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Elastic recoil detection analysis with atomic depth resolution
NOWB
Nuclear Instruments and Methods in Physics Research B79 (1993) 513-517 North-Holland
Beam Intemctions with Yaterlals & Atoms
Elastic recoil detection analysis with atomic depth resolution G. Dollinger Physik-Department E12, Technische Universitiit Miinchen, D-W-8046 Garching, Germany
Elastic recoil detection analysis (ERD) with swift heavy ions has been improved in view of depth resolution. Profiles of light elements could be measured in carbon near the surface with single atomic layer resolution using 5sNi, lz71 or lg7Au beams with energies between 0.5A MeV and 1A MeV and a Q3D magnetic spectrograph. Due to the specific ion optical arrangement of the Q3D a relative energy resolution of about 7 X 10m4 was achieved in ERD experiments although a large solid angle of 5 msr was used. This large acceptance guarantees good statistics even with low ion fluences. In this way the elemental concentration profiles are not essentially disturbed by irradiation damage effects and can be measured in the 0.1% level on an atomic depth scale.
1. Introductioa Elastic recoil detection analysis (ERD) using energetic heavy ions is a suitable method to measure quantitative depth profiles of light and medium heavy elements in thin films [l]. Using the Q3D magnetic spectrograph at the Munich tandem accelerator a depth resolution better than 1 nm could be achieved near the surface when analysing thin carbon films 121.Improvements were introduced now to further increase the depth resolution at the surface and also in deeper areas simultaneously with larger solid angles of detection.
2. Experimental
arrangement
AE = - 2 tan (p Acp + (tar&o - 1) A(p2 + $ tan cp Acp’ E tan2q) A(p4+ **. .
+i(l-
Therefore, a variation of scattering angle 4p= 15” by Arp = f3.2”, which are the conditions used at the Q3D, results in negative first order energy shift AE/E of 6%, 0.28% in the second order and 5 X 10m4 in the
and results
The experimental arrangement for ERD measurements with high depth resolution is shown in fig. 1. It is similar to that described in ref. [2]. In order to reduce irradiation damage effects ion beams of 60 MeV 5*Ni and lz71 were used in contrast to the previous experiments, where 120 MeV Au ions were utilized [2,3]. The ions hit the samples to be investigated at angles of 3.5” to 10” relative to the surface while the scattering angle was always 15”. The recoiled ions were detected with the Munich Q3D magnetic spectrograph [4]. An intrinsic energy resolution of 4 X lop4 was measured with a 212Po a-source (E, = 8.8 MeV) using a 1 m long ionisation chamber with two integrated position sensitive proportional counters as focal plane detector [5]. This high energy resolution can be obtained with 12 msr solid angle of detection. Large detection angles are required when high depth resolution of small elemental amounts is desired, otherwise the original depth profile is destroyed by irradi0168-583X/93/$06.00
ation damage before it is measured 123.But the strong kinematic energy shift AE(p) with scattering angle cp usually limits the solid angle of detection when high energy resolution is required [6-81. The kinematic shift for an angle variation Acp can be expanded in a Taylor series:
MULTIPOLE
--iiiDIPOLE2 I
.E 3
FOCAL P:ANE IONBEAM IONlSATbN CHAMBER
Fig. 1. Experimental arrangement for high resolution depth profiling.
0 1993 - Elsevier Science Publishers B.V. All rights reserved
VII. PIXE, MICROPROBES, . . *
G. Dollinger / Elastic recoil detection anabsk
514
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515
G. LMirtger / Elastic recoil detection analysti
third order of A4p. Only the fourth and higher orders can be neglected if energy resolutions better 10m3 are required. The multipole element of the Q3D, however, is an ion optical element which allows one to correct for the kinematic shift up to the third order. The effect of this correction is shown in fig. 2, where angle-energy spectra are shown, measured with the two position sensitive propo~ional counters of the focal plane detector. An uncorrected spectrum with full kinematic shift is shown in fig. 2a, measured for a 2.2 pg/cm’ thick carbon target, perpendicularily penetrated by 60 MeV 58Ni ions. In that case the recoiled carbon atoms transmitted the foil before detection with the consequence that only a small energy spread of 0.17% of the recoiled ions is expected due to energy loss processes. Due to the large kinematic energy variation a broad band can be located in the angle-~sition spectrum. The angle resolution of the focal plane detector is too low to enable kinematic shift correction with a software routine, because the poor angle resolution would be transferred into a poor energy resolution. The multipole element of the Q3D, however, allows one to correct ion optically for the kinematic shift. After first order correction with a quadrupole field in the multiple a parabola remains due to the second order kinematic shift (fig. 2b). Introducing an additional hexapole field an S-shaped curve remains (fig. 2c) which can be corrected with an additional octupole field (fig. 2d). A straight line in the angleposition dependence remains after these corrections and an energy resolution of about 7 x 10m4 could be measured for the whole experimental arrangement. But the accepted angle ~~endicular to the bending plane of the Q3D magnets had to be reduced to f 1.3” because the kinematic shift in that direction, which is only of second and higher orders, cannot be corrected with the multipole element. This portion reduces the accepted solid angle of the Q3D to 5 msr. With these adjustments the depth resolution was measured at the surface of a bulk graphite monocrystal. For example a i2C4+ signal is plotted versus the carbon depth in fig. 3. It was measured with 60 MeV 58Ni ions entering the surface at a flat angle of 3.5” while the scattering angle was still 15”. The spectrum rises within a relative energy loss of 7 x low4 from 13% to 87% of the first peak value. This slope corresponds to a carbon thickness of 0.34 nm for the given geometry, ion beam and sample conditions. Due to the slow variation of the charge yield (long tail of the dis~bution) this thickness is the depth resolution (FWHIvf) in this measurement which is equivalent to a resolution of about an interlayer spacing of the graphite (002) layers. Two seperated peaks near the surface can also be seen with a 0.34 nm distance and width which seems to be the signal of the first two graphite layers, where each gives a specific energy loss. The same
100 i
0 i
___.m
10
5 depth
5
[nm]
Fig. 3. 12C4* signal as an example of single layer depth resolution at the surface of a single graphite crystal obtained with 40 MeV s8Ni ion beam. The peak near the surface arises due to charge exchange effects of the scattered carbon atoms. The slope at the peak shows a single carbon layer depth resolution.
resolution could be obtained using 60 MeV 1271 ions. There the angle of incidence could be chosen to be 5” relative to the surface because the relative energy loss, which gives the depth resolution, is higher under these ion beam conditions. Another problem using ERD for depth profiling is that depth resolution decreases with increasing depth. Specially a geometrical energy broadening reduces depth resolution with increasing depth besides the noncorrectable effects of small angle scattering and energy loss scattering [6,8] when large detector angles are applied. This geometrical effect arises due to path length differences of the sample atoms detected under different angles. These path length differences lead to different detectable energies and increase linearly with depth. The ion optical performance of the Q3D, however, gives also the possibility to correct for this effect as soon as a hexapole field for the correction of the second order kinematic shift is introduced. When the adjustments of the multipole fields are done at a certain energy (here designated as energy shift zero) a rotation of the angle-energy dependence is obtained at different energies. This rotation can be measured again with a thin carbon foil in transmission geometry, where the path length differences are negligible, by varying all magnetic fields by the same relative amount. That is shown for five different variations in the same picture in fig. 4a. A rotation of the angle-energy spectrum in the opposite direction can be measured when e.g. the 13C profile of thin i3C layers in a 13C/ 12C multilayer sample is investigated in reflection geometry, where 60 MeV 58Ni ions impinged at an angle of 10” to the VII. PIXE, MICROPROBES,
. **
516
G. Dollinger / Elastic ret:oil detection analysis
surface and a scattering angle of 15” was used (fig. 4b). The path length differences induce a rotation in the opposite direction. The sum results in the spectrum in fig. 4b where each stripe corresponds to a 13C layer of the multilayer system. The correction of the path length effect is not perfect, but it can be optimized when an incidence angle of 7.5” is used. At last small angleposition dependences can be corrected for by software after the measurements without a loss of energy resolution.
3. Conclusion Depth resolution at the physical limit using large solid angles can be obtained in ERD measurements for the whole depth investigated using the Munich Q3D magnetic spectrograph with its specific ion optical arrangement. Due to the large solid angle of detection (5 msr) we are now able to measure elemental and isotopic depth profiles on an atomic scale below the 1% level, where the number of displaced atoms stays below
6
4
-4
-6
M
-6
1
-a + -1
0
5
relative
Lqy3shif+
6
7
[“g.]
Fig. 4. Angle-energy spectra (a) of l2C 6+ for a 2.2 &cm2 thick carbon foil at five different magnetic fields and (b) of 13C6+ signal of a 13C/ lzC multilayer system showing an opposite rotation in the angle-energy spectra compared to (a).
517
G. Dollinger / Elastic recoil detection analysis
10% during irradiation [2] and thus the depth profile should remain unchanged. Therefore a wide field is opened for quantitative investigations of surface and interfaces of thin films.
References [l] Proc. High Energy and Heavy Ion Beams in Materials Analysis Workshop, Albuquerque, 1989, eds. J.R. Tesmer, C.J. Maggiore, M. Nastasi, J.C. Barbour and J.W. Mayer (MRS, Pittsburgh, PA). [2] G. Dollinger, T. Faestermann and P. Maier-Komor, Nucl. Instr. and Meth. B64 (1992) 422.
[3] G. Dollinger, M. Boulouednine, T. Faestermann, P. Maier-Komor and H.-J. Kijmer, to be published in Radiat. Eff. Def. Sol. [4] M. LiZfler, H.-J. Scheerer and H. Vonach, Nucl. Instr. and Meth. 111 (1973) 1. IS] W. Mayer, thesis, Technical University of Munich (1985). [6] J.P. Stoquert, G. Guillaume, M. Hage-All, J.J. Grob, C. Ganter and P. Siffert, Nucl. Instr. and Meth. B44 (1989) 184. [7] D.O. Boerma, F. Labohm and J.A. Reinders, Nucl. Instr. and Meth. B.50 (1990) 291. [8] F. Paszti, E. Szil&yi and E. Kotai, Nucl. Instr. and Meth. B54 (1991) 507.
VII. PIXE, MICROPROBES,
...
Nuclear Instruments and Methods in Physics Research B79 (1993) 513-517 North-Holland
Beam Intemctions with Yaterlals & Atoms
Elastic recoil detection analysis with atomic depth resolution G. Dollinger Physik-Department E12, Technische Universitiit Miinchen, D-W-8046 Garching, Germany
Elastic recoil detection analysis (ERD) with swift heavy ions has been improved in view of depth resolution. Profiles of light elements could be measured in carbon near the surface with single atomic layer resolution using 5sNi, lz71 or lg7Au beams with energies between 0.5A MeV and 1A MeV and a Q3D magnetic spectrograph. Due to the specific ion optical arrangement of the Q3D a relative energy resolution of about 7 X 10m4 was achieved in ERD experiments although a large solid angle of 5 msr was used. This large acceptance guarantees good statistics even with low ion fluences. In this way the elemental concentration profiles are not essentially disturbed by irradiation damage effects and can be measured in the 0.1% level on an atomic depth scale.
1. Introductioa Elastic recoil detection analysis (ERD) using energetic heavy ions is a suitable method to measure quantitative depth profiles of light and medium heavy elements in thin films [l]. Using the Q3D magnetic spectrograph at the Munich tandem accelerator a depth resolution better than 1 nm could be achieved near the surface when analysing thin carbon films 121.Improvements were introduced now to further increase the depth resolution at the surface and also in deeper areas simultaneously with larger solid angles of detection.
2. Experimental
arrangement
AE = - 2 tan (p Acp + (tar&o - 1) A(p2 + $ tan cp Acp’ E tan2q) A(p4+ **. .
+i(l-
Therefore, a variation of scattering angle 4p= 15” by Arp = f3.2”, which are the conditions used at the Q3D, results in negative first order energy shift AE/E of 6%, 0.28% in the second order and 5 X 10m4 in the
and results
The experimental arrangement for ERD measurements with high depth resolution is shown in fig. 1. It is similar to that described in ref. [2]. In order to reduce irradiation damage effects ion beams of 60 MeV 5*Ni and lz71 were used in contrast to the previous experiments, where 120 MeV Au ions were utilized [2,3]. The ions hit the samples to be investigated at angles of 3.5” to 10” relative to the surface while the scattering angle was always 15”. The recoiled ions were detected with the Munich Q3D magnetic spectrograph [4]. An intrinsic energy resolution of 4 X lop4 was measured with a 212Po a-source (E, = 8.8 MeV) using a 1 m long ionisation chamber with two integrated position sensitive proportional counters as focal plane detector [5]. This high energy resolution can be obtained with 12 msr solid angle of detection. Large detection angles are required when high depth resolution of small elemental amounts is desired, otherwise the original depth profile is destroyed by irradi0168-583X/93/$06.00
ation damage before it is measured 123.But the strong kinematic energy shift AE(p) with scattering angle cp usually limits the solid angle of detection when high energy resolution is required [6-81. The kinematic shift for an angle variation Acp can be expanded in a Taylor series:
MULTIPOLE
--iiiDIPOLE2 I
.E 3
FOCAL P:ANE IONBEAM IONlSATbN CHAMBER
Fig. 1. Experimental arrangement for high resolution depth profiling.
0 1993 - Elsevier Science Publishers B.V. All rights reserved
VII. PIXE, MICROPROBES, . . *
G. Dollinger / Elastic recoil detection anabsk
514
. . . -_
r
n
m
. _.. ...
.
_.
. . ..
.
. .._. -_ _-_ .:
.D.
.
.
.
.
.
. .
d
.
.
.
.
I
.
-
.
~
-*--.
.
.
.
.
. .
.... .
.
*
_
.
.
.: : . ~ . . -m.. . . -.. . . :: -_. . . . . . . . . .:- -O ::. .::. :’
N
0
co1
‘;”
ml7
.
*
t
515
G. LMirtger / Elastic recoil detection analysti
third order of A4p. Only the fourth and higher orders can be neglected if energy resolutions better 10m3 are required. The multipole element of the Q3D, however, is an ion optical element which allows one to correct for the kinematic shift up to the third order. The effect of this correction is shown in fig. 2, where angle-energy spectra are shown, measured with the two position sensitive propo~ional counters of the focal plane detector. An uncorrected spectrum with full kinematic shift is shown in fig. 2a, measured for a 2.2 pg/cm’ thick carbon target, perpendicularily penetrated by 60 MeV 58Ni ions. In that case the recoiled carbon atoms transmitted the foil before detection with the consequence that only a small energy spread of 0.17% of the recoiled ions is expected due to energy loss processes. Due to the large kinematic energy variation a broad band can be located in the angle-~sition spectrum. The angle resolution of the focal plane detector is too low to enable kinematic shift correction with a software routine, because the poor angle resolution would be transferred into a poor energy resolution. The multipole element of the Q3D, however, allows one to correct ion optically for the kinematic shift. After first order correction with a quadrupole field in the multiple a parabola remains due to the second order kinematic shift (fig. 2b). Introducing an additional hexapole field an S-shaped curve remains (fig. 2c) which can be corrected with an additional octupole field (fig. 2d). A straight line in the angleposition dependence remains after these corrections and an energy resolution of about 7 x 10m4 could be measured for the whole experimental arrangement. But the accepted angle ~~endicular to the bending plane of the Q3D magnets had to be reduced to f 1.3” because the kinematic shift in that direction, which is only of second and higher orders, cannot be corrected with the multipole element. This portion reduces the accepted solid angle of the Q3D to 5 msr. With these adjustments the depth resolution was measured at the surface of a bulk graphite monocrystal. For example a i2C4+ signal is plotted versus the carbon depth in fig. 3. It was measured with 60 MeV 58Ni ions entering the surface at a flat angle of 3.5” while the scattering angle was still 15”. The spectrum rises within a relative energy loss of 7 x low4 from 13% to 87% of the first peak value. This slope corresponds to a carbon thickness of 0.34 nm for the given geometry, ion beam and sample conditions. Due to the slow variation of the charge yield (long tail of the dis~bution) this thickness is the depth resolution (FWHIvf) in this measurement which is equivalent to a resolution of about an interlayer spacing of the graphite (002) layers. Two seperated peaks near the surface can also be seen with a 0.34 nm distance and width which seems to be the signal of the first two graphite layers, where each gives a specific energy loss. The same
100 i
0 i
___.m
10
5 depth
5
[nm]
Fig. 3. 12C4* signal as an example of single layer depth resolution at the surface of a single graphite crystal obtained with 40 MeV s8Ni ion beam. The peak near the surface arises due to charge exchange effects of the scattered carbon atoms. The slope at the peak shows a single carbon layer depth resolution.
resolution could be obtained using 60 MeV 1271 ions. There the angle of incidence could be chosen to be 5” relative to the surface because the relative energy loss, which gives the depth resolution, is higher under these ion beam conditions. Another problem using ERD for depth profiling is that depth resolution decreases with increasing depth. Specially a geometrical energy broadening reduces depth resolution with increasing depth besides the noncorrectable effects of small angle scattering and energy loss scattering [6,8] when large detector angles are applied. This geometrical effect arises due to path length differences of the sample atoms detected under different angles. These path length differences lead to different detectable energies and increase linearly with depth. The ion optical performance of the Q3D, however, gives also the possibility to correct for this effect as soon as a hexapole field for the correction of the second order kinematic shift is introduced. When the adjustments of the multipole fields are done at a certain energy (here designated as energy shift zero) a rotation of the angle-energy dependence is obtained at different energies. This rotation can be measured again with a thin carbon foil in transmission geometry, where the path length differences are negligible, by varying all magnetic fields by the same relative amount. That is shown for five different variations in the same picture in fig. 4a. A rotation of the angle-energy spectrum in the opposite direction can be measured when e.g. the 13C profile of thin i3C layers in a 13C/ 12C multilayer sample is investigated in reflection geometry, where 60 MeV 58Ni ions impinged at an angle of 10” to the VII. PIXE, MICROPROBES,
. **
516
G. Dollinger / Elastic ret:oil detection analysis
surface and a scattering angle of 15” was used (fig. 4b). The path length differences induce a rotation in the opposite direction. The sum results in the spectrum in fig. 4b where each stripe corresponds to a 13C layer of the multilayer system. The correction of the path length effect is not perfect, but it can be optimized when an incidence angle of 7.5” is used. At last small angleposition dependences can be corrected for by software after the measurements without a loss of energy resolution.
3. Conclusion Depth resolution at the physical limit using large solid angles can be obtained in ERD measurements for the whole depth investigated using the Munich Q3D magnetic spectrograph with its specific ion optical arrangement. Due to the large solid angle of detection (5 msr) we are now able to measure elemental and isotopic depth profiles on an atomic scale below the 1% level, where the number of displaced atoms stays below
6
4
-4
-6
M
-6
1
-a + -1
0
5
relative
Lqy3shif+
6
7
[“g.]
Fig. 4. Angle-energy spectra (a) of l2C 6+ for a 2.2 &cm2 thick carbon foil at five different magnetic fields and (b) of 13C6+ signal of a 13C/ lzC multilayer system showing an opposite rotation in the angle-energy spectra compared to (a).
517
G. Dollinger / Elastic recoil detection analysis
10% during irradiation [2] and thus the depth profile should remain unchanged. Therefore a wide field is opened for quantitative investigations of surface and interfaces of thin films.
References [l] Proc. High Energy and Heavy Ion Beams in Materials Analysis Workshop, Albuquerque, 1989, eds. J.R. Tesmer, C.J. Maggiore, M. Nastasi, J.C. Barbour and J.W. Mayer (MRS, Pittsburgh, PA). [2] G. Dollinger, T. Faestermann and P. Maier-Komor, Nucl. Instr. and Meth. B64 (1992) 422.
[3] G. Dollinger, M. Boulouednine, T. Faestermann, P. Maier-Komor and H.-J. Kijmer, to be published in Radiat. Eff. Def. Sol. [4] M. LiZfler, H.-J. Scheerer and H. Vonach, Nucl. Instr. and Meth. 111 (1973) 1. IS] W. Mayer, thesis, Technical University of Munich (1985). [6] J.P. Stoquert, G. Guillaume, M. Hage-All, J.J. Grob, C. Ganter and P. Siffert, Nucl. Instr. and Meth. B44 (1989) 184. [7] D.O. Boerma, F. Labohm and J.A. Reinders, Nucl. Instr. and Meth. B.50 (1990) 291. [8] F. Paszti, E. Szil&yi and E. Kotai, Nucl. Instr. and Meth. B54 (1991) 507.
VII. PIXE, MICROPROBES,
...