Evaluating Dielectric Thin Film for Next-generation DRAM Capacitor using Al-doped TiO2

Abstract

The optimized metal-insulator-metal (MIM) capacitor consisted with Al-doped TiO2 (ATO) and RuO2 as dielectric and electrode, respectively, for next-generation dynamic random access memory (DRAM) device was investigated. First, the role of Al dopant in rutile-phased TiO2 films in the evaluation of the mechanism of leakage current reduction in ATO was studied in detail. The leakage current of the ATO film was strongly affected by the Al concentration at the interface between the ATO film and the RuO2 electrode. The conduction band offset of the interface increased with the increase in the Al dopant concentration in the rutile TiO2, which reduced the leakage current in the voltage region pertinent to the DRAM application. However, the Al doping in the anatase TiO2 did not notably increase the conduction band offset even with a higher Al concentration. The detailed analyses of the leakage conduction mechanism based on the quantum mechanical transfer-matrix method showed that Schottky emission and Fowler-Nordheim tunneling was the dominant leakage conduction mechanism in the lower and higher-voltage regions, respectively. The chemical analyses using X-ray photoelectron spectroscopy corroborated the electrical test results. Next, the effects of Pt and RuO2 top electrodes on the electrical properties of capacitors with ATO films were examined. The interface between top electrode and ATO was damaged during the sputtering process of the top electrode, resulting in the decrease in the dielectric constant. Post-metallization annealing at 400 oC was performed to mitigate the sputtering damage. During the post-metallization annealing, the ATO layer near the RuO2/ATO interface was well-crystallized because of the structural compatibility between RuO2 and rutile TiO2 while the ATO layer near the Pt/ATO interface still exhibited an amorphous-like structure despite the same thickness of the ATO films, therefore, the capacitors with RuO2 top electrodes show higher capacitance compared to the capacitors with Pt top electrodes. Eventually, an extremely low equivalent oxide thickness of 0.37 nm with low enough leakage current density (<1 × 10−7 A/cm2 at 0.8 V) and physical thickness of 8.7 nm for the next-generation dynamic random access memory was achieved from ATO films with RuO2 top electrodes. Finally, the chemistry of TiO2 atomic layer deposition (ALD) on Ru or RuO2, which is one of the most promising electrode materials in DRAM capacitors, was studied in detail. A series of the Ru-related layers with compositions ranging from Ru to RuO2 via RuOx (x: ~1.12) was used as a bottom electrode for the ALD growth of TiO2 films. It was found that the growth per cycle of TiO2 at the initial growth stage was drastically increased on RuOx (RuO2/Ru mixture) compared to Ru and RuO2. 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 RuOx 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.