Point-source approach of source-mask optimization

Abstract

As optical lithography feature size shrinks further, Resolution Enhancement Technologies (RETs) are pushed more aggressively and often have to be considered simultaneously. In this work, we explored a point-source approach for source-mask optimization (SMO) for use with complex low K1 mask patterns. The method consists of three steps: 1). optimizing the source by computing the image log slope (ILS) at multi-critical mask locations. 2). Using the optimized source to correct the mask for the edge placement error (EPE) utilizing standard OPC software. 3). Repeat 1) and 2) until the merit function reaches a certain condition. The image simulations were carried out by a physical model based lithography simulation where the resist process effect and the rigorous topography mask model are included. The process windows and the 3D resist profiles are simulated and used for lithography verifications. The motivation of the study is to explore the capacity of physical model based lithography process simulation software for SMO. We aim to find a simple approach for the determination of the best source shape for complex mask patterns. The simulation results demonstrate that the current commonly-used off-axis illumination shapes can be further optimized by considering multiple locations of a complex mask patterns.