Bismuth Confinement: A Strategy for Low Resistance and Good Thermal Endurance of Integrated Contacts to MoS2

Abstract

Two-dimensional (2D) transition metal dichalcogenides (TMDs) are preponderant candidates for advanced nanoelectronics owing to their atomically thin body, which could enable excellent electrostatic control. High current density NMOS transistors have been demonstrated with semimetal Bi and Sb contacts. Both these semimetals have low melting temperatures and thus limit their integration flow compatibility with the modern integrated circuit technologies. Integration of those metal layers requires exposing the transistors to 400 °C H₂ environment for extended periods of time. Using a Bi confinement strategy through AlO_x and/or TiN barriers, R_C values below 200 Ω·μm are demonstrated while preserving device performance after forming gas annealing at 400 °C for up to 10 min; this finding is validated through the characterizations of multiple devices. Furthermore, by using fab-like SiO₂ trench structures with TiN barrier and W plug in addition to the Bi, we confirm the thermal stability of the structures. The thermal stability is found to be good at 400 °C under several process environments, such as N₂, forming gas, and vacuum. Cross-sectional TEM and EDX confirm that Bi contacts remain fully confined without diffusion or outgassing, establishing the process compatibility of this confinement method. This study establishes a fab-compatible confinement strategy that enables low R_C monolayer MoS₂ transistors while maintaining thermal robustness.

Keywords: back-end-of-line (BEOL) integration; contact resistance; monolithic 3D integration; semimetal contacts; thermal stability; two-dimensional semiconductors.