Study on CoAl intermetallic compound films for advanced interconnect applications: Experimental and DFT investigations

Abstract

This paper reports the results of a materials study on co-sputtered CoAl thin films for interconnect applications based on both experiments and computations. We modulate the composition and microstructure of the CoAl films by controlling the deposition conditions (sputtering power, substrate temperature, and deposition time). The morphology and microstructure of the films are investigated using scanning electron microscopy, X-ray diffraction, and transmission electron microscopy. The measurement of the film resistivity reveals a strong stoichiometry effect: the lowest (∼30 μΩ∙cm) for the stoichiometric film. It also discloses a typical trend of size dependence: the resistivity increases rapidly with the thickness decreasing (from 38 μΩ∙cm for 43 nm to 72 μΩ∙cm for 13 nm). We also compute the resistivity dependence on the film thickness by solving the Boltzmann transport equation with inputs of the electronic and phononic structures from the first-principles density functional theory and the electron-phonon relaxation time modified according to the film thickness. Furthermore, we devised a simple model considering the effects of grain sizes and scattering at grain boundaries on the resistivity to describe the transport behavior in CoAl. Our calculated results were found consistent with our experimentally observed ones.