The bulk stability of the lens material in Deep-UV lithography (Fused Silica and Calcium Fluoride), as well as the susceptibility of the lens anti-reflection coating to a thin layer of chemical contamination during laser irradiation over long period of time, are the keys for advanced lithography systems lifetime.
Lens degradation impacts laser-based exposure systems’ performance and therefore affects the product quality. There is a need for careful monitoring and prediction of lens lifetimes. This paper describes a method to calculate the degradation rate of optics and the lifetime prediction of these systems, along with some possible mechanisms for imaging degradation and factors that accelerate the degradation process. Currently, 'Pulse count' methods are used for such calculation; here we describe a new 'Energy based' method equal to 'Cumulative energy' through the projection lens. The paper compares the two methods using actual cases and shows the benefits of using the proposed method. We also suggest some new ways to deal with the problem. In addition, we report the learning from a project which entails developing a software application for automatic continuous tracking of degradation rates as well as the lens lifetime prediction across all Intel's laser based lithography tools.