Active Matrix
`Liquid Crystal
`Displays
`Fundamentals and
`Applications
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`Contents
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`Preface ...................................................... xi
`Chapter One: Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
`1.1 Historical Perspective .......................................... 1
`1.2 Liquid Crystal Properties ........................................ 7
`1.3 Polarization, Dichroism, and Birefringence ......................... 11
`1.4 The Twisted Nematic Cell ..................................... 14
`1.5 Limitations of Passive Matrix Addressing .......................... 17
`References .................................................. 21
`
`Chapter Two: Operating Principles of Active Matrix LCDs ..................... 23
`2.1 The Case for Active Matrix .................................... 23
`2.2 Requirements for Active Matrix Switching Devices .................. 24
`2.3 The Thin Film Transistor ...................................... 29
`2.4 Thin Film Silicon Properties .................................... 32
`2.5 Amorphous Silicon TFTs ...................................... 34
`2.6 Poly-Silicon TFTs ............................................ 36
`2.7 Basic Pixel Circuit and Addressing Methods ........................ 39
`2.8 Diode-Based Displays ......................................... 43
`2.9 Plasma-Addressed LCDs ....................................... 47
`References .................................................. 48
`Chapter Three: Manufacturing of AMLCDs . .......................... 49
`3.1 Basic Structure of AMLCDs .................................... 49
`3.2 Thin Film Processing .......................................... 50
`3.3 Thin Film Properties .......................................... 61
`3.4 Amorphous Silicon TFT Array Processes .......................... 65
`3.5 Poly-Si TFT Array Processes .................................... 69
`3.6 Color Filter Array Process ...................................... 73
`3.7 LC Cell Assembly ............................................ 74
`3.8 Module Assembly ............................................ 77
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`3.9 Yield Improvements and Considerations .......................... 79
`3.10 Trends in Manufacturing ...................................... 83
`
`Chapter Four: AMLCD Electronics ................................. 87
`4.1 Drive Methods ............................................. 87
`4.2 Row Select and Column Data Drivers ............................ 93
`4.3 Timing Controllers, Display Controllers, and Interfaces .............. 99
`4.4
`Integration of Electronics on Glass ............................. 102
`4.5 Backlights ................................................ 105
`4.6 Power Consumption ........................................ 109
`References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110
`Chapter Five: Performance Characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . 113
`5.1 Basics of Photometry and Colorimetry ........................... 113
`5.2 Brightness and Contrast Ratio ................................. 117
`5 .3 Viewing Angle Behavior . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121
`5 .4 Color and Gray Scale Performance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123
`5.5 Response Time and Flicker ................................... 129
`5.6 Resolution and Size ......................................... 131
`5.7
`Image Artifacts ............................................ 134
`References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 7
`Chapter Six: Improvement of Image Quality in AMLCDs ................ 139
`6.1 Brightness Improvements .................................... 139
`6.1.1
`Increased Color Filter Transmission ....................... 140
`6.1.2 High-Aperture Ratio Designs ............................ 140
`6.1.3 Alternative Color Filter Arrangements .................... 144
`6.1.4 Brightness Enhancement Films .......................... 145
`6.2 Readability Under High Ambient Lighting Conditions .............. 146
`6.3 Color Gamut Improvements .................................. 149
`6.4 Wide Viewing Angle Technologies ............................. 150
`6.4.1 Compensation Films .................................. 151
`6.4.2
`In-Plane-Switching Mode .............................. 152
`6.4.3 Vertical Alignment ................................... 157
`6.4.4 A Comparison and Other Viewing Angle
`Improvement Methods ................................ 162
`6.5 Enhancement of Video Performance . . . . . . . . . . . . . . . . . . . . . . . . . . . . 166
`6.5.1 Response Time Compensation ........................... 167
`6.5.2 Emulation of an Impulse-Type Display ..................... 169
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`6.6 Large Size ................................................. 172
`References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 176
`Chapter Seven: Special AMLCD Configurations . ...................... 179
`7 .1 Ultra-High-Resolution Monitors and Improved Gray Scale ............ 179
`7.2 Reflective and Transflective Displays ............................. 182
`7.3 Field-Sequential Color LCDs .................................. 185
`7.4 Stereoscopic AMLCDs ....................................... 187
`7.5 Touch Screen Technologies .................................... 190
`References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 196
`
`Chapter Eight: Alternative Flat Panel Display Technologies . .............. 197
`8.1 Plasma Displays ............................................. 199
`8.2 Electroluminescent Displays ................................... 201
`8.2.1 TFEL Displays ........................................ 202
`8.2.2 Organic LED Displays .................................. 203
`8.2.3 Passive Matrix Organic LED Displays ...................... 205
`8.2.4 Active Matrix Organic LED Displays ....................... 206
`8.3 Electronic Paper and Flexible Displays ........................... 209
`8.4 Organic Thin Film Transistors .................................. 213
`8.5 Front and Rear Projection Displays .............................. 214
`References ................................................. 2 21
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`Chapter Nine: Active Matrix Flat Panel Image Sensors .................. 223
`9.1 Flat Panel Image Sensors ...................................... 223
`9.2 Direct Conversion Detectors ................................... 225
`9.3
`Indirect Conversion Detectors ................................. 228
`9.4 Applications of Flat Panel X-Ray Sensors ......................... 233
`References ................................................. 234
`Index . ..................................................... 235
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`ix
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`Preface
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`The meteoric rise of the active matrix LCD over less than two decades to undisputed
`dominance as a flat panel display has been breathtaking. The technology behind this
`remarkable progress will be summarized in this book. Manufacturing of AMLCDs is a
`more than $40 billion industry and now plays an important role in the economy of
`several Asian countries. This book will address the fundamentals of LCD operation and
`the principles of active matrix addressing. The reader will become familiar with the
`construction and manufacturing methods of AMLCDs, as well as with drive methods;
`performance characteristics; recent improvements in image quality; and applications in
`cellular phones, portable computers, desktop monitors, and LCD televisions.
`
`This book is on an introductory level and intended for students, engineers, managers,
`educators, IP lawyers, research scientists, and technical professionals. Emphasis has been
`placed on explaining underlying principles in the simplest possible way without relying
`extensively on equations. Last, but not least, the book is intended for those inquiring
`minds who, for many hours every day, look at the LCD displays of notebook computers,
`flat panel monitors, and televisions and simply wonder how they work.
`
`This book grew ouc of the course materials from seminars I presented several times at
`UCLA, at the gracious invitation of course organizer Larry Tannas. The course materials
`were further updated when the Society for Information Display kindly invited me to
`present Short Courses on AMLCDs at the annual SID Symposiums in 2002 and 2003.
`
`A book like this would not have been possible without frequent interaction and
`discussions with colleagues in the field. Working with them has always been a great
`pleasure and I would like to mention specifically some of my former coworkers at OIS
`Optical Imaging Systems, Inc. and at Planar Systems, Inc. They include Adi Abileah,
`Steve Aggas, Tom Baker, Yair Baron, Bill Barrow, Mike Boyd, Young Byun, Vin Cannella,
`Mark Friends, Pat Green, Tieer Gu, Chris King, Terrance Larsson, Alan Lien, Darrin
`Lowe, Yiwei Lu, Fan Luo, Tin Nguyen, Cheng~bin Qiu, Scott Robinson, Scott Smith,
`Scott Thomsen, Dick Tuenge, Victor Veerasamy, Mimi Wang, Moshe Yang, Mei Yen,
`Zvi Yaniv, and John Zhong.
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`Finally, I would like to thank the Elsevier Science organization for the opportunity to
`write this book and for their support during its production.
`
`Willem den Boer
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`CHAPTER 1
`
`Introduction
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`1.1 Historical Perspective
`
`Electronic displays have, for many years, been the window to the world in television as
`well as the primary human interface to computers. In today's information society, they
`play an increasingly important and indispensable role in communication, computing, and
`entertainment devices.
`
`The venerable cathode ray tube ( CRT) has been around for more than 100 years and has
`been the workhorse for television displays and, until recently, for computer screens. As
`one of the few surviving electronic devices based on vacuum tubes, the CRT can boast
`an unrivaled success as a low-cost color display with good image quality. Its large depth,
`weight, and power consumption, however, have limited its use to nonportable
`applications.
`
`From the early days of electronic display development, a flat panel display was considered
`a very attractive alternative to the bulky CRT. For decades, display engineers searched for
`flat panel display technologies to replace the CRT in many applications. In spite of many
`attempts to develop flat CRTs, plasma displays, and other low-profile displays,
`commercial success remained elusive for many years. Finally, by the 1990s several
`technologies were making significant inroads to achieve this goal. In particular, active
`matrix liquid crystal displays (AMLCDs) and plasma displays demonstrated large sizes
`and high image quality comparable to CRTs.
`
`The success of AMLCDs, the subject of this book, is the culmination of two significant
`developments: liquid crystal cell technology and large-area microelectronics on glass.
`For more than two decades these technologies have been refined and an extensive
`infrastructure for manufacturing equipment and materials has been established, especially
`in Asia.
`
`Liquid crystals were discovered in 1888 by the Austrian botanist Friedrich Reinitzer. He
`experimented with the cholesterol-type organic fluid cholesteryl benzoate and found that,
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`upon heating, it underwent a phase transition from a milky fluid to a mostly transparent
`fluid. This was later explained as a transition from an optically and electrically
`anisotropic fluid to an isotropic fluid. The anisotropy (i.e., the difference in dielectric
`constant and refractive index for different orientations of the molecules in the fluid) led
`to the analogy with the anisotropy of solid crystals, hence the name liquid crystal. The
`technological and commercial potential of liquid crystal was not realized until the 1960s,
`when RCA developed the first liquid crystal displays (LCDs) based on the dynamic
`scattering effect [1]. The twisted nematic (TN) mode of operation, on which many
`current LCDs are based, was first described by Schadt and Helfrich [2] in 1971 and,
`independently around the same time, by Fergason [3]. Twisted nematic LCDs appeared
`on the scene in the early 1970s in electronic wrist watches and in calculators.
`
`LCDs quickly dominated in small portable applications due to the compatibility of the
`simple reflective,type LCD with low,power CMOS driving circuitry and therefore with
`battery operation. In addition, high,volume manufacturing led to very low cost. The
`market for small, direct,driven, segmented, TN LCDs in portable devices increased
`rapidly during the 1970s. They were initially mostly used in a reflective mode, relying on
`ambient light for legibility. Since each segment in a direct,driven alphanumeric display
`needs to be separately connected to the control electronics, the information content of
`this type of display is very limited, usually to one or two lines of text or numbers. Other
`drawbacks of the reflective mode included the difficulty of implementing color, the
`dependence on ambient lighting, and the parallax caused by the separation of about
`0.5-1 mm between the back reflector and the LC layer.
`
`The mass market for electronic wrist watches, calculators, and other applications,
`however, allowed investments in manufacturing and further development.
`
`For displays with higher information content, the large number of picture elements
`(pixels) precluded the individual addressing of every pixel. This led to the development
`of matrix addressing in which an array of M X N pixels is addressed by applying pulses to
`each of its M rows and N columns. It reduces the number of interconnects to the
`external addressing circuitry from M x N to M + N. For example, a 100 x 100,pixel display
`now required 100 + 100 = 200 interconnections instead of 100 x 100 = 10,000. Such
`passive matrix displays are usually operated with a one,line,at,a,time addressing method
`called multiplexing. The TN LCD was limited to only about 10 rows of pixels because of
`its gradual transmission,voltage curve. Many improvements in passive matrix addressing
`have been proposed and implemented over the last fifteen years, notably the super,
`twisted nematic (STN) LCD. However, their performance in terms of viewing angle,
`response time, gray scale, and contrast ratio has generally fallen short of what was
`possible with a single direct,driven pixel.
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`Introduction
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`Early during the development of LCD technology, the limitations of direct multiplexing
`or passive matrix addressing were recognized. A solution was proposed by Lechner et al.
`[4] and by Marlowe and Nester [5]. By incorporating a switch at each pixel in a matrix
`display, the voltage across each pixel could be controlled independently. The same high,
`contrast ratio of more than 200, obtained in simple, direct,driven backlit displays, could
`then also be achieved in high,information,content displays.
`
`Peter Brody and coworkers [6] constructed the first so,called active matrix LCDs
`(AMLCDs) with CdSe thin film transistors (TFTs) as the switching elements. The TFTs
`in the array act merely as ON/OFF switches and do not have an amplifying function. For
`an electronic engineer, the term "active matrix" may therefore be inappropriate. It is
`now, however, commonly used and its definition has been extended to include arrays of
`switching elements other than TFTs, such as diodes.
`The CdSe TFTs used for the first AMLCDs turned out to be a temporary solution.
`Semiconductors such as CdSe are not compatible with standard processing in the
`microelectronic industry, which uses mainly silicon as the semiconductor material.
`Advanced photolithographic and etching processes have been developed over the years
`for silicon devices and this technology is not readily applicable to CdSe TFTs. Dr. Brody
`has, nonetheless, remained a strong and vocal proponent of CdSe,based AMLCDs
`over the years, even in the face of the overwhelming success of silicon thin film,based
`TFTLCDs.
`
`Polycrystalline Si materials and devices are more familiar to semiconductor process
`engineers and were developed for use in AMLCDs in the early 1980s. The first LC
`pocket television marketed by Seiko Epson in 1983 used a poly,Si TFT active matrix [7]
`and was the very first commercial application of AMLCDs. The early poly,Si TFTs
`required high,temperature processing and therefore used expensive quartz substrates.
`
`A color LCD was obtained by subdividing the pixel into three subpixels with red, green,
`and blue color filters. Since the color filters absorb a large portion of the light, these
`color LCDs required a backlight to operate in a transmissive rather than a reflective
`mode to be useful in most ambient lighting conditions.
`In parallel to the early development of LC cell technology and CdSe TFTs, thin film
`amorphous silicon (a,Si) was investigated in the 1970s. The rationale behind this
`interest was initially not its potential use for LCDs, but rather its promise for low,cost
`solar cells. A major development occurred at the University of Dundee in Scotland in
`1979, when LeComber et al [8] developed the first TFT with a,Si as the semiconductor
`material and suggested the active matrix LCD as one of its applications. Interestingly, a
`patent on the basic a,Si TFT was never filed, since this work was performed at an
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`academic institution. The University of Dundee had been a pioneer in the development
`and understanding of amorphous silicon materials under the leadership of Professor W.E.
`Spear and Dr. P.O. LeComber. Unfortunately, Dr. LeComber witnessed only the start of
`the tremendous growth in AMLCDs based on a-Si TFTs; he died at the age of 51 of a
`fatal heart attack while vacationing in Switzerland in 1992.
`
`Amorphous silicon can be deposited on low-cost, large-area glass substrates at a
`temperature below 300°C. It was more attractive than the early polycrystalline
`technology for active matrix LCDs, which needed much higher process temperatures and
`more process steps. A pocket television with a-Si TFTs was put on the market by
`Matsushita in 1984 [9].
`
`Soon after the first TFT LCD with amorphous silicon TFTs was introduced, the a-Si TFT
`overshadowed poly-Si TFTs as the semiconductor device of choice for AMLCDs. The
`first TFT LCDs had a diagonal size of 2-3 in. and were mainly used in small portable
`televisions. The performance of the color AMLCD was initially improved for high-end
`applications such as aircraft cockpit displays. In avionics, cost was a secondary concern
`and emphasis was placed on the highest possible performance in terms of legibility under
`any lighting conditions.
`
`While a market was established, volume production capability was gradually increased at
`several Japanese companies. This was accompanied by a scale-up in glass substrate size
`from wafer-like sizes to around 300x400 mm. A significant breakthrough occurred in the
`late 1980s, when the first laptop computers with 10-in. diagonal size TFT LCDs were
`marketed by several companies, including IBM, NEC, Sharp, Toshiba, and Hitachi. This
`scale-up was made possible by the gradual increase in manufacturing yields by process
`improvement methods borrowed from the semiconductor industry. Laptop and notebook
`computers turned out to be the killer application for active matrix liquid crystal displays.
`They addressed the need of traveling businessmen for lightweight computers with high
`image quality, and thin and low-power flat panel displays.
`
`By 1996 the manufacturing substrate size had grown to 550x650 mm and many
`improvements in processing and materials were implemented. In the mid-1990s Korean
`companies started mass production of AMLCD modules, followed a few years later by
`massive investments by several companies in Taiwan.
`
`The late 1990s also witnessed a revival of poly-Si TFT LCDs for small displays. Several
`companies succeeded in producing low-temperature poly-Si TFTs processed at
`temperatures below 600°C, compatible with lower-cost glass. The main attraction of
`poly-Si TFTs is their higher current-carrying capability, allowing the integration of some
`of the drive electronics on the glass.
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`Introduction
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`Application of a-Si TIT LCDs in notebook computers facilitated large investments in
`their manufacturing infrastructure. This, in tum, made possible the introduction of space(cid:173)
`and power-saving, flat-panel desktop monitors based on AMLCD panels with improved
`viewing angles.
`
`Several methods to improve the viewing angle, including compensation films and
`different LC modes ( in-plane-switching and multi-domain vertical alignment) were
`introduced after 1995 and implemented in 17-in. and larger monitors. In 2003 TIT
`LCDs surpassed CRTs in terms of revenue for computer monitors.
`
`Recent further improvements in brightness, color performance, viewing angle, and
`response time have led to the development of LCD television with superior image quality
`and progressively larger screens, now well beyond 40 in. in diagonal size. LCD television
`is the final frontier for the AMLCD and a number of companies have started production
`on glass substrates with 1-4 m2 size to participate in this rapidly growing market.
`
`Another application where AMLCDs have attained a large market share is handheld
`devices (i.e., in PDAs, digital cameras, camcorders, and mobile phones). With the
`introduction of 3G cellular phone service and built-in cameras, the demand for high(cid:173)
`contrast, video-rate color displays have allowed the AMLCD to replace many of the
`poorer-performing passive matrix LCDs.
`
`Parallel to the development of direct-view displays, microdisplays for projection have
`been developed since the late 1970s. It was realized early on that the LCD is a light valve
`that can act as an electronically controlled slide in a slide projector. Business-grade front
`projectors appeared on the scene in the early 1990s with three high-resolution poly-Si
`TFf LCDs. They are now commonplace in many meeting rooms and classrooms across
`the world and can weigh less than 3 lbs. Rear-projection, high-definition television based
`on reflective microdisplays (liquid crystal on silicon [LCOS]) have entered the
`marketplace as well.
`
`Microdisplays are also used in personal viewers such as viewfinders for digital cameras
`and camcorders.
`
`The success of AMLCD technology is the result of many years of close cooperation
`among scientists and engineers from different disciplines. They include organic chemists,
`physicists, optical, electrical, electronic, mechanical, packaging, and manufacturing
`engineers, all supported by increasing revenues from sales of LCDs.
`
`Figure 1.1 illustrates the exponential increase in the total market for AMLCDs. The
`development shows a remarkable parallel with the semiconductor industry in the 1980s
`and 1990s, also indicated in the plot. Some fluctuations in the LCD revenue curve can
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`Introduction
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`The AMLCD manufacturers are supported by a large infrastructure of equipment and
`material suppliers, which continue to improve efficiency and reduce cost. They include
`color filter, polarizer, and optical film manufacturers, driver IC and controller IC vendors,
`packaging firms, backlight manufacturers, and suppliers of materials such as LC fluids and
`alignment layers.
`
`The momentum the LCD industry has gained is difficult to supplant by alternative flat
`panel display technologies, even if, on paper, they may have advantages over LCDs.
`
`1.2 Liquid Crystal Properties
`After their discovery by Reinitzer in 1888, liquid crystals were, for many decades,
`considered interesting only from an academic point of view. Liquid crystals are an
`intermediate phase between crystalline solids and isotropic liquids, and combine certain
`characteristic properties of the crystal structure with those of a deformable fluid. Display
`devices utilize both their fluidity and the anisotropy associated with their crystalline
`character. The anisotropy causes the dielectric constant and refractive index of the LC
`fluid to depend on the orientation of its molecules.
`
`Nematic liquid crystals are commercially the most interesting type. Upon heating, most
`crystalline solids undergo a phase transition to an isotropic liquid. The nematic
`intermediate phase (mesophase) occurs in certain, mostly organic, substances in a
`temperature range between the solid and isotropic liquid state. Figure 1.3 shows an
`example of the molecular structure of a liquid crystal, p,methoxybenzylidene,p,n,
`butylaniline (MBBA), with its nematic temperature range.
`
`The liquid crystal (LC) molecules are generally elongated in shape and have a length of
`around 2 nm. Because of their "cigar" shape they tend to line up more or less parallel to
`each other in the lowest energy state. The average orientation axis along the molecules is
`a unit vector n, called the director. Nematic LC molecules are not polar, so there is no
`differentiation between n and -n. The dielectric constant and refractive index of the LC
`is different along the director and perpendicular to the director, as shown in Fig. 1.4,
`giving rise to dielectric and optical anisotropy, respectively. The dielectric anisotropy
`
`~N - -o -C4H9
`
`0
`H
`C /~ '
`3
`Figure 1.3: Example of molecular structure
`of LC - MBBA, with a nematic range of
`21-48°C.
`
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`Active Matrix Liquid Crystal Displays
`
`n
`
`~~~d~~
`~ c=:>~c:::=:> \\?=--~
`~?7
`
`c:::=::x=::::>
`
`,c:::::;;,
`
`(8)
`(A)
`(C)
`Figure 1.4: Orientation of LC molecules in (A)
`smectic, (B) nematic, and (C) isotropic phase.
`
`makes it possible to change the orientation of the LC molecules in an electric field,
`crucial for application in electro-optical devices. The optical anisotropy leads to
`birefringence effects (described later in this chapter) and is essential for the modulation
`of polarized light in display operation.
`
`In bulk, liquid crystal tends to form microdroplets. Within the droplets there is one
`director orientation, but the director can be different for adjacent droplets. Although
`most high-purity liquid crystals are transparent, this explains the milky appearance of
`bulk LC, as scattering of light occurs at the boundaries of the microdroplets.
`
`When nematic LC is heated beyond a certain temperature, a phase transition occurs to
`an isotropic liquid. The nematic-isotropic transition temperature is often referred to as
`the clearing point or clearing temperature because of the drastic reduction in light
`scattering that occurs when the fluid no longer consists of microdroplets with different
`directors. Just below the clearing point, the optical and dielectric anisotropy of the LC
`fluid starts to decline, until at the clearing point there is a single dielectric constant and
`refractive index in the isotropic state. Above the clearing point, the LC fluid no longer
`has the desirable optical and dielectric anisotropy, and display operation fails.
`
`When the temperature is lowered, the LC fluid undergoes another phase transition from
`the nematic to the smectic phase. In the smectic phase, the LC molecules form a layered
`structure and obtain a higher viscosity to become more grease-like. When approaching
`the smectic phase, the LC response to an electric field change becomes very sluggish, and
`below the transition temperature the display operation fails.
`
`A large variety of nematic liquid crystals has been synthesized over the years, each with
`its own molecular formula and nematic temperature range. For MBBA in the example of
`Fig. 1.3, the nematic range is 21-48°C. This range is too limited for practical
`applications. Commercial fluids generally consist of a mixture of two or more liquid
`crystal components and have a much wider nematic range.
`
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`(1.2)
`
`T = T0 cos2¢,
`where TO is the intensity of transmitted light through the first polarizer.
`For crossed ideal polarizers, <I> = 90° and transmission is zero. For parallel ideal polarizers,
`<I> = 0° and transmission is 50%.
`Practical polarizers do not follow Eqs. 1.1 and 1.2 exactly. Polarizers used in LCDs
`usually consist of stretched polymer films, such as polyvinylacetate (PVA), doped with
`iodine or other specific additives. The stretching process makes the polymer optically
`anisotropic. The iodine doping causes strong absorption of incident light along the
`optical axis, so that only one linearly polarized component of the incident light is
`transmitted.
`
`The optical absorption along the optical axis is referred to as dichroism.
`
`Polarizers are characterized by their transmittance and degree of polarization (also called
`polarization or polarizing efficiency).
`The polarization efficiency Eff is defined as
`
`Eff= jT1-T2
`T1+T2'
`where T 1 is the transmission of two parallel polarizers and T2 the transmission of two
`crossed polarizers.
`
`( 1.3)
`
`For LCDs using two polarizers, one outside each of its glass substrates, the maximum
`contrast ratio CRmax is limited by the polarization efficiency of the polarizers:
`
`- Tl
`CR
`max-y•
`2
`
`(1.4)
`
`The transmittance of practical single polarizers used in LCDs is typically 40-45%, so that
`T 1 = 32-40.5%, and polarizing efficiencies are 99.0-99.9%.
`Other types of polarization are circularly and elliptically polarized light. Circularly
`polarized light can be thought of as a combination of two linearly polarized light waves
`with equal amplitudes but with a phase difference of rr,/2 (90 degrees). In elliptically
`polarized light, the amplitudes can be different as well.
`All liquid crystal displays utilize the optical anisotropy L\n = n 11 - n .1 of the liquid crystal
`fluid, where n II is the refractive index parallel to the director and n .L is the refractive
`index perpendicular to the director. The optical anisotropy causes the polarization
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`Active Matrix Liquid Crystal Displays
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`light entering the LC layer is polarized parallel to the plane of Fig. 1.9, only the extra(cid:173)
`ordinary wave is propagated and the LC cell is said to be operating in the e-mode.
`
`As an example of how the polarization of light is changed by the LC layer, consider
`linearly polarized light with wavelength A perpendicularly entering an LC layer with
`uniform, homogeneous alignment (\jf = 90°) and thickness d = 1/411,/fl.n. In this case
`ne = n 11 and the LC slice acts, for all practical purposes, as a quarter-wavelength plate:
`when the polarization angle is at 45 degrees with the plane of Fig. 1.9, it converts linearly
`polarized light into circularly polarized light, because one of the two composing waves is
`retarded by <P = 90° (see Eq. 1.5). However, when the orientation of the LC fluid could
`be changed to homeotropic (\jf = 0°) by applying an electric field, the emerging light
`would remain unchanged and linearly polarized. This example shows the general
`principle of modifying the orientation of the LC optical axis ( the director) by electric
`fields in order to manipulate the polarization of light. Practical LCDs utilize variations on
`this basic concept.
`
`1.4 The Twisted Nematic Cell
`
`LCDs are nonemissive (i.e., they need some external lighting source to generate an
`image). This can be ambient lighting in the case of the reflective display in a calculator,
`or a backlight in the case of a transmissive display in a notebook computer. The electro(cid:173)
`optical behavior of the liquid crystal layer modulates the light from the external light
`source to form an image or pattern corresponding to the electronic data signal
`information supplied to the display pixels.
`
`The twisted nematic (TN) mode of operation, which forms the basis of many practical
`LCDs, was first described by Schadt and Helfrich [2] and Fergason [3] in 1971.
`
`The TN cell basically consists of two glass plates with transparent conductive electrodes
`patterned on their inner surfaces. A thin LC l