Abstract
Highly pure nanotwinned copper thin films which would be applied in semiconductor as interconnects were fabricated by using unbalanced magnetron sputtering (UBMS) system. In order to investigate the influence of different interlayers on microstructure and thermal stability of Cu film, CrN and TiN thin film was selected and respectively deposited onto a silicon wafer as an interlayer before coating Cu thin films. Then, the samples with different interlayers were subject to isothermally anneal at 250℃ in 5×10-6 torr for 30, 60 and 120 minutes, respectively, to study the thermal stability of Cu thin films. After deposition, the Cu thin film with high density nanotwin structure was observed in both interlayer samples, and all twin boundaries was found to be parallel to the Si substrate. However, some abnormal large grains were found in the samples with CrN interlayer and their grain size increased from 0.49μm to 60μm after annealing at 250℃ for 120 minutes. These huge grains were (200) grains and some huge grains were found to grow until penetrate the thickness of Cu thin film when annealing time increases. Furthermore, the direction of twin boundary in these huge grains was observed to tend to be perpendicular to Si substrate. On the other hand, there was no significant microstructure change in the samples with TiN interlayer after annealing for 120 minutes; the grain sizes of nano-twinned Cu thin film still remain and the boundary of nano-twin Cu is still parallel to the Si substrate. We proposed that the texture evolution from {111} to {200} of Cu film was resulted from the competition between surface/interface minimization and strain energy minimization, and strain energy dominates in Cu thin film with CrN interlayer which cause the {200} grains become stable. The CrN interlayer was found with a {200}-preferred orientation, which may cause many {200} Cu seeds nucleate onto the interlayer. During annealing process, lots of {200} Cu seeds become the driving force of abnormal grain growth, and the {200} abnormal grains appear. Regarding to the properties of nanotwinned copper thin films, the residual stresses, electrical resistivity and hardness of Cu thin films decreased for the samples with CrN interlayer due to abnormal grain growth of Cu thin film, whereas these properties of Cu thin films with TiN interlayer didn’t change a lot after 120 minutes of annealing time. The above results suggest that the thermal stability of nano-twinned Cu thin film is strongly related to the interlayer and the samples with TiN interlayer exhibits more thermal stability than CrN interlayer.