Direct Analysis of Trace Metal Impurities in High Purity Nitric Acid Using ICP-QQQ

Significance of the Topic

The advancement of semiconductor devices toward smaller feature sizes and higher integration densities intensifies the impact of trace metal contamination on manufacturing yield. Ultrapure nitric acid is critical in wafer cleaning and etching steps, and precise quantification of metal impurities at sub-ppb levels is essential for quality control and process reliability.

Objectives and Study Overview

This work evaluates a direct analysis approach for undiluted high-purity and electronic-grade nitric acid using triple quadrupole inductively coupled plasma mass spectrometry (ICP-QQQ). The goals were to simplify sample preparation, avoid contamination from dilution, and demonstrate compliance with SEMI C35-0708 Tier-B requirements (<1 µg/L for key elements).

Methodology and Instrumentation

Two nitric acid samples were investigated:

• 68 % high-purity grade
• 61 % electronic grade

Sample introduction required no dilution, minimizing contamination risk. Calibration employed the method of standard additions (MSA) with spike levels of 5, 10, 20, 30, and 40 ppt. Multi-tune acquisition allowed sequential optimization of reaction cell conditions using gases (He, H₂, O₂, NH₃) under both cool and hot plasma settings. Key instrumentation features:

• Agilent 8900 ICP-QQQ in Semiconductor configuration
• PFA-100 nebulizer with self-aspiration
• Peltier-cooled quartz spray chamber and quartz torch
• Platinum-tipped sampling and skimmer cones, s-lens interface
• Dry pump option and ball-type interface valve for corrosion resistance

Acquisition parameters included RF power of 600 W (cool) to 1500 W (hot), optimized sampling depth, gas flows, and MS/MS mode for interference removal.

Results and Discussion

Forty-nine elements were quantified in undiluted 68 % nitric acid. Detection limits ranged from sub-ppt to single-digit ppt, with linear calibration across 0–40 ppt. All SEMI target elements were measured well below the 1 ppb limit. Representative quantitative results for high-purity versus electronic-grade samples included:

• Li: < 0.061 ng/L vs. 0.19 ng/L
• B: 3.5 ng/L vs. 270 ng/L
• Fe: 1.1 ng/L vs. 270 ng/L
• Pb: 0.21 ng/L vs. 0.31 ng/L

Long-term stability was assessed over 6.5 hours with repeated analysis of a 30 ppt spike. Relative standard deviations for 21 measurements were between 0.4 % and 5.5 % for most elements, demonstrating robust performance.

Benefits and Practical Applications

This direct-analysis method offers:

• Elimination of dilution steps and associated contamination risk
• High throughput for routine monitoring of process chemicals
• Sub-ppb sensitivity meeting SEMI standards
• Automated multi-tune capability for comprehensive interference control

It supports quality assurance in semiconductor fabs and other industries requiring ultratrace elemental analysis.

Future Trends and Potential Applications

Ongoing developments may include advanced cone materials and valve designs for extended system durability, expanded use of automated external calibration across multiple acid grades, and application of multi-tune ICP-QQQ methods to other high-purity reagents such as hydrofluoric and phosphoric acids.

Conclusion

The Agilent 8900 ICP-QQQ platform delivers exceptional sensitivity, low background, and precise interference control for direct analysis of undiluted high-purity nitric acid. It enables reliable ultratrace metal quantification in compliance with semiconductor industry standards.

Reference

1. SEMI C35-0708, Specifications and guidelines for nitric acid (2008).