Researches Of Key Technologies For Extreme Ultraviolet Mask Blank Inspection Systems

In recent years,with the rapid development of extreme ultraviolet(EUV)lithography technology,the key problem in EUV lithography that defect free mask manufacturing has attracted much attention.This major technology for defect free mask manufacturing can't be realized by the existing technology level.Therefore,defect detection and repair technology is used to avoid the occurrence of device failure due to mask defects.Among the various detection methods,the method that actinic EUV mask blank inspection with dark-field imaging has the advantages of high resolution,high detection accuracy and high sensitivity,it has attracted wide attention of researchers.This thesis focuses on the optimization design of Schwarzschild optical dark-field imaging system,one of the key technologies in this typical actinic EUV lithography mask blank inspection system:1.Firstly,the spherical Schwarzschild optical imaging system with numerical aperture(NA)of 0.2,field of view of 0.5 mm × 0.5 mm and magnification of 26 is optimized.On this basis,a movable planar mirror and an off-axis spherical mirror are added,to achieve the optimization design of a high magnification system with a magnification of 1200 and good imaging in the center field of view.Then,the relationship between the scattered light intensity of different types of defects and the deflection angle of the mask blank is studied by the finite difference time domain method.By comparing the relationship between the scattered light intensity of different types of defects and the deflection angle of the mask blank,the amplitude defect and the phase defect can be distinguished.Thus,the optimization design of EUV lithography mask blank detection system is realized by integrating detection,high magnification and defect recognition functions.2.In order to improve the resolution of Schwarzschild system,the optimization design of aspherical Schwarzschild system with NA of 0.5 is carried out by using the method of getting the best bare mirror from the inversion of multilayer mirror.First,analyze the influence of Mo/Si multilayers with different structures on the imaging quality of the system.After comparison,it is found that the four-layer model multilayer is necessary for system design.On this basis,the reflector in the traditional optical design is regarded as the result of the interaction of multilayer and bare mirror.Based on the equivalent working surface model of multilayer,the best bare mirror can be inverted by multilayer mirror,then the optimization design of the aspherical Schwarzschild system with multilayers is achieved.3.In the aspherical Schwarzschild system,the finite element analysis method is used to analyze the influence of the residual stress of the Mo/Si multilayers on the shape of the mirror,and the influence of the residual stress of the multilayers on the imaging quality of the system is further quantified through optical design.It is found that the spherical aberration caused by the residual stress of the multilayers cannot be compensated only by optimizing the object distance and image distance.If the coating aperture of the substrate is larger than the effective working aperture,the high order spherical aberration can be well compensated by optimizing the object distance and image distance.Thus,the optimization design of aspherical Schwarzschild system considering the residual stress of the multilayers is realized.In conclusion,these researches provide a theoretical basis for solving some practical problems in the optimization design process of EUV lithography mask blank detection system,and lay a foundation for further realization of the optimization design of the complete optical system.