- Email: [email protected]
Defect imaging and channeling studies using channeling scanning transmission ion microscopy
Nuclear Instruments and Methods in Physics Research B 118(1996) 426-430 NlONil
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B
Seam Interactions with Materials 8 Atoms
!!l!J ELSEVIER
Defect imaging and channeling studies using channeling scanning transmission ion microscopy P.J.C. King a3*, M.B.H. Breese a, P.J.M. Smulders
b, P.R. Wilshaw
‘, G.W. Grime a
a SPM Unit, Nuclear Physics Laboratory, University of Oxford, Keble Road, Oxford, OXI 3RH, UK b Nuclear Solid State Physics, Materials Science Centre, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands ’ Departmenr of Materials. Universig of Oxford, Parks Road, Oxford, OX1 3PH. UK
Abstract The technique of channeling scanning transmission ion microscopy (CSTIM) can be used to produce images of individual crystal defects (such as dislocations and stacking faults) using the scanned, focused ion beam from a nuclear microprobe. As well as offering a new method for studies of crystal defects, this technique can be used to investigate the effects of single defects on the channeling process. This paper describes some of the characteristics of the CSTIM technique, such as its ability to image large areas of a crystal, to image under thin, amorphous surface layers and to image defects on the back surface of 50 p,m thick crystals. An unexpected effect seen in CSTIM images of stacking faults, namely the ability of faults to convert ions on blocked trajectories into well-channeled ones, is described to illustrate the use of CSTIM for channeled ion-defect interaction studies.
1. Introduction The nuclear microprobe adds to traditional, unfocused ion beam analysis techniques the ability to produce images, and this has recently led to its use in the field of crystal defect studies. Channeling investigations of crystals with a broad ion beam can provide information on defect depth distributions and, to a certain extent, can distinguish between different types of defects [ 11. The nuclear microprobe adds to such analyses the possibility of obtaining spatially resolved information, so that channeling images of individual defects can be produced. Images of single defects have been generated by detection of MeV ions transmitted through thinned crystals, a technique which has been called channeling scanning transmission ion microscopy (CSTIM). It has been shown that the CSTIM technique is able to image single and bunched misfit dislocations in Si, _,Ge,/Si crystals [2,3], and stacking faults in a silicon crystal [4,5]. This paper is a description of some of the characteristics of the CSTIM technique for defect imaging and will also demonstrate the technique’s value for studies of channeled ion-defect interactions.
Corresponding author. Tel. + 44 1865 273439, fax + 44 1865 273418, e-mail [email protected]. l
2. Description of the technique
The CSTIM technique exploits the reduced energy loss rate which channeled ions suffer as they pass through a crystal. A defect in a crystal locally disrupts the regular atomic arrangement and so will affect the channeling process, either producing dechanneling of initially channeled ions, or, as will be shown below, causing non-channeled ions to become channeled. In either case, the mean energy loss rate of the ions passing through the defect region is altered compared with that of ions passing through surrounding good crystal. In CSTIM, the mean energy of ions transmitted through a thinned crystal is measured and used to produce an image of the region scanned by the beam. The local variations in mean transmitted ion energy loss produced by the effects on channeling of the defects present are revealed in such images, enabling the defects to be seen. Most applications of the CSTIM technique to date have employed a MeV proton beam owing to the long range of these ions (90 km for 3 MeV protons in a silicon crystal). This enables crystals which are several tens of microns in thickness to be analysed. The samples described in this paper, silicon-based crystals, were prepared for analysis by thinning from their back, unpolished surface using wet and dry paper, followed by diamond lapping compound to remove large scratches. They were mounted in the cham-
0168-583X/%/$15.00 Copyright 0 1996 Elsevier Science B.V. All rights reserved SSDI 0168-583X(95)01091-2
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P_J.C. King et al./ Nucl. Instr. and Meth. in Phys. Rex B 118 f 1996) 426-430
ber of the Oxford nuclear microprobe, fixed either to a goniometer allowing rotation through a limited range of angles about two axes in the sample plane, or to a stage allowing unlimited rotation about a single axis. A semiconductor charged-particle detector was mounted behind the samples on the beam axis to measure transmitted protons. Images of the sample were produced by dividing the transmitted proton energy spectrum into energy slices. Images generated from the individual slices (showing the number of protons falling in given a slice at each pixel) were combined together to form a final image showing the mean transmitted proton energy loss. The images shown here are composed of 256 X 256 pixels and are printed in grey scale with darker greys representing higher mean transmitted energy loss. Fuller descriptions of the image production process can be found in Ref. [6].
3. Examples nique
and characteristics
of the CSTIM
tech-
3.1. Large area imaging and imaging under amorphous layers As mentioned above, the use of MeV protons for CSTIM analysis enables samples with thicknesses ranging from a few microns to over 50 p.m to be analysed. It is possible to thin and lap a 2 mm X 2 mm piece of silicon with initial thickness of about 500 urn to within this range in less than an hour, such that a large portion of the sample area can be imaged for defects. This is demonstrated in Fig. 1, which shows a montage of several CSTIM images combined to produce a single 325 X 735 pm2 image of the defects in a silicon crystal. The individual images were 200 p,rn wide and taken with the sample translated 150 km between each. The sample was tilted slightly between each image so that the beam remained channeled owing to the slight bend of the crystal produced by the thinning process. The image was taken with the incident beam aligned with a (111) planar channeling direction. The dark features which can be seen densely grey. “D”-shaped covering the image are stacking faults (see Fig. 3a) which had been deliberately introduced at the top, polished surface of the wafer. Two larger-scale contrast features can also be seen. Firstly, on going from the bottom of the image to the top, the image background changes from dark grey to very light grey. This was caused by a change in sample thickness across the sample, the lowest part in Fig. 1 being about 41 urn in thickness and the upper part about 38 p.m (the thicknesses were determined from the non-channeled proton energy loss). Secondly, there is a semi-circular region covering the centre of the image where the background is a darker grey. Within this region, the sample was covered by a layer of gold and palladium a few tens of nanometres in thickness. This layer caused some scattering
_.
___. ._____.
Fig. I. Montage of CSTIM mean energy loss images of a silicon crystal containing stacking faults. The sample area shown is 325 i( 735 em*. The darker grey, semi-circular region is the pan of the sample covered by a thin, amorphous metal layer.
of the incident protons, slightly increasing the divergence of the beam reaching the underlying silicon so that a smaller fraction of the protons became channeled. The mean energy loss within this region is therefore slightly increased over that produced by surrounding crystal. However, even within this region stacking faults are visible, demonstrating the ability of CSTIM to image defects under thin, amorphous surface layers.
VI. MICROBEAMS
428 3.2. Imaging
PJ.C. King et al./Nuci.
Instr. utul Meth in Phys. Res. B I 18 (1996) 426-430
of deep defects
With the incident beam aligned with a planar channeling direction of a crystal, some MeV protons can remain
50pm
channeled to large depths within the crystal. Shown in Fig. 2 is a montage of CSTIM images of a sample containing stacking faults similar to those shown in Fig. I. In this case, however, the sample was reversed so that the faults were close to the beam exit surface; the sample was tilted so that the beam was incident on the thinned side of the crystal [7]. The thinning was performed to produce a wedge-shaped sample, approximately 5 p.m in thickness at its thin end and extending to regions approximately 50 urn in thickness. In the image shown in Fig. 2, stacking faults on the back surface of this wedge are visible throughout the range of wedge thicknesses (15-50 pm), demonstrating that sufficient protons remained channeled 50 em below the sample surface to produce image contrast. The image was taken with the incident beam aligned with {I I I] planes, as these provide the most stable channeling in silicon; channeling in {I IO] planes was found to produce only faint contrast. It was also found necessary to use a transmitted proton detector with a restricted acceptance angle (0.4”, still larger than the planar channeling critical angle of 0.1”). This caused preferential detection of the well-channeled protons, increasing their relative contribution to the image contrast. The vertical dark and bright bands which can be seen are the damage on the beam entrance surface produced by the thinning process. The sloping background of the images produced by the variation in sample thickness has been removed so that it does not dominate the image contrast.
4. CSTIM
studies of channeled
ion-defect
interactions
As well as providing a method for characterising defects in crystals, CSTIM has enabled new features of the interaction between channeled ions and defects to be studied. In particular, studies of stacking faults in a silicon crystal revealed an unexpected phenomenon, namely that these defects were actually able cause channeling rather than producing dechanneling under some circumstances
[51. Fig. 3a shows the effects of a stacking fault on a crystal. At the fault plane the crystal lattice planes undergo a shift. A well-channeled ion meeting a fault may be dechanneled owing to the displacement of the planar channels in the crystal beyond the fault. Fig. 3b shows a CSTIM image of two faults in a silicon crystal, taken with
15pm
Fig. 2. CSTlM image montage of a wedge-shaped sample containing stacking faults, with the faults on the back surface. Image taken with the beam channeled in the (11 I} planes of the sample and a collimated detector. The sample varied in thickness from 15 km near the bottom of the image to about 50 p,rn near the top. The images have been smoothed and processed with histogram equaiisation to increase the fault contrast.
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Fig. 3. (a) Schematic of the effects of a stacking fault in a crystal. (b) 30 km wide (smoothed) CSTIM mean energy loss image of two faults taken with the incident beam (1 IO) planar channeled. (c) CSTIM linescans taken across the two faults in (b) with the beam at different tilt angles about the (I 10) planes. Each line scan has had the background energy loss level set to zero and indicated by a dashed horizontal iine. Successive linescans have been displaced vertically for clarity. The angle to (110) is given alongside each linescan. (d) Mean transmitted proton energy loss versus tilt angle from (1 IO) taken from the linescans of(c) for the part of the left hand fault closest to the sample surface (thicker line, triangles) and for surrounding good crystal (thinner line, squares). (e) Difference between the two curves of (d) showing the fault mean energy loss contrast. The offset of the curve minima from zero of -0.03” in (d) and (e) is a measure of the error in the location of exact channeling alignment. Curves produced from the right hand fault ate very similar to those shown in (d) and (e).
in Phps. Re.). B 118 (1996) 426-430
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the incident beam aligned with the (1 IO) planes. The faults in this crystal were deliberately produced near the top, polished surface (the fabrication process is described in Ref. [4] and the faults in Ref. [8]) and were lying on the four {I 1 I} planes of the crystal, each of which is inclined at 54.7” to the sample (001) surface plane. The straight edge of each fault is where it met the sample surface, and the curved edge where it ended inside the crystal. The faults appear dark in Fig. 3b as the incident beam was aligned with the channeling direction and the faults were causing some of the ions to be dechanneled. Fig. 3c shows a set of linescans taken across the faults shown in Fig. 3b at different tilt angles of the incident beam to the (I IO) planar channeling direction. The background mean energy loss level has been subtracted from each plot, and is shown in each case as a dashed horizontal line. For the linescan taken with the incident beam channeled, the faults were producing dechanneling so that they caused a local increase in the mean transmitted proton energy loss. They therefore appear above the background energy loss level; the same is true for a beam incidence angle of 0.05”. However, in the linescan taken at 0. IO”, the part of each fault closest to the sample surface produced a proton energy loss below that of the background level, and for tilt angles of 0.15” and greater the whole of each fault produced a mean energy loss below the backgrou,rd level. In a CSTIM image like that of Fig. 3b. the faults would appear bright rather than dark at these tilt angles. The channeling critical angle in this case is 0.1”. and Fig. 3c shows that the faults were causing a reduced proton energy loss with the beam five times this angle from alignment. Shown in Fig. 3d are plots of the energy loss versus tilt angle for the part of the crystal where the left-hand fault was near the surface and for perfect (virgin) surrounding crystal. The ‘ault started to produce a lower than background energy loss at a tilt angle of just greater than the channeling critical angle, and continued to do so for tilt angles greater than five times the critical angle. Fig. 3e shows the difference between the two curves of Fig. 3d (the fault “contrast”). The explanation of how a stacking fault can decrease the mean proton energy loss at tilt angles of just beyond the channeling critical angle has been given previously [5]. Protons incident just beyond the channeling critical angle have paths which, whilst they do not become channeled, are strongly affected by the steering action of the channel walls. The paths of such ions are described as blocked [9]. Their transverse energy (the component of their energy normal to the channeling planes) is just sufficient to carry them through the planes. At the stacking fault the planar potential undergoes a shift. This does rot affect an ion’s transverse kinetic energy, but its transverse potential energy can be either increased or reduced, depending on its position in the channel when it crosses the fault plane. For a blocked ion, it is possible for its transverse potential energy to be reduced at the fault so that its total transverse
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energy is low enough for it to become channeled beyond the fault plane. An increase in a blocked ions’ potential energy would only serve to increase the effective beam divergence. A significant fraction of the incident ions can therefore become channeled at a stacking fault with the incident beam just beyond the channeling critical angle, so that the fault produces a lower energy loss than the surrounding crystal. This explanation, based on the planar continuum model of channeling, has been confirmed by detailed Monte Carlo calculations. For tilt angles close to, but just beyond, the channeling critical angle, the part of the fault closest to the sample surface shows this blocking to channeling effect most strongly as can be seen from Fig. 3c (linescans between 0.10” and 0.25”.) This is because ions reaching deeper parts of the fault would have undergone scattering in the overlying crystal. Such scattering may increase an ion’s transverse energy so that it is too high for the potential shift at the fault to enable the ion to start channeling. At these tilt angles, deeper parts of the faults therefore caused fewer ions to become channeled than shallower parts. For larger tilt angles (linescans at 0.35” and beyond) the surface part of the fault is no longer showing the strongest contrast. It is proposed that at these tilt angles, the ions’ transverse energy is initially too high for the potential shift at the fault to produce channeling. However, the scattering suffered by ions reaching deeper parts of the fault may in this case reduce their transverse energy so that it is low enough for them to become channeled at a fault. The surface parts of the faults therefore no longer show the effect more prominently than the deeper parts at these larger angles.
5. Conclusions The CSTIM technique is capable of imaging individual defects in crystals several tens of microns in thickness which may be covered by thin, amorphous surface layers. By choice of channeling direction and use of a collimated detector, deep defects on the back surface of a 50 pm thick crystal have been imaged. This technique has the potential to be a very useful analytical tool for studies of crystal defects (it has been shown that CSTJM can mea-
sure local lattice plane rotation [2] and determine stacking fault translation vectors [4]), although further work is necessary to improve the spatial resolution and decrease the time to record an image (presently 15 minutes or greater). The ability to image defects enables studies of channeled ion-defect interactions which may continue to reveal new phenomena. Further work is needed to better characterise the effects on the CSTJM mean energy loss contrast of using a collimated detector, and experimental and simulated investigations into the effects of scattering on the blocking to channeling transition are required.
Acknowledgements
P.J.C.K. and P.R.W. would respectively like to thank the Royal Commission for the Exhibition of 185 1 and the Royal Society, for Fellowships. Thanks are due to Dr. R. Falster of Monsanto Electronic Materials Corporation for provision of the stacking fault samples, and to A. Amaku of the Department of Materials, University of Oxford, for preparing the stacking fault sample with the amorphous layer.
References
Ill L.C. Feldman, J.W. Mayer and S.T. Picraux, Materials Analysis By Ion Channeling (Academic Press, 1982). I21M.B.H. Breese, P.J.C. King, P.J.M. Smulders and G.W. Grime, Phys. Rev. B 5 1 (5) (1995)2742. [31 P.J.C. King, M.B.H. Breese, G.R. Booker, J. Whitehurst, P.R. Wilshaw, G.W. Grime, F. Watt and M.J. Goringe, Nucl. Ins@. and Meth. B 77 (1993) 320. [41 P.J.C. King, M.B.H. Breese, P.R. Wilshaw and G.W. Grime, Phys. Rev. B 51 (5) (1995) 2732. [51 P.J.C. King, M.B.H. Breese, P.R. Wilshaw and G.W. Grime, Phys. Rev. Lett. 74 (3) (1995) 411. 161M.B.H. Breese, P.J.C. King, J. Whitehurst, G.R. Booker, G.W. Grime, F. Watt, L.T. Roman0 and E.H.C. Parker, J. Appl. Phys. 73(6) (1993) 2640. (71 P.J.C. King, M.B.H. Breese, P.R. Wilshaw and G.W. Grime, Nucl. Instr. and Meth. B 103 (1995) 365. k41G.R. Booker and W.J. Tunstall, Phil. Mag. 13 (19661 71. ]91 C. Erginsny, Phys. Rev. Lett. 15 (8) (1965) 360.
__
B
Seam Interactions with Materials 8 Atoms
!!l!J ELSEVIER
Defect imaging and channeling studies using channeling scanning transmission ion microscopy P.J.C. King a3*, M.B.H. Breese a, P.J.M. Smulders
b, P.R. Wilshaw
‘, G.W. Grime a
a SPM Unit, Nuclear Physics Laboratory, University of Oxford, Keble Road, Oxford, OXI 3RH, UK b Nuclear Solid State Physics, Materials Science Centre, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands ’ Departmenr of Materials. Universig of Oxford, Parks Road, Oxford, OX1 3PH. UK
Abstract The technique of channeling scanning transmission ion microscopy (CSTIM) can be used to produce images of individual crystal defects (such as dislocations and stacking faults) using the scanned, focused ion beam from a nuclear microprobe. As well as offering a new method for studies of crystal defects, this technique can be used to investigate the effects of single defects on the channeling process. This paper describes some of the characteristics of the CSTIM technique, such as its ability to image large areas of a crystal, to image under thin, amorphous surface layers and to image defects on the back surface of 50 p,m thick crystals. An unexpected effect seen in CSTIM images of stacking faults, namely the ability of faults to convert ions on blocked trajectories into well-channeled ones, is described to illustrate the use of CSTIM for channeled ion-defect interaction studies.
1. Introduction The nuclear microprobe adds to traditional, unfocused ion beam analysis techniques the ability to produce images, and this has recently led to its use in the field of crystal defect studies. Channeling investigations of crystals with a broad ion beam can provide information on defect depth distributions and, to a certain extent, can distinguish between different types of defects [ 11. The nuclear microprobe adds to such analyses the possibility of obtaining spatially resolved information, so that channeling images of individual defects can be produced. Images of single defects have been generated by detection of MeV ions transmitted through thinned crystals, a technique which has been called channeling scanning transmission ion microscopy (CSTIM). It has been shown that the CSTIM technique is able to image single and bunched misfit dislocations in Si, _,Ge,/Si crystals [2,3], and stacking faults in a silicon crystal [4,5]. This paper is a description of some of the characteristics of the CSTIM technique for defect imaging and will also demonstrate the technique’s value for studies of channeled ion-defect interactions.
Corresponding author. Tel. + 44 1865 273439, fax + 44 1865 273418, e-mail [email protected]. l
2. Description of the technique
The CSTIM technique exploits the reduced energy loss rate which channeled ions suffer as they pass through a crystal. A defect in a crystal locally disrupts the regular atomic arrangement and so will affect the channeling process, either producing dechanneling of initially channeled ions, or, as will be shown below, causing non-channeled ions to become channeled. In either case, the mean energy loss rate of the ions passing through the defect region is altered compared with that of ions passing through surrounding good crystal. In CSTIM, the mean energy of ions transmitted through a thinned crystal is measured and used to produce an image of the region scanned by the beam. The local variations in mean transmitted ion energy loss produced by the effects on channeling of the defects present are revealed in such images, enabling the defects to be seen. Most applications of the CSTIM technique to date have employed a MeV proton beam owing to the long range of these ions (90 km for 3 MeV protons in a silicon crystal). This enables crystals which are several tens of microns in thickness to be analysed. The samples described in this paper, silicon-based crystals, were prepared for analysis by thinning from their back, unpolished surface using wet and dry paper, followed by diamond lapping compound to remove large scratches. They were mounted in the cham-
0168-583X/%/$15.00 Copyright 0 1996 Elsevier Science B.V. All rights reserved SSDI 0168-583X(95)01091-2
427
P_J.C. King et al./ Nucl. Instr. and Meth. in Phys. Rex B 118 f 1996) 426-430
ber of the Oxford nuclear microprobe, fixed either to a goniometer allowing rotation through a limited range of angles about two axes in the sample plane, or to a stage allowing unlimited rotation about a single axis. A semiconductor charged-particle detector was mounted behind the samples on the beam axis to measure transmitted protons. Images of the sample were produced by dividing the transmitted proton energy spectrum into energy slices. Images generated from the individual slices (showing the number of protons falling in given a slice at each pixel) were combined together to form a final image showing the mean transmitted proton energy loss. The images shown here are composed of 256 X 256 pixels and are printed in grey scale with darker greys representing higher mean transmitted energy loss. Fuller descriptions of the image production process can be found in Ref. [6].
3. Examples nique
and characteristics
of the CSTIM
tech-
3.1. Large area imaging and imaging under amorphous layers As mentioned above, the use of MeV protons for CSTIM analysis enables samples with thicknesses ranging from a few microns to over 50 p.m to be analysed. It is possible to thin and lap a 2 mm X 2 mm piece of silicon with initial thickness of about 500 urn to within this range in less than an hour, such that a large portion of the sample area can be imaged for defects. This is demonstrated in Fig. 1, which shows a montage of several CSTIM images combined to produce a single 325 X 735 pm2 image of the defects in a silicon crystal. The individual images were 200 p,rn wide and taken with the sample translated 150 km between each. The sample was tilted slightly between each image so that the beam remained channeled owing to the slight bend of the crystal produced by the thinning process. The image was taken with the incident beam aligned with a (111) planar channeling direction. The dark features which can be seen densely grey. “D”-shaped covering the image are stacking faults (see Fig. 3a) which had been deliberately introduced at the top, polished surface of the wafer. Two larger-scale contrast features can also be seen. Firstly, on going from the bottom of the image to the top, the image background changes from dark grey to very light grey. This was caused by a change in sample thickness across the sample, the lowest part in Fig. 1 being about 41 urn in thickness and the upper part about 38 p.m (the thicknesses were determined from the non-channeled proton energy loss). Secondly, there is a semi-circular region covering the centre of the image where the background is a darker grey. Within this region, the sample was covered by a layer of gold and palladium a few tens of nanometres in thickness. This layer caused some scattering
_.
___. ._____.
Fig. I. Montage of CSTIM mean energy loss images of a silicon crystal containing stacking faults. The sample area shown is 325 i( 735 em*. The darker grey, semi-circular region is the pan of the sample covered by a thin, amorphous metal layer.
of the incident protons, slightly increasing the divergence of the beam reaching the underlying silicon so that a smaller fraction of the protons became channeled. The mean energy loss within this region is therefore slightly increased over that produced by surrounding crystal. However, even within this region stacking faults are visible, demonstrating the ability of CSTIM to image defects under thin, amorphous surface layers.
VI. MICROBEAMS
428 3.2. Imaging
PJ.C. King et al./Nuci.
Instr. utul Meth in Phys. Res. B I 18 (1996) 426-430
of deep defects
With the incident beam aligned with a planar channeling direction of a crystal, some MeV protons can remain
50pm
channeled to large depths within the crystal. Shown in Fig. 2 is a montage of CSTIM images of a sample containing stacking faults similar to those shown in Fig. I. In this case, however, the sample was reversed so that the faults were close to the beam exit surface; the sample was tilted so that the beam was incident on the thinned side of the crystal [7]. The thinning was performed to produce a wedge-shaped sample, approximately 5 p.m in thickness at its thin end and extending to regions approximately 50 urn in thickness. In the image shown in Fig. 2, stacking faults on the back surface of this wedge are visible throughout the range of wedge thicknesses (15-50 pm), demonstrating that sufficient protons remained channeled 50 em below the sample surface to produce image contrast. The image was taken with the incident beam aligned with {I I I] planes, as these provide the most stable channeling in silicon; channeling in {I IO] planes was found to produce only faint contrast. It was also found necessary to use a transmitted proton detector with a restricted acceptance angle (0.4”, still larger than the planar channeling critical angle of 0.1”). This caused preferential detection of the well-channeled protons, increasing their relative contribution to the image contrast. The vertical dark and bright bands which can be seen are the damage on the beam entrance surface produced by the thinning process. The sloping background of the images produced by the variation in sample thickness has been removed so that it does not dominate the image contrast.
4. CSTIM
studies of channeled
ion-defect
interactions
As well as providing a method for characterising defects in crystals, CSTIM has enabled new features of the interaction between channeled ions and defects to be studied. In particular, studies of stacking faults in a silicon crystal revealed an unexpected phenomenon, namely that these defects were actually able cause channeling rather than producing dechanneling under some circumstances
[51. Fig. 3a shows the effects of a stacking fault on a crystal. At the fault plane the crystal lattice planes undergo a shift. A well-channeled ion meeting a fault may be dechanneled owing to the displacement of the planar channels in the crystal beyond the fault. Fig. 3b shows a CSTIM image of two faults in a silicon crystal, taken with
15pm
Fig. 2. CSTlM image montage of a wedge-shaped sample containing stacking faults, with the faults on the back surface. Image taken with the beam channeled in the (11 I} planes of the sample and a collimated detector. The sample varied in thickness from 15 km near the bottom of the image to about 50 p,rn near the top. The images have been smoothed and processed with histogram equaiisation to increase the fault contrast.
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Fig. 3. (a) Schematic of the effects of a stacking fault in a crystal. (b) 30 km wide (smoothed) CSTIM mean energy loss image of two faults taken with the incident beam (1 IO) planar channeled. (c) CSTIM linescans taken across the two faults in (b) with the beam at different tilt angles about the (I 10) planes. Each line scan has had the background energy loss level set to zero and indicated by a dashed horizontal iine. Successive linescans have been displaced vertically for clarity. The angle to (110) is given alongside each linescan. (d) Mean transmitted proton energy loss versus tilt angle from (1 IO) taken from the linescans of(c) for the part of the left hand fault closest to the sample surface (thicker line, triangles) and for surrounding good crystal (thinner line, squares). (e) Difference between the two curves of (d) showing the fault mean energy loss contrast. The offset of the curve minima from zero of -0.03” in (d) and (e) is a measure of the error in the location of exact channeling alignment. Curves produced from the right hand fault ate very similar to those shown in (d) and (e).
in Phps. Re.). B 118 (1996) 426-430
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the incident beam aligned with the (1 IO) planes. The faults in this crystal were deliberately produced near the top, polished surface (the fabrication process is described in Ref. [4] and the faults in Ref. [8]) and were lying on the four {I 1 I} planes of the crystal, each of which is inclined at 54.7” to the sample (001) surface plane. The straight edge of each fault is where it met the sample surface, and the curved edge where it ended inside the crystal. The faults appear dark in Fig. 3b as the incident beam was aligned with the channeling direction and the faults were causing some of the ions to be dechanneled. Fig. 3c shows a set of linescans taken across the faults shown in Fig. 3b at different tilt angles of the incident beam to the (I IO) planar channeling direction. The background mean energy loss level has been subtracted from each plot, and is shown in each case as a dashed horizontal line. For the linescan taken with the incident beam channeled, the faults were producing dechanneling so that they caused a local increase in the mean transmitted proton energy loss. They therefore appear above the background energy loss level; the same is true for a beam incidence angle of 0.05”. However, in the linescan taken at 0. IO”, the part of each fault closest to the sample surface produced a proton energy loss below that of the background level, and for tilt angles of 0.15” and greater the whole of each fault produced a mean energy loss below the backgrou,rd level. In a CSTIM image like that of Fig. 3b. the faults would appear bright rather than dark at these tilt angles. The channeling critical angle in this case is 0.1”. and Fig. 3c shows that the faults were causing a reduced proton energy loss with the beam five times this angle from alignment. Shown in Fig. 3d are plots of the energy loss versus tilt angle for the part of the crystal where the left-hand fault was near the surface and for perfect (virgin) surrounding crystal. The ‘ault started to produce a lower than background energy loss at a tilt angle of just greater than the channeling critical angle, and continued to do so for tilt angles greater than five times the critical angle. Fig. 3e shows the difference between the two curves of Fig. 3d (the fault “contrast”). The explanation of how a stacking fault can decrease the mean proton energy loss at tilt angles of just beyond the channeling critical angle has been given previously [5]. Protons incident just beyond the channeling critical angle have paths which, whilst they do not become channeled, are strongly affected by the steering action of the channel walls. The paths of such ions are described as blocked [9]. Their transverse energy (the component of their energy normal to the channeling planes) is just sufficient to carry them through the planes. At the stacking fault the planar potential undergoes a shift. This does rot affect an ion’s transverse kinetic energy, but its transverse potential energy can be either increased or reduced, depending on its position in the channel when it crosses the fault plane. For a blocked ion, it is possible for its transverse potential energy to be reduced at the fault so that its total transverse
VI. MICROBEAMS
430
PJ.C.
King et aL/Nucl.
Imr.
and Meth. in Phys. Res. B 118 (1996) 426-430
energy is low enough for it to become channeled beyond the fault plane. An increase in a blocked ions’ potential energy would only serve to increase the effective beam divergence. A significant fraction of the incident ions can therefore become channeled at a stacking fault with the incident beam just beyond the channeling critical angle, so that the fault produces a lower energy loss than the surrounding crystal. This explanation, based on the planar continuum model of channeling, has been confirmed by detailed Monte Carlo calculations. For tilt angles close to, but just beyond, the channeling critical angle, the part of the fault closest to the sample surface shows this blocking to channeling effect most strongly as can be seen from Fig. 3c (linescans between 0.10” and 0.25”.) This is because ions reaching deeper parts of the fault would have undergone scattering in the overlying crystal. Such scattering may increase an ion’s transverse energy so that it is too high for the potential shift at the fault to enable the ion to start channeling. At these tilt angles, deeper parts of the faults therefore caused fewer ions to become channeled than shallower parts. For larger tilt angles (linescans at 0.35” and beyond) the surface part of the fault is no longer showing the strongest contrast. It is proposed that at these tilt angles, the ions’ transverse energy is initially too high for the potential shift at the fault to produce channeling. However, the scattering suffered by ions reaching deeper parts of the fault may in this case reduce their transverse energy so that it is low enough for them to become channeled at a fault. The surface parts of the faults therefore no longer show the effect more prominently than the deeper parts at these larger angles.
5. Conclusions The CSTIM technique is capable of imaging individual defects in crystals several tens of microns in thickness which may be covered by thin, amorphous surface layers. By choice of channeling direction and use of a collimated detector, deep defects on the back surface of a 50 pm thick crystal have been imaged. This technique has the potential to be a very useful analytical tool for studies of crystal defects (it has been shown that CSTJM can mea-
sure local lattice plane rotation [2] and determine stacking fault translation vectors [4]), although further work is necessary to improve the spatial resolution and decrease the time to record an image (presently 15 minutes or greater). The ability to image defects enables studies of channeled ion-defect interactions which may continue to reveal new phenomena. Further work is needed to better characterise the effects on the CSTJM mean energy loss contrast of using a collimated detector, and experimental and simulated investigations into the effects of scattering on the blocking to channeling transition are required.
Acknowledgements
P.J.C.K. and P.R.W. would respectively like to thank the Royal Commission for the Exhibition of 185 1 and the Royal Society, for Fellowships. Thanks are due to Dr. R. Falster of Monsanto Electronic Materials Corporation for provision of the stacking fault samples, and to A. Amaku of the Department of Materials, University of Oxford, for preparing the stacking fault sample with the amorphous layer.
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