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A thin-film encapsulation process, featuring low-temperature steps, hermetic sealing (preliminary), and RF-compatible shell, is reported. Uniquely attractive as compared with the existing MEMS packaging approaches is its capability to monolithically package metal microstructures inside a microcavity on chip in one continuous surface-micromachining process. The key for this process is a technique to fabricate a large freestanding porous membrane on chip by postdeposition anodization of thin-film aluminum at room temperature. The porous-alumina membrane allows for the diffusion of gas or liquid etchants through the nanopores to etch away the sacrificial material underneath, freeing the movable microstructures encapsulated inside the cavity. To seal the package, a thin film is deposited over the alumina shell whose nanoscale pores of a high aspect ratio (> 30) do not allow any detectable penetration of the sealing material. The low-temperature (<300degC) encapsulation process produced a low-pressure seal (8 torr), monitored by a Pirani pressure gauge that also represents an encapsulated freestanding metal microstructure in the cavity. The thin-film package demonstrated a considerably low RF insertion loss of less than 0.1 dB up to 40 GHz.