Nathan Fritz

Packaging-compatible wafer level capping of MEMS devices

A cost-effective, wafer-level package process for microelectromechanical devices (MEMS) is presented. The movable part of MEMS device is encapsulated and protected while in wafer form so that commodity, lead-frame packaging can be used. A polymer epoxycyclohexyl polyhedral oligomericsilsesquioxanes has been used as a mask material to pattern the sacrificial polymer as well as overcoat the air-cavity. The resulting air-cavities are clean, debris-free, and robust. The cavities have substantial strength to withstand molding pressures during lead-frame packaging of the MEMS devices. A wide range of cavities from 20@mmx400@mm to 300@mmx400@mm have been fabricated and shown to be mechanically stable. These could potentially house MEMS devices over a wide range of sizes. The strength of the cavities has been investigated using nano-indentation and modeled using analytical and finite element techniques. Capacitive resonators packaged using this protocol have shown clean sensing electrodes and good functionality.

Domed and Released Thin-Film Construct—An Approach for Material Characterization and Compliant Interconnects

An approach for microfabricating precisely defined spherical 3-D domes through a simple low-temperature polymer reflow process was developed. Release of a thin metal film patterned over the domed structure was accomplished by the removal of the underlying polymer using two different methods: dry thermal decomposition and wet supercritical release. The domed shape impacted the effect of stiction during the release step and assisted in the release of large millimeter-size film geometries. The dome and release procedures were used to fabricate an experimental test specimen that functions as either a tensile or interfacial fracture test for thin films on rigid substrates. Other potential applications of the domed metal structures such as compliant electrical interconnects are discussed.

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.