Copper as metallization materials for semiconductor, is substituted for Aluminum, and electroplating is widely used to deposition of copper for many advantages. In case of electroplated Cu thin films, grain growth occurs at room temperature, and this phenomenon is called to "self-annealing". The causes of self-annealing have been reported, but exact mechanism is not known yet, and to make this problem clear, electroplated Cu film with various condition are performed, and characterization of self-annealing is accomplished.
After referring to various paper, Cu electroplating device was designed, and using this device, basic electroplating characteristics was analyzed. As electroplating time was increased, final grain size was increased, and electroplating temperature did not affect electroplated Cu film, It is well known that electroplated film is much affected by morphology of seed layer, but experiments show that thickness of seed layer and grain size of seed layer do not influence morphology of electroplated Cu film. And uniform plating was possible for high aspect ration trench or hole pattern if seed layer could be deposited.
Electroplated film was annealed at room temperature, and self-annealing characteristics was analyzed. It is known that when chlorine ion($Cl^-$) and PEG(Poly Ethylene Glycol) are added to electrolyte, self-annealing is accelerated. Temperature characteristics of self-annealing follows Arrhenius plot, and from this plot it is known that activation energy for self-annealing is about 0.98eV. This value is higher than activation energy for recrystallization or grain boundary diffusion of pure Cu, and it is thought that this high value is caused from volume size difference between PEG and Cu. Although activation energy is higher than that of pure Cu, grain growth occurs at lower temperature. This means that other driving forces which accelerate motion of Cu atoms, exist. Therefore, experiments about the role of $Cl^-$ and PEG which were considered to accelerate self-annealing when they were adsorbed in film, were performed. In case of $Cl^-$, it is predicted that existence of $Cl^-$ leads to acceleration of Cu atomic movement, because $Cl^-$ increases diffusivity of Cu.
Self-annealing is the phenomenon which occurs at only thick electroplated film whose thick is above 1μm, and to find the cause of this phenomenon, residual stress for electroplated film of various thickness measured. It is known that high stress is applied at the thick film whose thickness is above 1μm, and this stress is released during self-annealing, and it is higher than maximum theoretical stress by grain growth. Therefore it is thought that other factors which generate stress release, exist. This is originated from stress around PEG which is adsorbed in grain boundary, from misffting sphere model, it is known that strain energy of 27.1meV/atom is applied around PEG, and this energy is higher than any other energy of driving force for self-annealing, such as grain boundary energy, dislocation energy and impurity energy.
Consequently, the most influential driving force for self-annealing is locally high stress applied around PEG, this leads to grain size refinement and breaks bonds of PEG which acts as inhibitor to suppress grain boundary motion, makes high strain field around PEG, therefore plays a role of accelerating nucleation which is need to self annealing.
