Novel surface scanning inspection system for opaque and transparent substrates using laser Doppler velocimetry

摘要

This article presents a novel surface scanning inspection system (SSIS) for opaque and transparent substrates. In order to bypass the intrinsic difficulty of inspecting transparent substrates, a method is presented that uses Laser Doppler Velocimetry (LDV). This method enables to discriminate the front-side from the back-side of a transparent wafer based on a frequential signature. A probe volume consisting of interference fringes is produced using the crossing of two focused laser beams and projected at the surface of a substrate to be inspected. The substrate is scanned in a spiral shape, and any particle or defect at the surface of this wafer will cross the interference fringes and scatter light which is collected and directed towards a photosensitive sensor. The resulting temporal signal is a Doppler burst which frequency is known at each position, thus enabling a synchronous detection.