利用多層對準及奈米壓印技術製作與設計次波長彩色濾光極化片

Using Multi-layer Alignment and Nano-imprinting Technologies to Design and Fabricate Sub-wavelength Grating Based Wavelength Selected Polarizers

呂巧芸(Chiao-Yun Lu)

指導教授 ： 陳學禮

國立臺灣大學/工學院/材料科學與工程學研究所/碩士(2009年)

https://doi.org/10.6342/NTU.2009.02424

摘要

利用電子束直寫微影配合多層對準技術，我們得以製作次波長金屬光柵極化器，當非極化光垂直入射至金屬光柵時，能將電場振盪方向平行金屬線者(TE-mode)吸收，而使垂直金屬線方向者(TM-mode)穿透，並設計各種結構參數，以討論極化光穿透量與極化消散比的能力。此外藉由結構大小與入射光波長的關係，以共振腔結構篩選可通過的波段位置，使金屬光柵同時具備極化與彩色濾波的功能。
首先，我們討論單層鋁膜光柵之結構參數對極化光篩選的特性，設計不同鋁膜厚度、線寬大小、週期，以及金屬孔隙覆蓋比例等參數加以討論，得知厚度達4000Å，且週期越小者，對極化篩選的效果越好，且孔隙覆蓋比1：3者，雖降低TM光穿透量，但卻也有效抑制TE光的洩漏，使極化消散比大幅提昇。
但由於單層鋁膜不具有共振腔結構，無法對穿透波段作有效篩選，因此我們以多層對準技術製作雙層金屬光柵，且雙層鋁膜間的氧化矽層可作為極化光之共振腔，藉由層間相對平移量改變為週期的四分之一和二分之一，或是調控氧化矽層之厚度，可有效控制異常穿透波段，以及抑制長波段的穿透量。我們以時域有限差分法 (FDTD) 模擬近場光學現象並分析極化光穿透機制，且透過嚴格耦合電磁波分析法 (RCWA) 模擬證實實驗結果的可靠性，由此我們可得一結論為，改變入射光於結構內傳遞的光路徑，不論是藉由金屬膜相對平移量抑或是改變氧化矽共振腔的長度，皆可有效控制異常穿透的波段，使雙層金屬光柵成為通訊波段的濾波極化器。
本論文中，我們以雙層鋁膜厚度為「3000Å vs 1000Å」之最小結構，週期僅600nm，金屬孔隙覆蓋率為1:3者，得到於1500nm波段位置，其TM光穿透量高達60%且極化消散比將近3000的彩色濾波極化器。
而為了降低製程成本，我們也利用奈米轉印與壓印技術製作次波長極化器，且將基板置換為高分子材料，使極化片增加可撓曲的特性，可廣泛應用於各種領域。透過壓印參數設計，我們得到以19MPa壓印100nm之金膜可得到超過70的極化消散比，且增加斜向入射角度可使表面電漿共振波段發生紅移，而較長波段之TE光因遠離金之特徵波長，其穿透量有效被抑制，因此極化能力可獲得大幅提昇。

關鍵字

次波長金屬光柵 ； 極化器 ； 彩色濾波器 ； 表面電漿 ； 奈米壓印技術

並列摘要

The fabrication of sub-wavelength metal grating polarizers using e-beam direct lithography and multi-layered alignment technologies. As a non-polarized light is impinging into a metal grating, it will absorb the TE-mode whose electric field oscillation direction is parallel to the wire-grid. It will then perpendicularly transmit the TM-mode out of the wire. This thesis investigates the transmission of polarized light and its extinction ratio by designing the parameters of the grating structure. Besides from considering the relationship between structure size and the wavelength of the incident light, the resonant cavity can also filter out any undesired wavelength. Therefore, the metal grating has the ability to both polarize and filter.
First, this paper discusses the characteristics of polarization concerning the parameters of single layer aluminum film structure. By modulating the thickness of the aluminum, the width of the slits, the period of the grating and the filling factor, the project has been able to find and produce a substantially more effective grating structure. Once the thickness of aluminum film reaches 4000Å and the slits are smaller, the filter polarization becomes more effective. Although it will lower the TM transmission, this then is compensated by the suppression of the TE mode in the process creating a desired extinction ratio / filling factor of 1:3.
However, single layer grating systems are unable to contain a resonant cavity and therefore cannot filter out any undesired part of the incident light. To resolve this problem, we used multi-layered alignment technologies to construct a double layered metal grating system. Here, a silicon oxide layer was inserted between the metal layers to form a resonant cavity for the polarized light. The amounts of the shift between the two layers are 0.25 and 0.5 of the slits. By modulating the thickness of the middle oxide layer, we can control wavelength effectively so much so that we can suppress the transmission of longer wavelengths. We also use the finite-difference time-domain method (FDTD) to simulate the phenomenon of near-field optics and to analyze the mechanism of transmission. To confirm our result of the experiment, we used a rigorous-couple-wave-analysis (RCWA) simulation.
In conclusion, we modulated the optical path of the incident light in the structure by adjusting the amount of shift or by changing the length of the resonant cavity. These two processes allowed us to control the transmissible wavelength effectively while also enabling the double layered metal grating to become a filtered-polarizer for communication wavelength.
In the thesis, there is 60% of TM transmission at 1500nm of the double layers system the thickness control of [3000Å vs 1000Å] and the period is only 600nm with filling factor 1:3, and the extinction ratio is almost 3000. It is a high quality color-filtered polarizer.
In order to lower the cost of the process, we also used nano-imprinting and reversal imprinting lithography technologies to fabricate the sub-wavelength polarizer. We substituted the substrate as polymer material to increase its flexibility and feasibility. By using an imprint condition of 19MPa 100nm gold film, we obtained the extinction ratio of more than 70. Any increase in the degree of the oblique incident angle produces a red-shift on the surface plasma resonant wavelength. The TE-mode is much suppressed at the longer wavelength because it is far away from the characteristic wavelength of gold. Therefore, the ability of polarization can be improved dramatically.

並列關鍵字

sub-wavelength metal grating ； polarizers ； color filters ； surface plasma ； nano-imprinting techonologies.

參考文獻

31. 趙健皓，次波長金屬微結構之表面電漿特性與應用之研究，國立台灣大學碩士論文，(2004)。

45. 黃楷庭，次波長微結構在表面電漿元件及太陽能電池應用之研究，國立台灣大學碩士論文，(2006)。

12. 周冠文，單層介電質次波長極化分光器之設計、製作與量測，國立台灣大學碩士論文(2007)。

46. 王文昀，應用表面電漿共振效應設計具特性光學行為之次波長結構光電元件，國立台灣大學碩士論文，(2007)。

47. 郭珊珊，奈米直印金屬技術與低介電係數材料在半導體製程與光電元件應用之研究，國立台灣大學碩士論文，(2007)。