Sputtering yield of optical materials: Sigmund’s model and experimental results

The main drawback of the ion beam assistance during optical thin film deposition is the stoichiometry alteration due to the different sputtering yield of elements which compose the film itself. In optical materials such effect causes both an increase of absorption coefficient and undesirable variation of refractive index. In order to maintain the advantages of the ion beam assistance (i.e., enhancement of adhesion, compactness, good control of optical properties) the sputtering phenomena should be modeled to recognize the parameters of the ion beam (energy, angle of incidence, and ion mass) that minimize the stoichiometry alteration. The Sigmund’s model is an analytical approach widely used to treat the modelling of the sputtering phenomenon. In this work the Sigmund’s model was applied to fit the experimentally measured sputtering yield of several materials typically employed to fabricate optical coatings: silicon dioxide (SiO₂), yttrium oxide (Y₂O₃) and zinc selenide (ZnSe). The yield measurements were performed both at different angle of incidence of the beam (0°, 45°) and with different bombarding ion mass and energy. The experimental results appear well fitted by the relation of the sputtering yield (Sigmund’s model) at all considered angles and ion masses. Sputtering yields of the material components as for instance silicon (Y^Si_ion) and oxygen (Y^O_ion) for SiO₂ material, were modeled as well as their behavior with either ion energy or ion mass.