- Email: [email protected]
High-resolution depth profiling of ultrashallow boron implants in silicon using high-resolution RBS
Current Applied Physics 3 (2003) 9–11 www.elsevier.com/locate/cap
High-resolution depth profiling of ultrashallow boron implants in silicon using high-resolution RBS q Kenji Kimura a
a,*
, Yukitoshi Oota a, Kaoru Nakajima a, Temel H. B€ uy€ uklimanli
b
Department of Engineering Physics and Mechanics, Kyoto University, Kyoto 606-8501, Japan b Evans East, 104 Windsor Center Drive, Suite 101, East Windsor, NJ 08520-1407, USA Received 30 September 2002; accepted 14 November 2002
Abstract Depth profiles of ultralow energy (0.2–0.5 keV) B ion implants in Si(0 0 1) samples are measured by high-resolution Rutherford backscattering spectroscopy. The boron profile does not show a narrow surface concentration peak which is usually observed in the measurement of secondary ion mass spectroscopy. The obtained boron profiles roughly agree with TRIM simulation even at 0.2keV B ion implantation. Ó 2002 Elsevier Science B.V. All rights reserved. PACS: 82.80.Yc; 85.40.Ry; 61.72.Ss Keywords: RBS; High-resolution; Ultrashallow implantation; Boron profiling
1. Introduction Ion implantation at ultralow energies below 1 keV is a key process for the formation of ultrashallow junctions for device technologies with gate length below 100 nm. Depth profiling of such ultralow energy ion implants is an analytical challenge. While depth profile of implanted heavy atoms is easily measured by Rutherford backscattering spectroscopy (RBS), the profile of the implanted boron has never measured by RBS because of its low sensitivity to light elements. Boron depth profiling is generally performed by secondary ion mass spectroscopy (SIMS) [1]. There is always a narrow surface concentration peak at the topmost region in SIMS measurements [1]. This surface peak is usually considered as an artifact because SIMS cannot be accurate in the topmost 1–2 nm region. Recently, depth profile of 1-keV B ion implants in Si was measured using energy-filtered transmission electron microscopy (EFTEM) [2]. There was no narrow surface concentraq Original version presented at the 2nd International Workshop on Ion Beam Techniques for the analysis of Composition and Structure with Atomic Layer Resolution, 24–27 September 2002, Kyongju, Korea. * Corresponding author. Tel.: +81-75-753-5253; fax: +81-753-5253. E-mail address: [email protected] (K. Kimura).
tion peak in the observed profile but a smooth concentration peak was observed at a depth about 6 nm showing a good agreement with TRIM simulation [3]. This indicates that the narrow surface peak seen in SIMS is an artifact [2]. Sample preparation for EFTEM, however, is laborious. Moreover, EFTEM cannot provide absolute B concentration. Other simple technique able to measure quantitative boron depth profiles with sub-nm depth resolution is highly desired. We demonstrate for the first time that the depth profile of the implanted B can be measured using high-resolution RBS.
2. Experimental P-type silicon (0 0 1) wafers were treated with dilute HF solution to remove a native oxide layer and were implanted with 0.2 or 0.5-keV Bþ ions. The boron depth profiles of the implanted samples were measured by HRBS. The details of HRBS are described elsewhere [4]. Briefly, a beam of 400 or 500 keV Heþ ions was collimated to 2 2 mm2 and to a divergence angle less than 1 mrad. The beam was impinged to the Si sample which was mounted on a 5-axis goniometer. Heþ ions scattered at 50° were energy analyzed by a 90° magnetic spectrometer and detected by a one-dimensional position sensitive detector. The energy resolution of the
1567-1739/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. doi:10.1016/S1567-1739(02)00227-4
10
K. Kimura et al. / Current Applied Physics 3 (2003) 9–11
spectrometer was about 0.1% at an acceptance of 0.3 msr.
0.5–keV B+ implanted Si(001) Φ = 2 × 1015 cm–2 100
10
Si
3. Results and discussion
TRIM
O B
Fig. 1 shows the HRBS spectrum for 0.5 keV Bþ implanted Si (Bþ ion dose, U, was 2 1015 cm2 ) observed at an incident angle 39.8° (triangles). The arrows show the expected energies for He ions elastically scattered by Si, O, C and B surface atoms. The spectrum has an oxygen peak around 415 keV, indicating that a SiO2 layer was formed probably after implantation. Implanted boron is expected to appear around 380 keV, but it is very difficult to extract the boron signal from the present spectrum because a large silicon signal overlaps on the boron signal. In order to reduce the silicon signal, HRBS measurement was performed under channeling conditions. The solid circles show the observed [1 1 1] channeling spectrum. The Si signal is considerably reduced and a sharp peak at 391 keV and a broad peak around 376 keV can be seen. The energy of the sharp peak coincides with the energy of the He ion elastically scattered by a carbon atom (shown by the arrow labeled C), indicating that this peak corresponds to carbon atoms in a thin surface contamination layer. The broad peak appears at slightly lower energy than the energy for surface boron atoms (shown by the arrow labeled B), suggesting that this broad peak corresponds to the implanted boron atoms. In order to confirm this, a channeling spectrum was measured with a sample which was implanted at U ¼ 1 1013 cm2 . The observed spectrum is shown by open circles. Although there is a surface carbon peak, a broad peak seen in the heavily implanted sample (U ¼ 2 1015 cm2 ) disappears, indicating that the observed broad peak corresponds to the implanted B atoms. 500 keV He+
50
5
C 0
0
2
6
0 10
8
Fig. 2. Depth profiles of Si, O, B and C in the 0.5-keV Bþ ion implanted Si with a fluence of 2 1015 cm2 . The obtained boron profile agrees with the TRIM simulation (solid line). There is no surface narrow peak which is usually observed in SIMS measurement.
The depth profiles of Si, O, C and B in the sample of U ¼ 2 1015 cm2 were derived from the HRBS spectrum and are shown in Fig. 2. The B-profile has a peak at 3.5 nm and the peak concentration is about 7 at.%. There is no narrow surface concentration peak which was always observed in SIMS measurements. The B profile calculated by the TRIM code (SRIM 2000) is also shown by a solid curve for comparison. The agreement between the observed profile and the calculated one is reasonably good, showing that the TRIM simulation is accurate even for 0.5 keV B ion implantation. From a practical point of view, it is important to know down to what energy the TRIM simulation is accurate. To this end, the sample implanted with 0.2 keV Bþ ion at U ¼ 1 1015 cm2 was measured by HRBS. Fig. 3 shows the comparison between the observed boron profile (closed circles) with the TRIM simulation (solid line). The agreement is roughly good although the observed distribution has a slightly longer tail in the deeper region. This tail might be ascribed to
0.5–keV B+ implanted Si(001)
60000
0.2–keV B+ implanted Si(001) Φ = 1×1015 cm–2
Si
10
100
random
Si TRIM
40000
B
C O ×5
20000
5
50
B
× 10 [111]channeling
0 360
4
O C
380
400
420
440
460
Fig. 1. HRBS spectra observed under [1 1 1] channeling conditions for 0.5-keV Bþ ion implanted Si with a fluence of 2 1015 cm2 (solid circles) and 1 1013 cm2 (open circles). The random spectrum for the 2 1015 cm2 sample is also shown (triangles) for comparison.
0
0
1
2
3
4
5
6
7
0
Fig. 3. Depth profiles of Si, O, B and C in the 0.2-keV Bþ ion implanted Si with a fluence of 2 1015 cm2 . The observed boron profile has a longer tail than the TRIM simulation (solid line).
K. Kimura et al. / Current Applied Physics 3 (2003) 9–11
the channeling effect. The critical angle of Si[0 0 1] axial channeling for 0.2 keV Bþ is calculated to be about 12° using the formula given by Lindhard and Dan [5]. This suggests that the channeling effect is difficult to be avoided in this ultralow energy ion implantation.
11
Acknowledgements We are grateful to the members of Quantum Science and Engineering Center of Kyoto University for use of the 4 MV Van de Graaff accelerator. This work was supported in part by grant-in-aid for Scientific Research from Japan Society for the Promotion of Science.
4. Conclusion Depth profiles of ultralow-energy (down to 0.2 keV) B ion implants in Si(0 0 1) samples are successfully measured by high-resolution RBS. The observed boron profile does not show a narrow surface concentration peak which is usually seen in SIMS measurement. The obtained boron profiles roughly agree with TRIM simulation even at 0.2-keV B ion implantation although there is a longer tail in deeper region than the TRIM profile. This tail might be attributed to the channeling effect.
References [1] C.W. Magee, D. Jacobson, H.-J. Gossmann, J. Vac. Sci. Technol. B 18 (2000) 489. [2] T.-S. Wang, A.G. Cullis, E.J.H. Collart, A.J. Murrel, M.A. Foad, Appl. Phys. Lett. 77 (2000) 3586. [3] J.F. Ziegler, J.P. Biersack, U.L. Littmark, The Stopping and Range of Ions in Solids, Pergamon Press, New York, 1985. [4] K. Kimura, K. Ohshima, M. Mannami, Appl. Phys. Lett. 64 (1994) 2232. [5] J. Lindhard, K. Dan, Vidensk. Selsk. Mat.-Fys. Medd. 34 (1965) 14.
High-resolution depth profiling of ultrashallow boron implants in silicon using high-resolution RBS q Kenji Kimura a
a,*
, Yukitoshi Oota a, Kaoru Nakajima a, Temel H. B€ uy€ uklimanli
b
Department of Engineering Physics and Mechanics, Kyoto University, Kyoto 606-8501, Japan b Evans East, 104 Windsor Center Drive, Suite 101, East Windsor, NJ 08520-1407, USA Received 30 September 2002; accepted 14 November 2002
Abstract Depth profiles of ultralow energy (0.2–0.5 keV) B ion implants in Si(0 0 1) samples are measured by high-resolution Rutherford backscattering spectroscopy. The boron profile does not show a narrow surface concentration peak which is usually observed in the measurement of secondary ion mass spectroscopy. The obtained boron profiles roughly agree with TRIM simulation even at 0.2keV B ion implantation. Ó 2002 Elsevier Science B.V. All rights reserved. PACS: 82.80.Yc; 85.40.Ry; 61.72.Ss Keywords: RBS; High-resolution; Ultrashallow implantation; Boron profiling
1. Introduction Ion implantation at ultralow energies below 1 keV is a key process for the formation of ultrashallow junctions for device technologies with gate length below 100 nm. Depth profiling of such ultralow energy ion implants is an analytical challenge. While depth profile of implanted heavy atoms is easily measured by Rutherford backscattering spectroscopy (RBS), the profile of the implanted boron has never measured by RBS because of its low sensitivity to light elements. Boron depth profiling is generally performed by secondary ion mass spectroscopy (SIMS) [1]. There is always a narrow surface concentration peak at the topmost region in SIMS measurements [1]. This surface peak is usually considered as an artifact because SIMS cannot be accurate in the topmost 1–2 nm region. Recently, depth profile of 1-keV B ion implants in Si was measured using energy-filtered transmission electron microscopy (EFTEM) [2]. There was no narrow surface concentraq Original version presented at the 2nd International Workshop on Ion Beam Techniques for the analysis of Composition and Structure with Atomic Layer Resolution, 24–27 September 2002, Kyongju, Korea. * Corresponding author. Tel.: +81-75-753-5253; fax: +81-753-5253. E-mail address: [email protected] (K. Kimura).
tion peak in the observed profile but a smooth concentration peak was observed at a depth about 6 nm showing a good agreement with TRIM simulation [3]. This indicates that the narrow surface peak seen in SIMS is an artifact [2]. Sample preparation for EFTEM, however, is laborious. Moreover, EFTEM cannot provide absolute B concentration. Other simple technique able to measure quantitative boron depth profiles with sub-nm depth resolution is highly desired. We demonstrate for the first time that the depth profile of the implanted B can be measured using high-resolution RBS.
2. Experimental P-type silicon (0 0 1) wafers were treated with dilute HF solution to remove a native oxide layer and were implanted with 0.2 or 0.5-keV Bþ ions. The boron depth profiles of the implanted samples were measured by HRBS. The details of HRBS are described elsewhere [4]. Briefly, a beam of 400 or 500 keV Heþ ions was collimated to 2 2 mm2 and to a divergence angle less than 1 mrad. The beam was impinged to the Si sample which was mounted on a 5-axis goniometer. Heþ ions scattered at 50° were energy analyzed by a 90° magnetic spectrometer and detected by a one-dimensional position sensitive detector. The energy resolution of the
1567-1739/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. doi:10.1016/S1567-1739(02)00227-4
10
K. Kimura et al. / Current Applied Physics 3 (2003) 9–11
spectrometer was about 0.1% at an acceptance of 0.3 msr.
0.5–keV B+ implanted Si(001) Φ = 2 × 1015 cm–2 100
10
Si
3. Results and discussion
TRIM
O B
Fig. 1 shows the HRBS spectrum for 0.5 keV Bþ implanted Si (Bþ ion dose, U, was 2 1015 cm2 ) observed at an incident angle 39.8° (triangles). The arrows show the expected energies for He ions elastically scattered by Si, O, C and B surface atoms. The spectrum has an oxygen peak around 415 keV, indicating that a SiO2 layer was formed probably after implantation. Implanted boron is expected to appear around 380 keV, but it is very difficult to extract the boron signal from the present spectrum because a large silicon signal overlaps on the boron signal. In order to reduce the silicon signal, HRBS measurement was performed under channeling conditions. The solid circles show the observed [1 1 1] channeling spectrum. The Si signal is considerably reduced and a sharp peak at 391 keV and a broad peak around 376 keV can be seen. The energy of the sharp peak coincides with the energy of the He ion elastically scattered by a carbon atom (shown by the arrow labeled C), indicating that this peak corresponds to carbon atoms in a thin surface contamination layer. The broad peak appears at slightly lower energy than the energy for surface boron atoms (shown by the arrow labeled B), suggesting that this broad peak corresponds to the implanted boron atoms. In order to confirm this, a channeling spectrum was measured with a sample which was implanted at U ¼ 1 1013 cm2 . The observed spectrum is shown by open circles. Although there is a surface carbon peak, a broad peak seen in the heavily implanted sample (U ¼ 2 1015 cm2 ) disappears, indicating that the observed broad peak corresponds to the implanted B atoms. 500 keV He+
50
5
C 0
0
2
6
0 10
8
Fig. 2. Depth profiles of Si, O, B and C in the 0.5-keV Bþ ion implanted Si with a fluence of 2 1015 cm2 . The obtained boron profile agrees with the TRIM simulation (solid line). There is no surface narrow peak which is usually observed in SIMS measurement.
The depth profiles of Si, O, C and B in the sample of U ¼ 2 1015 cm2 were derived from the HRBS spectrum and are shown in Fig. 2. The B-profile has a peak at 3.5 nm and the peak concentration is about 7 at.%. There is no narrow surface concentration peak which was always observed in SIMS measurements. The B profile calculated by the TRIM code (SRIM 2000) is also shown by a solid curve for comparison. The agreement between the observed profile and the calculated one is reasonably good, showing that the TRIM simulation is accurate even for 0.5 keV B ion implantation. From a practical point of view, it is important to know down to what energy the TRIM simulation is accurate. To this end, the sample implanted with 0.2 keV Bþ ion at U ¼ 1 1015 cm2 was measured by HRBS. Fig. 3 shows the comparison between the observed boron profile (closed circles) with the TRIM simulation (solid line). The agreement is roughly good although the observed distribution has a slightly longer tail in the deeper region. This tail might be ascribed to
0.5–keV B+ implanted Si(001)
60000
0.2–keV B+ implanted Si(001) Φ = 1×1015 cm–2
Si
10
100
random
Si TRIM
40000
B
C O ×5
20000
5
50
B
× 10 [111]channeling
0 360
4
O C
380
400
420
440
460
Fig. 1. HRBS spectra observed under [1 1 1] channeling conditions for 0.5-keV Bþ ion implanted Si with a fluence of 2 1015 cm2 (solid circles) and 1 1013 cm2 (open circles). The random spectrum for the 2 1015 cm2 sample is also shown (triangles) for comparison.
0
0
1
2
3
4
5
6
7
0
Fig. 3. Depth profiles of Si, O, B and C in the 0.2-keV Bþ ion implanted Si with a fluence of 2 1015 cm2 . The observed boron profile has a longer tail than the TRIM simulation (solid line).
K. Kimura et al. / Current Applied Physics 3 (2003) 9–11
the channeling effect. The critical angle of Si[0 0 1] axial channeling for 0.2 keV Bþ is calculated to be about 12° using the formula given by Lindhard and Dan [5]. This suggests that the channeling effect is difficult to be avoided in this ultralow energy ion implantation.
11
Acknowledgements We are grateful to the members of Quantum Science and Engineering Center of Kyoto University for use of the 4 MV Van de Graaff accelerator. This work was supported in part by grant-in-aid for Scientific Research from Japan Society for the Promotion of Science.
4. Conclusion Depth profiles of ultralow-energy (down to 0.2 keV) B ion implants in Si(0 0 1) samples are successfully measured by high-resolution RBS. The observed boron profile does not show a narrow surface concentration peak which is usually seen in SIMS measurement. The obtained boron profiles roughly agree with TRIM simulation even at 0.2-keV B ion implantation although there is a longer tail in deeper region than the TRIM profile. This tail might be attributed to the channeling effect.
References [1] C.W. Magee, D. Jacobson, H.-J. Gossmann, J. Vac. Sci. Technol. B 18 (2000) 489. [2] T.-S. Wang, A.G. Cullis, E.J.H. Collart, A.J. Murrel, M.A. Foad, Appl. Phys. Lett. 77 (2000) 3586. [3] J.F. Ziegler, J.P. Biersack, U.L. Littmark, The Stopping and Range of Ions in Solids, Pergamon Press, New York, 1985. [4] K. Kimura, K. Ohshima, M. Mannami, Appl. Phys. Lett. 64 (1994) 2232. [5] J. Lindhard, K. Dan, Vidensk. Selsk. Mat.-Fys. Medd. 34 (1965) 14.