Positron lifetime measurements have been performed for Cu thin films in order to identify lattice defects. Theoretical positron lifetimes of vacancy clusters of Cu are also calculated for identification of the experimental values. Cu thin films were prepared by electro chemical deposition (ECD) and physical vapour deposition (PVD). All of the Cu thin films have longer positron lifetimes than bulk Cu does. This indicates that lattice defects are introduced during deposition. However composition analysis of positron lifetimes shows that whereas vacancy clusters exist in the ECD-Cu thin film, the positron lifetime of defect component in the PVD-Cu thin films is shorter than that of mono-vacancy. On the other hand, the positron lifetime of mono-vacancy was detected in PVD-Cu-0.5 at%Sb thin film prepared by PVD. In order to elucidate the effect of Sb, first-principles calculations have been performed for Cu including Sb and monovacancy. The result suggests that mono-vacancies are introduced because of the binding energy between a Sb atom and mono-vacancy. 1 Introduction The dual-damascene fabrication process is presently recognized as a standard Cu interconnection technique. In this technique, metallic barrier layers and Cu seed layers are deposited in successive order on the inside walls of via holes or trenches using a sputtering method (physical vapour deposition; PVD). The Cu interconnections are then embedded into via holes or trenches using an electrochemical deposition (ECD).As the diameter of the via holes becomes smaller and smaller, the relative embedding performance tends to decline. The high pressure anneal process is attracting attention because of its excellent capabilities in the filling performance of high aspect-ratio via holes and improved adhesion of wiring Cu interconnections. The PVD method is attractive from the viewpoint of simplifying the entire process and alloying of Cu-film to further enhance the electron migration resistance for example. However the PVDCu films requires higher temperatures than the ECD films to fill via holes completely by the high pressure annealing process [1,2]. On the other hand, it was found that the addition of Sb to the PVD-Cu lowers the required temperature. In the present study, positron lifetime measurement and its theoretical calculation have been performed for the ECD and PVD-Cu thin films in order to identity lattice defects, which can affects the filling behaviour.
As a part of the LSI interconnect fabrication process, a post-deposition high-pressure annealing process is proposed for embedding copper into trench structures. The embedding property of sputtered Cu films has been recognized to be improved by adding hydrogen to the sputtering argon gas. In this study, to elucidate the effect of hydrogen on vacancy formation in sputtered Cu films, normal argon-sputtered and argon–hydrogen-sputtered Cu films were evaluated by positron annihilation spectroscopy. As a result, monovacancies with a concentration of more than 10-4 were observed in the argon–hydrogen-sputtered Cu films, whereas only one positron lifetime component corresponding to the grain boundary was detected in the normal argon-sputtered Cu films. This result means monovacancies are stabilized by adding hydrogen to sputtering gas. In the annealing process, the stabilized monovacancies began clustering at around 300 °C, which indicates the dissociation of monovacancy-hydrogen bonds. The introduced monovacancies may promote creep deformation during high-pressure annealing.
An attempt to improve the reflow characteristics of sputtered Cu films was made by alloying the Cu with various elements. We selected Y, Sb, Nd, Sm, Gd, Dy, In, Sn, Mg, and P for the alloys, and “the elasto-plastic deformation behavior at high temperature” and “the filling level of Cu into via holes” were estimated for Cu films containing each of these elements. From the results, it was found that adding a small amount of Sb or Dy to the sputtered Cu was remarkably effective in improve the reflow characteristics. The microstructure and imperfections in the Cu films before and after high-temperature high-pressure annealing were investigated by secondary ion micrographs and positron annihilation spectroscopy. The results imply that the embedding or deformation mechanism is different for the Cu-Sb alloy films compared to the Cu-Dy alloy films. We consider that the former is embedded by softening or deformation of the Cu matrix, which has a polycrystalline structure, and the latter is embedded by grain boundary sliding.
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