Early stages of void formation in Al-Cu lines studied using positron annihilation

We have applied positron annihilation spectroscopy to examine the formation of voids in SiO2-passivated 1 μmx1 μm Al-0.5 wt. % Cu lines. Samples were heat-treated both ex situ and in the positron beam to monitor void formation as a function of time and temperature. By measuring the fraction of 3-γ annihilations (a sensitive indicator of large open volume defects) we have established that voids are present at the interface between the Al alloy lines and the SiO2 passivation before heat treatment. The 3-γ fraction then grows to a maximum in less than 1 h at a temperature of 300 degree C. Changes in the Doppler-broadening S parameter are also observed. Studies are underway to apply the same methodology to investigate electromigration