Kevin R. Coffey

This work outlines conditions suitable for the heteroepitaxial growth of Cr2O3(0001) films (1.5–20 nm thick) on a Ru(0001)-terminated substrate. Optimized growth is achieved by sputter deposition of Cr within a 4 mTorr Ar/O2 20% ambient at Ru temperatures ranging from 450 to 600 °C. The Cr2O3 film adopts a 30° rotated honeycomb configuration with respect to the underlying Ru(0001) substrate and exhibits a hexagonal lattice parameter consistent with that for bulk Cr2O3(0001). Heating to 700 °C within the same environment during film preparation leads to Ru oxidation. Exposure to temperatures at or above 400 °C in a vacuum, Ar, or Ar/H2 3% leads to chromia film degradation characterized by increased Ru 3d XPS intensity coupled with concomitant Cr 2p and O 1s peak attenuations when compared to data collected from unannealed films. An ill-defined but hexagonally well-ordered RuxCryOz surface structure is noted after heating the film in this manner. Heating within a wet Ar/H2 3% environment preserves the Cr2O3(0001)/Ru(0001) heterolayer structure to temperatures of at least 950 °C. Heating an Ru–Cr2O3–Ru heterostacked film to 950 °C within this environment is shown by cross-sectional scanning/transmission electron microscopy (S/TEM) to provide clear evidence of retained epitaxial bicrystalline oxide interlayer structure, interlayer immiscibility, and epitaxial registry between the top and bottom Ru layers. Subtle effects marked by O enrichment and O 1s and Cr 2p shifts to increased binding energies are noted by XPS in the near-Ru regions of Cr2O3(0001)/Ru(0001) and Ru(0001)/Cr2O3(0001)/Ru(0001) films after annealing to different temperatures in different sets of environmental conditions.

The electrodeposition of Ru was investigated from solutions of ruthenium(III) nitrosyl sulfate and ruthenium(III) chloride onto seed layers of epitaxial and polycrystalline Ru and epitaxial Au. Using both galvanostatic and potentiostatic deposition modes, metallic Ru was found to electrodeposit as a porous layer comprised of (0001) oriented Ru crystallites, the presence of which was discovered and confirmed by X-ray and scanning transmission and transmission electron microscope (S/TEM) analyses. This finding was independent of the Ru salt and seed layer used. Using X-ray reflectivity (XRR), the average film density ρ e f f of the porous electrodeposited Ru layer was measured as less than the density of bulk Ru ρ R u , b u l k (14.414 g cm⁻³). Increasing the magnitude of the applied current density from −100 μA cm⁻² to −10 mA cm⁻² in solutions of Ru nitrosyl sulfate increased the ρ e f f from 7.4 g cm⁻³ to 9.7 g cm⁻² while the current efficiency decreased from 9.4% to 4.3%.

Co electrodeposition was performed onto single crystal Ru(0001) and polycrystalline Ru films to study the influence of such seed layers on the growth of epitaxial Co(0001). The effect of misfit strain on the electrodeposited Co(0001) films was studied using 60 and 10 nm-thick Ru(0001) seed layers, where the misfit strains of the Co layer on the two Ru(0001) seed layers are 7.9% and 9.6%, respectively. Despite a large misfit strain of 7.9%, the planar growth of Co(0001) was achieved up to a thickness of 42 nm before a transition to island growth was observed. Epitaxial Co films electrodeposited onto 10 nm Ru(0001) showed increased roughness when compared with Co electrodeposited onto the 60 nm seed layer. Co electrodeposition onto polycrystalline Ru resulted in a rough, polycrystalline film with faceted growth. Electrochemical experiments and simulations were used to study the influence of [Co²⁺] and solution pH on the throughput of the electrodeposition process. By increasing [Co²⁺] from 1 to 20 mM, the deposition rate of Co(0001) increased from 0.23 nm min⁻¹ to 0.88 nm min⁻¹ at an applied current density of −80 μA cm⁻².

X-ray photoelectron spectroscopy (XPS) is used to analyze the chemistry of the Ru(0001) film surface and the Ru/SiO2 interfacial region at different annealing conditions. The XPS spectra are collected under ultrahigh vacuum (base pressure of ∼5 × 10−10 Torr) condition using a SPECS electron spectrometer with a PHOIBOS 100 hemispherical energy analyzer and an XR 50 Al Kα x-ray source (1486.67 eV). High-resolution spectra of O 1s, Ru 3d/C 1s, and Si 2p together with survey scans are presented. The presence of 1 × 1 low energy diffraction pattern, collected from a 950 °C Ar/H2 step-annealed Ru(0001) sample, confirms the hexagonal periodicity of Ru(0001) surfaces.

Experimentally measured resistivity of Co(0001) and Ru(0001) single crystal thin films, grown on c-plane sapphire substrates, as a function of thickness is modeled using the semiclassical model of Fuchs–Sondheimer. The model fits show that the resistivity of Ru would cross below that for Co at a thickness of approximately 20 nm. For Ru films with thicknesses above 20 nm, transmission electron microscopy evidences threading and misfit dislocations, stacking faults, and deformation twins. Exposure of Co films to ambient air and the deposition of oxide layers of SiO2, MgO, Al2O3, and Cr2O3 on Ru degrade the surface specularity of the metallic layer. However, for the Ru films, annealing in a reducing ambient restores the surface specularity. Epitaxial electrochemical deposition of Co on epitaxially deposited Ru layers is used as an example to demonstrate the feasibility of generating epitaxial interconnects for back-end-of-line structures. An electron transport model based on a tight-binding approach is described, with Ru interconnects used as an example. The model allows conductivity to be computed for structures comprising large ensembles of atoms (105–106), scales linearly with system size, and can also incorporate defects.

The resistivity size effect in nanoscale metals is of both scientific and technological interest, the latter due to its importance to interconnects between transistors in integrated circuits. In this work, the authors report the variation of resistivity with film thickness and with changes in surface scattering of ex situ annealed single crystal Ru thin films grown on sapphire substrates by sputter deposition. The room temperature deposition of SiO2 on the Ru sample surface was observed to increase the resistivity of films that had previously been subjected to annealing in a reducing gas ambient. These overcoated samples were also found to increase in resistivity as a result of an oxidizing anneal and reduce in resistivity as a result of a subsequent reducing gas (Ar + H2) anneal. To a large extent, the surface structure and electron scattering characteristics were found to be reversible between oxidizing and reducing gas anneals. The chemistry and structure of the Ru upper surface was characterized by low energy electron diffraction (prior to the SiO2 overcoat deposition), x-ray reflectivity, x-ray photoelectron spectroscopy, and resistivity measurements. The changes in surface structure and chemistry were related to the changes in the specularity of the Ru surface for electron scattering in the context of the Fuchs–Sondheimer semiclassical model of the resistivity size effect, and in this context a mostly specular metal/dielectric interface is reported.