Quality control of semiconductor acid baths as per ASTM E1655 – Time- and cost-efficient with NIRS

Significance of the topic

In semiconductor and printed electronics manufacturing, maintaining precise concentrations in mixed acid etch baths is critical to ensure consistent patterning on wafers and circuit boards. Traditional wet-chemical methods such as thermometric titration deliver reliable results but involve lengthy analysis times, complex titrant management, and recurring chemical costs. Near-infrared (NIR) spectroscopy offers a rapid, non-destructive alternative that dramatically reduces time-to-result and running expenses while upholding high analytical quality.

Objectives and Study Overview

This white paper evaluates NIR spectroscopy, in accordance with ASTM E1655, as an efficient technique for determining sulfuric, nitric, and hydrofluoric acid concentrations in semiconductor acid baths. Key goals include comparing analysis times and costs against thermometric titration, outlining NIR measurement and calibration workflows, and demonstrating the method’s suitability for routine quality control.

Methodology and Analytical Approach

NIR spectroscopy measures light-matter interactions in the 800–2500 nm region, detecting overtone and combination bands of functional groups (e.g., –OH, –CH). Transmission mode is used: samples in disposable vials are scanned, and spectra are acquired in under one minute without chemical reagents. Calibration follows ASTM E1655 guidelines, requiring:

• A representative calibration set (≥20 samples) with reference values from thermometric titration.
• Selection of sensitive wavelength ranges.
• Mathematical preprocessing (e.g., derivatives) to enhance signal.
• Model evaluation via figures of merit (correlation coefficient R > 0.9, SEC vs. SEP consistency).
• Validation on an independent sample set.

Used Instrumentation

• Metrohm NIRS DS2500 Liquid Analyzer with transmission module and disposable vials.

Main Results and Discussion

Time-to-result comparison shows that NIR spectroscopy quantifies all three acids in under one minute, versus approximately 42 minutes for thermometric titration when including standardization and blank determinations. Annual consumable costs drop from US $8,895 to US $2,250 (assuming 4,500 analyses/year), yielding savings of roughly US $6,645 per year and over US $66,000 across ten years. Spectral overlays confirm unsaturated NIR absorbance in mixed acids, supporting accurate predictions via multivariate calibration.

Benefits and Practical Applications

• Rapid turnaround enables near-real-time quality control.
• Elimination of chemical titrants reduces waste handling and disposal costs.
• Robust calibration models support multi-parameter analyses (acid concentrations, moisture content, acid number) in complex mixtures.
• Minimal operator training required for routine measurements once models are established.

Future Trends and Potential Applications

Deploying NIR spectroscopy in semiconductor manufacturing may evolve toward:

• In-line or at-line monitoring for continuous process control.
• Expanded model libraries covering diverse acid compositions and concentrations.
• Integration with digital platforms and predictive maintenance systems.
• Combination with other spectroscopic modalities (e.g., Raman) for broader chemical profiling.

Conclusion

NIR spectroscopy, implemented using instruments like the Metrohm NIRS DS2500, presents a validated, ASTM-compliant alternative to thermometric titration for acid bath quality control. It delivers significant gains in speed, cost efficiency, and simplicity without compromising analytical performance, making it an attractive solution for modern semiconductor and printed electronics workflows.

Reference

1. Wittmann J. Introduction to quality management in the semiconductor industry. CreateSpace Independent Publishing, 2016.
2. Metrohm AG. Determination of sulfuric acid, nitric acid, and hydrofluoric acid in etch solutions, Application Note H-114, 2011.
3. ASTM E1655-17. Standard Practices for Infrared Multivariate Quantitative Analysis, ASTM International, 2017.
4. Metrohm AG. Quality Control of Mixed Acids – Detection of phosphoric, sulfuric, nitric, and hydrofluoric acids, Application Note NIR-090, 2021.
5. Metrohm AG. Quality Control of Mixed Acids – Detection of acetic, hydrofluoric, and nitric acids, Application Note NIR-091, 2021.