Floating Electrode Microelectromechanical System Capacitive Switches: A Different Actuation Mechanism

The paper investigates the actuation mechanism in floating electrode MEMS capacitive switches. It demonstrated that in the pull-in state the device operation turns from voltage to current controlled actuation. The current arises from Poole-Frenkel mechanism in the dielectric film and FowlerNordheim in the bridge-floating electrode air gap. The pull-out voltage seems to arise from the abrupt decrease of Fowler-Nordheim electric field intensity. This mechanism seems to be responsible for the very small difference with respect to the pull-in voltage. The radio frequency (RF) microelectromechanical systems (MEMS) switches and varactors have been developed more than fifteen years ago for low loss switching/routing circuits and X-band to millimeter-wave (mm-wave) phase shifters, which have seen increasing applications in tunable filters, antennas and reconfigurable matching networks [1, 2]. Among the different designs, the capacitive switches proved to exhibit excellent RF performance and power handling [3, 4]. The performance of the capacitive switches depends on the down-state capacitance that can be limited by the finite roughness as well as the low planarity of both the dielectric layer and the beam [5, 6]. In order to diminish this effect and ensure a constant capacitance in the pull-in state, the deposition of an additional (electrically floating) metal layer on the dielectric layer was proposed [7, 8, 9]. Such devices are actuated through side actuation pads or by applying the bias directly to the transmission line. Among the two actuation methods, the former is similar to the one used in conventional capacitive switches and the pull-in condition has been analyzed in details in many papers including or not the charging effect, e.g. [1, 2, 10, 11. In these switches the actuation through the floating electrode has received no attention in spite of the dramatic change of bridge to floating electrode capacitance, hence E. Papandreou, et al 103 the potential difference, upon the transition from pull-out to pull-in state. The aim of the present work is to analyze the actuation mechanism of the floating electrode capacitive switch, demonstrate that in the pull-in state the conventional condition V ≥ Vpi cannot further hold and the device turns from voltage to current actuation. The switches used in present work were parallel single pole single through (SPST) cells. In the parallel (shunt) version of the SPST a metal membrane (movable air bridge) above the CPW substrate can electrically short the centre line to ground when electrostatically actuated, as shown in Figure 1a. Two side actuation pads were added in order to separate the DC bias from the RF signal on central line. These actuation pads were connected with polysilicon bias lines which were isolated with 300nm SiO2 film from the ground plane and coplanar transmission line (fig. 1a). In the present work the bias was applied only to coplanar waveguide transmission line (CPW) and not to the side actuation pads. Under the bridge the central line was constituted by a metal multilayer (Ti-TiNAl-Ti-TiN) covered by SiO2 dielectric film (Low Temperature Oxide, LTO) with a thickness of about 100 nm. A floating metallic (Au) contact (90μm × 150μm) was deposited on the top surface of the dielectric film to ensure a constant capacitance during pull-in state where the device must behaved like MIM capacitor with a capacitance of CMIM ≅ 4.66pF. The capacitance of the RF MEMS switches was measured at 1 MHz with a Boonton 72 B capacitance bridge that provided a resolution better than 0.5 fF. The current-voltage characteristic was measured at room temperature with a Keithley 6487 picoampere meter. All measurements were performed in vacuum and the surface humidity was removed by heating cycles at 140°C for two hours each time. Finally prior assessment the devices were stored in vacuum. Unipolar capacitance-voltage characteristic were obtained increasing the voltage from 0 to 50 V and then returning back to 0 V (fig. 1b). In all measurements the bias was applied to the transmission line with respect to the ground level. Unlike the conventional actuation without floating electrodes or by using the lateral pads, there was no apparent hysteresis. The two branches of CV curve for increasing and decreasing voltage are practically superimposed and the pull out and the pull in voltages are about the same. As already mentioned a metal film floating electrode was deposited on the dielectric surface in order to ensure primarily a constant capacitance during pullin. The presence of this metallic film cap leads to uniform charge injection and screens any potential fluctuation that may arise inside the insulating film. In absence of dielectric charging, if z0 is the air gap at equilibrium, k the spring constant, A the switch area, d the dielectric film thickness (d<< z0) and V the applied bias the pull-in voltage will be given by: Floating Electrode Microelectromechanical System Capacitive Switches 104