Chkhalo N I, Malyshev I V, Pestov A E, Polkovnikov V N, Salashchenko N N, Toropov M N, Soloviev A A
Appl Opt. 2016 Jan 20;55(3):619-25. doi: 10.1364/AO.55.000619.
Problems in the application of a null lens for surface shape measurements of aspherical mirrors are discussed using the example of manufacturing an aspherical concave mirror for the beyond extreme ultraviolet nanolithographer. A method for allowing measurement of the surface shape of a sample under study and the aberration of a null lens simultaneously, and for evaluating measurement accuracy, is described. Using this method, we made a mirror with an aspheric surface of the 6th order (i.e., the maximum deviation from the best-fit sphere is 6.6 μm) with the parameters of the deviations from the designed surface PV=5.3 nm and RMS=0.8 nm. An approximation of the surface shape was carried out using Zernike polynomials {Z(n)(m)(r,φ),m+n≤36}. The physical limitations of this technique are analyzed. It is shown that for aspheric measurements to an Angstrom accuracy, one needs to have a null lens with errors of less than 1 nm. For accurate measurements, it is necessary to establish compliance with the coordinates on the sample and on the interferogram.
以制造用于极紫外纳米光刻的非球面凹面镜为例，讨论了在非球面镜表面形状测量中使用零透镜的应用问题。描述了一种同时测量待测样品表面形状和零透镜像差以及评估测量精度的方法。使用该方法，我们制造了一个具有六阶非球面表面（即与最佳拟合球面的最大偏差为6.6μm）的镜子，其与设计表面的偏差参数PV = 5.3 nm和RMS = 0.8 nm。使用泽尼克多项式{Z(n)(m)(r,φ),m + n≤36}对表面形状进行了近似。分析了该技术的物理局限性。结果表明，对于精度达到埃级的非球面测量，需要一个误差小于1 nm的零透镜。为了进行精确测量，有必要确保样品和干涉图上的坐标一致。
