Tuneable RF MEMS capacitors based on electrostatically induced torsion

The paper presents the design, modelling, fabrication and testing of a new electromechanical structure for a tuneable RF MEMS capacitor based on an in-plane torsion generated by the in-plane forces developing during polarization of a parallel plate capacitor with partially overlapping electrodes. The designs explore the possibility to exploit an in plane rotation to extend the dynamic tuning range of a variable capacitor beyond the limits of conventional parallel plate capacitor and to determine the obtainable dynamic ranges and the limitations. These first prototypes adopt a butterfly structure suspended above the fixed electrodes. A small pivot point underneath the center of the mobile electrode acts as rest point after the pull-in and low friction pivot point for the rotation. Different suspension springs have been explored. A first pull-in followed by a parabolic branch with continuous tuning and a second pull-in characterize the C/V measurements. The measurements show a measured capacitance ratio ranging from 1.1 up to 1.6 with a 0–50V bias voltage. The capacitance values measured range from 286 fF up to 687 fF depending on the device type. Theoretical analysis shows that the capacitance ratio can easily be increased by a factor of 20 simply by reducing the initial overlap of the electrodes.