Electromigration Characteristics of Cu–Al Precipitate in AlCu Interconnection

The behavior of a CuAl precipitate during a direct-current stressing test in an Al–2% Cu interconnection was investigated from observations employing scanning electron microscopy and energy-dispersive X-ray microanalysis. Movement of a CuAl precipitate into the anode direction, accompanied with changes of shape and size, was frequently observed. Measurement of the drift velocity of CuAl precipitates was carried out for the first time, which revealed a value an order of magnitude less than that of solid solute Cu atoms. The obtained activation energy of 0.77 eV corresponds to Cu interface diffusion between a CuAl precipitate and Al lattice. Interaction between two adjacent CuAl precipitates was observed in such a manner that one precipitate became smaller while the other precipitate at the anode site became larger. It is considered that the Cu atoms which constituted the precipitate were transferred to the other precipitate at the anode site by solid-solute electromigration.