Electron scattering at transition metal surfaces and grain boundaries

Jog, Atharv
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Ozisik, Rahmi
Shi, Sufei
Sundararaman, Ravishankar
Gall, Daniel
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Materials engineering
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This electronic version is a licensed copy owned by Rensselaer Polytechnic Institute (RPI), Troy, NY. Copyright of original work retained by author.
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Electron transport in the mesoscopic regime is of great interest to the academic community and the semiconductor industry as a fundamental understanding of conduction in narrow wires is essential for the continued downscaling of integrated circuits and the development of interconnects for nanoelectronic devices and alternative computing approaches. As the interconnect line widths approach the electron-phonon scattering mean free path λ ~ 39 nm in Cu, the resistivity increases due to electrons scattering at surfaces and grain boundaries. Consequently, much of the recent literature has explored alternate metal solutions to alleviate this resistivity bottleneck in Cu by either (1) facilitating specular surface scattering, (2) high electron transmission at grain boundaries, or (3) a small electron phonon scattering mean free path which mitigates resistivity scaling. Despite the large industrial interest, direct measurements that aid to quantify and understand the fundamental physics describing electron surface and grain boundary scattering are absent and, consequently, resistivity scaling in alternate metals remains to be investigated. Thus, the goal of this thesis is to quantify (1) the electron-phonon scattering mean free path λ, (2) the surface specularity parameter p, and (3) the grain boundary reflection coefficient R for the two most promising transition metals Rh and Ir by studying the thickness dependent resistivity of epitaxial and polycrystalline metallic thin films. Furthermore, I investigate the effect of the addition of adlayers and adatoms on electron surface scattering for several epitaxial metals which reveals the breakdown of the semi-classical FS model and a need for a new 2D transport mechanism to explain the observed resistivity increase.In situ and low-temperature transport measurements on single-crystal Rh(001) layers deposited on MgO(001) substrates show a resistivity increase that is well described with the FS model and diffuse surface scattering, yielding an effective room-temperature mean free path eff = 9.5  0.8 nm and a temperature-independent product ρoλ = (4.5  0.4) ×10 16 Ωm2. Two-domain polycrystalline Rh(111) layers deposited on Al2O3("11" "2" ̅"0" ) have resistivity values higher than for the Rh(001) layers, which is attributed to electron scattering at the domain walls, indicate an electron reflection probability R = 0.16 ± 0.03. This value is for grain boundaries characterized by a 60° rotation about the <111> axis and matches the previously predicted R for 3 twin boundaries. The four times smaller λ for Rh in comparison to Cu suggests a much-reduced resistivity scaling which makes Rh a promising alternate interconnect metal. Secondly, epitaxial Ir(001) layers are sputter deposited on MgO(001) at 1000 °C. In situ and ex situ transport measurements at 295 and 77 K yield an effective electron mean free path λeff = 7.4 ± 1.2 nm and a temperature independent product ρoλeff = (3.8 ± 0.6)×10 16 Ωm2 which is in good agreement with first-principles predictions. However, dewetting is observed for layers with d < 19.5 nm due to the high surface energy of metallic Ir layers on ceramic MgO substrates. Ir layers deposited at Ts = 700 °C and stepwise annealed to 1000 °C exhibit a unique polycrystalline multi-domain microstructure with smooth renucleated 111-oriented grains that are >10 μm wide for d = 10 nm, resulting in a 26% lower ρoλeff. Ir(111)/Al2O3(0001) layers exhibit two 60°-rotated epitaxial domains with an average lateral grain size of 88 nm. The grain boundaries cause a thickness-independent resistivity contribution ρgb = 0.86 ± 0.19, indicating an electron reflection coefficient R = 0.52 ± 0.02 for this boundary characterized by a 60° rotation about the <111> axis. The Ir microstructure and surface morphology is strongly affected by deposition conditions and layer thickness, making direct quantification of the resistivity size effect challenging. Nevertheless, the measured ρoλeff for Ir is smaller than for any other elemental metal, and 69%, 43%, 25%, and 15% below reported ρoλ products for Co, Cu, Ru, and Rh, indicating that Ir is a promising alternate metal for narrow high-conductivity interconnects. In order to determine the effect of adlayers and adatoms on electron surface scattering, in situ transport measurements are performed on 10-nm-thick epitaxial metal layers during (1) the addition of Ti capping layers and (b) the exposure to oxygen. Epitaxial Cu(001) and Co(0001) layers exhibit a considerable resistance increase, but a five times smaller effect for Rh(001) on the addition of Ti adlayers and O2 exposure, suggesting a pronounced dependence on the conductor metal but no qualitative difference when changing the chemical identity of the capping layer. Furthermore, measurements on epitaxial layers of many different metals indicate a strong negative correlation between the metal electronegativity and the relative resistance increase Δρ/ρ during air exposure, suggesting that the magnitude of the surface potential perturbation due to charge transfer from metal to adatom is the primary parameter affecting electron surface scattering in thin metal layers. Thus, electronegative metals facilitate smooth surface potentials with specular electron reflection and a minimized resistance increase. They are therefore promising as conductors for highly scaled interconnect lines. Electron scattering at grain boundaries is investigated using polycrystalline Rh thin films. For this purpose, Rh layers with thickness d = 9 – 261 nm are deposited on SiO2/Si(001) substrates at Ts = 20 °C, 350 °C, and 350 °C followed by in situ stepwise annealing to 750 °C to yield three series of 111-textured layers with increasing average lateral grain sizes D. Electron backscatter diffraction maps show that D for annealed layers increases with layer thickness from D = 89–134 nm, matching the surface morphological lateral correlation length  = 86 – 154 nm measured by atomic force microscopy. The thickness and grain size dependence of the resistivity of polycrystalline layers is well described by the combined FS and MS model and indicates a Rh electron mean free path  = 9.5  0.8 nm and a reflection coefficient R = 0.41 ± 0.05 for grain boundaries characterized by a rotation about the <111> axis. As-deposited layers with Ts = 350 °C and 20 °C have considerably smaller grains, leading to a 1.8- and 3.5-times higher resistivity than annealed Rh layers for d = 10 nm, respectively. The overall results reveal that a large (>10 nm) grain size is essential to realize the conductivity advantage of Rh vs Cu for narrow interconnect lines. Finally, a combined experimental and first-principles study is performed to determine the most conductive metal among Cu, Co, Ru, Rh, and Ir in the limit of narrow interconnect lines. Transport measurements indicate that Rh and Ir are promising because their ρo×λ product is the smallest. However, their grain boundary reflection probability is 1.4- and 1.7-times larger than for Cu, negating some of the conductivity benefits of the small λ. Ru is also promising, because of its small ρo×λ value that is 24% below that of Cu but more importantly because of the smaller liner thickness which provides a larger cross-sectional area for narrow Ru interconnect lines. In summary, this thesis provides original discoveries and new knowledge on the growth and characterization of epitaxial and polycrystalline Rh and Ir layers. More importantly, it contributes to a more complete understanding of electron scattering at surfaces, interfaces, and grain boundaries of Rh and Ir.
July 2022
School of Engineering
Dept. of Materials Science and Engineering
Rensselaer Polytechnic Institute, Troy, NY
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