Perspective on high-entropy alloys and high-entropy nitrides as diffusion barriers for copper metallization

Abstract

The adoption of copper as the material of choice for electrical contacts in the semiconductor industry over three decades ago has been a key enabling factor for continuing the miniaturization trend that has led to faster and more efficient chips with specific functionalities. From the advanced packaging level where hybrid bonded structures are integrated in 3D with features sub-10 μm, to the front end of the line with dimensions sub-10 nm, the use of copper for signal and power transmission poses the risk of electromigration into the nearby dielectric or into silicon. This risk is currently mitigated by using diffusion barriers, thereby enabling significant improvements in the reliability and lifetime of the contacts. The diffusion barrier currently used in the semiconductor industry is a layer of TaN that can be at least ∼ 3 nm. However, TaN is not a substrate that naturally allows for the conformal growth of copper, so surfactants (liner materials) need to be added so that metallic copper can be deposited/electroplated into vias. While the dimensions and performance of the diffusion barrier are more critical close to the front of the line (FEOL), any material advances can also be leveraged to mitigate fabrication and alignment constraints faced at the back end of the line (BEOL) and the advanced packaging level. The present paper gives an account of the possible alternatives for the copper diffusion barrier, highlighting mainly high-entropy alloys (HEAs) and high-entropy nitrides (HENs). The promise of these materials as efficient barriers relies either on the absence of grain boundaries in amorphous HEAs that would act as fast diffusion avenues for copper, or on the presence of small interstitial spaces and high vacancy formation energies (in HENs) that hamper diffusion. We also mention a more drastic direction that may emerge in the future, in which copper could be replaced by other metals or intermetallic compounds.