Benefits and trade-offs of global source optimization in optical lithography

Abstract

Source optimization in optical lithography has been the subject of increased exploration in recent years [1-4], resulting in the development of multiple techniques including global optimization of process window [4]. The performance advantages of source optimization have been demonstrated through theory, simulation, and experiment. This paper will emphasize global optimization of sources over multiple patterns, e.g. co-optimization of critical SRAM cells and the critical pitches of random logic, and implement global source optimization into current resolution enhancement techniques (RETs). The effect on optimal source due to considering multiple patterns is investigated. We demonstrate that optimal source for limited patterns does work for a large clip of layout. Through theoretical analysis and simulations, we explain that only critical patterns and/or critical combinations of patterns determine the final optimal source; for example those patterns that contain constraints which are active in the solution. Furthermore, we illustrate, through theory and simulation, that pixelated sources have better performance than generic sources and that in general it is impossible for generic sources to construct a truly optimal solution. Sensitivity, tool matching, and lens heating issues for pixelated sources are also discussed in this paper. Finally, we use a RETs example with wafer data to demonstrate the benefits of global source optimization. ©2009 SPIE.