- Email: [email protected]
GaAs HBT by He+ ion implantation
December 2001
Materials Letters 51 Ž2001. 529–533 www.elsevier.comrlocatermatlet
Base resistance variation of AlGaAsrGaAs HBT by Heq ion implantation Il-Ho Kim Department of Materials Science and Engineering, Chungju National UniÕersity, 123 Komdan-ri, Iryu-myon, Chungju, Chungbuk 380-702, South Korea Received 12 October 2000; received in revised form 29 March 2001; accepted 30 March 2001
Abstract Reduction of base–collector junction capacitance has been intended by Heq ion implantation to enhance the maximum oscillation frequency of AlGaAsrGaAs heterojunction bipolar transistors ŽHBTs.. From the recovery of the base resistance by rapid thermal annealing ŽRTA. after ion implantation, it is found that optimum conditions are 140 keV and 5 = 10 12 –1 = 10 13 cmy2 for Heq ion implantation and 4008C and 20 s for rapid thermal annealing. q 2001 Elsevier Science B.V. All rights reserved. PACS: 72.80.Ga; 73.40.Cg; 61.72.Vv Keywords: Ion implantation; Helium ion; Gallium arsenide; Heterojunction bipolar transistor
AlGaAsrGaAs heterojunction bipolar transistors ŽHBTs. have been extensively investigated as promising electronic devices for high-speed digital and microwave circuits, such as monolithic microwave integrated circuit ŽMMIC. and optoelectronic integrated circuit ŽOEIC. w1,2x. They have been oriented toward communication systems that include high bit rate optical fiber transmission systems for the broadband integrated services digital networks ŽISDN.. Considerable attention has been also focused on HBTs in high-power microwave applications because of their inherent features, such as high-power handling capacity per unit chip area, ease of design for high breakdown voltage, and the class of operation with high efficiency. High-frequency microwave
E-mail address: [email protected] ŽI.-H. Kim..
circuits require high performance active devices since they determine the circuit performance. Such a high potential for superior microwave performance of HBTs has been limited by the fabrication technology that results to large extrinsic parasitics, such as extrinsic resistances, contact resistances and junction capacitances. However, these drawbacks could be overcome by advanced technologies. The extrinsic resistances may be greatly reduced by the self-aligned technology w3,4x. Extensive researches of various material systems have been performed to make good ohmic contacts w5,6x. The extrinsic junction capacitances can be reduced by proton or boron ion implantation that includes damage to compensate the donors in the collector region w7x. The maximum oscillation frequency Ž f max . is one of the important figures of merit for microwave
00167-577Xr01r$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 5 7 7 X Ž 0 1 . 0 0 3 5 0 - 0
530
I.-H. Kim r Materials Letters 51 (2001) 529–533
device performance. High values of f max that are suitable for millimeter-wave applications have been already demonstrated in GaAs-base HBTs w8–10x. In these devices, high f max is achieved primarily by reducing the base resistance and base–collector capacitance. A variety of approaches has been pursued for the maximization of f max . An essential issue is to eliminate the capacitance associated with the extrinsic base–collector junction, i.e. the area underneath the base contacts in the conventional HBT structure. Deep-implanted HBTs make use of ion implantation damage to render the collector and subcollector semi-insulating in the region underneath the base contact, thereby reducing the extrinsic base–collector junction capacitance Ž C bc .. Helium ion ŽHeq. can be deeply implanted with lower acceleration energy than boron ion ŽBq. or oxygen ion ŽOq. , and its propagation depth can be more easily controlled compared with proton ion ŽHq. . Moreover, protective layers such as Si 3 N4 or SiO 2 films are not necessarily in the rapid thermal annealing ŽRTA. process for the resistance recovery of the base damaged by ion implantation because post-implant annealing can be performed at low temperature. Therefore, Heq is most suitable for reducing C bc Žor increasing f max . of AlGaAsrGaAs HBTs by ion implantation. Two conditions should be satisfied to increase f max of HBTs by Heq ion implantation. First, suffi-
Fig. 1. Epitaxial layer structure grown by MOCVD for AlGaAsr GaAs HBT Žemitter layers are eliminated for ion implantation..
Fig. 2. Heq ion implanted region in AlGaAsrGaAs HBT ŽEM, emitter metal; BM, base metal; CM, collector metal..
cient ion dose and adequate ion energy are required to make the collector and subcollector semi-insulating. Secondly, the resistance of the base that sustains implantation damages simultaneously should be recovered by proper annealing treatment. In this study, these two Žimplantation and annealing. conditions are investigated and optimized for conventional AlGaAsrGaAs HBTs by Monte Carlo simulation and Heq ion implantation. Monte Carlo simulation program—transport of ions in matter ŽTRIM. w11x —was used to calculate the Heq ion distribution with implantation depth and the absorption energy by collision between implanted ions and matrix atoms. Ion implantation energy and dose for application to AlGaAsrGaAs HBTs were determined from simulation results. To avoid the channeling effect, Heq ions were implanted at an angle of 78 with the specimen at room temperature. Rapid thermal annealing ŽRTA. treatment was carried out at 350–4508C from 10 to 30 s to examine the variation of electrical properties with post-implant annealing. Fig. 1 shows the epitaxial layer structure grown by metal organic chemical vapor deposition ŽMOCVD. for AlGaAsrGaAs HBT, where emitter layers are eliminated for ion implantation. The base sheet resistance was measured by the transmission line model ŽTLM. method. After ultrasonic cleaning wafer with acetone, isopropylalcohol ŽIPA. and deionized ŽDI. water for 5 min, respectively, TLM patterns were made on the base surface layer by photolithography work using AZ5214-E photoresist and NMD-3 developer. Residual photoresist was removed by plasma dry stripping and HCl cleaning.
I.-H. Kim r Materials Letters 51 (2001) 529–533
Fig. 3. Damage distribution caused by Heq ion implantation ŽTRIM simulation..
531
532
I.-H. Kim r Materials Letters 51 (2001) 529–533
Fig. 4. Sheet resistance of the base with Heq ion implantation energy and dose.
˚ .rTi Ž300 Base ohmic contact metals of Pt Ž50 A ˚ .rPt Ž300 A˚ .rAu Ž800 A˚ . were deposited by A electron beam deposition and lift-off process. Heat treatment was not performed after deposition of
ohmic contact metals to exclude the temperature dependence of ohmic contact properties. Fig. 2 illustrates the vertical cross-sectional view of AlGaAsr GaAs HBT, where the Heq ion implanted region is
Fig. 5. Sheet resistance of the base with annealing condition after Heq ion implantation.
I.-H. Kim r Materials Letters 51 (2001) 529–533
indicated. In Fig. 2, the base metal ŽBM. should be deposited after ion implantation to suppress an increase in the contact resistance between the base surface layer and base metal by ion implantation. The damage distribution estimated with TRIM simulation is shown in Fig. 3, where straight lines in the base, collector and subcollector regions show the doping levels. Ion energy was changed from 100 to 200 keV and ion dose was varied from 1 = 10 12 to 5 = 10 13 cmy2 . Vacancy Ždamage. concentration increased with increasing ion dose at constant ion energy. Implantation damage have to reach to a depth greater than 1 mm through the collector and subcollector regions. It was found that ion dose higher than 5 = 10 12 cmy2 and ion energy of about 150 keV are required to make collector and subcollector semi-insulating Žisolating.. In this study, ion implantation was carried out at an acceleration energy of 140 keV with ion doses of 5 = 10 12 and 1 = 10 13 cmy2 . Fig. 4 shows the variation of the base sheet resistance with Heq ion implantation energy and dose. As expected, the base sheet resistance is increased by ion implantation, and it increased considerably with increasing ion dose. This results from larger lattice damage with increasing ion dose. It was also known that when the ion energy was lowered, the base resistance increased remarkably. This is why the base sustains larger lattice damage due to shallower penetration Žimplantation. depth for lower ion energy. Fig. 5 indicates the variation of the base sheet resistance with RTA after Heq ion implantation at an acceleration energy of 140 keV. It was found that the RTA treatment was needed at 400–4508C for 10 to 30 s to recover the base resistance. However, it is not proper to anneal at 4508C because optimum annealing temperature of good ohmic contacts is ranged from 3758C to 4008C in AlGaAsrGaAs HBT
533
fabrication. Therefore, it is reasonable that RTA should be performed at 4008C for longer than 10 s. In conclusion, reduction of the extrinsic base–collector junction capacitance has been tried by Heq ion implantation as a way to improve the microwave characteristics of AlGaAsrGaAs HBTs. Through the investigation into base resistance variation with Heq ion implantation and subsequent RTA treatment, it was found that ion energy of 140 keV and ion dose of 5 = 10 12 –1 = 10 13 cmy2 were required to apply to conventional AlGaAsrGaAs HBT structure, and that RTA process was needed at 4008C for 20 s for base resistance recovery.
References w1x K. Runge, D. Daniel, R.D. Standley, J.L. Gimlett, R.B. Nubling, R.L. Pierson, S.M. Beccue, K.C. Wang, N.H. Sheng, M.C.F. Chang, D.M. Chen, P.M. Asbeck, IEEE J. Solid-State Circuits 27 Ž1992. 1332. w2x M. Nakamura, Y. Imai, E. Sano, Y. Yamaguchi, O. Nakajima, IEEE J. Solid-State Circuits 27 Ž1992. 1421. w3x N. Nagano, T. Suzaki, M. Soda, K. Kasahara, K. Honjo, IEICE Trans. Electron. E76-C Ž1993. 883. w4x K. Hohkawa, S. Matsuoka, K. Hagimoto, K. Nakagawa, IEICE Trans. Electron. E76-C Ž1993. 68. w5x C.E. Chang, P.M. Asbeck, L.T. Tran, D.C. Streit, A.K. Oki, Proc. IEEE Int. Electron Devices Meet., 1993, p. 795. w6x H. Kroemer, Jpn. J. Appl. Phys. 20 Ž1981. 9. w7x M.Z. Martin, F.K. Oshita, M. Matloubian, H.R. Fetterman, W.J. Ho, N.L. Wang, F. Chang, D. Cheng, J. Appl. Phys. 76 Ž1994. 3874. w8x P.M. Asbeck, M.F. Chang, J.A. Higgins, N.H. Sheng, G.J. Sullivan, K.C. Wang, IEEE Trans. Electron Devices 36 Ž1989. 2032. w9x R.B. Nubling, N.H. Sheng, K.C. Wang, M.F. Chang, W.J. Ho, G.J. Sullivan, C.W. Farley, P.M. Asbeck, IEEE GaAs IC Symp. 1989, p. 125. w10x Y. Matsuoka, S. Yamahata, M. Nakatsugawa, M. Muraguchi, T. Ishibashi, Electron. Lett. 29 Ž1993. 982. w11x J.F. Ziegler, Handbook of Ion Implantation Technology, North-Holland, Amsterdam, 1992.
Materials Letters 51 Ž2001. 529–533 www.elsevier.comrlocatermatlet
Base resistance variation of AlGaAsrGaAs HBT by Heq ion implantation Il-Ho Kim Department of Materials Science and Engineering, Chungju National UniÕersity, 123 Komdan-ri, Iryu-myon, Chungju, Chungbuk 380-702, South Korea Received 12 October 2000; received in revised form 29 March 2001; accepted 30 March 2001
Abstract Reduction of base–collector junction capacitance has been intended by Heq ion implantation to enhance the maximum oscillation frequency of AlGaAsrGaAs heterojunction bipolar transistors ŽHBTs.. From the recovery of the base resistance by rapid thermal annealing ŽRTA. after ion implantation, it is found that optimum conditions are 140 keV and 5 = 10 12 –1 = 10 13 cmy2 for Heq ion implantation and 4008C and 20 s for rapid thermal annealing. q 2001 Elsevier Science B.V. All rights reserved. PACS: 72.80.Ga; 73.40.Cg; 61.72.Vv Keywords: Ion implantation; Helium ion; Gallium arsenide; Heterojunction bipolar transistor
AlGaAsrGaAs heterojunction bipolar transistors ŽHBTs. have been extensively investigated as promising electronic devices for high-speed digital and microwave circuits, such as monolithic microwave integrated circuit ŽMMIC. and optoelectronic integrated circuit ŽOEIC. w1,2x. They have been oriented toward communication systems that include high bit rate optical fiber transmission systems for the broadband integrated services digital networks ŽISDN.. Considerable attention has been also focused on HBTs in high-power microwave applications because of their inherent features, such as high-power handling capacity per unit chip area, ease of design for high breakdown voltage, and the class of operation with high efficiency. High-frequency microwave
E-mail address: [email protected] ŽI.-H. Kim..
circuits require high performance active devices since they determine the circuit performance. Such a high potential for superior microwave performance of HBTs has been limited by the fabrication technology that results to large extrinsic parasitics, such as extrinsic resistances, contact resistances and junction capacitances. However, these drawbacks could be overcome by advanced technologies. The extrinsic resistances may be greatly reduced by the self-aligned technology w3,4x. Extensive researches of various material systems have been performed to make good ohmic contacts w5,6x. The extrinsic junction capacitances can be reduced by proton or boron ion implantation that includes damage to compensate the donors in the collector region w7x. The maximum oscillation frequency Ž f max . is one of the important figures of merit for microwave
00167-577Xr01r$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 5 7 7 X Ž 0 1 . 0 0 3 5 0 - 0
530
I.-H. Kim r Materials Letters 51 (2001) 529–533
device performance. High values of f max that are suitable for millimeter-wave applications have been already demonstrated in GaAs-base HBTs w8–10x. In these devices, high f max is achieved primarily by reducing the base resistance and base–collector capacitance. A variety of approaches has been pursued for the maximization of f max . An essential issue is to eliminate the capacitance associated with the extrinsic base–collector junction, i.e. the area underneath the base contacts in the conventional HBT structure. Deep-implanted HBTs make use of ion implantation damage to render the collector and subcollector semi-insulating in the region underneath the base contact, thereby reducing the extrinsic base–collector junction capacitance Ž C bc .. Helium ion ŽHeq. can be deeply implanted with lower acceleration energy than boron ion ŽBq. or oxygen ion ŽOq. , and its propagation depth can be more easily controlled compared with proton ion ŽHq. . Moreover, protective layers such as Si 3 N4 or SiO 2 films are not necessarily in the rapid thermal annealing ŽRTA. process for the resistance recovery of the base damaged by ion implantation because post-implant annealing can be performed at low temperature. Therefore, Heq is most suitable for reducing C bc Žor increasing f max . of AlGaAsrGaAs HBTs by ion implantation. Two conditions should be satisfied to increase f max of HBTs by Heq ion implantation. First, suffi-
Fig. 1. Epitaxial layer structure grown by MOCVD for AlGaAsr GaAs HBT Žemitter layers are eliminated for ion implantation..
Fig. 2. Heq ion implanted region in AlGaAsrGaAs HBT ŽEM, emitter metal; BM, base metal; CM, collector metal..
cient ion dose and adequate ion energy are required to make the collector and subcollector semi-insulating. Secondly, the resistance of the base that sustains implantation damages simultaneously should be recovered by proper annealing treatment. In this study, these two Žimplantation and annealing. conditions are investigated and optimized for conventional AlGaAsrGaAs HBTs by Monte Carlo simulation and Heq ion implantation. Monte Carlo simulation program—transport of ions in matter ŽTRIM. w11x —was used to calculate the Heq ion distribution with implantation depth and the absorption energy by collision between implanted ions and matrix atoms. Ion implantation energy and dose for application to AlGaAsrGaAs HBTs were determined from simulation results. To avoid the channeling effect, Heq ions were implanted at an angle of 78 with the specimen at room temperature. Rapid thermal annealing ŽRTA. treatment was carried out at 350–4508C from 10 to 30 s to examine the variation of electrical properties with post-implant annealing. Fig. 1 shows the epitaxial layer structure grown by metal organic chemical vapor deposition ŽMOCVD. for AlGaAsrGaAs HBT, where emitter layers are eliminated for ion implantation. The base sheet resistance was measured by the transmission line model ŽTLM. method. After ultrasonic cleaning wafer with acetone, isopropylalcohol ŽIPA. and deionized ŽDI. water for 5 min, respectively, TLM patterns were made on the base surface layer by photolithography work using AZ5214-E photoresist and NMD-3 developer. Residual photoresist was removed by plasma dry stripping and HCl cleaning.
I.-H. Kim r Materials Letters 51 (2001) 529–533
Fig. 3. Damage distribution caused by Heq ion implantation ŽTRIM simulation..
531
532
I.-H. Kim r Materials Letters 51 (2001) 529–533
Fig. 4. Sheet resistance of the base with Heq ion implantation energy and dose.
˚ .rTi Ž300 Base ohmic contact metals of Pt Ž50 A ˚ .rPt Ž300 A˚ .rAu Ž800 A˚ . were deposited by A electron beam deposition and lift-off process. Heat treatment was not performed after deposition of
ohmic contact metals to exclude the temperature dependence of ohmic contact properties. Fig. 2 illustrates the vertical cross-sectional view of AlGaAsr GaAs HBT, where the Heq ion implanted region is
Fig. 5. Sheet resistance of the base with annealing condition after Heq ion implantation.
I.-H. Kim r Materials Letters 51 (2001) 529–533
indicated. In Fig. 2, the base metal ŽBM. should be deposited after ion implantation to suppress an increase in the contact resistance between the base surface layer and base metal by ion implantation. The damage distribution estimated with TRIM simulation is shown in Fig. 3, where straight lines in the base, collector and subcollector regions show the doping levels. Ion energy was changed from 100 to 200 keV and ion dose was varied from 1 = 10 12 to 5 = 10 13 cmy2 . Vacancy Ždamage. concentration increased with increasing ion dose at constant ion energy. Implantation damage have to reach to a depth greater than 1 mm through the collector and subcollector regions. It was found that ion dose higher than 5 = 10 12 cmy2 and ion energy of about 150 keV are required to make collector and subcollector semi-insulating Žisolating.. In this study, ion implantation was carried out at an acceleration energy of 140 keV with ion doses of 5 = 10 12 and 1 = 10 13 cmy2 . Fig. 4 shows the variation of the base sheet resistance with Heq ion implantation energy and dose. As expected, the base sheet resistance is increased by ion implantation, and it increased considerably with increasing ion dose. This results from larger lattice damage with increasing ion dose. It was also known that when the ion energy was lowered, the base resistance increased remarkably. This is why the base sustains larger lattice damage due to shallower penetration Žimplantation. depth for lower ion energy. Fig. 5 indicates the variation of the base sheet resistance with RTA after Heq ion implantation at an acceleration energy of 140 keV. It was found that the RTA treatment was needed at 400–4508C for 10 to 30 s to recover the base resistance. However, it is not proper to anneal at 4508C because optimum annealing temperature of good ohmic contacts is ranged from 3758C to 4008C in AlGaAsrGaAs HBT
533
fabrication. Therefore, it is reasonable that RTA should be performed at 4008C for longer than 10 s. In conclusion, reduction of the extrinsic base–collector junction capacitance has been tried by Heq ion implantation as a way to improve the microwave characteristics of AlGaAsrGaAs HBTs. Through the investigation into base resistance variation with Heq ion implantation and subsequent RTA treatment, it was found that ion energy of 140 keV and ion dose of 5 = 10 12 –1 = 10 13 cmy2 were required to apply to conventional AlGaAsrGaAs HBT structure, and that RTA process was needed at 4008C for 20 s for base resistance recovery.
References w1x K. Runge, D. Daniel, R.D. Standley, J.L. Gimlett, R.B. Nubling, R.L. Pierson, S.M. Beccue, K.C. Wang, N.H. Sheng, M.C.F. Chang, D.M. Chen, P.M. Asbeck, IEEE J. Solid-State Circuits 27 Ž1992. 1332. w2x M. Nakamura, Y. Imai, E. Sano, Y. Yamaguchi, O. Nakajima, IEEE J. Solid-State Circuits 27 Ž1992. 1421. w3x N. Nagano, T. Suzaki, M. Soda, K. Kasahara, K. Honjo, IEICE Trans. Electron. E76-C Ž1993. 883. w4x K. Hohkawa, S. Matsuoka, K. Hagimoto, K. Nakagawa, IEICE Trans. Electron. E76-C Ž1993. 68. w5x C.E. Chang, P.M. Asbeck, L.T. Tran, D.C. Streit, A.K. Oki, Proc. IEEE Int. Electron Devices Meet., 1993, p. 795. w6x H. Kroemer, Jpn. J. Appl. Phys. 20 Ž1981. 9. w7x M.Z. Martin, F.K. Oshita, M. Matloubian, H.R. Fetterman, W.J. Ho, N.L. Wang, F. Chang, D. Cheng, J. Appl. Phys. 76 Ž1994. 3874. w8x P.M. Asbeck, M.F. Chang, J.A. Higgins, N.H. Sheng, G.J. Sullivan, K.C. Wang, IEEE Trans. Electron Devices 36 Ž1989. 2032. w9x R.B. Nubling, N.H. Sheng, K.C. Wang, M.F. Chang, W.J. Ho, G.J. Sullivan, C.W. Farley, P.M. Asbeck, IEEE GaAs IC Symp. 1989, p. 125. w10x Y. Matsuoka, S. Yamahata, M. Nakatsugawa, M. Muraguchi, T. Ishibashi, Electron. Lett. 29 Ž1993. 982. w11x J.F. Ziegler, Handbook of Ion Implantation Technology, North-Holland, Amsterdam, 1992.