Determination of challenging elements in ultrapure semiconductor grade sulfuric acid by Triple Quadrupole ICP-MS

Significance of the Topic

The accurate determination of trace metal contaminants in ultrapure semiconductor grade sulfuric acid is critical for maintaining the performance and yield of advanced semiconductor manufacturing processes. Conventional single quadrupole ICP-MS faces challenges from stable polyatomic interferences in high sulfur matrices, resulting in limited detection capabilities for elements such as titanium, vanadium and chromium at the required parts-per-trillion levels.

Objectives and Study Overview

This study demonstrates the application of triple quadrupole ICP-MS (ICP-QQQ) operated in MS/MS mode for the removal of persistent spectral interferences and the quantification of challenging elements in 98 percent purity H2SO4. Key goals include achieving sub-ppt detection limits, evaluating reaction cell gases and defining optimal reaction product ions for reliable quantification.

Methodology and Instrumentation

Instrument:

• Agilent 8800 Triple Quadrupole ICP-MS with Octopole Reaction System ORS3
• Additional quadrupole mass filter (Q1) positioned before the cell and analyzer quadrupole (Q2)
• C-Flow PFA micro-concentric nebulizer and quartz torch with platinum sampling and skimmer cones

Approach:

• Use of MS/MS mode to pre-filter precursor ions in Q1 and perform controlled reactions in the cell
• On-mass and mass-shift strategies using NH3 or O2 reaction gases
• Product ion scans to identify optimal reaction pathways for each analyte
• Calibration by standard addition into 10× diluted acid matrix

Key Results and Discussion

Product ion scans for 48Ti in 9.8 percent H2SO4 with NH3 revealed multiple Ti-NH3 cluster ions. The 48Ti(NH3)3NH+ ion at m/z 114 provided the best background equivalent concentration and was selected for quantification. Mass-shift mode with O2 cell gas enabled interference-free detection of Cr, Ge, As and Se as MO+ species (16 amu shift). Vanadium, unreactive with O2, was measured on-mass at m/z 51 using NH3 gas. Calibration curves showed linear response down to 10 ppt, with sub-ppt detection limits. Method of standard addition corrected for residual matrix effects and yielded impurity levels: Ti 7.2 ng/kg, V 1.6 ng/kg, Cr 37 ng/kg, Ge 4.5 ng/kg, As 2.7 ng/kg, Se 2.0 ng/kg, Si 6800 ng/kg and P 800 ng/kg in the undiluted acid.

Benefits and Practical Applications

The MS/MS capability of ICP-QQQ significantly reduces spectral interferences that cannot be addressed by collision/reaction cell ICP-QMS. This enhances reaction efficiency, sensitivity and selectivity, enabling routine certification of ultrapure process acids at sub-ppt impurity levels. The method supports quality assurance in semiconductor chemicals and helps prevent device defects due to metal contamination.

Future Trends and Opportunities

• Expansion of MS/MS reaction chemistries for additional problematic elements and matrices
• Integration with automated sample introduction and inline monitoring in semiconductor fabs
• Adaptation of ICP-QQQ workflows for other high purity chemicals, such as phosphoric acid
• Further reduction of detection limits through optimized cell designs and alternative reaction gases

Conclusion

Triple quadrupole ICP-MS in MS/MS mode provides a robust solution for ultra-trace analysis in challenging sulfuric acid matrices. By leveraging dual quadrupole filtering and tailored reaction cell strategies, the method achieves sub-ppt detection limits and interference-free quantification of key semiconductor process contaminants.

Reference

1. Junichi Takahashi, Agilent Technologies, Determination of challenging elements in ultrapure semiconductor grade sulfuric acid by Triple Quadrupole ICP-MS, Application note 5991-2819EN, 2015