光波在介電質奈米結構的聚焦特性與其在光罩製程上的應用

微影技術在現代的科學研究和工業應用上，扮演著舉足輕重的角色，而光學微影又是微影技術中的主流研究，亦可說是科技發展的推手。然而光學微影技術受限於繞射限制，已逐漸面臨做到最小尺寸的極限。目前已知在近場範圍下，可突破此限制。因此，我們進一步研究出一套全新的近場微影技術，此技術是利用光在介電結構邊緣產生的繞射現象，透過有限時域差分法(FDTD) 的模擬以及近場光學顯微儀(NSOM)的量測方法，我們發現邊緣繞射光在很小尺寸的介電結構中會產生聚焦效果。利用此聚焦效果在微影上，可以在光阻層大量製作出次波長結構。這個技術的最大優點在於使用簡易、製作方便，而且製作出的次波長結構的品質很高，特別是光子能隙（PBG）結構。在本論文中，提出了微影方法上四種創新的構想和技術。透過四種不同的次波長光罩的設計，將之應用在近場微影上，而微影的結果也印證了我們技術的可行性。將此四種技術簡述如下： (1)我們發展出可以製作次波長尺寸且具有高深寬比的四角形和六角形週期性結構之技術。光罩被設計成四角形或六角形的柱狀結構，柱狀的厚度是0.2 微米，柱狀的直徑是0.3 微米，周期大約是在柱狀直徑的兩倍下，會有最佳的聚焦效果。在近場曝光下可以產生高深寬比的四角形或六角形次波長尺寸的光阻結構。研究結果也發現較高對稱性的幾何結構有較好的聚焦效果，即六角形柱狀結構的聚焦效果優於四角形柱狀結構。這個聚焦光束的長度可大於1 微米，寬度則可小於波長0.3 微米。 (2)我們發展出可以製作次波長尺寸的PBG 結構且可設計成任意缺陷形式之技術。目前廣受應用的2 維PBG 結構大部分都是被設計成破壞週期性結構的缺陷結構，然而要以光學的方式來製作缺陷的PBG 結構並不容易。我們設計的光罩是週期性的六角形柱狀結構，去除點或行變成缺陷形式的週期結構，在近場曝光下可以製作出缺陷形式的PBG 光阻結構。經由FDTD 的模擬和NSOM 的量測結果，可得知光罩 IV 上的缺陷結構在近場範圍下，並不影響其聚焦光束的特性，因此，透過近場微影的方法，可以在光阻層大量製作缺陷形式的PBG 結構，同時利用蝕刻的技術，可完美轉印在矽基板上。 (3)我們發展出可大量製造小於100 奈米線寬的近場微影技術。這個技術的成功關鍵在於光罩上小於100 奈米的介電結構的聚光效果與極化方向有直接的關聯。利用 FDTD 模擬與NSOM 的量測結果得知：小於100 奈米的介電結構的聚光效果是由TE 偏振光所造成。依據此結果，我們使用TE 偏振光在近場下曝光，可以做出80 奈米的線寬，當雷射光的波長為442 奈米時，可達到小於1/5 波長的解析度。 (4)我們發明一種等向性的蝕刻方法，可以製作六角最密堆積的次微米透鏡陣列，來當作近場曝光的光罩去大量製造3 維結構。這個構想是起源於研究柱狀結構與球狀結構對聚光效果的影響。此種方法是利用鎳金屬的柱狀結構當作蝕刻的光罩，使用乾式蝕刻，當玻璃基板含金屬雜質高時，會形成等向蝕刻玻璃和金屬光罩。在適當的蝕刻時間控制下，等向蝕刻玻璃會形成半球狀。此半球狀的六角最密堆積結構經由NSOM 量測和FDTD 的模擬結果，可得知具有次波長的聚焦效果。我們比較柱狀結構與球狀結構在2 維對聚光的效果來看，發現球狀結構聚焦效果不如柱狀結構的聚焦效果。但是以3 維的聚焦特性來看，球狀結構在周期500 奈米的條件下優於柱狀結構。因此，我們利用週期500 奈米的半球型光罩在近場曝光下，可在光阻層產生2 層六角形的次波長周期結構。

關鍵字

近場微影術；光子能隙結構；有限時域差分法；近場光學顯微儀；次微米透鏡陣列

並列摘要

We develop a near-field photomask lithographic method to fabricate high quality subwavelength patterns. This method uses edge-diffracted beams occur at the edges of subwavelength dielectric structures. According to finite-difference time-domain (FDTD) calculations and scanning near-field optical microscopy (NSOM) measurements, we find that light passes such small air-dielectric structures, those edge-diffracted beams will merge together at topographic higher regions and form subwavelength focused beams. Based on this novel focusing effect, a new approach for mass-production of subwavelength structures, especially the photonic bandgap (PBG) structures, is presented. The accomplished ideas of this dissertation include four different photomask designs for making subwavelength photolithographic patterns. Synopsis of these portions is as follows: (1) We develop a photolithographic approach to produce high aspect-ratio hexagonal and square arrays. The photomask is composed of hexagonal or square rod arrays with a thickness of 0.2μm and a rod size of 300nm. Illuminating the photomask with a blue laser generates periodically focused beams up to 1μm long and less than 300nm wide. Due to higher symmetry, hexagonal rod arrays exhibit better focused beams than the square ones. (2) Most PBG based devices need some designed defect patterns existed in the VI PBG arrays. Using a transparent photomask with periodic arrays and designed defects, we can fabricate subwavelength PBG structures with channel defects on the silicon substrate. The NSOM measurements and FDTD calculations confirm that the subwavelength focused beams are not affected by the neighboring defects in the near-field region. (3) We study a sub-100nm photolithographic approach by using TE-polarized wave in the transparent nanostructures. The optical near-field and its polarization anisotropy in transparent nano-structures are detected by a polarization near-field optical microscopy. According to experimental results and FDTD calculations, localized optical near-fields exist at topographic higher regions of nano-structures under TE polarized condition, while less localized near-fields for TM mode. We experimentally show these localized fields can produce photolithographic patterns with a feature size about 80nm by using a 442nm helium cadmium laser. The resolution is smaller than λ/5, far below the diffraction limit. (4) We study the geometrical effect for the subwavelength focusing beams. We invented a new method to fabricate the close-packed submicron lens array with a feature size close to optical diffraction limit. By controlling the size of rods in a nickel mask and the time of reactive dry etching, hemispherical lens array with submicron period can be directly made on a borosilicate glass. From NSOM measurements and FDTD calculations, the lens array can also make subwavelength optical spots near the lens surface. Although the focusing effect is not as good as in prior rod structures, the spots produced by the submicron lens array show periodic patterns in the propagation direction. By harnessing this optical property, 3D-PBG structures are possible to be made by the photolithographic method. In this dissertation, we demonstrate the fabrication of multilayer hexagonal structures with a period of 500nm.

並列關鍵字

near-field lithographic method ； PBG ； FDTD ； NSOM ； submicron lens array

參考文獻

[1] K.A. Valiev, The Physics of Submicron Lithography, Plenum Press, New York

with immersion lithography,” J. Microlitho. Microfab. Microsys. 3 (2004) (1), p.

electron beam column for high throughput maskless lithography,” J. Vac. Sci.

[5] M. Muraki and S. Gotoh et al., “New concept for high-throughput multielectron

spatial-phase-locked electron-beam lithography,” J. Vac. Sci. Technol. B 20 (2002)

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光波在介電質奈米結構的聚焦特性與其在光罩製程上的應用

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