Research On Ultra-precise Aspheric Surface Testing Based On Computer-generated Hologram

The extreme ultraviolet projection objective is a reflective optical system,which is limited by the reflectivity of the extreme ultraviolet multilayer film.The extreme ultraviolet projection objective must use the least reflective mirror to achieve the required optical imaging performance,and the use of aspherical elements is the only choice.At the same time,in order to achieve ultra-diffraction limit imaging performance,the surface shape error of aspherical elements is required to reach the order of deep sub-nanometers.With the increase of the aspheric element diameter and NA(Numerical aperture,NA)of the extreme ultraviolet lithography projection objective,the compensation mirror cannot meet the measurement accuracy requirements.Computer-Generated Hologram(CGH)has many advantages such as flexible design,small design residuals,simple structure,etc.,and can realize highprecision measurement of aspheric elements.Therefore,this paper uses ComputerGenerated Hologram as the zero compensator,and carries out the following research:1.Designing ultra-precision CGH.This part analysis the relationship between the test optical path structure parameters,distortion,stray light and CGH fringe density.Based on the relationship,a CGH design model was established,which greatly improved the design efficiency.2.CGH encoding method.Compared to other encode,this method can solve the contradiction between the encoding data volume and encoding precise of the equal-step encoding method.This method can dynamically select encoding step size according to the phase gradient,and it can simultaneously achieve the requirements of small amount of encode data and high encoding precise.3.Error analysis of aspheric testing and calibrating methods.This part analysis the substrate transmission wavefront error,etching error,radial distortion and the influence of the pressure and temperature fluctuation.The part gives the calibration methods of the substrate transmission wavefront error of CGH and builds a radial model to correct the imaging distortion of CGH precisely.4.The results comparing and experimental verification.A primary mirror of the extreme ultraviolet lithography two-mirror system is tested precisely using null lens and CGH separately.The precision of CGH is verified experimentally by comparing the two testing results.