Online monitoring of sulfuric acid and hydrogen peroxide using Raman spectroscopy

Importance of the Topic

In semiconductor manufacturing, wet etching processes rely heavily on precise control of chemical bath compositions. The use of sulfuric acid–peroxide mixes (SPM and DSP) demands accurate real-time monitoring to ensure optimal etch rates, selectivity, and uniformity. Conventional manual sampling poses safety risks and potential inaccuracies, highlighting the need for online analytical techniques.

Study Objectives and Overview

This application note describes the development and validation of an online Raman spectroscopy method using the PTRam Analyzer to simultaneously monitor sulfuric acid and hydrogen peroxide concentrations in SPM and DSP etching solutions. The goal is to enable real-time process control, improve safety, and reduce production variability.

Methodology

The study employs Raman spectroscopy to acquire real-time spectral data through PFA tubing directly in the wet bench. Calibration models are built by correlating Raman spectra with reference titrations, enabling quantitative analysis of both acid and peroxide in the mixed baths.

Used Instrumentation

• PTRam Analyzer (Metrohm Process Analytics): A compact 785 nm Raman spectrometer with automated calibration and performance validation.
• PFA tubing: Chemically resistant fiber-optic probe interface allowing measurements through transparent lines.

Key Results and Discussion

The Raman method demonstrated rapid and accurate determination of H₂SO₄ and H₂O₂ concentrations with minimal operator intervention. Spectral data correlated closely with titration references, enabling continuous monitoring and timely corrections. The small footprint of the PTRam Analyzer suits confined wet bench environments.

Benefits and Practical Applications

• Enhanced safety by eliminating direct handling of hazardous acids.
• Improved etch uniformity and process reproducibility through real-time feedback.
• Increased throughput and reduced waste by maintaining optimal bath composition.

Future Trends and Possibilities

Advancements may include integration with distributed control systems (DCS/PLC/SCADA) for automated dosing adjustments, expansion to other etching chemistries, and the development of more robust multivariate models to monitor additional bath parameters simultaneously.

Conclusion

Online Raman spectroscopy using the PTRam Analyzer offers a reliable, reagent-free solution for real-time monitoring of sulfuric acid and peroxide in semiconductor wet etching baths. This approach enhances safety, process control, and operational efficiency.

Reference

• Clews PJ, Nelson GC, Matlock CA et al. Sulfuric Acid/Hydrogen Peroxide Rinsing Study. Sandia National Laboratories.
• Dry Etching vs. Wet Etching - Differences and Applications. Xometry Blog, accessed May 3, 2023.