Diffusional breakdown of a Ag diffusion barrier in a Cu-Ag-Ni diffusion triple
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I.
INTRODUCTION
THE degradation
arising from interdiffusion, particularly in high temperature coating systems and in metallization processes for integrated circuits, has long been of interest. A diffusion barrier, or diffusion blocking layer, is usually a necessity for applications requiring either prevention or control of the interdiffusion between dissimilar phases in intimate contact. The thickness of such diffusion barriers can range from nanometers to micrometers, and the application temperatures cover a wide range reaching in excess of one thousand degrees Celsius. Therefore, a mechanistic study which can successfully interpret the behavior of one type of diffusion barrier is important, although it may not apply generally to all systems. An effective diffusion barrier should meet several criteria in a particular application; however, in general, three basic properties are critical to its performance: (i) The transport rate of components through the diffusion barrier should be small. (ii) The diffusion barrier should be thermodynamically stable with respect to the surrounding phases; in particular, there should be no rapid chemical reaction between the diffusion barrier and the contacting phases. (iii) The adhesion of the diffusion barrier to the contacting phases should be sufficiently strong to avoid a deterioration of the interface adhesion. There are trade-offs among these properties in applications. For example, the absence of chemical interaction usually indicates poor adhesion. Therefore, enhancing one property may possibly degrade another, and the compromise among properties depends upon the particular application. In a metallization system, most thin film diffusion barriers such as TiN have only several tens of nanometers thickness. 3 Such a thin film is needed because of the requirements for miniaturization and low resistivity in integrated circuit systems. In contrast, diffusion barriers beneath a high temperature coating are in the range of several micrometers thickness. 4 Because of the variety of environments and C. S. LIN is Metallurgist with Industrial Technology Research Institute, Taipei, Taiwan. R.A. RAPP and J.P. HIRTH are Professors with the Department of Metallurgical Engineering, The Ohio State University, Columbus, OH 43210. This paper is based on a presentation made at the "'Stephen R. Shatynski Memorial Symposium on Surfaces and Interfaces" held at the 114th annual AIME meeting in New York, February 24-28, 1985, under the auspices of the ASM-MSD Thermodynamic Activity Committee. METALLURGICAL TRANSACTIONS A
phase relations for barrier applications, a universal model to interpret the behavior of diffusion barriers will not apply. In thin-film applications, high diffusivity paths such as grain boundaries or interphase boundaries may play a vital role; 5 however, bulk diffusion is usually the dominant failure mechanism for high temperature coatings. Physical defects in the barrier can provide another major cause of failure. The categorization of kinetic behavior suggested by Nicolet6
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