High-aspect-ratio, ultrathick, negative-tone near-uv photoresist for MEMS applications

Detailed investigations of the limits of a new negative-tone near-UV resist (IBM SU-8) have been performed. SU-8 is an epoxy-based resist designed specifically for ultrathick, high-aspect-ratio MEMS-type applications. We have demonstrated that with single-layer coatings, thicknesses of more than 500 /spl mu/m can be achieved reproducibly. Thicker resist layers can be made by multiple coatings, and we have achieved exposures in 1200-/spl mu/m-thick, double coated SU-8 resist layers. We have found that the aspect ratio for near-UV (400 nm) exposed and developed structures can be greater than 18 and remains constant in the thickness range between 80 and 1200 /spl mu/m. Vertical sidewall profiles result in good dimensional control over the entire resist thickness. To our knowledge, this is the highest aspect ratio reported for near-UV exposures and resist thicknesses. These results will open up new possibilities for low-cost LIGA-type processes for MEMS applications. In addition, the SU-8 material has interesting mechanical properties which also makes it attractive for photoplastic device fabrication.