VACUUM SEALING OF MICROCAVITIES USING METAL EVAPORATION

A new encapsulation technique to seal a vacuum-tube microcavity hermet ically 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 wh ich a polycrystalline silicon anode is suspended on a silicon-rich nit ride layer. The diode cavity is cleared from the sacrificial oxide in buffered HF through the horizontal etch-access channels between the po lysilicon anode and the silicon-rich nitride isolation layer, Vacuum s ealing of the cavity using LPCVD polycrystalline silicon results in po lysilicon deposits (>50 nm) inside the cavity, and thus in a non-accep table degradation of the cathode-tip curvature. When sealing is perfor med using aluminium evaporation, no deposits inside the cavity are obs erved and pressures below 10(-3) Pa can be expected. Applications of t he technique presented are not restricted to micro vacuum diodes, but also include various types of hermetically seated micromechanical stru ctures, where deposits inside the sealed cavity are undesirable.