Control of thermo-mechanical wafer deformations in EUV lithography using an active wafer clamp

This paper deals with the modeling and control of thermo-mechanical deformations of a wafer, resulting from absorbed exposure power, in EUV lithography. To achieve correction of the induced deformations, an active wafer clamp concept is proposed. In this concept, between 1,000 and 30,000 actuators are incorporated into the electrostatic wafer clamp, such that forces can be applied directly onto the wafer. Due to dimensionality of the control problem, a distributed feedforward control approach is proposed which acts locally around the exposure slit, and reduces the number of design inputs by approximately 80%. The results obtained with this controller show an average reduction of a factor 10, of the thermo-mechanical deformations at the edge of the wafer as well as more towards the center of the wafer.