SEL EXHIBIT NO. 2028
`
`INNOLUX CORP. V. PATENT OF SEMICONDUCTOR ENERGY
`
`LABORATORY CO, LTD.
`
`|PR2013-00066
`
`

`

`201. W. L. Taylor, J. Geophys. Res. 83, 3,5?5—3,5&3 (1978).
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`tion. 011-19994209261, Guntersville, AL, 1999, pp. 460—463.
`
`LIQUID CRYSTAL DISPLAY TECHNOLOGY
`
`Gncaonv P. Cimwronn
`MICHAEL J. Escort
`Brown University
`Providence, RI
`
`INTRODUCTION
`
`As the display in most imaging systems is the final medium
`through which an image is rendered for manipulation
`and verification, an understanding of display technologies
`is essential to the imaging process. Because individuals
`
`LIQUID CRYSTAL DISPLAY TECHNOLOGY
`
`9555
`
`working in the field of imaging science and technology
`may spend more time looking at a display screen than at
`anything else in their office or laboratory, it is imperative
`that it be comfortable to use and appropriate for the
`particular context.
`Twenty years ago, system manufacturers often inte-
`grated the electronic display directly into the system
`to provide a complete package for a specified imaging
`application. Although this approach does not afford much
`flexibility, the display could be closely matched to a spe-
`ciflc application because user requirements were well
`defined. This custom design approach enabled optimiz-
`ing the graphics controller, system software, and user
`interface for the display, user, and application require-
`ments. Despite the positive attributes of this “black-box"
`approach, such as high performance and superior applica—
`tion specific image quality, closed architecture platforms
`tend to be more expensive and suffer from incompatibi-
`lity with peripheral add-one and software packages not
`supported by the system manufacturer.
`Today,
`the situation is dramatically different due
`to the continual evolution of the graphics controller
`interface. By mixing images with text and graphics.
`software developers require more from the display to
`support moving images without diminishing display per-
`formance for static images. The graphical capability of
`today’s standard computer platforms has now made it
`unprofitable for vendors of imaging systems to develop
`their own displays for system-specific tasks. End users
`now typically purchase a computer platform, display,
`and a variety of other peripherals from multiple vcn-
`dors and integrate them with ease (i.e., a plug-and"
`play philosophy). In such a marketplace. one must be
`well educated to match display technology to appli-
`cation needs. This article provides the reader with a
`fundamental knowledge of working principles of liquid
`crystal displays (LCDs), their capabilities, and their limi-
`tations.
`
`ADDRESSING DISPLAYS
`
`is
`it
`Before we delve into the operation of a LCD.
`important to understand how these displays are addressed
`and their
`impact on resolution,
`refresh rates, and
`image fidclity. Many treatises begin with material and
`device configurations, but we will first develop a basic
`understanding of electrical addressing schemes that
`apply to all LCDo. Our hope is that the reader will
`be better prepared to recognize the capabilities and
`limitations ofthc various display configurations presented
`afterward.
`
`A LCD with high—information content (e.g., computer
`or television screen) consists of a two—dimensional array of
`pixels, where a pixel is defined as tho Smallest switching
`element of the array. If the two-dimensional array has
`a total of N rows and M columns [N x M pixels), then
`in principle, there can he N x M electrical connections to
`control each pixel independently. This is known as direct
`addressing and is practical only for very low-resolution
`displays. For medium and highcr resolution displays,
`
`

`

`LIQUID CRYSTAL DISPLAY TECHNOLOGY
`
`addressing is accomplished through passiue- and active-
`matriac techniques. Both of these approaches require only
`N +M electrical connections, thereby greatly simplify the
`electronics, and make higher resolution possible.
`Luminance-«voltage plots for three hypothetical dis-
`plays are depicted in Fig. 1. This display characteristic
`ultimately dictates the type of addressing that can be
`used to create images by using a LCD. Luminance is the
`physical measure of brightness of a display or any surface
`and most commonly has units of candelas per squared
`meter [edfmah nits, or footlamberte (iL). The two mea-
`surable quantities from the luminance—voltage curve that
`have the greatest impact on display addressing are the
`threshold voltage VTH (the voltage at which the luminance
`begins to increase) and a parameter A (the additional volt
`age beyond V-m needed to cause the display to approach or
`reach its highest luminance). If a liquid crystal (LC) mate—
`rial does not start to respond to an electronic stimulus until
`it has reached a well—defined voltage, than it is said to have
`a threshold; otherwise, if the display material responds to
`all voltages, then it is said to be thresholdless (1).
`For simple direct addressing schemes, like the seven-
`seg‘ment digit electrodes shown in Fig. 2,
`the thres-
`holdlless) nature of the material is irrelevant because
`the segmented electrodes (or pixels) are independent 0f
`each other. The appropriate combinations of segments are
`addressed by dedicated logic circuitry (i.e., every pixel is
`independently driven by its own external voltage source),
`and the screen refresh rate is only as long as needed for a
`single pixel 1:0 BWltCh- Direct addressing is practical only
`for low-resolution displays (<50 pixels).
`In a passive addressing scheme (also known as
`multiplexing), one substrate has row electrodes, and the
`other substrate has column electrodes, as shown in Fig. 3.
`
`Luminance
`
`Threshold
`
`Voltage
`Figure 1. Luminance—voltage graph depicting the difi‘erence
`between materials that have Wellrdefined thresholds and those
`materials that have no threshold (thresholdless). Vm is defined
`as Lhe voltage at which the luminance begins to increase, and
`a parameter A is defined as the additional voltage beyond VT”
`needed to cause the diaplay to approach or reach its highest
`luminance.
`
` Concluding
`
`
`segment
`
`Figure 2. An example of direct addressing, where the segments
`are independently driven to create low-insolution information.
`
`Every pixel is uniquely determined by the region ofoverlap
`of a row and a column electrode, making it possible
`to access N x M pixels with only N +M connections. A
`display that has a passive matrix is driven one line
`at a time; that is, one row is selected for addressing,
`and all of the columns are addressed using voltages
`associated with the image for that row. At some time
`interval later, the next row receives an appropriate voltage
`pulse, and the columns again are addressed using the
`information required for that row. The net result is
`that a pixel
`is influenced only sufficiently to produce
`an optical effect when the time-averaged voltage [called
`the root-mean-square (rmsll across the row and column
`electrodes is beyond the threshold (VON a VTH). All of the
`rows not being updated are driven by row voltages that
`will not affect the image information already present.
`The catch is that there will always be a voltage on
`the non selected rows, and therefore that voltage must
`satisfy the relationship Vim: 5 V-m so as not to induce
`an optical change. Therefore, a well-defined threshold in
`the luminancemvoltage characteristic of the LCD material
`is necessary to prevent
`the non selected rows from
`being addressed (Len cross talk}. LC materials typically
`exhibit threshold behavior, which many displays to be
`multiplexed; display materials that are thresholdless, such
`as electrophoretics (2) and gyricon (3), cannot.
`An expression can be derived that completely specifies
`the maximum number of rows NMM that can he addressed
`in terms of the voltage threshold and the parameter A, a
`measure of the nonlinearity of the LC material:
`
`
`1
`
`A
`—<:
`Vrv — was
`
`(1)
`
`To maximize the number of addressable rows for higher
`resolution, one usually uses a material that has a very
`non-linear and steep luminance—voltage response, small
`A, rather than using materials that have a large Von.
`(which increases power consumption).
`Another important equation relates the rms voltages
`of the ON-statc van, and the OFF-state, VOW,
`to the
`maximum number of rows that is given by (foer :5," 1):
`
`
`“Vflgl+ 1
`Vans
`«WM '
`
`(2)
`
`

`

`Passive multiplexing: Amplitude modulation
`
`LIQUID CRYSTAL DISPLAY TECHNOLOGY
`
`957
`
` Pixel voltage
`
`(row-column)
`
`
`
`Figure 3. A simple example of multi—
`plexing on on N-row matrix showing
`the row addressing waveforms, column
`addressing Waveforms, and the corre-
`sponding pixel waveforms. Tins type
`of multiplexing, known as mplitude
`modulation, varies the amplitude ofthe
`pixel waveform to achieve the desired
`image. The normally black pixel is
`“ON" it' the rms average pixel voltage
`exceeds Vm. See color insert.
`
`Column signals
`
`where VON/Vow is referred to as the selection ratio.
`When NM“ is large, this ratio approaches unity, leading
`to poor contrast because the difference would decrease
`between the luminance in the ON- and OFF-states.
`Therefore, Eq. (2) dictates the optical contrast ratio (CR)
`of the display, which is defined as the ratio of the
`luminance in. the ON-state and the luminance in the
`OFF-state (CR =L0N/Lorp). Equations (1) and (2) were
`first derived by Alt and Pleshl-ro in 1974 and remain the
`fundamental expressions governing multiplexing (4). To
`increase resolution using passive addressing techniques,
`a technique known as “dual” scan can be implemented.
`In the dual-scan approach, two column drivers are used
`to address the upper N/Z rows and lower N/2 rows.
`Examination of Eq. (1) shows that the dual-scan approach
`improves the ratio A/V-m by a factor of c5 (5).
`Although some LC configurations that use the mul-
`tiplexing technique have reasonable contrast for large
`values of N, other limitations enter into consideration
`such as response time (the time taken for a pixel to be
`switched to the fully ON-stato plus the time needed to
`relax to a completely OFF-state], viewing angle (the maxi—
`mum polar angle fur which a display maintains reasonable
`contrast and minimal color degradation), and gray—scale
`issues (those that involve the operation of a pixel at a
`luminance which is intermediate between the ON— and
`OFF-states). The frame rate in a passively addressed dis-
`play is severely limited by the time it takes for a single
`row to switch. as well as the number of rows in the display,
`because the frame rate must be greater or equal to the
`product of the two. In only a few exceptions, displays that
`use passive addressing will not support full video Frame
`rates. However, this problem can be essentially olimlh
`natcd using an activcwmstrix addressing scheme, where a
`nonlinear element is used for pixel isolation.
`Activeamatrix addressing is recognised by the display
`industry as the ultimate solution for high fidelity, high
`
`full color, and significant gray-
`information content,
`scale applications. Additionally, this addressing approach
`can be used with thresholdlcss and large a materials
`because a discrete nonlinear switch is integrated into
`each pixel structure. An active-matrix LCD incorporates
`a two-dimensional circuit array (or matrix) to provide the
`eleotrical addressing of individual pixels (6). This matrix
`incorporates an active device in each pixel, usually a thin-
`fllm-transistor (TFT), positioned at one of the corners
`Almost all LCD pixels are essentially dielectric capacitors
`Whose leakage is minimal when a charge is placed on
`the electrodes through the transistor. And due to the
`electrical isolation afforded by the transistor, the voltage
`on one pixel remains constant while other pixel elements
`are subsequently addressed; therefore, the Alt—Pleshlro
`limitation expressed by Eq. (2) does not constrain the
`contrast ratio, as it does in passive-addressing schemes.
`is
`A schematic diagram of an active-matrix circuit
`shown in Fig. 4, where each pixel element is defined by the
`overlap of row and column bus lines. The circuit diagram
`shows that each pixel has one TFT and a LG capacitor
`formed between a top conducting surface, (typically indium
`tin oxide (ITOJ). and the active-matrix substrate. This
`approach is significantly more complex than passive
`addressing, as can be seen from the cross section of
`an active-matrix TFT display also shown in Fig. 4. The
`intricate underpinnings of active-matrix addressing and
`the complex processing required to create one are beyond
`our scope, but the basic operation can be understood as
`follows. The display is addressed one line at a time, as
`in passive addressing. When a row (called the scan or
`gate line) is addressed, a positive voltage pulse ofduration
`T/N (where N is again the number of rows and T is
`the Frame time) is applied to the line and turns on all
`transistors in the row. The transistors act as switches
`that transfer electrical charges to the LG cells from the
`columns (called data or source lines). When subsequent
`
`

`

`LIQUID CRYSTAL DISPLAY TECHNOLOGY
`
`aim,
`
`Glass
`
`
`._. Bleck matrix
`
`_|- Color llller
`
`Planarizaiion
`
`Passlvalion
`layer
`
`layer
`
`Source
`
`n+ tr-S'I
`
`
`
`
`Gate
`insulating
`capacitor
`
`electrode
`layer
`
`Figure 4- The driving circuit of an active-matrix thin-film transistor next to the cross section
`of an active-matrix pixel. As is apparent from the pixel structure becomes substantially more
`complex when active-matrix schemes are used. See color insert.
`
`W‘Si
`
`Polymer
`alignment
`layer
`
`Polarizer
`
`rows are addressed, a negative voltage is applied to the
`gate line that turns off all of the transistors in the row
`and holds the electrical charges in the LC capacitors for
`one frame until the line is addressed again. Alternating
`row-select voltages are required for most LCD materials,
`and the polarity of the data voltage is usually switched
`in alternate frames. The refresh rate in this addressing
`scheme is not limited by the number of rows but only by
`the responte time of the LC.
`A. nonlinear pixel element can be implemented by
`a variety of approaches, but we will mention only the
`two most prominent active-matrix types: amorphous
`silicon (or-Si) and polycrystalline silicon (poly-Si). Both
`of these involve complex fabrication of thin films of
`silicon structures (1:300 nml on glass or quartz substrates.
`The details of actiVe-matrix development are sufficiently
`complex to be far beyond the scope of this review. but
`we include a simple introduction to familiarize the reader
`with the basics.
`
`Pioneering work on the active matrix began in the
`early 1970s (7), and the now conventional ot-Si approach
`first proposed in 1979 (8), where the deposited
`thin film of silicon has a small grain structure and is
`randomly configured [9). In this approach,
`inexpensive
`glass substrates are typically procossod at ~800°C,
`using well established processes, including sputtering,
`photolithography (typically 5—8 photomaska), plasma-
`cnhanced chemical vapor deposition (PECVD), wot
`chemical etch. and reactive ion etching (RED. The
`resulting electron mobility is low («41.5 cmaN/s), but
`is more than adequate to form a useful
`switch
`the pixcl(9). However,
`there are two prominent
`limitations. First, the aperture ratio (the transmissive
`area of the pixel divided by the total pixel area) of the
`display is decreased to 50—75% due to the use of an
`absorbing light shield to overcome the photoconductivity
`of the o-Si TFT. Second. because the supporting row-
`and column—drivers must be mounted on additional IC
`chips bonded to the display substrate,
`the connections
`
`(oil—.000) increase fabrication complexity and decrease
`reliability. Nonetheless, o-Si active-matrix displays have
`been made in all sizes and are most popular as laptop
`monitors and other large-area LCDe. Current research
`issues include reducing the number of photoniasks (10),
`increasing display resolution (11), and increasing the
`aperture ratio (12).
`In contrast, a poly-Si substrate can yield TFTs whose
`electron mobility is much higher (~440 mail/We), but must
`be processed at significantly higher temperatures (6). Feb-
`rication typically involves the same oi-Si process described
`before on a more expensive quartz substrate. Additional
`processing at the higher temperatures leads to recrys-
`tallization in a furnace or by laser annealing (13,14).
`An intriguing approach uses a laser ablationr’annealing
`approach where CMOS—TFTs are fabricated at high tem-
`peratures using a quartz substrate end are subsequently
`transferred to flexible plastic substrates without any
`noticeable deterioration in poly-Si performancollli). In
`all approaches, the silicon grain becomes larger and more
`uniform and allows electrons to flow much more freely. The
`greatest benefits of these poly-Si substrates are the ability
`to fabricate row- and column~drivers directly on the periph-
`ery ofthe glass substrates and the reduction ofthe TFT size
`to *6 x 5 ion. Additionally. the aperture ratio can be made
`substantially higher. Disadvantages include the high pro—
`cess temperature. increased fabrication complexity (higher
`accuracy required in photolithography and ion implanta~
`tion}. and the higher off-leakage current. Integration of
`driver electronics onto the substrate,
`increased display
`brightness, lower power consumption, and the ability to
`form smaller pixels at higher densities (>200 dpi) make
`these poly-Si displays particularly useful for microdisplays
`and medium also displays.
`Display addressing directly impacts resolution and
`optical performance. Because most imaging applications
`require high resolution, active-matrhr addressing is the
`most prominent addressing approach in the imaging field.
`Because of the complexity of the substrate, active-matrix
`
`

`

`it enables
`addressing always involves more cost, but
`a high-resolution regime. A rule of thumb for LCD
`technology is that passive addressing can achieve NM,“ 5
`400, whereas active addressing can achieve NMM z 1, 000.
`
`PROPERTIES OF LIQUID CRYSTAL MATERIALS
`
`To understand the nuts and bolts of a LCD, it is worth—
`while to review briefly the material properties that make
`LCs functional for display applications. The liquid crystal
`phase is a state of matter that is intermediate between a
`solid crystal and an isotropic liquid; in fact, it has prop-
`erties of both. The geometry of a LG molecule is highly
`anisotropic; these molecules can be four to six times longer
`than they are wide and are often modeled as rigid rods (16).
`The nomatic phase is the simplest LC phase and is the
`most used in commercial display applications. This phase
`possesses only orientational order along the long axes of
`the elongated molecules, but no positional or bond oricna
`tational order. as in a conventional crystal. We discuss the
`ferroclcctrlc smectic C‘ liquid crystal phase later, so we
`will limit our discussion here to the nematic phase.
`The elastic properties of LCs are their most character-
`istic feature. At the display level, elastic theory is used
`to predict stable configurations and electric field-induced
`elastic deformations of the material that arc responsi-
`blc for the image. The elastic theory expression is often
`written in the following form:
`
`f = ilKulV * ")2 +ch(n-V x in)2
`
`I Kama x V a ma —suAs(E-nlzl.
`
`(3)
`
`is the free energy density; n is the nematic
`1"
`Here,
`director; E is the applied electric field; K11, K22, and
`K33 are lmown as the splay,
`twist, and bend elastic
`constants, respectively; and As is the dielectric anisotropy.
`The nematic director is denoted as n. and represents the
`average orientationnl symmetry axis of an ensemble of LC
`molecules. The elastic constants are typically of the order
`of 10"“ N, the dielectric anisotropy As is typically «45— 15
`for most display materials, and E is typically <1V/llm
`for conventional transmissive LCD technology. Display
`performance parameters can be predicted by minimizing
`Eq. (3) with respect to the boundary conditions imposed
`by the surfaces of the display substrates to obtain the
`configuration that has the lowest free energy. A number
`of mathematical representations can be used to calculate
`the device properties of LCDs (1'7). Equation (3) is used
`to predict the details of the field-aligned configuration of
`the LC material and its threshold voltage. The switching
`dynamics can be determined by balancing the elastic and
`electric field torques derived from Eq. (3) and the viscous
`torque of the material—y; (dri/df),whorc V1 is the rotational
`viscosity. The rotational viscosity V1 is typically from 1—2
`poise for display materials. The switching time is highly
`dependent on the choice of LC material and configuration
`but is typically in the 1—30 ms range. The reader is
`referred to the following references to explore basic LC
`properties further (18—30).
`
`LIQUID CRYSTAL DISPLAY TECHNOLOGY
`
`959
`
`The techniques involved in transforming the configu-
`rations dcrivcd from elastic theory and into the optical
`properties of LCDs have been treated extensively and will
`only be summarized here. In addition to their geometrical
`anisotropy, nlmatic LCs used in displays also possess
`optical anisotropy, or birefringence, that is responsible for
`the optical changes in contrast when one configuration is
`electrically switched to another configuration. The birc-
`fringence on. for LCs is in. the range MODE—0.3; on ~ 0.1
`is often used for display applications because it leads to a
`high contrast design known as the first minimum or muni-
`mum (see later). The 2 a 2 Jones matrix technique (31)
`can be used to calculate the transmission of light through
`a LCD at normal incidence but neglects Fresnel diffrac—
`tion and multiple reflection effects in thin layers. The
`more sophisticated 4 x d (32) method represents a com-
`plete solution of Maxwell’s equations and is used for the
`general case of light transmission through a LCD at any
`angle. Nematic LC configurations pertinent to displays arc
`most often modeled by this latter method by the display
`community (33).
`
`LIQUID CRYSTAL DISPLAY CONFIGURATIONS
`
`Having presented the basics of display addressing and the
`basic properties of nematic L05, we can now introduce the
`various configurations relevant to displays. It would be
`impossible to describe, oven briefly, every type of current
`LCD configuration, so we will discuss the operational
`principles of a few that are most relevant to imaging.
`Broadly speaking, LCDs can be classified as transmissive
`or reflective. A transmissive display is a light shutter
`that modulates a powerful backlight. Reflective displays
`take advantage of ambient lighting to reflect light back
`to the viewer efi'eetively and therefore do not require a
`backlight. TransflectiVe displays operate in both modes.
`Removing the backlight can result in significant power
`saving and therefore longer battery life. These are largely
`being developed for portable applications rather than
`high-resolution imaging. Therefore, we will
`focus on
`transmissive displays and end by presenting a promising
`low-power reflective display technology.
`
`Twisted Nematic (TN) Liquid Crystal Displays
`
`The most commercially successful LCD configuration,
`known as the twisted nematic (TN), uses cross—polsrizers
`and a molecular orientation of the molecules whose long
`axis twists through a 90° angle between the orienting
`substrates. A cross-section of a TN—LCD, presented in
`Fig. 5, includes two glass substrates that have polarizcrs
`laminated on the outer surfaces and an ITO conduct-
`ing layer on the inner surfaces covered by a polymer
`(e.g., polyimidc). The pclyimide layer is mechanically
`rubbed with a cloth to create microgroovos on the sur-
`face that uniformly align the long axis of the LC molecules
`at the surface (34). The alignment of the rub direction of
`the two substrates is placed parallel to the transmission
`axis of the respective laminated polarizcr, but perpendic-
`ular to each other. The surface alignment mechanisms
`introduced by the poly‘imidc then result in a LG layer that
`
`

`

`LIQUID CRYSTAL DISPLAY TECHNOLOGY
`
`
`
`
`
`
`
`quuld
`cryslal
`materlal
`
`Folarlzor
`
`Conducllng
`layer (ITO)
`
`Colorfllier
`m...
`
`.
`
`1.5.1.3345" ‘
`.r—"r‘F
`
`I ”7%pr
`_ AW_ .-
`
`rssolullon
`
`
`substrate
`
`
`
`Acllve matrlx
`for hlgh
`
`Polymer
`layer
`
`Polarlaer
`
`Backllght
`unpolarlzsd
`Ilghl
`
`
`
`Figure 5. The operation of the twisted nematic (TNJ configuration. which is currently the most
`common LCD mode. The center illustration shows the optical stack. of all of the components of the
`TN display cell. The adiacent insets show the field-on and field-off states of the normally white
`(NW) configuration. Note that the cross-section is not draWH to scale. Sea color insert,
`
`has a 90° twist sandwiched between crossed polarizers. A
`iii-urn cell gap is used for most TN displays. In the figure,
`the extraordinary axis (long axis} of the LC molecules at
`the surfaces is parallel to the transmission axes of the
`polarizers (called the e-mode). However, the o-mode, when
`the ordinary axis is parallel instead, is also used. These
`two configurations lead to slightly difi'erent transmission
`and viewing—angle characteristics (35). Finally. placing a
`red, green, and blue color filter array between the top sub-
`strate and ITO layer creates a full color display. known as
`spatial color synthesis.
`The TN principle of operation is as follows. After the
`first polarizcr, linearly polarized light will follow the LC
`twist and waveguide to an orthogonal linear polarization
`state. This process. sometimes called adiabatic following,
`enables light to escape through the second polarizer. This
`is valid if the twist angle (in this case 11/2) is much
`smaller than the retardation of the nematic (Brood/l),
`known as the Maug'uin condition. Using common LC
`Values, birefringence An = 0.10, cell thickness d = 5 pm,
`and wavelength J. = 510 am. this ratio is 1: MI. If this
`limit is not satisfied, then the light exiting the LC layer
`is elliptically polarized, and the output from the top
`polarizor will be reduced. This configuration is called
`normally white (NW), and describes the zero—voltage state
`as the transmissive one. This is most common in todagfs
`technology. The polarizers can also be arranged in a
`parallel configuration. so that the zero-voltage state is
`black and the voltage-on state is the transmissive one.
`
`The transmission ofa NW-mode TN-LCD can be derived
`by using the Jones formalism (35):
`
`13ln2[£v'1+flfi]
`l
`T=——~~——2——.
`2
`2
`1+u2
`
`(4)
`
`Here, a = Bahia/l, and it is assumed that the director a at
`the display substrates is parallel to the transmission axis
`of the polarizers. The 1/2 term indicates that the maximum
`transmission, as shown in Fig. 6, can be ideally 50% of the
`input light due to the loss in the first polarizer. Note that
`the maximum transmission occurs when the argument
`
`Transmissimllisl mmDD
`
`LD
`
`_a D GI
`
`u
`
`Figure I3. The transmission of the NW—TN cell in the field-oi?
`state as a function of the Mauguin parameter.
`
`

`

`of sinzl. . .] is an integral multiple of 2:, corresponding to
`u = J3 fig. «5—5., etc. The first maximum that occurs at
`J5 corresponds to the maximum brightness and contrast
`ratio. Optimizing around other maximums is also done
`(at m, the second maximum. for example), but this is
`not nearly as common. Nomatic domains can sometimes
`appear in the TN mode. which degrade the contrast but can
`be overcome by using a polyimide alignment layer which
`induces an out—ofiplane pretilt (typically in the range of
`2-10“).
`When a voltage is applied to the pixel, an electric field
`is created perpendicular to the substrates. Because the
`dielectric anisotropy of the LC is positive (As :- 0), the
`molecules tend to align parallel to the field. Equation (5)
`can be used to predict the actual threshold voltage of the
`twisted somatic configuration, which is the point where
`the molecules just begin to realign. The threshold voltage
`for :1 TN display is given by the following expression:
`
`
`K1.
`_
`VT“ _" ass i1 ll
`
`K1: ‘Mse Tl“
`4K1,
`)
`
`(5)
`
`Using typical values Kn = 10‘" N, Kym = 5.4 x 10‘12 N,
`K33 = 15.9 x 10'12 N, and As = 10.5, than VTH : 1.1 V, a
`common threshold for most nematic mixtures. Note that
`this threshold is the voltage where the LC starts to align
`due to the applied voltage and does not say anything about
`the director profile. Above Vm, the broken syrrunetry ofthe
`twist due to the out-of-plans reorientation of the nematic
`molecules align perpendicular to the substrates and light
`passes through teh LC layer without any change in
`polarization. In the NW configuration, the output polarizer
`then absorbs the light. Grayscale levels are attainable
`when intermediate voltages are used. In summary, the
`TIN-LCD therefore simply modulates the intensity of a
`powerful backlight by acting on the polarization state of
`the incident light.
`Notice that the display shmvn in Fig. 5 is addressed
`by an active matrix, which is common because the
`luminance-voltage curve for the twisted nematic LCD
`is not very steep and is not conducive to multiplexing
`schemes (36,37); therefore an active matrix is exclusively
`used to address the twisted nematic for high-end, high-
`resolution imaging. For example, if we consider Eq. (1)
`and substitute typical values for a TN material (A = 0.7
`and VT” = 1.8 V), the maximum number of rows that can
`be addressed is approximately six:
`thcreibre only very
`low resolution displays of this mode are possible using
`multiplexing schemes.
`The switching time (typically in the range of 10—30 ms)
`of the TN configuration is proportional to the viscosity
`and the square of the cell gap and therefore is very
`sensitive to the cell gap. Although thinner cells enable
`faster switching,
`they often compromise the Mauguin
`condition, which can reduce brightness and the contrast
`ratio. The art of display design consists, in large part, of
`balancing these parameters for maximum benefits.
`
`Supchisied Nematic (STN) Liquid Crystal Displays
`
`While the TN~TFT display configuration has become the
`standard in applications that require high pixel densities
`
`LIQUID CRYSTAL DISPLAY TECHNOLOGY
`
`961
`
`the supertwisted nematic
`and {nay-scale resolution,
`[STN) configul'ation(38,39) has been very successful
`in the low to medium resolution realm. The reasons
`for this primarily surround the ability of this display
`mode to be multiplexed (passive-matrix addressing).
`As discussed earlier, an N xM—pixel display requires
`N +M electrical connections using this passive scheme
`(the same number as for active-matrix addressing) but
`does not
`require 9. TFT matrix. And even though
`there are some optical performance trade-offs involving
`contrast and switching times, multiplexing Sllnpllfiea the
`manufacturing process tremendously and enables the
`fabricatio