High-Precision Planar Magnetic Levitation



These pictures show our linear-motor-levitated stage for positioning wafers for photolithography. This stage was developed by Dr. Won-jong Kim in his Doctoral thesis under the supervision of Prof. David L. Trumper of the MIT Department of Mechanical Engineering. The stage uses the novel levitation linear motors developed in Dr. Kim's thesis to both suspend and position a 5 kg platen in three translational (x,y,z) and three rotational (theta-x, theta-y, theta-z) degrees of freedom, with nanometer resolution. Such stages will be of increasing utility in advanced semiconductor production systems such as wafer steppers which require nanonmeter-resolution positioning in six degrees of freedom.



The first picture shows the stage sitting on its four linear motors. In the background is the laser interferometer which is used for position measurement for motions in the X-Y plane. This system has nanometer-level resolution. Motions out of the plane (Z) are measured with three capacitance gages which are located under the stage.

The stage is made of aluminum skins on an aluminum honeycomb core. A set of four magnet arrays are attached to the bottom of the stage. The triangular object on top of the stage is a mirror which reflects the laser beams to form the moving target in the interferomter.

When active, the stage is levitated 300 micrometers above the rest position via the four linear motors. Each linear motor is capable of providing a levitation force in the vertial direction, and a translational force in the X or Y directions. Two of the motors are oriented in the X-direction and two motors are oriented in the Y-direction. By coordinating the four motors, each with two independent forces, the six stage degrees of freedom can be controlled in a redundant fashion. The control calculations are carried out on a digital signal processing system at a 4 kHz update rate.



The second photo shows the four motor coil sets with the stage removed. In this photo you can see that the coils of two of the motors are oriented in X (right-left) and the coils of two of the motors are oriented in Y (front-back).