A New Flatness Reference Measurement System Based on Deflectometry and Difference Deflectometry

Abstract

A new optical flatness reference system for specimens with diameters of up to 700 mm is being set up at the Physikalisch-Technische Bundesanstalt (PTB). The system is designed for absolute optical flatness measurements with uncertainties in the sub-nanometre range. It is based on deflectometry as well as difference deflectometry procedures. Both principles use the straight propagation of light as a straightness reference, and thus no external reference surface with known topography is required. In the deflectometric procedure, the specimen is scanned by a pentaprism or double mirror. The angle of the reflected beam of the surface is measured with a high-resolution calibrated autocollimator. In contrast, the procedure of difference deflectometry measures angle differences between two points on the sample separated by a constant lateral shear chosen between some millimetres to centimetres. This has the advantage that the measurement will not be influenced by height shifts or tilts of the sample. Due to the extended shear, this procedure was named ESAD (Extended Shear Angle Difference measurement). With the existing ESAD set-up, initially built up almost a decade ago and improved by upgrades over the years, uncertainties in the range of 1 nm and below for specimens with a diameter of 150 mm have been reached [1]. The specimen dimensions are rather limited. The most severe restrictions of this system are the weight of the samples and the restriction to horizontally oriented samples. The set-up of the new system will be different from the existing system. It is designed with the highest accurate mechanical stages and improved optics. In this way, the uncertainty of the measurements will be even more reduced. Additionally, it will be more