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 σ, 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≤3D50 results from a change in the dominant diffusion mechanism with temperature, linewidth, and local interconnect ‘‘structure.’’

A new model for electromigration in thin metal film interconnects has recently been proposed which includes the effects of film microstructure and temperature on the components of mass transport. In this paper experimental data is presented which supports this model. Our results indicate that the grain structure of the film coupled with the temperature dependence of the lattice and grain boundary diffusivities plays an important role in determining the relative contributions of these diffusion components to mass transport. For line widths in the range of the median grain size the line width dependence of median fail time, '50, results from a change in the relative contribution of these components to the diffusional flux. The model correctly describes the experimental dependence of '50 and activation energy on line width.

Al 1.5% Cu interconnects with linewidths from 0.8 to 10 μm and median grain size of 3.0 μm were stressed at current densities from 2–3×106 A cm−2 and at film temperatures between 140 and 300 °C. The activation energy dependence of the linewidth and grain size distribution, along with evidence for electromigration damage at specific sites within the film grain structure provides support for a line segment model in which the mass transport mechanism is dependent on the microstructure of the film. The results suggest that the contribution of nongrain boundary diffusion mechanisms to mass transport is more significant than previously believed for lines having comparable grain size and linewidth dimensions. In the context of interconnect reliability in integrated circuits, the data indicates that interconnect design rules which are driven by reliability constraints must include the microstructural properties of the film for accurate assessment.

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Electromigration in Al-1.5%Cu interconnects was studied using a direct current (d.c.) induced 1/f2 noise spectrum. The interconnects were fabricated with a multi-step sputter deposition process, with deposition temperatures of 25°C, 300° C, and 475°C. The resulting microstructures were analyzed using SEM and TEM, and grain size distribution parameters were measured. The minimum temperature at which 1/f2 noise could be detected was found to depend on the deposition temperature; for the 25°C, 300°C, and 475°C depositions the lowest temperature at which 1/f2 noise could be measured was 190°C, 220°C, and 265°C, respectively. The activation energy of 1/f2 noise was measured and compared with the electromigration activation energy. The noise activation energy was found to depend on deposition temperature; 0.56 eV, 0.64 eV, and 0.96 eV for the 25°C, 300°C, and 475° C depositions, respectively. SEM analysis of the samples after measurement show initial stages of void and hillock formation. Corresponding electromigration activation energies, 0.56 eV, 0.69 eV, and 1.04 eV were also dependent on deposition temperature. Noise measurements of interconnects made prior to electromigration testing were highly correlated with the electromigration failure times.