Cracking of thin films bonded to elastic-plastic substrates

Abstract

Thin bonded films have many applications. In information storage and processing systems, for example, conducting, semiconducting and insulating films are used in integrated circuits, and thin magnetic films are used in disk storage systems. In many cases, thin bonded films are in a state of residual tension, which can lead to film cracking. Because cracking can alter desired film properties, methods for predicting it are needed. The geometry considered in this work is one in which cracks or flaws oriented normal to the film-substrate interface propagate (or “channel”) across the film. It is assumed that the film is brittle and the substrate is ductile. Plane strain fracture analyses are used to investigate the channel cracking of elastic thin films in residual tension in the presence of yielding in the substrate material. Although crack channeling induces yielding in the substrate, channel crack extension in the brittle film occurs under small scale yielding conditions. The case of an elastic film bonded to an elastic substrate has been considered in earlier work, and is used as the basis for the current study. A numerical model is used to extend the results from the fully elastic problem so that plastic yielding of the substrate is allowed. Results are presented for an elastic-perfectly plastic substrate and for substrates exhibiting strain hardening. A simple shear lag model of the problem without hardening in the substrate is discussed, which gives reasonable predictions for the dependence of dimensionless fracture quantities on the normalized loading over a wide range of material mismatches. In addition, a method is presented by which shear lag modeling can be extended to cases in which the substrate exhibits strain hardening.