Groupe de Physique Statistique/ Arbeitsgruppe Statistische Physik

Period expansion after annealing in tungsten/carbon (W/C) multilayer films has been observed by several authors. Although most results have emphasized that the carbon layers play the important role in such thermal evolution, it is important to clarify the structure of the carbon atoms in those multilayers both before and after annealing in order to explain such period expansion more clearly. In this paper, Raman scattering is carried out on the W/C multilayers fabricated by various methods, all of which showed different period expansion values ranging from 0% to 20%. We show that the initial carbon in those multilayers becomes graphitized differently depending on the fabrication methods. The carbon fabricated by rf sputtering under a reactive gas has the highest amount of graphite component as compared with those prepared under a pure argon gas atmosphere. After annealing at 1000°C, graphitization can be observed, but there are no obvious structural differences for the carbon in all the annealed W/C multilayer films. Such graphitization of the carbon layer causes a decrease in its density, and thus an increase in its thickness. Due to the different initial states of the carbon in the multilayers produced by the different methods and to the identical final state after annealing, the different carbon layer thickness expansions are expected and do agree with the small angle x-ray diffraction measurements.