2009 TIE Sen Micro LM Switch - Review

1314 IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 56, NO. 4, APRIL 2009

Microscale Liquid-Metal Switches—A ReviewProsenjit Sen and Chang-Jin “CJ” Kim, Member, IEEE

Abstract—Microelectromechanical systems (MEMS) have con-stituted an active R&D area over the last two to three decades, withone of the earliest application topics being microswitches. Typicaldesigns involve actuation of microscale flexural elements (e.g.,beams and membranes) to make a short or an opening in the trans-mission (signal) line. However, the problem of reliability of theseswitches persisted due to the presence of a solid–solid contact.Inspired by the regular mercury switches that use liquid–solid con-tact to solve the problems, several researchers have been exploringthe use of liquid metal (LM) in developing microscale switches.Over time, the following two different approaches have evolved:LM-wetted microswitches and LM-actuated microswitches. In thispaper, we summarize the progress of both approaches over the lastdecade by reporting a series of LM microswitches, each with themechanism, fabrication, and performance. In addition, the prop-erties of various LMs and LM alloys and the issues of fabricationand packaging involving LM are presented to help understand thereported developments as well as to assist in designing future LMmicroswitches.

Index Terms—Liquid metal (LM), mercury switch, micro-electromechanical systems (MEMS) switch, microswitch, reliabil-ity of microswitch.

I. INTRODUCTION

A. Mercury in Regular (Macroscale) Switches

AN ELECTRICAL switch is a device which can change theflow of current in an electrical circuit. This conceptually

simple device finds its use in many electrical appliances in oneform or another. Its diverse application has led to the develop-ment of several switching technologies, including mechanicalswitches, electromechanical relays, and transistors. One suchtechnology of our interest in this paper is the electromechanicalrelay, where an electrical signal is used to actuate a mechanicalelement to achieve switching. Despite being one of the earliesttechnologies, the electromechanical relays have several attrac-tive properties when compared to semiconductor switches: avery high on–off impedance ratio with OFF-state impedanceon the order of 1010–1014 Ω [1]. They also have low contactresistance (< 50 mΩ [1]), can handle large currents (power),and are relatively insensitive to environmental conditions (e.g.,temperature, humidity, and radiation). However, they are largeand slow (greater than 1 ms switching delay), and the presenceof a solid–solid mechanical contact leads to contact bounce,making them unsuitable for small-signal switching. Solid–solidmechanical contact also leads to contact arcing and welding,

Manuscript received May 21, 2008; revised September 5, 2008. First pub-lished October 31, 2008; current version published April 1, 2009. This workwas supported by the DARPA HERMIT program.

The authors are with the University of California, Los Angeles, CA 90095USA (e-mail: [email protected]; [email protected]).

Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/TIE.2008.2006954

causing surface damage and material transfer at the contact.This contact degradation limits the reliability and operationallife of these devices.

In contrast, semiconductor switches (e.g., transistors) arefast, with nanosecond switching times. The absence of solid–solid mechanical contact means that no problems are relatedto contact bounce or contact degradation, leading to very longoperational lives. They are also very small in size, integrate wellwith other circuit elements, and hence are cheaper. However,they have a high ON-state resistance of 2–6 Ω, low open-stateimpedance on the order of 105–107 Ω [2], and low power-handling capabilities. Also, their sensitivity to temperature andradiation limits their range of operating environments.

In order to obtain longer life and lower switching noise whileretaining other benefits of electromechanical relays, mercury-wetted relays were first developed at Bell Labs in the 1940s.After several years of research and improvement, these switchesconsisted of a movable and a fixed metallic contact in a sealedglass envelope containing a pool of doped mercury. Dopingwith Sn and Cu was used to protect against failure during long-term usage caused by the formation of amalgams, particularlyNiHg4 [3]. The solid contacts were wetted by mercury, whichwas drawn in the capillary formed by the closing gap betweenthe moving and the static contacts when the switch was actu-ated magnetically. The switch was hermetically packaged in ahydrogen environment to prevent arcing and mercury oxidation.These switches demonstrated bounce-free operation, low con-tact resistance (< 50 mΩ), long life (over 109 cycles [4]), fastrise time (on the order of 10 ps [5]), and, in some cases, highisolation voltage over 5 kV [6]. Mercury-wetted relays wereheavily commercialized and extensively used in telephony andother low-signal high-bandwidth applications [7]. However,these switches were slow, sensitive to gravity, and posed envi-ronmental hazard, as the quantity of mercury contained in eachswitch was significant. To solve the gravitational sensitivityproblem, mercury-film switches were developed [2], [8]. Inthese switches, contact surfaces were permanently wetted in athin film of mercury, and the mercury pool was not required.Despite several advantages, the risk of pollution due to spillor improper disposal caused this technology to be replaced byothers over the last two decades.

B. MEMS Switches

Microelectromechanical systems (MEMS), although thename was coined in the late 1980s, has been an active area ofresearch and development since the 1970s for several sensorsand actuators, with microswitches being one of the first appli-cations. One of the earliest switch implementations, a SiO2-based reed relay fabricated using MEMS surface and bulk

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SEN AND KIM: MICROSCALE LIQUID-METAL SWITCHES—A REVIEW 1315

micromachining techniques, was demonstrated by Petersen inas early as late 1970s [9], [10]. Like their macroscale ancestors,most of the MEMS switches have a moveable mechanicalelement actuated by various actuation mechanisms to achievea short or an open in the signal line. Electrostatic actuation(e.g., [11] and [12]) leads to a simple design and requiresalmost negligible power. These switches are fast, with typicalswitching speed on the order of tens of microseconds. However,this technique generally requires higher voltage (only specificdesigns demonstrating less than 20 VDC) and generates smalleractuation force in the range of tens to hundreds of micronew-tons, depending on the switch design and actuation voltage.Microswitches operated by the electrothermal [13]–[15]method, on the other hand, generate more closing force (inthe range of millinewtons [15]) and thus lead to low contact-resistance values (in the milliohm range [16]). Although theseswitches use low actuation voltage (less than 8 VDC), high-current requirement makes them power hungry (20–40 mW tomaintain ON-state [15]). Bistable designs have been demon-strated such that power is consumed during switching but not tomaintain state [13]. The typical reaction time is slow and usu-ally larger than several hundreds of microseconds. Electromag-netic actuation provides a large closing force and can be fast[17] but requires complex designs to facilitate coils or magnets.

Micromechanical switches enjoy the same benefits over thesolid-state switches as was enjoyed by their macroscale me-chanical switches. Reduced in size by MEMS technologies,the miniature mechanical switches are now becoming a moreviable substitute of the solid-state switches. Despite significantprogress in the MEMS switch technology (with some switchesdemonstrating more that 100 billion cycles [18], [19]), thereare still doubts about their long-term reliability under deployedconditions. Two common failure mechanisms observed aredielectric charging [20] and contact degradation [16]. Thecontact failure mechanism is similar to that observed in themacroscale electromechanical switches and is caused due toarcing and welding at the solid–solid contact, leading to surfacedamage and material transfer. This problem at contact hasled to the creation of many switch designs that avoid hotswitching. High-power operation accelerates the contact wear[19], thus severely limiting the power-handling capabilities ofthe MEMS switches. Large contact force is required to achievea proper metal–metal contact and low contact resistance butunfortunately accelerates the contact wear [16], resulting in atradeoff between the switch reliability and its contact resistance.To improve switch reliability against contact degradation, someswitches use prescribed actuation waveforms that minimize theimpact at the contact [21]. Taking a hint from their macroscalepredecessors in addressing the contact issue, some researchershave explored the use of liquid metal (LM) for the developmentof high-reliability microswitches.

C. The Review

The aim of this paper is to investigate the development ofLM-based microscale switches. In the next section, we willdiscuss the available choices of LMs and LM alloys for thedevelopment of the microscale switches. Their physical and

chemical properties will be discussed in brief. In Section III, wewill discuss the various fabrication technologies developed torealize LM microswitches. Owing to their high reactivity withother metals, LMs pose serious material compatibility issues.Innovative packaging technologies are also required to preventLM oxidation. The deposition of microscale LM droplets is adifficult process owing to their high surface tension; severaldeposition techniques suitable for different LMs are discussed.Next, we will discuss micro LM-switch devices developedfor electrical applications. Two distinct design approaches willbe reviewed. In Section IV, we will study LM-wetted micro-switches, where LM simply wets the contact surface of theswitching element. Finally, in Section V, we will look into LM-actuated microswitches, where LM not only moves directly asthe switching element but also provides a wetting contact.

II. LMs AND LM ALLOYS

A. Mercury

Mercury is one of the five elements (others include gallium,cesium, francium, and bromine) that are liquid at near roomtemperature. Its earliest use was in ointments and cosmetics,and its use in amalgamation with other metals was discoveredaround 500 B.C. Its symbol Hg is derived from its Greek nameHydrargyrum, which means “watery silver.” It is also known asArgentum vivum in Latin, meaning “quicksilver.” So strong isits history that mercury is the only metal that is still known byits alchemical planetary name.

Mercury is extracted from its naturally occurring mineralcalled cinnabar (HgS) by heating it to 800 ◦C in a flow of O2. Itsunique properties caught the interest of early scientists, leadingto the development of several scientific apparatuses such asthe thermometer, barometer, coulometer, and diffusion pump.Gaseous mercury is used in mercury-vapor lamps. Mercury issometimes used as a coolant in nuclear reactors, althoughsodium is preferred, as the high density of mercury makesits pumping an inefficient process. Mercury forms alloys withalmost all metals, and the alloys are called amalgams. Itsamalgamation with gold or silver has been used by historic met-allurgist for extraction and purification of the precious metals.

Why Is Mercury Liquid?: It is interesting to note that eventhough gold and mercury are neighbors in the periodic table,there is no other consecutive pair of metals with such hugedifference in physical and chemical properties. The differencesin melting points (−38.84 ◦C for Hg versus 1064.18 ◦C for Au)and in densities (13 550 kg/m3 for Hg versus 19 340 kg/m3 forAu) are greater than anywhere else. While Au is an excellentthermal and electrical conductor, Hg is only a fair conductor.The clue lies in understanding the phenomenon which leads toweaker Hg–Hg bonds. Relativistic effects lead to contraction ofthe outermost 6s orbital. Relativistically contracted 6s orbitalis filled with two electrons in Hg and hence cannot contributesignificantly in the formation of metal–metal bonds, thus be-having more like noble gases. It is thought that Hg–Hg bondingis mostly due to van der Waals forces and the weak interactionof 6p orbital, leading to a weaker bond and low melting point.A more complete explanation about the topic is discussedin [22].



Physical and Chemical Properties [23]: The atomic numberfor Hg is 80, and its atomic weight is 200.59. Mercury freezesat −38.84 ◦C and boils at 357 ◦C. Its density at 20 ◦C is13 550 kg/m3 and is a function of temperature. Denser than sev-eral solid metals (iron, copper, lead, etc.), mercury is the highestdensity liquid known at room temperature. The vapor pressurevaries from 0.16 Pa at 0 ◦C to 36.38 Pa at 100 ◦C. Mercury isalso known to have the highest surface tension of 485 mN/mfor a room-temperature liquid, with the next being 73 mN/mfor water. The specific heat of mercury is 1390 J/kg · K, andits thermal conductivity varies from 7.82 W/m · K at 0 ◦C to8.30 W/m · K at 20 ◦C and to 9.47 W/m · K at 100 ◦C. Its elec-trical resistivity is 0.96 μΩ · m, which is significantly higherthan that of gold (0.02 μΩ · m). The volume thermal expan-sion of mercury is given by V (t) = V ∗(1 + 1.82 × 10−4t +7.8 × 10−9t2), where t is in degrees Celsius and V is thevolume at 0 ◦C.

Toxicity: Over the past few decades, the use of mercuryhas been restricted significantly due to concerns about envi-ronmental hazards, although some doubt the risks estimatedfor common appliances. In recognition of the potential toxiceffects, the permissible exposure limit (PEL) has been set by theOccupational Safety and Health Administration at 0.01 mg/m3.Even though the PEL is indeed very low, it has been demon-strated that vapors produced by small spills are not hazardous.Vapors emitted from small droplets diffuse in air quickly overlarge distances, and the increase in concentration in the im-mediate vicinity is insignificant [24]. Mercury enters the bodymost critically through the lungs, with up to 90% of inhaledmercury being absorbed and significantly less through the skinand the digestive system [25]. Contrary to the common belief,mercury is not a cumulative toxin, having a half-life of threedays in the blood. However, mercury chemically bound to thetissue can easily have a half-life of 90 days. Kidney takes anactive part in the excretion of absorbed mercury, but excessivequantities can lead to renal failure. Mercury and several of itscompounds are insoluble and are less harmful when comparedto slightly soluble dimethyl mercury. Dimethyl mercury isknown to severely attack the central nervous system and causesthe often-fatal Minamata disease.

B. Gallium

With a melting point of 29.77 ◦C, gallium melts at tem-peratures slightly higher than room temperature, and indeed,body heat is sufficient to melt it. Gallium was discoveredspectroscopically and extracted by Lecoq de Boisbaudran in1875. Its name is derived from the Latin gallus, which means“a cock” (a translation of Lecoq). It is found in trace amountsin diaspore, sphalerite, germanite, bauxite, and coal. Metallicgallium finds its use in high-temperature thermometers. Whenpainted on glass, it wets the surface and forms a brilliant mirror.Gallium also finds extensive use in doping of semiconductors.Gallium arsenide can convert electrical energy to light directly,finding application in LEDs.

Physical and Chemical Properties [23], [26]: The atomicnumber of gallium is 31, and its atomic weight is 69.72. Witha melting point of 29.77 ◦C and a boiling point of 2205 ◦C,it is one of the metals with the largest liquid range. It has

a very low vapor pressure (9.31 × 10−36 Pa at 29.9 ◦C) evenat elevated temperatures (1 kPa at 1565 ◦C). A lower vaporpressure not only implies greater safety from accidental vaporinhalation but also favors miniaturization. As liquid dropletsbecome smaller in volume, their rate of evaporation per volumeincreases dramatically due to scaling laws. An inherent lowvapor pressure keeps the evaporation rate low. There is a strongtendency to supercool, i.e., remain liquid below its freezingpoint. Gallium expands approximately 3% on solidification,with a specific gravity of 5.904 (29.6 ◦C) in its solid phase anda specific gravity of 6.095 (29.6 ◦C) in its liquid phase. Puregallium is silvery in appearance. It does not crystallize to anysimple crystal structure and hence exhibits a conchoidal frac-ture similar to glass. The stable phase under normal conditionsis orthorhombic. There are many stable and metastable phasesas a function of temperature and pressure.

The surface tension of gallium measured at its melting pointin a hydrogen environment is 680 mN/m. The thermal con-ductivity of solid gallium is highly anisotropic and varies from88.4 W/m · K (parallel to the b-axis) to 16.0 W/m · K (parallelto the c-axis). Liquid gallium has a thermal conductivity of28.1 W/m · K at 302.93 K and 32.8 W/m · K at 373.2 K. Elec-trical resistivity of gallium is 0.13 μΩ · m (at 273 K). Galliumaggressively attacks nearly all metals (except tungsten and tan-talum) at all temperatures, making it difficult to use and limitingits application. However, this enhanced reactivity gives rise tothe several low-melting-temperature alloys discussed next.

C. Galinstan

In order to create an effective LM switch, the conductivemedium should remain liquid at temperatures well below 0 ◦C.There are several gallium-based alloys with low freezing pointsformed using a technique called “freezing-point depression”[27]. Freezing-point depression works on the principle that thedissolved metal impurities, having a different crystal structureand atomic size, in the gallium matrix inhibit crystallizationof the alloy. One of the most commercialized gallium alloyshaving a low freezing point is galinstan (from Geratherm Med-ical) as a eutectic alloy of 68.5% gallium, 21.5% indium, and10% tin [28]. Its name is derived from its constituents, withstannum being the Latin for tin. Galinstan is being widelyused for commercial thermometers as a safe replacement formercury. Due to its better reflectivity and lower density, it canbe used as a substitute of mercury in liquid-mirror telescopes.Due to the nontoxic nature of its constituent materials and its at-tractive properties, we consider galinstan a promising LM alloy,which will replace its toxic counterparts in the coming future.

Physical and Chemical Properties [29]: Galinstan has amelting point of −19 ◦C and a boiling point greater than1300 ◦C. It has a very low vapor pressure even at elevatedtemperatures (less than 10−6 Pa at 500 ◦C) and is much lighterthan mercury with a density of 6440 kg/m3. It has a thermalconductivity of 16.5 W/m · K, which is several times higherthan that of mercury, and an electrical resistivity of 0.43 μΩ · m.Although these properties are attractive, a major problem in thedevelopment of practical applications arises from the fact thatgalinstan very easily wets and adheres well to most surfaces.In development of commercial thermometers, the inner wall of



the tube is coated with gallium oxide to prevent the wetting ofgalinstan.

III. MICROFABRICATION AND PACKAGING TECHNOLOGIES

IN THE PRESENCE OF LMs

A. LM Material Compatibility

It is known that LMs can help in reducing the contact resis-tance and the contact wear of solid electrodes. They are alsoknown to improve the device operational life and remove manyunwanted characteristics of the usual solid-contact electro-mechanical relays including contact bounce. However, bothgallium and mercury have serious material compatibility issues.While designing LM devices, one has to take into considerationthe long-term effect due to material compatibility. Both galliumand mercury aggressively react with and dissolve most ofthe metals, thus forming alloys [30]–[32]. The solubility ofdifferent metals in mercury and gallium are shown in Fig. 1.

From Fig. 1, it is evident that only Cr, Ni, and Ti are resistantto mercury. Pt also has lower solubility when compared to Au,Ag, and Cu. However, Cr and Ti have a strong tendency toform surface oxide, resulting in a very high contact resistanceand making their use impractical. Mercury does not react orwet common dielectrics used in MEMS (e.g., silicon nitride orsilicon dioxide) and polymers. Compatibility of gallium withcommon MEMS materials is given in Table I, compiled from[33] and [34]. It can be easily seen that only a limited number ofmaterials are available for the design of MEMS switches usinggallium or gallium alloys. At room temperature, only titaniumand tungsten are compatible with gallium.

Another problem in the use of LMs arises due to their strongtendency to oxidize in air. When exposed to air, their surfacealmost instantaneously oxidizes, forming a thin membranearound the droplet. One consequence of oxide formation forgallium is that the oxide wets many materials that gallium doesnot. As a result, the gallium droplet becomes more resistantto motion due to its increased effective viscosity, slowing thedroplet motion. Dissolution of another metal in the LM matrixmay also increase the viscosity. Finally, these surface oxidesalso contribute to the increase of contact resistance. All theseproblems make it necessary to assemble and package LMdevices in an inert environment. Hermetic packaging is alsorequired for long-term operation.

B. LM Deposition Techniques

A critical technique in the fabrication of LM microswitchesis disposition of a target volume of LM on desired positions.Deposition through nozzles is not easy if the openings are inmicroscale because of the high surface tension of LM droplets.Manual positioning of such small LM droplets (tens of microm-eters) is also quite challenging.

Mercury: In 1996, Saffer et al. [35] developed an innovativetechnique to deposit microscale mercury droplets using selec-tive condensation of mercury vapor on micropatterned nucle-ation sites. The nucleation sites are gold dots micropatternedusing the standard photolithography process. Mercury vaporreacts with the very thin layer of gold and forms a liquid

Fig. 1. Solubility of different metals. (Top) In mercury at 298 K. (Bottom) Ingallium at 873 K [31], [32].

amalgam. Further condensation preferentially takes place atthe LM droplet, and the droplet grows on the lithographicallydefined positions.

Fig. 2 shows the mercury deposition setup of Latorre et al.[36], evolved from the original setup by Saffer et al. [35].It consists of a deposition chamber, a mercury source pool,and a sample holder. The deposition process starts by placingthe die on the sample holder. The shutter is closed, and themercury pool is heated to 140 ◦C. The sample is heated to100 ◦C. The shutter is then opened, and the sample is movedinto the deposition chamber while the sample heater is turned



TABLE ICOMPATIBILITY OF GALLIUM WITH METALS USED IN MEMS

Fig. 2. Schematic of the mercury droplet deposition setup using the selectivecondensation technique [36].

Fig. 3. Dependence of droplet size on the duration for which the chips areexposed to the mercury vapor [36].

off. The size of the droplet can be controlled by the exposuretime and the temperature. However, excessive exposure leads tounwanted nucleation and deposition at random positions. Fig. 3shows the average droplet diameter as a function of exposuretime. The setup was described in detail by Kim [37].

More recently (2006), a commercial liquid jetting system(JetLab from MicroFab Technologies, Inc.) has been used forthe deposition of mercury droplets by Wan et al. [38]. Thesystem jets small droplets, whose diameter mostly depends onthe jetting orifice diameter. Another parameter affecting thedroplet diameter is the jetting voltage waveform.

Gallium and Gallium Alloys: Although it is a liquid at nearroom temperature, gallium has one of the highest boiling pointsand the lowest vapor pressures among metals. These properties

Fig. 4. Screen printing of gallium alloy as LM [26].

make gallium thermally stable and extremely difficult to evap-orate. At room temperature, the vapor pressure of gallium is8 × 10−39 torr, making it almost impossible to evaporate. Asnoted by Truong [26] for gallium deposition through evapora-tion, gallium must be heated to at least 750 ◦C in a vacuumof approximately 7.6 × 10−6 torr. Despite these difficulties,very recently, in 2007, Cao et al. [39] successfully depositedgalinstan using thermal deposition techniques.

Truong [26] developed in 2000 a screen printing technologyfor the deposition of liquid gallium alloys, as shown in Fig. 4.The screen was fabricated using KOH bulk-etched siliconsubstrates. The idea of the whole process is to have selectivewetting regions on the substrate. After alignment of theprinthead to the substrate, the liquid alloy is bulged out of thescreen window by pressuring the printhead and makes contactwith the gold wetting pads on the substrate. When the screen isseparated from the substrate, the wetting pads retain the liquidalloy with enough adhesion force that the liquid breaks fromthe pool to form droplets. Truong was able to demonstrate theformation of droplets with screen opening sizes of 40–70 μm ata pressure of 5 lbf/in2. There has been no report of depositingmicroscale droplets of galinstan so far, which has becomeavailable only recently.

IV. LM-WETTED MICROSWITCHES

One of the two approaches to implement a liquid–solid con-tact, which truly mimics the regular (macroscale) wetted reedrelays, is to coat the contacts of the preexisting MEMS switcheswith LM. Several implementations have been developed anddemonstrated over the last decade. Overall, scaling implies thatsurface tension becomes dominant at micrometer scales overstructural forces. This explains why electrostatically actuatedmicrobeams, capable of generating small restoring force only,were quite slow when detaching from a mercury droplet [40].Better performance was demonstrated using an electrother-mal actuation capable of generating larger force to actuatethick beams, which command a large restoring force [41]. Incurrent developments, an LM-coated self-healing contact foran electrostatically actuated membrane is being explored byHoneywell Inc. for RF switch applications. We will discuss allthese developments in detail next.

A. Electrostatically Driven Microcantilever Relays With aStationary LM Contact

One of the earliest attempts to integrate LM into micro-switches was by Saffer et al. [35] in 1996. Their approachwas simple yet unique. As shown in Fig. 5, laterally actuated



Fig. 5. Schematic of the electrostatically driven microcantilever-basedmercury contact switch [35].

Fig. 6. Fabricated cantilever device with a mercury droplet deposited at thecontact point on signal electrode [35].

polysilicon cantilevers of 2 or 3 μm widths and 300–500 μmlengths were used. The driving electrodes were designed witha curved shape to achieve large actuation with a reasonabledriving voltage in a fashion often referred to as “zipper ac-tuation” [42]. Bumpers were designed to prevent the shortingof the actuated cantilever with the driving electrode. An LMdroplet with a diameter of ∼10 μm was deposited on thesignal electrode using the condensation technique discussed inSection III. A potential applied between the driving electrodeand the cantilever causes the cantilever to bend and makecontact with the deposited mercury droplet. The high surfacetension of mercury prevents the droplet from rolling even whenthe actuated cantilever makes contact with the droplet.

Fabrication: The polysilicon surface-micromachined devicewas fabricated using the MCNC Multi-User MEMS Process(MUMP). The cantilever (2 μm wide) and the driving electrodewere made using 2 μm thick Poly1 layer, and the signalelectrode was formed from 0.5 μm thick Poly0 layer. The2 μm thick sacrificial oxide layer was etched in HF to freethe beams using various releasing methods. A 10 μm2 metal(Cr/Au) patch on signal electrode was used as the preferentialcondensation site for the mercury droplet. As seen in Fig. 6, anLM droplet was formed at the contact.

Results: The fabricated devices achieved tip deflections of30 μm with a driving voltage of 60 VDC [35]. The measuredOFF-state resistance was greater than 200 MΩ. For contact-resistance measurements, the total resistance was measuredwith the actuated cantilever. The obtained resistance valueswere corrected for the device (interconnect) resistance bysubtracting resistance values measured from reference devicesfabricated with the actual devices on the same chip for thispurpose. The obtained poly-mercury-poly contact resistancewas in the range of 800–1000 Ω. The high contact resistancewas attributed to the poor wetting of the polysilicon surfaceby the mercury droplet [41]. However, this switch was notreliable because the cantilever was too flexible and significantlyaffected by the stiction with the droplet. Limited by the mi-crofabrication technology at the time, the cantilever was only

Fig. 7. Schematic of the electrostatic comb-drive-based mercury contactswitch [41].

Fig. 8. Fabricated comb-drive devices with mercury droplet at the contactpoint on signal electrode [41].

as thick (2 μm) as it was wide (2–3 μm), lacking the verticalrigidity needed against the out-of-plane bending [40].

B. Electrostatic Comb-Drive Microrelays With a StationaryLM Contact

To solve the stiction problem faced by the cantilever-basedrelays earlier, Simon et al. (1998) designed mercury contactmicroswitches using a folded beam driven by comb-drive ac-tuators [41], as shown in Fig. 7. The folded-beam structuregave a higher vertical rigidity than the single cantilever. Toswitch, the comb drive was actuated, and a central portion (i.e.,more stable) of the structure made contact with the mercurydroplet placed on the signal line. The widths of the beamswere designed to be 2 or 3 μm, while the lengths variedfrom 150 to 240 μm. The comb had 20 μm long fingers(26–56 counts, depending on the design) with a gap of 2 μmbetween the stationary and the movable fingers. Devices werefabricated using the MUMP technology, as described previ-ously. In postprocessing, the sacrificial layer was etched in HFto release the structures, and an LM droplet was placed usingthe condensation technique of Section III.

Results: A fabricated device is shown in Fig. 8. Switchingvoltage of 35 VDC was sufficient to achieve the required 4 μmtravel to close the contact. The maximum driving frequencyof the device was 4 Hz. At higher frequencies, however, thedeflection was not enough to obtain switching. Up to 11 mAswitching was reported [41]. Larger spring constant and, hence,the larger restoring force of the folded-beam structure were



Fig. 9. Schematic description of the bidirectional electrothermal electromag-netic actuator by Cao et al. [43].

cited as the reasons for the ability of these switches to detachfrom the mercury droplet reliably.

C. Electrothermally Actuated LM-Wetted Bistable Relays

Recently, in 2005, Cao et al. developed a bidirectional elec-trothermal electromagnetic actuator [43]. It has a simple design,with a beam clamped at both ends to bonding pads (see Fig. 9).When current is passed through the beam, Joule heating causesthe fixed–fixed beam to develop a compressive stress and buckleonce the stress passes the critical value. A NdFeB permanentmagnet is placed under the device to generate a constantmagnetic field of 0.2 T. The electromagnetic Lorentz forceexperienced by the current-carrying beam controls the directionof beam actuation. The direction of deflection is determined bythe direction of the current in the beam. The actuators wereable to generate greater than 100 μN forces for 2–3.5 μm thickbeams (fabricated using poly-MUMPs) and up to 20 mN for50 μm thick beams (fabricated using SOI wafers). While thethinner (2–3.5 μm) beams could be driven up to 4 kHz, thethicker (50 μm) beams were capable of a few tens of hertzoperation only. In this thermal device, response time and, hence,maximum driving frequency were determined by the coolingrate of the beams, which explains the lower driving frequency ofthe thicker beams. Based on the developed actuator, Cao et al.demonstrated an LM-coated SPDT and bistable switches(see Fig. 10) [39], [44]. The contacts were coated in galinstan.

Fabrication: A metal-MUMP process was first used fordevice fabrication. This process has eight thin-film layers pat-terned using six lithography steps with a 20 μm electroplatednickel serving as the structural layer. The actuation beamswere designed to be 950–1200 μm long, 10 μm wide, and20 μm thick. Relay actuators were built on a 25 μm trenchto provide better thermal isolation. Electrical isolation betweenthe actuators and the contact was achieved using a nitridebridge, as shown in Fig. 11. To achieve low contact resistance,the 2 μm gold overcoat layer in the metal-MUMP process isused to cover the top and sidewalls of the contacts.

For further reduction in contact resistance, they decided tocoat the contacts with LM. On a wetting contact surface, LMfills and smoothens all the microscale surface roughness, thus

Fig. 10. LM-coated switches based on the electrothermal electromag-netic actuation. (a) Single-pole double-throw switch. (b) Bistable switch byCao et al. [39].

Fig. 11. Cross-sectional view and SEM of the LM-wetted microswitch byCal et al., illustrating nitride bridge [44].

providing a larger true contact area and hence helping reducethe contact resistance [45], [46]. Considering the toxicity ofmercury, the authors decided to use galinstan. In postprocess-ing, the alloy was evaporated onto the metal-MUMP die usinga thermal evaporator. The highly directional nature of thermalevaporation caused most of the LMs to end up on the topsubstrate. However, by placing the die at an angle inside theevaporator and subsequent rotation of the die during the evap-oration process allowed the achievement of significant sidewallcoating so that a measurable difference in the contact resistancewas obtained. A good control of the coating process is required,as excessive coating of the contact could cause device failuredue to liquid bridging.

Results: Bistable relays were designed and fabricated.These relays, however, were not able to achieve bistabilitydue to residual stress in the nickel, which changed the shapeof the bistable beam such that they cannot achieve bistability.Fabricating the devices on a 25 μm trench led to a reductionin the heat loss to the substrate and hence improved the energyefficiency of the devices by three times. The relays wereoperated at 0.25–0.5 VDC, requiring 0.5–0.8 A of current. Thebreakdown voltage was measured to be greater than 200 VDC.The measured OFF-state resistance was greater than 100 MΩ.To measure the contact resistance, the authors used a techniquesimilar to that of Simon et al. [7]. First, the total resistancewas measured, and then, both the probe tips were placed onthe same contact pad to obtain the contact resistance of theprobe tips. Subtracting the contact resistance of the probe tipsfrom the total resistance gave the total resistance for the device.While interpreting the data, it is important to consider that suchtechniques are susceptible to significant experimental errors.Since the surface resistance of interconnects were insignificant



Fig. 12. Schematic of the electrostatically driven membrane switch with self-healing gallium contact by Honeywell [47].

(0.003 Ω/�), the authors assumed the measured deviceresistance to be the contact resistance of the device. Theuncoated gold–gold contact resistance measured by the authorswas 0.3–0.4 Ω. In comparison, LM-coated devices had contactresistance as low as 0.015 Ω. Even without LM, 50 VDC and1 A of power were hot switched. However, during high-power hot switching, the contacts deteriorated fast and failedcatastrophically with a huge increase in the contact resistance(several megaohms) within several hundreds to thousands ofcycles. In applications with large currents, the heat generatedat the contacts due to contact resistance was enough to actuatethe thermal actuators, limiting the current-carrying capacityof the devices. For example, less than 0.5 W of power wasrequired to actuate the switch, limiting the current to 1.3 Afor uncoated contacts (0.3 Ω). In comparison, coated contacts(0.02 Ω) could handle approximately 5 A without self-switching. When passing very large currents, contact heatingdeformed the actuating beams. One way to reduce this problemwas to make the fixed contacts very large, allowing excess heatto be lost to the substrate. Another method used a spring-loadedcontact design, as shown in Fig. 10(a), which thermally isolatesthe moving contact from the actuator. Without considering forself-actuation, even the uncoated contacts could handle 4–5 Aof current, but at this point, the beams were bent out of shape.For coated contacts, the test was limited to 4–5 A due to failureof the probe tips.

D. Self-Healing RF MEMS Switch With Gallium Contacts

One of the recent works using LM-wetted contacts for mi-croswitches was led by Honeywell for RF applications [47].The schematic of the switch is shown in Fig. 12. Under high-power operation, dc-contact RF MEMS switches are known tofail by surface erosion caused by localized welding and stiction.Honeywell’s solution for this problem was to wet the contactpoints of the switches with LM. However, during contact sepa-ration, necking of the LM takes place (i.e., formation of a verythin LM bridge). Even though gallium has a high boiling point(2205 ◦C), high current density through this narrow neck regioncauses high local resistive heating, leading to boiling, andcondensation of the evaporated droplets leads to the formationof satellite droplets. One of the main goals of this project was

to fully recover the evaporated LM during hot switching bycollecting the satellite droplets back to the desired positions.

Fig. 13 shows the SEM photographs of the fabricated mi-crodevice. Honeywell uses micropatterned wetting features tocollect the small satellite droplets (see Fig. 14). The featureswere designed such that surface tension causes the smallerdroplets to move toward the larger central droplet. For fabri-cation of the bridge, Honeywell uses tungsten, instead of thecommonly used gold, probably for two reasons. First, tungstenis one of the very few metals that do not react with gallium.Second, the high melting temperature and hardness of tungstenprovide extra contact reliability.

V. LM-ACTUATED MICROSWITCHES

The other approach to implement a liquid–solid contact is toactuate LM directly as the moving element in order to achieveswitching. This approach is different from the regular wettedreed relays and most microswitches, where what move are solidelements. To move or manipulate the LM for switching, severalactuation mechanisms have been developed, including thermal,electrostatic, and electrowetting. Recently, electrowetting-on-dielectric (EWOD) of LM droplets has also been demonstratedfor low-latency switching applications. In this section, we willdiscuss these developments in detail.

A. Thermal-Vapor-Bubble-Actuated LM Microrelays

In the earliest attempts to develop LM-based microswitches,Simon et al. [48], [49] actuated in 1996 an LM droplet in amicrochannel filled with a dielectric liquid using a pressuregenerated by thermal bubbles. The schematic of the deviceis shown in Fig. 15. The device design consisted of twobulk-micromachined reservoirs connected through a V-shapedchannel (called “V-groove throat”). The reservoirs had sus-pended heater elements to eliminate heat loss to the substrate.A signal electrode ran through the V-shaped channel but wasdisconnected inside the throat, as seen in the figure. An LM(mercury) droplet was deposited in the channel near the signalelectrodes. Actuating the LM droplet and positioning it overthe disconnected signal electrode caused the signal line to getconnected (i.e., switch on). Formation and expansion of thermalbubbles in a reservoir by passing current through the heaterelements generated the pressure, inducing a momentary flowof the dielectric liquid along the channel and moving the LMdroplet. It was noted that inclusion of an air cavity inside theDI-water-filled device helped protect the seals against breakingdue to pressure buildup [49].

Theoretical estimation of the retarding force, which givesthe actuation pressure required to move an LM droplet, iscomplicated, so a simple experiment was performed. Small LMdroplets of lengths ranging from 200 to 900 μm were intro-duced into glass tubes with 200 and 300 μm inner diameters.Pressure was applied at one end of the tube, while the otherend was maintained at atmosphere. The aim of the experimentwas to measure the pressure required to initiate the dropletmotion. Pressure was measured using an electrical transducerwhere the droplet motion was sensed using a CdS photodiode.For example, 1.1–1.5 lbf/in2 was required to move a mercury



Fig. 13. Fabricated devices. Courtesy of Youngner of Honeywell [47].

Fig. 14. (Top) Wetting structures with deposited gallium. (Bottom) Satellite droplets. Courtesy of Youngner of Honeywell [47].

Fig. 15. Schematic of the thermal-bubble-actuated mercury droplet switch asthe earliest LM microswitch by Simon et al. [49].

column in a tube of 200 μm diameter. Despite a significantscatter in the measured data, a general P ∝ 1/r relation wascorrectly reported. This observed relation could be explainedby considering contact-angle hysteresis as the retarding forceagainst the droplet motion.

Surface roughness, variation in surface chemical properties,and surface contamination cause the droplet interface to prefer

certain locations [7]. This results in adhesion of the droplet tothe surface, and energy is required to put the droplet in motion.The resulting retarding force on a droplet sitting on a substrateis given by

Fγ = 2r × γlv × cos π − θadv − cos π − θrec (1)

where r is the radius of the liquid–solid contact line, γlv isthe surface tension of the droplet, and θadv and θrec are theadvancing and receding contact angles, respectively. As seenin (1), the retarding force due to contact-angle hysteresis isproportional to the circumference (radius) and, hence, the 1/rrelation with the pressure. Contact-angle hysteresis stronglydepends on the surface properties, which leads us to suspectthat the large scatter observed in their data was due to surfaceoxidation of LM and poor control over the tube surface quality.

To have a better understanding of the driving mechanism, itis important to realize that due to the nonwetting nature on SiO2

(contact angle ∼145◦), mercury does not fill the sharp cornersof the triangular cross section (see Fig. 16) of the microchannel.



Fig. 16. Nonwetting droplet in a tube with different cross sections. (a) Round(glass) tube. (b) V groove made in silicon [49].

Fig. 17. SEM of the fabricated device (before LM droplet) [49].

The leaks hamper the pressure buildup, and the LM is drivenby the drag of the leaking medium. However, this leak was notnecessarily a bad thing because, along with the contact-anglehysteresis and high surface tension of mercury, this leak madethe bistable operation of the switch possible.

Fabrication: The devices were fabricated on silicon wafers[7], [49] (see Fig. 17). The V-shaped channels were formedusing KOH etching. A single metallization of chromium andnickel was used to define the heaters, the signal lines, and thecontact pads. To seal the device, a new microfabrication tech-nique was developed, in which Teflon was spin coated and pat-terned to form microgaskets. The reservoir (100–200 μm2) wasetched, and the heater elements were released in a single stepof XeF2 etching. A mercury droplet was deposited using the se-lective condensation technique of Section III. After wetting thedevice with DI water and placing a cover glass on top, the gapoutside the gasket was dried and sealed with UV-cured epoxy.

Results: The authors were able to achieve bubble nucleationand LM droplet actuation at 10–15 VDC and ∼100 mW of

Fig. 18. Oscilloscope trace showing bistable behavior and switching latencyof 10 ms [49].

Fig. 19. Thermally actuated LM switch by Kondoh et al. [50].

power input to the heater. Although their design could generateenough power, they had poor control over the bubble growthand location. The switch lag time observed was 10 ms, givinga maximum driving frequency of 100 Hz (see Fig. 18). Whenon, the device could handle 22 mA of current through mercury.Higher currents were not tested due to the fear of excessiveheating at the contacts in the presence of mercury. The contactresistance measured for a Ni–Hg contact using a four-pointtechnique was 120 Ω/μm2 (2.5 Ω for a 48 μm2 contact area) [7].Only a semibistable operation could be achieved. Once actu-ated, the device stayed on for several minutes before returningto the original (initial) position (switch off). This was probablydue to the back pressure caused by the cooling liquid. Theswitch could, however, be switched off by turning on the heaterin the other reservoir. The switch was able to achieve 40 dBisolation and less than 0.1 dB insertion loss at 2 GHz.

B. Thermally Actuated LM Microswitch

Another thermally actuated LM switch for RF switching wasdeveloped in the early 2000s at Agilent Laboratories in Japan byKondoh et al. [50]. The device had two reservoirs with heatingelements, as shown in Fig. 19. Although they used thermalexpansion like the one discussed in the previous section, inthis case, the expanding gas actuated mercury directly to cutor merge mercury slug in a microchannel to switch. As shownin Fig. 19, initially, contacts L and M were connected. When acurrent is passed through a heating element in a reservoir filled



Fig. 20. Device fabrication and assembly [50].

with gas, the expanding gas breaks the LM column and movesthe broken LM droplet to the other edge, thus connecting the Rand M contacts.

Fabrication: The 700 μm thick top glass had reservoirs,subchannels, and metal elements. The subchannels and thereservoirs were fabricated using sandblasting. The subchannelswere 70 μm wide and 50 μm deep, and the reservoirs were1.3 × 0.6 mm in size and about 100 μm deep. The metal padswere evaporated thin films of chromium, platinum, and gold.The intermediate glass was 100 μm thick and had sandblastedthrough-holes to define the reservoirs and the main channel.Cytop was spin coated for adhesion bonding and sealing. Thebottom ceramic substrate had sputtered TaN resistive elementsserving as heater elements. A 0.8 mg of mercury was put on thecontacts before bonding the plates together. Mercury wets thecontact pads and prevents gas leakage during switch operation.However, small leakage present helps in equalizing pressurein several tens of microseconds for true bistable operation,somewhat similar to the leakage discussion in Fig. 16. Thefabricated device is shown in Fig. 20.

Results: A four-point technique was used to measure thecontact resistance. A resistance of 55–59 mΩ was measuredas the combined value for mercury and the contact resistance.Kondoh et al. selected 0.8 ms pulse of 22.5 VDC as the standardoperating condition requiring less than 10 μJ. The switchingsignal is shown in Fig. 21, where switching latency of 0.92 msis observed. A high-speed camera was used to confirm theswitching latency observed. The insertion loss was better than1 dB, and isolation was better than 20 dB from dc to 18 GHz.Only a few-milliohm contact-resistance variation was observedfor operation up to 3 × 105 cycles. Switching operation wasconfirmed to over 108 cycles. The switch was able to handle 1 Aof dc current.

C. Electrostatically Actuated LM Droplet Switch in Cavity

Kim et al. [51] demonstrated in 2002 a microswitch with anLM droplet sliding by electrostatic attraction from a sidewallof a cavity. For switch design, it was necessary to have anestimate of the actuation forces required to move the droplet. Inmicroscale, a liquid droplet resting on a surface takes the shapeof a truncated sphere because of the reduced inertial effects at

Fig. 21. Switching latency and bistable operation. Source: Kondoh et al. [50].

Fig. 22. (a) Contact-angle definition. (b) Force equilibrium of a slidingdroplet [51].

Fig. 23. Experimental setup to measure contact-angle hysteresis with electro-static attraction by Latorre et al. [36].

microscale, as shown in Fig. 22(a). The contact angle is theresult of an equilibrium of the interfacial energies (γsl for solidto liquid, γsv for solid to vapor, and γlv for liquid to vapor) andis described mathematically by Young’s equation [52]

γlv cos θ + γsl = γsv. (2)

When a force is applied to the droplet parallel to the substratesurface, the droplet deforms, and the contact angle differsbetween the leading (advancing) and tailing (receding) edges[see Fig. 22(b)]. The static equilibrium is expressed by (1).This mechanical equilibrium is maintained until a critical forcerequired to slide the droplet is applied. The difference in theadvancing and receding contact angles at this critical momentis known as the contact-angle hysteresis and characterizes theminimum force required to move the droplet.

Fig. 23 shows the experimental setup used by Latorre et al.[36] to measure contact-angle hysteresis with electrostatic at-traction. A mercury droplet of known radius was placed ona SiO2 surface. A 5 μm wide nickel line was used to biasthe mercury droplet. The gap between the droplet and the



Fig. 24. Schematic of the electrostatically actuated LM microswitch in cavityby Kim et al. [51].

actuation electrode was first adjusted to a desired value. Then,the potential was increased until the LM droplet slides andsnaps to the actuation electrode. They reported contact-anglehysteresis measured from these experiments to be ∼6◦, whichimplied a driving force of 6.7 μN for a 300 μm diameter droplet.

Fabrication: The schematic of the devices designed byKim et al. [51] is shown in Fig. 24. It consists of a 100 μm deeppit to contain the LM (mercury) droplet. In order to patternthe driving and signal electrodes with less than 30 μm width,a new shadow masking process was developed. The shadowmask was formed using very thin (∼40 μm thick) siliconwafers. The thin wafers were bonded to Borofloat glass wafers(∼500 μm thick) using anodic bonding to simplify handling.After the desired pattern was transferred to the thin siliconwafers using photolithography and deep reactive ion etching(RIE), the carrier Borofloat wafer was dissolved in concen-trated HF.

Silicon wafer was etched in KOH to form the device cavity.Thermal oxide was grown as the passivation layer. A shadowmask was aligned to the wafer using a custom setup andtemporarily bonded using photoresist. Metal was evaporatedthrough the shadow mask to form the driving electrodes. A5000 Å silicon dioxide was deposited by plasma-enhancedchemical vapor deposition (PECVD) as the passivation layer.Another shadow mask was used to deposit the signal electrodes.Finally, the mercury droplet was placed manually or formed bythe selective condensation method of Section III. The devicewas packaged in air or with a dielectric liquid.

Results: The fabricated microswitch (see Fig. 25) was testedafter packaging it with silicone oil to ease the droplet motion byreducing contact-angle hysteresis. The larger dielectric constantof the oil compared with that of air somewhat increased theattraction force applied by the actuation voltage, which rangedfrom 100 to 150 VDC. This variation was probably due tothe varying gap between the droplet surface and the actuationelectrodes on the sidewall. Unfortunately, using oil had a speedpenalty on the device; a switching frequency of only 1 Hz wasdemonstrated. However, it was an acceptable speed for its targetapplication of reconfigurable circuits.

Fig. 25. Optical photograph of a device under test [51].

Fig. 26. Experiments to characterize effects of microstructures on adhesionforce by Shen et al. [57]. (a) Testing samples. A: pitch and B: line width.(b) Sliding force. (c) Detaching force.

D. Electrostatically Actuated LM Droplet Switch onStructured Surface

Realizing that the previously reported LM droplet switchesby Kim et al. [51] had to use high voltages and immerse thedroplet in oil because of the excessive adhesion in microscale,Shen et al. [53], [54] took a new approach of structuringthe surface. If nonwetting is maintained, a structured surface[Fig. 26(a)] reduces the liquid–solid contact area and, thus, theoverall adhesion forces [55], [56]. In order to have a quantitativeunderstanding of the reduction of adhesion forces due to phys-ical surface modification, Shen et al. carried out some simpleexperiments (see Fig. 26), using surfaces structured with micro-line patterns.

Pitch [Fig. 26(a)] was maintained constant at 10 μm, and theline width was varied to get various contact ratios (i.e., ratioof liquid–solid contact area with respect to a flat substrate)between 0.3 and 1. Two different sets of electrostatic actuationexperiments were carried out. The first set was to determine thehorizontal sliding forces [see Fig. 26(b)], and the second setwas to determine the vertical detaching forces [see Fig. 26(c)].In both experiments, at first, a known volume of the LM dropletwas deposited on the substrate, and then, the actuation electrodewas positioned to achieve a desired gap using a micropositioner.



Fig. 27. Electrostatic LM droplet switch of a planar design by Shen et al. [53].

Then, the actuation voltage was increased until the droplet slidor detached from the surface. For the sliding force, there wasa decrease in the overall surface adhesion, and hence, lowerdriving voltages were required with a decrease in the contactratio. For example, for a 550 μm diameter droplet with a gapof 20 μm, at a contact ratio of 1 (i.e., flat surface), the slidingvoltage is 115 VDC; at a contact ratio of 0.5, the sliding voltagewas 62 VDC; and at a contact ratio of 0.3, the sliding voltagewas 42 VDC [57]. From these results, the authors claimed thatthe surface adhesion can be controlled by physical surface mod-ification. The results of droplet detachment were more complexto analyze due to bridging of the LM and breakdown of theair between the LM and the electrode. However, the authorswere able to confirm, as predicted by their analytical analysis,that much higher voltages (greater than four times) and, hence,an order of magnitude larger forces are required to detach incomparison to slide [58]. This can be understood intuitivelyby realizing the fact that detachment involves destruction andcreation of surfaces, thus requiring larger energy also over ashorter period.

Encouraged by the reduced actuation voltage using a mi-crostructured surface, Shen et al. developed a planar design,i.e., with no cavity (see Fig. 27). In this design, the LMdroplet was always in contact with the common electrode,which is grounded. When sufficient potential was applied to thedriving electrode, the electrostatic force caused the droplet toroll and make contact with the signal electrode. There wereseveral advantages of a planar design. First, the LM dropletmotion was limited to sliding only, and droplet detachmentwas avoided, significantly reducing the actuation force requiredwhen compared to the droplet in the cavity design. Second, theactuation was self-limited, even though there was no wall tophysically limit the droplet motion, as seen in Fig. 27(a). At thebeginning [see Fig. 27(b)], the signal electrode was electricallyfloating and thus contributed to the electrostatic force applied

Fig. 28. Planar device postprocessed on CMOS circuit. The LM switch andunderlying circuit are integrated [53].

Fig. 29. Actuation and signal waveform of a planar switch with structuredsurface integrated with the CMOS driving circuit [53].

to the LM droplet. However, when the droplet made contactwith the signal electrode [see Fig. 27(c)], the signal electrodewas grounded. The grounded signal electrode shielded thefringing fields, which was responsible for the actuation force,thus restricting any further droplet motion. Third, this designallowed devices with much smaller switching gap (i.e., movingdistance required for switching) in comparison to the switchdesign with a droplet in a bulk-etched cavity. Finally, the planardesign simplified the fabrication and made the integration ofthe microswitch with the underlying CMOS IC circuit morecompatible, as demonstrated by the authors.

Fabrication: The planar microswitch was fabricated bypostprocessing on CMOS chips made using X-FAB Semicon-ductor Foundries. The driving electrodes and the passivationdielectric were integrated in the CMOS fabrication. Post-processing started by thinning the passivation dielectric usingRIE. In the next step, the dielectric was patterned to structurethe surface to reduce adhesion. Next, Cr/Ni was deposited forthe ground and signal electrodes and contact pads. A fabricateddevice before LM deposition is shown in Fig. 28. Finally,depending on the LM droplet size, the droplet is depositedmanually or using the selective condensation technique ofSection III.



Fig. 30. Electrowetting phenomenon on electrode passivated with a dielectriclayer (EWOD) [60].

Fig. 31. Experimental setup to measure contact-line speed under EWODactuation [60].

Fig. 32. Schematic design and operation of the EWOD-driven fast LM dropletswitch [60].

Results: By integrating various techniques aimed at reduc-tion of the driving voltages, the authors were able to achieveactuation at as low as 15 VDC. However, this comes at aprice of the droplet stability. Making the surface rough reducedthe actuation voltage required, but it also reduced the devicestability against shock and vibration. For example, an actuationvoltage of 80 VDC and a stability of 80 G were reported on

Fig. 33. Switching profile of a low-latency switch showing bounce-freeoperation [60].

a smooth surface. However, this stability deteriorates to 3 Gwhen the surface was structured to get an actuation voltageof 15 VDC. The measured performance is shown in Fig. 29,which shows switching latency of 1 ms and maximum drivingfrequency of 50 Hz.

E. Fast LM Droplet Switch Using EWOD

Many types of LM microswitches having desirable charac-teristics in their target application have been discussed untilnow. However, most of them have limited RF capabilities dueto their slow switching speeds on the order of milliseconds.Sen and Kim explored the use of recently developed EWOD ac-tuation of LM [59] for development of a low-latency switch in2007 [60].

Fig. 30 shows the phenomenon of electrowetting of a dropleton an electrode coated with thin dielectric layer or EWOD [61],[62]. On application of an electric field between the conductingliquid and the electrode, the liquid–solid interfacial energy de-creases, and the droplet spreads with an accompanying changein the contact angle, as shown in Fig. 30. The droplet beadsback to the initial shape when the electric field is removed. Theyused this contact-line motion, instead of moving the droplet, toexplore a fast low-latency LM droplet switch.



Fig. 34. LM-based micro RF switch demonstrated by Chen et al. [64]. (a) Side view. (b) Top view.

TABLE IISUMMARIZED CHARACTERISTICS OF VARIOUS LM-BASED MICROSWITCHES

In order to design a fast switch, they measured the slidingspeed of the liquid–solid contact line when the droplet is spreadby EWOD. The experimental setup used is shown in Fig. 31.A computer was used to generate step voltages of varying mag-nitudes, which were amplified using an amplifier to actuate thedroplet. A high-speed camera was used to capture the dropletdynamics at 10 000 ft/s. The same computer was used to triggerthe camera and synchronize it with the actuation voltage. Thecaptured movies were analyzed to extract the evolution of thedroplet. For a droplet with a radius ∼1 mm, the contact anglerapidly changed and stabilized to its final value within 500 μs,while the contact line continued to move. For an actuationvoltage of 100 VDC, sliding speeds up to 50 cm/s were observedduring the initial 100–200 μs of the contact-line motion.

It is important to realize that fast actuation speed alone is notenough to obtain fast switching. It is also necessary to obtain asmall droplet of a precise volume, hence a small and consistentswitching gap (the distance that the interface needs to moveto achieve switching). The goal was achieved by placing thedroplet in a lithographically defined microframe. The high sur-face tension of the LM droplet ensured that it is accurately posi-tioned and allowed a design with much smaller switching gapsthan possible without the microframe. Switches were designedfor 300 μm droplets with switching gaps of 10 and 20 μm. Theframe also provided vibrational stability against up to ∼16 G.The schematic of the design and switch operation is shown inFig. 32. When a potential is applied to the actuation electrode,the droplet spreads and makes contact with the signal electrode.

Fabrication: Chromium is patterned and wet etched to formthe actuation electrodes. A 3500 Å silicon nitride was depositedusing PECVD to serve as the passivation layer. The dielectricwas patterned and etched using RIE to open the contact pads.Lift-off Cr/Ni was used to form the ground and the signalelectrodes. The microframe was formed using a multicoatSU-8 process to yield 500 μm thick structures and was spincoated with a hydrophobic layer (Teflon AF). The hydrophobiclayer was then patterned and etched in O2 plasma to open thesignal electrodes. In the final step, a 600 μm diameter droplet(mercury) was placed manually on the device.

Results: The fabricated LM EWOD microswitch, althoughnot in a sealed package, was tested. The best switch-on voltageobserved was 75 VDC, with the best switch-off voltage being45 VDC. However, a device-to-device variation in the minimumswitch-on voltage was observed, which was attributed to theuncertainty in contact-angle hysteresis. Due to this uncertaintyin the switch-on voltage, the switching tests were done at100 VDC. The switching profile is shown in Fig. 33. Bounce-free operation is attained without any complicated circuit [63].The measured switch-on latency was 60 μs, and the switch-off latency was 150 μs. The signal rise and fall times werebetter than 5 μs. The contact resistance measured using a four-point technique was reported to be 2.35 Ω for a 50 × 50 μmNi–Hg contact. The large contact resistance was attributed toLM oxidation due to device packaging in air. In the preliminarytest, 1 W of hot switching was reported for tens of cycleswithout any visual damage.



F. LM-Based RF Capacitive Shunt Switches

Based on the actuation mechanism demonstrated byShen et al. [53], [54], an RF switch was demonstrated byChen et al. [64]. The schematic of the switch is shown inFig. 34. Using electrostatic actuation, the LM droplet is movedon or away from a coplanar waveguide (CPW). When theLM droplet is moved on the CPW, the RF signal is capac-itively shorted to ground, and the switch turns off. Movingthe LM droplet away from the CPW leads to a passage of theRF signal with low loss, and the switch turns on. The switchdemonstrated an insertion loss of 0.6 dB up to 20 GHz and anisolation of 21.1 dB at 20 GHz [64].

Chen et al. [65] also explored the galinstan droplet to replacemercury. Since galinstan reacts with most metals, the LMdroplet was encapsulated in a microchannel and immersedin a Teflon solution. However, no actuation mechanism wasdemonstrated, and the device was tested by moving the dropleton and off manually.

VI. SUMMARY AND CONCLUSION

We started this paper with a study of the various LM andLM alloys available for use in LM microswitches. A goodunderstanding of their properties was essential to design themicroswitches and develop appropriate fabrication techniques,as was discussed in Section III. Mercury, although easy touse in comparison to gallium, has restricted application dueto its toxicity. Gallium, on other hand, has serious materialcompatibility issues. It is also more difficult to integrate withexisting MEMS fabrication technologies. We have revieweda majority of the LM-based microswitches reported over thelast ten years, explaining the concept, fabrication, and results,and summarized their characteristics in Table II. LM-basedswitches using mercury have demonstrated very low contact-resistance values and demonstrated up to 1 A of hot switching,an order of magnitude larger than that of solid-based MEMSswitches. Even though many LM-based microswitches havebeen explored over the past decade, the field is still embryonic.Application-specific optimization (e.g., RF MEMS and high-power switching) of the LM-based microswitches will requirefurther research and development. There are several key tech-nologies that need to be developed in order to be able to designreliable devices and commercialize LM-based MEMS switches.One such technology is a low-temperature hermetic packagingto seal devices laden with LM droplets in an inert environ-ment. Another such required technology is accurate dropletdeposition. Although several deposition techniques have beendeveloped, improvement in deposition technology is requiredto achieve the accuracy demanded by commercial products.Development of new fabrication technology incorporating ma-terials compatible with other LMs or LM alloys will allowdevelopment of switches using other LMs.

ACKNOWLEDGMENT

The authors would like to thank Prof. L. Lin, Dr. A. Cao,Dr. W. Shen, and Dr. D. Youngner for their valuable comments.The authors would also like to thank A. Lee for her help inimproving the readability of this paper.

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Prosenjit Sen was born in Calcutta, India, in 1978.He received the B.Tech. degree in manufacturingscience and engineering from Indian Institute ofTechnology, Kharagpur, India, in 2000, and the Ph.D.degree in mechanical engineering from the Univer-sity of California, Los Angeles (UCLA), in 2007.

He is currently with the MicromanufacturingLaboratory, UCLA. His research interests includemicrofluidic systems, droplet dynamics, liquid-metal-based RF microelectromechanical systems,and reliability of electrowetting-on-dielectric devices.

Dr. Sen was the recipient of the Institute Silver Medal from the IndianInstitute of Technology.

Chang-Jin “CJ” Kim (S’89–M’91) received theB.S. degree from Seoul National University, Seoul,Korea, the M.S. degree from Iowa State University,Ames, and the Ph.D. degree in mechanical engi-neering in 1991 from the University of California,Berkeley.

Since joining the faculty of the University ofCalifornia, Los Angeles (UCLA), in 1993, he hasdeveloped several microelectromechanical systems(MEMS) courses and established a MEMS Ph.D.major field in the Mechanical and Aerospace Engi-

neering Department. Aside from directing the Micro and Nano ManufacturingLaboratory, he is also an IRG Leader for the NASA-supported Institute forCell Mimetic Space Exploration and a Founding Member of the CaliforniaNanoSystems Institute, UCLA. His research includes MEMS and nanotechnol-ogy, including design and fabrication of micro/nano structures, actuators, andsystems, with a focus on the use of surface tension.

Prof. Kim was the recipient of the TRW Outstanding Young TeacherAward, NSF CAREER Award, ALA Achievement Award, Samueli OutstandingTeaching Award, and Graduate Research Excellence Award. He has served onnumerous technical program committees, including Transducers and the IEEEMEMS Conference, and on the U.S. Army Science Board as a Consultant.He is currently chairing the Devices and Systems Committee of the ASMENanotechnology Institute and serving as a Subject Editor for the JOURNAL

OF MICROELECTROMECHANICAL SYSTEMS, on the Editorial Advisory Boardfor the IEEJ Transactions on Electrical and Electronic Engineering, and onthe National Academies Panel on Benchmarking the Research Competitivenessof the U.S. in Mechanical Engineering. He is also active in the commercialsector as board member, scientific advisor, consultant, and cofounder of startupcompanies.


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2009 TIE Sen Micro LM Switch - Review