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

• 61% and 137% increase in deposition rates of Nb2O5 and SiO2
• Rugate coating designs can be readily deposited.
• Nb2O5/SiO2 mixture films exhibited bulk or near-bulk density.
• Optimized process leads to stoichiometric and homogenous mixtures.
• Films are amorphous and suitable for low loss optical coatings.

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.