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Fundamentals of Active-Matrix Liquid-Crystal Displays
===== SID ‘01 SHORT COURSE =====
Sang Soo Kim, Ph.D.Vice President, AMLCD Div.Semiconductor BusinessSamsung Electronics Co., Ltd.Kyunggi-Do, Korea
(S-2)
(Sunday, June 3, 2001)
Fundamentals of Active-Matrix Liquid-Crystal DisplaysI. Introduction
II. Liquid Crystal Displays
III. Structure of Color TFT-LCDs
IV. Basic Operation Principles &Design of Color TFT-LCDs
V. Color TFT-LCD Fabrication Process
VI. Summary and Projections
I. Introduction
• What is Liquid Crystal ?
• Structure of L/C
• Alignment of L/C
• TN & STN Modes
• Normally White and Black Modes
F
F
FFluorine
F
C N
Flexible Part Rigid Part
Alkyl group Biphenyl group Terminal Group
C C C N
C C C NH N
What is Liquid Crystal ?
Figure 1. The structure of a L/C
Cyano Group
Terminal Group
C CH H
H H
C CH H
H H
H ...
Alkyl Chain
Solid Crystalline
Smectic Phase Nematic Phase Liquid Isotrope
Liquid Crystalline
Tm(Melting Point)
Tc(Clearing Point)
Temperature
Phases of L/C vs. Temperature
Figure 2. Phases of a Liquid Crystal
* Operating Temperature Range for Display Application
Birefringence: ∆n = -ne no
Dielectric Anisotropy: ∆ε = -εe εo
D(Director)
no εo ne εe
Structure of Liquid Crystal
D
E
DE
p-type (∆n >0) n-type (∆n <0)
C NF
Figure 3. Anisotropy of a L/C
F
C OO
OC2H5
ne > no ne < no
Nematic
n
Figure 4. Types of liquid crystal phases
n
nn
n
n
Cholesteric
Chiral Dopant
Intermolecular Attraction: Long Axis > Short Axis
move
Figure 5. Types of Liquid Crystal Phases
Smetic C(SmC)
n
Tilted to the layer
Smetic A(SmA)
n
Perpendicular to the layer
move
Intermolecular Attraction: Short Axis > Long Axis
Alignment Layer
Glass Substrate
L/C Molecule
Alignment of Liquid Crystal
Figure 6. Liquid crystal alignment layer
Interaction: L/C Molecule & Substrate
STN Mode
Φ = 90° 180°< Φ < 270°
TN and STN Modes
Figure 7. Orientation of L/C molecules in TN and STN cells
Mauguin’s Condition for TN : ∆n? p = ∆n? d x 2π/Θ > λ
D~ 5µm
Retardation for TN: ∆n? d = 0.3~0.5µm
TN Mode
( )T = sin2
u u
w u =n d
2 2 2π
λ
1 1
2
+
+
⋅
/
/ith
∆
Design of TN Cell
u
T
3 15 35
1st Minimum (∆n d ~ 0.48µm)
2nd Minimum (∆n d ~ 1.47µm)
3rd minimum
Normal Black Mode
Figure 8. Design of TN cell
d~ 5µm(1st min.)
Gooch-Tarry’s Law:
∆n=0.09~0.10 5µm 14.7µm
1.0 2.0 3.0 4.00
100
50
0
90% Trans.
10% Trans.
VsatVth
TN
Transmittance(%)NW-Mode
TN-Mode
V-T Characteristics
Figure 9. V-T curves for TN and STN cells in NW mode
L/C Voltage (V)
(V10/V90 ~ 1.6)
STN
STN-Mode(V10/V90 ~ 1.06)
Normal White (NW) Mode:• Higher C/R, True Black• Less Cell Gap Dependent
L/C
Polarizer(1)
Light On
Polarizer(2)
Light Off
BacklightBacklight
E
NW Mode TN Cell
Cross Nicols
Figure 10. Normally white mode TN cell
0 volt 5 volt
OpticalRotation
NoOptical
Rotation
L/C
Polarizer(1)
Light Off
Polarizer(2)
Light On
BacklightBacklight
E
NB Mode TN Cell
Figure 11. Normally black mode TN cell
0 volt 5 volt
II. Liquid Crystal Displays
• Passive and Active Matrix LCD’s
• Kinds of AMLCD’s
• TN (Twisted Nematic)
• STN(Super TN)
• DSTN(Double STN)
• FLC(Ferroelectric LC)
• GH(Guest-Host)
• DS(Dynamic Scattering)
• PDLC(Polymer Dispersed LC)
• VA(Vertical Alignment)
• IPS(In-plane Switching)
Liquid Crystal Operating Modes
Segment & Dot-Matrix Driving
Figure 12. Example of rendering an L/C image using direct driving
Segment Display (7-segment)
Dot-Matrix Display (5x7 matrix)
Multiplex Driving of Dot-Matrix Display
Figure 13. Example of rendering an L/C image by multiplex driving
y1
y2
y3
y4
y5
y6
y7
x1 x2 x3 x4 x5 x6
Signal Electrodes
ScanningElectrodes
x1 x2 x3 x4 x5 x6y1
y2
y3
y4
y5
y6
y7
• Projection Type: LCD Projector, OHP, Projection TV
• Direct View Type: Notebook PC, LCD Monitor, Potable TV, ViewCam
• Reflective Type: PDA, Cellular Phone, Game
• Transflective Type : PDA, etc.
Application of LCDs
Blue Red Green
LCD(1)
LCD(2)
LCD(3)
ProjectionLens
Mirror
Mirror
Screen
Dichroic Mirror
R G BI I I
3I
Dichroic Mirror
LCD Projector (3-Panel System)
Figure 14. LCD Projector using three black and white LCD’s
Composed Color Image
Lens System
Mirror
Screen
MirrorMirror
Lamp
LCD Panel(Color)
LCD Projection TV (Single-Panel System)
R (I/3)G(I/3)
B(I/3)
Figure 15. LCD projection TV using a color LCD
Spatially dividedColor Image
Color TFT-LCD Module (Direct View)
Figure 16. An example of direct view LCD’s
LDI Chip
Chassis UnitBEFDiffuser
LGPBacklight Lamp Reflector
LCD Panel
Kinds of AMLCD’s
Passive Matrix LCD (PMLCD)
Active Matrix LCD (AMLCD)
MIM-LCD
Diode-LCD
TFT-LCD
a-Si TFT-LCD
poly Si-LCD
Low Tem. poly-Si LCD
High Tem. poly-Si LCD
III. Structure of Color TFT-LCD
• Color TFT-LCD Panel
• Driving Circuit Unit
• Backlight and Assembly Unit
? LCD Panel
? TFT-Array Substrate
? Color Filter Substrate
? Driving Circuit Unit
? LCD Driver IC (LDI) Chips
? Multi-layer PCBs
? Driving Circuits
? Backlight & Chassis Unit
? Backlight Unit
? Chassis Assembly
? LCD Panel
? Driving Circuit Unit
? Backlight & Chassis Unit
PCB LDI
TCP
TFT-Array Substrate
Color FilterSubstrate
Chassis
LGPLamp
Structure of Color TFT-LCD
Figure 17. Structure of a color TFT-LCD module
Color Filter Substrate
TFT-Array Substrate
TFT
Storage CapacitorPixel Electrode(ITO)
Figure 18. The vertical structure of a color TFT-panel
Structure of Color TFT-Panel
Black Matrix
Color-Filter(Blue)
Common Electrode(ITO) Polarizer
Polarizer
L/C
Seal
Bonding PAD
Short
Spacer
AlignmentLayer
FPCConnectorSource PCB
Gate PCB
LCD Control ASIC
LDI Chip(Source)
LDI Chip(Gate)
LCD panel
Structure of Driving Circuit Unit
Figure 19. Assembly of LCD driving circuits
InterfaceConnector
Types of Backlight Units
Top-down
CCFL(Cold Cathode Fluorescent Lamp)Reflector
Light Diffuser
Figure 21. Two different types of LCD backlight systems
Edge-light DiffuserLight
CCFLLGPReflector
Types of LCD Module Package
Driving Circuit Unit
Chassis UnitFlat TCP
CCFLLGP ReflectorDiffuser
LCD Panel
Figure 20. Slim type LCD module package
CCFL(d<2.0mm)
Tapered LGP(t<2.5mm)
Chassis
Bent TCP
Driving Circuit Unit
Improvement of Backlight Brightness
Diffuser
Reflector SheetLamp Reflector LGP
CCFL
Prism Effect
Prism Sheets(BEF)
* BEF: Brightness Enhancement Film
Figure 22. Improvement of B/L brightness using BEF
IV. Basic Operation Principles andDesign of Color TFT-LCD
• Operation of TFT-LCD Pixels
• Gray Scale Generation
• Color Generation
• TFT Design
• Storage Capacitor Design
• Signal Bus-Line Design
• Aperture Ratio
• Design for Redundant
Break
Structure of Color TFT-Panel
n(600)
3m (800xRGB)
Bonding Pad
TFT-ArraySubstrate
(m x n) Resolution
(3m x n) active matrix
SVGA: 800 x RGB x 600
(2400 x 600) Matrix
Color-FilterSubstrate
Figure 23. Active matrix structure of a color TFT-panel
R G B
Unit Dot(R,G,B sub-pixels)
Resolution # of Dot # of PixelAspectRatio
Remark
320 x 240 76,800 230,400 4:3 Quarter VGA
640 x 400 256,000 768,000 16:10 EGA
640 x 480 307,200 921,600 4:3 VGA
800 x 480 384,000 1,152,000 15:9 Wide VGA
800 x 600 480,000 1,440,000 4:3 SVGA
1024 x 600 614,400 1,843,200 ~17:10 Wide SVGA
1024 x 768 786,432 2,359,296 4:3 XGA
1280 x 1024 1,310,720 3,923,160 5:4 SXGA
1400 x 1050 1,470,000 4,410,000 4:3 SXGA+
1600 x 1200 1,920,000 5,760,000 4:3 UXGA
1920 x 1200 2,304,000 6,912,000 16:10 Wide UXGA
2048 x 1536 3,145,728 9,437,184 4:3 QXGA
2560 x 2048 5,242,880 15,728,640 4:3 QSXGA
3200 x 2400 7,680,000 23,040,000 4:3 QUXGA
Figure 24. Resolution of color LCDs
Resolution of Color LCDs
Gate Bus-Line
Data Bus-Line
Figure 25. TFT-Array and its unit pixel
TFT-Array & Unit Pixel
TFT-Array Panel
Bonding Pad
Pixel Electrode(ITO) Storage Capacitor
(Cs)
TFT
Data Bus-Line
GatePixel Electrode
(ITO)
Drain
Storage Capacitor(Cs)
Source
TFT
Common Electrode (ITO)
Color-FilterBlack Matrix
ClcTFT
Data Bus-Line
Pixel Electrode(ITO)
Cs Clc
Common Electrode (ITO)
Gate Bus-Line
Unit Pixel & Equivalent Circuit
Figure 26. Vertical structure of a pixel and its equivalent circuit
T(Vlc)
T(Vlc)
VwhiteWhite Display
Vlc
White
Vblack
Vblack
Black Display
Black
Odd FrameEven Frame
AC Driving of TN-Mode
Figure 27. AC driving of a TN-mode L/C
<Vlc>eff = r.m.s. of (Vp-Vcom)
? TFT On : 27µsec(odd-frame)
Ion
Cs
Vcom(+5V)Clc
Pixel ElectrodeVd+(+8V)
Charge
Operation of Unit Pixel
Figure 28. Modeling of a unit pixel operation
+8V
? TFT Off :16.7msec(odd-frame)
+3V
Vcom(+5V)
Vd+(+8V)
Refresh
? TFT On : 27µsec(even-frame)
Ion -3V
+2V
Vcom(+5V)
Vd-(+2V)
? TFT Off :16.7msec(even frame)Vcom(+5V)
Vd-(+2V)
-3V
+3V
Gn-1
Gn
V1+
Pixel Electrode(ITO)
Storage Capacitor (Cs)
Gn+1
20V
-5V
-5V
On On On
Off Off Off
Off Off Off
V2- V3+
V1+ V2- V3+
Gate Selection
Line-by-Line Addressing
t
t
Active Addressing of (3x3) Matrix
Figure 29. An example of a (3x3) matrix pixel
Animation of a (3x3) Matrix
V11 V12 V13
V11 V12 V13
Odd Frame
G1
V21 V22 V23
V21 V22 V23
V11 V12 V13
Odd Frame
V31 V32 V33
V21 V22 V23
V11 V12 V13
V31 V32 V33
Odd Frame
G3
DC/DCConverter
ControlASIC
Data Signal
Source Driver IC’s
Gate D
river IC’s
Backlight Lamp
LCD PanelDC Power
LCD Module
Pixel Electrode
TFT
Inverter
Driving of LCD Panel
Figure 30. Driving of an LCD panel
LDI: LCD Driving IC
0 0 0 0 0 0 0 0 00 0 0 0 0 0 0 0 00 0 0 1 1 1 1 1 00 0 1 0 0 0 1 1 00 1 0 0 0 1 0 1 01 1 1 1 1 0 0 1 01 0 0 0 1 0 1 0 01 0 0 0 1 1 0 0 01 1 1 1 1 0 0 0 0
Bit Image in Memory
Pixels displayed on Screen
Representation of Image on LCD
Figure 31. Representation of an image on an LCD
multiplexing
W
Gate Bus-Line
SourceDrain Pixel ITO
Gate
Data Line
L
Parasitic Capacitance of TFT
Figure 32. Parasitic capacitors of a TFT
Data Bus-Line
Gate Bus-Line
Source Clc
Common Electrode(C/F Substrate)
Cs
Drain
• Staggered Structure• Process Margin
∆L Overlap (not avoidable)
Cds
CgdCgs
∆L
Odd Frame Even Frame
Vcom
Vg∆V On Off
Vp(t)
Vd
On
∆V = x V p-p(Clc + Cs + Cgd)Cgd
Kickback Voltage
Wave Forms of Pixel Driving Voltages
Figure 33. Driving a pixel and the effect of the parasitic capacitance
Voffset
Vd + Vd+ -
2
Tf = 1/60 sec
Vlc >Vcom
Vlc <Vcom
Tf = 1/60 sec
Vd + VdVoffset = - Vcom
+ -
2t=0
<Vlc>eff = { Vp(t)-Vcom} dt2 Tf
12Tf
? 2
∆V
Frame Inversion
H-Line Inversion
Dot Inversion
(Flicker free)
2nd frame 3rd frame1st frameDriving Method
Polarity Inversion Driving & Flickering
Figure 34. Polarity inversion driving methods
8 Gray-scale
L/C Voltage
Digital Data(3-bit)
D2
D1
D0
White Black
(101)(111) (000)
V1 V2 V3 V4 V5 V6 V7 V8
1 1 0 0 1 1 0 0
1 0 1 0 1 0 1 0
1 1 1 1 0 0 0 0
(011)
V1 V8V2 V7...
Transmittance
T1
T2
T8T7
Gray Scale Generation
Figure 35. A gray-scale example of the 3-bit LDI
L/C Voltage (V)
23 = 8 gray scales
T1 T2 T3 T4 T5 T6 T7 T8
# of Color = 2 (R) x 2 (G) x 2 (B) = 2
n= # of data bits of LDI chip
n n n 3n
3 bit = 8-gray/RGB = 512 colors
4 bit = 16-gray/RGB = 4,096 colors
ü6 bit = 64-gray/RGB = 262,144 colors
8 bit = 256-gray/RGB = 16,777,216 colors
Analog IC = Continuous gray-scale = full color
Total # of Colors
Figure 36. Total number of LCD colors
R G B
26=64218=262,144
5
6
5
5 1/4
5 2/4
5 3/4
6
1st 2nd 3rd 4th frame Average
6 5
Increasing Number of Gray Shades
Dithering
Frame RateControl(FRC)
5 3/4 5 2/4 5 1/4
Reduced Resolution
(2x2)Unit Pixel
Figure 37. Dithering and frame rate control driving methods
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
=
=
=
=
=
L/C Voltage (V)
Trans. (%)
1.0 2.0 3.00
100
V1V2V3
V4
V5
V6
V7
V8
0
50
Gray Scale with a Linear L/C Voltage
Figure 38. Gray-scale generation with a linear L/C voltage
Gray Scale
2 7
100
50
0
Trans. (%)
1 3 4 5 6 8
1.0 2.0 3.00
100
V1V2
V3
V4
V5
V6
V7
V9
V8
0
50
L/C Voltage (V)
Trans. (%)
Optimization of Gray Scale Curve
Figure 39. A gray-scale curve with an adjusted L/C voltage level
Gray Scale
2 7
100
50
0
Trans. (%)
1 3 4 5 6 8
16 32 48 64
100
50
0
Trans. (%)
Gray Scale
γ = 1.0
γ = 2.2
γ = 3.0
• Light Sensitivity of Human Eye
γ −Correction of Gray Scale
Figure 40. Gamma correction of the gray-scale curve
T = Tmax x ( gray # /Max. Gray)γ
Color Generation
Figure 41. The color generation of the LCD
400 500 600 700
Color-FilterSpectra
RGB
400 500 600 700
BacklightSpectra
400 600 700
500 600 700
400 500 600 700
RG
B
TransmittedLights
TFT-Array
Backlight
Color-Filter
R G B
• 10.4 inch VGA : 0.110mm x 0.330mm ( 77dpi )
• 12.1 inch SVGA : 0.1025mm x 0.3075mm ( 83dpi )
• 15.0 inch XGA : 0.099mm x 0.297mm (117dpi )
• 17.0 inch SXGA : 0.090mm x 0.270mm ( 94dpi )
• 21.3 inch UXGA : 0.090mm x 0.270mm ( 94dpi )
dpi : dot per inch
Sub-Pixel
~0.1mm
~0.3mm
R G B
Pixel Size and Resolving Power of Human Eye
Figure 42. Color mix of RGB sub-pixel in the LCD panel
θ ~ 0.02°
~30cm
~0.1mm
θ < 0.03° mixed colorRetina
Arrangement of RGB
Figure 43. Arrangement of the RGB color-filter
R G B
R G B
R G B
R G B
R G B
R G B
R
R
R R G B
R GB
R G B
R GB
R G B
R
B
R
B
RR G B
R G B
R G B
R G
R G B
R G B
G B
B
R
Simple SimpleSimplePoor / w. low res.
SimpleDifficultComplexGood
ComplexDifficultSimpleBest
R G B R G BR G BB R GG B R
Array DesignC/F Fab.Driving CKTColor Mix
Stripe Mosaic Delta
Representation of Color
Figure 44. The color coordinates
A color = rR + gG + bB• r = R /(R + G + B)• g = G /(R + G + B)• b = B /(R + G + B)
with r + g + b = 1
Color Coordinates
r
g
b(r, g, b)
R
G
B
Red (R)Green(G)Blue(B)
Primary Colors
(x, y)
X
Y
NTSC
(a)
R
G
B
(b)
(0.67, 0.33)
(0.21, 0.71)
(0.14, 0.08)
W
CIE Color Coordinates
Figure 45. The CIE color coordinates
Color Reproducibility of Display (a) =
Area of ∆ (a)
Area of ∆ (NTSC) X 100%
• Color Balance
• Color Reproducibility
or Color Saturation
• Color Temperature