Comparative study of a standard optical steel plate surface using ellipsometry and speckle interferometry

Defected areas purposely made in a standard steel plate of the type used in optical length metrology are investigated by ellipsometric and interferometric methods. The defects range from nano-metric scratches developed by routine work usage to heavy scratches. The optical constants n and k (refractive index and extinction coefficients) of the steel platen measured at different defected areas were obtained by ellipsometry and results were used to deduce the surface roughness of the studied areas. Surface roughness measurement by speckle interferometry based on interference fringes intensities is used to evaluate the ellipsometric measurements of the same defected areas. Results of both measurements allowed us to correlate the optical constants to the roughness degree and revealed that surface defects could be evaluated by ellipsometry. A source of error in the thickness measurement of thin film arising from surface imperfection can then be quantitatively evaluated by ellipsometry. Methods of measurement, analysis, and uncertainties are presented in details. © 2018, The Optical Society of India.