Three dimensional air-gap structures for MEMS packaging

Air-gap structures are of interest in a range of microelectronic applications especially in microelectromechanical systems (MEMS). In this work, we investigate the application of an unique trimaterial for MEMS packaging composed of polypropylene carbonate (PPC) as a sacrificial material, a photosensitive, hybrid inorganic/organic dielectric epoxycyclohexyl polyhedral oligomeric silsesquioxanes (POSS) as the overcoat material, and Al/Cr-Cu thin metal film as a hermetic seal. POSS was used both for patterning the PPC over the structures as well as a stable overcoat material thus reducing the complexity of the fabrication process. A wide range of device sizes and structures (from 20 × 100 µm to 600 × 1000 µm) were fabricated and the processing protocol was found to be compliant over these size/structure variations. Metal adhesion on the overcoat was substantially improved by using low power oxygen plasma for short durations. Cavity-strength was evaluated for different metals and thicknesses. An increase of 5.6 times in cavity-strength was observed for a thicker (3X) Al metal film. Current work is focused on implementing the wafer-level air-cavity package into a lead frame packaged MEMS device through injection and compression molding techniques.