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 pur-nsuit to stretch the applicability of immersion ArF lithography. Indeed, anspecific illumination source shape that is optimized for a particular designnleads to enhanced imaging results. Recently, freeformillumination has be-ncome available through pixelated diffractive optical elements or throughnASML’s programmable illuminator system (FlexRayTM) allowing for vir-ntually unconstrained intensity distribution within the source pupil. In thisnpaper, the benefit of freeform over traditional illumination is evaluated,nby applying source mask co-optimization (SMO) for an aggressive usencase and wafer-based verification. For a 22-nm node SRAM of 0.099 andn0.078 μm2 bit cell area, the patterning of the full contact and metal layerninto a hardmask is demonstratedwith the application of SMOand freeformnillumination. In this work, both pixelated diffractive optical elements andnFlexRay are applied. Additionally, the match between the latter two is con-nfirmed on wafer, in terms of critical dimension and process window