Meaning
Mass transport phenomenon describes the movement of metal atoms driven by a gradient in mechanical stress within an interconnect. Frequent stress migration occurs in the fine metal lines of integrated circuits during cooling from high fabrication temperatures. This process leads to the formation of voids that can eventually sever the electrical connection.
It stops once the internal stress is fully relaxed or the temperature drops below the diffusion threshold. Atomic flux is generally higher in materials with lower melting points.
Thermal Influence
Mismatch between the expansion coefficients of the metal and the surrounding dielectric creates the driving force. Atomic flux during stress migration moves from regions of high tension to regions of low tension. The rate of this movement follows an Arrhenius relationship with temperature.
Failure Analysis
Voids typically appear at the base of vias where the geometry causes a concentration of mechanical force. Evidence of stress migration is often found in devices that have undergone multiple thermal cycles without being powered. This differentiates the failure from electromigration which requires the presence of an electric current.
Quality Verification
Reliability testing involves baking components at constant temperatures for thousands of hours to observe resistance changes. Testing for stress migration is a mandatory part of the qualification process for industrial grade microprocessors. Failure to pass these tests prevents a product from receiving the necessary safety certifications for high-reliability applications.
