A micro-mechanics investigation on the bonding strength of heteroepitaxical film or strip on a hard substrate

The bonding strength of deposited metallic thin films or strips on a hard substrate is investigated through micro-strength tests and micro-mechanics analyses. The combination of Ti:W-Au/Cu deposited directly on SiO₂/Si substrate was taken as an example in the experiments conducted. Investigations and analyses resulted in a general model concerning the separation mechanism of thin ductile films from a hard brittle substrate. This separation model leads to some systematic methods of improving strip-substrate interconnection strength. Besides strip-substrate adhesion and internal stresses, strip stiffness turns out to be a key factor for the bonding strength of such a system. Hardening processes such as post-deposition heat treatment and work-hardening are proved to be effective in strengthening the bonding strength. These investigations reveal a principle of mechanical compatibility concerning interface micro-deformation. Instead of thin and flexible strips, thick and rigid strips have better compatibility with a hard substrate if the bonded combination is subjected to mechanical loading rather than thermal loading. As far as interface stresses are concerned, a stress distribution strategy is proposed based on the same principle. This original understanding will help to improve common bonding strength problems of various interconnection structures and materials combinations in manufacturing processes.