Electron scattering at anisotropic conductor surfaces

Abstract

The ongoing miniaturization of integrated circuits has reduced interconnect line widths to the sub-10 nm regime, causing a sharp increase in resistivity due to diffuse electron surface scattering and a corresponding increase in RC delay and power dissipation. The classical Fuchs-Sondheimer (FS) model describes this resistivity size effect using a phenomenological specularity parameter p, which quantifies the fraction of electron momentum parallel to the surface that is conserved during a scattering event. However, the FS model assumes a spherical Fermi surface and does not account for Fermi surface anisotropy. This assumption breaks down for highly anisotropic conductors, where Fermi surface topology fundamentally governs the strength of electron-surface coupling. This dissertation investigates electron surface scattering across a spectrum of metallic conductors: ruthenium (Ru), a hexagonal metal whose moderate anisotropy is well described within the standard FS framework, and the highly anisotropic conductors PtCoO2, PdCoO2, and CoSn, whose quasi-2D and kagome-lattice electronic structures intrinsically suppress electron-surface coupling.Electron scattering at Ru(0001) surfaces is quantified as a function of Ti cap layer chemistry and oxygen exposure. In situ four-point probe transport measurements are performed on 7-nm-thick epitaxial Ru(0001)/Al2O3(0001) films before and after deposition of 0.1–0.5 nm Ti cap layers and during subsequent O2 exposure at 0.05 mTorr. Ti cap deposition increases sheet resistance by 1.0–2.6%, corresponding to a specularity decrease of Δp1 = -0.08 to -0.20, attributed to localized Ru-Ti interface states causing diffuse scattering. Subsequent oxidation of the Ti cap reduces sheet resistance by passivating metallic interface states, partially recovering specular scattering at the Ru-TiOx interface. The measured specularity changes for Ru are 1.5-6× smaller than for Co and Rh, and 6-10× smaller than for Cu, demonstrating that Ru(0001) surfaces are only weakly sensitive to surface-state-induced scattering and that cap layer oxidation is an effective strategy for maintaining high conductivity in narrow Ru interconnects. The effect of atomic-scale surface roughness on the resistivity of epitaxial Ru(0001) thin films is examined using atomically smooth 6.5-nm-thick films that are roughened by room-temperature deposition of 0.5, 1.0, and 2.0 monolayers (ML) of additional Ru. In situ transport measurements show a 4–6% sheet resistance increase attributed to electron scattering at atomic-height surface steps. For 0.5 ML coverage, a resistivity penalty Δρ = 0.41 μΩ·cm is quantitatively described by a step-edge scattering model, yielding an average step separation ls = 2.7 nm in good agreement with ls = 3.6 ± 0.3 nm from atomic force microscopy. Increasing coverage to 1.0 ML reduces the penalty to Δρ = 0.38 μΩ·cm (ls = 3.0 nm), attributed to 2D island coalescence. Further deposition to 2.0 ML increases the penalty to Δρ = 0.49 μΩ·cm (ls = 2.3 nm), indicating a transition to 3D mound formation corroborated by progressive damping of Kiessig fringes in X-ray reflectivity. These results confirm that atomic-scale surface topography imposes significant, quantifiable resistivity penalties for deeply scaled interconnects. Electron scattering at PtCoO2(0001) and PdCoO2(0001) surfaces in contact with Si and SiOx cap layers is investigated. These metallic delafossites possess a quasi-2D cylindrical Fermi surface that suppresses the electron wavevector component perpendicular to the basal plane, intrinsically reducing electron-surface coupling at (0001) interfaces. In situ transport measurements on epitaxial films 3.1–28.4 nm thick, grown by molecular beam epitaxy (MBE), reactive sputtering, and metal-organic chemical vapor deposition (MOCVD), show that Si deposition universally increases sheet resistance scaling as 1/d, consistent with additive surface scattering from Si-induced interface states. The response is governed by growth-method-dependent surface termination: MBE- and sputter-deposited films show a monotonic resistance increase up to 4–8 ML of Si, while MOCVD-grown PdCoO2 saturates after 0.5 ML due to a stable CoO2 surface termination. Oxidation of the Si cap reduces the scattering penalty by 3.0% for MBE PtCoO2 and 11–12% for sputter-deposited PdCoO2. Benchmarking via a surface-specific scattering resistance Rs = Rs·d2 - ρo·d shows that MBE-grown PtCoO2 achieves the smallest Rs = 1.30 × 10-16 Ω·m2 of all compared materials under SiOx capping, at a bulk resistivity ρo = 2.1 μΩ·cm approaching that of Cu. These results establish that the quasi-2D Fermi surface topology of delafossites confers two intrinsic advantages, namely low bulk resistivity and low surface-scattering sensitivity, validating PtCoO2 and PdCoO2 as candidates for liner-free, all-oxide sub-10 nm interconnect architectures. The control of crystalline orientation of CoSn thin films deposited by DC/HiPIMS(Direct Current / High Power Impulse Magnetron Sputtering) co-sputtering on Al2O3(0001) and amorphous SiO2/Si substrates is explored using Ru and Co nucleation layers with thicknesses from 2 to 35 nm. CoSn is a kagome-lattice semimetal with highly anisotropic transport: flat bands near the Fermi level suppress in-plane mobility, while dispersive high-velocity bands along the c-axis facilitate metallic conduction, making c-axis alignment a prerequisite for suppressed sidewall scattering in vertical via interconnects. Direct CoSn deposition without a nucleation layer yields a near-randomly oriented film on Al2O3(0001) with a c-axis orientation ratio of 0.08. Ru nucleation layers achieve a complete ratio of 1.0 at just 2 nm, attributed to the strong wetting tendency of Ru on Al2O3(0001). Co nucleation layers exhibit a thickness-dependent transition, reaching 1.0 at 5 nm. Azimuthal φ-scans confirm full in-plane epitaxial alignment for both nucleation chemistries, with the common relationship CoSn[10\bar{1}0] ∥ Ru/Co[10\bar{1}0] ∥ Al2O3[11\bar{2}0]. Despite a larger lattice mismatch with CoSn (4.9% vs. 2.7% for Ru), a 13-nm-thick Co nucleation layer yields the narrowest rocking curve FWHM of 0.33°, surpassing all Ru-nucleated films. On amorphous SiO2/Si, Co nucleation layers promote c-axis-preferred fiber texture without in-plane alignment, with the orientation ratio increasing from 0.52 to 0.69 and FWHM narrowing from 9.39° to 4.61° as thickness increases from 5 to 35 nm. The room-temperature (300 K) and 77 K resistivities of a 5-μm-thick epitaxial CoSn film are 160.9 and 79.1 μΩ·cm, yielding a residual resistivity ratio of 2.03 and confirming bulk-like electronic quality. Collectively, these results demonstrate that: (i) Ru surfaces show weak sensitivity to surface-state-induced scattering and cap layer oxidation effectively maintains high conductivity; (ii) atomic-scale roughness causes quantifiable resistivity penalties in Ru accurately described by the step-edge scattering model; (iii) the quasi-2D cylindrical Fermi surface of PtCoO2 and PdCoO2 provides a structural suppression of electron-surface coupling inaccessible to metals with near-spherical Fermi surfaces; and (iv) CoSn films can be grown with controlled c-axis orientation on both crystalline and amorphous substrates through nucleation layer selection. These findings establish Fermi surface anisotropy as a key design principle for minimizing the resistivity size effect in next-generation narrow interconnect materials.