Reactive Gas Magnetron Sputtering of Lithium Hydride and Lithium Fluoride Thin Films

We report the preparation and structural characterization of lithium hydride and lithium fluoride thin films. These materials, due to their low absorption in the soft x-ray range, may have a role as spacer layers in multilayer mirrors. Theoretical reflection calculations suggest that an epitaxial crystalline multilayer stack of a nitride and a lithium compound spacer layer could produce respectable reflectance for short soft x-ray wavelengths (λ < 10 nm). Lithium targets were magnetron sputtered in the presence of hydrogen or ammonia to prepare the LiH films and nitrogen trifluoride to prepare the LiF films. The films were deposited on room temperature Si (100) or MgO (100) substrates. A near IR-Visible-UV spectrometer indicated a drop in reflectance at ~250 nm for a 100-nm-thick LiH film. This corresponds to a 5-eV band gap (characteristic of LiH). UV fluorescence indicated characteristic LiH defect bands at 2.5, 3.5, and 4.4 eV. The UV fluorescence characterization also indicated a possible lithium oxide (Li₂O) contamination peak at 3.1 eV in some of our thin films. Film surface morphology, examined by scanning electron microscopy, appeared extremely rough. The roughness size varied with reactive gas pressure and the type of substrate surface. A LiH/MoN multilayer was constructed, but no significant d spacing peak was seen in a low angle CuK_α XRD scan. It is believed that the roughness of the LiH film prevented smooth, uniform planar growth of the multilayer stack. Possible reasons of rough growth are briefly discussed.