Best focus shift mitigation for extending the depth of focus

Abstract

The low-k 1 domain of immersion lithography tends to result in much smaller depths of focus (DoF) compared to prior technology nodes. For 28 nm technology and beyond it is a challenge since (metal) layers have to deal with a wide range of structures. Beside the high variety of features, the reticle induced (mask 3D) effects became non-negligible. These mask 3D effects lead to best focus shift. In order to enhance the overlapping DoF, so called usable DoF (uDoF), alignment of each individual features best focus is required. So means the mitigation of the best focus shift.This study investigates the impact of mask 3D effects and the ability to correct the wavefront in order to extend the uDoF. The generation of the wavefront correction map is possible by using computational lithographic such Tachyon simulations software (from Brion). And inside the scanner the wavefront optimization is feasible by applying a projection lens modulator, FlexWave TM (by ASML). This study explores both the computational lithography and scanner wavefront correction capabilities.In the first part of this work, simulations are conducted based on the determination and mitigation of best focus shift (coming from mask 3D effects) so as to improve the uDoF. In order to validate the feasibility of best focus shift decrease by wavefront tuning and mitigation results, the wavefront optimization provided correction maps are introduced into a rigorous simulator. Finally these results on best focus shift and uDoF are compared to wafers exposed using FlexWave then measured by scanning electron microscopy (SEM).

Characterization and mitigation of 3D mask effects in extreme ultraviolet lithography

“…Several authors including ourselves have investigated the option to compensate the mask-induced phase effects and BF shifts in the DUV systems by dedicated wave aberrations of the projection lens [30][31][32]. However, the introduced wave aberrations of the projection lens can potentially have a negative impact on other features in the layout.…”

3D mask effects of absorber geometry in EUV lithography systems

“…5,6 In EUVL systems all of these 3D mask effects are still present [7][8][9][10][11][12] in addition to image placement error due to the oblique illumination. 13,14 Of course the effects of mask topography are not lens aberrations, but drawing analogies to traditional aberration theory informs potential pupil plane compensation strategies.…”