Micro-profile measurement of a transparent coating using windowed Fourier transform in white-light vertical scanning interferometry

Abstract

Owing to free of the phase ambiguity problem, white-light vertical scanning interferometry (WLVSI) is commonly used to inspect surface profiles of objects, especially for the micro-profile measurement of microelectromechanical systems (MEMS) devices. Although numerous algorithms have been proposed for the white-light interference signal analysis, most of them focused only on test objects with a single reflective surface. In this paper, a technique to measure the micro-profile of a transparent coating using windowed Fourier transform in WLVSI is described. A windowed Fourier filtering (WFF) operation is first adopted to roughly estimate the local peak positions of a recorded interference signal. Subsequently a least-square estimation based on the retrieved phase values of a filtered signal is separately applied into previously extracted local peak positions to achieve more accurate micro-profile results. The thickness of the coating can be finally estimated by the retrieved reflective surfaces. Experimental works, conducted on a flat plate and a MEMS device with a transparent coating on its substrate, demonstrate the validity of the proposed method. © 2012 SPIE.