Optimization of Ta-Si-N Thin Films for Use as Oxidation-Resistant Diffusion Barriers

We have demonstrated that the optimum Ta-Si-N compositions for use as oxygen diffusion barriers in stacked-capacitor DRAM structures are in the range Ta(20-25 at.%)-Si(20-45 at.%)-N(35-60 at.%). The choice to study Ta-Si-N as a barrier material was made due to its quasi-amorphous / naocrystalline microstructure, which remains stable at temperatures exceeding 900^o C. Twenty tw different Ta-Si-N compositions were evaluated, starting from six sputter deposited Ta-Si alloys of which four were reactively deposited in 2-8% nitrogen in an argon plasma. The barriers were considered for use between platinum metal electrodes and underlying, doped poly-silicon plugs which make contact to the active regions of the CMOS devices. Typically high dielectric constant materials, such as (Ba,Sr)TiO₃ (BST), are doposited on the electrode at high temperature in an oxygen ambient and/or post-annealed at high temperature in oxygen to achieve the high dielectric constant, perovskite phase. Therefore barriers were evaluated after an aggressive 650^o C/30 min. oxygen anneal to determine if they remained electrically conductive, prevented oxygen diffusion and formation of low dielectric constant oxides, and had minimal interaction with the Pt electrode and underlying Si plug. Rutherford backscattering spectroscopy (RBS), four point probe sheet resistance, through-film-resistance and x-ray diffraction (XRD) analysis techniques were used in the evaluation.