The replacement of aluminum by copper as interconnect metal in computer chips was and still is driven by the necessity to enhance the current density thus enabling higher packaging densities, a fact that correlates directly with faster, smaller, and less energy consuming devices. The usage of copper, however, leads to new technological challenges which are caused by its mechanical properties on one hand side and by its tendency to migrate into dielectric and/or semiconducting layers on the other hand side. To prevent such diffusion processes, very thin layers consisting of tantalum and tantalum nitride or titanium and titanium nitride are deposited.
A non-contact, non-destructive, short-pulse-laser-acoustic method is used to determine the mechanical properties of the barrier layers and of the copper layer. Mechanical waves are excited and detected thermoelastically using laser pulses of 70 fs duration. For metals this leads to wavelengths of 10 to 20 nm and the corresponding frequencies amount to 0.3 to 0.6 THz. Thin film measurements of buried diffusion layers are provided and compared with Scanning Electron Microscopy measurements (SEM), Transmission Electron Microscopy (TEM), and Rutherford Backscattering Spectroscopy measurements (RBS). Results of a thermo-elasto-mechanical simulation are presented.
Current limits of the presented method are discussed and future directions of the on-going research project are presented.