Crack Tip Plasticity Influence on Cracks Approaching Cu-Si Interface

摘要

Thin copper (Cu) films are widely used in numerous electronic applications where such films are frequently applied on silicon (Si) substrate. These devices can be exposed to severe loading conditions during their lifetime, induced by thermal, static and/or fatigue loading, which may lead to critical failure of such components due to cracking. Such failure may lead to a shortcut or disconnection of the electrical circuit which, in majority of cases, renders the whole device useless. Experimental studies showed that when short cracks propagate through Cu layer towards the Si interface, the crack tip plasticity tends to influence the crack driving force more when it approaches the Cu-Si interface, where the crack propagation speed is decreasing. This fact leads to possible decrease of the crack driving force (shielding effect) due to the Cu-Si interface. The change of the crack driving force in such systems was investigated in this study by the means of finite element calculations, whereas the cases of elastic-elastic and plastic-elastic transitions at the Cu-Si interface were considered and compared. The impact of the crack tip plastic region and its characteristic dimension (in comparison with the thickness of Cu film) on the crack driving force was quantified and related to the changes in crack driving force magnitude. Several magnitudes of loading (for models with the same yield stress) were considered to simulate different stages of evolution of the crack tip plastic zone. The results then divided these stages into several groups from no plasticity-induced influence on fully plastic Cu film. These findings can lead to better understanding of the crack propagation through the thin plastic films on elastic substrates and permit a better lifetime prediction of electronical or electro-mechanical devices.