以拉伸毛細管研製適用於次波長微影系統之長焦深光學頭

Abstract

貝索光束之次波長聚焦能力可望能突破光學系統中焦深與聚焦點大小的連動關係，取代近場光學聚焦點短且操作不易之缺點。近年來產生貝索光束的方法陸續被提出，而Ebbesen等人提出表面電漿效應可增強金屬週期性結構之穿透率，且透過此結構產生之光束具有指向性後，本研究團隊陸續提出不同結構之光學頭。近年提出單圓環孔徑結構製作之光學頭，可產生具有次波長尺度之貝索光束，此光學頭可應用在微影製程及雷射加工上，製作出高深寬比之結構。本研究基於前人之前導性研究，使用拉伸中空毛細管材料之方式，製作穿透率較高之光學頭。此光學頭之製作方式以非微機電製程為主，可降低光學頭之生產成本。 本研究以製作具有次波長聚焦能力之光學頭為主旨，由中空毛細管材料及規格之選擇開始，討論拉伸材料之方式與參數調教，將毫米尺度之中空毛細管，以熱融拉伸之方式，製作出尖端為微米尺寸之光學頭。拉伸完成之光學頭將進行內部及外部之屏閉，確保耦合進入光學頭之光束以波導之形式在光學頭內做傳遞，其屏蔽之材料與方式都經過模擬與實驗測試。為確保光學頭入射光及出射光之品質，此光學頭需進行二端之表面加工來達成表面光學品質之要求。 光學頭之聚焦效應將透過模擬做更多討論，本研究使用時域有限差分法電磁波模擬軟體，對光學頭製作之各種參數進行模擬，討論其最佳之設計方式。接著為光學頭出光之光強分佈實驗，以自製之光學顯微系統觀察其出光距離與光強分佈之關係，討論其聚焦效應，同時與模擬之結果比對。最後嘗試將此光學頭實際使用在微影系統上，使用自製之曝光系統在AZ4620光阻上製作高深寬比之結構，以驗證光學頭長焦深之聚焦能力。為量測光阻高深寬比之結構，本研究嘗試許多不同之量測方式，也將在本論文中進行討論，以便讓未來的研究者能夠依據這些基礎來規劃更合適的量測方法。

The sub-wavelength focusing ability of Bessel beam may provide us with a way to circumvent the closely interlocked relationship between the depth of focus and the spot size. In addition, Bessel beam may be able to bypass the short focal length associated with the light spot as well as the drawbacks of hard to manipulate light beams in near-field optics based systems. There are many methods proposed to generate Bessel beams in recent years. Ebbesen et al. found that illuminating a metallic film perforated with periodic subwavelength apertures led to unexpected large transmission and directional beaming effect. Following these studies, our research group proposed different structures of optical head. The subwavelength annular aperture SAA proposed in recent years formed the starting point of the research innovations disclosed in this thesis. The SAA optical head can be applied to micro lithography process and laser machining to produce high aspect ratio structures. After the preliminary study of using tapering capillary tube to produce optical head for better transmission the optical head developed in this thesis was fabricated by using non-MEMS process to reduce the cost. The subject of this research work is to fabricate the subwavelength focusing optical head. Started from choosing the material and defining the specification, this thesis discussed the way of tapering and the method adapted to set parameters. Heat melting method was implemented to fabricate the optical head with micron sized tips. The optical head’s inner and outer sides were blocked to ensure the tapered structure can act as a waveguide to couple light beam completely through the optical head. The materials used to cover the capillary were identified by using simulation and the methods were verified by experiment. To ensure the quality of incident and injecting light beams, both ends of the optical head were polished to achieve optical quality. The focusing phenomenon will be discussed first with simulations. These studies use finite-difference time-domain method to do the electromagnetic simulation. Different parameters associated with the optical head were then identified to fabricate the optical head. In this work, light intensity profile experiment and lithography experiment were set up to prove the high depth-of-focus property of the optical head generated Bessel beam. A home-made microscope was used to measure the intensity profile at different distances from the optical head tip. The intensity profile will also be discussed by using simulation results. The lithography experiment developed to perform exposures that led to high aspect ratio hole in AZ4620 photoresist. The method tried to measure these holes will also be discussed in this thesis so as to provide a strong basis for future researchers along the requirements of identifying proper metrology methods.