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