Determination of Transition Metals at PPT Levels in High-Purity Water and SC2 (D-clean) Baths

Importance of the Topic

The presence of trace transition metals in high-purity water and semiconductor cleaning baths can cause critical defects in device fabrication. Monitoring ppt-level metal contaminants such as Fe, Cu, Zn ensures process reliability and extends equipment life.

Objectives and Overview of the Study

This study aims to develop and validate an ion chromatographic method capable of quantifying transition metals at low ng/L (ppt) levels in ultrapure water and SC2 (D-clean) semiconductor baths. The focus is on achieving high sensitivity through preconcentration and selective detection.

Methodology

The method uses an IonPac CS5A column with a PDCA-based eluent to form stable anionic metal complexes. Postcolumn derivatization with 4-(2-pyridylazo)resorcinol (PAR) enables visible detection at 520–530 nm. Acidified samples (2 mM HCl) undergo preconcentration on a TCC-2 concentrator column at 2 mL/min for 5 to 15 minutes, depending on sample volume.

Used Instrumentation

• Dionex DX-500 Ion Chromatography system
• GP40 Gradient Pump (microbore)
• AD20 UV/Vis detector (10 mm cell)
• LC30 Chromatography Enclosure with Rheodyne valve
• Concentrator Pump DQP and RP-1 postcolumn reagent pump
• Knitted reaction coil and pressurizable reservoir chamber
• IonPac CS5A analytical and CG5A guard columns
• TCC-2 trace cation concentrator column

Main Results and Discussion

Chromatograms of a 1 µg/L standard concentrated from 30 mL show well-resolved peaks for seven transition metals. Ultrapure water blanks (10 and 30 mL) revealed iron background levels around 45 ng/L with RSDs below 2%. Analysis of SC2 bath samples indicated Fe3+ 80 ng/L, Cu2+ 75 ng/L, and Zn2+ 106 ng/L, reflecting contributions from bath chemicals.

Benefits and Practical Applications

This method provides reliable quantification of trace metals, supporting quality control in semiconductor manufacturing. It can be applied to monitor cleaning bath exhaustion, evaluate rinse water purity, and ensure compliance with industry water standards.

Future Trends and Potential Applications

Advancements may include coupling with mass spectrometry for enhanced selectivity, development of inline preconcentration modules for real-time monitoring, novel chelating agents for improved separation, and miniaturized systems for point-of-use analysis.

Conclusion

A robust ion chromatographic method with postcolumn PAR detection and sample preconcentration has been established for ppt-level analysis of transition metals in high-purity water and SC2 baths. The approach demonstrates high sensitivity, reproducibility, and practical applicability in semiconductor process control.

References

• Moreau WM. Semiconductor Lithography Principles, Practices, and Materials. Plenum Press, New York, 1988, pp. 270–280.
• SEMI. Suggested Guidelines for Pure Water Used in Semiconductor Processing. Document 2796, SEMI, 1998, pp. 1–3.
• Dionex. 2-mm Transition Metal System with Postcolumn Delivery Installation and Troubleshooting Manual, P/N 031355.