Effect of top coat and resist thickness on line edge roughness

193nm immersion lithography might have to incorporate a top layer coat to prevent leaching and contamination. Additionally, immersion and future lithography will require lowering the photoresist thickness. It has been reported in literature that the diffusion coefficient of small acid molecules reduces as the resist thickness is reduced below 200 nm. The goal of this paper is to understand how, the use of a top coat, changing resist thickness and changing the substrate affect line edge roughness (LER). The study is conducted using dry 193 nm lithography. It was found that the use of a top coat helps to improve LER for 193 nm dry resist process. Improvement in LER with the use of top coat can be explained by a change in intrinsic bias of the resist. LER was also studied as a function of resist thickness, by changing resist thickness from 790 angstrom to 2200 angstrom. It was found that LER is a strong function of resist thickness. At thickness less than about 1300 angstrom, LER increases, with a more pronounced effect as resist thickness is decreased further. LER was also studied as a function of substrate. Two substrates, organic bottom anti-reflection coating (BARC) and an inorganic silicon oxynitride film (SiON), were used in the present study, For ultra-thin resist films, less than 1300 angstrom thick, it was found that the SiON substrate produced greater LER compared with the organic BARC substrate. The data compiled provides a fundamental understanding of LER behavior and will eventually help in better control of LER for future generation devices.