Effect of atomic-scale surface roughness on the electrical resistivity of epitaxial Ru(0001) thin films

Abstract

The effect of atomic-scale surface roughness on electron transport is investigated using 6.5 nm-thick epitaxial Ru(0001) films, a promising metal for next-generation interconnects. Sputter deposition at 350 °C followed by stepwise vacuum annealing to 950 °C yields atomically smooth Ru(0001)/Al2O3(0001) films which are subsequently roughened by depositing ΘRu = 0.5, 1.0, and 2.0 monolayers (MLs) of additional Ru at room temperature. In situ four-point probe measurements show a resistivity ρ = 9.08–9.44 μΩ cm for 6.5 nm-thick films and a 4%–6% increase in sheet resistance upon roughening, attributed to electron scattering at atomic-height surface steps. The measured resistivity penalty Δρr = 0.41 μΩ cm caused by a ΘRu = 0.5 ML coverage suggests formation of 2D islands with an average step-edge separation ls = 2.7 nm, as quantified by a step-edge scattering model. This is in good agreement with ls = 3.6 ± 0.3 nm estimated from atomic force micrographs. Increasing ΘRu further to 1.0 ML causes a slight decrease in roughness and Δρr, indicating coalescence of 2D islands which increases ls to 3.0 nm. However, a ΘRu = 2.0 ML coverage causes 3D mound formation, an increased resistivity penalty Δρr = 0.49 μΩ cm, and a reduced ls = 2.3 nm, consistent with a significant damping of Kiessig fringes measured by x-ray reflectivity. These results demonstrate that atomic-scale topographical variations impose significant resistivity penalties for deeply scaled metallic interconnects.