Capacitors with an equivalent oxide thickness of <0.5 nm for nanoscale electronic semiconductor memory

The recent progress in the metal-insulator-metal (MIM) capacitor technology is reviewed in terms of the materials and processes mostly for dynamic random access memory (DRAM) applications. As TiN/ZrO₂-Al₂O ₃-ZrO₂/TiN (ZAZ) type DRAM capacitors approach their technical limits, there has been renewed interest in the perovskite SrTiO ₃, which has a dielectric constant of >100, even at a thickness ∼10 nm. However, there are many technical challenges to overcome before this type of MIM capacitor can be used in mass-production compatible processes despite the large advancements in atomic layer deposition (ALD) technology over the past decade. In the mean time, rutile structure TiO₂ and Al-doped TiO₂ films might find space to fill the gap between ZAZ and SrTiO₃ MIM capacitors due to their exceptionally high dielectric constant among binary oxides. Achieving a uniform and dense rutile structure is the key technology for the TiO₂-based dielectrics, which depends on having a dense, uniform and smooth RuO₂ layer as bottom electrode. Although the Ru (and RuO₂) layers grown by ALD using metal-organic precursors are promising, recent technological breakthroughs using the RuO ₄ precursor made a thin, uniform, and denser Ru and RuO₂ layer on a TiN electrode. A minimum equivalent oxide thickness as small as 0.45 nm with a low enough leakage current was confirmed, even in laboratory scale experiments. The bulk dielectric constant of ALD SrTiO₃ films, grown at 370 °C, was ∼150 even with thicknesses ≤15 nm. The recent development of novel group II precursors made it possible to increase the growth rate largely while leaving the electrical properties of the ALD SrTiO ₃ film intact. This is an important advancement toward the commercial applications of these MIM capacitors to DRAM as well as to other fields, where an extremely high capacitor density and three-dimensional structures are necessary.