Sub-nm topography measurement by deflectometry:: Flatness standard and wafer nanotopography

Current demands of the semiconductor industry on the measurement accuracy in lithography have reached nanometer and even sub-nanometer levels. We have built a novel scanning facility based on ESAD (Extended Shear Angle Difference) deflectometry for the ultra-precise and traceable measurement of large near-flat and slightly curved optical surfaces. One primary application of the device is to establish the improved standard for straightness and flatness with sub-nanometer accuracy at PTB. Transfer standards then will allow optical devices, e.g., interferometers and wafer mappers, to be calibrated with high accuracy. The measurement principle is based on the analysis of differences between reflection angles obtained at surface points with large lateral displacements (shears). The ESAD principle minimizes error influences and in first order is independent of stage errors and whole-body movements of the specimen. ESAD scanning does not rely on external reference surfaces of matched topography and allows the accurate calibration of the angle measuring device. The measurands are directly traceable to the SI units of angle and length. We will report in,detail on the new ESAD facility for the ultra-precise measurement of large (up to 500 mm in diameter) near-flat optical surfaces. We will present first measurements of a number of plane optical substrates with the device from various fields of application. The main error influences will be discussed, including the calibration of the high-resolution autocollimator used for angle measurement. Investigations into the repeatability, reproducibility and uncertainty of the topography measurement using the new facility at the sub-nanometer level will be presented.