Can Nanotechnology revive Field Emission Display Technology?

Can Nanotechnology revive
Field Emission Display
Technology?
Chenggang Xie
[email protected]
1.31.2005
Outline
Basic of Field Emission Display
Key Issues and components for FED
Nanotechnology for field emission technology
Depletion mode operation
Summary
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C.G. Xie
What is an FED?
Cathode Ray Tube
VIEWING
SCREEN
Field Emission Display
VIEWING
SCREEN
ELECTROMAGETIC
DEFLECTION COILS
CATHODE
A CRT is a vacuum tube in which electrons
from three hot cathodes are scanned across
a multicolor viewing screen to create a picture.
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C.G. Xie
FIELD
EMISSION
ARRAY
An FED is a vacuum tube in which electrons
from millions of tiny cathodes travel to a
multicolor viewing screen to create a picture.
The Business Case for FEDs
Superior 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
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C.G. Xie
20s
1960
1970
1990
15.3” FED demonstrated
Carbon Nanotube discovered
First high voltage full color FED
Spindt tip invented
R&D
F-N theory
Activities
Commercial
Product
First Matrix address monochrome FED
Milestones in development of Field
Emission Display
2000
Althoughno
noproduct
productwas
was
Although
produced,key
keytechnologies
technologies
produced,
weredeveloped
developedtotoprovide
provideaa
were
goodfoundation
foundationfor
fornext
next
good
generationcathode.
cathode.
generation
2005
2010
First generation cathode: Microtip
Second generation cathode: CNT &SE
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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
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World Largest Field Emission
Display
Prototype of a 15.3" full color
FED capable of running video
in real time
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Display System Requirements
Good color quality
Wide operating temperature range
Wide viewing angle
Good viewability under bright ambient light conditions
Lower power consumption
Thin package profile and lower weight
Competitive with AMLCDs and other FPD technologies
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C.G. Xie
Electron Devices - Common
Issues
Electron emission depends on the work function Φ of the emitter surface
Electron transport is ballistic and requires a sealed vacuum package
Uniformity of beam current density
Cathode geometry, beam trajectories
Electrode structures: diode, triode, tetrode, pentode, etc.
Cathode contamination and emitter life degradation
Effects of electron collectors like phosphors
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C.G. Xie
Basic Components of FED
Cost
& Manufactuability
Visual Quality
Uniformity
Resolution
Color purity
Reliability
Lifetime
HV stability
Back Plate
Spacer
(Cathode)
Electronic
Vacuum
(Seal & Getter)
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Face Plate
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(anode)
Power Efficiency
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 fabrication
Characterization techniques for FED.
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C.G. Xie
FED Spacer Technology
Materials Requirements
Should withstand high voltages across small gaps
Low secondary electron emission coefficients
Capable of bleeding deposited charge
Spacer Materials
Mainly engineered ceramics
Ceramics with special coatings
Spacer Technology Issues
Better understanding of the surface properties
Electron-spacer materials interactions
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C.G. Xie
Spacer- contamination
White spacer is caused by Na contamination on
spacer surface.
Spacer locat ed
8 .0 E-07
Subpixe l curre nt ( A)
7 .0 E-07
No spacer area
6 .0 E-07
5 .0 E-07
4 .0 E-07
3 .0 E-07
2 .0 E-07
1 .0 E-07
0.0E+00
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4 .0 E-0 3
C.G. Xie
6.0E-0 3
8.0E- 03
Time (s)
1 .0 E-02
1.2E-0 2
1 .4E -02
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
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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
High Voltage Operation Stability
Emitter damage due to uncontrolled emission current
Undesirable 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 materials
Dielectric breakdown between the cathode and anode
Excessive leakage between cathode to anode electrical paths
Particulate contamination and resulting damage
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Cathode Destruction due to
Uncontrolled Emission
Before Arc Damage
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After Arc Damage
C.G. Xie
Field Emission Display Package
Outline
Red
Blue
Green
Red
Field Emitter Cathode Array
Requires a hermetically sealed vacuum package
Maintenance of high vacuum during the package life
Spacers to reduce glass bowing
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Vacuum Seal
Phosphor Face Plate
C.G. Xie
Blue
Green
Spacer
Spacer
Black Matrix
Package Integration - Typical
Process Flow
Vacuum Packaging Process
Anode
Fabrication
Frame Assembly
Spacer Attach
Getter Placement
Vacuum
Seal
Cathode
Fabrication
Electronics Packaging
Package
Test
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Driver attach/
Board attach
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Module
Assembly
Final Test
Vacuum Technology Issues
VACUUM
AN D
GETTERS
ESD
OF
PHOSPHORS
GASEOUS
INTERACTIONS
VACUUM
ISSUES
ION
BOMBARDMENT
DAMAGE
CATHODE-ANODE
INTERACTIONS
OUTGASSING
AND GAS
DESORPTION
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Maintaining a Good Vacuum
Getters are used to maintain a good vacuum in the package
Major issues:
Better understanding of the outgassing characteristics of FED
materials
Reduction of desorption from phosphors
With increasing display size, gas flow through the narrow spacing
between the anode and cathode becomes a major problem
Distributed gettering will become important
Vacuum levels in the 10-6 to 10-7 Torr range are required for long life
Nanoparticles can significantly improve pumping rate of the getter
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Vacuum Technology Issues
Anode plate (glass, ITO, phosphor)
electron
emission
electron
stimulated
desorption
adsorption
surface diffusion
tip atoms
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field
desorption
C.G. Xie
Typical Gases inside FED Panels
5
Most of the residual gases are
typically H2O, O2, CO2, CO, H2,
hydrocarbons like CH4, process
residues, gases trapped inside
sputtered layers
With the metal and silicon
emitters, it is the oxygenic gases
we need to be concerned with.
4
3
N2
CO
-8
Ion Signal x10 (arb. units)
H2O
2
H2
OH
1
O
CH4
H
CO2
O2
C
0
0
10
20
30
40
50
Mass (amu)
Residual gas contents of a typical FED package
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Cathode Degradation and lifetime
model
Life model
Interaction of gases with the
cathode resulting in modified tip
surface work function Φ
Ion impact and sputter induced
tip shape modification
High field effects such as field
desorption and dissociation
Detail description of the model
was published in IEEE Trans on
Electron Device.
Basic Assumption
Partial coverage;Ionized species more
reactive; Self-clean capability
dI
= a − bI − cVa I 2
Idt
-dI/dt/I (1/hour)
1.0E-2
1.0E-3
1.0E-4
dI/dt/I = -1.09x1010I 2 + 2.76x103I
- 1.83x10-3
R2 = 0.9806
1.0E-5
0.0E+0 2.0E-7 4.0E-7 6.0E-7 8.0E-7 1.0E-6
Subpixel Current (A)
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FED Reliability - Fundamental
Issues
Vacuum Technology Issues
Outgassing of surfaces
Electron stimulated desorption
Vacuum seal integrity; Getter pumping
Materials
Cathode degradation, Phosphor degradation, Cathode-Phosphor
interaction
Charging of dielectrics and spacers
Dielectric breakdown
Design and Systems Engineering
Electron beam spot size issues
Beam focussing issues
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Display Visual Quality & Cathode
Characteristics
Color Purity
Resolution
Brightness
Contrast
& Gray Scale
Uniformity
Spot Size I-V Curve
RC delay
Variation
Material Engineering
DeviceEngineering
Engineering
Device
Understand the Issues: Spot Size,
Spatial Distribution, RC delay and
Current sensitivity
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C.G. Xie
How Can Nanotech help?
Nano material may be a
good material for gettering
material.
Nanoparticle phosphor can
improve the efficiency.
Nano material is a new
generation of field emission
cathode.
Basic Component of FED
Visual Quality
Back Plate
Reliability
Lifetime
HV stability
Spacer
(Cathode)
Vacuum
(Seal & Getter)
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Cost
& Manufactuability
Uniformity
Resolution
Color purity
C.G. Xie
Electronic
Face Plate
(anode)
Power
Efficiency
Surface Conduction Cathode - Nanoparticle
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 film
Create ~ 10 nm rupture on the PdO film
Field emission occurs when a voltage is
applied across the gap
Advantages
Simple device structure - Low manufacturing
costs
Screen printing - suitable for large panel
fabrication
Substrate
PdO
Electrode
Electrode
Canon's Surface Conduction Emitter
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C.G. Xie
E. Yamaguchi, et al, SID Digest, p. 52, (1997)
Canon SCE - Emission Characteristics
Anode current is about 0.2-0.5% of the total emitted current.
Low emission efficiency, comparable to Spindt FEAs.
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E. Yamaguchi, et al, SID Digest, p. 52, (1997)
Canon SCE - Emission Physics
Anode
Ie
Va
Vf
If
Electron tunneling through a barrier (like a double quantum well structure)
Electron scattering on the right edge
Some of these scattered electrons get towards the anode
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E. Yamaguchi, et al, SID Digest, p. 52, (1997)
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.
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C.G. Xie
Basics of Field Emission
Key obstacle
Emission current is very
sensitive to work function
and electric field
EV
-e2/2x
∆Φ
Φ
-eFX
Φeff
e-
EF
C urre nt De n sit y ov e r e m is s io n are a ( A /cm 2)
1.00E+12
FERMI ELECTRON SEA
Metal
Vacuum
0
J=
αE 2
Φt 2 ( y )
exp(( − B
Xc
X0
XF
3
2
Φ
ν ( y ) ) [ A / cm 2 ]
E
α , B = const .; y = 3.79 x10
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1.00E+08
1.00E+06
Work f unc tion dec reas e
1.00E+04
1.00E+02
1.00E+00
E = electric field at surface; Φ = work function
−4
1.00E+10
E
: image correction
Φ
C.G. Xie
1.00E+07
1.00E+08
1.00E+09
Ele ctr ic Fie ld (V/cm )
Work f unc tion=6 eV
Work f unction=5.5 eV
W ork func tion= 5 eV
Work f unc tion=4.5 eV
Work f unction=4 eV
W ork func tion=3.5 eV
Work f unc tion=3 eV
Work f unction=2.5 eV
Cross-section of nanotube device
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Emission Uniformity - Major Issue
Fowler-Nordheim plot
The I-V curve is consistent with
electron tunneling. The curve
produces a straight line on a
Fowler-Nordheim plot.
Uniformity
I/V2 (A/V2)
CNTs face the same challenges of
other traditional FE like microtips.
-13
10
10-14
10-15
Stability
Approach to achieve uniform
emission
10-16
-4
4 10
More emission sites to make each
pixel the same statistically.
New device structure.
New driving scheme.
C.G. Xie
-4
6 10
1/V (V-1)
ballast structure
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-4
5 10
-4
7 10
Current Saturation- may help
Current saturation is observed:
-2
10
In MWNTs by Saito et al.;
Collins and Zettl; Bonard et al.,
Xu and Brandes
IN MWNTs and SWNTs, and
individual SWNTs.
Current (A)
-4
10
-6
10
-8
10
10
-10
10
-12
* a single SWNT
1500
2000
2500
3000
Voltage (V)
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C.G. Xie
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
Stability of SWNT field emission in Gas
Ambients
UHV
Gas Exposure
UHV
10-6 torr
4
H
2
Current (µA)
2
0
10-7 torr
4
HO
2
2
0
10-7 torr
4
Ar
2
0
10-7 torr
4
O
2
2
0
0
10 20 30 40 50 60 70 80
Curves on same emitter
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Time (hours)
C.G. Xie
Ok with H2 and Ar.
Some degradation under H2O.
Serious degradation under O2.
More study under display
environment is needed.
10.477 s
10.978 s
11.979 s
13.247 s
13.480 s
14.248 s
12.746 s
14.678 s
field emission from adsorbate on top
Thermionic emission from wall
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Nanotube Deposition Techniques
Key Issues:
nanotubes must be “sticking up”!!
Nanotubes have high surface energy and they like
to stick to the substrate
Nanotubes must be bound down strongly and
reliably
⌧ just one loose nanotube can cause arcing
Sparsely distributed to avoid field screening
as far apart as they are tall
Low Temperature Deposition
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Ideal gated field emitter
Ideal but not practical
Carbon Nanotube
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What is the device structure best
suited for CNT?
Planar
Spindt
Above Gate
Cathode
Gate
Emission surface
Phosphor
SCE
Diode
Back Gate
Geometry
3D
Precision
fabrication
Critical
Pixel Cap
High
Emissive
efficiency
99%
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2.5D
“1D”
2D
Less critical
Not critical
Low
Less than 1%
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 voltage
Emission Controllability
New Driving Scheme - depletion mode operation
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Depletion Mode Operation
Detail discussion was published in IEEE Trans on Nanotechnology,
vol.3 no.3, Sept. 2004, pp404
Contrary 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 material
More stable, avoid edge emission.
Better uniformity
Work with planar structure
Small beam spread
Less critical requirement for emission layer alignment to the gate.
Disadvantages
Need highly selective deposition of emissive material
New Concept in field emission display.
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C.G. Xie
Anode field
355 µm
200 µm
140 µm
A-A’
A-A’
20 µm
100 µm
Current Density (A/mm^2)
1.0E-03
Conventional
scheme
Vg decreases
1.0E-06
Depletion
mode
1.0E-09
1.0E-12
0
2
4
6
8
10
x distance (micron)
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Vg=-5 volts
Vg=-1 volts
Vg=20 volts
Vg=30 volts
Vg=5 volts
C.G. Vg=50
Xie volts
Vg=10 volts
Anode voltage
produce
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=36V/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.
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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 voltage
operation 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 structure
Process to produce uniform emissive material.
1.31.2005
C.G. Xie