Investigation of thermal aging of Ni/C-multilayers by X-ray methods.

Thermally induced aging processes in Ni/C-multilayers with a double layer thickness on the scale of nanometers were investigated. The samples were prepared by pulsed laser deposition and annealed for 20 minutes at temperatures in the range of 50 degree Celsius to 400 degree Celsius. The characterization of the multilayers before and after thermal treatment was carried out by X-ray reflectometry under grazing incidence of Cu-K radiation. From the reflectivity depending on the angle of incidence layer thicknesses and the nickel density were determined by fitting of model calculations to the measured curves. The density of the amorphous carbon layers was derived from the fluorescence yield of a single carbon layer. To observe the changes of crystal structure, X-ray diffraction was used. Ni/C multilayers remain nearly unchanged up to 200 degree Celsius. Between 200 degree Celsius and 300 degree Celsius nickel carbide crystallizes. Above 350 degree Celsius the decomposition of the carbide a nd the crystallization of fcc nickel lead to a rapid increase of the double layer thickness and to deterioration of the reflectivity.