A microscopic rotating motor with corona drive
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A micro corona motor, a unique type of electrostatic motor, was studied. Sharp stator electrodes of this motor ionize air molecules and ionized charges transfer onto the rotor surface, resulting in rotating rotor motions by Coulomb forces. Important design parameters including number of corona electrodes, air gap spacing, and electrode tilt angle were investigated using an electrostatic field simulator. From estimated charge distributions on the stator and rotor, force and torque were estimated. This showed that the spatial phase difference between the charge distributions on the stator and rotor influences the torque-speed curve of the motor. For good performance, the stator's electrodes should be wide (axial) and have sharp tips. Therefore, X-ray lithography was adopted for precise, high aspect ratio characteristics. To avoid the fabrication difficulty of a membrane Xray mask, a built-on X-ray mask (conformal mask) technique was employed with negative toned SU-8 photoresist. SU-8 features X-ray fabrication compatibility, X-ray transparency and a large range of thickness. This technique may be suitable for fast fabrication of prototypes or very tall structures, which can be largely affected by printing gaps. Fabricated nickel structures showed good quality pattern transfer from the SU-8 pattern and smooth sidewalls. Speed and torque of a fabricated micro motor was measured. High rotating speed (> 410 RPM) was obtained with applied DC voltage of several hundreds. Using the spin-down method, the torque was estimated as order of 10 nNm. For over 20 million cycles, the motor showed good stability with no additional control action.