High-precision interferometric shape measurement of objects with areas of different reflectance

Abstract

Non-contact, high precision interferometric techniques like phase shifting interferometry (PSI), vertical scanning interferometry (VSI) and a VSI and PSI combination are commonly used for surface topography measurement. In order to obtain quality object surface data these techniques rely on both high fringe contrast and maximum intensity, which occur when the beams reflected from the reference and object surfaces are of equal intensity and when the fringe maxima are close to the saturation level of the detector. However, these conditions are difficult to attain when testing objects that have both high and low reflectance within the tested area such as ball grid arrays on a low-reflective substrate or a silver step on a glass substrate. Our proposals allow for obtaining better quality data when testing samples that have both high and low reflectivity areas. Separate modifications are suggested first for samples with different areas of reflectivity that are significantly separated in the vertical direction and second for samples with different reflectivity areas separated by less than about 10 microns in the vertical direction.