High speed precision X‐Y stage

A unique X‐Y stage has been developed for use in an electron‐beam lithography system. It is designed to accommodate wafer sizes up to 6 in. The stage uses conventional linear ways and ball bearings, but incorporates a unique capstan/swinging drive bar design to couple the servomotors to the X and Y stage elements. Major components of the stage are made from titanium to achieve maximum stiffness to weight ratio. The capstan drive gives very rigid coupling between the servomotor and the stage. This drive stiffness permits rapid and precise positioning of the stage by the servosystem. The rotary capstan uses a ferrofluidic seal, permitting the servomotors to be outside the vacuum chamber without requiring bellows seals. A prototype of the stage has extremely low vibration levels, and can slew at velocities up to 10 cm per s. It can step 2 mm and settle to 10 μm in 52 ms, and achieve submicron position accuracy in 70 ms. Maximum acceleration capability is 0.8 g. Stage position is sensed by a laser interferometer and associated pulse‐counting circuitry. A microprocessor reads the position counters every 0.5 ms. The motor drive currents are computed from feedbacks proportional to position error and observed velocity, and from feedforwards proportional to desired velocity and acceleration. Stiffness at low frequency is enhanced by feedback derived from the integral of position error. Low harmonic excitation of the system in which the stage resides is guaranteed by smoothly varying preprogrammed trajectories derived by triple integration of jerk (acceleration rate) commands. A powerful high‐level control strategy has been developed in which sequences of traversal time and traversal distance of contiguous segments are converted to appropriate jerk commands for smooth continuous motion. The strategy includes, as an integral part, constraints for maximum velocity and acceleration. Trajectory commands are interpreted by the same microprocessor which computes the servomotor currents. Desired position, velocity, and acceleration are updated by the microprocessor every 0.5 ms in accord with the sequence of jerk commands provided by the control computer, in synchronization with pattern generator operation.