A micromechanical switch with electrostatically driven liquid-metal droplet Joonwon Kim a,* , Wenjiang Shen a , Laurent Latorre b , Chang-Jin Kim a a Micromanufacturing Lab., Mechanical and Aerospace Engineering Department, University of California, Los Angeles 48-121 Engineering IV, 420 Westwood Plaza, Los Angeles, CA 90095-1597, USA b Laboratoire d'Informatique, de Robotique et de Microelectronique de Montpellier, Montpellier, France Received 3 July 2001; received in revised form 10 December 2001; accepted 11 December 2001 Abstract Thispaperpresentstheuseofamicroscaleliquid-metaldropletasacontactandmovingpartinamicromechanicalswitchwithelectrostatic actuation.Design,FEManalysis,fabricationandtestingofthedevicearereported.Thedropletisdrivenbyagivenvoltagebiasthatinduces electrostatic force between a grounded liquid-metal and an imbedded actuation electrode. The electrodes and the liquid-metal droplet are placedinsideofananisotropicallyetchedsiliconcavity.Anoveltechniquetomakeshadowmasksutilizingthinwafersisusedtopatternthe electrodes inside the silicon cavity. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Liquid-metal; Shadow mask; Electrostatic force; Microswitch; Droplet switch 1. Introduction Duetofunctionaladvantagesofmicromechanicalswitches over transistor switches, various types of micromechanical switcheshavebeenintroduced[1±10]sincemicromachining technologies were pioneered in 1980s. These micromecha- nical switches show a high on±off impedance ratio, wide operatingtemperaturerange,andradiationinsensibility.Gen- erally, they can also handle high voltages and currents. However, almost all micromechanical switches are designed withsolid-to-solidcontactsthatexhibitthesameproblemsas macroscale mechanical switches, such as contact surface degradationandsignalbounce.Theseproblemsaremagni®ed formicroswitchesduetotheincreasedimportanceofcontact surfaces in microscale. In order to solve the reliability problems of the solid-to- solid contact, mercury is used in some high-end electronic instruments.Mercury'slowcontactresistanceanditslackof signal bounce and contact wear greatly increase the instru- ments' performance. Moreover, in microscale, mercury droplets show a great physical stability since inertial forces become negligible while surface tension force gains its importance. To utilize from the above properties of mercury, a gap closing microcantilever microswitch that used a stationary mercury microdroplet at the point of contact (Fig. 1a) and a thermally driven mercury microswitch (Fig. 1b) were devel- opedandtested[11,12].Althoughthesedevicesdemonstrated successful operation, they are limited by the size of their actuationsources(i.e.,beamlengthandheatersize)aswellas large power consumption (on the order of mW for thermal actuation).Inordertodesignamicroswitchwithreducedsize, lowpowerconsumption,andsimplefabrication,electrostatic actuationofthedropletisconsideredwiththemercurydroplet as a moving part, following a recent theoretical study [13]. Different applications of electrostatic actuation with liquid droplets (not with liquid-metal droplets) on a solid surface have been introduced in recent literature [14,15]. Bycombiningthereliabilityandhighqualityofamercury contact along with the simple implementation of elec- trostatic actuation, we are developing micromechanical switchesthataresmallenoughtobeconsideredformemory chips. Our current project goal is integration of the micro- mechanical switch arrays directly on top of CMOS chips to realize recon®gurable circuits for space applications [16]. 2. Droplet actuation analysis 2.1. Theoretical approach Let us consider a liquid droplet resting on top of a solid surface (Fig. 2). Assuming that the droplet is small enough Sensors and Actuators A 97±98 (2002) 672±679 * Corresponding author. Tel.: 1-310-825-0267; fax: 1-310-206-2302. E-mail address: joonwon@ucla.edu (J. Kim). 0924-4247/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved. PII:S0924-4247(02)00022-5