2010
Anal. Chem. 1987, 59, 2018-2023
Microcharacterization of Trace Elemental Distributions within Individual Coal Combustion Particles Using Secondary Ion Mass Spectrometry and Digital Imaging X. B. Cox, 111,' Scott R. Bryan,2and Richard W. Linton* Department of Chemistry, University of North Carolina, Chapel Hill, North Carolina 27514
Dieter P. Griffis Engineering Research Services, North Carolina State University, Raleigh, North Carolina 27695
Secondary Ion Image depth profiling Is developed for appllcatlon to the chemkal characterization of IndIvMual mlcrometer-sized particleg, speclflcally coal combustlon fly ash collected by cascade Impaction. A dlgltal lmaglng system Interfaced to a Cameca IMS-3f Ion rnlcroscope permits the simultaneous a c q v l o n of spatially resolved mass spectral data for a number of single particles. Small reglons within each particle are chosen for computer-reconstructed local area depth profiles to minlmize oompllcatlons of particle geometry on sputter rate and useful Ion ykld. Sputtering rates are calibrated by use of SO2 standards and correlatlve SEM observation of the sputtered particles. Ion Intensities averaged over groups of particles are related to concentratlons by use of NBS standard fly ash samples. Substantial dlfferences are oflen found In the relatlve concentratlons and/or depsh protiles of selected elements ( ~ a pb, , SI, ~ hn, , u) from partkle to partlcle. The technique Is highly senslilve to trace elements within mlcrovolumes. For example, the estimated U detection llmit corresponds to approximately lo4 atoms In an analytlcal yplume of 0.1 pm3 wlthln a 20-pm3 single particle.
One of the most widely studied types of pollutant particles is coal fly ash, a byprodud of coal combustion ( I ) . It is known that these particles are composed of a complex mixture of elements, cqn span a broad range of sizes, and have various morphologids (2). It has also been shown (3) that numerous toxic trace elements tend to increase in bulk concentration in the fly ash as a function of decreasing particle size. Mechanisms that have been developed to support this trend indicate that smaller particles may form by the accretion and/or condensation of vapors in the cooler regions of the flue or stack ( 4 ) . These vapors, which are enriched in volatile elements and compounds, are also found to adsorb or condense as a surface layer on larger, solid particles (5). This surface layer is of special interest, since it contains certain toxic trace elements,,often in soluble form. Thus, fly ash particles interacting with biological systems may have an enhanced concentration of toxic species at the point of direct contact. In general, chemical analysis studies ( I , 5 , 6 ) have shown the presence of unique surface layers on environmental particles. In addition, element concentration levels in individual particles often show substantial variations from the average, even if the particles are morphologically similar (7,8). This indicates that various particles, even within a narrow size range, may Present address: Exxon R&D Laboratories, Baton Rouge, LA
70821.
Present address: Standard Oil Research Center, Cleveland, OH
44128.
Table I. Conditions Used for SIMS Analysis Primary Beam composition accelerating voltage current raster size
02+
15 kV (10.5 keV impact energy) 1-2 pA 150 pm by 150 pm
Secondary Beam transfer optics contrast diaphragm field aperture image field
150 pm 400 pm (150 pm used for imaging)
700 pm (1.8 mm used for imaging) 60 pm (150 pm used for imaging)
be derived from different sources or mechanisms. Very little research has been done, however, directly characterizing the microscopic interparticle vs. intraparticle variations of element concentrations, including trace species. Several analytical techniques have been used to study surface layers of and/or to analyze elemental concentrations as a function of depth into a collection of particles ( 1 , 5 ) . These techniques include X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), photoacoustic spectroscopy (PAS), and secondary ion mass spectrometry (SIMS) ( I ) . For studies of the depth distribution of trace elements, SIMS is the technique of choice. The inherent depth profiling capabilities of SIMS, along with its high sensitivity and periodic table elemental coverage, make it uniquely suited to the study of intraparticle trace element concentration gradients. When the imaging capabilities of a secondary ion microprobe or microscope are employed, an image depth profile (9)can be generated which yields information on three-dimensional elemental distributions within single, micrometer-sized particles. With an ion microscope, such information can be acquired simultaneously for a group of individual particles present in an image field typically on the order of 100 pm in diameter. Previous researchers have used SIMS to depth profile certain trace elements averaged over an array of particles (5, 6 ) or to perform bulk analyses on single particles (10,1 1 ) . However, to our knowledge, this research is the first publication utilizing image depth profiling to obtain the threedimensional distribution of selected trace elements in single particles only a few micrometers in diameter. EXPERIMENTAL SECTION Size-resolved fractions of particles from an Eastern U.S. coal-fired power plant were collected inside the stack by means of an Andersen Hi-Vol cascade impactor operated isokinetically at 280 "C, at a flow rate of 0.8 ft3/min. A single stainless steel plate from the impactor is used in this study. The particles had a mean effective aerodynamic diameter of 4.5 pm (corrected for particle density of 2.5 g/cm3). Particles from this size fraction also were characterized for size and morphology by scanning electron microscopy (SEM), both before and after the SIMS
0003-2700/87/0359-2018$01.50/0 0 1987 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 59, NO. 17, SEPTEMBER 1, 1987
2019
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Figure 2. Selected regions of the mass spectrum of fly ash particles plotted on a linear intensity scale approximately equal to counts per second. Peaks of interest are TI+ (203, 205), Pb+ (206, 207, 208), Th+ (232), and U+ (238).
IOE2 -
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RESULTS AND DISCUSSION Secondary Ion Mass Spectra. In order to minimize the introduction of artifacts due to sample preparation, in situ analysis was performed with the particles on the original stainless steel substrates used in the impactor. It was known a priori that the stainless steel would contribute secondary ion interferences because of its various transition-metal components. However, its choice was governed by prior considerations involving other analytical investigations as part of a larger study and to minimizing artifacts introduced by sample preparation techniques. T o demonstrate SIMS capabilities in this initial study, it was appropriate to emphasize heavy elements of environmental interest for which mass spectral interferences are negligible. Figure 1 is a comparison of mass spectra (100-200 and 200-250 daltons) from stainless steel areas without (top) and with (bottom) fly ash particles present. Molecular secondary ions were reduced in intensity by only detecting ions with an initial kinetic energy greater than 120 eV. While the expected significant number of interferences can be seen for the stainless steel blank, the region above 200 daltons is essentially free of interferences. The particle spectra generally appear to have a higher proportion of polyatomic ion components relative to the stainless steel blank. This may, a t least in part, reflect a charging artifact since the particles are nonconducting. Thus, the ion extraction field may be distorted resulting in a somewhat more efficient transmission of lower kinetic energy cluster ions through the mass spectrometer. Figure 1B compares the 200-250 dalton mass range for the particles on the stainless steel substrate to that of the stainless steel blank. Spectra for the particles show a number of intense peaks in this region, specifically peaks for T1 (mle 203, 205), P b (mle 206,207,208), T h (mle 2321, U (mle 238), and T h o ( m / e 248). All of these trace elements are of interest from an environmental standpoint as they represent potentially toxic or carcinogenic species. Prior SIMS studies of very large particles and agglomerates from other fly ashes suggested P b and T1 may be surface enriched (5), but no prior data were obtained for T h or U. T o further verify the atomic ion identities, complete low mass resolution spectra were acquired and plotted on a linear intensity scale for the 200-210 (Tl, Pb) and 230-240 (Th, U) dalton mass ranges (Figure 2). These plots show there is adequate resolution of neighboring nominal masses, there are
I 210
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IOE2 loEl
IOEO
210
220
230
240
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m/e Figure 1. Bar graph mass spectra of stainless steel substrate (top) and fly ash particles impacted on stainless steel substrate (bottom): (A) 100-200 dalton mass range; (B) 200-250 dalton mass range (of special interest in this study). Ion intensities are on a logarithmic scale in counts per second (cps).
experiments. The same particles were examined by both techniques. The SIMS analysis was done with a Cameca IMS-3f ion microscope, which has been described in detail elsewhere (12). A camera-based digital imaging system has been developed in-house and is supported by an extensive package of image acquisition and processing software (13,14). General conditions used for the analysis are listed in Table I. The SIMS digital imaging system is interfaced to a HP9845B computer which controls the ion microscope. This permits multielement image depth profiling in a completely automated fashion (9). Single particle analysis were performed by acquiring an image depth profile of a 150 ,um diameter area containing a number of single fly ash particles and particle agglomerates. The correlative SEM studies were performed with an IS1 DS-130 dual stage SEM equipped with an LaBs source and a Tracor Northern TN-5500 energy dispersive X-ray analysis and imaging system.
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ANALYTICAL CHEMISTRY, VOL. 59, NO. 17, SEPTEMBER 1, 1987 APPROXIMATE O E P l H
Table 11. Comparison of Isotope Ratios Determined by SIMS for Trace Elements in Fly Ash Particles with Natural Isotopic Abundances
element T1 Pb Th
U
%
%
abundance
abundance (natural)
isotope intensity, cps 203
205 206 201 208 232 238
1630 3260 10300 9240 21200 1480 890
(exptl) 33
29.5
61
10.5
25
23.6 22.6 52.3 100 99.3
23 52 100 100
(MICRONS)
Table 111. Bulk Concentrations of Elements in the NBS Standard Fly Ash and in the Fly Ash Used in This Study
concn, w t 70 element
NBS"
sample
A1 Fe Si
14 9.4 22.8 0.8
14.9* 2.P 26.4b
Ti
0.86
concn, r g l g element
NBS"
sample
Pb T1
12.4
Th U
24.1 10.2
54c 3c 39c 9C
5.1
a From NBS certificate of analysis for Standard Reference Material 1633a. *From ref 16. cThis work. ~
good signal-to-noise ratios for these trace elements, and the experimentally obtained isotope ratios are close to natural abundances (Table 11). The spectra illustrate the excellent detection sensitivity of SIMS since the concentrations of Pb, T1, Th, and U in fly ash are usually in the parts-per-million range (1-100 pg/g). Explicit quantitation using NBS standard fly ash for calibration will be discussed in the following section. SIMS Depth Profiles. T o determine which elements are surface enriched, and to test both the detectability and dynamic range of the above elements, conventional depth profiles were acquired from 60 pm diameter areas with relatively high fly ash particle loadings. The ion intensities of 'OaPb+, zsSi2+, 205Tl+,232Th+7and 238U+were monitored vs. sputter time. One analytical concern was the extent to which possible surface enrichments of elements may be attributable to ion yield transients prior to establishing a steady-state sputtering condition (15). However, since both the surface and bulk of inorganic fly ash particles are typically very highly oxidized, the implanted primary oxygen concentration should play a minor role on overall secondary ion yields. As a conservative definition of the depth required to achieve the steady-state condition, the Si2+signal was monitored until a constant signal (its%)was achieved. This required the sputtering of an estimated 0.08 pm of the particle surfaces, considerably larger than the mean projected range expected for a 10-keV implant in an SiOz substrate. This steady-state or sputter equilibrium condition was achieved by using a 02+primary ion dose of about 8 X 1017ions/cm2. Raw ion intensities for T1, U, and P b exhibit a continuing downward trend with sputter time indicating surface region enrichments of these elements. Since the Si2+ signal was constant, the decrease in the U+,Pb+,and TI+intensities with depth was not due to mass loss of the sample, "cover-up" due to redeposition of material from the substrate (16), or primary beam current drift. Figure 3 shows depth profile plots of U,
e
loo
ION DOSE trONS/C$
XIO")
Figure 3. Depth profiles averaged over a large number of particles for 20ePbf,205TI+,23*U+. Intensities (cps) are plotted vs. primary ion dose and corresponding depth. Concentrations in parts per million by weight are shown on the right side of the plots.
Pb, and T1 at primary ion doses greater than those estimated to achieve a steady-state or sputter equilibrium condition. The depth scales shown in Figure 3 were estimated based on calibrated rates for SiOz standards. The concentration scales were quantified by analyzing a NBS standard fly ash (Standard Reference Material 1633a) under identical instrument conditions, using Si as an internal standard. The nearly constant ion intensities near the end of the depth profiles were assumed to reflect bulk elemental concentrations. The compositions of the NBS standard and sample fly ashes were very similar; the concentrations of selected elements of interest in the two samples are listed in Table 111. Unlike the other trace elements (Pb, T1, U), T h did not show surface enrichment. The large area depth profiles reflect the average concentration of elements for a large number of particles. However, individual particles were expected to have considerable variations around the average, reflecting the contributions of
ANALYTICAL CHEMISTRY, VOL. 59. NO. 17. SEPTEMBER 1. 1987
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.* 4
Fbwe 4. Secondary electron image of an individual fly ash particle efler SIMS analysis (particle 8 in Figure 5).
various particle types and formation mechanisms (2). To establish the variations in chemical composition from one particle to another, as well as the variations in composition within a single particle, the imaging capabilities of SIMS were employed as follows. S I M S Image Depth Profiling. A major concern in the image depth profiling of isolated particles was the absolute detection sensitivity when the analytical volume is constrained to subvolumes within single 5-pm particles. Such particles have a total mass only on the order of g. Another question was the influence of the nonplanar particle surface on depth resolution and depth profile dynamic range. It is well-known that sputter yield (atoms sputtered/incident ion) is affected by the primary ion beam angle of incidence. Sputter yields are a t a minimum a t normal incidence, while they increase by a factor of 3-4 a t incident angles of 60-80° from normal (17). Since each point on a particle surface has a different orientation to the ion beam, variations in intraparticle sputter yields result in sputter induced topography changes. Figure 4 shows an individual fly ash particle after SIMS analysis. Although the particle was initially quite smooth and of spherical geometry, it has developed a conical shape oriented toward the primary ion beam. The area of the particle approximately normal to the primary beam sputtered more slowly to form the “cap” on the cone-shaped particle. The unsputtered ‘plateau” area of stainless steel under the particle was shadowed from the primary beam by the particle. The sputtered particle also has a “ledge” near the surface which resulted from an unfavorable glancing primary beam angle of incidence. I t may also reflect an inherently lower sputter yield of the surface layer relative to the particle interior. The net effect of the nonuniform sputter rate is a potential deterioration of depth resolution, since at any given time secondary ions are being generated from a range of depths. Therefore, a potential advantage of digital image depth protiling over conventional depth profiling is that postacquisition image processing readily allows local area depth profiles (LADP) to be reconstructed from a selected central area of individual particles rather than integrating the entire secondary ion signal over one or more particles. To evaluate the ion current and spectrometer conditions needed for imaging fly ash trace elements, a detailed analysis was performed initially for Ba, a minor element (0.01% by weight) having a high secondary ion yield. Reference to the earlier bar graph mass spectra (Figure 1A) indicated that the principal 13aBa+isotope had an intense peak, several orders of magnitude higher than the contribution from the stainless
B
‘w
Flgure 5. (A) Secondary ion image of lor a g w p of fly ash p a w s hnpacted on staMess steel substrate. (E) Ssumdary electmn image after SIMS analysis for the same group of fly ash particles.
steel blank. It should he noted that to retain high depth resolution, only a small number of elements can he profiled for a given set of particles, particularly if trace elements are involved. In all of the imaging experiments, it was deemed important to restrict the number of elements per profile such that each complete cycle of ion image acquisitions could be obtained over sputter depths on the order of 0.1 pm or less. Silicon was routinely monitored for all areas as a reflection of a major constituent of the refractory oxide matrix. Other elements characterized in selected areas included Ti, Ph, and
U. Figure 5A s h o w the ion image of Ti+ from an approximately 150 am diameter image field. The numbers on the image identify single particles, or occasional particle clusters, and are used when discussing specific particles of interest. A direct correlation also was established with a secondary electron micrograph of the same particles as shown in Figure 5B. Once the image depth profile data set was obtained, selected areas within single particles were evaluated for computing local area depth profiles. An inherent trade-off is involved in using
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ANALYTICAL CHEMISTRY, VOL. 59, NO. 17, SEPTEMBER 1, 1987
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2 SPUTTER
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Flgure 6. mpth profiles of 13e13a+from indMdual pixel areas of about
't
1 pm2 within a single fly ash particle (partlcle 8, Flgures 4 and 5). Range of digital Intensities is 45-148 per pixel. Total Sputtering time pixel in particle center; represents a depth of about 1 pm. Key: (-) (- .) one pixel to the rlght of center: (- -) one pixel to the left of center; (---) two pixels to the right of center; (--) two pixels to the left of center.
-
(mi")
B
A .,
I
0
J
10
SPUTTER T I M E
(min)
Flgure 8. Local area depth profiles from 3 X 3 pixel areas (9 pm2) of two fly ash particles (A and B). Range of digital intenskiis is 30-200 per pixel. Total sputtering time represents a depth of about 2 pm. Key: (- - -) 20apb+, (-) *esi+.
1
I
1
2 SPUTTER T I M E
3
4
5
(min)
Figure 7. Local area depth profiles from 3 X 3 pixel areas (9 pm2) within single fly ash particles. Range of digital intensities is 30-148 per pixel. Total sputtering time represents a depth of about 1 pm. Key: (A) particle 8 (Figures 4, 5, and 8). (B) particle 1 (Figure 5); (---) '%a+, (-) %i+, (- - -) 4aTi+.
the largest number of pixels for the benefit of statistical averaging, while restricting the number of pixels to the particle center such that depth resolution is not unduely compromised. As a working definition, the particle *center'' was considered to be the pixel with the highest secondary ion intensities resulting from the most favorable orientation with respect to the primary beam and the secondary ion extraction optics. Particle 8 (Figures 4 and 5 ) was examined to establish the number of pixels surrounding the particle "center" that could be averaged without a substantial loss in depth profile dynamic range. To illustrate, Figure 6 shows the depth profiles for single pixel areas located at the particle center, one pixel to
the right, one pixel to the left, two pixels to the right, and two pixels to the left of center. The latter cases begin to show loss of depth profile dynamic range as the particle edges are being approached. Pixels at a distance greater than two pixels from the particle center exhibited greatly reduced dynamic range. Thus, it was deemed appropriate to sum regions comprising up to 3 x 3 pixels in the particle center for local area depth profiles (LADP) without risking major losses in dynamic range and depth resolution. This corresponds to an area between 5 and 10 pm2 relative to a typical total particle area of about 20 Mm2. The 3 X 3 pixel areas used for the LADP's are illustrated by the black squares inside the particles for the Ti+ image as shown in Figure 5A. The LADP's of Ba+, Si+,and Ti+ from particle 8 (Figures 4 and 5 ) and particle 1 (Figure 5 ) are shown in parts A and B, respectively, of Figure 7. Zero sputter time for all LADP plots corresponds to a presputter ion dose of about 8 X 10'' ions/cm2 required to establish steady-state conditions as described in an earlier section. The intensities from each image were calculated by summing the pixel intensities within the selected 3 X 3 pixel area indicated in Figure 5A and then dividing the sum by 9 to get the intensity/pixel. The minimum and maximum intensities in parts A and B of Figure 7 range from 30 to 148 intensity units/pixel. The 8-bit digital imaging system has a range of 0 to 255 intensity units/pixel. The scale factor for converting image intensity units to secondary ion intensity depends on the particular detector gain and image acquisition time used. A prior paper has reviewed general dynamic range considerations for image depth profiling (14).
In general, the Ti+ and Si+LADP's remain relatively flat, while the Ba+ intensity undergoes a decrease of approximately a factor of 2 during the course of sputtering (Figure 7). The depth profiles for Ba+ decay to a steady intensity at around 2 min of sputter time, which corresponds to depths on the
ANALYTICAL CHEMISTRY, VOL. 59, NO. 17, SEPTEMBER 1, 1987 10-
t'
5
5 -
I
I
e
0 SPUTTER T I M E (rnin)
B
I
SPUTTER TIME (rnin)
Figure 0. Local area depth profiles from 3 X 3 pixel areas (9 pm2) of two fly ash particles (A and 6). Range of digital intensities is 30-80 per pixel. Total sputtering time represents a depth of about 1.5 pm. *'Si+. Key: (---) 238U+,(-)
order of 0.4 pm. This also is in reasonable agreement with approximation of surface layer depths reported by others for different fly ashes (5). It should be noted that the relative intensities of Ba+, Si+,and Ti+vary substantially from particle to particle. For example, the ratio of Si+ to Ti+ in particle 1 is roughly 3, while the ratio of Si+ to Ti+ in particle 8 is approximately 1 (Figure 7). Such variations in relative elemental concentrations indicate that variable particle compositions occur even within a rather narrow size fraction. Once the spectrometer had been optimized for image depth profiling, similar conditions were employed to analyze a different group of particles for Pb+ and Si+. LADP's from 3 X 3 pixel areas for two particles are shown in parts A and B of Figure 8. In general, the relative intensities of Pb and Si varied substantially between particles. In all cases, however, P b had a generally downward trend in concentration as a function of sputter time, while Si remained essentially constant. Figure 8B shows a sharp drop-off in Pb concentration, suggesting a more pronounced surface-enriched Pb layer in this particle. Another particle field was characterized for U+ and Si+. Two particles were chosen for generating LARP's (Figure 9A and Figure 9B, respectively). There is a general downward trend for U+ as a function of sputter time. The profiles for Si+ are again rather flat, with substantial variations of the relative U and Si intensities for different particles. The fact that U+ can be imaged at all is a testament to the high sensitivity of the Cameca IMS-3f ion microscope. The U is present in an average concentration of only about 10 ppm by weight, meaning that a 5-pm spherical particle may have only
2023
a few million U atoms in its entire volume. Taking into consideration that the analysis consumes no more than 50% of the particle volume, and that the useful ion yield (combination of spectrometer transmission and ionization effithen ciency) for U+ is expected to be no better than the total of the U images represents no more than a few hundred secondary U+ ions per particle. An alternative benchmark is that at the detection limit, the analytical volume of a single pixel within a fly ash particle for a U+ image is on the order of 0.1 pm3 and contains only about 3 X g of U corresponding to about 8000 U atoms. This calculation assumes that the single particles examined have U concentrations equal to the average value of 10 ppm determined by bulk analysis. It may be in error by as much as an order of magnitude judging by interparticle variations observed in U+/Si+ intensity ratios. In conclusion, the trace element image depth profiles show the viability of using SIMS to characterize single pollutant particles in terms of both inter- and intraparticle concentration variations. The downward trend with depth obtained for concentration profiles of some trace elements (Tl, Pb, U), as opposed to the flat profiles for refractory oxides (Si, Ti), indicates that profile shapes are not artifacts generated by loss of analytical volume or change in total ion yields as a function of primary ion dose.
ACKNOWLEDGMENT The authors thank David Lichtman (University of Wisconsin-Milwaukee) for providing the sample used in this research and for helping to arrange funding for the project. Registry No. Ba, 7440-39-3;Al, 7429-90-5; Fe, 7439-89-6; Si, 7440-21-3; Ti, 7440-32-6; Pb, 7439-92-1; T1, 7440-28-0; Th, 7440-29-1; U, 7440-61-1. LITERATURE CITED Linton, R. W.; Harvey, D. T.; Cabaniss, G. C. I n Analyticel Aspects of EnvironmentalChemistry; Natusch, D. F. S., Hopke, P. K., Eds.; Wiley: New York, 1983. Roy, W. R.; Thiery, R. G.; Schuller, R. M.; Suloway, J. J. Environmental Geology Note 9 6 Iliinols State Geological Survey: Champaign, IL, April 1981. Davison, R. L.; Natusch, D. F. S.; Wallace, J. R.; Evans, C. A., Jr. Environ. Sci. Techno/. 1974, 8 , 1107. Chung, F. H. In AtlParticulates Instrumentation and Analysis; Cheremisinoff, P. N., Ed.; Ann Arbor Science: Ann Arbor, MI, 1981; pp 89-117. Keyser, T. R.; Natusch, D. F. S.; Evans, C. A., Jr.; Linton, R. W. Environ. Sci. Techno/. 1978, 12, 768. Van Craen, M.; Natusch, D. F. S.; Adams, F. Anal. Chem. 1982, 5 4 , 1786. Bloch, P., Adams; F., van Landuyt, J.; van Goethem, L. I n First European Symposium on Physico-Chemical Behavior of Atmospheric Pollutants-Proceedings. 1979. Hwk, J. L.; Lichtman, D. Atmos. Environ. 1983, 17, 849. Bryan, S. R.; Woodward, W. S.; Linton, R. W.; Griffls, D. P. J . Vac. sci. rechnoi., A 1985, 3(6), 2102. McHugh. J. A.; Stevens, J. F. Anal. Chem. 1972, 4 4 , 2187. Characterization of Particles, Newbury, D. E., Heinrich, K. F. J., Eds.; NBS Speclal Publication 533; National Bureau of Standards: Washington, DC, 1980; Chapter 11. Lepareur, M. Rev. Tech. Thomson-CSF 1980, 12, 225. Bryan, S. R.; Woodward, W. S.; Griffis, D. P.: Linton, R. W. J . Microsc. 1985, 138, 15. Bryan, S. R.; Linton, R. W.; Griffis, D. P. J . Vac. Sci. Technoi., A 1988, 4(5), 2317. Bryan, S. R.; Linton, R. W.; Griffis, D. P. J . Vac. Sci. Technol., A 1988, 5 , 9. Lichtman, D.; Mroczkowski, S. Environ. Sci. Technol. 1985, 19, 274. Anderson, H. H.; Bay, H. L. In Sputtering by Particle Bombardment I , Behrisch, R., Ed.: Springer-Verlag: Berlin. 1981; pp 145-218.
RECEIVED for review March 2,1987. Accepted May 11,1987. Support was provided in part by the Electric Power Research Institute (Research Project 1625-1) and by the National Science Foundation (DMR-8107499).
