Fabricating 3D Nanostructures For Optical Application

Three-diemnsional(3D)nanostructures have drawn tremendous interest in the past decade due to their higher integration level and excellent mechanical and physical properties.Various devices based on 3D nanostructures can achieve lots of remarkable functions that beyond the ability of planar nanodevices,thus claim their applications in integrated circuit,nanoelectromechanical systems,bionics and nanooptics.Several work related to the reliable definition of 3D nanofabrication have been demonstrated.However,there are still some challenges in fabricating 3D nanostructures with high resolution,high aspect-ratio,material uniformity and arbitrary shape.Aiming at the fabrication of 3D nano-optical structrures,our work was focused on exploring novel 3D fabrication process by utilizing the high resolution and good flexibility of electron-beam lithography,and several optical applications of the obtained nanostructures was also demonstrated.The main research results are summarized as follows:(1)We developed a high-fidelity pattern transfer technique to reliably fabricate3D Si nanostructures with high resolution and high aspect-ratio,and the excellent antireflective properties of the special Si nanotubes were also demonstrated.Specifically,the high-resolution HSQ patterns were firstly defined by electron-beam lithography,and then dry etching of the bottom Si substrate was performed using the HSQ patterns as the hard mask.Systematic experiments have been done to investigate the influence of substrate power and gas flow rate ratio,two main etching parameters,on the etching results,and the mechanism of the mixed-mode cryogenic etching process was revealed.Various optimized 3D Si nanostructures with smooth and vertical sub-10 nm walls were realized based on complex patterns generated by electron-beam lithography.Further optical measurements show that the densely packed Si nanotubes offer lower reflectivity than commonly-used antireflection Si nanopillars with the same diameter and height,implying the potential application in photovoltaics.(2)High-quality Si nanocylindroids were fabricated by the developed method mentioned,and the corresponding visible-range polarization-dependendent scattering properties were systematically investigated at the single particle resolution.The experimental results show that the scattering properties of Si nanocylindroids could be effectively tuned by changing the size of structures and the polarization of incident light.Further numerical simulations clarified the magnetic dipole and electric dipole resonance modes in the spectra and revealed that the two dominant resonances spectrally overlapped in current prepared structures.The unique anisotropic optical property of Si nanocylindroids provides a promising candidate for on-chip control and manipulation of visible light with extra freedom for various applications such as bio-imaging,sensing,high-efficiency metasurfaces and integrated optical devices.(3)We directly fabricate the 3D all-dielectric nanostructures with high precision by a developed 3D electron-beam nanofabrication process based on HSQ/PMMA bilayer resist using the patterned PMMA sacr ificial layer.This method extends the application of electron-beam fabrication.A series of nanoholes were firstly defined by electron-beam lithography,followed by planarization via spin-coating HSQ resist.3D SiO_x structures were then obtained by an overlay exposure and the subsequent release of the sacrificial PMMA layer.All-dielectric 3D nanostructures with excellent mechanical properties and good stability were obtained,and the structures possess high resolution and low edge roughness.In addition,the eliminated capillary forces during the dry-release step enable the reliable fabrication of freestanding structures with high aspect ratio.(4)A kind of reflective color filters based on asymmetric Fabry-Perot cavities was designed based on 3D SiO_x nanostructures and their capability for colorimetric refractive index sensing was also demonstrated.The color filters could selectively reflect the incident light,resulting in specific structural color.By tuning the environment refractive index,the effect ive refractive index of the dielectric layer in the Fabry-Perot cavities changed due to the freestanding property of structures,resulting in the continuous tailoring of reflectance peak in the whole visible range.Such color filters can be used as a simple and intuitive colorimetic sensor.This 3D SiO_x nanostructures based colorimetric sensor has the potential as a visual platform in various fields including biochemical detection,anti-counterfeiting and smart active nano-optical devices.