Multiple focal plane exposure in 248nm lithography to improve the process latitude of 110nm contact

Abstract

Nowadays, RET (Resolution Enhancement Technology) is applied into lots of processes with special attention on the stage of development and manufacturing. Of the RET applications, FLEX (Focus Latitude Enhancement eXposure) [1- 2] is well known for 20 years and had shown that this method can enlarge the focus latitude on total window of DoF-EL (Depth of Focus and Exposure Latitude) through the benefit of gathering two or more exposure images with different focus planes. In double focal exposure, only focus level with even energy separation was considered in this study, and the image contrast flattening over wide focus range and contrast value lowering were demonstrated in this study by simulation. The lowering contrast level directly affects on physical resolution capability and proximity. But the area that is used to be a low contrast in single exposure has gained benefit from the image super-position, hence the variation of contrast over focus is much smaller by double focal exposure and wider DoF is achieved. As for triple image plane process, the process selections are more versatile than single and double exposure; for example, we can even superpose the images with different energy distributions. In this paper, several image plane combinations were first reviewed by contrast level and contrast variation through normalized focus by simulation for optimizing the process condition, and then experimental verifications were also carried out to compare the lithographic parameters, such as, depth of focus, exposure latitude, CD controllability and mask error enhancement factor, for our interesting 1-D contact.