Can Nanotechnology revive Field Emission Display Technology?

Chenggang Xiecgxie@cox.net

1.31.2005

1.31.2005 C.G. Xie

Outline

Basic of Field Emission DisplayKey Issues and components for FEDNanotechnology for field emission technologyDepletion mode operationSummary

1.31.2005 C.G. Xie

What is an FED?

A CRT is a vacuum tube in which electronsfrom three hot cathodes are scanned across a multicolor viewing screen to create a picture.

An FED is a vacuum tube in which electronsfrom millions of tiny cathodes travel to amulticolor viewing screen to create a picture.

CATHODE

VIEWINGSCREEN ELECTROMAGETIC

DEFLECTION COILS

Field Emission DisplayCathode Ray Tube

FIELD EMISSIONARRAY

VIEWINGSCREEN

1.31.2005 C.G. Xie

The Business Case for FEDsSuperior performance:

– Better than CRTs– Less than 1/10th the thickness and weight– Lower power and more rugged– No non-linearities or color errors

– Better than LCDs– Viewable from any angle with no change in brightness, contrast or color– Usable from -40°C to 85 °C with no change in performance!– Potentially lower power consumption than LCDs– Wider operating temperature range– Larger viewing angle– Sunlight readability

Potentially lower manufacturing costs– Fewer processing steps than Active Matrix LCDs

1.31.2005 C.G. Xie

Milestones in development of Field Emission Display

F-N

theo

ry

1970

Spin

dttip

inve

nted

Firs

t Mat

rix

addr

ess m

onoc

hrom

e FE

D

1990

Firs

t hig

h vo

ltage

full

colo

r FE

D

2000

First generation cathode: Microtip

1960

15.3

” FE

D d

emon

stra

ted

Car

bon

Nan

otub

edi

scov

ered

Second generation cathode: CNT &SE

R&D

2005 2010

Although no product was produced, key technologies were developed to provide a good foundation for next generation cathode.

Although no product was produced, key technologies were developed to provide a good foundation for next generation cathode.

Act

iviti

es

CommercialProduct

20s

1.31.2005 C.G. Xie

FEDs - Industry Status

At peak, TI, Motorola, Raytheon, Candescent, Micron, FED Corp., SIDT in the US; Pixtech in France, Samsung in Korea; Canon, Sony and Futaba in Japan investigated resources into the technology.The number of companies involved in FED development is down. Many companies keep low profile. Focus of cathode technology is switched from Microtip to carbon nanotube. Canon and Toshiba announced to invest $1.8 B in SED in DEC, 2004. Demonstrated 36 “ SED at CES 2005

1.31.2005 C.G. Xie

World Largest Field Emission Display

Prototype of a 15.3" full color FED capable of running video in real time

1.31.2005 C.G. Xie

Display System Requirements

Good color qualityWide operating temperature rangeWide viewing angleGood viewability under bright ambient light conditionsLower power consumptionThin package profile and lower weightCompetitive with AMLCDs and other FPD technologies

1.31.2005 C.G. Xie

Electron Devices - Common Issues

Electron emission depends on the work function Φ of the emitter surfaceElectron transport is ballistic and requires a sealed vacuum packageUniformity of beam current densityCathode geometry, beam trajectoriesElectrode structures: diode, triode, tetrode, pentode, etc.Cathode contamination and emitter life degradationEffects of electron collectors like phosphors

1.31.2005 C.G. Xie

Basic Components of FED

Cost& Manufactuability

Visual QualityUniformityResolution

Color purity

Power EfficiencyReliability

LifetimeHV stability

Back Plate(Cathode)

Electronic

Vacuum(Seal & Getter)

Spacer

Face Plate(anode)

1.31.2005 C.G. Xie

Key Accomplishments in 90s

Demonstrate invisible spacers in large display panel.Vacuum seal technique for thin glass panels.High voltage drivers and electronics.Knowledge of issues like cathode current degradation and phosphors degradation. Techniques to achieve high voltage operation stability.Device structure and pixel design.Understanding requirements of manufacturing for low cost and high yield product – 2D preferred and no precision fabricationCharacterization techniques for FED.

1.31.2005 C.G. Xie

FED Spacer Technology

Materials RequirementsShould withstand high voltages across small gapsLow secondary electron emission coefficientsCapable of bleeding deposited charge

Spacer MaterialsMainly engineered ceramicsCeramics with special coatings

Spacer Technology IssuesBetter understanding of the surface propertiesElectron-spacer materials interactions

1.31.2005 C.G. Xie

Spacer- contamination

White spacer is caused by Na contamination on spacer surface.

1.0E-07

2.0E-07

3.0E-074.0E-07

5.0E-07

6.0E-07

7.0E-07

8.0E-07

Time(s)

Subp

ixel

curr

ent (

A)

Spacer locat ed

No spacer area

4.0E-03 6.0E-03 8.0E-03 1.0E-02 1.2E-02 1.4E-02

0.0E+001.0E-07

2.0E-07

3.0E-074.0E-07

5.0E-07

6.0E-07

7.0E-07

8.0E-07

Time(s)

Subp

ixel

curr

ent (

A)

Spacer locat ed

No spacer area

4.0E-03 6.0E-03 8.0E-03 1.0E-02 1.2E-02 1.4E-02

0.0E+00

1.31.2005 C.G. Xie

Spacer - Effect of Charging

⊕⊕⊕⊕⊕⊕

⊕⊕⊕⊕⊕⊕

⊕⊕⊕⊕⊕⊕

Spacer location

The spacer surface is bombarded by high energy electrons (1k-3k eV). Those electrons create positive charges on the surface due to 2nd

electron emission yield of >1. Additional potential produced by those positive charges will change electron trajectory.∆V=ΓρΙp/ε

Some positive charges may dissipate until next period. But most of them stay on surface because of rough surface, low conductivity.

Positive charges on surface distort electron trajectory resulting in “dark” region

1.31.2005 C.G. Xie

High Voltage Operation Stability

Emitter damage due to uncontrolled emission currentUndesirable vacuum levels: Electron impact ionization of the residual gases, followed by the ion bombardment damage of the emitters.Electron stimulated desorption from surfaces and device degradation resulting from the gas - emitter surface interactions. E-beam induced desorption of gas from phosphor and subsequent adsorption on tips.Emission from only few sites, leading to current run off Joule heating, melting of the emitter materialsDielectric breakdown between the cathode and anodeExcessive leakage between cathode to anode electrical pathsParticulate contamination and resulting damage

1.31.2005 C.G. Xie

Cathode Destruction due to Uncontrolled Emission

Before Arc Damage After Arc Damage

1.31.2005 C.G. Xie

Field Emission Display Package Outline

Field Emitter Cathode Array

Spac

er

Phosphor Face Plate

Spac

er

Black Matrix Vacuum Seal

Red GreenBlue Red GreenBlue

Requires a hermetically sealed vacuum packageMaintenance of high vacuum during the package lifeSpacers to reduce glass bowing

1.31.2005 C.G. Xie

Package Integration - Typical Process Flow

Vacuum Packaging Process

Frame AssemblySpacer AttachGetter Placement

Anode Fabrication

CathodeFabrication

VacuumSeal

Driver attach/Board attach

ModuleAssembly

Package Test Final Test

Electronics Packaging

1.31.2005 C.G. Xie

Vacuum Technology Issues

VACUUMISSUES

CATHODE-ANODEINTERACTIONS

ESDOF

PHOSPHORS

VACUUMAN D

GETTERS

GASEOUS INTERACTIONS

OUTGASSINGAND GAS

DESORPTION

IONBOMBARDMENT

DAMAGE

1.31.2005 C.G. Xie

Maintaining a Good Vacuum Getters are used to maintain a good vacuum in the packageMajor issues:

Better understanding of the outgassing characteristics of FED materialsReduction of desorption from phosphorsWith increasing display size, gas flow through the narrow spacing between the anode and cathode becomes a major problemDistributed gettering will become important

Vacuum levels in the 10-6 to 10-7 Torr range are required for long lifeNanoparticles can significantly improve pumping rate of the getter

1.31.2005 C.G. Xie

Vacuum Technology Issues

Anode plate (glass, ITO, phosphor)

electronemission

electronstimulateddesorption

field desorption

surface diffusion

adsorption

tip atoms

1.31.2005 C.G. Xie

Typical Gases inside FED Panels

Most of the residual gases are typically H2O, O2, CO2, CO, H2, hydrocarbons like CH4, process residues, gases trapped inside sputtered layersWith the metal and silicon emitters, it is the oxygenic gases we need to be concerned with.

Mass (amu)0 10 20 30 40 50

Ion

Sig

nal x

10-8

(arb

. uni

ts)

0

1

2

3

4

5

H2O

N2

O2

CO2

H2

C

O

OH

CH4

CO

H

Residual gas contents of a typical FED package

1.31.2005 C.G. Xie

Cathode Degradation and lifetime model

Life modelInteraction of gases with the cathode resulting in modified tip surface work function ΦIon impact and sputter induced tip shape modificationHigh field effects such as field desorption and dissociationDetail description of the model was published in IEEE Trans on Electron Device.

Basic AssumptionPartial coverage;Ionized species more reactive; Self-clean capability

2IcVbIaIdtdI

a−−=

dI/dt/I = -1.09x1010I 2 + 2.76x103I - 1.83x10-3

R2 = 0.9806

1.0E-5

1.0E-4

1.0E-3

1.0E-2

0.0E+0 2.0E-7 4.0E-7 6.0E-7 8.0E-7 1.0E-6

Subpixel Current (A)

-dI/

dt/I

(1/h

our)

1.31.2005 C.G. Xie

FED Reliability - Fundamental Issues

Vacuum Technology IssuesOutgassing of surfacesElectron stimulated desorptionVacuum seal integrity; Getter pumping

MaterialsCathode degradation, Phosphor degradation, Cathode-Phosphor interactionCharging of dielectrics and spacersDielectric breakdown

Design and Systems EngineeringElectron beam spot size issuesBeam focussing issues

1.31.2005 C.G. Xie

Display Visual Quality & Cathode Characteristics

UniformityContrast

& Gray Scale

BrightnessResolution

I-V CurveRC delay

Spot Size

Variation

Color Purity

Device EngineeringDevice EngineeringMaterial Engineering

Understand the Issues: Spot Size, Spatial Distribution, RC delay and

Current sensitivity

1.31.2005 C.G. Xie

How Can Nanotech help?

Visual QualityUniformityResolution

Color purity

Power Efficiency

ReliabilityLifetime

HV stability

Cost& Manufactuability

Back Plate(Cathode)

Vacuum(Seal & Getter)

Spacer

Face Plate(anode)

Electronic

Nano material may be a good material for getteringmaterial.Nanoparticle phosphor can improve the efficiency.Nano material is a new generation of field emission cathode.

Basic Component of FED

1.31.2005 C.G. Xie

Surface Conduction Cathode - Nanoparticle

Electrode ElectrodePdO

Substrate

Canon's Surface Conduction Emitter

Surface emission display from Canon is based on surface conduction emission from nanoparticle films.Ink-jet deposit a droplet of PdO to form a 10 nm thick, 100 µm size filmCreate ~ 10 nm rupture on the PdO filmField emission occurs when a voltage is applied across the gapAdvantagesSimple device structure - Low manufacturing costsScreen printing - suitable for large panel fabrication

E. Yamaguchi, et al, SID Digest, p. 52, (1997)

1.31.2005 C.G. Xie

Canon SCE - Emission Characteristics

Anode current is about 0.2-0.5% of the total emitted current.Low emission efficiency, comparable to Spindt FEAs.

E. Yamaguchi, et al, SID Digest, p. 52, (1997)

1.31.2005 C.G. Xie

Canon SCE - Emission Physics

Vf If

Anode

Va

Ie

Electron tunneling through a barrier (like a double quantum well structure)Electron scattering on the right edgeSome of these scattered electrons get towards the anode

E. Yamaguchi, et al, SID Digest, p. 52, (1997)

1.31.2005 C.G. Xie

Why is Carbon Nanotube a promising candidate for FED?

They are very sharp! 2 um long and 10 Å wide! 2000:1 Aspect ratio. The electric field at the tip which pulls out electrons is ~ 2000 times greater than the parallel plate field!Basic material: Carbon. Unlimited resource.Key Issues

a traditional field emitter; follows FN plot.⌧Sensitive to surface conduction, geometry and

environment. ⌧Spotty emission

Uncontrolled growth.No device structure yet.Lifetime?Low voltage operation, <5-10 volts.

1.31.2005 C.G. Xie

Basics of Field Emission

Key obstacleEmission current is very

sensitive to work function and electric fieldΦ

Metal Vacuum

FERMI ELECTRON SEA

∆Φ

Φeff

EF

EV

0 Xc X0 XF

e-

-e2/2x

-eFX

1.00E+00

1.00E+02

1.00E+04

1.00E+06

1.00E+08

1.00E+10

1.00E+12

1.00E+07 1.00E+08 1.00E+09

Ele ctr ic Fie ld (V/cm )

Cur

rent

Den

sity

ove

r em

issi

on

are

a (A

/cm

2)

Work f unc tion=6 eV Work f unction=5.5 eV Work func tion= 5 eV

Work f unc tion=4.5 eV Work f unction=4 eV Work func tion=3.5 eV

Work f unc tion=3 eV Work f unction=2.5 eV

Work f unc tion dec rease

( )

correctionimageExyconstB

functionworksurfaceatfieldelectricE

cmAyE

Byt

EJ

:1079.3.;,

;

]/[)((exp)(

4

223

2

2

Φ==

=Φ=

Φ−

Φ=

−α

να

1.31.2005 C.G. Xie

Cross-section of nanotube device

1.31.2005 C.G. Xie

Emission Uniformity - Major Issue

10-16

10-15

10-14

10-13

4 10-4 5 10-4 6 10-4 7 10-4

I/V2 (A

/V2 )

1/V (V-1)

The I-V curve is consistent with electron tunneling. The curve produces a straight line on a Fowler-Nordheim plot.

CNTs face the same challenges of other traditional FE like microtips.

Uniformity

Stability

Approach to achieve uniform emission

ballast structure

More emission sites to make each pixel the same statistically.

New device structure.

New driving scheme.

Fowler-Nordheim plot

1.31.2005 C.G. Xie

Current Saturation- may help

Current saturation is observed:In MWNTs by Saito et al.; Collins and Zettl; Bonard et al., Xu and BrandesIN MWNTs and SWNTs, and individual SWNTs.

Saturation could be the greatest thing for FED. May help fix uniformity problem if we know how to implement it.Some kind of current limiting function is needed to compensate for non-uniform emission from CNT

10-12

10-10

10-8

10-6

10-4

10-2

1500 2000 2500 3000

Cur

rent

(A)

Voltage (V)

* a single SWNT

1.31.2005 C.G. Xie

Stability of SWNT field emission in Gas Ambients

Curves on same emitter

0 10 20 30 40 50 60 70 80

Time (hours)

0

2

4

0

2

4

0

2

4

0

2

4

UHV UHVGas Exposure

H2

O2

Ar

H2O

Cur

rent

(µA

)

10-7 torr

10-7 torr

10-7 torr

10-6 torr

Ok with H2 and Ar.Some degradation under H2O.Serious degradation under O2.More study under display environment is needed.

1.31.2005 C.G. Xie

11.979 s10.978 s10.477 s 12.746 s

14.678 s14.248 s13.480 s13.247 s

field emission from adsorbate on top

Thermionic emission from wall

1.31.2005 C.G. Xie

Nanotube Deposition Techniques

Key Issues:nanotubes must be “sticking up”!! Nanotubes have high surface energy and they like to stick to the substrateNanotubes must be bound down strongly and reliably⌧ just one loose nanotube can cause arcing

Sparsely distributed to avoid field screeningas far apart as they are tall

Low Temperature Deposition

1.31.2005 C.G. Xie

1.31.2005 C.G. Xie

Ideal gated field emitter

Ideal but not practical

Carbon Nanotube

1.31.2005 C.G. Xie

What is the device structure best suited for CNT?

Planar CathodeGate

Emission surfacePhosphor

DiodeSpindt Above Gate

SCE

Back Gate

Geometry 3D 2.5D “1D”2D

Precision fabrication

Less criticalCritical Not critical

High LowPixel Cap

Less than 1%99%Emissive efficiency

1.31.2005 C.G. Xie

Basic Device Requirement

Diode is the simplest but switching voltage is too high.Basic Requirements

No needs for alignment between gate and cathode⌧ self-aligned structure⌧ Gate and cathode on the same layer.

Low pixel capacitance-a must for large display⌧ no overlap between gate and cathode

Low switching voltageEmission Controllability

New Driving Scheme - depletion mode operation

1.31.2005 C.G. Xie

Depletion Mode Operation

Detail discussion was published in IEEE Trans on Nanotechnology,vol.3 no.3, Sept. 2004, pp404Contrary to the conventional driving scheme, in depletion mode, the gate is used to turn off emission not to turn on emission.

Electrons emit under anode field – require low field emission materialMore stable, avoid edge emission.Better uniformityWork with planar structureSmall beam spreadLess critical requirement for emission layer alignment to the gate.

DisadvantagesNeed highly selective deposition of emissive materialNew Concept in field emission display.

1.31.2005 C.G. Xie

20 µm

140 µm 355 µm

100 µm

200 µm

A-A’ A-A’

1.0E-12

1.0E-09

1.0E-06

1.0E-03

0 2 4 6 8 10

x distance (micron)

Curr

ent D

ensi

ty (A

/mm

^2)

Vg=-5 volts Vg=-1 volts Vg=5 volts Vg=10 voltsVg=20 volts Vg=30 volts Vg=50 volts

Vg decreases

Conventional scheme

Depletion mode

Anode field

Anode voltage produce

1.31.2005 C.G. Xie

The electric field on emission surface generated by applied anode voltage must be strong enough to extract electrons.

Anode voltage=3-6kV, gap=1mm; required operating electric field=3-6V/um.Carbon nanotube performance matches the requirement.

High electric field at center and low at edge. Avoid edge emission and achieve stable high voltage operation.Since lateral field is very small under emission conditions, beam spread is small, similar to that from diode.

1.31.2005 C.G. Xie

Summary

Investment in 90s established valuable infrastructure for next generation field emission display technology. Many challenging issues, such as spacer visibility, high voltageoperation stability, vacuum sealing, pixel layout and drivers, which was little known to us in 90s, now are better understood. Know more about manufacturing issue of field emission display and requirements of field emission cathode.Nanotechnology can have great impact on phosphor material, gettering material and specially emission material.Carbon nanotube has great potential to revive field emission display technology but difficult engineering challenges are still ahead.

New device structureProcess to produce uniform emissive material.