Research Of Measuring Technology For Wavefront Aberration Of EUVL Objective System

Extreme ultraviolet lithography (EUVL) technique is the next generationlithography technology which is based on the traditional lithography, it has inheritedthe developing achievements of the present lithography with maximum extent. Inorder to achieve lithography performance and control critical dimention, thewavefront aberration (RMS) of lithography objective which is one of the coreelements of the lithography machine must be less than λ/20. In this case, the generalmeasuring method is difficult to meet the above requirements. Foreign countrieshave engaged in related research for many years, in order to satisfy the urgentdemand of high accuracy measurement of EUVL objective system wavefrontaberration in our country, this article researches the measuring technology forwavefront aberration of EUVL objective system, and achieves the high repeatabilitymeasurement for optical system wavefront aberration.This paper mainly researches the measuring technology of EUVL objectivesystem wavefront aberration, including study of phase extraction algorithm, errorsources analysis, study of phase unwrapping algorithm, study of calibratingalgorithm for wavefront reference source, optimization of project design, and experimental data processing. The main contents are concluded as follows:1,The phase extraction algorithm is studied, and the error sources which couldaffect the measuring accuracy of EUVL objective system wavefront aberration areanalyzed. Using window function method to design a series of phase extractionalgorithms, the designed13buckets phase-shifting algorithm and5bucketsphase-shifting algorithm are used to analyze the error sources, the qualified ranges ofthe performance parameters of every component and environmental errors are givenby the analysis. At last, all the errors are added to the interferograms to simulate, thetheoretical accuracy and wavefront repeatability are analyzed. Through the erroranalysis, it shows that the error sensibility of13buckets phase-shifting algorithm islower than5buckets phase-shifting algorithm, so the13buckets phase-shiftingalgorithm is selected to be the phase extraction algorithm of this paper.2,The phase unwrapping algorithm is studied. The method of region growing isadopted to unwrap the wrapped phase, it can automatically identify the boundary andhas good noise suppression ability. Finally the13buckets phase-shifting algorithmand the method of region growing are used to process the13frame interferograms,better result is obtained.3,The calibrating algorithm of wavefront reference source is researched. Thecalibrating algorithm of wavefront reference source is designed, and test process ofthe wavefront reference source is given. Finally the calibrating algorithm issimulated, the initial conditions of the simulation are given, the rotating tolerances ofthe wavefront reference source which meet the calibrating accuracy are given by thesimulation.4,The project for measuring wavefront aberration of EUVL objective systemwhich uses the double-fiber point diffraction interferometer is optimized, theexperimental system is established. The principle and advantages of fiberphase-shifting point diffraction interferometer are analyzed, and the key componentsare selected and analyzed, the performance parameters and partial measurementresults are given. Finally, according to the optimized project, the fiber phase-shifting point diffraction interferometer is built.5,The wavefront aberration of an optical system are measured, and thewavefront repeatability is analyzed. The13buckets phase-shifting algorithm and5buckets phase-shifting algorithm are used to extract the phase from13frameinterferograms, and region growing method is used for phase unwrapping, thenZernike fitting is used and the tilt and defocus are removed, the results of13bucketsphase-shifting algorithm and5buckets phase-shifting algorithm are approximate, so13buckets phase-shifting algorithm is proved to be correct. The wavefrontrepeatability of8groups of measurements which one group is obtained by the meanof64times of measurements is0.47nm, it's less than λ/1000.