Anal. Chem. 1902, 64, 2942-2944
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Trace Determination of Uranium, Thorium, Calcium, and Other Heavy Metals in High-Purity Refractory Metal Silicides, Niobium, and Silicon Dioxide with Isotope Dilution Mass Spectrometry Peter Herzner and Klaus G. Heumann’ Institut fiir Anorganische Chemie der Uniuersitiit Regensburg, Uniuersitiitsstrasse 31, 0-8400Regensburg, Germany
A methodfor the determination of trace impuritles (U, Th, Ca, Fe, Cr, Ni, Cu, and Cd) in silicides of refractory metals, In niobium, and In silicon dioxide with isotope dliutlon mass spectrometry (IDMS) has been developed. Thls method enables uranium and thorium analyses down to the lowest picogram per gram level with hlgh precldon and accuracy, which is especlaily Important for the characterlration of microelectronic devices. The other elements can be determined down to the low nanogramper gram levelor-dependlng on the blank vaiues--in some cases less. Selective chromatographic, extractive, and electrolytic procedures for the tracematrix-separation were combined with positive thermal ionlzatbnmass spectrometry. Different samples of high-purtty materials (metal and silicide powders, compact dllclde, and silicon dioxlde powder) for advanced technoiogles were analyzed.
INTRODUCTION The properties of high-technology materials strongly depend on the composition and the purity of these substances. Whereas some properties are achieved by the addition of further elements in the percent range, modern technological applicationsrequire materialsof highest purity and, therefore, information about the exact composition at extremely low le~els.l-~ Refractory metals of high purity as well as their silicides have been increasingly used for gate electrodes, interconnectionsand diffusion barriers in integrated circuits as a result of recent developments in higher density VLSI (very large scale integration) systems. The impurities which affect the performance and the reliability of integrated circuits are the radioactive elements uranium and thorium and their decay products in particular, but also the alkaline and the alkaline earth metals, the heavy metals, and elements, which easily form oxides.4 Therefore,the demanded concentrations of these “problem”elements are placed in the low nanogram per gram range or even below.’ When Si02 is applied as filler material for integrated circuits, the same trace metals must be taken into ~onsideration.5~~ Besides this application, pure silicon dioxide is widely used in optics and telecommunications. Properties like color, transmission, and hardness are
* To whom correspondence should be addressed. (1)Ortner, H. M.;B l a o m , W.; Friedbacher, G.; Grasserbauer, M.; Krivan, V.; Virag, A.; Wilhartitz, P.; Wiinsch, G. Kontakte (Darmstadt) 1988,38-52. (2)Ortner, H. M.;Blijdorn, W.; Friedbacher, G.; Grasserbauer, M.; Krivan, V.; Virag, A.; Wilhartitz, P.; Wiinsch, G. Mikrochim. Acta 1987, I, 233-260. (3)Ortner, H. M.;Wilhartitz, P.; Grasserbauer, M. Key Eng. Mater. 1989,29-31, 21-46. (4)Sawada, S. In Proceedings of the 12th International Plansee Seminar; Bildstein, H., Ortner, H. M., Eds.; Verlagsanstalt Tyrolia: Innsbruck, 1990, Vol. 4,pp 201-221. (5)Baumann, H.; Pavel, J. Mikrochim. Acta 1989,3,413-422. Siemens (6)Aulich, H. A.; Eisenrith, K.-H.; Urbach, H.-P. Forsch.-Entwicklungsber. 1988,17, 154-158.
detrimentally influenced by traces of heavy metals (e.g. Fe, Cr, Ni, Cu) and alkaline oxides.’ Moreover, high-purity Si02 powders are primary materials for ceramics like SiaNd.’ Important fields of application for ultrapure niobium are superconductingmaterials*or the use of this metal in fission reactors.9 It is not easy to exactly determine nanogram per gram levels or even lower concentrations of uranium, thorium, and other heavy metals in the mentioned matrices with the analytical methods available today. Due to the necessary sensitivity, different mass spectrometric methods like secondary ion mass spectrometry (SIMS), glow discharge maas spectrometry (GDMS),and inductivelycoupled plasma mass spectrometry (ICPMS) are often the only techniques besides radiochemical neutron activation analysis (RNAA)lOwhich are able to detect such low concentration levels. However, the agreement of results between these methods is not necessarily acceptable for all important impurities aa some interlaboratory comparisons with refractory metals have shown in the past.*,” Recently, we therefore developed an alternative method of isotope dilution mass spectrometry (IDMS) to determine chlorine, uranium, thorium, and other metal traces in high-purity molybdenum and tungsten samples.12J3 IDMS is a reliable method which usuallyresults in accurate and precise data even at very low concentrations,14J5 required for further developments in high-technology materials. Another advantage of IDMS is the small influence of matrix interferences when thermal ionization mass spectrometry is used and the fact that no quantitative isolation of the elements to be determined is necessary after the isotope dilution step has taken place. Because refractory metal silicides are used more and more instead of the corresponding metals in VLSI systems, we have now also developed an IDMS method for these important materials. The necessary use of hydrofluoric acid for the sample decomposition made it possible to also analyze the technologically important materials of pure silicon dioxide and niobium with the identical analytical procedure. The silicide work was done within the frame of the ”COST 503/II” project of the European Communities. The further aims of (7)Broekaert, J. A. C.; Grade, T.; Jenett, H.; Talg, G.; Tsch8pe1, P. Freseniu’ 2.Anal. Chem. 1989,332,825-838. (8) Claassen, R. S.; Girifalco, L. A. Spektrum Wise. 1986,76-84. (9)Keller, C. Radiochemie, 1. Auflage, Verlag Diesterweg/Salle: Frankfurt, 1975;p 150. (10)Theimer, K.-H.; Krivan, V. Anal. Chem. 1990,62,2722-2727. (11)Graaserbauer, M.;Charalambous, P. M.;Jakubowski, N.; Stuewer, D.; Vieth, W.; Beske, H. E.; Virag, A,; Friedbacher, G. Mikrochim. Acta 1987,I , 291-319. (12)Herzner, P.; Heumann, K. G. Mikrochim. Acta 1992,106, 127135. (13)Heumann, K. G.; Herzner, P.; GPbler, H.-E. InProceeding8 of the 12th Znternutionul Plansee Seminar; Bildstein, H., Ortner, H. M., Eds.; Verlagaanstalt Tyrolia: Innsbruck, 1990,Vol. 4,pp 191-199. (14)Heumann, K. G.In Inorganic mass spectrometry; Ad”, F., Gijbels, R., van Grieken, R., Eds.; Wiley: New York, 1988; pp 301-376. (15)Fassett, J. D.;Paulsen, P. J. Anal. Chem. 1989,61,643A-650A. 0 1902 Amerlcan Chemical Soclety
ANALYTICAL CHEMISTRY, VOL. 64, NO. 23, DECEMBER 1, 1992
Table I. Isotope Ratios Measured from Isotope-Diluted Samples and Characteristic Data of the Used Spike Solutions spike solution isotope enrichment, element concn, element ratio isotope % atomdg of solution U 235U/238U 23SU 96.94 7.83 x 1014 Th 2Th/232Th 23Fh 98.68 5.62 x 1014 5.11 X 10l6 %a 94.71 Ca 40Ca/44Ca 95.24 9.81 X 1016 Fe 5eFe/51Fe 5'Fe Cr 52Cr/Y!r 53Cr 97.89 7.63 X 10l6 Ni mNi/62Ni 62Ni 98.25 1.83 X 10l6 99.64 2.94 X 10l6 cu ~3CU/WU 65Cu 3.12 x 1015 89.29 Cd l14Cd/116Cd 116Cd this series of material research projects are described in detail elsewhere.16J7
EXPERIMENTAL SECTION Chemicals. HN03 pa (pa = pro analysis)and HClpa (Merck) are purified by subboiling distillation in a quartz still. HF suprapure is distilled in a still made of PTFE. NH3 suprapure as well as diethyl ether pa are used without further treatments. Deionized water is additionally purified by double-distillation in a quartz still. The cation-exchanger resin Dowex 50 W-X8 (practical grade, Serva Biochemicals) is precleaned by repeated shaking with diluted HN03 and HC1. Isotope Dilution Technique. The positive thermal ionization techniques for mass spectrometric measurements used within this work are described elsewhere.12 The isotope ratios measured in the isotope-diluted samples and the characteristic data of the spike solutions used in this work are listed in Table I. The principles of IDMS are described in detail elsewhere.14 Sample Treatment. The sample treatment is similar to that described in a previous paper12for the HNOs/HF decomposition process of pure molybdenum and tungsten samples and is carried out in clean benches. Up to 3 g of a sample is weighed into a PE bottle cleaned by repeated shakingwithdiluted HN03and water. Samples (0.2-2.0 g) of each of the different spike solutions (see Table I) are then added. The decomposition of the samples is performed by 24 mL of a 1:l mixture of concentrated.HN03and concentrated HF. After decomposition the remaining sample solution is evaporated to dryness, dissolved in 6 mL of concentrated HC1, evaporated to dryness twice after another addition of 6 mL of concentratedHC1, and dissolved in 20 mL of HzOz (30% ). In the case of tantalum silicide and niobium as matrix, possible precipitations of the corresponding oxides are removed by using a centrifuge and subsequentdecanting. Afterward, HCl is added until a final concentration of 0.05 mol/L is reached. This solution is transferred on the top of a cation-exchanger column filled with Dowex 50 W-X8 for the subsequent trace-matrix separation. The trace elements are retained at the resin material, whereas the matrix refractory element passes the column. In the case of a silicon dioxide matrix the silicon is still evaporated as SiF4 during the decompositionstep with HF. The different separation steps for the trace elements by cation-exchange chromatography (Ca,Th,U),electrolyticdeposition(Cu,Cd,Ni, Cr),andextraction (Fe) are described in detail elsewhere.12 Blank Determination. Blank determinations are carried out parallel to all sample treatments by running the whole procedure without sample material. To avoid memory effects from the ion-exchanger resin, especially with respect to Th, the resin is removedfrom the columnafter each run. The blankcontributions are listed in Table 11. The mean blank values with standard deviations are always calculated from four independent determinations. For all analyzed elements the detection limit is restricted by the variation of the blank and not by the sensitivity or reproducibility of the mass spectrometric method. The determined blank values are relatively constant with standard (16) Ortner, H.M.; Wilhartitz, P.Fresenius' J. Anal. Chem. 1990,337,
686-700. (17)Gibbons, T.B.Ado. Mater. 1990,2,217-221.
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Table 11. Blank Values Received for the Described Sample Treatment for IDMS element blank value, np; element blank value, na U 0.056 f 0.001 Cr 23f5 Th 0.019 f 0.009 Ni 8.7 f 1.4 Ca 160 f 21 cu 9.4 f 0.8 Fe 201 f 5 Cd 4.83 f 0.08 Table 111. Trace Metal Concentrations Determined in Different Silicide Powders concn Mo silicide W silicide Ta silicide 34.00 f 0.30 1.42 f 0.17 0.93 f 0.15 7.51i 0.23 16.53 f 0.78 0.35 f 0.03 0.17 f 0.08
1.80 f 0.03 2.99 f 0.04 0.57 f 0.03 4.12 f 0.04 2.20 f 0.02 0.186f 0.004
