Control of roughness in Mo/Al multilayer film fabricated by DC magnetron sputtering

Multilayer Mo/Al films for soft X-ray optics have been fabricated by a DC magnetron sputtering method. The characteristics of multilayer films have been investigated using a transmission electron microscope for observing the cross-sectional microstructure, electron diffraction for determining the crystalline properties, and a small-angle X-ray diffractometer and synchrotron radiation reflectometer for obtaining refractivity. The roughness of the multilayer films was controlled by increasing the number of layers and applying a negative bias to the substrate. While the reflectivity of the reference multilayer film was 23.3% at a normal incidence angle of 5degrees, it increased to 29.2% as a result of the increase in the number of layers. The surface roughness of the multilayer film decreased and became similar to that of the aluminum monolayer, when the number of layers increased. This was the main reason for the increase in reflectivity. Bias sputtering reduced the roughness of the film and as a result, reflectivity increased to 33.5%, however, at higher bias voltages, probably due to the diffusion of Al or Mo through the interface, reflectivity did not increase any further.