Dynamic random access memory (DRAM) is used as the main memory of every modern computer, due to its high density, high speed and efficient memory function. For successful operation of DRAM, a large cell capacitance (~ 25 fF) and low leakage current (10^-7 A/cm² or 1 fA/cell) are required. In a traditional Si-based capacitor, the target cell capacitance has been achieved by increasing the surface area of the capacitor. More recently, innovations have been made in the component materials. A metal electrode, TiN or Ru, and a dielectric material with a higher-k value (k is the relative dielectric constant) than that of the SiO₂/Si₃N₄ layer (k ~ 6 - 7), such as HfO₂ (k ~ 25) and ZrO₂ (k ~ 40), are being explored in giga-bit scale DRAMs. The minimum achievable t_ox is ~ 0.7 nm for HfO₂ and ZrO₂ which are currently being used in the DRAM industry. However, the technology road map for memory devices states that t_ox less than 0.5 nm is necessary for the DRAMs with a design rule of < 40 nm. Perovskite-based dielectric films such as SrTiO₃ and (Ba,Sr)TiO₃ were reported to exhibit k values of several hundreds. However, growth of these films is extremely difficult with the atomic-layer-deposition (ALD) which is a method of choice for the growth of the dielectric films in microelectronic devices. In this presentation, the authors report Al-doped TiO₂ (ATO) dielectric thin films grown by an ALD method for application to next-generation DRAM capacitors. TiO₂ in rutile phase exhibits a k value ~100 and therefore can be used for DRAMs with a design rule of < 40 nm. However, the small energy gap of 3.1 eV implies that the control of leakage currents is not trivial. We achieve the specifications on the leakage currents, i.e., 10^-7A/cm² at 0.8V, through elaborate doping of TiO₂ films with Al atoms. In addition, it was found that this specific thin film material and process can meet all of the critical requirements.

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