Scaling Properties of Ru, Rh, and Ir for Future Generation Metallization

The downscaling of metal lines in CMOS devices to subnanometer sizes leads to an increase in their resistivity. Thus, the lower electron mean free paths of Ru, Rh, and Ir make them promising materials to replace Cu in conventional interconnected structures with sub-ten-nanometer dimensions. In this study, we investigate their scaling effects on the resistivities of metal lines with thicknesses of 4–32 nm. The resistivities of Ru, Rh, and Ir as-deposited films prepared via sputtering are lower than that of a Cu film with a sub-ten-nanometer thickness. Despite their similar electron mean free paths, the difference in their bulk resistivities lead to Ir and Rh having lower resistivities than Ru, even after annealing at 400 °C. Their resistivities before and after annealing were fitted with equations based on the Fuch–Sondheimer and Mayadas–Shatzkes models, which assess the contributions of surface and grain boundary scatterings, respectively. We determined that grain boundary scatterings have a significant effect on the resistivities of Ru, Ir, and Rh, whereas surface scatterings have a minimal effect. In addition, in the case of Ru, the effect of the surface roughness on the resistivity was also investigated by measuring the resistivity of Ru films deposited by atomic layer deposition. A high surface RMS roughness of 1–2 nm causes a significant increase in the resistivity at thicknesses below 10 nm that cannot be explained by the aforementioned models. These results present multiple points for consideration when applying short mean free path metals to finite size interconnects.