Research Status And Development Trend Of MEMS Switches: A Review - MDPI
micromachinesReviewResearch Status and Development Trend of MEMSSwitches: A ReviewTongtong Cao, Tengjiang Hu * and Yulong Zhao *State Key Laboratory for Manufacturing System Engineering, Xi’an Jiaotong University, Xi’an 710049, China;tammy0326@stu.xjtu.edu.cn* Correspondence: htj047@xjtu.edu.cn (T.H.); zhaoyulong@xjtu.edu.cn (Y.Z.) Received: 18 June 2020; Accepted: 16 July 2020; Published: 17 July 2020Abstract: MEMS switch is a movable device manufactured by means of semiconductor technology,possessing many incomparable advantages such as a small volume, low power consumption,high integration, etc. This paper reviews recent research of MEMS switches, pointing out the importantperformance indexes and systematically summarizing the classification according to driving principles.Then, a comparative study of current MEMS switches stressing their strengths and drawbacksis presented, based on performance requirements such as driven voltage, power consumption,and reliability. The efforts of teams to optimize MEMS switches are introduced and the applicationsof switches with different driving principles are also briefly reviewed. Furthermore, the developmenttrend of MEMS switch and the research gaps are discussed. Finally, a summary and forecast aboutMEMS switches is given with the aim of providing a reference for future research in this domain.Keywords: MEMS switches; driving principle; reliability; bistable mechanism1. IntroductionSwitches are essentially used to control the on–off state of circuits and are required to reactquickly and accurately to signals. The MEMS switch device is a tiny movable element withthree-dimensional structure fabricated by semiconductor technology. MEMS switches offer muchlower power consumption, much better isolation, and lower insertion loss compared to conventionalfield-effect transistors and p-i-n diode switches [1–3], and they possess advantages such as smallsize and high integration. The rise of MEMS switches provides strong technical support for thedevelopment of signal control systems. At present, the demand for MEMS switches mainly comes frommilitary security systems [4–7], the automobile industry [8,9], the wireless communication field [10–15],medical apparatus and instruments [9,16], micro-optical electromechanical systems (MOEMS) [17–19]and more. Over the last few decades, various types of MEMS switches have been developed. To befamiliar with the working mechanism and optimization direction of existing MEMS switches is ofgreat significance for the development of innovative MEMS switches. However, there is a lack ofa comprehensive classification of MEMS switches.MEMS switches can be classified in a variety of ways [20], such as according to whether thereis an additional driving source, and the existing MEMS switches can be segmented into passiveMEMS switches and active MEMS switches. Passive MEMS switches exploit their own system toinduce changes and absorb energy for inertial actuation [21,22]. This driving principle has betterlong-term storage performance and resistance to electromagnetic interference owing to no need forextra energy [23]. Active drive refers to the use of external energy to drive movable electrodes to changethe on–off state of switches. The drive of MEMS switches involves magnetic energy, electrical energy,photochemical energy and other energy fields, which are converted into mechanical energy to generatedisplacement [24,25].Micromachines 2020, 11, 694; chines
Micromachines 2020, 11, 6942 of 31MEMS switches can be roughly divided into silicon-based MEMS switches and non-silicon-basedMEMS switches according to the different processing materials. Silicon-based MEMS switches areusually fabricated on SOI (silicon-on-insulator) wafers with the advantages of high shape precisionand a simple process [26,27]. However, if the structural layer material of the silicon-based switch isused directly for contacts, the contact resistance will be too large compared with conductor materials,resulting in an unobvious signal. To reduce the contact resistance, it is necessary to apply a layer oflow-resistivity metal on the contact surface of the electrodes [28]. In addition, silicon is not suitablefor high impact and high load applications either as a structure layer or as a substrate [29]. On theother hand, non-silicon switches are mainly fabricated from LIGA (lithographie, galvanoformung andabformung) or ultra-precision processing technology. For metal-based switches, multi-layer suspendedmovable structures are usually fabricated from Ni via micro-electroplating [30–32]. In contrast to theproperties of silicon-based switches, metal structures provide excellent electrical conductivity, as wellas good mechanical properties and toughness. Although this switch solves the problem of high contactresistance, the maturity of metal microstructures manufacturing is relatively low. During processing,the structure is prong to deformation [33], leading to a low yield.What is more, according to the contact modes, MEMS switches can be grouped into resistiveswitches and capacitive switches, a classification quite common seen in RF (radio frequency) MEMSapplications. Capacitive switches are turned on or off through capacitance coupling [34], and thesetypes of switches are suitable for high-frequency (about 3 MHz to 30 MHz) applications [35,36]. On theother hand, resistive switches are generally used in the lower frequency band (about 30 KHz to300 KHz) of the radio frequency signal [37]. Low contact resistance, usually less than 1–2 ohms, is oneof the important performance requirements of MEMS switches [38].Of course, MEMS switches can be divided into laterally actuated switches [13,39–42] and verticallyactuated switches [43–45] The displacement of vertically actuated switches is out-of-plane while thatof laterally actuated switches is in-plane.This review aims to provide detailed insights into the structural design and performanceoptimization of MEMS switches, based on the literature of the last 20 years. In the second part, the keyperformance indexes of MEMES switches, especially the influencing factors of reliability, are pointedout. The third part, as the main body of the paper, introduces in detail the different principles ofswitches and the targeted performance optimization from the aspect of structure. Thereinto, bistablemechanism is used in almost every actuation as an effective method to enhance the contact effect andimprove the switching speed. In the design of active switch, there is also the problem of how to realizethe insulation between drive signal and switch signal, which has been also mentioned in each section.Each switch has its pros and cons, so designers have made specific improvements to the switches aftertrade-offs or analyzing the application requirements. Furthermore, the general development trend ofMEMS switches is predicted. This review serves the purpose of providing researchers in this field witha reference source.2. Performance Indicators of MEMS SwitchesIn the development of MEMS switches, their performance is constantly optimized. The keyperformance indicators of MEMS switches are driving voltage, switching time, power consumption,reliability and so on. Among them, the reliability of MEMS switches is a factor that must be consideredin performance design. The neglect of reliability is a major obstacle to the ultimate commercializationof switches. In order to improve the reliability of switches, possible failure modes of switches shouldbe analyzed first. Table 1 below is an analysis of common failure modes of MEMS switches.
Micromachines 2020, 11, 6943 of 31Table 1. Failure mode analysis of MEMS switches.Failure ModeFailure FactorsImprovement MethodsCreepTemperature, power, interior stressCreep resistant alloy;improving heat dissipation;StictionHumidity, adhesion force, powerReducing contact area;choosing harder contact materials;reducing the powerDielectric chargingElectric field intensity,temperature, humidityLower actuation voltage;signal isolation;changing the dielectricFractureRepeated loading, shockReducing stress;change the composition of alloys;shock absorptionWearRepeated contactIncreasing the hardness of the contactmaterialLayeredTemperature change, residualstress, microparticlesImproving temperature stability;transition layer to increase adhesionFailure of packageTemperature change, impactShock absorption;heat dissipationTo analyze the failure of MEMS switches in detail, capacitive switches and resistive switchesshould be considered separately. The main problem affecting the reliability of capacitive switchesis not the mechanical properties, but the charging issue. The rate of the C/V curve [46] and thestretched exponential for charging [47] can be used to evaluate the failure time of capacitor switches.Goldsmith [48] proposed an efficient accelerated life test method, where a continuous electrical signalis applied to the switch and detects the modulation signal generated by the switch action. The reasonfor this failure is assumed to be the continuous accumulation of electric charge in the dielectric layer,which eventually leads to the driving voltage drift or latch-up effect. The optimization of capacitiveswitches should solve the problem of charge accumulation [49]. On the one hand, it can be improvedby optimizing the dielectric material such as the dielectric layer material with high dielectric coefficientand low trap density [48–52]. A dielectric-less switch has proved to be an effective method [53]. On theother hand, the voltage can be optimized, such as using high voltage to drive the switch to close andlow voltage to maintain the closed state [54], or using bipolar control voltage [55].The failure of the resistive switches is due to contact fatigue. Mechanical stress causes deformationand wear of contact surfaces, while electrical stress mainly causes electromigration and melting ofcontact surfaces. Their combined action eventually leads to increased contact resistance or adhesion.Therefore, the choice of contact material is the key to the reliability of the switch, considering suchfactors as hardness, resistivity, melting point and sensitivity to organic pollutants [56,57]. Soft metals,such as gold, are suitable for reducing contact resistance, but their contact surfaces are prone tomicrowelding. Ke et al. [58] coated Au contacts with Ru to investigate placing harder materials ontop of softer materials for a lifetime enhancement. Yang et al. [59] showed Au–Ni alloy contacts resistmaterial transfer better than Au–Au contacts. In exchange, alloying Au with other metals also resultsin an increased resistivity. Yaglioglu et al. [60] examined the electrical contact properties of carbonnanotube (CNT)-coated surfaces. The high Young’s modulus and potential for low resistance of CNTsmakes them suitable candidates for micro-switch contacts. Experiments have shown that addinga small amount of Pd or Pt to the gold increases the lifetime of the device, but the contact resistanceincreases only a small amount [38]. In order to prevent the degradation of switch contact, apart frompreventing the mechanical damage of the contact surface, it is necessary to improve the sealing of thepackaging to prevent organic or inorganic pollution [61].
Micromachines 2020, 11, 6944 of 31Both residual stress and temperature affect the switching capabilities of MEMS switches.This means that changes in operating temperature or increases in residual stress may increasethe actuation voltage of MEMS switches. The actual driving voltage of the switch is often very differentfrom the design value. One of the reasons is the influence of residual stress. The residual stress inthe movable structure will accelerate the fatigue and reduce the durability of MEMS switches [62].Residual stress formed during micro-machining is the main factor that affects the reliability of MEMSswitches [63,64]. Thermal residual stress is generated during the thermal loading-unload cycles duringthe plasma etching stage. In surface micromachining process, multilayer metals are deposited on thesubstrate. The difference between the thermal expansion coefficient of different material layers leads tothe formation of residual stress. It affects the flatness of the fabricated switch, thus affecting the staticand dynamic characteristics of the switch. For example, the compressive stress increases the pull-involtage and reduces the switching time [65]. Temperature is the most common failure accelerationfactor. The experimental results show that the change in temperature accelerates the failure modes,such as charge capture, mechanical creep and contact degradation [66].The key performance of the switch, such as driving voltage and switching time, is closely relatedto the driving principles. Therefore, it is necessary to introduce the principles and optimizationperformance structurally of each driving mode, respectively and in detail.3. Classification of MEMS Switches Based on Driving PrinciplesAccording to the driving principles [67], MEMS mechanical switches can be roughly divided intopassive inertial switches, electrostatic switches, electro-thermal switches, electromagnetic switches,piezoelectric switches and shape memory alloy switches [68], etc. Table 2 provides a summary of thekey performance comparison of several mainstream MEMS mechanical switches.Table 2. Performance comparison of MEMS switches with different driven agneticPiezo-ElectricElectro-ThermalSize (µm)Fabrication processActuation voltage (V)Power consumption (mW)Switch speed (µs)The output force (µN)Durability 300021002200022002Simple/NZ 1300–1000Uncertain 2 106Simple20–200NZ 20050–1000108 1093002 20002Medium 1560–250300–10,000500–40001061 20002 60002Complex 10100–20020–100050–200 108 20002Complex3–20NZ10–30050–800 108NZ: near zero; 2 Uncertain: related to structure and acceleration.3.1. Passive Inertial SwitchesMEMS inertial switches are special acceleration sensors used to detect the thresholdacceleration [69]. The microinertial switch based on MEMS technique is generally designed with a flatplate structure, which is characterized by miniaturization, high reliability and a low cost.Under the premise that the gas damping and structural damping cannot be ignored in the dynamicresponse, the basic model of inertial switches can be simplified as a spring-mass -damping system,as shown in Figure 1a. When the acceleration applied in the sensitive direction of the switch is at orabove the threshold level, the movable electrode moves along the sensitive direction until the relativedisplacement reaches the distance d between the two electrodes, and the movable electrode contactswith the fixed electrodes to turn the switch on.
Micromachines 2020, 11, 694Micromachines 2020, 11, x5 of 315 of 31Figure 1.1. (a)(a) TheThe spring-massspring-mass -damping-damping systemsystem modelmodel ofof inertialinertial switches;switches; (b)(b) picturespictures ofof aa lerometer fabricated and its experimental setup built by Younis (2007 [70]).In this process, the equation of motion of the mass block can be described as:In this process, the equation of motion of the mass block can be described as:. kxmmxx ccxx kx mama(1)(1)where m, c, and k are the weight of the proof mass, the damping coefficient and the elasticitywhere m, c, and k are the weight of the proof mass, the damping coefficient and the elasticity coefficientcoefficient of the movable electrode, respectively. x is the relative displacement between the movingof the movable electrode, respectively. x is the relative displacement between the moving electrodeelectrode and the fixed electrode, and a represents the acceleration exerted by the outside world onand the fixed electrode, and a represents the acceleration exerted by the outside world on the switch.the switch. Inertial switches are discussed below from the aspects of acceleration threshold andInertial switches are discussed below from the aspects of acceleration threshold and contact effect.contact effect.Most inertial switches are passive devices, but sometimes switches are designed to be active in orderMost inertial switches are passive devices, but sometimes switches are designed to be active into regulate the threshold. Younis et al. [70] tested two commercial capacitive inertial switches fabricatedorder to regulate the threshold. Younis et al. [70] tested two commercial capacitive inertial switchesby Sentasa Technologies [71] (see Figure 1b). The test results showed that the acceleration threshold isfabricated by Sentasa Technologies [71] (see Figure 1b). The test results showed that the accelerationlinear with the DC voltage for the tunable threshold-acceleration switch. Besides meeting the functionthreshold is linear with the DC voltage for the tunable threshold-acceleration switch. Besides meetingof tunable threshold, this kind of switches also have shortcomings: increased volume and powerthe function of tunable threshold, this kind of switches also have shortcomings: increased volumeconsumption due to added power supply and vulnerability to external electromagnetic interference.and power consumption due to added power supply and vulnerability to external electromagneticTherefore, the passive acceleration switch still plays an irreplaceable role in some applications.interference. Therefore, the passive acceleration switch still plays an irreplaceable role in someFor different application requirements, uniaxial switches [72–75], biaxial switches [76–78],applications.tri-axial switches [79,80] gradually appeared. In the development of MEMS switches, not onlyFor different application requirements, uniaxial switches [72–75], biaxial switches [76–78], trithe number of acceleration directions have been expanded, but also the axial sensitivity of the switchaxial switches [79,80] gradually appeared. In the development of MEMS switches, not only thehas been improved [80]. For instance, Currano et al. [81] proposed a triaxial inertial switch basednumber of acceleration directions have been expanded, but also the axial sensitivity of the switch hason the symmetrical spiral springs, in which five switches are integrated. In 2014, Chen et al. [80,82]been improved [80]. For instance, Currano et al. [81] proposed a triaxial inertial switch based on thedesigned and fabricated an all-metal triaxial inertia switch. A triaxial inertial switch can be usedsymmetrical spiral springs, in which five switches are integrated. In 2014, Chen et al. [80,82] designedinstead of multiple uniaxial inertial switches to monitor acceleration in multiple directions and avoidand fabricated an all-metal triaxial inertia switch. A triaxial inertial switch can be used instead ofcomplex installations.multiple uniaxial inertial switches to monitor acceleration in multiple directions and avoid complexThe inertial switch can be divided into high-g inertial switches and low-g inertial switchesinstallations.according to the different load environment applied. On the one hand, the high-g inertial switchesThe inertial switch can be divided into high-g inertial switches and low-g inertial switchesgenerally refer to the inertial switches whose threshold acceleration range is from several hundredaccording to the different load environment applied. On the one hand, the high-g inertial switchesg to tens of thousands g. The high-g inertial switches are mainly applied in the harsh environmentgenerally refer to the inertial switches whose threshold acceleration range is from several hundred gto tens of thousands g. The high-g inertial switches are mainly applied in the harsh environment ofhigh load and high impact, such as in the military. A high-g switch also needs to have better anti-
Micromachines 2020, 11, 6946 of 31Micromachines 2020, 11, x6 of 31of high load and high impact, such as in the military. A high-g switch also needs to have consurfacemachiningtechnologyis oftenadoptedjammingabilityand hnologyis nertial switches. The structure materials and substrates with high strength are used to preventfracture failurefailure andand disengagementdisengagement ofXu etet al.al. [83][83] developeddeveloped aa multi-directionalmulti-directional MEMSMEMSfractureof bondbond wires.wires. tra-highgacceleration(about100,000g)in thetheinertial switch with shock-resistance. It can resist ultra-high g acceleration (about 100,000 g) inreverse sensitivesensitive direction.direction. TheThe schematicschematic diagramdiagram isis shownshown inin FigureFigure 2.The designdesign ofof thethe constraintconstraintreverse2. undoftheproofmass.Moreover,theinsulatingstructures can prevent false trigger caused by the rebound of the proof mass. Moreover, the insulatingquartz substratequartzsubstrate isis beneficialbeneficial toto witchisexpectedtobeinstalledindevicesinInternetof ThingsThingsg acceleration. The proposed MEMS switch is expected to be installed in devices in Internet ernalenvironment.Ontheother,low-gsystems (IoT) to monitor shock and vibration from the external environment. On the other, low-ginertial switches,switches, widelywidely usedused inin thethe aviationaviation andand automotiveautomotive industries,industries, havehave accelerationacceleration 4]proposedranging from several milli g to hundreds of g. Based on a feasibility study, Lior et al. [84] theproofmassandtosensetheacceleration,an idea of using a pair of bistable beams to suspend the proof mass and to sense the acceleration, inin developed anan nditcanwithstandunexpectedshocksofswitch with a threshold acceleration of no more than 10 g, and it can withstand unexpected shocks ofup toto 10001000 g,g, makingmaking itit suitablesuitable forfor harshupharsh militarymilitary environments.environments.Figure 2. The tri-axial MEMS inertial switch with shock-resistibilityshock-resistibility (Xu 2016 [83]): (a) the workingprinciple; (b) the photo; (c) the schematic of the test circuit.Rigid electrodeselectrodes ofof MEMSMEMS switchesswitches havehave thethe problemsproblems ofof shortshort contactcontact timetime andand signalsignal bounce.bounce.RigidTo improvein termsandToimprove contactcontact stability,stability, manymany methodsmethods havehave beenbeen proposedproposed interms ofof structurestructure designdesign elayMEMSswitchforsafetyandarmingsystem.materials selection. Huang et al. [6] proposed a time-delay MEMS switch for safety and armingAs shownFigurein3,Figurewhen 3,thewhenaccelerationreaches orexceedspredictedthreshold,the workingsystem.Asinshownthe accelerationreachesor theexceedsthe time,working fluid will flow toward the induction reservoir through the capillary valve. After the delaythe capacitancebetweenelectrodeschangeschangesand the andswitchturnedison.The measurementsshow thattime,the capacitancebetweenelectrodestheisswitchturnedon. The measurementsthe designedcan realizedelaytime of4.1 10.9s. Becauseof thewedge-shapeddesign,showthat theswitchdesignedswitchacanrealizea delaytimeof 4.1 10.9s. Becauseof the tastablechannel design, it is difficult for the droplet to flow back from the induction reservoir, so the plepreparationtechnologyandhighreliability,can output a stable switch-on signal. This microfluidic switch has simple preparation technology and C). Liu et al. [86], Yoo et al. [5],but theworkingbuttemperaturerangeof glycerolis narrow( 17.8–290highreliability,the workingtemperaturerangeof glycerolis narrow( 17.8 C–290 C). Liu et esbasedontheprincipleinertialflow.principleThey used[86], Yoo et al. [5], Li et al. [7] also designed micro-fluid inertial switches ofbasedon theofmercuryflow.or anTheyultra-lowtemperaturefluid as workingfluids. fluidAmongthem, mercuryinertialused mercuryor anconductiveultra-low temperatureconductiveas workingfluids.has excellentit is volatileand only ctricalmercuryconductivity,has excellentbutelectricalconductivity,it is forvolatileonlysuitable ct,butthechoiceofworkingfluidsandhow oftolow-g value environments. Liquid metal switches greatly enhance the contact effect, but thechoicemaintainfluidstheir statestabilityare trickytheirissues.workingand howto maintainstate stability are tricky issues.
Micromachines 2020, 11, 694Micromachines2020,2020,11,11,xxMicromachines7 of dropmicro-dropinertialswitch(Huanget2013(a) overview;theoverview;thepacking.(b)packing.the packing.theIn anotheranother edeformationof thethe ,process,thethedeformationof theflexibleelectrodes(i.e., collisioncontactbetweentheelectrodes,fixed electrode or the movable electrode) can provide a buffer for the collision contact between halowstiffness-fixedelectrodes, so as to prolong the contact time. Du [8] developed an inertial switch with a low stiffnessfixedelectrodefor extendingextendingthe contactcontactdurationin 20202020(seeFigure4). electrodeThe fixedfixedwaselectrodewaselectrodefor extendingthe contactdurationdurationin 2020 (seeFigure4).FigureThe UV-LIGA,inwhichthemethoddesigned in an arc to reduce its stiffness. The inertial switch was fabricated by UV-LIGA, in whichthemethodof widthwidth compensationcompensationwasadoptedtheto improveimprovetheaccuracy.fabricationaccuracy.The resultresultof widthcompensationwas adoptedwasto improvefabricationTheresult me,thethe contactcan reach260µsreachwhen260theμsdesignedswitchis triggeredby32 g. ialswitchwithaflexiblestructure.Thedesignedsame time, Xu et al. [87] proposed a vertically driven MEMS inertial switch with a flexible ctcontacttime atat 288288 ggByacceleration.By contrastcontrastexperiment,switchcan achieve125µsachievecontact125time288 g acceleration.contrast canbeachievedbyreducingthestiffnessofthethe conclusion has been proved that the extension of contact time can be achieved by thickness.fixed electrode,especiallyits thickness.stiffnessof the fixedelectrode,especially its thickness.Figure 4.4. TheThe contact-enhancecontact-enhance inertialinertial switchswitch (Du(Du etet al.al. 20202020 [8]):[8]): (a)(a) thethe crosscross sectionsection ofof thethe switchswitchFigureFigure4. Thecontact-enhance inertialswitch (Duet al.2020 [8]):(a) thecross sectionof ches.model;In additionaddition toto thethe methodmethod ofof utilizingutilizing flexibleflexible structurestructure toto extendextend thethe contactcontact timetime asas describeddescribedInInaddition tothe methodof utilizingflexible structureto extendthe contacttime above, somesome literatureliterature mentionedmentioned carboncarbon onon nanotubesnanotubes (CNTs)(CNTs) asas electrodeelectrode contactcontact materialsmaterialsabove,[9,88,89]. CNTsCNTs areare suitablesuitable forfor useuse asas contactcontact materialsmaterials duedue toto theirtheir excellentexcellent mechanicalmechanical andand[9,88,89].
Micromachines 2020, 11, 6948 of 31CNTs are suitable f
trade-o s or analyzing the application requirements. Furthermore, the general development trend of MEMS switches is predicted. This review serves the purpose of providing researchers in this field with a reference source. 2. Performance Indicators of MEMS Switches In the development of MEMS switches, their performance is constantly optimized .
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