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Anal. Chem. 1986, 58. 807-809
magnitude for every 2 orders of magnitude increase in the concentration of p, presuming shot noise to be the only noise source.
CONCLUSION Equation 1makes no distinction between various spectral series and does not take into account selection rules, state degeneracies, excitation mechanisms, or line broadening. If it is widely applicable, the differences between various elements must reside in the intensity of the brightest line in the spectrum, since the rate of increase in line number density takes no account of atomic structure. Based on the success of eq 1in fitting iron and rare gas emission data in the visible spectral region and its success in fitting the arsenic spectrum in the near-infrared region, one can conclude that there is some germ of truth in the relationship but that numerous spectral features do not conform to its constraints. The large numbers of weak lines predicted by eq 1 may occur in wavelength regions irrelevant to emission spectrochemical analysis. Without absolute calibration of spectrometer throughput and detector spectral response, there is some doubt that the fitting of the data represents the true behavior of the atomic system under study, but instead represents a complex convolution of instrumental and spectral effects. The availability of an interferometer of use throughout the spectral region employed for spectrochemical analysis (11)suggests that rigorous tests of eq 1 are feasible.
Perhaps the most disconcerting point about eq 1is that it is so similar to the equation for a Boltzmann plot used for estimating excitation temperature, yet asserts that spectral behavior is in many ways dependent only on the dynamic range of the observation system, not on the physics of the source or element involved. Both the goodness of fit to eq 1 of spectral data under some circumstances and the major deviations in other circumstances suggest that additional study is necessary to discover both the reasons why it is applicable in some instances and also what implications it may contain for the useful dynamic range of detection employing thermal excitation emission spectrochemical methods.
LITERATURE CITED Beaty, J. S.; Belmore, R. J. J . Test. Eva/. 1984, 72, 212-216. Wlrsz, D.; Blades, M. W. Anal. Chern. 1988, 5 8 , 51-57. Gray, A. L. Analyst (London) 1985, 770, 551-556. Butler, L. R. P. Spectrochlm. Acta, Part 8 1983, 388,913-919. (5) Learner, R. C. M. J . Phys. 8 1982, 75, L891-L895. ( 6 ) Howard, L. E.; Andrew, K. L. J . Opt. SOC.Am. 8 : Opt. Phys. 1985, 2, 1032-1077. (7) Brault, J. W. J . Opt. SOC.Am. 1978, 66, 1081. (8) Brault, J. W., personal communication. (9) Adams, D. L.; Whaling, W. J . Op.SOC.Am. 1981, 7 1 , 1036-1038. (IO) Andrew, K. L., personal communication. (11) Faires, L. M.; Palmer, 8. A,; Engeiman, R.; Niemczyk, T. M. Spectrochim. Acta, Part8 1984, 398, 819-826. (1) (2) (3) (4)
RECEIVEDfor review September 16,1985. Accepted January 2,1986. Financial support of the National Science Foundation (Grant CHE-81-21809) is gratefully acknowledged.
Determination of Carbon in Trichlorosilane by Metastable Transfer Emission Spectrometry R. R. Matthews* Semiconductor Division, Texas Instruments, Inc., Dallas, Texas 75265
L. A. Melton Department of Chemistry, University of Texas at Dallas, Richardson, Texas 75080
The detectbn lhnn for carbon impurtties in liquid trlchiorosiiane has been lowered to 3 ppb, an order of magnitude improvement over previous techniques, by use of metastable transfer emission spectrometry (MTES). The technique described here is rapid, requires lmle sample preparation, and is tolerant of harsh environments and, hence, should be readily adaptable for on-line process control.
As the trend to build complex semiconductor devices with denser packing of circuits continues, the impact of impurities in the silicon becomes more signficant. This work focuses on the detection of carbon impurities in trichlorosilane, a key intermediate in the purification of semiconductor grade silicon. Trichlorosilane (TCS) is formed by reacting the relatively impure metallurgical grade silicon with HC1 gas. The mixture is then carefully distilled to produce pure liquid TCS, which may be stored and subsequently reacted with hydrogen to produce highly pure polycrystalline silicon. This silicon is melted and single crystals are pulled from the melt; finally, 0003-2700/86/0358-0807$0 1S O / O
wafers are cut from the bulk crystal. These processed wafers are the basis for the further steps in semiconductor manufacturing, and the purity of the wafer is essential for reliable electrical performance of the semiconductor device. Carbon in the semiconductor grade silicon will form silicon carbide, and impurity levels of as little as 1-3 ppm cause stacking faults and oxygen nucleation (1). Previously GC/ mass spectrometry (GC/MS) (21, acid/base titration (3), and Fourier transform infrared spectroscopy (FTIR) (1)have been used to analyze for carbon impurities in the TCS. However, the first two techniques have detection limits of 0.3 and 10 ppm, respectively, which means that the carbon impurities cannot be followed to the electrically negligible level. The FTIR technique has a detection limit of 0.03 ppm, but is very slow since the TCS must be converted to single-crystal silicon before FTIR measurements are made. In related work, an inductively coupled plasma/mass spectrometer has been shown to have a detection limit of 0.05 ppm when used to determine carbon impurities in acids ( 4 ) , and MTES has been projected to have detection limits of 30, 20,1000,5, and 1ppb for the determination of Al, Cr, Fe, Mn, 0 1988 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 58, NO. 4, APRIL 1986
and T i in crystalline silicon (5). Reviews of the analytical use of microwave discharges are given by Zander and Hieftje (6) and Greenfield et al. (7). In the present work, metastable transfer emission spectrometry (MTES) is used; metastable argon atoms dissociate and excite molecules and atoms they collide with, and detection of the resulting emission from C2 molecules provides a rapid, sensitive analytical method. MTES can be used to detect 3 ppb carbon in TCS and is sufficiently rapid and robust to be used as the basis for a process control instrument.
EXPERIMENTAL SECTION The apparatus developed for this technique was derived from that of Capelle and Sutton (8-11) and is, in most respects, a routine microwave discharge apparatus. Minor modifications were made to accommodate the highly reactive TCS sample and to ensure reproducible discharge conditions. The argon gas entering the discharge flow tube passes through shutoff and metering valves in series. In this manner the gas flows and, hence, the discharge conditons can be made reproducible from run to run. In the quartz discharge tube, a cavity-driven microwave discharge (Opthos Instrument Co., McCarroll cavity, Kiva MPG-4 microwave generator) excites the argon atoms and the plasma exiting the cavity is then mixed with the analyte gas from the sample inlet port. The emission excited by the plasma is monitored with a small monochromator which views across the quartz tube just downstream of the sample introduction point (Instruments SA, H20V, 0.05 mm slits, dispersion 4 nm/mm), phototube (EM1 20-ER), electrometer system (Pacific Precision Instruments, Model 110), and xy recorder (Laboratory Data Control, Model 3301-M-Y). The plasma is exhausted through a liquid nitrogen trap to a mechanical vacuum pump. Since TCS attacks most common materials, and particularly rubbers, all valves were stainless steel with Teflon packing, so that no 0 rings were exposed to TCS. All tubing except in the sampler was stainless steel. New discharge tubes were treated with metaphosphoric acid prior to use (12). TCS is a liquid at room temperature (bp 31.8 "C), and the introduction of reproducible amounts of this easily hydrolyzable material was a major problem. A sampler was constructed from a Teflon union-tee valve (Norton Chemplast, Inc., Wayne, NJ), which had a third port at 90" to the original two. One of the inline ports was connected, with Teflon tubing, to a glass syringe, and the other was connected, again with Teflon tubing to the closed cup which contained TCS. As the syringe plunger was pulled, liquid TCS was drawn through the Teflon valve. The bore of the Teflon stopcock served as the sampling volume, and a reproducible sample of approximately 1 pL could then be delivered to the discharge system by turning the stopcock 90". Since the plasma pressure was approximately 4 torr, the TCS was flash vaporized and mixed with the argon plasma immediately. Other injection techniques, involving normal GC procedures, etc., resulted in plugged syringe needles, very high background signals from carbon impurities in the septa, and other unacceptable interferences. When the system was started up each day, it was first evacuated to 12 mtorr for a minimum of 30 min. UHP argon (>99.999%) was metered into the system at 4 torr pressure for 15 min prior to ignition of the discharge, which was then allowed to stabilize for several minutes. When hydrocarbons were injected into the argon plasma, an intense sky-blue color was observed, which originated in the Swan band emissions of the C2 molecule. In this work maximum Swan band emission was obtained within the argon pressure range of 3-4 torr; at other pressures, the plasma was striated and the emissions were not reproducible. The most intense Swan band, at 516.5 nm was used for the MTES carbon measurements. The atomic carbon transition at 247.9 nm was also considered for use in carbon determinations, but the atomic silicon transitions at 250.6 nm interfere. The argon plasma itself produced a minor background emission in the 500-nm region but did not provide any major intereferences. The intensity was taken as the maximum height of the C2 516-nm band above the base line. Deposits gradually form in the plasma discharge tube and attenuate the carbon emissions. These deposits, presumably pyrolyzed carbon compounds and/or silicon compounds, were
Table I. Typical MTES Response as Function of Added Carbon
(I
added carbon, ppm
intensity (arb units)
0.00 0.10 0.20 0.50 1.00
16.8 f 0.2 35.3 f 0.4 49.3 f 0.5 81.3 f 0.9 160.0 f 1.8
Base line fluctuations were typically *0.2 units.
removed by mixing oxygen and fluorine into the argon plasma. In typical operation, approximately 15 rnin of cleaning was required after 30 injections of TCS samples into the apparatus. In the future, it might well be possible to cycle the cleaning and injection operations so that continual samples can be run. No analytical measurements were made during cleaning cycles. Since neither carbon-freeTCS nor carbon-standard TCS were available, quantitative characterization of the MTES system was made using the method of standard additions (13). Typically, semiconductor grade TCS, either produced in-house or obtained from Union Carbide, was spiked with 1000 ppmw carbon (from ACS reagent grade chloroform), and serial dilutions were used to obtain samples spiked with as little as 0.1 ppmw carbon. The cleanup of the argon plasma after injection is rapid, and replicate injections can be made at intervals of 30 s. However, if the TCS is introduced upstream of the microwave discharge, the system may require as long as 45 min to return to a stable background reading.
RESULTS AND DISCUSSION Detection Limits. The emission intensity in the 516-nm band (defined as the intensity measured with a carbon-containing sample minus the intensity of the steady-state argon plasma background) was linear with added carbon over the range 0.10-100 ppmw carbon. In particular, the results for the carbon addition range from 0.00 to 1.00 ppmw, shown in Table I, have a linear correlation coefficient of 0.997 as calculated from least-squares regression (14). In order to determine the reproducibility of intensity measurements within a set of runs, repeated injections of spiked TCS samples were made; the relative standard deviations for these replicate injections were typically l-1.3% of the measured intensity in the 516-nm band. These fluctuations, as well as those in the argon plasma base line are also indicated in Table I. The first point in Table I is particularly noteworthy: injection of TCS with no added carbon gives a readily measurable signal, i.e., even the purest TCS we could obtain shows significant carbon contamination when examined with MTES. Since carbon-free TCS was not available, and with the confidence that intensity measurements on TCS spiked with carbon were linear over at least 3 orders of magnitude, the method of standard additions was used to determine that this TCS sample contained 0.13 f 0.01 ppmw carbon. With the slope of this MTES intensity curve established as 140 units/ppmw carbon contained in the sample, and with the criterion than the minimum detectable signal must be twice the root mean square fluctuations of the argon plasma background signal, the detection limit attainable with this MTES method can be calculated as approximately 0.003 ppmw carbon. This corresponds to 0.004 pg of carbon in the 1 ,uL injected TCS sample. Applicability to Other Carbon Sources. Since the TCS and analyte molecules are presumably completely broken up in the argon plasma, it is reasonable to hope that this MTES technique will yield a universal carbon detector, Le., give the same response per weight of added carbon regardless of the molecular form in which the carbon is added. Table I1 summarizes the results obtained when the signal produced by 5
ANALYTICAL CHEMISTRY, VOL. 58, NO. 4, APRIL 1986
Table 11. MTES Response for Differing Sources of Carbon compound
re1 responsep!
Halocarbons 1.12 0.98 0.99 0.96 1.00 1.04
1,l-dichloroethene
dichloromethane chloroform l,l,l-trichloroethane carbon tetrachloride trichloroethene tetrachloroethene
1.05
trichlorotrifluoroethane (new tube) trichlorotrifluoroethane (old tube)
1.08
2.69
Alkanes n-pentane n-hexane n-heptane n-nonane (peak broadened)
0.92 0.97
0.89 0.54
n-decane (plasma receded)
n-undecane (plasma receded) Aromatics
benzene toluene
ethylbenzene p-xylene p!
0.38 0.47
0.44 0.31
Peak height of 5 ppmw carbon from compound/peak height of
5 ppmw carbon from carbon tetrachloride.
ppmw of added carbon from various compounds is compared to that produced by 5 ppmw of added carbon tetrachloride. The results indicate that the MTES technique is a reasonable but not perfect universal carbon counter. Within *lo%, carbon from chlorinated hydrocarbons, n-pentane and nhexane contribute the same amount per carbon atom to the MTES signal. The deviations shown by the midweight alkanes-heptane and nonane-are probably due to their lesser volatility and not to any inherent chemical difference. A lesser rate of evaporation would easily explain the tailing peak obtained with nonane. The major deviations are shown by the fluorinated hydrocarbons and by the aromatics. The signals for the fluorinated compounds were dependent on the order in which the samples were run. If a new discharge tube was used, the relative signal per carbon atom from the fluorinated compound matched that of the chlorinated compounds, but if the fluorinated compounds were run after several nonfluorinated compounds had been tested, the relative signal per carbon atom increased markedly. It is believed that the fluorinated compounds etch additional carbon deposits off the discharge tube walls and thereby appear to have higher than expected relative intensities. The aromatics have low relative intensities per carbon atom; perhaps these more stable molecules are incompletely decomposed in collisions with metastable argon atoms. As previously noted, introduction of samples upstream of the discharge, which would allow more vigorous decomposition, results in very long purging times. The deviations found from behavior as a universal carbon counter are not a serious problem for the use of MTES in the manufacture and purification of TCS. The compounds that do not respond proportionately (aromatics, heavy hydrocarbons, and fluorocarbons) are also the least likely compounds to contaminate the distilled TCS liquid. Compounds that are
809
known or potential contaminants of TCS (2, 15) such as chloroform, carbon tetrachloride, dichloromethane, and pentane are detected proportionately. The MTES apparatus as developed and tested here represents a major advance in the available analytical methods. The detection limit is more than an order of magnitude less than that available with previous methods, and the sample measurement time has been reduced from hours or days to a few minutes. However, improvements are still possible. Even Teflon is attacked by TCS and contributes to the background at the levels discussed here. Samples should be taken in glass containers, rather than in Teflon containers, and the sampler should be constructed of stainless steel instead of Teflon. It is possible to modify the optical viewing region so that more of the C2 emissions are within the acceptance angle of the monochromator, and indeed, narrow band interference filters can be used to allow even more of the 516-nm emission into the detector system. Nonetheless, the major limitation on the lowering of the detection limit is the background noise contributed by the emissions from the argon plasma itself; unless some effemive means is found to deal with these intrinsic emissions, i t is unlikely that the detection limits can be lowered by more than a factor of 10 below the present result.
ACKNOWLEDGMENT R.R.M. wishes to acknowledge the support and assistance provided by Tom Ramsey and Ross Schraeder at Texas Instruments. Registry No. Carbon, 7440-44-0; 1,l-dichloroethane, 75-35-4; dichloromethane, 75-09-2; chloroform, 67-66-3; l,l,l-trichloroethane, 71-55-6; carbon tetrachloride, 56-23-5;trichloroethene, 79-01-6; tetrachloroethene, 127-18-4; trichlorotrifluoroethane, 26523-64-8; n-pentane, 109-66-0;n-hexane, 110-54-3;n-heptane, 142-82-5;n-nonane, 111-84-2;n-decane, 124-18-5;n-undecane, 1120-21-4; benzene, 71-43-2; toluene, 108-88-3; ethylbenzene, 100-41-4;p-xylene, 106-42-3;trichlorosilane, 10025-78-2. LITERATURE CITED (1) Keenan, J.; Larrabee, G. In “Microstructure Science”; Academic Press: New York, 1983; Vol. 6. (2) Agliulov, N.; Luchinkin, V.; Devyatykh, G. J . Anal. Chem. USSR(€ng/. Trans/.) 1968, 23, 830. (3) Schink, N. “Semiconductor Silicon, International Symposium on Silicon Materials Science and Technology”; Electrochemical Soclety: Princeton, NJ, 1969; p 85. (4) Neve, N.; Quan, E.; Boorn, A,; Canavan, M. “Analysis of Trace metals in Aclds by Inductively coupled Plasma Mass Spectrometry”; Proceedings of Second Annual Microcircuit Pure Materials Conference, San Jose, CA (August 13-14, 1985), p 190. (5) Sutton, D.; Galvan, L.; Melzer, J.; Heldner, R. F., I11 “Low Cost Solar Array Project: Composition Measurements by Analytical Photon Catalysis”, final project report #DOE/JPL-955201-79/4, Aerospace Corp., El Segundo. CA, 1979. (6) Zander, A.; Hieftje, G. Appl. Spectrtosc. 1981, 35,357. (7) Greenfield, S.; McGeachin, H.; Smith, P. Talanta 1975, 22, 1. (8) Sutton, D.; Westberg, K.; Melzer, J. Anal. Chem. 1979, 51, 1399. (9) Capelle, G.; Sutton, 0.Appl. Phys. Lett. 1977, 30,407. (10) Capelle, G.; Sutton, D. Rev. Sci. Instrum. 1978, 49, 1124. (11) Sutton, D.; Melzer, J.; Capelle, G. Anal. Chem. 1978, 50, 1247. (12) Anderson, J.; Margitan, J.; Kaufman, F. J . Chem. Phys. 1974, 60, 3310. (13) Willard, H. H.; Merritt, L. L.; Dean, J. A. “Instrumental Methods of Analysis”, 5th ed.; Van Nostrand: New York, 1974; p 379. (14) “TI Programmable 58/59 Personal Programming Manual”; Texas Instruments, Inc.: Dallas, TX, 1977; V-37. (15) Agafonov, 1.; Rachkov, V.; Agliulov, N.; Luchinkin, V. J . Anal. Chem. USSR (fngl. Trans/.) 1971, 26, 1417.
RECEIVED for review August 5 , 1985. Accepted October 25, 1985.
