The role of mechanical stress in electromigration of thin films is reviewed and discussed. The role of stress in the driving force and the diffusivity are seen both to be important. The concept of the activation volume is discussed with regard to information available from its measurement. It is seen that void nucleation by electromigration or stress voiding is an unsolved theoretical problem.

A brief review is given of models which propose a correlation between electromigration resistance and the mechanical strength of thin film interconnects. In an attempt to achieve metallurgical strengthening and improved electromigration resistance, aluminum films were implanted with oxygen ions. Preliminary electromigration tests on line arrays patterned from these films resulted in lifetimes comparable to the standard Al films. The lack of improvement is attributed to enhanced hillock/whisker growth during electromigration in the implanted interconnects. This behavior is coincident with a lower compressive strength in similarly treated continuous films at elevated temperatures as measured by the substrate curvature technique.

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 Al(1.6%Cu) lines.

Based on experimental findings obtained earlier by the authors [1], a model is devised in which the divergence of the vacancy flow created in the grain boundaries of Al-Si(1%) metallizations by high-density DC is attributed to temperature gradients and structural features. Void nuclei of critical size are created in areas of high vacancy concentration. These nuclei grow, fed by the electromigrational vacancy current. It is shown that stresses have only a negligible' effect on the vacancy diffusion but influence the nucleation process dramatically. Results expressed in terms of fraction of void area are calculated and then correlated with experimental results. Certain material parameters, whose values are not known, are derived from the experimental data. The physical significance of these parameters is discussed.

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 stress-migration and mechanical stresses on electromigration lifetime.

0.75–3 μm wide copper lines encased in a thin adhesion layer of Al or Cr were passivated at 300°C and annealed at 400°C. The passivation was then removed and the lines examined in a scanning electron microscope. The development of thermal stress induced voiding was found to depend primarily on aging conditions and adhesion.

Narrow Al lines were passivated at 300°C and subsequently annealed at 400°C for an hour. The passivation layer was then removed and the lines analyzed for thermal stress induced voids. Effects of the multiple high temperature process steps necessary in chip manufacturing were simulated by repeated anneals. Renewed void nucleation occurred during cool-down from each anneal, leading to a systematic increase in void density. In contrast, the maximum void size did not increase with number of anneals while it did increase with aging time for a single anneal.

Aluminum films deposited by partially»ionized beam were stressed by a current density of 2×106 A/cm2 at a temperature of 300°C. The films prepared at the acceleration voltage of 5 kV lived over 200 hours.