Effects of step slope on thickness measurement by optical interferometry for opaque thin films

Optical interferometry is a simple, quick and cheap method to measure the thickness of opaque thin films. The film edge, being formed as a step on the sample surface, is lighted with monochromatic light in an interference microscope, producing the interferogram that is recorded with a CCD camera. The film thickness (step height) is calculated by measuring offsets of the fringes across the step. However, the morphology of the film edge (step) significantly affects the thickness measurement, in some cases even yields false results. In this work, three kinds of methods were adopted to mask a part of the substrate surface during the deposition for fabrication of the step. The mask used was a thin silicon slice, a straight line of ink imprinted by a pen, or an Aluminum film. The step morphology recorded by a profilometer revealed large variation from one method to another. Accordingly, the accuracy of film thickness (step height) measurement by interferometry varies significantly. Results showed that large error occurs when the slope of the step is small and the step out spans the view field of the microscope. Therefore, the step should be fully visible in the view field of the microscope for reasonable measurement of thickness. A simple equation, in terms of geometrical configuration, is developed for this requirement.