大数值孔径物镜的波像差测量及其特殊问题

设计了基于振幅型棋盘光栅的二维剪切干涉仪,用于测量大数值孔径(NA)物镜的波像差。研究了棋盘光栅剪切干涉仪的基本原理,分析了大数值孔径物镜波像差测量时涉及的几个特殊问题。首先,根据棋盘光栅的远场衍射函数分析了棋盘光栅的衍射效率和衍射级分布,给出了剪切干涉图数据的处理方法。接着,根据球面光瞳坐标与平面探测器坐标的投影关系,分析了光瞳坐标畸变的影响;采用几何光线追迹方法,分析了光栅方程非线性对系统误差的影响。最后,推导了物镜光瞳边缘的相对照度与数值孔径的关系。试验结果表明:采用相同光瞳坐标,NA为0.6的显微物镜的波像差测量重复性(3σ)可达到3.7 mλ。对大数值孔径物镜测量过程中涉及的特殊问题进行了探讨,结果提示:测量大数值孔径物镜的波像差时,需要考虑光瞳坐标畸变、光栅方程引入的系统误差、光瞳边缘照度衰减的影响等。

Measurement of wavefront aberration for high NA objective and some special problems

A two-dimension shearing interferometer based on an amplitude chessboard grating was designed to measure the wavefront aberration of a high Numerical Aperture(NA) objective. The principle of Chessboard Grating Lateral Shearing Interferometer(CBGLSI) was researched, and several special problems involved in the measurement of wavefront aberration for the high NA objective were investigated. Firstly, the diffraction efficiencies and diffraction order distribution of the chessboard grating were analyzed by the far-field diffraction function, and the processing method of shearing interferogram was given. According to the projection relation between the spherical pupil coordinates and plane detector coordinates, the influence of pupil coordinates distortion was analyzed. Then, the effect of the nonlinearity of grating formula on systematic errors was analyzed by geometrical ray tracing method. Finally, the relation between the relative irradiance at the objective pupil edge and the numerical aperture was deduced. Experimental results indicate that the measuring repeatability(3σ) of wavefront aberration of a objective with the NA of 0.6 is 3.7 mλ under the same pupil coordinate. Several special problems involved in the measurement of wavefront aberration was discussed, and the results suggest that the influences of pupil coordinate distortion, systematic error due to grating formula, and the irradiance attenuation at pupil edge should be considered when the wavefront aberration of the high NA objective is measured.