Determination of chloride and sulfate in semiconductor-grade etchants comprised of acetic acid, nitric acid and phosphoric acid

The variable-capacity Dionex Cryptand A1 column was used for the determination of low-ppm levels of chloride and sulfate in etchants comprised of acetic acid, nitric acid and phosphoric acid. All possible ratios of the three acids could be analyzed for chloride and sulfate, if the samples were first diluted 1:100. However, a suitable eluent program was found to be needed for each mixture. A proprietary formulation was chosen to undergo this suitability determination. The resulting gradient was 10 mM KOH with a step to 30 mM NaOH at 15 min, flow-rate=0.5 ml/min; column temperature=29 °C; sample loop=7.5 μl. Under these conditions, a low-ppm calibration study (using the proprietary mix as the matrix) was performed and the associated prediction intervals were determined. At 50 ppm (in the original etchant), the ±prediction interval was ±7 ppm for chloride and ±20 ppm for sulfate, both at the 95% confidence level. This step gradient was found to be a good starting place for separating the five components in all other ratios of these three acids.