Influence of grain boundary type on electromigration in damascene copper lines

Abstract

The preceding trend of miniaturization in microelectronics industry leads to drastic increase of current density in the interconnect lines. Additionally, copper as new material and the double damascene technology poses new questions concerning the interconnect damage by electromigration (EM). In the work presented here, the influence of the grain boundary network on the formation of voids and hillocks in Cu interconnects is investigated. For this purpose, electromigration tests of unpassivated Cu lines (width 1 to 4 μm) were carried out in situ in a SEM, coupled with acquisition of their complete microstructural and orientation state by means of Electron Backscatter Diffraction (EBSD). After EM testing the line damages were investigated using a Focused Ion Beam device for studying the microstructural details in normal direction and around the defects. It was found that the strings of large angle grain boundaries in the electron flow direction are decisive for the formation of voids and hillocks. This was found for Cu lines produced by PVD as well as for electroplated lines, though they had rather different microstructure and texture. FIB cuts through hillocks showed that they have grown sometimes epitaxially, sometimes non-epitaxially, and they often consisted of more than one grain. The effect of misorientation state of a grain boundary on its transport properties is discussed and compared with former results on Al interconnects.