Performance impact of monolayer coating of polysilicon micromotors

This paper reports the impact on performance of flange-bearing polysilicon micromotors for different self-assembled monolayer coatings on the surface of released motors. Micromotors coated with Octadecyltrichlorosilane (OTS) show a stable rotor speed and minimum operating voltage during a nine month testing period. The experiments on gear ratio as a function of wobble cycles indicate that wear in a bearing without OTS coating is significant and results in changes in the gear ratio from the start of micromotor operation by as much as 40%, while the change of gear ratio is within 4% for near 80 million wobble cycles over a nine month testing period for motors with OTS coating. For motors coated with (3,3,3-trifluoropropyl) trichlorosilane (TFP), no stiction and no significant change of the gear ratio are observed for the testing duration. However, the study of gear ratio as a function of wobble cycles shows that the rotor speed fluctuates in the beginning and then stabilizes for wobble micromotors coated with TFP. OTS coating is found to decrease the flange frictional force/torque by a factor of about 1.5. This net reduction of the flange friction force/torque comes about from the combined action of increasing the frictional coefficient from 0.36 to 0.55 and decreasing the normal contact force associated with the rotor/flange contact friction from near 0.8 μN to near 0.3 μN.