- Email: [email protected]
Using accelerator techniques to verify details in SIMS profiles
Nuclear Instruments and Methods in Physics Research B 241 (2005) 321–325 www.elsevier.com/locate/nimb
Using accelerator techniques to verify details in SIMS profiles Steven W. Novak *, Charles W. Magee, Temel Bu¨yu¨klimanli Evans East, 104 Windsor Center Drive, East Windsor, NJ 08520, USA Available online 18 August 2005
Abstract Recent use of ultra-low energy (ULE) ion implantation in the semiconductor industry has placed increasing pressure on SIMS depth profiling capabilities. We have used nuclear reaction analysis (NRA) to calibrate implantation doses of boron ULE implants as an essential check of the SIMS accuracy. Comparison of NRA done in two different laboratories has revealed differences of up to 15%, making it essential to calibrate the measurements against an accurate standard. It has become evident in the last few years that the accepted SIMS depth profiling method for ULE B implants, low-energy oxygen bombardment with an oxygen leak, causes significant segregation of boron during the SIMS profile. High-resolution ERD measurements appear to be one of the few techniques other than SIMS that can resolve the nearsurface details of the boron distribution. Careful comparisons of the ERD and SIMS profiles allow us to refine our SIMS correction procedures to produce the most accurate measurement of the boron distribution. Similarly, ULE As implants show significant segregation of As toward the native oxide interface as a result of annealing. To ensure that we measure these distributions accurately, MEIS has been employed to examine the As distribution within 10 nm of the surface. 2005 Elsevier B.V. All rights reserved.
1. Introduction Since the late 1970s, accelerator-based techniques like Rutherford backscattering spectrometry (RBS) and nuclear reaction analysis (NRA) have been used to confirm or check details in SIMS depth profiles [1–3]. As the SIMS technique has progressed, more and more studies have compared the results from different techniques with SIMS *
Corresponding author. Tel.: +1 609 371 4800; fax: +1 609 371 5666. E-mail address: [email protected] (S.W. Novak).
profiles to check ion-implanted doses or profile shapes [4–6]. An area of intense recent interest in SIMS depth profiling is measurement of ultralow energy (ULE) implants in Si, particularly for characterizing B and As implants. Development and investigation of ULE implantation is driven by the need to make ever-shallower junctions as device dimensions shrink in semiconductor devices. Presently, ion implants are being investigated at energies of less than 1 keV, sometimes with polyatomic species, which places much of the implant within and immediately beneath the native oxide region of a silicon wafer. The transition from the
0168-583X/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2005.07.064
322
S.W. Novak et al. / Nucl. Instr. and Meth. in Phys. Res. B 241 (2005) 321–325
Table 1 Comparison of boron NRA doses and SIMS doses for ion implants measured at two different NRA laboratories Implant energy (keV)
Lab A
Lab B
Raw NRA
SIMS
1 2 3 8 14
10 70 40 0.5 3
1.06E15 4.70E15 1.11E15
9.24E14 4.09E15 9.58E15
14.5 14.9 15.8
1.22E16
1.06E16
14.9
Error (%)
SIMS
Error (%)
8.73E14 3.73E15 9.16E15 4.18E14 1.01E16
9.24E14 4.09E15 9.58E15 4.38E14 1.06E16
5.8 9.7 4.6 4.8 4.9
15.0 ± 0.5 (3%)
2. Using NRA to measure B dose One of the primary purposes of a SIMS depth profile is to provide an accurate total dose for the implant being measured. NRA has been used for a number of years to provide accurate total doses for ion implants [2,4]. The B dose is measured using the 11B(p, a)8Be reaction [6]. In our recent work, we need very accurate dose measurements in order to check that the SIMS profiles are providing accurate dose. Both the concentration scale and depth scale must be accurate to provide the correct dose in a SIMS profile. Because we know we can make the depth scale accurate to within a few percent, correlation of NRA with SIMS will check the accuracy of the concentration scale. To ensure accurate correlation of SIMS with NRA, we have calibrated four ion-implanted samples against NIST standard SRM-2137 (a 50 keV 10B implant with a nominal 1 · 1015 atoms/cm2 dose). These implants have high enough energies (>10 keV) that there is no error introduced due to the surface oxide or transient zone. With a demonstrated precision of less than 2%, and an accuracy of 3% for the NIST SRM,
Ave. 6.0 ± 1.9 (32%)
these measurements should provide B doses accurate to within 5%. These samples have been measured in two different NRA laboratories and the results are given in Table 1. Measurement conditions in lab B used a 660 keV proton beam with emitted alpha particles measured using a 3.5 cm surface barrier detector positioned 5 cm from the target at an angle of 150 from normal. The surface of the detector is covered with a mylar film that is thick enough to stop backscattered protons. Similar conditions were used in the other laboratory. The measurements show that one laboratory yields doses that are uniformly 15% low and another yields doses that are 5% higher than the SIMS doses. Each lab used its own internal standard. Subsequent measurement of the standard used in laboratory A showed the dose to be approximately 15% low. The NRA measurements demonstrate the
15
4.0x10
15
3.5x10
15
3.0x10
1:
native oxide to silicon is a particularly challenging region for quantitative SIMS, as has been discussed in numerous publications [7]. We have been developing SIMS protocols for quantifying ULE ion implants in the near-surface region over the past 10 years and have utilized several accelerator techniques to check ion implant dose or profile shape, thereby confirming the accuracy of the SIMS measurements. In this paper we will show these comparisons.
NRA Dose (at/cm2)
Ave.
Raw NRA
1
Sample
15
2.5x10
15
2.0x10
15
1.5x10
15
1.0x10
14
5.0x10 0.0
0.0
14
5.0x10
15
15
15
15
15
1.0x10 1.5x10 2.0x10 2.5x10 3.0x10
15
3.5x10
15
4.0x10
SIMS Dose (at/cm2)
Fig. 1. Comparison of doses measured by NRA with doses measured by SIMS for ULE B implants (500 eV BF3).
S.W. Novak et al. / Nucl. Instr. and Meth. in Phys. Res. B 241 (2005) 321–325
in the atomic percent range, which may be above the linear range of the SIMS calibration.
good precision of the technique, but also point out that the local standard used for calibration is important in providing accurate dose measurements. Once the higher energy calibrated SIMS standards are used to adjust the NRA measurements, we find excellent agreement between NRA and SIMS for a wide dose range of ULE implants (Fig. 1). In this set of measurements, the average difference between the NRA and SIMS measurements is 5%. The two highest dose samples have peak B concentrations
3. Comparison of ULE SIMS profiles with high-resolution ERD measurements
106
22
105
Si→
10
---as implanted 20
104
10
----annealed 19
103
10
18
10
100
17
10
10 0
5
(A)
10
15
20
25
DEPTH (nm)
22
106
10
B CONCENTRATION (atoms/cc)
Si Si→ Si→
21
105
10
20
104
10
---B (annealed) 19
10
103
--B (as implanted) implanted)
18
100
10
17
10
10 0
(B)
5
10
15
20
Si SECONDARY ION INTENSITY (counts/sec)
B CONCENTRATION (atoms/cc)
Si→
Si SECONDARY ION INTENSITY (counts/sec)
Although comparison of SIMS and NRA can verify that we measure the total dose correctly, it is also important to measure the shape of the
10
21
323
25
DEPTH (nm)
Fig. 2. As-implanted and annealed ULE B implant (1E15/cm2, 500 eV) analyzed by SIMS using oxygen bombardment (A) with oxygen flood and (B) without flood.
S.W. Novak et al. / Nucl. Instr. and Meth. in Phys. Res. B 241 (2005) 321–325
implant distribution correctly, both before and after annealing. Within the last 6 years, the preferred technique for measuring ULE B implants is SIMS using oxygen bombardment with oxygen leak, or the equivalent of normal incidence oxygen bombardment. However, recently it has become evident that this method distorts the shape of the B profile within the top 10 nm of the profile [7,8] evidently due to migration of B toward the interface with the native oxide (Fig. 2(A)). This effect is even more of a problem with annealed samples, in which the B strongly collects at the oxide/Si interface (Fig. 3). We have been developing a new SIMS method that does not use oxygen leak, which appears to eliminate the near-surface artifact of the oxygen leak technique (Fig. 2(B)). At this time it appears that high-resolution elastic recoil detection (ERD) is the only other technique that has sufficient sensitivity and depth resolution to check the accuracy of the SIMS profile in the native oxide/Si interface region (see Bergmaier and Dollinger, this proceedings). Fig. 3 shows a comparison of SIMS profiles acquired using the no-leak measurement with high-resolution ERD measurements [7,9]. It is evident that the shape of the B profile matches the ERD profile well within the native oxide and within the substrate; however the interval just beneath the sharp spike at the oxide interface is broader in the SIMS profile.
11B CONCENTRATION (atoms/cc)
1E+22
Tailing in this region may be due to a knock-on effect in the SIMS profile. This comparison strongly supports the use of SIMS at oblique incidence without oxygen flooding as the preferred method for measuring ULE B implants.
4. Comparison of ULE As profiles with MEIS We have recently developed a SIMS method for analysis of ULE As implants using Cs bombardment with negative secondary ion detection [10]. This method takes account of the changing ion yield between the native oxide and Si substrate and allows us to plot quantitative ULE As profiles through the surface oxide. To check that we are plotting the SIMS profile correctly we can compare with measurements done using medium energy ion scattering (MEIS) [11]. This low energy variation of RBS uses 100 keV protons as projectiles, which yields sufficient depth resolution (about 0.3 nm) and sensitivity for As to check the shape of an implant profile within 6 nm of the surface. Fig. 4 gives a comparison of SIMS with MEIS for a sample implanted with AsF5 at 5 keV (net energy of 2.2 keV). Although the data point spacing of the MEIS model is considerably less than that of SIMS, the concentrations and profile shapes match quite well and the doses differ
100
Oxygen ERD Oxygen SIMS Boron ERD
1E+21
10
Boron SIMS
1E+20
1
1E+19 0
50
100
150
O CONCENTRATION (atom%)
324
0.1 200
Depth (Å) Fig. 3. Annealed ion implant (500 eV B, 1E15/cm2) analyzed by SIMS (solid lines) and ERD (dotted lines).
S.W. Novak et al. / Nucl. Instr. and Meth. in Phys. Res. B 241 (2005) 321–325 23
of SIMS profiles with both techniques, provided proper correction procedures are used for the SIMS profiles.
100
10
SIMS dose = 8.1E14/cm2
Oxygen
10
F
10
As SIMS 21
10
As MEIS
1
20
10
19
10
0
10
20
30
40
50
60
70
O INTENSITY (arb. units)
As CONCENTRATION (cts/sec)
MEIS dose = 9.2E14/cm2 22
325
0.1 80
Acknowledgments We thank W. Vandervorst for providing the high-resolution ERD measurements. T. Gustafsson and E. Garfunkel provided the MEIS measurements.
References
DEPTH (Å)
Fig. 4. Comparison of SIMS and MEIS profiles for a 5 keV AsF5 1E15/cm2.
by only 14%. This comparison demonstrates the SIMS corrections properly account for ion yield changes in the native oxide region.
5. Conclusions We have used NRA, ERD and MEIS measurements to check the validity of recently developed SIMS protocols for measuring ULE B and As implants. The NRA measurements showed errors of up to 15% at different laboratories, however proper calibration yields data that agree with SIMS to within 5%. Both ERD and MEIS measurements have been used to check the shape of ULE B and As profiles, respectively. We find good agreement
[1] G.J. Clark, C.W. White, D.D. Allred, B.R. Appleton, C.W. Magee, D.E. Carlson, Appl. Phys. Lett. 31 (9) (1977) 1. [2] C.W. Magee, Nucl. Instr. and Meth. 191 (1981) 297. [3] W. Vandervorst, F.R. Shepherd, M.C. Swanson, H.H. Plattner, O.M. Wescott, I.V. Mitchell, Nucl. Instr. and Meth. B 15 (April) (1986) 201. [4] H. Kobayashi, W.M. Gibson, Nucl. Instr. and Meth. B 152 (1999) 365. [5] M. Tomita, M. Suzuki, T. Tachibe, S. Kozuka, A. Murakoshi, Appl. Surf. Sci. 203–204 (2003) 377. [6] C.W. Magee, D. Jacobson, Appl. Surf. Sci. 203–204 (2003) 310. [7] W. Vandervorst, T. Janssens, B. Brijs, T. Conard, C. Huyghebaert, J. Fruhauf, A. Bergmaier, G. Dollinger, T. Bu¨yu¨klimanli, J.A. VandenBerg, K. Kimura, Appl. Surf. Sci. 231–232 (2004) 618. [8] K. Kimura, Y. Oota, K. Nakijima, T. Bu¨yu¨klimanli, Curr. Appl. Phys. 3 (2003) 9. [9] G. Dollinger, C.M. Frey, A. Bergmaier, T. Faestermann, Nucl. Instr. and Meth. B 136–138 (1998) 603. [10] T. Bu¨yu¨klimanli, J. Marino, S.W. Novak, Appl. Surf. Sci. 231–232 (2004) 636. [11] T. Gustafsson, H.C. Lu, B.W. Busch, W.H. Schulte, E. Garfunkel, Nucl. Instr. and Meth. B 183 (2001) 146.
Using accelerator techniques to verify details in SIMS profiles Steven W. Novak *, Charles W. Magee, Temel Bu¨yu¨klimanli Evans East, 104 Windsor Center Drive, East Windsor, NJ 08520, USA Available online 18 August 2005
Abstract Recent use of ultra-low energy (ULE) ion implantation in the semiconductor industry has placed increasing pressure on SIMS depth profiling capabilities. We have used nuclear reaction analysis (NRA) to calibrate implantation doses of boron ULE implants as an essential check of the SIMS accuracy. Comparison of NRA done in two different laboratories has revealed differences of up to 15%, making it essential to calibrate the measurements against an accurate standard. It has become evident in the last few years that the accepted SIMS depth profiling method for ULE B implants, low-energy oxygen bombardment with an oxygen leak, causes significant segregation of boron during the SIMS profile. High-resolution ERD measurements appear to be one of the few techniques other than SIMS that can resolve the nearsurface details of the boron distribution. Careful comparisons of the ERD and SIMS profiles allow us to refine our SIMS correction procedures to produce the most accurate measurement of the boron distribution. Similarly, ULE As implants show significant segregation of As toward the native oxide interface as a result of annealing. To ensure that we measure these distributions accurately, MEIS has been employed to examine the As distribution within 10 nm of the surface. 2005 Elsevier B.V. All rights reserved.
1. Introduction Since the late 1970s, accelerator-based techniques like Rutherford backscattering spectrometry (RBS) and nuclear reaction analysis (NRA) have been used to confirm or check details in SIMS depth profiles [1–3]. As the SIMS technique has progressed, more and more studies have compared the results from different techniques with SIMS *
Corresponding author. Tel.: +1 609 371 4800; fax: +1 609 371 5666. E-mail address: [email protected] (S.W. Novak).
profiles to check ion-implanted doses or profile shapes [4–6]. An area of intense recent interest in SIMS depth profiling is measurement of ultralow energy (ULE) implants in Si, particularly for characterizing B and As implants. Development and investigation of ULE implantation is driven by the need to make ever-shallower junctions as device dimensions shrink in semiconductor devices. Presently, ion implants are being investigated at energies of less than 1 keV, sometimes with polyatomic species, which places much of the implant within and immediately beneath the native oxide region of a silicon wafer. The transition from the
0168-583X/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2005.07.064
322
S.W. Novak et al. / Nucl. Instr. and Meth. in Phys. Res. B 241 (2005) 321–325
Table 1 Comparison of boron NRA doses and SIMS doses for ion implants measured at two different NRA laboratories Implant energy (keV)
Lab A
Lab B
Raw NRA
SIMS
1 2 3 8 14
10 70 40 0.5 3
1.06E15 4.70E15 1.11E15
9.24E14 4.09E15 9.58E15
14.5 14.9 15.8
1.22E16
1.06E16
14.9
Error (%)
SIMS
Error (%)
8.73E14 3.73E15 9.16E15 4.18E14 1.01E16
9.24E14 4.09E15 9.58E15 4.38E14 1.06E16
5.8 9.7 4.6 4.8 4.9
15.0 ± 0.5 (3%)
2. Using NRA to measure B dose One of the primary purposes of a SIMS depth profile is to provide an accurate total dose for the implant being measured. NRA has been used for a number of years to provide accurate total doses for ion implants [2,4]. The B dose is measured using the 11B(p, a)8Be reaction [6]. In our recent work, we need very accurate dose measurements in order to check that the SIMS profiles are providing accurate dose. Both the concentration scale and depth scale must be accurate to provide the correct dose in a SIMS profile. Because we know we can make the depth scale accurate to within a few percent, correlation of NRA with SIMS will check the accuracy of the concentration scale. To ensure accurate correlation of SIMS with NRA, we have calibrated four ion-implanted samples against NIST standard SRM-2137 (a 50 keV 10B implant with a nominal 1 · 1015 atoms/cm2 dose). These implants have high enough energies (>10 keV) that there is no error introduced due to the surface oxide or transient zone. With a demonstrated precision of less than 2%, and an accuracy of 3% for the NIST SRM,
Ave. 6.0 ± 1.9 (32%)
these measurements should provide B doses accurate to within 5%. These samples have been measured in two different NRA laboratories and the results are given in Table 1. Measurement conditions in lab B used a 660 keV proton beam with emitted alpha particles measured using a 3.5 cm surface barrier detector positioned 5 cm from the target at an angle of 150 from normal. The surface of the detector is covered with a mylar film that is thick enough to stop backscattered protons. Similar conditions were used in the other laboratory. The measurements show that one laboratory yields doses that are uniformly 15% low and another yields doses that are 5% higher than the SIMS doses. Each lab used its own internal standard. Subsequent measurement of the standard used in laboratory A showed the dose to be approximately 15% low. The NRA measurements demonstrate the
15
4.0x10
15
3.5x10
15
3.0x10
1:
native oxide to silicon is a particularly challenging region for quantitative SIMS, as has been discussed in numerous publications [7]. We have been developing SIMS protocols for quantifying ULE ion implants in the near-surface region over the past 10 years and have utilized several accelerator techniques to check ion implant dose or profile shape, thereby confirming the accuracy of the SIMS measurements. In this paper we will show these comparisons.
NRA Dose (at/cm2)
Ave.
Raw NRA
1
Sample
15
2.5x10
15
2.0x10
15
1.5x10
15
1.0x10
14
5.0x10 0.0
0.0
14
5.0x10
15
15
15
15
15
1.0x10 1.5x10 2.0x10 2.5x10 3.0x10
15
3.5x10
15
4.0x10
SIMS Dose (at/cm2)
Fig. 1. Comparison of doses measured by NRA with doses measured by SIMS for ULE B implants (500 eV BF3).
S.W. Novak et al. / Nucl. Instr. and Meth. in Phys. Res. B 241 (2005) 321–325
in the atomic percent range, which may be above the linear range of the SIMS calibration.
good precision of the technique, but also point out that the local standard used for calibration is important in providing accurate dose measurements. Once the higher energy calibrated SIMS standards are used to adjust the NRA measurements, we find excellent agreement between NRA and SIMS for a wide dose range of ULE implants (Fig. 1). In this set of measurements, the average difference between the NRA and SIMS measurements is 5%. The two highest dose samples have peak B concentrations
3. Comparison of ULE SIMS profiles with high-resolution ERD measurements
106
22
105
Si→
10
---as implanted 20
104
10
----annealed 19
103
10
18
10
100
17
10
10 0
5
(A)
10
15
20
25
DEPTH (nm)
22
106
10
B CONCENTRATION (atoms/cc)
Si Si→ Si→
21
105
10
20
104
10
---B (annealed) 19
10
103
--B (as implanted) implanted)
18
100
10
17
10
10 0
(B)
5
10
15
20
Si SECONDARY ION INTENSITY (counts/sec)
B CONCENTRATION (atoms/cc)
Si→
Si SECONDARY ION INTENSITY (counts/sec)
Although comparison of SIMS and NRA can verify that we measure the total dose correctly, it is also important to measure the shape of the
10
21
323
25
DEPTH (nm)
Fig. 2. As-implanted and annealed ULE B implant (1E15/cm2, 500 eV) analyzed by SIMS using oxygen bombardment (A) with oxygen flood and (B) without flood.
S.W. Novak et al. / Nucl. Instr. and Meth. in Phys. Res. B 241 (2005) 321–325
implant distribution correctly, both before and after annealing. Within the last 6 years, the preferred technique for measuring ULE B implants is SIMS using oxygen bombardment with oxygen leak, or the equivalent of normal incidence oxygen bombardment. However, recently it has become evident that this method distorts the shape of the B profile within the top 10 nm of the profile [7,8] evidently due to migration of B toward the interface with the native oxide (Fig. 2(A)). This effect is even more of a problem with annealed samples, in which the B strongly collects at the oxide/Si interface (Fig. 3). We have been developing a new SIMS method that does not use oxygen leak, which appears to eliminate the near-surface artifact of the oxygen leak technique (Fig. 2(B)). At this time it appears that high-resolution elastic recoil detection (ERD) is the only other technique that has sufficient sensitivity and depth resolution to check the accuracy of the SIMS profile in the native oxide/Si interface region (see Bergmaier and Dollinger, this proceedings). Fig. 3 shows a comparison of SIMS profiles acquired using the no-leak measurement with high-resolution ERD measurements [7,9]. It is evident that the shape of the B profile matches the ERD profile well within the native oxide and within the substrate; however the interval just beneath the sharp spike at the oxide interface is broader in the SIMS profile.
11B CONCENTRATION (atoms/cc)
1E+22
Tailing in this region may be due to a knock-on effect in the SIMS profile. This comparison strongly supports the use of SIMS at oblique incidence without oxygen flooding as the preferred method for measuring ULE B implants.
4. Comparison of ULE As profiles with MEIS We have recently developed a SIMS method for analysis of ULE As implants using Cs bombardment with negative secondary ion detection [10]. This method takes account of the changing ion yield between the native oxide and Si substrate and allows us to plot quantitative ULE As profiles through the surface oxide. To check that we are plotting the SIMS profile correctly we can compare with measurements done using medium energy ion scattering (MEIS) [11]. This low energy variation of RBS uses 100 keV protons as projectiles, which yields sufficient depth resolution (about 0.3 nm) and sensitivity for As to check the shape of an implant profile within 6 nm of the surface. Fig. 4 gives a comparison of SIMS with MEIS for a sample implanted with AsF5 at 5 keV (net energy of 2.2 keV). Although the data point spacing of the MEIS model is considerably less than that of SIMS, the concentrations and profile shapes match quite well and the doses differ
100
Oxygen ERD Oxygen SIMS Boron ERD
1E+21
10
Boron SIMS
1E+20
1
1E+19 0
50
100
150
O CONCENTRATION (atom%)
324
0.1 200
Depth (Å) Fig. 3. Annealed ion implant (500 eV B, 1E15/cm2) analyzed by SIMS (solid lines) and ERD (dotted lines).
S.W. Novak et al. / Nucl. Instr. and Meth. in Phys. Res. B 241 (2005) 321–325 23
of SIMS profiles with both techniques, provided proper correction procedures are used for the SIMS profiles.
100
10
SIMS dose = 8.1E14/cm2
Oxygen
10
F
10
As SIMS 21
10
As MEIS
1
20
10
19
10
0
10
20
30
40
50
60
70
O INTENSITY (arb. units)
As CONCENTRATION (cts/sec)
MEIS dose = 9.2E14/cm2 22
325
0.1 80
Acknowledgments We thank W. Vandervorst for providing the high-resolution ERD measurements. T. Gustafsson and E. Garfunkel provided the MEIS measurements.
References
DEPTH (Å)
Fig. 4. Comparison of SIMS and MEIS profiles for a 5 keV AsF5 1E15/cm2.
by only 14%. This comparison demonstrates the SIMS corrections properly account for ion yield changes in the native oxide region.
5. Conclusions We have used NRA, ERD and MEIS measurements to check the validity of recently developed SIMS protocols for measuring ULE B and As implants. The NRA measurements showed errors of up to 15% at different laboratories, however proper calibration yields data that agree with SIMS to within 5%. Both ERD and MEIS measurements have been used to check the shape of ULE B and As profiles, respectively. We find good agreement
[1] G.J. Clark, C.W. White, D.D. Allred, B.R. Appleton, C.W. Magee, D.E. Carlson, Appl. Phys. Lett. 31 (9) (1977) 1. [2] C.W. Magee, Nucl. Instr. and Meth. 191 (1981) 297. [3] W. Vandervorst, F.R. Shepherd, M.C. Swanson, H.H. Plattner, O.M. Wescott, I.V. Mitchell, Nucl. Instr. and Meth. B 15 (April) (1986) 201. [4] H. Kobayashi, W.M. Gibson, Nucl. Instr. and Meth. B 152 (1999) 365. [5] M. Tomita, M. Suzuki, T. Tachibe, S. Kozuka, A. Murakoshi, Appl. Surf. Sci. 203–204 (2003) 377. [6] C.W. Magee, D. Jacobson, Appl. Surf. Sci. 203–204 (2003) 310. [7] W. Vandervorst, T. Janssens, B. Brijs, T. Conard, C. Huyghebaert, J. Fruhauf, A. Bergmaier, G. Dollinger, T. Bu¨yu¨klimanli, J.A. VandenBerg, K. Kimura, Appl. Surf. Sci. 231–232 (2004) 618. [8] K. Kimura, Y. Oota, K. Nakijima, T. Bu¨yu¨klimanli, Curr. Appl. Phys. 3 (2003) 9. [9] G. Dollinger, C.M. Frey, A. Bergmaier, T. Faestermann, Nucl. Instr. and Meth. B 136–138 (1998) 603. [10] T. Bu¨yu¨klimanli, J. Marino, S.W. Novak, Appl. Surf. Sci. 231–232 (2004) 636. [11] T. Gustafsson, H.C. Lu, B.W. Busch, W.H. Schulte, E. Garfunkel, Nucl. Instr. and Meth. B 183 (2001) 146.