A Method for Wafer-Scale Encapsulation of Large Lateral Deflection MEMS Devices

Packaging of microelectromechanical systems (MEMS) is a critical step in the transition from development to commercialized product. This paper presents a thin-film encapsulation process that allows varying trench widths suitable for MEMS devices with lateral deflections as large as 20 ??m. The process involves the deposition and planarization of a sacrificial-oxide layer of up to 23 ??m thick, the deposition of a 20 ??m epitaxial-silicon sealing cap, the release of structures using hydrofluoric acid (HF) vapor, and the sealing of the structure at low pressure. Devices produced using this encapsulation method are capable of surviving standard backend processes such as wafer singulation and wire bonding. Among the numerous types of devices encapsulated, two different types of silicon MEMS resonators were fabricated. These functioning resonators demonstrate the ability of the process to successfully encapsulate devices, taking advantage of both large and small trench widths. Such a generalized fabrication platform greatly expands the possibilities of the wafer-scale encapsulation to numerous MEMS devices and retains the robustness necessary for backend processing.