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Co-implantation of hydrogen and helium for thermal stabilization of lifetime in power devices
Nuclear Instruments and Methods in Physics Research B 186 (2002) 371–374 www.elsevier.com/locate/nimb
Co-implantation of hydrogen and helium for thermal stabilization of lifetime in power devices E. Ntsoenzok *, G. Blondiaux CNRS/CERI, 3A rue de la F erollerie, 45071 Orl eans Cedex 2, France
Abstract In order to provide engineering power devices with a good thermal stabilization of the lifetime with the temperature, we performed a proton implantation following by a helium one. The process resulted in a drastic improvement of the power rectifier parameters particularly a better stabilization versus temperature. While a lifetime control performed by gold or platinum resulted in a variation of lifetime by a factor of about 2 when the temperature increases from 25 to 125 °C, diodes processed with co-implantation gave a factor of about 1.4. Ó 2002 Published by Elsevier Science B.V. Keywords: Lifetime; Rectifier; Proton; Helium; Implantation
1. Introduction The control of lifetime in power devices is a subject which has been studied since many decades [1–5]. Devices such as power rectifiers are very useful and yet there is no new device which can replace them. They are often used as freewheeling diodes. The improvement of these devices is therefore very important since their failure can badly damage the circuit in which they are used. One of the most important problems are the losses induced by the diode. These losses must be reduced to a minimum if high efficiency is needed. They are usually reduced by controlling the lifetime of carriers. In power devices, lifetime is usually controlled by diffusion of metallic impurities (Pt, Au) [5–7] or *
Corresponding author. Tel.: +33-2-38517604; fax: +33-238620271. E-mail address: [email protected] (E. Ntsoenzok).
by irradiation (electrons, protons, helium) [3,4,8]. However, neither metallic diffusions nor irradiation provide the way to avoid the evolution of the lifetime of carriers with the temperature (at which the devices work). In addition, we can add that metallic impurities present many weaknesses including the contamination of the whole device. Actually, when using conventional techniques, the device speed decreases when the temperature increases. Only Hayashi et al. [9] obtained a good thermal stabilization of lifetime. They achieved it by combining iron and gold diffusion. In this study, we performed a co-implantation of hydrogen and helium.
2. Experimental The samples irradiated in this study consisted of Pþ NNþ rectifiers. The starting material was a {1 1 1} Nþ CZ silicon doped with the As at a level
0168-583X/02/$ - see front matter Ó 2002 Published by Elsevier Science B.V. PII: S 0 1 6 8 - 5 8 3 X ( 0 1 ) 0 0 9 4 5 - 4
372
E. Ntsoenzok, G. Blondiaux / Nucl. Instr. and Meth. in Phys. Res. B 186 (2002) 371–374
of about 2 1019 cm3 . The N layer was epitaxially grown on the CZ substrate while the Pþ anode was obtained by boron diffusion. The width and the doping concentration are, respectively, 40 lm and 3 1014 cm3 for the N layer, 9 lm and 2 1019 cm3 for the Pþ layer. The electrical contacts are made up of a 6 lm-Al layer on the anode side and a combination of titanium nickel and gold for the substrate side (see Fig. 1). The samples were irradiated by a 3.5 MV Van de Graaff and a Cyclotron accelerator for hydrogen and helium, respectively. The energy of protons chosen was equal to 2.2 MeV in order to þ obtain the implantation depth behind the N=N
interface (Nþ side). At this depth one can avoid the drawback effect induced by shallow donors. Shallow donors are usually reported to be induced at RP after a high dose hydrogen implantation and a thermal treatment [10]. For the helium, the energy 2 used, 7.4 MeV with a dose of 1012 He=cm , resulted in an RP in the second half of the N type layer where recombination centres are more efficient for lifetime control [11]. After implantation, samples were annealed at 400 °C in conventional furnaces under nitrogen. Lifetime measurements were then performed. Actually, we measured reverse recovery time ðTRR Þ and maximum reverse current (IRM ) (see Fig. 1). These two parameters reflect the speed of the diode and therefore the lifetime of minority carriers.
3. Results Figs. 2 and 3 show the evolution of TRR as a function of the temperature for the different configurations studied. Fig. 2 compares the relative evolution of TRR for gold and platinum diffusions with Heþ and Hþ þ Heþ implantation. Gold and platinum diffusion were performed at 970 and 910 °C, respectively. It clearly appears that devices which were diffused by gold or platinum present a strong evolution with the temperature. When gold or platinum diffusion is used, the results are coherent with what is usually reported as one of the major weaknesses of metallic diffusion. The in-
Fig. 1. Implanted device (top) and determination of TRR and IRM (bottom). For all measurements performed in this study dIF =dt ¼ 70 A=ls, VR ¼ 30 V.
Fig. 2. Relative evolution of TRR with the temperature of measurements. TRR0 is the value measured at room temperature.
E. Ntsoenzok, G. Blondiaux / Nucl. Instr. and Meth. in Phys. Res. B 186 (2002) 371–374
373
induces additional drawback effects including a decrease in VBR (reverse breakdown voltage).
4. Discussion The most important parameter which can explain the evolution of TRR with temperature is the capture cross-section (r) of the trap. s is given by n 1 X NTi ri vth s i¼1
Fig. 3. Comparison of TRR as extracted from Hþ þ Heþ implantation and platinum diffusion.
crease of TRR is strong even at low temperature. When implanted alone, helium presents a very slow increase of TRR with the temperature up to about 100 °C and then an important rise. When comparing diffusion by gold, platinum, and irradiation with helium it can be observed that helium presents the best stabilization versus temperature. Helium and platinum are competitive up to 100 and 50 °C, respectively, while gold diffusion is the less competitive with an increase in TRR from very low temperatures. Fig. 3 compares TRR obtained with platinum diffusion and co-implantation of hydrogen and helium. As reported by Figs. 2 and 3, when combining helium and hydrogen devices the best performances are achieved. The ratio (TRR at 125 °C)/ (TRR at 25 °C) is about 1.4 while this value is about 1.5 in the case of hydrogen and a value of about 1.8 when using platinum or gold in this study. Another important factor in the rectifiers is the behaviour of the reverse recovery. Co-implantation of hydrogen and helium also results in an improvement of this parameter. It is important to indicate that we did not use hydrogen alone because at these levels of speed, it
ð1Þ
or 1 NE1 rE1 vth ; sH
ð2Þ
where s is the lifetime of carriers. TRR is related to s. NTi and ri are the concentration and the capture cross-section of the ith center, respectively. Vth is the thermal velocity of carriers. sH is the lifetime at high level injection while NE1 and rE1 are the concentration and the capture cross-section of the most efficient center (E1) at high level injection. First of all it can be said that while the diffusion of gold or platinum induced only one acceptor level in the band gap, at Ec 0:23 eV for Pt [12] and Ec 53 eV for gold [13], irradiation with proton or helium results in various electron traps [14,15] including the A centre and divacancies. In the case of high injection level the A center, located at Ec 0:17 eV, is thought to be the main center for lifetime control when irradiation is used. Since Vth increases with temperature ðVth / T 1=2 Þ, TRR might decrease with temperature if r remains constant (NT can be considered as constant at these temperatures). However Bleichner et al. [16] pointed out that the cross-section of the A (see Table 1) centre decreases with temperature. A factor of about 0.5 is reported when the temperature increases from 25 to 125 °C. This leads to
Table 1 Thermal evolution of cross-sections of VO and divacancies as reported by Bleichner et al. [16] Center VO (A center) V22= V=0 2
Cn ðcm3 =sÞ 7
Validity (K) ðT =150Þ
6:4 10 e 1:6 1012 T 1:4 5:4 109 T 0:4
250–420 105–155 182–266
Cn (at 300 K) 8
8 10 – –
Cn (at 400 K) 3:89 108 – –
374
E. Ntsoenzok, G. Blondiaux / Nucl. Instr. and Meth. in Phys. Res. B 186 (2002) 371–374
an increase by a factor of 1.7 for the ratio (s at 125 °C)/(s at 25 °C). This factor is still higher than the one measured by our experiments. A complete study of the mechanism involved can be achieved by determining the thermal evolution of all centres induced by irradiation. There is still a lack of data in this regard (see Table 1).
5. Conclusion This study demonstrates that very competitive rectifiers can be built by a smart combination of proton and hydrogen implantation. In particular, a good thermal stability of devices has been achieved. However further investigations are needed to find out the actual process involved.
References [1] B. Holm, K.B. Nielsen, J. Appl. Phys. 78 (10) (1995) 5970. [2] N. Keskitalo, A. Hallen, L. Josyula, B.G. Swensson, Ion Beam Modification of Materials, 1996.
[3] E. Ntsoenzok, J.F. Barbot, P. Desgardin, J. Vernois, C. Blanchard, D.B. Isabelle, IEEE Trans. Nucl. Sci. 41 (1994) 1932. [4] W.D. Nowak, W. Wondrak, D. Silber, Arch. Elektrotech. 72 (1989) 133. [5] M.D. Miller, H. Schade, C.J. Nuese, J. Appl. Phys. 47 (6) (1976) 2569. [6] B. Deng, H. Kuwano, Jpn. J. Appl. Phys. 34 (1995) 4587. [7] H. Bleichner, P. Jonsson, N. Keskitalo, E. Nordlander, J. Appl. Phys. 79 (12) (1996) 9142. [8] A. Hallen, N. Keskitalo, F. Masszi, V. Nagl, J. Appl. Phys. 79 (8) (1996) 3906. [9] H. Hayashi, T. Mamine, T. Matsushita, IEEE Trans. Electron Devices ED-28 (3) (1981). [10] E. Ntsoenzok, P. Desgardin, M. Saillard, J. Vernois, J.F. Barbot, J. Appl. Phys. 79 (11) (1996) 8274. [11] P. Desgardin, Ph.D Thesis, University of Orleans, France, 1996. [12] K.P. Lisiak, A.G. Milnes, Solid-State Electron. 18 (1975) 533. [13] H. Weman, A. Henry, T. Begun, B. Monemar, O.O. Awadelkarim, J.L. Lindstr€ om, J. Appl. Phys. 65 (1) (1989) 137. [14] L. Palmetshofer, J. Reisinger, J. Appl. Phys. 72 (6) (1992) 2167. [15] B.G. Svensson, EMIS Data Reviews Series, Properties of Silicon, 1999. [16] H. Bleichner, P. Jonsson, N. Keskitalo, E. Nordlander, J. Appl. Phys. 79 (1996).
Co-implantation of hydrogen and helium for thermal stabilization of lifetime in power devices E. Ntsoenzok *, G. Blondiaux CNRS/CERI, 3A rue de la F erollerie, 45071 Orl eans Cedex 2, France
Abstract In order to provide engineering power devices with a good thermal stabilization of the lifetime with the temperature, we performed a proton implantation following by a helium one. The process resulted in a drastic improvement of the power rectifier parameters particularly a better stabilization versus temperature. While a lifetime control performed by gold or platinum resulted in a variation of lifetime by a factor of about 2 when the temperature increases from 25 to 125 °C, diodes processed with co-implantation gave a factor of about 1.4. Ó 2002 Published by Elsevier Science B.V. Keywords: Lifetime; Rectifier; Proton; Helium; Implantation
1. Introduction The control of lifetime in power devices is a subject which has been studied since many decades [1–5]. Devices such as power rectifiers are very useful and yet there is no new device which can replace them. They are often used as freewheeling diodes. The improvement of these devices is therefore very important since their failure can badly damage the circuit in which they are used. One of the most important problems are the losses induced by the diode. These losses must be reduced to a minimum if high efficiency is needed. They are usually reduced by controlling the lifetime of carriers. In power devices, lifetime is usually controlled by diffusion of metallic impurities (Pt, Au) [5–7] or *
Corresponding author. Tel.: +33-2-38517604; fax: +33-238620271. E-mail address: [email protected] (E. Ntsoenzok).
by irradiation (electrons, protons, helium) [3,4,8]. However, neither metallic diffusions nor irradiation provide the way to avoid the evolution of the lifetime of carriers with the temperature (at which the devices work). In addition, we can add that metallic impurities present many weaknesses including the contamination of the whole device. Actually, when using conventional techniques, the device speed decreases when the temperature increases. Only Hayashi et al. [9] obtained a good thermal stabilization of lifetime. They achieved it by combining iron and gold diffusion. In this study, we performed a co-implantation of hydrogen and helium.
2. Experimental The samples irradiated in this study consisted of Pþ NNþ rectifiers. The starting material was a {1 1 1} Nþ CZ silicon doped with the As at a level
0168-583X/02/$ - see front matter Ó 2002 Published by Elsevier Science B.V. PII: S 0 1 6 8 - 5 8 3 X ( 0 1 ) 0 0 9 4 5 - 4
372
E. Ntsoenzok, G. Blondiaux / Nucl. Instr. and Meth. in Phys. Res. B 186 (2002) 371–374
of about 2 1019 cm3 . The N layer was epitaxially grown on the CZ substrate while the Pþ anode was obtained by boron diffusion. The width and the doping concentration are, respectively, 40 lm and 3 1014 cm3 for the N layer, 9 lm and 2 1019 cm3 for the Pþ layer. The electrical contacts are made up of a 6 lm-Al layer on the anode side and a combination of titanium nickel and gold for the substrate side (see Fig. 1). The samples were irradiated by a 3.5 MV Van de Graaff and a Cyclotron accelerator for hydrogen and helium, respectively. The energy of protons chosen was equal to 2.2 MeV in order to þ obtain the implantation depth behind the N=N
interface (Nþ side). At this depth one can avoid the drawback effect induced by shallow donors. Shallow donors are usually reported to be induced at RP after a high dose hydrogen implantation and a thermal treatment [10]. For the helium, the energy 2 used, 7.4 MeV with a dose of 1012 He=cm , resulted in an RP in the second half of the N type layer where recombination centres are more efficient for lifetime control [11]. After implantation, samples were annealed at 400 °C in conventional furnaces under nitrogen. Lifetime measurements were then performed. Actually, we measured reverse recovery time ðTRR Þ and maximum reverse current (IRM ) (see Fig. 1). These two parameters reflect the speed of the diode and therefore the lifetime of minority carriers.
3. Results Figs. 2 and 3 show the evolution of TRR as a function of the temperature for the different configurations studied. Fig. 2 compares the relative evolution of TRR for gold and platinum diffusions with Heþ and Hþ þ Heþ implantation. Gold and platinum diffusion were performed at 970 and 910 °C, respectively. It clearly appears that devices which were diffused by gold or platinum present a strong evolution with the temperature. When gold or platinum diffusion is used, the results are coherent with what is usually reported as one of the major weaknesses of metallic diffusion. The in-
Fig. 1. Implanted device (top) and determination of TRR and IRM (bottom). For all measurements performed in this study dIF =dt ¼ 70 A=ls, VR ¼ 30 V.
Fig. 2. Relative evolution of TRR with the temperature of measurements. TRR0 is the value measured at room temperature.
E. Ntsoenzok, G. Blondiaux / Nucl. Instr. and Meth. in Phys. Res. B 186 (2002) 371–374
373
induces additional drawback effects including a decrease in VBR (reverse breakdown voltage).
4. Discussion The most important parameter which can explain the evolution of TRR with temperature is the capture cross-section (r) of the trap. s is given by n 1 X NTi ri vth s i¼1
Fig. 3. Comparison of TRR as extracted from Hþ þ Heþ implantation and platinum diffusion.
crease of TRR is strong even at low temperature. When implanted alone, helium presents a very slow increase of TRR with the temperature up to about 100 °C and then an important rise. When comparing diffusion by gold, platinum, and irradiation with helium it can be observed that helium presents the best stabilization versus temperature. Helium and platinum are competitive up to 100 and 50 °C, respectively, while gold diffusion is the less competitive with an increase in TRR from very low temperatures. Fig. 3 compares TRR obtained with platinum diffusion and co-implantation of hydrogen and helium. As reported by Figs. 2 and 3, when combining helium and hydrogen devices the best performances are achieved. The ratio (TRR at 125 °C)/ (TRR at 25 °C) is about 1.4 while this value is about 1.5 in the case of hydrogen and a value of about 1.8 when using platinum or gold in this study. Another important factor in the rectifiers is the behaviour of the reverse recovery. Co-implantation of hydrogen and helium also results in an improvement of this parameter. It is important to indicate that we did not use hydrogen alone because at these levels of speed, it
ð1Þ
or 1 NE1 rE1 vth ; sH
ð2Þ
where s is the lifetime of carriers. TRR is related to s. NTi and ri are the concentration and the capture cross-section of the ith center, respectively. Vth is the thermal velocity of carriers. sH is the lifetime at high level injection while NE1 and rE1 are the concentration and the capture cross-section of the most efficient center (E1) at high level injection. First of all it can be said that while the diffusion of gold or platinum induced only one acceptor level in the band gap, at Ec 0:23 eV for Pt [12] and Ec 53 eV for gold [13], irradiation with proton or helium results in various electron traps [14,15] including the A centre and divacancies. In the case of high injection level the A center, located at Ec 0:17 eV, is thought to be the main center for lifetime control when irradiation is used. Since Vth increases with temperature ðVth / T 1=2 Þ, TRR might decrease with temperature if r remains constant (NT can be considered as constant at these temperatures). However Bleichner et al. [16] pointed out that the cross-section of the A (see Table 1) centre decreases with temperature. A factor of about 0.5 is reported when the temperature increases from 25 to 125 °C. This leads to
Table 1 Thermal evolution of cross-sections of VO and divacancies as reported by Bleichner et al. [16] Center VO (A center) V22= V=0 2
Cn ðcm3 =sÞ 7
Validity (K) ðT =150Þ
6:4 10 e 1:6 1012 T 1:4 5:4 109 T 0:4
250–420 105–155 182–266
Cn (at 300 K) 8
8 10 – –
Cn (at 400 K) 3:89 108 – –
374
E. Ntsoenzok, G. Blondiaux / Nucl. Instr. and Meth. in Phys. Res. B 186 (2002) 371–374
an increase by a factor of 1.7 for the ratio (s at 125 °C)/(s at 25 °C). This factor is still higher than the one measured by our experiments. A complete study of the mechanism involved can be achieved by determining the thermal evolution of all centres induced by irradiation. There is still a lack of data in this regard (see Table 1).
5. Conclusion This study demonstrates that very competitive rectifiers can be built by a smart combination of proton and hydrogen implantation. In particular, a good thermal stability of devices has been achieved. However further investigations are needed to find out the actual process involved.
References [1] B. Holm, K.B. Nielsen, J. Appl. Phys. 78 (10) (1995) 5970. [2] N. Keskitalo, A. Hallen, L. Josyula, B.G. Swensson, Ion Beam Modification of Materials, 1996.
[3] E. Ntsoenzok, J.F. Barbot, P. Desgardin, J. Vernois, C. Blanchard, D.B. Isabelle, IEEE Trans. Nucl. Sci. 41 (1994) 1932. [4] W.D. Nowak, W. Wondrak, D. Silber, Arch. Elektrotech. 72 (1989) 133. [5] M.D. Miller, H. Schade, C.J. Nuese, J. Appl. Phys. 47 (6) (1976) 2569. [6] B. Deng, H. Kuwano, Jpn. J. Appl. Phys. 34 (1995) 4587. [7] H. Bleichner, P. Jonsson, N. Keskitalo, E. Nordlander, J. Appl. Phys. 79 (12) (1996) 9142. [8] A. Hallen, N. Keskitalo, F. Masszi, V. Nagl, J. Appl. Phys. 79 (8) (1996) 3906. [9] H. Hayashi, T. Mamine, T. Matsushita, IEEE Trans. Electron Devices ED-28 (3) (1981). [10] E. Ntsoenzok, P. Desgardin, M. Saillard, J. Vernois, J.F. Barbot, J. Appl. Phys. 79 (11) (1996) 8274. [11] P. Desgardin, Ph.D Thesis, University of Orleans, France, 1996. [12] K.P. Lisiak, A.G. Milnes, Solid-State Electron. 18 (1975) 533. [13] H. Weman, A. Henry, T. Begun, B. Monemar, O.O. Awadelkarim, J.L. Lindstr€ om, J. Appl. Phys. 65 (1) (1989) 137. [14] L. Palmetshofer, J. Reisinger, J. Appl. Phys. 72 (6) (1992) 2167. [15] B.G. Svensson, EMIS Data Reviews Series, Properties of Silicon, 1999. [16] H. Bleichner, P. Jonsson, N. Keskitalo, E. Nordlander, J. Appl. Phys. 79 (1996).