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.