Void Formation
Localized voiding at the interface between two dissimilar metals occurs when there is an inequality in the diffusion rates of the two metals. This process is kirkendall cavitation, which weakens the mechanical bond of the junction. The mismatch in atomic flux leads to a supersaturation of vacancies that coalesce into microscopic pores.
Sensor Degeneration
Electrical and mechanical connections in micro-sensors are highly sensitive to microscopic structural defects. When thin-film sensors undergo high temperatures, kirkendall cavitation can cause the resistive paths to narrow, resulting in a progressive increase in electrical resistance. In severe cases, the coalesced voids lead to delamination of the film from the substrate, resulting in complete failure of the sensor.
Structural Influence
The choice of materials and the introduction of barrier layers can suppress the transport mechanisms that drive vacancy accumulation. In inhibiting kirkendall cavitation, engineers use thin diffusion barriers, such as titanium or tantalum, to separate the reactive metal layers. These barriers slow down the atomic diffusion, ensuring that the flux mismatch remains low enough to prevent the nucleation of voids at the interface.
Experimental Characterization
Microscopic observation and depth profiling are used to monitor the progression of interface voids in metal junctions. The study of kirkendall cavitation employs cross-sectional scanning electron microscopy to visualize the size and distribution of the voids after thermal aging. By measuring the total void volume as a function of time and temperature, researchers determine the kinetic parameters of the diffusion process, allowing the prediction of the lifetime of the electrical contact.
This characterization is essential during the qualification of new metallization schemes, as it reveals whether the selected barrier materials can successfully prevent interface degradation under standard operating conditions.
