Influences of oxygen on the formation and stability of A15 β-W thin films

Thin tungsten films were produced by magnetron sputtering at room temperature in Ar and O₂ gas mixture. The influences of oxygen impurities by varying oxygen partial pressures on the formation and stability of the A15 β-W phase were investigated. The films were analyzed by X-ray photoelectron spectroscopy (XPS), electron energy-loss spectrometry (EELS), X-ray diffraction (XRD), and energy-filtered electron diffraction (EFED). It was found that the formation of the body-centered-cubic (bcc) α-W structure was favored when the oxygen content in the films was less than 3 at.%, while the A15 β-W phase was formed between 6 and 15 at.% oxygen. At higher oxygen partial pressures, the films deposited in this manner were found to be essentially amorphous. Phase transformation from A15 W to bcc W by higher temperature annealing (900 K) was accompanied by reduction of oxygen in the films. The driving force for the irreversible phase transition process, A15 β-W→bcc α-W by anneal, is discussed.