基于小孔点衍射的超精面形瞬态干涉测量技术

Transient Interferometry of Ultra Precision Surface Based on Small Hole Point Diffraction

为解决时域相移小孔点衍射干涉仪中移相依赖昂贵压电陶瓷相移器,且长时间采集易受环境干扰影响的问题,提出一种基于偏振同步相移的小孔点衍射瞬态干涉检测方法。构建反射式结构的小孔点衍射干涉测量光路得到两束正交的圆偏振光,利用棋盘位相光栅分光与偏振片阵列移相,在CCD上同时获得4幅不同相移量的干涉图像,对单次采集的4幅瞬时干涉图像进行处理即可直接获得被测面形信息。搭建实际测量系统,通过基于琼斯矩阵的偏振态分析及与ZYGO干涉仪的比对实验,从理论及实验两方面验证了所提方法的正确性,多次多组实验结果表明该系统稳定,有着良好的重复测量精度。

With the development of lithography technology in China from deep UV to extreme UV,the detection accuracy of lithography objective lens surface shape below nano or even sub nano is required. The detection accuracy of the commercial Tyman-Green and Fizeau interferometers is limited by their own standard reference mirror shape accuracy,which has been unable to meet the development needs of the above-mentioned ultra-precision detection. The point diffraction interferometry uses the micro aperture diffraction to produce an almost ideal spherical wave as the reference wave,which gets rid of the limitation of the measurement accuracy of the standard reference mirror,and opens up an effective way for the ultrafine surface shape detection in the field of lithography. The point diffraction method was proposed in the early 19th century and developed rapidly abroad,and the United States and Japan have developed molding equipment for point diffraction interferometers. The research on point diffraction interferometer in China started late,especially in the field of small hole point diffraction interferometry. At present,the research is mostly in the laboratory stage. One of the most important reasons restricting the improvement of its accuracy and the development of forming equipment is its phase shift. The existing small hole point diffraction interferometers mostly use high-precision piezoelectric ceramic phase shifter to drive the measured part to move for many times,and collect multiple interference images for phase extraction. The realization of this kind of time-domain multi-step phase-shifting technology relies on the import of expensive PZT on the one hand,and on the other hand,the interference image is easily affected by environmental interference during the long-term acquisition of interference images,resulting in low actual detection accuracy,especially low repeatability accuracy. In order to solve the problem that the phase shift depends on the piezoelectric ceramic phase shifter in the small hole point diffraction interferometer and the measurement is affected,a super precision surface transient interference detection method based on small hole point diffraction is proposed,and the system feasibility is verified from two aspects of theoretical analysis and comparative experiment. Two orthogonal circularly polarized beams are obtained by constructing a small hole diffraction interferometry optical path with reflective structure. The checkerboard phase grating is used to split the light and the polarizer array is phase-shifted,and four interference images with different phase shifts are simultaneously obtained on the CCD. The shape information of the measured surface can be obtained directly by processing the four instantaneous interference images collected at a single time. The theoretical analysis results of the system polarization state based on Jones matrix theory show that when the polarization angles of the polarization array are 0°,45°,90°and 135°,the introduced phase shifts are 0,π/2,π and 3π/2 respectively,which meet the requirements of the four-step phase extraction algorithm. The system can realize the transient detection by splitting the light through the chessboard phase grating and phase shifting with the polarizer array;Compared with the Zygo interferometer,the experimental results show that the measurement results of the proposed system are close to those of the Zygo interferometer,and the repeatability and accuracy of PV are better than those of the Zygo interferometer λ/200,the repeatability accuracy of RMS is better than λ/1 600.The experimental results show that the system is feasible and has good stability. The proposed system takes into account the advantages of high precision of point diffraction and strong anti-interference ability of synchronous phase shift. The principle is simple and easy to implement. The research results are conducive to promoting the process of small hole point diffraction interferometry from experimental research to instrument development.