Optical film thickness measurement of turbid materials using the fractional BiSpectrum noise-reduction technique

Abstract

A novel approach for deriving the thickness information from a rough and/or turbid thick film or coating is demonstrated. For the application where the roughness or inhomogeneity of the thickness layer produces a dominant speckle pattern, laser interferometry is often not a suitable technique for reliable thickness measurements. The implementation of a signal-to-noise ratio (SNR) enhancement algorithm, which can relate the spatial frequency information produced by the sample when illuminated by multiple lasers is demonstrated. Development of a metrology system using a specific laser interferometry setup to incorporate the noise-reduction technique is also presented. With respect to the detected information of the measurement system, this SNR enhancement technique allowed for the reliable identification of the proper frequency and thus the thickness of the sample.