Freeform illumination sources: an experimental study of source-mask optimization for 22-nm SRAM cells

Abstract

The use of customized illumination modes is part of the pursuit to stretch the applicability of immersion ArF lithography. Indeed, a specific illumination source shape that is optimized for a particular design leads to enhanced imaging results. Recently, freeform illumination has become available through pixelated DOEs or through FlexRay^TM, ASML's programmable illuminator system, allowing for virtually unconstrained intensity distribution within the source pupil. In this paper, the benefit of freeform over traditional illumination is evaluated, by applying source mask co-optimization (SMO) for an aggressive use case, and wafer-based verification. For a 22 nm node SRAM of 0.099 μm² and 0.078 μm² bit cell area, the patterning of the full contact and metal layer into a hard mask is demonstrated with the application of SMO and freeform illumination. In this work, both pixelated DOEs and FlexRay are applied. Additionally, the match between the latter two is confirmed on wafer, in terms of CD and process window.