Motion Control of Wafer Scanners in Lithography Systems: From Setpoint Generation to Multistage Coordination

Accurate pattern transfer, coupled with stringent requirements for throughput and quality in lithography systems, necessitates the wafer scanner to execute an aggressive motion with the tracking error in terms of nanometers. This underscores the need for advanced control technologies. The aim of this article is to identify motion control issues in wafer scanners taking the accuracy requirement and the mechatronic dynamics into account and then investigate the control methods for plants with diverse dynamics. Motion control aspects that will be discussed include: 1) setpoint generator that creates smooth motion profiles through trajectory planning and input shaping; 2) multi-degree of freedom (DOF) decoupling control with static and dynamic design; 3) single-DOF motion control utilizing the combination of linear/nonlinear feedback and model-/data-based feedforward; and 4) multistage coordination control for the synchronized motion between wafer stage (WS) and reticle stage (RS), short-stroke module (SSM), and long-stroke module (LSM), as well as LSM and balance mass (BM). The control challenges associated with these aspects mainly involve: 1) crosstalk behaviors among motion DOFs; 2) flexible modes in the mechatronic dynamics; 3) model uncertainties; 4) stochastic and nonstochastic disturbances/noise; and 5) inherent design tradeoffs such as Bode's sensitivity integral. Both long-known and next-generation motion control methods are investigated and discussed with appropriate insightful comments.