Phase Retrieval Algorithm for Surface Topography Measurement Using Multi-Wavelength Scattering Spectroscopy

We are currently developing a high-precision and wide-range in-process surface topography measurement system using the laser inverse scattering method. In the laser inverse scattering method, a monochromatic plane wave is illuminated perpendicular to the target surface and the surface topography is measured by retrieving the phase distribution of the reflected light. However, the dynamic range of this method is limited to the sub-micrometer range because of phase wrapping during phase retrieval. In this paper, we propose a laser inverse scattering method using a multi-wavelength light source based on the fact that the phase of light is inversely proportional to the wavelength with the propagation distance as a coefficient. We also constructed a surface profilometer based on the proposed method and measured the profile of a single rectangular groove with a width of 50 µm and a depth of 2 µm. The dimensions of the measured profiles agree well with the nominal dimensions of the rectangular groove.