# Instability and Drift Phenomena in Switching RF-MEMS Microsystems
## 3 Instabilities Due to Dielectric Charging  
Dielectric charging has been widely studied in microswitching devices [16; 17]. It is especially important in long-term actuation applications [18], but also plays a relevant role in many cycling experiments [19] because the effects of charging increase with time. The main effect of charging is a progressive drift of the release and actuation voltages, that can be either positive or negative and increases with bias time. The ultimate source of this problem is the presence of a dielectric between the bridge and the actuation electrodes, and it is therefore very important for capacitive switches [20]. Since a perfect dielectric does not exist, the real material becomes positively or negatively charged because of small defects in its structure, and this effect increases with actuation time. The effects of charging can generate several unwanted phenomena. The most important of these instabilities is caused by a progressive drift of the actuation and release voltages. Depending on the polarity applied and the nature of the charge traps, two different effects can be measured. In the first case, the release voltage drifts toward smaller values until it is so close to zero that the switch does not de-actuate anymore and remains stuck. In the second case, the actuation voltage can progressively increase to very high values until it is higher than the applied bias and the switch does not actuate. This behavior is schematically depicted in Figure 2. In real operation, stiction or actuation failure does not happen abruptly and without warning signs. In the first case, when the release voltage approaches zero, the adhesion forces between the surfaces can randomly prevent the release before the critical point, with a behavior that is largely unpredictable. In the other case, since charging phenomena are increased when the switch is actuated, and may slowly be reduced when the switch is de-actuated, an actuation voltage too close to the bias voltage can produce a continuous actuation-release mechanism not controlled by the external bias, which leads to a totally unstable switch. Different strategies have been developed to minimize charging phenomena either at material and design level or by applying specific voltage pulses [21]. The most promising ones work on the removal of the dielectric layer with the introduction of stopping pillars in order to avoid direct contact with the charged electrode [22]. These methods are quite effective and in fact, when most of the charge has been removed, other effects appear in the capacitance-voltage curve [23] evidencing complex charging effects either due to the inhomogeneous distribution of charge [24] or due to mechanical relaxation or creep [25; 26]. In this case, a more complex model including both shifting and narrowing of the actuation/release curves must be applied to the experimental data in order to characterize the switch behavior, and the voltage scan must be bipolar in order to take into account the drift of both positive and negative values. Unlike shift effects, narrowing effects in the capacitance-voltage curve are polarity independent and it is then quite hard to attribute them exclusively to dielectric charging. Shift and narrowing effects can be separated considering the average and the difference between the positive and negative actuation (and de-actuation) voltages [23; 27]. The change in the average voltage is a measure of the narrowing, whereas the difference is a direct measure of the shift. A typical example of this behavior is reported in Figure 3, where a dielectric-less capacitive switch was biased above the actuation value for twelve hours. The measurements were performed by sweeping the applied voltage from 0 to 60 V and then from 60 to \(-\)60 V and finally from \(-\)60 to 0 V before and after biasing the switch at 60 V for 12 hours. The sweeping process lasted a few seconds.