The effect of surface and grain boundary scattering on the resistivity of Cu thin films and nanowires is quantified using (i) in situ transport measurements on single-crystal, atomically smooth Cu(001) layers, (ii) textured Cu(111) layers and patterned Cu wires with independently varying grain size, thickness and line width, and (iii) in situ grown interfaces including Cu-Ta, Cu-MgO, Cu-SiO2 and Cu-oxygen. In addition, the electron surface scattering is also measured in situ for single-crystal Ag(001) and TiN(001) layers. These findings are important for the development of future generation narrow low-resistivity Cu interconnects and TiN metal gates.

 

Cu(001), Ag(001), and TiN(001) layers with a minimum continuous thickness of 4, 5 and 1.8 nm, respectively, are grown by ultra-high vacuum magnetron sputter deposition on MgO(001) substrates and are found to be atomically smooth single crystals by a combination of x-ray diffraction θ-2θ scans, ω-rocking curves, pole figures, reciprocal space mapping, Rutherford backscattering, x-ray reflectometry, transmission electron microscopy, and in-situ scanning tunneling microscopy. Polycrystalline Cu layers with a 111-texture are deposited on thermally grown SiO2, with and without Ta barrier layer. Subsequent in-situ annealing at 350oC followed by sputter etching in Ar plasma yields Cu layers with independently variable thickness and grain size. Cu nanowires, 50 to 150 mm long, 70 to 350 nm wide, and 45 nm thick, are patterned using electron beam lithography and sputter etching.

 

In-situ electron transport measurements at room temperature in vacuum and at 77 K in liquid nitrogen for single-crystal Cu and Ag layers is consistent with the Fuchs-Sondheimer (FS) model and indicates specular scattering at the 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 due to sub-monolayer oxidation. Electron transport measurements for polycrystalline Cu/Ta layers and wires show a ~10% and ~11% decrease in resistivity, respectively, when increasing the average lateral grain size by factor 2. In-situ deposition of 0.3 to 8 nm thick Ta barrier layers on Cu(001) leads to a resistance increase that indicates a transition from p = 0.8 to p = 0, independent of the Ta thickness. In-situ exposure of Cu(001) layers to O2between 10-3 and 105 Pa-s results in a sequential increase, decrease and increase of electrical resistance which is attributed to specular surface scattering for clean Cu(001) and for surfaces with a complete adsorbed monolayer, but diffuse scattering at partial coverage and after chemical oxidation.