高分辨率需求牵引极紫外光刻(EUVL)投影物镜向高数值孔径(NA)、自由曲面设计形式发展。传统的非球面EUVL物镜设计难以在高数值孔径下兼顾校正像差的需求, 往往造成遮拦, 破坏成像对比度。提出了一种高NA无遮拦自由曲面EUVL物镜设计方法。依据物镜形态参数判别引入自由曲面的最佳位置, 能够有效校正系统像差, 在不影响成像性能的情况下增大系统NA。应用该方法设计了一套高NA自由曲面EUVL投影物镜(PO)。与初始非球面物镜相比, 通过增加四面自由曲面, 将物镜数值孔径从0.3增大至0.35, 波像差均方根(RMS)值从1 nm减小至0.4 nm, 整个系统光路无遮拦。设计结果表明: 该方法有效提高了自由曲面EUVL物镜设计效率, 在不产生遮拦的情况下, 不但增大了系统NA且减小了系统波像差, 提高了物镜整体性能。

High numerical aperture(NA) projection objectives with freeform surfaces are demanded for extreme ultraviolet lithography(EUVL) with high resolution. The traditional aspherical EUVL lens design is difficult to meet the need of correcting aberrations under a large NA, which often causes obscuration and destroys the imaging contrast. A design method of a high NA EUVL objective with freeform surfaces and without obscurations was proposed. Lens-form parameters were used to determine the best position to insert freeform surface, which could effectively correct aberrations and increase NA of the system without affecting the imaging performance. A set of high NA EUVL projection objective(PO) with freeform surfaces was designed by this method. Compared with the initial aspherical objective, by adding four freeform surfaces, the objective NA was increased from 0.3 to 0.35 and wavefront error RMS was reduced from 1 nm to 0.6 nm, and there was no obscuration in the entire optical path. The design results indicate that the proposed method effectively improves the design efficiency of the freeform surfaces EUVL objective. In the case of no obscuration, the system not only increases the NA, but also reduces the wavefront error, which greatly improves the overall performance of the objective.