Optical inspection of flat reflective surfaces by a wave front sensor

The wave front sensor used in this paper is based on Makyoh method: the studied sample is illuminated by a collimated light beam and the reflected beam is collected by a camera. Previously it was demonstrated that this method enables the determination of surface flatness in the nanometer range. For this purpose the deformation of an initially planar wave front is detected and evaluated using patterns projected on the surface. This paper demonstrates that the sensor can also be used without patterns for characterization of surfaces flatness in the sub-micrometer and micrometer ranges. The intensity distribution image obtained can be interpreted in terms of topography as follows: convex areas of the studied surface defocus the beam (dark regions on the image) while the concave areas focus it (bright regions). The main result of this work is the development of a new approach for the fast assessment of the surface quality. This approach estimates the areas and the intensiti es of bright regions on the image and gives the value of the maximum concavity on the studied surface. For evaluation of data a simulation of the reflected from the given profile was made. The setup parameters, e.g. distances between the optical components, were optimized with the parameters obtained from the 2D simulation of the wave front sensor. This paper demonstrates the feasibility of wave front sensing for the topography analysis of reflective surfaces such as bare wafers' surfaces, metallic thin films, etc. used in semiconductor industry.