Deformable mirrors with thermo-mechanical actuators for extreme ultraviolet lithography: Design, realization and validation

In lithographic illumination systems, a nonuniform light distribution causes local deformations on the mirrors used. Active mirrors are a solution to correct these deformations by reshaping the surface. This paper presents the deformation of a mirror with thermo-mechanical actuators placed perpendicular to the surface. Two deformable mirrors are modeled, realized and validated: one with seven and one with 19 actuators. By placing the actuators on a thin back plate, the force loop is localized and therefore a lower actuator coupling is achieved. The thermo-mechanical actuators are free from mechanical hysteresis and therefore have a high position resolution with high reproducibility. Extensive Finite Element Analysis is done, to maximize actuator stroke and minimize input power. The mirrors are tested and validated with interferometer surface measurements and thermocouple temperature measurements. A mirror deflection of 0.68 nm/K is realized and no hysteresis is observed. Thermal step responses are fitted and both heating and cooling characteristic time constants are 2.5 s. The thermal actuator coupling from an energized actuator to its direct neighbor is 6.0%. The total actuator coupling is approximated around 10%, based on the good agreement between simulated and measured inter-actuator stroke.