Electrical and mechanical properties of micromachined vacuum cavities

This paper reports the measured electrical and mechanical properties of micromachined vacuum cavity resonators made on bonded silicon on insulator (SOI) wafers using the new "plug-up" process. With this novel process it is possible to fabricate microelectromechanical devices without the attack of hydrofluoric acid against the aluminium metallisation during the sacrificial oxide layer etching. Another merit of the new process is its improved immunity to stiction. The moving parts of the microresonator are constructed of single crystal silicon with very low internal stress. In surface micromachining the structures are typically made of CVD polysilicon, which has higher internal stress. Thus the mechanical properties of these resonators on SOI are governed by the elastic rigidity of the material used and the geometry of the structure which is affected by both design parameters and process variations. The mechanical losses of the cavity structure are caused by the supports at edges of resonator and the acoustic damping. The pressure during LPCVD process used for sealing determines the pressure remaining in the cavity. With the plug-up process one can fabricate vacuum cavities on SOI substrates without deteriorating the silicon material properties or its surface quality. These cavities can be utilised for realization of a wide variety of single crystal micromechanical devices and in monolithical integration of SOI/MEMS and integrated circuits.