Crystallite coalescence: A mechanism for intrinsic tensile stresses in thin films
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Crystallite coalescence: A mechanism for intrinsic tensile stresses in thin films W.D. Nix and B.M. Clemens Department of Materials Science and Engineering, Stanford University, Stanford, California 94305-2205 (Received 19 August 1998; accepted 10 May 1999)
We examined the stress associated with crystallite coalescence during the initial stages of growth in thin polycrystalline films with island growth morphology. As growing crystallites contacted each other at their bases, the side-walls zipped together until a balance was reached between the energy associated with eliminating surface area, creating a grain boundary and straining the film. Our estimate for the resulting strain depends only on interfacial free energies, elastic properties, and grain size and predicts large tensile stresses in agreement with experimental results. We also discuss possible stress relaxation mechanisms that can occur during film growth subsequent to the coalescence event. I. INTRODUCTION
Controlling the stresses in deposited thin films has become one of the more important challenges in modern technology. The fields of microelectronics, integrated optoelectronics, data storage technologies, and microelectromechanical systems (MEMS) all depend critically on the properties, behavior, and reliable performance of deposited thin films. In addition, coating technologies depend heavily on the reliable mechanical performance of deposited thin coatings. Residual stresses that develop in such films and coatings can lead to film cracking and peeling in the case of tension, and delamination and blistering in the case of compression. Thus, understanding the origins of such stresses and controlling them is of importance to the reliable manufacture of such thin film systems. In this study we focus on the development of tensile stresses that develop in deposited polycrystalline thin films through the mechanism of crystallite coalescence. We neglect the effects of thermal stresses, epitaxy, point defect annihilation processes, interface stresses and structural transformations that may occur within the film after deposition. These processes for generating stresses in thin films have been described in a number of reviews.1–4 Here we limit our attention to the role of crystallite coalescence in the development of intrinsic tensile stresses during the early stages of film deposition. In the island growth mode, the earliest stages of film growth occurs through nucleation and growth of individual crystallites at favorable sites on the surface of the substrate. As the crystallites grow, they begin to impinge on each other, eventually forming a continuous polycrystalline film. As first recognized by Hoffman and his coworkers,5–8 a reduction of interfacial energy occurs when J. Mater. Res., Vol. 14, No. 8, Aug 1999
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the surfaces of the crystallites come together to form a grain boundary. Indeed, they postulated that when two crystallite surfaces are brought int
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