## 3 Results and Discussion
SiO\({}_{2}\)/Nb\({}_{2}\)O\({}_{5}\) multilayer thin films were designed with Essential Macleod using the refractive index (\(n\)) of the single-layer SiO\({}_{2}\) and Nb\({}_{2}\)O\({}_{5}\) thin films. Figure 2: (**a**) XPS wide spectrum, (**b**) XPS narrow-scan spectrum of O1s, (**c**) XPS narrow-scan spectrum of Si2p, and (**d**) XPS narrow-scan spectrum of Nb3d of the SiO\({}_{2}\)/Nb\({}_{2}\)O\({}_{5}\) multilayer thin film. films measured on a spectroscopic ellipsometer. The refractive index of the SiO\({}_{2}\) thin film was similar to the refractive index of quartz (bulk SiO\({}_{2}\)). [PERSON] et al. reported a refractive index of 1.47 at 540 nm for a SiO\({}_{2}\) optical thin film deposited by the DC sputtering method [21]. The refractive index values of Nb\({}_{2}\)O\({}_{5}\) thin films deposited by sputtering methods were previously reported according to processing conditions and film thickness to be in the range of 2.14 to 2.44 at 530 nm [10; 22]. It is well known that the refractive index of optical thin films by the sputtering methods is higher than that of optical thin films by electron beam deposition methods. This indicates that the thin films deposited by sputtering methods are denser than the films prepared by electron beam deposition methods [23; 24]. In this work, single-layer SiO\({}_{2}\) and Nb\({}_{2}\)O\({}_{5}\) thin films deposited by the plasma-assisted reactive magnetron sputtering method indirectly show that a deposition process to obtain dense and durable thin films has been secured. Figure 4a shows cross-sectional and surface SEM images of the SiO\({}_{2}\)/Nb\({}_{2}\)O\({}_{5}\) multilayer thin films deposited by plasma-assisted reactive magnetron sputtering method on D263 substrates. The secondary electron (SE) image clearly shows 60 individual layers of alternating SiO\({}_{2}\) and Nb\({}_{2}\)O\({}_{5}\), since Nb has a higher atomic number than that of Si. The measured thickness of dark SiO\({}_{2}\) layers with a low index and bright Nb\({}_{2}\)O\({}_{5}\) layers with a high index is almost matching with the thickness designed with Essential Macleod. There was little difference in the total thickness between the designed and fabricated SiO\({}_{2}\)/Nb\({}_{2}\)O\({}_{5}\) multilayer thin films. The total thickness of the designed and fabricated thin films was 6.03 \(\upmu\)m and 6.10 \(\upmu\)m, respectively. Interfacial diffusion among thin films was not observed between each SiO\({}_{2}\) and Nb\({}_{2}\)O\({}_{5}\) layer. Hence, by constructing SiO\({}_{2}\)/Nb\({}_{2}\)O\({}_{5}\) multilayer thin films properly, one can reduce the cross-plane scattering without deterioration in band pass filtering. The surface with a grain boundary of 50 to 200 nm in size was observed on the top surface as shown in Figure 4b. Overall, a very smooth and flat surface was obtained. A cross-sectional image of 12 layers of the SiO\({}_{2}\)/Nb\({}_{2}\)O\({}_{5}\) thin films is displayed, where white indicates energy dispersive X-ray spectroscopy (EDS) line scanning, as shown in Figure 5a. EDS was used to indicate major elements composed of multilayer thin films. The EDS results showed Nb and Si elements and a small amount of other elements from the substrate. In order to represent the distribution of Nb and Si elements layer by layer, a small percentage of elements were ignored. The distribution results from the EDS line scan were displayed as shown in Figure 5b for the SiO\({}_{2}\)/Nb\({}_{2}\)O\({}_{5}\) multilayer thin films along the white solid line. The complementary EDS intensity of SiO\({}_{2}\) and Nb\({}_{2}\)O\({}_{5}\) is attributed to Si \(K\alpha 1\) and Nb \(L\alpha 1\). The line profiles show alternating sawtooth shapes. The relative contrast changes are reliable and clear periodic oscillations are observed from complementary elemental line profiles.