Interferometric Testing of Photomask Substrate Flatness

摘要

Conventional interferometric testing of the flatness of photomask substrates is rendered difficult by the long coherence length of the HeNe laser sources typically used in commercially available phase measuring interferometers appropriate for flatness testing. The Ritchey-Common configuration allows testing of flats in a spherical wavefront; this paper shows that, under appropriate conditions, high resolution surface flatness maps of photomask substrates may be obtained using instrumentation currently available in many optical shops.