Comparison of heavy metal analyses in hydrofluoric acid used in microelectronic industry by ICP-MS and thermal ionization isotope dilution mass spectrometry

Abstract

An isotope dilution mass spectrometric (IDMS) method with the thermal ionization (TI) technique has been developed for the determination of trace impurities of Cr, Fe, Ni, Cu, Zn, Ag, Cd, Tl, Pb, Th, and U in high-purity HF (50% by weight) used in the semiconductor industry. The evaporation step of the HF solution was carried out in an apparatus which did not significantly contribute to contaminations of the heavy metals to be analysed. This apparatus allowed fast evaporation of the HF solution of up to 200 ml/h and therefore also a fast trace heavy metal/matrix separation was carried out. The evaporation step was also used in connection with inductively coupled plasma mass spectrometry (ICP-MS) when applying the isotope dilution technique and an external calibration for quantification, respectively. The detection limits for TI-IDMS were (in pg/g): Cr=30, Fe=400, Ni=70, Cu=20, Zn=1100, Ag=70, Cd=10, Tl=1, Pb=16, Th=3, and U=1. With ICP-MS in combination with the evaporation step, detection limits of less than 50 pg/g have been achieved for Cr, Ni, and Zn and of <5 pg/g for the other elements except Fe, which could not be determined in concentrations less than 100 ng/g. On the other hand, the detection limits were much higher when the HF matrix was not removed before measuring by ICP-MS. A comparison of the different ICP-MS methods (isotope dilution technique and external calibration for both HF evaporated samples and those with HF matrix) with the results of TI-IDMS has been carried out. An excellent agreement was achieved between the results of TI-IDMS and the two ICP-MS methods using the HF evaporation step, whereas the ICP-MS techniques without HF evaporation essentially deviated from these results. Fe was the only trace element of all investigated heavy metals which could only be analysed by TI-IDMS in high purity HF in a concentration of about 3 ng/g. Although ICP-MS with isotope dilution and external calibration resulted in comparable analytical data, the ICP-IDMS method has some practical advantages such as time-saving and more reliable results.