DC and bipolar voltage stresses are used to isolate mechanical degradation of the movable electrode from charging mechanism in microelectromechanical capacitive switches. Switches with different metals as the movable electrode were investigated. In titanium switches, a shift in the pull-in voltages is observed after dc stressing whereas no shift occurs after the bipolar stressing, which is to be expected from charging theory. On switches with similar dielectric but made of aluminium, the narrowing effect occurs regardless if dc or bipolar stressing is used, which indicates the mechanical degradation as the mechanism responsible. (C) 2012 American Institute of Physics. (http://dx.doi.org/10.1063/1.4726116

Duane, Russell

Houlihan, Ruth

O'Mahony, Conor

Olszewski, Oskar Zbigniew

Ryan, Cormac

Applied Physics Letters

O\u27Mahony, Conor

Irish Universities

Experimental isolation of degradation mechanisms in capacitive microelectromechanical switches

Cork Open Research Archive

Title Experimental isolation of degradation mechanisms in capacitivemicroelectromechanical switchesAuthor(s) Olszewski, Oskar Zbigniew; Houlihan, Ruth; Ryan, Cormac; O'Mahony,Conor; Duane, RussellPublication date 2012Original citation Olszewski, Z., Houlihan, R., Ryan, C., O’Mahony, C. and Duane, R.(2012) 'Experimental isolation of degradation mechanisms in capacitivemicroelectromechanical switches', Applied Physics Letters, 100(23), pp.233505. doi: 10.1063/1.4726116Type of publication Article (peer-reviewed)Link to publisher'sversionhttp://aip.scitation.org/doi/abs/10.1063/1.4726116http://dx.doi.org/10.1063/1.4726116Access to the full text of the published version may require asubscription.Rights © 2012 American Institute of Physics.This article may bedownloaded for personal use only. Any other use requires priorpermission of the author and AIP Publishing. The following articleappeared in Viktorov, E. A., Erneux, T., Piwonski, T., Pulka, J.,Huyet, G. and Houlihan, J. (2012) 'Pump dependence of thedynamics of quantum dot based waveguide absorbers', AppliedPhysics Letters, 100(24), pp. 241108 and may be found athttp://aip.scitation.org/doi/abs/10.1063/1.4729155Item downloadedfromhttp://hdl.handle.net/10468/4299Downloaded on 2018-08-23T18:48:23ZExperimental isolation of degradation mechanisms in capacitivemicroelectromechanical switchesZ. Olszewski, R. Houlihan, C. Ryan, C. O’Mahony, and R. DuaneCitation: Appl. Phys. Lett. 100, 233505 (2012); doi: 10.1063/1.4726116View online: http://dx.doi.org/10.1063/1.4726116View Table of Contents: http://aip.scitation.org/toc/apl/100/23Published by the American Institute of PhysicsExperimental isolation of degradation mechanisms in capacitivemicroelectromechanical switchesZ. Olszewski,a) R. Houlihan, C. Ryan, C. O’Mahony, and R. DuaneTyndall National Institute, University College Cork, Dyke Parade, Cork, Ireland(Received 8 March 2012; accepted 19 May 2012; published online 6 June 2012)DC and bipolar voltage stresses are used to isolate mechanical degradation of the movableelectrode from charging mechanism in microelectromechanical capacitive switches. Switches withdifferent metals as the movable electrode were investigated. In titanium switches, a shift in thepull-in voltages is observed after dc stressing whereas no shift occurs after the bipolar stressing,which is to be expected from charging theory. On switches with similar dielectric but made ofaluminium, the narrowing effect occurs regardless if dc or bipolar stressing is used, which indicatesthe mechanical degradation as the mechanism responsible. VC 2012 American Institute of Physics.[http://dx.doi.org/10.1063/1.4726116]The reliability problems of capacitive microelectrome-chanical (MEM) switches are hindering commercializationof such devices for a range of microwave applications.1 Thecapacitance-voltage (C-V) curve of the switch changes overthe device lifetime due to reliability problems and thisreceived much attention in the literature.2–10 While chargingmechanism has been reported as the primary reliability prob-lem of capacitive switches, the C-V curve instability canalso occur due to mechanical degradation of the movableelectrode. The device cross section and ideal C-V curve of aswitch used in this work are shown in Fig. 1. The switch con-sists of the top movable electrode (membrane and springs)and the bottom electrode (coplanar waveguide, CPW) coatedby a thin layer of dielectric. When the membrane is in theup-state, the radio frequency (RF) signal passes through theCPW. The application of the bias above the pull-in (V>VPI)between the membrane and the signal line causes the mem-brane to collapse on the dielectric. This increases the capaci-tance of the device coupling the RF signal to ground. Torelease the membrane, the voltage has to be decreased tobelow the pull-out (V<VPO).The ideal C-V curve is symmetrical around zero bias dueto electrostatic principle of switch actuation, i.e., the electro-static force on the movable electrode is proportional to V2. Ifparasitic charge in the device (e.g., in the dielectric on top ofthe central line) or around the device (e.g., in substrate or inthe CPW gaps) due to bias stress exist and acts on the mova-ble electrode, the whole C-V curve can shift on the voltageaxis toward the positive or negative direction or narrow whenthe threshold voltages decreases in magnitude.2–6 The narrow-ing effect has also been attributed to mechanical degradationof the movable electrode.6–8 Currently, many capacitiveswitch technologies are presented in the literature where varia-tions in threshold voltages are observed. However, it is notalways clear if charging or mechanical degradation is thedominant reliability problem as this can depend on the switchtechnology, e.g., processing conditions, materials, and devicelayout. This work shows a method that isolates experimentallythe two physical degradation mechanisms.The pull-in voltage for positive and negative bias polar-ity in a capacitive MEM switch can be described byVPI ¼ﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ8k27e0Ag þ ded 3sþ f ðrÞ: (1)where k is the effective spring constant of the movable elec-trode, e0 and ed are the permittivities of the free space andthe dielectric, A is the area of overlap between the membraneand the central line, g is the air-gap height, d is the dielectricthickness, and ƒ(r) is the function describing the additionalpotential due to parasitic charge that appears in the device(or around the devices) due to bias stress over the device life-time. Considering Eq. (1), it can be noted that the pull-involtage can change from the initial value if (i) the change inthe parasitic charge is non zero as a results of applied bias,i.e., Dr= 0 and (ii) the effective spring constant of the de-vice changes due to mechanical degradation of the movableelectrode (membrane and springs) as a result of the mechani-cal forces during down-state, i.e., Dk= 0. In this considera-tion, we assume that A, d, e0, and ed cannot change over thetest times in this work and the air-gap height g can change asa result of mechanical degradation as described in (ii). Tak-ing into account the above conditions, it is proposed in thiswork that the bipolar and dc voltage stresses (Fig. 2) can beused to isolate the mechanical degradation from the chargingphenomenon. In bipolar stressing, the magnitude of bias lev-els (þV and V) is higher than the pull-in voltage of theswitch. Also, the transition time between bias levels (fromþV to V and vice versa) is significantly shorter than themechanical response time of the switch. In such conditions,the movable electrode cannot react mechanically to changesbetween bias levels and remains in the down-state over theentire stress time tbip. At the same time, because the biaschanges polarity at high frequency (in the range of kHz) andthe duty cycle is set to 50%, the charging mechanisms wouldbe significantly limited, i.e., the change in parasitic chargewill be close to zero, Dr  0. In dc stressing, the stress timeis set equal to the bipolar stress time, tdc¼ tbip. Thus,switches in both experiments undergo the same mechanicalstress as the total down-state times are the same. However,a)Author to whom correspondence should be addressed. Electronic mail:zbigniew.olszewski@tyndall.ie.0003-6951/2012/100(23)/233505/4/$30.00 VC 2012 American Institute of Physics100, 233505-1APPLIED PHYSICS LETTERS 100, 233505 (2012)during the dc stress the bias does not change polarity overthe entire stress time and the charging mechanism should besignificantly enhanced when compared with the bipolarstress conditions, i.e., Dr= 0. From the theory describedabove, it can be concluded that in the switch under the bipo-lar stress the change in the pull-in due to charging phenom-ena should be significantly lower than that observed duringthe dc stress on the same device. Also, if the changes in pull-in voltages are seen to be comparable regardless of the biasstress type, then it would indicate that the mechanical degra-dation of the moving electrode is the dominant reliabilityproblem. It must be noted that the charge injection and re-moval processes under positive and negative biases in bipo-lar stress are not identical.9,10 Therefore, bipolar stressingwill eventually result in charge build-up in the device. How-ever, significant differences after relatively short stress timesshould be observed when compared with the dc stress.To prove the theory above, we perform bipolar and dcstress experiments on two types of switches with layout asshown in Fig. 1. In both devices, the dielectric is 140 nmthick silicon oxide, substrate is low resistivity silicon passi-vated by 0.5 lm thick silicon oxide, and bottom electrode is0.5 lm thick layer of aluminium with 1% silicon. The dielec-trics have been deposited by plasma enhanced chemicalvapor deposition. The switches differ in terms of the mova-ble electrode composition, i.e., 0.5 lm thick titanium and1.0 lm thick aluminum, thus were fabricated in two separatefabrication runs. Moreover, the processing conditions duringthe final release step (i.e., removal of the sacrificial layer in aplasma environment) were different for both switches: the ti-tanium switches were released in a mixture of oxygen gas(O2—90%) and tetrafluoromethane gas (CF4—10%) at roomtemperature, whereas the aluminium switches were releasedin oxygen gas only at 220 C.The measurements of pull-ins are performed by C-Vsweeps using an Agilent B1500A Device Analyzer and bywafer probing on Cascade station in a dry environment andat room temperature. A typical time for a single pull-in mea-surement is 2 s. The dc bias stress was supplied to theswitches by Agilent B1500A whereas the bipolar stress bysignal generator Agilent 33220A with a signal amplified byFalco WMA-300 to the desired bias levels. The transitiontimes between bias levels in the bipolar signal were meas-ured with oscilloscope to be 0.2 ls, significantly lower thanthe switching speed (mechanical response time) of switchesused in this work, i.e., 100 ls. The bias in these experi-ments is applied to the membrane with respect the signalline. In experiments, three stress cycles have been per-formed; cycle 1: dc stress (0 Hz) for 10 min at 25 V,cycle 2: bipolar stress (1 kHz) for 10 min at625 V, and cycle3: bipolar stress (10 kHz) for 10 min at 625 V. During theexperiments, the relaxation intervals at no bias were usedbefore cycle 2 and cycle 3 to allow the pull-in voltages toreturn back to the initial values (60.1 V) after the stress inFIG. 2. The principle of (a) bipolar voltage stress and(b) dc voltage stress used in this work to isolate me-chanical degradation from dielectric charging.FIG. 1. (a) SEM image of a typicalcapacitive switch used in this work, themembrane area is 100 100lm2, (b)cross section along AA0 cut-line, and (c)an ideal C-V characteristic of the switch.233505-2 Olszewski et al. Appl. Phys. Lett. 100, 233505 (2012)previous cycle. It was observed that the relaxation interval of1 h was sufficiently long for pull-in voltages of aluminiumswitches to relax, while the relaxation of pull-in voltages oftitanium switch required around 10 h.The experimental data obtained on the titanium switchare shown in Fig. 3. Figure 3(a) shows the change in themagnitude of the negative and positive pull-in voltages afterdc and bipolar stress cycles. Note that a negative changemeans that the pull-in has decreased in magnitude whereas apositive change means that the pull-in has increased in mag-nitude. According to the theoretical basis for the experiment,the observed change in the pull-in voltages under dc stress(i.e., the C-V curve shifts in a positive direction as shown inFig. 3(b)) while exhibiting negligible pull-in voltage changeunder bipolar stress indicate that dielectric charging is thedominant reliability mechanism in titanium switches.The same set of experiments performed on an alumin-ium switch yield different results (see Fig. 4). In the alumin-ium switch, both positive and negative pull-in voltagesdecrease in magnitude (i.e., the C-V curve narrows as shownin Fig. 4(b)). This narrowing effect is in contrast to thecharging behaviour observed in titanium switches as itoccurs regardless of whether a dc or bipolar stress is used.According to the theoretical basis for the experiments, theseresults suggest that the dominant reliability mechanism foraluminium switch is mechanical degradation of the alumin-ium metal. This analysis agrees with our recent study of alu-minium based test structures published in Ref. 7. Thedifference between the positive and negative pull-in voltagechange under dc stress, which disappears under bipolarstress, provides evidence that the shift effect (charging) issuperimposed on the narrowing effect (mechanical degrada-tion). While the substrate, bottom metal and dielectric mate-rial and the applied dc stress is similar for the titanium andaluminium switches, the experiments indicate significantlyless charging in the aluminium devices compared with the ti-tanium devices. This difference may be due to the differencein the processing conditions for both devices during the finalrelease of the sacrificial layers.In the switches investigated in this work, the dominantdegradation mechanism depends on the metal of the movableelectrode. The results indicate that mechanical degradationdominates in aluminium switches and is responsible for thenarrowing effect independently of the bias type. From theswitch electrostatic theory, the pull-in is related with thespring constant of the switch by jVPI j! Hk (see Eq. (1)),and the decrease in the spring constant can be responsible forthe narrowing observer in aluminum switches. The physicalmechanism that can potentially be responsible for this effectis the material relaxation mechanism, which results fromviscoelastic phenomenon as described in Refs. 11 and 12.FIG. 3. (a) The change in the pull-in voltage magnitude after three stresscycles on the titanium switch. Note that a negative change in the magnitudeof the pull-in voltage means that the pull-in has decreased in magnitudewhereas a positive change in the magnitude of the pull-in voltage means thatthe pull-in has increased in magnitude. (b) The shift effect of the pull-incharacteristic due to dc stress.FIG. 4. (a) The change in pull-in voltage magnitude after three stress cycleson aluminium switch. Note that a negative change in pull-in voltage magni-tude means that pull-in voltage has decreased in magnitude. (b) The narrow-ing effect of pull-in characteristic due to dc stress.233505-3 Olszewski et al. Appl. Phys. Lett. 100, 233505 (2012)The results indicate that this mechanical degradation doesnot occur in titanium switches and this can be attributed tohigher melting point of titanium. In titanium switches, thecharging is observed to be dominant, which is responsiblefor the positive shift effect under the dc stress; whereas underthe bipolar stress, there is no shift as the net change in thedielectric charge is close to zero.The authors appreciate the sponsorship of the EuropeanSpace Agency (ESA) and Irish Research Council for Sci-ence, Engineering and Technology (IRCSET) through theNetworking and Partnership Initiative (NPI) and ScienceFoundation Ireland (SFI) under Grant No. SFI 10/RFP/ECE2883.1V. K. Varadan, K. J. Vinoy, and K. A. Jose, RF MEMS and Their Applica-tions (John Wiley & Sons, The Atrium, Southern Gate, Chichester, WestSussex, England, 2003).2J. R. Reid, in Proceedings of the Modeling and Simulation of Microsys-tems, San Juan, Puerto Rico, 2002, pp. 250–253.3D. Molinero and L. Castan˜er, Appl. Phys. Lett. 94, 043503 (2009).4X. Rottenberg, B. Nauwelaers, W. De Raedt, and H. A. C. 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Experimental isolation of degradation mechanisms in capacitive microelectromechanical switches

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