Development of LIGA-Like Process With Positive Photoresist and SU-8

Abstract

LIGA-Like techniques allows the fabrication of structures with high aspect ratio. SU-8 is a well-known thick photoresist used in this process. However, the remainder of highly cross-linked epoxy after photolithography process is difficult to remove without damage or alteration to electroplated metal due to the high aspect ratio of the structure. Different from SU-8, positive photoresist is easily removed, yet has limitation of fabricating structures with high aspect ratio. In this paper, we present novel LIGA-Like process with positive photoresist to improve above mentioned difficulties. In order to increase a height of positive photoresist, we have introduced SU-8 into the system. SU-8 is used for cast to control the thickness of positive photoresist, and AZ9260 (positive photoresis) is used for electroplating mold. In this suggested process, Donut-shaped SU-8 cast is patterned on the border of substrate to control and increase total height of positive photoresist. AZ9260, a kind of positive photoresist, fills the center of vacant area, and the thickness of the AZ9260 is controlled by height of SU-8 cast during the process of slow spin coating. After spin coating, the system is exposed to natural atmospheric condition for hours to improve surface planarization. The soft baking is carried out in two steps. The purpose of first prebaking is to evaporate large amount of solvent, and second prebaking is to enhance aspect ratios of photoresist. In the process of prebaking, the AZ9260 coated wafer is covered with chalet due to the fast dry process causes wrinkle and bubble on the surface. Through extensive experiments, we have established the condition of spin coating, exposure doses, prebaking and development process in each structure of 150, 200 and 240 μm thickness. Different from the case of SU-8, AZ9260 mold can be effectively removed after electroplating treatment. Finally, we obtain metal structures of 150, 200, 240μm heights corresponding to the mold height (AZ9260). It is shown that SU-8 can be successfully replaced by positive photoresist in LIGA-Like process. Therefore, the suggested fabrication method can be applied to materialize the positive photoresist structures with high aspect ratio in UV lithography process.