An experimental approach for studying the creep behavior of thin film/ substrate interfaces

Abstract

Large shear stresses often develop at the interface between dissimilar materials in microelectronic devices, when they are subjected to thermo-mechanical excursions. These stresses can facilitate diffusionally accommodated interfacial sliding, or creep. A driving factor for these stresses is the thermal expansion mismatch between the adjoining materials. For narrow thin film lines, these stresses may exist over a large fraction of the film-substrate interface. This thesis explores methodologies to measure the kinetics of interfacial creep at model Al thin film/silicon substrate interfaces. A method of sample production, which involved diffusion bonding a polished Si substrate to the surface of a thin Al film deposited on a second Si substrate was developed (Si/Al/Si sandwich). When loaded edge-wise in compression, the Al thin film - Si interface are loaded in shear. By measuring the relative displacements between the two Si substrates, the interfacial displacement rates at varying temperatures and stresses were experimentally determined. In accordance with previous results, the kinetics was given by a diffusional creep law with a threshold stress, and an activation energy representing interfacial diffusion. The activation energy was found to be unusually low, and further experimental and modeling studies are needed to better understand its origin.