Journal of the

A preview of the papers appearing in the March 2007 issue of the Journal of the SID. To obtain access to these articles on-line, please go to

Edited by Aris Silzars

Amorphous-silicon thin-film transistors made at 280°C on clear-plastic substrates by interfacial stress engineering

Ke Long
I-Chun Cheng
Alexis Kattamis
Helena Gleskova
Sigurd Wagner
James C. Sturm

Princeton University

Abstract — A process temperature of ~300°C produces amorphous-silicon (a-Si) thin-film transistors (TFTs) with the best performance and long-term stability. Clear organic polymers (plastics) are the most versatile substrate materials for flexible displays. However, clear plastics with a glass-transition temperature (Tg) in excess of 300°C can have coefficients of thermal expansion (CTE) much larger than that of the silicon nitride (SiNx) and a-Si in TFTs deposited by plasma-enhanced chemical vapor deposition (PECVD). The difference in the CTE that may lead to cracking of the device films can limit the process temperature to well below that of the Tg of the plastic. A model of the mechanical interaction of the TFT stack and the plastic substrate, which provides design guidelines to avoid cracking during TFT fabrication, is presented. The fracture point is determined by a critical interfacial stress. The model was used to successfully fabricate a-Si TFTs on novel clear-plastic substrates with a maximum process temperature of up to 280°C. The TFTs made at high temperatures have higher mobility, lower leakage current, and higher stability than TFTs made on conventional low-Tg clear-plastic substrates.

Experiments show that the TFT films crack when the film stress per unit length exceeds a critical interfacial force. The film stress developed in the device stack can be reduced by controlling the built-in strain of the PECVD nitride into tension, by reducing the thickness of the device stack, and by using a substrate with a low coefficient of thermal expansion. The PECVD deposition power can be used to control the built-in strain in the SiNx film to compensate for the thermal mismatch strain.


FIGURE 9 — Cracks in device films deposited at 250°C on substrate A. Film thicknesses were: buffer nitride, 500 nm; gate chromium, 100 nm; gate nitride, 350 nm; intrinsic a-Si, 200 nm; n+ a-Si, 50 nm.

Transflective TFT-LCDs with high optical performance

Lin Lin
Bau-Jy Liang
Han-Chang Lin

Feng Chia University

Abstract — In order to reduce panel cost, reduce power consumption, and minimize thickness, a single panel with dual functions for high-transmissive main displays and high reflective sub-displays is becoming the trend. Two novel RGB-W transflective 1.9-in. a-Si TFT- LCDs have been developed to meet the requirements. By using the traditional seven-mask dual-cell-gap structure, novel transflective tRGB-t/rW TFT-LCD and tRGB-rW TFT-LCD panels were fabricated with high transmittance and high reflectance, respectively. The optical clarity is excellent in both dark and bright conditions. Their superior optical performance is attributed to the high-efficiency "transflective white" subpixel or "reflective white" subpixel.

The two novel displays consist of a simple dual-cell-gap structure. The first cell gap is 4.2 μm, located in the T region. Then, a thick layer of organic material, 2.0 μm, is partially coated on the T region as a foundation for the second cell gap. The organic layer is a flat and smooth configuration without any beam-steering bump on it. Subsequently, an aluminum alloy is placed on top of the flat organic layer to serve as the reflector. Finally, the second cell gap for the reflective region, at 2.2 μm thick and supported by a photo spacer, is completed for the reflective region. The top and lateral views are shown in Figs. 1(a) and 1(b), respectively.


FIGURE 1 — The structures of a 1.9-in. QCIF + T/R tRGB-t/rW LCD and a tRGB-rW TFT-LCD. (a) The structure of the 1.9-in. QCIF + T/R tRGB-t/rW TFT-LCD. (b) Structure of a 1.9-in. QCIF + T/R tRGB-rW TFT-LCD.

Pore-free mold-pattern-transfer method for barrier ribs of HD PDPs

Yoo-Seong Kim
Woong-Sik Kim
Hak-Nyun Choi
Tae- Geum Koh
Yong-Seog Kim

Hongik University

Abstract — Among various barrier-rib manufacturing processes, the mold-pattern-transfer method has potential to reduce processing cost as well as the manufacture of high-resolution pixels. In this study, the effects of major processing variables of the mold-pattern-transfer process on the formation of air-trapped pores within barrier ribs were examined. The results indicated that with an optimum combination of the processing variables, barrier ribs without trapped defects can be produced, demonstrating the possibility of reducing the number of processing steps and costs of barrier ribs.

One of the potential applications of the mold-pattern-transfer process is the manufacturing complicated barrier ribs such as ribs with embedded electrodes. Figure 7 shows the morphology of barrier ribs produced by the mold-pattern-transfer process. In this structure, barrier ribs have trenches where the Ag electrodes can be embedded. The trenches were filled with Ag paste via capillary action and capped with a dielectric layer.


FIGURE 7 — SEM micrographs of a cross-sectional view of counterelectrode discharge cells using the mold-pattern-transfer route.

A dual-gap RGBW transflective TFT-LCD with adjustable color gamut

Kuo-Yung Hung
Tun-Chun Yang
Chih-Chun Pei
Chih-Jen Hu
Chih-Ming Chang

Mingchi University of Technology

Abstract — An adjustable-color-gamut dual-gap RGBW transflective liquid-crystal display that uses a four-color manufacturing process and a color-processing algorithm to achieve the appropriate color performance in both the transmissive and reflective modes is presented. Based on superior-color-transformation units, the total brightness and color gamut can be modified under different ambience. The highest NTSC color gamut in the reflective mode (reflectance, 4.4%) that has been fabricated successfully for a RGBW 1.5-in. dual-gap panel is 23% with a 7%, 17%, and 40% NTSC color gamut in the transmissive mode by using different algorithms. Compared to a typical RGB panel, it not only provides flexibility for any environment but also satisfies a variety of personal requirements. Based on personal preference, users have more choices to adjust the LCD settings such as color saturation, brightness, etc. The smart RGBW TRLCD will definitely become the developing trend towards sunlight-readable LCDs in the near future.

A transflective panel comprised of RGBW subpixels with variable transmission and reflection areas have been developed in order to achieve both high transmittance and reflectance. The brightness of TRLCDs can be improved by increasing the contribution of the white subpixels therein. However, it is hoped that the color saturation will not be influenced. By appropriating modification of the image-processing algorithms, bright full-color images can be achieved. Moreover, image-processing algorithms are able to transfer adjusted image data according to user demands and variant ambient light. No comparable research has been provided for implanting a RGBW subpixel system into transflective displays until now.


FIGURE 6 — Different image qualities by different environment illumination and algorithm function in transmission mode. Figures 6(a)–(c) are images in 54-klux environment illumination by algorithm 1–3. Figures 6(d)–(f) are images in 545-klux environment illumination by algorithm 1–3. Figures 6(g)–(i) are images in 0.8-klux environment illumination by algorithm 1–3.

Retardation-controlling color filter with two directly stacked retardation layers

Kanami Ikegami
Tomoya Kawashima
Shinji Hayashi
Norihisa Moriya

Dai Nippon Printing Co.

Abstract — A polymerizable liquid crystal (PLC), the orientation of which can be frozen, is useful for making retardation layers. In this paper, a new color filter (CF) with retardation-controlling layers made of PLC is reported. It has a positive A-plate and a negative C-plate, both directly stacked on a color-filter layer. These two retardation layers exhibit good orientation ability, and function well as retarders, even when they are only 1/10 or less as thick as ordinary retardation films. The new CF also has excellent thermal stability. The change in retardation after heat treatment at 200°C for 30 min is around 5%, and there is no observable peeling. A prototype VA-LCD made with a new CF provides good optical compensation, with the light leakage being extremely low in all azimuthal directions. This technology is very useful for making thin, highly reliable color filters for LCDs, even with other modes.

Figure 1 illustrates the concept of a VA-LCD with our retardation-controlling CF. First, a positive A-plate is formed on a CF, and then a negative C-plate is formed on top of that. These two layers are both composed of PLC, with the A-plate having a uniform uniaxial orientation and the C-plate having a cholesteric orientation. Because the fabrication process is completely isotropic, these films have some advantages over conventional ones, such as no residual internal stress and no undesirable in-plane retardation for the negative C-plate.


FIGURE 1 — Structure of new VA-LCD with directly stacked A-plate, C-plate, and CF.

Software-processed edge- and level-adaptive overdrive (SELAO) method for high-quality motion pictures

Masahiro Baba
Goh Itoh
Haruhiko Okumura

Toshiba Corp.

Abstract — In this paper, a software-processed edge- and level-adaptive overdrive (SELAO) method, which is a novel overdrive technique that utilizes not only a temporal change of gray levels but also a spatial edge intensity of motion pictures, is proposed. The SELAO method is a software video-processing technology to improve motion-picture quality rendered on LCDs more than is possible with a conventional SLAO method without edge-adaptive overdrive, and it works in real time on commonly used personal computers (PCs).

In the SELAO method, by emphasizing the writing level to LCDs calculated from two types of coefficients, one of them is determined based upon the response characteristic of the LCD and the other is calculated based on the edge intensity of each pixel in the motion picture, the response time, and the subjective motion-picture quality of LCDs can be improved.


FIGURE 4 — Block diagram of the SELAO algorithm.

Design and implementation of a wavelet-based addressing technique (WAT)

Amachavadi R. Shashidhara
Temkar N. Ruckmongathan

Raman Research Institute

Abstract — The design and implementation of a wavelet-based addressing technique capable of displaying gray shades is presented. The hardware complexity of the display drivers has been reduced by adding a few analog multiplexers that are common to the drivers. The controller was implemented by using a low-cost complex programmable logic device (CPLD) and it was demonstrated by displaying 16 gray shades in a liquid-crystal display.

It is advantageous to use wavelets for displaying gray shades in passive-matrix liquid-crystal displays. Wavelets with energies that are proportional to the binary weight of bits of gray-shade data are used to deliver energies that correspond to the gray shade of the pixels. The hardware complexity of the data drivers is reduced as compared to that of the amplitude modulation. Flicker in the display can be avoided even when the refresh rate is less compared to that of the frame modulation, i.e., frame-rate control.


FIGURE 11 — Typical waveforms of the wavelet-based addressing technique, (a) row waveform, (b) column waveform, and (c) waveform across a pixel.

Effect of gloss and diffuse reflectance of display frames on visual comfort

Marie-Claude Béland
Börje Andrén

Acreo AB

Abstract — A series of 12 display frames of different combinations of gloss levels and diffuse reflectance levels was mounted on a display to test the visual comfort of 31 test subjects. The frames were tested in a controlled environment where a light reflex was purposefully visible on the frame. The intensity of the light reflex was controllable by the user, and the dial setting was recorded for each frame. The frames were also shown in two different test office environments, one light and one dark. Qualitative comments about the frames were recorded. A gloss level of 30 gloss units corresponded to about 30% of test subjects that wanted no reflex at all to be visible on the frame. There was a significant effect of frame gloss on the acceptable light level of the reflex. When the acceptable-light-level dial setting was converted to luminance contrast, the acceptable luminance contrast on the frames decreased with increasing gloss, and this effect was more pronounced for black frames.

A study of eye-care professionals found that the most-common complaint of display users was of glare. Glare is "the sensation produced by luminances within the visual field that are sufficiently greater than the luminance to which the eyes are adapted to cause annoyance, discomfort, or loss in visual performance and visibility." Our goal was to determine the levels of front-frame gloss and diffuse reflectance that are acceptable from a user's point of view, given different characteristics of color, surface structure, curvature, and width, as well as lighting in different workplace environments.


FIGURE 7 — Luminance contrast as a function of the gloss level of the frames. The bars show 95% confidence intervals. At low gloss levels, there is no significant difference between the black, gray, and white frames. As gloss increases, the acceptable luminance contrast for the black frames decreases faster than for the white and gray frames.