High dielectric $(Ba,Sr)TiO_3$ capacitors using iridium-based electrode materials

Abstract

Barium strontium titanate (BST) is expected to be a good candidate material for storage capacitors of Gbit-scale dynamic random access memories (DRAMs) due to its high dielectric constant and good insulating properties. A platinum (Pt) thin film is commonly used as an electrode for high dielectric BST capacitors, because it exhibits low resistivity and good heat resistance, and leads to low capacitor leakage current. In spite of these advantages, Pt poses a number of problems with regard to its application. In this dissertation, iridium (Ir)-based thin films are proposed as new electrode materials for high dielectric BST capacitors. First, characteristics of Ir thin films are compared with those of other electrode materials. Electrical properties of BST capacitors using Ir electrodes are also characterized and demonstrated. The Ir electrode is found to be superior to the conventional Pt electrode in a number of aspects such as crystallinity, resistivity, adhesion and surface roughness. Furthermore, the possibility of chemical etching for Ir is confirmed by the investigations on the etch rates of the films with various reactive gases. The use of Ir bottom electrode instead of Pt is found to result in the major increase in the dielectric constant, maintaining a sufficiently low leakage current. It is considered to be due to the preferential oriental growth of BST film caused by $IrO_2$ nucleation sites formed on Ir bottom electrode. The minor increase in dielectric constant is also obtained by using Ir as a top electrode of BST capacitor, which is described by the stress effect of Ir on BST. The reliability of BST capacitors which is directly related with their practical applications to DRAMs is studied in detail, as the next topic. Another degradation phenomenon-dielectric degradation- in addition to resistance degradation has been found firstly and studied for the physical understanding of its nature. The dielectric degradation is explained by the increase in d...