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.