Roughness evaluation of very smooth surfaces using a novel method of scatter measurement

Abstract

A roughness measurement method based on the application of a photodiode integrator for scattered radiation gathering as well as results of experiments concerning this method are presented in the paper. The new integrator, contrary to known optical integrating devices, provides the possibility to get much less expensive and smaller instruments than traditional ones. Unfortunately, a decrease of the integrator dimensions could restrict its spatial frequency bandwidth causing measurement errors. Therefore, experimental works have been performed to find the relation between the range-of-acceptance angle of the integrator and measured rms roughness. They have been done by constructing a model of the integrator and a complete TIS instrument as well as a precision experimental set. For the purpose of making a direct comparison between the new instrument and the existing one, the Ulbricht sphere has also been used in the set. For very smooth surfaces, the value of the lower angle range is very critical and should be smaller than 2 deg. The upper limit can be about 30 deg. for measurement of smooth and isotropic surfaces. At such limits, the influence of the integrator flatness is stated to be non-substantial. Results of surface roughness measurements obtained from the new unit and the Ulbricht sphere show that both methods have similar issues. Taking into account the dimensions of the photodiode integrator, a small and inexpensive TIS instrument can be designed for measuring roughness and reflectance of isotropic smooth surfaces in the optical and electronic industries.