Resistivity Scaling of CuAl2-x Nanowires for Post-Cu Interconnects

Due to extreme miniaturization of integrated circuits, the dimensions of copper interconnects have decreased below copper's electron mean free path (40 nm), leading to detrimental increases in power dissipation and signal delays coming from the interconnect stacks. A potential route to solve this challenge is to find an alternative material with better nanoscale resistivity scaling than copper for M0 and M1 interconnect stacks in the Back-End-of-Line (BEOL) layers. CuAl2 is a promising candidate with its low bulk resistivity (6.5 μΩ·cm), small mean free path (14 nm), high electromigration resistance compared to copper, and demonstrated promising resistivity scaling in thin films. Here, we report the resistivity of CuAl2-x nanowires with diameters ranging from 30 nm to 70 nm. The CuAl2-x nanowires are fabricated using thermomechanical nanomolding. The nanowires possess a CuAl shell with a CuAl2 core due to the loss of Al during the mold etching process. The nanowire cores are also Al-deficient with the deficiencies ranging between 3-10%, observed by electron energy loss spectroscopy. At diameters below 45 nm, the resistivities range between 8 - 35 μΩ·cm while larger diameter nanowires show higher resistivities. Theoretical calculations show both Al vacancies and substitutional doping will lead to increased scattering and thus increased resistivity. Substitutional doping of Cu at Al sites is more likely than Al vacancies due to its lower formation energy. Our work shows that precise stoichiometric control is necessary to realize the promising properties of CuAl2 for interconnect applications.