Determination of ultratrace elements on silicon wafer surfaces using the Thermo Scientific iCAP TQs ICP-MS

Significance of the Topic

The rapid scaling of semiconductor devices demands ever higher purity of silicon wafers. Ultratrace metallic impurities on wafer surfaces can cause device failure, yield loss, and reliability issues. Accurate detection of ng·L-1 level contaminants is essential to meet SEMI guidelines and ensure process control.

Objectives and Study Overview

This application note aims to evaluate the performance of the Thermo Scientific iCAP TQs triple quadrupole ICP-MS for ultratrace analysis of silicon wafer VPD samples. The study focuses on minimizing background equivalent concentrations and improving detection limits through cold plasma operation, kinetic energy discrimination, and mass shift reaction modes. Key goals include demonstrating reproducible ppt-level measurements and rapid switching among analytical modes within a single sequence.

Methodology and Instrumentation

A simulated VPD sample was prepared by digesting silicon wafer material in hydrofluoric and nitric acids to yield a 200 mg·L-1 Si matrix. Standards at 25, 50 and 100 ng·L-1 were spiked directly into the matrix. Sample introduction employed a PFA concentric nebulizer, a double-pass PFA spray chamber, and a quartz torch with a sapphire injector. Platinum-tipped cones and a cold plasma extraction lens enhanced sensitivity. The iCAP TQs system operated in three modes: single-quadrupole KED (SQ-KED), single-quadrupole cold plasma with NH3 (SQ-CP-NH3), and triple-quadrupole with O2 reaction gas (TQ-O2). The Qtegra ISDS software autotune optimized gas flows and power settings.

Key Results and Discussion

Interferences from Si, O, F and Ar-based polyatomics were efficiently removed via mass shift (Ti to TiO+) and on-mass strategies (Ge) in TQ-O2 mode. Limits of detection ranged from 0.1 to 2.3 ng·L-1 for 26 elements. Spike recoveries fell between 90 % and 104 %, and reproducibility was better than 5 % RSD in a 200 mg·L-1 Si matrix. Calibration curves for Li, K, Ca, Ti, V and As exhibited excellent linearity in the low ppt range. The ability to switch seamlessly among hot/cold plasma and single/triple quadrupole modes enhanced analytical flexibility.

Benefits and Practical Applications

The iCAP TQs ICP-MS offers robust multi-element analysis at ultratrace levels, meeting stringent semiconductor purity requirements. Its modular approach ensures method adaptability across diverse matrices, high throughput, and reliable monitoring of wafer surface contamination in QA/QC workflows.

Future Trends and Potential Applications

Advances may include integration with in-line process control, automation of VPD sampling, expansion to new reaction gases, and AI-driven data processing for real-time decision support. Further refinement of interference removal strategies will extend ultratrace capabilities to emerging semiconductor materials.

Conclusion

The Thermo Scientific iCAP TQs ICP-MS system demonstrated exceptional sensitivity, accuracy, and flexibility for ultratrace analysis of silicon wafer VPD samples. Its performance in reducing background and detecting metal impurities at ng·L-1 levels supports rigorous semiconductor manufacturing standards.

Reference

Tomoko Vincent, Application Note 43359, Thermo Fisher Scientific, 2018.