A critical examination of submicron optical lithography using simulated projection images

Abstract

It may be possible to extend optical lithography well into the submicron regime, given a multilayer resist system that confines imaging to a thin layer, and that eliminates reflection from the substrate. The necessary exposure and focus tolerances are determined by simulating the projection of partially coherent, diffraction-limited images onto an ideal multilayer resist system. Exposure–defocus diagrams are then generated by requiring that critical dimensions satisfy a ±10% tolerance. We investigate the prospects for submicron lithography with 0.75 and 0.5 μm minimum features. Two lens systems are analyzed; a commercially available 0.28 NA lens operating at 436 nm, and an advanced 0.35 NA lens operating at 365 nm. The features studied include contact holes, rectangular lines and spaces, and equal line space gratings, and are representative of many typical IC mask patterns. Linewidth tailoring or biasing allows these features to be printed within tolerance simultaneously if the exposure can be controlled within ±6%. The corresponding depth of focus is about 3 μm for the 0.75 μm features, and 1.5 μm for the 0.5 μm features. More general shape correction may be needed to print objects of arbitrary complexity.