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Bipolar structure of carrier concentration in hydrogen pre-annealing Czochralski silicon wafer
ARTICLE IN PRESS
Physica B 340–342 (2003) 601–604
Bipolar structure of carrier concentration in hydrogen pre-annealing Czochralski silicon wafer Xuegong Yu, Deren Yang*, Ruixin Fan, Xiangyang Ma, Duanlin Que State Key Laboratory of Silicon Materials, Department of Material Science and Engineering, Zhejiang University, Zheda Lu 38, Hangzhou 310027, People’s Republic of China
Abstract The carrier concentration profiles in p-type Czochralski silicon wafers subjected to hydrogen or nitrogen hightemperature pre-annealing followed by a prolonged 450 C annealing have been investigated by spreading resistance profile. It is clarified that the carrier concentration profile in the samples subjected to hydrogen pre-annealing is characteristic of a NPN bipolar structure, while that it is just characteristic of a PN junction in the samples subjected to nitrogen pre-annealing. Furthermore, the location of the NPN structure in the wafer is dependent on hydrogen preannealing temperature. It is suggested that the formation of the NPN bipolar structure is due to the enhancement of thermal donors by hydrogen. r 2003 Elsevier B.V. All rights reserved. PACS: 61.72.Bb; 61.72.Yx; 61.72.Ss Keywords: Hydrogen annealing; Thermal donors; Czochralski silicon
1. Introduction Oxygen originated from the contamination of quartz crucibles is a main impurity in Czochralski (CZ) silicon, which strongly affects the properties and yield of electronic devices, as a result of oxygen aggregating to form thermal donors (TDs) in heat treatments in the temperature range of 300–500 C [1]. The concentration of the TDs ([TDs]) mainly depends on the interstitial oxygen concentration ([Oi]), and the carrier concentration profiles measured by spreading resistance probe *Corresponding author. Tel.: +86-571-8795-1667; fax: +86571-8795-2322. E-mail address: [email protected] (D. Yang).
(SRP) is generally used for the analysis of the distribution of the [TDs] and therefore the [Oi] in CZ silicon. On the other hand, it has been well known that atomic hydrogen incorporated into CZ silicon can act as a catalyst and significantly enhance the TD formation rate [2–8]. More interestingly, it is reported that after plasma hydrogenation at 400 C the hydrogen-enhanced region of initial p-type CZ silicon becomes n-type, and therefore a PN junction can be observed at the near-surface of wafer [9,10]. The depth of the PN junction is controlled by the duration of the plasma treatment, the dose of incorporated hydrogen ions from the plasma and oxygen concentration in wafer. Since the hydrogen diffusion coefficient is
0921-4526/$ - see front matter r 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2003.09.126
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very large, the hydrogen concentration in the bulk of wafer will reach its solubility at annealing temperature quickly [11]. Recently, we have reported that hydrogen annealing at high temperature can eliminate the crystal originated particles (COPs) to improve the gate oxide integrity [12,13]. In the paper, we found the bipolar (NPN) structure was formed in p-type CZ wafers subjected to hydrogen pre-annealing with a prolonged 450 C annealing, which cannot be formed in p-type CZ silicon subjected to nitrogen pre-annealing. It is suggested that the bipolar structure is corresponding to the enhancement of hydrogen on TDs. Based on our experiments, a formation model of bipolar structure is suggested.
2. Experimental P-type /1 0 0S CZ wafers with the resistivity of 8.5 O cm have been used in our experiments. The [Oi] of as-grown samples measured by the Fourier transmission infrared spectroscopy (FTIR) at room temperature with a calibration fact of 3.14 1017 cm 2 was about 6.7 1017 cm 3, and the concentration of carbon was below the detection limit of FTIR. The samples were preannealed in the temperature range of 1050–1200 C for 1–2 h in hydrogen or nitrogen ambient, respectively, and quenched to room temperature. Subsequently, the pre-annealed samples were annealed at 450 C for 80 h to generate TDs in nitrogen ambient. Finally, the carrier concentration profiles along the depth of the wafers were measured by SRP.
3. Results and discussion Fig. 1 shows the carrier concentration profiles at the sub-surface of the p-type CZ wafers subjected to 1150 C/2 h pre-annealing in nitrogen or hydrogen ambient respectively and followed by 450 C/ 80 h annealing. As it can be seen, two peaks appear in the wafer subjected to hydrogen pre-annealing (Fig. 1b), while only one appears in the wafer subjected to nitrogen pre-annealing (Fig. 1a). For the samples subjected to pre-annealing in nitrogen ambient and followed by 450 C annealing, the formation of PN junction is a result of the concentration compensation between TD formed during 450 C annealing and the inherent acceptors. As for the samples subjected to hydrogen preannealing and followed by 450 C annealing, the formation of bipolar structure is also a result of the compensation between the concentration of TDs and the inherent acceptor, however, the profile of [TDs] along the depth of the wafer was much different from that in the case of nitrogen preannealing. It is obvious that this bipolar structure is an NPN one, which will be elucidated later. Fig. 2 shows the location variation of the bipolar structure at the sub-surface of p-type CZ wafers with the hydrogen pre-annealing temperature. It can be seen that the location of the bipolar structure moved toward the bulk from the surface of the wafers with the increase of the annealing temperature. It indicates that the temperature of hydrogen pre-annealing has an influence on the location of bipolar structure. It has been well known that the out-diffusion of oxygen at the sub-surface of CZ wafers subjected
Fig. 1. The carrier profile at the sub-surface of the wafer subjected to nitrogen (a) and hydrogen (b) pre-annealing followed by 450 C/ 80 h annealing.
ARTICLE IN PRESS X. Yu et al. / Physica B 340–342 (2003) 601–604
to high temperature will take place whatever annealing ambient is, as shown in Figs. 3(a,b)-(1). According to diffusion theory, in the out-diffusion region, the [Oi] profile follows the complementary error function [14]. The bulk [Oi] will be somewhat higher than the initial [Oi] as a result of the grownin oxygen precipitation dissolving, while, the [Oi]
The depth of NPN (um)
30
20
10
0
1050 1100 1150 1200 Annealing temperature (°C)
Fig. 2. The location variation of the NPN structure at the subsurface of the wafer with hydrogen pre-annealing temperature.
603
at the surface is close to the oxygen solubility at annealing temperature. For the samples subjected to high temperature pre-annealing in nitrogen ambient, according to the [Oi] profile, the [TDs] profile can be schematically illustrated in Fig. 3a-(2) after a prolonged low temperature annealing, a result of TD formation depending on Oi, additionally, the inherent acceptor concentration is also illustrated as a horizontal line. It is clear that the region where the [TDs] is higher than the acceptor concentration will be of n-type by reason of compensation; on the contrary, the region is of p-type. So the ultimate carrier concentration profile is derived as shown in Fig. 3a-(3), and a PN junction is formed between the near-surface and the bulk regions. For the samples subjected to hydrogen preannealing, after pre-annealing, the [Oi] profile is similar to that subjected to nitrogen pre-annealing even though the hydrogen will enhance the outdiffusion of oxygen, as shown in Fig. 3b-(1), meanwhile, the hydrogen will diffuse into the bulk
Fig. 3. The formation model of the NPN structure at the sub-surface of the wafer subjected to hydrogen pre-annealing followed by 450 C annealing.
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from the surface of the wafer [11]. According to Henry’s Law, the concentration of hydrogen in the near-surface of wafer is proportional to the hydrogen ambient, which is much higher than that in the bulk, while, the concentration of hydrogen in the bulk of wafer will reach its solubility at the annealing temperature quickly, a result of hydrogen rapid diffusion in silicon. Additionally, since the samples are quenched, the hydrogen near the surface of wafer will not diffuse out during cooling. Fig. 3b-(1) shows the hydrogen concentration profile after hydrogen pre-annealing. During subsequent low temperature annealing the different concentration hydrogen in the sample will enhance the formation of the TDs under the condition of different [Oi]. That is, the [TDs] formed in the samples depends not only on the [Oi] but also the concentration of hydrogen. At the near-surface of the wafer, the [TDs] is much higher, as a result of higher concentration hydrogen existing. In the bulk the [TDs] is also higher because of higher [Oi]. Between the near-surface and the bulk of wafer, the [TDs] is much lower because both oxygen and hydrogen concentrations are lower. The [TDs] profile is shown in Fig. 3b(2). Accordingly, the ultimate carrier concentration profile is derived as shown in Fig. 3b-(3) after the compensation between TDs and the inherent acceptor. Three distinct regions respectively ascribed to n-, p- and n-types from the surface to the bulk region are generated, which is characteristic of a NPN transistor. Furthermore, it can be concluded that with the increase of hydrogen pre-annealing temperature the solubility concentration and diffusion rate of hydrogen will increase and the out-diffusion length of oxygen increases, therefore the profile of hydrogen concentration and Oi will change, which results in the location of NPN structure will move toward the bulk from the surface of the wafer (Fig. 2).
4. Conclusion In summary, we have reported the formation of a NPN structure in the p-type CZ-Si wafer
subjected to hydrogen pre-annealing at high temperature and subsequent prolonged low temperature annealing, while the wafer subjected to pre-annealing in nitrogen ambient, only a PN junction is formed at the sub-surface. Moreover, it is also clarified that the location of the NPN structure moves toward the bulk with the increase of hydrogen pre-annealing temperature. After taking into account the oxygen diffusion, the hydrogen enhancement on TD formation and compensation between TDs and inherent acceptors, a conceptual model is discussed.
Acknowledgements The authors would like to thank the Natural Science Foundation of China (Nos. 50032010 and 60225010) and 863 programs (No. 2002AA3Z1111) for financial supports.
References [1] P. Capper, A.W. Jones, E.J. Wallhouse, J.G. Wilks, J. Appl. Phys. 48 (1977) 1646. [2] E.M. Filangeri, T. Nishida, J. Appl. Phys. 75 (1994) 7931. [3] A.R. Brown, M. Clayborn, R. Murray, P.S. Nandra, R.C. Newman, J.H. Tucker, Semicond. Sci. Technol. 3 (1988) 591. [4] R. Murray, A.R. Brown, R.C. Newman, Mater. Sci. Eng. B 4 (1990) 299. [5] R.C. Newman, J.H. Tucker, A.R. Brown, S.A. McQuaid, Appl. Phys. 70 (1991) 3061. [6] H.J. Stein, S.K. Hahn, Appl. Phys. Lett. 56 (1990) 63. [7] H.J. Stein, S.K. Hahn, J. Appl. Phys 75 (1994) 3477. [8] H.J. Stein, S.K. Hahn, J. Electrochem. Soc. 142 (1995) 1242. [9] A.G. Ulyashin, Y.A. Bumay, R. Job, G. Grabosh, D. Borchert, W.R. Fahrner, A.Y. Diduk, Solid State Phenom. 57–58 (1997) 189. [10] A.G. Ulyashin, Y.A. Bumay, R. Job, W.R. Fahrner, Appl. Phys. A 66 (1998) 399. [11] A. Hara, M. Aoki, T. Fukuda, A. Ohsawa, J. Appl. Phys. 74 (1993) 913. [12] X. Yu, D. Yang, X. Ma, J. Yang, D. Que, J. Appl. Phys. 92 (2003) 188. [13] X. Yu, D. Yang, X. Ma, L. Li, D. Que, Semicond. Sci. Technol. 18 (2002) 399. [14] H.J. Ruitz, G.P. Pollack, J. Electrochem. Soc. 125 (1978) 128.
Physica B 340–342 (2003) 601–604
Bipolar structure of carrier concentration in hydrogen pre-annealing Czochralski silicon wafer Xuegong Yu, Deren Yang*, Ruixin Fan, Xiangyang Ma, Duanlin Que State Key Laboratory of Silicon Materials, Department of Material Science and Engineering, Zhejiang University, Zheda Lu 38, Hangzhou 310027, People’s Republic of China
Abstract The carrier concentration profiles in p-type Czochralski silicon wafers subjected to hydrogen or nitrogen hightemperature pre-annealing followed by a prolonged 450 C annealing have been investigated by spreading resistance profile. It is clarified that the carrier concentration profile in the samples subjected to hydrogen pre-annealing is characteristic of a NPN bipolar structure, while that it is just characteristic of a PN junction in the samples subjected to nitrogen pre-annealing. Furthermore, the location of the NPN structure in the wafer is dependent on hydrogen preannealing temperature. It is suggested that the formation of the NPN bipolar structure is due to the enhancement of thermal donors by hydrogen. r 2003 Elsevier B.V. All rights reserved. PACS: 61.72.Bb; 61.72.Yx; 61.72.Ss Keywords: Hydrogen annealing; Thermal donors; Czochralski silicon
1. Introduction Oxygen originated from the contamination of quartz crucibles is a main impurity in Czochralski (CZ) silicon, which strongly affects the properties and yield of electronic devices, as a result of oxygen aggregating to form thermal donors (TDs) in heat treatments in the temperature range of 300–500 C [1]. The concentration of the TDs ([TDs]) mainly depends on the interstitial oxygen concentration ([Oi]), and the carrier concentration profiles measured by spreading resistance probe *Corresponding author. Tel.: +86-571-8795-1667; fax: +86571-8795-2322. E-mail address: [email protected] (D. Yang).
(SRP) is generally used for the analysis of the distribution of the [TDs] and therefore the [Oi] in CZ silicon. On the other hand, it has been well known that atomic hydrogen incorporated into CZ silicon can act as a catalyst and significantly enhance the TD formation rate [2–8]. More interestingly, it is reported that after plasma hydrogenation at 400 C the hydrogen-enhanced region of initial p-type CZ silicon becomes n-type, and therefore a PN junction can be observed at the near-surface of wafer [9,10]. The depth of the PN junction is controlled by the duration of the plasma treatment, the dose of incorporated hydrogen ions from the plasma and oxygen concentration in wafer. Since the hydrogen diffusion coefficient is
0921-4526/$ - see front matter r 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2003.09.126
ARTICLE IN PRESS 602
X. Yu et al. / Physica B 340–342 (2003) 601–604
very large, the hydrogen concentration in the bulk of wafer will reach its solubility at annealing temperature quickly [11]. Recently, we have reported that hydrogen annealing at high temperature can eliminate the crystal originated particles (COPs) to improve the gate oxide integrity [12,13]. In the paper, we found the bipolar (NPN) structure was formed in p-type CZ wafers subjected to hydrogen pre-annealing with a prolonged 450 C annealing, which cannot be formed in p-type CZ silicon subjected to nitrogen pre-annealing. It is suggested that the bipolar structure is corresponding to the enhancement of hydrogen on TDs. Based on our experiments, a formation model of bipolar structure is suggested.
2. Experimental P-type /1 0 0S CZ wafers with the resistivity of 8.5 O cm have been used in our experiments. The [Oi] of as-grown samples measured by the Fourier transmission infrared spectroscopy (FTIR) at room temperature with a calibration fact of 3.14 1017 cm 2 was about 6.7 1017 cm 3, and the concentration of carbon was below the detection limit of FTIR. The samples were preannealed in the temperature range of 1050–1200 C for 1–2 h in hydrogen or nitrogen ambient, respectively, and quenched to room temperature. Subsequently, the pre-annealed samples were annealed at 450 C for 80 h to generate TDs in nitrogen ambient. Finally, the carrier concentration profiles along the depth of the wafers were measured by SRP.
3. Results and discussion Fig. 1 shows the carrier concentration profiles at the sub-surface of the p-type CZ wafers subjected to 1150 C/2 h pre-annealing in nitrogen or hydrogen ambient respectively and followed by 450 C/ 80 h annealing. As it can be seen, two peaks appear in the wafer subjected to hydrogen pre-annealing (Fig. 1b), while only one appears in the wafer subjected to nitrogen pre-annealing (Fig. 1a). For the samples subjected to pre-annealing in nitrogen ambient and followed by 450 C annealing, the formation of PN junction is a result of the concentration compensation between TD formed during 450 C annealing and the inherent acceptors. As for the samples subjected to hydrogen preannealing and followed by 450 C annealing, the formation of bipolar structure is also a result of the compensation between the concentration of TDs and the inherent acceptor, however, the profile of [TDs] along the depth of the wafer was much different from that in the case of nitrogen preannealing. It is obvious that this bipolar structure is an NPN one, which will be elucidated later. Fig. 2 shows the location variation of the bipolar structure at the sub-surface of p-type CZ wafers with the hydrogen pre-annealing temperature. It can be seen that the location of the bipolar structure moved toward the bulk from the surface of the wafers with the increase of the annealing temperature. It indicates that the temperature of hydrogen pre-annealing has an influence on the location of bipolar structure. It has been well known that the out-diffusion of oxygen at the sub-surface of CZ wafers subjected
Fig. 1. The carrier profile at the sub-surface of the wafer subjected to nitrogen (a) and hydrogen (b) pre-annealing followed by 450 C/ 80 h annealing.
ARTICLE IN PRESS X. Yu et al. / Physica B 340–342 (2003) 601–604
to high temperature will take place whatever annealing ambient is, as shown in Figs. 3(a,b)-(1). According to diffusion theory, in the out-diffusion region, the [Oi] profile follows the complementary error function [14]. The bulk [Oi] will be somewhat higher than the initial [Oi] as a result of the grownin oxygen precipitation dissolving, while, the [Oi]
The depth of NPN (um)
30
20
10
0
1050 1100 1150 1200 Annealing temperature (°C)
Fig. 2. The location variation of the NPN structure at the subsurface of the wafer with hydrogen pre-annealing temperature.
603
at the surface is close to the oxygen solubility at annealing temperature. For the samples subjected to high temperature pre-annealing in nitrogen ambient, according to the [Oi] profile, the [TDs] profile can be schematically illustrated in Fig. 3a-(2) after a prolonged low temperature annealing, a result of TD formation depending on Oi, additionally, the inherent acceptor concentration is also illustrated as a horizontal line. It is clear that the region where the [TDs] is higher than the acceptor concentration will be of n-type by reason of compensation; on the contrary, the region is of p-type. So the ultimate carrier concentration profile is derived as shown in Fig. 3a-(3), and a PN junction is formed between the near-surface and the bulk regions. For the samples subjected to hydrogen preannealing, after pre-annealing, the [Oi] profile is similar to that subjected to nitrogen pre-annealing even though the hydrogen will enhance the outdiffusion of oxygen, as shown in Fig. 3b-(1), meanwhile, the hydrogen will diffuse into the bulk
Fig. 3. The formation model of the NPN structure at the sub-surface of the wafer subjected to hydrogen pre-annealing followed by 450 C annealing.
ARTICLE IN PRESS 604
X. Yu et al. / Physica B 340–342 (2003) 601–604
from the surface of the wafer [11]. According to Henry’s Law, the concentration of hydrogen in the near-surface of wafer is proportional to the hydrogen ambient, which is much higher than that in the bulk, while, the concentration of hydrogen in the bulk of wafer will reach its solubility at the annealing temperature quickly, a result of hydrogen rapid diffusion in silicon. Additionally, since the samples are quenched, the hydrogen near the surface of wafer will not diffuse out during cooling. Fig. 3b-(1) shows the hydrogen concentration profile after hydrogen pre-annealing. During subsequent low temperature annealing the different concentration hydrogen in the sample will enhance the formation of the TDs under the condition of different [Oi]. That is, the [TDs] formed in the samples depends not only on the [Oi] but also the concentration of hydrogen. At the near-surface of the wafer, the [TDs] is much higher, as a result of higher concentration hydrogen existing. In the bulk the [TDs] is also higher because of higher [Oi]. Between the near-surface and the bulk of wafer, the [TDs] is much lower because both oxygen and hydrogen concentrations are lower. The [TDs] profile is shown in Fig. 3b(2). Accordingly, the ultimate carrier concentration profile is derived as shown in Fig. 3b-(3) after the compensation between TDs and the inherent acceptor. Three distinct regions respectively ascribed to n-, p- and n-types from the surface to the bulk region are generated, which is characteristic of a NPN transistor. Furthermore, it can be concluded that with the increase of hydrogen pre-annealing temperature the solubility concentration and diffusion rate of hydrogen will increase and the out-diffusion length of oxygen increases, therefore the profile of hydrogen concentration and Oi will change, which results in the location of NPN structure will move toward the bulk from the surface of the wafer (Fig. 2).
4. Conclusion In summary, we have reported the formation of a NPN structure in the p-type CZ-Si wafer
subjected to hydrogen pre-annealing at high temperature and subsequent prolonged low temperature annealing, while the wafer subjected to pre-annealing in nitrogen ambient, only a PN junction is formed at the sub-surface. Moreover, it is also clarified that the location of the NPN structure moves toward the bulk with the increase of hydrogen pre-annealing temperature. After taking into account the oxygen diffusion, the hydrogen enhancement on TD formation and compensation between TDs and inherent acceptors, a conceptual model is discussed.
Acknowledgements The authors would like to thank the Natural Science Foundation of China (Nos. 50032010 and 60225010) and 863 programs (No. 2002AA3Z1111) for financial supports.
References [1] P. Capper, A.W. Jones, E.J. Wallhouse, J.G. Wilks, J. Appl. Phys. 48 (1977) 1646. [2] E.M. Filangeri, T. Nishida, J. Appl. Phys. 75 (1994) 7931. [3] A.R. Brown, M. Clayborn, R. Murray, P.S. Nandra, R.C. Newman, J.H. Tucker, Semicond. Sci. Technol. 3 (1988) 591. [4] R. Murray, A.R. Brown, R.C. Newman, Mater. Sci. Eng. B 4 (1990) 299. [5] R.C. Newman, J.H. Tucker, A.R. Brown, S.A. McQuaid, Appl. Phys. 70 (1991) 3061. [6] H.J. Stein, S.K. Hahn, Appl. Phys. Lett. 56 (1990) 63. [7] H.J. Stein, S.K. Hahn, J. Appl. Phys 75 (1994) 3477. [8] H.J. Stein, S.K. Hahn, J. Electrochem. Soc. 142 (1995) 1242. [9] A.G. Ulyashin, Y.A. Bumay, R. Job, G. Grabosh, D. Borchert, W.R. Fahrner, A.Y. Diduk, Solid State Phenom. 57–58 (1997) 189. [10] A.G. Ulyashin, Y.A. Bumay, R. Job, W.R. Fahrner, Appl. Phys. A 66 (1998) 399. [11] A. Hara, M. Aoki, T. Fukuda, A. Ohsawa, J. Appl. Phys. 74 (1993) 913. [12] X. Yu, D. Yang, X. Ma, J. Yang, D. Que, J. Appl. Phys. 92 (2003) 188. [13] X. Yu, D. Yang, X. Ma, L. Li, D. Que, Semicond. Sci. Technol. 18 (2002) 399. [14] H.J. Ruitz, G.P. Pollack, J. Electrochem. Soc. 125 (1978) 128.