Gregory B. Thompson and Dr. David D. Allred, Physics and Astronomy
This research project reports the reactive gas sputtering deposition and structural characterization of lithium compound thin films (LiH and LiF). These materials, due to their low absorption, may have a role as spacer layers in soft x-ray multilayer reflectors. A multilayer is a periodic, alternating structure of a high density and low density (spacer layer) material stacked one on top of another. These materials reflect x-rays by the superposition of the multiple reflections that occur within the multilayer stack. These reflections can be modeled by Bragg’s condition, given as, n=2dsin, where n is an integer, is the wavelength, d is the bi-layer thickness of the multilayer stack, and is the incident angle.
Theoretical reflection calculations suggests that a crystalline multilayer stack of a nitride and a lithium compound spacer layer could produce respectable reflectance for short soft x-ray wavelengths (<10nm). These types of crystalline frame-works would have an alternating high and low density electron structure with atomically abrupt interfaces, i.e. sharp contrast between thin film layers. If the material is grown epitaxially, layer by layer, a bi-layer thickness on the order of the crystalline lattice parameters could be achieved.
The lithium compound thin films were deposited by magnetron sputtering. This method involves the bombardment of a metallic target (lithium) by positive ions extracted from a plasma source. The positive ions “collide and knock out” the target atoms by an acceleration to the target surface from a negative potential. A reactive gas was flowed into the sputtering chamber to allow the lithium atoms to bound and form a compound. The compound form would allow the lithium to be in a stable state in atmospheric conditions. Pure hydrogen (H ) and ammonia (NH ,) were used 2 3 as the hydride source. Nitrogen trifluol-ide (NF 3)was used as the fluoride source. The lithium compounds were deposited on room temperature Si and MgO substrates.
Unfortunately, the lithium compound thin films grown in this project showed extremely rough surface morphology. This may have been a result of sputtering conditions (small mean free path), oxide contamination within the sputtering chamber, and/or substrate hinderance. This type of rough growth is not suitable for the developing of uniform planar contrast needed within a multilayer stack. The thin films were chemically and structurally characterized by UV fluorescence, SEM, TEM, Electron Backscatter Diffraction, and X-ray Diffraction.
