As part of an investigation of transition metal nitrides formed by means of rapid thermal processing, the reactivity of 200/500-nm thick tantalum films deposited by electron beam evaporation on oxidized silicon substrates has been studied in the temperature range from 400 to 1200 degreesC under laminar gas flow of nitrogen or ammonia. The nitride phases formed were characterized by X-ray diffraction and secondary neutral mass spectrometry. In particular, the initial stages of nitride formation, the influence of film thickness and the reactivity of tantalum towards the silicon dioxide substrate were investigated. A classical formation sequence of tantalum nitride phases was observed with increasing nitrogen content in the order: alpha-Ta(N) --> beta-Ta2N --> TaN0.8 --> epsilon-TaN --> Ta3N5. The SiO2 layer on the silicon substrate acts not only as a diffusion barrier for nitrogen, but also as a source of oxygen, resulting in the formation of Ta-oxides/-oxynitrides at the Ta/SiO2 interface. (C) 2003 Elsevier Science B.V. All rights reserved.