Design, Fabrication, and Characterization of a Rotary Variable-Capacitance Micromotor Supported on Microball Bearings

Abstract

The design, fabrication, and characterization of a rotary micromotor supported on microball bearings are reported in this dissertation. This is the first demonstration of a rotary micromachine with a robust mechanical support provided by microball-bearing technology. One key challenge in the realization of a reliable micromachine, which is successfully addressed in this work, is the development of a bearing that would result in high stability, low friction, and high resistance to wear. A six-phase, rotary, bottom-drive, variable-capacitance micromotor is designed and simulated using the finite element method. The geometry of the micromotor is optimized based on the simulation results. The development of the rotary machine is based on studies of fabrication and testing of linear micromotors. The stator and rotor are fabricated separately on silicon substrates and assembled with the stainless steel microballs. Three layers of low-k benzocyclobutene (BCB) polymer, two layers of gold, and a silicon microball housing are fabricated on the stator. The BCB dielectric film, compared to conventional silicon dioxide insulating films, reduces the parasitic capacitance between electrodes and the stator substrate. The microball housing and salient structures (poles) are etched in the rotor and are coated with a silicon carbide film to reduce friction. A characterization methodology is developed to measure and extract the angular displacement, velocity, acceleration, torque, mechanical power, coefficient of friction, and frictional force through non-contact techniques. A top angular velocity of 517 rpm corresponding to the linear tip velocity of 324 mm/s is measured. This is 44 times higher than the velocity achieved for linear micromotors supported on microball bearings. Measurement of the transient response of the rotor indicated that the torque is 5.620.5 micro N-m which is comparable to finite element simulation results predicting 6.75 micro N-m. Such a robust rotary micromotor can be used in developing micropumps which are highly demanded microsystems for fuel delivery, drug delivery, cooling, and vacuum applications. Micromotors can also be employed in micro scale surgery, assembly, propulsion, and actuation.