Design Of High Reflective Vacuum Ultraviolet Multilayers With Layer-by-layer Method

Vacuum ultraviolet(VUV) reflecting films have found various applications in many high-tech fields including cosmic physics, life science, and synchrotron radiation, and the rapid enhancement of the economical and technological power of the world has made more and more demands for various VUV components these years. However, there still remain some problems in obtaining high reflectance VUV films, for almost all materials are strongly absorptive in this special spectral region, therefore it is of great importance to find a suitable design method to enhance the reflectivity in this spectral region. This paper summarizes the latest progress achieved in high reflective films design and fabrication, and trys to introduce the layer-by-layer(LBL) method, created by Masaki Yamamoto for high reflective x-ray stacks, into design of VUV stacks in order to achieve desired high VUV reflectance.The paper briefly explains the theory of LBL method and primary principle for the selection of stack materials, and derives Berning recurrence formula used for the calculation of the reflectivity of multilayers. The high reflective multilayers at wavelength between 50 and 200 nm are designed with the aid of analysis method and graphic representation in a Gaussian plane. The reflectance of multilayers of different material combinations including Si/C, Si/SiC, Si/W, Si/Ir and Si/Co on fused silica and Si substrates are calculated, and the optimum thickness and the total number of layers are therefore determined. The calculation indicates that the average reflectance of multilayers of Si and C on a fused silica substrate can reach as high as 64% in the whole VUV range, 85% in the region from 180~200 nm, slightly lower than that of Al/MgF2 combinations, and 50.4% in 50-110 nm wavelength region, much higher than 45% designed by sub-quarter-wavelength method. Compared to the single layer reflective films and conventional 1/4λmultilayers, this method can significantly enhance the reflectance of the multilayers used in VUV wavelength region. The influence of film thickness error on the reflectance is also studied. The result shows that the layer thickness does influence the VUV reflectance of the film.