Electromigration threshold in aluminum films

Abstract Electromigration threshold was measured in oil bths at the temperature range 100–280°C, approaching operation temperature of integrated circuits. For uncovered unannealed films, threshold was constant in the temperature range below 200°C, yielding a low threshold value of about 420 A/cm (42 μm × 105 A/cm2) for the product of current density and stripe length. Annealing does not affect temperature behavior, but threshold values grow with increasing annealing temperatures. Threshold does not depend on annealing atmospheres, such as N2, O2 and N2/H2. Different film compositions like Al, Al/Si and Al/Si/1% Cu are equivalent concerning threshold. Elevated, slightly temperature-dependent threshold values were measured using covering SiO2 films. The threshold of covered films depends on test conditions. Reduced current densities and stress times result in increasing values which may be beneficial for electromigration reliability. Moreover, electromigration test results can be influenced by threshold, which has to be considered. The discrepancy to former threshold results is assumed to be due to N2/H2 test ambients, reducing electromigration mass flow. The reason for threshold is suggested to proceed from different bond energies for the transition from grain boundaries to surface.