The effects of ion-assisted deposition on the mechanical, physical, chemical and optical properties of magnesium fluoride thin films.

Abstract

This dissertation investigates the results of ion assisted deposition (IAD) on various properties of magnesium fluoride thin films deposited on room temperature substrates. MgF₂ films deposited in this manner have increased abrasion resistance and increased adhesion comparable to that found in films deposited at the usual substrate temperature of approximately 300°C. IAD tends to drive the normal high tensile stress of non-IAD films to a more compressive state thereby reducing the overall stress. The IAD MgF₂ films have a higher index of refraction than non-IAD films, as high as 1.41, and the ultraviolet absorption edge in shifted to longer wavelengths beginning about 350 nm but no detectable absorption at visible wavelengths is seen in the films deposited with less than 250 eV bombardment energies. However, at higher IAD energies beginning at approximately 600 eV an absorption band is present in the red end of the visible spectrum making low energy bombardment the parameter of choice. Transmission electron microscopy and X-ray diffraction studies show that the IAD films have a more amorphous-like structure with fewer and smaller crystallites than non-IAD films deposited on either heated or unheated substrates. Rutherford backscattering spectroscopy (RBS) shows the bombarded films have fluorine depletion that roughly scales with the energy of bombardment with F:Mg ratios as low as 1.69 being found. Bombardment by fluorinated compounds, specifically C₂F₆ and SF₆, limit this depletion and in some instances super fluorinate the resulting compound. Additionally, RBS shows that IAD introduces a significant amount of oxygen throughout the film that is unaccountable as water take-up. X-ray photoelectron spectroscopy (XPS) indicates the presence of two compounds of oxygen that are attributed to MgO and Mg(OH)₂ or some oxy-fluoride complex similar to them and it is the introduction of these compounds which provide for the changes in the properties of IAD MgF₂ as compared to non-IAD films of MgF₂.