Improved bi-layer lift-off process for MEMS applications

Abstract

Conventional lift-off process is designed for defining very fine pattern or removing metals hard to etch and the process is limited to surface microfabrication due to the requirement on directional deposition. We improved the process for the step coverage requirement in 3-D MEMS. A novel bi-layer (top image resist Shipley S1808 and bottom lift-off resist Microchem LOR) lift-off process was built up for patterning 3-D devices. Resists were coated and patterned with an overhang profile on a substrate before the substrate was etched. The key feature of the new lift-off process is to create a resist undercut profile, which is deep enough to reduce step coverage and durable to aggressive substrate etchant. The two-step development method was effective for patterning 3-D microdevices with high aspect ratio trench or through-holes. The development time of the resist profile was optimized. Fine pattern of 2μm/5μm (line/space) was achieved on quartz microstructure with high aspect ratio (2.7) trench.