Determination of Trace Metal Impurities in Semiconductor Grade Phosphoric Acid by High Sensitivity Reaction Cell ICP-MS

Importance of the Topic

Phosphoric acid is widely used for wet etching of silicon nitride films in semiconductor manufacturing. Even trace levels of metallic impurities can cause wafer defects and reduce yield. As device critical dimensions continue to shrink, controlling sub-ppb metal contamination in high-purity process chemicals becomes essential for maintaining performance and reliability.

Study Objectives and Overview

This study demonstrates a robust analytical workflow for quantifying trace metals in 85% (w/w) semiconductor-grade phosphoric acid. The main goals were to meet SEMI C36-0301 purity specifications, achieve ppt-level detection limits, and simplify sample throughput by using an Agilent 7500cs ICP-MS with an octopole reaction system (ORS).

Methodology and Instrumentation

Sample Preparation:

• 100× dilution of 85% H3PO4 in ultrapure water to reduce viscosity.
• Standard additions calibration from 20 to 500 ppt; single-spike approach for subsequent external calibration.

Instrumentation:

• Agilent 7500cs ICP-MS equipped with ORS, ShieldTorch interface, and PFA Inert Kit.
• Cell gases: hydrogen (for reactive removal of plasma- and matrix-based interferences) and helium (for collisional dissociation and energy discrimination).
• Analysis modes: normal high-power plasma, helium collision mode, hydrogen reaction mode, and cool plasma.
• Automated data acquisition and report generation via ChemStation software.

Key Results and Discussion

Interference Removal and Sensitivity:

• ORS cell gases suppressed Ar, PO, POH, PCO, and other polyatomic overlaps on Ti, Co, Ni, Cu, and Zn.
• New extraction lens design reduced background equivalent concentrations (BECs) of easily ionized elements (Li, Na, Mg, Al, K) at 1500 W to levels comparable with cool plasma.

Analytical Performance:

• Detection limits (3σ) in 0.85% H3PO4 ranged from 0.06 to 28 ppt for SEMI-specified elements.
• Spike recoveries at 50 ppt were between 90% and 104% for key analytes.
• Short-term stability over 3 hours showed RSD <15% for all measured elements.

Comparison of Modes:

• High-power hydrogen mode and cool plasma mode both delivered low BECs and ppt detection limits, enabling flexible method deployment without sacrificing throughput.

Benefits and Practical Applications

This approach enables semiconductor fabs and analytical labs to:

• Directly measure trace metals in concentrated H3PO4 without complex sample cleanup.
• Maintain high sample throughput (≈4 min per analysis) using a single analytical run with automatic mode switching.
• Avoid internal standard addition, reducing contamination risk and simplifying workflows.

Future Trends and Opportunities

Ongoing advances may include:

• Integration of automated sample handling for real-time process monitoring.
• Further reduction of blank levels through ultra-clean acids and materials.
• Extension of reaction-cell ICP-MS to heavy matrix environments and new critical elements.

Conclusion

The Agilent 7500cs ICP-MS with ORS and ShieldTorch interface offers a powerful solution for ultratrace metal analysis in semiconductor-grade phosphoric acid. It delivers reliable ppt detection limits, robust interference removal, and efficient operation, meeting SEMI purity specifications and supporting high-yield manufacturing.

References

1. Werner Kern. Handbook of Semiconductor Wafer Cleaning Technology. William Andrew Publishing; 1993. Chap 2, section 2.3, p. 8.
2. K. Sakata and K. Kawabata. Reduction of Fundamental Polyatomic Ions in Inductively Coupled Plasma Mass Spectrometry. Spectrochimica Acta Part B 1994;49:1027.
3. N. Yamada, J. Takahashi, and K. Sakata. The Effects of Cell-gas Impurities and Kinetic Energy Discrimination in an Octopole Collision Cell ICP-MS under Non-Thermalized Conditions. Journal of Analytical Atomic Spectrometry 2002;17:1213–1222.
4. K. Kawabata, Y. Kishi, and R. Thomas. The Benefits of Dynamic Reaction Cell ICP-MS Technology to Determine Ultratrace Metal Contamination Levels in High-Purity Phosphoric and Sulfuric Acid. Spectroscopy 2003;18(1):16–31.