Coefficients of thermal expansion of thin metal films investigated by non-ambient X-ray diffraction stress analysis

Cu, Ni and Pd layers were deposited by DC-magnetron sputtering on Si wafer substrates. The mechanical stresses and the coefficients of linear thermal expansion (CTEs) were investigated by non-ambient (in-situ) X-ray diffraction measurements employing the sin2ψ crystallite group method. It was found that, in the as-produced state, the CTEs were larger than expected on the basis of literature values for the Cu and Ni layers, but not for the Pd layer. Upon annealing of the layers grain growth and decrease of the crystalline imperfection, monitored by in-situ X-ray diffraction measurements of the diffraction line broadening, occurred. The increase of the crystallite size was paralleled by a decrease of the CTE values for the Cu and Ni films. The grain-size dependence of the CTE is discussed in terms of the coordination (state of bonding) of atoms at grain boundaries and surfaces.