Electric micromotors on silicon

Abstract

A review of micromotor technology will be presented and micromotor design, fabrication, and operational characteristics will be discussed. Micromotors are submillimeter actuators capable of unrestrained motion in at least one degree of freedom. The micromotors described in this presentation are actuated by attractive electrostatic forces. For micromotor fabrication, silicon surface micromachining is used to create freely moving parts held to the surface of the silicon wafer with bearings. In this process, heavily phosphorus-doped polysilicon is used for the structural parts, deposited silicon dioxide is used for the scarificial layers, and silicon-rich silicon nitride is used for electrical isolation. Layers of the structural and sacrificial materials are deposited and patterned in a manner that the structural polysilicon parts (e.g., the rotor) are appropriately encapsulated by the sacrificial oxide layers. At the final stage of fabrication, the sacrificial oxide is selectively removed in hydrofluoric acid to free the micromotor structural parts. Operational rotary, variable-capacitance, salient-pole, and harmonic (or wobble) side-drive micromotors have been fabricated with two different bearing designs and typical diameters of 100- to 130-μm, rotor/stator gaps of 1.5 μm, and a rotor thickness of 2.2 μm. Electrical operation of the salient-pole and harmonic side-drive micromotors have been demonstrated in a number of gaseous environments including room air, oxygen, argon, and nitrogen, and in insulating liquids such as silicone lubricating oil and deionized water. Minimum operating voltages differ with the environment but can be as low as 30 V. Extended operation (i.e., beyond 100 h) of the micromotors in gaseous environments and in silicone lubricating oil has been demonstrated. Operational speeds of up to 15 000 and 1000 rpm have been achieved in gaseous environments for the salient-pole and harmonic side-drive micromotors, respectively. In silicone lubricating oil, the maximum operation speed has been below 125 rpm for both micromotor types. [The presentation reflects the results of a collaborative micromotor research program with Professor J. H. Lang and Professor S. D. Senturia of MIT. This research program is funded by the National Science Foundation.]