Design, fabrication, and characterization of a rotary micromotor supported on microball bearings

We report the design, fabrication, and characterization of a rotary micromotor supported on microball bearings. This is the first demonstration of a rotary micromachine with a robust mechanical support provided by microball-bearing technology. A six-phase bottom-drive variable-capacitance mi cromotor (phi = 14 mm) is designed and simulated using the finite-element (FE) method. The stator and the rotor are fabricated separately on silicon substrates and assembled with the microballs. Three layers of low-k benzocyclobutene polymer, two layers of gold, and a silicon microball housing are fabricated on the stator. Microball housing and salient structures (poles) are etched in the rotor and are coated with a silicon carbide film that reduces the friction without which the operation was not possible. A top angular velocity of 517 r/min, corresponding to the linear tip velocity of 324 mm/s, is measured at +/- 150-V and 800-Hz excitation. This is 44 times higher than the velocity previously demonstrated for linear micromotors supported on the microball bearings. A noncontact method is developed to extract the torque and the bearing coefficient of friction through dynamic response measurements. The torque is indirectly measured to be -5.62 +/- 0.5 mu N.m at +/- 150-V excitation which is comparable with the FE simulation results predicting -6.75 mu N.m. The maximum output mechanical power at +/- 150 V and 517 r/min was calculated to be 307 mu W. The bearing coefficient of friction is measured to be 0.02 +/- 0.002 which is in good agreement with the previously reported values. The rotary micromotor developed in this paper is a platform technology for centrifugal micropumps used for fuel-delivery and cooling applications.