Ion channeling effects in scanning ion microscopy with a 60 keV Ga+ probe

Nuclear Instruments and Methods 205 (1983) 299-309 North-Holland Publishing Company

ION CHANNELING

EFFECTS

IN SCANNING

299

ION MICROSCOPY

W I T H A 60 keV G a + P R O B E

*

R i c c a r d o L E V I - S E T T I , T i m o t h y R. F O X a n d K i n L A M The Enrico Fermi Institute and Department of Physics, The University of Chicago, Chicago, Illinois, 60637, U.S.A. Received 5 April 1982 and in revised form 7 July 1982

The operation of a 60 keV Ga + scanning ion microscope is described. This instrument has been shown capable of detecting ion-channeling phenomena in samples of brass and iron, through the observation of crystallographic constrast in images obtained with the secondary electron and secondary ion signals. The instrument also provides on-line quantitative information on surface amorphization and on channeling effects in sputtering. While the lattice orientation dependence of secondary electron and ion emission is generally comparable, several examples are discussed where anticorrelation of the two signal intensities is observed.

1. Introduction We report on the observation of pronounced crystallographic contrast in secondary electron (SE) and secondary ion (SI) iamges of crystalline materials obtained in a scanning ion microscope (SIM). The instrument makes use of a focused beam of 60 keV Ga + ions from a liquid metal source. The effect, studied here on polished samples of recrystallized a-brass, meteoritic iron, and steel, is due to the dependence of the SE and SI yields on lattice orientation relative to the incident ion beam. We interpret such dependence as being related to primary ion channelling and refer to the observed image constrast as ion channelling contrast (ICC). Regarding the physical processes yielding crystallographic information from bulk samples, the method presented here is the scanning analogue of the emission electron microscope (EEM) employing a primary ion beam. Since early results [1], the crystallographic contrast in E E M images of crystalline metal targets has been systematically investigated by the Toulouse group [2]. In our energy range, extensive measurements of the SE emission coefficient Ye with changing crystal orientation relative to the incident ion beam with monocrystalline targets have also been made by the above authors (e.g., ref. 3) and by the Moscow group (e.g., ref. 4) since the early 1960s. Fagot and Fert [2] have used the above image contrast for indexing individual crystallites, by mapping the emission dependence on incident beam direction. After some debate, due to the indirect dependence of the observed secondary effects on the funda* Work supported by the Air Force Office of Scientific Research (Contract F 49620-80-C-0074) and the National Science Foundation Ceramics Program (Grant DMR-8007978). 0167-5087/83/0000-0000/$03.00 © 1983 North-Holland

mental primary processes involved, the similarities of the anisotropies of ~'e with those of the sputtering yields Ys [5] eventually led to interpretations of the above observations in terms of primary ion channelling [3,6]. Crystallographic contrast in the E E M has also been observed with a primary electron beam [7] in experiments very similar to those of the Toulouse group. Comprehensive reviews of the EEM approach to material characterization are available [8,9]. Direct evidence of electron channelling and blocking in thick crystals was obtained in scanning electron microscope (SEM) experiments [10]. Electron channeling contrast and electron channeling patterns due to a primary process are obtained in the SEM with backscattered electrons (BSE). This method, which has developed rapidly to become a standard in crystallographic studies [11,12], has become more widespread than the EEM in practical applications. The vast literature on the subject has recently been summarized [13,14]. As will be shown here, the SIM presents a new option in the study of crystalline materials, in view of the magnitude of the primary ion-channelling effects on the secondary processes which are exploited for imaging purposes. Particularly relevant is the observation that when primary ion channelling is operative, even the copious SE signal may carry significant bulk target information.

2. The scanning ion microscope The instrument described here was originally developed as a scanning transmission ion microscope (STIM) [15], which used a field ionization (FI) source of hydrogen ions. In its present SIM version, the above STIM

300

R. Lei',i-Setti et a L / Ion channelling efJec'ts

was modified by replacing the FI tip with a cartridge containing a liquid metal (LM) source [16] and a closely spaced extraction electrode. The instrument is illustrated in fig. t. The LM source is a gallium-wetted tungsten needle, protruding from a ribbon heater. This preserves the point-like configuration of the FI source, thus providing the source brightness needed for high resolution SIM operation using heavy ions (see, for example, ref. 15). The extraction assembly serves two purposes. The closely spaced aperture increases the field at the gallium surface, lowering the voltage required for operation while preserving the optical position of the source with respect to the gun lens. Also, the enclosure between the two apertures confines the gallium within the cartridge. An extraction voltage of 6 - 7 kV suffices to obtain ion source currents of 2-10 #A; 4 /~A is an optimal current that minimizes the chromatic aberration resulting from the source energy spread ( - 8 eV fwhm at < 5 /~A) [17]. The ions are accelerated to - 60 keV and focused by a two-electrode lens to form a probe 100 nm wide in the specimen plane; the probe semiangle is 1.5 x 10 - 4 rad and the probe current is typically 50-100 pA. The SIM retains the electrostatic deflectors and stigmator of the original STIM. The SIM images result from detecting either sec-

ondary electrons or ions emitted from the top surface of the specimen. The detector is a large continuous-dynode electron multiplier (Galileo 4716), - 4 5 ° off-axis, biased at +45 V to collect secondary electron, or at - 3 0 0 0 V to collect secondary ions. For low detected currents ( < 2 MHz count rate), such as those collected in SI emission, we can display the detected partEcles individually (through a discriminator) as pulses on the camera C R T screen. For higher currents, it is more useful to display the signal through an analogue amplifier. The secondary electron signal is usually high enough to allow rapid scanning (7.4 vertical scans per second) for real-time display, which allows visual focusing, stigmation and specimen searching. A record of the signal intensities observeds in a line scan with either signal can be obtained with a multichannel scaler or oscilloscope. The LM Ga + source is particularly reliable and robust. We have operated the same tip without reloading for over 250 h. High voltage discharges, which would have destroyed an FI tip, did not affect the Ga + source. Useful currents are observed over a narrow range of extraction voltages above a short threshold. The turn-on threshold is higher than the turn-off threshold, usually by several hundred volts. The source

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R. Levi-Setti et al. / Ion channelling effects current may decrease during operation at a fixed voltage typically from 4/~A to 2 # A over 4 h. From day to day the operating extraction voltage may vary by not more than - 100 V. Heating the needle for an hour or so before applying voltage seems to improve the day to day reproducibility of the operating voltage. The entire SIM is designed as an ultra-high vacuum system ( - 10 -9 Torr) and is free of hydrocarbon contaminants. In these conditions, the ion beam effectively cleans the specimen surfaces in a few minutes by sputtering away absorbed or oxydation layers while scanning.

3. Crystallographic contrast in the SIM: physical considerations In this experiment we shall consider image contrast effects arising from the interaction of a 60 keV Ga + probe with polished samples of recrystallized c~-brass (70% Cu, 30% Zn), meteoritic iron and steel. The lattice structure of this brass is isomorphic with that of f.c.c. Cu; Fe is a b.c.c, crystal. It is relevant to examine from a physical standpoint those aspects of the beam-target interaction which may give rise to detectable image contrast, in upbeam observations at or close to normal incidence.

3.1. Contrast carrying signals We note first that since the colliding partners (Ga + on Cu and Fe) are very close in the periodic table, thus having about equal mass, primary ion backscattering is not allowed by kinematics for primary collisions. Thus only backscattering from multiple collision events and secondary processes will give rise to observable effects. All secondary yields ultimately reflect the mechanism and rate of energy transfer to the solid by the incident ions. For the particular projectile-target combination and energy range considered, the rate of energy transfer to atoms in the solid (nuclear stopping) [18] may become larger than that spent in electronic excitation (electronic stopping). Nuclear stopping initates an atomic collisional cascade, responsible for target atom sputtering, the related SI emission, and also recoil-induced SE emission. In the conditions of our experiment, disregarding lattice effects, it is estimated that up to 25% of the overall SE signal is due to recoil-induced emission [19], the remainder being attributed to primary excitations via electronic stopping. Since the total rate of energy loss for the incident ions, as well as the relative contribution of nuclear and electronic stopping to such rates are dependent on the physical and chemical bulk properties of the target, the SE and SI signals as well as the sputtering effects become effective carriers of structural information in the SIM.

301

3.2. 1on-channelling effects The relative magnitude of electronic and nuclear stopping during ion penetration is strongly affected by the lattice structure of the solid. When the incident ions become channelled in a crystal [20], even the rate of energy loss due to electronic stopping may differ substantially from the average value for a random direction. This gives rise to a strong dependence of the SE and SI yields on the lattice orientation relative to the incident beam, dependence which can be exploited to obtain crystallographic contrast in ion microscopy. We have been unable to find in the literature data on the SE yields from the bombardment of Cu and Fe crystals by 60 keV Ga + ions. However, some of the measurements reported by Colombie et al. [3] for noble gas ion bombardment of Cu and Fe approach the conditions of our experiment and are informative as to the magnitude of the contrast effects to be expected here. The yields are generally related to the lattice opacity. For example, the SE emission coefficients for 60 keV Kr + ions incident on Cu along the lattice axes (axial channelling) ~110), (100), ~111), and for polycrystalline Cu are respectively about 1.3, 1.7, 3.2, and 4.1 secondary electrons/incident ion. This implies that changes in signal level as large as - 3 : 1 may be available to yield crystallographic contrast, as indeed observed in the EEM [2]. An analogous progression of )'e values (ibid) for the principal axial directions of a b.c.c. crystal are found for 60 keV Ar + ions on Fe. To estimate possible crystallographic contrast in SI emission, no lattice orientation dependent data are availabe. A clue may be provided by the sputtering coefficients Ts, which for, e.g., 40 keV Kr + on Cu [5] are ~ 3.5, 5.0 and 11.0 sputtered atoms/incident ion in the case of axial channelling along the CII0), (100) and (111) directions respectively. We note that the yields quoted above reflect, although not in a strict proportional manner, the rates of energy deposition by the incident ions. On the basis of Firsov's [21] model, which is known to approximate well the electronic stopping (in the average over Z~ oscillations), both in channelling and non-channelling conditions, we obtain for d E / d x of 60 keV Ga + on ~110) Cu a value of 157 e V / n m . For amorphous Cu, the corresponding value is 484 e V / n m . The rate of energy loss due to nuclear stopping [18] for amorphous Cu amounts to 3100 e V / n m . Detailed calculations of the lattice orientation dependence of the SE and SI yields in our energy region and for the beam-target combination of interest here are still unavailable.

3.3. Critical channelling angles Relevant to the experimental observation of ionchannelling contrast is an estimate of the critical angles

R. Levi- Setti et al. / Ion channelling efjects

302

Table 1 Critical angles for the channelling of 60 keV Ga + ions in a Cu (f.c.c.) and a Fe (b.c.c.) lattice.

~2

t~2p

Axis:

(1 lO)

(lO0)

(l l l)

Cu Fe

8.3 ° 5.7 °

6.4" 7.5"

4.2 ° 8.3 °

Plane:

(110)

(100)

(111)

4.1. Secondary electron imaging

Cu Fe

5.4 ° 6.6 °

6.4 ° 5.5 °

6.9 ° 4.2 °

As a metallic specimen is first examined in the SIM, we note that the image darkens momentarily due to positive charging of insulating contamination layers (probably oxides) or adsorbates on the surface. The image evolves rapidly until a stable contrast configuration is reached after the surface contaminants have been sputtered away. Fig. 2a shows a region of brass exhibiting sharp contrast of twin structures and grain boundaries. For comparison, an SEM micrograph of a detail of one of the bright bands sandwiched between dark twins is shown in fig. 2b. Only the surface topography is represented in the SEM image, while the SIM image suggests the presence of contrast effects arising from the bulk structure of the specimen. That this is indeed the case and due to primary ion-channelling effects is demonstrated in figs. 2c and d. Here the same specimen area is viewed at normal incidence in fig. 2c and after rotating by 10 ° around a horizontal axis in fig. 2d. Such a tilt angle is larger than the critical channelling angles of table 1 for Cu and is sufficient to close particular channeling directions and open new ones among the randomly oriented crystallites in the sample. This results in contrast reversals dramatically evident in the above comparison for the two orientations. The actual crystallographic orientation of individual crystallites could be obtained, as done in the EEM [2], by mapping the SE emission signal in ,~ and 0. Other examples of contrast reversals in brass samples will be shown later, when comparing SE with SI emission. The magnitude of the maximum contrast observed between unchannelled (bright areas) and channelled (dark areas) conditions is obtained by measuring the intensity of the SE signal when the probe is directed sequentially on a pair of such preselected areas. The yield ratio observed is 3.2: 1, indeed the same ratio as obtained by the Toulouse group [3] for random Cu relative to (110) Cu under 60 keV Kr + ion bombardment. By ramping a retarding potential applied to the SE detector, we have also obtained integral energy spectra of the SE emitted in channelled and unchannelled conditions respectively. These spectra are shown in fig. 3. Although they originate from a rather primitive method of analysis, inadequate to observe possible discrete features, the spectrum from channelled ions does fall off somewhat more rapidly than that from unchannelled ions. This difference could be related to the

for channelling. In the classical description of the motion of fast ions in a lattice [20], ions travelling along major axes or planes may become constrained by harmonic forces to follow trajectories contained within axial channels or between atomic planes. This can occur only if the transverse ion motion has an energy smaller than a certain barrier energy for the channel. This condition translate into the requirement that the angle of the ion trajectory to the channel direction be less than a critical value. For the conditions of our experiment, the axial channelling condition is expressed [20] in terms of the critical angle

~2 =

crystalline structure smeared by the polishing process. The samples of ferrite from an iron meteorite (Toluca) were cut, polished and demagnetized for examination in the SIM. A sample of die steel (0.9% C, 1.1% Mn, 0.3% Si, 0.5% Cr, 0.5% W, 0.15% V) was recrystallized by annealing for one hour at 760°C, then sectioned and polished.

( 3a2TFZ1Z2e2 ) 1/4 Ed 3 ,

(1)

where E is the primary energy, d the atom spacing along the string direction,

aTz = ao" 0.8853( Z12/3 + Z2/3 ) - I/2 is the T h o m a s - F e r m i screening radius and a 0 is the Bohr radius. For planar channelling, the corresponding critical angle (see for example ref. 22) is expressed by ~2p = ~,N~p (a2vZ, Z 2 e 2 / E ) 1/3,

(2)

where N is the atomic number density and dp the relevant interplanar spacing. For 60 keV Ga + on Cu and Fe and for the lowest indices, such critical angles are given in table 1. The above values of the critical angles give an approximate measure of the half-width of the channelling dips which one should expect to observe in, e.g., SE and SI emission in our experiment. 4. The observation of crystallographic constrast in the SIM

The samples of a-brass were recrystallized by annealing in vacuum at 550°C for one hour. After sectioning and polishing, the specimen surface was lightly etched with nital (10% H N O 3 in ethyl alcohol) to expose the

R. Levi- Settiet al. / Ion channelling effects

303

Fig. 2. (a) Ion channelling contrast in SIM micrograph of a sample of recrystallized, polished, HNO3-etched brass. SE signal, 60 keV Ga ÷ UC-SIM, 160/xm full scale, 8 s scan. (b) Detail of top twin bright band, imaged with a 10 kV Coates and Welter SEM. 13 p.m full scale. Contrast due to surface topography. (c) SE images of another area of same brass specimen, obtained at normal incidence in the SIM. 64/~m full scale, 8 s scan. (d) Same specimen region as in (c), after rotation of the sample by 10° around a horizontal axis. Contrast reversals when compared to (c) demonstrate primary ion channelling effects in SE emission.

lattice orientation d e p e n d e n c e of the depth distribution of primary energy deposition, in turn affecting the energy lost during SE escape. Ferrite crystals in iron meteorites are known to exhibit characteristic twinning lines ( N e u m a n n bands) due to shock-induced damage [23]. These are typically 1-20 ~ m wide, may be several cm long, and are ordin-

arily rendered visible by chemical etching. Fig. 4 demonstrates the SIM detection of N e u m a n n bands in a polished section of ferrite from a large meteorite body (Toluca) [24]. Two pairs of micrographs are shown, taken respectively at normal incidence (a, c) and at 10°C tilt (b, d). The comparison again shows that ion-channelling is at the origin of the contrast effects

304

R. Levi-Settt et al. / ion channelling effects

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Fig. 3. Integral secondary electron energy spectra obtained with a variable retarding potential. 60 keV Ga + ions incident on brass in channelled (full points) and unchannelled (open points) conditions. observed. For a Fe crystal (b.c.c. lattice) the secondary electron yields for the principal.directions ((110), (100) and <111) are known [3] to differ by not more than 15% (about a mean of - 2 electrons/ion) under 60 keV Ar + ion bombardment. Maximum yields twice as large as these are observed instead along closely packed planar orientations, such as, e.g., (211). Remarkably, the twinning plane of the Neumann bands is the (211) plane. This leads to an interpretation of the brightest bands in fig. 4 as due to enhanced SE emission along such twinning planes. Patterns such as those of figs. 4c and d bear a superficial resemblance to the selected area channelling patterns (SACP) observed in the SEM with backscattered electrons [12]. Although not entirely unrelated, the patterns obtained here are not the SIM analog of the SEM SACP. Since they are obtained while scanning at a practically constant incident angle (~0 < 2 x 10 - 4 rad), our patterns are due to spatially localized crystal structures, rather than to non-localized Braggtype reflections as in SACP, which are obtained by varying the angles of incidence rather than the point of impact. Another example of ion-channelling contrast in iron is shown in figs. 5a, b. The sample is a polished section of die steel. Grains are clearly visible, and the constrast reversal for a 10 ° rotation again confirm the physical origin of the different levels of SE emission signals recorded. -

4.2. Sputtering

The lattice orientation dependence of the sputtering rates, due to ion-channelling effects, can be related to

SE crystallographic contrast by ion-beam wiriting in line scans. An example of this approach to the study of such relationship is provided in figs. 5c, d. In fig. 5c, an area of a brass sample exhibiting strong crystallographic contrast is located by SE imaging. Lines are then written at different speeds on the selected structure. The area is later examined at high magnification in the SEM, as shown in fig. 5d. Clearly the depth of the etched lines differs substantially for the crystallite area, which appear bright (unchannelled ions) and dark (channelled ions). More specifically, the raised plateau (dark in SE image) at the top of fig. 5d can be identified as a (100) plane, a characteristic of a chemical etching of Cu and brass [25]. This area, where (100> axial channelling occurs (at normal incidence), required a dose of 7.1 x 1017 i o n s / c m 2 for a visible etched line to appear (top line). Sputtering is much more effective in the furrowed band (bright in SE image). Here a dose of 2 . 4 x 1017 i o n s / c m 2 (bottom line) already produced damage comparable with that produced by a dose - 3 times as large in channeling conditions. We encounter here again a ratio of sputtering rates consistent with the axial results mentioned previously [5], which correlates with that observed for the SE yields. 4. 3. Secondary ion imaging

Crystallographic contrast is also present in SI images. In fig. 6 we compare images of the same area of brass as obtained at normal incidence from the SE (fig. 6a) and SI (fig. 6c) signals respectively, with the corresponding images (figs. 6b, d) taken after rotating the sample by 10 ° around a horizontal axis. Due to the lower SI yields, the micrographs for this signal have been taken with the pulse-mode type of display. For most crystallites, a direct correspondence is observed between SE and SI contrast. This implies that the ion channelling effects affect the secondary emissions in comparable manner, as anticipated from the known correlation of the SE and sputtering yields [26]. However, there are some exceptions to this general trend. For example, a triangular crystallite on the bottom right of the micrographs is dark in both SE and SI images only for incidence at 10 ° to the normal (figs. 6b and d), while for normal incidence (figs. 6a and c) the same crystallite appears bright in SE and dark in S! emission. This anomaly is further illustrated in fig. 7 where another brass region is analyzed in a similar fashion. Here we note again a general correspondence of SE and SI contrast at 10 ° incidence (figs. 7b and d) but a remarkable reversal between SE and SI contrast at normal incidence (figs. 7a and c) for several crystallites. Figs. 7e and f show the surface structure from SEM micrographs of some of the areas involved in this anticorrespondence. In particular, the flat-topped crystallite shown in fig. 7e can be attributed to the (100) plane (developed by nital etch), while

R. Levi-Setti et al. / l o n channelling effects

305

Fig. 4. SE images of polished ferrite crystal from the Toluca meteorite, obtained in the SIM. Contrast due to ion-channelling reveals the presence of twinning caused by shock (Neumann bands). (a, c) Normal incidence, 18/~m full scale. (b, d) Same areas as in (a, c) after rotation by 10° around horizontal axis.

fig. 7f shows twin boundary profiles (arrows) known to correspond to (111) twinning planes. The irregularly furrowed surfaces adjacent to the trace of the twinning planes are characteristic of ion etching of the (110) planes, the furrows developing along (100) directions [27]. We are then led to believe that the anticorrespondence observed in SE and SI emission does occur at least for orientations close to the (100) and (110) directions. Since this anomaly disappears by a 10 ° tilt, it must be related to primary ion channelling and not to

surface effects. We note that an effect such as observed could arise if the critical channelling angles, effective in SE and SI emission, were different and larger for SE than those in table 1. It should be noted in this regard that Colombie et al. [3] have consistently found that the widths of the channelling dips observed in SE emission are about twice as large as those predicted by Lindhard's theory [20] for channelled primary ions. Conversely, the critical axial channelling angles effective in sputtering were found to be consistent with l,indhard's theory [5],

306

R. Levi-Setti et al. / l o n channelling effects

Fig. 5. (a, b) SE images of the same area of polished, annealed die steel in the SIM. Contrast reversals between (a) (normal incidence) and (b) (10 ° incidence) due to primary ion channelling. 64/tm full scale. (c) SE image of brass sample in the SIM. Normal incidence, 32/tm full scale. (d) Magnified detail of bright band in c, after Ga + ion-beam-writing in single pass line scans. Beam doses were, for top line: 7.1 × 10 I~ ions/cm2: middle line: 2.4x 1018 ions/cm2; bottom line: 2.4× 10 ~7 ions/cm2. 10 kV Coates and Welter SEM. 13 ffm full scale.

although the shallow depths involved in the sputtering process may lead to regarding such agreement as fortuitous. It is perhaps naive to expect a close correspondence between the angular width for primary ion channelling and those of such secondary processes as SE and SI emission or sputtering. In fact, the latter occur over different ranges of penetration depths and are the result of quite different interaction mechanisms. Further experiments with the approach exemplified

here, but with a more detailed study of the angular d e p e n d e n c e of the SE and SI images, may help to clarify this preliminary evidence. 4.4. Loss o f S1 contrast After prolonged ion bombardments, crystallographic contrast in SI images disappears completely. The onset for this process, for 60 keV Ga + ions on brass, is

R. Levi-Setti et a L / Ion channelling effects

307

Fig. 6. Comparison of SE and SI images of brass crystallites in the SIM. For most crystallites, the two signals yield comparable contrast. (a) SE signal, normal incidence, 72 ~m full scale. (b) Same as in (a), at 10° incidence. (c, d) SI images of same areas as in (a, b) 200000 counts displayed in each micrograph.

observed for an overall dose in excess of 85< 1016 i o n s / c m 2. Even after SI contrast is lost, however, the SE images preserve u n c h a n g e d and indefinitely the original crystallographic contrast. It must be inferred that the loss of SI contrast is a surface effect and that secondary electrons originate from unaffected layers b e n e a t h the surface. Furthermore, since the surface is continuously eroded during scanning due to sputtering, an equilibrium must be reached so that the d e p t h of the af-

fected surface layer remains constant. The loss of SI contrast could be associated with the depletion of preferentially distributed reactive absorbates, responsible for the ionization of the sputtered atoms. Indeed, the loss of SI contrast is accompanied by an overall decrease of the SI signal. The persistence of SE contrast in any case is of value in d e m o n s t r a t i n g that this signal in the SIM carries information about the bulk structure of the material.

308

R. Levi-Setti et al. / l o n channelling efjects

Fig, 7. Comparison of SE and SI images of brass crystallites in the SIM. For several crystallites, the two signals yield opposite contrast at normal incidence, comparable contrast at 10° incidence. (a) SE signal, normal incidence, 64 ~m full scale. (b) Same as in (a), at 10° incidence. (c, d) SI images of same areas as in (a, b), (e) Magnified surface detail of dark band on top right of (a, b). 10 kV Coates and Welter SEM, 15 p~m full scale. (f) Detail of twin band at bottom of (a, b). Twin boundaries indicated by arrows. 10 kV Coates and Welter SEM, 15 /~m full scale.

R. Levi-Setti et al. / Ion channelling effects

309

5. Conclusions

References

We have shown that the observation of crystalline structures in the SIM can provide i n f o r m a t i o n o n the ion-channelling d e p e n d e n c e of kinetic SE emission, SI emission a n d sputtering. As to be expected, our preliminary observations by this a p p r o a c h agree in general with the results of previous systematic m e a s u r e m e n t s on monocrystals, alreadly available in the literature. F r o m this point of view, the use of the SIM to visualize such effects is to be regarded as a practical application of ion channelling which promises to be valuable in the study of materials. In particular, the large contrast effects intrinsic to the m e t h o d have obvious potential for the sensitive detection of defects and impurities in crystals especially at better spatial resolution than presented in this pilot study. There are, however, several unique aspects of the present a p p r o a c h as a method to investigate the i o n - s o l i d interaction proper. These originate in the capability to correlate directly on a local scale, different secondary processes. Thus far we have learned, as yet in a qualitative manner, relevant new information on two aspects of these processes. The observation of persistent crystallographic constrast in SE emission even after d i s a p p e a r a n c e of such contrast in SI emission favors a kinetic electron emission model where primary ion channelling i n f o r m a t i o n is transported to the surface irrespective of the conditions at the surface. A likely c a n d i d a t e for such energy transport is recoil-induced SE emission [19,28]. A n o t h e r i m p o r t a n t new aspect is the discovery of the occasional anticorrespondence between SE and SI emission, at variance with the correlation between SE a n d sputtering yields in crystals [26]. Clearly this calls for a detailed investigation of the lattice orientation dependence of the SI yields, as an issue separate from sputtering. A careful comparison must be m a d e of the widths of the channelling dips in SE and SI emission. Such a study m a y shed light on the critical impact parameters of relevance for the two emission processes a n d hence on the differences between the physical processes more p r o m i n e n t l y involved.

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We wish to t h a n k Dr. R.L. Seliger of Hughes Research Laboratories for kindly providing us with several G a liquid metal sources which m a d e this work possible. It is a pleasure to acknowledge Dr. P. Sigmund for an illuminating discussion of our early observations. We are indebted to Dr. E. Olsen of the Field M u s e u m of N a t u r a l History for providing us with several cut fragm e n t s of the Toluca meteorite. T h a n k s are due to Mr. K. Evans for help in the operation of the SIM a n d to Mr. J. Baralt for the p h o t o g r a p h i c reproductions. The Material Research L a b o r a t o r y of the James Franck Institute of the University of Chicago provided the o p p o r t u n i t y to use the Coates a n d Welter SEM.