Spectrochemical Analysis of Semiconductor Grade Boron Tribromide with a Plasma Jet A. Wayne Fagan and H. M. Klein Central Analysis & Characterization Laboratory, Texas Instruments Inc., Dallas, Texas
THERE is currently no satisfactory method for the rapid determination of trace metal impurities in certain reactive inorganic liquids critical to semiconductor manufacturing processes. Typical of these materials are SiHC13, SiC14, and BBr3, which are presently analyzed by an indirect method involving the hydrolysis of the sample, drying the precipitate, then spectrographically comparing it t o a series of graphitebased standards. At its best, this method is semiquantitative and is subject to many sources of error such as those caused by impure reagents, contact with glassware, and by volatilization of low boiling constituents during the drying process. I n this uncontrolled environment, the final analysis cannot be considered as an accurate reflection of the materials actual composition. The development of a rapid, quantitative, and sensitive method for the analysis of BBr3 was undertaken as representative of the problems involved in analyzing these low boiling, easily hydrolyzed liquids. BBr, is extensively used in the semiconductor industry as a diffusion dopant gas and thus must meet very stringent purity requirements. The goal was a method which would analyze BBr, at a maximum total impurity content of 0.1 ppm. Because of its inherent stability and extremely high temperatures (8,000-10,000 OK) the plasma jet is virtually the ideal source for solution analysis in emission spectrography. The principles and apparatus have been described by Scribner ( I ) , Mitteldorf (2), and Owen (3), and the instrument itself has been established as an analytical tool well adapted to the studying of aqueous and organic solutions ( 4 , 5). Because of the homogeneous samples and stable source, the precision of plasma-jet analyses is very good; sensitivities for most elements are about 0.1 ppm in these systems. No data is available on nonaqueous inorganic liquids. EXPERIMENTAL
High purity BBr3 is a clear, colorless, fuming liquid with a density of 2.65 and a boiling point of 91.7 “C. I t reacts violently with water to yield HBr and H3B03. Because of its reactive nature, all manipulations necessary for standards preparation were carried out in a stainless steel dry box purged with dry NP. The atmosphere was further dried by exposure to fresh P205. Ultimately, the internal atmosphere was dried until n o fumes were observed when BBr, was exposed t o it. Primary standards for Si and T i were prepared by volumetric methods. 100 pl of SiCI, and TiCL were added to 25 ml of pure BBr3 to make a stock solution of known concentration. Dilutions were then made every half order of
(1) B. F. Scribner and M. Margoshes, IX Colloquium Spectroscopium Internationale, Lyon, France, June 1961, p 309. (2) A. J. Mitteldorf and D. 0. Landon, Spex Speaker, 8 , 1 (1963). (3) L. E. Owen, Appl. Spectrosc., 15, 150 (1961). (4) P. A. Serin and K. H. Ashton, ibid., 18, 166 (1964). (5) E. H. Sirois, ANAL.CHEM., 36,2389 (1964).
Table I. Equipment and Operating Conditions Plasma jet assembly National Spectrographic Laboratory Atomizer bore Medium Atomizing gas Argon: about 5.0 liters/minute adjusted to give sample flow of 2.67 gm BBr3/ minute Tangential gas He: 47.5 liters/minute Current 20 amp while sample is being aspirated Pre-exposure 30 second Exposure 90 second Spectrograph 3 m Baird-Atomic Slit 25 P Sector 12.5 and 100% Wavelength range 2400-3700 A , order I Photographic plate Kodak SA-1 Table 11. Concentration Ranges and Spectral Lines Used in the Analysis Spectral Filter, Concentration Element line (A) transmittance range pg/g Aluminum 3092.71 100 0.05-4 Silicon 2881.58 12.5 0.04-2.5 Copper 3247.54 100 0.04-0.2 Iron 3020.64 100 0.1-6 Titanium 3234.52 100 0.01-0.75 3234.52 12.5 0.2-2.5
magnitude until the detection limits of Si and Ti were established. Secondary standards were made for F e and Al. Because FeBrs is slightly soluble and A1Br3 is very soluble (6) in BBr3, saturated solutions were prepared t o be used in preparing standards. The solutions were then filtered, and analyzed by atomic absorption methods for A1 and F e content. The solutions were then diluted in the same manner as the Ti and Si standards. The preparation of the copper standards was more difficult. It was found that neither cupric nor cuprous halides would dissolve in BBr3. However, copper was always observed spectrographically in crude BBr,. Accordingly, secondary standards were prepared from a lot of BBr, that was analyzed by atomic absorption spectroscopy. As before, dilutions were made every half order of magnitude until the detection limit was reached. The equipment used and the excitation conditions are shown in Table I. RESULTS AND DISCUSSION
In the development of this procedure, it was noted that the presence of water in the atomizing gas would soon cause clogging of the system. It was further noted that this liquid could not be fed into the source in a n open-topped vessel.
(6) W. F. Linke, Ed., “Solubilities of Inorganic and Inorganic Compounds,” Vol. I, part 1, Van Nostrand, Princeton, N. J., 1958, p 161. VOL. 40, NO. 13, NOVEMBER 1968
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T o overcome the difficulties of transferring this easily hydrolyzed material from the vessel to the source, a #22 thin-walled Teflon (DuPont) tube was attached to the atomizer. The other end was immersed in the fluid while the source was running. The positive pressure in the source allowed dry H e t o purge the feed tube of moisture bearing air. To further minimize the hydrolysis of the samples, it was required that they be handled in 50-ml vessels having a 5-ml neck. To readily aspirate this dense liquid, it was necessary t o maintain a head of less than 1 cm. The work here was performed with the fluid at that level.
Analytical curves for Cu, Al, Fe, Si, and Ti in BBr3 were prepared. In Table I1 the concentration range covered and the analytical line employed for the analyses are shown. The lowest concentrations at which quantitative determinations were made were those which, under our experimental conditions, gave a line having a maximum transmittance of 90%. The precision of the analysis is excellent. Based on fourteen determinations of Si and Ti in BBr3, the relative standard deviation at the 95% confidence level was shown t o be 4.6%.
RECEIVED for review May 27, 1968. Accepted July 15, 1968
Microdetermination of Mercury by the Oxygen Bomb Combustion Method Masahiko Fujita, Yasushi Takeda, Tadao Terao, Otomatsu Hoshino, and Tyunosin Ukita Faculty of Pharmaceutical Sciences, Unicersity of Tokyo, Hongo, Tokyo, Japan
ORGANICmercury compounds are widely used as fungicides
EXPERIMENTAL
against rice blast disease. I t is, therefore, very important from the standpoint of public health to determine the mercury residue in rice grains. In order to determine the mercury in rice, it is necessary to digest the organic material. The digestion method generally applied has been the wet digestion procedure ( I ) , but this procedure is tedious and takes a long time for complete digestion of the materials; also, it is sometimes accompanied by a loss of mercury during the digestion (2). The determination of mercury by a combustion method has been reported by Schoeniger (3) and several other investigators ( 4 ) . Though these methods are useful in their rapidity and accuracy, the amounts of samples available are too small, because the combustion is performed in a glass flask. In the present work, combustion has been carried out in an oxygen-filled bomb made of stainless steel to facilitate the treatment of larger amounts of organic material. By this method, satisfactory recovery of mercury from samples COP taining known quantities could be achieved. No interference from the metal bomb was observed in the mercury determination. The combustion was carried out in the presence of 1.ON nitric acid previously added to the bomb to absorb the oxidation products. Without the nitric acid, reproducibility of the results was unsatisfactory. The oxidation products were subsequently reduced by addition of hydroxylamine hydrochloride and urea solution. The mercury was extracted with dithizone solution, and the mercury dithizonate was submitted to column chromatography for separation from excess dithizone (5), and was determined colorimetrically.
Apparatus. Combustion was carried out in a bomb designed by Fujiwara and Narasaki (6). The spectrophotometric determination was carried out in a Beckman DU spectrophotometer using 1.00-X 0.50-cm quartz cells. Reagents and Chemicals. Solvents used were distilled before use. All chemicals used were of the highest purity obtainable. Dithizone solution: Dithizone (5.1 mg) was dissolved in 100 ml of carbon tetrachloride. Urea solution: Ten grams of urea was dissolved in 100 ml of deionized water and the solution was washed with dithizone solution and carbon tetrachloride successively until the extracts became colorless. Hydroxylamine hydrochloride solution: Forty grams of hydroxylamine hydrochloride was dissolved in 100 ml of deionized water. This solution was washed with dithizone solution and carbon tetrachloride as in the case of urea solution. Aluminum Oxide Column Chromatography. This was carried out by the method of Ishikura and Yokota ( 5 ) . The column (0.3- x 10-cm) was packed with 0.3 gram of aluminum oxide of activity grade 3 (7). Combustion of Samples and Preparation of Sample Solutions. The amounts of sample appropriate for one combustion are shown in Table 11. The samples were wrapped in a sheet of rice paper of 7- X 7-cm size and tied with cotton thread and put in the sample cup. In the case of oils, the sample was poured directly into the platinum cup and the end of the cotton thread was immersed in the sample. The remaining part of the thread was passed through the coil of platinum wire as a fuse. Forty milliliters of 1.ON nitric acid was then added to the bomb, wetting down the inside surface. The bomb was assembled and oxygen was passed into it until the pressure had built up to 25 kg/cm2. The sample was ignited by passing a small ac current through the platinum coil under a potential difference of 12 volts. After ignition, the bomb was shaken for 15 seconds and cooled in an ice
(1) Committee on Editing Methods of Analysis, “Official Methods of Analysis of the Association of Official Agricultural Chemists,” 10th Ed., Association of Official Agricultural Chemists, Washington, D.C., 1965, p 375. (2) Analytical Methods Committee, Analyst, 90, 515 (1965). (3) W. Schoeniger, Mikrochim. Acta, 1955, 123. (4) B. C. Southworth, J. H. Hodecker, and K. D. Fleischer, ANAL. CHEM.,30, 1152 (1958). (5) S. Ishikura and K. Yokota, Chem. P/iunn. Bull., 11,939 (1958). .
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(6) S. Fujiwara and H. Narasaki, Japan Analyst, 10,1268 (1961). (7) H. Brockmann and H. Schrodder, Ber., 74,73 (1941).
