Electromigration properties of multigrain aluminum thin film conductors as influenced by grain boundary structure
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D.P. Field School of Mechanical and Materials Engineering, Washington State University, Pullman, Washington 99164-2920 (Received 4 February 2000; accepted 18 April 2001)
Electromigration rates in polycrystalline interconnect lines are controlled by grain-boundary diffusion. As such, reliability of such interconnects is a direct function of the grain-boundary character distribution in the lines. In the present work, drift velocity experiments were performed on multicrystalline lines of pure Al to determine the electromigration activation energy of the lines. Lines cut from films processed by partially ionized beam deposition techniques were analyzed. One set of lines was analyzed in the as-deposited condition while the other film was annealed before testing. The measured drift velocities varied dramatically between these two types of films, as did the grain-boundary character distributions measured by orientation imaging. The data were analyzed based on Borisov’s equation to obtain mean grain-boundary energies. Grain-boundary energy of the film with poor electromigration performance was calculated to be that reported for random boundaries, while that for the more reliable film was calculated to be that reported for twin boundaries in Al. Percolation theory was used to aid explanation of the results based upon the fraction and connectedness of special boundaries in the films.
I. INTRODUCTION
It is well known that grain-boundary self-diffusion controls electromigration behavior in multigrain aluminum thin film conductors.1– 6 Therefore, grain-boundary structure must influence the measurable electromigration parameters such as electromigration lifetimes, activation energy, drift velocity of ions, and so on. The method of film deposition, purity, doping, and subsequent treatment influence the grain-boundary structures and hence their energy and diffusion characteristics. It is a small wonder then, that current literature on the subject quotes widely varying values of electromigration activation energy for a given alloy.7 Recently we showed that electromigration activation energy can be significantly changed even in the frame of a single deposition technique.8,9 Using self-ion-assisted deposition we have showed that simply increasing the bias applied to the substrate holder can increase electromigration activation energy in Al lines. Previous results suggest that an increase in the fraction of low-energy grain boundaries, such as low-angle and low-sigma coincident site lattice (CSL) boundaries, which possess lower diffusivity in comparison with random grain boundaries, is the underlying mechanism for this effect.9 2124
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J. Mater. Res., Vol. 16, No. 7, Jul 2001 Downloaded: 07 Oct 2014
It is unclear what fraction of low-energy grain boundaries is required to increase the electromigration resistance. In this paper we attempt to answer this question on the basis of percolation theory and empirical data. Also we make an estimation of the grain-boundary energy of those boundaries responsible
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