JOURNAL OF MICROELECTROMECHANICAL SYSTEMS, VOL. 15, NO. 4, AUGUST 2006 879 Electrostatically Actuated Metal-Droplet Microswitches Integrated on CMOS Chip Wenjiang Shen, R. Timothy Edwards, and Chang-Jin Kim, Member, IEEE Abstract—The dominance of surface tension over inertia in mi- croscale and favorable scale effect for electrostatic actuation allow electrostatically driven metal-droplet systems practical. Because of such potential advantages as low contact resistance, naturally bistable operation, and high switch density, the liquid-metal droplet switch is an excellent candidate for reconfigurable circuit interconnections. Following earlier droplet microswitch examples and related studies of metal-droplet behavior, we report the first functioning droplet switch directly integrated on top of a functional CMOS circuit. While the surface tension dominance makes the droplet switches practical as a mechanical system and also brings bistability, it also requires a high electric field to move the droplet. We implement the concept of physical surface modi- fication to lower the driving voltage to a value that a commercial CMOS process can provide. Unlike previous droplet switches, the reported device is planar-processed to allow the integration with the underlying CMOS circuits. The integrated switch is made functional by such provisions as self-limiting actuation and by optimizing the electrostatic force in the planar configuration and avoiding liquid-metal “flooding” into surface patterns. A fabri- cation process for low driving voltage and high compatibility is developed to integrate the droplet switch on the custom-developed CMOS chip. A packaging method adapted from well-established microelectronic packaging isolates the active switch space from the surrounding environment. Low driving voltage (as low as 15 V) and millisecond switching speed are achieved by the current on-chip device. While the current device uses m droplets for demonstration, additional theoretical and experimental results indicate that further miniaturization would lead to smaller devices and lower operation voltage. [1367] Index Terms—Droplet microswitch, liquid-metal droplet, mer- cury microswitch, microswitch, surface tension, electrostatic actu- ation. I. INTRODUCTION M EMS TECHNOLOGIES, with the ability to make moving structures in microscale with integrated circuit (IC) fabrication processes, have created an opportunity to build micromechanical switches along with ICs. Because of the clear application direction and process compatibility, the mi- croswitch has been an attractive research subject in MEMS from Manuscript received June 23, 2004; revised January 16, 2006. This work was supported by the National Science Foundation (NSF) CAREER Award ECS-9702875 and NASA research Grant NAG5-10397. Subject Editor C. H. Mastrangelo. W. Shen was with the Mechanical and Aerospace Engineering Department, University of California (UCLA), Los Angeles, CA 90095 USA. He is now with Innovative Micro Technology (IMT), Santa Barbara, CA 93117 USA (e-mail: WShen@imtmems.com). C.-J. Kim is with the Mechanical and Aerospace Engineering Department, University of California (UCLA), Los Angeles, CA 90095 USA (e-mail: cjkim@seas.ucla.edu). R. T. Edwards is with the Applied Physics Laboratory, Johns Hopkins Uni- versity, Baltimore, MD 21218 USA. Digital Object Identifier 10.1109/JMEMS.2006.878877 its inception [1]–[5]. Compared to semiconductor transistors, micromechanical switches are inherently slower in switching speed, because they involve the physical displacement of in- ertial mass. The electromechanical switching time is typically many microseconds or greater, which is substantially longer than that of semiconductor transistors. However, mechanical switches have many promising properties transistors lack. For example, micromechanical switches can be built to have a very high “off” resistance and a low “on” resistance, resulting in lower loss and less power dissipation Another advantage that micromechanical switches have over CMOS transistors is that they can be made more linear and stable with respect to a host of variable conditions such as voltage, current, temperature, pressure, and radiation. Because of these properties, MEMS switches can be used in programmable circuits and systems, in particular field-pro- grammable gate arrays, or FPGAs. FPGAs have become quite popular due to the ease of programming and fast turnaround time compared to custom ICs. Switches made from active devices in standard fabrication technologies (usually CMOS) are non-ideal in many ways. The worst property is the resis- tance through the switch. When CMOS transistors are used as switches, the resistance is typically in the kilo-ohm range, and densely populated applications (such as FPGAs) cannot afford to make the trade-off in area and capacitive loading by increasing the size of each device to reduce the switch resistance. The “antifuse” is another type of integrated switch with resistances as low as 20 [6]. However, the resistance is not well-controlled, and these devices can only be programmed once. Since present technology falls short of what is required from the configurable switches, designers must make compro- mises at the system level. Although MEMS technology creates an opportunity for programmable interconnection, there are still many limitations for traditional beam-type microswitches. First, the size is much larger than a typical IC device, making dense integration difficult. Second, the solid-to-solid contact resistance is not small enough, and the contact surfaces may degrade during operation because of surface wear, oxidation and contamination, which results in dramatic increase of the contact resistance. Finally, these microswitches will often in- terfere with the placement and arrangement of CMOS circuits underneath. These limitations of beam-type microswitches have led us to explore liquid-metal droplets [7]–[11]. Although mercury is used for convenience during develop- ment and demonstration of new ideas and concepts, it is ex- pected that other safer liquid metals (e.g., [12]) would be used for eventual commercial devices. A liquid-metal droplet in a MEMS microswitch system is expected to exhibit the following features for such applications as FPGAs. 1057-7157/$20.00 © 2006 IEEE