Extended tuning range RF MEMS variable capacitors using electrostatic and electrothermal actuators

In recent years, much efforts have been dedicated to the development of variable RF capacitors, a device which can take a clear benefit of MEMS technology. The most widely designed variable MEMS capacitors have an electrostatic force as actuation principle. This implies a limitation in the controlled tuning range due to the pull-in effect. In this paper we study and design three solutions in order to improve the controlled tuning range of RF MEMS variable capacitors, based both on electrostatic and electrothermal actuation principles and manufactured with the PolyMUMPSTM process. Measurements of a conventional electrostatically-actuated variable capacitor are compared to measurements of three variable capacitors with extended tuning range, based on the two above mentioned actuation principles, with the main purpose of improving the pull-in limitation and assessing and comparing their behaviour and, especially, their tuning ranges. The most important advantages and disadvantages of extendended tuning range capacitors are identified and are here reported and empirically characterized, focusing in device repeatability, understood as capacity deviation due to large capacity sensitivity to tuning voltage, for small gaps between electrodes, which arises from the strongly non-linear behaviour of the capacity vs the gap between electrodes.