DETERMINATION OF TRACE AMOUNTS OF ANIONIC IMPURITIES IN HYDROCHLORIC-ACID BY ION CHROMATOGRAPHY

The purity of the hydrochloric acid used in semiconductor processing has a direct impact on device reliability because ionic impurities adhere to the wafer's surface during processing, and the impurities must be monitored in order to avoid both unnecessary wastes and exceeding of the permitted levels. Ion-chromatographic procedures for the determination of bromide, nitrate, phosphate, selenate and arsenate in hydrochloric acid are described. The high chloride concentration and the pH of the sample cause major problems that, for monovalent anions, can be overcome by simply diluting the sample, while matrix elimination in the presence of potassium permanganate is effective in the determination of polyvalent anions. The optimum pH range, concentration of the matrix and of the eluent, and working conditions have been investigated. The use of different eluents has been suggested for the determination of bromide and nitrate after matrix dilution, as these ions elute very near to the large chloride peak. The procedures described have been successfully used for the routine analysis of hydrochloric acid batches used for cleaning semiconductors.