On-Wafer Monolithic Encapsulation by Surface Micromachining With Porous Polysilicon Shell

In this paper, we present a novel microfabrication technique that solves the main problems of existing monolithic on-chip encapsulation methods for polysilicon surface micromachining. The encapsulation technique includes the formation of a nanoporous polysilicon shell, creation of a cavity by removing the sacrificial layer through the pores in the shell, and sealing the cavity at a low pressure. Formed porous by postdeposition electrochemical etching on top of a sacrificial layer, the porous polysilicon is thick enough to free-stand when released, unlike the previously reported as-deposited permeable polysilicon. Benefiting from the dense pores through the polysilicon layer, the sacrificial material was removed in just one minute, and the vacuum sealing was achieved by a low-pressure chemical vapor deposition polysilicon as thin as 1000 Aring with no sealing material detected inside the cavity. The pressure inside the sealed cavity, measured by an encapsulated polysilicon Pirani gauge, was around 130 mTorr and showed no noticeable leak (<30 mTorr) over one year. To showcase the applicability, the proposed process was demonstrated through the common Multiuser MEMS Process (MUMPs) foundry service

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