Metrology of thin transparent optics using Shack-Hartmann wavefront sensing

The surface topography of thin, transparent materials is of interest in many areas. Some examples include glass substrates for computer hard disks, photomasks in the semiconductor industry, flat panel displays, and x-ray telescope optics. Some of these applications require individual foils to be manufactured with figure errors that are a small fraction of a micron over 10- to 200-mm lengths. Accurate surface metrology is essential to confirm the efficacy of manufacturing and sub- strate flattening processes. Assembly of these floppy optics is also facili- tated by such a metrology tool. We report on the design and perfor- mance of a novel deep-ultraviolet (deep-UV) Shack-Hartmann surface metrology tool developed for this purpose. The use of deep-UV wave- lengths is particularly advantageous for studying transparent substrates such as glass, which are virtually opaque to wavelengths below 260 nm. The system has a 1433143-mm 2 field of view at the object plane. Per- formance specifications include 350-mrad angular dynamic range and 0.5-mrad angular sensitivity. Surface maps over a 100 mm diam are accurate to ,17-nm rms and repeatable to 5 nm rms.