反射式單晶矽液晶面板邊際場效應及繞射效應之研究

Abstract

近年來，反射式單晶矽液晶(LCOS)元件一直是顯示產業持續注意的焦點，尤其是在投影顯示器的應用方面，其產品包括前投式投影機、背投式電視以及頭戴式虛擬顯示器等。由於單晶矽的電子漂移率甚高，因此LCOS元件可以擁有非常高的解析度，除此之外，其周圍的驅動電路也可以整合在單一的晶片上以減少生產成本。在製造方面，LCOS液晶面板乃建立在國內兩大產業的基礎上，即半導體與液晶顯示器產業，因此，這對於國內發展LCOS不啻為一大利基。由於矽基板的製造是屬於標準的半導體製程，所以具有低價格的潛力與優勢，也因此許多廠商已紛紛投入LCOS投影顯示器這個產業。 然而，目前LCOS顯示技術仍然存在著許多困難有待克服。在面板方面，由於要在微小的面板上做出高解析度的影像，其像素以及像素間距也相對地變得非常小。當相鄰像素的施加電壓不同時，其邊緣電場會被扭曲，使得此處的液晶分子產生不正常的排列，進而影響其光學特性，這就是所謂的邊際場效應。 此外，當解析度的要求提高而使得像素電極的大小接近可見光的波長時，其對光波的作用類似於一反射光柵。當光入射在面板上時會產生明顯的繞射效應，而斜向傳播的繞射光可能無法進入光學系統，因而造成嚴重的光損失。 在本論文中，我們探討八個常用液晶模態的邊際場效應，並且改變液晶盒參數，研究其對LCOS面板光學表現的影響。我們發現混和式扭轉向列型液晶模態的邊際場效應相對較弱，而扭轉向列型和垂直排列型液晶模態則會受到嚴重的邊際場效應影響而降低其影像品質。特別是對於垂直排列型液晶模態，其邊際場效應不但影響其靜態顯示的品質，同時也嚴重地拖慢了動態影像的切換速度。在液晶盒結構方面，根據電腦程式的模擬結果，我們也發現像素節距、液晶盒厚度、預傾角和電極斜率都是影響邊際場效應的重要參數。 為了要設計一個高對比度且同時不受邊際場效應影響的LCOS面板，我們針對擁有完美暗態的垂直排列型液晶盒做分析。根據電腦模擬出的液晶指向矢分佈，我們發現利用圓偏振光的特性，可以有效地保留因邊際場效應所損失的光效率進而提高影像的銳利度；同時，在動態響應上也解決了因緩慢切換過程而產生的影像模糊問題。相關的光學原理可以由著名的de Varies理論來解釋。 在探討LCOS面板的繞射效應方面，由於傳統的瓊斯矩陣法並無法分析光的繞射效應，因此我們將以往應用在光波導計算的光束傳播法延伸到LCOS元件的光學計算，而撰寫出有考慮繞射效應的光學模擬程式。利用此程式，我們針對垂直排列型液晶模態以及工研院電子所研發的FOP(finger-on-plane)模態進行分析。模擬結果顯示繞射效應對高解析度的LCOS元件影響甚巨；使用傳統瓊斯矩陣法會造成嚴重的誤差，唯有使用更嚴謹的光束傳播法才能正確的預估其光學行為。此外，我們發現稍微修改FOP模態的液晶盒結構能有效地降低在特定波段的繞射效應，若配合使用三片或雙片式的LCOS投影光學系統，則可以有效地提升因繞射效應所損失的光效率。

In recent years, the display industries keep showing great interests in liquid-crystal-on-silicon (LCOS) devices, especially in the application of projection display. The products of LCOS devices include data projectors, rear-projection TV and the head-mounted virtual display. Due to the advantage of intrinsic high electron mobility of crystalline silicon, LCOS devices can be fabricated with very high resolution. In addition, its peripheral driving circuits can be integrated on a single chip, which greatly reduces the cost of manufacturing. Technically, LCOS devices are based on two major domestic industries: the semiconductor and liquid crystal display industries. Since the fabrication of the silicon backplane is based on the standard manufacturing process of semiconductor, LCOS devices have great potential of low price. Therefore, many manufacturers have already invested in this industry. However, there are still many challenges in the LCOS industry. The two major issues of LCOS panels are the fringing-field and diffraction effects. As the resolution increases, the pixel size and the inter-pixel gap will become very small. When the applied voltages between adjacent pixels are different, the electric fields near the pixel edges will be distorted. Hence, the liquid crystal molecules near this region are aligned abnormally, which, in turns, degrades the optical performance of the device significantly. This is the so-called fringe field effect. In addition, as the pixel pitch becomes comparable to the wavelength of the visible light, the LCOS panel acts as a reflective grating. Therefore, obvious diffraction effect can be observed. The oblique diffracted light may not be able to enter the optical system, and consequently results in serious light loss. In this dissertation, we investigate the fringing-field effects of eight commonly used liquid crystal modes. We also investigate the influence of the LC cell structure on the optical performance of LCOS devices. It is found that the mixed-mode twist nematic (MTN) has weaker fringing-field effect while the twist nematic mode (TN) and vertically aligned mode (VA) suffer from the effect significantly. The fringing-field effect is particularly severe in VA mode. It not only degrades the static image qualities but also deteriorates the dynamic response of the LCOS panel. The pixel pitch, cell gap, pretilt angle and electrode slope are all found critical to the fringing-field effect. In order to design a high-contrast-ratio LCOS panel without fringing-field effect, we focus on the analyses of VA mode which possesses an excellent dark state. Based on the simulated results of the LC director profile, we find that, by utilizing the properties of circularly polarized light, the light loss caused by fringing-field effect can be preserved and the sharpness of the image can be enhanced dramatically. Moreover, the dynamic response is also improved and the imaging blurring effect is successfully eliminated. The results can be qualitatively illustrated by the de Vries theory. With regard to the effect of diffraction, a rigorous simulator is needed to investigate the optical performance of a high-definition LCOS panel. The conventional Jones matrix method is no longer suitable in this condition. We extend the beam propagation method (BPM), which is commonly employed in waveguide calculations, to the optical simulation of LCOS devices. Two promising LC operation modes are analyzed by BPM, i.e. VA and finger-on-plane (FOP) modes. The calculated light efficiencies by Jones matrix method and BPM with respect to the pixel pitch are compared. It is shown that the diffraction effect is critical to the light efficiency. Using Jones matrix method may give rise to significant miscalculation. By using BPM, it is found possible to reduce the diffraction effect for certain waveband by slightly modifying the FOP cell structure, suggesting that the light efficiency can be boosted effectively in a two- or three-panel LCOS projection system.