Thermal oxidation of tantalum films at various oxidation states from 300 to 700 degrees C

Abstract

This paper presents the combined use of mathematical modeling and Auger depth profiling to study and quantify the oxidation of Ta films over a wide range of temperatures. The thermal oxidation of tantalum films (similar to 700 nm) is studied using direct measurements of species concentration by means of Auger depth profiling. The oxidation temperature range of this study extends from 300 to 700 degrees C and the oxidation period varies from 5 s to 12.5 h. The Auger depth profiles revealed that the metallic film oxidizes to first form low valence oxides of Ta that progressively convert to tantalum pentoxide with increasing temperature and time. A first-order reaction diffusion model is used to quantify the diffusion of oxygen through a film that is evolving in composition. The Auger depth profiling and reaction-diffusion model are used to estimate the actual diffusivity values for oxygen in the evolving Ta/Ta-oxide thin-film matrix, rather than more conventional techniques that estimate either the initial diffusion of oxygen through a semi-infinite metal or give a depth- and time-integrated value for the diffusivity. A comparison between the actual diffusivity values estimated in this work and the depth- and time-integrated version using the same model revealed that the integrated values are higher than the actual diffusion values by greater than 300% for the temperature range tested. Moreover, these depth- and time-integrated values for diffusivity values match over the applicable temperature ranges the diffusivity values given in the literature, which are essentially integrated average values for Ta/Ta oxide matrix. Furthermore, using the Auger depth profiles, the oxide growth rates are quantified as a function of temperature and compared with available literature. The growth rate of the oxide that is observed to be logarithmic at 300 degrees C is seen to have a parabolic growth at 500 degrees C and then a multistep growth behavior (a combination of parabolic and linear growth) at 700 degrees C. These growth rates and the transition from one growth type to another strongly correlate to the change in surface and film morphology and also the transition from amorphous to crystalline Ta2O5.