A thick photoresist process for high aspect ratio MEMS applications

In recent years, increased demand for high aspect ratio MEMS structures has driven the need for thick photoresist fabrication processes. In this work, the optimization of a thick photoresist process using a negative tone resist (THB-151N) is described. A thickness of 85 μm is obtained with an aspect ratio of 17:1 in a single coating process, with a 5 μm pitch. Conventional UV lithography is used and its parameters are optimized in order to achieve straight and near vertical sidewall profiles. The developed patterns are used as a mold to electroplate high aspect ratio copper windings of micro-inductors and micro-transformers. A high aspect ratio yields a copper track with a large cross sectional area resulting in a lower DC resistance. This enables a further reduction in the footprint area allowing for a more efficient manufacturing process and smaller device size. Unlike other high aspect ratio resist such as SU-8, this resist does not need a post exposure bake and can be readily removed after metal electroplating.