Analysis of semiconductor grade TMAH by dynamic reaction cell ICP-MS

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Courtesy of PerkinElmer, Inc.

Tetramethylammonium hydroxide (TMAH) is a clear, water-soluble, strongly alkaline organic solvent which is widely used as a developer in the photolithography process for semiconductor and Liquid Crystal Display (LCD) manufacturing. Because of its wide use, the analysis of impurities in TMAHs becomes more critical in these demanding applications. SEMI Standard C46-03061 specifies limits for 25% TMAH, generally limiting contamination to less than 10 ppb for each element, although some users may require lower levels of impurities.

Inductively coupled plasma mass spectrometry (ICP-MS) traditionally has been an indispensable analytical tool for useful quality control because of its ability to rapidly determine analytes simultaneously at the ultratrace (ng/L or parts-per-trillion) level in various process chemicals. However, it is extremely important to address the significant matrix-derived polyatomic interferences which form, as well as matrix suppression effects due to carbon content. These issues arise when analyzing organic solvents directly.

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 technique which uses a quadrupole mass filter where both RF and DC voltages 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-product 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 TMAH can be measured using hot plasma conditions for all analytes during a single analysis.

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