The Fabrication of a UV Notch Filter by Using Solid State Diffusion

1. Introduction

Optical coatings with a combination of high and low refractive indices have attracted a significant amount of attention for their applications in distributed Bragg reflectors, notch filters, polarizers, antireflection coatings, and band-pass filters, among others. These optical coatings have excellent performance for optical system design if the refractive indices of each layer are appropriate. To make a layer with the correct refractive index, manufacturers have used co-evaporation [1], co-sputtering [2], adjusting the reactive gas [3], or depositing at an oblique angle [4]. In this study, the Kirkendall effect was applied to obtain a layer with an appropriate refractive.

The Kirkendall effect is a solid-state diffusion phenomenon in metallurgy. In 1942, the first experiment studying this phenomenon was performed by Kirkendall. This effect refers to a non-reciprocal mutual solid-state diffusion process by an interface between two metals, so that vacancy diffusion occurs to compensate for the inequality of the material flow. In a previous study, the modification of the Al₂O₃/ZnO refractive index has been investigated [5,6].

Notch filter describes a structure exhibiting a regular cyclic variation of refractive index resembling a sine or cosine wave. Such structures have the property of reflecting a narrow spectral region and transmitting all others. However, the manufacturing of notch filters is a difficult task. No material can be produced to a varying continuously refractive index; it is usually a trapezoidal discontinuous change. Conversely, an inhomogeneous layer can be deposited that exhibits a variation of refractive index by the co-evaporation method [7,8].

In this study, we found that the Kirkendall effect also occurs at the interface between Al₂O₃ and MgO. Mg atoms diffused into the Al₂O₃ layer and combined with the Al₂O₃ molecules to form the spine of the MgAl₂O₄ structures under specific annealing processes [9]. The effective refractive index of the MgO was decreased by the Kirkendall voids generated in the MgO layer. Figure 1 schematically shows the Kirkendall effect at the interface between MgO and Al₂O₃ [10]. The refractive index of MgO, with generated Kirkendall voids, was 1.55, while that of MgAl₂O₃ was 1.69. The differences in the indices between the layers were small. Owing to the inhomogeneity at the interface of MgO with void/MgAl₂O₄ and the small difference of the refractive indices of MgO with void and MgAl₂O₄ in comparison with that of MgO and Al₂O₃, it was possible to construct a notch filter based on Al₂O₃/MgO/Al₂O₃ multilayer deposition processes. The multilayer structure, (Al₂O₃/MgO)⁸ Al₂O₃, then behaved as a sinusoidal-like gradient layer (MgAl₂O₄/MgO with void/MgAl₂O₄)⁸ after post-annealing, and demonstrated its notch filter application potential [11,12].

2. Experimental

To form a spinel structure for the proposed design, we used Al₂O₃ and MgO. Al₂O₃/MgO/Al₂O₃ deposited on the quartz substrate using e-gun evaporation with ion-assisted deposition at 300 °C, gun power at 8 KV, and Ar flow rate at 13 sccm. The thicknesses of the layers measured by ellipsometry were 41, 38, and 40 nm respectively. A post-annealing process was implemented without additional gas injection, at 800 °C for 4 h at 1 atm. Inter-diffusions occurred during the post-annealing process through solid state diffusion. Therefore, the refractive indices of the layers changed by the Kirkendall effect. We also performed the deposition and post-annealing process of (Al₂O₃/MgO)⁸ Al₂O₃ multilayer as a reflection filter. The optical properties of the multilayer films were measured by a PerkinElmer Lambda 900 spectrometer (Waltham, MA, USA) prepared with an integrating sphere. The cross-sectional microstructures were inspected using a transmission electron microscopy (TEM) and scanning electron microscope (SEM). The composition depth profile of the three-layer structure was inspected by energy dispersive spectroscopy (EDS) line scan analysis and ellipsometry measurement [13]. After the annealing process, the effective refractive indices of the composite films were calculated using the Bruggeman effective medium approximation.

3. Results and Discussions

In Figure 2a, the cross-sectional SEM image of MgO/Al₂O₃ after annealing at 800 °C shows that, after the annealing process, the voids appeared at the interface of MgO/Al₂O₃. It is inferred that complete inter-diffusion reactions between the Al₂O₃ and MgO layers were achieved. The dimension of voids was about 10–20 nm, according to the TEM image in Figure 2b. After the annealing process, the total thickness of the multi-film structure did not change significantly. Figure 3 shows the XRD of Al₂O₃, MgO, and AMA thin films after a 4-h annealing process at 800 °C. The green line show the MgO single layer and the blue line shows the Al₂O₃ single layer after a 4-h annealing process at 800 °C. Compared with the red line and black line, the AMA thin-film as deposited (black line) shows non-crystalline MgAl₂O₄, and the AMA thin-film processes further show the crystalline structure of MgAl₂O₄, as labeled by the diamond mark.

Figure 4 shows the inhomogeneous index profile of MgAl₂O₄/MgO with void/MgAl₂O₄ when AMA was annealed at 800 °C for 4 h, as simulated by the ellipsometry measurement. It started with MgAl₂O₄ moving onto the middle layer MgO with voids, and ended with MgAl₂O₄. The middle layer was set by MgO, and voids were set with a Gaussian distribution. The middle layer (MgO with voids) exhibited an average porosity of 19.73% by the ellipsometry measurement [14]. The composition depth profile analysis of MgAl₂O₄/MgO with voids/MgAl₂O₄ by TEM EDS line scan analysis was illustrated by the inter-diffusion mechanism between Al₂O₃ and MgO, as shown in Figure 5. The atomic ratio of Al and Mg were found to be constant in both the top and bottom layers. Additionally, we deposited the (Al₂O₃/MgO)⁸ Al₂O₃ multilayer stack and performed the post-annealing process at 800 °C for 4 h. After the post-annealing process, the multilayer stack changed from (Al₂O₃/MgO)⁸ Al₂O₃ to (MgAl₂O₄/MgO with voids/MgAl₂O₄)⁸. Figure 6 show the transmittance spectrum as deposited (black line) and after annealing (red line). There was no reflection band in the transmittance spectrum before the post-annealing process. The blue spectrum line in Figure 6 is the transmittance of (MgAl₂O₄/MgO with voids/MgAl₂O₄)⁸ simulated with a 0.14 refractive index difference between MgAl₂O₄ and MgO (with voids) at a wavelength of 300 nm. The transmittance spectrum of the multilayer stack after annealing (red line) was close to the simulated result (blue line). This shows that thin film fabrications of (Al₂O₃/MgO)⁸ Al₂O₃ multilayers can be applied as notch filters through solid state diffusion. Solid state diffusion can make the diffusion index difference more gradual in a multilayer structure, which is advantageous in fabricating rugate filters [15,16].

4. Conclusions

In this study, MgAl₂O₄ and porous MgO layer were successfully fabricated as an AMA structure. The average porosity of MgO with voids was 19.73%, and the refractive index difference between the high (MgAl₂O₄) and low (MgO with voids) index materials was increased to 0.14 by an 800 °C post-annealing process due to solid-state diffusion. This allowed simply using a structured multilayer, (Al2O3/MgO)⁸ Al₂O₃, and post-annealing to form a sinusoidal-like gradient layer for a UV notch filter. The possibility of solid-state diffusion in multilayer films was demonstrated and a notch filter with a (MgAl₂O₄/MgO with voids/MgAl₂O₄)⁸ multilayer structure was successfully fabricated.