Paper EM-TuM1
Resistivity Increase due to Electron Surface Scattering in Nanoscale Metal Films

Tuesday, October 19, 2010, 8:00 am, Room Dona Ana

The resistivity increase due to electron surface scattering can be reduced by facilitating specular scattering, as demonstrated by in-situ transport measurements on single crystal Cu, Ag, and TiN layers. However, metal barrier layers and surface exposure to oxygen/air perturb the periodic surface potential, causing diffuse electron scattering which increases the resistivity. These findings are important for the development of future generation narrow low-resistivity Cu interconnects and TiN metal gates. Epitaxial Cu(001), Ag(001), and TiN(001) layers exhibit a minimum continuous thickness of 20, 50, and 1.8 nm, respectively, when grown on MgO(001) substrates by ultra-high vacuum magnetron sputter deposition at 80, 140, and 750 ° C, respectively, while Cu grown on a 2-nm-thick TiN(001) buffer layer at room temperature shows a minimum continuous thickness of 4 nm. X-ray diffraction θ-2θ scans, ω-rocking curves, and pole figures show the layers are single crystals with a cube-on-cube epitaxy. The surface and interface roughness, and layer thickness were determined by Rutherford backscattering spectrometry, x-ray reflectometry, and in-situ scanning tunneling microscopy. In-situ electron transport measurements at room temperature show a resistivity increase with decreasing thickness d, from (i) 1.7 to 6.4 μΩ-cm for Cu layers with d = 1400 to 4 nm, (ii) 13 to 150 μΩ-cm for TiN layers with d = 760 to 1.8 nm, and (iii) 1.6 to 2.1 μΩ-cm for Ag layers with d = 1250 to 50 nm. The data for Cu and Ag layers is consistent with the Fuchs-Sondheimer model and indicates specular scattering at metal-vacuum boundary with an average specularity parameter p = 0.6 and 0.4, respectively. In contrast, layers measured ex-situ show completely diffuse surface scattering (p = 0) due to sub-monolayer oxidation.