Properties of ultrathin molybdenum films for interconnect applications

Abstract

The structural and electrical properties of Mo thin films with thicknesses between 3 and 50 nm, deposited by physical vapor deposition, have been evaluated in order to assess the potential of Mo as an alternative to Cu or W for nano-interconnect applications. Mo films deposited on SiO2/Si (100) were polycrystalline with randomly oriented grains close to the interface and the progressive formation of a (110) texture above 5 nm film thickness. Adhesion between Mo and low-κ dielectrics was strong with adhesion energies above 5 J/m2. The films showed intrinsic tensile stress of ∼1 GPa, which decreased with increasing thickness. The Mo resistivity showed a weaker thickness dependence than Cu, which rendered Mo competitive with conventional TaN/Cu/TaN metallization below metal thicknesses of 8 nm. Semiclassical resistivity modeling found that the thin film resistivity was limited by grain boundary scattering with a reflection coefficient of R = 0.46 ± 0.03. Furthermore, the effects of Mo deposition on different dielectric and metallic substrates and of post-deposition annealing up to 950ºC were investigated. Crystallinity, impurity concentration, and resistivity were all affected by the substrate. Post-deposition annealing in H2 led to continuous grain growth, followed by recrystallization at 860ºC. Furthermore, annealing at 650°C led to compressive stress in the films. By contrast, annealing in H2/N2 led to the incorporation of N and ultimately to Mo nitride formation above 500ºC. The results indicate that Mo can be a promising candidate for advanced interconnect metallization schemes if surface oxidation as well as impurity incorporation can be kept under control.