Periodic Nanostructure Stacking Technology for Grin Antireflection Layer

Antireflection technology has been widely researched and various methods have been raised to reduce the Fresnel reflection losses from the mismatch of the optical index on the interface of two different materials [1-2]. One of a solution is placing a medium layer with gradual-vary optical index to improve the material discontinuity on the boundary. Some previous studies show that by building a layer of tapered nanostructure [3] or a gradient-index (GRIN) layers [4], a more continuous boundary can be achieved to reduce the energy loss. In our previous work, multiple layers of periodic nanostructures have been successfully built in a single stacking, and the material properties of each layer can be control individually. Here, we demonstrate the results of this technology and a potential application as an antireflection medium.

In the proposed fabrication technology, the periodic nanostructure is first patterned by phase-shift lithography process in which a monolayer of hexagonal-close-packing nanospheres is employed as the near-field phase mask. A thin shell of ceramic layer is then deposit onto the nanostructures by the atomic layer deposition (ALD) method as both the protection of the underlying layer. The material and thickness of the ALD layer can be precisely controlled to achieve the desired optical properties. The further photoresist patterning and ALD processes are allowed to build multiple layers of nanostructures on the existing pattern, and an ALD thin-shell stacking can be made after removing the photoresist by high temperature. The fabrication process is illustrated in Figure 1(a). Figure 1(b) shows resulting thin-shell nanolattice stacking, the EDX image shows the material and geometry of each layer can be fabricated individually, in this case, the layers are made by zinc oxide, aluminum oxide, zinc oxide, titanium oxide, zinc oxide respectively.

In the initial stage, a multiple layers of nanolattice thin-shell stacking with gradient index built on a silicon substrate is used for the optical measurement. The ALD thickness conditions for each layer from bottom to top are 40 nm, 25 nm, 15 nm and 10 nm of titanium oxide, and the corresponding optical index are 1.69, 1.40, 1.21 and 1.18 respectively for red light (633 nm). This GRIN suppresses 60% to 90% of the specular reflectance under the incident condition between 4 to 70 degree. Figure 2(a) and (b) show the TE and TM specular reflectance for 633 nm laser. In both cases, the specular reflectance reduces as the layer number of nanolattices increases. The propose stacking material also demonstrates nice antireflection property over wide band spectrum. Figure 2(c) shows the specular reflectance for the light in the range of 400 nm to 2400 nm is between 1% to 6% at 15^o incident. More optical properties as the diffuse reflectance and the numerical model of the stacking structures will be further explored and presented. This novel technology shows a breakthrough of the hybrid of multiple functional nanostructures in a single stacking.