Evolution behavior of helium bubbles and thermal desorption study in helium-charged tungsten film

In order to investigate the evolution behavior of the formation and growth of He bubble, helium-charged W films have been prepared by radio frequency (RF) magnetron sputtering in a mixed atmosphere of He and Ar as well as the annealing were performed at different temperatures at 500 °C, 700 °C and 1000 °C for 2 h, respectively. Microstructure of the films before/after the annealing were characterized by using X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). There are a number of He bubbles in the films both before and after the annealing. The bubble size increases with increasing the annealing temperature but the bubble sizes in all samples are less than 3 nm. Nanohardness demonstrates a decrease tendency with increasing the annealing temperature. There are four He desorption peaks in thermal desorption spectroscopy (TDS) spectra within the range of 1000 °C for the as-deposited film, while they disappear completely or shift toward high temperature for the samples annealed at 700 °C and 1000 °C. The prominent TDS peaks can be attributed to the release of the interstitial He atoms from the trapping sites in the distorted lattice in the helium-charged W films.