Stress relaxation and void formation in passivated Al(Cu) line structures

Abstract

Relaxation of hydrostatic stresses in passivated Al lines can lead to void formation. We have investigated the long-term stress relaxation behavior of passivated Al(1 wt. % Cu) lines with 3, 1, and 0.5 μm linewidths up to 20 days using a bending beam technique. The results revealed two distinct kinetic regimes for stress relaxation: one due to plastic flow which occurs during an initial short period of annealing, and the other due to stress-induced voiding during long-term annealing. At 250 °C anneal, stress relaxation was dominated by the short-term process, in contrast to the 200 and 150 °C anneals where a clear transition from the short-term to the long-term relaxation was observed. Void density for different linewidths has been measured by TEM and SEM as a function of aging time and temperature. A good correlation between stress relaxation rate and void density was observed for 1, and 0.5 μm lines but not for the 3 μm lines. One micron wide lines show most voiding, whereas 3 and 0.5 μm wide lines show less voiding. This can be attributed to the differences in grain structure where the 1 μm wide lines show a near bamboo structure with some embedded polycrystalline clusters while the 0.5 μm wide lines show an almost perfect bamboo structure. The lower void density for the 0.5 μm wide line suggests that bamboo grain structure reduce the mass transport for voiding and retard void formation. A lower incidence of voiding in 3 μm wide lines can be attributed to a low stress driving force for void formation.