# Instability and Drift Phenomena in Switching RF-MEMS Microsystems
## 1 Introduction  
Microbeams are widespread in Micro-Electro-Mechanical Systems (MEMS), and they are in fact one of the most common building blocks encountered in MEMS design and fabrication. Microengineering books usually have a very large section dealing with the analytical modeling of suspended microbeams, including cantilevers, double-anchored bridges, circular structures, and various combinations of these basic structures [1, 2].  
These books are rather encouraging because it seems that these microstructures are quite easy to design and fabricate and, more importantly, their performances are quite easy to calculate and predict.  
Of course, as always happens when comparing theory or simulation with experiment, some details may not fit perfectly, or not fit at all. The final cause of these discrepancies is some assumption made in the design or simulation phase that is not completely true or, alternatively, the purposeful neglect of some phenomenon that was considered unimportant but then turned out to be relevant [3]. The examples discussed will be taken mainly from the field of Radio-Frequency MEMS (RF-MEMS) and will deal with RF-MEMS switching structures, because this is a field where electrically driven instabilities are not only very common but also the basis of the device functioning [4, 5].  
RF-MEMS technology rapidly emerged around twenty years ago as an enabling technology that yields small, low-weight, and high-performance components [6] to replace some of the bulky, expensive, and low-performing passives largely employed in wireless technologies [7, 8]. Applications include wireless personal communication systems, wireless local area networks (WLAN), satellite communications, and some automotive electronics, such as systems for adaptive cruise control or parking aid.  
RF-MEMS can be used for switching, RF frequency selection, tuning and matching in the RF front end in communication transceivers [9], and for switching in phase shifters used in automotive and satellite communications radar systems. They are technologically very attractive not only for their very small footprint but also due to their extremely low loss, high linearity, and virtually no power consumption [10].  
In spite of all these advantages, RF-MEMS are not widespread in the market yet. The reason for this delay can be mostly found in important reliability problems [11].  
A lot of research work has been done in the last few years to improve the reliability of these devices, some important results have been obtained, and most of the relevant failure mechanisms have been understood.  
RF-MEMS switches are the basic blocks for more complex switching circuits, and the switching mechanism is at the core of the reliability problem. An RF-MEMS switch is an electromechanical switch and features a moving microbeam that collapses when the applied bias reaches a threshold value. In this sense, its mechanical functioning is based on an electrical instability. The reliable control of this instability will be discussed in detail in the different sections of this paper, outlining the most common sources of deviation from the predicted behavior, together with the limits, and the consequences of these deviations and the countermeasures and the mitigating actions that the research work has developed in the last years.