Ruthenium Nanocrystals And Its Application In Non-volatile Flash Memory

With downscaling of the device feature size,conventional poly-silicon floating gate flash memories are facing a severe challenge that scaling of the tunneling oxide thickness will degenerate retention characteristics.As potential substitutes,floating gates composed of discrete nanocrystals as opposed to continuous thin films have been proposed for next generations of flash memories,resulting in improved retention characteristics and more aggressive scaling ability for the tunneling oxide to achieve small operating voltages and fast program/erase speeds.Growth of Ru nanocrystals on atomic-layer-deposited Al₂O₃ film has been studied using ion-beam sputtering followed by post-deposition annealing.The Ru nanocrystals with a high density of 1.6×10¹¹cm^-2 and a mean diameter of 20 nm have been achieved for the PDA at 900℃for 15s.Scanning electron microscopy measurements show that well-defined spherical Ru nanocrystals are not formed till the PDA temperature is raised to 900℃.The mean diameter of the nanocrystals enlarges with increasing PDA temperature whereas the nanocrystal density decreases, which is attributed to coalescence process between adjacent nanocrystals.It is further illustrated that the resulting nanocrystal size and density are weakly dependent on the annealing time,but are markedly influenced by the decomposition of RuO₂.X-ray photoelectron spectroscopy analyses reveal that RuO₂ and Ru co-exist before annealing,and part of RuO₂ is reduced to metallic Ru after PDA process.The growth of Ru nanocrystals on the SiO₂ and Al₂O₃ films has been investigated comparatively using ALD method,indicating that much higher density Ru nanocrystals are grown on the Al₂O₃ film compared with the SiO₂ film.RuCp₂ and O₂ are chosen as precursors in our experiment and substrate temperature was kept at 300℃while deposition.Ru nanocrystals with a density of 9×10¹⁰ cm^-2 and a mean diameter of 14 nm are obtained on the Al₂O₃ film after 200 ALD cycles.The typical nanocrystal height increases with ALD cycles,however,the resulting nanocrystal density decreases for long deposition time(e.g.,600 cycles) on the Al₂O₃ film,going with inferior size uniformity.Post deposition annealing treatments at both 800 and 900℃cause a decrease in the nanocrystal density and an increase in the medial transverse dimension of nanocrystals.Prolonged annealing time at 900℃leads to a shrinkage of the medial transverse dimension of nanocrystals due to the formation of ball-like nanocrystals driven by minimizing the total surface energy.X-ray photoelectron spectroscopy analyses reveal that metallic Ru nanocrystals are surrounded by RuO₂ which is attributed to Ru oxidation in the air.Further,the Al₂O₃/Ru-nanocrystals/Al₂O₃ stack is constructed using atomic-layerdeposition technique,and the metal-oxide-silicon capacitors with this structure are electrically measured,exhibiting obvious memory effects such as a large hysteresis memory window of 3.4V for the sweeping gate voltage of-2.5/+8V and a significant fiat-band voltage shift of 3.2V under the programming of 10 V/1 ms,i.e.an effective electron injection rate as fast as 1.78×10^-6 e/cm².ms.This relates to the program mechanism of direct tunneling and a large potential well depth.The leakage current density is as low as 1×10^-6Acm^-2 at the gate bias of 6V.