The Design,Fabrication And Application Of 3D Optical Functional Resist Micro/Nanostructures

The three-dimensional （3D） functional micro-nano structures can achieve numerous excellent optical function that beyond the ability of planar micro-nano structures,thus various devices based on 3D optical functional micro-nano structures significantly promote the development and engineering applications of high-performance equipment related to aerospace,augmented reality,stealth materials,etc.In order to obtain stable 3D optical functional micro-nano structures for different practical applications,numerous 3D optical functional micro-nano structures fabricating methods are proposed.However,the existed fabrication approaches still have critical challenges in terms of material uniformity,structural sensitivity,pattern resolution,process complexity,and compatibility.In this thesis,aiming at the innovative design and the reliable fabrication of 3D functional micro-nano structures for optical application and benefiting from the good flexibility and high resolution of electron beam lithography,we developed a series of new approaches to directly obtain 3D optical functional micro-nano structures based on electron beam resist.We also demonstrated the related optical applications of the achieved micro-nano structures.The main research contents of this thesis are list as follows:（1）A synthetic platform for transmissive color filter array based on Fabry-Pérot（FP）interference effect was demonstrated.The color is determined by adjusting thickness of each 3D nanostructure in the z-axis direction and its filling density in the x-and y-axis directions.Meanwhile,the thickness variation of those FP nanocavities has been fully deployed as an alternative degree of freedom by grayscale lithography,yielding vivid colors with wide gamut and excellent saturation.Experimental results show that the color rendering capability of the pixelated nanocavities can be still retained as pixels are miniaturized to 500 nm.Crosstalk between closely-spaced pixels of a Bayer color filter arrangement was calculated,showing minimal crosstalk for 1μm² square sub-pixels.The results provide an approach to design and fabricate ultracompact color filter arrays for various potential applications,including stained-glass microprints,microspectrometers and high-resolution image sensing systems.（2）In order to address the problem of the contrast of the developer in the electron beam grayscale lithography caused nonlinear change of resist structure size in the z-axis direction,a template-confined thermal reflow approach is proposed to fabricate monolithic color filters based on silver/polymethyl methacrylate/silver（Ag/PMMA/Ag）3D configuration.Using hydrogen silsesquioxane（HSQ）resist frames as a template-confined thermal reflow of periodic PMMA nanogratings with different filling density,the thickness of PMMA dielectric layer in each pixel can be independently controlled,enabling the spectral tunability across the whole visible light region with high transmittance and narrow linewidth.The approach we proposed is able to control the thickness of dielectric film via a single exposure step and combined with nanoimprint lithography for high-throughput production in practical application such as image sensors and color displays.（3）Based on PMMA resist melting under high temperature conditions,we developed a directed self-assembly approach to fabricate metallic nanogaps through glass-transition-induced micro-reflow of Au/PMMA hybrid structures.Firstly,electron-beam lithography process is used to obtain initial PMMA 3D micro-nano structures.Then Au film was deposited onto the pre-patterned micro-nano structures.Finally,the tiny metal nanogaps were obtained by using the acetone solvent-assisted thermal reflow process.The experimental results show that the flowing orientation of polymer/metal hybrid nanostructures undergoing glass transition can be determined via several asymmetrical geometrical parameters including shapes and initial nanogaps.In addition,mechanical simulations based on thermo-mechanical coupled finite element method are used to qualitatively understand the mechanism of the deterministic thermal reflow phenomenon.It is demonstrated that this method is effective to fabricate sub-10-nm metallic nanogaps in gold nanostructures,which may have potential applications in plasmon-enhanced light-matter interactions,ultrafast nanotransistors,nanoelectronics and molecular electronics.（4）To solve the dilemma of morphology deformation based on 3D polymer structures under harsh conditions such as high temperature and solvent environment during the fabrication applications.We report a simple approach to improve the stability of 3D polymer micro-nano structures by coating a conformal ultrathin oxide film via atomic-layer deposition technology.By taking PMMA as a demonstration,we investigated the performance of PMMA devices above the glass transition temperature of PMMA.The results show that thermal reflow of PMMA polymer can be avoided under a high temperature even much higher than its glass transition temperature.Meanwhile,the 3D PMMA structures coated with a sub-10 nm titanium dioxide（TiO₂）film can effectively prevent dissolving PMMA structures in acetone,therefore having a good morphology under high power irradiation.Furthermore,the mechanism of the verified thermal stability is analyzed via mechanical simulations.Such an effective approach is supposed to significantly extend the application of polymer nanostructures as functional elements for optical structures or devices which require excellent thermal and chemical stability.