A mechanical stage and electronic drive servos have been built which provide six degrees of freedom of motion in positioning. The stage is used in conjunction with a mask-wafer alignment sensing system to align a semiconductor wafer with to 0.1 micrometer tolerance in a masked ion beam exposure system. Because the exposure system utilizes a step and repeat mode of operation, the stage was designed to operate at high speed to achieve high wafer throughput. A backlash free capstan drive system is used on the X, Y, and Theta servos to achieve the high precision, high speed performance. The wafer to be exposed is mounted on top of an X-Y stage, which is, in turn, mounted on a rotary Theta stage, which is then mounted on a platform. The platform is supported from a base by three deformable diaphragms which allow limited vertical Z or tilt motion of the entire stage assembly. In step and repeat operation, the X and Y motions are relatively large from the chip-to-chip, while the Theta and Z motions are very small. Mounting the X-Y stage on the top of the stage assembly provides the design with the lowest mass to be accelerated in stepping, and has the additional advantage that the algorithms for providing mask-wafer alignment error signals remain the same no matter where the chip being aligned is located on the wafer. Dedicated microprocessors for each of the X,Y,and Theta axes, incorporating digital error integration, provide the required precision of motion.
Prepared in collaboration with Hughes Research Labs., Malibu, CA.