X-ray photoelectron spectroscopy and time-of-flight secondary ion mass spectrometry study of the role of Ti and TiN caps on the cobalt/SiO2 interface

Abstract

Continuous down scaling of device features and increases in operation frequency of integrated circuit (ICs) requires a low electric resistance of interconnects to transistors. Due to its low resistivity and high thermal stability, the integration of CoSi2 into ultralarge scale ICs is becoming the main stream. In this study, a 15 or 20 nm Co film was grown on SiO2 and Ti or TiN top layers were deposited on the Co layer without breaking the vacuum. The samples were annealed in N2 ambient and the specimens were analyzed by x-ray photoelectron spectroscopy and time-of-flight secondary ion mass spectrometry in depth profile mode. Significantly different depth profiles were obtained depending on the nature of the cap layer and on the annealing temperature. The multilayer with a top TiN layer presents a profile corresponding to sharp interfaces with only a limited diffusion of Ti inside the Co layer and no differences in chemistry of the interfaces. The Co depth profile has a very symmetrical shape. On the contrary, the presence of a Ti cap layer induces very strong modification of the interface reactions. An important diffusion of the Ti is observed through the Co layer and an accumulation of Ti occurs at the Co/SiO2 interface. At the interface, Ti is observed in an oxidized form and reduces the top of the SiO2 layer. When the annealing temperature is reduced to 650 °C no pile up of Ti is seen at the interface and the silicon profile is similar to the profile obtained with the TiN cap layer.