Useful way to compensate for intrinsic birefringence caused by calcium fluoride in optical lithography systems

Calcium fluoride is widely used in optical lithography lenses and causes retardation that cannot be ignored. However, few studies have been conducted to compensate for the retardation caused by calcium fluoride in optical lithography systems. In this Letter, a new index based on orientation Zernike polynomials is established to describe the value of retardation. Then, a method of retardation compensation is described. The method is implemented by clocking calcium fluoride lens elements, and the optimal rotation angles are calculated using a population-based stochastic optimization algorithm. Finally, an example is provided to validate the method.