Anal. Chem. 1989. 67, 1946-1948
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Factors Affecting Precision and Accuracy in Quantitative Analysis by Secondary Ion Mass Spectrometry Ray-Chern Deng and Peter Williams* Department of Chemistry, Arizona State University, T e m p e , Arizona 85287-1604
I n quantltatlve analysls by secondary ion mass spectrometry (SIMS) uslng external standards, precislon and accuracy are affected by sample misallgnment and Merent secondary Ion energy dlstrlbutlons. To Investigate thls, depth profiles of four pleces of a Blmplanted slllcon wafer were obtained wlth samples mounted flat, or Intentionally mlsallgned. The B+ signal was referenced to 'S I+ and '*SI3+. I n comparative analyses of four nomlnally ldentlcal samples, the relatlve standard devlatlon (RSD) was 5.9 % for nominally flat samples compared to 13% for misaligned samples for the 30Sl+ reference Ion. When '%I3+ was used as the reference specles, Internal agreement was much worse (150 % for mlsallgned samples compared to 13 YO for flat samples). The best results (RSD 1.9%) were obtained for analyses from the center of one large sample wlth 'OSI' as a reference. This study has shown that random sample misalignments lead to discrimination between analyte and reference Ion spedes wlth differing energy dlstrlbutlons, whlch cause slgnlflcant errors In the comparlson of two samples.
INTRODUCTION Secondary ion mass spectrometry (SIMS) has been used widely for depth profiling ion-implanted impurities in semiconductor materials in the electronics industry. For quantification, the average count rate in the implant profile is compared with an average concentration over the analyzed depth, calculated from the implant dose (1). When one implanted sample is used as a standard for a second sample, it is usual to normalize the implant count rate to a signal for the matrix element, to minimize errors arising from differing instrumental sensitivity (transmission) between samples. It is generally felt that SIMS analyses obtained in this way should be accurate to about f10% (standard deviation of a single comparison); detailed measurements using replicate analyses gave a precision of 5-7% (2), again for a single comparison. The question arises: what is the source of this sample to sample variation? Counting statistics on most depth profiles should yield errors much less than 1%. Matrix effects and ion yields should be constant for a given dilute impurity in, say, Si, and these effects should not contribute any error in the final results. We suspected that the sample to sample variation arises from small misorientations of the samples with respect to the secondary ion optic axis of the instrument, together with differences in energy distribution between implant and matrix ion species. Together, these effects can result in a misregistration of the secondary ion beams for the two species at the entrance aperture of the secondary ion mass spectrometer, so that the entrance aperture, if aligned to maximize the matrix ion signal, may not be aligned accurately for the implant ion signal. Note that the secondary ions move slowest in the initial portion of their trajectory, so that electric fields near the sample have a particularly strong effect on ion
* To whom correspondence should be addressed.
trajectories. Thus a random fraction of the signal for the implanted element may be rejected by the entrance aperture which leads to random errors in the results. If the secondary mass spectrometer must be operated at highest sensitivity to detect a weak impurity signal, the signal for the matrix element is often too high to be measured with an electron multiplier. Under such circumstances, one often looks for a matrix-related signal of lower intensity; cluster ions, e.g. Si3+,Si;, or multiply charged ions, e.g. Si2+,are often used. If the initial kinetic energy distribution for the reference ion is markedly different from that for the analyte species, as is the case for the cluster ions, then the effects of sample misorientation are increased. For negative ion analysis, multiply charged ions are not formed, so that cluster ions offer the only low-intensity matrix reference signal. To evaluate the effects of sample misorientation and initial kinetic energy differences on analytical precision, we have compared depth profies for a set of samples cut from the same boron-implanted silicon wafer. Four mounting conditions were compared. Sample set I used our normal four-hole sample holder. This sample holder has a thin metal faceplate with four holes against which samples are pressed by small springs; some slight distortion of the faceplate, and small misorientation of the samples was therefore possible. For set I1 the samples were deliberately misaligned using small shims. For sample set I11 analyses from different areas on a large single wafer piece were compared to simulate identically aligned samples. Finally (set IV), four samples were mounted in a one-hole sample holder separately and introduced into the instrument successively for analysis. %i+ and %i3+ were used as reference signals.
EXPERIMENTAL SECTION All the depth profiles were obtained by using a Cameca IMS 3f secondary ion mass spectrometer. An 02+ primary ion beam with an impact energy of 8 keV was used. Secondary ion signals were taken from a 62 km diameter circular area in the center of a 250 X 250 pm raster area, eliminating signals from the crater edge. The chosen sample was a B-implanted silicon wafer with an implant energy of 40 keV and a nominal dose of 1 X l O I 5 ions/cm*. For each set of analyses, pieces slightly larger than 1 cm square were cut from adjacent areas on the wafer. The different sample sets were analyzed under different sets of conditions as follows. Sets I and 11. For these analyses our standard four-position sample holder for multiple sample holding was used. This sample holder has a thin (0.5 mm) tantalum faceplate containing four 1 cm diameter holes. For set I, four samples taken from adjacent areas were placed one over each hole and pressed against the rear of the faceplate by small springs. This mounting procedure could induce some small sample misorientations if the tantalum faceplate flexed slightly, and such misorientations were suspected as the source of sample-to-samplevariation encountered in normal analyses. For set 11, two of the four samples were deliberately misaligned by loading with a 0.5 mm shim (another piece of the wafer) inserted between the sample holder and one end of the sample as shown in Figure 1. These two samples were tilted in opposite directions, each by an angle between ' 4 and 6'. Sample sets I and I1 were each analyzed by using two reference ion species, 30Si+and '*Si,+, leading to the results labeled conditions 1-4 in Table I.
0003-2700/89/0361-1946$01.50/0Q 1989 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 61, NO. 17, SEPTEMBER 1, 1989
Tantalum faceplate
J Shim
Sampies
Shim
Flsure 1. Detail of sample holder showing the Ta faceplate and mounting of the tilted samples. The angle ofthe tilting is beiween 4'
and . ' 6 Table I. Analytical Results for the Four Sample Sets condition 1
2 3 4 5 6 7
sample set and loading I, flat I, flat 11, tilted 11, tilted 111, one piece 111, one piece IV, flat
reference ion % RSD %Si+ 28Si + SO&
*Si3+ Wi+ 30Si+ 30Si+
5.9 13 13 150 3.9 1.9
2.9
% RSDM 1.7
3.8 3.8 44 1.0 0.55 0.84
Set 111. Sample set I11 was actually a single flat piece of wafer, about 2 X 2 cm, cut from the starting wafer and attached with silver paint to a flat copper mounting block held in a sample holder without a faceplate. The height difference measured from each edge of this sample to the back of the copper block was less than 75 bm. Analyses in four regions of this sample corresponding to the four analysis positions in the four-hole sample holder were intended to simulate analyses of identically aligned samples (condition5). %i+ was used as the reference ion species. A further set of analyses was obtained from a small region near the center of this sample to avoid any electric field perturbations near the edges of the sample holder (condition 6). Again 30Si+was used as the reference ion species. Set IV. These samples (again taken from adjacent areas on the wafer) were each mounted separately in a sample holder having a single 1 cm diameter hole in the center, and introduced successively into the instrument for analysis. Analyses were obtained from regions close to the center of the sample holder. 30Si+was used as the reference ion species (condition 7). Depth profiles were taken in each case after the secondary current was maximized for the reference ion by adjusting the primary ion beam position and the mass spectrometer entrance aperture. At least three depth profies were taken for each sample. Each depth profile was obtained in 10 min. Integrals of the B+ signal were normalized to the reference ion signals before the relative standard deviations were calculated. Simulation of the secondary ion trajectories was performed by using the SIMION PC/PS2 computer program (Idaho National Engineering Laboratory, Idaho Falls, ID).
RESULTS AND DISCUSSION In the Cameca IMS 3f secondary ion mass spectrometer ions are accelerated by a 4500-Velectrostatic potential applied between the sample and a grounded extraction plate. Leaving the acceleration space through a hole in the extraction plate, the beam is focused by a pair of einzel lenses (transfer optics) to a beam waist or crossover in the plane of the mass spectrometer entrance aperture. The width of the crossover is a function of the initial trajectories and kinetic energies of the secondary ions; its mean position reflects the alignment of the primary ion impact area with the optical axis of the transfer optics, and the alignment of the sample surface normal to this axis (3). The circular entrance aperture is known also as the contrast diaphragm (CD), because it allows improvement of contrast and lateral resolution in the secondary ion image by limiting the angular and energy spread of the ions entering the mass spectrometer. The smallest CD (diameter 20 pm) was used for the present study; this CD transmitted an estimated 0.1% of the sputtered ions. It was noted that to maximize the reference ion signal when moving from sample to sample in set I, the primary beam position and/or the CD
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position needed to be adjusted. This effect was much stronger for the intentionally tilted samples (set 11). Table I contains the results of the seven study conditions and shows the relative standard deviations (RSD) of the normalized ratios of the integrals, calculated for 12 replicate analyses (three from each of four sample positions in the set). Also shown are the relative standard deviations of the means (RSDM) of the seven data sets. Data from the intentionally misaligned samples were treated as a single set as they would be in a normal analytical situation where misalignment was not suspected. As expected, the agreement for the misaligned samples was particularly bad when %i3+ was used as a reference species. The use of %i+ as a reference for this sample set gave much better internal agreement (13% RSD rather than 150%). For the nominally flat set of four separate samples (set I), the results were significantly better. Again, the use of the %Si3+reference gave demonstrably worse results (RSD 13%) than did 30Si+(RSD 5.9%). Mounting samples separately in the single-hole sample holder and restricting analyses to the center of the holder gave even better results (RSD 2.9%, condition 7), even though the possibility remained of sample-to-sample variations in alignment. For the single, large sample (III), analyses at points distant from the center of the sample (RSD 3.9%, condition 5) were better than for individual samples a t these positions (RSD 5.9%, condition 1) but were less accurate than analyses that were restricted to the center of the sample holder. Analyses on sample I11 performed at the center of the sample gave the best results in the set (RSD 1.9%, condition 6). This study clearly shows that sample misalignment is a significant source of error in SIMS analyses that use an external standard. The results also substantiate our initial suggestion that the error arises from a misregistration of the secondary ion beam crossovers for the analyte and reference species at the position of the contrast diaphragm. This causes discrimination against the analyte signal if, as is usually the case, the CD and primary ion beam positions are adjusted to maximize transmission of the reference signal. Alternatively, if the analyte signal were maximized, discrimination against the reference signal would result. The effect of the misregistration is exaggerated if the analyte and reference signal have markedly different initial kinetic energy distributions. The effect was observed in reductions of the B+ signals by as much as 40% for conditions 3 and 4, where the 28Si3+signal was maximized for the tilted sample. Although initial kinetic energy differences are most often encountered when an atomic analyte signal is compared to a cluster reference signal, the effect can also occur even if two atomic signals are compared. H+ and F+ in particular have energy distributions very different from the majority of atomic secondary ions ( 4 , 5 ) . In some cases it is necessary to operate the secondary ion mass spectrometer at high mass resolving power to distinguish the analyte signal from that of an interfering cluster ion. A typical entrance slit width for such analyses is comparable to the size of the contrast aperture used in the present study (20 pm diameter). Depending on the quality of the magnet power supply, it can be difficult to switch reproducibly between the analyte and reference peaks, so that an additional source of error is introduced. If, instead of peak switching, the analyte signal alone is monitored, with the reference signal being recorded only a t the end of the analysis, then the stability of the primary ion current may limit the analytical accuracy. In general, the analytical accuracy would be expected to degrade a t high mass resolution, but the extent of this degradation will vary depending on the stability of the instrument being used. The effects of sample tilt examined here would also be present to about the same extent in high mass resolution analyses. In addition, a tilted sample can
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ANALYTICAL CHEMISTRY, VOL. 61, NO. 17, SEPTEMBER 1, 1989
degrade the mass resolution of the mass spectrometer, because ions are initially directed away from the ion-optical axis of the instrument. The crystal orientation of the sample is not expected to affect the analytical results for semiconductor samples because the ion bombardment rapidly amorphizes the sample surface to a depth on the order of the ion projected range. For heavy ion bombardment (e.g. Ozf) the dose required to fully amorphize the surface region of a silicon sample at room temperature is 1015ions/cm2; such a dose sputters no more than one to two monolayers of silicon, so that over essentially the entire analysis the sample surface is amorphous. The good results from the individually loaded samples (condition 7), which were loaded with random orientations, confirm that variations in sample orientation for silicon do not introduce major errors. For metals, many of which do not amorphize fully under ion bombardment, changes in sample orientation may well introduce sizable errors in analyses which use external standards. It is not clear where the residual 3.9% RSD for widely spaced analyses on the flat sample (condition 5) arises. Counting statistics in the B+ and silicon signals would account for only a 0.3 7’0 RSD. A portion of the variation might result from nonuniform implantation across the wafer. The wafer used was 10 cm in diameter, and implant doses should be uniform to at least 5% across the wafer area, assuming proper operation of the implanter. A 5% nonuniformity could produce a 170variation across the 2 cm wide sample. A possible source of error is misalignment of the sample holder in the instrument, which could have caused the entire sample to have been tilted and changed the path length through the acceleration field from one side of the sample to the other. We believe that the major source of variation is the finite extent of the sample holder which gives rise to fringe field effects that slightly distort the acceleration field. The sample holder has a finite size (approximate 2.8 cm X 3.4 cm) and is situated 5 mm from an extraction electrode a t ground potential. Within about 4-6 mm of the edge of the sample holder, the accelerating field is not accurately normal to the sample surface, and this can cause deviations of the ion trajectories which vary depending on the initial kinetic energies of the different ion species. Trajectory simulations using the SIMION ray-tracing program of the secondary ions ejected from the edge and from the center of the sample holder show that the distorted acceleration field near the edge does alter the
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ion trajectories to some extent. When a set of replicate measurements were all obtained in the central region of the large flat sample (condition 6), the reproducibility improved to 1.9% RSD, suggesting that samples should be centered in the sample holder for the most accurate SIMS analysis. The relative standard deviation of t h e m e a n for condition 6 was only 0.6% (0.8% for condition 7 and 1.7% for condition I), so that the precision of a set of 12 measurements compares well with other precise microanalytical or thin film techniques (electron probe or Rutherford backscattering spectrometry). The results of this study suggest that for the most accurate quantitative SIMS analysis, the sample should be mounted as flat as possible in the center of a sample holder having a rigid faceplate, and the analyzing area should be restricted to the center of the sample to minimize the effects of nonuniform accelerating fields near the edges of the sample holder. In situations where sample orientation is not under ideal control, for example if the sample itself is not flat, or is very small, it is worth noting that careful selection of the reference ion species to have a closely similar energy distribution to the analyte species gives the best chance of analytical accuracy.
CONCLUSIONS Quantitative analysis by SIMS is an important requirement, particularly in the electronic industry, and because standards are always required to calibrate elemental sensitivities in each sample type, it is important to know the factors affecting the accuracy with which two samples can be compared. This study has shown that differences in initial kinetic energy distributions between analyte and reference species, combined with small, random sample misalignments, may be a significant source of error in the comparison of two samples. When care is taken to minimize sample misalignments, and analyte and reference ion species are chosen to have similar initial energy distributions, the present results demonstrate that the precision of a comparative SIMS analysis can be better than &3% for a single comparison of standard and unknown.
LITERATURE CITED (1) Leta, D. P.; Morrison, G. H. Anal. Chem. 1980, 52, 514-519. (2) Williams, P.; Baker, J. E.; Davies, J. A.; Jackman, T. E. Nucl. Instrum. Methods 1981, 197, 316-322. (3) Slodzian, G. Surf. Sci. 1975, 48, 161-186. (4) Wittmaack, K. Phys . Rev. Len. 1979, 43, 872-875. ( 5 ) Williams, P. Phys. Rev. B : Condens. Matter 1981, 23, 6167-6190.
RECEIVED for review January 26,1989. Accepted May 25,1989.
