Online monitoring of sulfuric acid and hydrogen peroxide using Raman spectroscopy

Significance of the Topic

Wet etching processes such as sulfuric acid-peroxide mix (SPM) and dilute sulfuric acid-peroxide mix (DSP) are critical steps in semiconductor fabrication for removing material layers. Accurate control of acid concentrations directly impacts etching rate, selectivity, and uniformity, while manual sampling poses safety risks and can disrupt production.

Objectives and Study Overview

This application note demonstrates an online, reagent-free Raman spectroscopy approach for simultaneous monitoring of sulfuric acid and hydrogen peroxide concentrations in SPM and DSP solutions. The aim is to enable real-time process control, enhance safety, and improve analytical accuracy compared to conventional manual titration methods.

Methodology and Instrumentation

The study employs a 785 nm Raman probe connected to the PTRam Analyzer, leveraging the spectrometer's ability to measure through transparent PFA tubing. The built-in IMPACT software processes spectral data, while results can be transmitted to Distributed Control Systems (DCS), PLCs, or SCADA networks for automated feedback.

Used Instrumentation

• PTRam Analyzer from Metrohm Process Analytics featuring self calibration and automated performance validation
• 785 nm Raman excitation source
• PFA tubing for chemical resistance and direct in situ measurements
• IMPACT software for data processing
• Connectivity to DCS, PLC, and SCADA systems

Main Results and Discussion

The online Raman system achieved accurate, simultaneous quantification of both sulfuric acid and hydrogen peroxide in mixed acid baths, matching reference titration methods. The compact analyzer facilitated installation in restricted wet bench areas without process interruption, and continuous monitoring provided high temporal resolution.

Benefits and Practical Applications

• Enhanced safety by eliminating direct operator exposure to corrosive reagents
• Improved process control through real time concentration measurements
• Reduced sampling errors and downtime associated with manual methods
• Optimized wafer cleaning efficiency, increased throughput, and reduced waste

Future Trends and Opportunities

Integration of Raman spectroscopy with advanced chemometric modeling and artificial intelligence could further enhance predictive maintenance and process optimization. Miniaturization of probes, expansion to additional etchant chemistries, and integration within Industry 4.0 frameworks will drive broader adoption.

Conclusion

Reagent-free online Raman spectroscopy using the PTRam Analyzer provides a robust, safe, and efficient solution for monitoring sulfuric acid and hydrogen peroxide concentrations in wet etching processes. Its implementation supports improved product quality, operational safety, and process economics.

References

1. Dry Etching vs. Wet Etching - Differences and Applications. Xometry Blog. Accessed 2023-05-03.
2. Clews PJ, Nelson GC, Matlock CA et al. Sulfuric Acid/Hydrogen Peroxide Rinsing Study, Sandia National Laboratories.