Applications of Two-Dimensional Layered Materials in Interconnect Technology

Copper (Cu) has been used as the main conductor in interconnects due to its low resistivity. However, because of its high diffusivity, diffusion barriers/liners (tantalum nitride/tantalum; TaN/Ta) must be incorporated to surround Cu wires. Otherwise, Cu ions/atoms will drift/diffuse through the inter-metal dielectric (IMD) that separates two distinct interconnects, resulting in circuit shorting and chip failures. The scaling limit of conventional Cu diffusion barriers/liners has become the bottleneck for interconnect technology, which in turn limits the IC performance. The interconnect half-pitch size will reach ~20 nm in the coming sub-5 nm technology nodes. Meanwhile, the TaN/Ta (barrier/liner) bilayer stack has to be > 4 nm to ensure acceptable liner and diffusion barrier properties. Since TaN/Ta occupy a significant portion of the interconnect cross-section and they are much more resistive than Cu, the effective conductance of an ultra-scaled interconnect will be compromised by the thick bilayer. Therefore, two dimensional (2D) layered materials have been explored as diffusion barrier alternatives owing to their atomically thin body thicknesses. However, many of the proposed 2D barriers are prepared at too high temperatures to be compatible with the back-end-of-line (BEOL) technology. In addition, as important as the diffusion barrier properties, the liner properties of 2D materials must be evaluated, which has not yet been pursued.

The objective of the thesis is to develop a 2D barrier/liner that overcomes the issues mentioned. Therefore, we first visit various 2D layered materials to understand their fundamental capability as barrier candidates through theoretical calculations. Among the candidates, hexagonal-boron-nitride (h-BN) and molybdenum disulfide (MoS₂) are selected for experimental studies. In addition to studying their fundamental properties to know their potential, we have also developed techniques that can realize low-temperature-grown 2D layered materials. Metal-organic chemical vapor deposition (MOCVD) is adopted for the synthesis of BEOL-compatible MoS₂. The electrical test results demonstrate the promises of integrating 2D layered materials to the state-of-the-art interconnect technology. Furthermore, by considering not only diffusion barrier properties but also liner properties, we develop another 2D layered material, tantalum sulfide (TaS_x), using plasma-enhanced chemical vapor deposition (PECVD). The TaS_x is promising in both barrier and liner aspects and is BEOL-compatible. Therefore, we believed that the conventional TaN/Ta bilayer stack can be replaced with an ultra-thin TaS_x layer to maximize the Cu volume for ultra-scaled interconnects and improve the performance. Furthermore, Since via resistance has become the bottleneck for overall interconnect performance, we study the vertical conduction of TaS_x. Both the intrinsic and extrinsic properties of this material are investigated and engineering approaches to improve the vertical conduction are also tested. Finally, we explore the possibilities of benefiting from 2D materials in other applications and propose directions for future studies.