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

Importance of the topic

Quality control of acid baths used in semiconductor and printed electronics etching is critical to ensure consistent pattern reproduction on wafers. As demand for microelectronics and PCBs grows, rapid and reliable monitoring of acid concentrations becomes essential to maintain throughput and minimize downtime.

Objectives and overview of the study

This white paper evaluates near-infrared (NIR) spectroscopy as a fast alternative to thermometric titration for determining sulfuric, nitric, and hydrofluoric acid levels in etching baths according to ASTM E1655. It examines time-to-result, running costs, and method performance.

Methodology and used instrumentation

The study employs a Metrohm NIRS DS2500 Liquid Analyzer operating in transmission mode with disposable vials for sample handling. Method development follows four steps: selection of relevant wavelength regions, mathematical preprocessing of spectra, calibration model generation using a representative sample set with reference titration values, and validation of the prediction model with independent data. Calibration guidelines adhere to ASTM E1655 criteria including figures of merit such as correlation (R), standard error of calibration (SEC), and standard error of prediction (SEP).

Main results and discussion

• NIR spectroscopy delivers all three acid concentrations in under one minute, compared to approximately 12 minutes per test for nine thermometric titrations (or up to 42 minutes including calibration and blank determinations).
• Annual consumable costs drop from $8,895 (titration) to $2,250 (NIR), yielding roughly $6,645 in yearly savings and over $66,000 across ten years.
• NIR spectra remain unsaturated in the 780–2500 nm range, allowing accurate quantification in aqueous acid mixtures up to 15% water content.
• Calibration models achieve high correlation (R>0.9) with SEC and SEP values comparable to the titration method.

Benefits and practical applications

• Rapid, non-destructive analysis requiring minimal sample preparation.
• Elimination of chemical reagents for routine measurements.
• Streamlined workflows with reduced operator training needs.
• Potential integration into production lines for real-time quality control.

Future trends and applications

Advances may include expanded prediction models for additional acid combinations, automated model maintenance, integration with process analytical technology (PAT) frameworks, and incorporation of machine learning to further improve predictive accuracy and adaptability.

Conclusion

NIR spectroscopy offers a time- and cost-efficient solution for monitoring mixed acid etching baths in semiconductor manufacturing. By conforming to ASTM E1655, it provides reliable quantification with substantial savings in consumables and faster analysis times, supporting increased throughput and sustainable laboratory practices.

References

1. Wittmann, J. Introduction to quality management in the semiconductor industry. CreateSpace Independent Publishing/Amazon Media EU S.à.r.l.: Luxembourg, 2016; Vol. 1.
2. Metrohm AG. Determination of sulfuric acid, nitric acid, and hydrofluoric acid in etch solutions, Metrohm AG: Herisau, Switzerland, 2011. Application Note H-114.
3. ASTM International. E1655-17 Standard Practices for Infrared Multivariate Quantitative Analysis; ASTM International: West Conshohocken, PA.
4. Metrohm AG. Quality Control of Mixed Acids – Fast and reliable detection of phosphoric, sulfuric, nitric and hydrofluoric acids, Metrohm AG: Herisau, Switzerland, 2021. AN-NIR-090.
5. Metrohm AG. Quality Control of Mixed Acids – Fast and reliable detection of acetic, hydrofluoric, and nitric acids, Metrohm AG: Herisau, Switzerland, 2021. AN-NIR-091.