Refractive index determination of SiO₂ layer in the UV/Vis/NIR range: Spectrophotometric reverse engineering on single and bi-layer designs

In this paper we use spectrophotometric measurements and a Clustering Global Optimization procedure to determine the complex refractive index of SiO₂ layer from 250 nm to 1250 nm. A special commercial optical module allows the reflection and transmission measurements to be made under exactly the same illumination and measurement conditions. We compare the index determination results obtained from two different single layer SiO₂ samples, with high and low index glass substrates, respectively. We then determine the refractive index of SiO₂ for a bi-layer design in which the first deposited layer is Ta₂O₅. The corresponding solutions are discussed and we show that the real part of the complex refractive index obtained for a bi-layer is slightly different to that found for a single layer investigation. When SiO₂ is included inside a thin film stack, we propose the use of an index determination method in which a bi-layer is used for the real part of the complex refractive index, and single layer determination is used for the imaginary part of the refractive index in the UV range.