Basic concepts of moving-sidewall tuneable capacitors for RF MEMS reconfigurable filters

This paper presents for the first time MEMS tuneable filters, where the reconfiguration of the filter is achieved by moving the sidewalk of a 3D micromachined transmission line. The sidewalls of the transmission line are moved by MEMS electrostatic actuators completely integrated into the ground layers of a thick-film coplanar waveguide. Multi-step actuators have been utilized for achieving a tuning range of up to 2.41 for the tuneable capacitance elements. Two different 3D transmission line metallization schemes and two different concepts for tuning the capacitive load have been investigated for constructing filters based on this novel tuning mechanism. Measurements of fabricated devices have revealed that 3D transmission lines with top metallization only, and capacitors avoiding the routing of the RF signal over mechanical-spring meanders achieve the best results. A successfully implemented filter based on this configuration is shown, with a passband insertion and return loss of 5 and 12 dB, respectively, at a center frequency of 20 GHz. Various MEMS actuators designs with spring constants from 3.5 to 95 N/m have been implemented, resulting in actuation voltages of 15.4 to 73 V. The self-actuation power simulated in a non-linear Agilent Advanced Design System model has been estimated to 40 and 50 dBm for the soft and the stiff spring actuators, respectively. The fabrication is done by a single-mask silicon-on-insulator RF MEMS process.