Helium-charged titanium films deposited by direct current magnetron sputtering

Helium charging (or trapping) in the Ti films deposited by direct current (DC)-magnetron sputtering with a He/Ar mixture was studied. Helium atoms with a surprisingly high concentration (He/Ti atomic ratio is as high as 56 at.%) incorporate evenly in deposited film. Apart from the relative helium content of sputter gas, the trapped amount of helium also depended on the substrate temperature, total sputter pressure, sputtering rate, discharge voltage, and applied bias. However, at a lower bias voltage, the trapped helium appears to trend to saturate. The introduction of the helium with no extra irradiation damage is expected by choosing suitable deposition conditions. It was also found that because of the formation of nanocrystalline Ti film, a relative high helium flux for bubble formation is needed and the amount of the retained helium in sputtering Ti films is much higher than that in the coarse-grain Ti films. The nanocrystalline Ti film can accommodate larger concentration of trapped sites to helium, which results in a high density and small size of the helium bubbles. With increasing helium irradiation flux, the grain size of Ti film decreased and the lattice spacing and width of the X-ray diffraction (XRD) peaks increased due to helium effects, corresponding to the increase of defects and disorder in the films.