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On the mechanism of ion-implanted As diffusion in relaxed SiGe
Applied Surface Science 224 (2004) 59–62
On the mechanism of ion-implanted As diffusion in relaxed SiGe S. Eguchia,*, J.J. Leeb, S.J. Rheeb, D.L. Kwongb, M.L. Leec, ˚ bergd, J.L. Hoytd E.A. Fitzgeraldc, I. A a
Semiconductor and Integrated Circuit Group, Hitachi Ltd., Ext. T07, Hitachi Ltd., 111 Nishiyokote, Takasaki, Gumma, 370-0021, Japan b Microelectronics Research Center, University of Texas at Austin, Austin, TX 78712, USA c Materials Science and Engineering Department, MIT, Cambridge, MA 02139, USA d Microsystems Technology Laboratories, EECS, MIT, Cambridge, MA 02139, USA
Abstract The diffusion behavior of ion-implanted arsenic in strained Si/relaxed Si0.8Ge0.2 structures is studied during rapid thermal processing in oxygen (interstitial injection) and ammonia (vacancy injection). During rapid thermal processing in an oxygen ambient, arsenic diffusion in SiGe is reduced, while an ammonia ambient produces strong enhancement, especially for short times. The results suggest that arsenic diffusion in SiGe has a stronger vacancy component than arsenic diffusion in Si. Vacancy injection is also observed to enhance the diffusivity of Ge from relaxed SiGe into strained Si. # 2003 Published by Elsevier B.V. PACS: 61.71Ji; 61.72.Tt Keywords: Silicon Germanium; Arsenic; Diffusion
1. Introduction High performance strained Si/relaxed SiGe MOSFETs require the fabrication of shallow source/ drain junctions. The formation of nþp junctions suffers from the dramatically enhanced diffusion of arsenic in SiGe [1,2]. In this paper, we focus on arsenic (As) diffusion during rapid thermal processing in nitrogen, oxygen, and ammonia. To gain insight into the diffusion mechanism for As in SiGe oxidation and nitridation of a top strained Si layer are employed to inject interstitials and vacancies into the underlying relaxed *
Corresponding author. Tel.: þ81-27-360-2238; fax: þ81-27-360-2166. E-mail address: [email protected] (S. Eguchi). 0169-4332/$ – see front matter # 2003 Published by Elsevier B.V. doi:10.1016/j.apsusc.2003.08.029
SiGe layer. High dose As implants, typical of nþp source/drains, are used to investigate the possibility of controlling diffusion during source/drain formation using annealing ambient. The effective diffusivity of As in Si and SiGe is extracted by fitting secondary ion mass spectrometry (SIMS) data to simulated As profiles using the TSUPREM-4 process simulator. The As diffusion model in Si used in TSUPREM-4 is described in detail in reference [3]. We apply the same model to As diffusion in SiGe under equilibrium conditions n 0 DAs ðSiGeÞ ¼ g f D þ D As i eff i i ni ðSiGeÞ n þ fv D0v þ D (1) v ni ðSiGeÞ
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where n is the electron concentration. D0i and D i represent the contributions from neutral and singlenegatively charged interstitial defects, respectively. D0v and D v represent the contributions from neutral and single-negatively charged vacancy defects, and the factors fi and fv represent the fractions of interstitial and vacancy mediated diffusion, respectively. In Eq. (1) the intrinsic carrier concentration in SiGe (niðSiGeÞ), is given by DEg ni ðSiGeÞ ¼ ni ðSiÞ exp (2) 2kT where the DEg between Si and relaxed SiGe (Ge 20%) is roughly 0.08 eV [4]. The diffusion coefficients (D0i , D i , etc.) used in Eq. (1) are the same values used in Si. In this work, the parameter gAs is varied in the simulations to fit the observed SIMS profiles in SiGe. Using the extracted enhancement factors gAs and ni(SiGe), the effective diffusivities for As in SiGe under equilibrium conditions can be evaluated at any given value of n using Eq. (1).
temperature range from 950 to 1050 8C. The oxide layers were then stripped with dilute HF. For the RTN and RTO experiments, strained Si/relaxed SiGe (Si/SiGe) and CZ samples were capped with LTO and a 150 nm thick Si3N4 layer deposited by PECVD at 450 8C. These two cap layers were patterned lithographically to produce both bare (exposed to the ambient) and capped (inert) regions on the wafers during rapid thermal processing. The SIMS data reported here is for the samples that were open to the processing ambient. These samples were subject to rapid thermal processing in one of N2, O2, or NH3. The thin SiO2 or SiON layers produced during rapid thermal processing were removed with dilute HF prior to SIMS analysis. In all cases CZ Si samples were processed along with each SiGe sample, to serve as controls. As profiles were measured using Csþ (2 keV) by Quadrupole SIMS. The SIMS data were compared with TSUPREM-4 simulations. The PD. FERMI model in TSUPREM-4, which invokes a diffusion coefficient that is dependent upon the doping concentration, was used to extract effective, time-averaged diffusivities.
2. Experiment 3. Results and discussion Fig. 1 shows the As SIMS profiles for annealing in nitrogen at 950 8C in Si and SiGe. For relatively long time annealing, for instance 20 s, As diffuses faster in
As Concentration (atoms/cm 3)
The Si1xGex samples have a 2 mm thick graded, relaxed Si1xGex layer, and a 2 mm thick Si0.8Ge0.2 top layer, grown on <1 0 0> Czochralski silicon. The Si1xGex layers were in situ doped with boron (B) to 1017 cm3. The Si0.8Ge0.2 samples designed for rapid thermal nitridation (RTN) and rapid thermal oxidation (RTO) were capped with an 18 nm thick strained Si layer during epitaxial growth. During oxidation or nitridation, only this strained Si layer reacts with the annealing ambient. Limiting reaction to the strained Si layer enables control of interstitial or vacancy injection into the SiGe, since it is well known that oxidation (nitridation) of Si injects interstitials [5] (vacancies [6]), while such reactions with SiGe are not yet well characterized. In this work, the Ge concentration is 20% for all SiGe samples. Arsenic ion implants were performed at 15 keV with doses of 1015 and 3 1015 cm2, at a 78 tilt and 228 rotation. After ion implantation, a 50 nm thick low temperature oxide (LTO) cap layer was deposited at 400 8C on all wafers. To study As diffusion behavior under transient diffusion conditions, relaxed SiGe and CZ Si samples were rapid thermal annealed (RTA) in an N2 ambient at the
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As: 15keV 3x1015cm-2, 950oC SiGe Si 1sec. 5sec. 20sec. 1,5,20sec.
as implanted 0
0.02
0.04 0.06 Depth(um)
0.08
Fig. 1. Arsenic SIMS profiles in Si and in SiGe after annealing in an N2 ambient at 950 8C.
1000 Normalized Point Defect
As 950oC 15keV 3x1015cm-2
2
As effective diffusivity(cm /sec)
S. Eguchi et al. / Applied Surface Science 224 (2004) 59–62
SiGe -13
10
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SiGe Si Si
TED 20
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-2
Interstitial (CI/CI*)
10 1 0.1
Vacancy(Cv/Cv*)
0.01 Simulation for Si
70
0.001
0
40 80 120 Annealing Time(min)
Fig. 3. Simulation results of annealing time dependence of CI and CV after As ion implant.
SiGe than in Si. However, for short time annealing, e.g. 1 s, the As SIMS profiles and junction depths in SiGe are similar to those in Si. These results suggest the possibility of arsenic transient enhanced diffusion (TED) in Si or arsenic transient retarded diffusion (TRD) in SiGe. To evaluate the As effective diffusivity under transient diffusion conditions, As SIMS profiles are fitted by varying the enhancement factor ‘‘gAs’’ in Eq. (1) for the diffusion coefficients. Fig. 2 shows the resulting time dependence of the extracted time averaged effective arsenic diffusivities in Si and SiGe. For relatively long time annealing, arsenic effective diffusivity in SiGe is much higher than that in Si. However, for short time annealing, the effective arsenic diffusivity in Si is enhanced by ion implant damage while arsenic diffusion in SiGe appears to be retarded compared to the diffusion observed for longer times. The effective diffusivity under transient diffusion conditions can be described as [3]: " # CI CV eff DAs ¼ DAs fI þ fV (3) CI CV
while the vacancy concentration (CV) is suppressed compared to its equilibrium value. After a long time annealing, both interstitial (CI/CI ) and vacancy (CV/CV ) ratios approach to unity. The time dependence of CV/CV in Fig. 3 displays the same trend as the As effective diffusivity in SiGe illustrated in Fig. 2. This suggests the possibility that As diffusion in SiGe is mediated by a strong vacancy component. Fig. 4 shows a comparison of As SIMS profiles in Si and strained Si/relaxed SiGe under RTA, RTO, and RTN conditions. Compared to As SIMS profile after RTA, As profiles after RTO are slightly retarded in both Si and SiGe samples. During rapid thermal nitridation (vacancy injection), As diffusion in both
where DAs is the equilibrium diffusivity measured under inert conditions. CI and CV are equilibrium Si interstitial and vacancy concentrations, respectively. Fig. 3 illustrates the simulated Si interstitial (CI/CI ) and vacancy (CV/CV ) ratios in Si. Under N2 ambient, just after ion implant, the concentration of interstitial (CI) is increased by the ion implant damage,
As Concentration (atoms/cm 3)
Fig. 2. Time dependence of As effective diffusivities in Si and SiGe.
As 15keV 1x15 cm
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20
10
19
-2
o
1000 C 10 sec.
as implanted
Si
RTN RTA RTO Strained-Si SiGe
Si/SiGe 10
18
0
0.04 0.08 Depth(um)
0.12
Fig. 4. Comparison of arsenic SIMS profiles after 1000 8C for 10 s annealing in N2, O2, and NH3 ambients.
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25 Ge Concentration (%)
Ge in RTN 20 15
Ge in RTA as implanted
10
Strained-Si SiGe
5
As 15keV 1x1015cm-2 1000 C annealed for 5-20sec. o
0
0
0.01
0.02 0.03 Depth(um)
0.04
Fig. 5. Comparison of Germanium SIMS profiles under N2 and NH3 ambients.
relaxed SiGe and CZ Si samples is observed to be enhanced. For times of 5–10 s, the extracted diffusivity enhancement factor is on the order of 7–10 for the SiGe samples, and is slightly larger than the enhancement factor extracted from the Si samples (5–10). These results suggest that As diffusion in SiGe may have a stronger vacancy component than that in Si. Another issue for the processing of strained Si MOSFETs is the need to minimize Ge diffusion into the strained Si channel layer. Fig. 5 compares Ge SIMS profiles in strained Si/relaxed SiGe samples after annealing in nitrogen and ammonia. Ge diffusion from relaxed SiGe into strained Si is dramatically enhanced by vacancy injection during RTN. This is consistent with the Ge self-diffusion results reported in [7].
In summary, transient retarded diffusion of arsenic is observed in SiGe during annealing under N2 ambient, while transient enhanced diffusion is observed for arsenic in Si. Enhancement of As diffusion in SiGe is observed during vacancy injection under transient diffusion conditions. These results suggest that As diffusion is strongly vacancy mediated in Si0.8Ge0.2 in the temperature range from 950 to 1050 8C. Vacancy-injecting processes such as RTN must be avoided during strained-Si channel on relaxed-SiGe MOSFET fabrication because of the rapid diffusion of Ge into the strained Si layer.
Acknowledgements The authors from MIT acknowledge the support of Hitachi Ltd., SRC and DARPA.
References [1] S. Eguchi, J.L. Hoyt, C.W. Leitz, E.A. Fitzgerald, Appl. Phys. Lett. 80 (2002) 1743. [2] S. Eguchi, C.W. Leitz, E.A. Fitzgerald, J.L. Hoyt, in: W. En, E.C. Jones, et al. (Eds.), MRS Symposium Proceedings, vol. 686, p. 33, 2002. [3] TSUPREM-4 User’s Manual, 2001.2 version, Synopsys. [4] R. Braunstein, A.R. Moore, F. Herman, Phys. Rev. B 109 (3) (1958) 695–710. [5] P. Fahey, G. Barbuscia, M. Moslehi, R.W. Dutton, Appl. Phys. Lett. 46 (1985) 784. [6] P.A. Packan, J.D. Plummer, J. Appl. Phys. 68 (1990) 4327. [7] P. Fahey, S.S. Iyer, G.J. Scilla, Appl. Phys. Lett. 54 (1989) 843.
On the mechanism of ion-implanted As diffusion in relaxed SiGe S. Eguchia,*, J.J. Leeb, S.J. Rheeb, D.L. Kwongb, M.L. Leec, ˚ bergd, J.L. Hoytd E.A. Fitzgeraldc, I. A a
Semiconductor and Integrated Circuit Group, Hitachi Ltd., Ext. T07, Hitachi Ltd., 111 Nishiyokote, Takasaki, Gumma, 370-0021, Japan b Microelectronics Research Center, University of Texas at Austin, Austin, TX 78712, USA c Materials Science and Engineering Department, MIT, Cambridge, MA 02139, USA d Microsystems Technology Laboratories, EECS, MIT, Cambridge, MA 02139, USA
Abstract The diffusion behavior of ion-implanted arsenic in strained Si/relaxed Si0.8Ge0.2 structures is studied during rapid thermal processing in oxygen (interstitial injection) and ammonia (vacancy injection). During rapid thermal processing in an oxygen ambient, arsenic diffusion in SiGe is reduced, while an ammonia ambient produces strong enhancement, especially for short times. The results suggest that arsenic diffusion in SiGe has a stronger vacancy component than arsenic diffusion in Si. Vacancy injection is also observed to enhance the diffusivity of Ge from relaxed SiGe into strained Si. # 2003 Published by Elsevier B.V. PACS: 61.71Ji; 61.72.Tt Keywords: Silicon Germanium; Arsenic; Diffusion
1. Introduction High performance strained Si/relaxed SiGe MOSFETs require the fabrication of shallow source/ drain junctions. The formation of nþp junctions suffers from the dramatically enhanced diffusion of arsenic in SiGe [1,2]. In this paper, we focus on arsenic (As) diffusion during rapid thermal processing in nitrogen, oxygen, and ammonia. To gain insight into the diffusion mechanism for As in SiGe oxidation and nitridation of a top strained Si layer are employed to inject interstitials and vacancies into the underlying relaxed *
Corresponding author. Tel.: þ81-27-360-2238; fax: þ81-27-360-2166. E-mail address: [email protected] (S. Eguchi). 0169-4332/$ – see front matter # 2003 Published by Elsevier B.V. doi:10.1016/j.apsusc.2003.08.029
SiGe layer. High dose As implants, typical of nþp source/drains, are used to investigate the possibility of controlling diffusion during source/drain formation using annealing ambient. The effective diffusivity of As in Si and SiGe is extracted by fitting secondary ion mass spectrometry (SIMS) data to simulated As profiles using the TSUPREM-4 process simulator. The As diffusion model in Si used in TSUPREM-4 is described in detail in reference [3]. We apply the same model to As diffusion in SiGe under equilibrium conditions n 0 DAs ðSiGeÞ ¼ g f D þ D As i eff i i ni ðSiGeÞ n þ fv D0v þ D (1) v ni ðSiGeÞ
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where n is the electron concentration. D0i and D i represent the contributions from neutral and singlenegatively charged interstitial defects, respectively. D0v and D v represent the contributions from neutral and single-negatively charged vacancy defects, and the factors fi and fv represent the fractions of interstitial and vacancy mediated diffusion, respectively. In Eq. (1) the intrinsic carrier concentration in SiGe (niðSiGeÞ), is given by DEg ni ðSiGeÞ ¼ ni ðSiÞ exp (2) 2kT where the DEg between Si and relaxed SiGe (Ge 20%) is roughly 0.08 eV [4]. The diffusion coefficients (D0i , D i , etc.) used in Eq. (1) are the same values used in Si. In this work, the parameter gAs is varied in the simulations to fit the observed SIMS profiles in SiGe. Using the extracted enhancement factors gAs and ni(SiGe), the effective diffusivities for As in SiGe under equilibrium conditions can be evaluated at any given value of n using Eq. (1).
temperature range from 950 to 1050 8C. The oxide layers were then stripped with dilute HF. For the RTN and RTO experiments, strained Si/relaxed SiGe (Si/SiGe) and CZ samples were capped with LTO and a 150 nm thick Si3N4 layer deposited by PECVD at 450 8C. These two cap layers were patterned lithographically to produce both bare (exposed to the ambient) and capped (inert) regions on the wafers during rapid thermal processing. The SIMS data reported here is for the samples that were open to the processing ambient. These samples were subject to rapid thermal processing in one of N2, O2, or NH3. The thin SiO2 or SiON layers produced during rapid thermal processing were removed with dilute HF prior to SIMS analysis. In all cases CZ Si samples were processed along with each SiGe sample, to serve as controls. As profiles were measured using Csþ (2 keV) by Quadrupole SIMS. The SIMS data were compared with TSUPREM-4 simulations. The PD. FERMI model in TSUPREM-4, which invokes a diffusion coefficient that is dependent upon the doping concentration, was used to extract effective, time-averaged diffusivities.
2. Experiment 3. Results and discussion Fig. 1 shows the As SIMS profiles for annealing in nitrogen at 950 8C in Si and SiGe. For relatively long time annealing, for instance 20 s, As diffuses faster in
As Concentration (atoms/cm 3)
The Si1xGex samples have a 2 mm thick graded, relaxed Si1xGex layer, and a 2 mm thick Si0.8Ge0.2 top layer, grown on <1 0 0> Czochralski silicon. The Si1xGex layers were in situ doped with boron (B) to 1017 cm3. The Si0.8Ge0.2 samples designed for rapid thermal nitridation (RTN) and rapid thermal oxidation (RTO) were capped with an 18 nm thick strained Si layer during epitaxial growth. During oxidation or nitridation, only this strained Si layer reacts with the annealing ambient. Limiting reaction to the strained Si layer enables control of interstitial or vacancy injection into the SiGe, since it is well known that oxidation (nitridation) of Si injects interstitials [5] (vacancies [6]), while such reactions with SiGe are not yet well characterized. In this work, the Ge concentration is 20% for all SiGe samples. Arsenic ion implants were performed at 15 keV with doses of 1015 and 3 1015 cm2, at a 78 tilt and 228 rotation. After ion implantation, a 50 nm thick low temperature oxide (LTO) cap layer was deposited at 400 8C on all wafers. To study As diffusion behavior under transient diffusion conditions, relaxed SiGe and CZ Si samples were rapid thermal annealed (RTA) in an N2 ambient at the
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As: 15keV 3x1015cm-2, 950oC SiGe Si 1sec. 5sec. 20sec. 1,5,20sec.
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0.08
Fig. 1. Arsenic SIMS profiles in Si and in SiGe after annealing in an N2 ambient at 950 8C.
1000 Normalized Point Defect
As 950oC 15keV 3x1015cm-2
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As effective diffusivity(cm /sec)
S. Eguchi et al. / Applied Surface Science 224 (2004) 59–62
SiGe -13
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Retarded
SiGe Si Si
TED 20
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n=4x10 cm 0
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0.01 Simulation for Si
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Fig. 3. Simulation results of annealing time dependence of CI and CV after As ion implant.
SiGe than in Si. However, for short time annealing, e.g. 1 s, the As SIMS profiles and junction depths in SiGe are similar to those in Si. These results suggest the possibility of arsenic transient enhanced diffusion (TED) in Si or arsenic transient retarded diffusion (TRD) in SiGe. To evaluate the As effective diffusivity under transient diffusion conditions, As SIMS profiles are fitted by varying the enhancement factor ‘‘gAs’’ in Eq. (1) for the diffusion coefficients. Fig. 2 shows the resulting time dependence of the extracted time averaged effective arsenic diffusivities in Si and SiGe. For relatively long time annealing, arsenic effective diffusivity in SiGe is much higher than that in Si. However, for short time annealing, the effective arsenic diffusivity in Si is enhanced by ion implant damage while arsenic diffusion in SiGe appears to be retarded compared to the diffusion observed for longer times. The effective diffusivity under transient diffusion conditions can be described as [3]: " # CI CV eff DAs ¼ DAs fI þ fV (3) CI CV
while the vacancy concentration (CV) is suppressed compared to its equilibrium value. After a long time annealing, both interstitial (CI/CI ) and vacancy (CV/CV ) ratios approach to unity. The time dependence of CV/CV in Fig. 3 displays the same trend as the As effective diffusivity in SiGe illustrated in Fig. 2. This suggests the possibility that As diffusion in SiGe is mediated by a strong vacancy component. Fig. 4 shows a comparison of As SIMS profiles in Si and strained Si/relaxed SiGe under RTA, RTO, and RTN conditions. Compared to As SIMS profile after RTA, As profiles after RTO are slightly retarded in both Si and SiGe samples. During rapid thermal nitridation (vacancy injection), As diffusion in both
where DAs is the equilibrium diffusivity measured under inert conditions. CI and CV are equilibrium Si interstitial and vacancy concentrations, respectively. Fig. 3 illustrates the simulated Si interstitial (CI/CI ) and vacancy (CV/CV ) ratios in Si. Under N2 ambient, just after ion implant, the concentration of interstitial (CI) is increased by the ion implant damage,
As Concentration (atoms/cm 3)
Fig. 2. Time dependence of As effective diffusivities in Si and SiGe.
As 15keV 1x15 cm
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o
1000 C 10 sec.
as implanted
Si
RTN RTA RTO Strained-Si SiGe
Si/SiGe 10
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0.04 0.08 Depth(um)
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Fig. 4. Comparison of arsenic SIMS profiles after 1000 8C for 10 s annealing in N2, O2, and NH3 ambients.
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25 Ge Concentration (%)
Ge in RTN 20 15
Ge in RTA as implanted
10
Strained-Si SiGe
5
As 15keV 1x1015cm-2 1000 C annealed for 5-20sec. o
0
0
0.01
0.02 0.03 Depth(um)
0.04
Fig. 5. Comparison of Germanium SIMS profiles under N2 and NH3 ambients.
relaxed SiGe and CZ Si samples is observed to be enhanced. For times of 5–10 s, the extracted diffusivity enhancement factor is on the order of 7–10 for the SiGe samples, and is slightly larger than the enhancement factor extracted from the Si samples (5–10). These results suggest that As diffusion in SiGe may have a stronger vacancy component than that in Si. Another issue for the processing of strained Si MOSFETs is the need to minimize Ge diffusion into the strained Si channel layer. Fig. 5 compares Ge SIMS profiles in strained Si/relaxed SiGe samples after annealing in nitrogen and ammonia. Ge diffusion from relaxed SiGe into strained Si is dramatically enhanced by vacancy injection during RTN. This is consistent with the Ge self-diffusion results reported in [7].
In summary, transient retarded diffusion of arsenic is observed in SiGe during annealing under N2 ambient, while transient enhanced diffusion is observed for arsenic in Si. Enhancement of As diffusion in SiGe is observed during vacancy injection under transient diffusion conditions. These results suggest that As diffusion is strongly vacancy mediated in Si0.8Ge0.2 in the temperature range from 950 to 1050 8C. Vacancy-injecting processes such as RTN must be avoided during strained-Si channel on relaxed-SiGe MOSFET fabrication because of the rapid diffusion of Ge into the strained Si layer.
Acknowledgements The authors from MIT acknowledge the support of Hitachi Ltd., SRC and DARPA.
References [1] S. Eguchi, J.L. Hoyt, C.W. Leitz, E.A. Fitzgerald, Appl. Phys. Lett. 80 (2002) 1743. [2] S. Eguchi, C.W. Leitz, E.A. Fitzgerald, J.L. Hoyt, in: W. En, E.C. Jones, et al. (Eds.), MRS Symposium Proceedings, vol. 686, p. 33, 2002. [3] TSUPREM-4 User’s Manual, 2001.2 version, Synopsys. [4] R. Braunstein, A.R. Moore, F. Herman, Phys. Rev. B 109 (3) (1958) 695–710. [5] P. Fahey, G. Barbuscia, M. Moslehi, R.W. Dutton, Appl. Phys. Lett. 46 (1985) 784. [6] P.A. Packan, J.D. Plummer, J. Appl. Phys. 68 (1990) 4327. [7] P. Fahey, S.S. Iyer, G.J. Scilla, Appl. Phys. Lett. 54 (1989) 843.