Enhancement in Electrical Properties of (Ba,Sr)TiO 3 Films Using Tailored Oxide Electrodes
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dielectric constant and
the leakage
current. The
interface
characteristics between the BST film and the electrode were examined in order to interpret the electrical properties of BST films. INTRODUCTION In order to realize the next generation Giga-bit random access memories, one of the key issues is to seek a high dielectric capacitor material. To date, it is a widely accepted idea that the (Ba.Sr .•jTiO 3[hereafter referred to as BST] is a leading candidate capacitor material for DRAM application because of its chemical stability and good dielectric properties.[1-3] It is also of great importance to select a proper electrode material for BST because the process condition of BST film is extremely severe relative to the conventional dielectric materials such as SiO, or SiN. To be a good electrode material, several requirements should be met: good conductivity, thermal and chemical stability, minimum reaction with dielectric materials, ease in etching, etc. The critical issues related to the BST capacitor and the electrode are shown in Fig. 1.
Pt has been the most widely used electrode for dielectric materials due to its excellent 77 Mat. Res. Soc. Symp. Proc. Vol. 574 © 1999 Materials Research Society
conductivity and relatively good stability in high temperature and oxidizing ambients.[4-6] However, it is very difficult to etch platinum and it is necessary to use a barrier/adhesion layer, which makes the fabrication complicated. [7] Electrode - Conductivity - Patterning - Oxidation - Interaction with BSlT - Surface roughness (hillock)
B BS'I -Uniformity -T'hickness -Composition Grain size/orien. - Step coverage
I
.'
I
•Plug resistance
Iarricr - 0, in Si diffusion Conductivity - Contact resistant - Interactions with BST and plug
Plug resistance _-
Fig. 1. Schematic cross-section of BST capacitor structure. Many researchers are interested in looking for new electrode materials to replace the Pt electrode. Among them, RuO 2 which is a conductive oxide, has been the most intensively investigated. It is reported that the RuO 2 electrode has a relatively low resistivity and acts as an effective diffusion barrier against oxygen.[8-9] RuO 2 can be easily etched by the CH 4 and Ht,gas mixture.[10] But the practical use of RuO 2 is not realized because it results in high leakage current.[ I1] Therefore, there have been many efforts to reduce the leakage current. In this study, we explored new RuO,-base electrode materials by adding elements to RuO, from the recognition that the high leakage current associated with BST films on the RuG 2 electrode can be originated from the structural mismatch, incompatible chemical composition, and relatively small work function difference between the two materials. A systematic approach has been attempted to solve these problems. Since it was demonstrated that SrRuO3 and CaRuO, crystallize into the perovskite structure, firstly, we selected and evaluated the (Ca,Sr)RuO 3[hereafter referred to as CSR] as an electrode that can structurally match with BST. Different atomic ratio
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