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Anal. Chem. 1091, 63,788-792
High Mass Resolution Secondary Ion Mass Spectrometry via Simultaneous Detection with a Charge-Coupled Device David S. Mantus, Gary A. Valaskovic, and George H. Morrison*
Baker Laboratory of Chemistry, Cornell University, Ithaca, New York 14853-1301
A method for high mass resolution secondary ion mass spectrometry (SIMS) Is presented. A chargecoupled devke Is employed as a llnear array detector, vla blnning, to slmultaneously detect a small portlon of the mass spectrum from a commerclai SIMS Instrument. The technlque can be used for trace analysls and depth profiling. The method Is compared to the conventional twogilt technique, and the Wmlt of resolutbn of the simultaneous method is explored. Spectra from trace constttuents in a glass matrlx are presented with mass resolutions exceeding 10 500 M / A M . Detection ilmtts well below 1 ppm are determined for a number of elements. Depth profiles of "P- in SI, acqulred at a mass resolutlon of 8000 M I A M , and "As- in Ge, at a mass resoMlon exceeding 10500 M / A M , are presented. The limits of the method, as well as potential applications and improvements, are dlscussed.
INTRODUCTION Secondary ion mass spectrometry (SIMS) is a useful tool for the determination of elemental and isotopic distributions in solid materials and has seen application in a wide variety of fields (1-3). One of the major limitations of SIMS, however, has been the presence of mass interferences in the complex spectra produced. Mass interferences arise when an undesired ion has the same nominal mass-to-charge ratio ( m / z )as the analyte ion of interest. These artifacts can appear due to singly or multiply charged matrix ions, cluster ions from the matrix, oxides, hydrides, etc. As samples increase in complexity, the need for high mass resolution SIMS, in which the technique's selectivity is markedly improved, will continue to grow. Over the past several years, a number of methods have been developed to take full advantage of the mass resolving power of one of the most widely employed SIMS instruments, the Cameca IMS-3f/4f ( 4 ) . This instrument has a nominal maximum mass resolving power of 10000 M / A M , where M is the m/z of the ion of interest and AM the difference between M and the m / z of the interfering ion. However, in order to increase the transmission of the instrument, most analyses are performed a t a resolution below 1000 MIAM. In the standard procedure for high mass resolution analysis, both the entrance and exit slits of the mass spectrometer are narrowed. The image of the entrance slit is then carefully scanned across the exit slits by varying the magnet voltage, with single-channel detection performed by an electron multiplier (EM). Stingeder et al. (5, 6) employed a peakmatching algorithm with this method to produce depth profiles of 31P-in Si, demonstrating a mass resolution of 4500 M I A M . Peak matching is necessary due to fluctuations of the magnet voltage; slight drifts in the position of the entrance slits seriously alter the monitored signal in the two-slit experiment and could introduce artifacts in depth profiles. A number of groups have employed the spectrographic mode of the Cameca IMS-3f for high mass resolution analyses. In this mode, the image of the entrance slit is projected onto the microchannel plate/fluorescent screen (MCP/FS) assembly, which is normally used to detect ion images in SIMS
imaging, or ion microscopy. In this way, a small portion of the mass spectrum is simultaneously recorded. Odom et al. (7) used a resistive anodic encoder (RAE) to record slit images from a dual MCP. A pulse-counting device, the RAE reproduces images electronically and has a somewhat limited dynamic range. Bryan et al. (8)constructed a depth profile from a series of such images by using a video camera system for acquisition. The extraordinary potential of the spectrographic method for high resolution SIMS was demonstrated by Thorne and Degreve (9). By imaging the entrance slits, they were able to achieve high mass resolution results on transient signals, while exceeding the nominal limit of resolution for the Cameca instrument. All of these imaging methods require the storage of large quantities of data, and significant postacquisition processing, to produce spectra and depth profiles. Recently, our laboratory has developed a slow-scan charge-coupled device (CCD) camera system for the acquisition of images from the MCP/FS (10, 11). The unique flexibility of CCDs allows this system to be used as a spectral detector. This is achieved by using the technique of binning (12), in which CCD pixels are summed in an analog manner, on-chip. Binning can provide a unique signal-to-noise advantage in addition to converting a two-dimensional image detector into a sensitive linear array. The signal-to-noise advantage is achieved for low-intensity peaks by reducing the number of times a signal is sampled, by both the on-chip amplifier of the CCD and the analog-to-digital converter (ADC) of t,he detection system. Binning has helped to make CCDs powerful tools for optical and Raman spectroscopy (13-1 5). By combining spectrographic operation of the Cameca IMS-3f with spectral acquisition via a binned CCD, a fast and flexible method for high mass resolution SIMS is produced. This technique makes full use of the resolving power of the spectrometer, allows for high mass resolution spectra of trace constituents to be recorded, and is easily adapted for the acquisition of multielement, high mass resolution depth profiles. Details of the method and examples are described below. EXPERIMENTAL SECTION Instrumentation. A Cameca IMS-3f ion microprobe was used for all SIMS analyses. This instrument has been detailed by Rouberol and co-workers (4). The general procedures for high mass resolution operation of the Cameca instrument have been outlined by Thorne and Degreve (9). The 150-pm contrast aperture and the 750-pm field aperture were used to optimize the size and quality of the entrance slit image. The energy slits were closed to a -30-eV window to further sharpen the slit image. The entrance slit was then narrowed to resolve neighboring peaks. Detection sensitivity was varied by control of the bias across the MCP, ranging from 300 to 1350 V. The primary beam conditions and transfer optics varied depending on sample parameters and desired results. The CCD detection system is based on a Photometrics Ltd., Inc., Series 200 camera and has been described previously (IO, II). The system maintains the CCD at -50 O C to minimize thermal noise. The device within the camera is a Thomson CSF-TH7882 CCD, with 576 X 384 pixels, each of which is 22-pm X 22-pm square. Pixel intensities are digitized with a 14-bit, 50-kHz ADC. Camera control is maintained via an IEEE-488
0003-2700/91 /0383-0788$02.50/0 0 199 1 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 63, NO. 8, APRIL 15, 1991
0.0 0
50 100 150 200 250 m/z
Flgure 1. Approximate mass range projected on the MCP/FS versus m l z . The line represents a second-order polynomial fit of the data.
interface (National Instruments), by a PDP-l1/34A (Digital Equipment Corp.) and Trapix 55/32 (Recognition Concepts, Inc.) based data processing system (16). Transfer of data to an HP 935 Workstation (Hewlett-Packard Co.) for spectral analysis and display is also possible. Methods for Simultaneous Detection. By using the CCD control software, the format of the device can be varied freely. In order to align the entrance slit image, the CCD was operated without binning. Once the image was vertical and centered on the MCP/FS, the CCD was reformated for spectral acquisition. A central portion of the device, 256 X 130 pixels in size, was used with a binning format of 1 X 130. This effectively converts the CCD into a linear array with 256 rectangular 'super" pixels. These pixels are parallel to the image of the entrance slits. Addition of thermal noise, or dark current, limited exposure times to less than -70 s. Dark current spectra were subtracted by the PDP-l1/34A based system and the final spectrum transferred to the H P 935 Workstation. The inhomogeneous response of the MCP was recorded by using %Si+flat field illumination to allow for the correction of spectral intensities. The maximum variation in sensitivity due to this effect was -20%. The appearance of peak shapes varied slightly across the field of view. Toward the higher mass portion of the spectrum, peaks are typically shorter and wider; toward lower masses, they are generally taller and more narrow. The effect is not serious but should be considered in rigorously quantitative applications. The variation in peak heights is linear with increasing count rates, and by using 60-s integration times, it is possible to record peaks at 1count/s. The variation in mass across the field of view is also linear. The width of the spectrum projected on the MCP increases with increasing mlz. Figure 1illustrates this behavior. It is not quite linear, and the line in Figure 1 represents a fit of the data using a second-order polynomial. It should be noted that the width of the spectrum exceeds 0.7 amu at the highest masses. The CCD is able to view approximately 85% of this range when formated for spectral acquisition. The exact mass determination of peaks is a difficult problem that has been addressed in the past. If, in an individual spectrum, two known peaks are visible, the masses of additional peaks are easily calculated (17). If no such reference peaks are available, the spectrum from an external standard can be used, if both the reference and analyte spectra are acquired under identical conditions (9). With prior knowledge of a sample, it is often possible to infer the identity of peaks by using tables of known mass interferences. Recording of weak spectral lines in close proximity to bright peaks is a classic difficulty in spectroscopy. When detected simultaneously with the CCD, intense mass spectral peaks are observed to be significantly broadened at their bases. However, by using one edge of the exit slits, bright peaks can be blocked, allowing weak lines to be recorded with improved signal-to-noise. Such blocking eliminates the increased baseline from the interfering peak. This indicates that the observed broadening is an artifact of converting the ion image of the slits to a photon image for detection, rather than an inherent ion optical effect. This broadening of peaks is most likely due to the behavior of the MCP/FS or the optics between this assembly and the CCD. Depth Profiling Methods. The software developed to acquire individual high mass resolution spectra with the CCD was easily adapted for depth profiling applications. Spectra were acquired
789
sequentially, dark current spectra recorded and subtracted, and the final spectra stored. By switching masses between acquisition sequences,multielement profdes could be produced. The resultant profiles consisted of a large series of individual spectra. These data were transferred to the HP 935 for display and analysis. The entire series of spectra could be visualized as a three-dimensional surface or a conventional depth profile constructed from desired peaks. Fluctuation in the position of peaks did not influence these profiles, for a window about each peak was integrated for each spectrum in a series. Samples and Analysis Conditions. In order to quantitatively compare the two-slit method to simultaneous acquisition, a resolution standard consisting of 10% (w/w) TiOs in Cu was used in the form of a pressed pellet. A useful doublet appears in the mass spectra of this sample at m / z 65, arising from 4qi160+and &Cu+. The difference in the actual masses is 0.015 amu, and the peaks require a mass resolution of 4300 MIAM for separation. This sample was analyzed by using a 1-pA, 12-kV Os+primary beam rastered over an area of 250 pm X 250 pm and the 150-pm transfer optics. The limit of mass resolution of the simultaneous acquisition technique was evaluated by using a sample consisting of a mixture of Ge and Fe, pressed into a pellet. At m/z 72, the 7%e+/mFe1%+ doublet requires a resolution of 13 700 M/AM for separation. A 1-pA, 12-kV 02+ primary beam rastered over an area of 250 pm X 250 pm and the 400-pm transfer optics were used to analyze this sample. As an example of trace element analysis in a complex matrix, the National Bureau of Standards (NBS) SRMs 614, 612, and 610 were analyzed. These standards consist of a glass (Sios,CaO, Na20,A1203)doped with a number of elements a t nominal concentrations of 1,50, and 500 ppm. For calculations, certified or informational concentrations were used when available. Singly charged ions from the matrix, cluster ions, and oxide ions present a number of interferences between m/z 23 and m / z 60. The sample was coated with A1 to facilitate analysis. Analyses were performed by probing cracks and holes in this coating. SIMS primary beam analysis of this sample employed a 1-pA, 10-kV 02+ rastered over an area of 500 pm X 500 pm and the 150-pm transfer optics. In order to demonstrate the ability to produce depth profiles at high mass resolution, two samples were ion implanted by using an Accelerators Incorporated Model 300R ion implanter a t the Cornel1 National Nanofabrication Facility. A Si wafer (General Diode) was implanted with 31P+at an energy of 200 kV and a total dose of 10l5atoms/cm2. Ge (General Diode) was implanted with 75As+at an energy at 250 kV and a total dose of 2.5 X 1015 atoms/cm2. These examples were chosen for both their technical importance and the mass interferences associated with SIMS analysis of the analyte. Signal from 30SiH-interferes with the analysis of 31P-and requires a resolving power of 3960 M/&U for separation. The 75As- peak is overlapped by 74GeH-and requires a resolution of 10500 M/AM for separation. Both samples were analyzed by using a 10-kV Cs+ beam, a 500-pm X 500-pm raster, negative ion detection, and the 150-pm transfer optics. The 31P-profile used a 500-nA primary beam; the 75Asprofile was analyzed by using 150 nA of primary current. Crater depths were measured with a Tencor Alpha Step surface profilometer.
RESULTS AND DISCUSSION Comparison of Conventional and Simultaneous Acquisition. A direct comparison of the conventional two-slit
method of high mass resolution analysis and the spectrographic method with CCD detection was possible using the Ti02/Cu sample. All instrumental conditions were optimized for the two-slit method. The exit slits were narrowed and suitable acquisition times chosen. Once the best possible spectrum was acquired, the exit slits were opened and a spectrum acquired simultaneously. Figure 2 shows the simultaneously acquired spectrum a t m/z 65. The acquisition time was 2.0 s, and the MCP voltage was 750 V. A virtually identical spectrum was acquired by using the two-slit experiment, but a total scan time of 188 s was required. While the sensitivity of each pixel in the CCD array is comparable
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ANALYTICAL CHEMISTRY, VOL. 63, NO. 8, APRIL 15, 1991
m/z 65A0.06
Flgure 2. Simultaneously acquired, high mass resolution spectrum at mlz 65, from a TI0,lCu pellet. The integration time was 2.0 s; the voltage to the MCP was 750 V.
m/z
Figure 4. Estimated smallest resolvable mass difference, A#, and the mass resolving power, M I A M , as a function of m l z , for the simultaneous method. I
m/z 72i0.07
Flgure 3. Simultaneously acquired, high mass resoiutlon spectrum at mlz 72, from an Fe/Ge pellet. The necessary resolution for peak separation is 13700 MIAM. The integration time was 20.0 s; the voltage to the MCP was 1000 V.
to the EM, the practical sensitivity of the array is superior, because it integrates in parallel across the spectrum. When measured using the 10% valley definition of mass resolution, the resolution of the spectrum in Figure 2 is 11000 MIAM. The identical measurement on the two-slit spectrum resulted in a resolution of 9600 MIAM. This difference is due to the width of the exit slits, which must have been slightly larger than both the resolution channel of the magnet's voltage scan and the width of a pixel on the CCD. These latter two parameters are, coincidentally, very close in effective value. In summary, the spectrographic method with CCD detection provides a spectrum of equal quality or better and acquires it 100 times faster than conventional methods. Mass Resolution Limit. In order to consider the practical resolution limits of the spectrographic technique, the Fe/Ge sample was analyzed. Figure 3 shows a spectrum from this sample a t m / z 72; the acquisition time was 20.0 s, and the MCP voltage was 1000 V. The two peaks arise from 72Ge+ and =Fel60+,with a difference in mass of -0.005 amu. The required resolution for separation is 13 700 M /AM. The total count rate measured for the entire spectrum with the EM was -5 counts/s. An estimate of the mass resolution limit as a function of mlz can be determined by using Figure 3 as an example of a high-quality, high resolution spectrum and by using the data from Figure 1 on mass range viewed versus mlz. The full width at half maximum of the peaks in Figure 3 is -8 pixels. Assuming this width to be a typical value, it can be converted to AM, the smallest resolvable mass difference, at each mass. This is achieved by multiplying the mass range projected on the CCD by 81256. Figure 4 shows both AM and MIAM versus M, the fit of the data in Figure 1 was used to create the curves. The maximum resolving power occurs in the region of the transition elements. It should be noted that a smaller pixel size would not improve resolution, as long as the most narrow usable peak was many pixels in width. In order to improve resolution, the width of a usable slit image must be decreased so that pixel width becomes the limiting factor. T r a c e Elements in Glass. The SRM series 610,612, and 614 provides a good example for high mass resolution trace
-
m/z 59h0.06
Figure 5. Simultaneously acquired, high mass resolution spectrum at m l z 59 from the NBS SRM 612.
elemental analysis. Figure 5 illustrates the complexity of a typical spectrum from SRM 612. The spectrum was integrated for 10.0 s, a t an MCP voltage of 1000 V, and has a nominal mass resolution of 9OOO MIAM. The nominal concentration of Co in this sample is 35.5 ppm. The three peaks to the right of 59C0+arise from the glass matrix and do not vary in relative intensity between SRMs. The middle peak is difficult to identify, although the calculated mass for the peak matches reasonably well with @Ca180H+. Regardless, the %Co+is well separated from the potential interferences. Figure 6 contains three spectra at m / z 48 from all three SRMs. All three spectra were acquired at an MCP voltage of 1000 V, and the integration times were 30.0, 1.0 and 0.5 s for parts of a, b, and c, respectively. The spectra have been normalized such that the @Ca+peak has the same intensity in each case. The 48Ti+ signal is resolved from @Ca+in all three samples. The small peak to the right of %a+ varies in height between samples and is most likely due to 32S160+; the SRMs are doped with S. The required resolution for the separation of the 48Ti+/48Ca+doublet is 10 500 M /AM, and the Ti concentrations in the three samples are nominally 3.1, 50.1, and 437 ppm. This allows the estimation of a detection limit, assuming a maximum exposure time of 60.0 s and a desired signal-to-noise ratio of 2. For Ti, a t a mass resolution exceeding 10 500 M / AM, the detection limit for the method is 1.3 ppm. Detection limits for five additional elements, at a mass resolution of 9000 MIAM,were determined similarly and are presented in Table I. As a reference between samples, the 43Ca+signal was recorded to correct for small differences in primary beam current and the size of the sampled area. The relatively high detection limit for Fe is not indicative of a limited sensitivity for this element. The baseline noise at mlz 56 is unusually high due to the intense signal from ?3i2+. These examples demonstrate the utility of simultaneous detection for the SIMS analysis of trace elements in complex samples and the potential of the method for quantitative analyses. High Mass Resolution Depth Profiles. A high mass resolution depth profile of implanted P in Si was obtained
ANALYTICAL CHEMISTRY, VOL. 63, NO. 8, APRIL 15, 1991
791
0
m/z 48i0.05
a 0
Flgure 7. Three-dimensionalsurface representing a portion of the data from a high mass resolution depth profile at m l z 31. The mass resolution of each spectrum is 8000 MIAM.
' 104
n
-
100 0.00
0.25 0.50 0.75 Depth (v)
1.00
Figure 8. Constructed hi h mass resolution depth profile of 31P-(X) and 30Si-(A)in Si. The "P- data are taken from Figure 7.
, ._ m
m/z 48*0.05
10-2
100
::
C
Flgure 6. Simultaneously acquired, high mass resolution spectra at m / z 48 from (a, top) NBS SRM 614 (3.1 ppm Ti), (b, middle) NBS SRM 612 (50.1 ppm Ti), (c, bottom) NBS SRM 610 (437 ppm Ti).
+ .0
.->
lo-' I .
i I
W
E
Table I. Estimated Detection Limits for Seven Elements in NBS Glass Standards, Using High Mass Resolution SIMS Analysis"
mass resol,
analyte
species detected
M/AM
Ti Mn Fe
Ti+ &Mn+ 66Fe+
>10500
Ni co
66Ni+
cu
WO+ 83Cu+
9000 9000 go00 9000 9000
detection limit, ppm 1.3 0.1 6.0 0.3 0.4 0.1
Mass resolution given is the resolution for which the detection limit has been determined. by using simultaneous detection. Spectra were acquired a t m / z 30 (0.2-9 integration) and m/z 31 (6.0-s integration), every 5.0 s for 60 cycles. Due to data transfer times and the recording of dark current spectra, the total analysis time was 1700 s. A portion of the data at m / z 31 is shown in Figure 7 as a three-dimensional surface. The separation of 31P-from 31SiH-is clear and the near-Gaussian shape of the P implant visible. The approximate mass resolution of each of these spectra is 8000 MIAM. The data from Figure 7 were converted into a more conventional depth profile by integration of a window about the 31P peak in each of the 60 spectra. This profile, along with the monitored 30Si- signal, is shown in Figure 8. In order to compare the high mass resolution profile to more conventional results, an analogous profile was per-
-
10-3
Y
i 0-4
.-3 10-2 v)
W +
= 10-3
10-5 0.00 0.25 0.50 0.75 1.00 Depth ("
Figure 9. Comparison between a hi h mass resolution (X) and low mass resolution (A)depth profile of 'P- in Si. See text for details.
3
formed by using standard methods. This profile monitored the same mlz's, at a nominal mass resolution of 300 MIAM, with 80 data points taken over 3100 s of sputtering time. The signal from m / z 31 was normalized to that from m / z 30 for both depth profiles to allow for a comparison between the high and low resolution results. A direct comparison is shown in Figure 9. The loss in detail and quality of information is obvious in the low resolution data. A three-dimensional surface, representing a portion of the high mass resolution depth profile a t m/z 75 of the As-implanted Ge, is shown in Figure 10. Fifty spectra were acquired at both m / z 70 (0.2-s integration) and m / z 75 (5.0-9 integration) in rapid sequence, for a total analysis time of 810 s. Figure 10 shows the clear separation of 75As-from 74GeH-, which requires a mass resolution of 10 500 M /AM. Figure 11 is the high mass resolution depth profile of 75As-constructed from these data. Once again, a conventional depth profile was acquired, at low mass resolution, at both m / z 70 and mlz 75. It consisted to 120 cycles over 900 s. Again, the high and low resolution results were referenced to 70Ge-for normalization and comparison. This comparison is shown in Figure 12. The
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ANALYTICAL CHEMISTRY, VOL. 63, NO. 8, APRIL 15, 1991 74GeH-
7
Figure 10. Three-dimensionalsurface representing a portion of the data from a high mass resolution depth profile at mlz 75. The resolution required to separate the two peaks is 10500 MIAM.
100
-
0.00
0.10 Depth
0.20
0.30
(m)
Flgure 11. Constructed high mass resolution depth profile of 75As- (X) and 'OGe- (A)in Ge. The 76As- data are taken from Figure 10.
In addition, binning of the CCD makes profile construction quite simple. A large number of images do not need to be stored, retrieved, and analyzed. This allows more data points to be acquired, improving depth resolution. Other potential applications of simultaneous detection include the acquisition of spectra over a series of masses. These spectra could be combined to produce a high mass resolution fingerprint spectrum of a sample. The quantitative application of simultaneous detection must carefully consider the behavior of the MCP/FS, studied previously ( I I , 1 8 ) . In addition to inhomogeneous response, the mass dependence of the MCP must be included in spectral corrections. This increases the need for improved understanding of the response of the MCP to multiply charged species, cluster ions, etc. The ultimate sensitivity and mass resolution of the method would require further instrumental enhancements. By fiber optically coupling the FS to the CCD, broadening of peaks due to isotropic fluoroscence and optical aberrations would be minimized. In addition, a CCD cooled with liquid nitrogen would improve signal-to-noise performance and allow significantly longer integration times. The current system cools the CCD to -50 "C; thermal noise builds at a rate of - 5 electrons/(pixel s). At 100 K, thermal noise is reduced to electrons/ (pixel s). The height of the thermal baseline limits the usable dynamic range and integration times. As the baseline grows, the random noise within it increases, degrading signal-to-noise and potential sensitivity. A liquid nitrogen cooled CCD improves both of these situations by considerably lowering the thermal noise baseline.
-
ACKNOWLEDGMENT We thank Alisa M. Alma for preparing the Fe/Ge resolution sample and Mike Skvarla of the Cornel1 National Nanofabrication Facility for his assistance with the ion implantations. LITERATURE CITED
:-
10-4
1 0.00
1 0.10 Depth
1
1
0.20
0.30
(m)
Flgure 12. Comparison between a high mass resolution (X) and low mass resolution (A)depth profile of 75As-in Ge. See text for details.
low resolution data provide virtually no information concerning the As implant. In this case, the intensity of the interfering peak and its proximity to the analyte signal require the use of the spectrographic mode of operation coupled with simultaneous acquisition. CONCLUSIONS The data presented illustrate the great potential of simultaneous spectral acquisition with a CCD for high mass resolution SIMS. The power of the method is derived from a combination of simplicity, flexibility, and practical sensitivity. The conventional two-slit method for mass resolution is tedious and time consuming; it requires the careful alignment of two slits and the narrowing of the exit slits to a precise width. If the width is too large, the ultimate mass resolution that would be gained from control of the magnet is lost. If the width is too small, transmission, and therefore, sensitivity, is lost without an increase in resolution. Simultaneous detection simplifies the process considerably and reduces acquisition time by a factor of 100. The flexibility of the method is attested to by its easy adaptation for depth profiling. Magnet fluctuations are not critical as a spectrum covering a range of masses is acquired.
Morrison, 0. H.; Slodzian, 0. Anal. Chem. 1875, 47. 932A-943A. Rudenaur, G. G. Surf. Interface Anal. 1884, 6 , 132-139. Benninghoven, A.; RiSdenauer, F. G.; Werner, H. W. Secondary Ion Mass Spectrometry : Basic Concepts , Instrumental Aspects , Applications and Trends;John Wiley 8 Sons: New York, 1987. Rouberol, J. M.; Lepareur, M.; Autier, 6.; Gougot, J. M. 8th International Conference on X-ray Optics and Microanalysis and 12th Annual Conference of the Microbeam Analysis Society, Boston, MA, 1977; pp 133A- 1330. Stingeder, G. Anal. Chem. 1988, 60,1524-1529. Stingeder, G.; Piplits, K.; Gara, S.; Grasserbauer, M.; Budii, M.; Potzl, H. Anal. Chem. 1989, 67, 412-416. M o m , R. W.; Furman, B. K.; Evans, C. A., Jr.; Bryson, C. E.; Petersen, W. A.; Kelly, M. A.; Wayne, D. H. Anal. Chem. 1983, 55, 574-578. Bryan, S. R.; Griffis, D. P.; Linton, R . W.; Hamilton, W. J. I n Secondary Ion Mass Spectrometry-SIMS V; Benninghoven, A., Colton, R. J., Simons, D. S., Werner, H. W., Eds.; Springer Verlag: Berlin, 1986; pp 239-24 1. Thorne, N. A.; Degreve, F. Surf. Interface Anal. 1888, 1 7 , 189-197. Mantus, D. S.; Turner, L. K.; Ling, Y.-C.; Morrison, G. H. I n Secondary Ion Mass Spectrometry-SIMS V I I ; Benninghoven, A., Evans, C. A., McKeegan, K. D., Storms, H. A., Werner, H. W., Eds.; John Wiley 8 Sons: Chichester, 1990; pp 921-924. Mantus, D. S.; Morrison, G. H. Anal. Chem. 1990, 62, 1148-1155. Epperson, P. M.; Denton, M. B. Anal. Chem. 1989, 6 7 , 1513-1519. Sweedler, J. V.; Bilhorn, R. 6.; Epperson, P. M.; Sims, 0 . R.; Denton, M. B. Anal. Chem. 1988, 60, 282A-291A. Bilhorn. R. 6.; Sweedler, J. V.; Epperson, P. M.; Denton, M. B. Appl. Spectrosc. 1987, 41, 1114-1124. Pemberton, J. E.; Sobocinski, R. L.; Bryant, M. A,; Carter, D. A. Spectroscopy (Eugene, Oreg.) 1980, 5 , 26-36. Ling, Y . 4 . ; Bernius, M. T.; Morrison, G. H. J . Chem. Inform. Comput. Sci. 1987, 27, 86-94. Bakaie. D. K.: Colbv. 8. N.: Evans. C. A,. Jr. Anal. Chem. 1875. 4 7 . 1532- 1537. Ling, Y.-C.; Turner, L. K.; Bernius, M. T.; Morrison, G. H. Anal. Chem. 1988. 6 7 , 66-73.
RECEIVEDfor review October 19,1990. Accepted January 11, 1991. This work was supported by the National Science Foundation and the Office of Naval Research.
