Analysis of impurities in nitric acid by dynamic reaction cell ICP-MS
Nitric acid (HNO3) is widely used in the semiconductor industry. Semiconductor devices are currently being designed with smaller line widths and are more susceptible to low level impurities. In more critical processes, the impurities in HNO3 need to be monitored for continuous performance at desired and achievable levels of quality. SEMI Standard C35-0708 specifies the maximum concentration of metal contaminants by element and tier for nitric acid.
Inductively coupled plasma mass spectrometry (ICP-MS) traditionally has been an indispensable analytical tool for quality control because of its ability to rapidly determine analytes simultaneously at the ultratrace (ng/L or parts-pertrillion) level in various process chemicals. However, it should be pointed out that under conventional plasma conditions, argon ions combine with matrix components to generate polyatomic interferences. Some of the common interferences are 38Ar1H on 39K, 40Ar on 40Ca, 40Ar16O on 56Fe.
While cold plasma has been shown to be effective in reducing argon-based interferences, it is even more prone to matrix suppression than hot plasma. Additionally, because of the low plasma energy, other polyatomic interferences which are not seen under hot plasma conditions, may be preferentially formed. Collision cells using multipoles and low reactive gases have proven useful in reducing polyatomic interferences. This approach necessitates the use of kinetic energy discrimination to remove the unwanted by-products. However, kinetic energy discrimination results in the loss of sensitivity, which is an issue when analyzing ng/L levels. Additionally, sensitivity loss is more significant for lighter analytes.
The Dynamic Reaction Cell (DRC™) is another correction technique which uses a quadrupole mass filter where both RF and DC voltage can be applied. The advantage of this configuration is that ions of a specific mass range pass through the cell, while ions outside of this range are ejected from the cell. This process is known as Dynamic Bandpass Tuning (DBT). As a result of this capability, undesirable by-products ions do not form within the cell, even when very reactive gases are used, such as NH3 and O2.
This application demonstrates the DRC’s ability to easily remove interferences so that trace levels of impurities in HNO3 can be measured using hot plasma conditions for all analytes during a single analysis.