Enhanced Iterative Learning Control with Applications to A Wafer Scanner System

This thesis addresses the improvement of wafer scanner technology from the controls aspect. In particular, the thesis proposes to enhance basic Iterative Learning Control (ILC) in several ways. ILC is a feedforward control strategy used to improve the performance of a system that performs a repeated task by considering the error from previous iterations of the process. The existence of non-repeating events, however, degrades the performance of ILC. Furthermore, the selection of the Q-filter and learning function in the iterative control law also limits the performance of ILC. To relax these limitations, we first introduce a disturbance observer into the learning scheme. As a result, we are able to reduce the effect of non-repetitive disturbances on the ILC scheme. The combination of ILC and DOB information when performing one task can also be used to provide valuable information for selecting initial ILC effort for a different task. By doing so, we can improve the convergence rate of the ILC algorithm for the new task. Studies performed in this thesis will show that by properly selecting the initial ILC effort, one can reduce the number of iterations before ILC achieves high tracking accuracy.In addition to the ILC effort, we also apply a pre-designed optimal feedforward control input to minimize the initial tracking error. An optimization strategy is presented to obtain this optimal feedforward input as well as the optimal feedback controller for integration with the ILC scheme. By using this optimal feedback-feedforward controller combination, which replaces the standard PID feedback controller, it becomes possible to utilize a simple P-type ILC algorithm without compromising performance. This thesis also investigates the synchronization problem to ensure small alignment mismatch between the wafer stage and reticle stage. The challenge is to meet tracking requirements of each individual stage as well as maintaining the relative positioning between the two stages in the presence of cross-coupling dynamics. Therefore, to achieve the ultrahigh precision motion control requirements demanded for such scanners, a synchronization ILC algorithm is proposed and designed to reduce the synchronization error while minimizes the individual tracking error.

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