Automated Analysis of Ultratrace Elemental Impurities in Isopropyl Alcohol

Importance of the Topic

Controlling trace metal contamination in process chemicals is critical for semiconductor manufacturing. Inorganic impurities at ppt levels can degrade device yield and reliability. High‐purity isopropyl alcohol (IPA) used for wafer cleaning must meet stringent SEMI grade 4 specifications with contaminant levels below 100 ppt for each element.

Objectives and Study Overview

This work describes an automated method for ultratrace elemental analysis in IPA that removes the need for manual spike additions and complex sample handling. Key goals:

• Implement automated online method of standard additions (MSA) using the IAS Automated Standard Addition System (ASAS).
• Use Agilent 8900 Triple Quadrupole ICP-MS (ICP-QQQ) to quantify 47 elements, including 22 SEMI-specified impurities, at ppt levels.
• Assess detection limits, background equivalent concentrations (BECs), and spike recovery performance under clean-room conditions.

Methodology and Instrumentation

High‐purity IPA was distilled in-lab and introduced undiluted via self-aspiration in a Class 10,000 clean room. A 1 µg/L mixed multi‐element standard was prepared in IPA with 1 % ultrapure HNO₃ and connected to the ASAS.

Automated Standard Addition System (ASAS):

• Microflow syringe pump loads standard into a 700 µL loop.
• Excess standard is flushed to waste; an air bubble then traverses two optical sensors to measure sample uptake flow (typically 200 µL/min).
• Software calculates and injects spike volumes at 0, 5, 10, 20, and 50 ppt into the sample stream.
• Mixed sample passes directly to the ICP-QQQ nebulizer without further manual handling.

Instrumental setup:

• Agilent 8900 ICP-QQQ with semiconductor configuration.
• Glass concentric nebulizer with 0.3 mm id PFA tubing; Peltier-cooled quartz spray chamber at –5 °C; narrow-injector quartz torch (1.5 mm); platinum sampling/skimmer cones; S-lens interface.
• RF power 1500 W; cool plasma achieved by optimizing make-up gas flow; collision/reaction cell gases (He, NH₃, H₂, O₂) in MS/MS and no-gas modes.

Results and Discussion

• 47 elements measured, including all SEMI C41-0705 impurities.
• Detection limits ranged from 0.000 ppt (Hf, Re) up to a few ppt; all SEMI elements were below 100 ppt, many < 1 ppt.
• BEC values also met grade 4 requirements; Cu was confirmed by secondary isotope to rule out interference.
• Automated spike recoveries at 20 ppt showed 91–108 % accuracy with 1.6–8.9 % RSD (n = 10).
• Cool plasma plus NH₃ cell gas in MS/MS removed carbon‐based interferences for Mg, Al, Cr; O₂ mass‐shift mode to PO⁺ at m/z 47 provided P determination with DL 2.6 ppt and BEC 43 ppt.

Benefits and Practical Applications of the Method

• Fully automated standard additions reduce manual handling and contamination risk.
• Simplified workflow lowers analyst skill requirements and potential errors.
• Compact ASAS integrates seamlessly into clean-room environments.
• Accurate, reproducible quantification meets or exceeds SEMI grade 4 specifications.
• Suitable for routine QA/QC of semiconductor process chemicals and high-purity solvents.

Future Trends and Applications

• Demand for even lower detection limits will grow with advanced device nodes.
• Integration with continuous online monitoring systems (e.g., CSI) for real-time quality control.
• Extension of automated MSA to other high-purity solvents and complex matrices.
• Further automation guided by software and AI could enhance throughput and reliability.

Conclusion

The combination of the IAS ASAS and Agilent 8900 ICP-QQQ delivers a robust, fully automated approach for ultratrace analysis of elemental impurities in IPA. Sub‐ppt detection limits, excellent spike recovery, and streamlined workflows ensure compliance with current and future SEMI purity standards while minimizing contamination risk and operator intervention.

References

1. SEMI C41-0705, Specifications and Guidelines for 2-Propanol, 2005.
2. J. Takahashi and K. Mizobuchi, Asia Pacific Winter Conference on Plasma Spectroscopy, 2008.
3. K. Mizobuchi, N. Yamada, and M. Yukinari, Japan Society for Analytical Chemistry, 66th Nenkai, 2017.