IPR2015-00863
`Petition for Inter Partes Review of U.S. Patent 7,202,843 - EXHIBIT 1009_Page 1
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` I
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`2001
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`Typeset in 10/ 12pt Times by Thomson Press (India) Ltd., Chennai
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`Page 3
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`Contents
`
`Foreword
`
`Preface
`
`About the Author
`
`I
`
`Introduction
`
`2 Liquid Crystal Materials and Liquid Crystal Cells
`2.1
`Properties of Liquid Crystals
`2.1.1
`Shape and phases of liquid crystals
`2.1.2
`Material properties of anisotropic liquid crystals
`22 The Operation of a Twisted Nematic LCD
`2.2.1
`The electro—optical effects in transmissive twisted nematic LC-cells
`2.2.2
`The addressing of LCDs by TFTs
`
`3 Electra-optic Effects in Untwisted Nematic Liquid Crystals
`3.1 The Planar and Harmonic Wave of Light
`3.2 Propagation of Polarized Light in Birefringent
`Untwisted Nematic Liquid Crystal Cells
`3.2.1
`The propagation of light in a Fréedericksz cell
`3.2.2
`The transmissive Fréedericksz cell
`3.2.3
`The reflective Fréedericksz cell
`3.2.4
`The Fréedericksz cell as a phase-only modulator
`3.2.5
`The DAP cell or the vertically aligned cell
`3.2.6
`The HAN cell
`The 71' cell
`3.2.7
`3.2.8
`
`Switching dynamics of untwisted nematic LCDs
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`Page 4
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`4.2.3 The mixed mode twisted nematic cell (MTN cell)
`4.2.4 Reflective TN cells
`
`4.3 Electronically Controlled Birefringence for the Generation of Colour
`
`5 Descriptions of Polarization
`5.1 The Characterizations of Polarization
`
`5.2 A Differential Equation for the Propagation of Polarized
`Light through Anisotropic Media
`Special Cases for Propagation of Light
`5.3.1
`Incidence of linearly polarized light
`5.3.2
`Incident light is circularly polarized
`
`5.3
`
`6 Propagation of Light with an Arbitrary Incident
`Angle through Anisotropic Media
`6.1 Basic Equations for the Propagation of Light
`6.2 Enhancement of the Performance of LC Cells
`6.2.1 The degradation of picture quality
`6.2.2 Optical compensation foils for the enhancement of picture quality
`The enhancement of contrast
`
`6.2.3
`
`Compensation foils for LC molecules with different optical axis
`Suppression of grey shade inversion and the preservation
`of grey shade stability
`6.2.4 Fabrication of compensation foils
`6.3 Electro-optic Effects with Wide Viewing Angle
`6.3.1 Multidomain pixels
`6.3.2
`In-Plane switching
`6.3.3 Optically compensated bend cells
`6.4 Polarizers with Increased Luminous Output
`6.4.1 A reflective linear polarizer
`6.4.2 A reflective polarizer working with circularly polarized light
`6.5 Two Non-birefringent Foils
`
`7 Modified Nematic Liquid Crystal Displays
`7.1
`Polymer Dispersed LCDs (PDLCDs)
`7.1.1 The operation of a PDLCD
`7.1.2 Applications of PDLCDs
`7.2 Guest-Host Displays
`7.2.1 The operation of Guest-Host displays
`7.2.2 Reflective Guest-Host displays
`
`8 Bistable Liquid Crystal Displays
`8.1 Ferroelectric Liquid Crystal Displays (FLCDs)
`8.2 Chiral Nematic Liquid Crystal Displays
`8.3 Bistable Nematic Liquid Crystal Displays
`8.3.1 Bistable twist cells
`8.3.2 Grating aligned nematic devices
`8.3.3 Monostable surface anchoring switching
`
`Page 5
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`10 Addressing Schemes for Liquid Crystal Displays
`
`11 Direct Addressing
`
`12 Passive Matrix Addressing of TN Displays
`12.1
`The Basic Addressing Scheme and the Law of Alt and Pleshko
`12.2
`Implementation of PM Addressing
`12.3
`Multiple Line Addressing
`12.3.1 The basic equations
`12.3.2 Waveforms for the row selection
`12.3.3 Column voltage for MLA
`12.3.4
`Implementation of multi-line addressing
`12.3.5 Modified PM addressing of STN cells
`Decreased levels of addressing voltages
`Contrast and grey shades for MLA
`Two Frequency Driving of PMLCDs
`
`12.4
`
`13
`
`14
`
`Passive Matrix Addressing of Bistable Displays
`13.1
`Addressing 0f Ferroelectric LCDs
`13.1.1 The V—‘rmin addressing scheme
`13.1.2 The V— 1/7' addressing scheme
`13.1.3 Reducing crosstalk in FLCDs
`13.1.4
`Ionic effects during addressing
`Addressing of Chiral Nematic Liquid Crystal Displays
`
`13.2
`
`Addressing of Liquid Crystal Displays
`with a-Si Thin Film Transistors (a-Si-TFTs)
`14.1
`Properties of a-Si Thin Film Transistors
`14.2
`Static Operation of TFTs in an LCD
`14.3
`The Dynamics of Switching by TFTs
`14.4
`Bias-Temperature Stress Test of TPTs
`14.5
`Drivers for AMLCDs
`14.6
`14.7
`14.8
`
`The Entire Addressing System
`Layouts of Pixels with TFT Switches
`Fabrication Processes of a—Si TFTs
`
`15 Addressing of LCDs with Poly Si-TFTs
`15.1
`Fabrication Steps for Top-Gate and Bottom-Gate Poly—Si TFTs
`15.2
`Laser Crystallization by Scanning or Large Area Anneal
`15.3
`Lightly Doped Drains for Poly-Si TFTs
`15.4
`The Kink Effect and its Suppression
`15.5
`Circuits with Poly—Si TFTs
`
`16
`
`Liquid Crystal Displays on Silicon
`16.1
`Fabrication of LCOS with DRAM-Type Analog Addressing
`16.2
`SRAM—Type Digital Addressing of LCOS
`16.3
`Microdisplays Using LCOS Technology
`
`Page 6
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`I9
`
`20
`
`21
`
`22
`
`and Optical, Plasma, Laser and e-beam Techniques
`
`Colour Filters and Cell Assembly
`19.1
`Additive Colours Generated by Absorptive Photosensitive
`Pigmented Colour Filters
`Additive and Subtractive Colours Generated by Reflective
`Dichroic Colour Filters
`
`19.2
`
`19.3
`19.4
`
`Colour Generation by Three Stacked Displays
`Cell Assembly
`
`Projectors with Liquid Crystal Light Valves
`20.1
`Single Transmissive Light Valve Systems
`20.1.1 The basic single light valve system
`20.1.2 The field sequential colour projector
`20.1.3 A single panel scrolling projector
`20.1.4
`Single light valve projector with angular colour separation
`20.1.5
`Single light valve projectors with a colour grating
`Systems with Three Light Valves
`20.2.1
`Projectors with three transmissive light valves
`20.2.2 Projectors with three reflective light valves
`20.2.3 Projectors with three LCOS light valves
`Projectors with Two LC Light Valves
`A Rear Projector with One or Three Light Valves
`A Projector with Three Optically Addressed Light Valves
`
`20.3
`20.4
`20.5
`
`20.2
`
`Liquid Crystal Displays with Plastic Substrates
`21.1
`Advantages of Plastic Substrates
`21.2
`Plastic Substrates and their Properties
`21.3
`Barrier Layers for Plastic Substrates
`21.4
`Thermo-Mechanical Problems with Plastics
`21.5
`
`Fabrication of TFTs and MIMs at Low Process Temperatures
`21.5.1
`Fabrication of a—Si:H TFTs at low temperature
`21.5.2 Fabrication of low temperature poly-Si TFTs
`21.5.3 Fabrication of MIMs at low temperature
`21.5.4 Conductors and transparent electrodes for plastic substrates
`
`Printing of Layers for LC-Cells
`22.1
`Printing Technologies
`22.1.1
`Flexographic printing
`22.1.2 Knife coating
`22.1.3
`Ink jet printing
`22.1.4 Silk screen printing
`Printng of Layers for LCDs
`Cell Building by Lamination
`
`22.2
`22.3
`
`Page 7
`
`

`

`References
`
`Index
`
`Page 8
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`Page 9
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`
`
`Liquid Crystal Materials
`and Liquid Crystal Cells
`
`2. I Properties of Liquid Crystals
`
`2.1.1 Shape and phases of liquid crystals
`
`Most liquid crystals consist of molecules shaped like the rod in Figure 2.1(a). The direction
`of the long axis is called the director, given by the vector 71', which is an apolar vector as if
`and —fi are equivalent. Rod-shaped molecules are also termed calamitic. Other shapes of
`molecules are disc-like or discotic, as in Figure 2.1(b), and lath—like.
`We focus on calamitic (Bahadur, 1990; Demus et al., 1998a,b) liquid crystals as they are
`the most important for applications. Below the melting point Tm they are solid, crystalline
`and anisotropic, whereas above the clearing point with temperature TC > TIn they are a clear
`isotropic liquid. In the mesophase in Figure 2.2 in between Tm and To, the material has the
`appearance of a milky liquid, but still exhibits the ordered phases shown in Figure 2.2. These
`phases are now described in the sequence given by increasing temperature. The first phase
`above Tm is the smectic C phase (smectic is derived from the Greek word for soap). As all
`smectic phases, it is ordered in two dimensions. The molecules are arranged with random
`deviations tilted to the plane of the layer. In the smectic A phase the directors of the
`molecules are again with random deviations perpendicular to the plane of the layer. Next
`to the clearing point, the nematic phase appears with only a one-dimensional order (nematic
`in Greek means a thread, indicating the thread—like defects in the material). All members of
`the mesophase are anisotropic, as is the solid phase.
`Some more phases of minor importance for display applications are below the smectic C
`phase, the smectic Bhex phase (hexatic B phase), with the same layers as smectic C but a short
`range close packed hexagonal structure, in Figure 2.3(a) seen against the director ii; in this
`direction, the smectic C phase exhibits the irregular structure in Figure 2.3(b). The phases J,
`G, E, K and H are located above Tm, and are smectic-like soft crystals with a long range order.
`
`Page 10
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`(0
`
`(b)
`
`(a) Road—like or calamitic liquid crystal molecule with director n; (b) disclike or discotic
`Figure 2.1
`liquid crystal molecules
`
`The smectic C* phase (chiral smectic C phase) in Figure 2.4 possesses a layered smectic
`structure in which the parallel directors of the molecules are rotated from layer to layer on
`the surface of a cone, resulting in a helix.
`If chiral compounds such as cholesterol esters are added, the nematic phase changes to the
`cholesteric phase in Figure 2.5, which exhibits a helical structure in which, again, the
`director is rotated from layer to layer.
`An as yet poorly understood peculiarity are the blue phases which occur in a small
`temperature range between the cholesteric and solid anisotropic phase.
`More than 20 000 calamitic compounds are known.
`Liquid crystals, the phases of which change with temperature, are called thermotropic.
`Those which change with the concentration of solvents and temperature are lyotropic.
`Calamitic and thermotropic liquid crystals are important for LCDs. Their nematic phase
`is the basis for both the most widely used Twisted Nematic (TN) cell with active matrix
`addressing, and for the SuperTwist Nematic (STN) cell with passive matrix addressing.
`Further LCDs based on calamitic and thermotropic nematic phases are Polymer Dispersed
`
`Page 11
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`C T
`
`‘5’6"}"$“:'
`
`”tit/Till
`
`. ’
`
`I
`
`,
`
`Liquid crystalline mesophases
`Solid {————> quwd
`
`crystalline
`
`T
`
`ITI
`
`Smectic C
`
`Smectic A
`
`Nematic
`
`T
`
`Figure 2.2 Phases of LC materials versus temperature
`
`
`
`Figure 2.3 Top view of (a) the close packed hexagonal structure of the smectic Bhex phase, and (b) of
`the smectic C phase
`
`Liquid Crystals (PDLC) and guest-host—LCDs. The smectic A and smectic C* phases pro-
`vide bistable ferro-electric LCDs with passive matrix addressing. The cholesteric phase gave
`rise to the Stabilized Cholestericr Texture (SCT) with bistability at zero field. LCDs based on
`these phases will be discussed later.
`To better understand electro—optical effects and electronic addressing, some materials
`properties have to be presented (Bahadur, 1990; Demus et (11., 1998a,b).
`
`2.1.2 Material properties of anisotropic liquid crystals
`
`The rod-like molecules have a head and a tail, which is, however, not taken into account
`
`by the direction of ii. Molecules in an unordered alignment exhibit an average director.
`
`Page 12
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`

`

`pitch
`
`is zero
`
`p
`
`Figure 2.4 The helix in a layered structure of chiral smectic C liquid crystals with polarization is,
`perpendicular to if
`
`The individual molecules have an angle 6) to this average director. The order parameter S of
`a phase is defined by (Tsvetkov, 1942)
`
`l2
`
`$2 (300529 — 1)
`
`where the bracket indicates that the average over a large number of molecules with angles 9
`is taken. In a perfectly ordered state, 8 =0, and hence 521. A completely unordered phase
`has S =0. In typical nematic phases, S lies in the region of 0.4 to 0.7, indicating that the
`molecules are rather disordered.
`
`The energy needed for a phase transition, e.g., from smectic A to smectic C, is character—
`ized by a transition enthalpy in kJ/mol. Extensive investigations of phase transitions have
`revealed the temperature dependance of physical parameters such as the helical pitch, the
`viscosity or the elastic coefficients.
`Due to the ordered structure, all phases between Tm and Tc are anisotropic, meaning that
`all dielectric, optical and mechanical properties depend upon the direction.
`The dielectric constant is 5 25,50, where 50 = 8.854- 10 _ 14 F/m stands for the permittivity
`in vacuum and at for the relative dielectric constant. This means, as shown in Figure 2.1,
`6,25” in the direction parallel to the director and erzeL perpendicular to the director,
`leading to the dielectric anisotropy
`
`A5 = a” — EL.
`
`Page 13
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`

`

`
`- A
`
`
`.0 Y—N V‘\‘.Qt:&\\
`
`
`
`\ mt-
`
`
`
`
`
`
`
`\
`
`
`
`
`,_A
`7‘
`
`.7—
`§.'_ "C
`
`unseat—i213
`
`
`efisgv
`
`
`
`Figure 2.5 Helix of the cholesteric phase
`
`Materials with A5 > O are called p-type; their molecules align with the director parallel to
`the electric field, whereas in n-type materials with A5 < 0, they align perpendicular to the
`field. This holds independent of the direction of the field vector. Values for A5 are found in
`the range from —O.8 to —6 and from 2 to 20. The addition of cyanogroups enlarges A5,
`Whereas fluorine atoms in materials with A5 < 0 lower A5 even further. Values for four
`materials are listed in Table 2.1.
`
`The optical anisotropy An concerns the refractive indices no for the ordinary beam of
`light, where the vector of the electrical field oscillates perpendicular to the optical axis that is
`perpendicular to the director and the refractive index 113 for the extraordinary beam of light,
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`Page 14
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`7] [mmzls] (20°C)
`”0:”;
`
`nean
`An (589 nm, 20°C)
`
`20
`1.4672
`
`1.5188
`0.0516
`
`32
`1.4554
`
`1.5034
`0.0480
`
`45
`1.469
`
`1.506
`0.037
`
`where the field vector oscillates in parallel to the director. Hence we obtain
`
`and
`
`and the optical anisotropy
`
`no 2 ni,
`
`”e = nu»
`
`.
`
`An = n” — nL 7— ne — mg.
`
`More explanation about the optic axis and the ordinary beam will be given in Chapter 6. The
`refractive index n is based on optical frequencies which are very high. Therefore,
`equation known from Maxwell’s theory (Born and Wolf, 1980)
`
`provides for frequencies approaching infinity:
`
`n = {5:
`
`for Mr 2 1
`
`and
`
`{5er : nfi:
`
`em, = n1
`
`Asm = nfi — ni.
`
`The refractive indices depend upon the wavelength A. Values for An lie in the range
`A116 [0.04, 0.45]; some values are listed in Table 2.1. As a rule, materials with a high
`An are not stable to UV light. Due to the optical anisotropy, the material is birefringent.
`The speed of light is (Born and Wolf, 1980)
`
`
`
`,
`
`Page 15
`
`

`

`Vi : m,
`
`where the E—vector oscillates parallel and perpendicular to the director, are different and
`dependent on the wavelength.
`This is the key for the electro-optical effects in liquid crystal cells.
`The direction with the larger refraction index n” exhibits the smaller speed, and hence is
`called the slow axis, whereas ”L defines the fast axis.
`
`The dynamic behaviour of LC materials is affected greatly by the viscosity- Too high
`viscosities at lower temperatures slow down the movement of the molecules and yield the
`lower temperature limit of LC cells. The proximity to TC provides the upper temperature
`limit. The dynamic viscosity 77,, is defined as
`
`F d
`N
`m2?” in m—jzpas,
`
`where F is the force needed to shift a body with the area A with the velocity v over a viscous
`layer with a thickness d. For displays, the kinematic viscosity
`
`is used, where 6 is the density of the viscous material. As for most LC materials, 6 is around
`lez/mm4; the values for W and 7] do not differ much. The viscosity depends upon the
`orientation of the directors. For a random orientation, the bulk or turbid kinematic viscosity
`is given’ in Table 2.1. The rotational viscosity is measured according to Figure 2.6, where the
`vector of the rotation is perpendicular to the director. Values for dynamic rotational vis-
`cosities of LCs are 0.02 Pa 5 to about 0.5 Pa s. This viscosity is important for the movement
`of the director in an electric field.
`
`The elastic constants belong to restoring torques if the field of directors is deformed. The
`three deformations from the equilibrium are splay, twist and bend, with the elastic constants
`K11, K22 and K33, as shown in Figure 2.7. The dimension is a force. The values are very
`small in the range of lO-lO’ZN. These elastic forces determine the equilibrium in the
`presence of electric and magnetic fields.
`A large variety of chemical compounds exhibit the properties of liquid crystals. The basic
`structure with rings, linking groups and terminal groups is shown in Figure 2.8. Rings can be
`cyclohexyl, pyridine, dioxane, phenylcyclohexane or phenyldioxane. Fluorinated com-
`pounds have a high specific resistance p=5~1015 9cm. The characteristic temperatures of
`LC compounds can be shifted by additive ingredients. By this means, Merck’s nematic
`compounds reached the wide temperature range of operation, from —40°C to 120°C, which
`is very suitable for automotive application. In Table 2.2 properties of LC materials with this
`wide temperature range are listed.
`
`Page 16
`
`

`

`Equilibrium
`configuration
`
`
`
`Elastic constant K22
`(b)
`
`Elastic constant K33
`(C)
`
`Figure 2.7 Equilibrium configuration; the elastic deformati0ns splay (a), twist (b) and bend (c)
`
`Page 17
`
`

`

`F
`
`F
`
`VW—“W—“W—JW—J
`
`Terminal
`group
`
`Ring
`
`Linking
`group
`
`Ring
`with F as
`lateral
`substitute
`
`Terminal
`group
`
`Figure 2.8 The basic structure of a calamitic LC molecule
`
`Table 2.2 Properties of nematic LC materials with a wide temperature range
`
`MLC-1390000
`MLC-13800100
`MLC-1380000
`E.“—
`
`Transition temp.
`smectic—nematic
`Clearing pt TC
`Rotational
`
`< — 40°C
`110°C
`
`< —40°C
`111°C
`
`< —40°C
`110.5°C
`
`235 mPas
`151 mPas
`228 mPas
`viscosity, 20°C
`+8.3
`+5.0
`+8.9
`A5 1kHz,20°C
`1.4816
`1.4832
`1.4720
`no=nL
`1.5888
`1.5735
`1.5622
`naan
`
`
`
`
`+0.0902 +0.0903 +0.1073An +0.1081
`
`2.2 The Operation of a Twisted Nematic LCD
`
`The liquid crystals used are calamitic and thermotropic in the nematic phase. The operation
`of this most widely applied LCD will be phenomenologically described in order to give an
`overview over the entire flat panel display system, including the addressing scheme (Demus
`et al., 1998a; Kaneko, 1987; Lueder, 1998a). This alleviates the more analytical and detailed
`treatments which follow.
`
`2.2.1 The eiectro-opticai effects in transmissive
`twisted nematic LC-ceils
`
`Figure 2.9 depicts the top View of a display panel with the conducting rows and columns
`terminating in the contact pads. The rectangular pixels can only be electrically addressed
`from those contact pads.
`A colour VGA display, as used in laptops, has 480 rows and 3 x 320 columns forming
`triple dots for the three colours red, green and blue. An NTSC TV display has 484 rows and
`3 X 450 columns corresponding to 653 400 pixels, whereas an HDTV display has
`
`Page 18
`
`

`

`—}
`E
`me
`emme
`
`llllllllllllllllu
`lllllllllllllllnl
`IIIIIIIIIIIIIIIII
`IIIIIIIIIIIIIIIII
`IIIIIIIIIIIIIIIII
`IIIIIIIIIIIIIIIII
`IIIIIIIIIIIIIIIII
`IIIIIIIIIIIIIIIII
`IIIIIIIIIIIIIIIII
`III-IIIIIIIIIIIII
`IIIIIIIIIIIIIIIII
`IIIIIIIIIIIIIIIII
`IIIIIIIIIIIIIIIII
`IIIIIIIIIIIIIIIII
`IIIIIIIIIIIIIIIII
`IIIIIIIIIIIIIIIII
`IIIIIIIIIIIIIIIII-
`
`Figure 2.9 Top View of the rows, columns, pixels and contact pads of a display panel
`
`standardized formats,
`
`1080-3 - 192027 320 800 pixels. For more
`Appendix 1.
`Figure 2.10 shows a pixel of a transmissive twisted nematic LC—cell with no voltage
`applied. The white back light f passes the polarizer a. The light leaves it linearly polarized
`in the direction of the lines in the polarizer, and passes the glass substrate b, the transparent
`electrode 6 out of Indium-Tin—Oxide (ITO) and the transparent orientation layer g. This
`layer, made of an organic material such as polyimide, 100nm thick, is rubbed to generate
`grooves in the direction of the plane of the polarized light. In these grooves the rod—like LC
`molecules are all anchored in parallel, but, as shown in Figure 2.11, with a pretilt angle 040 to
`the surface of the orientation layer. The sequence of layers is the same on the second glass
`plate. A typical thickness of the cell in Figure 2.10 is d=3.5 p to 4.5 u. The grooves on the
`second plate are perpendicular to those on the first plate. This forces the liquid crystal
`molecules to twist on a helix by [3:90" from one plate to the other without the addition
`of chiral compounds. All twist angles are called [3.
`Due to the birefringence, the components of the electric field vector of the light in parallel
`and perpendicular to the directors travel with different speeds, which depend upon the
`wavelength. They superimpose along their path between the two glass plates first to ellipti-
`cally polarized light, in the distance d/2 from the input to circularly polarized light, then
`again to an elliptic polarization, and if
`
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`Orientation layer
`
`Figure 2.“ LC molecules with pretilt angle do on top of the orientation layer
`
`they reach the analyser again linearly polarized, but with the polarization plane rotated by
`90°. If the analyser is crossed with the polarizer, the light can pass the analyser. The pixel
`appears white. This operation is termed the normally white mode. If the analyser is rotated
`by 90°, a parallel analyser, the light is blocked in the analyser. The pixel is black. This is
`called the normally black mode. A useful visualization of what happens to the light while
`travelling through the cell is as follows: the planes of the various polarizations follow the
`twist of the helix. This is, however, only true if Equation (2.15) holds. The explanation is
`also only true for light travelling and viewed perpendicular to the plane of the substrate. If
`Viewed under a different angle, light perceived by the eye has travelled in a different path
`with different angles to the director and a different cell thickness d.
`If a voltage VLC of the order of 2 V is applied across the cell, as shown in Figure 2.10(b),
`using the two transparent ITO—electrodes 100 nm thick, the resulting electric field attempts to
`align the molecules for A5 > 0 parallel to the field. This holds independent of the sign of the
`vector of the electrical field, as already pointed out in Section 2.1.2. Hence, the following
`effects are not dependent on the polarity of VLc- Due to the anchoring forces, a thin LC layer
`on top of the orientation layers maintains its position almost parallel to the surfaces. A
`threshold voltage Vm is needed to overcome intermolecular forces before the twisted mole—
`cules start to rotate. A uniform start over the plane of the panel is favoured by a pretilt angle
`around 3°, which seems to avoid strong differences in the anchoring forces. Only at a
`saturation voltage Vmax several times Vm with a value around 10V have all molecules
`besides those on top of the orientation layers aligned parallel to the electric field, as depicted
`in Figure 2.10(b). In this state the vector of the electrical field of the incoming light oscillates
`perpendicular to the directors, and encounters only the refractive index nL. Hence, no
`bi-refringence takes place and the wave reaches the crossed analyser in the same linearly
`polarized form as at the input. The analyser blocks the light and the pixel appears black. This
`is an excellent black state as it is independent of the wavelength, resulting in a blocking of the
`light. This black state is gradually reached from the field-free initial state by increasing the
`voltage VLC from OV over an intermediate voltage up to Vmax, which is also gradually
`rotating the molecules in Figure 2.12 from the initial twisted state with directors parallel
`to the surfaces (Figure 2.10(a)) over an intermediate state with the director already tilted
`down with tilt angle a (Figure 2.12(b)) to the final state with directors parallel (a = 90°) to the
`electric field. The transmitted luminance, also termed transmittance, of the light is shown in
`Figure 2.13 for the normally white mode discussed so far. In the normally black mode, the
`analyser
`is parallel
`to the polarizer and allows
`the light
`to pass at
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`0.3
`
`luminanceL
`Transmitted
`
`0.0
`
`0.5
`
`1.0
`
`1.5
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`Reduced voltage
`
`2.0
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`VLC
`0
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`2.5
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`Figure 2.13 Transmitted luminance versus the reduced voltage VLC across the LC cell for the
`normally white and normally black modes
`
`Vth S VLC S Vmax. For this mode the transmitted luminance is also depicted in Figure 2.13.
`Only in this mode is the threshold voltage Vth visible, as in the normally white mode a small
`change in luminance at a high value of the luminance cannot be perceived by the eye.
`Luminance is the correct term for ‘brightness’. The physical meaning and dimensions of
`luminance and other display-related units are explained in Appendix 2.
`The blocking of light in the analyser as described by Equation (2.15) is only valid for one
`wavelength for which, as a rule, yellow light with /\:505 nm is chosen. As other wave-
`lengths can still pass the analyser, the black state is not perfect. As a rule, it has a bluish tint.
`The imperfect black state can be improved by compensation foils, as discussed later.
`The Contrast Ratio CR of a display is defined by
`
`CR
`
`
`_ highest luminance in the pixels, Lmax
`lowest luminance in the pixels,Lmin
`
`The measurement should be performed without the interference of reflected ambient light,
`i.e. in darkness. If the black state in the denominator of Equation (2.16) is increased by the
`imperfect blocking of the light, contrast falls in any case. This is the case in the normally
`black state, whereas the normally white state described above yields an excellent contrast
`due to a much lower value of the denominator in Equation (2.16).
`Grey shades of a pixel are controlled by the voltage VLC in Figure 2.13, which modulates
`the luminance from a full but imperfect black up to a full white. Luminance differs when the
`display is viewed under angles different from perpendicular to the glass plates. Contrast
`decreases the more oblique the angles become.
`The TFT addressing circuit will be placed on the glass next to the backlight in Figure 2.10.
`In a colour display, the glass plate facing the viewer carries the pixellized colour filter,
`
`Page 23
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`

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`
`
`(a)
`
`(b)
`
`Figure 2.14 The geometrical arrangement of colour pixels for red R, green G, and blue B (a) in
`triangles, (b) in stripes and (c) in diagonal form
`
`shown in Figure 2.14. The pixels for red, green and blue are covered with a compound which
`absorbs all wavelengths originating from the white backlight besides red, green and blue,
`respectively. The saturation of the colours is individually controlled for each pixel by the
`voltage VLC in the same way as for grey shades.
`The geometrical arrangement of the colour pixels in triangles in Figure 2.14(a) and along
`diagonals in Figure 2.l4(c) are recommended for moving TV pictures, whereas the colour
`stripes in Figure 2.14(b) are preferred for computer displays often presenting rectangular
`graphs.
`The cross-section of a colour filter in Figure 2.15 contains the colour materials for R, G
`and B, an absorptive layer with a low reflection in between the pixels, a so-called black
`matrix, an overcoat layer and, in the case of TFT addressing, an unpixellized ITO electrode
`over the entire display area. For other addressing schemes, the ITO layer is no longer .
`unstructured. The ITO electrode on the TFT-carrying plate is pixellized. The black matrix
`prevents light between the pixels, which is neither controlled by the voltage VLC at the ITO
`electrodes nor exhibits the desired colour, from seeping through the cell. This light would
`lighten up the black state, and would thus degrade contrast and the saturation of colour. A
`suitable material for a black matrix is an organic material with carbon particles exhibiting a
`reflectivity of only 4percent, whereas the previously used Cr-oxide has a reflectivity of
`40percent. The overcoat layer (e.g. out of a methacrylate resin solution) equalizes the
`different heights of the colour pixels and protects them.
`
`2.2.2 The addressing of LCDs by TFTs
`
`So far we know that we have to control the grey shade individually in each pixel by applying
`the appropriate pixel voltage VLC, but by only using the external contact pads in Figure 2.9.
`
`Page 24
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`

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`
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`L Glass substrate
`
`Figure 2.15 Cross-section of a colour filter for TFT addressed LCDs
`
`Columns
`
`
`/
`
`.
`'ii-<—
`Gate-
`
`impuls
`V.
`= V 1
`Video information
`Video information
`V
`Video
`d
`T
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`9
`,9
`f
`n
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`Vg l
`Lines
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`T
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`\ ID
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`EEK —L
`VLC l
`_
`CLC T03
`
`
`
`Figure 2.16 TFT addressing of the pixels in a row
`
`The TFT—addressed LCD, usually called an Active Matrix LCD (AMLCD), solves this task
`as depicted in Figure 2.16. It shows two pixels of a row of pixels, with the row— and column-
`conductors and ground represented by the unstructured ITO electrode on the colour plate in
`Figure 2.15. The TFTs are n-channel Field Effect Transistors (FETs) fabricated with thin
`film technology. They operate as switches in the pixels. All TFTs in a row are rendered
`conductive by a positive gate impulse Vg. TFTs in other rows are blocked by grounding the
`rows. The video information is fed in through the columns and the conducting TFTs into all
`the pixels of a row simultaneously. More specifically, the video voltage Vd corresponding to
`a desired grey shade charges the LC-capacitor CLC and an additional thin-film storage
`capacitor CS up to the voltage Vd. This constitutes an amplitude modulation. The operation
`addresses one line at a time, as opposed to one pixel at a time, of the e-beam in CRTs.
`During the charging time, the storage capacitor connected to the succeeding line n—i—l
`grounded, and hence connected in parallel to CLC. As this is no more true during other
`
`Page 25
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`

`

`Tr = Tf/N-
`
`The waveform of the pixel-voltage VLC is depicted in Figure 2.17. In the time Tr, the storage
`capacitors are charged with the time constant
`
`T
`Ton = (CLc + C,)1eon g 0.1Tr = 0.1 —I%
`
`where Ron is the on—resistance of the TFT. The inequality guarantees that at the end of T,, the
`voltage VLC is only 1 percent below the desired voltage Vd in Figure 2.16. The TFTs need to
`be fast enough to make sure that even if their properties fluctuate, as ind