Rigorous EM simulation of the influence of the structure of mask patterns on EUVL imaging

Abstract

Rigorous electromagnetic scattering simulation is used to characterize mask diffraction of various EUVL mask structures. Planar multilayer masks with three kinds of absorber stacks and new subtractive (binary, att-PSM, alt-PSM) etched multilayer masks are studied for lithography performance for line and space features in the 45 nm node. The simulation results for process window show that the combined depth of focus of the Cr/SiO₂ absorber stack masks is about 320 nm maximum with sigma of 0.8 when mask CD biases are applied. The biased etched binary mask has a higher DOF of 360 nm at sigma = 0.6. The line width variations caused by the absorber thickness changes are shown to be about 2 nm due the interference effects and image placement errors (IPE) are 4-7 nm with a 0.4 nm variation. The consideration of a full 2% EUV illumination bandwidth decreases the interference effects by 50%. The etched binary mask is not affected by such phenomenon and provides better contrast. The H-V bias and IPE results of absorber stacks show a linear dependence of absorber thickness. The etched binary multilayer mask has less H-V bias and IPE due to its smaller effective thickness. A simple alternating PSM structure created by the subtractive multilayer mask patterning technique is also proposed and it shows a larger 722 nm DOF.