Optimization And Analysis Of The Point Diffraction Interferometer For The High Precision Testing Of Spherical Surfaces

The high-precision measurement technology is of great importance to assist and guarantee the high-precision optical processing and optical system alignment. Among plenty of the optical measurement methods, the point diffraction interferometer (PDI), which enables the measurement precision in the order of subnanometer, has become one of the most promisinghigh-precision optical measurement methods. Due to the convenience of in-suit testing in different working wavelength and obtaining the high-precision reference wavefront in a large numerical aperture, the pinhole-type diffraction interferometric system has become a significant research focus in the high-precision optical testing field. However, the domestic research on the PDI is still at the starting stage. In order to establish the high-precision detection equipment to meet the development requirements of domestic photolithographic techniques, especial to meet the high-precision testing requirements of spherical surfaces widely used in optical systems, optimization and analysis on key issues affecting the testing ability of the pinhole-type PDI for the high precision testing of spherical surfaces is carried out in the dissertation based on the previous work.The major content of this dissertation includes:the important supporting role of PDI system as a promising high-precision spherical surface testing technology in modern optical processing and alignment is discussed. After a review over the current PDI systems at home and abroad, especially those used in the field of high-precision detection such as the detection of the extreme ultraviolet lithography projection objective, the necessity of the research on the pinhole-type point diffraction interferometric testing system used for high-precision spherical surface testing is put forward.The light-path layout and overall system solution of the pinhole-type PDI for spherical surface testing are introduced. The main error factors of the proposed PDI system are discussed. Work carried out previously is summarized and further optimization aspects needed by the system are proposed.Three-dimensional finite-difference time-domain simulation model is established to analyze the pinhole diffraction wavefront. Vector diffraction theory is applied to analysis for the influence of the numerical aperture and the aberration of the microscope objective on the quality of the pinhole diffraction wavefront, providing a reference for the design of the actual pinhole diameter and the selection of the microscope objective. So that the established PDI system is ensured to satisfy the precision requirement of the spherical surface testing within certain numerical aperture range.Against the problem that the system errors increase when the PDI system tests the high-numerical-aperture spherical surface, the method of model simulation is adopted to analyze the reasons for error increasing and the system parameters impacting on this error. Corresponding aberrations calibration method, called systematic positon aberration cancellation (SPAC) method, is proposed and verification by both computer simulation and experiments are carried out to demonstrate the feasibility of the proposed method.By introducing polarizer, a pinhole-type PDI with adjustable fringe contrast for testing spherical surfaces is designed to overcome the limitations of the testing accuracy of the original system, the interferogram contrast of which tends to be poor when the surface under test is of low reflectivity. The system principle is presented. A simulation model for the system is established and optimal design for processing parameters and positions of key elements is studied. The system error is analyzed and corresponding aberrations calibration method based on the recovery model optimization is proposed.An experimental, point diffraction interferometric system for high-precision spherical surface testing is established, with high-numerical-aperture testing ability for the spherical surface and with adjustable fringe contrast.. Compared with the testing results of ZYGO interferometer, three testing experiments for the spherical surfaces have been carried out to verify the research results of this dissertation. Different combinations of the pinhole and the microscope objective are used in the PDI system to test the same spherical surface and the best combination is chosen. The spherical surface with the high-numerical-aperture of 0.5 is measured and the accuracy is better than 0.0100λ PV and 0.0010λ RMS on the basis of the corresponding error correction method proposed in this dissertation,. Spherical surfaces respectively with high and low reflectivity (96% and 4%) are tested both by the original PDI system and the system with adjustable fringe contrast. Experiments show that with the fringe contrast adjustable, the measurement accuracy of the PDI system for the spherical surface with low reflectivity is improved. The accuracy better than 0.0100λ PV and 0.0010λ RMS is realized with the PDI system with adjustable fringe contrast for the testing of spherical surface with both high and low reflectivity.