UV-LIGA process for high aspect ratio structure using stress barrier and C-shaped etch hole

SU-8 is a well-known thick. photoresist used in fabrication of high aspect ratio Micro EectroMechanical System (MEMS) structure. However, it is hard to get fine patterns in the high aspect ratio structures because of high internal stress and difficulty of removing SU-8. The purpose of this paper is to set up the process condition for both low film stress and high aspect ratio and to find design rules that make the pattern be less dependent on stress problem. Firstly, the mold fabrication process is optimized by adjustment of exposure and heat treatment. These two conditions control the amount of cross-linkage in polymer structure, which is the most important parameter of both pattern generation and remaining stress. Heat treatment is dealt with soft bake and post-exposure bake (PEB). Temperature and time duration of each step are varied with heat treatment condition. Exposure time is varied with exposure condition. Secondly, stress barrier and C-shaped etch hole are proposed for layout design scheme. The stress barrier is introduced to minimize stress from the large area polymerized SU-8 film. The etch hole designed to be the shape of character "C" instead of conventional square or wall type etch hole. Some test patterns are fabricated to evaluate the proposed process. Ultra Violet (UV)-LIGA process was performed by nickel electroplating with the mold fabricated through the proposed process to confirm the SU-8 as a good electroplating mold. (C) 2000 Elsevier Science S.A. All rights reserved.