Improvement of ICP-MS detectability of phosphorus and titanium in high purity silicon samples using the Agilent 8800 Triple Quadrupole ICP-MS

Significance of the Topic

This application study addresses the critical need to accurately quantify trace levels of phosphorus and titanium in high-purity silicon matrices used in semiconductor manufacturing. Controlling metallic impurities at ultratrace concentrations is vital for device performance and yield. Traditional single-quadrupole ICP-MS methods struggle with polyatomic interferences in silicon-rich samples, while cool plasma modes suffer from matrix suppression and cone fouling at higher silicon levels. The introduction of triple quadrupole ICP-MS (ICP-QQQ) with MS/MS capability offers a robust solution to these challenges.

Objectives and Study Overview

The primary goals of this work were to:

• Evaluate the Agilent 8800 Triple Quadrupole ICP-MS in MS/MS mode for interference-free detection of P and Ti in silicon matrices.
• Compare the effectiveness of hydrogen and oxygen reaction gases for phosphorus analysis.
• Determine detection limits, background equivalent concentrations (BECs), and long-term signal stability in two representative silicon concentrations (30 ppm and 2000 ppm).

Methodology and Instrumentation

Sample Preparation:
A vapor phase decomposition (VPD) approach was simulated by dissolving bulk silicon cleaned with HF and HNO3 to produce stock solutions at two matrix levels: 30 ppm Si (natural oxide) and 2000 ppm Si (thermally oxidized wafer equivalent). Final solutions contained 3.8 % HF and 6.8 % HNO3.

Instrumentation:

• Agilent 8800 Triple Quadrupole ICP-MS operated in MS/MS mode.
• Ultralow-flow C-flow 50 µL/min PFA nebulizer with PFA spray chamber and demountable torch.
• Pt sampling and Pt/Ni skimmer cones.
• Reaction cell gases: H2 for phosphorus (PH4+ at m/z 35) and O2 for phosphorus (PO+ at m/z 47) and titanium (TiO+ at m/z 64).
• Auto-tuned ion lens voltages and optimized plasma conditions (1600 W RF, elevated carrier and makeup gas flows) for 2000 ppm Si.

Main Results and Discussion

Calibration and Sensitivity:
P calibration in H2 reaction mode yielded a BEC as low as 0.35 ppb (compared with 5.86 ppb in O2 mode). In a 2000 ppm Si matrix, P detection limits reached 210 ppt, with a BEC of 350 ppt. Ti analysis in O2 mode achieved a detection limit of 2.0 ppt and a BEC of 2.5 ppt under high-matrix conditions.

Comprehensive Multielement Performance:
Additional key elements (alkali, alkaline earth, transition and noble metals) exhibited detection limits from sub-ppt to low ppt and BECs below 10 ppb in 30 ppm Si and below 250 ppb in 2000 ppm Si. MS/MS mode reliably separated target product ions from overlapping polyatomic species.

Long-Term Stability:
Continuous analysis of the 2000 ppm Si solution over three hours showed signal variations within ±20 % for representative P (4 ppb), S (40 ppb) and Ti (500 ppt) spikes, confirming robust matrix tolerance and low cone fouling under optimized hot plasma tuning.

Benefits and Practical Applications of the Method

• Unambiguous quantification of P and Ti at ultratrace levels in silicon-rich samples.
• Minimal interference from Si-based polyatomics due to mass filtering in Q1 and selective reaction chemistry in the cell.
• Enhanced matrix robustness and long-term stability compared with cool plasma techniques.
• Applicability to semiconductor quality control for wafer contamination monitoring and high-purity chemical profiling.

Future Trends and Potential Applications

Advances in triple quadrupole ICP-MS will likely focus on expanding reaction gas chemistries to target additional difficult analytes in complex matrices, further miniaturization of sample introduction systems for limited-volume analysis, and integration with automated wafer sampling tools. Real-time monitoring of semiconductor process streams and inline contamination control are emerging applications.

Conclusion

The Agilent 8800 ICP-QQQ operated in MS/MS mode effectively overcomes severe polyatomic interferences in silicon matrices, delivering low detection limits and stable long-term performance for phosphorus, titanium, and a broad suite of trace elements. This methodology enhances confidence in impurity control critical to semiconductor manufacturing.

References

1. Junichi Takahashi, Katsuo Mizobuchi and Masakazu Yukinari; Third Asia-Pacific Winter Conference on Plasma Spectrochemistry; Tsukuba, Japan; 2008.