Abstract
In microsensors or microactuators which use an electrostatic capacitance change or electrostatic force as a driving force, a structure with an insulation layer on a movable electrode or its opposing electrode is often used in order to prevent short circuiting of the electrodes when they contact each other.
Model samples for evaluating surface force in such micro devices were fabricated by a silicon micromachining technique. Each sample was composed of a movable electrode supported by cantilever beam and opposing electrodes. Insulation films of thermal oxide were formed on the surface of the movable silicon electrode. Surface force between the insulation film and the opposing electrode which arise after applying voltages between the electrodes was evaluated.
The surface force depended on magnitude, applied time and especially polarity of the applied voltage. Such a surface force is considered to originate in the electrostatic force due to the electric charge stored in the insulation film or the residual polarization of the insulation film arising from the large applied electric field.
The surface force of the model sample was significantly decreased by forming shield electrodes, which have the same electric potential as the movable electrode, at opposed portions of the opposing electrode to the insulation film. In such a structure, the electric field applied to the insulation film was much reduced compared with that without the shield electrode. This structure can be applied to various kind of microsensors or microactuators to reduce the surface force which lowers performance of such devices.
