Advanced characterisation of thermo-mechanical fatigue mechanisms of different copper film systems for wafer metallizations

In this study, two different electrodeposited 5 μm thick copper films were subjected to thermal cycling. The microstructural evolution of both films was studied with a site specific tracking technique. It was observed that the initially similar microstructures develop differently upon cycling with respect to grain size and texture. In detail, a Cu film which contains residual inorganic species from the organic additives used during Cu plating, in the ppm regime, showed a constrained surface roughness evolution and marginal grain growth coupled with a stable twin boundary network. Furthermore, voiding in the film interior was observed. In comparison, a high-purity Cu film exhibited strong surface roughening in conjunction with pronounced grain growth promoted by twin boundary migration. Moreover, the film showed a (100) texture intensity increase as a result of strain energy minimization upon cycling and no void formation. These observations underline the influence of film processing related impurities on the thermo-mechanical behaviour of Cu thin films.