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Wafer Level Thin Film Encapsulation for MEMS

Because they have moving parts, or because they need to work in a specific atmosphere (vacuum, inert gas, etc.), micro-electromechanical systems (MEMS) are not compatible with standard integrated circuit (IC) packaging technologies. Specific packaging needs represent a great part of the final manufacturing cost of such devices. This article presents a solution to encapsulate RF MEMS devices in hermetically sealed cavities, using only standard IC manufacturing technologies at wafer level. A polymer sacrificial layer is deposited on the devices, and then covered with a dielectric cap. The cap is perforated with a standard lithography/etching process to enable the removing of the sacrificial layer by a dry process. The cap is finally vacuum sealed by a dielectric deposit. This thin film pre-packaging process enables MEMS protection and greatly reduces the cost of final packaging. A simplified device was designed and encapsulated under vacuum with the presented process. It allows to measure pressure evolution inside the cavity and by this way to check the hermeticity of the package