Quasi-2D Materials for Ultra-Low Resistance Electrical Interconnects

The dramatic increase in the resistivity of interconnect lines with decreasing dimensions presents a significant bottleneck for further downscaling of integrated circuits.¹ This is because current interconnects use 3-dimensional metals which experience increased interface electron scattering as the interconnect dimensions approach their electron mean free path. A possible solution is to use metals with much lower electron mean free paths such as: W, Mo, and Ru. Metallic delafossite oxides are an alternative solution because of their inherent advantages over traditional metals such as, ultra-low room temperature resistivity, potential mitigation of interface/surface scattering due to their 2D Fermi surface, potentially decreased likelihood of electromigration, and potentially better compatibility with low-K oxide dielectrics. Metallic delafossite can prove to be a disruptive new material for ultra-scaled electrical interconnects.<br/><br/>Delafossites are layered oxides with the formula ABO₂ where A is a metal cation that forms 2D sheets separated by the BO₂ transition-metal oxide octahedra. In this study we focus on metallic delafossites PtCoO₂ and PdCoO₂ because of their ultra-low room temperature resistivity of 2.1 μΩ.cm and 2.6 μΩ.cm, respectively, which is comparable to the current semiconductor industry standard interconnect metal, Cu.² The metallic delafossite structure has an anisotropic nature with resistivity along the c-axis a factor of 1000 higher than resistivity within the Pt/Pd sheet. Due to the layered crystal structure, the Fermi surface of the metallic delafossites is cylindrical as for a 2D metal. This quasi-2D crystal structure can potentially mitigate interface and surface scattering since the electron Fermi velocity does not have components perpendicular to the Pd/Pt sheets. This can potentially overcome the resistivity penalty encountered by conventional 3D metals in ultrathin films (&lt; 20 nm). Additionally, the unique Fermi surface topology allows for an electron-phonon coupling constant that is a factor of 3 lower than copper.³<br/><br/>In this study we investigate the structural and electrical properties of MBE-grown PdCoO₂ delafossite films. High-resolution X-Ray diffraction (HRXRD) confirms that the films are phase-pure. XRD phi scans, however, reveal in-plane twinning in these films, and the lateral size of the rotational twin domains are ~17 nm based on skew-symmetric rocking curves. We measured the resistivity of the films using a van der Pauw geometry and modelled the resistivity scaling with film thickness using Fuchs-Sondheimer (FS) and Mayadas-Shatzkes (MS) model. The upshot is that a 50 nm thick PdCoO₂ film has a resistivity of 5 µΩ.cm. Based on our resistivity fitting we find that the twin boundaries have a very low electron reflection coefficient of ~5%. This is extremely encouraging since atomic layer deposition (ALD) which is a back-end-of-the-line (BEOL) compatible synthesis technique will likely yield highly twinned delafossite thin films.<br/><br/>¹ D. Gall, J. Appl. Phys. <b>127</b>, 050901 (2020).<br/>² V. Sunko, P.H. McGuinness, C.S. Chang, E. Zhakina, S. Khim, C.E. Dreyer, M. Konczykowski, M. König, D.A. Muller, and A.P. Mackenzie, Phys. Rev. X <b>10</b>, 021018 (2020).<br/>³ C.W. Hicks, A.S. Gibbs, A.P. Mackenzie, H. Takatsu, Y. Maeno, and E.A. Yelland, Phys. Rev. Lett. <b>109</b>, 116401 (2012).