Implanted He retention and release from boronized layers

Abstract

He has been implanted at an energy of 3 keV into amorphous hydrogenated boron-carbon (a-BC) films deposited by rf sputtering onto single crystal Si substrates. The initial composition of the films was analyzed by nuclear-enhanced backscattering spectrometry to be B2C with ∼20% H and ∼10% O. The areal density of the implanted and retained 3He was measured in situ by a new ion beam analysis technique using the 3He(3He, pp) three-body nuclear reaction. The He trapping or pumping efficiency at room temperature is only 3.4% for low fluence implants and the a-BC layer saturates with He at a fluence of 5×1017 He/cm2. At this saturation fluence, only 3.1×1015 He/cm2 is retained in the film. Isochronal annealing of the implanted samples reveals a distributed release of implanted He at ∼200°C, which corresponds to a trap activation energy of 1.65±0.25 eV. 3He was trapped less efficiently at 250°C than at room temperature and exhibited a saturated retention of 8.6×1014 He/cm2. These results indicate that wall pumping should play only a minor role in the interpretation of the Textor He-pump experiment carried out earlier this year. The results also show that the unintentional deposition of a-BC, onto He pumping plates could adversely affect the operation of such devices, and should therefore be avoided.