Outer-rotor polysilicon wobble micromotors

Abstract This paper reports the development of a new class of polysilicon surface-micromachined wobble micromotors in which the rotor outer radius is exposed for mechanical coupling and the first instance of a polysilicon wobble micromotor directly driving another mechanical component. These motors are fabricated in a three-mask process which results in a flange bearing. The rotor and stator are fabricated from heavily phosphorus-doped polysilicon, while the bearing is made of an insulating material such as polyimide or silicon-rich silicon nitride. Rotor diameters of 150 to 250 μm and rotor/stator thicknesses of 3 to 5 μm are typical. The 5 μm thick micromotors with silicon nitride bearings have operated smoothly and reproducibly in room air for more than two years after release. Minimum operating voltages are 14 V, while maximum rotor speeds are 125 rpm, which is limited by our power-supply switching speed. These micromotors have gear ratios over 300 and estimated motive torques on the order of 10 −9 N m for 100 V excitations, which is over two orders of magnitude larger than previously reported motive torques for polysilicon micromotors. Mechanical coupling is demonstrated using a 500 μm diameter outer-rotor micromotor with gear teeth driving another similar motor operated as a passive gear.