Soldered sealing process to assemble a protective cap for a MEMS CSP

Capping and hermetic sealing of MEMS devices is a key factor in enabling them to be processed with further packaging steps like TO can, ceramic package or molded plastic. Aside from mounting a hermetic cap, the soldering of a front- and backside structured cap wafer allows as well to realize a chip scale package type of device protecting the internal MEMS structures and providing direct electrical interconnect structures for subsequent surface mount assembly, omitting additional packaging steps. In order to forego the cost and process issues around wafer feed through contacts, standard, established wire bond process is used to make a contact from the MEMS device to the backside structured cap wafer having a contact layer deposited on top. Solder balls terminate the electrical interconnects, allowing a seamless integration into a SMD compatible process. The paper presents the concept, the technical realization of this approach and the extension to a full wafer level chip scale MEMS package.