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 um can be achieved reproducibly. Thicker resist layers can be made by multiple coatings, and we have achieved exposures in 1200-um-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 um. 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.