照明物镜数值孔径对微孔衍射波前质量影响分析

Analysis of Effect of Numerical Aperture of Illumination Objective Lens on Quality of Wavefront Diffracted by Tiny Pinhole

通过微孔衍射产生高质量的球面波是进行夏克一哈特曼波前传感器（SHWS）高精度标定的关键。采用时域有限差分（FDTD）法，进行了193nm准分子激光照明下有限厚度有限电导率微孔的衍射仿真计算，分析了照明物镜数值孔径（NA）对微孔衍射波前质量的影响，确定了满足SHWS高精度标定所需微孔直径和照明物镜NA的大小。分析计算得出，采用直径为200nm的微孔及NA为0．6～0．75的照明物镜时，衍射波前均方根（RMS）偏差为3．50×10^-3λ，强度均匀性为0．10，微孔透射率约为0．15，满足SHWS进行纳米精度波像差检测对参考球面波质量的要求。

High quality spherical wave, which is generated by the pinhole diffraction, is the core for the calibration of the Shack-Hartmann wavefront sensor （SHWS） with high-accuracy. The diffraction of the tiny pinhole under the illumination of the excimer laser with wavelength of 193 nm is calculated based on finite-difference time- domain （FDTD） method. The finite thickness and real conductivity of the pinhole are considered. The effect of numerical aperture （NA） of the illumination objective lens on the quality of the wavefront diffracted by the tiny pinhole is analyzed. The diameter of the tiny pinhole and the NA of the illumination objective lens needed to calibrate the SHWS with high accuracy are determined. The calculation and analysis show that, to obtain the wavefront whose quality meets the requirement of wavefront error measurement by using SHWS method with nanometer accuracy, the diameter of the tiny pinhole should be 200 nm and the NA of the illumination objective lens should be in the range of 0.6 to 0.75. In this situation, the RMS deviation of the wavefront diffracted by the tiny pinhole is 3.50×10^-3λ; the intensity uniformity is 0.10; the transmission of the tiny pinhole is about 0.15.