Analysis of Mixtures of Acetic, Nitric, and Hydrofluoric Acids H. G. Griffin, Jr., and W. E. Sonia, Jr. Texas Instruments, Inc., P . 0. Box 5012, Dallas, Texas 75222
IN THE SEMICONDUCTOR industry, a wide variety of chemical etchants are used, many of which contain critical ratios of acetic, nitric, and hydrofluoric acids. The analyses of these mixtures to determine their chemical compositions have always presented a rather perplexing problem. Each acid must be determined quantitatively in the presence of the others in a reasonable length of time to be of any value as a control method. The procedure is complicated by the fact that most materials are attacked by the etchants. This consideration rules out many instrumental and physical methods unless prior treatments are undertaken. Although several methods are cited in the literature for the determinations (1-3), they are unsuitable because of time consumption. A method has been developed that involves two major steps. The acetic acid is esterified in methyl alcohol t o form methyl acetate and allow the remaining nitric and hydrofluoric to be titrated potentiometrically with a standard base. The potentiometric titration curve, after esterification has two breaks, the first nitric and the second hydrofluoric acid. The total acid concentration is then determined for all three acids and all components can then be calculated. EXPERIMENTAL
The titrations were performed using a Sargent Recording Titrator, Model D, equipped with a Sargent combination electrode filled with a n electrolyte of saturated methanolic potassium chloride. Initially, the electrolyte in the combination electrode is removed, the electrode is dried, and replaced with a saturated methanolic potassium chloride solution. If this is not done, the noise level of the recording titrator is so great the titration breaks cannot be reproduced. Two 1.5-ml samples are weighed into polyethylene weighing vials. One sample is transferred into a 250-ml wide-mouth (1) S.A. Long, ANAL.CHEM., 26,1968 (1954). (2) J. W. Jones and J. M. Dendy, Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, March 1951. (3) L. P. Morgenthales, Unpublished work; Procedure of Western Electric Co., Inc., Engineering Research Center, Princeton, N. J.
900 I
800 700
600
2
6
500
2-J -I
5
400
300
MILLILITERS
Figure 1. Potentiometric titration curves of etchant mixtures
(I) Curves for total acid of hydrofluoric, nitric, and acetic acid etchants a . Nitric acid b. Hydrofluoric plus acetic acid (11) Curves after esterification for determination of hydrofluoric and nitric acid a. Nitric acid c. Hydrofluoric acid Erlenmeyer flask containing 100 ml of methyl alcohol. The other vial is transferred to a tall-form 200-ml beaker containing 100 ml of methyl alcohol. The sample in the flask is refluxed for 30 minutes using a standard reflux apparatus. The total acid concentration of the sample in the beakers is determined potentiometrically (Figure 1). The other sample is allowed to cool after refluxing, transferred to a 200-ml tall-form beaker, chilled for 15 minutes in a cold bath, and titrated potentiometrically. The first break in the titration curve, after esterification, is nitric and the second is hydrofluoric acid (Figure 1).
~
Table I. Precision Study on Potentiometric Titration Method for Analysis of Etchants Wt found, Re1 std Etchant Component Wt std 30 analyses dev, Planar HF 4.5 4.5 2.1 "0%
CP-5
CH3COOH HF
23.1 47.5
23.0 47.7
1.9 0.5
12.4 28.9 31.7 3.0 7.2 83.9 4.5 63.6
1.7 2.8 1.5 1 .o 1.8 0.2 2.3 0.9
12.0 53.0
1.6 1.3
10-1
HF "01
12.6 28.7 31.6 3.0 7.1 83.7 4.5 63.6
3- 1
HF HNOp
11.8 53.2
"03
Special
1488
CH3COOH HF HNOi CHaCOOH
ANALYTICAL CHEMISTRY
DISCUSSION AND RESULTS
Refluxing the sample in a n excess of methyl alcohol converts the acetic acid to methyl acetate by the reaction:
CH3COOH
H+ + CHIOH e CH~COOCHI + H?O
This method of esterification, introduced by E. Fisher (4), calls for the addition of a mineral acid catalyst. An abundance of mineral acid is present in the sample, and it is obviously unnecessary to add more. Although the reaction is reversible, the yield of pure ester is high because an unbranched acid is being esterified with a primary alcohol. The large excess of methanol assures that the reaction is driven to completion for all practical purposes. This as(4) Fieser and Fieser, "Introductory to Organic Chemistry," D. C. Heath and Co., Boston, 1952, pp 137-41.
surance is substantiated by the experimental data obtained. The titration of the esterified sample is performed after the sample is chilled to prevent saponification of the ester which could possibly render less distinct end point. The sample sizes are critical in this method. If the etchant being analyzed contained less than 60% acetic acid, the sample sizes must be maintained between 0.8 and 1.4 grams. For etchants containing greater than 60% acetic acid, the sample sizes should be approximately 4 grams. Large errors in volumes of titrant used result when smaller sample sizes are used. Larger sample sizes require excess titrant which has a leveling effect and results in poor titration breaks. Five standard etchants were prepared and analyzed by this method. The samples were analyzed by three different analysts for a total of ten times each. The results of these analyses are shown in Table I. The method has a maximum relative standard deviation of 2.8 %. The results obtained
for each etchant compared well with the standard with a maximum relative error of 1.7%. This method, used for two years in our laboratory to analyze 200-250 samples per week, has proved to be reliable and accurate, and has resulted in time-saving over other methods mentioned in the literature. An entire analysis of a threecomponent etchant can be performed in 45 minutes. ACKNOWLEDGMENT The authors thank the personnel of the Analytical Service Laboratory of Texas Instruments Incorporated for technical assistance rendered. RECEIVED for review March 10, 1969. Accepted June 16, 1969. Presented at the Pittsburgh Conference, Cleveland, Ohio, March 1969.
Determination of Acid-Soluble and Acid-Insoluble Tin in Tough-Pitch Copper Using Wet Chemical Techniques C. H. McMaster Mineral Sciences Division, Mines Branch, Department of Energy, Mines and Resources, Ottawa, Canada THENONFERROUS STANDARDS Committee of the Spectroscopy Society of Canada (SSC) prepared four copper standards, similar in composition to commercial grades of tough-pitch copper, by the addition of various impurities to high-purity electrolytic copper. The metal was worked into rods approximately jl16-inch in diameter by 12 inches in length. After homogeneity tests, using spectrochemical techniques, the samples were distributed to participating laboratories for certification analyses. Details of the methods used in fabrication of the rods and the results of the homogeneity tests have been published ( I ) . Polarography was one of the wet chemical techniques used at the Mines Branch in the certification program, and was applied to the determination of bismuth, cadmium, lead, tellurium, tin, and zinc in the concentration range 0.1-50 ppm. The polarographic and spectrochemical results were in close agreement except in the case of tin. The spectrochemical values, which were the averages of large numbers of tests carried out by numerous laboratories, showed higher tin contents in three of the four standards. Certain chemical separations were required prior to the polarographic determination of tin but preliminary work using synthetic samples indicated that the method was applicable to the amounts present in the SSC standards. Also, the close agreement obtained with one of the standards seemed to suggest that the discrepancies were not due to the analytical procedure. The results of the homogeneity tests revealed that one sample was heterogeneous with respect to tin and, as this sample contained the largest amounts of tin and oxygen, it was proposed that the inhomogeneity might be due to the presence of tin(1V) oxide (2). (1) J. L. Dalton, R. Thomson, and A. H. Gillieson, Can. Spectrmc., 12, 58 (1967). (2) A. H. Gillieson, Mineral Sciences Division, Mines Branch, Department of Energy, Mines and Resources, Ottawa, Canada,
personal communication, 1966.
The existence of this compound can account for the low polarographic results as refractory tin oxide is insoluble in most acids. The term “acid-insoluble tin,” as used throughout this paper, refers to tin compounds that are insoluble in acids commonly used for the dissolution of copper and copperbase alloys. This interpretation excludes hot, concentrated hydriodic acid which is capable of dissolving tin(1V) oxide (3). As a qualitative test for tin oxide, a 30-gram sample of the standard having the highest tin content was dissolved in 1:1 nitric acid, the insoluble portion was separated from the solution by means of a centrifuge, washed with water, dried at 110 “C, and examined by X-ray diffraction. The residue was identified as tin(1V) oxide accompanied by a small amount of copper(I1) oxide. This paper describes the technique used in separating the acid-soluble tin from the interfering elements in tough-pitch copper, the treatment of the insoluble residue, and the polarographic determination of the tin in each fraction. EXPERIMENTAL Apparatus. A linear sweep K-1000 Cathode Ray Polarograph, manufactured by Southern Analytical Limited, Camberley, Surrey, England, was used in all analytical measurements. Cells of conventional design, as supplied with the polarograph, and capable of holding about 5 ml of solution were employed. Peak potentials are referred to the mercury pool anode. Sampling. The rods were cut into segments, each weighing about 5 grams; one segment was used for each determination. After weighing, the sample was treated with hot 1 :3 hydrochloric acid to remove surface contamination, and washed with water. Reagents. Ammonium pyrrolidinedithiocarbamate (APDTC), 0.2p7, wjv aqueous solution was prepared fresh daily. (3) S. L. Phillips, ANAL.CHEM., 32, 1062 (1960). VOL. 41, NO. 11, SEPTEMBER 1969
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