A Robust Bilayer Cap in Thin Film Encapsulation for MEMS Device Application

Thin-film encapsulation (TFE) is one of the promising wafer-level packaging techniques for microelectromechanical systems (MEMS) devices. One of the drawbacks of TFE is that it is difficult to encapsulate large MEMS devices due to the downward deformation of the cap layer during the release of the TFE. This paper presents a robust bilayer cap made out of aluminum nitride (AlN)/nickel (Ni) layers, which can solve the problem of downward deformation of the cap layer. This bilayer cap provides both strength and flexibility to the TFE. This bilayer cap is simulated using ANSYS and experimentally demonstrated by encapsulating cavities as large as 1200 μm × 1200 μm with 0.5 μm AlN/0.7 μm Ni, whereas even 400 μm × 400 μm-sized TFE cannot be fabricated without cracks with a solitary 1-μm-thick AlN cap layer. Standard microfabrication processes are used for the fabrication of this bilayer cap.