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

Abstract

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.