Study Of Optical Properties And Damage Resistance On Ni,B₄C And Other XFEL Thin-film Mirrors

Over the last two decades,the fourth-generation light sources of extreme ultraviolet（EUV）/X-ray pulses free electron laser（FEL）have been built and developed around the world.FELs not only provide cutting-edge experimental tools for basic scientific researches,but they also open up a plethora of new opportunities for industrial development.Thin-film mirrors,playing important roles in beam deflection,monochromatizing and focusing,are indispensable optical components used in EUV/X-ray FEL facilities.To maintain the excellent beam qualities of EUV/X-ray FELs,such as high brightness and full coherence,thin-film mirrors with excellent optical properties and high damage resistance must be developed.Currently,B₄C and other large-scale thin-film mirrors are successfully applied in FEL facilities around the world,but the researches on high-performance,damage-resistant XFEL thin-film mirrors are still very lacking in China.For the development demands of XFEL project in China,the researches on optical properties and damage resistance of Ni,B₄C,Ru and other XFEL thin-film mirrors were carried out and reported in this thesis.Firstly,considering the urgent demands on thin-film mirrors with excellent optical properties for EUV/X-ray FEL light sources,nitrogen-reactive magnetron sputtering method was used to fabricate Ni monolayer mirror with improved optical properties.The main mechanism for improving optical properties is revealed to be the inhibition of grain growth by nitrogen atom doping.The optimal ratio N₂ partial pressure ratio in the sputtered gas was determined to be 8%.In this case,the smallest average grain size and the smallest surface roughness were achieved,which results in the best optical performance under grazing incidence geometry and the low intrinsic stress of Ni monolayer film.This study indicates that nitrogen-reactive magnetron sputtering is an effective method to improve the optical properties and to reduce the intrinsic stress of XFEL thin-film mirrors.Second,the following studies have been conducted in order to achieve the damage resistances and the damage mechanisms of B₄C,Ru and B₄C/Ru thin-film mirrors:1)The effect of nitrogen doping on the damage resistance of B₄C monolayer film was investigated.It was found that the damage threshold of B₄C monolayer film prepared under 15%proportional nitrogen partial pressure was 0.89 J/cm²,which was lower than that of B₄C sample prepared under pure argon.It is due to the formation of metastable boron nitride and an increase of absorption coefficient;2)Ru monolayer and B₄C/Ru bilayer film mirrors were designed for FEL-I beamline at Shanghai HIgh repetitio N rate XFEL and Extreme light facility（SHINE）.It was discovered that the damage threshold of B₄C/Ru bilayer film was only 0.24 J/cm² compared with that of 0.32 J/cm² for Ru monolayer film.Thermal stress dominated the damage mechanisms for both films;3)For the first time in the world,a nanosecond EUV pump-femtosecond infrared laser probe imaging system was established.The nanosecond EUV induced damage dynamics on B₄C/Ru bilayer film was investigated using this system.The self-recovery phenomenon was observed on portion of the damaged area.Accroding to experimental and theoretical studies,it has been proved that the intrinsic stress of Ru film induced by EUV irradiation causes the bilayer film to peel off from the substrate and a bulge structure was formed.Furthermore,the self-recovery phenomenon is related to the elastic deformation induced by the thermal stress.Finally,to investigate the rule of multi-shot irradiation damage,EUV multi-shot damage experiments were performed on a Mo/Si multilayer sample,and the multi-shot accumulation effect was discovered.The formation of Mo Si₂ caused by the diffusion of Mo atoms into Si layers explains the crater-shaped damage morphology and the multi-shot accumulation effect.