Static friction and surface roughness studies of surface micromachined electrostatic micromotors using an atomic force/friction force microscope

Abstract

A technique to measure the static friction forces (stiction) encountered in surface micromachined micromotors using a commercial atomic force microscope (AFM)/friction force microscope has been developed and is described. An AFM tip is pushed against a rotor arm of the micromotor so as to generate lateral deflection (torsion) of the tip, which is measured by the AFM. The maximum value of the lateral deflection obtained prior to rotor movement (rotation) is a measure of the static friction force of the micromotors. This technique was employed to study the effect of humidity and rest time on the static friction force of polysilicon motors, both unlubricated and lubricated using perfluoropolyether lubricants. Surface roughness parameters (rms, peak-to-valley distance, skewness, and kurtosis) and microscale friction properties of the various surfaces of the motor were measured. Dramatic differences between the roughness of the underside and top surfaces of the rotor and between the surface beneath the rotor and adjacent areas were observed. The mechanisms responsible for stiction in such devices are discussed. Lubrication methods to minimize friction problems are also presented.