Online monitoring of sulfuric acid and hydrogen peroxide using Raman spectroscopy

Etching is used during semiconductor fabrication to chemically remove layers from the surface of the wafer substrate. Etching is a critically important process, and every wafer undergoes multiple etching steps before it is complete. To ensure optimal etching is performed, strict quality control measures must be in place to determine the acid etchant concentration in the varied mixed acid solutions. Depending on the wafer substrate and etching step, SPM (sulfuric acid-peroxide mix, also known as piranha solution), DSP (dilute sulfuric acid-peroxide mix), or DSP+ (dilute sulfuric acid-peroxide-hydrofluoric acid mix) are typically used as etching solutions. Maintaining the correct balance of acid concentrations in these mixtures plays a critical role in optimizing the etch rate, selectivity, and uniformity of the etching process.

This Process Application Note presents a method to measure sulfuric acid and hydrogen peroxide online in SPM and DSP solutions simultaneously using Raman spectroscopy with the PTRam Analyzer from Metrohm Process Analytics.

In the semiconductor industry, there are two types of etching processes used: wet and dry etching. Dry etching uses reactive gases (e.g., plasma) to remove the unwanted portions of the semiconductor material. Wet etching is a process that involves the selective removal of material from a substrate by using chemical solutions. These processes are widely used in various industries including electronics, semiconductors, and metalworking.

Wet and dry etching processes are utilized based on the unique requirements of the device being manufactured. However, in semiconductor manufacturing processes, wet etching has been more commonly used than dry etching, particularly for the removal of large amounts of wafer material and the ease of handling [1].

Depending on the material or layer being etched and the intended outcome, several kinds of chemical baths can be utilized for the wet etching of semiconductors. Sulfuric acid-peroxide mix, also known as piranha solution (SPM), and dilute sulfuric acid-peroxide mix (DSP) are typically used to produce silicon wafers [2].

Successful wet etching requires precise control of the reagent concentration in the bath solution. The determination of the acid concentration in mixed acid etching baths is a critical quality control step that can affect the outcome of the etching process.

Both SPM and DSP are potentially toxic solutions that must be handled with extreme caution. When working with these chemicals, personal protective equipment (PPE) should be worn, and all waste materials should be disposed of in accordance with local regulations. This demands online analysis to reduce employee exposure as much as possible and avoid accidents.

Aside from the hazards already mentioned, manual sampling of mixed acid baths is undesirable because of time constraints, potential inaccuracies, and production interruptions. Manual sampling can result in inconsistent sampling depths and locations, potentially providing inaccurate data about the bath's actual condition. To control the process in real time, reduce operational disruptions, enhance safety, and obtain more accurate and representative data, a better solution is necessary.

A safer, more efficient, and faster way to monitor multiple parameters simultaneously in mixed acid baths is via reagent-free online analysis with Raman spectroscopy. The PTRam Analyzer from Metrohm Process Analytics (Figure 1) is an ideal solution for this kind of challenging situation. This Raman analyzer enables the comparison of «real-time» spectral data from the process to a reference method (e.g., titration, ion chromatography) to create a simple yet indispensable calibration model for all etching bath constituents.

There is minimal space available to install an analysis system within the wet bench area (Figure 2a). Therefore, the PTRam Analyzer is the ideal solution for restricted spaces due to its small dimensions. Thanks to the embedded IMPACT software and the variety of industrial communication protocols, results can be transmitted in the same format to any Distributed Control System (DCS), a Programmable Logic Controller (PLC), or a Supervisory Control And Data Acquisition (SCADA) system for further actions (e.g., to adjust chemical dosing).

Perfluoroalkoxy (PFA) tubing is a popular choice for applications with aggressive media due to its excellent chemical resistance, which is greatly useful in this application. Moreover, Raman spectroscopy allows the measurement through transparent and semi-transparent materials. This application takes full advantage of this property by having the sample measured through PFA tubing (Figure 2b).

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The use of Raman spectroscopy, specifically the PTRam Analyzer from Metrohm Process Analytics, presents a reliable and efficient method for online monitoring of the sulfuric acid and hydrogen peroxide content in etching solutions such as SPM and DSP. This online analysis technique enables real-time measurement of multiple parameters simultaneously, providing precise control over the reagent concentrations in the mixed acid baths. With the ability to compare spectral data to reference methods, Raman spectroscopy can be part of a safer production environment by eliminating operator exposure to hazardous chemical reagents. Additionally, using Raman spectroscopy to monitor the etching bath composition enhances wafer cleaning efficiency and increases product throughput, reproducibility, production rates, and overall profitability by minimizing wastage and ensuring optimal etching processes.

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