| Deep ultraviolet projection lithography machine is currently the main stream equipment for high-end semiconductor integrated circuit manufacturing.Among them,the lithography objective system is the most precise and complex subsystem in the lithography machine.With the increasing of the numerical aperture NA,the number of aspheric surfaces with high steepness and large deviation is increasing accordingly,together with the difficulty of manufacturing.It becomes an urgent issue for settlement to actively develop various new principles,new processes,and new algorithms to achieve aspherical sub-nano-precision,ultra-smooth surface manufacturing.In which,the combined process of magnetorheological finishing(MRF),smoothing polishing(SP)and ion beam figuring(IBF)adopted by Carl Zeiss Company has represented the highest level of optical manufacturing.MRF,with its unique flexible shear mechanism,has the advantages of extremely high convergence efficiency,super-smooth surface manufacturing capacity and no subsurface damage,etc.,has been considered to be one of the most innovative technologies in the past two decades and preferred by both domestic and foreign precision optical manufacturing companies.Taking MRF precision polishing process with high-efficiency,high-precision,high-quality for the large deviation aspherical surface of deep ultraviolet lithography objective as background,nanometer precision and super-smoothing surface quality manufacturing as the goal,and the research of high stability MRF removal function and the method of error compensation as premise,this article systematically studied the key theoretical and technological issues such as nanometer precision algorithm,error control and compensation theory,mid-frequency error suppression method and super-smoothing polishing,aiming to achieve nano-meter class precision and super-smoothing surface convergence for the high steepness and large deviation of aspheric surfaces,and in the mean time make the mid-frequency error index meet the input conditions of ion beam figuring(IBF)to achieve the sub-nanometer accuracy.This research results were of great significance and value to the development of next generation deep ultraviolet lithography objective system for our country.The theory research and analysis in this paper mainly included:(1)Conducted experimental analysis on the influencing factors of the removal efficiency and stability during MRF process,including processing parameters,and material differences.Combining with the process experiments,obtained the optimal processing parameters of the MRF process;Performed error compensation on the removal efficiency change caused by the material difference between the removal function component and the aspherical component.(2)Based on the CCOS polishing theory,explained the deterministic process of MRF,and developed the nano-precision algorithm and error compensation theory.Through compared the iterative algorithm and the linear matrix algorithm process,proposed pulse iterative correction algorithm based on the MRF small size of beam spot.Carried out MRF removal function offset compensation for improving convergence accuracy and reduced mid-frequency error;Analyzed the stability of the magnetorheological edge removal function and developed the post-processing algorithm of edge error based on the linear time-invariant removal function;Carried out the velocity calculation of the grid path and the spiral path of the magnetorheological polishing;Established the spatial posture calculation model of the polishing pad according to the aspheric surface equation and finished experimental verification.(3)Introduced the evaluation and suppression theory of mid-frequency error,and evaluated the uncertainty of surface convergence ratio during MRF process,and analysed the factors that affect the accuracy of the MRF system on the change of mid-frequency error,including the responsing capability of spatial spectrum about the beam spot size of removal function,the effect of removal function stability on mid-frequency error,the effect of removal function offset correction on mid-frequency error,the effect of positioning accuracy error on mid-frequency error as well as the effect of polishing head speed on mid-frequency error.The experiment to suppress the mid-frequency error by combing CCOS with MRF process is carried out.(4)Carried out the research on the super-smoothing polishing process of MRF.By analyzing the influence of MR-fluid comprised 0.1μm and 0.5μm cerium oxide abrasive size on the surface roughness and surface defects of optical components,analyzed the influence of other MRF parameters,such as immersion depth,fluid viscosity and magnetic field strength on the surface roughness.By optimizating super-smoothing process parameters,carried out experimental verification ofsuper-smoothing manufacturing on high-steep optical spherical component.The experiments proved that the MRF could achieve large removal efficiency through the optimization of the process to achieve the improvement of surface accuracy and(sub)surface defect elimination,and could also reduce the removal efficiency to achieve super-smoothing surface manufacturing while controlling the surface accuracy.(5)Integrating the studying theories and technologies in this paper,the nano-meter class precision,super-smooth surface experiment based on MRF precision polishing was carried out on a large deviation aspheric surface to verify the feasibility of correction algorithm,error compensation theory,mid-frequency error suppression method and super-smoothing manufacturing as suggested,which demonstrated the significance and value of the magnetorheological precision polishing technology in the aspheric precision manufacturing of deep ultraviolet lithography objective system. |
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