# Instability and Drift Phenomena in Switching RF-MEMS Microsystems
## 5 Instabilities Due to Fabrication Uncertainties and Material Wear  
When designing a device with suspended beams, an ideal fabrication process is normally assumed. In addition to that, design simplifying assumptions are often made, in order to reduce analytical modeling or simulation complexity. The typical RF-MEMS capacitive switch consists of two actuating electrodes separated by a central underpass line where the signal travels, covered by a suspended movable beam clamped at both ends (see Figure 1a). This means that there are at least two actuation pads separated by the central line. In an ideal device, the beam is rigid and only the anchors bend when actuating. The electrodes and the line have the same height and the collapse on the electrodes occurs exactly when the beam contacts the dielectric above the line. Moreover, the effects of surface roughness are negligible. In a real device, the beam can be flexible, and the surface roughness can limit the contact surface between the membrane and the line [33; 34]. Consequently, to assure a reliable actuation, the central line  
Figure 5: Micrograph (**a**) and surface profile (**b**) of the mobile membrane of a MEMS capacitive switch at different temperatures. The surface profile was measured with an optical profiler along the red dashed line (**a**). must be fabricated sensibly higher than the actuation electrode. This is especially true for dielectric-less switches with stopping pillars, a common design strategy aimed at reducing charging [22]. As a result, the line can be consistently higher than the actuation electrodes, with important consequences on the actuation and de-actuation properties of the switch. An example of this behavior is reported in Figure 6 for the same device depicted in Figure 5.  
As can be seen from Figure 6, the switch presented not one but two collapse points as the applied bias increased. The first one was the actuation on the central line, which is the true mechanical actuation. The second one was the collapse on the actuation electrodes, and it usually happens a few volts after the first one. This double actuation was seen in the capacitance-voltage curve as a small \"step\", as reported in the inset of Figure 7. As can be seen, mechanical and electrical actuation were 5 V apart in this case. Figure 6: Evolution of the suspended beam shape at different (increasing) voltages for the RF-MEMS switch reported in Figure 5. The lines have been recorded with an optical profiler while biasing the switch. Figure 7: C–V curve of a capacitive MEMS switch. The inset reports the same curve expanded in the vertical direction. The first and the second collapse are clearly visible. This behavior can be understood by taking into account the quality of the electrical contact. When dealing with microswitches, the quality of the contact is always an issue and is of paramount importance in the case of ohmic switches [35]. In MEMS devices, the contact forces are usually very low because of the small contact dimensions and a low contact force may result in a bad electric contact and bad switch performance. In the case reported above, after the first actuation, there is a poor electrical contact between the line and the beam. This happens because the contact force depends strongly on the height difference between the central line and the electrode [36]. For this reason, the second actuation where the beam collapses on the electrode is usually detected as an electrical actuation in the C-V curve. In this case, the contact force is much higher and the electrical contact is very good. On the other hand, this means that the experimental actuation voltage is normally higher than the simulated one by at least a few volts, depending on the switch geometry and on the flexibility of the membrane. Unfortunately, the continuous actuation or a high number of switching cycles can also affect the quality of the electrical contact, which slowly degrades generating another type of performance drift and, eventually, another actuation failure.