Determination of the Pull-Off Forces and Pull-Off Dynamics of an Electrostatically Actuated Silicon Disk

To increase the mechanical sensitivity of micromachined yaw rate sensors, the proof mass can be designed as an electrostatically levitated flywheel. Therefore, the flywheel is kept in its levitated position by an electrostatic field exerted by an array of electrodes above and underneath the flywheel. Once the flywheel is in contact with either the bottom electrodes or the top electrodes, adhesion forces arise which might prohibit the flywheel from being lifted back to its levitated operation position. The subject of this paper is the investigation of the pull-off forces emerging during the start-up phase of an electrostatically actuated silicon disk, which is a test structure for such a levitated flywheel gyroscope. For the experimental determination of the pull-off forces, a test device based on a levitated flywheel gyroscope design is fabricated featuring soft polymeric and thus insulating springs for the inhibition of the lateral movement of the disk. Furthermore, the surfaces of the silicon disk and the electrode contact areas are modified to reduce the amount of the adhesion forces. For such textured surfaces, a pull-off force of 0.25 plusmn 0.05 mN is measured for a circular disk with a diameter of 3 mm. This corresponds to an adhesion energy per area of 3.5 ldr 10^-6 mJ/m². Thus, a repeatable pull-off of the flywheel at moderate actuation voltages is achieved.