Preparation of SiO 2 :TiO 2 for High-Performance Double Layer Anti-Reflection Coating

Abstract


Introduction
Optical coatings are thin film layers deposited between two media in order to alter the behavior of light striking the interface of the structure.It may consist of a series of surfaces that are the boundaries between different materials.Coatings are applied to optical components intended for use at wavelength regions between UV and far-IR.The most common and important optical coatings are Antireflection Coatings (ARC), which still surpass all other filters and coatings [1].The basic principle of such coating is reflecting specific wavelengths and largely canceling others by destructive interference and internal reflection which is related to the critical angle of the surface [2].Antireflection coatings can range from a single layer with nearly zero reflection at just one wavelength to a multilayer system of many layers having virtually zero reflection over a broad spectral range.The performance of an optical coating depends on the number of layers, total thickness, and refractive index [3][4][5].Anti-reflection coatings are often used for applications in medicine and various new applications in lenses, eyeglasses, lasers, and mirrors.Mixed materials composed of a host oxide and a small percentage of a similar oxide compound have been introduced to create a material with a desired refractive index.In addition, mixed materials promote the growth of dense film layer structures because the incorporation of the additive discourages the growth of multiple crystalline phases [6,7].This new mixed material can be predicted by following the Lorentz-Lorenz theory [8,9].
This work introduces a novel wide-band anti-refraction coating (ARC) composed of a double layer that combines a thin layer of pure SiO2 and a mixed dielectric thin layer.The mixed dielectric layer is composed of TiO2 (n = 2.6) and SiO2 (n = 1.46) such that (SiO2(x): TiO2(1-x)) where X = 0.45.This ARC was deposited onto a glass substrate at a predetermined concentration ratio.The mixed layer was synthesized by the chemical spraying method.The SiO2-TiO2 mixed coatings' structure has previously been investigated [10].
Optimum results (low reflectivity) were obtained for the double-layer antireflection coating in the spectral region (400-1000) nm.A comparison was made between the experimental results and those of the theoretical work that relied on the calculations of the characteristic matrix theory.The adopted technique depends on preparing a coating consisting of a mixture of materials (SiO2 and TiO2) and calculating its molar refractivity according to the Lorentz -Lorenz formula.This technique helps overcome manufacturing problems and limitations in the availability of materials.This kind of ARC is used in many scientific applications and contemporary industrial settings.

Theoretical background
The optical performance of antireflection coatings depends on calculating the characteristics matrix to determine the spectral of reflectance (R) profile for a double layer structure on a substrate.The characteristics matrix of a composition of q thin film layers is the product of the individual matrices of the individual layers of the assembly taken in the correct order, as is given below [2]: where: ] is the characteristic matrix of the assembly, C and B are normalized total tangential electric and magnetic fields, respectively, at the input surface, ᵣ is the refractive index, q is the number of layers next to the substrate, and ᵣ is the phase thickness specified by the equation: where: dᵣ is the physical thickness of a layer, and λ is the design wavelength.
The reflectance (R) of the multilayer system is given as follows: where: n ̥ is the refractive index of air (or incident medium).

Experimental work
In this study, dielectric materials with a large difference in their refractive index were employed for the mixed layer, specifically, (SiO2(x): TiO2(1-x)) where X= 0.45.
A chemical sprayer (homemade) was used to deposit both the composite and pure layers on glass substrates at room temperature.Using the chemical approach, 10 ml of hydrochloric acid (40%) was used as the solvent to make the solution.A clear solution was obtained by combining SiO2 solution (at a molar ratio of 0.5) with TiO2 solution (at a molar ratio of 0.1) in a beaker and continuously swirling the mixture with a magnetic stirrer for 15-30 minutes.An ultrasonic cleaner and a mix of ethanol and deionized water were used to remove contaminants from the glass substrates' surfaces.
Different samples were prepared by spraying different numbers of SiO2 sprays (2, 4, 6) on 12 sprays of the mixed dielectric layer, onto glass substrates.The time between sprays was 15 seconds, and the sprayer nozzle was 27 centimeters above the substrate.
To calculate the refractive index of the mixture, the Lorentz -Lorenz formula was used [8] n = [ where: ,  1 and  2 are the refractive indices of the two materials, and ρ 1 and ρ 2 are their densities ( ρ SiO 2 = 2.65gcm −3 , ρ TiO2 = 4.23gcm −3 ) .The new refractive index was found to be equal to 1.8.

Optical properties
The optical performance (reflectance spectra) of a double layer was determined using UV-Vis spectroscopy, as shown in Fig. 1.This figure demonstrates that the 6:12 double layer showed the lowest reflectance.The optical performance of the ARC is less than about 2 % for the 6:12 double layer along the Visible -NIR region.This result showed the optimum performance of ARC with only a two-layer deposition.Increasing the number of sprays led to an increase in thickness, and the last led to a low reflection.This behavior is attributed to the destructive interference due to the phase difference at wavelengths (400-1100nm), as shown in Fig. 1.

Figure 1: Reflectance as a function of wavelength for the double layer (SiO2: Mix) samples.
The comparison between the experimental results and theoretical results is presented in Fig. 2. The theoretical results were computed using Eq. ( 4) for the double layer of SiO2(n=1.46)and the mixed layer of (SiO2(x): TiO2(1-x)) where (x=0.45)(nH =1.8), using the MATLAB software.The experimental result chosen for the comparison was for the 6:12 (6 SiO2 sprays: 12 sprays of the mix) because this sample gave the optimal optical performance.Fig. 2 shows the possibility of fabricating this antireflection coating (one with two layers only) in the Visible -NIR region.

X-Ray diffraction (XRD)
An XRD diffractometer (XRD-6000, Shimadzu) was used in a step scan mode with a measuring range of (10-80).The JCPDS card was used to measure the scan range XRD at 40 kV and 30 mA.
The Scherrer equation was used to calculate the crystal size.[11]: where: D: is particle size (cubic meter), K: shape factor of the particle, typically ranging from 0.9 to 1.2, λ: wavelength of the radiation used (meter), : full width at half maximum (FWHM) of the diffraction peak (radians), θ: diffraction angle between the incident radiation and the sample (radians) The peak width of a strong diffraction plane was used to calculate the values of D, which were within the nanostructure range.The average crystalline size of TiO2, SiO2, and TiO2:SiO2 are listed in Table 1.

Atomic force microscopic analysis (AFM)
The AFM three-dimensional images in Fig. 4 show the surface morphology of (SiO2: mix) thin layers deposited by the chemical deposition method on glass substrates for a different number of SiO2 layers (2,4,6).AFM parameters (averaged diameter, root mean square, and average roughness) for these samples are listed in Table 2. From the table, it can be noted that increasing the number of sprays from 2 to 6 caused a decrease in the average diameter at the surface from 26.45 to 6.33 nm and a decrease in the average roughness from 3.70 to 1.2 nm with the increase of the SiO2 ratio.

Conclusions
A wide-band antireflection coating (ARC) of optimum optical performance was fabricated with only two thin layers.The achieved structure coating is characterized by low reflectivity over a wide range of the electromagnetic spectrum, which includes (VIS-NIR) regions.These coatings are used in scientific applications and advanced industries.Results showed that the high reflectance from the substrate surface decreased by adopting the coating mixture (SiO2 and TiO2) calculated according to the Lorentz -Lorenz formula.This technique helps overcome manufacturing problems and limitations in the availability of materials.This gives higher flexibility in manufacturing specialty optical filters in the future.

Figure 2 :
Figure 2: The comparison of the optical performance between theoretical results and experimental results for ARC.

Figure 4 :
Figure 4: 3D AFM images and their granularity accumulation distribution for double layer pure and mix (TiO2:SiO2) composite thin films at different number of sprays.