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18 March 2016 Multi-mirror adaptive optics for control of thermally induced aberrations in extreme ultraviolet lithography
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The imaging quality of the projection optics of an extreme ultraviolet lithography scanner degrades under the influence of thermally induced deformations of its mirrors. Wavefronts of different reticle points encounter different parts of the deformed mirrors, resulting in a field dependent wavefront error. This paper presents how ideas from multi-conjugate adaptive optics can be used to reduce these thermally induced aberrations. To this end a generic deformable mirror model is implemented. Linear actuator sensitivities are derived directly, based on nominal ray locations and directions, enabling fast prototyping. An integrated opto-thermo-mechanical mirror heating model is used to determine the evolution of thermally induced abberations over time. This transient simulation is used to analyze four different adaptive optics configurations and two different control algorithms. It is shown that by employing the multi-objective goal-attainment method, it is possible to improve the optical performance significantly when compared to minimizing the ℓ2-norm of the total residual wavefront error vector.
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Michel Habets, Joni Scholten, Siep Weiland, and Wim Coene "Multi-mirror adaptive optics for control of thermally induced aberrations in extreme ultraviolet lithography", Proc. SPIE 9776, Extreme Ultraviolet (EUV) Lithography VII, 97762D (18 March 2016);

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