Ultra deep SU-8 manufacturing and characterization for MEMS applications

The Micro Systems Engineering Team (mSET) at Louisiana State University (LSU) utilizes microfabrication for a number of heat and mass transfer devices. These include cross flow heat exchangers, mechanical seals with integrated micro heat exchangers, catalytic converters, and micro reactors. In all of these applications, micro honeycomb arrays provide increased surface area per unit volume which significantly enhances heat and mass transfer. In the past, it was only possible to fabricate SU-8 structures approximately 1.5 mm tall. Furthermore, qualitatively, it is much more difficult to fabricate close packed feature arrays than sparse arrays. For many of the previously mentioned applications, it is important to both increase the height of the features and to produce considerably more closely packed features. The goal of this research is to develop a greatly enhanced capability to lithographically define SU-8 features with heights that are on the order of 2-3 mm, with characteristic widths that are on the order of a few hundred micrometers, and, equally important, close packed. The major discovery that was ascertained in an attempt to achieve this goal was the diffusion of acid into unexposed regions prior to and during post bake is THE important physical parameter that governs all SU-8 processing steps. From this central idea, all SU-8 processing steps were altered to limit diffusion. The main process modification that allowed for this accomplishment was the new casting procedure that permitted for low uniform solvent content. The resulting new processing procedure led to SU-8 samples with heights between 2-4.5 mm and with a high density of SU-8 structures