Acceleration feedback in a lithographic tool

Acceleration feedback increases the mass of a system as experienced by disturbance forces. A high bandwidth is required to avoid phase lag in the position control loop. This article splits the acceleration controller in a forward and backward path to create the original process behavior for the position controller, removing the high-bandwidth requirement. Digital controller effects, incorporation of plant dynamics, and implications for setpoint feedforward are discussed. The method is implemented in the stage of a lithographic scanner operating at nanometer accuracy in semiconductor manufacturing. It is shown that the low-frequent disturbance rejection is considerably improved without impacting higher-frequency performance. --------------------------------------------------------------------------------