Boundary structure of Mo/Si multilayers for soft X-ray mirrors

Characterizations of Mo/Si multilayers for soft X-ray mirrors have been carried out by high-resolution transmission electron microscopy (TEM) and X-ray scatterings. The crystallite size of oriented Mo in the direction normal to the (110) plane has been found to be approximately equal to the designed thickness of the Mo layer below 8 nm. However, when the Mo layer thickness becomes greater than 8 nm, the crystallite size deviates from the designed thickness and saturates at about 10 nm. The Si layer thickness is smaller than the expected one, which is calculated from the periodic length and the Mo layer thickness, indicating that the mixed layer at the inter-face is formed in the Si layer. The thickness and density of the mixed layer at the Mo-on-Si inter-face are larger than those at the Si-on-Mo interface. The sum of the thicknesses of the mixed layer is about 1.4 nm, irrespective of the Mo/Si composition. Moreover, the density of the mixed layer at the Mo-on-Si interface becomes larger with the increase of the Mo content.