Multilaver Mirrors For 182Å

Abstract

Multilayer coatings have been designed for peak reflectivity at 182.2A and 45° angle of incidence. An optimizing computer code was used to find the best layer thicknesses and total number of layers in the structure. Silicon was chosen as the spacer material and molybdenum for the absorber. Mirrors were produced both by sputtering and by electron-beam evaporation in ultra-high vacuum (UHV). The mirrors fabricated by sputtering were made from high-purity source materials by moving a substrate table over the sources. The UHV evaporated samples were produced in a silicon/metals MBE system with a base pressure of 5X10-11 torr. The layers were formed by alternately opening shutters over the two electron-beam evaporators. The deposition rates were feedback stabilized and the shutter timing was computer controlled. The substrate was rotated to produce uniform layers and heated to 200°C for one sample. Rutherford Backscattering Spectroscopy (RBS), x-ray diffraction (XRD), and scanning tunneling. microscopy (STM) were used for characterization. Synchrotron radiation from SURF-II was used to measure the reflectivity as a function of wavelength in the design region. All of the mirrors have amorphous Si layers and polycrystalline Mo layers. The measured reflectivity at 45°, scaled for 10096 S-polarized light, ranges from 29% for a sputter-deposited mirror to 47% for the mirror produced in the MBE system at a substrate temperature of 200°C.