Mechanisms Active during Fracture under Constraint
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Mechanisms Active during Fracture under Constraint Robert F. Cook and Z. Suo Abstract Many advanced technologies center on devices of small feature sizes made of diverse materials. Internal stresses that arise in the devices during fabrication and use can result in fracture. Fracture of an individual feature in such a device may impair the function of the device. The materials surrounding the feature have a constraining effect on the elastic energy available to drive the fracture, the plastic flow associated with the fracture, and sometimes even the atomic processes at the crack tip. This article reviews fracture behavior in small structures, several distinct roles played by plasticity, and bond-breaking kinetics. Research challenges are also outlined. Keywords: adhesion, brittle materials, ductility, fracture, stress modeling, thin films.
Introduction Fracture at small length scales is a concern in many advanced technologies. Microelectronic, magnetic storage, and photonic devices make great use of oxide, nitride, and carbide dielectrics in submicrometer multilayer thin-film form. Liquid-crystal displays and optical fibers consist of glasses in sheet, particle, and fiber form. Hightemperature turbines and engines rely on ceramic thermal-barrier coatings for blades and cylinders. Structural composites of all sorts use particles, whiskers, fibers, and platelets. These constrained geometries localize cracking so that fracture may not compromise the structural integrity of the entire component, but it may still impair the component’s electrical, magnetic, optical, or thermal functions. For example, localized fracture of a dielectric film adjacent to a conducting line in a microelectronic chip-interconnection structure leads to electrical failure of the chip (which is of great importance), but not structural failure (which is of minor importance). Thin-film fracture is a widely studied example of constrained fracture. Several cases are shown in Figures 1a–1d, including film splitting under tension, in which cracking occurs perpendicular to the film– substrate interface; film delamination under tension, in which cracking occurs parallel to the film–substrate interface, either at the interface (adhesive failure) or in the
MRS BULLETIN/JANUARY 2002
substrate (cohesive failure); and film delamination under compression, in which cracking occurs at the film–substrate interface and is accompanied by the formation of spherical or “telephone cord” buckled film caps. These examples highlight a feature common to fracture under constraint: The driving force usually derives not from a global stress applied by an external agency, but instead from an internal reaction stress to an imposed strain mismatch. Such strain mismatches arise from coefficient of thermal expansion (CTE) differences (giving rise to “thermal” stresses), phase transformations, mass transport (e.g., electromigration), and nonequilibrium processing conditions (giving rise to “athermal” stresses).1 The challenge in understanding constrained fracture is twofold: (
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