Paper TF-TuP21
Chemistry of Active Oxygen in RuO_x and Its Influence on the Atomic Layer Deposition of TiO₂ Films

Tuesday, December 9, 2014, 4:00 pm, Room Mauka

Atomic layer deposition (ALD) is a highly intriguing thin-film growth technique with self-regulating and self-terminating properties due to the saturated chemical reactions between the adsorption sites and the chemically adsorbing precursor molecules. In such ALD processes, the substrate plays a crucial role by taking part in the ALD-specific chemical reactions. The surface of the substrate must provide the chemical adsorption sites, which would anchor the incoming metal or non-metal precursors via the ligand exchange reactions. There are other cases, however, where the oxygen ions (or atoms) inside the bottom layer film play the crucial role in the film growth. Typical example is Ru (and RuO₂), which are the material of choice as the capacitor electrode in futuristic DRAM. Recent reports on the inducing of rutile phase formation in the atomic layer deposition (ALD) of TiO₂ films, which have the highest dielectric constant among the binary dielectric oxides, by adopting Ru or RuO₂ as the bottom electrode invokes an even higher interest in this material. While both Ru and RuO₂ electrode have similar electrical properties, the initial stage of deposition of TiO₂ film by ALD exhibited quite different behavior on each substrate due to their different oxygen contents. In this study, the initial growth behavior of ALD TiO₂ film on Ru and RuO₂ substrate with various oxygen contents has been investigated in a greater detail, which has not been explored previously. Ru, RuO_x (x ~ 1) and RuO₂ films were grown by the cycling pulsed chemical vapor deposition process at a substrate temperature of 230 ^oC using RuO₄ and H₂(5%)/N₂(95%) as the Ru-precursor and reducing agent. The phase control was achieved by controlling the injection time of H₂(5%)/N₂(95%) gas for the given RuO₄ injection condition.

It was found that the growth per cycle of TiO₂ at the initial growth stage was drastically increased on RuO_x (RuO₂/Ru mixture) compared to Ru and even RuO₂, although it has been reported that higher oxygen content film (RuO₂) promotes the initial growth of TiO₂. This is attributed to the drastic increase in the chemical activity of oxygen in the mixture film of RuO₂/Ru. The catalytic decomposition of RuO₂ with the help of Ru in the film played the crucial role for the increase in the active oxygen. While RuO₂ and Ru mostly retained their structures during the ALD of TiO₂ or chemical etching using O₃ gas, the RuO_x film, which was composed of 56% RuO₂ and 44% Ru, drastically changed its phase composition during the ALD of TiO₂ at 250 °C and became almost Ru. Other chemical effects depending on the chemical composition and phase structure were also examined in detail.