Formation of WO3 by remote plasma assisted oxidation of tungsten thin films

- WO3 was formed by remote plasma oxidation of metallic tungsten (W) films on Si.
- Crystalline WO3 films are obtained at 500 °C and pressures of 1.33 and 33.3 Pa.
- Under these conditions, oxidation in O2 without plasma assistance was incomplete.
- WO3 formation is diffusion-limited progressing with a sharp oxidation front into W.

In this work, we have investigated the formation of WO3 by remote plasma oxidation of pulsed laser deposited tungsten thin films. The effects of experimental parameters, such as pressure, temperature and duration on the process, were investigated and the oxidized layers were characterized by X-ray diffraction, Rutherford back-scattering and scanning electron microscopy. Irrespective of the pressure used (1.33 or 33.3 Pa), remote plasma activation was found to lead to an efficient and controllable process for the formation of crystalline WO3, whereas oxidation using O2 gas was incomplete after 1 h of oxidation of 20 nm thick films kept at 500 °C. The remote plasma was also found to initiate the oxidation process at lower temperature (425 °C) but at a much slower rate and with lesser crystalline quality of the films. Remote plasma assisted oxidation experiments performed as a function of time indicate that the formation of WO3 is dictated by a diffusional process progressing with a sharp oxidation front into the W layers. The degree of control over the oxidation process and crystallinity of the films as well as the possibility of performing large area oxidation make this method attractive for the formation of WO3.