On-line Raman Monitoring of the Composition of Etchant Solution through Existing Teflon Tubing

Abstract

An NIR (near-infrared) spectral collection scheme, employing direct transmission through Teflon tubing, has been recently studied for the measurement of etchant solution. Alternatively Raman spectroscopy can be used for the same purpose since it could provides more selective and diverse spectral information compared to NIR spectroscopy, especially for the components in etchant solutions, such as H2O2 and inorganic acids. For Raman measurement, we have developed a novel Raman collection scheme, using Teflon (PFA: perfluoroalkoxy fluorocarbon) tubing as an effective and synchronous external standard. Therefore, the resulting spectrum is the sum of spectral features from the sample and the Teflon tubing which allows for the non-overlapping Teflon bands to be used to correct for variations in the Raman intensity. We also built a pilot-scale chemical wet station that can simulate a real cleaning bench and generate continuous flow and Raman spectra were then directly collected from on-line samples. We employed an MAE (Metal Aluminum Etchant) solution (composed of H3PO4, HNO3, CH3COOH and water), one of the most frequently utilized etchants, for this study. PLS calibration models were built for each component and their on-line prediction selectivities were evaluated by spiking individual components into a mixture sample. Additionally NIR spectra were simultaneously collected along with Raman measurement for comparison. NIR discrimination between H3PO4 and HNO3 feature was somewhat difficult; while, Raman spectral features were distinct for these compounds. Overall, Raman spectroscopy also would be an simple and selective tool for the monitoring of diverse etchant solutions.