Investigation on near room temperature self-annealing of electrochemical deposited (ECD) blanket copper films

Self-annealing of ECD blanket copper films on an IMP Cu (200nm)/IMP TaN (30nm)/ PECVD SiO2 (500nm)/Si multilayer structure with varying ECD Cu thickness from 0.5 mum to 3 mum is described and investigated. Sheet resistance, stress and surface topography evolution as a function of time have been monitored to provide a better understanding to the grain growth and surface agglomeration aspects of the self-annealing process. Resistivity changes, which exhibit a sigmoidal curve, are closely related with the grain size growth, which occurs during the recrystallization. However, this microstructural evolution should not be correlated to the stress relaxation effect on the films because of the difference in time dependence. The self-annealing process of ECD Cu films lasts from a few hours to days and weeks depending on the plated thickness of the Cu film. The initiation of self-annealing is accompanied by an abrupt rise in the standard deviation of the sheet resistance, reflecting the possibility of abnormal grain growth during the process. Changes in the time dependence of this decrease with film thickness can be understood in view of the physical constraint of a two-dimensional grain growth at the film boundary for thinner films. The coalescence mechanism of small grains until a full recrystallization of the layer into large twinned grains, which is used to explain the sheet resistance drop is supported by an increase in surface roughness of the ECD Cu film characterized by AFM. Diffusion or desorption of contaminants/additives in the relatively dirty process of ECD into the copper films could be a mechanism for the unusual stress release. It is found that the increase in the rate of stress relaxation scales with the thickness of the ECD Cu film. A change of the dominant factor for stress release from curvature dependent to film thickness dependent is speculated for this behavior.