U.S. patent application number 11/467880 was filed with the patent office on 2007-01-04 for electron emitting composite based on regulated nano-structures and a cold electron source using the composite.
Invention is credited to Zhidan L. Tolt.
Application Number | 20070003472 11/467880 |
Document ID | / |
Family ID | 37572716 |
Filed Date | 2007-01-04 |
United States Patent
Application |
20070003472 |
Kind Code |
A1 |
Tolt; Zhidan L. |
January 4, 2007 |
ELECTRON EMITTING COMPOSITE BASED ON REGULATED NANO-STRUCTURES AND
A COLD ELECTRON SOURCE USING THE COMPOSITE
Abstract
A field emission electron source includes a substrate, a first
conductive electrode terminated to provide electrons, an emitting
composite layer for emitting electrons, and a second electrode
insulated from the emitter layer and terminated to extract
electrons through vacuum space. The emitting composite layer lies
between and parallel to the said first and the second electrodes,
and comprises nano-structures embedded in a solid matrix. One end
of the nano-structures is truncated and exposed at the surface of
the emitter layer so that both the length and the apex of the
nano-structure are regulated and the exposed nano-tips are kept
substantially the same distance from the gate electrode. The
embedding material is chosen to form triple junctions with the
exposed tip to further enhance the field.
Inventors: |
Tolt; Zhidan L.; (San Jose,
CA) |
Correspondence
Address: |
DECHERT LLP
P.O. BOX 10004
PALO ALTO
CA
94303
US
|
Family ID: |
37572716 |
Appl. No.: |
11/467880 |
Filed: |
August 28, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10807890 |
Mar 24, 2004 |
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11467880 |
Aug 28, 2006 |
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60457210 |
Mar 24, 2003 |
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60491570 |
Jul 30, 2003 |
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60521135 |
Feb 24, 2004 |
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Current U.S.
Class: |
423/447.3 ;
445/51; 977/842 |
Current CPC
Class: |
B82Y 10/00 20130101;
H01J 9/025 20130101; H01J 2201/30469 20130101; H01J 1/304 20130101;
H01J 3/022 20130101 |
Class at
Publication: |
423/447.3 ;
445/051; 977/842 |
International
Class: |
H01J 9/04 20060101
H01J009/04; D01F 9/12 20060101 D01F009/12 |
Claims
1. A method of fabricating an electron source, the method
comprising: providing a substrate; depositing on the substrate a
first conductive layer; depositing on the first conductive layer an
emitter layer composed of an embedding material and one or
plurality of nano-structures embedded therein; truncating and
exposing the ends of the nano-structures and the interface between
the nano-structures and the embedding material; depositing an
insulator over the polished surface of the emitter layer;
depositing second conductive layer over the insulator; and removing
portions of second conductive layer and insulator to form apertures
therein and expose the ends of the nano-structures for
emission.
2. A method as recited in claim 1, wherein depositing an emitter
layer includes: depositing on the first conductive layer a thin
catalyst layer; growing an array of from the catalyst; and
conformally depositing a material to embed the nano-structures, the
embedding material and nano-structures forming the emitter layer
with a surface at which ends of the nano-structures are to be
exposed.
3. A method as recited in claim 2, where the grown nano-structures
are in alignment and with controlled spacing between the
nano-structures.
4. A method as recited in claim 1, wherein the nano-structures
conductive, wherein the embedding material is an insulator.
5. A method as recited in claim 4, where the nano-structure include
carbon nanotubes, carbon nanofibers, carbon nanocones, and carbon
nanoplane,
6. A method as recited in claim 4, wherein the conductive
nano-structure is formed from an insulator core and a conductive
shell.
7. A method as recited in claim 6, where the insulator core is made
of a wide band gap semiconductor including diamond, BN, AlN, GaN,
AlGaN and SiC.
8. A method as recited in claim 4, wherein the embedding insulator
is SiO.sub.2.
9. A method as recited in claim 1, wherein the nano-structures is a
wide band gap semiconductor including AlN, GaN, AlGaN, BN, and SiC,
wherein the embedding material is a conductive material including
refractory metals and alloys, conductive ceramics, conductive
composite and doped semiconductors.
10. A method as recited in claim 1, wherein the nano-structures are
further etched after truncation such that the nano-structures are
slightly recessed from the surface of the emitter layer.
11. A method as recited in claim 1, wherein depositing the emitter
layer includes: dispersing pre-fabricated nano-structures in a
slurry to form an uniform mixture; depositing the uniform mixture
on the first conductive layer; drying the uniform mixture; and
heating at a high temperature to form the emitter layer with a
surface at which ends of the nano-structures are to be exposed.
12. A method as recited in claim 1, wherein depositing the emitter
layer includes: dispersing pre-fabricated nano-structures in a
precursor solution; coating the first conductive layer with the
solution; and condensing the precursor solution to form the emitter
layer with a surface at which ends of the nano-structures are to be
exposed.
13. A method as recited in claim 1, wherein the truncation is
performed by polishing.
14. A method as recited in claim 13, where the polishing is
performed by chemical-mechanical planarization.
15. A method as recited in claim 1, further comprising the step of
depositing and patterning an etch-stopper prior to truncating and
exposing the ends of the nano-structures by polishing the surface
of the emitter layer.
16. A method as recited in claim 1, further comprising the step of
a surface treatment after truncating and exposing the ends of the
nano-structures for enhanced field emission of the
nano-structures.
17. A method as recited in claim 1, wherein the embedding material
is comprised of at least two layers; and further comprising:
depositing the first layer by vapor deposition; and depositing the
second layer by disposing a fluidic precursor onto the substrate
and curing or condensing the precursor by heating or
illumination.
18. A method as recited in claim 1, wherein the truncation is
performed by a combination of lithography and chemical etch.
19. A method of forming an electron emitting composite, the method
comprising: forming a composite from an embedding material and one
or plurality of nano-structures embedded therein; forming an
emitter layer by polishing the composite to form a surface at which
the ends of the nano-structures are truncated, and the ends of the
nano-structure and the interfaces between the nano-structures and
the embedding material are exposed; wherein the presence of the
embedding material increases the intensity of externally applied
electrical field at the interface between the exposed ends of the
nano-structures and the embedding material.
20. A method as recited in claim 19, wherein the composite is
formed from growing one or more the nano-structures on a substrate
and comformally depositing an embedding material to encapsulate the
nano-structures.
21. A method as recited in claim 20, wherein the nano-structures
are grown in alignment and with controlled spacing between the
nano-structures.
21. A method as recited in claim 19, wherein the nano-structures
are conductive, wherein the embedding material is an insulator.
22. A method as recited in claim 21, wherein the conductive
nano-structures include carbon nanotube, carbon nano-fiber, carbon
nano-cone, and carbon nano-plane.
23. A method as recited in claim 19, wherein the nano-structures
have at least an insulator core and a conductive shell.
24. A method as recited in claim 19, wherein the nano-structure is
an insulator, wherein the embedding material is conductive.
25. A method as recited in claim 24, wherein the insulator
nano-structure is made of a wide band gap material including AlN,
AlGaN, GaN, BN, and SiC.
26. A method as recited in claim 24, wherein the nano-structures
have at least a conductive core and an insulator shell.
27. A method as recited in claim 24, wherein the conductive
embedding material include the refractive metals and alloys,
conductive ceramics, conductive composites, and doped
semiconductors.
28. A method as recited in claim 19, wherein the embedding material
is made of at least two different materials.
29. A method as recited in claim 19 further comprises of a step of
etching back the nano-structures after polishing so that the
nano-structures slightly recessed from the surface of the emitting
layer.
30. A method as recited in claim 19, wherein forming the composite
further comprising: dispersing pre-fabricated nano-structures in a
binder containing slurry to form a uniform mixture; casting the
mixture; drying the cast; heating the cast at high temperature to
remove the binder.
31. A method as recited in claim 19, wherein the polishing is
performed by chemical mechanical planarization.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application claims priority to and incorporates by
reference each of the following applications:
[0002] U.S. Provisional Application titled ELECTRON SOURCE BASED ON
REGULATED NANO-STRUCTURED MATERIALS, Ser. No. 60/457,210, filed
Mar. 24, 2003;
[0003] U.S. Provisional Application titled COLD ELECTRON SOURCE
BASED ON REGULATED NANO-STRUCTURED MATERIALS, Ser. No. 60/491,570,
filed Jul. 30, 2003; and
[0004] U.S. Provisional Application titled ELECTRON EMITTING
COMPOSITE BASED ON REGULATED NANO-STRUCTURES AND A COLD ELECTRON
SOURCE USING THE COMPOSITE, Ser. No. 60/521,135, filed Feb. 24,
2004.
[0005] This application also claims the benefit of U.S. patent
application Ser. No. 10/807,890, titled "ELECTRON EMITTING
COMPOSITE BASED ON REGULATED NANO-STRUCTURES AND A COLD ELECTRON
SOURCE USING THE COMPOSITE," filed Mar. 24, 2004.
BACKGROUND OF INVENTION
[0006] The present invention relates generally to the emission of
electrons from nano-structured materials and more particularly to
the controlled emission of electrons from a cold electron source
using nano-structures as the emitters.
[0007] It is a long strived for goal of various applications to use
a cold electron source that can be modulated by low applied
voltage, is highly uniform, addressable with high resolution,
capable of generating high current density and has a collimated
beam and long lifetime.
[0008] A cold electron source operates in a vacuum environment,
where an electrical field is applied to an emitter to generate
electrons. The emitter is connected to a negative cathode
electrode. A positive gate or anode electrode extracts electrons
from the emitter through a vacuum gap. In order for emission to
occur, a field on the order of 5.times.10.sup.9 V/m is required. A
common scheme to induce emission with a reduced external field is
to use a low work function material for the emitter and to enhance
the local field at the emitter by sharpening it to a high aspect
ratio. The higher the aspect ratio is, the lower the threshold
field, generated from the electrodes, is for an emitter to emit
electrons.
[0009] With a given emitter, the closer the gate is to the cathode,
the lower the voltage that is required to modulate the emission. In
order to achieve a modulation voltage of 10 V, the distance has to
be close to one micrometer. Only a gate that is integrated onto the
substrate by micro-fabrication can meet this requirement. And only
this type of integrally gated electron source that is suitable for
many applications and can achieve reasonable energy efficiency and
be modulated by inexpensive and small COMS drivers.
[0010] The widest known field emission electron source is the
Spindt type field emitter, where a conical or pyramid micro-tip of
high aspect ratio is used as an emitter. The high cost and low
yield associated with its fabrication process, and the high
susceptibility of the tip to contamination and ion bombardment have
effectively prevented this technology from wide application.
[0011] Recently, various nano-structures, such as nano-tubes and
nano-wires, have been successfully synthesized. They are typically
a few tens of nanometer in diameter and a few micrometers in
length, thus a high aspect ratio. One nano-structure that has
emerged as the leading candidate for field emitter is carbon
nano-tube (CNT). Its extraordinary high electrical and thermal
conductivity as well as strong chemical and mechanical stability
make it an ideal field emitter. Numerous studies have reported its
advantageous emission properties.
[0012] There has been great progress achieved in the growth of
nano-structures, particularly CNT. It can be grown by various
processes with transition metal or compound as catalyst. Through a
combined scheme of selective passivation and catalyst application,
one can control precisely where the CNT grows. The growth of CNT
can be random in direction or aligned vertically to the surface.
The spacing between aligned CNT and the tube diameter are largely
determined and can be controlled by the catalytic particle spacing,
size, and catalyst film thickness. Highly oriented and regularly
spaced nano-tube or nano-wire arrays can also be grown on a
template surface with pre-etched nano-pores. In summary, the growth
of nano-structures, particularly CNT, has reached a high degree of
controllability.
[0013] Despite the advantageous field emission properties of a
single CNT and the ease of growing CNTs in abundance, a densely
populated CNT forest can hardly emit any electrons, dashing the
hope that such a CNT array would make a practical electron source
with high emission site and current densities over large areas. The
result has been attributed to the electrostatic screen effect
between CNTs. The presence of neighboring CNTs reduces the field
penetration down into the CNT film and counter acts the field
enhancement factor derived from the high aspect ratio at the CNT
tips. Even for a growth with a relatively lower density, where
electrostatic effect is negligible, there is an ever-existing
variation between CNTs in their height and apex. Those that have a
higher aspect ratio and are closer to the extracting electrode
experience a higher local field and always emit first, preventing
the adjacent ones to do the same.
[0014] As a result, the hope of deriving an electron source of high
emission site density from a vertically aligned CNT array has
largely unfulfilled. Alternatively, a randomly oriented CNT film
has provided a better solution. Emission is believed to result from
those tips that protrude from the surface. There are two ways to
apply these random oriented CNTs to a surface. One is to grow them
on the surface. Two major problems plague these random CNT films.
First, emission is inhomogeneous and often dominated by strong
sites so that the emission site density is still relatively low
despite of improvement over vertically aligned CNTs. Second, it is
difficult to fabricate a gate electrode on top of these films
without degrading their emission properties. The emitter cannot,
therefore, be modulated with low voltage even though the film has a
low threshold field. A conductive grid is often mechanically
mounted on top of these films to modulate electron emission. A
typical distance from the grid to the emitting film is 100 pm. Such
a large distance results in high driving voltage, and the emission
from underneath the grid goes directly to the grid without
contributing to the output current. The difficulty in accurately
and uniformly mounting and maintaining a fine grid over large area
with a small gap is another hurdle. The second way to apply random
CNTs to a surface is to mix pre-fabricated CNT with a conductive
paste and then screen print them onto a surface, or mix the CNT
with an organic resin and spin the mixture onto the surface. This
approach has been more widely adopted for the advantages of low
cost and not having to expose the substrate to the high temperature
of CNT growth. However, the emission performance from these films
is worse than when the film is directly grown on the surface.
Although it is possible to pre-fabricate a gate electrode on the
substrate before screen print the CNT paste, the gate distance has
to be sufficiently large to prevent the conductive paste and CNT
from shorting the electrodes. The electron beams generated from
both types of the emitter are also highly divergent due to the
random directions in which the CNT emit.
[0015] Integrally gated CNT field emission electron sources by
conventional micro-fabrication have also been reported. Apertures
are formed in the gate electrode and aligned to the emitter for a
low driving voltage. A bunch of CNTs were then directly grown into
the gate hole without further processing. However, there exist
large variations among the CNTs, both in the same gate hole as well
as from gate hole to gate hole, in their length (thus tip-to-gate
distance) and aspect ratios. Such variation, plus the electrostatic
screening effect between CNTs in the same gate hole, causes an
emission dominated by the ones closer to the gate in each gate
hole, and changes the emission threshold field from gate hole to
gate hole, resulting in a source of little practical use.
[0016] In another type of integrally gated device, there was only
one CNT in each gate hole, and the CNTs were at least 5 .mu.m
apart. The device still does not address the problem of a varying
CNT in its height and aspect ratio in each gate hole and there is
little emitter redundancy built in. If one CNT is missing or is not
grown properly, which happens often, there would be no emission
within 100 .mu.m.sup.2. Also, due to the large inter-tube spacing,
CNT grow thick, reducing the aspect ratio and therefore increasing
the driving voltage.
[0017] It should also be pointed out that one of the most common
and persistent problems that plagues the fabrication of all types
of gated field emitter, whether it is micro-tip or nano-structure
type, is the tip-to-gate or cathode-to-gate shorting. To use fully
exposed conductive nano-structures as emitters, as are with all the
prior art, the uncontrolled length and their fragility can cause
serious electrical shorting problems during fabrication and
operation. Nano-structures also tend to stick together, to the wall
of the gate hole or to the substrate surface upon exposure to
moisture or a wet process due to electrostatic force, diminishing
their emission performance. Full exposure also subjects the
nano-structures to greater ion bombardment during operation,
causing faster emitter erosion and re-deposition of the conductive
material, and, therefore, shorter lifetime and further possibility
of shorting electrodes.
SUMMARY OF INVENTION
[0018] It is an objective of the current invention to: a) provide a
means for inducing a greater and consistent local field enhancement
at the emitting tip to counteract the electrostatic screening
effect between densely packed nano-structures, b) for minimizing
the variation in tip-to-gate distance and tip apex for each
potential emitter in the array, c) for eliminating the divergence
of the emitting directions, and d) for enabling many
nano-structures that are most desirable and yet previously not
feasible for use as field emitters. It is another objective of the
invention to provide an electron source using nano-structures as
emitter that: a) is integrally gated in order to modulate at a low
voltage, b) is of high emission site and current density, c) is
substantially collimated, d) is able to sustain severe ion
bombardment, e) has consistent emission performance and long
lifetime, and f) is easy to fabricate with high production
yield.
[0019] An important aspect of the invention is an emitter layer
that includes one or an array of nano-structures, CNT for example,
embedded in a matrix material. The tip of the nano-structure is
truncated and exposed at the surface of the emitter layer, and kept
parallel to the cathode and gate electrode. The embedding material
forms triple junction with the nano-structure, and vacuum space at
the surface of the emitter layer. A triple junction is defined as a
contact between a conductor, an insulator or a wide-gap
semiconductor, and a vacuum space. At the junction, local electric
field can become so high that electrons readily tunnel through
barrier and inject into vacuum. Formation of such a triple junction
is another major mechanism of enhancing local field other than by
the geometric factor of high aspect ratio. Examples of such
mechanism at work include the frequently observed flashover near
the triple junction in vacuum electron beam devices. Since the
nano-structure array is truncated at the surface of the emitter
layer, each one of the potential emitters is kept at the same
distance from the gate electrode and the same direction
perpendicular to the emitting surface. The wall of a nano-tube is
also sharper than its tip radius. Therefore, the truncated
nano-tips not only have a regulated apex, but also have a higher
aspect ratio and hence a higher field enhancement. The combined
effect of triple junction and a higher aspect ratio counteracts
with the electrostatic effect among CNTs, allowing higher packing
density of the nano-structures and still keeping low the threshold
field for the emitter. Because both the triple junction effect and
the tip aspect ratio change little with the tip erosion from ion
bombardment, the field enhancement factor for each tip is expected
to be consistent throughout its lifetime. A field emitter of such
characteristic is expected to have a long lifetime and steady
performance. Furthermore, the solid and smooth surface of the
emitter layer allows easy fabrication of an integrated gate
electrode for the purpose of low voltage emission modulation.
[0020] In accordance with one embodiment of the present invention,
the electron source includes a substrate, a cathode, an emitter
layer, an insulator, and a gate electrode. The cathode is disposed
over the substrate and provides, in operation, a source of
electrons. The emitter layer is disposed over the cathode and is
formed from a composition of a dielectric embedding material and a
plurality of conductive nanostructures embedded therein. The
emitter layer has a surface at which ends of the nano-structures
are truncated and exposed for emitting electrons. The insulator is
disposed over the emitter layer and has a plurality of apertures
for exposing the ends of the nano-structures. The gate electrode is
disposed over the insulator and has a plurality of apertures that
are aligned with the apertures in the insulator. The gate
electrode, in operation, controls the emission of electrons through
the apertures from the exposed nano-structures. When an electrical
field is applied between the electrodes, the gate extracts
electrons from the emitter layer through a vacuum gap.
[0021] Since the emitter is embedded in an insulator and the gate
electrode is further insulated from the emitter layer, the chance
for shorting between the gate and the cathode, as well as between
gate and the emitting tip is substantially decreased, resulting in
a much higher fabrication yield.
[0022] When multiple cathodes, each electrically independent, and
multiple gate electrodes, each electrically independent and
intersects with the multiple cathodes, are used, an addressable,
planar electron source is formed.
[0023] The nano-structure is defined as anything that has at least
one dimension that is in the nanometer scales. Examples are
nano-tube, nano-wire, nano-plane, nano-fiber, nano-cone, and
nano-particles.
[0024] The conductive nano-structure includes those made of
conductive material and those of an insulator core and a conductive
shell. To ensure sufficient electrical contact of the emitter with
the cathode, these nano-structures need to be grown on the
substrate.
[0025] The embedding insulator is defined as any solid material
that insulates the nano-structures from each other. It includes
those made of a single insulating material and those of multiple
layers of different materials.
[0026] Another embodiment of the emitter layer is to embed an
insulator nano-structure in a conductive matrix. Alone, these
nano-structures cannot be used as emitters because of their
insulating nature. The conducting matrix forms triple junction with
the insulator nano-structure at the surface of the emitter layer.
Due to enhanced local field at the junction, electrons tunnel from
the matrix through the exposed and truncated nano-tip at the
emitting surface directly into vacuum space. There are additional
advantages to the embodiment. First, since the embedding matrix
provides electrical conduction from the cathode to the
nano-structure, the emitter does not have to have a physical
contact with the cathode. As a result, pre-fabricated
nano-structures can be dispersed in a conductive medium and then
deposited onto the cathode, and, nano-particles can be used as
emitter as well. Second, nano-structures made of wide gap
semiconductors, such as diamond, BN and AIN, that have the most
desirable emission properties of low work function, superb thermal
conductivity, high chemical and mechanical stability, and robust to
ion bombardment, can be used as emitters. Nano-particles of these
wide-band-gap materials are already abundant. Growth of BN
nano-tubes has also been widely reported. Third, the conductive
matrix can effectively spread the heat away from the emitter,
further extending its lifetime. Although there is no need to
regulate the length of nano-particles, the emitter layer in the
current invention, still allows one to regulate their emission
direction. By embedding them in a conductive matrix, the current
invention provides a means to enhance their local field and an
effective path to electrical and thermal conduction, as well as a
way to assemble and adhere them to the substrate.
[0027] Using pre-fabricated nano-structures has some unique
advantages. Not only is the material and processing much cheaper,
and there are a large quantity and variety available, but also it
leaves tremendous flexibility to the fabrication of an electron
source. The nano-structures can be applied to a substrate in many
ways. First, they can be uniformly dispersed in such precursor
solutions as of conductive oxides, or colloidal suspension of a
conductive composite, and applied onto a substrate by spinning or
dipping. When the precursor solution or suspension is cured or
condensed at high temperature or by UV illumination, a conductive
solid matrix and the nano-structures fuse together forming the
emitter layer. Second, they can be first dispersed in a slurry
composed of a conductive material, (conductive ceramic, metal or
carbon particles for instance), and organic or ceramic binders
(glass frit for instance), surfactants, solvent and additives, and
then applied to the substrate as a film by printing, doctor blade
or extrusion coating. After the film is dried, organic additives
burned off and the film heated or sintered at high temperature, a
composite film forms with nano-structures embedded in a metal or a
conductive composite matrix. CMP or an etch back is then applied to
the surface of these films to expose the nano-structures.
[0028] A thick composite film of hundreds of micrometers or even a
freestanding composite slab can be formed by the second method.
These composites are particularly suitable for high power
applications, being able to withstand severe ion bombard. They are
also suitable for large area planar illumination since they can be
produced inexpensively and conveniently in large quantity. Such a
free standing emitting slab can be used to build an electron source
without the need of a substrate and cathode electrode layer.
[0029] Embedding wide gap semiconductor nano-structures in an
electrically and thermally conductive matrix of high thermal and
chemical stability and low ion sputter yield, as are some
conductive oxides and conductive composite, is the most desirable
material combination for a field emitter.
[0030] A further embodiment of the emitter layer is to embed a
conductive nano-structure in a conductive matrix. The embodiment is
similar to the one described above in that a pre-made
nano-structure can also be utilized, and a thick film or
freestanding emitting composite can be made. There may not be the
benefit of additional field enhancement from the formation of
triple junction between the nano-structure and the embedding
matrix. It, therefore, may seem advantageous to coat or grow a
layer of insulating material outside of these nano-structures
before embedding them in the conductive matrix. However, when the
embedding material is a conductive composite of conductive
particles, such as graphitic carbon, dispersed in a nonconductive
matrix, such as glass, the conductive particles provide the
conduction between the emitting nano-structures and the cathode,
while the nonconductive glass may still form triple junction with
the nano-structures at the emitting surfaces. A special case of
this scenario is a high volume percent of pre-fabricated conductive
nano-structure mixed with a nonconductive material, where some of
the internally embedded conductive nano-structures provide
electrical contact to others that are exposed at the surface of the
emitter layer.
[0031] It should also be pointed out that even without the benefit
of triple junction field enhancement, the embodiment still
regulates the length, apex, and emitting direction of the embedded
nano-structures, and, therefore, is still superior to the prior art
in which the CNTs are dispersed in a conductive medium and then
screen-printed or spun onto the substrate.
[0032] The desired material characteristics for the nano-structure
as an emitter include. a) low work function for reduced emission
threshold field, b) excellent thermal conductivity, c) high thermal
stability, d) strong chemical and mechanical stability, e) good
electrical conductivity if the embedding matrix is not conductive,
and f) resistance to ion sputtering. Typical conductive
nano-structure materials include, but are not limited to: carbon,
such as CNT; refractory metals, such as Mo and W; doped
semiconductors, such as p or n type Si or GaAs; Conductive
ceramics, such as Indium Tin Oxide (ITO), or a combination of the
above. Typical insulator nano-structure materials include: wide
band-gap semiconductors, such as diamond, AIGaN, AIN, BN and SiC;
semiconductors such as undoped GaAs or Si; oxides, such as
ZrO.sub.2 and AI.sub.2O.sub.3; nitrides, such as Si.sub.3N.sub.4;
carbides, such as HfC, and ZrC; or a combination of the above. The
nano-structures can be multi-layered. It is particularly
advantageous for the nano-structure to have an alternating
conducting-and-insulating layer, with each interface forming a
triple junction at the exposed tip.
[0033] The following material characteristics are the major factors
in selecting an embedding material: a) excellent thermal
conductivity, b) strong chemical and mechanical stability, and c)
low ion sputtering yield. Typical embedding insulators include, but
not limited to, ferroelectric materials, such as BaTiO.sub.3;
oxides, such as Al.sub.2O.sub.3, ZrO.sub.2, Ta.sub.2O.sub.5, and
SiO.sub.2: Nitrides, such as AIN, GaN, c-BN, Si.sub.3N.sub.4;
carbides, such as SiC, WC, ZrC; diamond; diamond-like carbon;
un-doped semiconductors, such as Si, GaAs; insulating ceramics,
glasses, organically modified glasses, insulating composites; and
cured organic resins. Typical embedding conductive materials
include, but are not limited to: metals, such as Mo, W, and their
alloys; doped semiconductors, such as doped amorphous or
polycrystalline Si; conductive ceramics, such as CrO.sub.2 and
Indium-Tin-Oxide (ITO); conductive polymeric or ceramic composites,
such as carbon dispersed in glass or silver dispersed in epoxy.
[0034] The embedding material can be applied to the surface by
chemical or physical vapor depositions, sol-gel processes,
printing, or other liquid-film coating and curing or condensing
processes, such as Metal Organic Decomposition (MOD) or a
combination of the above. It is sometimes advantageous to apply a
layer of embedding material by a vapor deposition process, such as
thermal evaporation or chemical vapor deposition (CVD), and to
apply second layer of embedding material with liquid coating and
curing or condensation processes. Depending on the selected
material, a third layer can further be applied by a vapor
deposition process.
[0035] It can be advantageous for either the nano-structure or the
embedding matrix, or the both, to be made of a combination of
materials for the purpose of tailoring their electronic,
electrical, mechanical, chemical, and thermal properties to the
optimum or for the easiness of fabrication. The materials
adjustment can be stepwise or at atomic level.
[0036] For some applications, it is advantageous to have the
nano-tips slightly recessed into the embedding matrix surface. This
way, the section of the hollow wall formed above the nano-tip can
act as a focusing lens for the electrons emitted from the emitting
tips or as a shield to protect the nano-structure from direct ion
bombardment.
[0037] Yet, for other applications, it is advantageous to have the
nano-tips slightly protruding from the embedding matrix to prevent
charging from building up in the embedding insulator matrix.
[0038] The surface of the emitter layer can be treated to modify
the exposed tips of the nano-structure for a better or more
reliable emission performance. Such treatment includes, plasma
treatments, UV radiation, thermal annealing, and other chemical
processes.
[0039] To achieve even higher emission uniformity, a ballast
resistor layer can be inserted between the cathode electrode and
the emitter layer.
[0040] The cathode, emitter layer and the gate can also be arranged
laterally, in which the cathode and the emitter layer are in
contact with each other by their sides, and the gate electrode
faces the emitter layer. The preferred embodiment for this
configuration is to selectively grow horizontally oriented CNT on
the appropriate sidewall of the cathode. The advantage of this
configuration is that it ensures an equal tip-to-gate distance for
each nano-tip without resorting to sophisticated photolithography
processes. Also, the emitters are not facing directly the pathway
of high-energy ions.
[0041] The source of the current invention can be simplified to
include a substrate, the first conductive cathode electrode and the
emitter layer on the substrate, and be operated by either mounting
a conductive grid on top to modulate electron emission, or in a
diode mode, where an anode mounted in a distance is used to
modulates and accelerates the electrons. Another simplified
embodiment of the source comprises only an emitter layer on a
conductive substrate, where the conductive substrate serves also as
the cathode electrode. A further simplification of the source is to
include only a slab of a conductive and freestanding emitting
composite, as is described above. The slab functions as the
substrate, cathode electrode and the emitter layer.
[0042] For the purpose of further focusing, deflecting or scanning
the electron beam, The source of the current invention can include
one or more additional integrated electrodes, each insulated by
another insulating layer and having apertures, both in the
electrode and its insulator, aligned to those in the gate electrode
and the first insulator layer.
[0043] One advantage of the present invention is that an integrated
gate electrode can be conveniently built on top of the fragile
nano-structures.
[0044] Another advantage is that a lower modulation voltage is
derived from the integrated gate structure and additional field
enhancement at the emitting tip from the formation of triple
junctions and higher aspect ratio.
[0045] Yet another advantage is that a higher emission site and
current density are derived from the higher density of
nano-structures embedded in a matrix with substantially equalized
tip to gate distance and tip apex.
[0046] Other advantages include: (a) a higher degree of collimation
of the electron beam derived from regulated emission direction from
each emitter; (b) longer lifetime and consistent emission
performance derived from a consistent local electrical field at
each emitting tip as the tip erodes from ion bombardment,
mechanical protection, thermal and electrical conduction
enhancement from the embedding matrix, and lower ion sputter yields
of the selected nano-structure and embedding materials; and (c) a
higher production yield and an increase in ease of fabrication
derived from embedding the nano-structure in an insulating matrix
and protection of the nano-structure from the matrix.
BRIEF DESCRIPTION OF DRAWINGS
[0047] The accompanying drawings, which are incorporated in and
form a part of this specification, illustrate embodiments of the
invention and, together with the description, serve to explain the
principles of the invention:
[0048] FIGS. 1A-1B show an electron source from the current
invention;
[0049] FIG. 1A shows a section view from the front; and
[0050] FIG. 1B shows a top view;
[0051] FIGS. 2A-2G illustrate the fabrication steps of a preferred
embodiment of the current invention;
[0052] FIG. 2A shows a substrate;
[0053] FIG. 2B shows the deposition of cathode metal and
catalyst;
[0054] FIG. 2C shows the growth of CNT;
[0055] FIG. 2D shows the deposition of an embedding insulator;
[0056] FIG. 2E shows the formation of the emitter layer by
polish;
[0057] FIG. 2F shows the deposition of a gate insulator and gate
metal; and
[0058] FIG. 2G shows formation of gate aperture.
[0059] FIGS. 3A-3I show three types of embodiments of an emitter
layer.
[0060] FIGS. 3A-3C show a grown conductive nano-structure embedded
in an insulating matrix;
[0061] FIG. 3A shows a conductive grown nano-structures embedded in
an insulating matrix;
[0062] FIG. 3B shows a grown nano-structure with an insulator core
and a conductive shell embedded in an insulating matrix; and
[0063] FIG. 3C illustrates a nano-structure with an insulator core
and a conductive shell;
[0064] FIGS. 3D-3G shows insulator nano-structures embedded in a
conductive matrix;
[0065] FIG. 3D shows a grown insulator nano-structure embedded in a
conductive matrix;
[0066] FIG. 3E shows a pre-made insulator nano-structure embedded
in conductive matrix;
[0067] FIG. 3F shows an insulator nano-particles embedded in a
conductive matrix; and
[0068] FIG. 3G shows a detail of the emitter layer shown in FIG.
3F.
[0069] FIGS. 3H-3I shows conductive nano-structures embedded in a
conductive matrix;
[0070] FIG. 3H shows grown conductive nano-structure embedded in a
conductive matrix; and
[0071] FIG. 3I shows pre-made nano-structure embedded in a
conductive matrix;
[0072] FIGS. 4A-4C show nano-tip positions relative to the surface
of an emitter layer;
[0073] FIG. 4A shows a nano-tip slightly recessed from the surface
of the embedding matrix;
[0074] FIG. 4B shows a nano-tip that slightly protrudes from the
surface of the embedding matrix; and
[0075] FIG. 4C shows a nano-tip at the same surface level as of the
embedding matrix;
[0076] FIGS. 5A-5B show an addressable electron source of present
invention;
[0077] FIG. 5A shows a section view from the front;
[0078] FIG. 5B shows a section view from the left;
[0079] FIG. 6 is an illustration of the current invention with a
lateral configuration.
[0080] FIGS. 7A-7D are a flow chart setting forth the steps for
fabricating an electron source.
[0081] FIG. 7A sets for the main steps for fabricating an electron
source;
[0082] FIG. 7B sets forth the steps of a first alternative for
depositing the emitter layer;
[0083] FIG. 7C sets forth the steps of a second alternative for
depositing the emitter layer; and
[0084] FIG. 7D sets forth the steps of a third alternative for
depositing the emitter layer.
[0085] Reference designators used herein are: 20--CNT,
22--nano-tip, 26--cathode electrode, 28--gate electrode, 30--gate
aperture, 32--composite emitter layer, 34--gate insulator,
36--substrate, 38--conductive nano-structure directly grown on the
substrate, 40--insulating embedding matrix, 42--insulator
nano-structure directly grown on the substrate, 44--pre-fabricated
insulator nano-structure, 46--conductive embedding matrix,
48--pre-made nonconductive nano-particles, 50--pre-made conductive
nanostructure, 52--catalyst, 54--nano-structure, 56--embedding
matrix, 58--insulating core of a nano-structure, 60--conductive
shell of a nano-structure, 62--composite nano-structure with
alternating insulating-to-conductive layers.
DETAILED DESCRIPTION
[0086] FIG. 1 illustrates an electron source by current invention
using vertically aligned CNT 20. It includes a substrate 36, a
cathode 26 deposited on the substrate, an emitter layer 32, a gate
electrode 28 insulated from the emitter layer by a gate insulator
34. Apertures 30 are opened in the gate electrode and gate
insulator to extract electrons from the exposed CNT tips in the
emitter layer 32.
[0087] FIG. 2 depicts one of possible fabrication process flows for
the source shown in FIG. 1. The process starts with a substrate 36
shown in FIG. 2A. Examples are Si or glass. A first conductive
cathode layer 26 and a thin catalyst layer 52 are then deposited,
as is shown in FIG. 2B. Examples of the first conductive layer and
catalyst are Cr and Ni respectively. The conductive layer should be
chosen so that it does not prohibit CNT growth with the presence of
catalyst. The catalyst layer is made of many Ni dots of nanometer
size. An array of vertically aligned CNT 20 is then grown, as shown
in FIG. 2C, on the catalyst 52. Preferably, the CNTs have a
diameter less than 100 nm, wall thickness less than 30 nm, spacing
less than 5 .mu.m and a length less than 5 .mu.m. A conformal layer
of insulator 40, Ta.sub.2O.sub.5 for instance, is then deposited,
shown in FIG. 2D, to embed the CNT. Chemical-mechanical
planarization (CMP) is then applied, shown in FIG. 2E, to remove
and polish the non-uniform part of the surface until CNTs are
truncated and their tips are exposed, forming the composite emitter
layer 32 of about one or a few .mu.m thickness. In order to control
the polish uniformity, an "etch stopper" can, optionally, be
deposited and patterned before the polishing. A gate insulator 34
and a gate electrode layer 28, SiO.sub.2 and Cr, respectively for
instance, are then deposited, as is shown in FIG. 2F. The thickness
of SiO.sub.2 is preferred to be comparable to the diameter of the
gate aperture to ensure that distances of each nano-tip inside to
the edge of the aperture are not significantly different and yet
the modulation voltage at the gate will remain low. Gate apertures
30 are then formed in the gate electrode 28, shown in FIG. 2G,
through the gate insulator 34 until nano-tips 22 in the emitter
layer are exposed. The embedding dielectric can be deposited by MOD
process, or reactive sputter. In case a thick embedding layer is
needed, one can apply a relatively thin layer, 300 nm, for example,
of Ta.sub.2O.sub.5 first by either MOD or reactive sputter and then
a spin-on material, such as an organically modified spin-on-glass
(SOG), for the remaining embedding layer, or to sandwich the SOG in
between two layers of Ta.sub.2O.sub.5.
[0088] There can be many embodiments for the emitter layer
described above. FIGS. 3A through 3I depict three types of
embodiments. Selection depends on the desired device performance,
process and material compatibility, and fabrication cost.
[0089] Referring to FIGS. 3A through 3C, one or an array of
conductive nano-structures 38, grown directly on the substrate, is
embedded in an insulating matrix 40. The nano-structure can be made
either of entirely conductive material, such as CNT and W nanowire,
as shown in FIG. 3A, or of an insulator core 58 and a conductive
shell 60, such as a GaN nano-wire with a W shell, as is shown in
FIG. 3C. The composite nanostructure 62 can even be made of
multiple alternating insulating-and-conductive layers to fit to the
needs of a good field emitter. Since the nano-structures themselves
have to carry electrons from the cathode to the emitting tips, they
have to have a strong electrical contact to the cathode. Therefore,
they need to be grown directly on the substrate. Aligned growth
with a controlled spacing between the structures is preferred, as
is shown in the figures. However, it is also possible to use
randomly grown structures provided that they don't cause cross talk
between cathodes in an addressable source and there is a reasonably
uniform distribution of the nano-tips in the resulting emitter
layer.
[0090] FIGS. 3D through 3F represent another type of embodiment of
the emitter layer, where one or an array of insulator
nano-structure is embedded in a conductive matrix. In FIG. 3D, the
insulator nano-structure 42 grown directly on the substrate is
embedded in a conductive matrix 46. Again, an aligned growth is
preferred as is shown in the figure. However, random growth should
not be completely excluded. Examples include an array of vertically
aligned GaN nano-wires, or BN nano-tubes embedded in CrO.sub.2. In
FIG. 3E, pre-made insulator nano-structures 50 are embedded in a
conductive matrix 46. These nano-structures can be first dispersed
in a medium, such as a slurry containing carbon or conductive
ceramics particles, or a conductive oxide precursor solution, and
then applied to the surface by printing or spin coating. The film
applied to the surface as such is then dried and condensed, or
burned off and heated or sintered at a higher temperature, forming
the composite emitter layer. Examples of the embodiment are AIN or
other wide band gap semiconductor nano-wires embedded in a
conductive composite of graphitic carbon power dispersed in a
glass, or AIN embedded in W. In FIG. 3F pre-made insulator
nano-particles 48 are embedded in a conductive matrix 46. FIG. 3G
shows the details of the emitter layer in FIG. 3F. Examples of this
embodiment are diamond and other wide band gap semiconductor
nano-particles embedded in a conductive ceramic, such as ITO, or a
conductive composite, such as graphitic carbon dispersed in
glasses, or a metal, such as W.
[0091] FIGS. 3H and 3I depict the third type of embodiment of the
emitter layer, where conductive nano-structures, such as CNT,
embedded in a conductive matrix 46. In FIG. 3H, the nano-structures
38 are grown directly on the substrate, while in FIG. 31, they are
pre-made 50.
[0092] For some applications, it can be advantageous to have the
nano-tips slightly recessed, as shown in FIG. 4A, from the
embedding matrix to either protect the nano-tip from ion
bombardment or to further collimate the electron beam from the
emitter by the extra length of the embedding matrix from the
emitter tip. For some other applications, it can be advantageous to
have the nano-tip protrudes from the embedding matrix, as shown in
FIG. 4B, to prevent the matrix from charging. Any one of the wet or
dry etch processes known to the skilled in the art could be used
for a slight etch back of either the embedding matrix or the
nano-tips.
[0093] For other applications, it can also be advantageous to treat
the surface of the emitter layer for such purposes as introducing a
particular atomic bonding to the truncated nano-tips, or for
cleaning the exposed nano-tips. Typical processes include plasma
treatments, UV radiation, thermal annealing or other chemical
treatment. As an example, the surface of an emitter layer with
diamond nano-particles embedded in a conductive film is treated
with hydrogen plasma to hydrogenate the dangling carbon bond at the
surface of the exposed nano-particle. Such hydrogenated diamond
surface is believed to have a lower work function, making electron
emission easier.
[0094] The electron source shown in FIG. 1A and FIG. 1B become
addressable when the cathode electrode is configured as multiple
parallel rows of electrodes, each supplies an independent source of
electrons; the gate electrode is configured as multiple columns of
electrodes, each intersects the multiple rows of the cathodes and
have one or a plurality of apertures at the intersection that are
aligned to the apertures in the insulator to extract electrons.
Activation of a selected row of cathode and a selected column of
gate electrode determines an intersection that emits electrons.
FIG. 5A and FIG. 5B show one of these addressable sources. Although
the nano-structures in the figures are shown in patches, they can
be distributed over the entire cathode.
[0095] FIG. 6 shows a lateral configuration of the current
invention. This configuration has two notable advantages. First,
all the nano-tips are kept at exactly the same distance from the
gate electrode. Second, the emitter layer does not directly face
the high-energy ions in the free space so that the tip erosion from
ion bombardment is minimized. For devices such as an x-ray
generator, where the ion energy can be as high as 100 keV, this
configuration presents a unique way to protect the emitter,
especially when the nano-tip is recessed into the embedding
matrix.
[0096] It is to be understood that the embodiments described above
are illustrative of only a few of the many possible specific
embodiments that can represent applications of the principles of
the invention. Numerous and varied other arrangements can be
readily devised by those skilled in the art without departing from
the spirit and scope of the invention.
* * * * *