EFFECT OF TEMPERATURE AND TIME OF BAKING PHASE ON SU-8 PR FILM USED AS A HARD MASK FOR A DEEP LITHOGRAPHY

Abstract

The surface profile of Fresnel lens during the fabrication process depends on many factors
including the thickness, flatness, and pattern clarity of a photoresist (PR) film used as a hard
mask for the Si lithography. SU-8 PR with a high viscosity, which is capable of creating large
thickness films, can be used as a hard mask in the Fresnel lens fabrication processes. However,
it is not easy to achieve a film thickness of about 10 µm with the requirement of a flat surface,
free of air bubbles after the baking phases. This paper presents the effect of temperature and
baking time on the quality of SU-8 PR film. Accordingly, to achieve the required quality of PR
film, a two-step baking phase was employed. The time and temperature for a soft-baking
corresponds to 60 minutes and 60°C, respectively, while a post exposure baking (PEB) phase
must be kept at 90°C for about one hour. The experimental results show that the defects
including PR cracking, bubble problem on the PR layer are eliminated. Thus, these baking
parameters are feasible for the application used a thick PR film as a hard mask in a deep
lithography process.