Effect of the silicon nitride passivation layer on the Cu/Ta/SiO₂/Si multi-layer structure

Microstructure, composition and thermal stability of the SiN_x/Cu/Ta/SiO₂/Si (hereinafter 'passivated') multi-layer sample were investigated after annealing at different temperatures and compared with those of the Cu/Ta/SiO₂/Si (hereinafter 'unpassivated') multi-layer sample. Observed in this study were formation of new phases, movement of element atoms across the interface, failure of a tantalum (Ta) barrier layer, agglomeration of Cu grains and change in surface morphologies of nitride and Cu layers by Rutherford backscattering spectrometry, X-ray diffraction and scanning electron microscopy. The Cu layer of the passivated sample did not show a severe agglomeration phenomenon at high annealing temperature due to protection of the Cu surface from oxygen in the annealing ambient. In addition, there were no such phases as Cu₂O and Ta₂O₅ that were detected in the unpassivated sample. Although the thermal stability of the Ta layer was improved by addition of the passivation layer, Ta atoms diffused out to the passivation layer during annealing process at 750 °C, resulting in formation of Ta₂N at the SiN_x/Cu interface.