Magnetic disturbance compensation for a reticle stage in a lithographic tool

The wafer stage and reticle stage in a lithographic tool, used to manufacture integrated circuits, operate at nanometer accuracy during a scanning motion. Magnetic interaction with the environment causes disturbance forces that result in a 5 nm moving-average filtered scanning error of the reticle stage.Stage motor magnet interaction with ferromagnetic material in the vicinity causes a position-dependent disturbance force, and eddy currents in electrically conductive metals cause a position-dependent damping force which is proportional with velocity.The total disturbance force can be estimated from control-loop signals, and by moving the stage in both directions both disturbance types can be distinguished from each other and recorded in compensation tables. Applying these tables as a feedforward reduces the scanning position error from 5 nm to 1.8 nm in a production reticle stage system.