Focus considerations of design pitches and absorber choice for EUV random logic

Abstract

Background: In EUV lithography, the absorber material determines the amplitude and phase of the diffracted orders, which define best focus (BF) and depth of focus (DOF). Control of BF and DOF is needed for mid and high numerical aperture (NA) systems. Aim: Generate recommendations for EUV absorber classes and explain the best focus shifts that occur for bright and dark field mask imaging. Approach: We anchor simulations with experiments using a TaBN absorber, then investigate the BF and DOF for various absorbers using simulation. We use a simple analytical model to show that the analytically predicted diffraction orders behave similarly in magnitude and phase to EMF simulations. Results: We find good prediction for BF experiments and simulations using a very simple resist model, and fair prediction in DOF. We explain the BF shift depending on simple absorber parameters using an analytical model. The exact BF shift needs to be calculated using EMF simulations. Conclusions: Best focus shifts are more pronounced for dark than clear field masks, and more so in attPSM materials than in more binary absorbers. Magnitude and phase of the 0th order plays an important role in BF shifts. We are encouraged to explore low-reflection attPSM materials as mask absorbers.