X-ray study of strains and dislocation density in epitaxial Cu/Ni/Cu/Si(001) films

The strain state of epitaxial Cu(50 Å)/Ni(t_Ni)/Cu(2000 Å)/Si(001) films as a function of the nickel film thickness (30Å<~t_Ni<~2000Å) has been studied using Bragg diffraction and grazing-incidence diffraction with a synchrotron x-ray source. For 30Å<~t_Ni<~150Å both the in-plane and out-of-plane nickel strains show a phenomenological (1/t)^2/3 power dependence, which is significantly different from the 1/t law commonly accepted in the literature. The Matthews’ theory, including the effect of the copper capping layer, is used to account for the equilibrium strains of the nickel layer. The 500 and 2000 Å films show larger strains than that predicted by the theory, consistent with other studies. The ratio of the nickel in-plane to out-of-plane strains is -1.18±0.05, very close to the expected nickel bulk value of -2c₁₂/c₁₁=-1.28.