Thermodynamics of Oxide Systems Relevant to Alternative Gate Dielectrics

Abstract

The search for gate dielectric materials superior to amorphous silica requires the in- put of thermodynamic data to assess materials compatibility with silicon and stability against crystallization or unwanted phase transformation during processing and/or subsequent use. Because superior dielectric properties are generally associated with materials containing heavy ions of large size and high charge, emphasis has been on trivalent and tetravalent oxides containing Ti, Zr, Hf, and the rare earths. Further- more, the materials must be insulating (disqualifying ions of variable valence and making TiO2 somewhat questionable) and the oxide must be less reducible than SiO2 so that it does not oxidize silicon. Within these constraints, an amorphous film is considered superior to a polycrystalline one with grain boundaries, so it is important to understand the persistence, controlled by both thermodynamic and kinetic factors, of amorphous and glassy materials. The purpose of this paper is to summarize rele- vant thermodynamic data, to provide a thermodynamic and structural framework for considering new compositions and their likely properties, and to present some new calorimetric data on bulk and thin film systems based on ZrO2 and HfO2.