Hydrogen permeation through silicon nitride films

Amorphous silicon nitride films, 500 and 700 nm thick, were deposited on Eurofer substrates by applying reactive radio-frequency magnetron sputtering from pure Si targets in an argon/nitrogen atmosphere. The hydrogen permeation through such double-layered, 40 mm diameter membranes at 400 °C and 1 bar upstream pressure involved the use of a conventional technique with enhanced sensitivity. The extremely high barrier efficiency for these films with respect to hydrogen, expressed as a permeation-reduction factor in excess of 2000, was only achieved with films containing 6–7 at.% of hydrogen. The achieved permeation-reduction factor at 400 °C corresponds to the permeability of silicon nitride, which is as low as P = 1 × 10−17 mol H2/m s Pa0.5. The hydrogen concentration was determined with an Elastic Recoil Detection Analysis, which indicated that this high concentration represents only the strongly bound hydrogen that is not mobile at this low temperature, but impedes the mobility of the diffusive hydrogen. A silicon nitride film with a low hydrogen content is a far less efficient barrier, which supports the role of the strongly bound hydrogen.