Multivariable feedback control in stage synchronization

Synchronization in lithographic wafer scanners involves a wafer-to-reticle stage controller. However, in view of the increasing wafer stage bandwidths the need for an additional reticle-to-wafer stage controller becomes more evident, i.e. a cross-coupling controller. By itself, this has severe implications for the stability and control design of the synchronized system, which becomes multivariable. In this work, an H8 controller is synthesized that can effectively deal with the synchronization problem while guaranteeing robust closed-loop stability. An application of the H8 design will be given through measurement results obtained from an industrial wafer scanner