Device-Level Vacuum Packaging for RF MEMS

Abstract

For specific RF applications, where the use of MEMS is highly attractive, cost-effective reliable packaging is one of the primary barriers to commercialization. Many RF MEMS devices require a hermetic or vacuum operation environment. This paper presents a post-CMOS-compatible method for vacuum packaging of RF MEMS devices by growing an encapsulation layer during the device fabrication. The resulting MEMS devices are surrounded by a vacuum cavity and can then be placed in a conventional low-cost circuit package. This is a low-temperature area-efficient device-level encapsulation for MEMS devices. RF MEMS resonators in a fixed-fixed configuration were used as the test bed since their quality factor can be used as a measure of the package quality. The encapsulation process is based on a double-sacrificial-layer surface micromachining technique, which is used to create a cavity under and above the resonator. Polyimide was used as the sacrificial layer, followed by the deposition of a packaging layer with trench cuts, which facilitate the sacrificial layer removal. The trench cuts were then sealed at a low pressure, forming a cavity around the device at the sealant layer deposition pressure. Extensive RF characterization and reliability tests were performed on the packaged resonators.