Cobalt advanced barrier metallization: A resistivity composition analysis

Abstract

Cobalt is a high-potential material applicable as liner and seed replacement in BEoL metallization schemes. To enable seedless plating, the resistivity of thin cobalt liners (<5nm) has to be controlled. In the presented paper, the impacts of impurities, grain boundary scattering, surface scattering and roughness on the resistivity of thin cobalt layers is evaluated and modeled. For analysis, i-PVD TaNx barrier films are covered by MOCVD cobalt films with thicknesses in the range of 2–90nm. Film stacks are deposited in a 300mm AMAT Endura2™ cluster deposition tool without breaking the vacuum. An optional heat treatment at 400°C is applied to initiate morphology modification. The amounts of precursor induced impurities quantified by in situ ARXPS are found to be independent of the deposited film thickness. Ex situ characterizations of cobalt films are performed by four point probe measurement, XRR, AFM and TEM revealing resistivity, thickness, roughness and morphology of the cobalt films. A comparison of the measured data with established models for thin film resistivity from Fuchs–Sondheimer, Mayadas–Shatzkes and Namba shows the tremendous impact of the film roughness on resistivity. With implementing a modification on the Namba model to include thickness dependent roughnesses it is possible to reproduce the measured data of the annealed samples very well. Due to the good correlation between measured and calculated resistivity values and the reduced number of fitting parameters, the measurement method can be also used as a simple way to control thin film thicknesses.