Multi-position large tuning-range digitally tuneable capacitors embedded in 3D micromachined transmission lines

This paper reports for the first time on multi-position RF MEMS digitally tuneable capacitors with large tuning range which are integrated inside a coplanar transmission line and whose tuning is achieved by moving the sidewalls of the 3D micromachined transmission line, with the actuators being completely embedded and shielded inside the ground layer. Devices with symmetrical two and three-stage actuators have been fabricated in an SOI RF MEMS process. A tuning range of C_max/C_min=2.41 with a total of 7 discrete tuning steps from 44 to 106 fF was achieved for the three-stage tuneable capacitors. The symmetrical integration in the transmission line and a low parasitic inductance result in a high resonance frequency of 77 GHz. Devices with actuator designs of different mechanical stiffness, resulting in actuation voltages of 16 to 73 V, were fabricated and evaluated. The robustness of the actuator to high-power signals has been investigated by a nonlinear electromechanical model, which shows that self actuation occurs for high-stiffness designs (73 N/m) not below 50 dBm, and even very low-stiffness devices (9.5 N/m) do not self-actuate below 40 dBm.