The oxidation and resistance of tantalum-nitride thin films

Abstract

The oxidation of reactively-sputtered, tantalum—nitride thin films has been studied between 473 and 773 K in air. Films with thicknesses that correspond to sheet resistances of 43, 75, and 150 Ω/□ were evaluated in this study. X-ray diffraction revealed that the films and the oxidation products were amorphous. The oxidation products were characterized with AES and XPS. The oxidation process was monitored by measuring the change in sheet resistance with time. Sheet resistance measurements were performed with a four-point probe. The oxidation kinetics can be represented by the equation ΔR/R n =k′t. ΔR/R is sheet resistance change in %, t is time, and k′ is the rate constant. The oxidation kinetics, for the thickest film, were parabolic above about 598 K, and approximately quartic below this temperature. The 75 Ω/□ films deviated from parabolic kinetics below 603 K, and approached approximate cubic kinetics at 498 K. However, the data were forced to fit parabolic kinetics with good correlation. For the thinnest film, no deviation from parabolic kinetics was observed. The rate constants obeyed the Arrhenius relation. An activation energy of 147 kJ/mole (1.52eV) was calculated, for the thickest film, during parabolic kinetics. This value is approximately equal to the activation energy obtained during quartic kinetics. A model that explains the deviation from parabolic to quartic oxidation kinetics at lower temperatures is presented. In this model, the change in resistance as a function of time was due to simultaneous change in film resistivity, as a result of oxygen dissolution into the film, and oxide-scale growth. The temperature at which this deviation occurs should be a function of film thickness.