离轴照明是现代投影光刻中的一种重要的分辨率增强技术。针对不同的照明模式要求，提出了一种利用自由曲面来实现离轴照明的方法。根据入射面和目标面之间的坐标拓扑和能量守恒关系,利用数值求解法得出自相应的折射式自由曲面并对结果进行模拟，分别设计了环形、二极和四极照明，照明效率均达到90%以上，照明均匀性均优于93%。结果表明，可以满足193 nm投影光刻照明系统的设计要求。

Off-axis illumination is an important application of resolution enhancement technology in projection lithography. An approach of designing freeform lens for off-axis illumination (OAI) in optical lithography is propose. Based on the mapping between the source and the target and conservation law of energy, a set of partial differential equations are obtained and numerically solved, and then optical performances of the freeform lenses are simulated. With the method, free form surfaces are designed to achieve an annular, dipole, quadrupole lighting. The shape and intensity of illumination distribution on the target surface can be controlled precisely. Also, the illumination efficiency is over 90% and good uniformity can be achieved.