Cu behaviors induced by aging and their effects on electromigration resistance on Al-Cu lines

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Abstract
The amount of the segregated Cu in AlCu films detected by energy dispersive X-ray spectroscopy after an aging treatment at 250 °C showed the same trend with aging time as that for the extension in time-to-failure (TTF) for AlCu/TiN lines indicating that the segregated Cu plays a central role in improving electromigration(EM) resistance. A model proposed by Rosenberg et al. which intensifies the importance of the decrease in the defect density at grain boundaries due to Al-Cu bonding was supported as a mechanism for the correlation between the segregated Cu and the TTF extension. Differences in TTF between AlSiCu and AlSi lines were plotted as a function of temperature to obtain the activation energy as 0.67 eV. This value interpreted to represent the preferential diffusion of Cu was close to that for the grain boundary diffusion of Cu, 0.6 eV, indicating that the preferential diffusion of Cu proceeds along grain boundaries. A model which includes both the role of the preferential diffusion of Cu and the segregated Cu was proposed. In this model, the segregated Cu is kept at grain boundaries during the preferential diffusion period to restrict EM of Al atoms. Based on this model, the aging treatment was proposed to be a promising technique which accumulates Cu atoms to grain boundaries in AlCu lines to extend the preferential diffusion period. In addition, estimate of diffusion length of Cu told an importance to eliminate aging phenomena during EM testing for precise EM life evaluation, especially for AlCu lines with W-studs where depletion of Cu near W-plugs has a crucial effect on the TTF’s.

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