Grating interferometer for metrology of transparent flats

Abstract

Transparent flats are difficult to measure because of the reflections from the front and back surfaces of the flat. The metrology is even more difficult if the surface deformations are too large for conventional testing, as is often the case for plastic hard disk blanks, flat panel displays and silicon wafers. This paper describe an interferometer that successfully separates the front and back surface reflections while working at an equivalent wavelength of 12μm, thus solving both problems with one system.