Determination of an Anionic Fluorochemical Surfactant in a Semiconductor Etch Bath

Significance of the Topic

Perfluorinated surfactants are critical wetting agents in semiconductor acid etch baths. They prevent air entrapment during pattern engraving in silicon dioxide, ensuring precise etch profiles and avoiding device defects such as electrical shorts. Accurate quantification of these high value chemicals at low mg/L levels is essential for process control, cost efficiency and yield optimization.

Objectives and Article Overview

This study presents an ion chromatography method to determine trace levels (low mg/L) of an anionic fluorochemical surfactant FC-93 in a hydrofluoric acid/ammonium fluoride etch bath. The approach integrates on line sample cleanup with high sensitivity suppressed conductivity detection, aiming to deliver robust, reproducible quantification for semiconductor manufacturing quality control.

Methodology and Used Instrumentation

The method employs a two column setup: an IonPac NG1 guard concentrator column for on line matrix elimination, followed by an OmniPac PAX-500 analytical column configuration with both reversed phase and anion exchange retention characteristics. Key instrumentation includes the Dionex DX-500 IC system with GP40 gradient pump, CD20 conductivity detector with ASRS 4 mm, DQP rinsing pump, and rear loading injection valves. Samples (100 µL) are loaded in reverse flow onto the NG1 column, rinsed with deionized water (2 mL/min for 20 min) to remove the HF matrix, and then eluted using a 55:45 mixture of 20 mM sodium hydroxide and acetonitrile at 1 mL/min.

Main Results and Discussion

The system maintains a conductivity background of 0.3–3 µS and peak to peak noise of approximately 10 nS. Five replicate injections of 5 mg/L FC-93 in an etch bath yielded an average area count of 29 625 with area %RSD of 0.49 and retention time %RSD of 2.16. The surfactant peak is well resolved from fluoride and system peaks. Linearity was confirmed over 5–15 mg/L (R² = 0.9998), with similar calibration curves in pure water and etch matrix. A minimum 20-minute rinse is critical to prevent fluoride breakthrough and ensure chromatographic stability.

Benefits and Practical Applications of the Method

This approach enables on line removal of aggressive acid matrices and sensitive detection of fluorochemical surfactants without manual cleanup, reducing sample handling and contamination risk. The method supports real time monitoring of etch bath surfactant concentration, optimizing bath lifetime, reducing chemical costs, and enhancing etch process reliability.

Future Trends and Potential Applications

Future developments may include integration of smaller centralized sample loops to improve detection limits, use of alternative detectors for enhanced selectivity, and adaptation to other perfluorinated compounds. Automation and miniaturization of the cleanup module could accelerate analyses and enable inline process analytics in semiconductor fabrication.

Conclusion

The described ion chromatography protocol offers a robust, sensitive, and automated solution for quantifying low µg/mL levels of anionic fluorochemical surfactants in hydrofluoric acid based etch baths. Its high reproducibility and linearity make it well suited for semiconductor process control and QA/QC applications.

References

• Dionex Corporation OmniPac Guidebook P N 34517
• Dionex Corporation Application Note 85 Determination of Trace Anions in Isopropyl Alcohol
• Kaiser E Wojtusik M J J Chromatogr A 1994 671 253 258