直流離子濺鍍機於液晶配向的應用及其配向特性之研究

Abstract

液晶配向技術早已廣泛的應用在液晶顯示科技領域。在這段期間，由於傳統摩刷配向方法本身所具有的缺點限制了顯示相關技術品質的提昇，因此有許多新的配向技術陸續被開發出來。本論文係利用一台構造簡單的直流離子濺鍍機來產生離子源，以提供液晶配向處理之用。利用電漿放電的不同區域來進行表面處理，能提供不同的液晶配向效果。分別利用偏光顯微鏡與聚光干涉儀來鑒別所組合成之液晶盒的配向效果，並且進一步量測液晶盒的表面定向錨定強度與預傾角。此外，利用原子力/磁力顯微鏡、表面化學成分分析儀、紫外/可見光譜儀、超導量子干涉磁量儀及接觸角量測系統深入探討造成表面配向與預傾角的機制是本論文的研究重點。 首先，我們發現不同的離子束能量可以使液晶產生兩種配向效果，包括水平配向及垂直配向。經證實於高能量離子轟擊的狀況下，會在配向膜表面堆積一定量的氧化鐵。也因此，我們提出一種利用單一步驟濺鍍所形成的薄膜來產生垂直配向方法。此種薄膜經由表面分析證實是由許多顆粒大小約三十奈米的粒子所組成，且表面具有磁化強度的異向性。由表面錨定強度與飽和磁化強度的比較結果，我們推測垂直配向的效果是起因於磁性薄膜所產生的磁場。因此我們假設一個簡單的表面模型，描述兩兩相鄰但所具備磁矩的方向相反的磁區所產生的磁場空間分布。經由與液晶間的力矩作用所產生的能量計算，進一步推得表面錨定能的大小並與實驗量測值做比較。 此外，我們利用偏振紫外光照射與離子轟擊一種新型帶有含氟羰基側鏈的感光性配向膜。並觀察含氟側鏈的不同濃度對於液晶配向效果的影響。發現經由子外光照射過的表面有明顯較大的預傾角。相反地，離子轟擊過後的表面所得到的預傾角很小。經由表面分析證實，含CF2的側鏈結構含量的確在影響表面預傾角的大小上扮演重要的角色。此外，在經離子轟擊過後的表面發現含氧量的大幅增加，這也解釋了極性表面能的增加現象。在這份工作中，表面的極性力被證實是影響液晶預傾角的主要因素。 最後，我們利用輝光放電區對聚醯亞胺膜表面進行處理，同時濺鍍氧化鐵於其上。發現在所加偏壓低於七百伏特的條件下，所處理過的薄膜表面對液晶產生品質與穩定度皆不錯的水平配向效果。此外，預傾角能透過調變偏壓或處理時間來改變其大小。其產生預傾角的機制也在本論文探討的範疇內。

Liquid crystal alignment technologies have been widely used in liquid crystal display industry in the past decades. Meanwhile, numerous alignment methods have been intensely developed to replace the conventional rubbing method due to its intrinsic shortcoming. In this thesis, the liquid crystal alignments on surfaces treated by ion bombardments are achieved by utilizing a direct-current ion sputter with diode-type electrodes. Various conditions of plasma discharge offer a variety of alignment effects. The alignment effects of nematic liquid crystal on plasma/ion-treated surfaces are characterized using polarizing optical microscope and conoscope. The surface polar anchoring strength and pretilt angle have also been studied. Furthermore, the corresponding mechanisms of pretilt angle and alignment effect are studied using atomic/magnetic force microscope (AFM/MFM), x-ray photoelectron spectroscopy (XPS), ultraviolet-visible spectroscopy (UV-VIS), super conducting quantum interference device (SQUID), and apparatus of contact angle measurement. First of all, we found that two alignment modes, homogeneous alignment and homeotropic alignment, can be induced by bombardments of ion accelerated by different electric bias. According to the discovery of iron oxide on the surface bombarded by high-energy ion, a method for homeotropic alignment of liquid crystals using a one-step, ion beam sputtering on glass substrates is proposed. The surveyed surface morphology reveals that the films are composed of granular nanoparticles with dimensions around 30 nm. Anisotropy of magnetization is also found on the sputtered ferric films. Polar anchoring strength and saturation magnetization of the coated films of different thicknesses are compared with each other. Accordingly, we deduce that the homeotropic alignment is achieved probably due to the orientation of the diamagnetic nematogenic molecules in the magnetic field caused by the □-Fe2O3 ferrimagnetic thin films. Therefore, a simple model of alternatively distributed magnetic moments with opposite direction is proposed. The profile of magnetic field strength near the surface is then evaluated and taken into the calculation of polar anchoring strength. Besides, surface treatments accomplished through the ultraviolet irradiation and ion bombardment on a newly developed copolymer materials with fluorinated carbonyl groups of different mole fraction and their effects on liquid crystal surface alignments have been studied. The surface alignment with high pretilt angle is achieved by the ultraviolet treatment. The induced pretilt angle by ion bombardments on this photo-reactive polymer film, however, is relatively small. It is confirmed that the content of CF2 grafted side chains plays a dominant role while determining the pretilt angle. A significant increase of oxygen content has also been found to be responsible for the increase of polar surface free energy in ion bombardments. Consequently, the polar force contributed to the surface tension has been proved to dominate the induced pretilt angle. Finally, we demonstrate that the polyimide surfaces are treated by oblique plasma irradiation with simultaneous coating of maghemite nanoparticles. Homogeneous alignments of liquid crystals with good quality and reliability are obtained on the polyimide surfaces treated with plasma energy lower than 700 V. The pretilt angle can be controlled by different plasma energies and treating time. The mechanism in determining the pretilt angle has also been discussed.