Thickness measurement of thin films on curved surfaces with ellipsometry

Ellipsometry is a proven method for measuring layer thicknesses of flat, specularly reflective surfaces from the angstrom up to the micrometer range. At the Fraunhofer IOSB a new measuring system has been developed which allows the application of imaging ellipsometry on curved surfaces. The light beam is reflected twice off the sample surface, hence the ellipsometry measurements change but they are directly related to the measurements of conventional single wavelength ellipsomety. Several problems arise when interpreting the ellipsometry measurements from the new measurement system, like unknown angle of incidence, few measurements per pixel and often unknown materials. In this article these problems are identified and some steps are proposed to be able to apply imaging ellipsometry on curved surfaces for special applications. Currently, the focus remains on isotropic samples consisting of a single layer on a substrate.