Meaning
Physical phenomena involving the atomic migration of metal atoms away from regions of high compressive stress toward areas of lower stress influence the longevity of microelectronic interconnects. This stress gradient drift occurs primarily in thin film metallization used in integrated circuits. The movement of atoms leads to the formation of voids in some areas and hillocks in others.
Diffusional Flow
Atomic transport is driven by the chemical potential difference created by mechanical pressure within the metal lattice. In the context of stress gradient drift, the flux of vacancies in the opposite direction of the atoms causes structural thinning. This thinning increases the electrical resistance of the line over time.
Thermal Variance
Fluctuations in temperature accelerate the rate of material displacement by increasing the mobility of the atoms. Since different materials in the package expand at different rates, stress gradient drift is more pronounced at the interfaces between metal and dielectric layers. Sustained high temperatures can cause the metal lines to fail prematurely.
Reliability Assessment
Prediction of failure rates depends on understanding the relationship between the stress distribution and the material properties. Engineers model stress gradient drift to determine the safe operating limits for current density and temperature. This analysis is a prerequisite for certifying the reliability of advanced semiconductor nodes.
