Research on the Performance of Vacuum Ultraviolet Reflective Films and Related Technology
|School||University of Science and Technology of China|
|Course||Synchrotron Radiation and Applications|
|Keywords||vacuum ultraviolet (VUV) reflective layer VUV reflective mirror VUV reflectance Au film Ir film Al film covered with MgF2 reflectometer multi-fractal spectrum (MFS)|
Vacuum ultraviolet (VUV) reflector has found various applications in many hi-tech fields including solar physics, cosmic physics, life science, and synchrotron radiation. The rapid enhancement of the economical and technological power of our country has made lots of demand for various VUV components including VUV reflector. So it is quite necessary to have a systematical and intensive study on the fabrication of VUV reflector.The main purpose of this thesis is to study the reflective performance of several commonly used film material in 50～200 nm wavelength region and the influence of various fabrication parameters on the reflectivity. Based on these work, optimum fabrication parameters can be presented and VUV reflector with high reflectance can be made. The thesis contains three parts of work: 1. Research on the performance of VUV reflective filmBased on the achievements acquired by other researchers, Au and lr are selected as reflective film material for the wavelength region from 50 nm to 100 nm, and Al covered with MgF2 for the wavelength region from 110 nm to 200 nm. The theoretical models for a single metal layer and a dual-absorbing-Iayer on absorbing substrate were established. The reflectance of Au, Ir, and Al film covered with MgF’2 of different thickness on fused silica, Bk7 glass, and Si wafer substrate, which are commonly used in optical engineering, was calculated, and optimum thickness was therefore determined.The different metal films of various thickness were deposited on various substrate with different fabrication parameter and deposition method (ion beam sputtering and electron beam evaporation). Their reflectance in the wavelength region from 115 nm to 140 nm was measured continuously by the reflectometer located in the National Synchrotron Radiation Libratory (NSRL) at the University of Science and Technology of China (USTC). The relationship between the reflective performance of the films and various fabricating parameters was investigated systematically and intensively. The results indicate:1) The substrate material has an important impact on the VUV reflectance of the film. Fused silica and Bk7 glass are proved to be suitable substrate materials which shows little difference, while Si wafer is not. In the wavelength region from 115nm to 140 nm, the highest normal incidence reflectance acquired is 24% for Au film, nearly 30% for Ir film, and 75% for Al film covered with MgF2.2) The layer thickness has a significant influence on the VUV reflectance of the film. In the case of pre-ion-cleaning, the optimum layer thickness on fused silica is about 30 nm for Au layer, and 12 nm for Ir layer, while on Bk7 glass substrate, a little bit thicker thickness is needed (Au: 30～40 nm, Ir: 12～18 nm). For Si wafer, the thicker the layer, the better the reflectance. As to Al covered with MgF2, the layer thickness of Al should be no less than 60 nm, while that of MgF2 should be optimized according to the incident wavelength.3) Bombardment of the substrate with ion beam of suitable energy before deposition can significantly enhance the VUV reflectance.4) The sputtering ion energy has a significant influence on the VUV reflectance. There does exist a optimum ion energy. The VUV reflectance will considerably decrease when the sputtering ion beam energy is lower than the optimum, while the higher energy contributes little to the enhancement of the reflectance.5) Compared with electron beam evaporation, the reflective film acquired by ion beam sputtering technique has average smaller grain size, better adhesion to the substrate, simpler and stabler deposition process, and easier control of layer thickness.6) The binding layer used for the enhancement of adhesion between film and the substrate should be as thin as possible. The material used for binding layer impacts the reflectance little.7) A layer of very thin film has a noticeable "averaging" effect on substrate surface, i.e. a rather rough surface could be "smoothened", while a rather smooth surface could be rough after covered with a layer of thin film. The surface roughness of film has much bigger influence on VUV reflectance than that of the substrate. So the surface roughness of 1 nm is reasonable for the substrate when it was grinded, better surface roughness is not necessary.8) The annealing of the film does benefit to the release of the tension, but it also causes the increase of average size of the grain and degradation of the reflectance.9) Compared with Au film, Ir layer can get higher VUV reflectance. As far as the adhesion is concerned, Ir layer performs better as well.10) Different solvent cleaning procedure before deposition has little impact on the VUV reflectance.2. Measuring error analysis of the reflectometerA detailed measuring error analysis is given to the reflectometer. Some effective measures are taken to ensure the accuracy and reliability of the measurement.3. Evaluation of optical surface roughness with multi-fractal spectrum(MFS)For the first time MFS was introduced into the evaluation of optical surfaceroughness in order to get rid of some shortcomings such as the variation of the evaluation with sampling size and position, which were generally encountered by conventional evaluation method including power spectrum density (PSD) and root-mean-square (rms) height. The MFS of the surfaces of fused silica, Bk7 glass, and Si wafer substrate were calculated, and the result indicates that MFS can distinguish different surface topographic condition from one another and can be hardly influenced by the sampling size and position.The main innovations of this paper include: for the first time the influence of various fabricating parameters on the VUV reflectance was studied theoretical and experimental, and optimum parameters for the fabrication of high reflective VUV layer were given; a intensive measuring error analysis is carried out for the reflectometer, and for the first time MFS was introduced into evaluation of optical surface roughness and some explorative conclusion was drawn.