Portable angle-resolved scattering system to measure high spatial frequency roughness for ultra-smooth surfaces

Ultra-smooth surfaces require stringent quality control of their high spatial frequency roughness (HSFR). Angle-resolved scattering (ARS) allows for fast, non-contact roughness measurement that is insensitive to environmental disturbances. However, there is a tradeoff between the roughness measurement bandwidth and the measurement speed for the current ARS methods. We propose a rapid ARS method for HSFR measurement. A prototype instrument is developed using 405 nm laser illumination and a CMOS sensor to capture the scattering distribution. By solving the inverse scattering problem under the Kirchhoff approximation, the power spectral density function can be obtained, from which the root mean square roughness is calculated. The method allows measurement of HSFR from 1 μm−1 to 2.5 μm−1 over a 2 mm area in one minute. Experiments on polished metal flats showed excellent agreement between our method and atomic force microscopy.