Research On Technologies Of Microspherical Target Entire Topography Inspection By Phase Shifting Diffraction Interferometer

Microsphere is one of the most commonly used components forms in microoptics, micromechanics and other fields. Its micro topography of surface has important influence on its optical property, mechanical characteristics, physical property and so on. For instance, in the inertial confinement fusion(ICF) experiment, a microspherical target with diameter between several hundreds micron and a few millimeters is one of the most critical components. Any tiny defect on its surface may lead serious consequence of low experiment efficiency or even failure.Traditional inspection methods of spherical surface such as profilometer, roundness tester, and atomic force microscope(AFM) may scratch the tested surface, work in low efficiency or miss some isolated defect points. Interferometry provides a quite ideal way for microsphere surface topography inspection because of its outstanding advantages such as high accuracy, high efficiency, high sampling density, non-touching and so on. Currently, only few relevant research departments in the United States and France had carried out some studies in this field, using the scheme of phase shifting diffraction interferometer(PSDI) or digital holographic microscopy. However, our research on microsphere inspection is still at the level of AFM rotating scanning at present.This dissertation does research primarily about the testing method and key technologies of target entire topography inspection based on the principle of phase shifting diffraction interferometry. The main contents of this paper are as follows:The inspection method by PSDI is researched at first, and the optical-mechanical structure of testing system is designed. A method of target entire surface topography inspection by phase shifting diffraction interferometry based on polarization control is proposed. Stray light beams are filterd from the interference field, so phase shifting and high contrast interference signal can be achieved. By this method, the problems of short-coherence light source dependency and its poor signal contrast in traditional method are sloved, which makes the new method also suitable for the low reflectivity surface testing. Meanwhile, a point diffraction calibration method of test wave distortion is used for improving the measurement accuracy.Linear phase shifting error is a common error source existed in mechanical type phase shifting system, so its control method is studied, and a phase extraction method based on error complementary correction is proposed. Because the constructed five frames algorithm and classic Hariharan algorithm have the response characteristics whose size is near and sign is opposite at the same phase point with equal linear phase shifting error, the phase extraction error is controlled greatly by error complementary. The new algorithm can significantly improve the accuracy of phase information extraction, and avoid the generation of imaginary outliers that commonly existed in arbitrary equal steps algorithm. Meanwhile, for solving the problem that the iterative least square phase unwrapping method is not suitable for the usage in circular domain, the algorithm is improved by leading in a multiple binarization template for boundary limitation.Traditional correction method of eccentricity errors may generate greater residual errors, so a correction method that is suitable for microsphere testing is proposed. By the analysis and higher approximation of the optical path difference model, eccentricity errors are mapped more accurately with Zernike error terms, so the correction accuracy of eccentricity errors can be improved. Then, the residual errors leaded by normalized polar path approximation in factors fitting process are also analyzed, and corresponding revised method is provided.Spatial location arrangement of subapertures is designed according to the principle of subaperture arrangement and overlapping stability, so the rotating angle in target scanning is confirmed. For the mass stitching data, combination of 2D and 3D matching method is employed. Based on mapping image matching, an intra-annular subaperture stitching method is proposed, which is more efficient by reducing the computation greatly. During the adjacent ring stitching step, point cloud matching method based on ICP algorithm is used for enhancing its capability of disposing errors. For overcoming the application limitation of ICP algorithm, a matching method based on data horizontal compression with a virtual tiny radius is used for making the algorithm more suitable for microstructure point cloud matching.According to the principle of target inspection, a phase shifting diffraction interferometer facility is built. Actual experiments for accuracy testing are carried out by using a ruby probe and a GDP target as objects. The single-aperture testing result is agreed with that from a Wyko scanning white light interferometer(SWLI), and the achieved PV and RMS value of comparative region are 66.17 nm and 16.89 nm, respectively. The measured roughness of ruby probe is 0.0149μm, which agrees with its nominal value presented from its manufacturer. The cross-section profile from stitched hemisphere is in basic accordance with the scanning result from AFM. So, the feasibility of proposed PSDI system has been demonstrated with the experimental results.