Optimization and stability of CD variability in pitch 40 nm contact holes on NXE:3300

Abstract

Controlling the Global and Local CD uniformity (GCDU and LCDU) of Contact Holes (CH) and the associated edge placement errors are important for the implementation of EUV lithography in high-volume production at memory chip manufacturers. The GCDU describes the average CH CD variability within and between the fields on the wafer, while the LCDU comprises the CH-to-CH variation between neighboring CHs. We have experimentally measured these parameters on a representative memory layer to understand the current performance, and suggest possibilities and pathways for future improvement. We report on an extensive experimental imaging study of a 40 nm pitch square CH array use case, using ASML's NXE:3300 EUV exposure tool at imec. We decompose the GCDU into Intra-Field and Intra-Wafer signatures, and the LCDU into systematic and stochastic components. Through this decomposition, we can assess the contribution of mask, scanner and resist process. A 10-month monitor evaluates the changes over time of these respective components and the relation between GCDU and LCDU. The mask contribution to LCDU was further examined by a CH-to-CH comparison of mask and wafer measurements. LCDU improvements could be obtained by optimizing the source for a better contrast through focus (6% LCDU improvement w.r.t. a Standard Quasar source shape at best focus, up to 30% in defocus) as well as by a resist stack optimization. Optimized resist stacks delivered 15% improvements in a lower LCDU on one hand, or a lower dose-tosize on the other hand. The results of this pitch 40 nm contact hole study lead to a better understanding of the needs for mask and scanner for the memory use case at 0.33 NA EUV lithography.