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

A model for electromigration in thin metal film interconnects is presented 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 patterned planar interconnects. Interconnect structure is defined in terms of single- and polycrystalline line segments, which result from the local grain microstructure for a patterned interconnect line. The dependence of the diffusional flux on the length and type of line segment is included in the model. The results indicate that the grain structure of the film plays an important role in determining the relative contribution of the diffusion components to mass transport. The model assumes that the length and type of interconnect line segment determines the relative contribution of grain boundary and lattice diffusion components, and provides a means for extrapolating accelerated test results for planar interconnects by taking into consideration the temperature dependence of the diffusion mechanisms, and the effect of the local microstructure on diffusion. The model also indicates that extrapolations made using Black's equation may result in an overestimate of safe operating conditions. Calculations show that the effective activation energy depends on the median grain size and its distribution parameter, D50 and sigma, respectively, and the interconnect linewidth W. Model calculations of electromigration lifetime t50 were compared to experimental results obtained on patterned interconnects using sputter-deposited Al-1.5% Cu alloy films. The experimental data support a linewidth-dependent electromigration activation energy and show that the dependence of t50 on linewidth for W less-than-or-equal-to 3 D50 results from a change in the dominant diffusion mechanism with temperature, linewidth, and local interconnect ''structure.''