- Email: [email protected]
Stopping cross section measurements of Ti thin films
BoamIntomdons
with Ysteriats & Atoms
Nuclear Instruments and Methods in Physics Research B 136-138 (1998) 109-l 13
ELSWIER
Stopping cross section measurements of Ti thin films A. Climent-Font
a,*, J. R$is%nen b, E. Rauhala b
a Universidad Autonoma de Madrid, Departamento de Fisica Aplicada C-12. Campus de Cantohhnco. ES-28049 Madrid, Spain b Department ofPhysics, Accelerator Laboratory. Universiiy of Helsinki, Siltavuorenpenger 20. 00170 Helsinki, Finlund
Abstract
The stopping cross section for 4He projectiles of Ti thin films have been measured with the backscattering method using a multi-compound marker layer deposited between the test film and the substrate. Problems derived from the nonuniformity of the marker layer as well as corrugations of the films under test have been solved with the help of a computer code to synthesize Rutherford Backscattering Spectrometry (RBS) spectra. Stopping cross section values are obtained with an estimated uncertainty of 4%. 0 1998 Elsevier Science B.V.
1. Introduction Ti is a material of great technological interest in microelectronics. It can form silicides with selective etching properties useful for contact formation in metal-oxide-semiconductor (MOS) devices by means of a process known as self-aligned silicide (SALICIDE) [ 11. Rutherford backscattering spectrometry (RBS) has been a widely used analytical tool in the study of silicide formation [2]. We have performed the present study of the stopping cross section using the backscattering method. Normally these kinds of measurements require very good samples with well defined interfaces in order to be able to extract the information in a straightforward manner from the experiment. The use of molecular beam epitaxy (MBE) grown samples is, therefore, recommended whenever
*Corresponding author. Tel.: 34 1 397 5264; fax: 34 I 397 3969: e-mail: [email protected]. 0168-583X/98/$19.00 0 1998 Elsevier Science B.V. All rights reserved. PIISO168-583X(97)00661-7
available [3]. In some cases MBE methods are not available or simply not suitable for the growth of the specific test film. It is then quite difficult to obtain samples of such a good quality, and one has to cope with problems derived from poorly defined interfaces. In such circumstances the help of computer codes in simulating RBS spectra can be quite useful for the interpretation of the results. We have used this latter situation for the study of the energy loss of 4He ions in Ti thin films grown by magnetron sputtering, and we will show the procedure adopted to extract useful information from samples with non-sharp interfaces.
2. Experimental 2.1. Thin jilnu The samples were prepared at the Department of Applied Physics of the University Autonoma of Madrid. Substrates of polished vitreous carbon
and commercially available silicon wafers were used. A marker layer consistent on a multi-elemental very thin film containing mainly Bi, Sr, Ca, Cu and 0, was deposited on the substrate with a lowrate deposition process, such as ion beam sputtering (Kauffman ion gun 1 kV, 4 x lo-” mbar Ar). The substrates coated with the marker layer were partially covered by Ti thin films with DC magnetron sputtering in an Ar atmosphere ( 1.5 x 10-j mbar). No Ar was detected in the films in subsequent RBS measurements. In order to have samples of good stability, several months elapsed from the deposition of the marker layer to the deposition of the Ti films, and still more time (more than one year in fact) till the stopping experiments were performed. Besides, in the results shown there are two series of measurements made on the same samples with an interval of seven months. 2.2. Stopping meusurements
(experimental
method)
Stopping cross section measurements were made with the 2.5 MV Van de Graaff accelerator of the University of Helsinki. The energy calibration of the beam analyzing magnet was based on the “N(p, CLy)‘lC, 17Al(p,y)‘sSi and ‘3C(p,y)‘4N resonance reactions at E,, = 429, 992, and 1747 keV. respectively. Measurements were made with a “He beam at energies 1.4, 1.6, 1.8, 2.0 and 2.2 MeV. The procedure to measure the stopping values was to measure the shift in energy of the signal peaks from an element in the marker layer for two consecutive RBS measurements obtained with same beam energy and different tilts of the sample as shown in the inset of Fig. 1. Using a multi-elemental marker layer allows one to measure the stopping at several energies with a single incident beam energy. The ingoing paths through the film in our arrangement are the same in both measurements, and the difference in outgoing paths is Nd( (l/ cos 20) - l), where N{i is the area1 density of the test film. The average stopping cross section is then the energy separation between the two peaks divided by the difference in outgoing paths, and it is evaluated at an energy given by the average position of the two signal peaks. After several trials it was found that a satisfactory value of B was 20”. With this value the separation between
the peaks was around 25 or 30 keV. The beam spot size was 0.5 mm diameter and the uniformity of the layer was satisfactorily checked taking RBS spectra from points 1 mm apart from the aimed point. Leblanc et al. [4] have used a very similar method to measure stopping powers. They use gold as a marker layer and instead of tilting the sample they have two detectors at different positions detecting particles with different outgoing paths in one single measurement.
3. Results and discussion When performing stopping cross section measurements using the method described above, the knowledge of the area1 density of the film is the big issue. Obtaining this value means having a good measurement of the product of the irradiation dose and the detector solid angle. This product was measured with a beam chopper system and a reference Cu sample. The stopping cross section for Cu in an energy range of 1.5-2.0 MeV is considered to be accurately known [5] and was checked using the computer code to simulate RBS spectra GISA 3.84 [6]. This code offers the possibility of using different stopping power tables to simulate RBS spectra. It was checked for the values currently used by RUMP [7] and for those of TRIM 91 [8], and the agreement was better than 2%. For the range of energies used (1.4-2.2 MeV), the Ti signal appears in regions of energy higher than the maximum of the stopping cross section curve. A very accurate determination of the area1 density of Ti atoms in the film can be obtained from the integral of the Ti signal, considering a linear dependence for the stopping cross section with energy, and correcting for the energy dependence of the scattering cross section and the screening effect as described in Ref. [9]. The result is fairly independent of the stopping cross section values assumed. When an overlap of signals from the marker layer with the Ti layer occurred, subtraction of the unwanted signals was made relying on RBS spectra taken from a part of the sample uncovered by the film under test. Using this method the area1 density of the Ti films was determined with an uncertainty of ?2’%. RBS was able to de-
III
A. Clirnent-Font et al. I Nucl. Instr. and Meth. in Phys. Rex B 136-138 (1998) 109-113
tect a small concentration of about 1% of oxygen throughout the films, and a thin surface layer (?z 30 x 10” atom/cm2) of TiOz. To test how sharp the interfaces of the samples were the RBS spectra were fitted with simulations made with RUMP. In all cases the amount of Ti that needed to get a good fit of the RBS spectra was within 5% of the value obtained using the method of Ref. [9]. Fig. 1 shows the experimental (dotted) and simulated (continuous line) spectra for the Ti sample with the marker film. The Ti signal is located between channels 280 and 375 and, for the actual incident energy of the 4He ion (1.6 MeV) it overlaps the Sr and Cu signal of the marker layer. The analysis of the spectra with RUMP showed that the interfaces were not sharp. This can be explained by assuming that the deposited layers are corrugated, as shown schematically in Fig. 2. This shows how in an RBS experiment the surface corrugation affects the inner interfaces. This corrugation of the surface was already detected during the profilometer measurements made to estimate the thickness of the films. Comparison of RBS spectra from a sample with portions covered and uncovered by the test film also gave evidence of the corrugation of the film. These spectra also showed that the multi-elemental marker layer did not have a constant composition profile.
Energy
Simulation10obtain reference points in the signal peaks of the
marker layer
Fig. 2. Schematic representation of the effect of the corrugation of the surface in the inner interface and model to obtain reference points in the signal peaks of the marker layer. See text for details.
To locate reference points in the peaks of the marker layer we adopted the following procedure, sketched in Fig. 2. Using RUMP, from the edited sample which gave a satisfactory fit of the experimental RBS spectrum, we removed all the layers from the surface inward that did not have elements of the marker layer, e.g. Bi. The simulated RBS spectrum of the new edited sample with some layers removed, will have the marker element near the surface and its surface position can be located in the signal peak. The ratio h/H can then be obtained as shown in Fig. 3. This ratio is dependent
Energy
(MeV) 1 OxlO
/
1 25 /
1 30 /
1 35 /
(MeV) 1 40
1 45
I 50 /
l;‘::~~
420
200
3bo
440
Channel
Channel Fig. 1. 1.6 MeV RBS spectrum of Ti on carbon substrate with a marker layer of Bi, Sr. Cu. Ca, and 0 (thickness of the Ti film 1010 x lOI Ti atom/cm? +2’%,).
Fig. 3. Procedure to obtain the reference points in the signal peak from a modified edition of the sample (spectrum in continuous line) and allocation of the point in the signal of the experimental spectrum.
112
A. Climmt-Font
rt al. I Nucl. Instr. and Meth. in Phys. Rex B 136-138 (1998) 109%113
on the tilting position of the sample and is used to locate the reference point in the signal peak of the marker layer of the experimental RBS spectrum as shown in Fig. 3 (dotted spectrum). We found that the energy separation between maxima of the peaks for the two tilt positions is larger, in more than lo%, than the separation according to the h/H ratio. Fig. 4 shows the stopping cross section values so obtained for Ti films. Also shown in the figure are values given by TRIM 91, Ziegler and Chu [lo] and G&z and Gartner [I 11. Correction for the detector’s dead layer was made for energies of 400 keV and lower. A similar method to the one used for obtaining the stopping using the peaks in the marker layer can be applied using the edge of the carbon substrate. In this case the reference point is found from the simulated RBS spectrum after removing in the edition of the sample all the layers that do not contain carbon. Table 1 lists the values of Fig. 4 indicating which peak of the marker layer was used to measure the stopping cross section. The energy of the 4He incident beam is indicated in parenthesis. The inaccuracy of the present measured stopping cross sections is due to the uncertainty in measuring the energy shift of the peaks (detector calibration, non-linearity of the detector response, localization of the reference points, positioning of the sample) which is less than 3%, dose calibration, 2%, and area1 density, 2% for the Ti film. Combi-
Table 1 Measured values of the stopping cross section of Ti thin films for 4He ions with indication of the marker peak element used as reference. C is for the carbon substrate. Indicated in parenthesis is the energy in MeV of the incident beam Energy
(keV)
241 300 352 391 403 433 466 545 621 691 1130 1320 1520 1525 1701 1710 1904
Ti (IO-” 85.9 81.3 87.4 95.4 89.6 96.5 91.2 98.1 96.3 104.7 92.8 83.0 77.8 79.6 84.2 77.1 79.4
eV cm’/at)
Marker
element
C( 1.4) C(1.6) C(1.8) O(1.4) C(2.0) C(2.2) O(1.6) O(l.8) O(2.0) O(2.2) Bi(1.4) Bi(1.6) Bi( 1.8) Sr(2.0) Sr(2.2) Bi(2.0) Bi(2.2)
nation of these uncertainties gives an inaccuracy of 4%.
4. Conclusions The stopping cross section of 4He in Ti thin films has been measured in the energy range 2401900 keV using the backscattering method with a multi-element marker layer to have values at several energies from one incident energy setting. The difficulties arising from the corrugated character of the samples and the non-uniformity of the composition of the marker layer have been solved with the help of a computer code to simulate RBS spectra. The values obtained for the Ti film are close to those given by Ziegler and Chu [lo].
TilOO.01
Energy/keV I
Thiswork
+
Zegler-Chu --Gotz-Gartner'-TRIM
Fig. 4. Stopping values measured ed uncertainty of 4%.
91
for ‘He in Ti with an estimat-
Acknowledgements
We wish to thank Mr. Raimo Ingren for his assistance during the experiments with the Van de
A. Climent-Font et al. I Nucl. Instr. and Mrth. in Phys. Rex B 136-138 (1998) 109-113
Graaff accelerator. Support from the Local Government of Madrid through the grant CAM AE00140194 is gratefully acknowledged.
References [I] W. Kaplan, S-L. Zhang, H. Norstrom, M. bstling, A. Lindberg. Applied surface Science 53 (1991) 338. [2] A.J. Barcz, M. Bartur, MRS Symp. Proc. 25 (1984) 33. [3] D. Niemann. G. Konac, S. Kalbitzer, Nucl. Instr. and Meth. B 118 (1996) 11.
113
[41 L. Leblanc, G.G. Ross. Nucl. Instr. and Meth. B 118 (1996) 19. 151 K. Vakeviiinen, Nucl. Instr. and Meth. B 122 (1997) 187. 161 J. Saarilahti. E. Rauhala, Nucl. Instr. and Meth. B 64 (1992) 734. [71 L. Doolittle. Nucl. Instr. and Meth. B 9 (1985) 344. PI J.P. Biersack, L.G. Haggmark, Nucl. Instr. and Meth. 174 (1980) 257. [91 A. Climent-Font, Nucl. Instr. and Meth. B 61 (1991) 541. UOI J.F. Ziegler, W.K. Chu. Atom. Data Nucl. Data Tables 13 (1974) 481. 1111 G. Gotz. K. Gartner (Eds.), High Energy lon Beam Analysis of Solids. Akademie Verlag. Berlin. 1988.
with Ysteriats & Atoms
Nuclear Instruments and Methods in Physics Research B 136-138 (1998) 109-l 13
ELSWIER
Stopping cross section measurements of Ti thin films A. Climent-Font
a,*, J. R$is%nen b, E. Rauhala b
a Universidad Autonoma de Madrid, Departamento de Fisica Aplicada C-12. Campus de Cantohhnco. ES-28049 Madrid, Spain b Department ofPhysics, Accelerator Laboratory. Universiiy of Helsinki, Siltavuorenpenger 20. 00170 Helsinki, Finlund
Abstract
The stopping cross section for 4He projectiles of Ti thin films have been measured with the backscattering method using a multi-compound marker layer deposited between the test film and the substrate. Problems derived from the nonuniformity of the marker layer as well as corrugations of the films under test have been solved with the help of a computer code to synthesize Rutherford Backscattering Spectrometry (RBS) spectra. Stopping cross section values are obtained with an estimated uncertainty of 4%. 0 1998 Elsevier Science B.V.
1. Introduction Ti is a material of great technological interest in microelectronics. It can form silicides with selective etching properties useful for contact formation in metal-oxide-semiconductor (MOS) devices by means of a process known as self-aligned silicide (SALICIDE) [ 11. Rutherford backscattering spectrometry (RBS) has been a widely used analytical tool in the study of silicide formation [2]. We have performed the present study of the stopping cross section using the backscattering method. Normally these kinds of measurements require very good samples with well defined interfaces in order to be able to extract the information in a straightforward manner from the experiment. The use of molecular beam epitaxy (MBE) grown samples is, therefore, recommended whenever
*Corresponding author. Tel.: 34 1 397 5264; fax: 34 I 397 3969: e-mail: [email protected]. 0168-583X/98/$19.00 0 1998 Elsevier Science B.V. All rights reserved. PIISO168-583X(97)00661-7
available [3]. In some cases MBE methods are not available or simply not suitable for the growth of the specific test film. It is then quite difficult to obtain samples of such a good quality, and one has to cope with problems derived from poorly defined interfaces. In such circumstances the help of computer codes in simulating RBS spectra can be quite useful for the interpretation of the results. We have used this latter situation for the study of the energy loss of 4He ions in Ti thin films grown by magnetron sputtering, and we will show the procedure adopted to extract useful information from samples with non-sharp interfaces.
2. Experimental 2.1. Thin jilnu The samples were prepared at the Department of Applied Physics of the University Autonoma of Madrid. Substrates of polished vitreous carbon
and commercially available silicon wafers were used. A marker layer consistent on a multi-elemental very thin film containing mainly Bi, Sr, Ca, Cu and 0, was deposited on the substrate with a lowrate deposition process, such as ion beam sputtering (Kauffman ion gun 1 kV, 4 x lo-” mbar Ar). The substrates coated with the marker layer were partially covered by Ti thin films with DC magnetron sputtering in an Ar atmosphere ( 1.5 x 10-j mbar). No Ar was detected in the films in subsequent RBS measurements. In order to have samples of good stability, several months elapsed from the deposition of the marker layer to the deposition of the Ti films, and still more time (more than one year in fact) till the stopping experiments were performed. Besides, in the results shown there are two series of measurements made on the same samples with an interval of seven months. 2.2. Stopping meusurements
(experimental
method)
Stopping cross section measurements were made with the 2.5 MV Van de Graaff accelerator of the University of Helsinki. The energy calibration of the beam analyzing magnet was based on the “N(p, CLy)‘lC, 17Al(p,y)‘sSi and ‘3C(p,y)‘4N resonance reactions at E,, = 429, 992, and 1747 keV. respectively. Measurements were made with a “He beam at energies 1.4, 1.6, 1.8, 2.0 and 2.2 MeV. The procedure to measure the stopping values was to measure the shift in energy of the signal peaks from an element in the marker layer for two consecutive RBS measurements obtained with same beam energy and different tilts of the sample as shown in the inset of Fig. 1. Using a multi-elemental marker layer allows one to measure the stopping at several energies with a single incident beam energy. The ingoing paths through the film in our arrangement are the same in both measurements, and the difference in outgoing paths is Nd( (l/ cos 20) - l), where N{i is the area1 density of the test film. The average stopping cross section is then the energy separation between the two peaks divided by the difference in outgoing paths, and it is evaluated at an energy given by the average position of the two signal peaks. After several trials it was found that a satisfactory value of B was 20”. With this value the separation between
the peaks was around 25 or 30 keV. The beam spot size was 0.5 mm diameter and the uniformity of the layer was satisfactorily checked taking RBS spectra from points 1 mm apart from the aimed point. Leblanc et al. [4] have used a very similar method to measure stopping powers. They use gold as a marker layer and instead of tilting the sample they have two detectors at different positions detecting particles with different outgoing paths in one single measurement.
3. Results and discussion When performing stopping cross section measurements using the method described above, the knowledge of the area1 density of the film is the big issue. Obtaining this value means having a good measurement of the product of the irradiation dose and the detector solid angle. This product was measured with a beam chopper system and a reference Cu sample. The stopping cross section for Cu in an energy range of 1.5-2.0 MeV is considered to be accurately known [5] and was checked using the computer code to simulate RBS spectra GISA 3.84 [6]. This code offers the possibility of using different stopping power tables to simulate RBS spectra. It was checked for the values currently used by RUMP [7] and for those of TRIM 91 [8], and the agreement was better than 2%. For the range of energies used (1.4-2.2 MeV), the Ti signal appears in regions of energy higher than the maximum of the stopping cross section curve. A very accurate determination of the area1 density of Ti atoms in the film can be obtained from the integral of the Ti signal, considering a linear dependence for the stopping cross section with energy, and correcting for the energy dependence of the scattering cross section and the screening effect as described in Ref. [9]. The result is fairly independent of the stopping cross section values assumed. When an overlap of signals from the marker layer with the Ti layer occurred, subtraction of the unwanted signals was made relying on RBS spectra taken from a part of the sample uncovered by the film under test. Using this method the area1 density of the Ti films was determined with an uncertainty of ?2’%. RBS was able to de-
III
A. Clirnent-Font et al. I Nucl. Instr. and Meth. in Phys. Rex B 136-138 (1998) 109-113
tect a small concentration of about 1% of oxygen throughout the films, and a thin surface layer (?z 30 x 10” atom/cm2) of TiOz. To test how sharp the interfaces of the samples were the RBS spectra were fitted with simulations made with RUMP. In all cases the amount of Ti that needed to get a good fit of the RBS spectra was within 5% of the value obtained using the method of Ref. [9]. Fig. 1 shows the experimental (dotted) and simulated (continuous line) spectra for the Ti sample with the marker film. The Ti signal is located between channels 280 and 375 and, for the actual incident energy of the 4He ion (1.6 MeV) it overlaps the Sr and Cu signal of the marker layer. The analysis of the spectra with RUMP showed that the interfaces were not sharp. This can be explained by assuming that the deposited layers are corrugated, as shown schematically in Fig. 2. This shows how in an RBS experiment the surface corrugation affects the inner interfaces. This corrugation of the surface was already detected during the profilometer measurements made to estimate the thickness of the films. Comparison of RBS spectra from a sample with portions covered and uncovered by the test film also gave evidence of the corrugation of the film. These spectra also showed that the multi-elemental marker layer did not have a constant composition profile.
Energy
Simulation10obtain reference points in the signal peaks of the
marker layer
Fig. 2. Schematic representation of the effect of the corrugation of the surface in the inner interface and model to obtain reference points in the signal peaks of the marker layer. See text for details.
To locate reference points in the peaks of the marker layer we adopted the following procedure, sketched in Fig. 2. Using RUMP, from the edited sample which gave a satisfactory fit of the experimental RBS spectrum, we removed all the layers from the surface inward that did not have elements of the marker layer, e.g. Bi. The simulated RBS spectrum of the new edited sample with some layers removed, will have the marker element near the surface and its surface position can be located in the signal peak. The ratio h/H can then be obtained as shown in Fig. 3. This ratio is dependent
Energy
(MeV) 1 OxlO
/
1 25 /
1 30 /
1 35 /
(MeV) 1 40
1 45
I 50 /
l;‘::~~
420
200
3bo
440
Channel
Channel Fig. 1. 1.6 MeV RBS spectrum of Ti on carbon substrate with a marker layer of Bi, Sr. Cu. Ca, and 0 (thickness of the Ti film 1010 x lOI Ti atom/cm? +2’%,).
Fig. 3. Procedure to obtain the reference points in the signal peak from a modified edition of the sample (spectrum in continuous line) and allocation of the point in the signal of the experimental spectrum.
112
A. Climmt-Font
rt al. I Nucl. Instr. and Meth. in Phys. Rex B 136-138 (1998) 109%113
on the tilting position of the sample and is used to locate the reference point in the signal peak of the marker layer of the experimental RBS spectrum as shown in Fig. 3 (dotted spectrum). We found that the energy separation between maxima of the peaks for the two tilt positions is larger, in more than lo%, than the separation according to the h/H ratio. Fig. 4 shows the stopping cross section values so obtained for Ti films. Also shown in the figure are values given by TRIM 91, Ziegler and Chu [lo] and G&z and Gartner [I 11. Correction for the detector’s dead layer was made for energies of 400 keV and lower. A similar method to the one used for obtaining the stopping using the peaks in the marker layer can be applied using the edge of the carbon substrate. In this case the reference point is found from the simulated RBS spectrum after removing in the edition of the sample all the layers that do not contain carbon. Table 1 lists the values of Fig. 4 indicating which peak of the marker layer was used to measure the stopping cross section. The energy of the 4He incident beam is indicated in parenthesis. The inaccuracy of the present measured stopping cross sections is due to the uncertainty in measuring the energy shift of the peaks (detector calibration, non-linearity of the detector response, localization of the reference points, positioning of the sample) which is less than 3%, dose calibration, 2%, and area1 density, 2% for the Ti film. Combi-
Table 1 Measured values of the stopping cross section of Ti thin films for 4He ions with indication of the marker peak element used as reference. C is for the carbon substrate. Indicated in parenthesis is the energy in MeV of the incident beam Energy
(keV)
241 300 352 391 403 433 466 545 621 691 1130 1320 1520 1525 1701 1710 1904
Ti (IO-” 85.9 81.3 87.4 95.4 89.6 96.5 91.2 98.1 96.3 104.7 92.8 83.0 77.8 79.6 84.2 77.1 79.4
eV cm’/at)
Marker
element
C( 1.4) C(1.6) C(1.8) O(1.4) C(2.0) C(2.2) O(1.6) O(l.8) O(2.0) O(2.2) Bi(1.4) Bi(1.6) Bi( 1.8) Sr(2.0) Sr(2.2) Bi(2.0) Bi(2.2)
nation of these uncertainties gives an inaccuracy of 4%.
4. Conclusions The stopping cross section of 4He in Ti thin films has been measured in the energy range 2401900 keV using the backscattering method with a multi-element marker layer to have values at several energies from one incident energy setting. The difficulties arising from the corrugated character of the samples and the non-uniformity of the composition of the marker layer have been solved with the help of a computer code to simulate RBS spectra. The values obtained for the Ti film are close to those given by Ziegler and Chu [lo].
TilOO.01
Energy/keV I
Thiswork
+
Zegler-Chu --Gotz-Gartner'-TRIM
Fig. 4. Stopping values measured ed uncertainty of 4%.
91
for ‘He in Ti with an estimat-
Acknowledgements
We wish to thank Mr. Raimo Ingren for his assistance during the experiments with the Van de
A. Climent-Font et al. I Nucl. Instr. and Mrth. in Phys. Rex B 136-138 (1998) 109-113
Graaff accelerator. Support from the Local Government of Madrid through the grant CAM AE00140194 is gratefully acknowledged.
References [I] W. Kaplan, S-L. Zhang, H. Norstrom, M. bstling, A. Lindberg. Applied surface Science 53 (1991) 338. [2] A.J. Barcz, M. Bartur, MRS Symp. Proc. 25 (1984) 33. [3] D. Niemann. G. Konac, S. Kalbitzer, Nucl. Instr. and Meth. B 118 (1996) 11.
113
[41 L. Leblanc, G.G. Ross. Nucl. Instr. and Meth. B 118 (1996) 19. 151 K. Vakeviiinen, Nucl. Instr. and Meth. B 122 (1997) 187. 161 J. Saarilahti. E. Rauhala, Nucl. Instr. and Meth. B 64 (1992) 734. [71 L. Doolittle. Nucl. Instr. and Meth. B 9 (1985) 344. PI J.P. Biersack, L.G. Haggmark, Nucl. Instr. and Meth. 174 (1980) 257. [91 A. Climent-Font, Nucl. Instr. and Meth. B 61 (1991) 541. UOI J.F. Ziegler, W.K. Chu. Atom. Data Nucl. Data Tables 13 (1974) 481. 1111 G. Gotz. K. Gartner (Eds.), High Energy lon Beam Analysis of Solids. Akademie Verlag. Berlin. 1988.