The stress of damascene Cu integrated with silicon dioxide (SiO2) and a low-k material was analyzed by x-ray diffraction, and that of the via-line structure was evaluated using finite element analysis. In both cases, the hydrostatic stress of the Cu line embedded in SiO2 was greater than that of the Cu line with low-k dielectric, whereas the opposite was true for the von Mises stress. In particular, the von Mises stress in the via embedded in the low-k dielectric was large. It was also shown that the yield strength of the via embedded in the low-k material is severely reduced compared with that of the via embedded in SiO2. Therefore, the deformation of the via, due to high von Mises stress and low yield strength, is expected to be the important failure mode in the interconnects made with low-k dielectrics which have higher coefficient of thermal expansion and lower elastic modulus.
Damascene Cu interconnects show significant differences in both their microstructural and stress behavior as compared to those of Al interconnects patterned using the etching process. Thermal stresses build up during the successive thermal cycles due to the differences in the coefficients of thermal expansion of the component materials. Other than these thermal stresses, growth stresses originating from grain growth develop in damascene Cu interconnects as well. In this study, the linewidth dependence of the stress in damascene Cu was examined experimentally, as well as by numerical simulation. The stresses of damascene Cu with widths ranging from 0.13to2μm were measured using x-ray diffraction, and the measured hydrostatic stress was found to increase with increasing linewidth, in contrast to the typical behavior of Al interconnects. Microstructure analysis using transmission electron microscopy revealed that the grain sizes increased with increasing line dimensions. The increase in stress in the interconnect with increasing dimensions is attributed to the larger grain size, which induces higher growth stress in addition to the thermal stress. The contribution of the growth and thermal stresses of the damascene lines were quantified based on the grain size data utilizing finite element analysis. In this way, the linewidth dependence of the hydrostatic stress of damascene Cu was clearly explained. Finally, the effect of growth stress on the stress-related reliability is discussed.
The line width dependence of stress in damascene Cu was examined experimentally as well as with a numerical simulation. The measured hydrostatic stress was found to increase with increasing line width. The larger stress in an interconnect with large dimension is attributed to the larger grain size, which induce higher growth stress in addition to thermomechanical stress. A stress model based on microstructure was constructed and the contribution of the growth and thermal stress of the damascene lines were quantified using finite element analysis. It was found that the stress of the via is lower than that of wide lines when both the growth stress and thermal stress were considered. This stress gradient between via and line, which is the driving force of vacancy diffusion, is larger when the low-k with lower stiffness and higher thermal expansion is used for dielectric layer. For this reason, the Cu/low-k can be more vulnerable to stress-induced voiding.
The anisotropic spread of the central peak in a (111) pole figure by x-ray diffraction (XRD) was observed for damascene Cu lines of 0.18-2 µm in width and 0.5 µm in depth. The spread originates from the existence of slightly tilted (111) grains because of inclined sidewalls. The tilted (111) orientation is favorable only for polygranular clusters whose sidewall energies can be minimized simultaneously. Consequently, bamboo grains have an exact <111> orientation, while the polygranular clusters have a tilted <111> orientation. Using this concept, the volume fraction of the bamboo grains and polygranular clusters in the damascene Cu lines were quantified using the XRD pole plots.
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