Electromigration-resistant Cu-Pd alloy films

Abstract

Copper is a prospective material for metal interconnects in microelectronic circuits since it has a high resistance to electromigration damage (EMD) and a low electrical resistivity. In the present work, a still higher resistance to EMD was achieved by alloying Cu with a small amount of Pd. Cu-Pd alloy films consisting of 0.5, 1.0 and 1.6 wt.% Pd were made on oxidized silicon wafers by vacuum evaporation. These films were EMD tested in a high vacuum system by an isothermal resistance change measurement method using a thin film thermistor technique. The activation energies of Cu, Cu-0.5 wt.% Pd, Cu-1.0 wt.% Pd and Cu-1.6 wt.% Pd films were found to be 0.79 eV, 1.01 eV, 1.26 eV and 1.15 eV, respectively. Scanning electron microscopy was used to characterize the Cu and Cu-Pd alloy films after EMD testing. Many of the hillocks that formed had crystalline facets. Film thinning and voiding were also observed, the former being more prominent in the alloy films than in pure Cu. X-ray energy spectroscopy showed that the hillocks on the Cu-Pd alloy films had a measurably lower Pd concentration than the hillock-free areas. Thus Cu appears to diffuse faster than Pd when electromigration takes place. These results show clearly that adding small amounts of Pd to Cu improves the EMD resistance of the stripe.