Anal. Chem. 2001, 73, 1106-1111
Dual-Vessel Integrated Microwave Sample Decomposition and Digest Evaporation for Trace Element Analysis of Silicon Material by ICPMS: Design and Application Ye Han,† H. M. Kingston,*,† Robert C. Richter,† and Camillo Pirola‡
Department of Chemistry and Biochemistry, Duquesne University, Pittsburgh, Pennsylvania 15282, and Milestone s.r.l, Sorisole (BG), Italy
A dual-vessel apparatus using closed-vessel microwave sample preparation is designed. A protocol that includes in situ reagent purification, sample decomposition, and digest evaporation has been developed. The essential chemistry mechanisms will be discussed in detail. Application of the method for analysis of trace amounts of chromium, nickel, copper, and zinc in polycrystalline silicon using inductively coupled plasma mass spectrometry (ICPMS) is demonstrated. Over the past five years, computer chips and electronic devices have become smaller and faster. The major limiting factor for the size of a computer chip and its speed is the ultratrace (pg/g level) metal contamination of the silicon used in manufacturing. These ultratrace metal impurities lead to higher dark currents that influence the chip’s ability to carry current.1,2 Therefore, such ultratrace metal impurities must be determined and controlled. The most common methods for determining impurities in surface layers of semiconductor materials are X-ray fluorescence (XRF),3-7 neutron activation analysis (NAA),8-10 or photoconductivity spectrometry.11 The impurities in bulk semiconductor materials can be determined by spectroscopic methods, such as atomic emission spectrometry (AES), atomic absorption spec* Fax: 412-396-5359. E-mail:
[email protected]. † Duquesne University. ‡ Milestone s.r.l. (1) Close, K. J.; Yarwood, J. An Introduction to Semiconductors; Heinemann: London, 1971. (2) Ghandi, B. K. VLSI Fabrication Principles; Wiley: New York, 1997. (3) Berneike, W.; Knoth, J.; Schwenke, H.; Weisbrod, U. Fresenius’ J. Anal. Chem. 1989, 333, 524-526. (4) Comin, F.; Navizet, M.; Mangiagalli, P.; Apostolo, G. Nucl. Instrum. Methods Phys. Res., Sect B. 1999, B150, 538-542. (5) Fan, Q. M.; Liu, Y. W.; Li, D. L.; Wei, C. L. Fresenius’. J. Anal. Chem. 1993, 345, 518-520. (6) Liu, Y. W.; Fan, Q. M.; Wu, Y. G.; Wei, C. L.; Xiao, H. Guangpuxue Yu Guangpu Fenxi 1997, 17, 96-99. (7) Nishihagi, K.; Kawabata, A.; Taniguchi, T.; Ikeda, S. Proc. -Electrochem. Soc. 1990, 90, 243-250. (8) Huang, R. M.; Huang, R. S. J. Electrochem. Soc. 1986, 133, 2605-2608. (9) Ortega, C.; Siejka, J.; Vizkelethy, G. Nucl. Instrum. Methods Phys. Res., Sect. B. 1990, 626, B45622-626. (10) Takeuchi, T.; Nakano, Y.; Fukuda, T.; Hirai, I.; Osawa, A.; Toyokura, N. J. Radioanal. Nucl. Chem. 1997, 216, 165-169. (11) Andreev, B. A.; Golubev, V. G.; Kropotov, G. I.; Maksimov, G. A.; Shmagin, V. B. J. Anal. Chem. 1998, 53, 1046-1052.
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trometry (AAS) and mass spectrometry (MS), after decomposing bulk samples by the mixture of hydrofluoric acid (HF) and nitric acid (HNO3)12 or their vapors.13,14 In the acid decomposition methods, sulfuric acid (H2SO4)15 or phosphoric acid (H3PO4)14 is often heated with final digests as post decomposition procedures to evaporate silicon-related matrix components such as hydrofluosilicic acid (H2SiF6). However, the use of H2SO4 or H3PO4 introduces new problems and interferences for ICPMS analysis. These acids do not completely decompose in the plasma and adhere to the interface components and ion lenses, resulting in signal instability. Additionally, their higher impurity levels, when compared to nitric acid, result in an increase in the analytical blank. If the magnitude and uncertainty of the analytical blank is not controlled, the analysis of pg/g level impurities is impossible. Closed vessel microwave sample preparation has been acknowledged as one of the best solutions for clean chemistry applications. The use of closed-vessel microwave sample preparation techniques minimizes the analytical blank by minimizing the amount of reagents used and controlling the decomposition environment, as well as through augmentation of the operator’s skills.16,17 Recently, microwave-assisted evaporation has been shown to be an alternate postdigestion process that eliminates matrix effects associated with sample decomposition.18 In this paper, we present a novel closed-vessel microwave sample decomposition method for semiconductor-grade silicon. This method uses a dual-vessel design that minimizes contamination while eliminating the siliconrelated matrix interference in the final digest. The performance of this design is demonstrated by the determination of trace amounts of chromium, nickel, copper, and zinc in polycrystalline silicon using ICPMS. (12) Vasilevskaya, L. S.; Muravenko, V. P.; Kondrashina, A. I. Zh. Anal. Khim. 1965, 20, 540. (13) Morachevskii, Y. V.; Zil’bershtein, K. I.; Piryutko, M. M.; Nikitina, O. N. Vestink LGU 1962, 22, 140. (14) Kojima, I.; Jinno, F.; Noda, Y.; Iida, C. Anal. Chim. Acta 1991, 245, 35-41. (15) Myshlyaeva, L. V.; Krasnoshchekov, V. V. Analytical Chemistry of Silicon; John Wiley & Sons: New York, 1972. (16) Kingston, H. M.; Jassie, L. B. Introduction to Microwave Sample Preparation: Theory and Practice; American Chemical Society: Washington, DC, 1988. (17) Kingston, H. M.; Haswell, S. J. Microwave Enhanced Chemistry: Fundamentals, Sample Preparation, and Applications; American Chemical Society: Washington, DC, 1997. (18) Link, D. D.; Kingston, H. M. Anal. Chem. 2000, 72 (13), 2908-2913. 10.1021/ac001236o CCC: $20.00
© 2001 American Chemical Society Published on Web 02/16/2001
Figure 2. Diagram of the dual-vessel apparatus for microwaveassisted evaporation. Figure 1. Diagram of the dual-vessel apparatus for in situ microwave reagent purification and sample decomposition.
EXPERIMENTAL SECTION Reagents and Standards. High-purity HNO3 and HF (Optima grade), as well as regular HNO3 and HF (certified ACS grade), were purchased from Fisher Scientific (Pittsburgh, PA). (CAUTION: HF is particularly corrosive and a contact and inhalation hazard.19) Distilled deionized water (DDI water) (18 MΩ‚cm) was purified with a Milli-Q system (Millipore Corp.). Sub-boiled distilled HNO3 and sub-boiled DDI water were prepared in a class1000 clean room (Duquesne University) by a duoPUR sub-boiling distillation system (Milestone GmbH, Germany) from certified ACS grade HNO3 and 18 MΩ‚cm DDI water, respectively. After preparation, all reagents were stored in acid-cleaned PTFE fluoropolymer bottles in a class-100 clean area. Working standard solutions were prepared by diluting multielement calibration standards (SPEX CertiPrep) with 5% Optima HNO3 in sub-boiled DDI water. Instrumentation. An Ethos 1600 microwave system (Milestone GmbH, Germany) in the class-1000 clean room was used for sample decomposition and evaporation. The dual-vessel design used in conjunction with a microwave closed vessel is illustrated in Figure 1. The outer vessel (Milestone GmbH, Germany) is made of PFA fluoropolymer. The inner vessel (Milestone GmbH, Germany) is manufactured from high-purity TFM fluoropolymer prepared in class-100 conditions and shipped in sealed nitrogenfilled bags. The outer vessel is 100 mm in height with an inner diameter of 36 mm. The inner vessel is 65 mm in height with an inner diameter of 28 mm. The solid polycrystalline silicon sample and high-purity reagent (HNO3) were placed in the inner vessel. Alternatively, the reagents that contain higher concentrations of impurities, such as HF, are (19) Sax, N. I. Dangerous Properties of Industrial Materials; Van Nostrand Reinhold Co.: New York, 1979.
placed in the outer vessel. In this configuration, the reagents in the outer vessel are further purified in situ during the sample decomposition procedures. This dual-vessel design was also applied for evaporation of the digests following decomposition. The evaporation apparatus shown in Figure 2 is similar to that used for sample decomposition. The outer vessel is placed inside a sheath made of Weflon (Milestone GmbH, Germany), a carbonimpregnated fluoropolymer material that converts microwave energy into heat. The sheath heats the outer vessel, thereby increasing the rate at which the evaporation proceeds. During the evaporation, the vapor is removed using a vacuum/scrubber system (Milestone GmbH, Germany). Final measurement of the elements of interest in the samples was performed using a HP 4500 ICPMS (Agilent Technologies, USA and Yokogawa Analytical System Inc., Japan) in the class-1000 clean room. To achieve the appropriate sensitivity and precision for ultratrace analysis, the ICPMS conditions were optimized using 1.0 ng/g multi-element tuning solution. A set of platinum sampling and skimmer cones and a quartz concentric nebulizer were used. The integration time was 6 s/mass. METHODS SECTION Sample Decomposition. Within a custom-built class-10 clean hood (Duquesne University) in the class-1000 clean room, a 0.50-g polycrystalline silicon sample and 5.0 mL of high-purity HNO3 were placed in the inner vessel. HF (10.0 mL) was placed separately in the outer vessel. The dual-vessel unit was sealed inside the class-10 clean hood, then transferred to the Ethos 1600 microwave unit in the class-1000 clean room for sample decomposition. The unique characteristics and advantages of the dualvessel design will be discussed in detail. A sample decomposition program was developed on the basis of a temperature-feedbackcontrolled microwave decomposition profile. During the decomposition, the temperature of the reagents in the outer vessel was Analytical Chemistry, Vol. 73, No. 6, March 15, 2001
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measured by a thermocouple. The temperature was first raised to 120 °C in 2 min, then was continuously increased to 240 °C in 60 min, and finally, was cooled to room temperature. The temperature-feedback-controlled decomposition profile could be adapted to a microwave-power-controlled decomposition profile by calculating microwave energy consumed in each 60-second period during the decomposition. Continuously increasing temperature to 240 °C ensured that a sufficient amount of HF, a limiting reagent, was purified and condensed into the HNO3 in the inner vessel. A slow-temperature ramp helps control the vigorous reactions that occur among silicon, HF, and HNO3. Slow temperature ramping is also needed in order to prevent the digestate from being physically transferred from the inner vessel to the outer vessel. After decomposition, the final mass of a sample digest in the inner vessel was typically ) where γ is the surface tension of the solution and r is the radius of the liquid drop. A small liquid drop is in a ball shape on the concave bottom, and has a smaller radius than when on the flat bottom; therefore, the liquid drop builds the stronger pressure barrier (∆P) at its surface to prevent aerosols from escaping. (Refer to the Supporting Information section for the proposed evaporation process.) Because the volatile chlorides and fluorides have similar boiling points (refer to the Supporting Information section for details), the total retention of 2.5 ng volatile chlorides in 10 mL 20% (v/v) HCl using the custom evaporation unit with dual-vessel design was demonstrated, and the results were summarized in Table 2. (22) Atkins, P. Physical Chemistry (6th ed.); W. H. Freeman and Company: New York, 1999.
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elements volatile as chloride species18,24
recovery (%)a
Ti V Cr Mo Sn Sb Os Hgb
103 ( 4 105 ( 15 89 ( 23 98 ( 6 102 ( 12 92 ( 8 95 ( 6 101 ( 4
a Uncertainties expressed as 95% confidence intervals, with n g 3. Volatile as elemental form.
Clean Chemistry Optimization. To accurately determine the impurities in high-purity silicon materials, the determination must be carried out under special clean working conditions. Factors include the cleanliness of the laboratory, specific preparation of the sample for analysis, selection of vessels, and purification and storage of reagents.17,23 The sample preparation and analysis instruments are placed in the class-1000 clean room. All sample manipulations are exclusively processed in the inner vessel in the class-10 clean hood. Closed-vessel microwave sample preparation minimizes airborne contamination during sample decomposition and evaporation. The contamination introduced by the vessel is reduced because the inner vessel is manufactured in a clean environment and has 60% less inner surface area than the outer vessel in which sample decomposition typically proceeds. The unique dual-vessel design reduces the contamination introduced by reagents, such as HF. Reagent impurities are further minimized through the use of sub-boiled DDI water. (Refer to the Supporting Information section for details.) Analysis of Selected Polycrystalline Silicon Samples. The developed method was applied to a polycrystalline silicon sample. Optima HNO3, Optima HF, and sub-boiled DDI water were used in the sample preparation protocol. Final sample digests or analytical blanks were diluted with 5% Optima HNO3 to 5 g in the inner vessel in the class-10 clean hood and analyzed by ICPMS directly. The concentration of the analytical blank was the element concentration of the reagent blanks that were diluted in 5 g 5% Optima HNO3 solution after being processed through the sample preparation protocol. The detection limit (DL) here is defined as 3 times the standard deviation of the element concentration of the analytical blanks in 5 g 5% Optima HNO3 solution. The mean values of six replicate samples, along with the uncertainties at the 95% confidence intervals, are summarized in Table 3. To demonstrate a unique advantage of the microwave dualvessel design, which allows the in situ purification of outer vessel reagent during sample decomposition as well as the cost-effective advantage of such capability, reagent grade HF and sub-boiled distilled HNO3 were placed in the outer and inner vessels, respectively. The results are summarized in Table 4. No statistically significant differences were observed in the determination (23) Richter, R.; Link, D. D.; Han, Y.; Kingston, H. M. Presented at PITTCON’2000, New Orleans, March, 2000. (24) Kingston, H. M. http://www.sampleprep.duq.edu/sampleprep; Duquesne University, 1995-2000.
Table 3. Analysis Results for Detection of Impurities in Polycrystalline Silicon Sample Using Optima HNO3 in the Inner Vessel and Optima HF in the Outer Vessela element
impur. in silicon pg/g silicon
anal. blanksb pg/g solution
DL for blanksc pg/g solution
Cr Ni Cu Zn
770 ( 35 970 ( 85
