Download 國立中山大學 物理學系研究所 碩士論文 - eThesys 國立中山大學學位

國立中山大學
物理學系研究所
碩士論文
液晶分子方位三維控制之視角可切換顯示器
Viewing angle switchable displays based on three-dimension
control of liquid-crystal orientation
研究生：陳嘉偉 撰
指導教授：林啟湟
中華民國
九十九年七月
摘要
我 們 在 這 篇 研 究 裡 提 出 一 種 有 視 角 可 切 換 (switchable
viewing-angle)能力的顯示器，此顯示器的對比值(contrast ratio)主要依
賴於亮態和暗態的穿透度。藉由改善畫素電極結構來切換此顯示器的
廣窄視角顯示模式。依據所提出的三通道電極(three-electrode)結構，
由控制液晶分子指向(liquid-crystal orientation)在大角度方向下產生不
同漏光(light-leakage)情況，來切換此顯示器之視角。我們提出的結構
能簡單的達到控制顯示器的視角，而且具有輕薄的外框和較低的製作
成本，適合用於個人隱私保護的應用上。
i
Abstract
In this study, a liquid-crystal display (LCD) with the capability of
switchable viewing-angle is proposed. The contrast ratio of the LCD
mainly depends on the transmittance of the bright state and the dark state.
By using the improved structure of the pixel electrodes, the viewing angle
of the LCD can be switched between the wide viewing-angle mode and
the narrow-viewing angle mode. Based on the proposed three-electrode
structure, the liquid-crystal orientation of the LCD is controlled to
generate different light-leakage conditions at large viewing angles, thus,
resulting in the switch of the viewing angle of the LCD. The proposed
structure, which has the slim outline and low-cost of the LCD, is a simple
way to control the viewing angle of the LCD. Potential applications for
the protection of personal privacy are emphasized.
ii
Acknowledgement
首先要感謝的是我的指導教授，林啟湟博士，老師給予我在研究
上相當多的指導，使我學習到對事情的看法 更多元化，讓我對於許多
事情的態度與見解更有想法與解決能力。也感謝我的直屬學長：禮璿，
不管在研究討論還是運動休閒下，給予不少的幫助與歡樂。接著要感
謝的是我求學生涯中同學們和學弟們：老郎、正偉、奐甫、鋁罐、老
蘇、學志和明岦、和宜、炳堯、郁茵，志欣、謝博、小A和政瀚，感
謝你們的協助以及陪伴，讓我在這兩年的研究所生涯中更快樂充實。
最後要感謝我最親愛的家人對我無私的付出與關心，在生活和物質上
讓我不虞匱乏，使我能夠專心的完成學業，非常感謝！
iii
Contents
摘要----------------------------------------------------------------------------------i
Abstract-----------------------------------------------------------------------------ii
Acknowledgement ---------------------------------------------------------------iii
Contents----------------------------------------------------------------------------iv
List of Figures --------------------------------------------------------------------vi
Chapter 1 Introduction
1.1 Liquid crystal display technology ---------------------------1
1.2 Wide viewing angle technology -----------------------------3
1.2-1In-plane switching ----------------------------------------4
1.2-2 Another wide viewing technology ---------------------6
1.3 Viewing angle switching technologies -----------------------9
1.4 Motivation and objective -------------------------------------10
1.5 Organization of this thesis -----------------------------------10
Chapter 2 Principles
2.1 Physical properties of liquid crystal ------------------------11
2.2 Anisotropy physical properties ------------------------------13
2.3 Optical birefringence -----------------------------------------14
2.4 Dielectric anisotropy -----------------------------------------16
2.5 Phase retardation effect --------------------------------------18
2.6 Viewing angle characteristics -------------------------------21
2.7 TechWiz LCD simulation software ------------------------22
iv
2.8 Summary -------------------------------------------------------25
Chapter 3 Devices fabrication and measurement
3.1 Devices fabrication -------------------------------------------26
3.2 Measurement system -----------------------------------------29
3.3 Summary -------------------------------------------------------30
Chapter 4 Simulation and experiment results
4.1 Design of three-terminal electrode configuration --------31
4.2 Operation principle and viewing angle control -----------36
4.3 Electrical potential distribution estimation ----------------39
4.4 WVA mode and NVA mode ----------------------------------43
4.5 Simulation results of viewing angle switching ------------47
4.6 Inter-digitated electrode width and separation gap -------54
4.7 Experimental results ------------------------------------------60
4.8 Summary -------------------------------------------------------65
Chapter 5 Conclusions and future work
5.1 Conclusions ----------------------------------------------------66
5.2 Future work ----------------------------------------------------66
Reference -------------------------------------------------------------------------67
v
List of Figures
Figure 1.1-1 Scheme of transmissive LCD structure.
Figure 1.2-1 An IPS mode using a fringing field on a homogeneous aligned cell.
Figure 1.2-2 Demonstration of viewing angle characteristics on IPS vs VA modes.
Figure 1.2-3 Device structure and LC orientation of the fringing field switching mode
in a voltage ON state.
Figure 1.2-4 Operation principle of multi-domain vertical alignment LCD.
Figure 2.1-1 Mesophases lie between a crystalline solid and liquid with various
temperatures, T m and T c , melting and cleaning temperature.
Figure 2.1-2 Scheme of nematic LC orientation director.
Figure 2.2-1 Anisotropy physical properties of nematic LC.
Figure 2.3-1 Light travels at birefringence material.
Figure 2.3-2 Index anisotropic of uniaxial liquid crystal.
Figure 2.5-1 Scheme of drawing of the transverse field mode LCD with normally
black operation, where V is the applied voltage and V th is the threshold
voltage.
Figure 2.6-1 Polar coordinate system for viewing angle definition.
Figure 2.7-1 Flow chart of the TechWiz LCD.
Figure 2.7-2 3D-structure file from the 2D-layout of TechWiz Layout.
Figure 2.7-3 Variation condition of Transmission module according to voltage
condition of LC Analysis.
Figure 3.1-1 Flow fabrication of inter-digitated electrode.
Figure 3.1-2 Cell fabrication process.
Figure 3.2-1 (a) Conoscope 80 (Autronic Melchers), (b) Scheme of the Conoscope
vi
system in transmissive mode.
Figure 4.1-1 Schematic of this cell with three-terminal electrode.
Figure 4.1-2 Viewing angle characteristics for (a) dark state for wide view and (b)
dark state for narrow view.
Figure 4.1-3 Schematic of electrode width w and gap l with LC cell gap d.
Figure 4.1-4 Top view of the sample with photo-lithography and dry etcher.
Figure 4.2-1 Schematic of great dark state with LC homogeneous alignment under
crossed polarizers.
Figure 4.2-2 Schematic of great bright state for both wide-viewing angle mode and
narrow-viewing angle under crossed polarizers.
Figure 4.2-3 Schematic of dark state for narrow-viewing angle mode with longitudinal
fringe field under crossed polarizers.
Figure 4.3-1 Approximation IPS electrode configuration.
Figure 4.4-1 Dark state for WVA mode with LC homogeneous alignment under
crossed polarizers.
Figure 4.4-2 Viewing characteristics in the normal viewing direction and at the
oblique viewing angle of IPS electrode direction.
Figure 4.4-3 Dark state for NVA mode with longitudinal fringe field.
Figure 4.4-4 Viewing characteristics in the normal viewing direction and at the
oblique viewing angle of IPS electrode direction of NVA mode.
Figure 4.5-1 Schematic of Cell configuration for simulation with three-terminal
electrode.
Figure 4.5-2 Cell configuration for simulation with three-terminal electrode.
Figure 4.5-3 Voltage dependent transmittance curve with electrode width w (w + =
w - =w) and electrode separation gap l (l) are both 8 μm. The LC cell gap
d (d) is 7.5 μm.
vii
Figure 4.5-4 Iso-transmittance contour with applied voltage V= 4.2 V rms .
Figure 4.5-5 Voltage dependent transmittance curve with electrode width w (w + =
w - =w) and electrode separation gap l (l) are both 8 μm. The LC cell gap
d (d) is 7.5 μm.
Figure 4.5-6 Iso-transmittance contour with applied voltage V= 37 V rms .
Figure 4.5-7 The dark state of WVA mode under crossed polarizers.
Figure 4.5-8 Iso-Contrast ratio contour for WVA mode under crossed polarizers.
Figure 4.5-9 Iso-Contrast ratio contour for NVA mode under crossed polarizers.
Figure 4.6-1 The LC profile cross-section of bright state with applied voltage V = 4.2
V rms on y-z plane.
Figure 4.6-2 Computer-simulated potential distribution of the IPS electrode with
applied voltage V = 4.2 V rms on y-z plane.
Figure 4.6-3 Twist angle of LC director between the electrode gap with applied
voltage V = 4.2 V rms .
Figure 4.6-4 The LC profile cross-section of dark state for NVA mode with applied
voltage V = 37 V rms on y-z plane.
Figure 4.6-5 Computer-simulated potential distribution of the proposed electrode
structure with applied voltage V = 37 V rms on y-z plane.
Figure 4.6-6 Twist angle of LC director of dark state for NVA mode between the
electrode gap with applied voltage V = 4.2 V rms .
Figure 4.6-7 Tilt angle of LC director of dark state for NVA mode between the
electrode gap with applied voltage V = 37 V rms .
Figure 4.6-8 Iso-contrast ratio with variations of (a) electrode width and (b) electrode
gap.
Figure 4.7-1 Photography are in different voltage conditions. (a) Bright state for WVA
mode, (b) Dark state for NVA mode.
viii
Figure 4.7-2 Light leakage of dark state for NVA mode with applied voltage V = 20
V rms .
Figure 4.7-3 Viewing angle characteristics of the proposed structure are measured by
Conoscope. (a) WVA mode (b) NVA mode.
ix
Chapter 1
Introduction
1.1 Liquid crystal display technology
With the popularization of computer and wireless communication,
multimedia application displays and mass information interchange
become important parts of people, and the demand of technologies of
display mass information in contents and pictures are getting more
imperative. Liquid-crystal displays (LCDs) are now applied to electric
products, such as LCD TVs and portable displays with advantages
include light weight, slim volume, high resolution and low power
consumption. In order for thin-film-transistor liquid-crystal display
(TFT-LCD) technology to claim an even larger share of the large display
market, manufacturers must continue to improve image quality.
Figure 1.1-1 Scheme of transmissive LCD structure.
1
In general, in the viewpoint of light sources there are two types of
LCDs. The first type is the transmissive LCD which needs backlight to
emit light. A transmissive type color LCD was demonstrated by Sharp
Corporation in 1989. The LC is placed between two glass substrates
which are coated with an indium tin oxide (ITO) electrode. Applied
voltage can control liquid crystals twist angle, and amount of transmitted
light can be modulated. The transmissive LCDs configuration is show in
Figure 1.1-1. Transmissive LCDs consume high power because of
backlight system requirements. Furthermore, the devices in the LCD
module such as the color filter, polarizer, LC cell, pixel electrode, and
analyzer will absorb and block light transmittance. Therefore, a typical
transmissive LCD device transmits only about 8% of total light.
The second type is reflective LCD which does not need backlight but
uses ambient light, such as sunlight. Typically, the transmissive LCD
provides higher brightness and better contrast indoors. On the contrary,
the reflective LCD has higher contrast when seeing outdoors, but while
seeing indoors the brightness of reflected image is not high enough. There
are currently several technologies to create the reflective type LCDs. For
example, super-twisted nematic LCDs (STN-LCDs) for cellular phones,
mixed-mode twisted nematic LCDs (MTN-LCDs) for personal digital
assistants, Cholesteric LCDs (Ch-LCDs) for e-papers and billboards, and
electrowetting display for e-books.
2
1.2 Wide viewing angle technology
The viewing angle is a critical problem for large panel liquid crystal
displays. Several technologies have been developed to widen the viewing
angle of transmssive LCDs. We can roughly categorize them as the
external, internal and combination methods. In the external method, the
LCD structure is a conventional twisted nematic (TN) cell with a phase
compensation film or diffusive optics is laminated outside the LCD panel.
In the internal method, the LCD structure is modified internally like
in-plane switching (IPS), multi-domain vertical alignment (MVA) and
fringe field switching (FFS). For example, in the in-plane switching
method, the electrodes are arranged in the same substrate such that the
longitudinal, rather than transversal, electric field is used to drive the LC
molecules. The combination method integrates internal structure change
and also adds compensation films [1-7].
3
1.2-1 In-plane switching
In-plane switching technology was developed by Hitachi in 1996 to
solve the two main limitations of TN-matrices at the time, those being
small viewing angles and low-quality color reproduction. The name
“In-Plane Switching” comes from the crystals in the cells of the IPS panel
lying always in the same plane and being always parallel to the panel’s
plane. The inter-digital electrodes are arranged in the same substrate such
that the generated fringing field is in the transverse plane.
Figure 1.2-1 An IPS mode using a fringing field on a homogeneous aligned cell.
Figure 1.2-1 shows an early version of the Hitachi IPS cell using a
negative dielectric anisotropy LC in homogeneous alignment. The front
polarizer is parallel to the LC directors and rear analyzer is crossed. In the
voltage-off state, the incident light experiences no phase retardation so
that the outgoing beam remains linearly polarized and is absorbed by the
crossed polarizer. In a voltage-on state, the fringing field reorients the LC
4
directors, causes phase retardation to the incoming light and modulates its
transmittance through the analyzer.
Initial IPS technology was plagued with slow response time and a
low contrast ratio but later evolutions have made marked improvements
to these shortcomings. Because of its wide viewing angle and accurate
color reproduction (with almost no off-angle color shift), IPS is widely
employed in high-end monitors aimed at professional graphic artists,
although with the recent fall in price it has been seen in the mainstream
market as well. Prototypes of Apple's 4th generation iPhone are believed
to be using IPS technology for their screen too. The original IPS
technology became a foundation for several improvements: Super-IPS
(S-IPS), Dual Domain IPS (DD-IPS), and Advanced Coplanar Electrode
(ACE). The latter two technologies belong to IBM (DD-IPS) and
Samsung (ACE) and are in fact unavailable in shops. Super-IPS panels
are mostly produced by LG.Display (previously LG.Philips) and have
gone through several generations since their inception. Initiall S-IPS built
upon the strengths of IPS by employing an advanced “multi-domain”
liquid crystal alignment.
5
Figure 1.2-2 Demonstration of viewing angle characteristics on IPS vs VA modes.
(Images from LG.Display website)
1.2-2 Another wide viewing technology
Another way to improve the aperture ratio while retaining a wide
viewing angle is the fringing field switching (FFS) mode developed by
Hyundai. The basic structure of FFS is similar to that of the IPS except
for the much smaller electrode gap. In a FFS mode, the transmittance is
about 95% of the normal TN cell. By contrast, the IPS mode
transmittance is about 60% of the normal TN cell. The viewing angle
characteristics of the FFS mode are very similar to the IPS mode. Figure
1.2-3 shows a FFS structure with a homogeneous alignment and positive
dielectric anisotropy LC mixture. The fringing field covers both the
electrodes and gaps.
6
Figure 1.2-3 Device structure and LC orientation of the fringing field switching mode
in a voltage-ON state.
Another display technology, Multi-domain vertical alignment (MVA)
technology, was later developed by Fujitsu in 1998 as a compromise
between TN film and IPS technologies. On the one hand, MVA provided
a full response time of 25 milliseconds (that was impossible at the time
with IPS), and on the other hand, MVA mode had wide viewing angles of
160° - 170°, and thus can compete with IPS in that parameter. The
viewing angles are also very good in the vertical field (an area where TN
panels suffer a great deal) as well as the horizontal field. MVA technology
also provides high contrast ratios, which IPS and TN Film couldn't quite
meet and a decent black depth. In MVA panels, the molecules in the
domains are oriented differently, so if one domain lets light pass through,
the neighboring domain will have the molecules at an angle and will
shutter the light (of course, save for the display of white color, in which
case all the molecules are placed almost in parallel to the plane). Figure
1.2-4 shows the demonstration of four-domain vertical alignment cell.
The protrusions were formed before coating the alignment layer. No
rubbing on the alignment layer is needed. The domains are automatically
7
controlled by the slopes of the protrusions.
Figure 1.2-4 Operation principle of multi-domain vertical alignment LCD.
The conventional IPS LCDs utilize opaque metals for inter-digital
electrodes. Thus, the aperture ratio is reduced to nearly half that of the TN
cell. This leads to a lower contrast and brightness of the matrix.
Super-IPS (S-IPS) panels are mostly produced by LG.Display (previously
LG.Philips) and have gone through several generations since their
inception. Initial S-IPS built upon the strengths of IPS by employing an
advanced “multi-domain” liquid crystal alignment. S-IPS panels have
gained the widest recognition, mostly due to the efforts of another joint
venture LG.Philips LCD (now known as LG.Display), which is
outputting rather inexpensive and high-quality 19”, 20”, 24" and 30"
matrices.
8
1.3 Viewing angle switching technologies
Due to an explosive increase of mobile electronic devices such as the
mobile phone and notebook, privacy protection has recently become a
crucial factor is applications. Users can decide whether to disclose or
share information with others in public places. For this purpose, the
narrow-viewing angle mode is required for mobile applications. Recently,
various viewing-angle switching displays from wide-viewing angle
(WVA) mode to narrow-viewing angle (NVA) mode have been
developed.[8-11] One of the approaches is using additional liquid-crystal
switching panel outside the main display panel.[8] Another approach is
associated with dual backlight system.[9] The other approach is divided
one pixel into two subpixels such that the first pixel is for displaying the
main
image
and
the
second
pixel
is
for
controlling
the
viewing-angle.[10-11] However, these approaches are not suited for
portable applications. Among these approaches, both the additional
switching panel and dual backlight system increase panel thickness,
manufacture cost, and power consumption, and the pixel division
decreases the aperture of main pixel, which results in decreased
brightness of the display.
9
1.4 Motivation and objective
In the past, various LCD modes such as IPS, MVA, FFS, and
advanced super view (ASV) have been extensively developed for wide
viewing angle characteristics, particularly in large flat panel display
applications. Due to an explosive increase of mobile electronic devices
such as the mobile phone and notebook, privacy protection has recently
become a crucial factor is applications. Users can decide whether to
disclose or share information with others in public places. As a result, the
main objective of this thesis is to propose a novel fabrication method of
three-terminal electrode structure based on IPS mode devices. This novel
structure has the simplest way of the cell fabrication, minimization of cell
thickness, and without any additional optical component, those
advantages are suited to mobile applications in IPS cell by applying
fringing field, respectively. Besides, the discussion of simulation results
of director distribution will also be given in this thesis.
1.5 Organization of this thesis
The thesis is organized as following：Physical properties of liquid
crystal, the principles of the phase retardation, and the 3D simulator is
presented in Chapter 2. After that, in Chapter 3, the fabrication process
of viewing angle switching IPS cell in detail. The optimal simulation
results and experimental results, including contrast ratio and discussions
will be in Chapter 4. Finally, the conclusions of this thesis will be
presented in Chapter 5.
10
Chapter 2
Principles
In this chapter, several principles are described for liquid crystal and
display, such as physical and optical anisotropy of liquid crystal, phase
retardation and viewing angle characteristics of display. For building the
model of LCD structure and optical analysis of simulation, the TechWiz
LCD was used to calculate. Finally, a summary will be given.
2.1 Physical properties of liquid crystal
Liquid crystal (LC) is a matter that is intermediate between the
crystalline solid and the amorphous liquid. This intermediate state was
first observed in 1888 in cholesteryl benzoate, a crystalline solid. Many
organic compounds, such as hexylcyanobiphenyl (6CB) and sodium
benzoate, exhibiting this behavior are known and used extensively in
electric and electronic displays. A direct consequence of the ordering of
the anisotropic molecules is the anisotropy of mechanical, electric, and
optical properties. General speaking, there are three phases of LC, known
as smectic phase, nematic phase, and cholesteric phase. For the sake of
clarity, we assume that the LC is made of rodlike molecule.
11
Figure 2.1-1 Mesophases lie between a crystalline solid and liquid with various
temperatures, T m and T c , melting and cleaning temperature.
The scheme of molecular order characteristics of nematic liquid
crystals is shown in Figure 2.1-1, where n is defined as the director axis,
which represents the preferred orientation of LC molecules. Nematic
liquid crystals provide two features in this figure. There is a long range
orientation order, the LC molecules tend to align parallel to each other.
The nematic phase is fluid, and there is no long range correlation of the
molecular center of mass positions.
In the thermal equilibrium state, the nematic phase has infinite
symmetry and is uniaxial. The direction of the principle axes n is
arbitrary in space.
Figure 2.1-2 Scheme of nematic LC orientation director.
12
2.2 Anisotropy physical properties
The physical tendency of LC molecules points along the director and
its behavior is anisotropic. It means the properties of a material depend on
the direction that be measured. All of the LC possesses the specifically
regular arrangement and the important feature is the long axes of LC
molecules are parallel to each other. Due to the specifically regular
arrangement, the values of refractive index (n), dielectric constant (ε),
permeability (χ), conductivity (σ), viscosity (η), etc… are different in
direction of parallel and perpendicular to long axes of LC molecules. For
the anisotropic physical properties, LC also named anisotropic liquid.
Figure 2.2-1 shows the anisotropic physical properties of nematic LC.
Figure 2.2-1 Anisotropy physical properties of nematic LC.
13
2.3 Optical birefringence
The important feature of LC is birefringence. It means that LC
material possesses two different indices of refraction. One index of
refraction corresponding to the light polarized along the director of LC
and the other corresponding to the light polarized perpendicular to the
director of LC. When light get into birefringence material, such as LC,
that modeled in terms of the light separated the fast (ordinary ray) and
slow (extraordinary) components. The two components travel at different
velocities, the waves get out of phase. When the rays are recombined as
they exist birefringence material the polarization state are changed due to
phase difference shown in Figure 2.3-1.
Figure 2.3-1 Light travels at birefringence material.
The birefringence material is characterized by the difference, ∆ n, in
the indices for the ordinary and extraordinary lights. The definition is
given as follows:
∆n = ne − no
(2.3-1)
The condition n e > n o describes a positive uniaxial material and condition
n e < n o describes a negative uniaxial material. The typical nematic LCs
14
are positive uniaxial as shown in Figure 2.3-2.
Figure 2.3-2 Index anisotropic of uniaxial liquid crystal.
In classical dielectric theory, the macroscopic refractive index is
related to the molecular polarizability at optical frequencies. The
existence of the optical anisotropy is due mainly to the anisotropic
molecular structure. Most liquid crystals with rodlike molecules exhibit
positive birefringence ranging from 0.05 to 0.45. The optical
polarizability is due mainly to the presence of delocalized electrons not
participating in chemical bonds and of π electrons. This is the reason that
LC molecules composed of benzene rings have higher values of ∆ n than
do the respective cyclohexane counterparts. The optical anisotropy plays
an essential role in changing the polarization state of light in liquid crystal
display.
15
2.4 Dielectric anisotropy
As a result of the uniaxial symmetry, the dielectric constants differ in
value along the preferred axis (ε // ) and perpendicular to this axis (ε ⊥ ). The
dielectric anisotropy is defined as Δε = ε // - ε ⊥ . The sign and magnitude
of the dielectric anisotropy Δε are of the utmost importance in the
applicability of the LC material in liquid crystal displays using one of the
various electrooptic effects. To illustrate this, we consider a nematic LC
cell in the xy-plane. The z-axis is chosen to be perpendicular to the cell.
Initially, the LC molecules are aligned parallel to the xy-plane. An electric
field is then applied in the cell. First, we consider the elastic energy
density due to the redistribution of the directors.
In liquid crystal displays, an electric field is often applied to cause a
reorientation of the molecules. It’s the balance between the electric torque
and the electric restoring torque that determines the LC’s static
deformation pattern, which can be divided into a combination of three
basic deformations, are splay, twist, and bend. Follow the notation of
Oseen-Frank theory, the elastic energy density of a deformed LC can be
written []
=
U
 2

 2

 2
1
k1 ∇ ⋅ n + k2 n ⋅ ∇ × n + k3 n × ∇ × n 

2 
(
)
(
)
(
)
(2.4-2)
where k1 , k2 , k3 are the splay, twist, and bend elastic constants and n is
a unit vector representing the director distribution in the cell.
As a result of anisotropy, the induced dipole moment of the
16
molecules is not parallel to the applied electric field, except when the
molecular axis is parallel or perpendicular to the electric field. This
creates a net torque that tends to align the molecules along the direction
of the electric field for most rodlike molecules. In a homogeneous
medium the displacement field vector D is independent of the orientation
of the liquid crystal. Letting θ be the angle between the director and the z
axis, we can express z component of the displacement field vector as
=
Dz
(ε cos θ + ε
2
||
⊥
sin 2 θ ) E
(2.4-3)
Thus the electrostatic energy can be written as
1
Dz2
U=
2 ε || cos 2 θ + ε ⊥ sin 2 θ
(2.4-4)
For liquid crystal with positive dielectric anisotropy (ε // > ε ⊥ ), lowest
electrostatic energy occurs at θ = 0 when the director is parallel to the
applied electric field.
If an electric field is applied to the cell along the y-axis, the LC
molecules are aligned toward the y-axis. The distribution of the director
twist angle Φ(z) is a result of the balance between the elastic restoring
force and the electric force , as well as the boundary conditions. The
elastic energy density can be written
1  dφ 
U = k2 

2  dz 
2
(2.4-5)
where Φ is the twist angle measured from the x-axis and k2 is the twist
elastic constant. The net change in the electromagnetic energy density due
to the reorientation of the LC director can be written as
17
U
∆=
1
1
ε ⊥ E 2 − (ε // sin 2 φ + ε ⊥ cos 2 φ ) E 2
2
2
(2.4-6)
2.5 Phase retardation effect
When a plane wave is incident normally or obliquely to a uniaxial
liquid crystal layer sandwiched between crossed polarizers, the outgoing
bean will experience a phase retardation δ due to the difference
propagation velocities of the extraordinary and ordinary rays inside the
liquid crystal layer and the cell gap.
δ
=
2π d
λ
( ne − no )
(2.5-1)
Dark state is especially important for any normally black operation
LCD. A display with low contrast implies that the dark state is not
sufficient black. When a homogeneous cell is sandwiched with two
polarizers, the normalized light transmittance is governed by the
following equation
β 
T =−
cos 2 χ sin 2 β sin 2 ( β − χ ) sin 2  
2
(2.5-2)
Where χ is the angle between the polarizer and analyzer, β is the angle
between the polarizer and the LC directors, and δ is the phase retardation.
The effective phase retardation depends on the wavelength. For a 90° TN
LCD, the transmission property can be investigated by using the Jones
matrix method. We mentioned earlier that waveguiding occurs as the
beam propagates in the LC cell. And the transmitted beam is linearly
polarized with a rotation of the plane polarization by 90°. Strictly
18
speaking, the waveguiding is valid only in the Mauguin condition
Φ <<
2π
λ
∆nd
(2.6-3)
where Φ is the total twist angle of the LC cell.
Figure 2.5-1 Scheme of drawing of the transverse field mode LCD with normally
black operation, where V is the applied voltage and V th is the threshold voltage. (a) In
the field-OFF state, the LC director is parallel to the transmission axis of the polarizer.
The transmission is zero, due to the crossed polarizers. (b) In the field-ON state, the
director is aligned parallel to the electric field. Maximum transmission occurs with a
proper choice of the cell thickness.
Referring to Figure 2.5-1, we consider another switching mode of the
homogeneous anti-parallel aligned nematic cell for LCD. In the 90° TN
cell, the applied electric field is in the longitudinal direction. In this case,
the applied electric field is in the xy-plane parallel to the LC layer surface.
The optical wave traversing through the LC cell is an extraordinary wave
whose polarization state remains unchanged. This leads to a zero
19
transmission due to the crossed polarizer configuration. When an electric
field is applied to the LC cell, the LC molecules are aligned toward the
electric field (along the y axis). The LC directors are rotated in the plane.
This leads to a new director distribution with a twist Φ(z) in the xy-plane.
As a result of the boundary condition due to the anchoring of LC
molecules with the rubbed directions, the twist angle Φ is a function of
position z. Except for a small pretilt, the tilt angle θ is zero since the
electric field is in plane. Unlike the TN LC cells, the twist here is not a
simple function of z. Thus, a simple transmission formula is not available
for this case. However, if the applied field is sufficiently strong enough to
ensure alignment of most of LC molecules along the direction of the field,
the LC cell is approximately in a homogeneously anti-parallel alignment,
by neglecting the thin boundary layers at the surfaces. In this
approximation, the transmission can be written
=
T
δ
1 2
sin (2∆φ )sin 2 ( )
2
2
(1-2.4.4)
where ∆φ is the twist angle relative to the transmission axis of the
polarizer due to the applied field, and δ is the phase retardation of the
LC cell. In the field-OFF state, ∆φ =
0 , the transmission of T = 0.
Selection of a proper cell thickness such that δ = π , and a proper
reorientation of the LC director such that ∆φ = 45° , the transmission of
1
T= .
2
20
2.6 Viewing angle characteristics
Figure 2.6-1 Polar coordinate system for viewing angle definition.
To describe the orientation of the polarization and the director of the
liquid crystal, we define the azimuth Φ as the angle in the xy-plane
measured from the x axis in Figure 2.6-1. We define the tilt θ as the angle
between the director and the xy-plane, measured from the xy-plane. With
this definition, Φ = 0° is parallel to the x axis, Φ = 90° is parallel to the y
axis, θ = 0° is parallel to the z axis, and θ = 90° is parallel to the xy-plane.
As a result of the different optical path, the birefringence, and distribution
of LC director n(z), the transmission of the display is intrinsically
dependent on the angle of incidence (θ, Φ). The transmission properties
of the LCD as functions of horizontal and vertical viewing angles at
various applied voltages are shown by Yeh and Gu.
21
2.7 TechWiz LCD simulation software
This section describes the main frame and flow charts of TechWiz
LCD. Moreover, flow charts of each module and various results will be
described.
Figure 2.7-1 Flow chart of the TechWiz LCD.
TechWiz
LCD
provides
various
modules
which
simulate
electro-optical characteristics of the LC cell. Material DB module to
manage various material for the simulation. A material database contains
the material name, type and property informations. Material types are
classified into LC, Insulator, Metal, Polarizer, Retarder, and Color Filter
and each material type has different kinds of material properties.
22
TechWiz LAYOUT is a layout tool that represents the masks that are
used to generate 3D mesh structures for computational calculation
concerned with physics of semiconductor. TechWiz LAYOUT describes
a layout design in terms of libraries, cells, instances, and mask primitives.
Figure 2.7-2 3D-structure file from the 2D-layout of TechWiz Layout.
LC Analysis module is for the transient analysis of electric potential,
electric field distribution, orientation direction of LC molecules, parasitic
capacitances, and so on. Optical Analysis module for analyzing optical
characteristics of designed cell can analyze transmittance, contrast ratio,
chromaticity, color difference, gamut, Poincare sphere, etc. In case that
the Variation is Voltage, Transmission module analyzes transmittance
with director distribution data at Final time of Operating Time in LC
Analysis. In case that the Variation is Time, Transmission module
23
analyzes transmittance with director distribution data for time variation.
The data is decided by Final Time, Times Step, and Output Step in LC
Analysis module.
Figure 2.7-3 Variation condition of Transmission module according to voltage
condition of LC Analysis.
24
2.8 Summary
In chapter 2, the phase retardation can be calculated by considering
the LC director distribution with incident light in polar angle system.
Physical properties also play an important role in director distribution
with applying electric field. According to these characteristics, we then
preceded some research objectives to present the transmittance in this
thesis.
25
Chapter 3
Devices fabrication and measurement
In this chapter, the fabrication process and measurement instruments
are presented in the following sections. The proposed cell configuration
was an inter-digitated type electrode on one glass substrate and was
combined with an opaque plane electrode on the other glass substrate.
Additionally, the optical measurement system and optical microscope can
obtain cell data and evaluate cell viewing angle performance.
3.1 Devices fabrication
To fabricate the cell, the first priority process is to pattern the
inter-digitated type electrode with photolithography method. The
fabrication steps are listed below, and the substrate pretreatment is shown
schematically in Figure 3.1-1 and Figure 3.1-2.
I.
Inter-digitated type pattern electrode
i.
Substrate pretreatment：For the display application, the glass is
widely used as a substrate. The opaque ITO thin film was
uniformly
coated
on
the
glass
substrate.
Before
photo-lithography process, the ITO glasses need to be cleaned
definitely.
26
ii.
Photo-lithography：First of all, positive photoresist was coated on
clean ITO glass. Then the ITO glass was exposed under
ultra-violet (UV) light source through a patterned mask by Mask
Aligner and Exposure System. Consequently, the pattern on the
mask was transformed to the positive photoresist after
developing.
iii.
Dry etching：After the pretreatment of exposure and development,
the ITO layer on the glass can be etched by Inductive Couple
Plasma Etcher (ICP). Removing the residues photoresist, the
patterned ITO substrate with inter-digitated pattern was obtained.
Figure 3.1-1 Flow fabrication of inter-digitated electrode. (a) Clean ITO glass
substrate, (b) spin-coating positive photoresist upon the ITO surface, (c) using
lithography technique to obtain the latent image, (d) etching to produce the desired
ITO pattern, and (e) eliminating the remaining photomask by acetone.
27
II. Cell fabrication process
i.
Alignment layer coating：Polyimide thin film was spin-coated
upon the ITO layer and baked by heating. Thereafter, the solid
thin film was treated by rubbing technique to perform the
substrate preparation.
ii.
Cell process ： One glass substrate with inter-digitated type
electrode was glued with one clean ITO glass substrate
containing plastic ball spacers. The diameter of spacer with
specific value is required for maintaining the cell gap and optical
performance. Finally, the space between two substrates was
filled with nematic liquid crystal.
Figure 3.1-2 Cell fabrication process. (a) Clean inter-digitated type electrode, (b)
spin-coating polyimide alignment layer on substrate, (c) combining two substrates
with gel containing spacers, (d) filling in liquid crystal between two substrates.
28
3.2 Measurement system
In order to inspect the quality of the IPS cell and evaluate the
variation of the optical performance, the optical microscope and the
Conoscope were utilized in this thesis. The Conoscope system applies
Fourier transform lens to transfer light beams and emitted from the test
area to a CCD array. Therefore, the angular properties can be easily
measured on the CCD sensor plane. The CCD array consists of various
directional CCD sensors which detect brightness, color, and angular
distribution of transmissive light. Besides, not only transmissive type but
also reflective type can be measured through the operation mode.
The working principle of transmissive mode is depicted in Figure
3.2-1, where the first lens provides a Fourier transform image of the
display surface. Each light emitted from the test area at incident angle, θ,
will be focused on the focal plane at the same azimuth and at a position
x=F(θ). The sample angular characteristics are thus measured simply and
quickly, without any mechanical movement.
29
(a)
(b)
Figure 3.2-1 (a) Conoscope 80 (Autronic Melchers), (b) Scheme of the Conoscope
system in transmissive mode.
3.3 Summary
In order to simplify the simulation and evaluate the designed optical
performance, the Conoscope system, which can measure the test sample
and angular luminance distribution, was adopted. Then the measurement
result was imported into simulation results.
30
Chapter 4
Simulation and experimental results
In this chapter, the simulation for switching the viewing
characteristics of IPS LCDs with three-terminal electrode configuration
by some factors：LC cell gap (d), IPS electrode width (w ± ) and electrode
separation gap (l). By using the simulation software, the simulation
results of the designed configuration were built and are shown in this
chapter.
4.1 Design of three-terminal electrode configuration
As section 1.2-1 mentioned that the general issues of conventional
in-plane switching (IPS) LCD with inter-digital type electrode and the
characteristic of wide viewing angle. Because our objective was to
control the viewing angle switching of wide viewing angle mode and
narrow viewing angle mode without any additional switching panel. Thus,
the applied third planar electrode into conventional IPS cell was
necessary. In this study, we proposed three-terminal electrode
configuration to control the LC director orientation due to fringe field
distribution for viewing angle switching. As showed in Figure 4.1-1, the
structure was suited for LCD application whether the portable device or
not, thus the fabrication was simple for any size of display panel. The
idea for this design using the inter-digitated type electrode with the high
31
transmittance of viewing characteristic due to the LC molecules were
in-plane twisted mostly. It can be considered as the bright state of
conventional in-plane switching (IPS). The operation principle of viewing
angle switching is to control the LC orientation from homogeneous
alignment to homeotropic alignment with longitudinal fringe field.
Therefore, the electrodes and cell conditions were the crucial problems of
this design. The viewing angle characteristic of this structure is showed in
Figure 4.1-2.
Figure 4.1-1 Schematic of this cell with three-terminal electrode.
32
(a)
(b)
Figure 4.1-2 Viewing angle characteristics for (a) dark state for wide view and (b)
dark state for narrow view.
To evaluate the suitable value of dark state for narrow viewing angle,
the relationship of electrode width w, electrode separation gap l and LC
cell gap d, were considered as illustrated in Figure 4.1-3.
Figure 4.1-3 Schematic of electrode width w and gap l with LC cell gap d.
33
The fabrication uniformity is shown in Figure 4.1-4. Because the
patterned electrode were etched by photo-lithography method, the
uniformity of electrode was not easy to be great for large size sample in
academic community due to the defects of electrode etching. Therefore,
the measurement of this study should be avoided the defect area of the
sample.
Figure 4.1-4 Top view of the sample with photo-lithography and dry etcher. The
bright area of the ITO electrode was etched and the dark area was maintained to be
the patterned ITO electrode.
In order to define the viewing angle characteristics, the definition of
the diagram is shown in Figure 4.1-5. Where polar angle θ and azimuthal
angle Φ are the angle between the normal direction in the vertical and
horizontal planes, respectively.
34
Figure 4.1-5 Diagram of the definition of viewing angle.
In this study, the three-terminal electrode LCD was measured by
Conoscope [] in transmissive mode. Then the measures data is important
to compare with simulation results. The next section will show the
operation principle of wide-viewing angle mode and narrow-viewing
angle mode.
35
4.2 Operation principle and viewing angle control
In this study, the cell was fabricated with LC homogeneous
alignment under crossed polarizers. When the applied voltage was turned
OFF, the linear polarization light cannot transmit the other polarizer due
to zero phase retardation. The state is a great dark state for wide-viewing
angle mode and all LCD applications. It’s shown in Figure 4.2-1.
Figure 4.2-1 Schematic of great dark state with LC homogeneous alignment under
crossed polarizers.
When voltage applied on the bottom IPS electrode, the liquid crystals
were rotated with transverse electric field between electrode separation
gaps. When the linear polarization light passed through the LC layer, the
polarization of the incident light was changed into circular polarization or
36
elliptical polarization. Finally, the incident can transmit the other
polarizer under crossed polarizers. This state is great to be the bright state
for both wide-viewing angle mode and narrow-viewing angle mode. The
schematic of this state is shown in Figure 4.2-2.
Figure 4.2-2 Schematic of great bright state for both wide-viewing angle mode and
narrow-viewing angle under crossed polarizers.
When the voltage applied on the third electrode, the liquid crystals
were tilted by the longitudinal electric field with whole LC layer. The
longitudinal field was not uniform near the edge of the IPS electrode.
Therefore, the liquid crystals near the bottom IPS electrode were rotated
by the fringe field between the IPS electrode separation gap areas. Thus,
it will produce low light leakage near the edge of bottom electrode in the
normal viewing direction and large leakage will occur at the oblique
37
viewing direction under crossed polarizers. This state was used to be the
dark state for narrow-viewing angle mode. The schematic is shown in
Figure 4.2-3.
Figure 4.2-3 Schematic of dark state for narrow-viewing angle mode with longitudinal
fringe field under crossed polarizers.
In this study, we proposed two different conditions of dark state and
share the same bright state by applying two distinct electric fields for
switching with wide-viewing angle (WVA) mode and narrow-viewing
angle (NVA) mode. The liquid crystal with different orientation due to
two distinct electric field distributions will have influence to optical
properties such as transmittance in the large viewing direction.
38
4.3 Electrical potential distribution estimation
Ignoring the inter-digitated IPS electrode separation gap, one can
approximate the electrode configuration using the 2D model as shown in
Figure 4.3-1.
Figure 4.3-1 Approximation IPS electrode configuration.
Neglecting the LC dielectric anisotropy, the electrical potential
V ( x, y ) above the inter-digital type electrode is governed by the Laplace
Equation：
∇ 2 V=0
By using separation variables V ( x, y ) =X ( x ) Y ( y ) , and applying the
symmetry condition V ( x, y=) V ( − x, y ) , the periodic condition
 W +W
V  x+ +
+Ws , y  =V ( x, y ) ,
2


and the boundary conditions
39
W
≤
x
≤
0
,
0

2
V ( x, 0 ) = 
,
W
W
W
+
V ,
+ Ws ≤ x ≤
+ Ws +
 c
2
2
2
once can show that from Laplace Equation
Y ( y)
d 2 X ( x)
d 2 Y ( y)
+
X
x
=
0,
(
)
d 2x
d2y
2
2
1 d X ( x)
1 d Y ( y)
+
=
0,
X ( x) d 2x
Y ( y) d 2 y
1
1
X′′ ( x ) +
Y′′ ( y ) =
0.
X ( x)
Y ( y)
For the equation with separating two different variables, we obtain
ordinary differential equation, and every each part is constant. We first
calculate the derivate with respect to x variable
 ′′

d  X ( x) 
= 0,
dx  X ( x ) 


we obtain the following form：
X′′ ( x )
X ( x)
= Constant .
To satisfy the equation, the solution must be X′′ = −k x 2 X , and we can
get homogeneous second order constant coefficient linear ordinary
differential equation
X′′ + k x 2 X =
0.
Using the boundary condition X′ ( 0 ) = 0 and X′ ( W ) = 0 , where
40
W≡
W+ +W+Ws , the correspond to the real value solution is
2
X ( x ) = Cx Cos[k x x] ,
 nπ  2
kx2 = 
 =k ,
W
kx =
nπ
.
W
Then we calculate the derivate with respect to y variable
 ′′

d Y ( y)
= 0 , we also can get the differential equation Y′′ − k y 2 Y =
0 , it
dx  Y ( y ) 


satisfy the relation k x 2 + k y 2 =
0 , and the corresponding solution to this y
variable differential equation must be Harmonic Function,
Y=
( y ) C y e- ky + Dy eky ,
according to boundary condition y → ∞, Y ( ∞ ) =0 , the function can be
written as
Y ( y ) = C y e- ky .
By combining above two solutions of different variables, we can
obtain the electrical potential
Vn ( x, y ) =X ( x ) Y ( y ) = anCos[k x x]e- ky ,
using boundary condition V =
( x, 0 ) Vc ,
WW
W
+ Ws ≤ x ≤ - + Ws + + , the
2
2
2
electrical potential can be written as
V ( x, 0 ) = Vc =
∞
∑ a Cos[kx],
n =1
n
WW
W
+ Ws ≤ x ≤ - + Ws + + .
2
2
2
First we calculate the coefficient an of electrical potential
41
an =
=
W
2
W
∫
Vc Cos[kx] dx
W+Ws
2
1
2
Vc  Sin ( kx )
W  k
W
W+Ws
2



,

  W 
+Ws   
 Sin ( kW ) -Sin  k 
 
  2

2V
 W

= - c Sin  k  - +Ws  
nπ

  2
=
2Vc
kW
the potential can be shown
V
 nπ  W

 nπ
−2∑ c Sin   - + Ws   Cos 
V ( x, y ) =
W

n =1 nπ
W  2
∞
nπ
 − y
x  e W + C0 ,

then we need to calculate the coefficient C0 by considering the boundary
condition
=
Vc V ( x, 0 ) ,
WW
W
+ Ws ≤ x ≤ - + Ws + + ,
2
2
2
2 W
V ( x, 0 ) dx
W
W ∫ 2- +Ws
2V W
= c ∫W- dx
W 2 +Ws
.
2Vc W+
=
W 2
W+
=2Vc
W+ +W- +2Ws
C0 =
The electrical potential distribution can be shown
V ( x, y ) =
2Vc
∞
V
 nπ  W
W+

 nπ
− 2∑ c Sin   - + Ws   Cos 
W+ +W- +2Ws
W

n =1 nπ
W  2
nπ
 − y
x e W .

Therefore, as y is increased, V(x, y) decays exponentially. The
characteristics decay length can be controlled by selecting a suitable W
42
for the IPS electrodes. For an LC cell gap larger than a few multiples of
W, only the alignment of the LC molecules close to, but not those far
away from, the IPS electrodes are directly affected by V (x, y).
4.4 WVA mode and NVA mode
In order to control the viewing angle switching between two different
modes, we should realize the dark state for narrow-viewing angle (NVA)
mode in detail. Here we take the dark state for WVA mode in considering
compare to the dark state for NVA mode. As shown in Figure 4.4-1, the
dark state for WVA mode due to zero phase retardation is perfect dark
whether in the normal viewing direction or not.
Figure 4.4-1 Dark state for WVA mode with LC homogeneous alignment under
crossed polarizers.
43
We can choose two conditions to know the dark state for WVA mode.
One condition is in the direction of the IPS electrode and the other
condition is perpendicular to the direction of the IPS electrode at oblique
viewing angle. Those two conditions are shown in Figure 4.4-2.
Figure 4.4-2 Viewing characteristics in the normal viewing direction and at the
oblique viewing angle of IPS electrode direction.
As shown in Figure 4.4-2, the optical properties of dark state at
oblique viewing direction are perfect dark. The contrast ratio values at
oblique viewing direction maintain high quality with high contrast. It can
be considered as conventional IPS structure with wide viewing angle.
Compare to dark state for WVA mode, light leakage of the dark state
for NVA mode with longitudinal fringe field distribution due to LC
44
orientation occurs in the large viewing angle of the direction of the
inter-digitated electrode under crossed polarizers. The LC distribution is
shown in Figure 4.4-3.
Figure 4.4-3 Dark state for NVA mode with longitudinal fringe field.
According to Figure 4.4-3, the light leakage will occur in the large
viewing angle of the direction of the inter-digital electrode under crossed
polarizers. Light leakage resulted from LC phase retardation will reduce
the contrast ratio at oblique viewing angle.
The viewing characteristic is shown in Figure 4.4-4. Compare to the
viewing property in the normal viewing direction, the light leakage due to
the phase retardation result from tilted LC distribution will decrease the
contrast ratio. Fringe-field distribution can be control by some factors
including inter-digitated IPS electrode width and separation gap. Those
corresponding relationship will discussion in the later section.
45
Figure 4.4-4 Viewing characteristics in the normal viewing direction and at the
oblique viewing angle of IPS electrode direction of NVA mode.
46
4.5 Simulation results of viewing angle switching
To ensure the optical properties of the proposed three-terminal
electrode structure with viewing angle switching characteristics, we use
commercial LCD simulator TechWiz LCD (Sanyi) to compute
numerically. The simulation configuration is shown in Figure 4.5-1.
d
w−
w+
l
Figure 4.5-1 Schematic of Cell configuration for simulation with three-terminal
electrode. The electrode width are w + and w - . The electrode separation gap is l . The
LC cell gap is d.
First we use a simple configuration to confirm the property of the
proposed structure. By using TechWiz Layout to set a structure file with
electrode width w (w = 8μm), electrode separation gap l (l = 8μm) and LC
47
cell gap d (d = 7.5μm). The LC material we used here is ZTO-5024 (Δn =
0.113 at 550nm). The LC was filled in the structure with homogeneous
alignment by the rubbing angle to electrode γ (γ= 3°).
Figure 4.5-2 Cell configuration for simulation with three-terminal electrode. The
electrode width w (w + = w - =w) and electrode separation gap l (l) are both 8 μm. The
LC cell gap d (d) is 7.5 μm.
First, we set voltage condition to find the bright state of this structure
with maximum transmittance. By sweeping the voltage range from 0 V rms
to 6 V rms , the maximum transmittance occurs in 4.2 V rms . Voltage
dependent transmittance curve is shown in Figure 4.5-3. The
transmittance contour is shown in Figure 4.5-4.
48
0.14
Transmittance (a.u.)
0.12
0.10
0.08
0.06
0.04
0.02
0.00
0
1
2
3
4
5
6
Applied voltage (Vrms)
Figure 4.5-3 Voltage dependent transmittance curve with electrode width w (w + =
w - =w) and electrode separation gap l (l) are both 8 μm. The LC cell gap d (d) is 7.5
μm.
Figure 4.5-4 Iso-transmittance contour with applied voltage V= 4.2 V rms .
49
Then we sweep the dark state for NVA mode with the voltage range
from 0 V rms to 50 V rms , the minimum transmittance occurs in 37 V rms .
Voltage dependent transmittance curve of dark state for NVA mode is
shown in Figure 4.5-5. The transmittance contour is shown in Figure
4.5-6.
0.07
Transmittance (a.u.)
0.06
0.05
0.04
0.03
0.02
0.01
0.00
0
10
20
30
40
50
Applied voltage (Vrms)
Figure 4.5-5 Voltage dependent transmittance curve with electrode width w (w + =
w - =w) and electrode separation gap l (l) are both 8 μm. The LC cell gap d (d) is 7.5
μm.
50
Figure 4.5-6 Iso-transmittance contour with applied voltage V= 37 V rms .
Figure 4.5-7 The dark state of WVA mode under crossed polarizers.
The definition of Contrast ratio (CR) is that
51
Contrast ratio (CR) =
Bright state
Dark state
If we choose the bright state which is shown in Figure 4.5-4 to divide
with the dark state which is shown in Figure 4.5-7, the corresponding
contrast ratio (CR) is shown in Figure 4.5-8. The wide-viewing angle
(WVA) mode is defined by Figure 4.5-8. But we choose the other dark
state which is shown in Figure 4.5-6, the corresponding CR is shown in
Figure 4.5-9. The narrow-viewing angle (NVA) mode is defined by
Figure 4.5-9. According to Figure 4.5-9, the horizontal viewing angle is
about 26°.
Figure 4.5-8 Iso-Contrast ratio contour for WVA mode under crossed polarizers.
52
Figure 4.5-9 Iso-Contrast ratio contour for NVA mode under crossed polarizers.
53
4.6 Inter-digitated electrode width and separation gap
In order to find the optimal viewing angle characteristic, we should
consider the quality of dark state for NVA mode and quality of bright
state for both two viewing modes. Let’s go back to the structure with
electrode width and electrode gap (w = l = 8μm, d = 7.5μm), we can
extraction the LC profiles from the structure due to the fringe field
distribution. Figure 4.6-1 shows the cross-section of bright state with
applied voltage V = 4.2 V rms on y-z plane. We can find the distribution of
LC that the most rotated liquid crystal is in the area between electrode
gaps due to transverse filed.
Figure 4.6-1 The LC profile cross-section of bright state with applied voltage V = 4.2
V rms on y-z plane.
In this condition, we can extract the potential distribution from the
structure. The potential distribution is shown in Figure 4.6-2. From the
potential distribution, the transverse electric field between the gap is
54
basically in the horizontal direction. Near the edges of the electrodes the
fields are curved, and at the center the fields are flat.
Figure 4.6-2 Computer-simulated potential distribution of the IPS electrode with
applied voltage V = 4.2 V rms on y-z plane.
Figure 4.6-3 Twist angle of LC director between the electrode gap with
applied voltage V = 4.2 V rms .
55
Because the LC molecules are rotated in-plane, its viewing angle is
wider and more symmetric than of the TN cell. In general, the fringe field
strength within the LC layers is dependent on the electrode gap and width,
and the LC cell gap. However, right above the center of the electrodes,
the fringe field is nearly symmetric and the LC directors remain
untwisted. As a result, the transmittance is reduced right above the
electrodes. Thus, widening the electrode gap would improve the aperture
ratio, however, at the expense of increased operating voltage.
By considering the dark state for NVA mode with longitudinal fringe
field, the light leakage will increase in the normal viewing direction with
widening the electrode gap. As shown in Figure 4.6-4 and Figure 4.6-5,
the LC director profile cross-section and potential distribution are
dependent on the electrode gap and width, and LC cell gap.
Figure 4.6-4 The LC profile cross-section of dark state for NVA mode with
applied voltage V = 37 V rms on y-z plane.
56
Figure 4.6-5 Computer-simulated potential distribution of the proposed electrode
structure
with applied voltage V = 37 V rms on y-z plane.
Figure 4.6-6 Twist angle of LC director of dark state for NVA mode between the
electrode gap with applied voltage V = 4.2 V rms .
57
Figure 4.6-7 Tilt angle of LC director of dark state for NVA mode between the
electrode gap with applied voltage V = 37 V rms .
From the longitudinal fringe field distribution and the LC director
orientation as shown in Figure 4.6-4 and Figure 4.6-5, the fringe filed
near the edges of the electrodes are curved. The LC will twist and tilt
between the electrode gap due to fringe field strength and potential
distribution. Near the edge of electrodes, the phase retardation will
produce by the twisted and tilted LC result in decreasing the contrast ratio
in the normal viewing direction rarely. But in the large viewing direction,
massive light leakage occurs due to the effective phase retardation at
oblique angle. The experimental results will show in the latter sections.
When we change the electrode parameters with electrode width and
gap and LC cell gap, the transmittance of the dark state and the bright
state for both two modes may increase or decrease due to the fringe field
58
distribution will affect the contrast ratio. As shown in Figure 4.6-8, the
variation of electrode width and gap will affect the contrast ratio in the
normal viewing direction. The optimal structure can be pointed out from
the figure.
(a)
(b)
Figure 4.6-8 Iso-contrast ratio with variations of (a) electrode width and (b) electrode
gap.
59
4.7 Experimental results
In this section, the optical results including bright state and dark state
for each viewing mode will be shown in photography and Conoscope
system. The cell structure of the sample here we used is a LC cell gap d =
4.8 μm, electrode width and gap w = l = 5 μm, and the LC directors are at
near 3° with respect to the long edges of inter-digitated electrodes. The
LC materials used for experiment have Δε = +9.4, Δn = 0.113 at 550nm,
and homogeneous alignment. The photography for different applied
voltage conditions with bright state and two dark states for WVA and
NVA mode are shown in Figure 4.7-1.
0V
1V
2V
3V
4V
4.2V
5V
6V
7V
8V
9V
10V
(a)
60
0V
1V
2V
3V
4V
5V
6V
7V
8V
9V
10V
20V
30V
40V
50V
(b)
Figure 4.7-1 Photography are in different voltage conditions. (a) Bright state for WVA
mode, (b) Dark state for NVA mode.
61
The dark state for NVA mode was shown in Figure 4.7-2 in different
viewing angles with applied voltage V = 20 V rms . The light leakage
occurs in the large horizontal viewing directions.
(a)
(b)
(c)
Figure 4.7-2 Light leakage of dark state for NVA mode with applied voltage V = 20
V rms . (a) Vertical viewing angle from top to bottom. (b) Right viewing angle. (c) Left
viewing angle.
62
The viewing-angle characteristics measured by the Conoscope
system (Autronic-Melchers GmbH) are shown in Figure 4.7-3.
(a)
63
(b)
Figure 4.7-3 Viewing angle characteristics of the proposed structure are measured by
Conoscope. (a) WVA mode (b) NVA mode.
64
4.8 Summary
According to Figure 4.6-8, the contrast ratio will decrease
corresponding to the increased electrode width due to the transmittance
of the bright state for WVA mode. But widen the electrode gap will
increase the light leakage in the normal viewing direction, and then the
contrast ratio is decreased for NVA mode.
65
Chapter 5
Conclusions and future work
5.1 Conclusions
In this study, the optical properties for WVA and NVA mode are
shown in simulation and experiment results. For the WVA mode, the
contrast ratio relies on the electrode gap and electrode width. But for the
NVA mode, the light leakage increased due to the electrode gap from
phase retardation near the edge of the electrodes in the normal viewing
direction. The fringe field will rotate the LC molecules between
electrodes. The dark state for NVA mode was not black enough to
increase the contrast ratio. The uniformity of the cell structure with
photolithography process and dry etching may cause the patterned ITO
electrode defects. The LC cell gap is also the problem to control the
optical performance. The structure condition will affect the viewing angle
characteristics directly.
5.2 Future work
In this study, the NVA mode was control in the direction of IPS
electrode. If we can find another patterned electrode to produce the
three-dimensional fringe field distribution, the phase retardation in the
large viewing direction may be produced through the directions of the
crossed polarizers.
66
[1] C. H. Park, S. H. Lee, J. K. Jeong, K. J. Kim, and H. C. Choi, Appl. Phys. Lett. 89,
101119 (2006).
[2] A. Takeda, S. Kataoka, T. Sasaki, H. Chida, H. Tsuda, K. Ohmuro, Y. Koike, T.
Sasabayashi, and K. Okamoto, Soc. Info. Display Symp. Digest, 29, 1077 (1998).
[3] S. G. Kim, S. M. Kim, Y. S. Kim, H. K. Lee, S. H. Lee, G. D. Lee, J. J. Lyu, and
K. H. Kim, Appl. Phys. Lett. 90, 261910 (2007).
[4] S. H. Lee, S. H. Park, S. T. Oh, M. H. Lee, and G. D. Lee, Appl. Phys. Lett. 86,
031108 (2005).
[5] M. Oh-e, and K. Kondo, Jpn. J. Appl. Phys. 36, 6798 (1997).
[6] S. H. Lee, S. L. Lee, and H. Y. Kim, Appl. Phys. Lett. 73, 2881 (1998).
[7] M. S. Kim, S. M. Seen, and S. H. Lee, Appl. Phys. Lett. 90, 133513 (2007).
[8] E. Jeong, Y. J. Lim, J. M. Rhee, S. H. Lee, G. D. Lee, K. H. Park, and H. C. Choi,
Appl. Phys. Lett. 90, 051116 (2007).
[9] K. W. Chien, Y. J. Hsu, and H. M. Chen, SID Int. Symp. Digest Tech. Papers 37,
1425 (2006).
[10] H. S. Jin, H. S. Chang, J. K. Park, S. K. Yu, D. S. Lee, and I. J. Chung, SID Int.
Symp. Digest Tech. Papers 37, 729 (2006).
[11] Y. J. Lim, E. Jeong, M. H. Chin, S. H. Ji, G. D. Lee, and S. H. Lee, J. Phys. D:
Appl. Phys. 41, 085110 (2008).
67