Influence of indenter tip diameter and film thickness on flat indentations into elastic-plastic films by means of the finite element method

Large imprint depths on Back End Of Line bond pad stacks during wafer testing and wire bonding may lead to mechanical failure. In order to reduce or avoid this failure it is essential to know how input geometrical parameters, namely the indenter tip radius and the film thickness, affect the imprint depth. The elastic-plastic behavior of the pad layer leads to unpredictable stresses generated ahead of the indenter, which hinders the possibility of an analytical approach. In this work, we investigate finite element models of an imprinting scenario of a flat cylindrical indenter into a thin aluminum elastic-plastic film on a rigid substrate. The ratio between indenter tip diameter and film thickness appears to have a large effect on the load-displacement curves which has not been described in detail so far. Previous analytical approaches are critically reviewed. The model results are compared with existing analytical equations and with experiments.