MICROSTRUCTURAL EVOLUTION OF AL-CU THIN-FILM CONDUCTING LINES DURING POST-PATTERN ANNEALING

This work reports a statistical analysis of the evolution of polygranu lar segment lengths during high-temperature annealing of Al(Cu) thin-f ilm interconnects with quasi-bamboo microstructures. To create samples of Al(Cu) lines that could be imaged by transmission electron microsc opy without breaking or thinning, the lines were deposited on electron -transparent silicon nitride films (the ''silicon nitride window'' tec hnique). The microstructures of the lines were studied as a function o f annealing time and temperature. In particular, the distribution of p olygranular segment lengths was measured. The results show that the lo nger polygranular segments are preferentially eliminated during post-p attern annealing. As a consequence, the segment-length distribution na rrows monotonically during annealing, and changes in shape. The prefer ential loss of the longest polygranular segments leads to a dramatic i ncrease in resistance to electromigration failure.