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Dielectric and electrode thin films for stack-cell structured DRAM capacitors with sub 50-nm design rules

Cheol Seong Hwang ,  Seong Keun Kim ,  Kyu Jin Choi 

Seoul National University (SNU), School of Mat. Sci. Eng., Seoul 151742, Korea, South

Abstract

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 tox 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 tox 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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Related papers

Presentation: Invited at E-MRS Fall Meeting 2007, Symposium C, by Cheol Seong Hwang
See On-line Journal of E-MRS Fall Meeting 2007

Submitted: 2007-05-12 07:29
Revised:   2009-06-07 00:44