Interconnect Technologies in The Ultrathin Limit: Performance of Standard and 2D Barriers and Liners in The Nanometer Thickness Regime

Copper wires in the back-end-of-line (BEOL) interconnects typically require two layers surrounding them: a diffusion barrier preventing Cu from diffusing into the dielectrics, and a liner, ensuring cohesive Cu wires and smooth interfaces. The current industry standards for barrier and liner applications include TaN and Ta, respectively, which then form a bilayer with a typical thickness of ~4 nm. However, with the advent of the 3-nm technology nodes, interconnect half-pitches have reached the scales of 10 nm. Consequently, the barrier/liner bilayer would represent a significant fraction of the wire cross-section and increase the wire resistance and cause significant RC delays. The polycrystalline nature of the conventional TaN/Ta bilayer films prevents formation of a continuous and effective film in the ultrathin regime. Moreover, their performance as a function of the film thickness is poorly understood, especially in ultrathin films.<br/>In recent years, this expected limitation of conventional barriers and liners has motivated intense efforts to replace them with 2D materials, as these can be continuous yet atomically thin. However, no approach reported thus far fulfills the requirements for BEOL integration, namely low temperature (&lt;450 °C), uniform and scalable growth, thickness control and film conformality to the substrate. We have recently developed a novel approach to growing layered films of WS₂ that fulfills all of these requirements. Our WS₂ films function as bifunctional liners and Cu diffusion barriers. We demonstrate these functionalities through electrical measurements that assess Cu film conductivity in the thin film limit, where we see a significant improvement of several orders of magnitude in Cu conductivity for Cu films of 10 nm. Their performance as diffusion barriers was assessed through time-dependent breakdown measurements, where we see the projected device lifetime increases by one order of magnitude through the utilization of single layers of WS₂. To benchmark the performance of this new class of 2D barrier/liner layers, we compare the level of performance as a function of the film thickness both in 2D layered WS₂ films and in polycrystalline TaN films, in regimes from 4-nm thicknesses down to the ultrathin ~1 nm limit. This approach provides an ability to identify and study the thickness regime in which 2D-layer films are expected to outperform the standard TaN layers. We will also discuss measurements and modelling approaches toward evaluation of temperature-dependent diffusion barrier properties for standardized barrier evaluation in the ultrathin film limit.