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ELECTROMIGRATION DAMAGE BY CURRENT INDUCED COALESCENCE OF THERMAL STRESS VOIDS P. Borgesen*, M. A. Korhonen*, T. D. Sullivan", D. D. Brown%, and C.-Y. Li* "Department of Materials Science & Engineering, Cornell University, Ithaca, 14853 NY "IBM General Technology Division, Essex Junction, Vermont 05452 ABSTRACT According to a new electromigration model, small thermal stress induced voids remain trapped at grain and phase boundaries, growing under an electric current. When they reach a 'threshold' size, migration and coalescence leads to rapid line failure. Predictions are compared to experimental results for Al, Al(0.6%Si), and AI(l.6%Cu) lines. INTRODUCTION Thermal stress induced voiding and electromigration damage are among the most serious reliability problems facing the next generations of microelectronic circuits. Unless new remedies can be found, both phenomena are expected to severely limit lifetimes as line widths are reduced to a few tenths of a micron. We have recently presented a new model directly relating thermal stress and thermal stress induced voiding to electromigration damage in passivated narrow lines [1, 2]. According to this model, voids nucleate before, or shortly after the beginning of electromigration testing. Small voids are trapped and grow at grain boundaries and matrix-precipitate interfaces under the influence of simultaneously developing local stress distributions. Voids exceeding a critical size migrate and coalesce, eventually leading to line severance. In refs. [1, 2] the model was described with particular emphasis on 'near-bamboo' structure lines, where voids might get trapped at blocking grain boundaries associated with the bamboo grains and lattice diffusion becomes a limiting factor. In the following, we shall extend our model to lines with slightly smaller grain sizes, where grain boundary diffusion dominates. MODEL The thermal mismatch between the metal and its surroundings usually leads to thermal stress induced voiding in narrow, passivated aluminum based lines [3], as well as in passivated copper lines (4, 5]. Even if void nucleation is somehow suppressed, however, the combined effects of a current and the high residual stresses will then lead to rapid nucleation [1J. An electric field, E, generally exerts a driving force, Z eE, on the atoms of a metal line. eZ* is here the 'effective charge' for the corresponding transport path (surface, grain boundary or lattice). In the absence of trapping, a cylindrical void of radius Mat. Res. Soc. Symp. Proc. Vol. 239. (1992 Materials Research Society

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Rv will then migrate with a velocity of approximately [6] v = -D(,,Dsn/R.kT)

eEZ8

(1)

Here q is the effective number of atoms per unit surface area, D. is the surface diffusion coefficient, n the atomic volume, k is Boltzmann's constant, and T the temperature. The smaller voids thus move faster than, and catch up with, the larger ones, leading to a relatively rapid coalescence and line severance. As described in detail elsewhere [1, 7] we evaluate the duration of the coalescen