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Total width of line is occupied by asingle grain Atom on grain boundary isunable to move in thedirection of the force. Electron flowOverall electromigration force.
Grain growth

The grain structure observed is influenced by both the original production and subsequent grain growth. Grain growth is most significant at temperatures more than half the melting temperature i.e. TTm.

During grain growth, some grains grow larger whilst others shrink and become smaller, eventually disappearing. Overall , the average grain size increases with time.
Note: Animation of grain growth is only a schematic replication of the actual process. Several factors are involved to determine which grains grow and which shrink.
Grain growth within metallization lines

Metallization lines are very narrow (approximately 0.2µm or less), and are becoming ever narrower. This results in the formation of a bamboo-like microstructure, which is desirable for its electromigration resistance.
Bamboo Structure
Atoms along the grain boundary is most mobile. Due to the electromigration force acting parallel to the grain boundary, no motion can take place. This reduces the amount of electromigration flux, thus protecting the line from electromigration-induced damage.