TFT/LCD: liquid-crystal displays addressed by thin-film transistors

172 Pages·1996·3.78 MB·English

by  Toshihisa Tsukada

TFT/LCD
Japanese Technology Reviews
Editor in Chief
Toshiaki Ikoma, University of Tokyo
Section Editors
Section A: Electronics
Toshiaki Ikoma, University of Tokyo

Section B: Computers and Communications
Tadao Saito, University of Tokyo
Kazumoto Iinuma, NEC Corporation, Kawasaki

Section C: New Materials
Hiroaki Yanagida, University of Tokyo
Noboru Ichinosé, Waseda University, Tokyo

Section D: Manufacturing Engineering
Fumio Harashima, University of Tokyo

Section E: Biotechnology
Isao Karube, University of Tokyo
Reiko Kuroda, University of Tokyo

Section A: Electronics
Volume 2
MMIC-Monolithic Microwave Integrated Circuits
Yasuo Mitsui
Volume 4
Bulk Crystal Growth Technology
Shin-ichi Akai, Keiichiro Fujita, Masamichi Yokogawa, Mikio Morioko and Kazuhisa Matsumoto
Volume 8
Semiconductor Heterostructure Devices
Masayuki Abe and Naoki Yokoyama
Volume 11
Development of Optical Fibers in Japan
Hiroshi Murata
Volume 12
High-Performance BiCMOS Technology and Its Applications to VLSIs
Ikuro Masuda and Hideo Maejima
Volume 13
Semiconductor Devices for Electronic Tuners
Seiichi Watanabe
Volume 19
Amorphous Silicon Solar Cells
Yukinori Kuwano
Volume 20
High Density Magnetic Recording for Home VTR: Heads and Media
Kazunori Ozawa
Volume 29
TFT/LCD
Toshihisa Tsukada
TFT/LCD
Liquid-Crystal Displays Addressed by Thin-Film Transistors
Toshihisa Tsukada
Hitachi Ltd.
Tokyo, Japan
Gordon and Breach Science Publishers
Australia • Canada • France • Germany • India • Japan • Luxembourg
Malaysia • The Netherlands • Russia • Singapore • Switzerland
Copyright © 1996 OPA (Overseas Publishers Association) N.V.
Published by license under the Gordon and Breach Science Publishers
imprint.
All rights reserved.
First published 1996
This edition published in the Taylor & Francis e-Library, 2003.
Second printing 2000
No part of this book may be reproduced or utilized in any form or by any means, electronic or mechanical, including
photocopying and recording, or by any information storage or retrieval system, without permission in writing from the
publisher. Printed in Malaysia.
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British Library Cataloguing in Publication Data
Tsukada, Toshihisa
TFT/LCD: Liquid-Crystal Displays
Addressed by Thin-film Transistors.—
(Japanese Technology Reviews. v. 29)
I. Title II. Series
621.38152
ISBN 0-203-41326-1 Master e-book ISBN
ISBN 0-203-41331-8 (MP PDA Format)
ISBN 2-919875-01-9 (Print Edition)
Copyright © 2002/2003 Mobipocket.com. All rights reserved.
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Contents
Preface to the Series
Preface
1. Introduction
2. TFT/LCD
2.1. TFT/LCD Configuration
2.2. Pixel Design
2.3. Design Analysis
2.3.1. Charging of pixel capacitance
2.3.2. Voltage offset
2.3.3. Gate delay
2.3.4. Colorimetric design
2.4. TFT Panel Fabrication
References
3. Thin-Film Transistors
3.1. Hydrogenated Amorphous Silicon Thin-Film Transistors
3.2. TFT Characteristics
3.2.1. Current-voltage characteristics
3.2.2. Parasitic effects
3.3. Threshold Voltage Shift
3.4. Process-Related Issues
3.4.1. Aluminum gate TFT
3.4.2. Self-alignment process
3.5. Computer Simulation of a-Si:H TFT
References
4. Hydrogenated Amorphous Silicon
4.1. Amorphous Semiconductors
4.2. Density of States of a-Si:H
4.3. Conductivity and Carrier Mobility
4.4. Absorption Spectra of a-Si:H
4.4.1. Optical absorption
4.4.2. IR absorption
References
5. Twisted-Nematic Cell
5.1. Threshold Voltage of TN Cell
5.2. C-V Characteristics of TN Cell
5.3. Optical Properties of TN Cell
5.4. Super-Twisted Nematic (STN) Cell
References
6. Liquid Crystal
6.1. Dielectric Permittivity and Refractive Index
6.2. Elastic Constants
6.3. Viscosity
References
General References
Index
Preface to the Series
Modern technology has a great impact on both industry and society. New technology is first created by pioneering
work in science. Eventually, a major industry is born, and it grows to have an impact on society in general.
International cooperation in science and technology is necessary and desirable as a matter of public policy. As
development progresses, international cooperation changes to international competition, and competition further
accelerates technological progress.
Japan is in a very competitive position relative to other developed countries in many high-technology fields. In some
fields, Japan is in a leading position: for example, manufacturing technology and micro-electronics, especially
semiconductor LSIs and optoelectronic devices. Japanese industries lead in the application of new materials such as
composites and fine ceramics, although many of these new materials were first developed in the United States and
Europe. The United States, Europe and Japan are working intensively, both competitively and cooperatively, on the
research and development of high-critical-temperature superconductors. Computers and communications are now a
combined field that plays a key role in the present and future of human society. In the next century, biotechnology will
grow, and it may become a major segment of industry. While Japan does not play a major role in all areas of
biotechnology, in some areas such as fermentation (the traditional technology for making sake), Japanese research is of
primary importance.
Today, tracking Japanese progress in high-technology areas is both a necessary and rewarding process. Japanese
academic institutions are very active; consequently, their results are published in scientific and technical journals and
are presented at numerous meetings where more than 20,000 technical papers are presented orally every year.
However, due principally to the language barrier, the results of academic research in Japan are not well-known
overseas. Many in the United States and in Europe are thus surprised by the sudden appearance of Japanese high-
technology products. The products are admired and enjoyed, but some are astonished at how suddenly these products
appear.
With the series Japanese Technology Reviews, we present state-of-the-art Japanese technology in five fields:
Electronics
Computers and Communications
New Materials
Manufacturing Engineering
Biotechnology
Each tract deals with one topic within each of these five fields and reviews both the present status and future prospects
of the technology, mainly as seen from the Japanese perspective. Each author is an outstanding scientist or engineer
actively engaged in relevant research and development.
The editors are confident that this series will not only give a deep insight into Japanese technology but will also be
useful for developing new technology of interest to our readers.
As editor-in-chief, I would like to sincerely thank the members of the editorial board and the authors for their
contributions to this series.
TOSHIAKI IKOMA
Preface
This is the first book describing in full detail liquid-crystal displays addressed by thin-film transistors (TFT/LCDs):
their design, fabrication, characteristics, material issues, as well as their basic operation. Since the fields relevant to
TFT/LCDs are quite diverse, the topics covered in this book are interdisciplinary. The two most important materials
discussed are amorphous semiconductors (a-Si:H) and liquid crystals. The unique features of TFT/LCDs arise from the
combination of these two materials. In this book, device characteristics and material issues are described in terms of
how they relate to display technology. Therefore, themes not directly related to TFT/LCDs but quite important in other
applications were left to the other excellent books listed below. In conventional books, the opening chapters deal with
basic studies and later chapters emphasize application. However, this book starts with an overview of the TFT/LCD,
including its design, fabrication and characteristics; then, the following chapters describe devices, materials and their
basic principles. This arrangement will make it easier for the reader to fully understand the contents of this book by
first explaining the overall subject matter.
This book will be useful to those who are engaged in the research and development of TFT/LCDs. It will be of great
interest especially to those who are working in the fields of consumer electronics and computer and communication
engineering, since the display quality of the man-machine interface is critical in these applications. Those who are
studying solid-state physics, mesophase chemistry, and electronic devices and systems, will also find information of
great interest.
I would like to express my gratitude for the support that I have received from Hitachi Ltd. and from my many
colleagues at the Central Research Laboratory, Hitachi Research Laboratory, and Mobara Works of Hitachi Ltd.
Particular thanks go to E.Kawamata who helped to prepare the manuscript of this book.
T.Tsukada
Kokubunji 1994
CHAPTER 1
Introduction
The TFT/LCD, an abbreviation of thin-film-transistor-addressed liquid-crystal display, is a flat-panel display in which
the display medium is liquid-crystal and each picture element (pixel) is controlled by a thin-film transistor. The
TFT/LCD is creating a whole new world of technology in consumer electronics and in computer and communication
systems. The market for TFT/LCDs is now growing much faster than expected and impacting new application fields,
as well as conventional fields.
The concept of the TFT/LCD is not new, but rather old. As early as 1966, Weimer 1 mentioned the possibility of using
TFTs as display switches. A more detailed concept was described by Lechner et al. 2 in 1971, where the use of diodes
or triodes (transistors) was discussed as switches for active-matrix liquid-crystal displays. The use of storage
capacitors implemented in parallel with the liquid-crystal cell capacitor was also mentioned. Preceding this discussion,
Heilmeier et al. 3 proposed nematic liquid-crystal as a material for a flat-panel display. A sandwich cell consisting of a
transparent front electrode, a reflecting back electrode, and nematic liquid-crystal in between, was prepared (reflective
mode). When there was no applied field, the cell appeared black. When a dc voltage was applied, the liquid-crystal
became turbulent and scattered light: the cell appeared white. This phenomenon was termed "dynamic scattering", and
was applied to demonstrate the first liquid-crystal display both in reflective and transmissive mode operations. A
3.5"×4" alphanumeric display was fabricated and a maximum contrast ratio higher than 20 to 1, was demonstrated.
Brody et al. 4 applied the CdSe TFT to the active-matrix liquid-crystal panel. This display panel consisted of 14,000
transistors, storage capacitors and the twisted-nematic (TN) liquid-crystal cell. Although TFTs were made of CdSe
rather than a-Si:H, the configuration is essentially the same as today's TFT/LCD panels. The TN liquid-crystal cell was
first proposed by Schadt and Helfrich 5 and featured low-voltage operation, low power consumption, and fast response
time. In the TN cell, the average direction (director) of the liquid-crystal molecules is twisted 90° as they go from the
back to the front glass substrate. The polarization of the light is also rotated 90° as the light passes though the liquid-
crystal cell. When the front polarizer is set parallel to the rear polarizer, the light is not transmitted as there is no
applied voltage. When the ac voltage is applied to the cell, the director of liquid-crystal molecules becomes
perpendicular to the substrate. In this state, the TN cell becomes optically inactive and linearly polarized light travels
through the cell without any rotation of polarization. Therefore, the light is transmitted through the front polarizer.
Since its development, the TN cell has played a very important role as the display medium for TFT/LCDs.
Hydrogenated amorphous silicon (a-Si:H) was a late arrival in TFT technologies. However, it had a great influence in
achieving practical TFT/LCDs. Since the first report by the Dundee group 6 , a-Si:H TFT has been recognized as the
most suitable device for TFT/LCDs. The mobility of a-Si:H has proved to be just enough to charge liquid-crystal
capacitance and storage capacitor. The off-current of a-Si:H TFT has turned out to be on a sufficiently low level not to
discharge the pixel capacitance during the frame time, owing to the high resistivity of undoped a-Si:H. Another
important feature of a-Si:H is that it can be deposited over a large area. Uniformity over the area, and reproducibility
from run to run can be obtained relatively easily. Also important is that it can be deposited at low temperatures, so
glass substrates can be used. Moreover, good interface properties between a-Si:H and other thin films, like metals,
insulators, and semiconductors can be obtained. Due to these properties, the hydrogenated amorphous silicon TFT has
acquired wide acceptance among the many candidates to make TFT/LCDs.
As for liquid-crystal, more than one hundred years have passed since its discovery. In 1888, an Austrian botanist, F.
Reinitzer reported that cholesteric benzoate which he had purified, showed a strong birefringence at a temperature
range between 145.5°C and 178.5°C. He sent the sample, which became a turbulent liquid in this temperature range, to
a German physicist O.Lehmann. Lehmann found that this material showed an optical anisotropy when observed under
a crossed Nikol and coined the term "liquid-crystal" for the first time. It took a long (eighty-year) elapse of time after
this discovery until the concept of liquid-crystals appeared as a medium for displays. Even after this, about twenty
years were necessary to develop the practical TFT/LCD. However, we are now at the stage to benefit from this long
history of research and development, which will be described in the following chapters.
References
1. Weimer, P.K. (1966). Thin film transistors. In Field Effect Transistors , edited by J.T.Wallmark and H.Johnson.
New Jersey: Prentice Hall.
2. Lechner, B.J., Marlowe, F.J., Nester, E.O., and Tults, J. (1971). liquid-crystal matrix displays. Proc. IEEE , 59,
1566–1579.
3. Heilmeier, G.H., Zanoni, L.A., and Barton, L.A. (1968). Dynamic scattering: a new electrooptic effect in certain
classes of nematic liquid-crystals. Proc. IEEE , 56, 1162–1171.
4. Brody, T.P., Asars, J.A., and Dixon, G.D. (1973). A 6×6 inch 20 lines-per-inch liquid-crystal display panel. IEEE
Trans. Electron Devices , ED-20, 995–1001.
5. Schadt, M. and Helfrich, W. (1971). Voltage-dependent optical activity of a twisted nematic liquid-crystal. Applied
Physics Letters , 18, 127–128.
6. LeComber, P.G., Spear, W.E., and Gaith, A. (1979). Amorphous-silicon field-effect device and possible application.
Electronics Letters , 15, 179–181.
CHAPTER 2
TFT/LCD
The thin-film-transistor-addressed liquid-crystal display (TFT/LCD) is a flat-panel display which is used in a variety
of products, including consumer electronics, computers, and communication terminals. However, the application of
TFT/LCD to such products is only a recent development. For more than forty years, cathode-ray tubes (CRTs) have
dominated the display world. The CRT's most dramatic success has been in monochrome and then color TVs. With the
expansion of the computer industry, computer terminal displays have also provided a large market for CRTs. As a
result, the word "display" has become almost synonymous with "CRT".
However, the situation is now changing rapidly. Developments in transistors and integrated circuits have made it
possible to greatly reduce the size of computers due to improvements in the performance of miroprocessors,
semiconductor memories, and other devices. As a result, main-frame computers have given way to minicomputers, to
workstations, and to personal computers. This evolution hastened the arrival of flat-panel displays, including
TFT/LCDs. Not only do these displays avoid the curvature normally associated with CRTs, they are also lightweight
and consume little power. These features make them ideal for use in modern electronic systems.
TFT/LCDs are generally characterized by the diagonal length of the panel and their resolution. Figure 2.1 shows the
correspondence between display area and the number of pixels (picture elements). The numerical examples shown in
this figure correspond to actual video-data terminals. In computer display applications each pixel generally consists of
three colored stripes (one red, one green, and one blue). The inserts in this figure show the relative sizes of this
configuration, assuming the pixels are square. A resolution of 640 (horizontal) by 480 (vertical) pixels corresponds to a
video-graphic-array (VGA) monitor, and if a pixel size of 0.33 mm square is assumed, diagonal of VGA monitors
becomes 26.4 cm or 10.4". In panels with a resolution of
Figure 2.1 Number of pixels vs. area of flat-panel displays. The number of pixels in rows and columns shown in this
figure by H and V correspond to computer-terminal-display applications. At each resolution, an example is shown for
the panel and pixel size for an assumed substrate size of 460×360 mm2.
1,280 (H) by 1,024 (V), the number of pixels exceeds one million, and there are more than three million RGB dots or
subpixels. Thus, more than three million TFTs must be fabricated on these panels.
2.1. TFT/LCD Configuration
The basic configuration of a TFT/LCD is shown in Figure 2.2. Liquid-crystal is encapsulated between two glass
substrates, a TFT substrate and a color-filter substrate. The color-filter substrate is also called the common electrode
substrate. The transparent common electrode on this substrate is made of ITO (Indium Tin Oxide), and is deposited on
top of the color filter. In order to obtain good display quality, the cell gap of the liquid-crystal (i.e., the spacing
between the two glass substrates) has to be precisely controlled to a specific value, e.g., 5 um. This gap has to be
uniform over the whole display area and reproducible from
Figure 2.2 Configuration of TFT/LCD. The TFT and the color-filter substrates are two parallel sheets of glass with
liquid crystal injected between them. A crossed-polarizer system is shown here, corresponding to a normally-white
display.
run to run. Therefore, transparent spacers such as plastic beads are placed on the surface of the glass substrate.
The liquid-crystal cells are twisted-nematic type (see Chapter 5) in which the director (orientation) of the liquid-
crystal molecules is twisted 90° between the TFT substrate and the common electrode substrate. In Figure 2.2, the
crossed-polarizer system is shown, in which the first polarizer works as a backlight polarizer and the other acts as an
analyzer. In this system, light passes through the analyzer when there is no applied voltage on the cell, and is blocked
when the applied voltage is high enough to align the liquid-crystal molecules vertically. The liquid-crystal is anchored
on the surface of the glass substrates so that its molecules are oriented to a proper direction. In order to set the
anchoring direction, the glass substrate is coated with an organic film such as a polyimide film and the surface of the
film is rubbed with a fabric in a specific direction. The liquid-crystal molecules are tilted several degrees with respect
to the glass surface. This tilt angle is called the pretilt angle and plays an important role in determining the electrical