Spectropho'tometric Determination of Traces of Boron in Silicon Tetrtrchloride Utilizing an Extractive Separation SIR: A need for the quantitative estimation of boron content in silicon tetrachloride arose with the first commercial production of hyperpure silicon by the Pigments Dept. of the Du Pont Co. at its Newport, Del., plant. The presence of boron in concentrations as low as 0.5 p.p.b. influenced the properties of semiconductor grade silicon. Research on silicon tetrachloride analysis in the above laboratory led to the development of the method herein described. This method has since been in use in connection w t h the commercial manufacture of hyperpure silicon a t the Newport, Del., and Brevard, N.C., plants. In the method, the boron in the silicon tetrachloride sample is quantitatively extracted by means of a quinalizarinsulfuric acid extract on reagent. An aliquot of the extract is taken, a small amount of water added to develop the color, and the transmbtance is measured on a spectrophotometw a t a wavelength of 620 mh. The boron content of the sample is then es5mated from a standardization curre. Since the system does not conform exactly to Beer's law, calibration points must be determined at small intervals over the working range. The method appears to be accurate in the range of 0 to 2000 p.p.b. EXPERIMENTAL
Reagents. The extraction reagent is prepared by disscllving 10 mg. of quinalizarin (Eastman #2787) in 2.0 liters of concentrated (sp. gr. 1.84) sulfuric acid. This reagent is stable for only 3 days and should not be unnecessarily exposed to light or air. The 2.0 liters of concentrated acid should be taken from a newly opened bottle. The bottle containing the prepared reagent mu5 t be kept tightly closed. A standard boron solution is prepared from C P boric acid (HaB03) by dissolving 0.5716 gram in water and diluting to 1000 ml. in a volumetric flask. One hundred nilliliters of this solution is diluted to 11300 ml. in a volumetric flask to obtain a solution containing 0.00001 gram of boron per ml. Apparatus. The solvent extractions are done in Squibb-type separatory funnels with T e Ion TFE-fluorocarbon resin stopcoclrs on which the stems have been shoi-tened to within 2 cm. of the stopcocktr. Note that all glassware except the spectrophotometer cells must be immersed for 5 t o 10 minutes in a 5% riqueous sodium hydroxide solution inimediately after use to eliminate silicic acid gel formation. After cleaning, the glassware is
rinsed with demineralized water and dried in an oven. All glassware must be thoroughly dried before use since silicon tetrachloride forms gel on contact with moisture. The necessary gentle shaking action can be obtained from an ExtractoMatic shaker or equivalent. Transmittance measurements are made with a Beckman DU spectrophotometer using a matched set of four 10-mm. cells. Procedure. Carry two reagent blanks through the procedure. The blanks will contain all of the reagents, b u t no silicon tetrachloride, and will undergo the same manipulations as the samples. By means of a dry, 25-ml. graduated cylinder transfer 20 ml. of quinalizarinsulfuric acid reagent to a 60-ml. separatory funnel. The graduated cylinder should be rinsed with quinalizarinsulfuric acid reagent just before use. It is not advisable to work with more than 8 funnels a t one time because of the excessive time delay on the individual sample. Using a dry 25-m1. graduated cylinder, add 20 ml. of silicon tetrachloride sample into the funnel. Shake the funnel gently on an Extracto-Matic shaker for 3 minutes. Then allow the funnel and contents to stand undisturbed for 15 minutes to permit separation of any gel which has formed. Draw off the quinalizarin-sulfuric acid extract into a dry 25- X 83-mm. glass vial and securely cap it with a closure lined with Teflon TFEfluorocarbon resin. Leave a small amount of reagent in the funnel to ensure separation. Discard the contents of the funnel. Allow the vial to stand for 1 hour. If, after 15 minutes, it is observed that silica gel and gas bubbles are dispersed in the sample, it is sometimes helpful to shake the vial vigorously and again allow the vial to stand for 1 hour. Pipet 1.0 ml. of demineralized water into a dry vial (as above). Then pipet 10 ml. of the quinalizarin-sulfuric acid extract to the vial containing the 1.0 ml. of water. Particular care must be taken so as not to dram gel or bubbles into the pipet. Because of the viscous nature of the extraction reagent, the pipet should be carefully wiped with a soft cloth before the contents are discharged into the vial containing 1.0 ml. of water; and after delivery of the contents, a t least 30 seconds should be allowed for complete drainage of this pipet. Firmly cap the vial with a closure lined with Teflon TFE-fluorocarbon resin and invert several times t o completely mix the contents. Place the vial in a water bath a t room temperature and cool for 1 hour. Fill a IO-mm. spectrophotometer cell with the reagent extract wing a
dropping pipet. Avoid any particles of silica gel in the transfer. Determine transmittance for blanks and samples using 620-mp wavelength and 0.04-mm. slit width on a Beckman DU spectrophotometer. Use distilled water in the reference cell. Determine milligrams of boron present in the sample by reference to the standardization curve prepared as described below. STANDARDIZATION CURVE.Place 200 ml. of pure silicon tetrachloride and 200 ml. of concentrated (sp. gr. 1.84) sulfuric acid in a dry 500-ml. separatory funnel and extract the boron from the silicon tetrachloride by gently shaking the funnel for 3 minutes. Draw off and discard the sulfuric acid layer. Boron-free silicon tetrachloride, which may be used for preparing standards, will remain in the funnel. A series of standards is then prepared by placing 20 mi. of the boron-free silicon tetrachloride and 20 ml. of quinalizarinsulfuric acid reagent in each of five 60-ml. separatory funnels. Carry out the extractions as in the procedure outlined above. Add standard boron mlution and water to five sample vials according to the schedule in Table I. This replaces the 1 ml. of water used in the regular procedure. Proceed with the remainder of the procedure. The calibration should be carried out in duplicate and the average of the transmittance values a t each boron concentration plotted using semilog coordinates. DISCUSSION
The addition of boric acid to a polyhydroxy aromatic compound in reasonably concentrated sulfuric acid causes a definite color change which may be used to estimate the amount of boron present (1). The change is due to the formation of a chelate ring. The most sensitive compound of this type is 1,2,5,8-tetrahydroxyanthraquinone (quinalizarin), which dissolves in concentrated sulfuric acid to give an intense bluish violet solution. With the addition of water, this solution changes color from the bluish violet to red. However, if the water or the acid
Table 1. Standards for Boron Calibration Curve Boron solution, mi. 1 .o
Water, ml. 0
Boron, mg. 0.0100
VOL. 36, NO. 1, JANUARY 1964
e
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solution contains borate, the change toward red is less, and if sufficient boron is present there is a reverse, so that the resulting color is more blue. This color change is the basis for the method. The method as described is remarkably free from interferences. Germanium is the only ion that presents a similar color change; however, approximately 200 times as much germanium as boron is required to produce a given color change. Aside from the possibility of interference from germanium, there are two variables to
be considered: concentration of sulfuric acid and the temperature at which measurements are made. Both of these variables, in view of the extremely small n-orking range over which the absorption measurements are made, appear to have small effect on the sensitivity of the method. The reproducibility of the method, notwithstanding variable room temperatures, appears to be satisfactory. It might be of interest to point out that while the method indicateq the impurity content of the silicon tetrachloride, an intermediate in the silicon production process, it does not neceq-
sarily define the impurity content of the final silicon product in the process. LITERATURE CITED
(1) Johnson, E. A,, Toogood, M. J., Analyst 79, 493-6 (1954). C. S. HAAS R. A. PELLIN~ E. I. du Pont de Nemours & Co. Newport, ljel. M. R. EVERINGHAM E. I. du Pont de Nemours & Co. Brevard, N. C. 1 Present address, Motorola, Ino., Phoenix, Ariz.
Spectrophotometric Determination of Traces of Phosphorus in Silicon Tetrachloride Utilizing an Extractive Separation the extraction must be cleaned with ammonium bifluoride after each analysis to remove silica deposits. .411 glrtssware must be thoroughly dried before use. Procedure. Two reagent blanks are carried through the procedure. They are not exposed to silicon tetrachloride but undergo all the manipulations of the procedure. Add exactly 0.50 ml. of concentrated sulfuric acid t o a 125-nil. borosilicate glass flask from an Ultramax buret (Fischer and Porter Co.). Place a TeflonTFE-fluorocarbon resincoated magnetic stirring bar in the flask. Add 25 ml. of silicon tetrachloride from a dry graduated cylinder. Immediately place a Teflon stopper in the neck to prevent loss of fumes. Stir magnetically for 30 minutes. Then allow the mixture to stand for 15 minutes. Decant the SiC14, being careful that EXPERIMENTAL no acid is lost. In this step, the stirring Reagents. AMMONIUM VANADATE. bar is held on the bottom of the flask by a magnet held on the outside surface Mix 11.75 grams of ammonium meta of the flask. Immediately after devanadate and 68 ml. of 60% percantation, add to the flask the assembly chloric acid in 4 liters of water and of distilling head and separatory funnel, filter through #40 Whatman filter containing 50 ml. of demineralized paper. water, as shown in Figure 1. Open AMMONIUM MOLYBDATE. Mix 390.65 the stopcock of the separatory funnel grams of ammonium molybdate in 4 t o allow water to pass down t o the liters of water and filter through #40 flask, wash the bottom of the Teflon Whatman paper. STANDARDPHOSPHORUS SOLUTION. stopper and allow the washings to enter the separatory funnel. When it Use 0.4263 gram of anhydrous diis seen that the water is being definitely ammonium phosphate per liter of soludrawn into the flask, note the time tion. Dilute 10 ml. of this solution t o 1 and allow to digest for 20 minutes liter with water. (1 ml. = 0.001 mg. of with occasional shaking to mix the phosphorus.) contents. Use only demineralized water (10 Wash off the outer surface of the megohms) for mixing reagents and assembly a t the joint of the separatory throughout the analysis for rinsing funnel and the distilling head. Dry by residues, etc. wiping off gently with paper tissue. Apparatus. A critical point in the Remove the separatory funnel and determination is the prevention of wash down the inner surfaces into the loss of phosphorus during the dilution distilling head. Allow the remaining of the sulfuric acid extract. The apassembly to drain while the top is paratus as shown in Figure 1 was used. covered with a clean 50-ml. beaker. Transmittance measurements are Clean the outer portion of the remainmade with a Beckman DU spectroing joint between the flask and distilling photometer using a matched set of 100head as above. Remove the distilling mm. cells. head; with the aid of a magnet a t the The borosilicate glass flasks used for
SIR: The level of phogphorus in silicon tetrachloride is of interest because of the effect of trace amounts on semiconductor properties of elemental silicon. Silicon tetrachloride was first used in the zinc reduction process for commercial production of hyperpure silicon a t the Du Pont Co.’s Newport, Del., plant. Research was therefore undertaken to develop a method to measure trace amounts of phosphorus in silicon tetrachloride. I t was found that phosphorus could be quantitatively extracted from silicon tetrachloride with sulfuric acid, oxidized with perchloric acid to the pentavalent state, and combined with two colorforming agents to give a yellow complex. The intensity of the yellow color was measured with a spectrophotometer.
246
ANALYTICAL CHEMISTRY
Figure 1 .
Dilution apparatus
A. Separatory funnel, 60 ml., 2 4 / 4 0 v 8. Distillation head, 2 4 / 4 0 T C. Erlenmeyer Aark, 1 2 5 ml., 24/40-$
