Routine determination of ultratrace elements in semiconductor grade nitric acid by the Thermo Scientific iCAP RQ ICP-MS

Significance of the Topic

The semiconductor industry requires ultrapure reagents to avoid device defects. Nitric acid used for wafer cleaning and etching must be free of metal contaminants at sub-ng·L⁻¹ levels. Reliable ultratrace analysis methods ensure process control and yield optimization.

Objectives and Study Overview

This study evaluates the Thermo Scientific iCAP RQ ICP-MS for routine determination of ultratrace metals in semiconductor-grade nitric acid. It compares three operating modes—hot plasma, cold plasma, and hot plasma with kinetic energy discrimination (KED)—to achieve low background equivalent concentrations (BEC) and detection limits (LoD).

Methodology and Instrumentation

Samples and calibration standards were prepared in precleaned PFA bottles using 1% HNO₃. Multielement standards (50–1000 ng·L⁻¹) enabled calibration, and a 10 ng·L⁻¹ spike recovery test assessed accuracy. The iCAP RQ ICP-MS configuration included:

• MicroFlow PFA-100 nebulizer
• Peltier-cooled quartz spray chamber (3 °C)
• 2.0 mm sapphire injector and quartz torch
• Platinum sampler and skimmer cones
• Dynamic frequency RF generator for hot (1550 W) and cold (550 W) plasma
• QCell collision/reaction cell with He KED at 3.5 mL·min⁻¹

Main Results and Discussion

In 1% HNO₃, 64 elements were measured in mixed mode with automated switching. Typical LoD values ranged from 0.01 to 2.5 ng·L⁻¹ and BEC from 0.01 to 5.2 ng·L⁻¹. Spike recoveries at 10 ng·L⁻¹ were 94–114%, demonstrating accuracy. Cold plasma operation reduced system-derived contamination and enhanced sensitivity for first- and second-group metals and transition elements.
Improved cold plasma stability allowed direct analysis of up to 7% HNO₃ without additional dilution. In this matrix, LoD and BEC for twelve key elements (Li, Na, Mg, Al, K, Ca, Cr, Mn, Fe, Ni, Co, Cu) were below 0.5 ng·L⁻¹ and 5 ng·L⁻¹, with spike recoveries of 92–108% and linear calibrations from 1 to 5 ng·L⁻¹.

Benefits and Practical Applications of the Method

• Sub-ng·L⁻¹ detection limits for a broad multielement panel
• Reduced contamination and low backgrounds via cold plasma
• Automated mode switching increases throughput and robustness
• Minimal sample handling decreases risk of external contamination

Future Trends and Potential Applications

• Expansion to other high-purity acid matrices (HF, HCl) in semiconductor processes
• Integration with automated sample preparation for inline quality control
• Advanced cell chemistries to further suppress polyatomic interferences
• Application to nuclear and geoscience ultratrace challenges

Conclusion

The iCAP RQ ICP-MS delivers the sensitivity, interference control, and operational flexibility needed for ultratrace metal analysis in semiconductor-grade nitric acid. Cold plasma and KED enhance performance, enabling reliable sub-ng·L⁻¹ quantification without cleanroom installation or extensive sample dilution.

References

• Vincent T., Hsu J., Wu V., Wills J. Routine determination of ultratrace elements in semiconductor grade nitric acid by the Thermo Scientific iCAP RQ ICP-MS. Application Note 43176. Thermo Fisher Scientific; 2017.