The Benefits of Dynamic Reaction Cell ICP-MS Technology to Determine Ultratrace Metal Contamination Levels in High-Purity Phosphoric and Sulfuric Acid

摘要

This study investigates the applicability of a quadrupole-based ICP-MS fitted with a dynamic reaction cell (DRC) to analyze high-purity phosphoric and sulfuric acid used in the semiconductor industry. It compares the DRC approach with traditional ICP-MS background reduction techniques to compensate for phosphorus- and sulfur-based interferences and presents data that suggests that the DRC technology can reach the next generation of semiconductor purity levels for these chemicals. The drive for more-compact integrated circuits and smaller electronic devices has put very stringent demands on suppliers of high-purity chemicals used in the semiconductor manufacturing process. To reduce defects and increase yield, lower trace element contamination levels are being required in all process chemicals, especially as initiatives like the International Technology Roadmap for Semiconductors (ITRS) (1) are setting the course for the next generation of semiconductor devices. Of particular significance are contamination issues associated with high-purity acids, because their aggressive dissolution properties make them prone to acquire metal from other sources. In addition, they are often used in large quantities with other chemicals to dissolve metals from the surface of silicon wafers, to build a layer of silicon dioxide on top of the silicon substrate, or as an etchant. For these reasons, trace element purity levels in concentrated mineral acids must be monitored very closely. Two of the most important acids used in the manufacture of semiconductor devices are phosphoric and sulfuric acids. Even though trace element purity levels are not as strict as for acids such as hydrofluoric and hydrochloric, they are still low enough to present problems for the analytical techniques being used. The major difficulties are related to their extremely high acid concentration — ～85% w/v for phosphoric acid and ～98% w/v for sulfuric acid — which produces corrosion problems and very high specific gravity — 1.70 g/mL for phosphoric acid and 1.83 g/mL for sulfuric acid — which in turn produces sample viscosity effects. In addition, when inductively coupled plasma-mass spectrometry (ICP-MS) is required to analyze the highest purity acids, the phosphate and sulfate matrices generate a significant number of plasma-based ionic species, which interfere with many of the analyte elements.