Micromechanical resonant switches (“resoswitches”) that harness the high-Q resonance and nonlinear dynamical properties of micromechanical structures are demonstrated that can achieve higher switching speed, better reliability (even under hot switching), and lower actuation voltage, all by substantial factors, over existing RF MEMS switches. Various mechanical structures (low to high resonance frequencies) with different structural and contact materials (medium to low resistances) are designed and fabricated to verify the advantages predicted by theory. Ample amounts of data were gathered and analyzed and new phenomena have been discovered. The first prototype resoswitch was based on a 61MHz wine-glass mode disk constructed in doped polysilicon. As the pioneer demonstration vehicle, this device requires an actuation voltage of only 2V to switch with sub-nanosecond switching times. It further achieved over 16.7 trillion hot-switched cycles, made possible in part by a large restoring force (>160mN) afforded it orders of magnitude larger stiffness than conventional RF MEMS switches. The contact resistance is on the kilo ohm range but reasonable considering the fact that doped polysilicon is the contact interface. The already impressive 16.7 trillion hot-switched cycle life time might have been much longer if not for undesirable impacting at the drive port that occurred with this initial device. An immediate solution to the high contact and series resistances of the polysilicon resoswitch is to construct everything in metal instead. Therefore, an electroplated nickel surface micromachining process was developed that allows for unequal electrode-to-resonator gap spacings in order to eliminate drive port impacting. The process uses only metal materials to keep the process temperature below 80 Celsius in an effort to permit post CMOS process compatibility. This process led to demonstration of a 25MHz wine-glass mode nickel disk resoswitch that delivered 17.7dB of power gain in a simple power amplifier topology, where the power gain comes about mainly through substantial reduction of contact resistance. Despite the introduction of unequal electrode-to-resonator gaps, endurance did not improve, mainly because the method used introduced an asymmetry that lowered the pull-in voltage of these devices. To address this issue, a 153MHz digitally specified displacement amplifier was then designed that employs asymmetrical mechanical coupling to generate larger displacement along the switch axis than that along the drive axis. The prototype device used polysilicon structural material, so did not have a lower enough contact resistance to demonstrate an actual power amplifier. However, the device did greatly extend the hot-switched cycle life time, which now achieves 173.9 trillion cycles using an open-loop drive circuit. If driven by a closed-loop drive circuit, an even larger count is expected. Finally, by synthesizing a group of comb-driven resoswitches, MEMS-based charge pumps were demonstrated with promise to eventually remove the diode voltage drop and junction breakdown issues that plague conventional transistor versions, allowing them to transfer charge with any input voltage level and achieve much higher voltages, perhaps eventually as high as 200V. A number of different topologies were successfully implemented, including single and multiple stages of Dickson’s and charge transfer series volt-age doubling designs, which proves the feasibility and agility of this technology.
December 31, 2014
Lin, Y. (2014). Micromechanical Resonant Switches ("Resoswitches") and Resonant Power Converters. United States: University of California, Berkeley.