- Email: [email protected]
epitaxial layer grown by molecular-beam epitaxy
NIM B Beam Interactions with Materials & Atoms
Nuclear Instruments and Methods in Physics Research B 242 (2006) 509–511 www.elsevier.com/locate/nimb
Ion-cut of Si facilitated by interfacial defects of Si substrate/epitaxial layer grown by molecular-beam epitaxy Lin Shao a,*, J.K. Lee a, T. Ho¨chbauer a, M. Nastasi a, Phillip E. Thompson b, I. Rusakova c, H.W. Seo c, Q.Y. Chen c, J.R. Liu c, Wei-Kan Chu c a
c
Material Science and Technology Division, Los Alamos National Laboratory, Los Alamos, NM 87545-1663, United States b Code 6812, Naval Research Laboratory, Washington, DC 20375-5347, United States Texas Center for Superconductivity and Advanced Materials, University of Houston, Houston, TX 77204-5506, United States Available online 27 October 2005
Abstract We have shown that the Si/Si interface produced by molecular-beam epitaxy (MBE) growth of Si on a Si substrate can significantly enhance the efficiency of ion cutting. MBE growth is performed at 650 C. Samples are then implanted at room temperature by 62 keV H to a dose of 7 · 1016 ions/cm2. The implantation energy locates H-peak in the vicinity of the Si/Si interface, which is 600 nm below the Si surface. Scanning electron microscopy shows that, after post-implantation annealing at 300 C for 50 min, the H implanted MBE Si has bubbles formed with an average diameters of 33 lm, which is around one order of magnitude larger than that observed in the control bulk silicon sample. It is also observed that the area covered with blisters is a factor of 2 larger for the MBE samples, a trend that is systematically observed for anneals carried out in the range of 300–550 C. 2005 Elsevier B.V. All rights reserved. PACS: 61.72.Tt; 61.72.Ss; 61.72.Qq Keywords: Hydrogen; Blistering; Ion cutting; Interface
1. Introduction
2. Experiment
In the ‘‘ion-cut’’ technique of layer splitting, implanted hydrogen atoms (at a dose of a few 1016 cm 2) create hydrogen-terminated cavities upon annealing that finally evolve into cracks parallel to the surface that allow the surface layer to be completely separated [1]. This technique has become one of the mainstreams in the fabrication of silicon-on-insulator (SOI) wafers, a multiple-layer semiconductor/dielectric structure for advanced Si devices [2]. However, the process is far from fully optimized and additional research is needed to reduce the required hydrogen dose, to minimize the thermal budget and to further improve ion cutting efficiency.
In this study, a layer of Si was grown by molecular-beam epitaxy (MBE) on Si(1 0 0) p-type (500 X cm) substrates. The substrate was cleaned with 2.5% hydrofluoric acid dip. The Si molecular beam was obtained from an elemental source in an electron-gun evaporator. The Si growth rate was 0.1 nm/s. The thickness of the deposited Si layer was 600 nm. The MBE wafers were implanted at room temperature with 7 · 1016 H ions/cm2 at 62 keV, which locates the H-peak in the vicinity of the interface. Samples were then annealed in a vacuum furnace at temperatures ranging from 300 to 550 C. Scanning electron microscopy (SEM) was performed to study bubble and blistering formation. Elastic recoil detection (ERD) analysis was performed to measure the hydrogen depth distribution using a 2.0 MeV 4He+ analyzing beam. The analyzing beam was oriented 75.25 from the sample normal and the detector
*
Corresponding author. Tel.: +1 505 667 2841; fax: +1 505 667 8779. E-mail address: [email protected] (L. Shao).
0168-583X/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2005.08.192
510
L. Shao et al. / Nucl. Instr. and Meth. in Phys. Res. B 242 (2006) 509–511
was positioned 150.5 from the incident beam. To avoid detection of forward-scattered a-particles, a 20 lm thick Mylar foil was placed between the sample and the hydrogen detector. Some of the samples were also measured by channeling Rutherford backscattering spectrometry (RBS) using 2 MeV He+ ions. 3. Results and discussion
Fig. 1. SEM top views of Si surfaces of H implanted MBE Si (a) and virgin Si (b) after annealing at 300 C for 50 min. H ions are implanted at 62 keV to a dosage of 7 · 1016 cm 2.
Our previous transmission electron microcopy studies show that the interface of as-grown Si MBE samples contains dislocation loops originating from the interface [3]. The dislocation density decreased dramatically with increasing distance from the interface, with the majority of loops distributed in a narrow region less than 10 nm thick. Existence of interfacial dislocation loops for homoepitaxial growth is related to impurities at the interface. The impurities play a blocking role where sites are excluded from the growth process [4]. Fig. 1 presents SEM images of surface blisters from the control Si sample and MBE sample after H implantation and 300 C/50 min annealing. Both samples were implanted with 7 · 1016 cm 2, 62 keV H. As shown in Fig. 1(a), H implanted MBE Si has bubbles formed with an average diameters of 33 lm, while Fig. 1(b) shows the averaged bubble diameters are only 3.5 lm in the control bulk Si sample, an order of magnitude smaller than the MBE samples. This trend was systematic for samples annealed at elevated temperatures. Fig. 2 compares the integrated bubble covered areas for annealing temperature at 300 and 550 C. It shows that the area covered with blisters is a factor of 2 larger for the MBE samples. Fig. 3 shows the ERD profiles of H distribution after annealing at 300 C for 15 min. As a comparison, open
1
(a)
(b)
MBE Si Virgin Si
MBE Si Virgin Si
Covered Area
Averaged Diameter (µm)
40
20
0.1
0 300
350
400
450
500
550
300
350
400
450
500
550
o
Temperature ( C) Fig. 2. Averaged bubble diameters (a) and the bubble covered areas (b) measured from SEM for H implanted MBE and virgin Si after annealing at 300 C or 550 C for 50 min.
L. Shao et al. / Nucl. Instr. and Meth. in Phys. Res. B 242 (2006) 509–511
[5]. The Si/Si interfaces provide two benefits here: (1) it provides dangling bonds for the formation of hydrogenrelated defects; (2) it provides H trapping sites in a well defined depth which facilitates the network of microcracks. The case is very similar to strain-relaxed SiGe/Si heterostructure which shows the interface enhances H trapping during plasma hydrogenation [6]. Further studies on the role of interfacial defects are underway. Although there is no conclusive explanation on the mechanism at current stage, our finding does show a promising way to facilitate blistering by introducing interfacial defects.
o
Virgin Si + 100 keV H, 7e16, 300 C/15m o MBE Si + 100 keV H, 7e16, 300 C/15m
1000
estimated interface posit.
100
Hydrogen Yield (arb. unit.)
10
o
1000
511
Virgin Si + 60keV H, 7e16, 300 C/15m o MBE Si + 60keV H, 7e16, 300 C/15m
100
10
4. Conclusion o
Virgin Si + 30keV H, 7e16, 300 C/15m o MBE Si + 30keV H, 7e16, 300 C/15m
1000 100 10 1 0
200
400
600
Channel Fig. 3. ERD H distribution for H implanted MBE and virgin Si after annealing at 300 C for 15 min. H implantation are performed at 30, 60 and 100 keV with a dosage of 7 · 1016 cm 2, respectively.
circles represent H profile in control sample and solid line represents that in MBE samples. H implantations in both samples were performed with three different energies, 30, 60 and 100 keV, respectively. By controlling implantation energy, H peak locations either shallower or deeper than the interface location were investigated. There are no visible differences in H peak locations after annealing in control sample and MBE sample. We did not observe any sign of long range H migration and trapping at the interface. However, this does not exclude the possibility of short range H trapping. H migration can be affected by the defects in the MBE samples. The densities of these defects are very sensitive to the growth temperatures. Therefore, we expect that H migration and trapping phenomena can be different for samples grown at different temperatures. It is well known that hydrogen ion implantation introduces hydrogen-related defects, including the formation of hydrogen platelets, which evolve into hydrogen microbubbles with high internal pressure. Coalescence of these micro-bubbles during annealing will lead to layer splitting
We have shown that the interface defects of Si/Si layer grown by molecular-beam epitaxy can significantly enhance ion cutting efficiency. The H implanted MBE Si forms bubbles in a diameter one order of magnitude larger than that in the control Si sample. The area covered with blisters is a factor of 2 larger for the MBE samples. The finding shows a promising approach to improve the efficiency of ion cutting. Acknowledgements The research was supported in part by the State of Texas through the Texas Center for Superconductivity and Advanced Materials at the University of Houston, and in part by the DOE, Office of Basic Energy Science and in part by the Office of Naval Research. Portion of W.K.C.Õs research activity is also supported by the Robert A. Welch Endowment. References [1] M.K. Weldon, V.E. Marsico, Y.J. Chabal, A. Agarwal, D.J. Eaglesham, J. Sapjeta, W.L. Brown, D.C. Jacobson, Y. Caudano, S.B. Christman, E.E. Chaban, J. Vac. Sci. Technol. B 15 (1997) 1065. [2] G.K. Celler, S. Cristoloveanu, J. Appl. Phys. 93 (2003) 4955. [3] L. Shao, X.M. Wang, I. Rusakova, H. Chen, J.R. Liu, P.E. Thompson, W.K. Chu, Appl. Phys. Lett. 83 (2003) 934. [4] A. Ishazaka, K. Nakagawa, Y. Shiraki, in: 2nd International Symposium on Molecular-Beam Epitaxy and Related Clean Surface Techniques, Tokyo, Japan, 27–30 August 1982. [5] T. Ho¨chbauer, A. Misra, M. Nastasi, J.W. Mayer, J. Appl. Phys. 92 (2002) 2335. [6] P. Chen, P.K. Chu, T. Ho¨chbauer, M. Nastasi, D. Buca, S. Mantl, N.D. Theodore, T.L. Alford, J.W. Mayer, R. Loo, M. Caymax, M. Cai, S.S. Lau, Appl. Phys. Lett. 85 (2004) 4944.
Nuclear Instruments and Methods in Physics Research B 242 (2006) 509–511 www.elsevier.com/locate/nimb
Ion-cut of Si facilitated by interfacial defects of Si substrate/epitaxial layer grown by molecular-beam epitaxy Lin Shao a,*, J.K. Lee a, T. Ho¨chbauer a, M. Nastasi a, Phillip E. Thompson b, I. Rusakova c, H.W. Seo c, Q.Y. Chen c, J.R. Liu c, Wei-Kan Chu c a
c
Material Science and Technology Division, Los Alamos National Laboratory, Los Alamos, NM 87545-1663, United States b Code 6812, Naval Research Laboratory, Washington, DC 20375-5347, United States Texas Center for Superconductivity and Advanced Materials, University of Houston, Houston, TX 77204-5506, United States Available online 27 October 2005
Abstract We have shown that the Si/Si interface produced by molecular-beam epitaxy (MBE) growth of Si on a Si substrate can significantly enhance the efficiency of ion cutting. MBE growth is performed at 650 C. Samples are then implanted at room temperature by 62 keV H to a dose of 7 · 1016 ions/cm2. The implantation energy locates H-peak in the vicinity of the Si/Si interface, which is 600 nm below the Si surface. Scanning electron microscopy shows that, after post-implantation annealing at 300 C for 50 min, the H implanted MBE Si has bubbles formed with an average diameters of 33 lm, which is around one order of magnitude larger than that observed in the control bulk silicon sample. It is also observed that the area covered with blisters is a factor of 2 larger for the MBE samples, a trend that is systematically observed for anneals carried out in the range of 300–550 C. 2005 Elsevier B.V. All rights reserved. PACS: 61.72.Tt; 61.72.Ss; 61.72.Qq Keywords: Hydrogen; Blistering; Ion cutting; Interface
1. Introduction
2. Experiment
In the ‘‘ion-cut’’ technique of layer splitting, implanted hydrogen atoms (at a dose of a few 1016 cm 2) create hydrogen-terminated cavities upon annealing that finally evolve into cracks parallel to the surface that allow the surface layer to be completely separated [1]. This technique has become one of the mainstreams in the fabrication of silicon-on-insulator (SOI) wafers, a multiple-layer semiconductor/dielectric structure for advanced Si devices [2]. However, the process is far from fully optimized and additional research is needed to reduce the required hydrogen dose, to minimize the thermal budget and to further improve ion cutting efficiency.
In this study, a layer of Si was grown by molecular-beam epitaxy (MBE) on Si(1 0 0) p-type (500 X cm) substrates. The substrate was cleaned with 2.5% hydrofluoric acid dip. The Si molecular beam was obtained from an elemental source in an electron-gun evaporator. The Si growth rate was 0.1 nm/s. The thickness of the deposited Si layer was 600 nm. The MBE wafers were implanted at room temperature with 7 · 1016 H ions/cm2 at 62 keV, which locates the H-peak in the vicinity of the interface. Samples were then annealed in a vacuum furnace at temperatures ranging from 300 to 550 C. Scanning electron microscopy (SEM) was performed to study bubble and blistering formation. Elastic recoil detection (ERD) analysis was performed to measure the hydrogen depth distribution using a 2.0 MeV 4He+ analyzing beam. The analyzing beam was oriented 75.25 from the sample normal and the detector
*
Corresponding author. Tel.: +1 505 667 2841; fax: +1 505 667 8779. E-mail address: [email protected] (L. Shao).
0168-583X/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2005.08.192
510
L. Shao et al. / Nucl. Instr. and Meth. in Phys. Res. B 242 (2006) 509–511
was positioned 150.5 from the incident beam. To avoid detection of forward-scattered a-particles, a 20 lm thick Mylar foil was placed between the sample and the hydrogen detector. Some of the samples were also measured by channeling Rutherford backscattering spectrometry (RBS) using 2 MeV He+ ions. 3. Results and discussion
Fig. 1. SEM top views of Si surfaces of H implanted MBE Si (a) and virgin Si (b) after annealing at 300 C for 50 min. H ions are implanted at 62 keV to a dosage of 7 · 1016 cm 2.
Our previous transmission electron microcopy studies show that the interface of as-grown Si MBE samples contains dislocation loops originating from the interface [3]. The dislocation density decreased dramatically with increasing distance from the interface, with the majority of loops distributed in a narrow region less than 10 nm thick. Existence of interfacial dislocation loops for homoepitaxial growth is related to impurities at the interface. The impurities play a blocking role where sites are excluded from the growth process [4]. Fig. 1 presents SEM images of surface blisters from the control Si sample and MBE sample after H implantation and 300 C/50 min annealing. Both samples were implanted with 7 · 1016 cm 2, 62 keV H. As shown in Fig. 1(a), H implanted MBE Si has bubbles formed with an average diameters of 33 lm, while Fig. 1(b) shows the averaged bubble diameters are only 3.5 lm in the control bulk Si sample, an order of magnitude smaller than the MBE samples. This trend was systematic for samples annealed at elevated temperatures. Fig. 2 compares the integrated bubble covered areas for annealing temperature at 300 and 550 C. It shows that the area covered with blisters is a factor of 2 larger for the MBE samples. Fig. 3 shows the ERD profiles of H distribution after annealing at 300 C for 15 min. As a comparison, open
1
(a)
(b)
MBE Si Virgin Si
MBE Si Virgin Si
Covered Area
Averaged Diameter (µm)
40
20
0.1
0 300
350
400
450
500
550
300
350
400
450
500
550
o
Temperature ( C) Fig. 2. Averaged bubble diameters (a) and the bubble covered areas (b) measured from SEM for H implanted MBE and virgin Si after annealing at 300 C or 550 C for 50 min.
L. Shao et al. / Nucl. Instr. and Meth. in Phys. Res. B 242 (2006) 509–511
[5]. The Si/Si interfaces provide two benefits here: (1) it provides dangling bonds for the formation of hydrogenrelated defects; (2) it provides H trapping sites in a well defined depth which facilitates the network of microcracks. The case is very similar to strain-relaxed SiGe/Si heterostructure which shows the interface enhances H trapping during plasma hydrogenation [6]. Further studies on the role of interfacial defects are underway. Although there is no conclusive explanation on the mechanism at current stage, our finding does show a promising way to facilitate blistering by introducing interfacial defects.
o
Virgin Si + 100 keV H, 7e16, 300 C/15m o MBE Si + 100 keV H, 7e16, 300 C/15m
1000
estimated interface posit.
100
Hydrogen Yield (arb. unit.)
10
o
1000
511
Virgin Si + 60keV H, 7e16, 300 C/15m o MBE Si + 60keV H, 7e16, 300 C/15m
100
10
4. Conclusion o
Virgin Si + 30keV H, 7e16, 300 C/15m o MBE Si + 30keV H, 7e16, 300 C/15m
1000 100 10 1 0
200
400
600
Channel Fig. 3. ERD H distribution for H implanted MBE and virgin Si after annealing at 300 C for 15 min. H implantation are performed at 30, 60 and 100 keV with a dosage of 7 · 1016 cm 2, respectively.
circles represent H profile in control sample and solid line represents that in MBE samples. H implantations in both samples were performed with three different energies, 30, 60 and 100 keV, respectively. By controlling implantation energy, H peak locations either shallower or deeper than the interface location were investigated. There are no visible differences in H peak locations after annealing in control sample and MBE sample. We did not observe any sign of long range H migration and trapping at the interface. However, this does not exclude the possibility of short range H trapping. H migration can be affected by the defects in the MBE samples. The densities of these defects are very sensitive to the growth temperatures. Therefore, we expect that H migration and trapping phenomena can be different for samples grown at different temperatures. It is well known that hydrogen ion implantation introduces hydrogen-related defects, including the formation of hydrogen platelets, which evolve into hydrogen microbubbles with high internal pressure. Coalescence of these micro-bubbles during annealing will lead to layer splitting
We have shown that the interface defects of Si/Si layer grown by molecular-beam epitaxy can significantly enhance ion cutting efficiency. The H implanted MBE Si forms bubbles in a diameter one order of magnitude larger than that in the control Si sample. The area covered with blisters is a factor of 2 larger for the MBE samples. The finding shows a promising approach to improve the efficiency of ion cutting. Acknowledgements The research was supported in part by the State of Texas through the Texas Center for Superconductivity and Advanced Materials at the University of Houston, and in part by the DOE, Office of Basic Energy Science and in part by the Office of Naval Research. Portion of W.K.C.Õs research activity is also supported by the Robert A. Welch Endowment. References [1] M.K. Weldon, V.E. Marsico, Y.J. Chabal, A. Agarwal, D.J. Eaglesham, J. Sapjeta, W.L. Brown, D.C. Jacobson, Y. Caudano, S.B. Christman, E.E. Chaban, J. Vac. Sci. Technol. B 15 (1997) 1065. [2] G.K. Celler, S. Cristoloveanu, J. Appl. Phys. 93 (2003) 4955. [3] L. Shao, X.M. Wang, I. Rusakova, H. Chen, J.R. Liu, P.E. Thompson, W.K. Chu, Appl. Phys. Lett. 83 (2003) 934. [4] A. Ishazaka, K. Nakagawa, Y. Shiraki, in: 2nd International Symposium on Molecular-Beam Epitaxy and Related Clean Surface Techniques, Tokyo, Japan, 27–30 August 1982. [5] T. Ho¨chbauer, A. Misra, M. Nastasi, J.W. Mayer, J. Appl. Phys. 92 (2002) 2335. [6] P. Chen, P.K. Chu, T. Ho¨chbauer, M. Nastasi, D. Buca, S. Mantl, N.D. Theodore, T.L. Alford, J.W. Mayer, R. Loo, M. Caymax, M. Cai, S.S. Lau, Appl. Phys. Lett. 85 (2004) 4944.