Copper thin films have been considered as alternative materials to Al in ULSI metallization. Under various processing conditions, Cu films experience thermal stress that often cause stress-induced failure. Although cavity formation has been reported in Cu thin films, its mechanism has not been understood up to now. The present work is aimed at understanding the cavitation mechanism in relation to deformation and stress concentration mechanisms. Samples consist of layer of Cu/Ta/SiO2/Si. A Ta layer was deposited as a diffusion barrier layer. Stress change was measured in copper thin films during thermal cycling at heating and cooling rates of 0.056 K/s in a temperature range of R.T. to 723 K. Morphological change was observed by SEM and TEM. Analysis of stress-temperature curves indicated that the stress state is in tension during cooling and in compression during heating. The obtained stress-temperature curve was compared with a calculated deformation mechanism map. It was found that deformation occurs mainly by a dislocation-glide creep mechanism during cooling and by a grain-boundary-diffusion creep mechanism during heating. Microstructure observation revealed that cavities were formed at twin/twin and twin/grain-boundary intersections. Based on the obtained results, the cavity formation mechanism can be understood as follows. Elastic anisotropy of neighboring twin variants gives rise to the concentration of shear stress at twin interfaces. This, in turn, causes the twin interfaces to be preferential dislocation glide planes, leading to dislocation pile up and cavitation at the intersection.
|Number of pages
|Nippon Kinzoku Gakkaishi/Journal of the Japan Institute of Metals
|Published - 2000