Direct Measurement of Metallic Impurities in 20% Ammonium Hydroxide by 7700s/7900 ICP-MS

Importance of the Topic

Ammonium hydroxide (NH4OH) is a key cleaning reagent in semiconductor fabrication, used in mixtures such as Standard Clean 1 (SC-1) to remove particles and contaminants from silicon wafers. Trace metal impurities in NH4OH can introduce defects and compromise device yields, making sensitive, reliable analysis of undiluted 20% NH4OH essential for quality assurance.

Objectives and Study Overview

This study demonstrates the direct determination of metallic impurities at the single-digit parts-per-trillion (ppt) level in undiluted 20% NH4OH using an Agilent 7700s/7900 ICP-MS. The goals were to avoid sample dilution or matrix removal, maintain plasma stability, and achieve robust quantification by the method of standard additions (MSA).

Methodology and Measurement Approach

– Calibration was performed by sequentially spiking undiluted NH4OH with multi-element standards at 10, 20, 50, and 100 ppt.
– Agilent’s Octopole Reaction System (ORS) was used in multiple modes (no gas, helium collision, hydrogen reaction, and cool plasma) to remove spectral interferences across all analytes.
– A single-visit, multi-mode acquisition combined data from each mode into one quantitative report per sample vial, with an 8 min 20 s cycle including uptake and rinse.

Used Instrumentation

• Agilent 7700s ICP-MS with high-speed frequency-matching RF generator for instantaneous plasma stabilization during aspiration of high-vapor matrices.
• Inert sample introduction: PFA double-pass spray chamber, demountable torch with 1.5 mm sapphire injector, platinum interface cones, and PFA concentric nebulizer.
• ORS enabling no gas, He collision, H2 reaction, and cool plasma modes for optimized interference removal.

Main Results and Discussion

Detection limits (3σ) for 48 elements were in the single-digit ppt or sub-ppt range, with background equivalent concentrations similarly low (except Ca at 11 ppt). Calibration plots for vanadium (He mode) and iron (cool plasma) exhibited excellent linearity at ppt levels. A long-term stability test over ~7 hours for 22 elements spiked at 100 ppt showed ~5% RSD, confirming minimal precipitation or adsorption in the alkaline matrix and stable MSA performance.

Benefits and Practical Applications of the Method

Direct ICP-MS analysis of undiluted 20% NH4OH eliminates matrix removal steps, reducing analysis time, risk of contamination, and potential loss of volatile species. This approach streamlines semiconductor chemical quality control and supports high-throughput laboratory workflows.

Future Trends and Potential Applications

Advances may include automated sample handling for increased throughput, application to other high-vapor matrices in semiconductor processing, integration with process monitoring systems, and coupling with separation techniques for speciation analysis.

Conclusion

The Agilent 7700s/7900 ICP-MS, with its high-speed RF generator and versatile ORS interference removal, enables routine, direct quantification of metallic impurities in undiluted 20% NH4OH at ppt levels. Reliable MSA calibration and excellent signal stability make this method a robust tool for semiconductor chemical analysis.

References

1. Takeda K.; Ikushima S.; Okuzaki J.; Watanabe S.; Fujimoto T.; Nakahara T. Direct determination of trace metals in ammonium hydroxide by ICP-MS following matrix removal. Analytica Chimica Acta 426(1), 2001, 105–109.