Research On Absolute Testing Of The Optical Surface Figure

Higher and higher level of optical testing is required with the rapid developmentof society, especially in the lithographic objective lens system. It requires the surfacefigure to nanometer RMS even sub-nanometer RMS. One of the key technologies isthe absolute surface figure testing method. The requirement of the surface in EUVlithography objective reaches0.2nm RMS, which is a great challenge for opticalmetrology. In the widely used Fizeau and Twyman-Green interferometers, thereshould be a reference surface acting as a standard in the testing. But the normalquality of the reference surface of commercial transmission flat or sphere is aboutλ/20PV (about6nm RMS).So the absolute calibration of the reference surface isalways necessary for sub-nanometer metrology. Several techniques have beendeveloped to obtain the absolute surface. Absolute testing methods for sphericalsurfaces are mainly focused on the two-sphere method and ball averaging method.For plano surfaces, three-flat method and its extensions are widely used. This articlepresents a novel calibration method: shift-rotation method using Zernikepolynomials. Uncertainty of the absolute testing method, including shift-rotationmethod, two-sphere method is analyzed. The uncertainty of shift-rotation method isanalyzed detailed; the main uncertainty source of two-sphere method: the centerdetermination of the cat s-eye position is analyzed theoretically. Next step of work isto decrease the uncertainty of the shift-rotation method and study a new stable andintuitionistic evaluation method of the surface figure.