液態晶體表面配向之新方法及其特性研究

Abstract

玻璃基板表面在液晶物理扮演一個重要角色。 被摩刷過的聚合物層被廣泛的使用來控制液晶分子的排列。新一代的液晶設備與當前設備比較 , 其體積將會進一步縮減並且基板將會更薄和精美。 傳統摩刷方法不能做小區域或不同區域(微米範圍)的配向。 所以 , 新的表面配向方法將是極必要的。 另外 , 摩擦刷的過程產生的纖維殘渣和殘留靜電可能會限制住良率的提升。如果能以一般的半導體微影製程和活性離子蝕刻(RIE)方式來做基板配向處理 , 液晶的應用將可進一步延伸。 利用此製程 , 不但可達到小區域或不同區域的配向 , 而且可提高包含液晶組件的微機電系統(MEMS)的功能。

我們研究線狀5CB液晶（4`-n-Octyl-4-Cyanobiphenyl）在表面有平行溝槽的玻璃基板上的配向性質。我們是以半導體活性離子蝕刻的方法(Reactive Ion Etch) ，直接在玻璃基板蝕刻出不同深度與週期的平行U型溝槽。再以原子力顯微鏡檢視其表面溝槽寬度、深度、及形狀。然後將兩片具有相同方法處理且有平行溝槽的玻璃基板做成上下平行的液晶盒，灌入有左旋添加物5CB液晶（4`-n-Octyl-4-Cyanobiphenyl）。利用光學方法測量表面方位角定向強度 , 研究玻璃基板對液晶分子的配向能力 ,觀察溝槽週期、深度對配向強度的影響。我們觀察到溝槽間距少於4微米和深度大於50奈米時 ,表面方位角定向強度可達10-4 J/m2。 我們也研究溫度對5CB液晶方位角定向強度的影響。 我們使用光學方法 ,在5CB液晶相溫度範圍 , 測量液晶扭轉角度以得到不同溫渡的方位角定向強度。 為了避免一些特別實驗條件(尤其是波長)的限制 , 我們使用二個不同波長的雷射光。 我們發現 , 當溫度增加時表面方位角定向強度穩地減少。 我們也發現了液晶扭轉角度和螺距在很寬溫度範圍中不改變, 因此表面方位角定向強度與K22 是成比例的, 除非溫度很靠近臨界溫度.

我們也研究其他配向方法 , 譬如原子力量顯微鏡來修飾鍍有配向膜的玻璃的方法。我們使用原子力量顯微鏡修飾表面鍍有聚合物配向膜的玻璃基板。再以原子力顯微鏡檢視其表面結構。 我們研究被修飾過表面的玻璃基板對液晶分子配向性質與修飾條件的關係。 我們發現修飾密度是一個控制配向的主要條件。

The surface of glass substrate plays an important role in LC physics. Rubbed polymer layers are widely used to control the alignment of LC molecules. New generations of LC device will be much further reduced and the substrates will be much thinner and more delicate compare to present devices. The conventional rubbing method can not pattern orientations over small areas or multi-domain. New surface alignment methods will be necessary in urgent. The fiber residues and static charges introduced by rubbing process can cause trouble for devices with fine patterns. The application of LC will be much further extended, if the alignment can be achieved with common semiconductor lithography process. With this process, the small area and multi-domain alignment can be achieved and the functionality of Micro-electromechanical system (MEMS) can be increased by including LC components.

The alignment property of the nematic liquid crystal (LC) 4’-n-pentyl-4-cyanobiphenyl (5CB) on glass substrates with parallel grooves are studied. The U-shaped grooves with a variety of depths and periods are prepared by the reactive ion etch method. Surface morphology of the grooved glass is examined by Atomic Force Microscope. Two parallel grooved substrates with chiral doped 5CB sandwiched in between were used to form a LC cell. The alignment quality for LC is studied by measuring its surface azimuthal anchoring strength using an optical method. The effect of the groove period and depth are studied. Strong anchoring strength of 10-4 J/m2 is observed for groove spacing less than 4 □m and depth large than 50 nm.

Temperature dependence of the azimuthal anchoring strength of the nematic liquid crystal 5CB on parallel grooved glass substrates has been also studied. We measured the azimuthal anchoring strength in the nematic temperature range by measuring the twist angle in the LC cells using an optical method. Two lasers with different wavelengths were used to avoid the limitations of either wavelength in particular conditions. We found the anchoring strength decreases steadily with increasing temperature. We found that the twist angle and pitch of LC do not change significantly in a wide temperature range, resulting in that the anchoring strength is proportional to K22, unless the temperature is close to Tc.

Other alignment methods, such as atomic force microscope (AFM) modifying method has been used to modify the polyimide films on glass substrates. The surface morphology of the PI films was then probed with the AFM operating in the non-contact mode. The properties of these films for liquid crystal alignment and their relations to the modifying conditions have been studied. The modifying density is a dominant factor in the conditions we have studied.