Fabrication of Nb₂O₅/SiO₂ mixed oxides by reactive magnetron co-sputtering

This paper investigates niobia/silica mixed oxide thin films deposited by reactive pulse-DC/RF magnetron co-sputtering of Nb and Si metal targets at room temperature. The reactive gas flow during the sputtering processes was either controlled by direct mass flow rate (i.e. constant reactive gas flow) or by an active feedback process control system. 61% and 137% higher deposition rates of Nb₂O₅ and SiO₂ layers, respectively, were obtained using the latter technique as compared to constant reactive gas flow. Films exhibited bulk or near-bulk density. All mixture films produced in this study had an amorphous structure. A volume law of mixtures was used to determine the coating composition. It is shown that the fraction of SiO₂ or/and Nb₂O₅ has a linear dependency on sputter target power density. On this basis, rugate filter coating designs can be easily deposited, where refractive index gradually varies between that of pure Nb₂O₅ and pure SiO₂. Substantially less inhomogeneity of coating mixtures was found in films produced using a reactive sputtering process with feedback-control.