Thermomechanical stress studies for advanced copper metallization and integration
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Thermomechanical stresses in the copper interconnects are directly related to void formation and interfacial delamination. In this study, the stress behavior of electroplated copper films was investigated using surface curvature measurement, with an emphasis on the passivation effect. It was found that the passivation introduces a stress barrier only above which the deformation mechanisms can take place. X-ray diffraction was then employed to measure the tri-axial stresses in the copper lines with a line width down to 0.25 micron. For the copper/TEOS interconnects, it can be concluded that the plastic processes are almost suppressed due to the passivation effect and possibly the lack of dislocations. In additional to the experiment, a 3D FEA model with half of the periodic unit was developed and justified by comparing with the model containing multiple lines. The FEA results are in reasonable agreement with the experiments. The parametric studies show that a rigid confinement on the copper lines generally results in a high stress level. As the line width is scaled down, the stress in the copper lines becomes more hydrostatic, indicating an increasing driving force for void formation. FEA simulation also reveals a highly hydrostatic tensile stress state present in the SiLK dielectric. Studies on the dual damascene copper interconnect structure indicate that the out-of-plane stress in the vias becomes dominant. In an effort to address the adhesion issue for copper/Low k integration, the 3D FEA model was extended to calculate the driving force for interfacial delamination. The stability of the solution was verified by comparing the energy release rate for different crack length. Various interfaces within the interconnect structure were studied. It can be concluded that interfacial delamination is unlikely to take place for the single damascene copper line structures.