Microstructure study of electroplated copper films for ULSI metal interconnections

Microstructure control of electroplated Cu film is an issue of vital importance in Cu manufacturing process and interconnect reliability. This research explores the microstructure control in electroplated Cu films. Factors that affect the microstructure of the Cu films and underlying mechanisms have been identified, and their correlations have been investigated in both theoretical and experimental approaches.; The large grain size of electroplated Cu films results from a remarkable secondary grain growth at room temperature after deposition, so-called “self-annealing” or “room-temperature recrystallization.” During the self-annealing, the grain size increases by about an order of magnitude, from ∼0.1 μm to ∼1 μm, accompanied by a reduction in sheet resistance by about 20%. Degradation of (111) texture, enhancement of (200) texture, and development of stress towards the tensile direction are also observed in the film during the self-annealing process. The rate of the self-annealing process increases drastically as the plating current density is increased. Strong correlation between stress and texture development was found for all electroplated Cu films under the self- and thermal annealing. This correlation is explained by the surface/interface energy and strain energy in anisotropic metal films. Texture remains unchanged until when the biaxial stress in the film exceeds a critical value, and over the critical stress, the (111) texture starts to degrade and the (200) start to enhance to reduce the strain energy in the film. The existence of a high density of dislocation loops in the pre-recrystallization grains of as-deposited films were demonstrated as a driving force of the abnormally fast grain growth during self-annealing at room temperature. Plating with a higher deposition rate produces more dislocation loops in the Cu film, leading to faster self-annealing rate. The kinetic calculation with the inclusion of the dislocation loop energy demonstrates a good correspondence with the experimental observations. The grain size is small in the Cu film plated on a rough underlying seed layer surface due to the significant retardation of the self-annealing process. Strong texture in the electroplated film is observed only when plated on a strongly textured and smooth seed layer.