Principles in design and microfabrication of variable‐capacitance side‐drive motors

This paper presents a detailed discussion of the critical issues it the design and fabrication of polysilicon, rotary, variable‐capacitance, side‐drive, electric micromotors. Three different side‐drive motor architectures with stator pole number to rotor pole number ratios of 3:1; 3:2, and 2:1 are considered. For each architecture, output torque characteristics of typical microfabricated motors are simulated using two‐dimensional finite‐element solutions in the plane of the substrate. The 3:2 design is shown to provide superior torque coverage with higher minimum torque values as compared to the other two designs. An examination of the contribution of the axial fringing fields shows that, for typical micromotors, the rotor–stator capacitance is more directly a function of the rotor–stator thickness and not of the vertical rotor–stator pole‐face overlap. Furthermore, since the rotor–stator capacitance is not very sensitive to a vertical offset between the rotor and the stator, electric forces tending to vertically align the rotor to the stator are significantly smaller than would be predicted from a simple parallel‐plate capacitance calculation. A standard and a localized oxidation of silicon (LOCOS)‐based side‐drive micromotor fabrication process are described. The standard process is used as a case study to provide a detailed discussion of practical issues that need to be considered in the development of a polysilicon surface‐micromachined motor fabrication process. Specific motor design examples are described and a brief history of our experimental findings is presented. Typical 3:2 micromotors have been operated with bipolar excitations as low as 37 V across 1.5 μm gaps and at speeds as high as 15 000 rpm.