Characterization of Ta and TaN diffusion barriers beneath Cu layers using picosecond ultrasonics

In computer chips, aluminum is being replaced with copper in order to produce smaller, faster and more efficient electronic devices. The usage of copper allows higher current densities and thus higher packaging densities than aluminum. However, copper leads to new challenges and problems. It has different mechanical properties and a tendency to migrate into the surrounding dielectric and/or semiconducting layers. These diffusion processes can be prevented by so called diffusion barriers. A diffusion barrier is a very thin layer consisting of tantalum and tantalum nitride or titanium and titanium nitride, deposited between the copper and the substrate. A pump-probe setup is used to determine the mechanical properties of the barrier layers and of the copper layer. This short-pulse-laser-acoustic method is contact-free and non-destructive. Mechanical waves are excited and detected thermoelastically using laser pulses of 70 fs duration. Thin film measurements of buried diffusion layers are provided and compared with scanning electron microscopy (SEM), transmission electron microscopy (TEM), and Rutherford Backscattering Spectroscopy measurements (RBS). Results of a thermo-elasto-mechanical simulation are presented and a short overview of the simulation procedure is given. Current limits of the presented method are discussed and future directions of the on-going research project are presented.