Development of a novel magnetically levitated surface motor

To realize high speed and high precision positioning for photolithography, a novel scheme of six degree of freedom magnetically levitated precision positioning stage is designed, where 12 air core electromagnetic coils and 36 permanent magnets are employed as the stator and mover respectively, and the levitation and linear drive parts of the stage are separated, thus the coupling between them is minimized and the nonlinearity due to the material ferromagnetism is avoided. Based on Lorentz law, some parameters, including permanent magnets length, currents and levitation height, which may affect the stability are analyzed to determine the equilibrium position. With the control of voltage, the equilibrium position between stator and mover can be controlled with inherently infinite resolutions. A dynamical model was modeled as a third-order system with parameter variation-stiffness and damping vary with respect to the z-directional displacement. Therefore, a simple robust control algorithm based on the mixed sensitivity control is applied to improve the control performance. With the aid of robust control, this system provides good performance. This validates the design of the system and demonstrates the feasibility of the developed controller. The stage is especially suitable for semiconductor photolithography requiring ultra precision positioning and super cleanness.