Electromigration in aluminum damascene lines

Abstract

Four different metallization processes for AlCu dual damascene are investigated with respect to their electromigration lifetime and compared to a metallization using subtractive metal etching. It is shown that the electromigration reliability lifetime depends strongly on the metal stack and the metallization process used. Three fill processes that employ PVD Ti and PVD Ti/TiN liners result in poor electromigration, while a process using CVD TiN leads to superior electromigration performance. In this particular case the current density had to be increased to values as high as 2.5 MA/cm2 in order to induce a statistically relevant number of fails in certain structures. Two of the most important parameters that determine the electromigration lifetime in damascene lines are (1) the quality of the Al sidewalls as determined for instance by the presence of a TiAl3 reaction layer and (2) the amount of TiAl3 formation that reduces the bulk AlCu thickness. The texture of the AlCu matrix is of less importance. In fact, the damascene lines with the longest lifetime have no significant TiAl3 formation and almost random texture. This is in contrast to electromigration of metal RIE structures where randomly textured lines generally have short electromigration lifetime. The excellent electromigration performance of the optimum damascene samples is attributed to the absence of an etched Al-SiO2 interface and the suppression of Ti-Al reactions. This results in reduced number of defects and void nucleation sites, and a low material diffusivity.