The National Metrology Institute of Technology provides a flatness calibration service for large flat substrates using a Fizeau interferometer with 10 nm (k = 2) uncertainty over a measurement range of 300 mm. In the Fizeau interferometer, the surface profile of the reference flat is a measurement error factor because the measurand is the gap distance between the reference flat and the specimen. We have reduced uncertainty by constructing a surface profile map of the reference flat based on the three-flat test; however, it is difficult to manufacture a highly accurate reference flat and create a correction surface profile map in order to measure even larger flat substrates. Thus, we developed a three-dimensional scanning deflectometric profiler (3-D SDP) that does not require a reference flat and can directly measure a surface profile. Measuring devices based on deflectometry have been developed by many laboratories for highly accurate straightness profile measurement, but measurement by such systems is limited to a line (two-dimensional) profile. To solve this problem, we developed a novel method of measuring the surface topography by calculating the topography from radial lines obtained by rotating the specimen. In this study, we compared measurements between the Fizeau interferometer and 3-D SDP. We also report the results of measuring an optical flat with a diameter of 300 mm.
The flatness is one of the important geometric quantity such as a silicon wafer, a photo mask and ultra-precise optics for an extreme ultraviolet lithography, synchrotron radiation facilities and a gravitational wave interferometer. A Fizeau interferometer is in a high accurate flatness measurement equipment. In a Fizeau interferometer, the measurand is a gap distance between a reference flat and a specimen. The measurement accuracy of the Fizeau interferometer is limited by the flatness of the reference flat. The flatness of commercially reference flats is about λ/40 to λ/20 (15 nm - 30 nm). There is strong demand for developing that the flatness of reference flat is less than 10 nm. Thus, we developed a λ/100 (6.3 nm) reference flat which is attachable to a commercial Fizeau interferometer. The measurable diameter of the optical flat was φ100 mm. In the factory fields, the flatness of the reference flat mounted on a commercial Fizeau interferometer is required. We designed the reference flat using the finite element method (FEM) to decrease a deformation by mounting. The developed reference flat was evaluated by a scanning deflectometric profiler (SDP) and an ultra-high accurate Fizeau interferometer. Finally, the reference flat was validated through the evaluation results.
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