AN ELECTROMIGRATION MODEL THAT INCLUDES THE EFFECTS OF MICROSTRUCTUREAND TEMPERATURE ON MASS-TRANSPORT

A model for electromigration in thin metal film interconnects is prese nted that includes two components of diffusion. The grain-boundary and lattice components of mass transport are considered in terms of their temperature dependence and the metallurgical ''structure'' of pattern ed planar interconnects. Interconnect structure is defined in terms of single- and polycrystalline line segments, which result from the loca l grain microstructure for a patterned interconnect line. The dependen ce of the diffusional flux on the length and type of line segment is i ncluded in the model. The results indicate that the grain structure of the film plays an important role in determining the relative contribu tion of the diffusion components to mass transport. The model assumes that the length and type of interconnect line segment determines the r elative contribution of grain boundary and lattice diffusion component s, and provides a means for extrapolating accelerated test results for planar interconnects by taking into consideration the temperature dep endence of the diffusion mechanisms, and the effect of the local micro structure on diffusion. The model also indicates that extrapolations m ade using Black's equation may result in an overestimate of safe opera ting conditions. Calculations show that the effective activation energ y depends on the median grain size and its distribution parameter, D50 and sigma, respectively, and the interconnect linewidth W. Model calc ulations of electromigration lifetime t50 were compared to experimenta l results obtained on patterned interconnects using sputter-deposited Al-1.5% Cu alloy films. The experimental data support a linewidth-depe ndent electromigration activation energy and show that the dependence of t50 on linewidth for W less-than-or-equal-to 3 D50 results from a c hange in the dominant diffusion mechanism with temperature, linewidth, and local interconnect ''structure.''