Improving the Process Capability of SU-8, Part III

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. As applications using SU-8 move into production, reproducibility and purity of the materials becomes of major importance. In most resist systems the resin is the major cause of batch to batch inconsistency. In SU-8, the photoacid generator has been the primary reason for the resist variation. The PAG used in the standard formulation is a sulfonium salt widely used in industrial applications. However, it is a multi component mixture and the composition varies widely from batch to batch. Metal ion contamination is also a concern as the PAG contains a metal ion which could be deleterious to semiconductor devices.
This work allowed us to identify single component PAG structures having acceptable lithographic performance compared to standard SU-8 resist. All PAGs identified show lower sensitivity compared to the mixed triarylsulfonium salt currently used. No usable metal ion free PACs were found, however.