Microstructural evolution and interfacial reactions in copper-magnesium thin films on silicon dioxide

Cu-alloy films are being explored for integrated circuits, for creating low-resistivity interconnects with stabilized metal/dielectric interfaces via solute segregation and interfacial reaction. A combination of in situ sheet resistance measurements, transmission electron microscopy (TEM), X-ray diffraction, and Rutherford backscattering spectrometry have been used to follow the pathways of microstructure evolution in supersaturated Cu- 5–13 at.% Mg films and phase formation at the Cu/SiO₂ interface during annealing.; As-deposited films consist primarily of a Cu-Mg solid solution with trace amounts of orthorhombic CuMg₂. Upon annealing to 400°C, Mg segregates to the surface and the Cu-Mg grains grow from an average size of 22 to 68 nm, resulting in a ∼25–40% decrease in film resistivity. The amount of surface-segregated Mg is independent of initial Mg concentration. In the same temperature region, CuMg₂ partially dissolves and fcc Cu₂Mg forms. Energy-dispersive spectroscopy used in conjunction with TEM shows that at ≃400C small quantities of Cu and Mg diffuse into SiO₂ with no observable crystalline phase formation.; Upon annealing to higher temperatures, Mg segregates to the film/silica interface, decreasing the Mg content in the film to concentrations close to that of the equilibrium solid-solubility of Mg in Cu at that temperature as well as eliminating virtually all intermetallics present at 400°C. The Mg transported to the interface reduces SiO₂ and forms fcc MgO on the silica side of the interface. Electron energy loss spectroscopy and high-resolution scanning-TEM measurements reveal the formation of binary Cu-Mg silicates as well. The Si released by this interfacial reaction diffuses into the metal film resulting in a ∼40–190% increase in resistivity, for films with 8–12 at.% Mg. Interfacial Mg segregation and subsequent Si release into the film increase with the as-deposited Mg content.; A Kissinger analysis of sheet resistance characteristics from Cu-Mg/SiO ₂ and Mg/SiO₂ structures reveal that the increase in sheet resistance due to Si release is limited by the Mg reduction of SiO₂ with an activation energy of 2.7 ± 0.1 eV. Further analysis reveals that Si diffuses through the bulk of the film with an activation energy of 2.2 ± 0.1 eV.