A low-loss MEMS tunable capacitor with movable dielectric

This paper reports a MEMS tunable capacitor with a new actuation principle. The new design adopts electrostatic actuation of an electrically floating movable dielectric. This enables us to achieve a high Q factor by eliminating the loss associated with springs in the RF signal path. Also, the design can achieve a high tuning range, by using additional actuation combs and thus eliminating the pull-in effect. Since no DC bias is applied on the plates of variable capacitor, the parallel plates 1/3 gap limitation does not apply. The designed devices were fabricated in a SOI MEMS process, with a 25 µm thick device layer and minimum gap of 2 µm. Measurement results show that the tunable capacitor has a 135 fF initial capacitance with a tuning range of 367% and a Q factor of 56 at 1GHz by bidirectional actuation.