(618bj) Versatile Low-Cost Air-Gap Structures for MEMS Packaging

Recent advances in microelectromechanical systems (MEMS) technology have expanded their possible applications and potential market use. However, MEMS packaging presents a pivotal challenge to the evolving MEMS industry because it is costly and overly specialized to each MEMS device or family of devices. For today’s typical MEMS-based products, packaging expenses can be up to 20% to 40% of the product’s total material and assembly cost. [1] The aim of this project is to create a wafer-level packaging technology for families of MEMS devices that would transform the MEMS device into a form compatible with IC packaging processes. In this work, a polymer-based Air-gap structure process is being investigated to provide a cost efficient method for lead frame packaging of MEMS devices.       

The air gap process uses an overcoat material to cover a patterned sacrificial polymer, polycarbonate (PC), which protects the MEMS device during packaging. Once the overcoat is in place, the sacrificial polymer is thermally decomposed freeing the MEMS structure while the overcoat dielectric provides mechanical and chemical protection from the environment.  Improvements in the sacrificial materials and the introduction of an inorganic/organic epoxy-POSS overcoat has improved cavity design for clean removal of the PC material without damaging the cavity formed during the process.  These developments have demonstrated that air-gaps with rigid overcoats can be used to encapsulate MEMS devices in a wafer-level packaging process.       

The packaging structures can be designed for a range of MEMS device sizes and operating environments including fluidic, hermetic and vacuum conditions. However, the air-cavity structures need additional rigidity to withstand chip level packaging conditions, which are dependent on cavity size and molding pressure. Current work is focused on implementing a wafer level air-cavity package into a lead frame packaging scheme for MEMS devices. Metalized air-gap packages have been tested and optimized to withstand injection molding conditions necessary for chip-level packaging. A second approach will look at a size independent, semi-hermetic package (no metal). This process will look to combine the thermal steps of the sacrificial release and the molding process in to one step. This design will provide improved mechanical stability by prevention of cavity deformation during decomposition and molding simultaneously. 

References:

1. Grace,  R.;  Maher,  M. A.  “MEMS: Think outside the chip... at the package level,” Hearst Electronic Products 2010, http://www2.electronicproducts.com/PageSearch.aspx?     FName=farc_softmems_nov2010.html.