Electrostatically-driven MEMS liquid-metal droplet microswitches

In a mercury microswitch system, in which a mercury droplet acts as moving and contact part under electrostatic actuation, the liquid-to-solid contact brings many advantages over conventional microswitch system. The stability of mercury droplet in microscale, the low contact resistance of liquid-to-solid contact, and relatively easy integration with CMOS make the mercury switch an excellent candidate for the reconfigurable circuit interconnect.;However in microscale the high surface tension of mercury droplet requires a high voltage to actuate it. In order to reduce the actuation voltage, the contact properties of mercury droplet on solid surfaces need to be evaluated. This work reports that the adhesion forces of mercury droplet on a solid surface can be controlled or designed by physical modification of the solid surface; that is controlling adhesion force by controlling relative contact area between mercury droplet and solid surface. Relative forces to actuate a mercury droplet on different structured surfaces are theoretically analyzed and experimentally tested. Results show that the smaller contact ratio between mercury droplet and solid surface, the smaller adhesion force between them. The criteria for detaching a mercury droplet from solid surface was predicted and verified by experimental result. These results provide a good understanding in designing a mercury droplet microswitch with a lower driving voltage.;The physical modification of device solid surface is adopted to reduce the actuation voltage as well as control the stability of mercury droplet. A planar design and fabrication process of the switch are proposed in this project. The planar design simplifies the fabrication process and is compatible with standard IC process. Design issues have been carefully considered to improve the performance of mercury microswitches. The driving voltage of the planar device is reduced from more than 150V to around 80V by surface modification technology.;Post fabrication process has been developed successfully to integrate mercury switch with CMOS chip. The packaging method adapted from standard microelectronic packaging can seal mercury droplets inside the packaging chamber. With the improved post-CMOS fabrication process, a low driving voltage (15V) and millisecond switching speed have been achieved for the on-chip mercury droplet microswitches.