Electromigration in Aluminum Based Interconnects of VLSI-Microcircuits, with and without Preceding Stress-Migration Dama
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ELECTROMIGRATION IN ALUMINUM BASED INTERCONNECTS OF VLSI-MICROCIRCUITS, WITHOUT PRECEDING STRESS-MIGRATION DAMAGE
WITH AND
M.A. Korhonen, P. Bergesen, and Che-Yu Li Department of Materials Science and Engineering, Cornell University, Bard Hall, Ithaca, NY 14853 ABSTRACT In the narrow, confined metal interconnects used in the chip level, electromigration flux is resisted by the evolution of mechanical stresses in the interconnects. Recently there has been a growing concern on the effects of preceding stress-migration on the subsequent electromigration damage. We present here an electromigration model, which is able to address the effects of stressmigration and mechanical stresses on electromigration lifetime. INTRODUCTION High tensile stresses arise during fabrication of microcircuits because of the differential thermal expansion between aluminum and silicon. These stresses can relax by a void growth mechanism known as stress-migration (SM). If voids are not formed the thermal stresses remain very high. Electromigration (EM) is of serious concern in the interconnects because the current densities in them can be extremely high 3while the line dimensions are so small that minimal voids, on the order of .1 pm , may lead to line severance [1]. The relatively good EM resistance of the confined metal lines, as contrasted to the unconfined, is ascribed to the retarding action owing to the stress gradients due to EM [2], the evolution of which will be considered below. Below we delineate the most feasible combinations
of SM and EM damage:
1)
In case of relatively large SM voids, voids are likely to migrate during EM testing which leads to coalescence of them, and to a fast line failure [1].
2)
In case of small SM voids, possibly too small to be detected by microscopy, a growth stage by EM must follow. The tensile stress state from SM will be relieved, at least partly, by voids growing during current load which enhances void growth. The tensile stress also increases the equilibrium concentration of vacancies which enhances diffusive fluxes. The final line severance may take place by the continued growth of the voids through the cross section of the line, or by migration and coalescence of them as described in
1). 3)
In case of no voids after SM, the stresses are extremely high, on the order of 500 - 700 MPa at room temperature [3,4]. These high stresses, combined with the local stresses created by EM, lead to void nucleation after relative short periods under current loading. The situation is now reverted to that described in 2).
In order to carry out the analyses according to the above program in a systematic way, we start from the description of our electromigration model. ELECTROMIGRATION MODEL In accordance with the analysis of Blech and Herring [2] we assume that in an interconnect the sources and sink for vacancies are so effective that the vacancy concentration C, is always in equilibrium with the stress a, or C, = C,, exp(Oo/kT),
(I)
where C•, is the equilibrium vacancy concentration in the absence of str
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