Information
-
Patent Grant
-
6204597
-
Patent Number
6,204,597
-
Date Filed
Friday, February 5, 199925 years ago
-
Date Issued
Tuesday, March 20, 200123 years ago
-
Inventors
-
Original Assignees
-
Examiners
- Patel; Nimeshkumar D.
- Hopper; Todd Reed
Agents
- Pickens; S. Kevin
- Koch; William E.
-
CPC
-
US Classifications
Field of Search
US
- 313 310
- 313 309
- 313 336
- 313 351
- 313 495
- 313 422
-
International Classifications
-
Abstract
A field emission device (110, 210, 310, 410) includes an electron emitter (124), a first dielectric focusing layer (122) defining a first aperture (127), and a second dielectric focusing layer (123) defining a second aperture (133). Second dielectric focusing layer (123) is disposed on first dielectric focusing layer (122). The dielectric constant of second dielectric focusing layer (123) is less than the dielectric constant of first dielectric focusing layer (122). During the operation of field emission device (110, 210, 310), electron emitter (124) emits an electron beam (134), which is focused as it travels through first aperture (127) and then through second aperture (133).
Description
FIELD OF THE INVENTION
The present invention relates, in general, to field emission devices having focusing structures for focusing electron beams.
BACKGROUND OF THE INVENTION
Field emission displays are well known in the art. A field emission display includes an anode plate and a cathode plate that define a thin envelope. The anode plate and cathode plate can be separated by dielectric spacer structures. The cathode plate includes column electrodes and gate extraction electrodes, which are used to cause selective electron emission from electron emitters, such as Spindt tips or emissive surfaces.
The separation distance between the anode plate and the cathode plate has a lower limit. The minimum distance is determined by the break down voltage of the dielectric spacer structures and by the need to avoid arcing between the anode plate and the cathode plate. Especially at high anode voltages, the minimum separation distance can result in electron beams that have unacceptably large cross-sections at the anode plate. It is known in the art to use additional electrically conductive layers or electrically resistive layers for the purpose of focusing the electron beams to achieve a desired cross-section at the anode plate. Benefits, such as improved resolution of a display image, can be realized by the focusing.
It is also known in the art to use an electrically conductive or electrically resistive layer, which circumscribes an emissive surface, for reducing leakage currents at the gate extraction electrode.
However, the use of these additional layers can result in problems, such as suppression of the electric field at the electron emitter as well as unacceptable, spurious electron emission from the additional material.
Accordingly, there exists a need for a field emission display having an improved focusing structure, which overcomes at least some of these shortcomings.
BRIEF DESCRIPTION OF THE DRAWINGS
Referring to the drawings:
FIG. 1
is a cross-sectional view of a field emission device, in accordance with a preferred embodiment of the invention;
FIG. 2
is a computer model representation of one half of the cross-sectional view of the field emission device of
FIG. 1
;
FIG. 3
is a computer model representation of one half of a cross-sectional view of a prior art field emission device;
FIG. 4
is a graphical representation of electric field strength versus position along the x-axis for the structures of
FIGS. 2 and 3
;
FIG. 5
is a cross-sectional view of a field emission device having an edge emitter, in accordance with another embodiment of the invention;
FIG. 6
is a cross-sectional view of a field emission device having a dielectric focusing structure disposed above a gate extraction electrode, in accordance with yet another embodiment of the invention; and
FIG. 7
is a cross-sectional view of a field emission device having a dielectric focusing structure that defines apertures of dissimilar sizes, in accordance with a further embodiment of the invention.
It will be appreciated that for simplicity and clarity of illustration, elements shown in the drawings have not necessarily been drawn to scale. For example, the dimensions of some of the elements are exaggerated relative to each other. Further, where considered appropriate, reference numerals have been repeated among the drawings to indicate corresponding elements.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Herein, the term “dielectric” is used to describe materials having a resistivity greater than or equal to 10
10
ohm-cm, and the term “non-dielectric” is used to describe materials having a resistivity less than 10
10
ohm-cm. Non-dielectric materials are divided into electrically conductive materials, for which the resistivity is less than 1 ohm-cm, and electrically resistive materials, for which the resistivity is within a range of 1 ohm-cm to 10
10
ohm-cm. These categories are determined at an electric field of no more than 1 volt/pm.
The invention is for a field emission device having a dielectric focusing structure. The dielectric focusing structure of the invention is a multi-layer structure. Each of the layers of the multi-layer structure is made from a dielectric material. The dielectric constants of the layers decrease in the direction of electron flow. The dielectric focusing structure of the invention provides improved electric field strength at the electron emitter or, equivalently, lower operating gate voltage, when contrasted with the focusing structures of the prior art. The dielectric focusing structure of the invention is also useful for focusing the electron beams to provide low leakage current at the gate extraction electrode. In the application of a field emission display, improved display image resolution can be achieved.
FIG. 1
is a cross-sectional view of a field emission device (FED)
110
, in accordance with a preferred embodiment of the invention. Although the embodiment of
FIG. 1
is a display device, the scope of the invention is not limited to display devices. Rather, the invention can be embodied by other types of electronic devices, such as field effect transistors. FED
110
includes a cathode plate
112
and an anode plate
114
, which define an interspace region
135
therebetween.
Cathode plate
112
includes a substrate
116
, which can be made from glass, silicon, and the like. A column electrode
118
is disposed upon substrate
116
. Column electrode
118
is made from an electrically conductive material, such as aluminum, molybdenum, and the like. Column electrode
118
is connected to a first voltage source
131
, V
1
.
A ballast resistor layer
120
is disposed on column electrode
118
. Ballast resistor layer
120
is made from an electrically resistive material, such as a phosphorus-doped, amorphous silicon. The resistivity of ballast resistor layer
120
is about 10
7
ohm-cm.
An electron emitter
124
is formed on ballast resistor layer
120
. In the embodiment of
FIG. 1
, electron emitter
124
defines an emissive surface
125
.
The resistivity of ballast resistor layer
120
is greater than that of column electrode
118
and is selected to cause uniform electron emission over emissive surface
125
.
In FED
110
, electron emitter
124
is a layer of an electron-emissive material. Preferably, the electron-emissive material is characterized by a turn-on field of less than 100 volts/pm. In general, the turn-on field is the electric field at the emissive surface, which causes the material to emit at a current density of 10
−4
amps/cm
2
. The electron-emissive material can be selected from materials having low work functions, such as diamond-like carbon, diamond, partially graphitized nanocrystalline carbon, and the like.
In accordance with the invention, cathode plate
112
further includes a dielectric focusing structure
121
. In the embodiment of
FIG. 1
, dielectric focusing structure
121
is disposed on electron emitter
124
. Dielectric focusing structure
121
includes a first dielectric focusing layer
122
and a second dielectric focusing layer
123
.
First dielectric focusing layer
122
is disposed on electron emitter
124
and has a surface
129
, which defines a first aperture
127
. Second dielectric focusing layer
123
is disposed on first dielectric focusing layer
122
and has a surface
130
, which defines a second aperture
133
. First aperture
127
and second aperture
133
partially define an emitter well
128
. In accordance with the invention, the dielectric constant of first dielectric focusing layer
122
is greater than the dielectric constant of second dielectric focusing layer
123
.
The scope of the invention is not limited to a dielectric focusing structure having only two dielectric focusing layers. More than two dielectric focusing layers can be employed. In accordance with the invention, the dielectric constants of the layers decrease with distance from the electron emitter.
Preferably, first dielectric focusing layer
122
is made silicon nitride, which has a dielectric constant of 7.9, and second dielectric focusing layer
123
is made from silicon dioxide, which has a dielectric constant of 3.9. However, the scope of the invention is not limited to these dielectric materials.
Cathode plate
112
further includes a gate extraction electrode
126
, which is disposed on second dielectric focusing layer
123
. Gate extraction electrode
126
defines a third aperture
137
, which further defines emitter well
128
. A second voltage source
132
, V
2
, is connected to gate extraction electrode
126
. First, second, and third apertures
127
,
133
, and
137
are disposed to allow passage therethrough of an electron beam
134
.
In accordance with the invention, the thickness and the dielectric constant of each of first and second dielectric focusing layers
122
and
123
are selected to cause the focusing of electron beam
134
. Electron beam
134
is focused at least to an extent sufficient to avoid receipt of electron beam
134
by gate extraction electrode
126
.
Anode plate
114
is disposed to receive electron beam
134
. Anode plate
114
includes a transparent substrate
136
made from, for example, glass. An anode
138
is disposed on transparent substrate
136
. Anode
138
is preferably made from a transparent, electrically conductive material, such as indium tin oxide. A third voltage source
146
, V
3
, is connected to anode
138
.
A phosphor
140
is disposed upon anode
138
. Phosphor
140
is cathodoluminescent. Thus, phosphor
140
emits light upon activation by electron beam
134
. Methods for fabricating anode plates for matrix-addressable field emission displays are known to one of ordinary skill in the art.
Cathode plate
112
is fabricated using convenient deposition and patterning methods known to one skilled in the art. In the embodiment of
FIG. 1
, electron emitter
124
can be formed by a deposition technique, such as vacuum arc deposition, plasma enhanced chemical vapor deposition, other forms of chemical vapor deposition, spin-on techniques, various growth techniques, and the like.
The operation of FED
110
includes the step of applying potentials at column electrode
118
and gate extraction electrode
126
, which are useful for causing electron emission from emissive surface
125
. A potential is applied to anode
138
for attracting the electrons to anode
138
.
FIG. 2
is a computer model representation of one half of the crosssectional view of FED
110
of FIG.
1
.
FIG. 2
does not include anode plate
114
. Rather, a simulation boundary
154
is utilized in the computer model. Simulation boundary
154
represents a voltage of
150
volts. The abscissa represents a position, x, along electron emitter
124
. The ordinate represents the axis of symmetry of the structure. A first distance, x
1
, on the abscissa is equal to about 2 micrometers, and a first distance, y
1
, on the ordinate is equal to about 1.0 micrometer. Simulation boundary
154
is positioned at a second distance, y
2
, which is equal to approximately 10 micrometers, on the ordinate.
Further illustrated in
FIG. 2
are a plurality of equipotential lines
148
and a plurality of electron trajectories
150
generated by the computer model, for the following conditions: a gate voltage at gate extraction electrode
126
of about 100 volts, electron emitter
124
at ground potential, and a potential at simulation boundary
154
of about 150 volts.
Illustrated in
FIG. 2
is the warping or shaping of the of the electric field within emitter well
128
due to the dissimilar dielectric properties of first and second dielectric focusing layers
122
and
123
. The shaping of the field is sufficient to direct electron beam
134
in a direction toward the axis of symmetry of emitter well
128
. This focusing ameliorates the impingement of electrons upon gate extraction electrode
126
and upon surfaces
129
and
130
of first and second dielectric focusing layers
122
and
123
, respectively.
FIG. 3
is a computer model representation of one half of a cross-sectional view of a prior art field emission device (FED)
160
. Prior art FED
160
includes an electron emitter
162
, a non-dielectric layer
164
disposed on electron emitter
162
, a dielectric layer
166
of silicon dioxide disposed on non-dielectric layer
164
, and a gate extraction electrode
168
formed on dielectric layer
166
.
FIG. 3
illustrates a plurality of equipotential lines
169
and a plurality of electron trajectories
170
generated by the computer model, using the distances (x
1
, y
1
, y
2
), simulation boundary
154
, and operating voltages described with reference to FIG.
2
. Also, non-dielectric layer
164
is at ground potential. Contrasting equipotential lines
169
of
FIG. 3
with equipotential lines
148
of
FIG. 2
, it is evident that prior art non-dielectric layer
164
(
FIG. 3
) suppresses the electric field at the emissive surface to a greater extent than does dielectric focusing structure
121
(
FIG. 2
) of the invention.
FIG. 4
is a graphical representation of electric field strength, E, versus position, x, along the electron emitter for the structures of
FIGS. 2 and 3
. A graph
190
is a general representation of electric field strength at the emissive surface of prior art FED
160
. Although prior art FED
160
focuses the electron beam, the focusing effect is due to warping of the field lines caused by field retardation because the normal field at the edge of non-dielectric layer
164
is forced to zero by non-dielectric layer
164
.
In contrast, the normal field at the edge of first dielectric focusing layer
122
is not forced to zero, as illustrated by a graph
180
of electric field strength, E, versus position, x, for FED
110
. Consequently, the electric field strength is greater for FED
110
than prior art FED
160
for all positions along the emissive surface. In accordance with the invention, reduced field suppression at emissive surface
125
allows the use of a reduced operating voltage at gate extraction electrode
126
.
FIG. 5
is a cross-sectional view of a field emission device (FED)
210
, in accordance with another embodiment of the invention. In the embodiment of
FIG. 5
, electron emitter
124
defines an emissive edge
225
, rather than an emissive surface. Emissive edge
225
is located at a distance above the bottom surface of emitter well
128
. This configuration provides improved electric field properties at emissive edge
225
.
The fabrication of FED
210
includes the fabrication steps described with reference to FIG.
1
and further includes the step of removing electron-emissive material from the bottom of emitter well
128
. The fabrication of FED
210
also includes the step of selectively and partially etching ballast resistor layer
120
, so that the bottom surface of emitter well
128
lies below a plane defined by electron emitter
124
.
One of the advantages of the configuration of FED
210
is that fewer contaminant ions per unit area impinge upon the generally vertical walls of emitter well
128
, than upon the bottom surface of emitter well
128
. By reducing ionic bombardment at emissive edge
225
, the lifetime of the device can be increased.
FIG. 6
is a cross-sectional view of a field emission device (FED)
310
, in accordance with yet another embodiment of the invention. In the embodiment of
FIG. 6
, dielectric focusing structure
121
is disposed above gate extraction electrode
126
, rather than below. Also, in the embodiment of
FIG. 6
, electron emitter
124
defines an emissive tip
325
. In FED
310
, electron emitter
124
can be a Spindt tip electron emitter.
In the embodiment of
FIG. 6
, gate extraction electrode
126
is separated from column electrode
118
by a third dielectric layer
312
, which is preferably made from silicon dioxide. FED
310
further includes an electrode
314
disposed on dielectric focusing structure
121
. A fourth voltage source
316
, V
4
, is connected to electrode
314
. In the operation of FED
310
, the potential applied to electrode
314
is greater than the potential applied to gate extraction electrode
126
.
Methods for fabricating Spindt tip electron emitters are known to one skilled in the art. In the fabrication of FED
310
, dielectric focusing structure
121
and electrode
314
are fabricated subsequent to the formation of the Spindt tip electron emitters, by using convenient deposition and etch techniques.
FIG. 7
is a cross-sectional view of a field emission device (FED)
410
, in accordance with a further embodiment of the invention. In the embodiment of
FIG. 7
, dielectric focusing structure
121
defines apertures of dissimilar sizes. Preferably, the size of first aperture
127
is less than the size of second aperture
133
. Most preferably, each of first aperture
127
and second aperture
133
has a circular cross-section, and the diameter of first aperture
127
is less than the diameter of second aperture
133
.
The fabrication of FED
410
includes the fabrication steps described with reference to FIG.
1
and further includes, subsequent to the step of forming second aperture
133
in second dielectric focusing layer
123
, the step of forming a sidewall on surface
130
. The sidewall is formed prior to the step of forming first aperture
127
in first dielectric focusing layer
122
. The thickness of the sidewall at the upper surface of first dielectric focusing layer
122
defines half of the difference between the diameters of first aperture
127
and second aperture
133
. After the step of forming first aperture
127
, the sidewall is removed.
In summary, the invention is for a field emission device having a dielectric focusing structure. The device of the invention provides at least the benefit of lower operating gate voltage over that of the prior art.
While we have shown and described specific embodiments of the present invention, further modifications and improvements will occur to those skilled in the art. For example, the ballast resistor layer can be omitted. As a further example, one of the dielectric focusing layers can be made from barium titanate.
We desire it to be understood, therefore, that this invention is not limited to the particular forms shown and we intend in the appended claims to cover all modifications that do not depart from the spirit and scope of this invention.
Claims
- 1. A field emission device comprising:an electron emitter designed to emit an electron beam; a first dielectric focusing layer defining a first aperture and characterized by a first dielectric constant, wherein the first aperture is disposed to allow passage therethrough of the electron beam; and a second dielectric focusing layer defining a second aperture and characterized by a second dielectric constant, wherein the second dielectric focusing layer is disposed over the first dielectric focusing layer, wherein the second aperture is disposed to allow passage therethrough of the electron beam, and wherein the second dielectric constant is less than the first dielectric constant.
- 2. The field emission device as claimed in claim 1, wherein the first dielectric focusing layer comprises silicon nitride, and wherein the second dielectric focusing layer comprises silicon dioxide.
- 3. The field emission device as claimed in claim 1, wherein the electron emitter defines an emissive surface.
- 4. The field emission device as claimed in claim 3, wherein the electron emitter comprises an electron-emissive material characterized by a turn-on field of less than 100 volts/μm.
- 5. The field emission device as claimed in claim 1, further comprising a gate extraction electrode disposed on the second dielectric focusing layer, wherein the gate extraction electrode defines a third aperture disposed to allow passage therethrough of the electron beam.
- 6. The field emission device as claimed in claim 5, wherein the first dielectric focusing layer has a first thickness; wherein the second dielectric focusing layer has a second thickness; and wherein the first dielectric constant, the second dielectric constant, the first thickness, and the second thickness are selected to cause the electron beam to be focused to an extent sufficient to avoid receipt of the electron beam by the gate extraction electrode.
- 7. The field emission device as claimed in claim 1, further comprising a gate extraction electrode, wherein the gate extraction electrode defines a third aperture disposed to allow passage therethrough of the electron beam, and wherein the first dielectric focusing layer is disposed on the gate extraction electrode.
- 8. The field emission device as claimed in claim 1, wherein the first dielectric focusing layer is characterized by a resistivity of not less than 1010 ohm-cm.
- 9. The field emission device as claimed in claim 1, wherein the first aperture of the first dielectric focusing layer has a size, wherein the second aperture of the second dielectric focusing layer has a size, and wherein the size of the first aperture is less than the size of the second aperture.
- 10. A field emission device comprising:an electron emitter designed to emit an electron beam; a first dielectric focusing layer defining a first aperture and characterized by a first dielectric constant; and a second dielectric focusing layer defining a second aperture and characterized by a second dielectric constant, wherein the first aperture and the second aperture are disposed to allow passage therethrough of the electron beam in a direction from the first aperture to the second aperture, and wherein the second dielectric constant is less than the first dielectric constant.
- 11. The field emission device as claimed in claim 10, wherein the first dielectric focusing layer comprises silicon nitride, and wherein the second dielectric focusing layer comprises silicon dioxide.
- 12. The field emission device as claimed in claim 10, wherein the electron emitter defines an emissive surface.
- 13. The field emission device as claimed in claim 10, wherein the electron emitter comprises an electron-emissive material characterized by a turn-on field of less than 100 volts/pm.
- 14. The field emission device as claimed in claim 10, further comprising a gate extraction electrode disposed on the second dielectric focusing layer, wherein the gate extraction electrode defines a third aperture disposed to allow passage therethrough of the electron beam.
- 15. The field emission device as claimed in claim 14, wherein the first dielectric focusing layer has a first thickness; wherein the second dielectric focusing layer has a second thickness; and wherein the first dielectric constant, the second dielectric constant, the first thickness, and the second thickness are selected to cause the electron beam to be focused to an extent sufficient to avoid receipt of the electron beam by the gate extraction electrode.
- 16. The field emission device as claimed in claim 10, further comprising a gate extraction electrode, wherein the gate extraction electrode defines a third aperture disposed to allow passage therethrough of the electron beam, and wherein the first dielectric focusing layer is disposed on the gate extraction electrode.
- 17. The field emission device as claimed in claim 10, wherein the first dielectric focusing layer is characterized by a resistivity of not less than 1010 ohm-cm.
- 18. The field emission device as claimed in claim 10, wherein the first aperture of the first dielectric focusing layer has a size, wherein the second aperture of the second dielectric focusing layer has a size, and wherein the size of the first aperture is less than the size of the second aperture.
- 19. A field emission device comprising:an electron emitter designed to emit an electron beam; a first dielectric focusing layer defining a first aperture and characterized by a first dielectric constant; and a second dielectric focusing layer defining a second aperture and characterized by a second dielectric constant, wherein the second dielectric focusing layer is disposed on the first dielectric focusing layer, wherein the first aperture and the second aperture are disposed to allow passage therethrough of the electron beam in a direction from the first aperture to the second aperture, and wherein the second dielectric constant is less than the first dielectric constant.
- 20. The field emission device as claimed in claim 19, wherein the first dielectric focusing layer comprises silicon nitride, and wherein the second dielectric focusing layer comprises silicon dioxide.
- 21. The field emission device as claimed in claim 19, wherein the electron emitter defines an emissive surface.
- 22. The field emission device as claimed in claim 19, wherein the electron emitter comprises an electron-emissive material characterized by a turn-on field of less than 100 volts/pm.
- 23. The field emission device as claimed in claim 19, further comprising a gate extraction electrode disposed on the second dielectric focusing layer, wherein the gate extraction electrode defines a third aperture disposed to allow passage therethrough of the electron beam.
- 24. The field emission device as claimed in claim 23, wherein the first dielectric focusing layer has a first thickness; wherein the second dielectric focusing layer has a second thickness; and wherein the first dielectric constant, the second dielectric constant, the first thickness, and the second thickness are selected to cause the electron beam to be focused to an extent sufficient to avoid receipt of the electron beam by the gate extraction electrode.
- 25. The field emission device as claimed in claim 19, further comprising a gate extraction electrode, wherein the gate extraction electrode defines a third aperture disposed to allow passage therethrough of the electron beam, and wherein the first dielectric focusing layer is disposed on the gate extraction electrode.
- 26. The field emission device as claimed in claim 19, wherein the first dielectric focusing layer is characterized by a resistivity of not less than 1010 ohm-cm.
- 27. The field emission device as claimed in claim 19, wherein the first aperture of the first dielectric focusing layer has a size, wherein the second aperture of the second dielectric focusing layer has a size, and wherein the size of the first aperture is less than the size of the second aperture.
US Referenced Citations (6)
Foreign Referenced Citations (2)
Number |
Date |
Country |
0848406 |
Jun 1998 |
EP |
0 795 622 |
Sep 1997 |
GB |