Improving the process capability of SU-8

Abstract

SU-8 negative resists designed to produce uniform thick films in a single spin-coating step are widely used in the fabrication of MEMS devices. A potential limitation in using resists of this type in large-scale production is the relatively long processing cycle time, which is determined by the choice of solvent and bake conditions. A well-known disadvantage of SU-8 is its inability to wet low surface energy substrates which can result in non-uniform coating. In this paper we present the results of the first part of a study aimed at improving SU-8 process capability in which the focus is on solvent replacement. Some of the important trade-offs that should be considered in the selection of a practical solvent are discussed. The results of an investigation that led to the selection of cyclopentanone to replace gamma-butyrolactone are presented, the relative drying rates are compared and the drying mechanism is discussed. This study led to the commercialization of a new family of cyclopentanone based resists, SU-8 2000 (referred to as SU-8 CP in this paper) which shows significantly improved wetting, faster drying for film thicknesses up to about 50 microns and clean edge bead removal without the need for an intermediate bake step.