Paper TF-WeA2
Phase Evolution in Sputter Deposited Hafnia-Titania Nanolaminates with Changing Architecture and Thermal Annealing

Wednesday, October 22, 2008, 2:00 pm, Room 302

Thin film HfO₂ is a candidate for a high dielectric constant replacement material for SiO₂ in integrated circuits. The addition of TiO₂ to HfO₂ has shown promise for producing a Hf_1-xTi_xO₂ ternary with an even higher dielectric constant than pure HfO₂ while maintaining thermal stability with Si. A convenient way of combining TiO₂ with HfO₂ in thin films is to sequentially sputter deposit them in a nanolaminate structure. In such a structure, interfaces are important in determining phase composition. The bulk pseudobinary HfO₂-TiO₂ temperature-composition phase diagram shows low miscibility between the end-point oxides. In this respect, the HfO₂-TiO₂ system is typical of an oxide nanolaminate in which there is a driving force for the formation of an interfacial mixed cation compound but there is no obvious kinetic path to achieve this structure via the formation of an interfacial crystalline substitutional solid solution. In this paper we examine the moderate temperature annealing behavior of HfO₂-TiO₂ nanolaminates with many different bilayer architectures. Multilayer stacks of HfO₂ and TiO₂ bilayers were grown on unheated fused silica substrates in a rf-excited multiple cathode reactor. The substrates were sequentially positioned under Hf and Ti targets and sputtered in 20 mtorr 80% Ar-20% O₂ discharges to build up the films. Four sequential annealing stages were carried out in laboratory air at the following temperatures: (I) 573 K, (II) 673 K, (III) 773 K, (IV) 973 K for 1 h each. The films were furnace cooled to room temperature and analyzed by double angle x-ray diffraction between annealing stages to obtain crystallographic data. The results show that the as-grown films are nanocrystalline and contain a mixed cation interface. This interface develops upon annealing into an orthorhombic HfTiO₂ phase adjacent to a titania layer, followed by an unusual metastable phase, monoclinic Hf_1-xTi_xO₂ more remote from the interface. In nanolaminates with thicker HfO₂ layers, a simulaneous development of monoclinic HfO₂ occurs. The lattice parameters of this nanocrystalline intralayer phase, however, are greater than the expected bulk value, indicating dipole-dipole repulsion at the surface of a HfO₂ nanocrystal might be occurring.