Stress, texture and electromigration in damascene copper interconnects

Recently Al was replaced by Cu as an interconnecting material. The primary objective of the present research was to investigate the mechanism of texture and microstructure evolution and to study its influence on electromigration in damascene Cu interconnects. For this purpose electromigration experiments were performed on the Cu interconnects in vacuum in the SEM. The in-situ electron back-scatter diffraction (EBSD) investigation of Cu interconnect lines before and after the electromigration failure helped to identify the orientations which were associated with electromigration defect nucleation. A mechanism was proposed which shows the correlation between the texture and electromigration damage formation.The second part of the study involved investigation of texture and microstructure in damascene Cu interconnects. The experiments were designed so as to allow an analysis of both pre and post-CMP (chemical mechanical polishing) annealing on texture and microstructure evolution not only as a function of line width but also line spacing. In order to investigate the mechanism of texture evolution in the damascene Cu interconnects in-situ EBSD study of Cu films was carried out during their thermal treatment in SEM. A similar separate experiment carried out on freestanding Cu film helped to ascertain the role of substrate in inducing the texture transformation at high temperature. Some models were proposed which establish the combined effect of dislocation activity and substrate on texture evolution. Both EBSD and X-ray diffraction (XRD) methods were used to measure the texture. Finally the residual stresses in the damascene lines were measured using XRD to evaluate their impact on the mechanical reliability of the chip interconnect system.The EBSD and XRD studies helped to identify the mechanisms which govern the texture and microstructure evolution in Cu interconnect lines during annealing performed before and after CMP. The role of surface/interface and strain energy on formation of energy minimizing textures was established. The mechanism of texture evolution was explained explicitly as function of principal and shear stress, dislocation density, trench aspect ratio and top passivation layer.Thus, from the electromigration studies it was possible to screen out the undesirable grain orientations. The XRD and EBSD investigations revealed the mechanism of texture and microstructure evolution in damascene lines and were used to explain the impact of various factors like stresses, dislocations, line width, line spacing, trench aspect ratio, passivation layer, substrate on texture evolution. The knowledge gained from these two studies opens a new door giving an opportunity to design the texture and microstructure in the damascene interconnect lines so as to eliminate undesirable grain orientations which are vulnerable to electromigration and mechanical failure. Since the mechanism of texture evolution is now known, one could vary the process parameters like say current density, barrier and passivation layer material and thickness, Cu seed and electroplated layer thickness, additive content in electroplating bath, annealing conditions etc. to obtain an optimum texture and microstructure which provides best reliability against electromigration and mechanical failures