Optical coupling of lens, liquid and resist in immersion lithography: rigorous model and assessment

Abstract

This paper develops a rigorous and rapid model for the simulation of 3D optical imaging in resist in Liquid Immersion Lithography (LIL) by using full vector, Fourier Optics approach. The wave exiting the lens is decomposed as a set of vector plane waves that are incident upon resist stack. The transmitted and reflected waves in resist are calculated by solving the simplified boundary conditions in matrix form. Then the field in resist is constructed by Fourier transform. The partially coherent illumination is modeled by discretizing the source into many point sources, calculating the fields due to every source and then adding the light intensities together. Based on this model, this paper compares the performance of 193nm dry and water immersion lithography. Water LIL has better Depth-of-Focus (DOF) and contrast, but exaggerates the polarization effect. The paper further evaluates the impacts of extreme NA, polarization, and partial coherence on the image quality in 193nm water LIL with binary mask by simulating the latent images of 70nm equal line/space. The profiles and contrast of TE and TM images are compared, assuming 0.85 NA and 0.7 σ. The optimum σ is observed from contrast curves, and the process window is obtained. The major concerns of LIL are small DOF and image degradation due to TM waves. Possible solutions are discussed.