Characterization of electroplated copper films with laser-generated surface acoustic waves

Elastic properties of electroplated copper films fabricated under different plating and annealing conditions have been characterized with laser-generated surface acoustic waves. For acoustic wavelengths longer than the characteristic copper grain size, the effective elastic modulus of copper was found to be unaffected by variations of the plating current as well as by the grain size variation due to different annealing temperatures. Consequently, copper thickness measurements with surface acoustic waves have been found to be more stable with respect to process variations than electrical sheet resistance measurements. For short acoustic wavelengths (similar to2 mum), grain growth results in a significant increase of acoustic attenuation and a shift in the measured velocity. This finding opens an opportunity for copper structure monitoring by surface wave-based optoacoustic metrology.