Performance-Based Active Wafer Clamp Design for Wafer Heating Effects in EUV Lithography

Abstract

To improve overlay and focus errors in lithographic applications, an active wafer clamp (AWC) concept is proposed. Using this concept, mechanical actuators deform the wafer to compensate thermally induced deformations. In [1] a localized feedforward (FF) control algorithm for the AWC concept was introduced. In this paper we aim to improve this algorithm and discuss practical implications for the design of an AWC concept, motivated by the optimal performance that can be achieved using the FF control scheme. This leads to a design, where the actuators are placed below the clamping electrode, and 3-degree-of-freedom (DOF) actuators can be reduced to 2-DOF actuators with negligible performance loss. Considering the fact that the AWC concept contains tens of thousands of actuators, this leads to a significant reduction. Finally, other methods are presented to further reduce the computational complexity to control the AWC.