Improvement of smooth surface of RuO₂ bottom electrode on Al₂O₃ buffer layer and characteristics of RuO₂/TiO₂/Al₂O₃/TiO₂/RuO₂ capacitors

Ruthenium oxide (RuO₂) thin films, which are deposited by plasma-enhanced atomic layer deposition (PE-ALD) with a Ru(EtCp)₂ precursor and oxygen plasma, exhibit a smoother surface [root mean square (RMS) roughness <1 nm] on ionic Al₂O₃ and TiO₂ buffer layers than on a covalent SiO₂ buffer layer (RMS roughness of RuO₂: 2.5 nm). The Al₂O₃ and TiO₂ buffer layers which have some charges enable us to prolong the duration time of the Ru(EtCp)₂ precursor on the buffer layer and cause the nucleation of RuO₂ to occur uniformly. The RuO₂ film deposited on the Al₂O₃ buffer layer by PE-ALD (hereafter "PE-ALD-RuO₂") was used as the bottom electrode for a metal-insulator-metal with a TiO₂/Al₂O₃/TiO₂ (TAT) insulator. RuO₂/TAT/RuO₂ capacitors on the Al₂O₃ and TiO₂ buffer layers had a low enough leakage current density (J) (on the order of ∼10^-8 A/cm²), unlike RuO₂/TAT/RuO₂ capacitors on the SiO₂ buffer layer and TiN/TAT/TiN capacitors. These results suggest that the different J properties must be related to the surface roughness of the bottom electrode rather than the work function of RuO₂ and TiN. Furthermore, the overall dielectric constant of TAT in RuO₂/TAT/RuO₂ capacitors reasonably ranged from 50 to 30 as the Al₂O₃ interlayer thickness increased from 0 to 5 nm. During conductive-atomic force microscopy measurements, the TAT/TiN stack structure exhibited several leakage points, while no such leakage points were observed in the case of TAT/RuO₂ on the Al₂O₃ buffer layer. Thus, PE-ALD-RuO₂ on the Al₂O₃ buffer layer is a candidate bottom electrode material for future dynamic random access memory.