Mode switching control in stages with synchronized motion

During the chuck swap process of a wafer scanner, i.e. the exchange of the measured chuck with the exposed chuck, a control mode switch occurs between two measurement systems. As a result of this switch, unwanted transients occur in the wafer stage system: the long-stroke (LoS) and the short-stroke (SS) wafer stage modules. By means of initial value compensation, an impulsive force is used to counteract these transients. But in view of actuator limitations, this appears insufficient and the following solution is proposed. Upon occurrence of a mode switch, the setpoint is adjusted with a third-order profile that limits the required actuator forces by limiting the jerk levels. The profile is scaled with the velocity difference measured over the mode switch. As a result, a continuous and smooth transition of the position signals is obtained. In addition, the SS feedforward controller is replaced by a feedback term in the multivariable and synchronized wafer stage system. This removes the interaction term from the LoS to the SS module, which decouples the synchronization performance from mode switching, and which renders the control design multi-loop SISO.