Analysis of semiconductor grade mineral acids

Importance of the Topic

High purity mineral acids such as hydrochloric, sulfuric and nitric acids are essential in semiconductor manufacturing for wafer cleaning and etching steps. Trace metal contaminants in these reagents can deposit on silicon surfaces and reduce yield. As device geometries shrink, allowable impurity levels fall into the sub-part-per-trillion range, demanding highly sensitive and accurate analytical methods.

Objectives and Study Overview

This study evaluates the performance of a high-resolution inductively coupled plasma mass spectrometer for quantifying trace metals in semiconductor-grade mineral acids. Key objectives include establishing detection limits, assessing background equivalent concentrations, verifying spike recoveries in matrix-matched samples and demonstrating reproducibility under different acid strengths.

Methodology

The analytical protocol comprised:

• Sample preparation by 1:10 mass-to-mass dilution of concentrated acid in ultrapure water without internal standard addition to minimize contamination.
• Quantification via standard addition at two spike levels (5 and 10 pg/g) across eighteen isotopes to correct for any residual matrix effects.
• Use of hot plasma conditions to reduce polyatomic interferences and ensure full elemental coverage in a single run.
• Measurement of blank, spiked and calibration solutions in triplicate to assess precision and accuracy.

Instrumental Setup

The analytical platform consisted of:

• Thermo Scientific Element 2 high resolution ICP-MS
• PFA micro-flow self-aspirating nebulizer and PFA spray chamber with O-ring-free endcap
• Sapphire or platinum injector, quartz torch and platinum-tipped cones for inert sample introduction
• Hot plasma operation and mass resolution settings up to R = 4000 for interference removal
• Data acquisition software configured for rapid resolution switching (<1 s) and low dark noise (<0.2 cps).

Main Results and Discussion

Detection limits (3σ) below 1 pg/g were achieved for 17 of 18 target elements in 10% m/m HCl. Background equivalent concentrations remained under 10 pg/g for all isotopes. Spike recovery experiments in high purity water and hydrochloric acid matrices yielded recoveries between 90% and 110%, demonstrating method accuracy. Reproducibility studies over a six-hour period showed relative standard deviations below 10% for all elements. Comparison of cold versus hot plasma modes highlighted minimal trade-offs: hot plasma maintained low backgrounds while avoiding non-spectroscopic interferences encountered at reduced power.

Benefits and Practical Applications

• Sub-50 fg/g detection capability supports specification compliance for ultra-trace analysis in semiconductor process chemicals.
• High resolution mode ensures unequivocal isotope identification and multi-element coverage without matrix separation steps.
• Inert sample introduction minimizes contamination, critical for reproducible trace measurements.
• Standard addition calibration compensates for residual matrix effects in concentrated acid media.

Future Trends and Potential Applications

Continued development of plasma-based mass spectrometry will focus on higher throughput, lower sample consumption and expanded interference removal techniques. Integration with chromatographic separation and automation of sample preparation could support real-time process monitoring. Advances in detector technology and data processing are expected to further lower detection limits and improve robustness for next-generation semiconductor manufacturing.

Conclusion

The described high-resolution ICP-MS method provides the sensitivity, accuracy and reproducibility required to quantify sub-pg/g trace metals in semiconductor-grade mineral acids. Its multi-element capability and robust interference control make it an effective tool for ensuring chemical purity and optimizing manufacturing yields.