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A plasma immersion implantation system for materials modification
Surface and Coatings Technology 136 Ž2001. 138᎐141
A plasma immersion implantation system for materials modification Michael I. Current a,U , Wei Liu a , Ian S. Rotha , Albert J. Lamma , William G. Ena , Igor J. Malik a , Lucia Feng a , Michael A. Bryan a , Shu Qin a , Francois J. Henley a , Chung Chan b, Nathan W. Cheung c b
a Silicon Genesis, Campbell, CA, USA Northeastern Uni¨ ersity, Boston, MA, USA c UC Berkeley, Berkeley, CA, USA
Abstract A plasma immersion ion implantation ŽPIII. system is described which provides the capability to bridge the range between research exploration and commercial applications for materials modification of electronic materials, with a particular focus on layer transfer processes. The Silicon Genesis PIII system is capable of operation at high plasma densities Žf 5 = 10 11 ionsrcm3 at the wafer. with high purity, mono-species ionization Ž) 99% Hq ions with a hydrogen plasma.. The first generation of Silicon Genesis PIII systems is equipped to use 200-mm wafers Žthrough an automated loadlock. and pulsed potentials up to 50 kV. Use of the mono-species ionization characteristic of the Silicon Genesis PIII system provides the capability to precisely vary the characteristics of surface layers through implantation of atoms and damage creation at well-controlled depths in the materials of choice. The Silicon Genesis PIII system is designed for efficient production of SOI and other layer transfer-generated materials and can be adapted for materials modification of more complex structures and work pieces. 䊚 2001 Elsevier Science B.V. All rights reserved. Keywords: Plasma immersion ion implantation; Hydrogen plasma; Layer transfer; Silicon-on-insulator wafers; Protonic mode
1. Introduction: materials modification applications Plasma immersion ion implantation ŽPIII., after more than a decade of research and development, is rapidly maturing towards the level of use for practical applications w1x. Although doping of shallow junctions in CMOS transistors has received the most attention w2x, applications for materials modification of surfaces, such as hydrogenation of amorphous-Si layers w3x, direct implantation of oxygen for formation of buried oxide layers w4x are also well suited for the capabilities of
U
Corresponding author. Silicon Genesis, 590 Division Street, Campbell, CA 95008 USA. Tel.: q1-408-871-3082; fax: q1-408-8718607. E-mail address: [email protected] ŽM.I. Current..
plasma immersion systems. The development of PIII for layer transfer methods of fabrication of silicon-oninsulator wafers w5x opens another promising application. A survey of the typical ion dose and wafer bias voltage for PIII is shown in Fig. 1. 2. System design The initial Silicon Genesis PIII tool described here is a ‘stand alone’ Žcompared to a ‘clustered’. chamber equipped to operate at wafer pulse voltages up to 50 kV and fixtured to handle 200-mm wafers. As such, this tool is named the SA-50r200. The chamber is scaled to operate with pulses up to 100 kV at plasma densities above 10 11 ionsrcm3 and to be fixtured for 300-mm wafers without major design changes. The core of the
0257-8972r01r$ - see front matter 䊚 2001 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 7 - 8 9 7 2 Ž 0 0 . 0 1 0 4 3 - 4
M.I. Current et al. r Surface and Coatings Technology 136 (2001) 138᎐141
139
Fig. 2. Photo of the plasma chamber, control electronics, loadlock and wafer transport stage for the SiGen PIII system. Fig. 1. Ion dose and wafer bias conditions for applications of PIII for electronic materials and devices.
system, the process chamber and control electronics is a compact design that is 1.07 m wide, 1.6 m deep and 1.6 m high. The wafer pulse and RF plasma excitation power systems are remote stand-alone units, as are the optional power and water cooling control panels. A cassette loadlock and wafer handler was integrated into the system to allow for fully automated, recipe-driven operation to facilitate an efficient transfer of processes from research to pilot line production. The overall system design was driven towards the goals of design flexibility through the use of modular components. The plasma chamber is ‘oversized’ for many of the expected applications in order to allow for fitting of a wide range of plasma diagnostic tools. The plasma chamber, control panel and 200-mm wafer handling components of the SA-50r200 system are shown in Fig. 2.
3. Operational characteristics The plasma is energized with a RF power supply at 13.56 MHz. By appropriate application of confining magnetic fields, the power match to the plasma can be tuned so that plasma densities in excess of 10 11 ionsrcm3 can be obtained with a hydrogen discharge in the 0.133-Pa range at plasma power of the order of 5 kW. The wafer bias can be pulsed with potentials up to 50 kV over time periods of 5᎐50 s. The pulser used in this system can be operated at repetition rates of up to 200 Hz with a peak current of the order of 500 A. A typical net current pulse from the pulser, a 19-s 40-kV pulse with a 300-A peak as the plasma sheath is stabilizing, is shown in Fig. 3. This measured net current is a combination of the ion current implanted into
the wafer, secondary electrons from the wafer surface and assorted inductive effects w1,6,7x.
4. Protonic Mode TM For layer-transfer applications, it is often critical to have a buried implant profile with a known peak concentration at a well-controlled depth. This is in contrast to the profile control requirements for doping of shallow junctions, where a useful profile can be one with its peak concentration at or near the implanted surface plane with a monotonic decrease of implanted atom concentration with increasing depth into the target. In the ‘surface doping’ process, a wide mix of ion charge states, molecular fractions and ion acceleration potentials can be tolerated as long as the total net profile is shallower than the desired final junction depth. To achieve a buried profile, the PIII ion flux must have many of the characteristics of a conventional implantation ion beam; single charge state, single-form
Fig. 3. Net drain current on the pulser for a 19-s pulse.
140
M.I. Current et al. r Surface and Coatings Technology 136 (2001) 138᎐141
molecular composition and a well-defined acceleration potential. The single charge state and single-form molecular composition can be achieved for a hydrogen plasma by breaking a large number of hydrogen molecular bonds and stripping electrons from a large fraction of the single hydrogen atoms, while only weakly ionizing the hydrogen molecular population. When these conditions are achieved in the SA-50r200 plasma chamber, the fraction of Hq ions exceeds 99.9% and the plasma density at the wafer surface is f 5 = 10 11 ionsrcm3. Plasma operation with this high fraction of Hq ions, referred to as Protonic ModeTM , is highly stable and characterized by a strong ruby plasma color. A mass spectra sampled near the wafer location of the ionized hydrogenic species during Protonic ModeTM operation is shown in Fig. 4. Another requirement for the use of PIII for formation of buried layers is a stable, well-defined acceleration potential. This is achieved by operating so that a large fraction of the implanted ions are accelerated across a steady-state plasma sheath. For hydrogen plasma densities of 10 11 ionsrcm3 and a pulse potential of 35 kV, the time to establish steady-state sheath conditions is under 0.5 s. By using a wafer bias pulse of f 20 s, steady-state sheath conditions are obtained for greater than 97% of the pulse cycle. The typical current pulse during the growth of the plasma sheath is twice the steady-state current, so that the fraction of ions with a mix of acceleration energies is approximately f 5% under these conditions. The measurement of hydrogenic atomic profiles in Si by secondary ion mass spectroscopy, SIMS, is complicated by numerous ionization and diffusion effects w8x. However, the general nature of a buried hydrogen implant from PIII operated in the Protonic ModeTM is clearly seen in Fig. 5 for a 40-keV implant into Si
Fig. 5. SIMS profile of hydrogen after PIII implantation at 40 keV into Si though a 40-nm surface oxide at a dose of 6.7= 10 16 Hrcm2 .
through a 40-nm surface oxide. The high ionization rate of hydrogen in SiO 2 is evident as well as the segregation of hydrogen at the SiO 2rSi interface and the ledge at mid-range attributed to the fraction of implanted Hq 2. The hydrogen atomic profile closely follows the calculated Si lattice damage distribution and not the ion range distribution calculated by methods such as the Monte-Carlo code, TRIM. This effect, observed many times for hydrogen implantation into electronic materials w9x, indicates the dynamic character of the behavior of implanted hydrogen, with significant hydrogen diffusion occurring during and after the implantation and strong chemical interactions at surfaces and buried interfaces.
5. Layer transfer for SOI wafer fabrication A number of approaches have been successfully used to separate thin, wafer-scale layers of device quality Si and to bond them with Si and other materials to form a variety of SOI wafers w5,10,11x. While using a significantly different set of procedures and mechanisms than earlier methods w11x, the Silicon Genesis PIII system, operated in the Protonic ModeTM , has been used to fabricate a variety of SOI structures. The device-Si layer thickness control on these wafers is within f 3% for layers ranging from 100 to 250 nm. An example of an optical thickness map of a device-Si layer bonded to form a 200-mm SOI wafer is shown in Fig. 6.
Fig. 4. Mass spectrum of the ionized hydrogenic species sampled near the wafer position with the SiGen PIII operated in the Protonic ModeTM .
6. Summary Layer transfer methods for formation of heteroge-
M.I. Current et al. r Surface and Coatings Technology 136 (2001) 138᎐141
141
surface coatings for tribology improvements, doping of non-planar transistor and memory device structures and fabrication of MEMS devices, are under investigation at Silicon Genesis and other PIII development centers. References
Fig. 6. An optical thickness map of a device-Si layer transferred following PIII processing to form a 200-mm SOI wafer. The mean thickness of the Si layer was 209.2 nm with a total thickness variation over the wafer of 5.9 nm. The optical map was assembled from a sample of 3000 locations across the wafer.
neous multilayer structures on Si and other electronic materials are a new and important application of PIII techniques. The Silicon Genesis SA-50r200 is the first PIII system to be specifically developed for these applications. Other applications, such as direct implantation of oxygen to create buried oxide layers, formation of
w1x P.K. Chu, S. Qin, C. Chan, N.W. Cheung, L.A. Larson, Mater. Sci. Eng. R17 Ž1996. 207᎐280. w2x P.K. Chu, S.B. Felch, P. Kellerman, F. Sinclair, L.A. Larson, B. Mizuno, Solid State Technol. 42 Ž9. Ž1999. 55᎐60. w3x S. Qin, Y. Zhou, T. Nakatsugawa, C. Chan, Nucl. Instrum. Methods B124 Ž1997. 69᎐75. w4x J. Min, P.K. Chu, Y.C. Cheng et al., Mater. Chem. Phys. 40 Ž1995. 219᎐222. w5x I.S. Roth, M.A. Bryan, W. Liu, S. Qin, A.J. Lamm, C. Chan, Electrochem. Soc. Proc. 99-3 Ž1999. 107᎐110. w6x S. Qin, Z. Jin, C. Chan, Nucl. Instrum. Methods B114 Ž1996. 288᎐292. w7x W. En, N.W. Cheung, IEEE Trans. Plasma Sci. 24 Ž3. Ž1996. 1184᎐1187. w8x R.M. Wallace, P.J. Chen, L.B. Archer, J.M. Anthony, J. Vac. Sci. Technol. B17 Ž5. Ž1999. 2153᎐2162. w9x C. Ascheron, H. Bartsch, A. Setzer, A. Schindler, P. Paufler, Nucl. Instrum. Methods B28 Ž1987. 350᎐359. w10x R. DeJule, Semicond. Int. 22 Ž13. Ž1999. 67᎐74. w11x M. Bruel, Nucl. Instrum. Methods B108 Ž1996. 313᎐319.
A plasma immersion implantation system for materials modification Michael I. Current a,U , Wei Liu a , Ian S. Rotha , Albert J. Lamma , William G. Ena , Igor J. Malik a , Lucia Feng a , Michael A. Bryan a , Shu Qin a , Francois J. Henley a , Chung Chan b, Nathan W. Cheung c b
a Silicon Genesis, Campbell, CA, USA Northeastern Uni¨ ersity, Boston, MA, USA c UC Berkeley, Berkeley, CA, USA
Abstract A plasma immersion ion implantation ŽPIII. system is described which provides the capability to bridge the range between research exploration and commercial applications for materials modification of electronic materials, with a particular focus on layer transfer processes. The Silicon Genesis PIII system is capable of operation at high plasma densities Žf 5 = 10 11 ionsrcm3 at the wafer. with high purity, mono-species ionization Ž) 99% Hq ions with a hydrogen plasma.. The first generation of Silicon Genesis PIII systems is equipped to use 200-mm wafers Žthrough an automated loadlock. and pulsed potentials up to 50 kV. Use of the mono-species ionization characteristic of the Silicon Genesis PIII system provides the capability to precisely vary the characteristics of surface layers through implantation of atoms and damage creation at well-controlled depths in the materials of choice. The Silicon Genesis PIII system is designed for efficient production of SOI and other layer transfer-generated materials and can be adapted for materials modification of more complex structures and work pieces. 䊚 2001 Elsevier Science B.V. All rights reserved. Keywords: Plasma immersion ion implantation; Hydrogen plasma; Layer transfer; Silicon-on-insulator wafers; Protonic mode
1. Introduction: materials modification applications Plasma immersion ion implantation ŽPIII., after more than a decade of research and development, is rapidly maturing towards the level of use for practical applications w1x. Although doping of shallow junctions in CMOS transistors has received the most attention w2x, applications for materials modification of surfaces, such as hydrogenation of amorphous-Si layers w3x, direct implantation of oxygen for formation of buried oxide layers w4x are also well suited for the capabilities of
U
Corresponding author. Silicon Genesis, 590 Division Street, Campbell, CA 95008 USA. Tel.: q1-408-871-3082; fax: q1-408-8718607. E-mail address: [email protected] ŽM.I. Current..
plasma immersion systems. The development of PIII for layer transfer methods of fabrication of silicon-oninsulator wafers w5x opens another promising application. A survey of the typical ion dose and wafer bias voltage for PIII is shown in Fig. 1. 2. System design The initial Silicon Genesis PIII tool described here is a ‘stand alone’ Žcompared to a ‘clustered’. chamber equipped to operate at wafer pulse voltages up to 50 kV and fixtured to handle 200-mm wafers. As such, this tool is named the SA-50r200. The chamber is scaled to operate with pulses up to 100 kV at plasma densities above 10 11 ionsrcm3 and to be fixtured for 300-mm wafers without major design changes. The core of the
0257-8972r01r$ - see front matter 䊚 2001 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 7 - 8 9 7 2 Ž 0 0 . 0 1 0 4 3 - 4
M.I. Current et al. r Surface and Coatings Technology 136 (2001) 138᎐141
139
Fig. 2. Photo of the plasma chamber, control electronics, loadlock and wafer transport stage for the SiGen PIII system. Fig. 1. Ion dose and wafer bias conditions for applications of PIII for electronic materials and devices.
system, the process chamber and control electronics is a compact design that is 1.07 m wide, 1.6 m deep and 1.6 m high. The wafer pulse and RF plasma excitation power systems are remote stand-alone units, as are the optional power and water cooling control panels. A cassette loadlock and wafer handler was integrated into the system to allow for fully automated, recipe-driven operation to facilitate an efficient transfer of processes from research to pilot line production. The overall system design was driven towards the goals of design flexibility through the use of modular components. The plasma chamber is ‘oversized’ for many of the expected applications in order to allow for fitting of a wide range of plasma diagnostic tools. The plasma chamber, control panel and 200-mm wafer handling components of the SA-50r200 system are shown in Fig. 2.
3. Operational characteristics The plasma is energized with a RF power supply at 13.56 MHz. By appropriate application of confining magnetic fields, the power match to the plasma can be tuned so that plasma densities in excess of 10 11 ionsrcm3 can be obtained with a hydrogen discharge in the 0.133-Pa range at plasma power of the order of 5 kW. The wafer bias can be pulsed with potentials up to 50 kV over time periods of 5᎐50 s. The pulser used in this system can be operated at repetition rates of up to 200 Hz with a peak current of the order of 500 A. A typical net current pulse from the pulser, a 19-s 40-kV pulse with a 300-A peak as the plasma sheath is stabilizing, is shown in Fig. 3. This measured net current is a combination of the ion current implanted into
the wafer, secondary electrons from the wafer surface and assorted inductive effects w1,6,7x.
4. Protonic Mode TM For layer-transfer applications, it is often critical to have a buried implant profile with a known peak concentration at a well-controlled depth. This is in contrast to the profile control requirements for doping of shallow junctions, where a useful profile can be one with its peak concentration at or near the implanted surface plane with a monotonic decrease of implanted atom concentration with increasing depth into the target. In the ‘surface doping’ process, a wide mix of ion charge states, molecular fractions and ion acceleration potentials can be tolerated as long as the total net profile is shallower than the desired final junction depth. To achieve a buried profile, the PIII ion flux must have many of the characteristics of a conventional implantation ion beam; single charge state, single-form
Fig. 3. Net drain current on the pulser for a 19-s pulse.
140
M.I. Current et al. r Surface and Coatings Technology 136 (2001) 138᎐141
molecular composition and a well-defined acceleration potential. The single charge state and single-form molecular composition can be achieved for a hydrogen plasma by breaking a large number of hydrogen molecular bonds and stripping electrons from a large fraction of the single hydrogen atoms, while only weakly ionizing the hydrogen molecular population. When these conditions are achieved in the SA-50r200 plasma chamber, the fraction of Hq ions exceeds 99.9% and the plasma density at the wafer surface is f 5 = 10 11 ionsrcm3. Plasma operation with this high fraction of Hq ions, referred to as Protonic ModeTM , is highly stable and characterized by a strong ruby plasma color. A mass spectra sampled near the wafer location of the ionized hydrogenic species during Protonic ModeTM operation is shown in Fig. 4. Another requirement for the use of PIII for formation of buried layers is a stable, well-defined acceleration potential. This is achieved by operating so that a large fraction of the implanted ions are accelerated across a steady-state plasma sheath. For hydrogen plasma densities of 10 11 ionsrcm3 and a pulse potential of 35 kV, the time to establish steady-state sheath conditions is under 0.5 s. By using a wafer bias pulse of f 20 s, steady-state sheath conditions are obtained for greater than 97% of the pulse cycle. The typical current pulse during the growth of the plasma sheath is twice the steady-state current, so that the fraction of ions with a mix of acceleration energies is approximately f 5% under these conditions. The measurement of hydrogenic atomic profiles in Si by secondary ion mass spectroscopy, SIMS, is complicated by numerous ionization and diffusion effects w8x. However, the general nature of a buried hydrogen implant from PIII operated in the Protonic ModeTM is clearly seen in Fig. 5 for a 40-keV implant into Si
Fig. 5. SIMS profile of hydrogen after PIII implantation at 40 keV into Si though a 40-nm surface oxide at a dose of 6.7= 10 16 Hrcm2 .
through a 40-nm surface oxide. The high ionization rate of hydrogen in SiO 2 is evident as well as the segregation of hydrogen at the SiO 2rSi interface and the ledge at mid-range attributed to the fraction of implanted Hq 2. The hydrogen atomic profile closely follows the calculated Si lattice damage distribution and not the ion range distribution calculated by methods such as the Monte-Carlo code, TRIM. This effect, observed many times for hydrogen implantation into electronic materials w9x, indicates the dynamic character of the behavior of implanted hydrogen, with significant hydrogen diffusion occurring during and after the implantation and strong chemical interactions at surfaces and buried interfaces.
5. Layer transfer for SOI wafer fabrication A number of approaches have been successfully used to separate thin, wafer-scale layers of device quality Si and to bond them with Si and other materials to form a variety of SOI wafers w5,10,11x. While using a significantly different set of procedures and mechanisms than earlier methods w11x, the Silicon Genesis PIII system, operated in the Protonic ModeTM , has been used to fabricate a variety of SOI structures. The device-Si layer thickness control on these wafers is within f 3% for layers ranging from 100 to 250 nm. An example of an optical thickness map of a device-Si layer bonded to form a 200-mm SOI wafer is shown in Fig. 6.
Fig. 4. Mass spectrum of the ionized hydrogenic species sampled near the wafer position with the SiGen PIII operated in the Protonic ModeTM .
6. Summary Layer transfer methods for formation of heteroge-
M.I. Current et al. r Surface and Coatings Technology 136 (2001) 138᎐141
141
surface coatings for tribology improvements, doping of non-planar transistor and memory device structures and fabrication of MEMS devices, are under investigation at Silicon Genesis and other PIII development centers. References
Fig. 6. An optical thickness map of a device-Si layer transferred following PIII processing to form a 200-mm SOI wafer. The mean thickness of the Si layer was 209.2 nm with a total thickness variation over the wafer of 5.9 nm. The optical map was assembled from a sample of 3000 locations across the wafer.
neous multilayer structures on Si and other electronic materials are a new and important application of PIII techniques. The Silicon Genesis SA-50r200 is the first PIII system to be specifically developed for these applications. Other applications, such as direct implantation of oxygen to create buried oxide layers, formation of
w1x P.K. Chu, S. Qin, C. Chan, N.W. Cheung, L.A. Larson, Mater. Sci. Eng. R17 Ž1996. 207᎐280. w2x P.K. Chu, S.B. Felch, P. Kellerman, F. Sinclair, L.A. Larson, B. Mizuno, Solid State Technol. 42 Ž9. Ž1999. 55᎐60. w3x S. Qin, Y. Zhou, T. Nakatsugawa, C. Chan, Nucl. Instrum. Methods B124 Ž1997. 69᎐75. w4x J. Min, P.K. Chu, Y.C. Cheng et al., Mater. Chem. Phys. 40 Ž1995. 219᎐222. w5x I.S. Roth, M.A. Bryan, W. Liu, S. Qin, A.J. Lamm, C. Chan, Electrochem. Soc. Proc. 99-3 Ž1999. 107᎐110. w6x S. Qin, Z. Jin, C. Chan, Nucl. Instrum. Methods B114 Ž1996. 288᎐292. w7x W. En, N.W. Cheung, IEEE Trans. Plasma Sci. 24 Ž3. Ž1996. 1184᎐1187. w8x R.M. Wallace, P.J. Chen, L.B. Archer, J.M. Anthony, J. Vac. Sci. Technol. B17 Ž5. Ž1999. 2153᎐2162. w9x C. Ascheron, H. Bartsch, A. Setzer, A. Schindler, P. Paufler, Nucl. Instrum. Methods B28 Ž1987. 350᎐359. w10x R. DeJule, Semicond. Int. 22 Ž13. Ž1999. 67᎐74. w11x M. Bruel, Nucl. Instrum. Methods B108 Ž1996. 313᎐319.