Substrate temperature effect on the refractive index and a two-step film method to detect small inhomogeneities in optical films.

Nb2O5 films were deposited by a reactive magnetron sputtering technique. The average refractive index was found to increase with the rise of substrate temperature. Modulated interference transmittance spectra were observed in the two-step films, which were prepared by stopping the deposition process in the middle of the designed sputtering time, and then, after a full cooling down to room temperature, starting the same deposition process again to complete the whole preparation of the films. A linearly graded-index model was used to explain the interference behavior. It was proved that the two-step film method was sensitive to the small inhomogeneity in the films. We also suggest that the inhomogeneity of sputtered films can be minimized by controlling the substrate temperature at a constant value.