Thin film deposition of metastable face-centered cubic cobalt and ruthenium
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Authors
Thakral, Anshuman
Issue Date
2025-08
Type
Electronic thesis
Thesis
Thesis
Language
en_US
Keywords
Materials engineering
Alternative Title
Abstract
The downscaling of devices leads to an increase in the resistivity of interconnect wires, causing resistance-capacitance delay. This has become a prominent roadblock in further advancement of microelectronics. The increase in the wire’s resistivity with decreasing line widths is primarily attributed to electron scattering from surfaces and gran boundaries. This effect is well explained by the Fuchs and Sondheimer and Mayadas and Shatzkez models for surface and grain boundary scattering, respectively. Theoretical predictions propose metastable face-centered cubic metals, such as cobalt and ruthenium, as potential replacements for copper, due to their predicted less pronounced resistivity increase at small dimensions. This motivates this study which delves into firstly synthesizing the metastable materials, followed by quantifying their electron transport. Firstly, I investigated synthesis of metastable fcc Co thin films. The phase composition of Co layers deposited by magnetron sputtering was studied as a function of processing gas (Ar or N2), temperature Ts = 100-600 °C, and substrate [Al2O3(0001), MgO(001) and SiO2/Si] in order to determine the kinetics for synthesis of metastable face-centered cubic (fcc) cobalt. N2 processing gas resulted in residual nitrogen in Co layers for Ts ≤ 200 °C but facilitated the growth of nitrogen-free fcc Co for Ts ≥ 300 °C. Templating with hexagonal Al2O3(0001) led to nucleation and growth of epitaxial hexagonal close-packed (hcp) Co(0001) in Ar. However, N2 caused stacking faults and the formation of a coherent mixed hcp/fcc epitaxial microstructure. Co on MgO(001) nucleated in the fcc phase, resulting in epitaxial fcc Co(001) layers in both Ar or N2 atmospheres. Density functional calculations confirmed the experimental observations, indicating that nitrogen facilitates formation of the cubic phase, with a predicted hcp-to-fcc transition at 10 at% N for T = 0 K and only 1.7 at.% N at 300 °C. They also suggest a negligible hcp(0001)/fcc(111) phase-boundary energy which facilitated the experimentally observed mixed hcp/fcc microstructure. The overall results demonstrates that the introduction of N2 gas during Co thin film deposition represents an effective approach to synthesize metastable fcc Co.
After recognizing the required kinetics barrier that led to formation of metastable fcc Co, I next quantified its electron transport within the vicinity of the Fuchs and Sondheimer model. In order to do this, I deposited face centered cubic Co thin films by reactive magnetron sputtering in 5 mTorr N2 at 400 °C followed by vacuum annealing at 500 °C. The resulting phase-pure Co(001)/MgO(001) layers contained negligible nitrogen, exhibited a surface roughness < 0.8 nm and a cube-on-cube epitaxial relationship with the substrate with Co[100] || MgO[100]. The measured resistivity vs thickness d = 10 - 1000 nm indicates a bulk resistivity ρo = 6.4 ± 0.3 µΩ-cm for fcc Co at room temperature and ρo = 1.3 ± 0.1 µΩ-cm at 77 K, and an effective electron phonon mean free path λ = 27 ± 2 nm and 79 ± 6 nm at 295 and 77 K, respectively. The resulting ρo × λ benchmark quantity is 3-5 times larger than predicted from first principles, suggesting a breakdown of the Fuchs-Sondheimer model at small dimensions. The overall results indicates that fcc Co exhibits no intrinsic conductance benefit over stable hcp Co nor conventional Cu for narrow interconnects.
Next, I investigated the effect of the phase change on magnetic properties of epitaxially grown cobalt layers of similar thicknesses. Fcc Co(001)/MgO(001) and hcp Co(0001)/Al2O3(0001) were epitaxially grown using an ultra-high vacuum magnetron sputtering system. X-ray diffraction analysis confirmed a single orientation epitaxial layer with negligible strain while a large mosaic spread particularly in the fcc layer. Atomic Force Microscopy (AFM) showed a rough topology for the hcp layer in comparison to the fcc, while Magnetic Force Microscopy (MFM) data shows clear large magnetic domains for the hcp layer (2 x 0.5 μm wide), while no domains are observed for fcc within the measured scan range. Surface Magneto-Optic Kerr Effect (SMOKE) measurements indicate coercivity for fcc and hcp Co as 48 Oe and 220 Oe respectively, suggesting a soft magnet like behavior for the fcc layer, in agreement with the MFM measurement. The measured saturation magnetization for fcc and hcp is 250 Oe and 500 Oe respectively. Magneto Resistance (MR) measurements on the two layers show a 0.4% change in Rs for the fcc layer while nearly 0.7% change for the hcp layers, suggesting electron scattering from magnetic domain boundaries is more pronounced for the hcp layers. This study portrays partial tuneability of the magnetic properties of Co via hcp to fcc transition.
Finally, I used the novel developed nitridation mechanism to synthesize another metastable fcc material, namely ruthenium. A combination of thin film deposition and first-principles calculations are employed to explore the effect of N2 gas on the phase formation of hexagonal close-packed (hcp) Ru, metastable face-centered cubic (fcc) Ru, and zincblende (zb) RuN. Sputter deposition in 20 mTorr Ar / N2 gas mixtures at 25 °C led to 1000-nm-thick phase-pure hcp Ru films for N2 partial pressures PN2 = 0 mTorr but a transition to fcc Ru and zb RuN with increasing PN2, indicating that nitrogen facilities the formation of metastable fcc Ru. This is confirmed by first-principles calculations on the Ru1-xNx formation energies which predict transitions from hcp Ru for x ≤ 0.077 to fcc Ru for 0.077 ≤ x ≤ 0.185 and zb Ru1-xNx for 0.204 ≤ x ≤ 0.500. This is in qualitative agreement the measured 0.073 ≤ x ≤ 0.241 for layers with phase-pure fcc Ru(N) and 0.241 ≤ x ≤ 0.496 for zb RuN. The fcc Ru lattice parameter afcc increases linearly with residual nitrogen content, with dafcc/dx = 0.009 Å/% and 0.01 Å/% from experiment and simulations, respectively. Deposition at higher temperature (Ts = 100 °C) led to no residual nitrogen within the Ru lattice and a phase change from zb (Ts = 25 °C) to hcp Ru (Ts = 100 °C). Alternatively, annealing experiments performed on the fcc Ru layer at Ta = 100 and 400 °C led complete loss of nitrogen by ta = 24 hours and 0.5 hour respectively, as well as a phase change from fcc to hcp Ru in both cases. The overall results demonstrate the use of nitrogen to facilitate synthesis of metastable fcc Ru.
Description
August2025
School of Engineering
School of Engineering
Full Citation
Publisher
Rensselaer Polytechnic Institute, Troy, NY