What is the phenomenon of stress migration?

Stress Migration (SM) is a common reliability failure phenomenon in integrated circuits. It refers to the phenomenon in which atomic migration, local cavities, or cracks occur due to stress concentrations (such as mismatched thermal expansion coefficients) after manufacturing or encapsulation, ultimately leading to a fracture of the conductive path or an abnormal increase in resistance.

In actual observation, the formation of voids can be observed on some metal wires without applying electric current, and the formation of voids or even complete disconnection can be observed on some metal wires. This phenomenon is stress migration, which is thought to be the result of stress release of the metal itself.

0040-31980 GAS BOX EC WXZ

Causes and physical mechanisms

Stress migration is a diffusion process caused by mechanical stress. Mechanical stress is generated by high-temperature process steps in the manufacturing process of integrated circuits. Due to the different coefficients of thermal expansion between metal and insulating layers, these high-temperature processes introduce greater stresses in the metal layer (aluminum or copper).

Specifically, copper has a much higher coefficient of thermal expansion (about 17 ppm/°C) than low k media or silica (about 0.5 ppm/°C). During temperature changes, copper "stretches" more, resulting in strong mechanical stresses with the surrounding medium. At the same time, copper generates "built-in stress" during deposition and cooling, and over time, copper atoms migrate from high-stress zones to low-stress zones driven by stress in order to "decompress", resulting in the accumulation of atomic vacancies in high-stress zones and eventually the formation of cavities.