Ultramicroscopy 77 (1999) 65}75
An experimentally convenient con"guration for electron channeling contrast imaging B.A. Simkin, M.A. Crimp* Department of Materials Science and Mechanics, 3513 Engineering Building, Michigan State University, East Lansing, MI 48823-1226, USA Received in "nal form 5 August 1998
Abstract Electron channeling contrast imaging (ECCI) has historically employed a highly tilted sample with a forward-scattered electron detector for viewing near-surface crystal defects in thick (electron opaque) samples. This paper demonstrates that ECCI may be performed successfully using more conventional backscattered electron detectors and a sample surface oriented near normal to the electron beam. The dislocation contrast characteristics seen using this low-tilt con"guration are found to be comparable to the dislocation contrast characteristics using the high-tilt con"guration. Additionally, dislocation contrast characteristics as a function of deviation from the exact Bragg channeling condition are presented. 1999 Elsevier Science B.V. All rights reserved.
1. Introduction Shortly after the discovery of electron channeling by Coates in 1967 [1], the concept of using this phenomenon for the imaging of near-surface crystal defects in the scanning electron microscope (SEM) was put forward by Booker et al. [2]. In spite of early enthusiasm, the actual implementation of electron channeling contrast imaging (ECCI) as a practical experimental tool for the SEM has taken much longer to develop, principally due to the exacting experimental conditions required. Although many early scanning transmission electron microscopy (STEM) experiments carried out using thin foils served to establish the basic feasibility of the technique [3,4], practical applica-
* Corresponding author.
tion of ECCI to electron opaque samples is considerably more challenging due to the dominance of the backscattered electron (BSE) signal by the electrons backscattered from the interaction volume, as opposed to electrons scattered from the primary beam which carry channeling contrast information [5]. It was recognized early on that bulk sample ECCI would require high signal collection e$ciencies to overcome the low signal-to-noise ratio resulting from the dilution of the electron channeling contrast (ECC) signal by the addition of this strong background. Early ECCI studies by Pitval et al. and Morin et al. [6,7] examined bulk samples using high brightness sources and a sample normal tilted approximately 503}703 from the beam axis to improve both the total number of `backscattera events, as well as to increase the fraction of highenergy backscattered electrons in the total BSE yield, following the methodology of Wells [8].
0304-3991/99/$ } see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 3 9 9 1 ( 9 9 ) 0 0 0 0 9 - 1
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Collection of the BSE signal was achieved through the use of an energy "ltering, large solid angle collection detector. The energy "lter was used to exclude all but the high-energy backscattered electrons to improve the relative ECC signal strength [9]. By energy "ltering, the signal produced was limited in interaction depth, and so had essentially the signal characteristics of the earlier STEM experiments. Presumably due to the perceived rigor required for this experimental setup, ECCI lay dormant for several years. In 1990, Czernuszka et al. [10] began performing ECCI using the tilted sample and forward-scatter detector con"guration used previously by Pitaval et al. [7] and Morin et al. [6], but used long collection times and post-collection signal processing to enhance the signal-to-noise ratio, rather than the cumbersome energy "ltering detector. At approximately the same time, Joy [11] demonstrated the feasibility of ECCI at low tilt angles on thin foils of MoS . Although well submerged in the paper, there is also brief mention made of dislocation imaging in the thick foil regions; this is the "rst mention of low-tilt bulk sample ECCI. Analogous to the case of transmission electron microscopy (TEM) di!raction contrast images, channeling contrast image characteristics are subject to their channeling conditions, which are controlled by the crystal orientation relative to the electron beam, and only weakly related to the orientation of the sample surface normal. The e!ect of the speci"c channeling contrast planes, de"ned by their normal vector (u) chosen for ECC, has been previously noted to control the visibility of dislocations based on the u . b criteria [7,10]. Additionally, a reversal of the sense of contrast for dislocation images taken using channeling conditions corresponding to opposite sides of the channeling band (that is, $u) has also been reported [7]. To date, however, no reports have speci"cally examined the role of the deviation from the exact Bragg condition (the deviation parameter, s). This paper explicitly examines the experimental practicality of performing ECCI on bulk samples at both low and high sample tilts. This is done by comparing images taken using the two experimental setups, one following the con"guration of Czernuszka et al., the other using low sample tilts
and a commercially available polepiece mounted BSE detector. Further, an attempt is made here to characterize the contrast behaviour observed for single dislocations as a function of s.
2. Experimental procedure A single crystal of semiconductor grade silicon that had been previously deformed in compression was cut approximately parallel to the traces of the most prominent slip planes, then bolted between two brass blocks so that the freshly cut surface was #ush with the block surfaces (see Fig. 1(a)). This assembly was in turn attached to a 12 mm rod (suitable for mounting in the microscope stage) with the surface of interest tilted roughly 403 away from the rod axis. The entire assembly was then mechanically ground and polished using SiC and Al O polishing media in a manner previously de termined to cause minimal near-surface defects in Si [12]. The purpose of the tilted sample surface was to minimize stage maneuvering geometry problems while still allowing imaging of the same region of the sample at both low and high tilt conditions using a tilt}rotate stage. All microscopy was conducted using a CamScan 44FE SEM equipped with a Schottky thermal "eld emission gun operating at 25 keV. The electron probe parameters used were: probe current of 1.8}2.9 nA; beam convergence angles (2a) of 5.0}6.4 mrad; and spot sizes in the range of 20}50 nm. Working distances were between 11 and 13 mm. ECC imaging was performed at low tilt (with the sample surface normal within &103 of the microscope axis) using a conventional four quadrant Si diode type polepiece-mounted BSE detector (with &0.6p str solid angle of collection for these working distances), as in Fig. 1(b). ECC imaging at high sample tilts (with the sample normal at greater than &453 from the microscope axis) employed a side-mounted, retractable scintillator}light}pipe} photomultiplier BSE detector of the type used by recent ECCI researchers (see, for example, [13]), as in the layout in Fig. 1(c). Shifting between the two imaging con"gurations was achieved by rotation of the sample holder by 1803 about its axis, with subsequent changes in the stage tilt. Using this
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method, surface normal tilts of approximately !453 to #803 (where positive tilts are de"ned as towards the side mounted BSE detector) were obtainable, at the expense of access to signi"cant portions of angular space. All recorded micrographs were collected as recursively "ltered 32 frame images with a 0.85 second frame time using the image processing equipment integral to the microscope. Some images were further processed after scanning of the negatives. Samples were aligned to approximate channeling contrast conditions through the use of selected area channeling patterns (SACPs). Final orientation of the region of interest to the approximate Bragg condition was typically performed by observing the region of interest in regular scanning mode while tilting until a di!use channeling band edge was centered upon the region of interest.
3. Results While it was possible to obtain ECCI images of the dislocations using both experimental con"gurations, signi"cant di!erences in the image quality and characteristics were noted. In general, the quality of the images taken using the low-tilt con"guration was found to be superior. Good contrast of the dislocations in these images allowed easy identi"cation of these features. Fig. 2(a) shows an example of one of these images. Al O polishing media from the surface preparation appears as bright specks, useful for reference purposes, and is labeled `pa in the "gure. Dislocations are readily observed throughout the "eld of view; dislocations lying close to parallel with the surface of the sample are labeled `da, while the paired bright/dark specks (labeled `ea) are believed to be dislocations oriented near normal to the surface, seen end-on.
䉳&&&&&&&&&&&&&&&&&&&&& Fig. 1. Sample holder con"guration and detector positions for the ECCI con"gurations described in this study. (a) Prespective drawing of the sample holder with the Si sample; (b) sample orientation and detector position used for low-tilt ECCI; (c) sample orientation and detector position used for high-tilt ECCI.
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Fig. 2. High- and low-tilt ECC images of near-surface dislocations in silicon. (a) Low-tilt image (&13 tilt); `pa"polishing media, `da"dislocations near parallel to the surface, `ea"end-on dislocations near normal to the surface. (b) High-tilt image (&653 tilt). (c) Channelling conditions for (a), u"2 02. (d) Channeling conditions for (b), u"022. The two-axis scale bars in (a,b) re#ect the e!ect of foreshortening.
Fig. 2(b) is an image of a similar area of the same Si sample using the the high-tilt experimental con"guration of Czernuszka et al. [13] which shows some of the disadvantages of imaging in this mode. In general, it was found that the contrast of the dislocations was lower in these images. Furthermore, images taken of highly tilted samples su!er from foreshortening, leading to a compression of the image information perpendicular to the tilt axis. (In this case, there is a compression factor of &2.4 in the vertical direction.) From an experimental standpoint, ECCI using the highly tilted con"guration is much more arduous. Beyond the much greater care required in maneuvering of the sample imposed by the close proximity of the detector, di!erences in the information carried by the SACPs used to set up the
imaging conditions also play an important role. Fig. 2(c) and Fig. 2(d) show the SACPs used to form Fig. 2(a) and Fig. 2(b), respectively. Using the lowtilt con"guration, the full angular range of the SACP has usable channeling information as opposed to high sample tilts, where the SACP only has usable channeling information over a narrow angular range. This narrow range results in less information in the SACP, making it signi"cantly more di$cult to identify and establish ECC conditions. When operating at low sample tilts, the appearance of dislocations in ECC images is dominated by subtle changes in the channeling conditions, as is shown in Fig. 3(a)}Fig. 3(c). The channeling conditions for imaging are de"ned by the trajectory of the incident beam relative to the SACP from the
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Fig. 3. ECC images taken using di!ering deviations from the perfect Bragg condition, using u"220: (a) s'0; (b) s+0; (c) s(0. (d) An illustration of the approximate beam direction relative to the crystal channeling pattern for each case. Circle diameters represent the beam convergence angle, 2a+5.7 mrad.
crystal under examination, in many ways akin to the incident beam orientation relative to the Kikuchi patterns for di!raction contrast imaging in the TEM. The channeling conditions for Fig. 3(a)}Fig. 3(c) are demonstrated in Fig. 3(d), where an SACP is displayed with the beam trajectories noted. If the incident beam trajectory lies within a channeling band, the beam meets the lattice planes at less than the Bragg angle, and (following TEM convention) the deviation parameter s is less than zero. Under these conditions, dislocations are in weak contrast against a bright background (Fig. 3(a)). When the beam is oriented on the edge of the channeling band (s"0), the dislocations appear in bright/dark contrast with respect to the background (Fig. 3(b)). When s'0 (beam outside of channeling band), the dislocations appear as bright
features against a dark background (Fig. 3(c)). In general, dislocations display the most striking contrast for the condition s+0. For the instances where s+0 and ECC dislocation images appear as twin bands of bright and dark contrast relative to the background, it was noted that the side of the dislocation core having the bright or the dark contrast switches upon changing the sign of u. This is shown in Fig. 4, which shows a $u pair of the same area of sample. Both "gures employ s+0, so the principal di!erence between images is due to the sign of u. For example, the features noted as `aa, `ba, and `ca in Fig. 4(a) undergo a clear sense of contrast reversal when viewed using the !u of Fig. 4(b). This inversion of contrast with the sign of u is the same for the low-tilt con"guration as that found for the
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u"220 channeling contrast condition. Feature `ba is similarly visible for both u"220 and 202, while not in contrast for u"02 2.
4. Discussion
Fig. 4. ECC images taken using $u channeling conditions of the same sample region at (a) u"220 and (b) using u"2 2 0, s+0, showing the switch across the dislocation core in contrast for $u. Dislocation structures `aa, `ba, and `ca are examples of this switch, and are referred to in the text.
The results presented in this study demonstrate that ECC imaging is possible using a low sample tilt con"guration to achieve comparable results to the high tilt con"guration, using less specialized experimental equipment. The image quality of these low-tilt images is equivalent (if not superior) to their high-tilt counterparts. Furthermore, orientation and positioning of the sample for the lowtilt con"guration is considerably less challenging than for the high-tilt case, for two reasons: channeling information, and sample}detector geometry. As demonstrated in Fig. 2(c) and Fig. 2(d), less channeling information is available in SACPs taken at high sample tilts than in patterns taken at low sample tilts. This is likely due to two separate e!ects: the "rst and dominant being the dependence of the overall backscattered electron yield (g) on beam}surface angle, the second related to signal collection e$ciency. As noted in a prominent basic SEM text [14], g is a function of tilt angle W, which can be approximated by 1 g" (1#cos W)N
high-tilt, energy-"ltering conditions used by Morin et al. [7]. While the sense of the dislocation contrast is a!ected by the deviation parameter s, dislocation contrast as a whole is a function of the lattice planes giving rise to the channeling contrast. These planes can be de"ned by their normals using their reciprocal lattice vector u. Examination of Fig. 5, which shows low-tilt ECC images taken using the three +2 2 0,-type u-vectors around the pole of the (1 1 1 ) plane (near to parallel with the surface), reveals a number of dislocations that go out of contrast for one of the channeling conditions. Some examples of this are features `aa and `ba. Feature `aa is strongly visible using the u"202 and 02 2 channeling contrast conditions, while out of contrast for the
(1)
where p"9/(Z (Z"at. number). Because of the sensitivity of g to tilt, the background component of g (that is, the component of g that is not related to channeling) will change by several percent over the &173 rocking angle of the SACP at high tilts, but will remain almost unchanged across the "eld at low tilts. The plot of Eq. (1) in Fig. 6 shows this dependence of g on surface tilt for silicon, along with the rate of change *g/*W. As the variation in the background g over the rocking angle of the SACP is typically greater than the variation of g due to channeling e!ects, then contrast levels su$cient to see the ECC component of g will cause saturation of the majority of the pattern for the high-tilt case, as is seen in Fig. 2(d).
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Fig. 5. ECC images of the same sample region taken using di!ering u vectors: (a) u"220; (b) u"202; (c) u"02 2; (d) shows the same region as an E}T image. Features `aa and `ba are dislocations referred to in the text.
The second reason for the lower amount of information available in SACPs taken at high sample tilts is related to signal collection e$ciency. At low tilts, the angular distribution of g is approximately related to the cosine of the surface emission angle [15], so that the largest fraction of g is emitted normal to the surface. Because of the geometries necessary to gain the large rocking angle necessary to form SACPs, working distances are short, with the result that for a relatively large range of sample tilts the broad intensity maximum of g will intersect a polepiece-mounted BSE detector. As opposed to this, at high sample tilts the majority of the `backscattereda electrons are strongly forward-scattered, with an average trajectory at a shallow angle to the sample surface [15]. In consequence, e$cient collection of these electrons can only be accomplished by placing a detector close (&0.05}2.0 mm) to this
surface (Fig. 7). Because the angular distribution of the backscattered electrons is strongly peaked for this high tilt case, changes in the angle at which the incident beam intersects the sample surface will also change the direction of this preferred trajectory. If the beam rock used to collect the SACP is su$cient to cause the preferred trajectory to miss the detector (as in Fig. 7(b)), then a strong signal loss will occur, causing a loss in intensity for the SACP over that angular region. Another consequence of the close proximity required between the detector and the sample surface is that maneuvering of the sample is much more likely to impact the detector. This tends to make tilting the sample much more time consuming, while strongly hampering the use of more complicated stage assemblies, such as in-situ testing equipment, that might impact or obscure the detector.
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Fig. 6. The variation of the (a) total backscattered electron yield, g (after [14]), and (b) the rate of change with sample surface tilt relative to the incident beam, plotted for silicon.
The dependence of the sense of the channeling contrast for dislocations on the sign of s can be understood by considering the local orientation of the atomic planes used for channeling contrast in
relation to the beam trajectory. This is illustrated by considering two separate portions of the sample separately: the regions of the sample away from the defects, and the defected regions. At high
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Fig. 7. Schematic representation of the backscattered electron distribution for a highly tilted sample and the spatial relationship with the detector for high-tilt ECCI. (a) The condition for the normal scanning beam, close to the microscope axis; (b) the case for a beam tilted o!-axis, as happens when collecting an SACP.
magni"cations, the beam trajectory can be considered as approximately constant over the "eld of view. (For a 10 lm "eld width at 10 mm working distance, the total change in trajectory is &1 mrad, which is signi"cantly less than the typical beam
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convergence angle of 5}6 mrad.) Thus, the dominant contribution to channeling contrast at these magni"cations will come from the changes in the orientation of the di!erent regions of the sample relative to the beam. For the regions of crystal signi"cantly far away from any defects, alterations of crystal orientation by the defect strain "elds will be minimal, so that these (background) regions will be expected to have the same orientation relative to the beam throughout the "eld, and would consequently have the same backscatter intensity g. Regions of the crystal close to the defects, however, will in general be more strongly a!ected by the defect strain "elds, and will di!er in orientation relative to the beam, as compared to the background crystal. As SACPs are formed by plotting the collected fraction of g as a function of beam angle relative to the microscope axis, g for the background region is then expected to be the same as for the center of a hypothetical SACP, where the beam is parallel to the microscope axis. In turn, g for a defected region then corresponds to the intensity at the center of an SACP that has been shifted slightly to correspond to the rotation (or tilt) of the crystal in the vicinity of the defect. The e!ect of this is demonstrated in Fig. 3(a)}Fig. 3(c), where the electron beam trajectory relative to the bulk crystal changes from s'0 in (a) to s(0 in (c). For the s"0 condition of (b), the bulk crystal is at the Bragg condition, so that the atomic planes to opposite sides of the dislocation core will have opposite signs for s . By examination of Fig. 3(d), The region of the crystal with s '0 can then be expected to appear dark, while the region with s (0 can be expected to appear bright, as is seen for Fig. 3(b), where the dislocations appear as bright-dark line pairs. This same e!ect is seen in the cases where sO0, in 3(a) and Fig. 3(c). The channeling conditions for Fig. 3(a) are such that the bulk crystal is at s'0, so that only the side of the dislocation where s (0 shows contrast, because the intensity of the SACP at s0 is approximately constant. This same argument also applies in converse for the case where the bulk orientation is such that s(0, as in Fig. 3(c). Employing this same method of interpretation, the dislocation image extinctions of Fig. 5 are
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expected to occur for the case of u . b+0, and should therefore allow contrast analysis for the determination of Burgers vectors for dislocations. In the present case, however, as only a single extinction condition is recorded for each of the candidate dislocations in Fig. 5, no de"nite assignments for b based on these images is possible. This brings forward one of the principal limitations still inherent to ECCI, which is the greater di$culty in setting up multiple extinction conditions for a single defect in order to perform a full Burgers vector analysis, as compared to TEM analysis. The region of angular space conveniently amenable to low-tilt ECCI analysis is typically on the order of &303 or less o! the surface normal, which is not as convenient for establishing the low-order channeling contrast conditions necessary for strong ECC in the SEM as it is for setting up convenient di!raction conditions in the TEM. Possibly it is in this application that low-angle and high-angle ECCI could be pro"tably combined. It should be noted that contrast analysis using ECCI is more operator dependent than is the case in the TEM, due to the ease of misalignment from the s+0 condition. Whereas dislocation contrast is not lost for small misalignments in the TEM, total loss of ECC can occur for reasonably small deviations from the s"0 condition, which may lead to erroneous conclusions.
5. Conclusions The use ECC for the imaging of crystal defects in bulk samples does not require the highly tilted sample con"gurations commonly used in ECCI studies of defected materials. Provided that adequate probe current and a high detector collection e$ciency are used (in addition to the usual ECCI requirement for a small beam convergence angle) ECCI may be successfully performed using a conventional polepiece-mounted BSE detector at near-normal incidence angles using only the digital image processing capabilities typical of a modern SEM. The advantages of the low-tilt con"guration are principally: (1) greatly decreased mechanical interference between the detector and stage assemblies, allowing easier maneuvering of the sample
and the use of in-situ testing apparati; (2) access to a larger choice of ECC conditions due to the more advantageous detector placement; and (3) the elimination of the need for specialized detectors, with the concurrent advantage of freeing up chamber space by using a general-purpose polepiece-mounted BSE detector.
Acknowledgements This work was supported through National Science Foundation Grant DMRC9257826. The CamScan 44FE was purchased under National Science Foundation Grant DMRC9302040 and funds from Michigan State University.
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Electron Microscopy and X-Ray Microanalysis, 2nd ed., Plenum Press, New York, 1992, p. 95, citing: F. Arnal, P. Verdier, P.-D. Vincinsini, C.R. Acad. Sci. Paris 268 (1969) 1526. [15] J.I. Goldstein, D.E. Newbury, P. Echlin, D.C. Joy, A.D. Roming Jr., C.E. Lyman, C. Foiri, E. Lifshin, Scanning Electron Microscopy and X-Ray Microanalysis, 2nd ed., Plenum Press, New York, 1992, pp. 98}99.