Formation of Electromigration Defects in Small Damascene Cu Interconnects with Respect to the Microstructure

Electromigration processes in Cu metallization are of increasing importance in microelectronics. In order to study correlations between microstructural details and localization of electromigration defects generated in narrow electroplated Cu lines, microstructure of entire line was recorded by EBSD measurements with high lateral resolution, and then electromigration test was performed in high vacuum under direct SEM observation. To do this, the lines had to be unpassivated.Short electroplated Cu lines (length of 20 and 50 {mu}m, width of 230 to 130 nm) were loaded at 250 deg. C and 20 MA/cm2 with in-situ-SEM observation, and EBSD maps of the line with stepsize, i.e. point distance of 20 nm were acquired before and after test. In the result, a number of hillocks were found to have grown in the vicinity of (100) grains having at least one high angle grain boundary around it. Fatal voids often formed at the cathode end and produced dielectric cracking in some cases. Furthermore, voiding started from the sidewall of interconnect trenches mostly at such places where (111) grains with high angle grain boundaries were located. Microstructure of lines down to 130 nm was analysed with respect to electromigration defects, and the electromigration behavior was found to be rather similar compared to larger lines.