Experimental verification of source-mask optimization and freeform illumination for 22-nm node static random access memory cells

摘要

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 diffractive optical elements or through ASML's programmable illuminator system (FlexRay™) 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 and 0.078 μm~2 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 diffractive optical elements and FlexRay are applied. Additionally, the match between the latter two is confirmed on wafer, in terms of critical dimension and process window.