| Precise optical systems are widely used in aerospace,high-end equipment manufacturing and other high-precision fields,and are of great significance to China's scientific and technical level and the comprehensive national power development.The optical component is the basic unit of optical systems,and its fabrication quality is the main factor of restricting the system performance.Therefore,requirements of high-accuracy and high-precision measurements of optical surfaces are becoming more and more strict.In the case of the extreme ultraviolet lithography machine,the surface shape accuracy of a single optical component in the projection lithography objective is required to be in the sub-nanometer range.However,the ZYGO interferometer,which is regarded as the current industry testing standard,can only reach the measurement accuracy of ?/40(?=632.8nm._Therefore,based on a near-ideal spherical reference wavefront emerging from a microhole by diffraction effect,the point diffraction interferometry(PDI)has been developed.The PDI breaks the limitations of fabrication quality of standard reference mirrors on the system measurement accuracy in the traditional interferometry.Consequently,it is theoretically possible to achieve sub-nanometer accuracy for testing surface.However,there are still some problems in the implementation of this technique,such as low phase retrieval accuracy,insufficient light intensity contrasts of low-reflectivity mirrors,and non-common path interference imaging errors,which affect the realization of its theoretical accuracy.In terms of above problems,this thesis has conducted researches on the key technique optimization and error correction of high-accuracy pinhole point diffraction interferometry.The principle of a PDI system is constructed.Key structural parameters of pinholes and nanowire waveguides are studied,analyzed and optimized to generate high-quality point diffraction spherical waves.The system simulation model based on self-made ray-tracing programs is proposed,which lays a theoretical foundation for the following researches on the system optimization.The Zernike wavefront fitting technique based on the Gram-Schmidt orthogonalization is discussed to achieve the phase reconstruction of discrete sampling points.The phase shift error introduced by inaccuracy of phase shifters and environmental disturbances would lead to a decrease in the phase reconstruction accuracy.To solve this problem,a self-correction phase retrieval algorithm based on the linear correlation is proposed.The optimal linear combination coefficients of difference intensity maps are searched for by solving correlation coefficients.Then the phase shift and the phase under test can be obtained on the basis of them.The algorithm does not need phase shifts as known parameters,so it reduces requirements for the phase shifter performance and the environmental stability.Compared with other self-correction algorithms,this algorithm has no complicated mathematical transformation and iteration during the whole phase retrieval process,and therefore it can quickly and accurately achieve the high-accuracy phase reconstruction.A polarization PDI contrast enhancement technique,adopting a specially designed even aspheric quarter-wave plate(EAQWP)to transform the beam polarization in the test path,is proposed to improve the insufficient light intensity contrast for testing low-reflectivity and high-numerical-aperture spherical surfaces.Conventional wave plates,which are usually used in parallel light paths,would introduce distortion aberrations into high-numerical-aperture spherical wave light paths.Therefore,a plano-convex lens base wave plate with convex even aspheric surface is optimized,and detailed wave aberration analysis is carried out.A difference restoration model is built to calibrate wave aberrations introduced by pose errors of the EAQWP.The calibrated EAQWP is fixed at the designed ideal position,and there is no need to repeat the calibration with the change of the numerical aperture of the spherical surface under test,which facilitates the system alignment and avoids introducing random errors in practical optical shop testing.For aberrations of the imaging lens in non-common path interference,a free-lens point diffraction interferometry(FLPDI)technique and an reverse diffraction wavefront reconstruction algorithm are proposed.After the imaging lens being removed,the relationship between the spherical surface under test and the imaging plane of CCD is the diffraction imaging instead of the conjugate imaging.The interferogram with obvious diffraction rings can no longer reflect the true distribution of the spherical surface under test.Therefore,a diffraction tracing model based on a virtual lens is built to theoretically derive the diffraction relationship.Then the complex amplitude of the spherical surface under test can be obtained accurately through the iterative reverse diffraction from the interferogram on CCD.As a result,the full-aperture surface shape without diffraction effects is reconstructed.Utilizing a virtual lens to remove the spherical wave phase factor,this technique directly adopts the angle spectrum method of plane waves,which solves the sampling difficulty in the diffraction propagation of spherical waves.A calibration method of system principle errors based on the difference Zernike coefficient vector is proposed,and the CCD tilt error is theoretically analyzed and controlled.Experimental results verify the validity and accuracy of the proposed system and techniques.In the first experiment,phase demodulations are carried out for the interferograms of straight fringes,circular fringes and complex fringes,and the residual errors RMS are 0.0296rad,0.0617rad and 0.0314rad,respectively,which verifies the accuracy of the LCA algorithm.In the next experiment,pose errors of the designed wave plate are calibrated by the difference restoration model.When the longitudinal deviation,lateral deviation and tilt are controlled within 0.017 mm,0.004 mm and 0.660',respectively,the residual error PV can be better than 0.001 ?.Then,a spherical surface with a reflectivity of 0.04 and a numerical aperture of 0.5 is tested.As a result,compared with the ZYGO interferometer,the PDI system can achieve the residual errors PV and RMS of only 0.0167? and 0.0025?.It is verified that the polarization contrast enhancement technique can effectively improve the measurement accuracy for low-reflectivity and high-numerical-aperture spherical surfaces.In the last experiment,with the imaging.lens removed,the free lens PDI measures a spherical surface with a 0.05 numerical aperture to an accuracy of PV 0.1825? and RMS 0.0300?,in contrast with the ZYGO interferometer by implementing the iterative reverse diffraction algorithm to obtain the surface shape without diffraction effects. |
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