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This work reports a statistical analysis of the evolution of polygranular segment lengths during high-temperature annealing of Al(Cu) thin-film interconnects with quasi-bamboo microstructures. To create samples of Al(Cu) lines that could be imaged by transmission electron microscopy without breaking or thinning, the lines were deposited on electron-transparent silicon nitride films (the “silicon nitride window” technique). The microstructures of the lines were studied as a function of annealing time and temperature. In particular, the distribution of polygranular segment lengths was measured. The results show that the longer polyglranular segments are preferentially eliminated during post-pattern annealing. As a consequence, the segment-length distribution narrows monotonically during annealing, and changes in shape. The preferential loss of the longest polygranular segments leads to a dramatic increase in resistance to electromigration failure.