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total reflection X-ray fluorescence in the silicon semiconductor manufacturing environment
Spectrochimica Acta Part B 54 Ž1999. 1399]1407
Application of vapor phase decompositionrtotal reflection X-ray fluorescence in the silicon semiconductor manufacturing environment q Gayle Buhrer U Micron Technology, Inc., 8000 S. Federal Way, M.S. 632, P.O. Box 6, Boise, ID 83707-0006, USA Received 21 November 1998; accepted 28 May 1999
Abstract Total reflection X-ray fluorescence ŽTXRF., in combination with vapor phase decomposition ŽVPD., provides an efficient method for analyzing trace metal contaminants on silicon wafer surfaces. The progress made in applying these techniques to the analysis of silicon wafers in a wafer fabrication cleanroom environment is reported. Methods of standardization are presented, including the preparation and characterization of VPD standards. While the VPD wafer preparation process increases the sensitivity of the TXRF measurement by at least one order of magnitude, inherent uncertainties associated with the VPD technique itself are apparent. Correlation studies between VPDrTXRF and VPDrinductively coupled plasma mass spectrometry ŽICP-MS. are presented. Q 1999 Elsevier Science B.V. All rights reserved. Keywords: Semiconductor; Trace metal contamination; Vapor phase decomposition ŽVPD.; Total reflection X-ray fluorescence ŽTXRF.; Inductively coupled plasma mass spectrometry ŽICP-MS.
1. Introduction The stringent demands of contamination conq This paper was presented at the 7th Conference on ‘Total Reflection X-Ray Fluorescence Analysis and Related Methods’ ŽTXRF98. held in Austin, Texas, September 1998, and is published in the Special Issue of Spectrochimica Acta Part B, dedicated to that Conference. U Corresponding author. Tel.: q1-208-368-5280; fax: q1208-368-5165. E-mail address: [email protected] ŽG. Buhrer.
trol during the processing of high-performance, reliable, low-cost semiconductor devices are well known. Metallic impurities present in the surface oxide of the starting silicon wafers or introduced during processing can lead to poor electrical properties and adversely affect overall yields. The needs of the manufacturing team include resolving the contamination problem at hand while, at the same time, minimizing downtime to the fabrication line. Therefore, characterizing and understanding the source of a wide range of metallic impurities in an efficient and timely manner has
0584-8547r99r$ - see front matter Q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 5 8 4 - 8 5 4 7 Ž 9 9 . 0 0 0 8 4 - 1
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G. Buhrer r Spectrochimica Acta Part B: Atomic Spectroscopy 54 (1999) 1399]1407
been important to the analytical team as it works with the manufacturing team. Total reflection X-ray fluorescence ŽTXRF. has been an effective tool for multi-element trace analysis and wafer mapping over the past decade. In the production environment, where quick turnaround of sample analysis is essential, TXRF alone can be slow and cumbersome. Multiple measurement sites and longer measurement times are necessary for good statistical evaluation of the wafer using TXRF. Moreover, with ever-increasing line density, smaller space critical dimensions, and shrinking die sizes, the detection limits of analytical techniques such as TXRF may not be adequate w1x. Vapor phase decomposition ŽVPD. is a somewhat newer chemical preparation of silicon wafers that has enhanced the sensitivity and lent efficiency to the TXRF technique by permitting trace analysis of an entire wafer surface in a single-point measurement. The combination of these techniques, called VPDrTXRF, has been effectively employed in the resolution of metallic contamination problems encountered in wet and dry etch, diffusion, and chemical mechanical polish ŽCMP., as well as in many of the cleaning steps of the device manufacturing process. Analysis time is minimized, and a quick response can be delivered to the process engineer in an efficient report format. Over time, the VPDrTXRF measurement data collected can be used in statistical process control methods incorporating control charts to monitor the fabrication process. As demonstrated in much of the literature, well-characterized analytical techniques are correlated with other analysis methods to substantiate the technique in question w2]6,11x. Comparisons between TXRF and other techniques such as vapor phase decompositionrdroplet surface etchingrgraphite furnace atomic absorption spectroscopy ŽVPDrDSErGFAAS., heavy ion backscattering spectrometry ŽHIBS., Rutherford backscattering spectrometry ŽRBS., secondary ion mass spectrometry ŽSIMS., and inductively coupled plasma-mass spectrometry ŽICP-MS. have been made w2]6x. In this paper, correlations between VPDrTXRF and VPDrICP-MS were
made using NIST W calibration standards for ICP. The VPD method was better characterized with collection efficiency studies into the various chemistry scenarios of the collection droplet. Characterization of appropriate VPD drop standards was performed to further verify the quantification value of the VPDrTXRF method. However, it was not demonstrated that VPD could enhance TXRF detection limits to the 10q8 atomsrcm2 range, as discussed by Neumann et al. w7,8x.
2. Experimental methods 2.1. Reagents The VPD method involves dissolving the native oxide layer of a wafer surface and collecting the dissolved material into a collection droplet. Much work has been done to automate the VPD method of wafer preparation, and some units are commercially available w8x. For our purposes, VPD was accomplished by placing the wafers in specifically designed polypropylene etch chambers in which rails suspend the wafers a few inches above a small container of high purity HF. The accumulation of HF vapors within the closed chamber decomposes the native oxide to the state where the wafer surface becomes hydrophobic. The reaction of the surface layer oxide is as: SiO 2 q 6HFª H 2 SiF6 q 2H 2 O The reaction components are then collected manually using a 200-ml droplet of a water-based solution of 3:1 H 2 O 2rHF. The chemistry of the collection droplet was determined from in-house collection efficiency and recovery tests and verified with atomic absorption ŽAA.. Our collection droplet chemistry is HF Ž1%.rH 2 O 2 Ž3%.rH 2 O Ž96%.. This chemistry compares favorably with the chemistry and collection efficiency results discussed by Neumann et al. w8x. Previous accounts have found copper to be particularly troublesome in terms of collection efficiency, but uptake of Cu can be improved by adding hydrogen peroxide to
G. Buhrer r Spectrochimica Acta Part B: Atomic Spectroscopy 54 (1999) 1399]1407
the dilute acid solution w3,8x. The collection of Cu has not been found to be a problem in our work. VPD wafer preparation was performed under class 100 cleanroom conditions in a locally manufactured chemical flow hood. The collected material suspended in the microdroplet was then dried onto the wafer surface using a heat lamp adjusted
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such that the droplet did not boil or spatter as it evaporated. To ensure that the surface oxide layer was completely collected, the wafers were monitored using a Censor ANS instrument. The instrument produced light point defect ŽLPD. maps of the wafer surface. From these maps ŽFig. 1., it could
Fig. 1. Censor ANS LPD maps of the wafer surface after VPD was used to determine the efficiency of microdroplet collection. These wafers were prepared in-house. Žd. VPD droplet was spread out during drying. This wafer was prepared in an automated VPD system available commercially. The darker areas indicate incomplete oxide removal.
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G. Buhrer r Spectrochimica Acta Part B: Atomic Spectroscopy 54 (1999) 1399]1407
Fig. 2. VPD drop angle scan. Fe curve of the residue shows angle independence with fluorescence intensity. Si curve shows the wafer substrate.
be easily determined whether the collection procedure was complete or the manual VPD procedure needed modification. 2.2. Apparatus The instrument used in this work was a model
TREX610T, which was manufactured by Technos W in Japan. The instrument is equipped with a rotating tungsten anode and a molybdenum tube X-ray source, an 80 mm2 SiŽLi. energy dispersive detector, a multichannel analyzer, and a Micron power station loaded with Technos software. To excite the wafer substrate under total reflection
Fig. 3. Ža. Potassium VPD standard. Plot is intensity Žcps. vs. concentration Žatomsrcm2 .. Regression statistic R 2 s 0.9947. Žb. Nickel VPD standard. Plot is intensity Žcps. vs. concentration Žatomsrcm2 .. Regression statistic R 2 s 0.9983.
G. Buhrer r Spectrochimica Acta Part B: Atomic Spectroscopy 54 (1999) 1399]1407
conditions, the tungsten Lb1 X-ray is most frequently employed. It has been shown that the VPD residue on the wafer will not participate in the interference phenomenon caused by total reflection and is therefore independent of the angle of incidence, if below the critical angle w8x. Angle scans of wafers prepared in our laboratory verified this angle independence ŽFig. 2.. Thus, an incident angle of 0.058 was selected and has been used consistently throughout this work. The TREX610T is located in a class 100 cleanroom in an area adjacent to the wafer preparation area. 2.3. Calibration Until recently, quantification of the TXRF measurement has been based on calibration curves generated from plated standards made by the GeMeTec Company of Germany. However, previous studies have suggested that calibration of the instrument for the VPD technique of wafer contamination measurements should be based on standards of similar microscopic nature, i.e. droplet residues of known concentrations of the elements of interest w8,9x. Several attempts were made in our laboratory to produce VPD standards that would exhibit statistically consistent measurements and suitable linear calibration curves based on the method of least squares. Wafers that finally met acceptable criterion were produced using NIST W-traceable standard solutions for ICP. Solutions containing a single element such as nickel were prepared and diluted to scaled concentrations close to the elemental coverage typically observed on sample wafers. In the case of nickel, dilutions were pre-
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pared in concentrations of ; 1, 2.5, 5, 7.5, 10 and 20 mgrl. The solution concentrations were verified with ICP-MS. Ten microdroplets of each solution were pipetted onto a pre-cleaned wafer in 200-ml volumes and allowed to dry under a heat lamp; they were closely observed for spattering. The x, y-coordinate location of the dried spots was determined with an in-house tool designed specifically for this purpose w13x. The sample areas were measured with TXRF, along with measurements of a blank solution dried in similar fashion for background determination. The measurement results were plotted as countsrs vs. units of concentration in atomsrcm2 of measured area Žcalibration curves in Fig. 3.. The units of concentration in Fig. 3 were converted from the stock solution concentrations in units of mgrl as: for a 200-mgrl drop volume: Ž200 = 10y6 l solution. Ž1000 g solutionr1 l. Ž10y9 g elementr1 g solution. Žmolrelement atomic mass. ŽAvogadro’s atomsrmol.. Once an acceptable calibration line was obtained, the values were placed into the Technos W software calibration file to measure samples of unknown contamination levels.
3. Results: verification r correlation To further verify our TXRF measurements, correlations were made between VPDrTXRF and VPDrICP-MS. Contaminated wafers that were prepared through the VPD process as usual were studied, but the collection droplet volume was split such that an aliquot of the sample could be
Table 1 Correlation between VPDrTXRF and VPDrICP-MS for various elements using a paired t-test to compare measurement delta significance a Element
K
Ca
Cr
Fe
Ni
Cu
Zn
Mean d S.D. t-Value Crit. value
2.6Eq 9 1.39Eq 10 0.59 2.26
7.8Eq 9 7.53Eq 9 3.28 2.26
2.2Eq 9 5.22Eq 9 1.30 2.26
2.8Eq 9 9.85Eq 9 0.91 2.26
9.3Eq 8 1.01Eq 9 2.89 2.26
5.4Eq 7 3.23Eq 8 0.53 2.26
y2.4Eq 7 6.31Eq 7 1.20 2.26
a
Mean d, mean of the sample differences; S.D., standard deviation; t-value derived from the t-test; crit. value, critical value.
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measured on the ICP-MS and the remainder could be dried on the wafer for measurement with TXRF. Ten deliberately contaminated wafers were processed in this manner and measured. TXRF element concentrations were based on calibration of the instrument using the VPD standard wafers described in the calibration section of this paper. Agreement between the two techniques for most elements was good, i.e. the two methods did not differ significantly at a confidence level of Ps 0.05. This was true for the elements K, Cr, Fe, Cu and Zn, where the critical value 2.26 was greater than the t-value. The results for Ca and Ni did differ significantly, as the critical value 2.26 was less than the t-value. Correlation was determined using a paired t-test, comparing the differences between sample measurements, referred to as the sample deltas w11x. Results appear in Table 1. From these correlation studies, it seemed that the TXRF and ICP-MS instruments correlated reasonably well. On further reflection, however, it
became obvious that the sample measurements fell very near the detection limits of both instruments. Thus, further study was undertaken to purposely contaminate wafers with higher concentrations of contaminants. A set of NIST W traceable standard solutions was prepared and mixed to create a multi-element solution. Three dilutions of this multi-element solution were made at concentrations of approximately 1, 10, and 50 mgrl. Ten 200-ml droplets of each solution were dried on separate clean wafers for TXRF. The remainder of each solution was then measured on the ICP-MS to verify the concentration of each solution. The three wafers with dried droplets were measured with TXRF for comparison to the results of the ICP-MS measurements. In general, the results of the TXRF measurements trended lower than the ICP-MS measurements for all of the mixed element solutions. Fig. 4 presents a comparison of the element coverage Žin atomsrcm2 . and the element type as a function of each technique. Approximately 200 pg of
Fig. 4. Ža. 90% confidence intervals ŽCI. for concentration vs. machine and element. This graph is for the ; 1 mgrl mixed element solution. Žb. 90% confidence intervals for concentration vs. machine and element. This graph is for the ; 1 mgrl mixed element solution.
G. Buhrer r Spectrochimica Acta Part B: Atomic Spectroscopy 54 (1999) 1399]1407
each element were deposited on the wafers via a 200-ml droplet of a mixed element solution. The concentration of the ; 1 mgrl solution was determined by ICP-MS. A confidence interval ŽCI. of 90% was used to create the error bar charts. TXRF element values were based on the calibration of the instrument using the VPD standard wafers described in Section 2.3. From the VPD drop standards produced, detection limits were calculated as DL s ŽC stdrpeak cps. = square root Žblank cpsrt . = 3, where: v
v
v
v
C std s known coverage Žin atomsrcm2 . of the element of interest in the VPD drop; peak cps s counts per second measured over the known concentration drop site; blank cps s counts per second measured over the blank VPD solution drop site; and time s measurement time in seconds.
A comparison of detection limits between one of the plated standards manufactured by GeMeTec of Germany and the two VPD drop standards prepared in-house, as calculated from the formula for our instrument, is presented in Table 2. Only two VPD drop standard wafers were available at the time of this writing.
4. Discussion To meet the demands of high-volume production, we have successfully employed VPDrTXRF as an analytical measure of wafer surface contamination and as a process or tool monitor over the past 3 years. However, there are several factors that affect the accuracy and sensitivity of this technique. Sample preparation is critical in VPD. The process must be conducted in a clean environment with strict attention to ensuring that all of the surface area oxide on the wafer is collected in the sample droplet, so that contamination can be accurately measured. Once collection is complete, the samples must be dried in such a way that a minimal amount of material is lost to evaporation. If the droplet spatters or boils, the contami-
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Table 2 Comparison of detection limits Žatomsrcm2 . between GeMeTec plated standards and VPD drop standards produced in-house
K Ni
Plated standards
VPD standards
Not available 1.0Eq 10
1.2Eq 10 7.2Eq 9
nation contained within the drop may be lost or spread out beyond the 80-mm2 field of vision window of the detector. It is also imperative that the samples are not unintentionally contaminated in handling, as the TXRF measurement is sensitive enough to detect most contaminants introduced by humans andror tooling. Other problems can arise in the TXRF measurement of the droplet residue. The residue is not a smooth, consistent surface and most likely causes some X-ray scattering w12x, as seen in Fig. 5. It has been found that the detector must be precisely located directly over the residue for maximum signal response. Mislocation by even tenths of a millimeter can reduce signal and lead to inconsistent measurements. Because the residue is not a smooth surface and may be crystalline, the internal planes within the residue structure could cause refraction and reflection of the primary beam. It is apparent from the SEM micrographs in Fig. 5 that the VPD residue tends to form islands of material. These islands can reach heights greater than 1 mm w14x. Under total reflection conditions, typical X-ray penetration ˚ w3,8]10x. depth in TXRF is of the order 30]50 A Some of these islands may shadow inner islands and, due to the geometry and penetration depth of the incoming X-ray beam, reduce fluorescence yields. This phenomenon, as well as other scattering effects, could contribute to the inconsistent readings observed as the sample coordinates are changed with respect to the fixed detector. These factors may also contribute to the general trend toward lower concentration readings as obtained by TXRF compared to ICP-MS, as observed in Fig. 4. The VPD method of collecting the wafer surface information into one site, while lending efficiency to the TXRF measurement, comes at
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Fig. 5. Ža]d. Scanning electron micrographs of VPD dried collection droplet. Formation of islands within the drop site may account for inconsistent TXRF signal intensity and lower readings as compared to ICP-MS.
the expense of lost information apropos to the homogeneity of the contamination across the wafer surface. It has been found, however, that with VPD, the gains in sensitivity of the TXRF measurement are essential and outweigh the loss of information with respect to the location of the contaminants across the wafer. Our VPDrTXRF data is reported as an average over the total wafer surface area. The multiple sites TXRF measurement, without VPD, has been found more useful for evaluating samples from ‘dirtier processes’ } from etchers, furnaces, and ovens } and for qualifying new tools in the fabrication area. In such cases, information regarding the location of contaminants on the wafer surface Žwafer mapping. can be useful in determining whether a specific part
of the equipment is suspected as a contamination source. Another disadvantage of the VPD method is that it compromises the sample. On silicon wafers, the native silicon dioxide layer is decomposed in the etch chamber and consequently stripped in the VPD collection droplet. While the wafers subjected to VPDrTXRF analysis cannot be used as product wafers, they can be reclaimed with wet chemical cleaning agents for use as test wafers or particle monitors.
5. Conclusions In the manufacturing environment, VPDr TXRF has been found to be more efficient than
G. Buhrer r Spectrochimica Acta Part B: Atomic Spectroscopy 54 (1999) 1399]1407
multiple measurement TXRF alone. With VPD, the TXRF measurement sensitivity is increased to the mid-10q9 atomsrcm2 range for most elements. Unfortunately, due to detector limitations, the Technos W TXRF instrument cannot detect aluminum and sodium contamination down to the levels required for high-yield integrated circuit manufacturing. By comparison, VPDrICP-MS is capable of aluminum and sodium detection as well as lower detection limits in the 10q8 atomsrcm2 , as reported by staff chemists. Correlation between the two techniques has been demonstrated near the detection limits of VPDrTXRF. Uncertainties, which may be better controlled in automated systems, remain in the VPD method. Most likely, the greatest source of error is attributable to the drying step where sample material may be lost or not have the structural integrity necessary for accurate measurement with TXRF. Over the years, with the advancements made in tooling and chemical processes, many of the cleaning steps in the device manufacturing process have been refined and now introduce orders of magnitude less contamination to product wafers than was once the case. To this end, TXRF has made a significant contribution to the measurement of metallic contaminants, thus leading to the elimination of their sources. However, with ever-increasing line density, smaller space critical dimensions, and shrinking die sizes, the detection limits of analytical techniques such as TXRF will be the determining factor of that technique’s applicability in the manufacturing environment of semiconductor silicon.
Acknowledgements
The author gratefully acknowledges the experiments and the VPDrICP-MS work of David Palsulich and the TXRF work of Vivian Miller, Tracee Milburn, and Tom Mendiola of Micron Technology, Inc. Their dedicated efforts con-
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tributed greatly to this paper. Also appreciated are the many thoughtful discussions with fellow colleague Mike Canavan, which provided direction in the writing of this paper. References w1x G.B. Other, Personal Communications with Micron Technology, Inc., Design Engineers, 1998. w2x M.R. Frost, W.L. Harrington, D.F. Downey, S.R. Walther, Surface metal contamination during ion implantation: comparison of measurements by SIMS, TXRF and VPD used in conjunction with GFAA and ICPMS, J. Vac. Sci. Technol. B 14 Ž1. Ž1996. 329]335. w3x A.C. Diebold, P. Maillot, M. Gordon, J. Baylis, J. Chacon, R. Witowski, H.F. Arlinghaus, J.A. Knapp, B.L. Doyle, Evaluation of surface analysis methods for characterization of trace metal surface contaminants found in silicon integrated circuit manufacturing, J. Vac. Sci. Technol. A 10 Ž4. Ž1992. 2945]2952. w4x R.S. Hockett, W. Katz, Comparison of wafer cleaning processes using TXRF, J. Electrochem. Soc. 136 Ž11. Ž1989. 3481]3486. w5x L.H. Hall, J.A. Sees, B.L. Schmidt, Characterization and application of VPD technique for trace metal analysis on silicon oxide surfaces, Surf. Interface Anal. 24 Ž1996. 511]516. w6x M. Weling, C. Gabriel, Oxide etch induced silicon damage evaluation using minority carrier lifetime, Electrochem. Soc. Proceedings 94-33, The Electrochemical Society, Pennington, NJ, 1994, pp. 143]150. w7x H. Ryssel, L. Frey, N. Streckfuss, R. Schork, F. Kroninger, T. Falter, Contamination control and ultrasensitive chemical analysis, Appl. Surf. Sci. 63 Ž1993. 80. w8x C. Neumann, P. Eichinger, Spectrochim. Acta 46B Ž10. Ž1991. 1372. w9x L. Fabry, S. Pahlke, L. Kotz, Accurate calibration of TXRF using microdroplet samples, Fresenius J. Anal. Chem. 354 Ž1996. 266]270. w10x A. Prange, H. Schwenke, Trace element analysis using total-reflection X-ray fluorescence spectrometry, in: C.S. Barrett ŽEd.., Advances in X-Ray Analysis, vol. 35, Plenum Press, New York, 1992, p. 918. w11x J.C. Miller, J.N. Miller, Statistics for Analytical Chemistry, 3rd ed., Prentice Hall, Chichester, West Sussex, England, 1993. w12x A.C. Diebold, Calibration issues for total reflection X-ray fluorescence analysis of surface metallic contamination on silicon, J. Vac. Sci. Technol. A 14 Ž3. Ž1996. 1923. w13x G. Buhrer, Z. Drussel, Tool design, patent pending. w14x E.J. Mori, L.W. Shive, Analysis of defects on the surface of bare unpatterned silicon wafers by SEM-EDS, Electrochem. Soc. Proceedings 92-21, The Electrochemical Society, Pennington, NJ, 1992, pp. 158]160.
Application of vapor phase decompositionrtotal reflection X-ray fluorescence in the silicon semiconductor manufacturing environment q Gayle Buhrer U Micron Technology, Inc., 8000 S. Federal Way, M.S. 632, P.O. Box 6, Boise, ID 83707-0006, USA Received 21 November 1998; accepted 28 May 1999
Abstract Total reflection X-ray fluorescence ŽTXRF., in combination with vapor phase decomposition ŽVPD., provides an efficient method for analyzing trace metal contaminants on silicon wafer surfaces. The progress made in applying these techniques to the analysis of silicon wafers in a wafer fabrication cleanroom environment is reported. Methods of standardization are presented, including the preparation and characterization of VPD standards. While the VPD wafer preparation process increases the sensitivity of the TXRF measurement by at least one order of magnitude, inherent uncertainties associated with the VPD technique itself are apparent. Correlation studies between VPDrTXRF and VPDrinductively coupled plasma mass spectrometry ŽICP-MS. are presented. Q 1999 Elsevier Science B.V. All rights reserved. Keywords: Semiconductor; Trace metal contamination; Vapor phase decomposition ŽVPD.; Total reflection X-ray fluorescence ŽTXRF.; Inductively coupled plasma mass spectrometry ŽICP-MS.
1. Introduction The stringent demands of contamination conq This paper was presented at the 7th Conference on ‘Total Reflection X-Ray Fluorescence Analysis and Related Methods’ ŽTXRF98. held in Austin, Texas, September 1998, and is published in the Special Issue of Spectrochimica Acta Part B, dedicated to that Conference. U Corresponding author. Tel.: q1-208-368-5280; fax: q1208-368-5165. E-mail address: [email protected] ŽG. Buhrer.
trol during the processing of high-performance, reliable, low-cost semiconductor devices are well known. Metallic impurities present in the surface oxide of the starting silicon wafers or introduced during processing can lead to poor electrical properties and adversely affect overall yields. The needs of the manufacturing team include resolving the contamination problem at hand while, at the same time, minimizing downtime to the fabrication line. Therefore, characterizing and understanding the source of a wide range of metallic impurities in an efficient and timely manner has
0584-8547r99r$ - see front matter Q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 5 8 4 - 8 5 4 7 Ž 9 9 . 0 0 0 8 4 - 1
1400
G. Buhrer r Spectrochimica Acta Part B: Atomic Spectroscopy 54 (1999) 1399]1407
been important to the analytical team as it works with the manufacturing team. Total reflection X-ray fluorescence ŽTXRF. has been an effective tool for multi-element trace analysis and wafer mapping over the past decade. In the production environment, where quick turnaround of sample analysis is essential, TXRF alone can be slow and cumbersome. Multiple measurement sites and longer measurement times are necessary for good statistical evaluation of the wafer using TXRF. Moreover, with ever-increasing line density, smaller space critical dimensions, and shrinking die sizes, the detection limits of analytical techniques such as TXRF may not be adequate w1x. Vapor phase decomposition ŽVPD. is a somewhat newer chemical preparation of silicon wafers that has enhanced the sensitivity and lent efficiency to the TXRF technique by permitting trace analysis of an entire wafer surface in a single-point measurement. The combination of these techniques, called VPDrTXRF, has been effectively employed in the resolution of metallic contamination problems encountered in wet and dry etch, diffusion, and chemical mechanical polish ŽCMP., as well as in many of the cleaning steps of the device manufacturing process. Analysis time is minimized, and a quick response can be delivered to the process engineer in an efficient report format. Over time, the VPDrTXRF measurement data collected can be used in statistical process control methods incorporating control charts to monitor the fabrication process. As demonstrated in much of the literature, well-characterized analytical techniques are correlated with other analysis methods to substantiate the technique in question w2]6,11x. Comparisons between TXRF and other techniques such as vapor phase decompositionrdroplet surface etchingrgraphite furnace atomic absorption spectroscopy ŽVPDrDSErGFAAS., heavy ion backscattering spectrometry ŽHIBS., Rutherford backscattering spectrometry ŽRBS., secondary ion mass spectrometry ŽSIMS., and inductively coupled plasma-mass spectrometry ŽICP-MS. have been made w2]6x. In this paper, correlations between VPDrTXRF and VPDrICP-MS were
made using NIST W calibration standards for ICP. The VPD method was better characterized with collection efficiency studies into the various chemistry scenarios of the collection droplet. Characterization of appropriate VPD drop standards was performed to further verify the quantification value of the VPDrTXRF method. However, it was not demonstrated that VPD could enhance TXRF detection limits to the 10q8 atomsrcm2 range, as discussed by Neumann et al. w7,8x.
2. Experimental methods 2.1. Reagents The VPD method involves dissolving the native oxide layer of a wafer surface and collecting the dissolved material into a collection droplet. Much work has been done to automate the VPD method of wafer preparation, and some units are commercially available w8x. For our purposes, VPD was accomplished by placing the wafers in specifically designed polypropylene etch chambers in which rails suspend the wafers a few inches above a small container of high purity HF. The accumulation of HF vapors within the closed chamber decomposes the native oxide to the state where the wafer surface becomes hydrophobic. The reaction of the surface layer oxide is as: SiO 2 q 6HFª H 2 SiF6 q 2H 2 O The reaction components are then collected manually using a 200-ml droplet of a water-based solution of 3:1 H 2 O 2rHF. The chemistry of the collection droplet was determined from in-house collection efficiency and recovery tests and verified with atomic absorption ŽAA.. Our collection droplet chemistry is HF Ž1%.rH 2 O 2 Ž3%.rH 2 O Ž96%.. This chemistry compares favorably with the chemistry and collection efficiency results discussed by Neumann et al. w8x. Previous accounts have found copper to be particularly troublesome in terms of collection efficiency, but uptake of Cu can be improved by adding hydrogen peroxide to
G. Buhrer r Spectrochimica Acta Part B: Atomic Spectroscopy 54 (1999) 1399]1407
the dilute acid solution w3,8x. The collection of Cu has not been found to be a problem in our work. VPD wafer preparation was performed under class 100 cleanroom conditions in a locally manufactured chemical flow hood. The collected material suspended in the microdroplet was then dried onto the wafer surface using a heat lamp adjusted
1401
such that the droplet did not boil or spatter as it evaporated. To ensure that the surface oxide layer was completely collected, the wafers were monitored using a Censor ANS instrument. The instrument produced light point defect ŽLPD. maps of the wafer surface. From these maps ŽFig. 1., it could
Fig. 1. Censor ANS LPD maps of the wafer surface after VPD was used to determine the efficiency of microdroplet collection. These wafers were prepared in-house. Žd. VPD droplet was spread out during drying. This wafer was prepared in an automated VPD system available commercially. The darker areas indicate incomplete oxide removal.
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Fig. 2. VPD drop angle scan. Fe curve of the residue shows angle independence with fluorescence intensity. Si curve shows the wafer substrate.
be easily determined whether the collection procedure was complete or the manual VPD procedure needed modification. 2.2. Apparatus The instrument used in this work was a model
TREX610T, which was manufactured by Technos W in Japan. The instrument is equipped with a rotating tungsten anode and a molybdenum tube X-ray source, an 80 mm2 SiŽLi. energy dispersive detector, a multichannel analyzer, and a Micron power station loaded with Technos software. To excite the wafer substrate under total reflection
Fig. 3. Ža. Potassium VPD standard. Plot is intensity Žcps. vs. concentration Žatomsrcm2 .. Regression statistic R 2 s 0.9947. Žb. Nickel VPD standard. Plot is intensity Žcps. vs. concentration Žatomsrcm2 .. Regression statistic R 2 s 0.9983.
G. Buhrer r Spectrochimica Acta Part B: Atomic Spectroscopy 54 (1999) 1399]1407
conditions, the tungsten Lb1 X-ray is most frequently employed. It has been shown that the VPD residue on the wafer will not participate in the interference phenomenon caused by total reflection and is therefore independent of the angle of incidence, if below the critical angle w8x. Angle scans of wafers prepared in our laboratory verified this angle independence ŽFig. 2.. Thus, an incident angle of 0.058 was selected and has been used consistently throughout this work. The TREX610T is located in a class 100 cleanroom in an area adjacent to the wafer preparation area. 2.3. Calibration Until recently, quantification of the TXRF measurement has been based on calibration curves generated from plated standards made by the GeMeTec Company of Germany. However, previous studies have suggested that calibration of the instrument for the VPD technique of wafer contamination measurements should be based on standards of similar microscopic nature, i.e. droplet residues of known concentrations of the elements of interest w8,9x. Several attempts were made in our laboratory to produce VPD standards that would exhibit statistically consistent measurements and suitable linear calibration curves based on the method of least squares. Wafers that finally met acceptable criterion were produced using NIST W-traceable standard solutions for ICP. Solutions containing a single element such as nickel were prepared and diluted to scaled concentrations close to the elemental coverage typically observed on sample wafers. In the case of nickel, dilutions were pre-
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pared in concentrations of ; 1, 2.5, 5, 7.5, 10 and 20 mgrl. The solution concentrations were verified with ICP-MS. Ten microdroplets of each solution were pipetted onto a pre-cleaned wafer in 200-ml volumes and allowed to dry under a heat lamp; they were closely observed for spattering. The x, y-coordinate location of the dried spots was determined with an in-house tool designed specifically for this purpose w13x. The sample areas were measured with TXRF, along with measurements of a blank solution dried in similar fashion for background determination. The measurement results were plotted as countsrs vs. units of concentration in atomsrcm2 of measured area Žcalibration curves in Fig. 3.. The units of concentration in Fig. 3 were converted from the stock solution concentrations in units of mgrl as: for a 200-mgrl drop volume: Ž200 = 10y6 l solution. Ž1000 g solutionr1 l. Ž10y9 g elementr1 g solution. Žmolrelement atomic mass. ŽAvogadro’s atomsrmol.. Once an acceptable calibration line was obtained, the values were placed into the Technos W software calibration file to measure samples of unknown contamination levels.
3. Results: verification r correlation To further verify our TXRF measurements, correlations were made between VPDrTXRF and VPDrICP-MS. Contaminated wafers that were prepared through the VPD process as usual were studied, but the collection droplet volume was split such that an aliquot of the sample could be
Table 1 Correlation between VPDrTXRF and VPDrICP-MS for various elements using a paired t-test to compare measurement delta significance a Element
K
Ca
Cr
Fe
Ni
Cu
Zn
Mean d S.D. t-Value Crit. value
2.6Eq 9 1.39Eq 10 0.59 2.26
7.8Eq 9 7.53Eq 9 3.28 2.26
2.2Eq 9 5.22Eq 9 1.30 2.26
2.8Eq 9 9.85Eq 9 0.91 2.26
9.3Eq 8 1.01Eq 9 2.89 2.26
5.4Eq 7 3.23Eq 8 0.53 2.26
y2.4Eq 7 6.31Eq 7 1.20 2.26
a
Mean d, mean of the sample differences; S.D., standard deviation; t-value derived from the t-test; crit. value, critical value.
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measured on the ICP-MS and the remainder could be dried on the wafer for measurement with TXRF. Ten deliberately contaminated wafers were processed in this manner and measured. TXRF element concentrations were based on calibration of the instrument using the VPD standard wafers described in the calibration section of this paper. Agreement between the two techniques for most elements was good, i.e. the two methods did not differ significantly at a confidence level of Ps 0.05. This was true for the elements K, Cr, Fe, Cu and Zn, where the critical value 2.26 was greater than the t-value. The results for Ca and Ni did differ significantly, as the critical value 2.26 was less than the t-value. Correlation was determined using a paired t-test, comparing the differences between sample measurements, referred to as the sample deltas w11x. Results appear in Table 1. From these correlation studies, it seemed that the TXRF and ICP-MS instruments correlated reasonably well. On further reflection, however, it
became obvious that the sample measurements fell very near the detection limits of both instruments. Thus, further study was undertaken to purposely contaminate wafers with higher concentrations of contaminants. A set of NIST W traceable standard solutions was prepared and mixed to create a multi-element solution. Three dilutions of this multi-element solution were made at concentrations of approximately 1, 10, and 50 mgrl. Ten 200-ml droplets of each solution were dried on separate clean wafers for TXRF. The remainder of each solution was then measured on the ICP-MS to verify the concentration of each solution. The three wafers with dried droplets were measured with TXRF for comparison to the results of the ICP-MS measurements. In general, the results of the TXRF measurements trended lower than the ICP-MS measurements for all of the mixed element solutions. Fig. 4 presents a comparison of the element coverage Žin atomsrcm2 . and the element type as a function of each technique. Approximately 200 pg of
Fig. 4. Ža. 90% confidence intervals ŽCI. for concentration vs. machine and element. This graph is for the ; 1 mgrl mixed element solution. Žb. 90% confidence intervals for concentration vs. machine and element. This graph is for the ; 1 mgrl mixed element solution.
G. Buhrer r Spectrochimica Acta Part B: Atomic Spectroscopy 54 (1999) 1399]1407
each element were deposited on the wafers via a 200-ml droplet of a mixed element solution. The concentration of the ; 1 mgrl solution was determined by ICP-MS. A confidence interval ŽCI. of 90% was used to create the error bar charts. TXRF element values were based on the calibration of the instrument using the VPD standard wafers described in Section 2.3. From the VPD drop standards produced, detection limits were calculated as DL s ŽC stdrpeak cps. = square root Žblank cpsrt . = 3, where: v
v
v
v
C std s known coverage Žin atomsrcm2 . of the element of interest in the VPD drop; peak cps s counts per second measured over the known concentration drop site; blank cps s counts per second measured over the blank VPD solution drop site; and time s measurement time in seconds.
A comparison of detection limits between one of the plated standards manufactured by GeMeTec of Germany and the two VPD drop standards prepared in-house, as calculated from the formula for our instrument, is presented in Table 2. Only two VPD drop standard wafers were available at the time of this writing.
4. Discussion To meet the demands of high-volume production, we have successfully employed VPDrTXRF as an analytical measure of wafer surface contamination and as a process or tool monitor over the past 3 years. However, there are several factors that affect the accuracy and sensitivity of this technique. Sample preparation is critical in VPD. The process must be conducted in a clean environment with strict attention to ensuring that all of the surface area oxide on the wafer is collected in the sample droplet, so that contamination can be accurately measured. Once collection is complete, the samples must be dried in such a way that a minimal amount of material is lost to evaporation. If the droplet spatters or boils, the contami-
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Table 2 Comparison of detection limits Žatomsrcm2 . between GeMeTec plated standards and VPD drop standards produced in-house
K Ni
Plated standards
VPD standards
Not available 1.0Eq 10
1.2Eq 10 7.2Eq 9
nation contained within the drop may be lost or spread out beyond the 80-mm2 field of vision window of the detector. It is also imperative that the samples are not unintentionally contaminated in handling, as the TXRF measurement is sensitive enough to detect most contaminants introduced by humans andror tooling. Other problems can arise in the TXRF measurement of the droplet residue. The residue is not a smooth, consistent surface and most likely causes some X-ray scattering w12x, as seen in Fig. 5. It has been found that the detector must be precisely located directly over the residue for maximum signal response. Mislocation by even tenths of a millimeter can reduce signal and lead to inconsistent measurements. Because the residue is not a smooth surface and may be crystalline, the internal planes within the residue structure could cause refraction and reflection of the primary beam. It is apparent from the SEM micrographs in Fig. 5 that the VPD residue tends to form islands of material. These islands can reach heights greater than 1 mm w14x. Under total reflection conditions, typical X-ray penetration ˚ w3,8]10x. depth in TXRF is of the order 30]50 A Some of these islands may shadow inner islands and, due to the geometry and penetration depth of the incoming X-ray beam, reduce fluorescence yields. This phenomenon, as well as other scattering effects, could contribute to the inconsistent readings observed as the sample coordinates are changed with respect to the fixed detector. These factors may also contribute to the general trend toward lower concentration readings as obtained by TXRF compared to ICP-MS, as observed in Fig. 4. The VPD method of collecting the wafer surface information into one site, while lending efficiency to the TXRF measurement, comes at
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Fig. 5. Ža]d. Scanning electron micrographs of VPD dried collection droplet. Formation of islands within the drop site may account for inconsistent TXRF signal intensity and lower readings as compared to ICP-MS.
the expense of lost information apropos to the homogeneity of the contamination across the wafer surface. It has been found, however, that with VPD, the gains in sensitivity of the TXRF measurement are essential and outweigh the loss of information with respect to the location of the contaminants across the wafer. Our VPDrTXRF data is reported as an average over the total wafer surface area. The multiple sites TXRF measurement, without VPD, has been found more useful for evaluating samples from ‘dirtier processes’ } from etchers, furnaces, and ovens } and for qualifying new tools in the fabrication area. In such cases, information regarding the location of contaminants on the wafer surface Žwafer mapping. can be useful in determining whether a specific part
of the equipment is suspected as a contamination source. Another disadvantage of the VPD method is that it compromises the sample. On silicon wafers, the native silicon dioxide layer is decomposed in the etch chamber and consequently stripped in the VPD collection droplet. While the wafers subjected to VPDrTXRF analysis cannot be used as product wafers, they can be reclaimed with wet chemical cleaning agents for use as test wafers or particle monitors.
5. Conclusions In the manufacturing environment, VPDr TXRF has been found to be more efficient than
G. Buhrer r Spectrochimica Acta Part B: Atomic Spectroscopy 54 (1999) 1399]1407
multiple measurement TXRF alone. With VPD, the TXRF measurement sensitivity is increased to the mid-10q9 atomsrcm2 range for most elements. Unfortunately, due to detector limitations, the Technos W TXRF instrument cannot detect aluminum and sodium contamination down to the levels required for high-yield integrated circuit manufacturing. By comparison, VPDrICP-MS is capable of aluminum and sodium detection as well as lower detection limits in the 10q8 atomsrcm2 , as reported by staff chemists. Correlation between the two techniques has been demonstrated near the detection limits of VPDrTXRF. Uncertainties, which may be better controlled in automated systems, remain in the VPD method. Most likely, the greatest source of error is attributable to the drying step where sample material may be lost or not have the structural integrity necessary for accurate measurement with TXRF. Over the years, with the advancements made in tooling and chemical processes, many of the cleaning steps in the device manufacturing process have been refined and now introduce orders of magnitude less contamination to product wafers than was once the case. To this end, TXRF has made a significant contribution to the measurement of metallic contaminants, thus leading to the elimination of their sources. However, with ever-increasing line density, smaller space critical dimensions, and shrinking die sizes, the detection limits of analytical techniques such as TXRF will be the determining factor of that technique’s applicability in the manufacturing environment of semiconductor silicon.
Acknowledgements
The author gratefully acknowledges the experiments and the VPDrICP-MS work of David Palsulich and the TXRF work of Vivian Miller, Tracee Milburn, and Tom Mendiola of Micron Technology, Inc. Their dedicated efforts con-
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tributed greatly to this paper. Also appreciated are the many thoughtful discussions with fellow colleague Mike Canavan, which provided direction in the writing of this paper. References w1x G.B. Other, Personal Communications with Micron Technology, Inc., Design Engineers, 1998. w2x M.R. Frost, W.L. Harrington, D.F. Downey, S.R. Walther, Surface metal contamination during ion implantation: comparison of measurements by SIMS, TXRF and VPD used in conjunction with GFAA and ICPMS, J. Vac. Sci. Technol. B 14 Ž1. Ž1996. 329]335. w3x A.C. Diebold, P. Maillot, M. Gordon, J. Baylis, J. Chacon, R. Witowski, H.F. Arlinghaus, J.A. Knapp, B.L. Doyle, Evaluation of surface analysis methods for characterization of trace metal surface contaminants found in silicon integrated circuit manufacturing, J. Vac. Sci. Technol. A 10 Ž4. Ž1992. 2945]2952. w4x R.S. Hockett, W. Katz, Comparison of wafer cleaning processes using TXRF, J. Electrochem. Soc. 136 Ž11. Ž1989. 3481]3486. w5x L.H. Hall, J.A. Sees, B.L. Schmidt, Characterization and application of VPD technique for trace metal analysis on silicon oxide surfaces, Surf. Interface Anal. 24 Ž1996. 511]516. w6x M. Weling, C. Gabriel, Oxide etch induced silicon damage evaluation using minority carrier lifetime, Electrochem. Soc. Proceedings 94-33, The Electrochemical Society, Pennington, NJ, 1994, pp. 143]150. w7x H. Ryssel, L. Frey, N. Streckfuss, R. Schork, F. Kroninger, T. Falter, Contamination control and ultrasensitive chemical analysis, Appl. Surf. Sci. 63 Ž1993. 80. w8x C. Neumann, P. Eichinger, Spectrochim. Acta 46B Ž10. Ž1991. 1372. w9x L. Fabry, S. Pahlke, L. Kotz, Accurate calibration of TXRF using microdroplet samples, Fresenius J. Anal. Chem. 354 Ž1996. 266]270. w10x A. Prange, H. Schwenke, Trace element analysis using total-reflection X-ray fluorescence spectrometry, in: C.S. Barrett ŽEd.., Advances in X-Ray Analysis, vol. 35, Plenum Press, New York, 1992, p. 918. w11x J.C. Miller, J.N. Miller, Statistics for Analytical Chemistry, 3rd ed., Prentice Hall, Chichester, West Sussex, England, 1993. w12x A.C. Diebold, Calibration issues for total reflection X-ray fluorescence analysis of surface metallic contamination on silicon, J. Vac. Sci. Technol. A 14 Ž3. Ž1996. 1923. w13x G. Buhrer, Z. Drussel, Tool design, patent pending. w14x E.J. Mori, L.W. Shive, Analysis of defects on the surface of bare unpatterned silicon wafers by SEM-EDS, Electrochem. Soc. Proceedings 92-21, The Electrochemical Society, Pennington, NJ, 1992, pp. 158]160.