Advances in display technologies

Hoi-Sing Kwok

Center for Display ResearchHong Kong University of Science and Technology

Display symposium November 2008

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Displays – historyFirst generation – CRT

Braun tube was invented in 1897RCA patented color picture tube in 1954Still has a market of US$17B in 2007

Second generation – LCDLC effect was discovered in 1888Field effect was invented in 1968 by RCA researchersActive matrix LCD commercialized in 1990

Third generation – OLED Invented in 1982 by C W Tang (邓青云)Passive matrix display commercialized in 2000Active matrix display commercialized in 2003Large scale deployment expected in 2009

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Display Market

As a comparison, the semiconductor market is about $250B

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Active matrixRegardless of display technology used, the active matrix backplane is essentialPassive matrix – multiplex driving

LCD – time multiplexing reduces selection ratioLimited multiplexing to 240 lines (STN)OLED – time multiplexing increases current loadingLimited to 100 lines

Active matrix – one or more transistors per pixelIndividual voltage or current control of pixels with no crosstalk

Thin film transistors (TFT)Amorphous silicon (a-Si)Polycrystalline silicon (poly-Si)

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Display manufacturing

Active matrix array process :

growth of TFT on glass

Backend :

deposition of OLED

or fabrication of LCD cell

Module manufacturing: driver IC, PCB, backlight in case of TFT LCD

Passive matrix : simple etching of ITO glass

PMOLEDPMLCDPassive matrix

AMOLEDAMLCDActive matrix

OLEDLCD

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FPD technology• Display research encompasses

electronics, optics, material and vacuum technologies

• FPD has obvious advantages over other technologies in thickness, weight, and power consumption

• High-resolution, full-color and video FPD’s need active-matrix addressing

• SOP or SOG technology is required for versatile, compact and super high resolution mobile displays DisplayDisplay

Flat-Panel display

AM-FPD

SoP

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FPD mega trend

*From Samsung Electronics

Both require LTPS

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Thin film transistors90% of TFT fabs use a-Si technologyPoly-Si is gaining market share

HTPS – used in LCD projectorsLTPS – can be grown on glass

LTPS (low temperature poly-Si) is needed forCircuits on glass – integrated drivers and scanners and multiplexer – reduces use of external IC and connectors to panelMuch smaller TFT ⇒ larger aperture ratio – mobileapplications Much more stable than a-Si under high current loading (OLED needs current to drive)

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Array processGen 10 for a-Si. Talk of Gen 11.Mostly Gen 3 for LTPS. Samsung and Sony are building Gen 5 and Gen 6 lines

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Silicon technologies

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Comparison of a-Si, poly-Si TFT

Mobility : ~500 cm2/VsN/P-channel TFT

(CMOS)< 1 ㎛

Mobility : 50~200 cm2/Vs

N/P-channel TFT (CMOS)2~3 ㎛

Mobility : 0.5~1 cm2/Vs

N-channel TFT3~5 ㎛

System on panelPoly-Si TFTa-Si TFT

Gate Driving IC

Source PCB (Controller, converter, gamma, DC-DC)

Data Driving IC

Controller PCB(Controller, Y-source,

Amp, DC-DC)

Data Driving IC

Control IC, Peripheral circuits

Data Driving IC

Poly-Si TFT-OLED

System on Panel

a-Si TFT-LCD

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AMOLED bottleneckAMOLED is now in the same situation as LCD in 19901990 – introduction of TFT array process – active matrix – AMLCD quickly replaced STN in notebook computersOLED lacks a TFT that can bring it to the next level -current TFT is developed for LCD Difference: LCD needs voltage driving and OLED needs current drivingPotential LTPS technologies for OLED:

MIC, SLS, CGS, GGS, MICC, MILCBG-TIGR (HKUST proprietary)

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LTPS – external connectorsCommon practice: LTPS line scanner plus data multiplexerFor example, use 1/10 multiplexer for VGA display, then external connection is reduced from 1120 to 110Greatly improves packaging yield and cost

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LTPS – aperture ratioLTPS TFT can be much smaller than those with a-SiFor example 3” VGA，a-Si ～50％，LTPS～80％Larger aperture ratio means smaller backlight and lower power consumptionImportant for mobile displays

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LTPS – data processingLTPS TFT is much faster than a-Si TFTFast data loading needed for field sequential colorCan also incorporate multiple transistors on each pixel for extra functions such as sensors, touch panels

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LTPS market2.2” QCIF － 30％ LTPS2.4” QVGA － 70％ LTPS3-4” VGA(cell phones, PDA) － 100％ LTPSSmall palmtop － 100％ LTPSIn general, the division between LTPS and a-Si TFT is 150 dpiMore and more LTPS LCD for mobile displays

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Active matrix LCD and OLED

Scan Scan lineline

Data lineData line

AddressAddress TFTTFT

Liquid Liquid crystal crystal

cellcell(C(CLCLC))

CCss

Address TFTAddress TFT

CCss

OLEDOLED

Data lineData linePower linePower line

DriveDrive TFTTFT

LC PixelLC Pixel OLED PixelOLED Pixel

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Recent results on SLS poly-Si TFT

SID 2008 9.2: Sequential Lateral Solidification (SLS) Process for Large Area AMOLEDLCD Business, Samsung Electronics Co., LTD,

6 TFT + 1 capacitor for pixel circuit

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9.2: Sequential Lateral Solidification (SLS) Process for Large Area AMOLED (From 08SID)

LCD Business, Samsung Electronics Co., LTD,

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Recent results on SPC TFT OLED

9.3: 15-inch AMOLED Display with SPC TFTs and symmetric drivingLG Display R&D Center, Anyang-shi, Gyeonggi-do, Korea

W/L of 6 µm/18 µm,The threshold voltage, -2 V sub-threshold swing, 0.6 V/decMobility, 30 cm2/Vs,

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Excimer laser annealingDeveloped for LCDVariations – CGS (continuous grain silicon) of Sharp, SLS (single crystal like silicon) of SamsungProblems : too expensive and mura defects

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Metal Induced crystallization of a-Si

Metal Induced Lateral crystallization (MILC)/Metal Induced Unilaterally crystallization (MIUC)

Disk-like domain poly-Si or giant grain (GGS) poly-Si

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Metal induced lateral/unilaterally crystallization

Low temperature crystallization ( < 600 ℃) glass substrate applicableLarge grain size compared to SPCHigh mobility, high uniformity

Advantages

Disadvantages

High nickel concentrationin some areaAlignment misplacement caused by the shrinkage of the glass substrate during the crystallization process

Crystallization inducing holeNi

MILC

a-Si MILC MIC

High Ni Concentration at the MILC

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Disk-like domain poly-Si or GGS poly-Si

Can be realized using solution-based MIC (S-MIC) or sputtering Nickel on silicon-nitride (SiNx) cap layer.

Disadvantages:The random distribution of crystallization nuclei Lack of control over the crystallization time Long annealing time

Advantages:Low nickel density Continuous film No alignment misplacement

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Defined grain (DG) Poly-Si

30µm Nickela-Si

Substrate

Silicon Oxide

Crystalline Nuclei Lines

Poly-Si

Cross section of the DG structure

Aims: Reduce nickel residualReduce crystallization timeImprove film uniformity

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GGS and DG poly-Si

Optical microscopy images of amorphous silicon film after one hour annealing at 590℃ in N2 atmosphere employing (a) GGS technology and (b) CZD technology.

30µm 30µm

(a) (b)

GGS – spontaneous, random

DG – patterned and controlled

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0

20

40

60

80

100

0 50 100 150 200 250Annealing time(mins)

Ave

rage

cry

stal

lizat

ion

frac

tion

(%)

CZD-PSGGS-PS

This figure shows the average crystallization fraction over large area substrate of the film employing GGS and DG technology as a function of the annealing time at 590℃. 100% crystallization fraction can be obtained using DG technology at 60 mins, while that is just about 40% of the GGS technology.

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microscope photo of TFTW/L=30µm/10µm

500nm SiO2

500nm LTO

Schematic cross-section of a P-channel CZD poly-Si TFT

Metal Gate

S/D

poly-Si

Al-S/D electrode

Substrate

TFT structure and plan-view photos

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DG grains

Controlled grain locationsTIGR – transistor in grainVery uniform and high quality TFT

100 micron

TFT 5x5 micron

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Latest invention－BG TFTBG – bridged grainHKUST patented technology

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MIC-TFT & MIC-BG-TFT (p-channel)

1.E-12

1.E-11

1.E-10

1.E-09

1.E-08

1.E-07

1.E-06

1.E-05

1.E-04

1.E-03

1.E-02

-40 -30 -20 -10 0 10

Vgs(V)

Ids(

A) [

Vds

=-5V

]

0

20

40

60

80

100

120

140

160

180

200

µFE(

cm2 /V

s)

MIC

Active layer: 45nmW/L=30µm/10µmTox=100nmVds=-5V

B+ Implantationdosage: 4E15/cm2

MIC-BG

MIC-TFT & MIC-BG-TFT (p-channel)

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UNIFORMITY – BG MIC TFT

Position of TFT across substratePosition of TFT across substrate

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SummaryLTPS technology is increasing in importanceHKUST BG-TIGR technology is very competitivePilot line in Foshan being constructed

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Back to realityLCD is not exactly standing stillIt is being improved constantly – cost downWhat needs to be improved?

Energy consumption – light utilization is only 5-8%Brightness, viewing angle, speed, contrast, color saturation, cost

How can LCD be improved? Light recyclingField sequential colorLED dynamic backlight

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Fast LCD modesOCB and NBBThin cell gap TN, VAFerroelectric FlexoelectricOthers

OCB requires a bias voltageNBB – no-bias bend mode, made possible with high pretilt angle

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Ferroelectric LC

Already in production for LCOS Binary switching. Frame rate modulation to obtain grayscale (just like DLP)Traditional FLC are surface stabilizedNew development – V-shape monostable FLC

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FLC based FSC display

Fujitsu SID 2000

And SID 2007

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VA mode

J Anderson, SID 2002

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Traditional FSC-DisplaysActive Matrix addressedOCB mode (Toshiba)

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HKUST invention – NBB modeNo bias bend modeTrue bend cell without bias voltagePossible due to a new nano-alignment layerAny pretilt angle can be obtained – impossible previously

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Conventional pi-cell (OCB mode)

Splay Bend

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Splay to Bend transition is tricky

TMD SID 2004

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Critical holding voltage

A critical voltage is needed to stabilize the bend state for π-cellsTraditional OCB cell has a pretilt of a few degreesSince we can vary pretilt angle easily, can study the dependence of Vc on pretilt, as well as other properties

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Dependence of Vc on θο

Bend cellSplay cell

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No-bias bend (NBB) cell If the pretilt angle is large enough, no critical holding voltage is needed.The stable configuration is the bend state, not the splay state.NBB has smaller effective birefringence and fasterswitching than OCBOptimization of NBB optical properties

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New alignment layerMixing of vertical and horizontal alignment PI

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Surface morphology

AFM5µm

V

High resolution Microscope50µm

HV

H

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Any pretilt angle can be obtained

0

10

20

30

40

50

60

70

80

90

0.000% 5.000% 10.000% 15.000%

Percentage of JALS2021

Pretilt angle

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Fast response times of NBB50° pretilt angle π-cell (d = 5 µm)

Off time: 2ms On time: 70µs

1.00ms/div 50µs/div

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Gray-to-gray switching speed of NBB cell

(µs)

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Latest development in NBBNew Alignment method is development for large area uniform high pretilt angle generations

67ο, 52ο, 39ο, 27ο

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Optical performance is improved By using our new alignment layers, the brightness performance of NBB is improved by 20% without any compensation film applied

Pretilt: 52 degCell gap: 5.25µm

Δn: 0.2Δε:+7

Pol/Anal=+-45degFor both Cells A, B

Α

Β

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Response time of Improved NBBThe Response time is 2.7ms which is fully compatible with FSC requirement

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SummaryImproved NBB show much better EO performance than traditional one. Further increase of ∆ε to 14 can reduce the driving value to <5V.

2.7ms2.1msResponse Time

80%60%Brightness75.327Contrast

Modified NBB

Traditional NBB

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ConclusionsDisplay is in a transition to third generationCore technology – LTPS TFTMobile display will be dominated by LTPS technologiesMany LTPS technologies are near commercial successFast LCD still has a bright future, especially for large displays

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Thank you!