Structure control of sputter deposited noble metal thin films using surfactant

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Noble metal thin films with very smooth surface morphology and high crystal orientation are desired for use as capacitor electrodes of DRAM (Dynamic Random Access Memory) and Ferroelectric memory (FeRAM), and as spin valve films, which are applied for magnetic recording devices. We aimed to improve the surface morphology and crystal orientation of noble metal thin films by introducing a small amount of active gas which act as a surfactant.Ru thin films, which are used as capacitor electrodes of DRAM and FeRAM were deposited by RF magnetron sputtering system. Effects of substrate temperature, deposition rate and composition of sputtering gas (Ar+O_2) on surface morphology and crystal orientation of Ru thin films were studied. Very flat and highly c-axis single oriented Ru films with surface roughness of about 1nm and XRD rocking curve width of 1.2° were obtained under optimum deposition conditions, which are substrate temperature of 500℃, deposition rate below 10 nn/min, and sputtering … More gas flow ratio, O_2/(Ar+O_2) of several %. It is thought that O_2 gas acts as a surfactant and decrease the surface free energy of Ru c-plane and furthermore, O_2 gas is expected to inhibit aggregation of Ru atoms and improve surface morphology.Next, effects of deposition conditions of dielectric films on the surface roughness of Ru films were studied, because it is very important for the application of Ru films to DRAM capacitors. Ta_2O_5 films were sputter deposited under various substrate temperatures on Ru films. It was found that the surface and interface of Ta_2O_5/Ru multilayer remain flat if the Ta_2O_5 films were deposited below 400℃, however, those of the multilayers become rough at a deposition temperature of 500℃. The reason for the surface and interface roughness was found to be the oxidation of Ru films and the formation of RuO_2. It is clarified that deposition temperature of dielectric films should be lower than 400℃ to avoid the oxidation of the Ru film and maintain smooth surface morphology. Less