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

The dielectric charging constitutes a major problem that still inhibits the commercial application of RF MEMS capacitive switches. The effect arises from the presence of the dielectric film (Fig.1a), which limits the displacement of the suspended electrode and determines the device pull-down state capacitance. Macroscopically, the dielectric charging is manifested through the shift (Fig.1b) (Rebeiz 2003, Wibbeler et al. 1998, Melle et al. 2003, Yuan et al. 2004) or/and narrowing (Czarnecki et al. 2006, Olszewski et al. 2008) of the pullin and pull-out voltages window thus leading to stiction hence the device failure. The first qualitative characterization of dielectric charging within capacitive membrane switches and the impact of high actuation voltage upon switch lifetime was presented by C. Goldsmith et al. (Goldsmith et al. 2001) who reported that the dependence of number of cycles to failure on the peak actuation voltage follows an exponential relationship. Particularly it was reported that the lifetime improves by an order of a decade for every 5 to 7 V decrease in applied voltage. The lifetime in these devices is measured in number of cycles to failure although experimental results have shown that this tests do not constitute an accurate figure of merit and the time the device spends in the actuated position before it fails is a much better specification to judge device reliability (Van Spengen et al. 2003). The aim to improve the reliability of capacitive switches led to the application of different characterization methods and structures such as the MIM (Metal-Insulator-Metal) capacitors that allowed to determine the charging and discharging times constants (Yuan et al. 2004, Lamhamdi 2008) as well as to monitor the various charging mechanisms (Papaioannou 2007a), since these devices marginally approximate the capacitive switches in the pull-down state. A method that approximates more precisely the charging process through asperities and surface roughness in MEMS and allows the monitoring of the discharging process is the Kelvin Probe Force Microscopy (Nonnenmacher 1991). This method has been recently employed for the investigation of the charging and discharging processes in capacitive switches (Herfst 2008, Belarni 2008). The charging of the dielectric film occurs independently of the actuation scheme and the ambient atmosphere (Czarnecki et al. 2006). Up to now the effect has been attributed to the charge injection during the pull-down state (Wibbeler et al. 1998, Melle et al. 2003, 14