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(14) McCrery, D. A,; Ledford, E. E.; Gross, M. L. Anal. Chem. 1982, 54, 1435-1437. (15) Comisarow, M. B. Int. J . Mass Spectrom. Ion Phys. 1081, 37, 251-257. (16) Comlsarow, M. 0.; Grassi, V.; Parisod, G. Chem. Phys. Lett. 1078, 57, 413-416. (17) McLuckey, S. A.; Saiians, L.; Cody, R. B.; Eurnier, R. C.; Verma, S.; Freiser, E. S.; Cooks, R. G. Int. J . Spectrom. Ion Phys. 1982, 4 4 , 215-229. (18) Comlsarow, M. E.; Marshall, A. G. J . Chem. Phys. 1076, 8 4 , 110-119, (19) White, R . L.; Ledford, E. E., Jr.; Ghaderi, S.; Wiikins, C. L.; Gross, M. L. Anal. Chem. 1980, 52, 1527-1529. (20) Burnier, R. C.; Cody, R. E.; Freiser, E. S. J . Am. Chem. SOC., In
press. (21) Henis, J. M. S. J . Am. Chem. SOC. 1968, 00, 844-651. (22) Freas, R. B.; Ridge, D. P. J . Am. Chem. SOC. 1980, 702, 7 13 1-7 132,
(23) Cariin, T . J.; Wise, M. 0.; Freiser, 2743-2745.
B. S.;
Inorg. Chem. 1981, 20,
T. J. Carlin B. 5. Freiserh Department of Chemistry Purdue University West Lafayette, Indiana 47907 RECEIVED for review September 14,1982. Accepted November 29, 1982. Support for this research was provided by the Department of Energy (Grant No. De-AC02-80ER10689)and the National Science Foundation (Grant No. CHE-8002685), which provided funds to purchase the FTMS.
Quantitative Image Acquisition System for Ion Microscopy Based on the Resistive Anode Encoder Sir: Secondary ion mass spectrometry (SIMS) is a surface microandytical technique which provides sensitive elemental analysis of material surfaces with high lateral and in-depth resolution (I, 2). As a result of these capabilities, SIMS ion microanalysis has been extensively applied to the analysis of semiconductor devices (31, advanced materials systems ( 4 ) , and biological tissues (5). Two instrumental configurations have evolved for performing quantitative surface analysis using the SIMS process and both of these configurations provide some form of ion imaging capability. The two techniques are referred to as the SIMS ion microprobe (1) and ion microscope (21, and these designations differentiate between the instrumental processes of generating either quantitatively or qualitatively an image of the lateral distribution of the secondary ions formed in the sputtering process. Since this sputtering process samples the near surface region (-30 A in depth) of the material, the ability to quantitatively record the intensity of this lateral distribution coupled with the capability of storing multiple images on a suitable computer system would provide both two- and three-dimensional quantitative representations of the elemental constituents in the near surface region of a material. In the ion microprobe technique, a small diameter (1-3 pm) primary beam bombards the sample surface and the secondary ions formed from this beam are energy and mass analyzed and detected on a Faraday cup or electron multiplier (I). The ion microprobe produces qualitative ion images of the analyzed area by synchronously rastering the electron beam of a CRT with the primary ion beam, while the intensity of the rastered electron beam is modulated by the intensity of a mass selected secondary ion formed from the bombarded area. Quantitative ion images can be generated with the ion microprobe technique by counting the secondary ions from each point on the sample on an electron multiplier and storing this quantitative image information in a computer system. The ion microscope technique utilizes a large diameter (up to -300 pm) primary beam to irradiate the sample surface while the secondary ions formed from this large primary beam are extracted in a planar electrostatic field such that the lateral distribution of these secondary ions is retained through a double-focusing mass spectrometer system. The resulting mass and energy-filtered ion image may then be either quantitatively measured by an electron multiplier or Faraday cup or projected onto an image detector (consisting of a microchannel plate and fluorescent screen) to be visually observed or qualitatively recorded on film. The instrumental configuration of the ion microscope technique is exemplified
in the CAMECA IMS-3f ion microanalyzer which is illustrated schematically in Figure 1. The implementation of quantitative image acquisition capabilities for the secondary ion images produced in the sputtering process is more easily achieved with the ion microprobe technique and is exemplified by the work of Schilling (6) and Steiger and Rudenhauer (7). Although secondary ion image acquisition on the ion microscope compared to the ion microprobe offers the inherent advantages of shorter acquisition times (order of magnitude shorter because each area element, or pixel, is sputtered simultaneously rather than sequentially (8)), the higher spatial resolution of the ion microscope ( ~ 0 . pm), 3 and its higher practical detection sensitivity, the ion microscope with its present image detector cannot quantitatively record ion intensities and establish lateral distributions within the sample area without extensive off-line processing. In order to implement quantitative image acquisition for ion microscopy, Morrison and co-workers (8, 9) have developed a real-time image acquisition system for the first generation ion microscope (IMS-300) in which the ion image intensity is recorded on a vidicon camera and the analog video output is digitized and stored on a real-time image processing system. Although this image acquisition system represents the state-of-the-art in real-time image processing, the system does not provide direct quantitation of the incident ion image intensity (in counts per second, cps) and the recorded image intensity or “brightness”. As a consequence, the correspondence between image brightness and ion intensity requires calibration curves relating these two quantities. In this correspondence, we report our results on the use of a resistive anode encoder (RAE) as a quantitative image detector for the CAMECA IMS-3f. The RAE is a pulse counting, position computing device which provides a direct means of both localizing ion signals in the image plane of the detector and providing pulse counting capabilities of the ion intensity in this image (IO). The results reported herein are sufficiently encouraging to warrant further investigation of this type of ion imaging device in ion microanalysis applications.
EXPERIMENTAL SECTION A commercial version of the resistive anode encoder manufactured by Surface Science Laboratories (Mountain View, CA) was utilized in this work. The operation of the RAE is quite simple conceptually and a schematic diagram of this ion imaging device is illustrated in Figure 2. An incoming ion strikes the front surface of a chevron style, dual microchannel plate (MCP) electron multiplier and produces a pulse of lo6 electrons out the exit
0 1983 American Chemical Society 0003-2700/83/0355-0574$01.50/0
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ANALYTICAL CHEMISTRY, VOL. 55, NO. 3, MARCH 1983 575 and an “enable” pultie to indicate a pulse has been processed; and (3) a TTL pulse chain which represents the true ion arrival rate independent of the 3 to 10 ps position computing dead time of the detector electronics. The position computing dead time of the detector electronics refers to the time required for the electronic circuitry to process (calculate) the position of the incident ion signal and this dlead time limits the maximum ion intensity which can be positionally processed to 100000 cps. The single microchannel plate/ fluorescent screen detector on the IMS-3f was removed for these experiments and the RAE was mounted in the same location on the IMS-3f by using a special high vacuum adapter housing. ‘The performance specifications for the RAE device from SSL (Model 239G) include an active area of 25 mm diameter, a background count rate of 10 cps (total area), a maximum count rate of IO6 cps, and a resolution of 4 lp/mm. A new version of the 239 has a resolution of 16 lp/mm which would provide -0.6 pm lateral resolution of the ion image for an image field of 150pm magnified by a factor of 100 onto the input microchannel plate of the RAE. The output from the encoder was both displayed on an oscilloscope ( X , Y , and intensity) and recorded in a multichannel analyzer (MCA). Polaroid photographs were taken of the oscilloscope image and the MCA CRT (the MCA had just arrived in our laboratory and the X-Y recorder interface was not in operation). A copper grid structure having a 25 pm center to center spacing on an aluminum substrate was used as the test ions were used to sputter sample. Twelve kiloelectronvolts 02+ this sample surface, and positive ion spectroscopy was evaluated. The initial tune-up of the ion optics of the IMS-3f required drastically attenuating the matrix ion (Al+)intensity in order to prevent saturating tlhe RAE. The ion image signal displayed on the oscilloscope at 100000 cps was relatively difficult to optimize because the scope phosphor was not as persistent as the phosphor on the conventional MCP/fluorescent screen detector. However, the instrument tune-up was relatively easy to perform, after becoming familiar with this new ion image display mode.
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lyzer. face of the dual MCP. This charge pulse impacts a layer of resistive material and the charge is taken off the anode by four electrodes oriented at right angles to each other. The magnitude of the charge collected at each of these electrodes depends on the position of the charge pulse impacting the resistive material, and the position of this incident pulse can be calculated by using specialized electronic circuitry to resolutions approaching 16 line pairs/mm (lp/mm). The RAE system developed by Surface Science Laboratories (SSL)provides the following output signals: (1)analog x , y , and z (intensification) voltages for an oscilloscope display of the incident flux position and intensity much as in an X-ray or Auger electron spectrometric “dot-map”; (2) a calculation of the x and y position of the charge pulse impact with eight to ten bit precision
RESULTS AND DISCUSSION The first series ojf photographs (Figure 3) demonstrate the ion microscope nature of the IMS-Sf. All of these images are of the Al+ ion produced by 02+ bombardment of the grid and were taken with the instrument operating in a large diameter, static primary beam (top photo), point probe (center photo) and partial beam rmter (bottom photo) mode. It is important
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Schematic diagram of the Surface Science Laboratories resistive anode encoder.
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Figure 3. Oscilloscope display 01 the RAE output 01 AI' secondaIY ions under different primary beam condiions (seated for discussion).
Flp8# 4. Oscilloscope display of the RAE output lor low level Signa$ of Li' and Na+ secondary ions (see text for discussion).
to stress that the IMS-3f is an ion microscope and not an ion microprobe and, as such, can produce spatially resolved secondary ion images from an area 3 to 300 pm in diameter with a static primary beam while maintaining 1pm resolution. The primary beam is rastered during depth profiling in order to produce flat-bottomed cratere in the sample which results in improved depth resolution. The transfer ion optics of the microscope in conjunction with contrast diagrams and field apertures in the ion image path determine the intensity and field of new of the sample surface. The ion image produced hy the static primary beam (the field of view for all of these images was 150 pm) illustrates the complete 150 pm image field of the instrument with the corresponding transfer optic settings. The distortion in the lower right portion of the image reflects primarily the nonuniformity of the grid structure, although the oscilloscope may also add a distortion to the view image. The unrastered probe beam illustrated in Figure 3 is 25 pm in diameter and is of nominally uniform intensity a c r w its diameter. The bottom photo shows the ion image when this primary beam is rastered over a 50 pm X 50 r m area. The 25-pm center to center grid spacing is well illustrated in this image. All of these images were obtained with a secondary nAl+ beam intensity of approximately 100ooO cps, and a
camera exposure time of 1s. This high signal intensity was near the saturation level for the RAE, and for this reason spurious counts can be seen outside of the field of new. The fast exposure and turn around times for these qualitative ion images are a strong positive feature of the RAE detector since it is possible to rapidly obtain informative ion images with minimum sample consumption. The secondary ion images displayed in Figure 4 are of lithium (top and center photo) and sodium contamination on the grid material and demonstrate the high sensitivity of the RAE detector. The top photograph required only 700 total Li+ ions incident on the detector. This very low ion signal can be easily viewed and photographed (1 s exposure time) and clearly demonstrates the advantage of coupling a pulse counting, position sensitive detector to the ion microscope. By way of comparison, a similar image of a low level signal could be obtained by using the pulse counting detector of an ion microprobe. However, since the ion microprobe uses a small diameter primary beam (-1 pm) a i d rasters this beam over the sample area in order to generate the ion image, the time required to acquire an image with the microprobe is orders of magnitude longer than the image acquisition time of the ion microscope. For example, an ion microprobe
ANALYTICAL CHEMISTRY, VOL. 55,
analpis of an area 150 pm in diameter using a 1pm diameter probe beam would require rastering the beam through 2.2 X 10' beam p i t i o n s or pixels. If the primary ion beam current density, sputtering rate, and useful yield (number of ions detected/atoms sputtered) were equivalent in the ion microprobe and ion microscope, it would take the microprobe approximately 20000 times longer than the ion microscope to accumulate the same total ion count. An ion image similar to the low intensity Li+ image in Figure 4 obtained in 1 s on the ion microsmpe would require more than 5 h analpis time on an ion microprobe! The RAE deteetor coupled with the ion microscope technique represents a significant advance in rapid, qwntitatiue ion imaging since it combines the analysis speed of the ion microscope with the pulse counting capability of the ion microprobe. Considering only qualitative (photographic) image acquisition on the IM53f. the RAE repreaents a significant advance in terms of sensitivity, speed of processing, a sample consumption. Ion images are currently obtained in cur laboratory with a 35-mm camera and 400 ASA speed film. An image similar to the 700 cps Li+ image in Figure 4 would require an exposure time of 480 s. The film would then have to be processed, so the analyst does not have immediate access to the image. In addition, during the time of exposure of the 35mm fh, the sample material is continually being removed by sputtering and, in the case of thin fh or ahallow i m p h t a , can be totally consumed during the image acquisition time. For example, a 480-9 film exposure a t a sputtering rate of 1 A/s removes about 500 A of sample material during exposure. Since many thin films and epitaxial layers are significantly less than 500 A ( 2 2 ) in thickness, all of the sample material would he consumed during the imaging of only one trace component. The long exposure time necessary for low secondary ion signals could of course be reduced by using higher speed film (and more expensive processing) or replacing the single microchannel plate/fluorescent screen intensifier with a dual microchannel plate version. Neither of these adaptations would add pulse counting capabilities, however, and therein lies the true advantage of the RAE. Experiments with the RAE were also performed in the high maas resolution mode of operation on the IM53f. High mass resolution analysis is necessary in a number of SIMS surface analysis procedures mainly because of molecular ion interferences at the same nominal m&58 of a given atomic ion (22). Common analvses which reauire hieh m m resolution include 31Pin silicon @SiH interfe;en~e),~~As in silicon (%igSi'Bo interference), and MFe in silicon ('%i2 interference). The problems agsociated with conventional high mass resolution SIMS analysis include setup difficulties, complete masking of the ionic interference by the exit slit of the mass spectrometer, and efficient collection of the mass resolved ion on the electron multiplier. The RAE provides a means of reducing or eliminating these problems and Figure 5 illustrates the high mass resolution performance of the RAE with the instrument tuned to transmit mass 29. The total ion intensity a t this mass was approximately 500 cps and Figure 5 illustrates a photograph of the oscilloscope screen (top photo) and photographs of the MCA CRT in a linear (center photo) and log (bottom photo) intensity scale after integrating the images for 10 8. The instrument was operating a t a maea resolution of approximately 3500 and the three slit images in the CRT screen of the MCA were identified as (from left to right) %i*, %iH*, and WHO+. There was a very weak slit image near the 12CHO+peak which appears as a shoulder on the MCA log d e spectrum This peak is most probably nAlH2+. The high m m resolution mode was relatively easy to implement with the resistive anode encoder image. The simultaneous coupling
npur 5.
NO. 3. MARCH 1983 577
Osciibscape and MCA displays of RAE output lor high mass at mBss 29 (see text lor discussion,,
of the RAE output to the multichannel analyzer and the oscilloscope display allows one to view the image while counting the signal intensity. The analytical importance of this capability cannot he overstated, since one critical limitation of high mass resolution depth profiling is the drift in the magnetic field which causes the ion collection efficiency to vary and hence reduces the precision of the profile. The RAE detector operating with a multichannel analyzer or the spectrometry module from SSL (essentially a MCA) permits viewing and integrating the ions collected a t each horizontal pixel in several slit (i.e., high resolution) images. The use of the MCA for data acquisition and transfer in high mass resolution analyses will reduce the effects of signal intensity fluctuations resulting from short-term magnetic field drift or hysteresis due to large AM peak switching. The major drawback of the resistive anode encoder is its limited count rate capacity (-loK cps). This limitation is a very serious one since it does not permit tuning the ion optics of the instrument with a matrix level species a t the primary currents commensurate with the analytical sputtering rates (count rates >lo' cps). and prevents the analysis of high concentration, l d i z e d regions, or particles. In the majority of SIMS analytical work, the ion optics are tuned on a matrix
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ion since the intensitv variations of these mecies are small and the instrumental I;arameters can be o p t i k e d . However, since the position computing electronics of the RAE saturate above lo5 cps, any attempt to focus the instrument on a matrix level species is impossible. It is of course possible to adjust the ion optics on trace component signals, but this procedure is fraught with difficulties and potential artifacts. This issue of dynamic range limitation is not unique to the RAE but is common to any instrumental system when detection is required over 6 to 9 decades in intensity. We are presently investigating methods for extending the count rate capacity of the RAE to the lo6 cps range and are also pursuing methods to incorporate both an RAE and a microchannel plate/fluorescent screen image detector on the IMS-3f. This dual detector configuration would permit matrix ion tune-up of the instrument and provide pulse counting, position computing imaging of those signal intensities which are below the count rate capacity of the RAE.
LITERATURE CITED Liebi, H. "Proceedings of Workshop on SIMS and Ion Microprobe Analysis"; Heinrich, K. F. J.. Newbury, D. E., Eds.; National Bureau of Standards: Washington, DC, 1974; NBS Spec. Publ. No. 427. Morrison, G. H.; Siodzian, G. Anal. Chem. 1975, 4 7 , 932A. Wilson, R. G.;Vasudev, P. K.; Jamba, D. M.; Evans, C. A., Jr.; Deiine, V. R. Appl. Phys. Lett. 1980, 36, 215. Huggins, R. A,, Ed. "Annual Review of Materials Science"; Annual Reviews, Inc.: Paio Alto, CA, 1980; Vol. 10. Burns-Bellhorn, M. S. I n "Microbeam Analysis in Biology"; Lechence, C. P., Warner, R. R., Eds.; Academic Press: New York, 1979; pp 129-15 1. Schilling, J. H.; Buger, P. A. Int. J. Mass Spectrom. Ion Phys. 1978. 2 7 , 283. Rudenauer, F. G.;Steiger, W. Mlcrochim. Acta 1981, 11, 375. Furman, F. K.; Morrison, G. H. Anal. Chem. 1980, 5 2 , 2305. Patkin, A. J.; Morrison, G. H. Anal. Chem. 1882, 5 4 , 2.
(10) LamDton. M.: Paresce. F. Rev. Sei. Instrum. 1974. 45. 1098. and refeiences therein. (11) Poate, J. M.; Tu, K. N.; Mayer, J. W. "Thin Films-Interdiffusion and Reactions"; Wiiey-Interscience: New York, 1978. (12) Williams, P.; Evans, C. A., Jr. "Proceedings of Workshop on SIMS and Ion Microprobe Analysis"; Heinrich, K. F. J., Newbury, D. E., Eds.; National Bureau of Standards: Washington, DC, 1974; NBS Spec. Pubi. No. 427.
'
Present address: IBM Corporation, P.O. Box 390 South Road, Bidg. 052, Dept. A46, Poughkeepsie, NY 12602.
Robert W. Odom* Bruce K. Furman' Charles A. Evans, Jr. Charles Evans & Associates 1670 South Amphlett Boulevard, Suite 120 San Mateo, California 94402
Charles E. Bryson William A. Petersen Michael A. Kelly Surface Science Laboratories, Inc. 1206 Charleston Road Mountain View, California 94043
Donald H. Wayne Cameca Instruments, Inc. 2001 West Main Street, Room 105 Stamford, Connecticut 06902
RECEIVED for review July 8, 1982. Accepted November 12, 1982. We wish to gratefully acknowledge support for this research from a National Science Foundation/Small Business Innovative Research (NSF/SBIR) Grant No. DMR-8113779.
Evaluation of Lutetium for Volumetric Calibration by Isotope Dilution Mass Spectrometry Sir: One of the problems faced by safeguards is that of determining the quantities of fissile material in holding tanks. It is difficult to measure the volume of the contents of the irregularly shaped, partially filled vessels that hold radioactive waste generated in the nuclear fuel cycle. Through the choice of a suitable element, the double spike technique of isotope dilution analysis is a viable method for addressing this problem. A suitable element must have at least two naturally occurring isotopes and an isotopically enriched spike must be available. Addition of this element to the solution to be sampled must not interfere with subsequent chemical processing and the element must be amenable to mass spectrometric analysis. Two elements being considered for this purpose are lithium and magnesium. Neither of these elements is ideal for the purpose. Both are very common elements, making it necessary to take stringent precautions against contamination. Because both are light in mass, isotopic fractionation during mass spectrometric analysis can be a serious problem. This can largely be surmounted by use of standards and meticulous attention to procedural details, but the entire procedure is cumbersome and time-consuming. Considerations such as these led us to consider using lutetium for this purpose. It meets all the requirements listed above. Although produced in fission, the quantities are not large enough to be measurable. Rare earth elements in general are highly amenable to thermal emission mass spectrometry;
lutetium has the lowest ionization potential of the rare earths (5.3 eV) (1)and is thus particularly suited for this application. Its isotopic masses (175 and 176) are high enough that fractionation is controllable without the extensive analysis of standards required by lithium.
EXPERIMENTAL SECTION The mass spectrometer used in this experiment is a two-stage instrument with two 30 cm radius, 90° sector magnets ( 2 ) . It is equipped with a pulse-countingdetection system, making possible analysis of subnanogram samples. The instrument is equipped with an ion source of new design that yields improved performance in comparison to the one previously used (3). The technique of using anion resin beads for introduction of uranium and plutonium samples into the mass spectrometer has been described in previous publications ( 4 , 5 ) . The mechanism of the bead-sample-filament interaction has been the subject of a study by secondary ion mass spectrometry (6). Lutetium samples as small as 0.2 ng were analyzed with no difficulty. A resin bead was added to 1pL of sample solution in the V-shaped rhenium filament (7); this allowed us to take advantage of the benefits of forming metal ions from resin beads without performing a chemical separation. The isotopic composition of the lutetium spikes were as follows: natural Lu, 97.393% 175Lu,2.607% '16Lu; enriched Lu, 28.577% lI6Lu, 71.423% l16Lu. The high abundance of 176Luin the naturally occurring element is advantageous for its use in this application. Substantial quantities of the original spike are required for direct spiking of the contents of the tank, making use of an
0003-2700/83/0355-0578$01.50/00 1983 American Chemical Society
