CRACKING OF THIN-FILMS BONDED TO ELASTIC-PLASTIC SUBSTRATES

Thin bonded films have many applications. In information storage and p rocessing systems, for example, conducting, semiconducting and insulat ing films are used in integrated circuits, and thin magnetic films are used in disk storage systems. In many cases, thin bonded films an 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 crack s or flaws oriented normal to the film-substrate interface propagate ( or ''channel'') across the film. It is assumed that the film is brittl e and the substrate is ductile. Plane strain fracture analyses are use d to investigate the channel cracking of elastic thin films in residua l tension in the presence of yielding in the substrate material. Altho ugh crack channeling induces yielding in the substrate, channel crack extension in the brittle film occurs under small scale yielding condit ions. The case of an elastic film bonded to an elastic substrate has b een considered in earlier work, and is used as the basis for the curre nt study. A numerical model is used to extend the results from the ful ly elastic problem so that plastic yielding of the substrate is allowe d. Results are presented for an elastic-perfectly plastic substrate an d 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 frac ture quantities on the normalized loading over a wide range of materia l mismatches. In addition, a method is presented by which shear lag mo deling can be extended to cases in which the substrate exhibits strain hardening. Copyright (C) 1996 Elsevier Science Ltd