 |
 |
 |
 |
|
Comments? Suggestions? Send us your feedback.
MicroMagazine.com |
ANALYTICAL TECHNOLOGIESUsing two-dimensional ion chromatography to measure contaminants in ultrapure chemicals(First in a series)Jian-Ge Chen and ManLi Wu, Olin Research Center Because the purity of process chemicals plays an important role in device reliability, the continuing increase in the scale of integration for semiconductor devices has led to the need for monitoring trace levels of ionic contaminants.1 Parts-per-billion-level cationic contaminants in acids can be analyzed by inductively coupled plasma mass spectrometry;2 however, quantitating anionic contaminants at that level of sensitivity has been an analytical challenge because of the high concentration of the anion matrix. To eliminate matrix interference, concentrated acid samples are often diluted, digested, or pretreated with off-line cartridges. Unfortunately, dilution reduces detection sensitivity, and digestion or the use of cartridges often introduces contaminants. Ion chromatographs (ICs) have been used successfully to measure ionic contaminants in a wide range of matrices.3 Other researchers have reported detection limits for quantitation of common anions in concentrated hydrofluoric acid (HF) by direct injection and matrix elimination IC methods of 10 ppm and 250 ppb, respectively.4 This article describes the development of a sensitive and reliable IC procedure for the analysis of chloride, sulfate, phosphate, and nitrate in concentrated HF. The procedure employs a two-dimensional ion chromatographic system; one IC is used for preseparating anionic contaminants from their matrix prior to the analytical separation by the other IC. During the development project, several experimental parameters were optimized to achieve the desired level of sensitivity. System decontamination, and the choice of HF concentration for sample analysis, fractionation interval from the first IC, and quantitation technique are discussed here. With the optimized conditions, we were able to achieve quantitation limits of 2 ppb for chloride and sulfate, 5 ppb for nitrate, and 10 ppb for phosphate in 24.5% HF injected samples. The results of an analysis of several commercial HF solutions are also presented. Two-Dimensional Ion Chromatography The instrument setup for the new procedure is shown in Figure 1. The approach is to reduce the F— ion matrix by introducing the sample directly into the first IC, where the anions of interest are separated from HF via a preseparation. The effluent containing the anions of interest and some F— is then fractionated using an automated switching valve and introduced into the second IC for analysis. Because the preseparation significantly reduces the F— concentration, the anion analysis proceeds with minimum matrix interference. The benefit of using two ICs can be seen in Figure 2. The top chromatogram, from the first IC, shows the interference caused by the F— matrix, whereas in the bottom chromatogram, from the second IC, the anions of interest are clearly detected.
Figure 1: Schematic of the two-dimensional ion chromatography system.
Figure 2: Anionic separations from the two ionic chromatographs that make up the two-dimensional system. The elimination of the anion matrix by the first IC increases the sensitivity achievable by the second. Minimizing System Contamination Trace analysis of chloride is particularly challenging because this contaminant is commonly present in the air and on the glassware used for sample preparation. During our initial experiments, chloride was observed even after the glassware had undergone a thorough cleaning and rinsing with ultrapure Mill-Q water. We then determined that HF caused chloride to leach out of the sample containers, flasks, and vials. Subsequently, when samples were prepared in plasticware that was pretreated prior to use, test results showed minimum chloride contamination from the labware. Selecting the Optimal HF Concentration Injecting concentrated HF (49%) directly into the IC system can minimize the need for sample handling and improve sensitivity; however, it also can overload the system, thereby decreasing the resolution of other anions. On the other hand, diluted samples (with lower percentages of HF) might not give the desired analytical sensitivity. Figure 3 presents overlaid chromatograms obtained from the first IC, showing how the F— matrix can interfere with the detection of the anions of interest.
Figure 3: Overlaid chromatograms from the first IC, indicating the influence of HF concentration on anion analysis. In order to determine the optimal HF concentration for analysis, we injected samples of 49, 24.5, 12.25, and 6.13% HF. It was found that prolonged injections of 49% HF solutions caused significant contaminant bleeding from the IC column and the injection vials as well as severe damage to the column. Fortunately, the detection limits of anionic contaminants were found to be adequate with lower concentrations of HF. As indicated in Table I, the 24.5% HF samples provided the best detection limits and therefore that concentration level was chosen for further study.
Table I: Detection limits of the anions of interest in various dilutions of 49% HF. Optimizing the Fractionation Interval The reliability and sensitivity of the two-dimensional IC anion analysis method is also dependent on fractionation time—i.e., the total time that the effluent from the first IC is fractionated into the second IC. Fractionation times ranging from 1.5 to 5 minutes were studied using a mixture of anion standards. Figure 4 shows the measured peak areas of the four anions of interest in 24.5% HF samples with various fractionation times; the arrow is pointing at the time that resulted in the most sensitive anion detection with the least interference from F—. When the fractionation extended beyond 3.5 minutes, increases in sensitivity were offset by increases in F—.
Figure 4: Fractionation time versus peak areas for the anions of interest and F- in 24.5% HF samples. Choosing the Best Quantitation Technique Using the two-dimensional IC system, anions can be quantified by the external calibration method (ECM) or the standard addition method (SAM). Normally used when the matrix is relatively clean, ECM offers a shorter analysis time than SAM. However, when analyses were run on five different HF samples using the two methods we found that ECM underestimated the amount of anions present. The test results, shown in Table II, were supported by the poor spike recoveries obtained using ECM, which were less than 50% in most cases (also shown in Table II). SAM, on the other hand, yielded good spike recoveries on all anions. At the 20-ppb level for chloride and sulfate and 10-ppb level for phosphate and nitrate the spike recoveries were 106 ± 9.3% (relative standard deviation), 107 ± 11.2%, 90.2 ± 9.6%, and 121 ± 1.3%, respectively.
Table II: Comparison of the standard addition method (SAM) and external calibration method (ECM) for the analysis of various anions in concentrated HF. Errors given for SAM are one standard deviation; spike recovery (20-ppb) with ECM is given in percent in parenthesis. Analysis of HF Samples Once the optimized experimental conditions were defined, we analyzed five 49% HF samples from three different suppliers using those parameters. Two of the HF solutions were manufactured by an overseas producer (supplier A) and three by U.S. producers. Figure 5 shows typical calibration curves obtained for the anions of interest, and Table III lists the quantitative results for all five samples. These results are the average of three analyses. In all cases, the average relative standard deviations of the analyses were below 10%. As the table indicates, anion concentrations in the HF solutions from different suppliers varied significantly. In particular, the HF from supplier B contained much higher concentrations of chloride and nitrate than the other samples.
Figure 5: Typical calibration curves for the anions of interest quantified by the standard addition method. Error bars are one standard deviation.
Table III: Anionic contaminants detected in samples of concentrated HF from three suppliers. Conclusion A two-chromatograph system reliably analyzes trace anion contaminants such as chloride, sulfate, phosphate, and nitrate in ultrapure HF. The technique allows for the direct injection of concentrated HF (24.5%) with minimum sample handling, which maximizes the sensitivity and reproducibility of the anion analysis. The quantitation limits are 2 ppb for chloride and sulfate, 5 ppb for nitrate, and 10 ppb for phosphate. The procedure should be useful in comparing products from various suppliers to ensure that semiconductor device reliability is not adversely affected by ionic impurities in process chemicals. Acknowledgments
Part of this paper was presented at the UCPSS '96 Symposium held in Antwerp, Belgium, September 23—25, 1996. The authors wish to acknowledge the contributions of their colleagues at Olin's central analytical department: Pat Turley and Sonia Oberson provided helpful suggestions and George Osae conducted tests of the IC separation columns. Olin References 1. Schafer H, and Budde KJ, "Study of Contamination Mechanism at Silicon Surface during Wet Chemical Processes," presented at the International Ion Chromatography Symposium, Baltimore, MD, September 1993. 2. Volosin MT, "Quality Control of High Purity Process Chemicals by ICP/MS," Spectroscopy, 7(4): 44—47, 1992. 3. Heberling S, Kaiser E, and Riviello J, "Advances in the Application of Ion Chromatography for the Determination of Ultratrace Ions in Semiconductor Pure Water and Chemicals," in Proceedings of the 14th Semiconductor Pure Water and Chemicals Conference, Santa Clara, CA, Balazs Analytical Laboratory, pp 207—36, 1995. 4. Siriraks A, Pohl CA, and Toofan M, "Determination of Trace Anions in Concentrated Acids by Means of a Moderate Capacity Anion-Exchange Column," Journal of Chromatography, 602:89—95, 1992. Jian-Ge Chen, PhD, is a senior research chemist in the central analytical department of Olin Corp., Cheshire, CT. He specializes in trace analysis using microseparations, including multidimensional chromatography. He received his BS in chemistry (1990) from the State University of New York at Stony Brook and his PhD in analytical chemistry (1995) from the University of Pittsburgh. He has authored or coauthored more than 10 research papers, a book chapter, and a review article. ManLi Wu, PhD, is senior research associate in Olin's central analytical department. Her field of specialization is liquid chromatographic analysis of polyurethanes, biocides, and industrial chemicals. Recently, she has also been involved with GLP studies of biocides in product chemistry and environmental fate areas. She received her BS from Tamkang University and her MS from Tsing Hwa University (both in Taiwan) and her PhD in analytical chemistry from the University of Connecticut. (Wu can be reached at 203/271-4298.)
MicroHome |
Search | Current Issue | MicroArchives Questions/comments about MICRO Magazine? E-mail us at cheynman@gmail.com. © 2007 Tom Cheyney |
||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
