Barrier layers for copper metallization

Abstract

Semiconductor device miniaturization as proposed by Moore’s law, results in the demand for new materials to replace Aluminium (Al) interconnect for such devices. Copper (Cu) was chosen as the new interconnect and so barrier layers like Ta/TaN suitable for Al interconnect can no longer work efficiently to prevent diffusion or as a seed layer for Cu deposition.

Ruthenium-Titanium Nitride (Ru-TiN) was proposed as a viable barrier for Cu metallization. Pure Ru barrier layer requires no seed layer for the deposition of Cu. Nitrogen (N) incorporated in Ru barrier film offers even better barrier properties in curbing Cu diffusion but once N effusion occurs at 275oC, the barrier fails as Cu diffusion through easy pathways is possible. Subsequently to improve the thermal stability and suppressing the diffusion of N, it was recommended that a strong nitride former like Ti could be added into the film.

Barrier properties, compositions and topography were obtained through X-ray Diffraction (XRD), Atomic Force Microscopy (AFM), X-ray Photoelectron Spectroscopy (XPS) and sheet resistance measurement. XRD provides evidence in the amorphous and crystallization process of barriers at different temperatures. XPS data indicates the presence of elements and compounds on the surface of the sample. It shows that Ti did form bond with N and Ru is in the metallic state. Ru crystallization process can be observed from the sheet resistivity test and XRD results. AFM images illustrate the topography of the samples.

Samples (Set 1 and Set 2) sputtered with different N2 concentration and Ti power densities demonstrate different film thickness and resistivities at the as-deposited state however after annealing, all samples show similar changes to the microstructure.

It was found that Ru-TiN barrier without Cu top layer is a feasible barrier up to 700oC. No silicides were detected at all elevated temperatures. However, with Cu layer deposited on Ru-TiN, silicide formations were being detected at lower temperatures.