Determination of total acids in highly acidic etch solutions

Significance of the Topic

Precise determination of total acid concentration in nitric–hydrofluoric etch solutions is critical for ensuring consistent silicon wafer etching, protecting equipment, and maintaining product quality. Thermometric titration offers a fast and robust approach well suited to highly acidic, corrosive mixtures.

Study Objectives and Overview

The application note describes a method for quantifying total acids in concentrated etch solutions by direct thermometric titration with a standard sodium hydroxide solution. The goal is to achieve accurate, reproducible results with minimal interference from hydrofluoric acid.

Methodology and Instrumentation

The method involves:

• Weighing approximately 1 g of etch solution into a PFA titration vessel and diluting with 30 mL of deionized water.
• Titrating with 2 mol/L NaOH delivered at a rate of 2.5 mL/min.
• Detecting a single exothermic endpoint via a thermometric sensor and second-derivative analysis.
• Applying a data smoothing factor of 55 and stirring at speed setting 6 using an 802 stirrer.

Used Instrumentation

• Automated Titrotherm system with Dosino dispenser protected by a soda-lime guard tube.
• PFA titration vessel (10–90 mL).
• Thermometric endpoint sensor and 802 stirrer module.

Main Results and Discussion

Analysis of a 70 % HNO3-based etch solution yielded a total acid content of 69.30 ± 0.04 % w/w HNO3 (n = 9). A single, well-defined thermometric endpoint was observed, reflecting the low HF concentration. Blank titrations demonstrated excellent linearity (R2 = 1.000) with a y-intercept of 0.0351 mL, confirming baseline stability and method precision.

Benefits and Practical Applications

• Rapid endpoint detection without visual indicators.
• High accuracy and repeatability in strongly acidic matrices.
• Minimal interference from trace HF.
• Ideal for routine quality control in semiconductor etch bath preparation.

Future Trends and Potential Applications

This thermometric titration approach can be extended to other aggressive acid mixtures and adapted for inline process monitoring. Development of miniaturized or flow-through thermometric systems may enable real-time control. Integration with speciation techniques could allow differentiation of individual acid components in complex samples.

Conclusion

Thermometric titration with 2 mol/L NaOH provides a straightforward, reliable method for determining total acid content in highly acidic etch solutions. Its robustness and precision make it an effective tool for quality assurance in silicon substrate processing.

Reference

Metrohm Application Note H-100, 2009