Impact of alternative mask stacks on the imaging performance at NA 1.20 and above

Abstract

The lithographic performance of current state-of-the-art resolution enhancement techniques (RET) will become critical at hyper numerical aperture (NA>1) due to mask 3D effects. We have studied the impact of the mask material on the lithographic performance at NA 1.2 and above. The assessment, both by rigorous simulations and experiments, involves the standard mask stacks, Cr binary mask (BIM) and MoSi 6% attenuated phase shift mask (attPSM), as well as alternatives such as thick Cr BIM, Ta/SiO₂ 1% and 6% attenuated PSM, and Ta/SiON 1% attenuated PSM. Using the rigorous electro-magnetic field (EMF) and lithographic process simulations (IISB DrLiTHO) the mask structure is optimized taking into account the trade_off with mask error enhancement factor (MEEF). Next, a throughpitch evaluation of the 45nm half-pitch (HP) node at NA1.2-1.35 is carried out examining maximum exposure latitude (EL), depth-of-focus (DOF), best focus shifts, and MEEF behavior for the various mask stacks. For the validation of the simulation methodology a correlation is made between scanner (ASML XT:1700Fi), AIMS (Zeiss AIMS^TM45-193i), and simulation results indicating the importance of the mask quality and mask properties. Based on the lithographic performance and the mask manufacturability we put together a ranking of the commercially available mask stacks for the 45nm HP node at NA 1.2 and 1.35.