A systematic study of the dip in the CD through-pitch curve for low-k1 processes

Abstract

Optical proximity correction has been widely used to correct line width variation in various different environments. The most important correction will be the CD through-pitch variation. For deep-UV (DUV) photo processes, it is observed that the CD will have a reduced trend at certain intermediate pitch range around 1.1 to 1.4 wavelength / NA (numerical aperture), also called "forbidden pitch". The process windows within this pitch range are small. In this case, even though we can use OPC to print the CD correctly, the process window can still be limited, which can generate a bottleneck for the entire process. In order to make OPC more effective, we find it necessary to be able to design an optimized process with enough process windows for all pitches. Although this may mean that we need to map out the entire parameter space spanned by relatively unknown parameters in resist, exposure tool quality, mask tolerance, etc, recent developments in the understanding of the effect of illumination selection, scanner lens aberration, and resist blur have provided us with new hints in realizing it. Such new developments include the optimization of off-axis illumination (OAI) condition, the characterization of the effect of lens aberration, and the selection of resists with appropriate effective acid diffusion length. We have studied the effect of illumination, lens aberration, and resist diffusion to the CD and process window at the above described intermediate pitch range both in theory and experiment. We have found that the effective resist diffusion, whose range is from 10nm to 50nm, can affect the process window at the intermediate pitch range, to as much as a few tens of nanometers. We will show that, in general, longer diffusion correlates to a deeper "dip". However, according to the experience in the use of photo resists, short diffusion length can also affect process window through the reduction of depth of focus. Therefore it is important to find an optimized resist diffusion length under various ground-rule and illumination conditions. But there has been no studies reported so far as to how much diffusion that can be tolerated for a given process at the intermediate pitch range. We have also performed experiments on the effect of the scanner lens aberration, we found that the lens aberration, which may be largely ignored in the past, may affect the process performance, causing mask error factor to rise significantly. In this paper, we will present the result of our experiments and theoretical investigations in how much resist diffusion and lens wave front error that can be tolerated for a given photolithographic process with certain CD tolerance. Insight will be provided for the choice of illumination conditions, resolution enhancement techniques, and the resist in realizing the best CD through-pitch performance under any given ground-rule condition.