Improving the Process Capability of SU-8. Part II.

Abstract

SU-8 Resists are used widely in the development and fabrication of MEMS devices [1-4]. The resist's images are the product of photochemical and thermal cationic processes and result in vertical sidewalls and high aspect ratio features. These are among SU-8's most desirable attributes. The process, which creates the desirable images, however, generates considerable internal stress, which produces cracked features. It would be beneficial, therefore, to investigate modifications of the SU-8 resist formulation which incorporate materials that will attenuate stress cracking without significant image degradation.Work reported previously by IBM focused on compositions containing SU-8 targeted for applications in which lithographic performance could be sacrificed for high flexibility [5]. In this paper we present our results of studies directed toward the selection of materials compatible with SU-8, which will significantly reduce cracking and also retain high aspect ratios images and vertical sidewalls. To this end, we examined mono and polyfunctional epoxides from the following chemical classes; siloxanes, urethanes, cycloaliphatics, a bicycloaliphatic copolymer with phenol, bisphenol-A derivatives, a long chain aliphatic polyol and some long chain aliphatic esters.This work allowed us to identify compositions having improved crack resistance and adhesion over standard SU-8 resist, without sacrificing lithographic performance, which may provide benefits for applications such as a dielectric barrier.