Optimum design of MEMS/RF-MEMS: an iteration flow with the simulation phase

Design of RF-MEMS is an essential step in defining the features and characteristics of new MEMS/RF MEMS devices, prior to their manufacturing. However, the design phase of Microsystems can play an important role also for what concerns the path leading to the realization and prototyping of MEMS/RF-MEMS new concepts, able to satisfy the requested specifications and performance. As it is well known, the design optimization of novel MEMS and RF-MEMS devices is not an easy and straightforward task, because of the multi-physical behaviour characterizing them [1]. Regardless of the sensing or actuation function a certain MEMS device is supposed to realize, its behaviour always involves the coupling of the mechanical domain with physical magnitudes belonging to some other domains. For example, a capacitive MEMS accelerometer couples the mechanical domain (displacement of the proof mass proportional to the imposed acceleration) to the electrical domain (capacitance variation proportion to the proof mass displacement) [1]. The case of RF-MEMS is even more complex, as the coupling takes place between three different domains, namely, the mechanical and electrical for what concerns the actuation and controlling of the MEMS movable parts and membranes, and the electromagnetic one, concerning the filtering and conditioning of one or more RF signals operated by the Microsystems-based device or network. Of course, each physical domain involved brings in additional Degrees Of Freedom (DOFs) that have to be accounted for in the design phase, as well as increasing trade-offs between the device characteristics. For example, in designing an RF-MEMS ohmic switch a target could be reducing the pull-in voltage (i.e. the activation voltage) to make the device compatible with the typical bias levels of CMOS circuitry. Such a target can be obtained by acting on the design of the flexible suspensions, in such a way to reduce their effective elastic constant. However, a low actuation voltage means also a reduced contact pressure on the ohmic contacts when the switch is actuated, leading to a worse ohmic contact and to the subsequent increase of the RF signal attenuation, operated by the micro-relay (in the closed configuration). The just mentioned case demonstrates that the links and trade-offs between the characteristics of MEMS and RF-MEMS belonging to the involved physical domains are numerous and must be carefully identified and managed