室溫製程及軟性基板超穎介面元件之研究

Abstract

近年來，超穎介面製程技術已日漸成熟，除了平面的應用場合外，可撓式超穎介面尚可透過施加外部機械應力調控光學性質。現有的軟性超穎介面製作方式大多在剛硬基板上製作奈米柱陣列，接著再填覆有機材，使奈米柱陣列嵌入其中並進行轉置製作，最常見的有機材為聚二甲基矽氧烷(Polydimethylsiloxane, PDMS)，照光固化後，再將樣品剝離剛硬基板，即可取得軟性超穎介面。雖然此方式可有效製作，但製作流程較繁瑣且材料受限，因此本論文研究於室溫條件下，將高折射率的二氧化鈦(TiO2)超穎介面直接沉積製作於高透明、軟性的聚醯亞胺(Polyimide)基板，簡化製作流程。 本研究的製作流程雖與平面超穎介面大致相似，但聚醯亞胺基板不耐高溫，不適合進行高溫退火，以獲得較佳之二氧化鈦薄膜之折射率等光學性質，因此在濺鍍過程中，除了通氬氣之外，額外加入適量氧氣時則可以室溫製程獲得高折射率及透明度。濺鍍完成後以電子束微影定義超穎介面圖案，接著進行乾蝕刻，製作出側壁陡直的奈米柱陣列，最終以濕蝕刻移除金屬遮罩(Hard mask)。然而，由於水具有高表面張力，在乾燥過程中奈米柱(nanopillars)之間會產生毛細作用力且聚醯亞胺基板容易吸水與翹曲，造成奈米柱大範圍倒塌。因此，我們透過濕蝕刻後引入臨界點乾燥(Critical Point Drying, CPD)方式，並降低奈米柱深寬比，可有效改善奈米結構大範圍倒塌的情況。所製作之軟性基板超穎介面透過光學量測系統驗證超穎介面元件之光學特性與模擬結果吻合。

In recent years, metasurface fabrication technology and its application have been developed. Flexible metasurfaces, are also attracting attention with their optical properties tuned by applying external mechanical stress. Current fabrication of flexible metasurface mostly involve fabricating nanopillar arrays on rigid substrates, followed by filling them with organic materials to embed the nanopillar arrays. The most common organic material is polydimethylsiloxane (PDMS). After photocuring, the sample is then lifted off the rigid substrate, resulting in a flexible metasurface. While this method is effective, the fabrication process is more complicated. Therefore, this study aims to directly fabricates high-refractive-index titanium dioxide (TiO2) metasurfaces on highly transparent, flexible polyimide substrates at room temperature, simplifying the fabrication process. While the fabrication process in this study is similar to those planar metasurfaces, the polyimide substrate is not heat-resistant and unsuitable for high temperature annealing, hindering preparation of high refractive index and highly transparent titanium dioxide film on PI substrate. Thus, in addition to argon sputtering gas, we add an appropriate amount of oxygen in sputtering to achieve room temperature preparation of high refractive index and high-transparency TiO2 film. After sputtering, the metasurface pattern was defined using electron beam lithography, followed by dry etching to create an array of nanopillars with steep sidewalls. Finally, wet etching was used to remove the hard mask. However, due to the high surface tension of water, capillary forces were generated between the nanopillars during the drying process, and the polyimide substrate easily absorbed water and warped, causing large-scale collapse of the nanopillars. To mitigate this, we introduced critical point drying (CPD) after wet etching and reduced the aspect ratio of the nanopillars. This effectively reduced the large-scale collapse of the nanostructure. The optical measurement of the metasurface on PI substrate verified that optical properties of fabricated metasurfaces matched the simulation results.