirror and window w5x. After the SiO 2 Žglass or crystal. samples were exposed to the argon excimer laser light, a beam pattern was observable on the surface by optical inspection. The surfaces of the irradiated SiO 2 samples were investigated using XPS, Raman scattering, XRD, and RHEED measurements. We compare the results taken from irradiated and non-irradiated areas. XPS analyses were performed with an XPS spectrometer ŽESCA-1000; Shimadzu. using a MgK a X-ray source Ž E s 1253.8 eV.. For calculation of the binding energy, the peak position of the 1 s core level emission from oxygen contaminants was assumed to be 532.8 eV. The minimum size of the analyzed area is 0.2 mm in diameter. Raman scattering spectra were taken at room temperature by a Raman spectrometer ŽNR-1000; JASCO. in the quasi-backscattering configuration. The excitation light was the 514.5 nm line of an argon ion laser with a power of 300 mW. The
79
diameter of the incident light beam on a sample surface was about 200 mm. The scattered light was analyzed using a double monochromator in conjunction with a conventional photon counting system. The spectral resolution was several cmy1 . We also performed a line analysis in Raman scattering measurements. In order to investigate the surface structures of the irradiated SiO 2 , we used an X-ray diffractometer ŽRINT-1500; Rigaku. with an X-ray tube operating at 50 kV, with a tube current of 200 mA. At a sample stage, we used a thin film attachment in order to enhance sensitivity, because only a thin surface region is to be observed. We also took an RHEED pattern with an incident electron beam accelerated at 100 kV. The analytical depth of the RHEED measurement is the order of a nm. Since the penetration depth of the argon excimer laser light in SiO 2 Žthe order of 100 nm. is thicker than the analytical depth, RHEED is suitable for investigating the thin surface layer of the laser irradiated samples.
3. Results and discussion Fig. 2 shows Si-2p core level emission spectra. Curve 1 shows a spectrum from a non-irradiated part, namely a virgin SiO 2 surface, curve 2, is one from the area inside a laser-induced beam pattern. Curve 1 consists of one peak located at 103.4 eV, which corresponds to the binding energy of Si-2p in SiO 2 . In curve 2, a sub peak appears at 99.3 eV. The peak is identified as a peak from Si-2p core level emission in bulk silicon. Curve 2 implies the presence of silicon as well as silicon dioxide in the surface layer. Fig. 3 shows Raman spectra taken from Ža. the non-irradiated part and Žb. the irradiated part. The wave number region is from 100 to 1100 cmy1 . The Raman spectrum in Fig. 3Ža. is a typical spectrum from glass SiO 2 . A broad peak assigned as Si–O bonds was observed at 400 cmy1 . On the other hand, a Raman spectrum from the irradiated area shows a sharp peak at 520 cmy1 . The same peak is obtained from a silicon single crystal. From the evidence above, we conclude that the laser irradiation induced a chemical change from SiO 2 to elemental silicon.
80
M. Ohmukai et al.r Applied Surface Science 137 (1999) 78–82
Fig. 2. X-ray photoelectron spectra of Si-2p core level emission. Curve 1 and 2 are obtained from non-irradiated and irradiated areas of SiO 2 , respectively.
Fig. 3. Raman spectra from a non-irradiated area Ža. and an irradiated area Žb. of a glass SiO 2 . Laser irradiation induced a drastic change in Raman spectra.
In order to explore a spatial distribution of the Raman peak in the beam pattern, we took Raman spectra from several points separated by 0.25 mm on a line through the center of the beam pattern. Fig. 4 shows integrated intensities of the peak at 520 cmy1 plotted against the distance. The intensities are zero in the central part of the beam pattern as well as the outside of the beam pattern. The shape of the area where the peak intensity is noticeable, is donut-like. The shape is similar to the intensity profiles of the argon excimer laser w6x. This result indicates the positive correlation between the laser intensity and the amount of the precipitated silicon. It suggests that intensity control of the argon excimer laser is of importance. A full width at half maximum ŽFWHM. in Raman spectrum tells us whether the material is crystalline or amorphous. A Raman spectrum from crystalline silicon gives us a sharp peak ŽFWHMs 4 cmy1 . at 520 cmy1 . On the other hand, a Raman spectrum from amorphous silicon consist of a broad peak at 480 cmy1 , whose FWHM is over 73 cmy1 . In this case, the spectra do not include a peak at 520 cmy1 . In Fig. 3 a peak at 520 cmy1 has the FWHM of 4 cmy1 . This indicates that the precipitated silicon is crystalline. In order to confirm that the precipitated silicon is crystalline, we obtained X-ray reflection peak at 2 u s 28.5 degree ŽFig. 5., corresponding to Ž111. reflection of a bulk silicon crystal. The measurement took 8 h because silicon had precipitated only in the
Fig. 4. Spatial distribution of Raman intensity at 520 cmy1 .
M. Ohmukai et al.r Applied Surface Science 137 (1999) 78–82
Fig. 5. X-ray diffraction peak with a grazing incidence beam. The peak corresponds to a Ž111. reflection from crystalline silicon.
thin surface layer. We also performed RHEED measurements as a surface-sensitive investigation. Fig. 6 shows RHEED pattern obtained from the irradiated SiO 2 sample surface. We observed several weak spots which were arranged on a vertical line as well as a halo pattern. The halo pattern was derived from electrons scattered in SiO 2 glass. The weak spots correspond to a micro-crystalline structure of the sample surface. From the fact that the RHEED spots are only observed on a vertical line, precipi-
81
tated silicon consists of silicon polycrystals which have a preferential orientation normal to the surface. We showed enough evidence of elemental silicon precipitation at the surface. The precipitated silicon is polycrystalline and preferentially oriented. The silicon-precipitated region appears to be limited to several nanometer of depth from the surface. It is because Ž1. the penetration depth of the laser light in SiO 2 is the order of nm, and Ž2. the conversion from SiO 2 to Si is based on the photochemical bond breaking of SiO 2 . The phenomenon is characterized by two points. The silicon precipitation occurred only in the thin surface layer and needed no reactive gas or solution. The latter is attractive because such a combustible gas as silane is unnecessary as a starting material. This precipitation can be used for a safe and contamination-free crystal growth. Oxidation of silicon is widely performed as a dry process in manufacturing semiconductor devices. In order to remove the silicon oxide layer, wet etching or ion etching is usually used. The advantages of the irradiation of an argon excimer laser is also welcome
Fig. 6. RHEED pattern obtained with a 100 keV electron beam. It shows several spots, which indicates the precipitated silicon is crystalline and preferentially oriented. A halo pattern derives from electrons scattered by SiO 2 glass.
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M. Ohmukai et al.r Applied Surface Science 137 (1999) 78–82
to the removal process of silicon oxide as a contamination-free dry etching. On the other hand there are some problems in the application of the precipitation to the formation of a thin silicon layer. An argon excimer laser can not afford a sufficient repetition frequency at present. It is difficult to obtain a well-controlled growth of silicon crystals by the one-shot laser pulse. It is because the flat surface of samples is waved owing to a thermal effect. In order to make use of this precipitation mechanism as a clean and simple method to form silicon crystallites, further development of an argon excimer laser is unavoidable.
4. Conclusion Argon excimer laser light has the ability to convert SiO 2 to elemental silicon without any reactive gas or solution. The precipitated silicon is polycrystalline and preferentially oriented. The phenomenon
will be applied to a formation of silicon with the help of further development of an argon excimer laser. Further research is expected to explore the grain size and detailed structure of silicon crystallites.
References w1x W. Sasaki, K. Kurosawa, P.R. Herman, E. Fujiwara, Y. Kato, in: C. Yamanaka ŽEd.., Short-Wavelength Lasers and Their Applications, Springer-Verlag, Berlin, 1988, p. 316. w2x K. Kurosawa, W. Sasaki, M. Okuda, Y. Takigawa, K. Yoshida, E. Fujiwara, Y. Kato, Rev. Sci. Instrum. 61 Ž1990. 728. w3x Y. Takigawa, K. Kurosawa, W. Sasaki, K. Yoshida, E. Fujiwara, Y. Kato, J. Non-Cryst. Solids 116 Ž1990. 293. w4x Y. Takigawa, K. Kurosawa, W. Sasaki, M. Okuda, K. Yoshida, E. Fujiwara, Y. Kato, Y. Inoue, J. Non-Cryst. Solids 125 Ž1990. 107. w5x K. Kurosawa, Y. Takigawa, W. Sasaki, M. Katto, Y. Inoue, Jpn. J. Appl. Phys. 30 Ž1991. 3219. w6x M. Katto, R. Matsumoto, K. Kurosawa, W. Sasaki, Y. Takigawa, M. Okuda, Rev. Sci. Instrum. 64 Ž1993. 319.
Polycrystalline silicon precipitates on SiO 2 using an argon excimer laser Masato Ohmukai a
a,)
, Yasuo Takigawa b, Kou Kurosawa
c
Department of Electrical Engineering, Akashi College of Technology, Uozumicho-Nishioka, Akashi, Hyogo, 674-8501, Japan b Department of Electronics, Osaka Electro-Communication UniÕersity, Neyagawa, Osaka, 572-8530, Japan c Department of Electrical Engineering, Miyazaki UniÕersity, Miyazaki, 889-2192, Japan Received 28 May 1998; accepted 17 July 1998
Abstract We are developing an argon excimer laser which oscillates at 126 nm Ž9.8 eV.. Since the photon energy of the laser is as high as 9.8 eV, the laser can induce bond breaking in most of materials without any reactive gas or solution. We performed irradiation of an argon excimer laser on crystal and glass SiO 2 , and then investigated the surfaces by means of X-ray photoelectron spectroscopy, Raman scattering, X-ray diffraction and reflection of high energy electron diffraction measurements. The results indicate that polycrystalline silicon precipitates on the surface with a preferential orientation. q 1999 Elsevier Science B.V. All rights reserved. PACS: 82.50.Fv Keywords: Growth; Radiation effect; Silicon
1. Introduction We are developing an argon excimer laser w1x as a high-fluence and short-wavelength laser. The wavelength of the laser is 126 nm Ž9.8 eV. which is in the vacuum ultraviolet region. The largest fluence attained up to now is 80 mJ Ž16 MW for a duration of 5 ns. w2x. Since photon energy of the argon excimer laser is no less than the binding energy of SiO 2 bonds Ž9 eV., the irradiation of argon excimer laser light can induce bond breaking and chemical changes of SiO 2 in the surface layer w3,4x.
)
Corresponding author. Tel.: q81-78-946-6124; Fax: q81-78946-6138; E-mail: [email protected]
In this article we report further results on irradiation effects of an argon excimer laser on crystal and glass SiO 2 . We investigated the surface chemical changes by means of X-ray photoelectron spectroscopy ŽXPS. and Raman scattering measurements. We evaluated the physical properties of the surface layer by means of X-ray diffraction ŽXRD. and reflection of high energy electron diffraction ŽRHEED..
2. Experimental details The argon excimer laser developed by us is illustrated in Fig. 1. The laser oscillates between a reflec-
0169-4332r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 Ž 9 8 . 0 0 3 7 1 - 7
M. Ohmukai et al.r Applied Surface Science 137 (1999) 78–82
Fig. 1. Schematic illustration of an argon excimer laser. A crystal or glass SiO 2 is fitted up as a reflector.
tor and an output mirror which are located at both ends of an anode pipe. Inside the anode pipe, a laser medium of argon gas was charged at a pressure of 35 atm. The output mirror and window are made of MgF2 plates. An SiO 2 sample was put as a reflector in an argon gas environment. During the laser oscillation, several pulses of intense laser light irradiate an SiO 2 sample. A repetition frequency is about 1 shotrh. The beam size is 5 mm in diameter. From the fact that the output energy of the laser was about 10 mJ, the energy fluence was estimated to be 125 mJrcm2 on the SiO 2 sample surface. The value was calculated from the transmittance of the MgF2 mirror and window w5x. After the SiO 2 Žglass or crystal. samples were exposed to the argon excimer laser light, a beam pattern was observable on the surface by optical inspection. The surfaces of the irradiated SiO 2 samples were investigated using XPS, Raman scattering, XRD, and RHEED measurements. We compare the results taken from irradiated and non-irradiated areas. XPS analyses were performed with an XPS spectrometer ŽESCA-1000; Shimadzu. using a MgK a X-ray source Ž E s 1253.8 eV.. For calculation of the binding energy, the peak position of the 1 s core level emission from oxygen contaminants was assumed to be 532.8 eV. The minimum size of the analyzed area is 0.2 mm in diameter. Raman scattering spectra were taken at room temperature by a Raman spectrometer ŽNR-1000; JASCO. in the quasi-backscattering configuration. The excitation light was the 514.5 nm line of an argon ion laser with a power of 300 mW. The
79
diameter of the incident light beam on a sample surface was about 200 mm. The scattered light was analyzed using a double monochromator in conjunction with a conventional photon counting system. The spectral resolution was several cmy1 . We also performed a line analysis in Raman scattering measurements. In order to investigate the surface structures of the irradiated SiO 2 , we used an X-ray diffractometer ŽRINT-1500; Rigaku. with an X-ray tube operating at 50 kV, with a tube current of 200 mA. At a sample stage, we used a thin film attachment in order to enhance sensitivity, because only a thin surface region is to be observed. We also took an RHEED pattern with an incident electron beam accelerated at 100 kV. The analytical depth of the RHEED measurement is the order of a nm. Since the penetration depth of the argon excimer laser light in SiO 2 Žthe order of 100 nm. is thicker than the analytical depth, RHEED is suitable for investigating the thin surface layer of the laser irradiated samples.
3. Results and discussion Fig. 2 shows Si-2p core level emission spectra. Curve 1 shows a spectrum from a non-irradiated part, namely a virgin SiO 2 surface, curve 2, is one from the area inside a laser-induced beam pattern. Curve 1 consists of one peak located at 103.4 eV, which corresponds to the binding energy of Si-2p in SiO 2 . In curve 2, a sub peak appears at 99.3 eV. The peak is identified as a peak from Si-2p core level emission in bulk silicon. Curve 2 implies the presence of silicon as well as silicon dioxide in the surface layer. Fig. 3 shows Raman spectra taken from Ža. the non-irradiated part and Žb. the irradiated part. The wave number region is from 100 to 1100 cmy1 . The Raman spectrum in Fig. 3Ža. is a typical spectrum from glass SiO 2 . A broad peak assigned as Si–O bonds was observed at 400 cmy1 . On the other hand, a Raman spectrum from the irradiated area shows a sharp peak at 520 cmy1 . The same peak is obtained from a silicon single crystal. From the evidence above, we conclude that the laser irradiation induced a chemical change from SiO 2 to elemental silicon.
80
M. Ohmukai et al.r Applied Surface Science 137 (1999) 78–82
Fig. 2. X-ray photoelectron spectra of Si-2p core level emission. Curve 1 and 2 are obtained from non-irradiated and irradiated areas of SiO 2 , respectively.
Fig. 3. Raman spectra from a non-irradiated area Ža. and an irradiated area Žb. of a glass SiO 2 . Laser irradiation induced a drastic change in Raman spectra.
In order to explore a spatial distribution of the Raman peak in the beam pattern, we took Raman spectra from several points separated by 0.25 mm on a line through the center of the beam pattern. Fig. 4 shows integrated intensities of the peak at 520 cmy1 plotted against the distance. The intensities are zero in the central part of the beam pattern as well as the outside of the beam pattern. The shape of the area where the peak intensity is noticeable, is donut-like. The shape is similar to the intensity profiles of the argon excimer laser w6x. This result indicates the positive correlation between the laser intensity and the amount of the precipitated silicon. It suggests that intensity control of the argon excimer laser is of importance. A full width at half maximum ŽFWHM. in Raman spectrum tells us whether the material is crystalline or amorphous. A Raman spectrum from crystalline silicon gives us a sharp peak ŽFWHMs 4 cmy1 . at 520 cmy1 . On the other hand, a Raman spectrum from amorphous silicon consist of a broad peak at 480 cmy1 , whose FWHM is over 73 cmy1 . In this case, the spectra do not include a peak at 520 cmy1 . In Fig. 3 a peak at 520 cmy1 has the FWHM of 4 cmy1 . This indicates that the precipitated silicon is crystalline. In order to confirm that the precipitated silicon is crystalline, we obtained X-ray reflection peak at 2 u s 28.5 degree ŽFig. 5., corresponding to Ž111. reflection of a bulk silicon crystal. The measurement took 8 h because silicon had precipitated only in the
Fig. 4. Spatial distribution of Raman intensity at 520 cmy1 .
M. Ohmukai et al.r Applied Surface Science 137 (1999) 78–82
Fig. 5. X-ray diffraction peak with a grazing incidence beam. The peak corresponds to a Ž111. reflection from crystalline silicon.
thin surface layer. We also performed RHEED measurements as a surface-sensitive investigation. Fig. 6 shows RHEED pattern obtained from the irradiated SiO 2 sample surface. We observed several weak spots which were arranged on a vertical line as well as a halo pattern. The halo pattern was derived from electrons scattered in SiO 2 glass. The weak spots correspond to a micro-crystalline structure of the sample surface. From the fact that the RHEED spots are only observed on a vertical line, precipi-
81
tated silicon consists of silicon polycrystals which have a preferential orientation normal to the surface. We showed enough evidence of elemental silicon precipitation at the surface. The precipitated silicon is polycrystalline and preferentially oriented. The silicon-precipitated region appears to be limited to several nanometer of depth from the surface. It is because Ž1. the penetration depth of the laser light in SiO 2 is the order of nm, and Ž2. the conversion from SiO 2 to Si is based on the photochemical bond breaking of SiO 2 . The phenomenon is characterized by two points. The silicon precipitation occurred only in the thin surface layer and needed no reactive gas or solution. The latter is attractive because such a combustible gas as silane is unnecessary as a starting material. This precipitation can be used for a safe and contamination-free crystal growth. Oxidation of silicon is widely performed as a dry process in manufacturing semiconductor devices. In order to remove the silicon oxide layer, wet etching or ion etching is usually used. The advantages of the irradiation of an argon excimer laser is also welcome
Fig. 6. RHEED pattern obtained with a 100 keV electron beam. It shows several spots, which indicates the precipitated silicon is crystalline and preferentially oriented. A halo pattern derives from electrons scattered by SiO 2 glass.
82
M. Ohmukai et al.r Applied Surface Science 137 (1999) 78–82
to the removal process of silicon oxide as a contamination-free dry etching. On the other hand there are some problems in the application of the precipitation to the formation of a thin silicon layer. An argon excimer laser can not afford a sufficient repetition frequency at present. It is difficult to obtain a well-controlled growth of silicon crystals by the one-shot laser pulse. It is because the flat surface of samples is waved owing to a thermal effect. In order to make use of this precipitation mechanism as a clean and simple method to form silicon crystallites, further development of an argon excimer laser is unavoidable.
4. Conclusion Argon excimer laser light has the ability to convert SiO 2 to elemental silicon without any reactive gas or solution. The precipitated silicon is polycrystalline and preferentially oriented. The phenomenon
will be applied to a formation of silicon with the help of further development of an argon excimer laser. Further research is expected to explore the grain size and detailed structure of silicon crystallites.
References w1x W. Sasaki, K. Kurosawa, P.R. Herman, E. Fujiwara, Y. Kato, in: C. Yamanaka ŽEd.., Short-Wavelength Lasers and Their Applications, Springer-Verlag, Berlin, 1988, p. 316. w2x K. Kurosawa, W. Sasaki, M. Okuda, Y. Takigawa, K. Yoshida, E. Fujiwara, Y. Kato, Rev. Sci. Instrum. 61 Ž1990. 728. w3x Y. Takigawa, K. Kurosawa, W. Sasaki, K. Yoshida, E. Fujiwara, Y. Kato, J. Non-Cryst. Solids 116 Ž1990. 293. w4x Y. Takigawa, K. Kurosawa, W. Sasaki, M. Okuda, K. Yoshida, E. Fujiwara, Y. Kato, Y. Inoue, J. Non-Cryst. Solids 125 Ž1990. 107. w5x K. Kurosawa, Y. Takigawa, W. Sasaki, M. Katto, Y. Inoue, Jpn. J. Appl. Phys. 30 Ž1991. 3219. w6x M. Katto, R. Matsumoto, K. Kurosawa, W. Sasaki, Y. Takigawa, M. Okuda, Rev. Sci. Instrum. 64 Ž1993. 319.