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Channeling STIM and its applications
NOMB
Nuclear Instruments and Methods in Physics Research B77 (1993) 184-187 North-Holland
Beam Interactions with Materials&Atoms
Channeling STIM and its applications M. Cholewa ‘, A. Saint, G.J.F. Legge and D.N. Jamieson Micro Analytical Research Centre (MARC), School of Physics, The University of Melbourne, Parkuille, fit. 3052, Australia
T. Nishijima Electrotechnicat Laboratory, I-l-4, Ume.zorw, Tsukuba, Ibaraki Japan
Channeling scanning transmission ion microscopy (CSTIM) has been used to explore transmission channeling in thin Si and Sic crystals. The CSTIM technique was applied to the investigation of the quality of - 30 pm thick Sic crystal. In this work the results from channeling contrast microscopy (CCM) were combined with CSTIM. The channeling STIM (CSTIM) technique is almost 100% efficient, which reduces the analysis time, and the beam causes negligible damage, compared to backscattering channeling contrast microscopy (CCM). CSTIM is capable of very high resolution (50 nm). These features can be successfully applied to the investigation of crystal damage and small size imperfections in samples transparent to the beam.
1. Introduction
Silicon carbide (SE) is an excellent materiaf for high temperature semiconductor devices, because of its electronic properties, such as its wide band gap and high electron mobility. It is then expected that Sic electronic devices will be insensitive to radiation damage [l]. In this paper we used a combination of channeling contrast microscopy [2] and channeling STIM [3] techniques to investigate Sic crystal. Ion beam techniques are important in the formation and in the analysis of semiconductor devices and many new materials. For the last decade, since Inga~ield et al. [4] commenced channeling with a microbeam of alpha particles and MacCaIlum et al. [Z] developed CCM, these techniques have been utilised in the laboratory to study the properties of individual small crystals and in particular the damage caused by ion bombardment. Although CCM is a valuable technique, the large beam currents required (N I nA) limit the spatial resolution to a few microns and limit the amount of information that can be gathered from one location of the specimen before damage induced by the analyzing beam becomes si~fi~nt. These matters were discussed by Williams et al. [5] and Jamieson et al. [6], and modifications to previous estimates were made more recently by Dooley and Jamieson [7]. With thin specimens, channeling of ion beams can also be examined in transmission [8,9] (0’ scattering). The required beam currents are then very low due to 1 Permanent address: institute of Nuclear Physics, Cracow, Poland. Old-583X/93/$06.~
the high detection efficiency. We showed recently [3] that scanning transmission ion microscopy @TIM) [IO] and CCM could be combined to carry out high resoluEion channeling measurements with negligible accompanying damage to the specimen. That first demonstration of channeling STIM (CSTIM) utilised a 3.9 MeV proton beam of less than 0.5 fA to channel through a 58 km thick crystal of silicon and required a very low charge dose (- 200 fC) for the examination. This is much less than the 5.8 PC charge dose needed to produce observable damage when channeling in the (11.1) channel direction - an energy dose consistent with that found in other work [5-71. In this work we apply the CSTIM technique to thin crystals of silicon (- 3 Frn) and thick crystals of silicon carbide (- 30 Frn) and compare results with those obtained with CCM (which here denotes channeling RBS). Data collected with the SiC sample, both CSTIM and CCM, show extreme sensitivity of the first technique to the channeling properties.
2. Expedients
and
results
The crystal of silicon was about 3 p.m thick and aligned in the (111) channeling direction. A beam of 2.3 MeV alphas was focussed to a spot of less than 200 nm diameter with beam current less than 1 L4. Transmitted alpha particles were detected at 0” by a surface barrier detector with energy resolution of 15 keV full width at half maximum (FWHM). A collimator in front of the detector limited the acceptance angle to O-23 mrad.
0 1993 - Elsevier Science Publishers B.V. All rights reserved
185
M. Cholewa et al. / CSTIM and its applications case of this thick crystal, 6000
Silicon
B50
400
2.3 MeV He+
h
450
500 550 600 CHANNELNUMBER
650
700
750
Fig. 1. STIM spectra for 2.3 MeV alpha beam transmitted through thin ( - 3 km thick) silicon crystal, taken in random and channeled directions.
The Melbourne microprobe was operated in continuous scanning mode and the data were collected as three-word events (energy, and x-y position on the specimen) by total quantitative scanning analysis (TQSA) 1111 into a 3D data block. No windowing restrictions were placed on the data when collected. Fig. 1 shows two spectra. One was collected in the (111) channel direction and the other in random orientation of the thin silicon sample. A large difference between the two spectra is seen. The CSTIM technique can thus be used very successfully to position the sample in the channel direction without significant damage to the specimen. Because of the high efficiency of this technique, a sample can be oriented very quickly. Another sample of Sic, also prepared at ETL, was oriented in the (100) direction. Because of the sample thickness a 3 MeV proton beam had to be used. In all this CSTIM work, the detector operates in bright-field mode (i.e. at 0”) with a collimator in front. In general, the tighter this collimation the cleaner the separation of the channeling and nonchanneling peaks. In the
we would like to separate the effect caused by the difference in thickness of the sample with this caused by channeling using both the CSTIM and CCM techniques. Fig. 2a shows CSTIM spectra for random and channel orientations of the sample. Fig. 2b shows a comparison between spectra for channel and random directions when the CCM technique was used. It also shows a predicted random spectrum, which takes into account the presence of a nearby scattering resonance in silicon and is based on cross section measurements for natural targets by Amirikas et al. [12]. The selectivity (for mapping requirements) is much better in the CSTIM case than in the CCM case and would have been better still if the 23 mrad collimator had been closed down to the 11 mrad sometimes used [3]. Also the channeling, as evidenced by the CCM spectrum of fig. 2b, was only planar rather than axial, with the result that there was only a small difference in energy loss between particles transmitted through channels and those transmitted in the random direction. Despite this, it was possible to use the CSTIM technique to image variations in crystal quality by windowing the energy loss peak in the CSTIM spectrum. Fig. 3 (top left) shows a STIM map of the median energy loss for each of the 256 x 256 pixels in the 152 X 136 micron scanned area for the sample aligned in the random direction. The CSTIM map of the same area is shown top right, for the sample aligned in the (100) planar direction as discussed above. Even with such a low contrast, the nonuniformities in the channeling properties of this sample are visible - the right image is much more nonuniform than the left. The grey scale bars of fig. 3 refer to the energy of fig. 2a. They show that the readily identified thin region (black spot) of fig. 3 is 112 keV thinner than the average crystal thickness of 1025 keV, and the thick (white) region in the centre is only 25.3 keV thicker
02 ENERGY (keV)
Fig. 2. (a) STIM
spectra
1500
2000 Energy
for 3.0 MeV H+ transmitted through - 30 km of silicon carbide random direction; (b) shows the RBS spectra for the same orientations
2500
3000
CkeV)
crystal oriented as in (a).
in channeled
and
IV. IMAGING
700 600
,
,
,,I,
Sic ( Random
,
)
,
3 MeV H+
Sic
( Channeled
)
3 MeV H+
ENEZGY (keV)
Fig. 3. Image from CSTIM of silicon carbide oriented in random (top left corner) and channeled (top right comer) directions. Scan size is 152 x 136 pm and data were not smoothed. Grey scale bars refer to the energy of fig. 2a. The spectra beneath each image are extracted from the regions outlined on the right image.
than average. Of greater interest is the fine network of crystal imperfections apparent in the CSTIM image on the right. The quantitative significance of this image is
seen in the random and channeled spectra taken from the two regions of interest outlined on the channeled image. These spectra are shown below their respective
M. Cholewa et al. / CSTIM and its applications
images. The two random spectra are identical, showing that the two regions are of the same thickness; but the two channeled spectra confirm that region B has greater median energy loss and hence has more crystal defects, as seen in the channeled map.
3. Conclusion It is important to note that in this work the order of data collection was aligned STIM, aligned backscattering, random STIM and random backscattering. This ensured that the alignment for CSTIM and CCM was identical. Only CSTIM is able to visualise the difference in channeling properties of the thick sample on such a small (submicron) scale. The CSTIM technique can be applied to thin and thick samples, as long as the beam is able to pass through them. Based on the detection of changes in an ion’s energy loss in the transmitting sample, CSTIM is extremely sensitive to the presence of crystal defects like interstitial atoms, vacancies and complexes of simple defects. Because of the very low currents required, this technique creates negligible crystal damage compared to backscattering CCM.
187
References
[II I. Nashiyama, T. Nishijima, E. Sakuma and S. Yoshida, Nucl. Instr. and Meth. B33 (1988) 599.
PI J.C. MacCallum, C.D. McKenzie, M.A. Lucas, K.G. Rossiter, K.T. Short and J.S. Williams, Appl. Phys. Lett. 42 (1983) 827. [31 M. Cholewa, G. Bench, G.J.F. Legge and A. Saint, Appl. Phys. Lett. 56 (1990) 1236. [41 S.A. Ingarfield, CD. McKenzie, K.T. Short and J.S. Williams, Nucl. Instr. and Meth. 191 (1981) 521. 151J.S. Williams, J.C. MacCallum and R. Brown, Nucl. Instr. and Meth. B30 (1988) 480. [61 D.N. Jamieson, R.A. Brown, C.G. Ryan and J.S. Williams, Nucl. Instr. and Meth. B54 (1991) 213. 171 S.P. Dooley and D.N. Jamieson, Nucl. Instr. and Meth. B66 (1992) 369. Bl A.R. Sattler and G. Dearnaley, Phys. Rev. 161 (1967) 161. [91 B.R. Appleton, C. Erginsoy and W.M. Gibson, Phys. Rev. 161 (1967) 330. 1101 R.M. Sealock, A.P. Mazzolini and G.J.F. Legge, Nucl. Instr. and Meth. 218 (1983) 217. [ill G.J.F. Legge and I. Hammond, J. Microsc. 117 (1979) 201. 1121 R. Amirikas, D.N. Jamieson and Sean P. Dooley, these Proceedings (3rd Instr. Conf. on Nuclear Microprobes, Uppsala, Sweden, 1992) Nucl. Int. and Meth. B77 (1993) 110.
Acknowledgement
This work was supported by a grant from the Australian Research Council.
IV. IMAGING
Nuclear Instruments and Methods in Physics Research B77 (1993) 184-187 North-Holland
Beam Interactions with Materials&Atoms
Channeling STIM and its applications M. Cholewa ‘, A. Saint, G.J.F. Legge and D.N. Jamieson Micro Analytical Research Centre (MARC), School of Physics, The University of Melbourne, Parkuille, fit. 3052, Australia
T. Nishijima Electrotechnicat Laboratory, I-l-4, Ume.zorw, Tsukuba, Ibaraki Japan
Channeling scanning transmission ion microscopy (CSTIM) has been used to explore transmission channeling in thin Si and Sic crystals. The CSTIM technique was applied to the investigation of the quality of - 30 pm thick Sic crystal. In this work the results from channeling contrast microscopy (CCM) were combined with CSTIM. The channeling STIM (CSTIM) technique is almost 100% efficient, which reduces the analysis time, and the beam causes negligible damage, compared to backscattering channeling contrast microscopy (CCM). CSTIM is capable of very high resolution (50 nm). These features can be successfully applied to the investigation of crystal damage and small size imperfections in samples transparent to the beam.
1. Introduction
Silicon carbide (SE) is an excellent materiaf for high temperature semiconductor devices, because of its electronic properties, such as its wide band gap and high electron mobility. It is then expected that Sic electronic devices will be insensitive to radiation damage [l]. In this paper we used a combination of channeling contrast microscopy [2] and channeling STIM [3] techniques to investigate Sic crystal. Ion beam techniques are important in the formation and in the analysis of semiconductor devices and many new materials. For the last decade, since Inga~ield et al. [4] commenced channeling with a microbeam of alpha particles and MacCaIlum et al. [Z] developed CCM, these techniques have been utilised in the laboratory to study the properties of individual small crystals and in particular the damage caused by ion bombardment. Although CCM is a valuable technique, the large beam currents required (N I nA) limit the spatial resolution to a few microns and limit the amount of information that can be gathered from one location of the specimen before damage induced by the analyzing beam becomes si~fi~nt. These matters were discussed by Williams et al. [5] and Jamieson et al. [6], and modifications to previous estimates were made more recently by Dooley and Jamieson [7]. With thin specimens, channeling of ion beams can also be examined in transmission [8,9] (0’ scattering). The required beam currents are then very low due to 1 Permanent address: institute of Nuclear Physics, Cracow, Poland. Old-583X/93/$06.~
the high detection efficiency. We showed recently [3] that scanning transmission ion microscopy @TIM) [IO] and CCM could be combined to carry out high resoluEion channeling measurements with negligible accompanying damage to the specimen. That first demonstration of channeling STIM (CSTIM) utilised a 3.9 MeV proton beam of less than 0.5 fA to channel through a 58 km thick crystal of silicon and required a very low charge dose (- 200 fC) for the examination. This is much less than the 5.8 PC charge dose needed to produce observable damage when channeling in the (11.1) channel direction - an energy dose consistent with that found in other work [5-71. In this work we apply the CSTIM technique to thin crystals of silicon (- 3 Frn) and thick crystals of silicon carbide (- 30 Frn) and compare results with those obtained with CCM (which here denotes channeling RBS). Data collected with the SiC sample, both CSTIM and CCM, show extreme sensitivity of the first technique to the channeling properties.
2. Expedients
and
results
The crystal of silicon was about 3 p.m thick and aligned in the (111) channeling direction. A beam of 2.3 MeV alphas was focussed to a spot of less than 200 nm diameter with beam current less than 1 L4. Transmitted alpha particles were detected at 0” by a surface barrier detector with energy resolution of 15 keV full width at half maximum (FWHM). A collimator in front of the detector limited the acceptance angle to O-23 mrad.
0 1993 - Elsevier Science Publishers B.V. All rights reserved
185
M. Cholewa et al. / CSTIM and its applications case of this thick crystal, 6000
Silicon
B50
400
2.3 MeV He+
h
450
500 550 600 CHANNELNUMBER
650
700
750
Fig. 1. STIM spectra for 2.3 MeV alpha beam transmitted through thin ( - 3 km thick) silicon crystal, taken in random and channeled directions.
The Melbourne microprobe was operated in continuous scanning mode and the data were collected as three-word events (energy, and x-y position on the specimen) by total quantitative scanning analysis (TQSA) 1111 into a 3D data block. No windowing restrictions were placed on the data when collected. Fig. 1 shows two spectra. One was collected in the (111) channel direction and the other in random orientation of the thin silicon sample. A large difference between the two spectra is seen. The CSTIM technique can thus be used very successfully to position the sample in the channel direction without significant damage to the specimen. Because of the high efficiency of this technique, a sample can be oriented very quickly. Another sample of Sic, also prepared at ETL, was oriented in the (100) direction. Because of the sample thickness a 3 MeV proton beam had to be used. In all this CSTIM work, the detector operates in bright-field mode (i.e. at 0”) with a collimator in front. In general, the tighter this collimation the cleaner the separation of the channeling and nonchanneling peaks. In the
we would like to separate the effect caused by the difference in thickness of the sample with this caused by channeling using both the CSTIM and CCM techniques. Fig. 2a shows CSTIM spectra for random and channel orientations of the sample. Fig. 2b shows a comparison between spectra for channel and random directions when the CCM technique was used. It also shows a predicted random spectrum, which takes into account the presence of a nearby scattering resonance in silicon and is based on cross section measurements for natural targets by Amirikas et al. [12]. The selectivity (for mapping requirements) is much better in the CSTIM case than in the CCM case and would have been better still if the 23 mrad collimator had been closed down to the 11 mrad sometimes used [3]. Also the channeling, as evidenced by the CCM spectrum of fig. 2b, was only planar rather than axial, with the result that there was only a small difference in energy loss between particles transmitted through channels and those transmitted in the random direction. Despite this, it was possible to use the CSTIM technique to image variations in crystal quality by windowing the energy loss peak in the CSTIM spectrum. Fig. 3 (top left) shows a STIM map of the median energy loss for each of the 256 x 256 pixels in the 152 X 136 micron scanned area for the sample aligned in the random direction. The CSTIM map of the same area is shown top right, for the sample aligned in the (100) planar direction as discussed above. Even with such a low contrast, the nonuniformities in the channeling properties of this sample are visible - the right image is much more nonuniform than the left. The grey scale bars of fig. 3 refer to the energy of fig. 2a. They show that the readily identified thin region (black spot) of fig. 3 is 112 keV thinner than the average crystal thickness of 1025 keV, and the thick (white) region in the centre is only 25.3 keV thicker
02 ENERGY (keV)
Fig. 2. (a) STIM
spectra
1500
2000 Energy
for 3.0 MeV H+ transmitted through - 30 km of silicon carbide random direction; (b) shows the RBS spectra for the same orientations
2500
3000
CkeV)
crystal oriented as in (a).
in channeled
and
IV. IMAGING
700 600
,
,
,,I,
Sic ( Random
,
)
,
3 MeV H+
Sic
( Channeled
)
3 MeV H+
ENEZGY (keV)
Fig. 3. Image from CSTIM of silicon carbide oriented in random (top left corner) and channeled (top right comer) directions. Scan size is 152 x 136 pm and data were not smoothed. Grey scale bars refer to the energy of fig. 2a. The spectra beneath each image are extracted from the regions outlined on the right image.
than average. Of greater interest is the fine network of crystal imperfections apparent in the CSTIM image on the right. The quantitative significance of this image is
seen in the random and channeled spectra taken from the two regions of interest outlined on the channeled image. These spectra are shown below their respective
M. Cholewa et al. / CSTIM and its applications
images. The two random spectra are identical, showing that the two regions are of the same thickness; but the two channeled spectra confirm that region B has greater median energy loss and hence has more crystal defects, as seen in the channeled map.
3. Conclusion It is important to note that in this work the order of data collection was aligned STIM, aligned backscattering, random STIM and random backscattering. This ensured that the alignment for CSTIM and CCM was identical. Only CSTIM is able to visualise the difference in channeling properties of the thick sample on such a small (submicron) scale. The CSTIM technique can be applied to thin and thick samples, as long as the beam is able to pass through them. Based on the detection of changes in an ion’s energy loss in the transmitting sample, CSTIM is extremely sensitive to the presence of crystal defects like interstitial atoms, vacancies and complexes of simple defects. Because of the very low currents required, this technique creates negligible crystal damage compared to backscattering CCM.
187
References
[II I. Nashiyama, T. Nishijima, E. Sakuma and S. Yoshida, Nucl. Instr. and Meth. B33 (1988) 599.
PI J.C. MacCallum, C.D. McKenzie, M.A. Lucas, K.G. Rossiter, K.T. Short and J.S. Williams, Appl. Phys. Lett. 42 (1983) 827. [31 M. Cholewa, G. Bench, G.J.F. Legge and A. Saint, Appl. Phys. Lett. 56 (1990) 1236. [41 S.A. Ingarfield, CD. McKenzie, K.T. Short and J.S. Williams, Nucl. Instr. and Meth. 191 (1981) 521. 151J.S. Williams, J.C. MacCallum and R. Brown, Nucl. Instr. and Meth. B30 (1988) 480. [61 D.N. Jamieson, R.A. Brown, C.G. Ryan and J.S. Williams, Nucl. Instr. and Meth. B54 (1991) 213. 171 S.P. Dooley and D.N. Jamieson, Nucl. Instr. and Meth. B66 (1992) 369. Bl A.R. Sattler and G. Dearnaley, Phys. Rev. 161 (1967) 161. [91 B.R. Appleton, C. Erginsoy and W.M. Gibson, Phys. Rev. 161 (1967) 330. 1101 R.M. Sealock, A.P. Mazzolini and G.J.F. Legge, Nucl. Instr. and Meth. 218 (1983) 217. [ill G.J.F. Legge and I. Hammond, J. Microsc. 117 (1979) 201. 1121 R. Amirikas, D.N. Jamieson and Sean P. Dooley, these Proceedings (3rd Instr. Conf. on Nuclear Microprobes, Uppsala, Sweden, 1992) Nucl. Int. and Meth. B77 (1993) 110.
Acknowledgement
This work was supported by a grant from the Australian Research Council.
IV. IMAGING