Electron microprobe analysis of the Ta-O system

The detection system of an electron microprobe was modified to increase its sensitivity to soft x-rays, absorbed oxygen in a tantalum single crystal being readily detected. The oxygen concentration within about 400 Å of the surface was determined for a number of samples by comparison with a set of standards. The standards were sputtered tantalum films with oxygen concentrations between 8·3 and 71·4 at%, as determined from anodization efficiencies. For the single crystal the oxygen concentration was approximately 8 at%, but ion-scattering spectrometry showed that the oxygen concentration decreases with increasing distance from the surface. The energy of the O Kα radiation was 6·0 ± 1·4 eV higher than for Ta2O5, and the oxygen may be present interstitially in the tantalum crystal. Similar results were obtained for a sputtered tantalum film with the BCC structure. For a sputtered β-Ta film the oxygen concentration was approximately 10 at% and did not decrease with increasing depth. The oxygen is probably incorporated during deposition and the O Kα energy was the same as for Ta2O5. Tantalum foil annealed in oxygen at temperatures up to 600 °C increased in oxygen content according to an activation energy of 0·47 ± 0·08 eV. At 350 °C, the surface concentration was 30 at%, but no second phase was observed. Segregation of a second phase, with an oxygen concentration of 58 at%, was observed at 400 °C, and Ta2O5 was formed at higher temperatures.