以中空微管製作次波長圓環孔徑結構於曝光微影系統之模擬與研究驗證

Abstract

近年來，在近場光學研究領域中，對於如何將近場的光學特性延伸到遠場操作，進而取代近場光學因光點小使聚焦短難以操控的缺點，有許多的研究與探討，而研究發現許多方法可以產生具有長焦深特性與突破繞射極限可能性的貝索光束，其中週期性結構為較常見的方法之一，傳統上是在同心圓的周期性結構上鍍上能產生表面電漿的金屬膜以提升出光效率及指向性。而本研究團隊改良傳統做法，提出利用微機電技術製作單圓環次波長圓環孔徑的光學頭，產生具有次波長光點大小的貝索光束並整合至奈米直寫儀系統作曝光微影及雷射鑽孔的應用，也證實置換鍍膜為不能產生表面電漿效應的鍍膜金屬也可產生類似貝索光束。 故本研究承襲先前團隊研究利用中空微管加熱拉針的方式製作光學頭，從材料、內外徑選擇、拉針機參數調整、鍍膜方式、光學頭前端與末端的光學表面處理確保入射光與出射光品質到中空部分阻塞使光能在圓環材質中經由波導方式傳遞，製作出具有高穿透率及微米尺寸的單圓環光學頭。此種利用非微機電的製作方式無疑能大大減少傳統利用微機電技術所花的製作成本與時間。 此外，本研究利用有限時域差分法光學模擬軟體模擬針對傳統次波長圓環孔徑結構到實際製作的光學頭進行各種參數模擬，包含實際內外徑的討論以確保選擇較好的內外徑比材料、外壁鍍膜與表面電漿效應的關係、中心阻塞有無對出光的效應及圓環出光的狹縫理論，目的是希望能找出最佳的製作方式。而分析實際光學頭模擬出光光強結果，顯示其可產生次波長光點大小與長焦深的貝索光束。 接著，將光學頭整合至自行架設的光強與曝光系統，並利用自製六軸顯微鏡與曝光夾具在人機軟體操控下進行光強與曝光的實驗。在光強實驗中，分析不同類型光學頭的優缺點與各位置的出光光強分布，討論聚焦效應並與模擬結果比較，了解實際出光與模擬出光的相同與相異性，證明光學頭能產生與模擬相符合的貝索光束特性。最後在曝光實驗中，成功利用正光阻紀錄光束樣貌，證實光束具有高深寬比結構，同時也證明光學頭不僅出光具有長焦深貝索光束特性，其聚焦能量也足以被光阻紀錄，且為了量測高深寬比結構，本研究也利用各種量測方式，進行優缺點討論與比較，使能夠用適當的量測技術得到完整的曝光資訊。

Recently, many studies have explored the approaches to circumvent the difficulty of short depth-of-focus (DOF) associated with near-filed optical system. Researchers found that the long DOF of Bessel beam is one convenient method to achieve long DOF while maintaining the small spot size as many methods exist to generate the Bessel beam. One method is to use concentric periodic structures, which traditionally were made of coated surface plasmonic metallic film, to enhance emitted light beam efficiency and directivity. The NTU Nano-Bio-Mems group proposed that the single Sub-wavelength Annular Aperture (SAA) structure based plasmonic metallic or non-plassmonic metallic film structure can serve as an optical head to generate sub-wavelength focus spot and long DOF Bessel beam, which can then be integrated to create an nano-writter system so as to facilitate exposure for lithography and many other applications. Taking previous researches that utilized hollow tube (HT) to form tapered micro-annular optical head by pulling as a starting point in this study, various design parameters include choice of materials, the inner and outer diameters, puller processing parameters, coating methods, tip and end of optical head optical surface processing methods to maintain emitted light beam quality, and hollow part blocking approaches were tried with an attempt to optimize light beam transmission through the annular material by HT formed waveguide. This optical head not only has high transmission property but also is low cost since no MEMS processeses were involved in fabricating it. Finite Difference Time Domain (FDTD) method was used to perform a series of simulations on the possibilities of forming SAA by tapered micro-annular optical head. Discussions on optimizing the HT design parameters such as inner and outer diameter ratio, relationship of outer coating and the existence of plasmonic effects, effect of hollow part blocking process, verifications of slit theory and influence of SAA emitted beam interference, etc. were presented. All these simulations were performed with an attempt to find the best way of making an optical head . The simulation results performed by using the real micro-annular optical head parameters demonstrated sub-wavelengh focal spot and long DOF Bessel beam was achieved. In addition, the newly developed tapered micro-annular optical head was used to create a home-made optical intensity metrology system, an exposure lithography system, etc. by using a home-made microscope and an exposure holder. These newly developed systems were controlled by using LabVIEW. In the intensity metrology experiment, the advantages and disadvantages of using different materials to create the optical head were analyzed. Intensity distribution at different positions were also measured. The results showed that the intensity data recorded by CCD matched well with the simulation results. A slilght difference on the DOF was identified. Finally, the exposure experiments showed that structures with aspect ratio higher than 10 were successfully fabricated within the AZ4620 photoresist. These results showed that the optical head not only creates long DOF Bessel beam but also emits focused light beam with energy strong enough to expose photoresist. In order to measure the high aspect ratio structures fabricarted, various methods and techniques were attempted in this study. The advantages and disadvantages of these techniques were compared with an attempt to identify the proper metrology technique for verifying the geometry of the structures exposed, which is important for full exposure experiments in the future.