Copyright (C) JCPDS-International Centre for Diffraction Data 1999 794 Issues With TXRF Angle Scans and Calibration Dennis Werho, Stephen N. Schauer, and George F. Carney, Motorola, Inc., AZ Abstract Previous work has indicated very close agreement between vendor-certified values on TXRF reference calibration standards and the results from an ion beam technique heavy ion backscattering spectrometry (HIBS) which does not require standards for accurate analysis. However, when any one of these standards are utilized to calibrate a TXRF system, serious discrepancies (up to 100% relative) are noted when the rest of the standards are analyzed as though they were samples. Since there is very high confidence in the HIBS results, these selfinconsistencies among standards suggest that there may be something unique to the TXRF analysis which is responsible for the discrepancies. Although there may be other possibilities for the disagreement among the TXRF results, this paper focuses on the possible effects of errors in angle registration and possible errors due to the location of the contaminating element relative to the sample surface. Errors in angle registration, at least for the instruments studied, were found to be significant but nowhere near the magnitude required to fully explain the discrepancies. The results of the angle scans determined that even though all the standards were produced by the same vendor using the same process, very significant differences in the forms of the contamination could be inferred from the shapes of the angle scans. Specifically, as much as 50% of the deliberately added contamination was found to be present in the form of particulate matter rather than in the expected form of a very thin (plated) film. At the analyzing angle used, this would almost completely account for the discrepancies observed. Author to whom correspondence should be addressed.
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Copyright (C) JCPDS-International Centre for Diffraction Data 1999 795 Introduction In a previous paper 1 we reported on the analysis of commercially prepared calibration standards using total reflection X-ray fluorescence (TXRF) and heavy ion backscattering spectrometry (HIBS) and compared them to the vendor s certified values. This was precipitated by the observation that different TXRF instruments within the company were providing significantly different results for the same sample. Our initial suspicion had been that the reference calibration standards were incorrectly quantified by the vendor; however, the HIBS analysis results generally agreed with the vendor certified values to within about 10 % relative. Thus, as determined by HIBS the set of TXRF standards were self-consistent and correct. Unfortunately, when one of the TXRF instruments was calibrated using any one of those standards, and when the other standards were then analyzed as samples, discrepancies as large as 100 % relative were noted between the vendor certified (and HIBS verified) values and the TXRF results. These discrepancies are demonstrated in the TXRF sensitivity curves developed from the HIBS-analyzed standards and shown in Figure 1. Each curve was arrived at by first determining the sensitivity factor (TXRF cps / HIBS-determined concentration in 10 10 atoms/cm 2 ) for the one element in the single element standard and then calculating the sensitivity factors for the other elements. Thus, each single-element calibration standard gave rise to one of the curves shown in Figure 1. It can be seen that there is serious disagreement among the standards. FIGURE 1: TXRF Sensitivity Curves Based on HIBS Results. The units for the sensitivity factors are as TXRF counts per second divided by HIBS-determined concentrations in 10 10 atoms/cm 2. The Lab # denotes the owner of the standard. The Universal curve is the average of the other curves and was used to adjust the vendor-certified values in an attempt to make them all self-consistent. The fact that the vendor-certified and HIBS-determined values for the calibration standards agree quite well and that the TXRF results are self-inconsistent relative to the HIBS or vendor values suggests that there may be something unique to the TXRF analysis which is responsible for the discrepancies. One of the possible explanations for the HIBS/vendor values vs. TXRF discrepancies may be related to the different analysis depths of the two techniques and the location of the contaminating species relative to the wafer surface. For polished samples TXRF generally detects contaminants no deeper than about 50 Å below the surface, whereas the HIBS analysis depth can be as deep as 1000 Å. Thus, any portion of the contaminants which migrate to greater than 50 Å but less than 1000 Å below the surface will no longer be detectable by TXRF, but will still be accounted for in the HIBS analysis. However, significant diffusion of the elements used as TXRF standards at room temperature in silicon is most unlikely. Another possibility would be limitations in the software provided with the TXRF instrument which could be giving
Copyright (C) JCPDS-International Centre for Diffraction Data 1999 796 rise to errors; however, without access to the source code our ability to check out this possibility is somewhat limited. Two possibilities that are open to investigation are errors in incident angle determination and errors due to differences in the physical location of the contaminating elements relative to the surface; i.e., whether the contaminating material is present below the surface in the bulk, above the surface as particulate matter, or plated as a thin layer on the surface. This paper is limited to the study of these last two possibilities. TXRF Experimental All data was acquired on Technos TREX 610S and 610T instruments using the tungsten rotating anode target with the incident beam monochromatized to the W Lβ line by a crystal monochromator. For the angle registration and angle scan data the target was operated at 30 kv and the acquisition time was 300 sec. The current used was either 30 or 40 ma, depending on the particular instrument. For all quantitative data, the instrument was operated at its normal full power levels of 30 kv and 200 ma with data acquisition times of 1000 sec. The TXRF instruments used in this study were routinely calibrated and operated at an angle of 0.09. Like many other TXRF users we find ourselves in circumstances in which we are not able to produce our own standards, and thus must rely on calibration standards acquired from an outside source. Calibration standards which had been purchased by three different labs within Motorola were used in this study. All standards were single crystal silicon wafers which had been purchased from a single supplier and which had been fabricated by the spin coat contamination method; 2 the vendor-certified values had been obtained by either Rutherford Backscattering Spectrometry (RBS) or vapor phase decomposition-graphite furnace atomic absorption (VPD-GFAA) techniques. In addition the standards from Labs 1 and 3 were obtained at the same time and came from the same vendor lots. Theory The basic principles of TXRF have been well described in the literature, 3-5 so there is no need to present them here; however, it is probably appropriate to present the theoretical intensity vs. incident angle profiles that one would expect for the different types of contamination that might be present on a wafer surface. Figure 2 presents the theoretical profiles that would be expected for contamination present in the bulk, as particulates above the surface, and as plated material in a thin layer on the surface. 5 4 critical angle 3 2 Bulk Particulate Plated 1 0 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 Angle of Incidence ( )
Copyright (C) JCPDS-International Centre for Diffraction Data 1999 797 FIGURE 2: Theoretical TXRF Angle Scan Profiles. Theoretical curves for the angular dependence of the fluorescent intensity for different types of contamination, calculated for W Lβ (9.67 kev) excitation on a mirror-quality silicon surface and assuming zero beam divergence. The theoretical derivation of these profiles has been presented in several sources 6-8, and the reader is directed to these for the details. It will be noted in Figure 2 that for the case of a contaminant present in the bulk substrate the fluorescent intensity remains very low at low incident angles and then rises dramatically in the region of the critical angle. Since the probability of the spin-on contaminant elements having diffused into the bulk is very unlikely, this situation will not be considered any further in this paper. The two important cases that will be discussed are for the particulate and plated forms of contamination. The goal when preparing a spin-on contaminant standard is to produce a uniformly distributed sub-monatomic layer of the element of interest plated on the surface. As can be seen in Figure 2 the intensity profile for this case results in a monotonically increasing fluorescent intensity signal until the critical angle is reached, at which point the intensity rapidly drops and at high angles approaches about one quarter of its peak value. For a particulate contaminant the intensity profile begins at its maximum value and remains there for increasing angles until the critical angle is reached, at which point it rapidly drops and at high angles approaches about one half of its maximum value. For W Lβ source radiation on a Si substrate an isosensitive point is reached for the plated and particulate cases at about 0.13. The plated form of contamination is the type predominantly encountered on most semiconductor samples, so ideally calibration standards having only plated contamination should be used to ensure accurate quantitation. Thus, the expectation when purchasing calibration standards is that all of the contamination on the spin-on calibration standards would be present as plated material; however, the possibility exists both for real world samples and standards that the contamination may not necessarily be present as just one type, but may be a mixture. In those cases the expected angle scan profiles would change depending on the amount of each type of contaminant present. Figure 3 below shows the effect when some arbitrary amounts of particulate contamination replace what was originally 100% plated contamination. FIGURE 3: Effect of Mixing Particulate and Plated Contamination. Theoretical curves showing the effect on angle scans when both particulate and plated contamination are present on a wafer. Again, the curves were calculated for W Lβ (9.67 kev) excitation on a mirror-quality silicon surface. Angle Registration Note in Figure 2 that in the range of angles for which TXRF analyses are typically performed (~0.05 to 0.13 ), the slope of the plated curve is quite large. Thus, for the case of plated contamination an error in the incident angle will give rise to a higher or lower intensity,
Copyright (C) JCPDS-International Centre for Diffraction Data 1999 798 which will in turn cause errors in the quantitation. In 1992 Hockett et al. reported a standard deviation of 0.024 degrees for angle reproducibility for the 24 different instruments included in their study, although a subset of 10 instruments achieved a much better standard deviation of 0.014 degrees. 9 Referring to the plated angle scan in Figure 2 one can calculate the expected errors in intensity introduced by these levels of errors in angle registration. At 0.09 degree, which has been the incident angle used by our labs, an error of 0.024 degree would translate to a +60% or -46% error in intensity, depending in which direction the angle error occurred, and an error of 0.014 degree would result in errors of +34 to -29% in intensity. Since these are errors resulting from a single standard deviation unit, the actual errors could be much greater and could potentially account for the discrepancies observed among our TXRF calibration standards; i.e., if some standards are consistently being analyzed at an angle other than the expected angle, either due to wafer warpage or some other instrumental problem, then this could account for the differences noted. In order to determine the angle registration and reproducibility the peak position of the angle scan was employed as the key parameter. One will notice in Figure 3 that even when both particulate and plated contamination are present in varying amounts, the peak height of the angle scans may differ substantially, but the peak position changes only very slightly; thus, monitoring peak position should provide a reasonable estimate of peak position registration. Also, one will notice in the angle scans in Figure 3 that on both sides of the critical angle the curves approach linearity. We made use of this characteristic in accurately determining the peak positions of the various acquired angle scans by fitting the data on both sides of the critical angle to a straight line; the angle at the point of intersection was taken to be the peak position. Although one might hope to determine the peak position by counting at very small increments near the critical angle and then choosing the angle that has the highest peak position, normal fluctuations in the intensity and the fact that the beam dispersion is non-zero make this method inherently less accurate as can be seen in Figure 4 below. Intensity (cps) 8 6 4 Cu Loc. 1 #1 Cu Loc. 1 #2 Cu Loc. 1 #3 Cu Loc. 1 #4 Cu Loc. 2 #1 Cu Loc 2 #2 Cu Loc. 2 #3 2 0 0 0.1 0.2 0.3 0.4 Angle of Incidence ( ) FIGURE 4: Typical Angle Scans. A typical set of angle scans on a 5e12 copper standard wafer showing the repeatability at two different sites on the wafer. Location 1 was at the wafer center and Location 2 was approximately halfway from center to edge. The wafer was cycled in and out of the instrument between replicate scans. Angle scans were performed at the center position and occasionally at a periphery position on several nickel and copper calibration standard wafers using instruments from two different labs, and the peak positions were determined as described above. The data was acquired over the course of about one month, and in all cases the wafers were cycled in and out of the instrument
Copyright (C) JCPDS-International Centre for Diffraction Data 1999 799 between replicate scans. A typical set of angle scans is presented in Figure 4 above and generally reveals good reproducibility. There is a small but noticeable concentration difference between the center and peripheral locations, but there is only a slight difference in the peak positions. The data for the angle reproducibility study is summarized in Figure 5 below in which all of the results are presented on the same angle of incidence scale. The vertical axis is used solely to describe the particular conditions standard and instrument tested. Only higher concentration standards (e12 and higher) were used, as the profiles from lower level standards were too noisy to determine the peak position accurately. FIGURE 5: Peak Position Results. Peak positions for angle scans determined for different copper and nickel calibration standards on different instruments. All angle scans were acquired at the wafer center except those noted by an asterisk, which were acquired approximately halfway from the center to the edge. The data indicated that for the same location on the same standard wafer analyzed on the same or different instruments, peak positions on the angle scans agreed to within ± 0.007 at the three sigma level. This translates to a possible error in quantitation of about ± 15 % relative at an analyzing angle of 0.09. When a variety of standards are analyzed on the same instrument or the same standard is analyzed at different locations on the same instrument, peak positions on the angle scans are not quite as reproducible. In those cases the agreement is to within ± 0.012 at a three sigma level. This translates to a possible error in quantitation of about ± 27% relative at 0.09. These differences in errors can probably be partly explained by the method used by the instrument manufacturer to determine the incident angle. A procedure is used which defines the zero angle position from one edge of a wafer to the opposite edge; thus, points in the center of the wafer may or may not be perfectly aligned with the edge-to-edge alignment due to wafer warpage, etc. But at least it is done reproducibly so that the same analysis site analyzed even on different instruments gives very good agreement in angle registration. However, analyzing different sites on the same wafer or the same site on different wafers is subject to more error, as wafer warpage can introduce differences from site to site, and even at the same analysis sites different wafers can exhibit different angles relative to the edge-to-edge alignment. In any case, the differences observed on the various standards and instruments cannot completely account for the discrepancies noted among the calibration standards.
Copyright (C) JCPDS-International Centre for Diffraction Data 1999 800 Angle Scans The situation that originally made us consider the effect of angle scans and exactly how and where the contamination resided on the wafer surfaces occurred after a successful round robin between lab instruments had been completed using an analyzing angle of 0.09. In order to reconcile the discrepancies between TXRF calibration standards, the certified values for the calibration wafers had been reassigned such that they were all consistent with the Universal curve noted in Figure 1. Indeed, the round robin results showed that this was quite successful, as all the instruments taking part in the study were able to provide results that agreed to ±10 % relative. 1 The problem came about when a sample wafer happened to be analyzed on two different lab s instruments at an angle other than 0.09 with results that disagreed substantially. To investigate this unexpected development, one instrument was calibrated at three different analyzing angles 0.05, 0.09, and 0.12 using one standard (Lab 1 Ni), and then the other calibration standards were analyzed as though they were samples at all three angles. Because the standards had been previously adjusted to agree at 0.09, there was little surprise at that analyzing angle; however, the results at the other two angles showed serious disagreement as can be seen in Figure 6 below. FIGURE 6: Angle Dependence of TXRF Standards. Relative concentrations at two different analyzing angles 0.05 and 0.12 after standard values were adjusted to agree at 0.09 and after calibrating on the Lab 1 Ni standard. All of the TXRF calibration standards were analyzed as a function of incident angle to characterize the nature of the spin coated surface. The expectation was that the angle scans would all correspond closely to the theoretical plated profile or at least that all of the profiles would be quite similar. In fact, the results showed a wide range of profiles, indicating that a significant portion of the contamination was present in particle form for many of the standards. This situation is best illustrated by a series of nickel standards presented in Figure 7 below. It will be noted that the angle scan for the standard from Lab 2 corresponds closely to the theoretical plated profile, whereas the angle scans for the standards from Labs 1 and 3 vary significantly from the theoretical profile and between themselves. Once again, it should be pointed out that all standards were acquired from the same vendor, and in addition, the standards from Labs 1 and 3 were from the same lot.
Copyright (C) JCPDS-International Centre for Diffraction Data 1999 801 4 3.5 3 2.5 2 1.5 1 0.5 Lab 1 Ni (5e13) Lab 2 Ni (5e13) Lab 3 Ni (5e13) Plated (Theor.) 0 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 Angle of Incidence ( ) FIGURE 7: Angle Scans for Nickel Standards. Plot of intensity as a function of analysis angle of three Ni standards, along with a theoretical plot of plated contamination. All scans were normalized to 1 at a high angle of incidence. An alternate way of looking at the data is presented in Figure 8 below. In this case, the profiles have been normalized at 0.09 rather than at a high angle of incidence. This was essentially what had been done when the reference values for the standards were reassigned so that they were all self-consistent when analyzed at 0.09. After this renormalization, all the intensities necessarily agree at 0.09, but there is serious disagreement among the intensity readings at both 0.05 and 0.12. In fact, at 0.05 there is greater than a 100 % relative difference between the high and low intensities. Had three different instruments been calibrated using these three standards and even if they all agreed perfectly at 0.09, even a relatively minor shift in the analysis angle would produce significant differences in the relative signal intensity and concentration. 3.5 Relative Intensity 3 2.5 2 1.5 0.05 0.12 Lab 1 Ni (5e13) Lab 2 Ni (5e13) Lab 3 Ni (5e13) 1 0.5 0 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 Angle of Incidence ( ) FIGURE 8: Ni Angle Scans Normalized at 0.09. Similar plot as Figure 7, except now the angle scans for the nickel standards have been normalized to unity at 0.09.
Copyright (C) JCPDS-International Centre for Diffraction Data 1999 802 Using a simple method utilizing the intensities at two angles critical angle/2 and critical angle/6 which has been suggested in the literature to calculate the amount of plated and particulate contamination present, 5,10 the standards were found to contain anywhere from less than 5% to just over 50% particulate matter. In addition, the percentage of contamination present in particulate form was found not to be uniform across the wafer surfaces. For the most nonuniform case, the percentage of contamination present in particulate form varied from 52% at the center to 36% near the periphery. The situation of having a set of standards which have varying amounts of plated vs. particulate contamination on them introduces major problems in trying to achieve accurate quantitation, not only between instruments, but even for a single instrument. Ideally, the standards should be as similar as possible to the samples, which for the semiconductor industry are typically plated. If an instrument is calibrated using a standard which has one type of contamination on it, and then a wafer which has a different type of contamination is analyzed, the results will be inaccurate by an amount that will be dependent on the dissimilarity among standards. Conclusion Our results indicate that errors in angle registration can introduce errors of ± 15 % relative when the same sample is analyzed at the same site or as much as ± 27 % when different sites on the same wafer are analyzed or when different wafers are analyzed. These errors were not sufficiently large to account for the 100% relative discrepancies noted between the commercially obtained standards. Although the angle reproducibility observed was significantly better than what had been reported previously, it should be noted that this current work involved far fewer instruments and that these instruments were of later vintage than those involved in the earlier study. The profiles of the angle scans for the various standards indicated that several of them were not primarily composed of the plated form of contamination, but contained as much as 50 % of the total contamination in particulate form. These differences in plated vs. bulk contamination in the standards were sufficient to explain the large discrepancies between the HIBS (and vendorcertified) values and the TXRF results. In view of the importance of having standards which are composed of plated-only contamination, all standards should be angle scanned prior to use to ensure that the standards conform to the plated-only condition; in fact, considering the price of commercially available standards, it would seem reasonable to demand that the vendor provide an angle scan with each standard. As a result of this study all standards having more than ~ 5% of the contamination present in the particulate form have been removed from the set of calibration wafers, and a new Universal sensitivity curve based on the HIBS results has been arrived at. A new round robin is currently underway which it is anticipated will result in agreement among the company s TXRF instruments at essentially all analyzing angles.
Copyright (C) JCPDS-International Centre for Diffraction Data 1999 803 REFERENCES 1. D. Werho, S. N. Schauer, X. Liu, G. F. Carney, J. C. Banks, J. A. Knapp, B. L. Doyle, and A. C. Diebold, Heavy Ion Backscattering Analysis of TXRF Calibration Standards, in Advances in X-ray Analysis, Vol. 40, in publication. 2. H. Hourai, T. Naridome, Y. Oka, K. Murakami, S. Sumita, N. Fujino and T. Shiraiwa, Jpn. J. Appl. Phys., 27(12), L2361 (1988). 3. H. Schwenke, W. Berneike, J. Knoth, and U. Weisbrod, in Advances in X-Ray Analysis Vol. 32, J. V. Gilfrich, C. S. Barrett, T. C. Huang, R. Jenkins and P. K. Predecki, eds., p. 105, Plenum Press, New York (1989). 4. P. Eichinger, H. J. Rath, and H. Schwenke, in Semiconductor Fabrication: Technology and Metrology ASTM STP 990, D. C. Gupta, ed., p. 305, American Society for Testing and Materials, Philadelphia (1989). 5. W. Berneike, Spectrochim. Acta, 48B, 269 (1993). 6. J. Knoth, H. Schwenke, and U. Weisbrod, Spectrochim. Acta, 44B(5), 477 (1989). 7. L. Torcheux, B. Degraeve, A. Mayeux, and M. Delamar, Surface and Interface Analysis, 21, 192 (1994). 8. R. Klockenkämper, Total-Reflection X-ray Fluorescence Analysis, Vol 140 in Chemical Analysis series, J. D. Winefordner, ed., Wiley, New York (1997). 9. R. S. Hockett, S. Ikeda, and T. Taniguchi, Round Robin Results for TXRF, Abstract 340, p. 498, The Electrochemical Society Extended Abstracts, Vol. 92-2, Toronto, Ontario, Canada, Oct. 11-16, 1992. 10. H. Schwenke and J. Knoth, Part. Surf. [Proc. Symp.], Meet., 1992, pp 311-323 (1995).