Formation of Surface Sulfide Layer on Liners for Reducing Interface Scattering of Ruthenium Interconnect

Ruthenium with a low bulk resistivity, a high melting point and a short mean free path of electron is of great potential to replace copper as the next-generation metallization for low-resistance interconnects. However, the drastic resistivity scaling with the continual decrease of linewidth due to the scattering of electrons at the interfaces with dielectrics or liners needs to be minimized. Much specular scattering is expected when a layer of two-dimensional materials such as disulfides is applied to the interface, retaining the original electron density of state of the metals. Hence in this study, three types of liner materials (tantalum-titanium alloy, nitride and oxide) were deposited on silicon substrates by physical vapor deposition. Instead of complicated transfer process, sulfurization was applied onto the surface of the liner layers in a tube furnace at different temperatures, simply by using sulfur vapor or hydrogen sulfide gas, to form an ultrathin sulfide layer. Thin ruthenium films with different thickness were then deposited on the sulfurized liners for investigating the influence of the sulfide layer on interface scattering. The microstructure, chemical composition and bonding configuration of the sulfurized surface were characterized, and the resistivity scaling of the ruthenium films was examined with implementation of the Fuchs-Sondheimer and Mayadas-Shatzkes models. Experimental results indicate that, with the sulfur vapor and particularly the hydrogen sulfide gas at an appropriate temperature, an ultrathin sulfide layer of only a few nanometers thick was successfully formed on the top surface of the liners and changed the binding states from a metallic, nitride or oxide state to a sulfide state with altered binding energy. Microscopic observations revealed a crystalline structure of tantalum-titanium binary sulfide being formed, similar to the structure of two-dimensional sulfide materials. With the surface sulfide layer on the liners, the interface diffuse scattering was effectively reduced to elevate the permeability of electrons from 0.2 to 0.9, and the resistivity scaling was accordingly lessened for at least 40% at a ruthenium film thickness of below 10 nm, without a significant loss of interfacial adhesion strength.