Process of opto-mechanical design and assembly for reflective mirror subsystem of lithographic projection lens

Considering the system performance of the projection lens, not only surface quality of the optics shall be concerned, misalignment between each optics and the wavefront distortion contributed by the mounting stress and gravity are also the factors degraded the optical performance. This article introduces the opto-mechanical design and stress-free assembly process of the reflective mirror subsystem with 300 mm in outer diameter of an I-line lithographic projection lens. The flexure with mounting position pass through the center gravity of the mirror can be adopted as supporting mechanism to prevent the gravity distortion. The distortion due to temperature difference can be avoided by adopting CLERACREAM® -Z glass ceramic and INVAR for material of reflective mirror and supporting flexure respectively. The adjustment mechanism of the mirror subsystem integrates the concepts of Kinematic and exact constraint to provide six degrees of freedom (6DoF) of posture adjustment of the mirror. Furthermore, the assembly process of the flexure which minimizes the mounting stress on the mirror is presented. In the end of this article, interferometric performance test of the reflective mirror after opto-mechanical assembly compared with the measurement result in manufacturing stage is also presented. With the proposed opto-mechanical design and stress-free mounting process of the mirror, the surface distortion contributed by the amount of mounting stress and gravity effect is less than P-V 0.02 wave @632.8 nm.