Vacuum sealing of microcavities using metal evaporation

A new encapsulation technique to seal a vacuum-tube microcavity hermetically at low pressures, based on aluminium evaporation, is presented and its performance is compared to conventional low-pressure chemical vapour deposition (LPCVD) reactive sealing. The microdiode consists of an in-cavity recessed single-crystalline silicon cathode tip above which a polycrystalline silicon anode is suspended on a silicon-rich nitride layer. The diode cavity is cleared from the sacrificial oxide in buffered HF through the horizontal etch-access channels between the polysilicon anode and the silicon-rich nitride isolation layer, Vacuum sealing of the cavity using LPCVD polycrystalline silicon results in polysilicon deposits (>50 nm) inside the cavity, and thus in a non-acceptable degradation of the cathode-tip curvature. When sealing is performed using aluminium evaporation, no deposits inside the cavity are observed and pressures below 10(-3) Pa can be expected. Applications of the technique presented are not restricted to micro vacuum diodes, but also include various types of hermetically seated micromechanical structures, where deposits inside the sealed cavity are undesirable.