Abstract
This work studies the coupled problems of electromigration (EM)- and thermomigration (TM)-induced anomalous atomic flows through experiments for demonstrating EM and TM tests and the theoretical formulation describing the atomic behavior. A two-dimensional unpassivated Au interconnect, having a protrusion nose area where atoms are accumulated, was used for observing hillock formation under current stressing and heating of the substrates. The hillock formation locations depend on the experimental conditions, including current density and temperature involved with the driving forces of EM and TM. The relationship between coupling the EM- and TM-induced driving forces was clarified through the experimental observation of the hillock formation location and the proposition of the equilibrium current density. Considering the magnitude relationship of the EM- and TM-induced driving forces, it was proved that an equilibrium current density, below or above which EM or TM is dominant, respectively, exists. The theoretical equilibrium current density was formulated to estimate the equilibrium state of EM and TM, deducing the behavior of hillock formations by EM and TM. Hillocks form at the nose edge due to EM-induced forward flow in the case of lower current density and higher temperature. Conversely, hillocks form at an area slightly away from the nose end due to TM-induced backward flow in the case of higher current density and lower temperature.
Highlights
A metallic hillock, the protruding formation of an interconnect in electronic circuits, is formed due to the accumulation of metallic atoms driven by atomic migration phenomena, threatening the deterioration of an electronic circuit
The hillock formation locations depend on the experimental conditions, including current density and temperature involved with the driving forces of EM and TM
When we consider only EM, without TM, the interconnect forms hillocks at the nose edge of the interconnect, similar to the atomic behavior in a Blech structure which is a typical interconnect structure exemplified as a 1D interconnect for the EM-test
Summary
A metallic hillock, the protruding formation of an interconnect in electronic circuits, is formed due to the accumulation of metallic atoms driven by atomic migration phenomena, threatening the deterioration of an electronic circuit. Modern electronics, including largescale integration with metallic interconnects in the back end of the line and C4 flip-chips with solder bumps in packaging, have many corners and dissimilar metallic joints, which cause a temperature increase due to current crowding, and the temperature distribution in interconnects is very complicated. This complicated non-uniform distribution of temperature and crucial temperature increases due to current crowding, drastically accelerating EM and TM based on thermal activation. EM, driven by the electron wind force, acts to diffuse metallic atoms along with electron flows
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