Micro texture dependence of both the mechanical and electrical properties of electroplated copper thin films used for interconnection

Both mechanical and electrical properties of electroplated copper thin films were investigated experimentally with respect to changes in their micro texture. Clear recrystallization was observed after the annealing even at low temperature of about 150°C. The fracture strain of the film annealed at 400°C increased from the initial value of about 3 % to 15 %, and at the same time, the yield stress of the annealed film decreased from about 270 MPa to 90 MPa. In addition, it was found that there were two fatigue fracture modes in the film annealed at the temperatures lower than 200°C. One was a typical ductile fracture mode with plastic deformation and the other was brittle one. When the brittle fracture occurred, the crack propagated along weak or porous grain boundaries which remained in the film after electroplating. The brittle fracture mode disappeared after the annealing at 400°C. These results clearly indicated that the mechanical properties of electroplated copper thin films vary drastically depending on their micro texture. Next, the electrical reliability of electroplated copper thin film interconnections was discussed. The interconnections used for electromigration (EM) tests were made by damascene process. The width of the interconnections was varied from 1 μm to 10 μm. An abrupt fracture mode due to local fusion appeared in the as-electroplated films within a few hours during the test. Since the fracture rate increased linearly with the increase of square of the applied current density, this fracture mode was dominated by local Joule heating. It seemed that the local resistance of the film increased due to the porous grain boundaries and thus, the local temperature around the porous grain boundaries increased drastically. On the other hand, the life of the interconnections annealed at 400°C was improved significantly. This was because of the increase of the average grain size and the improvement of the quality of grain boundaries in the annealed films. The electrical properties of the electroplated copper films were also dominated by their micro texture. However, the stress migration occurred in the interconnections after the annealing at 400°C. This was because of the high residual tensile stress caused by the constraint of the densification of the films by the surrounding oxide film in the interconnection structures during the annealing. Finally, electroplating condition was controlled to improve the electrical properties. Both the resistance of electromigration and electrical resistivity were improved significantly. However, electromigration of copper atoms still occurred at the interface between the electroplated copper and the thin tantalum (Ta) layer sputtered as base material. Therefore, it is very important to control the crystallographic quality of electroplated copper films and the interface between different materials for improving the reliability of thin film interconnections.