Anal. Chem. 1994,66, 2279-2284
Particle- and Vapor-Phase Contributions to Metallic Impurities in Electronic-Grade Chlorine Tracey Jacksier,' Richard Udischas, and Hwa-Chi Wang Air Liquide, Chicago Research Center, Countryside, Illinois 60525 Ramon M. Barnes Department of Chemistty, Lederle Graduate Research Center, Universiw of Massachusetts, Box 345 10, Amherst, Massachusetts 0 1003-4510
Techniques are described to quantify both the particulate and vapor phase impurities in electronic-gradechlorine. A particle counter was used in tandem with the sealed inductively coupled plasma system to establishparticle size, particle concentration, and elemental composition of impurities present in a chlorine gas stream. The total particle volume (deduced from particle counts) and the total impurity emission were correlated. Al, C, Ca, and Sn were present in both particle and vapor phases, while Ti was present solely as particles. More than 65%of elemental impurities occur as particles with 35% as volatile impurities. Carbonaccounts for 80%of these volatile impurities (or 21%of the total impurity content). Electronic specialty gases (ESGs), such as chlorine, play a significant role in the production of integrated circuits of very large scale integration (VLSI), particularly for chemical vapor deposition (CVD) and reactive ion etching (RIE). The purity of ESGs influences the yield and reliability of devices produced. For example, the electrical performance of semiconductor films can be affected by metallic contamination on the wafers during processing.' Despite the importance of metal contaminants, their determination in corrosive gases is limited by the analysis techniques. Trapping particles from gases with microfilters and subsequent analysis of the trapped particles2 and hydrolysis methods3 are the primary analysis approaches. The particle composition in anhydrous hydrogen chlorine (HCl) has been investigated by passing largevolumes of HCl through a membrane filter with subsequent analysis of the filter by graphite furnace atomic absorption spectroscopy (GFAAS).2 However, the gaseous contributions to the total impurity content are undetermined.4 Hydrolysis methods for chlorine parallel hydrolysis techniques for HCl. However, owing to the limited solubility of gaseous chlorine in water,5 hydrolysis products may not represent the impurities present because they pass unreacted through the aqueous trap. In addition, both filtration and hydrolysis techniques are indirect methods, time consuming, and prone to contamination. (1) Cooper, D. W. Aerosol Sci. Technol. 1986, 5, 287. (2) Faix, W. G.;Schramm, W.;Vix, F.;Weichbrod, G.;Hendelmann, R. Fresenius Z . Anal. Chem. 1988, 329, 847. (3) Bridenne, M.;Carre, M.;Coffre, E.;Marot, Y.;Simondet, F. Presented at the 1992Winter Conferenceon Plasma Spectrochemistry,San Diego, CA, January 1992; poster WP35. (4) Schram, J. Fresenius Z . Anal. Chem. 1992, 343, 727. (5) Handbook of Chemistry and Physics, 61st ed.; Weast, R. C., Ed.; Chemical Rubber Co.: Cleveland, OH, 1980.
0003-2700/94/0366-2279$04.50/0 0 1994 American Chemlcal Soclety
A direct field usable technique to monitor metallic impurities in ESGs is needed. Particle counting technology in ESGs, recently developed6and implemented at Air Liquide, represents a viable approach. However, the partition of metallic impurities between vapor and particulate phases must be known for particle counting to reflect the total impurity concentration in the gas accurately. Comparison of particle counts with metallic impurities was attempted with conventional techniques7 However, the correlation was inconclusive, probably due to the limitation of the experimental design that required off-line metal analyses. In addition, the noise background level of the particle counter employed was high. To examine the complexity of particle- and vapor-phase impurity analysis, two independent real-time analyses were applied in this study: a sealed inductively coupled plasma (SICP) and a laser-based particle counter. A pure, low-flow atmospheric pressure chlorine discharge in the SICP can be formed and has been applied to the impurity analysis of chlorine.8-1° Additionally, optical particle counters have been widely used to determine particle concentration and size distribution from gas streams."-l3 The general operating principle of the particle counter involves passing a particleladen gas stream into a sensing volume illuminated by a laser and detecting the scattered light by a photodetector. Particle concentration is deduced from the frequency of the abovenoise pulses, while the size distribution is determined from the intensity distribution with a set of predetermined calibration data. The ability to measure particle size distributions is important to many industries. However, the field of microcontamination does not share these measurement needs. Differential size distribution data are less important than the cumulative concentration totals at sizes above a threshold value where chip failure will occur.14 Therefore, in microelectronics success hinges on the ability to count the total particles present rather than their distribution. However, since (6) Wang, H-C.; Udischas, R. Solid State Technol., in press. (7) Flaherty, E. T.; Johns, L.; Amato, A. F. Solid Stare Technol. 1992, I , S1. (8) Jacksier, T.; Barnes, R. M. Presented at the Pittsburgh Conferencc and Exposition, Chicago, IL, 1994; paper 1186. (9) Jacksier, T.; Barnes, R. M. Spectrochim. Acta, Parr E, in press. (10) Jacksier, T.; Barnes, R. M.Appl. Specrrosc. 1994, 48, 382. (1 1) Wang, H.-C.; Doddi,G.; Jurick, B.;Nishikawa, Y .Microcontamination1993,
25. 83.
4,
(12) Hardy, T. K.; Christman, D. D.; Shay, R. H. Solid State Technol. 1988,10, (13) Wang, H.-C.; Wen, H. Y . ; Kasper, G. Solid Stare Technol. 1989, 5. 155. (14) Knollenberg, R.; Veal, D. L. J. Inst. Emiron. Sci. 1992, 3, 64.
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correlation between emission and particles is desired, it is necessary to convert particle counts into volunie of particles. It is possible to convert particle counts into total particle volume by assuming the particles are spherical and the geometric diameter is the same as the optical equivalent diameter (4/ 3ar3). Particle counting in inert gases is part of the continuous quality control today in the semiconductor industry owing to the availability of particle counters with low noise, accurate sizing, and high counting efficiency.l* Particle counting in specialty gases has required additional consideration of material compatibility and flow control.16 These two techniques operated in tandem yield both emission signals and particle number (particle volume) and size distribution of the same sample. Preliminary results for impurity contributions to electronic-grade chlorine are given.
EXPERIMENTAL SECTION Sealed Inductively Coupled Plasma. An enclosed plasma developed as an analytical spectrochemical source for the analysis of toxic and reactive ga~es17-l~ exhibits advantages that suggest its suitability for the analysis of chlorine. Low chlorine flow rates (C100 mL/min) limit sample consumption without dilution. No sample preparation is required because chlorine is a gas at room temperature; therefore, the possibilities for external contamination are eliminated. The discharge is formed with a plasma gas and sample enclosed inside a quartz container. In this arrangement the discharge is initiated in pure argon under flowing conditions, and chlorine (Electronic Grade, Carbon Steel Cylinder, Air Liquide, Walnut Creek, CA) is added to the argon stream. The argon flow is slowly decreased until the chlorine concentration is approximately 50%. After the plasma stabilizes (approximately 30 s), the argon flow is stopped and a chlorineplasma forms at atmosphericpressure. A minimum of 800 W (at 40.68 MHz) is required to maintain the chlorine discharge. Techniques developed for the analytical applications of the SICPs have been described elsewhere,18J9and the direct analysis of chlorine in the SICP is u n d e r ~ a y . ~ . ~ The chlorine discharge was maintained at 40.68 MHz (Model HFS 5000D, 0-5 kW, RFPlasma Products, Marlton, NJ) and 1.0-kW foward power, by using a 65-mm, quartz, SICP container.20 Details of experimental equipment and operation were reported previously.18J9 Particle Counter. A particle counter (Particle Measuring Systems, Boulder, CO) was used in this study (Table 1). The wetted portions of the counter are made from stainless steel, glass, and a Kalrez O-ring. Therefore, gases compatible with these materials can be measured with this sensor. Additionally, the optimal flow rate through the detector at atmospheric pressure is 20 mL/min. Gas entering the particle counter passes through a glass capillary where a laser diode beam (780 nm) crosses orthogonally. Both the light scattered by the glass capillary ~
(15) Wang,H.-C.; Wen,H. Y.;Kasper,G.RocccdingsofInatituteofEnvironmmtsl Science, 1989 Technical Meeting, Anaheim, CA, 1989; pper 419. (16) Wang, H.-C.;Udischas.R. Microcontamination 1993ConferenccROceedings, San Jose, CA, 1993: p 465. (17) Jahl, M. J.; Jachier, T.; Barnes, R. M. J . AMI. Ar. Specfrom. 1992, 7, 653. (18) Jachier, T.;Barnes, R. M. J. AMI. At. Spectrom. 1992, 7, 839. (19) Jahl, M. J.; Barnes, R. M. J. And. AI. Specrrom. 1992, 7, 833. (20) Jachier, T.;Barnes, R. M. Spccfrochfm.Acro 1993.18B. 944.
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Table 1. Partkle Counter Inrtrumentrl
particle counter size range channefihresholds optical design laser source environment spectrometer
Particle Measuring Systems, Model CGS-200 >0.2 um 0.20,0.25,0.30,0.35,0.40, 0.45,0.50,0.55,0.60,0.70, 0.80.0.90, 1.0, 1.5,2.0,3.0 passive, glass capillary
laser diode, 780 nm inert or reactive gases up to 207 bar PLPS
and particles in the flowing gas are detected by a photodiode. There are three types of background for this counter: electronic, light scattering from the surface of the glass capillary, and particles generated by the sampling system. Obtaining a low background is important to ensure that detected particles are from the gases to be sampled and not particles generated within the whole sampling system.I3 A transient signal is observed for the particles as they travel with the gas through the laser beam. The transient signal is measured above a constant background signal. This restricts the detection limit of the particle diameter to approximately 0.2 pm. The counter background noise is less than 1 particle/ standardcubic foot (
