In Depth Study of In-Plane Thermal Conductivity of Thin Diluted AlCu Alloy Films as Metallic Interconnects

Understanding the thermal conductivity trends in metallic thin films has attracted tremendous interest over the past decades due to their wide applications as interconnects in integrated circuits of electronic devices. As the dimension of the metallic interconnects has decreased comparable to the electron mean free path, size effects on the thermal transport process becomes of high importance. Reduction in size also leads to a reduction in thermal conductivity of the thin metallic films. Such a reduction will then worsen the heat transfer in the circuits and, in most cases, lead to the burnout of the circuit following by the device failure. Upon reduction in size, thermal and electrical conductivity of these interconnects are governed by emerging phenomena that challenge our understanding of the thermal and electrical conductivity of bulk materials. Matthiessen’s rule combines all the non-ideal effects together through a summation rule to calculate the electronic mobility and thermal conductivity of systems subject to multiple scattering sources. Therefore, it can be readily applied to estimate the relative magnitude of different scattering mechanisms. Matthiessen’s rule has been widely used to predict the phonon thermal conductivity of dielectric solids and the electrical conductivity or electronic thermal conductivity of metallic materials.<br/>In this work we address the limitations of Matthiessen’s rule in estimating thermal conductivity of Al_99.5Cu_0.5 (AlCu) films at nanometer length scales and as a result of alloying. We employ ultrafast laser spectroscopic techniques such as time-domain thermoreflectance (TDTR) to directly measure the in-plane thermal conductivity of AlCu films ranging from 24 to 175 nm. We investigate the in-plane thermal conductivity trend with film thickness down to thicknesses comparable to the electron mean free path of Al which is ~19 nm. In principle, one can attribute deviations from Matthiessen’s rule to three causes: (a) changes in the band structure and the phonon spectrum due to alloying (b) additional temperature-dependent scattering processes associated with solute atoms, in particular phonon-assisted impurity scattering processes, where electrons and phonons are scattered by solute atoms, and (c) the “two-band” effect where two or more groups of electrons and phonons with different relaxation times contributing to the conductivities. In addition to the experimental thermal conductivity measurements and in order to gain a better understanding of the role of electron-phonon scattering in total thermal conductivity, we measure scattering rate with IR-VASE, an spectroscopic ellipsometry technique. All the provided characterization techniques enable us to validate the contributing share of the electron-electron, electron-phonon, or electron-system boundaries scattering in total thermal conductivity of these films.<br/><br/>SNL is managed and operated by NTESS under DOE NNSA contract DE-NA0003525.