Automated analysis of etch acid mixtures using the 859 Titrotherm and the 814 USB Sample Processor

Importance of the Topic

The precise quantification of nitric acid, hydrofluoric acid and hexafluorosilicic acid in silicon etching solutions is critical for process control in semiconductor wafer fabrication and solar cell production. Automated thermometric titration offers a rapid and reliable alternative to classical methods, minimizing manual handling of highly corrosive reagents while maintaining high throughput and data quality.

Objectives and Overview of the Study

This study presents a fully automated protocol for determining the concentrations of HNO3 (200–600 g/L), HF (50–160 g/L) and H2SiF6 (0–185 g/L) using the Metrohm 859 Titrotherm coupled with the 814 USB Sample Processor. Two linked titration sequences are described for freshly prepared binary mixtures (HNO3 + HF) and for “used” etch solutions containing H2SiF6. Results are processed by the tiamo software to yield individual acid concentrations.

Methodology and Instrumentation

Thermometric titration relies on constant addition of titrant and direct measurement of the solution temperature with a Thermoprobe. Endpoints are detected as inflections on the temperature curve. The protocol involves:

• First titration with 2 mol/L NaOH to determine combined acid content, followed by back-titration with 2 mol/L HCl to quantify H2SiF6.
• Second titration with 0.5 mol/L Al(NO3)3 to isolate the HF concentration.
• Alternative two-step titration for fresh HNO3 + HF mixtures using NaOH followed by Al3+ titration.

Reagents include TRIS buffer, deionized water, standard NaF solution, potassium/sodium acetate buffer and specified titrants. Samples are prepared by diluting 1 mL of etch mixture to 30 mL or 15 mL of water, followed by programmed titration sequences.

Main Results and Discussion

Calibration of titrants via regression yielded molarities of 2.01 mol/L for HCl, 1.994 mol/L for NaOH and 0.5 mol/L for Al(NO3)3 with R2 values of 1.000, demonstrating excellent linearity. Method blanks were determined to correct for system offsets. Analysis of synthetic mixtures showed recoveries within ±1.0 g/L for HNO3 and HF, and ±0.5 g/L for H2SiF6 over six replicates. Representative thermometric and second derivative curves confirmed clear and reproducible endpoints.

Benefits and Practical Applications

• Fully automated workflow enhances laboratory safety by reducing manual handling of corrosive acids.
• High throughput analysis with minimal sample and reagent volumes conserves resources.
• Robust statistical calibration and blank correction improve accuracy for quality control in semiconductor and photovoltaic manufacturing.

Used Instrumentation

• 859 Titrotherm with 10 mL Dosino
• 814 USB Sample Processor
• 800 Dosino units and dosing units (5, 10 and 20 mL)
• Stirring propeller, sample tubes, sample rack and reagent organizer
• FEP tubing and stacking frame

Future Trends and Possibilities for Use

The integration of thermometric titration with robotic sample preparation and advanced data analytics will enable real-time process monitoring and control. Expansion of this approach to other aggressive or multi-component systems, combined with sensor miniaturization and IoT connectivity, promises broader industrial adoption. Further development of chemometric endpoint detection may enhance sensitivity and reduce analysis time.

Conclusion

The automated thermometric titration method using the 859 Titrotherm and 814 USB Sample Processor provides precise, reproducible and high-throughput quantification of HNO3, HF and H2SiF6 in etch acid mixtures. Its implementation strengthens quality assurance protocols and promotes safer handling of hazardous materials in semiconductor and solar cell manufacturing.