A tunable RF MEMS transformer on silicon

We present a tunable RF MEMS transformer fabricated on silicon based on the bimorph effect of two materials. The suspended coil is composed of an amorphous silicon and aluminum bilayer and due to the stress developed in the layers, outer turns reach hundreds of microns of vertical displacement. The mutual coupling of the transformer is tuned by applying a DC voltage to control the distance between the windings. By applying an actuation voltage of 1.25 V, the mutual inductance can be tuned by 24%. This device is fabricated in a sub-150 degrees C process opening up the possibility of post-CMOS integration of RF MEMS devices on silicon.