- Email: [email protected]
New method for boron removal from silicon by electron beam injection
Materials Science in Semiconductor Processing 18 (2014) 42–45
Contents lists available at ScienceDirect
Materials Science in Semiconductor Processing journal homepage: www.elsevier.com/locate/mssp
Short Communication
New method for boron removal from silicon by electron beam injection Yi Tan a,b, Shiqiang Qin a,b, Shutao Wen a,b, Jiayan Li a,b,n, Shuang Shi a,b, Dachuan Jiang a,b, Dayu Pang a,b a b
School of Materials Science and Engineering, Dalian University of Technology, Dalian 116023, China Key Laboratory for Solar Energy Photovoltaic Systems of Liaoning Province, Dalian 116023, China
a r t i c l e in f o
abstract
Available online 12 November 2013
A new method for boron removal from silicon using electron beam injection (EBI) is proposed. After thermal oxidation on monocrystalline silicon (100) wafer at 1000 1C for 1 h, EBI was used to induce thermal and negative charging effects to enhance boron diffusion in the oxide film and the silicon substrate. This facilitates boron removal from the silicon substrate. The boron concentration in samples was measured by secondary ion mass spectrometry. The results show that EBI reduced the boron concentration in the silicon substrate by 4.83%. & 2013 Elsevier Ltd. All rights reserved.
Keywords: Electron beam injection Silicon Solar energy materials Boron Thin films
1. Introduction Metallurgy is believed to be an effective and energysaving approach for producing solar-grade silicon. The method involves removing impurities from metallurgicalgrade silicon, including metals such as iron, calcium, and aluminum, and nonmetals such as phosphorus and boron. These impurities have a negative effect on the electrical properties and efficiency of solar cells. Metals can be removed effectively by directional solidification [1,2]. Phosphorus can be removed by electron beam melting [3,4]. Boron has stable physical and chemical properties in silicon, so it is not easy to remove. Researchers have studied this problem for many years. The JFE company in Japan used a plasma beam to ionize H2O and generate O– and OH– to react with boron to facilitate its escape from silicon [5]. Other methods include slagging [6] and Si–Al alloy formation [7]. However, there has been no research
n Corresponding author at: Dalian University of Technology, School of Materials Science and Engineering, No 2 Linggong Road, Ganjingzi District, Dalian City, Liaoning province 116023, China. Tel./fax: þ 86 411 84707583. E-mail address: [email protected] (J. Li).
1369-8001/$ - see front matter & 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.mssp.2013.10.006
on the use of electron beam injection (EBI) to remove boron from silicon. Here we propose EBI as a new method for boron removal from silicon. We measured boron concentrations in silicon and SiO2 film before and after EBI. EBI reduces the boron concentration since thermal oxidation causes boron redistribution from the silicon to the oxide film [8]. Boron can be electropositive in oxide films since it can form O–B–O bonds when it diffuses in amorphous SiO2 [9]. The SiO2 film can be negatively charged during EBI due to the negative charging effect [10]. The negatively charged oxide film can induce diffusion of electropositive boron from inside the substrate to the surface. The thermal effect of the electron beam can also enhance boron diffusion. Thus, EBI facilitates boron removal from silicon. 2. Experimental The samples used were monocrystalline silicon (100) wafers of 450 μm in thickness. The wafers were polished on one side and had a high boron doping concentration. They were cut into squares with an area of 1 cm2. Each sample was cleaned ultrasonically in deionized water for 30 min and then dried in an oven.
Y. Tan et al. / Materials Science in Semiconductor Processing 18 (2014) 42–45
SiO2 films were generated by thermal oxidation of the polished side in a tubular resistance furnace at 1000 1C for 1 h under a continuous flow of oxygen gas. A silicon wafer with a homogeneous oxide film was then selected for EBI. The procedure was carried out using a 30-keV electron beam at 20 mA for 30 min in an electron beam melting furnace with a vacuum o5 10 2 Pa. Field-emission scanning electron microscopy (FE-SEM) was used to investigate the surface morphology of the samples. The structure of the oxide film was characterized by X-ray diffraction (XRD). An ellipsometer was used to measure the thickness of the oxide film before and after EBI process. Boron concentrations were measured by secondary ion mass spectrometry (SIMS). 3. Results and discussion Fig. 1a–c shows SEM images of the surface morphology. The surface of the SiO2 film obtained by thermal oxidation has a much more homogeneous structure than for the original silicon surface. However, after EBI the oxide film has many cracks. This indicates that electron beam bombardment causes cracks on the surface of the oxide film. XRD patterns for the samples are shown in Fig. 1e. The only peak for the original silicon wafer (69.2351) corresponds to the silicon (400) plane, which is in the same crystal plane family as (100). This confirms that the original silicon wafer was monocrystalline. XRD patterns for the oxide film before and after EBI have no peak, indicating that the SiO2 films are both amorphous and the structure is not affected by EBI. The thickness and color of the oxide film before and after EBI are shown in Table 1. The color did not change, while the thickness decreased to 7.829 nm after EBI. This is
43
probably because electron beam bombardment removes some surface SiO2 under the vacuum environment. Fig. 2 shows the boron concentration profile according to SIMS. The boron concentration in the original silicon wafer is almost uniform since boron is doped into the silicon during crystal growth. The boron concentration profile after thermal oxidation is in general agreement with the theoretical curve for impurity uptake by oxide [8]. The surface boron concentration is lower than in the original silicon because of volume expansion when silicon forms SiO2. The boron concentration is higher on the SiO2 side than on the silicon side because of the segregation of boron from silicon to oxide [8]. The Si (raw silicon counts) curve in Fig. 2 represents the silicon ion intensity during SIMS. This curve indicates the transition from SiO2 film to the silicon substrate since the rate of Si ionization is much higher in SiO2 than in silicon. The curve breaks at 98 nm and then decreases rapidly to a stable value at 149 nm, indicating a transition between SiO2 and silicon in this area rather than a clear interface. We call this area the transition region. The boron concentration gradually changes in the transition region, which differs from the theoretical curve. The boron concentration in the silicon substrate after thermal oxidation was integrated over depth from 155 to Table 1 Results for oxide film thickness and color before and after EBI.
Before EBI After EBI
Thickness (nm)
Color
134.785 126.956
Light blue Light blue
Fig. 1. SEM images and XRD patterns for the samples. (a) Surface of the original silicon wafer. (b) Surface of the SiO2 film before EBI. (c) Surface of the SiO2 film after EBI. (d) Higher magnification of the SiO2 film after EBI. (e) XRD patterns for the original silicon wafer and the oxide films before and after EBI.
44
Y. Tan et al. / Materials Science in Semiconductor Processing 18 (2014) 42–45
Fig. 3. Method for boron removal by EBI.
Fig. 2. SIMS profile of the boron concentration.
690 nm. The initial boron concentration in silicon was also integrated from the surface to the same depth. The results show that thermal oxidation reduced the boron concentration by 27.88%. After EBI, the surface boron concentration decreases because some boron atoms separate from the oxide under the vacuum environment when the electron beam bombards the surface. The boron concentration in the oxide film also decreases. The effect of EBI on the oxide film can be explained as follows. (i) The surface boron concentration is reduced, which induces a boron concentration gradient from the oxide film to the surface. Therefore, boron in the oxide film diffuses to the surface. (ii) The boron diffusivity in the oxide increases. The electron beam has a thermal effect and the sample temperature increases. The diffusivity increases with temperature as there is a linear relation between the reciprocal of temperature and the logarithm of boron diffusivity in SiO2 [11]. Thus, EBI enhances the ability of boron to diffuse. (iii) The negatively charged SiO2 film induces boron diffusion in the oxide film. A schematic of the process is shown in Fig. 3. Boron is a network former element and interacts with peroxy linkage defects (PLDs), which are abundant in vitreous SiO2, and forms O–B–O bridging molecules when it diffuses in the oxide film [9]. Once an O–B–O bond is formed, the boron atom is electropositive since oxygen is more electronegative. The electron beam injects electrons into the oxide film and makes it negatively charged [10]. As a result, the negatively charged oxide film induces diffusion of electropositive boron and helps it move towards the surface. In the transition region, boron piles up near the silicon substrate, even in the substrate, due to EBI cessation. The negative charging effect stops when the electron beam is dropped to 0 mA within 2 s after 30 min of EBI. The temperature stops increasing, so the diffusivity no longer increases. However, boron continues to diffuse because the temperature is still high. Since the negative charging effect stops, boron that diffused into the transition region does not have sufficient energy to move into the oxide film, leading to a boron-enriched area around the silicon side of the transition region.
The maximum boron concentration decreased and the minimum concentration increased after EBI. The curve has a tendency to approach the theoretical curve for fast impurity diffusion in oxide [8].This indicates that EBI accelerates boron diffusion. Integration revealed that EBI reduced the boron concentration in the silicon substrate by 4.83% EBI (compared to the thermal oxidation curve). The SIMS detection limit for boron is 1 1014 atoms/cm3 and the order of magnitude for the decrease in boron concentration in silicon after EBI is 1018 atoms/cm3, so the removal rate of 4.83% can be attributed to EBI and not a SIMS detection error. 4. Conclusion We investigated the effect of EBI on boron removal from silicon. EBI applied to an amorphous SiO2 film formed by thermal oxidation reduced the boron concentration in the silicon substrate by 4.83%. Three reasons for this result are proposed: a reduction in the surface boron concentration, an increase in boron diffusivity in the oxide film, and the effect of the negatively charged oxide film. Boron redistribution in the transition region after EBI indicates that the negatively charged oxide film may be the most important factor in enhancing boron diffusivity in the SiO2 film. Overall, EBI is effective in removing boron from silicon. This new method can be used to refine silicon.
Acknowledgment We gratefully acknowledge financial support from the National Natural Science Foundation of China (Grant Nos. 51074032 and U1137601) and the National Key Technology R&D Program (Grant No. 2011BAE03B01). References [1] Y. Delannoy, J. Cryst. Growth. 360 (2012) 61–67. [2] M.A. Martorano, J.B. Ferreira Neto, T.S. Oliveira, T.O. Tsubaki, Mater. Sci. Eng. B. 176 (2011) 217–226. [3] D.C. Jiang, Y. Tan, S. Shi, W. Dong, Z. Gu, R.X. Zou, Mater. Lett. 78 (2012) 4–7.
Y. Tan et al. / Materials Science in Semiconductor Processing 18 (2014) 42–45
[4] T. Kemmotsu, T. Nagai, M. Maeda, High. Temp. Mater. Process. 30 (2011) 17–22. [5] N. Nakamura, H. Baba, Y. Sakaguchi, et al., J. Jpn. Inst. Metals 67 (2003) 583–589. [6] L. Zhang, Y. Tan, F.M. Xu, J.Y. Li, H.Y. Wang, Z. Gu, Separ. Sci. Technol. 48 (2013) 1140–1144.
[7] [8] [9] [10] [11]
45
T. Yoshikawa, K. Morita, Metall. Mater. Trans. B 36 (2005) 731–736. A.S. Grove, O. Leistiko, C.T. Sah, J. Appl. Phys. 35 (1964) 2695–2696. R.B. Fair, J. Electrochem. Soc. 144 (1997) 709. W.Q. Li, H.B. Zhang, Acta Phys. Sin. 57 (2008) 3219–3222. H.F. Wolf, Silicon semiconductor data, 1st ed. Pergamon, London, 1969.
Contents lists available at ScienceDirect
Materials Science in Semiconductor Processing journal homepage: www.elsevier.com/locate/mssp
Short Communication
New method for boron removal from silicon by electron beam injection Yi Tan a,b, Shiqiang Qin a,b, Shutao Wen a,b, Jiayan Li a,b,n, Shuang Shi a,b, Dachuan Jiang a,b, Dayu Pang a,b a b
School of Materials Science and Engineering, Dalian University of Technology, Dalian 116023, China Key Laboratory for Solar Energy Photovoltaic Systems of Liaoning Province, Dalian 116023, China
a r t i c l e in f o
abstract
Available online 12 November 2013
A new method for boron removal from silicon using electron beam injection (EBI) is proposed. After thermal oxidation on monocrystalline silicon (100) wafer at 1000 1C for 1 h, EBI was used to induce thermal and negative charging effects to enhance boron diffusion in the oxide film and the silicon substrate. This facilitates boron removal from the silicon substrate. The boron concentration in samples was measured by secondary ion mass spectrometry. The results show that EBI reduced the boron concentration in the silicon substrate by 4.83%. & 2013 Elsevier Ltd. All rights reserved.
Keywords: Electron beam injection Silicon Solar energy materials Boron Thin films
1. Introduction Metallurgy is believed to be an effective and energysaving approach for producing solar-grade silicon. The method involves removing impurities from metallurgicalgrade silicon, including metals such as iron, calcium, and aluminum, and nonmetals such as phosphorus and boron. These impurities have a negative effect on the electrical properties and efficiency of solar cells. Metals can be removed effectively by directional solidification [1,2]. Phosphorus can be removed by electron beam melting [3,4]. Boron has stable physical and chemical properties in silicon, so it is not easy to remove. Researchers have studied this problem for many years. The JFE company in Japan used a plasma beam to ionize H2O and generate O– and OH– to react with boron to facilitate its escape from silicon [5]. Other methods include slagging [6] and Si–Al alloy formation [7]. However, there has been no research
n Corresponding author at: Dalian University of Technology, School of Materials Science and Engineering, No 2 Linggong Road, Ganjingzi District, Dalian City, Liaoning province 116023, China. Tel./fax: þ 86 411 84707583. E-mail address: [email protected] (J. Li).
1369-8001/$ - see front matter & 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.mssp.2013.10.006
on the use of electron beam injection (EBI) to remove boron from silicon. Here we propose EBI as a new method for boron removal from silicon. We measured boron concentrations in silicon and SiO2 film before and after EBI. EBI reduces the boron concentration since thermal oxidation causes boron redistribution from the silicon to the oxide film [8]. Boron can be electropositive in oxide films since it can form O–B–O bonds when it diffuses in amorphous SiO2 [9]. The SiO2 film can be negatively charged during EBI due to the negative charging effect [10]. The negatively charged oxide film can induce diffusion of electropositive boron from inside the substrate to the surface. The thermal effect of the electron beam can also enhance boron diffusion. Thus, EBI facilitates boron removal from silicon. 2. Experimental The samples used were monocrystalline silicon (100) wafers of 450 μm in thickness. The wafers were polished on one side and had a high boron doping concentration. They were cut into squares with an area of 1 cm2. Each sample was cleaned ultrasonically in deionized water for 30 min and then dried in an oven.
Y. Tan et al. / Materials Science in Semiconductor Processing 18 (2014) 42–45
SiO2 films were generated by thermal oxidation of the polished side in a tubular resistance furnace at 1000 1C for 1 h under a continuous flow of oxygen gas. A silicon wafer with a homogeneous oxide film was then selected for EBI. The procedure was carried out using a 30-keV electron beam at 20 mA for 30 min in an electron beam melting furnace with a vacuum o5 10 2 Pa. Field-emission scanning electron microscopy (FE-SEM) was used to investigate the surface morphology of the samples. The structure of the oxide film was characterized by X-ray diffraction (XRD). An ellipsometer was used to measure the thickness of the oxide film before and after EBI process. Boron concentrations were measured by secondary ion mass spectrometry (SIMS). 3. Results and discussion Fig. 1a–c shows SEM images of the surface morphology. The surface of the SiO2 film obtained by thermal oxidation has a much more homogeneous structure than for the original silicon surface. However, after EBI the oxide film has many cracks. This indicates that electron beam bombardment causes cracks on the surface of the oxide film. XRD patterns for the samples are shown in Fig. 1e. The only peak for the original silicon wafer (69.2351) corresponds to the silicon (400) plane, which is in the same crystal plane family as (100). This confirms that the original silicon wafer was monocrystalline. XRD patterns for the oxide film before and after EBI have no peak, indicating that the SiO2 films are both amorphous and the structure is not affected by EBI. The thickness and color of the oxide film before and after EBI are shown in Table 1. The color did not change, while the thickness decreased to 7.829 nm after EBI. This is
43
probably because electron beam bombardment removes some surface SiO2 under the vacuum environment. Fig. 2 shows the boron concentration profile according to SIMS. The boron concentration in the original silicon wafer is almost uniform since boron is doped into the silicon during crystal growth. The boron concentration profile after thermal oxidation is in general agreement with the theoretical curve for impurity uptake by oxide [8]. The surface boron concentration is lower than in the original silicon because of volume expansion when silicon forms SiO2. The boron concentration is higher on the SiO2 side than on the silicon side because of the segregation of boron from silicon to oxide [8]. The Si (raw silicon counts) curve in Fig. 2 represents the silicon ion intensity during SIMS. This curve indicates the transition from SiO2 film to the silicon substrate since the rate of Si ionization is much higher in SiO2 than in silicon. The curve breaks at 98 nm and then decreases rapidly to a stable value at 149 nm, indicating a transition between SiO2 and silicon in this area rather than a clear interface. We call this area the transition region. The boron concentration gradually changes in the transition region, which differs from the theoretical curve. The boron concentration in the silicon substrate after thermal oxidation was integrated over depth from 155 to Table 1 Results for oxide film thickness and color before and after EBI.
Before EBI After EBI
Thickness (nm)
Color
134.785 126.956
Light blue Light blue
Fig. 1. SEM images and XRD patterns for the samples. (a) Surface of the original silicon wafer. (b) Surface of the SiO2 film before EBI. (c) Surface of the SiO2 film after EBI. (d) Higher magnification of the SiO2 film after EBI. (e) XRD patterns for the original silicon wafer and the oxide films before and after EBI.
44
Y. Tan et al. / Materials Science in Semiconductor Processing 18 (2014) 42–45
Fig. 3. Method for boron removal by EBI.
Fig. 2. SIMS profile of the boron concentration.
690 nm. The initial boron concentration in silicon was also integrated from the surface to the same depth. The results show that thermal oxidation reduced the boron concentration by 27.88%. After EBI, the surface boron concentration decreases because some boron atoms separate from the oxide under the vacuum environment when the electron beam bombards the surface. The boron concentration in the oxide film also decreases. The effect of EBI on the oxide film can be explained as follows. (i) The surface boron concentration is reduced, which induces a boron concentration gradient from the oxide film to the surface. Therefore, boron in the oxide film diffuses to the surface. (ii) The boron diffusivity in the oxide increases. The electron beam has a thermal effect and the sample temperature increases. The diffusivity increases with temperature as there is a linear relation between the reciprocal of temperature and the logarithm of boron diffusivity in SiO2 [11]. Thus, EBI enhances the ability of boron to diffuse. (iii) The negatively charged SiO2 film induces boron diffusion in the oxide film. A schematic of the process is shown in Fig. 3. Boron is a network former element and interacts with peroxy linkage defects (PLDs), which are abundant in vitreous SiO2, and forms O–B–O bridging molecules when it diffuses in the oxide film [9]. Once an O–B–O bond is formed, the boron atom is electropositive since oxygen is more electronegative. The electron beam injects electrons into the oxide film and makes it negatively charged [10]. As a result, the negatively charged oxide film induces diffusion of electropositive boron and helps it move towards the surface. In the transition region, boron piles up near the silicon substrate, even in the substrate, due to EBI cessation. The negative charging effect stops when the electron beam is dropped to 0 mA within 2 s after 30 min of EBI. The temperature stops increasing, so the diffusivity no longer increases. However, boron continues to diffuse because the temperature is still high. Since the negative charging effect stops, boron that diffused into the transition region does not have sufficient energy to move into the oxide film, leading to a boron-enriched area around the silicon side of the transition region.
The maximum boron concentration decreased and the minimum concentration increased after EBI. The curve has a tendency to approach the theoretical curve for fast impurity diffusion in oxide [8].This indicates that EBI accelerates boron diffusion. Integration revealed that EBI reduced the boron concentration in the silicon substrate by 4.83% EBI (compared to the thermal oxidation curve). The SIMS detection limit for boron is 1 1014 atoms/cm3 and the order of magnitude for the decrease in boron concentration in silicon after EBI is 1018 atoms/cm3, so the removal rate of 4.83% can be attributed to EBI and not a SIMS detection error. 4. Conclusion We investigated the effect of EBI on boron removal from silicon. EBI applied to an amorphous SiO2 film formed by thermal oxidation reduced the boron concentration in the silicon substrate by 4.83%. Three reasons for this result are proposed: a reduction in the surface boron concentration, an increase in boron diffusivity in the oxide film, and the effect of the negatively charged oxide film. Boron redistribution in the transition region after EBI indicates that the negatively charged oxide film may be the most important factor in enhancing boron diffusivity in the SiO2 film. Overall, EBI is effective in removing boron from silicon. This new method can be used to refine silicon.
Acknowledgment We gratefully acknowledge financial support from the National Natural Science Foundation of China (Grant Nos. 51074032 and U1137601) and the National Key Technology R&D Program (Grant No. 2011BAE03B01). References [1] Y. Delannoy, J. Cryst. Growth. 360 (2012) 61–67. [2] M.A. Martorano, J.B. Ferreira Neto, T.S. Oliveira, T.O. Tsubaki, Mater. Sci. Eng. B. 176 (2011) 217–226. [3] D.C. Jiang, Y. Tan, S. Shi, W. Dong, Z. Gu, R.X. Zou, Mater. Lett. 78 (2012) 4–7.
Y. Tan et al. / Materials Science in Semiconductor Processing 18 (2014) 42–45
[4] T. Kemmotsu, T. Nagai, M. Maeda, High. Temp. Mater. Process. 30 (2011) 17–22. [5] N. Nakamura, H. Baba, Y. Sakaguchi, et al., J. Jpn. Inst. Metals 67 (2003) 583–589. [6] L. Zhang, Y. Tan, F.M. Xu, J.Y. Li, H.Y. Wang, Z. Gu, Separ. Sci. Technol. 48 (2013) 1140–1144.
[7] [8] [9] [10] [11]
45
T. Yoshikawa, K. Morita, Metall. Mater. Trans. B 36 (2005) 731–736. A.S. Grove, O. Leistiko, C.T. Sah, J. Appl. Phys. 35 (1964) 2695–2696. R.B. Fair, J. Electrochem. Soc. 144 (1997) 709. W.Q. Li, H.B. Zhang, Acta Phys. Sin. 57 (2008) 3219–3222. H.F. Wolf, Silicon semiconductor data, 1st ed. Pergamon, London, 1969.