Structural stability of beat-treated CoN/CN soft X-ray multilayers fabricated by dual-facing-target sputtering

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 2.00-2.04 nm 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 high-temperature annealing results imply that the destructive threshold of the CoN/CN multilayers is 100-200 degrees C higher than that of Co/C multilayers. The small-angle X-ray diffraction of CoN/CN soft X-ray multilayers indicates that the period expansion of the multilayers is only 4% at 400 degrees C, and the interface pattern still exists even if they were annealed at 700 degrees C. The large-angle X-ray diffraction and transmission electron microscopy analysis reveal that the crystalline process is significantly retarded if doped with N atoms, leading to a smaller grain size at higher annealing temperatures. The significant improvement of the thermal stability can be interpreted with Raman spectroscopy and X-ray photoelectron spectroscopy analysis. The Raman spectra give the evidence that the formation of the sp(3) bonding in the CN sublayers can be suppressed effectively by doping with N atoms, and thus the period expansion resulting from the changes in the density of CN layers can be decreased considerably. The X-ray photoelectron spectra give information about existence of the strong covalent bonding between N atoms and the ionic bonding between Co and N atoms, which can slow down the tendency of the structural relaxation. The interstitial N atoms decrease the mobility of Co atoms, and thus the fee Co and hcp Co coexist even though the annealing temperature is much higher than the phase transformation temperature of 420 degrees C, leading to the suppression of the grain growth.