can nanotechnology revive field emission display technology?
TRANSCRIPT
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.