Physics of Charging in Dielectrics and Reliability of Capacitive RF-MEMS Switches

Abstract

The present chapter attempts to provide a better insight on the dielectric charging and reliability of RF-MEMS capacitive switches. It has been shown that the dielectric material properties play a key issue role in the dielectric charging process. In ionic materials the ionic/dipolar polarization as well as the space charge polarization is the dominant charging mechanisms. In the case of the well established stoichiometric Si3N4 and SiO2 dielectric materials the covalent bonds prevent the dipolar polarization. On the other hand, the low temperature deposition conditions, which are suitable for MEMS capacitive switches, lead to materials that are Si-rich and the significant deviation from stoichiometry gives rise to the formation of Si nanoclusters which allow the formation of defects that exhibit dipole properties, hence giving rise to dipolar polarization in addition to the space charge one. Presently, the assessment of dielectric charging is manly based on cycling the devices between the pull-down to pull-up states. Additional assessment is performed through Metal-Insulator-Metal capacitors but these devices can provide information only on the bulk electrical properties of the dielectric film. These devices cannot be considered similar to MEMS switches since they luck free surface which interacts with ambient and accumulate surface charges. Moreover, the top electrode is in excellent contact with the top surface a fact that is not encountered in MEMS switches. A reasonable equivalent to the contacting of bridge with the dielectric film is achieved with the aid of Kelvin Probe Force Microscope (KPFM) since the contacting and charge injecting tip can reasonably simulate the surface roughness and asperities of the metal bridge. This technique has been successfully used to simulate the decay of injected charges. Moreover, it allowed the recording of charge injected in a stressed switch. The results were excellent and the preliminary evaluation of the charge decay allowed the determination of the diffusion coefficient of charges in silicon nitride. Regarding the lateral diffusion, this is still an open issue since there are no concrete results supporting such an effect. For this reason the injected charges in capacitive switches are still considered to be directly collected by the rigid electrode. Temperature dependence of dielectric charging constitutes a key issue assessment tool. This is because the observed relaxations in disordered materials such as the dielectrics used in MEMS can be distributed over more than 11 to 12 decades. This broad distribution, as