SU-8 surface micromachining using polydimethylglutarimide (pmgi) as a sacrificial material

Microelecromechanical systems (MEMS) are mechanical structures that are fabricated using similar techniques used to fabricate integrated circuits. MEMS were originally fabricated using silicon based technologies; however, a more diverse set of materials, including polymers, are being now being used to fabricate MEMS. Polymer micromachining is less expensive than silicon micromachining and can be performed in less time. SU-8 is finding greater use as a structural polymer MEMS material due to its biocompatibility, mechanical properties, and low cost. The goal of this work is to expand the use of SU-8 through the creation of SU-8 based multi-user surface-micromachining processes, which use polydimethylglutarimide (PMGI) as a sacrificial layer. PMGI is a deep UV (DUV) positive-tone resist used mainly for bilayer lift-off processes; however, PMGI has found some application as a sacrificial material for MEMS. PMGI is a good sacrificial layer candidate, as it can be spun on at a wide variety of thicknesses, is photopatternable, and has a glass transition temperature of 180°C, which is greater than the processing temperatures required for SU-8. This work describes the characterization and use of PMGI (MicroChem Corp. SF series) as a sacrificial material for SU-8 surface-micromachining processes. PMGI is shown to be useful for fabricating single layer devices with patterned metal, single layer devices with multiple different thicknesses and two layer devices. Many different devices were fabricated with the developed processes in two different device classes. The first class of devices fabricated are compliant mechanisms, including bent-beam actuators, thermal isolation platforms, and out-of-plane grippers. The second class of devices fabricated are freely moving devices such as hinged plates and gears, which require the use of true kinematic joints, such as scissor hinges, staple hinges, and pin joints. This work describes in detail the application of PMGI as a sacrificial material for a multi-user SU-8 surface-micromachining technology, and provides examples of fabrication.