陽極氧化鋁薄膜基板及具溝槽之PDMS基板用於液晶配向及其配向特性之研究

Study on the Alignment Properties of Liquid Crystal on the Substrate with Anodic Aluminum Oxide Films and the Grooved PDMS Substrate

湯宗達(Tsung-Ta Tang)

指導教授 ： 潘犀靈 ； 趙如蘋

國立交通大學/電機學院/光電工程系所/博士(2009年)

https://doi.org/10.6842/NCTU.2009.01031

摘要

液晶顯示器為目前市面上最為廣泛使用的顯示器。為了提供液晶分子在未驅動狀態下能夠整齊排列，通常使用傳統的磨刷聚亞醯胺薄膜作為液晶配向薄膜。由於傳統磨刷方式會產生靜電殘留、雜質顆粒污染及損壞驅動薄膜電晶體，且聚亞醯胺薄膜容易被強背光模組中的紫外光波段改變其配向特性。因此研究新穎的非接觸式配向法或無機的配向材料是最為迫切的研究主題。
本論文利用具多孔性的陽極處理氧化鋁薄膜做為液晶顯示元件的配向膜。陽極處理氧化鋁薄膜為一種無機且具有奈米孔洞陣列的透明薄膜。利用表面結構的液晶配向機制，此陽極處理氧化鋁薄膜具有很高的潛能可以作為液晶顯示元件的配向膜。本論文成功利用此一薄膜使液晶分子產生垂直配向。利用改變陽極處理電壓進而改變薄膜上之奈米孔洞大小，此薄膜之極角錨定強度也隨之改變。其極角錨定強度約為15×10-6 J/m2，約略小於傳統的垂直配向膜DMOAP之極角錨定強度38×10-6 J/m2。
除了改變奈米孔洞大小外，本論文中也利用蝕刻一次陽極處理氧化鋁薄膜並控制蝕刻時間，得到具有相同孔洞密度且具有不同孔洞寬深比之奈米孔洞陣列。由實驗發現具有較高寬深比之奈米孔洞陣列可以提供較大的極角錨定強度。此外在特定蝕刻時間下所產生的陽極處理氧化鋁薄膜可提供液晶分子水平配向能力。關於此一現象之配向機制，更多的研究及探討仍須進行。
此外利用兆赫波時域分析系統，對於陽極處理氧化鋁薄膜的光學特性，包括折射率與吸收特性進行分析。本論文研發一套配合兆赫波時域分析系統的光學係數分析程式。利用此程式可直接由兆赫波時域訊號計算出待測物的折射率及吸收特性，並大幅消除待測物所造成的多重反射雜訊。陽極處理氧化鋁薄膜的折射率會隨孔洞密度而改變且在兆赫波波段並沒有明顯的吸收峰。因此陽極處理氧化鋁薄膜也適合作為兆赫波下液晶元件的配向薄膜。
雖然水平錨定強度對於垂直樣品並不是指標參數，但本論文也討論對於水平錨定強度的量測修正。利用量測楔形樣品的強度變化條紋所計算出來之旋性液晶週期，來計算水平錨定強度可大幅減少因雙折射係數所造成的誤差由10%至1%。
除了利用多孔洞陽極處理氧化鋁薄膜作為垂直配向薄膜，本論文也嘗試使用奈米壓印技術，於PDMS基板上轉印出U型微溝槽結構，並使用氧電漿作表面改質作為水平配向薄膜。此一方式可以成功製作出可橈式的配向基板。
最後本論文已經成功嘗試將陽極處理氧化鋁薄膜製作於ITO透明導電薄膜上，並製作出可電控的液晶顯示元件。未來可以利用此一方式來製作以陽極氧化鋁薄膜作為配向膜之元件，並量測其電性反應或利用外加電場來量測其極角錨定強度。

關鍵字

液態晶體 ； 液晶配向 ； 陽極氧化鋁薄膜 ； 兆赫波 ； 聚雙甲基矽氧烷 ； 溝槽基板

並列摘要

In the past decades, the liquid crystal displays (LCDs) have been widely used in different applications. The rubbed polyimide thin film is the most common alignment layer for aligning liquid crystal without any applied field. The rubbing method usually introduces the static charge, dust contamination, and damages the driving thin-film transistors (TFT) devices. Because of the chemical structure of the polyimide, it is easy to be modified by the UV-light or the back light module. Therefore, the novel alignment method, which is non-contact and uses the inorganic alignment material, is desired.
In this thesis, the porous anodic aluminum oxide (AAO) thin film is used as the alignment layer of the LCDs. The AAO thin film is an inorganic and transparent thin film with nanopores array. Because of the nanopores array, the AAO thin film is an excellent candidate of the alignment layer. By controlling the anodizing voltage, the pore diameter of the AAO thin film can be varied between 15 nm and 65 nm, and the polar anchoring strength can also be modified. The polar anchoring strength of the AAO thin film is around 15×10-6 J/m2, which is smaller than one of the DMOAP, 38×10-6 J/m2.
Besides controlling the anodizing voltage and changing the pore diameter, the one-step AAO thin film can be etched with different etching time. Because the etching solution etches both the top and the wall of the AAO thin film, the etched AAO thin film has different aspect ratio with the same pore density. The AAO thin film with higher aspect ratio has the higher polar anchoring strength. In some specific etching time, the etched AAO thin film performs as a homogenous alignment layer. Further works are progressing to understand the mechanism of the homogenous alignment on the etched AAO thin film.
The optical properties of the AAO thin film, such as the complex refractive constants and the attenuation constant, are investigated by using the Terahertz Time-Domain Spectroscopy (THz-TDS). The optical constants analysis program is developed for calculating the optical constants by analyzing the time-domain signal directly. The multiple reflections have been considered in the program to cancel out the Febry-Perot effect. The refractive constant of the AAO thin film depends on the pore density of the AAO thin film. There is no obvious absorption peak in THz region. Therefore, the AAO thin film is suitable for THz application.
Although the azimuthal anchoring strength is not an important indication of the vertical alignment cell, it is also discussed in this thesis. The pitch value used in calculating the azimuthal anchoring strength can be modified by counting the fringes of the wedge cell. By using the modified pitch value, the error of the azimuthal anchoring strength can be reduced from 10% to 1%.
On the other hand, the grooved PDMS substrates are used as the homogenous alignment substrates. The PDMS substrates with the U-shape groove are imprinted by the nanoimprinting technology, and then treated by O2 plasma to change the surface property. The flexible LCDs are demonstrated by using the grooved PDMS substrates as the alignment substrates.
Finally, the prototypical electrically controllable LCDs with the AAO thin film as the alignment layer have been demonstrated in this thesis. In the future, the electric performance and the polar anchoring strength can be measured by applying the electric field.

並列關鍵字

liquid crystal ； liquid crystal alignment ； anodic aluminum oxide ； Terahertz ； PDMS ； grooved substrate

參考文獻

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