标题: 反射式单晶矽液晶面板边际场效应及绕射效应之研究
Study on the Fringing-Field and Diffraction Effects of LCOS Panels
作者: 范姜冠旭
Kuan-Hsu Fan-Chiang
王淑霞
吴诗聪
Shu-Hsia Chen
Shin-Tson Wu
光电工程学系
关键字: 反射式单晶矽液晶元件;边际场效应;绕射效应;投影显示器;光束传播法;Liquid-crystal-on-silicon;Fringing-field effect;Diffraction effect;Projection display;Beam propagation method
公开日期: 2004
摘要: 近年来,反射式单晶矽液晶(LCOS)元件一直是显示产业持续注意的焦点,尤其是在投影显示器的应用方面,其产品包括前投式投影机、背投式电视以及头戴式虚拟显示器等。由于单晶矽的电子漂移率甚高,因此LCOS元件可以拥有非常高的解析度,除此之外,其周围的驱动电路也可以整合在单一的晶片上以减少生产成本。在制造方面,LCOS液晶面板乃建立在国内两大产业的基础上,即半导体与液晶显示器产业,因此,这对于国内发展LCOS不啻为一大利基。由于矽基板的制造是属于标准的半导体制程,所以具有低价格的潜力与优势,也因此许多厂商已纷纷投入LCOS投影显示器这个产业。
然而,目前LCOS显示技术仍然存在着许多困难有待克服。在面板方面,由于要在微小的面板上做出高解析度的影像,其像素以及像素间距也相对地变得非常小。当相邻像素的施加电压不同时,其边缘电场会被扭曲,使得此处的液晶分子产生不正常的排列,进而影响其光学特性,这就是所谓的边际场效应。
此外,当解析度的要求提高而使得像素电极的大小接近可见光的波长时,其对光波的作用类似于一反射光栅。当光入射在面板上时会产生明显的绕射效应,而斜向传播的绕射光可能无法进入光学系统,因而造成严重的光损失。
在本论文中,我们探讨八个常用液晶模态的边际场效应,并且改变液晶盒参数,研究其对LCOS面板光学表现的影响。我们发现混和式扭转向列型液晶模态的边际场效应相对较弱,而扭转向列型和垂直排列型液晶模态则会受到严重的边际场效应影响而降低其影像品质。特别是对于垂直排列型液晶模态,其边际场效应不但影响其静态显示的品质,同时也严重地拖慢了动态影像的切换速度。在液晶盒结构方面,根据电脑程式的模拟结果,我们也发现像素节距、液晶盒厚度、预倾角和电极斜率都是影响边际场效应的重要参数。
为了要设计一个高对比度且同时不受边际场效应影响的LCOS面板,我们针对拥有完美暗态的垂直排列型液晶盒做分析。根据电脑模拟出的液晶指向矢分布,我们发现利用圆偏振光的特性,可以有效地保留因边际场效应所损失的光效率进而提高影像的锐利度;同时,在动态响应上也解决了因缓慢切换过程而产生的影像模糊问题。相关的光学原理可以由着名的de Varies理论来解释。
在探讨LCOS面板的绕射效应方面,由于传统的琼斯矩阵法并无法分析光的绕射效应,因此我们将以往应用在光波导计算的光束传播法延伸到LCOS元件的光学计算,而撰写出有考虑绕射效应的光学模拟程式。利用此程式,我们针对垂直排列型液晶模态以及工研院电子所研发的FOP(finger-on-plane)模态进行分析。模拟结果显示绕射效应对高解析度的LCOS元件影响甚巨;使用传统琼斯矩阵法会造成严重的误差,唯有使用更严谨的光束传播法才能正确的预估其光学行为。此外,我们发现稍微修改FOP模态的液晶盒结构能有效地降低在特定波段的绕射效应,若配合使用三片或双片式的LCOS投影光学系统,则可以有效地提升因绕射效应所损失的光效率。
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.
URI: http://140.113.39.130/cdrfb3/record/nctu/#GT008924504
http://hdl.handle.net/11536/78280
显示于类别:Thesis


文件中的档案:

  1. 450401.pdf

If it is a zip file, please download the file and unzip it, then open index.html in a browser to view the full text content.