Alow Temperature Vacuum Package Utilizing Porous Alumina Thin Film Encapsulation

We report a monolithic thin-film encapsulation method that satisfies the most popular requirements for on-wafer packaging: low temperature, low cost, hermetic, and RF compatible. The key for this new nano-porous thin-film encapsulation method is the technique to produce a large free-standing porous alumina membrane on-chip by post-deposition anodization of aluminum at room temperature. A porous alumina membrane allows for the diffusion of gas or liquid etchants through the nano-pores into the cavity to etch the sacrificial material, freeing the movable structures encapsulated inside. Subsequent vacuum sealing was achieved by depositing a thin film over the nano-porous alumina shell in a vacuum deposition tool, with no detectable penetration of the sealing material owing to the nano-pores with a high aspect ratio (> 30). The process, done at a low temperature, produced a hermetic vacuum seal and demonstrated an exceptionally low RF insertion loss: < 0.1 dB up to 40 GHz.