Direct detection and quantification of molecular surface contaminants by infrared and Raman spectroscopy

Abstract

Detection and identification of surface molecular contamination is important for improving process and product yields in a wide range of industrial applications. In particular, molecular compounds can easily deposit on the surface of hardware, optical components, semiconductor devices, medical devices, etc. with the risk of impairing their functionality. In this work, qualitative and quantitative metrological methodologies for surface molecular contamination detection based on Fourier transform infrared spectroscopy and micro-Raman spectroscopy are presented. The specificity of detection of two ubiquitous industrial contaminants i.e. poly(methylphenylsiloxane) and paraffin oil by infrared and Raman fingerprints is first demonstrated. Moreover, in order to obtain homogeneously contaminated surfaces that can be used as standard materials for Raman calibration, films of different thicknesses of contaminants were prepared on calcium fluoride windows, within a contamination range of 70–900 ng cm⁻², by a spin coating method. The amount of contaminants spread on the surface was quantified by applying a pre-set FT-IR calibration curve in accordance with the European Cooperation for Space Standardization procedure (ECSS-Q-ST-70-05C) and it was subsequently used to calibrate the Raman equipment which demonstrated a sensitivity up to 10⁻⁸ g cm⁻². A real case study of industrial contaminated surfaces i.e. a glass lens for a laser cutting machine is also presented to assess the applicability of both techniques in molecular contamination monitoring. In the discussed case Raman analysis turned out to be particularly useful for punctual investigation of the surface, especially when the sample is not transparent to the infrared radiation.