Stress migration (SM), also known as stress voiding or stress-induced voiding (SIV), is a phenomenon involving the atomic diffusion of metal atoms within a wire to balance mechanical stress. In stress migration, there is a net flow of atoms into regions experiencing tensile forces (stretching) and a flow of metal atoms out of regions under compressive stress (compression). This results in atomic diffusion occurring in the direction of the negative mechanical tension gradient, as depicted in Figure.
The primary reasons for the emergence of mechanical stress as the driving force behind stress migration in metal wires are:
- Thermal Expansion: Differences in the coefficients of thermal expansion (CTE) between the metal, dielectric, and packaging materials, combined with temperature variations during fabrication, storage, and operation, result in mechanical stress.
- Electromigration (EM): Internal mechanical stress can be generated by electromigration processes, such as those involving the movement of metal atoms due to high current densities.
- Deformation Through Packaging: The packaging process and assembly can introduce mechanical stress into the metal wires.
In metal lattices, there are vacancies, which are atomic positions within the lattice that are unoccupied. Vacancies occupy less space than atoms, so a crystal lattice with vacancies has a smaller volume than the same lattice with all positions filled. Vacancies play a significant role in stress migration.
The stress gradient within the metal lattice drives atoms from regions under high pressure (compressive stress) to regions with tensile stress (tensile stress). Conversely, vacancies are pushed from regions of tensile stress to regions of compressive stress. This migration process can be likened to a highly viscous fluid that responds slowly to an external pressure gradient. Temperature is a crucial factor in this process, as it facilitates the movement of atoms and vacancies.
When external mechanical stress is applied, the crystal lattice either stretches or compresses, depending on the type of stress. Atoms are more likely to migrate to the stretched regions, while atoms in compressed regions are pushed outward, increasing the number of vacancies. This results in atomic flux from regions of compressive stress to regions of tensile stress until a state with no stress gradient is achieved.
If the stress is internally generated by migration processes, such as electromigration (EM), there will be a higher concentration of vacancies in regions experiencing tensile stress. Stress migration will then occur to reach a steady state where the atomic flux due to EM is balanced by stress migration.
In cases where the number of vacancies induced by external or internal stress exceeds a threshold, vacancies may combine to form voids due to vacancy supersaturation. This phenomenon, known as void nucleation, can result in the formation of cracks in the material.
In summary, stress migration in metal wires is driven by mechanical stress gradients caused by factors like thermal expansion, electromigration, and packaging-induced deformation. This phenomenon involves the migration of metal atoms and vacancies to balance the mechanical tension, and it can lead to the formation of voids when vacancy concentrations exceed certain thresholds. Understanding and managing stress migration is crucial for ensuring the reliability and longevity of electronic devices and circuits.
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