Some mechanics for the adhesion of thin films

Abstract

There has recently been an increased interest in the mechnical properties of thin films, owing to their importance in electronic packaging applications. A review of some of the failure mechanisms associated with these films are presented in this paper. Films may fail by a number of different mechanisms depending upon the geometry and the stresses in the film. These include buckling, “mud-cracking”, and/or delamination. A complete understanding of film failure requires consideration of the forces that drive crack growth, those that resist it, and an appreciation of what determines the mode of failure. Some of these aspects are explored in this paper by a discussion of the decohesion of tensile films from a brittle substrate Although failure may occur by delamination of the interface, often it is the substrate that cracks, rather than the interface: the stresses in the film tend to drive the crack into the substrate. This tendency may be counteracted, however, if a weak interface makes it energetically favourable for failure to occur at the interface. As well as influencing the preferred crack trajectory, the stresses may also influence the apparent fracture toughness of the interface. This is considered in the last section of this paper and is illustrated by a discussion of some model experiments.