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Enhanced diffusion of oxygen in silicon due to resonant laser excitation of local vibrational mode
ARTICLE IN PRESS
Physica B 376–377 (2006) 66–68 www.elsevier.com/locate/physb
Enhanced diffusion of oxygen in silicon due to resonant laser excitation of local vibrational mode H. Yamada-Kaneta, K. Tanahashi Atsugi Laboratories, Fujitsu Ltd., 10-1 Morinosato-Wakamiya, Atsugi 243-0197, Japan
Abstract We have found that the diffusion of the oxygen impurity atoms in silicon crystals is significantly enhanced by irradiation with infrared laser light whose wave number is close to the frequency of the local vibrational mode of the oxygen. This effect is considered to be ascribed to the optical excitation of the oxygen-localized vibrations at the Si–O–Si units in the silicon crystals. r 2005 Elsevier B.V. All rights reserved. PACS: 66.30.Jt; 41.75.Jv; 63.20.Pw; 85.40.Ry Keywords: Oxygen; Silicon; Diffusion; Laser
1. Introduction The structures of the impurity-incorporated regions in the CMOS transistors become finer and more complicated. Now we need a new technique of the impurity diffusion that enables (1) selectivity for the impurity species, (2) quick switching on and switching off, and (3) lowtemperature diffusion. Here we propose a completely new technique of impurity diffusion. By irradiating the sample with the laser light that optically excites the local vibrational mode of the impurity, we selectively enhance the diffusion of the particular impurity species only. Switching off of the laser stops the diffusion quickly. Our preliminary experiment actually suggested that the diffusion of oxygen can be enhanced by resonant laser (optical) excitation of the local vibrational mode of oxygen [1]. 2. Experiment The tube of the annealing furnace was made from the high-purity quartz tube of the radius of nearly 100 mm and the thickness of about 5 mm. The length was nearly 650 mm, including the detachable cap at the front end of Corresponding author. Tel.: +81 46 250 8276; fax: +81 46 248 3473.
E-mail address: [email protected] (H. Yamada-Kaneta). 0921-4526/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2005.12.018
the furnace. The front cap of the furnace tube was equipped with ZnSe optical window through which the laser beam was introduced into the interior of the furnace tube. The furnace tube was purged by the argon gas. The sample holder illustrated in Fig. 1, together with the loading boat made of quartz, was inserted into the midfurnace tube, and was heated up by the three flash lamps equipped just outside of the furnace tube. The sample holder was made from a silicon single crystal grown by the Czochralski method. The side faces of the dimension of 31 mm 18 mm, and the top face of the sample holder were irradiated with the flash lamps. The square-shaped silicon sample with the edge length of 17 mm and the thickness of 0.5–2.0 mm was inserted into the sample holder, as shown in Fig. 1. We optimized the structure of the experimental apparatus, e.g., location of the flash lamps and the sample holder, and the shape of the sample holder, so that the light of the flash lamps do not irradiate the face of the sample. The sample was heated (annealed) through the heat contact to the sample holder. The maximum annealing temperature of the sample was nearly 1250 C under the maximum-power operation of the three flash lamps. During the anneal, a part of the sample face was irradiated with the laser beam guided into the furnace tube, as illustrated in Fig. 1. We adopted the CO2 gas laser. We
ARTICLE IN PRESS H. Yamada-Kaneta, K. Tanahashi / Physica B 376–377 (2006) 66–68
67
Fig. 1. Illustration of the sample inserted into the sample holder. The SIMS measurement points for the depth profiles of oxygen concentration are indicated: the point A inside the spot irradiated with the laser and the point B without laser irradiation.
mainly employed the laser line of the wave number of 1085:0 cm1 , which just coincides with the strong infrared absorption line of the 18O impurities (28 Si–18 O–28 Si centers) in silicon. The other laser line we adopted here was 933 cm1 . All the laser irradiation experiments were carried out with the power of the beam emitted from the laser apparatus fixed at 25 W. In this case, the power of the laser beam that reached the sample face was nearly 7 W. The radial distribution of the laser beam intensity was Gaussian-like, and the full-width (diameter) at half-power was 3.8 mm. In order to investigate how the laser irradiation affects the speed (extent) of phenomenon driven by the thermal anneal, e.g., the oxygen diffusion, we compare the speed of the phenomenon between the laser-irradiated point A and the point B without laser irradiation (Fig. 1). 3. Enhanced diffusion due to resonant laser excitation of local vibrational mode In order to prepare the sample used for the present diffusion experiment, we first annealed the floating-zonegrown sample in the 18 O2 gas ambient, to incorporate 18 O into the near-surface region of the sample. We first put the sample into the quartz ampoule, and evacuated the air in the ampoule using a vacuum pump. Then we injected the 18 O2 gas into the ampoule. The pressure of the 18 O2 gas in the ampoule was adjusted to 0.3 atmospheric pressure at the room temperature. We finally plugged up the inlet for the 18 O2 gas (outlet for the ampoule air) by melting the inlet (outlet) pipe of the ampoule. The ampoule containing the sample and the 18 O2 gas was heat-treated at 1050 C for 3 h in the horizontal-type furnace for the wafer processes. After removing the surface oxide, Sið18 OÞ2 , we measured the depth profile of the 18 O atoms diffused into the sample, by the secondary-ion mass spectroscopy (SIMS). To obtain this depth profile, a Czochralski-grown sample was used, and the pressure of
Fig. 2. The depth profile of 18 O concentration measured for the Czochralski-grown silicon crystal annealed in 18 O2 gas at 1050 C for 3 h. The pressure of the 18 O2 gas in the ampoule was 0.2 atmospheric pressure at the room temperature. The sharp dip of the profile in the nearsurface region is due to the out-diffusion during the slow cooling of the sample at the end of the heat treatment. The expected profile near the surface in case of ideal rapid quench is drawn in the dotted line.
the 18 O2 gas in the ampoule was adjusted to 0.2 atmospheric pressure. The result is shown in Fig. 2. We regard this depth profile of 18 O as the initial one. We next annealed the 18 O-incorporated floating-zone-grown sample to investigate the effect of laser irradiation on the diffusion of oxygen, as explained in the previous section (Fig. 1). We inserted this 18 O-containing sample into the sample holder (Fig. 1), and subjected to the heat treatment at 1173 C for 2 h. One should note that the value of the temperature here, 1173 C, is the temperature of the laser-irradiated point A measured by the radiation thermometer. We confirmed, however, that the difference in temperature between the laser-irradiated point A and the point B without laser irradiation was at most 10–15 C for all experiments performed here. Fig. 3 compares the depth profile of 18 O concentration measured for the laser-irradiated point A with that for the point B without laser irradiation. From the figure, we see that the diffusion coefficient at the laser-irradiated point was significantly increased as compared to that at the point without laser irradiation. According to the numerical analysis by Kakimoto et al. [2], the diffusion coefficient at the laser-irradiated point is nearly three times that at the point without irradiation. As mentioned above, the temperature increase at the laser-irradiated point was at
ARTICLE IN PRESS 68
H. Yamada-Kaneta, K. Tanahashi / Physica B 376–377 (2006) 66–68
of the optical excitation by the laser, and is not due to the heating up of the crystal lattice (heat bath) by the laser irradiation. This was also confirmed by the fact that the degree of enhancement depends on the laser frequency: for the laser light of the wave number 933 cm1 , the enhancement was smaller by 25% [2] as compared to the abovementioned one for the 1085 cm1 . The stretching vibration of the oxygen in the Si–O–Si unit is known to cause strong optical absorptions in the range 1000–1200 cm1 , whereas the lattice absorption is weak enough here. This suggests that the laser light in this wave number range causes the local heating of the vibration of oxygen in the Si–O–Si unit. In other words, the resonant irradiation of laser light optically excites the thermal vibration of the oxygen, the lattice temperature remained almost unchanged. This is why the diffusion of oxygen is enhanced by the irradiation of laser light whose wave number is close (resonant) to the frequency of the local vibrational mode of the oxygen. Acknowledgments
Fig. 3. The depth profiles of 18 O concentrations measured for the 18 Oincorporated floating-zone-grown sample subjected to the heat treatment at 1173 C for 2 h: (a) The depth profile measured for the point A irradiated by the laser, and (b) that measured for the point B with no laser irradiation. The laser irradiation continued all through the duration of the heat treatment. Prior to this heat treatment, the sample was annealed at 1050 C for 3 h in 18 O2 gas to form the initial concentration profile shown in Fig. 2.
most 10 C, relative to the point without laser irradiation. This indicates that the observed enhancement is the result
The work was partly supported by Grant-in-Aid for The Creation and Innovation through Business-Academy-Public Sector Cooperation from The Japanese Ministry of Education, Science, Sports and Culture. References [1] H. Yamada-Kaneta, K. Tanahashi, in: Proceedings of the Fourth International Symposium on Advanced Science and Technology of Silicon Materials, November 22–26, 2004, Kona, Hawaii, USA, pp. 305–308. [2] K. Kakimoto, to be presented elsewhere.
Physica B 376–377 (2006) 66–68 www.elsevier.com/locate/physb
Enhanced diffusion of oxygen in silicon due to resonant laser excitation of local vibrational mode H. Yamada-Kaneta, K. Tanahashi Atsugi Laboratories, Fujitsu Ltd., 10-1 Morinosato-Wakamiya, Atsugi 243-0197, Japan
Abstract We have found that the diffusion of the oxygen impurity atoms in silicon crystals is significantly enhanced by irradiation with infrared laser light whose wave number is close to the frequency of the local vibrational mode of the oxygen. This effect is considered to be ascribed to the optical excitation of the oxygen-localized vibrations at the Si–O–Si units in the silicon crystals. r 2005 Elsevier B.V. All rights reserved. PACS: 66.30.Jt; 41.75.Jv; 63.20.Pw; 85.40.Ry Keywords: Oxygen; Silicon; Diffusion; Laser
1. Introduction The structures of the impurity-incorporated regions in the CMOS transistors become finer and more complicated. Now we need a new technique of the impurity diffusion that enables (1) selectivity for the impurity species, (2) quick switching on and switching off, and (3) lowtemperature diffusion. Here we propose a completely new technique of impurity diffusion. By irradiating the sample with the laser light that optically excites the local vibrational mode of the impurity, we selectively enhance the diffusion of the particular impurity species only. Switching off of the laser stops the diffusion quickly. Our preliminary experiment actually suggested that the diffusion of oxygen can be enhanced by resonant laser (optical) excitation of the local vibrational mode of oxygen [1]. 2. Experiment The tube of the annealing furnace was made from the high-purity quartz tube of the radius of nearly 100 mm and the thickness of about 5 mm. The length was nearly 650 mm, including the detachable cap at the front end of Corresponding author. Tel.: +81 46 250 8276; fax: +81 46 248 3473.
E-mail address: [email protected] (H. Yamada-Kaneta). 0921-4526/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2005.12.018
the furnace. The front cap of the furnace tube was equipped with ZnSe optical window through which the laser beam was introduced into the interior of the furnace tube. The furnace tube was purged by the argon gas. The sample holder illustrated in Fig. 1, together with the loading boat made of quartz, was inserted into the midfurnace tube, and was heated up by the three flash lamps equipped just outside of the furnace tube. The sample holder was made from a silicon single crystal grown by the Czochralski method. The side faces of the dimension of 31 mm 18 mm, and the top face of the sample holder were irradiated with the flash lamps. The square-shaped silicon sample with the edge length of 17 mm and the thickness of 0.5–2.0 mm was inserted into the sample holder, as shown in Fig. 1. We optimized the structure of the experimental apparatus, e.g., location of the flash lamps and the sample holder, and the shape of the sample holder, so that the light of the flash lamps do not irradiate the face of the sample. The sample was heated (annealed) through the heat contact to the sample holder. The maximum annealing temperature of the sample was nearly 1250 C under the maximum-power operation of the three flash lamps. During the anneal, a part of the sample face was irradiated with the laser beam guided into the furnace tube, as illustrated in Fig. 1. We adopted the CO2 gas laser. We
ARTICLE IN PRESS H. Yamada-Kaneta, K. Tanahashi / Physica B 376–377 (2006) 66–68
67
Fig. 1. Illustration of the sample inserted into the sample holder. The SIMS measurement points for the depth profiles of oxygen concentration are indicated: the point A inside the spot irradiated with the laser and the point B without laser irradiation.
mainly employed the laser line of the wave number of 1085:0 cm1 , which just coincides with the strong infrared absorption line of the 18O impurities (28 Si–18 O–28 Si centers) in silicon. The other laser line we adopted here was 933 cm1 . All the laser irradiation experiments were carried out with the power of the beam emitted from the laser apparatus fixed at 25 W. In this case, the power of the laser beam that reached the sample face was nearly 7 W. The radial distribution of the laser beam intensity was Gaussian-like, and the full-width (diameter) at half-power was 3.8 mm. In order to investigate how the laser irradiation affects the speed (extent) of phenomenon driven by the thermal anneal, e.g., the oxygen diffusion, we compare the speed of the phenomenon between the laser-irradiated point A and the point B without laser irradiation (Fig. 1). 3. Enhanced diffusion due to resonant laser excitation of local vibrational mode In order to prepare the sample used for the present diffusion experiment, we first annealed the floating-zonegrown sample in the 18 O2 gas ambient, to incorporate 18 O into the near-surface region of the sample. We first put the sample into the quartz ampoule, and evacuated the air in the ampoule using a vacuum pump. Then we injected the 18 O2 gas into the ampoule. The pressure of the 18 O2 gas in the ampoule was adjusted to 0.3 atmospheric pressure at the room temperature. We finally plugged up the inlet for the 18 O2 gas (outlet for the ampoule air) by melting the inlet (outlet) pipe of the ampoule. The ampoule containing the sample and the 18 O2 gas was heat-treated at 1050 C for 3 h in the horizontal-type furnace for the wafer processes. After removing the surface oxide, Sið18 OÞ2 , we measured the depth profile of the 18 O atoms diffused into the sample, by the secondary-ion mass spectroscopy (SIMS). To obtain this depth profile, a Czochralski-grown sample was used, and the pressure of
Fig. 2. The depth profile of 18 O concentration measured for the Czochralski-grown silicon crystal annealed in 18 O2 gas at 1050 C for 3 h. The pressure of the 18 O2 gas in the ampoule was 0.2 atmospheric pressure at the room temperature. The sharp dip of the profile in the nearsurface region is due to the out-diffusion during the slow cooling of the sample at the end of the heat treatment. The expected profile near the surface in case of ideal rapid quench is drawn in the dotted line.
the 18 O2 gas in the ampoule was adjusted to 0.2 atmospheric pressure. The result is shown in Fig. 2. We regard this depth profile of 18 O as the initial one. We next annealed the 18 O-incorporated floating-zone-grown sample to investigate the effect of laser irradiation on the diffusion of oxygen, as explained in the previous section (Fig. 1). We inserted this 18 O-containing sample into the sample holder (Fig. 1), and subjected to the heat treatment at 1173 C for 2 h. One should note that the value of the temperature here, 1173 C, is the temperature of the laser-irradiated point A measured by the radiation thermometer. We confirmed, however, that the difference in temperature between the laser-irradiated point A and the point B without laser irradiation was at most 10–15 C for all experiments performed here. Fig. 3 compares the depth profile of 18 O concentration measured for the laser-irradiated point A with that for the point B without laser irradiation. From the figure, we see that the diffusion coefficient at the laser-irradiated point was significantly increased as compared to that at the point without laser irradiation. According to the numerical analysis by Kakimoto et al. [2], the diffusion coefficient at the laser-irradiated point is nearly three times that at the point without irradiation. As mentioned above, the temperature increase at the laser-irradiated point was at
ARTICLE IN PRESS 68
H. Yamada-Kaneta, K. Tanahashi / Physica B 376–377 (2006) 66–68
of the optical excitation by the laser, and is not due to the heating up of the crystal lattice (heat bath) by the laser irradiation. This was also confirmed by the fact that the degree of enhancement depends on the laser frequency: for the laser light of the wave number 933 cm1 , the enhancement was smaller by 25% [2] as compared to the abovementioned one for the 1085 cm1 . The stretching vibration of the oxygen in the Si–O–Si unit is known to cause strong optical absorptions in the range 1000–1200 cm1 , whereas the lattice absorption is weak enough here. This suggests that the laser light in this wave number range causes the local heating of the vibration of oxygen in the Si–O–Si unit. In other words, the resonant irradiation of laser light optically excites the thermal vibration of the oxygen, the lattice temperature remained almost unchanged. This is why the diffusion of oxygen is enhanced by the irradiation of laser light whose wave number is close (resonant) to the frequency of the local vibrational mode of the oxygen. Acknowledgments
Fig. 3. The depth profiles of 18 O concentrations measured for the 18 Oincorporated floating-zone-grown sample subjected to the heat treatment at 1173 C for 2 h: (a) The depth profile measured for the point A irradiated by the laser, and (b) that measured for the point B with no laser irradiation. The laser irradiation continued all through the duration of the heat treatment. Prior to this heat treatment, the sample was annealed at 1050 C for 3 h in 18 O2 gas to form the initial concentration profile shown in Fig. 2.
most 10 C, relative to the point without laser irradiation. This indicates that the observed enhancement is the result
The work was partly supported by Grant-in-Aid for The Creation and Innovation through Business-Academy-Public Sector Cooperation from The Japanese Ministry of Education, Science, Sports and Culture. References [1] H. Yamada-Kaneta, K. Tanahashi, in: Proceedings of the Fourth International Symposium on Advanced Science and Technology of Silicon Materials, November 22–26, 2004, Kona, Hawaii, USA, pp. 305–308. [2] K. Kakimoto, to be presented elsewhere.