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Low-damage surface smoothing of laser crystallized polycrystalline silicon using gas cluster ion beam
NIM B Beam Interactions with Materials & Atoms
Nuclear Instruments and Methods in Physics Research B 257 (2007) 658–661 www.elsevier.com/locate/nimb
Low-damage surface smoothing of laser crystallized polycrystalline silicon using gas cluster ion beam H. Tokioka a
a,*
, H. Yamarin a, T. Fujino a, M. Inoue a, T. Seki b, J. Matsuo
b
Advanced Technology R&D Center, Mitsubishi Electric Corporation, 8-1-1, Tsukaguchi-Honmachi, Amagasaki, Hyogo 661-8661, Japan b Quantum Science and Engineering Center, Kyoto University, Gokasho, Uji, Kyoto, Japan Available online 14 February 2007
Abstract Surface smoothing of laser crystallized polycrystalline silicon (poly-Si) films using gas cluster ion beam (GCIB) technology has been studied. It is found that both SF6-GCIB and O2-GCIB decrease the height of hillocks and reduce the surface roughness of the irradiated films. The mean surface roughness value of poly-Si films was reduced from 10.8 nm to 2.8 nm by SF6-GCIB irradiation at 80. Ultraviolet reflectance measurement reveals that GCIB irradiation causes damage near-surface of the poly-Si films. Formation of the damage, however, can be suppressed by using GCIB irradiation at high incident angle. Effect of GCIB irradiation in a metal–insulator–semiconductor (MIS) capacitor has also been investigated. The capacitance–voltage curves of MIS capacitor with SF6-GCIB irradiation are distorted. On the contrary, the distortion is reduced by O2-GCIB irradiation at 80, which suggests that electrical-activated damage of the films can be decreased by using O2-GCIB irradiation. 2007 Elsevier B.V. All rights reserved. PACS: 68.55.a Keywords: Polycrystalline silicon; Thin films; Cluster; Surface; Atomic force microscopy; Ultraviolet spectroscopy
1. Introduction Polycrystalline silicon thin-film transistors (poly-Si TFTs) have been developed for their application in active matrix flat panel display (AMFPD) [1]. Recently, poly-Si TFTs have been utilized for embedding various driving circuits on glass substrate [2,3]. These circuits require high performance poly-Si TFTs to achieve low voltage, highspeed operation. Poly-Si films applied as active layers in the poly-Si TFTs were formed by using a laser annealing technique that has several advantages to realize high performance poly-Si TFTs with low threshold voltage and high field-effect mobility. The technique, however, causes rough surface that consists of ridges and hillocks on the poly-Si films created during lateral crystallization of the sil*
Corresponding author. Tel.: +81 6 6497 7520; fax: +81 6 6497 7549. E-mail address: [email protected] (H. Tokioka). 0168-583X/$ - see front matter 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2007.01.269
icon [4]. The poly-Si-gate oxide interface roughness induces weak scattering of carrier that affects the field-effect mobility. The oxide field is enhanced by the rough morphology of the interface, which causes device degradation during TFT operation [5]. This means the oxide thickness cannot be scaled down, which make it difficult to realize high performance poly-Si TFT. Because the thickness of the laser crystallized poly-Si film is very thin and the damage in the film lowers TFT mobility, surface smoothing without etching and damaging the films is important for downscaling of the TFTs. In this work, we investigated surface roughness of the poly-Si films smoothed by gas cluster ion beam (GCIB). Both SF6-GCIB and O2-GCIB were applied to reduce the roughness of the poly-Si films. The thickness of the GCIBirradiated film was determined using spectroscopic ellipsometry (SE) method. Atomic force microscopy (AFM) measurements were carried out to examine surface roughness of the poly-Si film. Damage of the films created by
H. Tokioka et al. / Nucl. Instr. and Meth. in Phys. Res. B 257 (2007) 658–661
659
the GCIB irradiations was evaluated by using ultraviolet reflectance (UVR) method and SE method. The etching depth dependence of surface roughness was investigated at several incident angles of SF6-GCIB and O2-GCIB. The influence of the damage on electric characteristics of GCIBirradiated silicon films was investigated by capacitance– voltage (C–V) method. 2. Experimental Poly-Si films and single silicon wafers were irradiated by either SF6-GCIB or O2-GCIB. Poly-Si thin films were prepared by laser annealing of amorphous silicon films. The amorphous silicon films were deposited on glass substrates using plasma-enhanced chemical-vapor deposition (PECVD). The typical value of the film thickness was 100 nm. Amorphous silicon films were irradiated by pulsed laser and transformed into poly-Si films. The resulting poly-Si films had rough surface due to ridges and hillocks. Single crystal wafers samples of n-type h1 0 0i orientation were also used. All GCIB irradiation experiments were carried out using a GCIB apparatus, as described elsewhere [6]. The SF6-GCIB irradiation was performed at 5 keV and 20 keV and ion fluences of 2 · 1014 ions/cm2, 5 · 1014 ions/cm2, 1 · 1015 ions/cm2 and 5 · 1015 ions/cm2. The source gas was a mixture SF6 and He, and total gas pressure was 4000 Torr. The mean cluster size was 550 molecules per cluster. The incident angle were 7, 67 and 80. Fig. 1 shows GCIB irradiation of samples at incident angle h. The O2-GCIB irradiation was performed at 20 keV and ion fluences were varied from 5 · 1014 ions/cm2, 2 · 1015 ions/cm2. The incident angle was 80. The source gas was an O2 and He mixture, and total gas pressure was 8000 Torr. The mean cluster size was 3000 molecules per cluster. The ionization energy and the emission current of the ionizer were fixed at 300 eV and 300 mA at all gas sources.
Fig. 2. Structure and fabrication of MIS capacitor.
AFM measurements were performed to investigate the surface roughness of the poly-Si films. Scan area and resolution were 10 · 10 lm2 and 256 · 256 pixels, respectively. The spectroscopic ellipsometry (SE) measurements were carried out to determine the thickness of the GCIB-irradiated poly-Si films using SOPRA ES4G over the spectral range of 300–800 nm in 10 nm steps. The incident angle of the probing light was 75. Spectra calculated from an optical model were fitted to the measured data to obtain the film thickness. A Bruggeman effective medium approximation (BEMA) was applied to determine the dielectric function of poly-Si layer in the optical model. Thickness of silicon oxide layer generated by GCIB irradiation was also estimated by SE method. UVR spectroscopy measurements were carried out to evaluate damage in the near-surface region of GCIB-irradiated sample over the spectral range of 200–350 nm. All reflected lights from the sample were collected by using an integrating sphere. Capacitance–voltage (C–V) measurements were carried out to explore the influence of the damage caused by GICB irradiation on electrical properties of silicon. Metal–insulator–semiconductor (MIS) capacitors were fabricated for the C–V measurement using GCIB-irradiated silicon wafers. Silicon nitride (SiN) films were applied as insulator. Thickness of the SiN films was typically 200 nm. Fig. 2 demonstrates the structure and fabrication of MIS capacitor. We used conventional C–V measurement equipment to measure C–V characteristics of the MIS capacitors. The ac signal applied to the MIS capacitor was 0.1 V rms and signal frequency was 100 Hz. 3. Result and discussion
Fig. 1. Schematic diagram of GCIB irradiation with incident angle h.
Surface roughness reduction of the poly-Si film using GCIB was examined by AFM. Fig. 3 shows typical AFM images of the film without and with SF6-GCIB irradiation, respectively. It is clear that surface roughness was reduced by GCIB irradiation. To confirm the result mentioned above, relationship between etching depth and mean surface roughness (Ra) was examined. It is found that O2-GCIB irradiation oxidized the poly-Si films, whereas SF6-GCIB irradiation etched the films. Therefore, we defined total etching depth as a sum of sputtering depth and the silicon oxide thickness. Both the sputtering depth and oxide thickness were
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H. Tokioka et al. / Nucl. Instr. and Meth. in Phys. Res. B 257 (2007) 658–661
Fig. 3. AFM image of poly-Si film (a) before GCIB irradiation, and (b) etched by SF6-GCIB irradiation with an acceleration voltage of 20 keV and an ion fluence of 2.5 · 1015 ions/cm2.
evaluated using SE measurement. The total etching depth dependence of Ra of the films is shown in Fig. 4. The mean surface roughness of the film without GCIB irradiation is 10.8 nm. In all cases, the Ra of the films tends to decrease with increasing the etching depth and approached a constant value. The acceleration voltage of GCIB hardly affected the mean roughness of the films. Ra of the films with O2-GCIB irradiation seems to be slightly smaller than the films with SF6-GCIB irradiation. This is considered due to lateral sputtering effect of GCIB irradiation which enhances selective removal of ridges and hillocks of films. It is known that the lateral sputtering effect becomes stronger for larger-size cluster beam. As described above, the mean cluster size of O2-GCIB is larger than that of SF6GCIB. At an etching depth of 10 nm at a 80 incident angle, the mean surface roughness was reduced to about 2.8 nm. It was found that the Ra of the film with 80 irradiation tends to be smaller than the film with 67 irradiation at the same etching depth. Because the poly-Si films used in the poly-Si TFTs is very thin, surface roughness reduction of the films at a shallower etching depth was desirable. Therefore, GCIB irradiation at an incident angle
of 80 is suitable for surface roughness reduction of poly-Si films. We measured the UVR spectra of the silicon wafers to investigate damage in the near-surface region of the wafer before and after GCIB irradiation. In the reflectance spectra of single crystal silicon, reflectance increases at a wavelength of 280 nm, which is caused by optical interband transitions at 280 nm. In amorphous silicon, however, interband transition does not occur, so reflectance does not increase. Therefore, increments of reflectance at 280 nm can be used to evaluate damage in the near-surface of the films [7]. Damage caused by GCIB irradiation was examined by using UVR method. The incident angles were 7, 67 and 80. The incident angle dependence of UVR spectra is illustrated in Fig. 5. The UVR around 280 nm increased with the incident angle. In the case of 80, the reflectance spectra was the same spectra of crystal silicon without the irradiation. Therefore, GCIB irradiation at high incident angle is suitable for reducing the damage to the GCIB-irradiated sample.
Fig. 4. Relations between etching depth and mean roughness of poly-Si films.
Fig. 5. Incident angle dependence of UV reflectance spectra of c-Si wafers with SF6-GCIB irradiation.
H. Tokioka et al. / Nucl. Instr. and Meth. in Phys. Res. B 257 (2007) 658–661
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4. Conclusion
Fig. 6. C–V characteristics of MIS capacitor.
We evaluated electrical degradation of poly-Si induced by the damage using C–V measurements of the MIS capacitors. If damage generated by GCIB irradiation influences electrical characteristics of the irradiated samples, C–V curve of a MIS capacitor will be distorted. This is because the damage acts as trap states of carriers. The trap states are allowed energy states in which electrons are localized in the near-surface of material [8]. Fig. 6 shows C–V characteristics of GCIB-irradiated silicon MIS capacitor. The C–V curve of 80 SF6-GCIB-irradiated silicon was distorted at a weak inversion region. This means that damage induced by SF6-GCIB irradiation is electrically activated, although the damage caused by SF6-GCIB irradiation was not detected by UVR measurement. On the contrary, the distortion does not exist on the C–V curve of O2GCIB-irradiated silicon, which suggest that O2-GCIB cause less damage than SF6-GCIB. This result agrees with our previous result that thickness of amorphous layer generated by O2-GCIB irradiation is thinner than that generated by SF6-GCIB irradiation [9]. We consider that the oxide layer near the surface of the film probably prevents formation of damage by O2-GCIB irradiation.
Surface smoothing of the poly-Si films was carried out using SF6-GCIB and O2-GCIB irradiation technique. It was confirmed that the GCIB irradiation decreased the surface roughness of the poly-Si films. Total etching depth dependence of surface roughness suggests that smoothing surface roughness of the films with small etching depth is achieved by GCIB irradiation at high incident angle. The damage caused by GCIB irradiation is reduced by GCIB irradiation at high incident angle. The C–V measurement of MIS capacitor also shows that the electrical-activated damage generated by GCIB irradiation was suppressed by using O2-GCIB. We conclude that low-damage surface smoothing of the poly-Si films with small etching depth is realized by using O2-GCIB irradiation at high incident angle. These results appear to demonstrate that GCIB is promising technology for future TFT fabrication. Acknowledgement This work was supported by the New Energy and Industrial Technology Development Organization (NEDO) of Japan. References [1] Mark A. Crowder, A.T. Voutsas, S.R. Droes, M. Moriguchi, T. Mitani, IEEE Trans. Electron Dev. 51 (2004) 560. [2] Y. Nakajima, Y. Kida, M. Murase, Y. Toyoshima, Y. Maki, SID04 Digest (2004) 864. [3] M. Tai, A.M. Hamano, S. Yamaguchi, T. Noda, S.K. Park, T. Shiba, M. Ohkura, IEEE Trans. Electron Dev. 51 (2004) 934. [4] D.J. McCulloch, S.D. Brotherton, Appl. Phys. Lett. 66 (1995) 2060. [5] N.A. Hastas, C.A. Dimitriadis, J. Appl. Phys. 92 (2002) 4741. [6] I. Yamada, J. Matsuo, Z. Insepov, M. Akizuki, Nucl. Instr. and Meth. B 106 (1995) 165. [7] G. Harbeke et al., J. Appl. Phys. 59 (1986) 1160. [8] E.H. Nicollian, J.R. Brews, MOS Physics and Technology, Wiley, New York, 1982. [9] H. Tokioka, T. Fujino, H. Ymarin, M. Inoue, Extended Abstract of 6th Workshop on Cluster Ion Beam and Advanced Quantum Beam Process Technology, 2005, p. 113.
Nuclear Instruments and Methods in Physics Research B 257 (2007) 658–661 www.elsevier.com/locate/nimb
Low-damage surface smoothing of laser crystallized polycrystalline silicon using gas cluster ion beam H. Tokioka a
a,*
, H. Yamarin a, T. Fujino a, M. Inoue a, T. Seki b, J. Matsuo
b
Advanced Technology R&D Center, Mitsubishi Electric Corporation, 8-1-1, Tsukaguchi-Honmachi, Amagasaki, Hyogo 661-8661, Japan b Quantum Science and Engineering Center, Kyoto University, Gokasho, Uji, Kyoto, Japan Available online 14 February 2007
Abstract Surface smoothing of laser crystallized polycrystalline silicon (poly-Si) films using gas cluster ion beam (GCIB) technology has been studied. It is found that both SF6-GCIB and O2-GCIB decrease the height of hillocks and reduce the surface roughness of the irradiated films. The mean surface roughness value of poly-Si films was reduced from 10.8 nm to 2.8 nm by SF6-GCIB irradiation at 80. Ultraviolet reflectance measurement reveals that GCIB irradiation causes damage near-surface of the poly-Si films. Formation of the damage, however, can be suppressed by using GCIB irradiation at high incident angle. Effect of GCIB irradiation in a metal–insulator–semiconductor (MIS) capacitor has also been investigated. The capacitance–voltage curves of MIS capacitor with SF6-GCIB irradiation are distorted. On the contrary, the distortion is reduced by O2-GCIB irradiation at 80, which suggests that electrical-activated damage of the films can be decreased by using O2-GCIB irradiation. 2007 Elsevier B.V. All rights reserved. PACS: 68.55.a Keywords: Polycrystalline silicon; Thin films; Cluster; Surface; Atomic force microscopy; Ultraviolet spectroscopy
1. Introduction Polycrystalline silicon thin-film transistors (poly-Si TFTs) have been developed for their application in active matrix flat panel display (AMFPD) [1]. Recently, poly-Si TFTs have been utilized for embedding various driving circuits on glass substrate [2,3]. These circuits require high performance poly-Si TFTs to achieve low voltage, highspeed operation. Poly-Si films applied as active layers in the poly-Si TFTs were formed by using a laser annealing technique that has several advantages to realize high performance poly-Si TFTs with low threshold voltage and high field-effect mobility. The technique, however, causes rough surface that consists of ridges and hillocks on the poly-Si films created during lateral crystallization of the sil*
Corresponding author. Tel.: +81 6 6497 7520; fax: +81 6 6497 7549. E-mail address: [email protected] (H. Tokioka). 0168-583X/$ - see front matter 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2007.01.269
icon [4]. The poly-Si-gate oxide interface roughness induces weak scattering of carrier that affects the field-effect mobility. The oxide field is enhanced by the rough morphology of the interface, which causes device degradation during TFT operation [5]. This means the oxide thickness cannot be scaled down, which make it difficult to realize high performance poly-Si TFT. Because the thickness of the laser crystallized poly-Si film is very thin and the damage in the film lowers TFT mobility, surface smoothing without etching and damaging the films is important for downscaling of the TFTs. In this work, we investigated surface roughness of the poly-Si films smoothed by gas cluster ion beam (GCIB). Both SF6-GCIB and O2-GCIB were applied to reduce the roughness of the poly-Si films. The thickness of the GCIBirradiated film was determined using spectroscopic ellipsometry (SE) method. Atomic force microscopy (AFM) measurements were carried out to examine surface roughness of the poly-Si film. Damage of the films created by
H. Tokioka et al. / Nucl. Instr. and Meth. in Phys. Res. B 257 (2007) 658–661
659
the GCIB irradiations was evaluated by using ultraviolet reflectance (UVR) method and SE method. The etching depth dependence of surface roughness was investigated at several incident angles of SF6-GCIB and O2-GCIB. The influence of the damage on electric characteristics of GCIBirradiated silicon films was investigated by capacitance– voltage (C–V) method. 2. Experimental Poly-Si films and single silicon wafers were irradiated by either SF6-GCIB or O2-GCIB. Poly-Si thin films were prepared by laser annealing of amorphous silicon films. The amorphous silicon films were deposited on glass substrates using plasma-enhanced chemical-vapor deposition (PECVD). The typical value of the film thickness was 100 nm. Amorphous silicon films were irradiated by pulsed laser and transformed into poly-Si films. The resulting poly-Si films had rough surface due to ridges and hillocks. Single crystal wafers samples of n-type h1 0 0i orientation were also used. All GCIB irradiation experiments were carried out using a GCIB apparatus, as described elsewhere [6]. The SF6-GCIB irradiation was performed at 5 keV and 20 keV and ion fluences of 2 · 1014 ions/cm2, 5 · 1014 ions/cm2, 1 · 1015 ions/cm2 and 5 · 1015 ions/cm2. The source gas was a mixture SF6 and He, and total gas pressure was 4000 Torr. The mean cluster size was 550 molecules per cluster. The incident angle were 7, 67 and 80. Fig. 1 shows GCIB irradiation of samples at incident angle h. The O2-GCIB irradiation was performed at 20 keV and ion fluences were varied from 5 · 1014 ions/cm2, 2 · 1015 ions/cm2. The incident angle was 80. The source gas was an O2 and He mixture, and total gas pressure was 8000 Torr. The mean cluster size was 3000 molecules per cluster. The ionization energy and the emission current of the ionizer were fixed at 300 eV and 300 mA at all gas sources.
Fig. 2. Structure and fabrication of MIS capacitor.
AFM measurements were performed to investigate the surface roughness of the poly-Si films. Scan area and resolution were 10 · 10 lm2 and 256 · 256 pixels, respectively. The spectroscopic ellipsometry (SE) measurements were carried out to determine the thickness of the GCIB-irradiated poly-Si films using SOPRA ES4G over the spectral range of 300–800 nm in 10 nm steps. The incident angle of the probing light was 75. Spectra calculated from an optical model were fitted to the measured data to obtain the film thickness. A Bruggeman effective medium approximation (BEMA) was applied to determine the dielectric function of poly-Si layer in the optical model. Thickness of silicon oxide layer generated by GCIB irradiation was also estimated by SE method. UVR spectroscopy measurements were carried out to evaluate damage in the near-surface region of GCIB-irradiated sample over the spectral range of 200–350 nm. All reflected lights from the sample were collected by using an integrating sphere. Capacitance–voltage (C–V) measurements were carried out to explore the influence of the damage caused by GICB irradiation on electrical properties of silicon. Metal–insulator–semiconductor (MIS) capacitors were fabricated for the C–V measurement using GCIB-irradiated silicon wafers. Silicon nitride (SiN) films were applied as insulator. Thickness of the SiN films was typically 200 nm. Fig. 2 demonstrates the structure and fabrication of MIS capacitor. We used conventional C–V measurement equipment to measure C–V characteristics of the MIS capacitors. The ac signal applied to the MIS capacitor was 0.1 V rms and signal frequency was 100 Hz. 3. Result and discussion
Fig. 1. Schematic diagram of GCIB irradiation with incident angle h.
Surface roughness reduction of the poly-Si film using GCIB was examined by AFM. Fig. 3 shows typical AFM images of the film without and with SF6-GCIB irradiation, respectively. It is clear that surface roughness was reduced by GCIB irradiation. To confirm the result mentioned above, relationship between etching depth and mean surface roughness (Ra) was examined. It is found that O2-GCIB irradiation oxidized the poly-Si films, whereas SF6-GCIB irradiation etched the films. Therefore, we defined total etching depth as a sum of sputtering depth and the silicon oxide thickness. Both the sputtering depth and oxide thickness were
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H. Tokioka et al. / Nucl. Instr. and Meth. in Phys. Res. B 257 (2007) 658–661
Fig. 3. AFM image of poly-Si film (a) before GCIB irradiation, and (b) etched by SF6-GCIB irradiation with an acceleration voltage of 20 keV and an ion fluence of 2.5 · 1015 ions/cm2.
evaluated using SE measurement. The total etching depth dependence of Ra of the films is shown in Fig. 4. The mean surface roughness of the film without GCIB irradiation is 10.8 nm. In all cases, the Ra of the films tends to decrease with increasing the etching depth and approached a constant value. The acceleration voltage of GCIB hardly affected the mean roughness of the films. Ra of the films with O2-GCIB irradiation seems to be slightly smaller than the films with SF6-GCIB irradiation. This is considered due to lateral sputtering effect of GCIB irradiation which enhances selective removal of ridges and hillocks of films. It is known that the lateral sputtering effect becomes stronger for larger-size cluster beam. As described above, the mean cluster size of O2-GCIB is larger than that of SF6GCIB. At an etching depth of 10 nm at a 80 incident angle, the mean surface roughness was reduced to about 2.8 nm. It was found that the Ra of the film with 80 irradiation tends to be smaller than the film with 67 irradiation at the same etching depth. Because the poly-Si films used in the poly-Si TFTs is very thin, surface roughness reduction of the films at a shallower etching depth was desirable. Therefore, GCIB irradiation at an incident angle
of 80 is suitable for surface roughness reduction of poly-Si films. We measured the UVR spectra of the silicon wafers to investigate damage in the near-surface region of the wafer before and after GCIB irradiation. In the reflectance spectra of single crystal silicon, reflectance increases at a wavelength of 280 nm, which is caused by optical interband transitions at 280 nm. In amorphous silicon, however, interband transition does not occur, so reflectance does not increase. Therefore, increments of reflectance at 280 nm can be used to evaluate damage in the near-surface of the films [7]. Damage caused by GCIB irradiation was examined by using UVR method. The incident angles were 7, 67 and 80. The incident angle dependence of UVR spectra is illustrated in Fig. 5. The UVR around 280 nm increased with the incident angle. In the case of 80, the reflectance spectra was the same spectra of crystal silicon without the irradiation. Therefore, GCIB irradiation at high incident angle is suitable for reducing the damage to the GCIB-irradiated sample.
Fig. 4. Relations between etching depth and mean roughness of poly-Si films.
Fig. 5. Incident angle dependence of UV reflectance spectra of c-Si wafers with SF6-GCIB irradiation.
H. Tokioka et al. / Nucl. Instr. and Meth. in Phys. Res. B 257 (2007) 658–661
661
4. Conclusion
Fig. 6. C–V characteristics of MIS capacitor.
We evaluated electrical degradation of poly-Si induced by the damage using C–V measurements of the MIS capacitors. If damage generated by GCIB irradiation influences electrical characteristics of the irradiated samples, C–V curve of a MIS capacitor will be distorted. This is because the damage acts as trap states of carriers. The trap states are allowed energy states in which electrons are localized in the near-surface of material [8]. Fig. 6 shows C–V characteristics of GCIB-irradiated silicon MIS capacitor. The C–V curve of 80 SF6-GCIB-irradiated silicon was distorted at a weak inversion region. This means that damage induced by SF6-GCIB irradiation is electrically activated, although the damage caused by SF6-GCIB irradiation was not detected by UVR measurement. On the contrary, the distortion does not exist on the C–V curve of O2GCIB-irradiated silicon, which suggest that O2-GCIB cause less damage than SF6-GCIB. This result agrees with our previous result that thickness of amorphous layer generated by O2-GCIB irradiation is thinner than that generated by SF6-GCIB irradiation [9]. We consider that the oxide layer near the surface of the film probably prevents formation of damage by O2-GCIB irradiation.
Surface smoothing of the poly-Si films was carried out using SF6-GCIB and O2-GCIB irradiation technique. It was confirmed that the GCIB irradiation decreased the surface roughness of the poly-Si films. Total etching depth dependence of surface roughness suggests that smoothing surface roughness of the films with small etching depth is achieved by GCIB irradiation at high incident angle. The damage caused by GCIB irradiation is reduced by GCIB irradiation at high incident angle. The C–V measurement of MIS capacitor also shows that the electrical-activated damage generated by GCIB irradiation was suppressed by using O2-GCIB. We conclude that low-damage surface smoothing of the poly-Si films with small etching depth is realized by using O2-GCIB irradiation at high incident angle. These results appear to demonstrate that GCIB is promising technology for future TFT fabrication. Acknowledgement This work was supported by the New Energy and Industrial Technology Development Organization (NEDO) of Japan. References [1] Mark A. Crowder, A.T. Voutsas, S.R. Droes, M. Moriguchi, T. Mitani, IEEE Trans. Electron Dev. 51 (2004) 560. [2] Y. Nakajima, Y. Kida, M. Murase, Y. Toyoshima, Y. Maki, SID04 Digest (2004) 864. [3] M. Tai, A.M. Hamano, S. Yamaguchi, T. Noda, S.K. Park, T. Shiba, M. Ohkura, IEEE Trans. Electron Dev. 51 (2004) 934. [4] D.J. McCulloch, S.D. Brotherton, Appl. Phys. Lett. 66 (1995) 2060. [5] N.A. Hastas, C.A. Dimitriadis, J. Appl. Phys. 92 (2002) 4741. [6] I. Yamada, J. Matsuo, Z. Insepov, M. Akizuki, Nucl. Instr. and Meth. B 106 (1995) 165. [7] G. Harbeke et al., J. Appl. Phys. 59 (1986) 1160. [8] E.H. Nicollian, J.R. Brews, MOS Physics and Technology, Wiley, New York, 1982. [9] H. Tokioka, T. Fujino, H. Ymarin, M. Inoue, Extended Abstract of 6th Workshop on Cluster Ion Beam and Advanced Quantum Beam Process Technology, 2005, p. 113.