Fabrication and optimization of three-dimensional metamaterials for terahertz energy-harvesting
Metamaterials typically consist of metallic and dielectric repeating structures. Electrodeposition of copper is the preferred approach to fabricating the metallic part of the metamaterials of interest in this study. The highly-variant topography requires chemical additives, like chloride ions, 3-mercapto-1-propanesulfonic acid (MPSA), polyethylene glycol (PEG) and polyvinylpyrrolidone (PVP) to enhance bottom-up superfilling while maintaining terrace flatness. This study focuses on both experimental and computational investigations of the degradation potential of the additives and their adsorption mechanism in a highly-acidic copper electrolyte in order to optimally parameterize the copper electrodeposition process. Results show Cl-MPSAPEG-PVP additives perform well, but substitution of PVP with Janus Green B provides better terrace leveling. Additionally, NMR data show a quick and complete conversion of MPSA to bis(3-sulfopropyl) disulfide (SPS) in the acidic copper bath. Finally, FEM simulations further show that the accelerator species may initially accumulate and be transported vertically until overplating, whereby they are transported laterally. Our study also show that the electromagnetic performance of metamaterial is dependent on SU-8 related parameters, including pillar height, bottom side length and spacing between pillars. The metamaterial geometry is successfully optimized and it can localize the surface plasmon with the peak frequency of 1.2 THz to the nano-antenna and the highest terahertz absorption is 30%.
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