Electrostatic actuators are important components in the field of Microelectromechanical systems (MEMS). FIG. 1(a) shows a conventional electrostatic actuator, which features a base substrate 10 on which a drive electrode 12 is mounted in a stationary position, while a movable MEMS structure 14 carrying or containing a second electrode is suspended at a spaced distance D above the drive electrode, which is biased to a drive voltage VD. The application of drive voltage VD creates an electric field incident on the second electrode of the MEMS structure 14, whereby electrostatic attraction of the second electrode of the MEMS structure toward the stationary drive electrode draws the MEMS structure toward the drive electrode and underlying base substrate. By varying the drive voltage, this displacement of the MEMS structure can be controlled. So the drive voltage, i.e. the voltage that generates the electric field attracting the movable second electrode toward the stationary drive electrode doubles as a control voltage, as it is the variation of this drive voltage that controls the displacement of the MEMS structure.
Such conventional electrostatic actuators suffer pull-in after displacing only approximately ⅓ of the electrode separation distance D, as described in below cited Hung et al. [1], thereby limiting the controllable displacement range. In other words, the controllable stroke of the MEMS structure is limited to a pull-in distance Dpull-in, which occurs at VDmax and is approximately ⅓ of D. As a result, the drive electrode must be placed significantly distant from the MEMS structure when large controllable stroke is required. However, this leads to necessity of a significantly elevated driving voltage, since the electrostatic force is proportional to the square of the separation distance. In below-cited Seeger et al. [2], a capacitor in series with the electrode power supply was explored to avoid pull-in. However, this still suffers from the requirement for larger voltage as the series capacitor is charged in the effort to mitigate positive feedback of MEMS motion.
Accordingly, there remains room for improvement in the design of electrostatic actuators, and particularly a desire to reduce the control voltage required by such actuators.