In situ TEM study of stability of TaRhx diffusion barriers using a novel sample preparation method

The atomic diffusion mechanisms associated with metallurgical failure of TaRh_x diffusion barriers for Cu metallizations were studied by in situ transmission electron microscopy (TEM). The issues related to in situ heating of focused ion beam (FIB) prepared cross-sectional TEM samples that contain Cu thin films are discussed. The Cu layer in Si/(13 nm)TaRh_x/Cu stacks showed grain growth and formation of voids at temperatures exceeding 550 °C. For Si/(43 nm)TaRh_x/Cu stacks, grain growth of Cu was delayed to higher temperatures, i.e., 700 °C, and void formation was not observed. Extensive surface diffusion of Cu, however, preceded bulk diffusion. Therefore, a 10 nm film of electron beam evaporated C was deposited on both sides of the TEM lamellae to limit surface diffusion. This processing technique allowed for direct observation of atomic diffusion and reaction mechanisms across the TaRh_x interface. Failure occurred by nucleation of orthorhombic RhSi particles at the Si/TaRh_x interface. Subsequently, the barrier at areas adjacent to RhSi particles was depleted in Rh. This created lower density areas in the barrier, which facilitated diffusion of Cu to the Si substrate to form Cu₃Si. The morphology of an in situ annealed lamella was compared with an ex situ bulk annealed sample, which showed similar reaction morphology. The sample preparation method developed in this study successfully prevented surface diffusion/delamination of the Cu layer and can be employed to understand the metallurgical failure of other potential diffusion barriers.