Determination of Hydrofluoric Acid in Silicon Etch solutions

Importance of the Topic

Hydrofluoric acid is widely used in silicon etching processes but poses significant safety and quality-control challenges due to its corrosive nature and low working concentrations. Accurate quantification of fluoride content in etch mixtures is essential for process optimization, environmental safety and regulatory compliance.

Objectives and Overview of the Study

This application note presents a thermometric titration method for rapid and precise determination of fluoride ions in industrial silicon etch solutions. The goal is to demonstrate method robustness in high-nitric acid matrices containing low levels of hydrofluoric acid.

Methodology

The approach relies on the exothermic reaction between fluoride ions and boric acid under acidic conditions. Key reaction:

H₃BO₃ + 3H⁺ + 3F⁻ → HBF₃(OH) + 2H₂O

Only this fast reaction is observed when fluoride is analyte and boric acid is titrant, yielding a stoichiometry of three moles of fluoride per mole of boric acid. The procedure involves:

• Reagent preparation: 0.7 mol/L boric acid solution
• Sample preparation: Weigh ~20 g of etch solution and dilute to ~30 mL with deionized water
• Titration parameters: Titrant delivery rate 2.5 mL/min, one exothermic endpoint, data smoothing factor 80
• Equilibration: 120 s pre-titration delay under stirring to stabilize temperature
• Endpoint detection: Thermometric signal and its second derivative pinpoint the exothermic inflection

Used Instrumentation

• PFA plastic titration vessel (e.g., 6.1450.210, 10–90 mL) to avoid glass corrosion
• Fluoride-resistant Thermoprobe (6.9011.040) for precise temperature measurements
• Thermometric titrator with endpoint detection based on temperature and derivative curves

Main Results and Discussion

Replicate analyses of a used silicon etch solution yielded hydrofluoric acid contents of 0.049 %, 0.047 %, 0.047 % and 0.046 %. A blank determination showed titrant consumption of 0.14–0.24 mL, confirming negligible background interference. The thermometric titration plot displays a clear temperature rise (red curve) and a sharp second derivative peak (black curve) at the endpoint. Good linearity (R² = 0.9837) was observed between titrant volume and sample mass despite the small titration volumes involved.

Benefits and Practical Applications

This thermometric titration method offers several advantages:

• Rapid analysis with a single exothermic endpoint
• High precision at low fluoride concentrations
• Resistance to corrosion by using plastic vessels and specialized probes
• Simple sample preparation suitable for routine QA/QC in semiconductor manufacturing

Future Trends and Potential Applications

Advances may include integration with automated sample handling, miniaturized titration cells for micro-volumes, and coupling with data-analysis software for real-time process monitoring. The compatibility of thermometric endpoints with inline sensors could enable continuous control of etch bath composition.

Conclusion

Thermometric titration with boric acid provides a robust, accurate and corrosion-resistant technique for determining hydrofluoric acid in silicon etch solutions. Its simplicity and repeatability make it ideal for industrial applications requiring precise monitoring of fluoride levels.

References

Metrohm Application Note No. H-098 (2009) Determination of Hydrofluoric Acid in Silicon Etch Solutions