November 21, 2024
Materials Discovery using Machine Learning and DFT for Interconnects
Lily Jade Joyce, Materials Science & Engineering, Rensselaer Polytechnic Institute
Modern transistor density on microchips requires nano-scale transistors, which necessitates the use of nano-scale wires. At this scale, traditional wires such as copper hinder performance due to a massive increase in resistance as the wire dimensions decrease. As shown previously, materials with Fermi velocities directed primarily along the direction of current can combat the resistivity scaling due to side-wall scattering[1]. We predict the resistivity scaling for thousands of materials utilizing machine learning (ML) models trained on the data from the previous study. For materials whose predicted resistivity scaling is competitive with copper, we validate the ML predictions using DFT. From there we compute electron-phonon interactions for candidates with competitive DFT-predicted resistivity scaling to estimate the resistivity as a function of dimension. We identify three materials with potential to outperform copper at dimensions less than 10 nanometers. [1] S. Kumar, C. Multunas, B. Defay, D. Gall and R. Sundararaman, “Ultralow electron-surface scattering in nanoscale metals leveraging Fermi-surface anisotropy”, Phys. Rev. Mater. 6, 085002 (2022)
An Oxidation-Resistant High Entropy Alloy for Aqueous Aluminum-Battery Chemistries
Apurva Anjan, MANE, Rensselaer Polytechnic Institute
Today rechargeable Lithium (Li)-ion batteries are ubiquitous from portable electronics to electric vehicles and grid energy storage. However, Li-ion technology may not be sustainable in the long-run; Li is scarce and comprises < 0.0065% of the earth’s crust. Aluminum (Al) on the other hand is the most earth-abundant metal and offers an outstanding specific capacity of ~2,980 mAh g-1 due to three electron transfer per Al atom. Despite this promise, there are no commercial Al-based batteries today. Traditional aqueous Al-metal batteries face a major obstacle: the formation of a passivating Al₂O₃ layer that blocks Al³⁺ ion movement. My talk will introduce an innovative solution—a specially designed Al-based high-entropy alloy (HEA) that enables efficient Al³⁺ transport while stabilizing the Al-metal/water interface. First-principles calculations reveal that the solid-solution structure of the HEA leads Al atoms to transfer electrons to neighboring elements, which reduces oxidation thermodynamically. Additionally, the HEA’s oxidation process is slower compared to pure Al, keeping the alloy/water interface open for Al³⁺ transport with minimal overpotential. Taking advantage of this, we demonstrate a high-performance aqueous Al–selenium (Al–Se) battery that leverages this unique chemistry.
October 17, 2024
Surface-Induced Effects in Ferroelectric BaTiO3 Thin Films
Anoop Kumar Kushwaha, Materials Science and Engineering, Rensselaer Polytechnic Institute
Integrating ferroelectric thin films into microelectronic devices raises critical questions regarding their ferroelectric characteristics compared to bulk materials, particularly the surface effects on these properties. Using molecular dynamics simulations and the core-shell model of atomic interactions, we investigated the surface-induced effects on strain, stress, and polarization fields for in-plane and out-of-plane polarized BaTiO3 free-standing thin films. Our analysis revealed that surface effects are more pronounced in out-of-plane polarized films, with strain and stress propagating approximately 4 nm (10-unit cells) into these films. Notably, 4 nm-thick film behaves like linear paraelectric, while 12 and 20 nm films exhibit strong non-linearity without a hysteresis loop. We attribute this behavior to surface-reduced polarization switching barrier. Conversely, in-plane polarized films exhibit surface effects only within several atomic planes adjacent to the surfaces, with ~ 12 nm-thick film already displaying a clear polarization hysteresis loop. These findings highlight the substantial impact of surface effects on the ferroelectric behavior of thin films, which is crucial for their application in microelectronics.
Investigation of Electron Scattering at the Metal-Liner Interface
Sadiq Nishat, Materials Science and Engineering, Rensselaer Polytechnic Institute
Electron scattering at the interface between interconnect metals and liners is quantified using in situ transport measurements to examine the impact of surface chemistry and determine optimal materials that minimize the interconnect resistance and maximize back-end-of-line energy efficiency. The sheet resistance Rs of epitaxial 7-nm-thick Ru(0001)/Al2O3(0001) films with and without Ti cap layers (0.08-3.0 nm thick) is measured continuously during low-pressure 0.05 mTorr O2 exposure to simulate Ti-liners in contact with an oxidizing dielectric. Similarly, 10-nm-thick Cu(001)/MgO(001) films capped with varying thicknesses of Co (0.05-0.8 nm thick) are subjected to oxygen exposure with a linearly increasing oxygen partial pressure. Oxygen exposure causes only a slight (1%) increase in Rs for Ru(0001) surfaces but a substantial 28% increase for Cu(001). Both Ti caps on Ru(0001) and Co caps on Cu(001) cause an increase in the sheet resistance due to partially diffuse electron scattering caused by localized defect states at the interface. However, O2 exposure results in a decreasing resistance as Ti oxidation reduces the localized defect states, facilitating specular surface scattering and a reduced Ru resistivity. Conversely, Co capped Cu films exhibit a resistance increase during oxygen exposure which is attributed to atomic-level roughening during Co/Cu surface oxidation. The overall results suggest that engineering the interface chemistry and the interfacial density of states can be utilized to maximize the conductivity of narrow interconnect lines.
September 19, 2024
Single Micro-projectile Impact Testing: A New View on Extreme Rate Impacts
Ian Dowding, Department of Materials Science and Engineering, Massachusetts Institute of Technology
Extreme strain rate deformations are seen across many fields of science and engineering; from meteorite impacts and impact induced crystallographic phase changes to high-speed machining and additive manufacturing. Despite the range of applications, many common high-rate impact experiments are intrinsically limited to strain rates of only 104 s-1 before complicating the material deformation with a superimposing state of shock due to high impact pressures. However, recent advances in in-situ single micro-projectile impact testing have provided a new quantitative look into both unit deposition processes in metal additive manufacturing as well as extreme mechanics of materials, at rates above 106 s-1 and well below the onset of shock effects.
Here, using single microparticle impacts of Al particles on Al targets and Ti particles on Ti targets, we show that there exists a discernable particle size effect for the onset of impact induced metallic bonding. We then adapt this technique to become a dynamic microindentation test by using hard ceramic particles on Cu, Ti, and Au targets. Combining the impact experiments with post mortem electron and laser scanning confocal microscopy of each impact crater, we are able to assess the plasticity of metals under very high-rate deformations. These complimentary characterization methods provide multiple independent quantitative measurements of strength of the metal substrates. We then extended these experiments beyond ambient conditions to elevated temperatures, offering further insights into the role of thermal energy in extreme-rate mechanics.