THE MECHANISM OF ELECTROMIGRATION FAILURE OF NARROW AL-2CU-1SI THIN-FILM INTERCONNECTS

This work is principally concerned with the microstructure of electrom igration failure in narrow Al-2Cu-1Si conducting lines on Si. Samples were patterned from 0.5-mum-thick vapor-deposited films with mean grai n size of 2.4 mum, and had linewidths of 1.3 mum (W/G almost-equal-to 0.5), 2 mum (W/G almost-equal-to 0.8), and 6 mum (W/G almost-equal-to 2.5). The lines were tested to failure at T=226-degrees-C and j=2.5X10 (6) mu/cm2. Other samples were tested over a range of substrate temper atures and current densities to test the effect of these variables, an d 1.3 mum lines were tested after preaging at 226-degrees-C for variou s times to change the Cu-precipitate distribution prior to testing. Th ree failure modes were observed: The 6 mum specimens failed by separat ion along grain boundaries with an apparent activation energy of 0.65 eV; the 1.3 mum specimens that were preaged for 24 h failed after very long times by gradual thinning to rupture; all other narrow lines fai led by the transgranular-slit mechanism with an activation energy near 0.93 eV. Microstructural studies suggest that the transgranular-slit failure mechanism is due to the accumulation of a supersaturation of v acancies in the bamboo grains that terminate polygranular segments in the line. Failure occurs after Cu has been swept from the grain that f ails. Failure happens first at the end of the longest polygranular seg ment of the line, at a time that decreases exponentially with the poly granular segment length. Preaging the line to create a more stable dis tribution of Cu lengthens the time required to sweep Cu from the longe st polygranular segment, and significantly increases the time to failu re. In the optimal case the transgranular-slit failure mechanism is su ppressed, and the bamboo grain fails by diffuse thinning to rupture. P reaging is particularly effective in increasing the lifetimes of lines that contain very long polygranular segments, and has the consequence that the time to first failure in an array of lines is much longer th an predicted by a log-normal fit to the distribution of failure times.