Determination of trace anions in hydrofluoric acid, ammonium fluoride, and a buffered oxide etchant

Importance of the Topic

Concentrated hydrofluoric acid (HF) and buffered oxide etchant (BOE) are critical in semiconductor manufacturing for removing oxide layers. Impurities such as chloride, sulfate, nitrate and phosphate at trace levels can damage devices. Industry standards set by SEMI require HF purity with anion concentrations below specified limits to ensure process reliability and yield.

Objectives and Study Overview

This work presents an improved method for determining trace anions in high-purity HF, ammonium fluoride, and BOE. It aims to achieve low detection limits in samples with high fluoride content without overloading chromatography columns, using an automatic heart-cutting ion-exclusion technique combined with ion chromatography and method of standard additions for accurate quantification.

Methodology

An ion-exclusion separation on a Dionex IonPac ICE-AS1 column isolates strong acid anions from excess fluoride. The excluded fraction is diverted to a high-capacity concentrator (AG11-HC) and then analyzed by ion chromatography on an AS11-HC column with suppressed conductivity detection. A dual-pump system automates sample loading, fraction collection, analytical separation and ICE column cleaning using periodic formic acid washes. Calibration is performed by standard additions directly in the sample matrix to account for matrix effects.

Instrumentation Used

• Dionex ICS-3000 ion chromatography system with DP dual pump and DC detector module
• EG eluent generator module (EGC II KOH or EGC III KOH)
• Chromeleon chromatography data system
• ASRS-300 self-regenerating suppressor external water mode
• Dionex IonPac columns ICE-AS1, ATC-HC trap, AG11-HC concentrator and guard, AS11-HC analytical
• Dionex AS autosampler and AM automation manager with 10-port valve

Main Results and Discussion

The method delivered detection limits of 1–10 µg/L in 12% HF, 8.3% ammonium fluoride and BOE (10× dilution), meeting SEMI specifications. Calibration was linear (R2 > 99.5%) and recoveries exceeded 90% for all anions in spiked samples. Retention time reproducibility (RSD < 0.7%) confirmed system robustness. Using 12% HF and a 500 µL injection reduced HF consumption and extended column lifetime.

Practical Benefits and Applications

• Meets industry purity requirements for semiconductor chemicals
• Reduces sample consumption and improves sensitivity
• Automated workflow minimizes manual steps and risk of contamination
• Applicable to HF, ammonium fluoride and BOE matrices

Future Trends and Opportunities

Advances in column technology and eluent generation may further lower blanks and detection limits. Integration with online process monitoring and microfluidic platforms could enable real-time control of etch processes. Adaptation to other aggressive acid matrices and coupling with mass spectrometry are potential extensions.

Conclusion

The optimized heart-cutting ion-exclusion and ion chromatography method offers a reliable, sensitive and automated solution for trace anion analysis in challenging high-fluoride matrices, supporting semiconductor quality control and process monitoring.

Reference

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