Agglomeration and dissociation of vacancies in electroless deposited Cu films studied by monoenergetic positron beams

Positron annihilation is used to probe vacancy-type defects in electroless deposited (ELD) Cu films on Ta/TaN/SiO₂/Si. Doppler broadening spectra of the annihilation radiation and lifetime spectra of positrons are measured for ELD-Cu fabricated with different residual impurity concentrations using a novel electrolyte. For as-deposited films, the major species of vacancy-type defects are identified as vacancy complexes (V₃-V₄) and larger vacancy clusters (~V₁₀). After annealing around 200°C, they diffuse toward the surface and aggregate. The same trend is observed for sulfur, suggesting the formation of complexes between sulfur and vacancies. Although the defects near the surface anneal out above 300°C, the defect concentration near the Cu/barrier-metal interface is high even after annealing above 600°C, suggesting an accumulation of vacancy-impurity complexes. The observed defect reactions are attributed to the suppression of the vacancy diffusion to sinks by the formation of impurity-vacancy complexes. Through careful control of the concentration of residual impurities and their species, these electroless Cu films have a high potential to suppress the formation of voids/hillocks caused by defect migration.