Takeru Kato

Minimization of residual stress in TSV interconnections by controlling their crystallinity

The crystallinity of electroplated copper thin-film interconnections varies drastically depending on its manufacturing process, and thus, their mechanical and electrical properties change significantly depending on their micro texture. These changes should cause the variation of the residual stress in the interconnections, and thus, electronic performance of devices and the lifetime of interconnections should vary depending on the amplitude of the residual stress around TSV structures. The main reasons for high residual stress is attributed to not only thermal stress but also the shrinkage of the interconnections during their thermal history. Since the crystallinity of the interconnections varies drastically depending on their electroplating process, the residual stress after high temperature annealing is a strong function of the crystallinity. In this paper, the dominant process factors for changing the crystallinity and the residual stress in the electroplated interconnections were investigated by varying the electroplating process parameters systematically. Finally, it was found that the control of the crystallinity of a seed layer material used for the electroplating is the most important factor for controlling the crystallinity and long-term reliability of thin electroplated interconnections.

Effect of the crystallinity of a grain boundary on the self-diffusion of copper in thin-film interconnections

In this study, the effect of the crystallinity and the characteristics of grain boundaries on the grain boundary diffusion was investigated experimentally. The crystallinity and characteristics of the grain boundaries were evaluated by using an EBSD (Electron Back-scattered Diffraction) method. The atomic diffusion of the grain boundary was also observed as the change of the surface morphology of interconnections by SPM (Scanning Probe Microscopy). It was found that the main diffusion paths during EM (Electro Migration) was random grain boundaries. The grain boundaries with low quality, regardless of crystallographic orientation, were also the main atomic diffusion paths.

Effect of the crystallinity on the long-term reliability of electroplated copper thin-film interconnections

In this study, the crystallinity of electroplated copper thin-film interconnections on Cu and Ru seed layers before and after EM (Electro migration) test was evaluated by an EBSD (Electron Back-scattered Diffraction). The EM resistance of the interconnection on the Ru seed layer was improved drastically compared with that on the Cu one. EBSD analyses revealed that the crystallinity of the interconnection on the Ru seed layer was much higher than that on the Cu one. In particular, the density of random grain boundaries with low crystallinity decreased in the interconnection on the Ru seed layer. It was concluded, therefore, that the improvement of the crystallinity of the electroplated copper thin-film interconnection by using the seed layer that can reduce the lattice mismatch between Cu and a barrier layer material is effective for assuring the high EM resistance and thus, the long-term reliability of the interconnection.