Low-Cost MEMS Packaging Using Polymer-Based Air-Gaps

Packaging of microelectromechanical systems (MEMS) is vital to maintaining device performance and minimizing the impact of environmental threats. Packaging accounts for a substantial portion of the total cost of the MEMS device. In the approach taken here, the movable  MEMS component is encapsulated by a sacrificial polymer in order to protect the device during processing. The component is then packaged in a traditional way, such as by overmolding on a substrate or lead-frame packaging. Thermal decomposition of the sacrificial polymer releases the MEMS device either during or after component packaging. A thermally decomposable sacrificial material, polypropylene carbonate (PPC) and overcoat material BCB (benzocyclobutene) was used for protection of the MEMS device during packaging. PPC decomposition products permeate through the overcoat to leave an air cavity. Control of the thermal decomposition of polypropylene carbonate is an integral part of the process because the MEMS device needs to be released at a critical point in the process.

In this study, the thermal decomposition temperature of the PPC was modified by kinetically inhibiting the thermal decomposition via end-capping. Reacting the hydroxyl-end group of polypropylene carbonate with chemical moieties such as organohalides and mono/difunctional acids alters the thermal stability of these materials. Furthermore, additives such as metal halides and multifunctional alcohols can also alter the thermal decomposition PPC.  In this presentation, the status of these modified polymers in this cavity-forming MEMS packaging process will be discussed.