The structural characterization of heat-treated CN films fabricated by dual-facing-target sputtering for soft X-ray multilayer mirrors was performed by means of X-ray diffraction (XRD), Raman spectroscopy (RS) and X-ray photoelectron spectroscopy (XPS). The XRD analyses indicate a graphization process in the CN films during thermal annealing. The Raman analyses imply that the primary bonding in the CN films is sp2. In other words, the formation of the sp3 bonding in the CN films can be suppressed effectively by doping with N atoms, and thus the thickness expansion resulting from the changes in the density of CN films during annealing can be decreased considerably. This result is also clarified by the increased conductivity measured. The XPS results give the information of the existence of the strong covalent bonding between N and C atoms, which can slow down the tendency of the structural relaxation during annealing. These results suggest that CN films suitable for soft X-ray multilayers used at high-temperature environments can be obtained by reactive dual-facing-target sputtering. With the X-ray diffraction measurements, we do observe the enhanced thermal stability of CoN/CN multilayers.

The structural stability of CoN/CN soft X-ray multilayers has been investigated by using complementary measurement techniques. Annealing in the low-temperature range of 473-523 K, we find three interdiffusion features, different from Co/C multilayers [H.L. Bai et al.: Thin Solid Films 286, 176 (1996)]. (1) The interdiffusion critical wavelengths were calculated as 2.00-2.04nm at temperatures ranging from 473 to 523 K, which is equal to those of Co/C multilayers within the experimental error, indicating that the interdiffusion behaviours in the CoN/CN multilayers are still decided by the thermodynamic properties of the Co-C system. (2) The effective interdiffusivities and macroscopic diffusion coefficients are smaller. (3) The activation energy for diffusion is larger. The features imply that it is possible to improve the thermal stability of Co/C multilayers by doping with N atoms.

The thermal stability of Co/C soft x-ray multilayers is improved by 100-200 centigrade degrees through doping with N. The low-angle x-ray diffraction of CoN/CN soft x-ray multilayers indicates that their period expansion is only 4% at 400℃, and the interface pattern of the multilayers still exists even if they are annealed at 700℃. High-angle x-ray diffraction and transmission electron microscopy analyses reveal that this crystalline process is significantly retarded by doping with N atoms, leading to a smaller grain size at higher annealing temperatures. Raman spectroscopy analyses of the multilayers give evidence that the formation of the sp3 bonding is suppressed effectively by doping with N atoms, and thus the period expansion is decreased considerably. The strong covalent bonding between N atoms and the ionic bonding between Co and N atoms can slow down the structural relaxation. The significant suppression of grain growth is believed to be attributable to the coexistence of hcp and fee Co structures at annealing temperatures higher than 500℃.

Period expansion of Co/C and CoN/CN soft x-ray multilayers has been investigated by x-ray diffraction and Raman spectroscopy. Below the anneal temperature of 400℃, the period expansion (<12%) of Co/C multilayers is mainly caused by the graphitization of the amorphous carbon layers. By 500 oC, the crystallization and agglomeration of Co layers induce an enormous period expansion (～40%). The period expansion of CoN/CN multilayers is only 4% at 400℃, which is much smaller than that of Co/C multilayers. The interface patterns of the CoN/CN multilayers still exist even if they were annealed at 700℃ The Raman spectroscopy analyses indicate that the formation of the sp3 bonding can be suppressed effectively by doping N atoms, and thus the period expansion is decreased considerably at annealing temperatures below 600℃. The significant suppression of grain growth above 600℃ is believed to be attributed to the coexistence of hcp and fcc Co structures induced by interstitial N atoms, which cause the high-temperature period expansion decrease. The results also imply that the structural stability of Co/C soft x-ray multilayers can be significantly improved through doping N atoms.

low 400℃, the upward shift of D and G Lines in Raman spectra indicates that the amorphous carbon layers are changing from ones with bond-angle disorder and fourfold-bonding only to ones containing threefold-bonding. In the crystalline stage, the amorphous carbon layers in the as-deposited multilayers crystallize to graphite crystallites in the annealing temperature range of 500-600℃. The rapid increase in the intensity ratio of D line to G line and dramatic decrease in linewidth further confirm this substantial structural change. In the grain growth stage, the specimens are annealed at temperatures higher than 700℃. The decrease in the intensity ratio implies a growth in the graphite crystallite dimensions, which is consistent with the XRD and TEM results.