No Magic Needed to Levitate Rotors

Edited by John R. Gyorki

Stephn Nichols
Manager, Magnetic Bearing Technology
Magmotor Co.
Worcester, Mass.

Process-equipment manufacturers continually look for faster, better, and less-expensive ways to manufacture semiconductor wafers. They also constantly integrate new technologies into chipmaking devices to improve yield and throughput. One of their biggest challenges is to protect process chambers from contamination-producing components.

Major offenders are ball-bearing systems in wafer-processing machines. Metal contact between mechanical components produce minute particles that eventually contaminate the wafers. Lubrication systems in vacuum chambers also emit particles, and their contaminating outgassing can force the line to shut down for unscheduled maintenance. Lost wafers and machine downtime from these problems quickly inflate processing costs.

Clean solutions
A solution is Magmotor’s new Integral Suspension and Motor (ISAM), a two-piece device that rotates wafers in a closed chamber without contaminating the environment. One component, a magnetically levitated, noncontacting, speed-controlled rotor is suspended inside the chamber and can lock in place for wafer transfer. The rotor is simply a ring with an open center. It handles larger diameter wafers, and works over wider operating temperature ranges at higher speeds than conventional motor control systems. The metal rotor, which can be coated for protection in caustic environments, does not contain any magnets or electromechanical elements.

The second component is the stator which mounts to the outside of the chamber. It modulates the magnetic field around the integral bearing that supports and rotates the rotor. The stator is confined to a ring structure outside the chamber and has no overhanging features that can prevent access to the rotor. All active components for the system are in the stator.

The stator is made as a single structure free of stresses and is not subjected to any motion. Routine maintenance is eliminated, and reliability is much higher than it is in conventional support systems.

Speed control, too
Signals from displacement sensors provide position feedback so the controller can regulate rotor float in the wafer chamber. The controller can regulate acceleration and speed to within 0.5%. Also, the acceleration profile can be shaped to reduce the impulsive momentum that is often transferred to the wafer. Using such schemes, the wafer and its carrier can reach high operating speeds without losing positive mutual contact.

But levitating and moving the rotor without mechanical contact with the stator is only one critical function of the control system. To facilitate wafer loading and unloading, the motor controller switches automatically from speed regulation to a position servoloop when receiving a command to stop rotation. While the present system comes to a locked position in any one of sixteen discrete positions, the next generation controller will be able to select an absolute position reference on the rotor to provide repeatable indexing to the same location.

Another advantage of a noncontacting rotor is that it allows technicians to quickly and easily remove and replace the wafer carrier for maintenance. Should a wafer break during processing, bringing the system back on line does not involve complicated disassembly procedures.