Study of stress relaxation and electromigration in Cu/low-k interconnects

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Yoon, Sean Jhin

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In this study, the stress relaxation and electromigration behaviors of Cu/low-k interconnects were studied with special emphasis on the effect of interface diffusion. The effect of passivation was studied using various passivation materials such as SiN/CoWB, SiN, SiCN, and SiC, which were incorporated into blanket copper films and line structures. The thermal cycling stresses of passivated and unpassivated blanket copper films showed higher yield strengths for passivated copper films, implying that the passivation restrained the plastic deformation in the copper films. The stress relaxation results of the same blanket copper films showed that the SiN/CoWB-passivated copper films had the smallest stress relaxation, followed by the SiN, SiCN, and SiC-passivated films while the unpassivated copper films had the largest relaxation. These results indicate that the mass transport with the SiN/CoWB passivation was the slowest among the samples studied. Huang's kinetic model [31] was used to determine interface diffusivities for various passivation layers. Electromigration lifetimes for Cu/low-k interconnects were measured at 300, 325, and 350°C. FTEOS was used as a low-k material while SiN and SiC were implemented as passivation materials. The activation energies from Black’s equation [20] were calculated to be 0.96±0.09 and 0.74±0.11eV for SiN and SiC passivations respectively, indicating that the diffusion mechanism was dominated by interface diffusion. The correlation between interfacial diffusion and electromigration damage was investigated experimentally and confirmed by the void growth prediction from Korhonen’s analysis. Stress relaxations of Cu/OSG line structures were also measured. Three different line widths (0.12, 0.18, and 0.36µm) and two different passivations (SiN and SiCN) were incorporated into 0.3µm thick single damascene structures. The isothermal stress relaxations of Cu/OSG line structures were studied at 160, 200, and 240°C. Between the two passivations, the SiN-passivation provided a smaller stress relaxation, suggesting a slower interfacial mass transport than the SiC passivation. The results did not show a pronounced difference between line structures with different linewidths due to the low measurement sensitivity for the line samples.