Analysis and experiments of the thermal-optical performance for a kinematically mounted lens element.

In order to evaluate the thermal-optical performance of a kinematic mounting applied in lithographic projection lens, the optical surface figure and wavefront changes of the lens element under a certain thermal load are investigated with both experimental and numerical simulation methods. From the experimental and numerical results, the temperature on the edge of the lens element rises up to 22.51°C, and the center of lens is 5.3°C higher. As a result, this thermal nonuniformity leads to a 9.622 nm RMS change of the optical surface figure and 71.905 nm RMS change of the index inhomogeneity, consisting mainly of Z4, Z9, and Z16. Because of the radial flexibility of the supporting legs of the kinematic mounting, aberrations such as pri trefoil and sec trefoil are less than 2% of the total wavefront changes, and other nonaxisymmetric aberrations are negligible. The Zernike coefficient differences between experiments and simulation are less than 2 nm, which supports the correctness of our method. The kinematic mounting shows good thermal adaptability, and the method for evaluation of the thermal-optical characteristics is proved effective.