Electric lamp and discharge devices – With luminescent solid or liquid material – Vacuum-type tube
Reexamination Certificate
2000-09-19
2003-08-26
Patel, Ashok (Department: 2879)
Electric lamp and discharge devices
With luminescent solid or liquid material
Vacuum-type tube
C313S309000, C313S351000
Reexamination Certificate
active
06611093
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to field emission displays, and more particularly to field emission displays having transparent cathodes.
BACKGROUND OF THE INVENTION
A type of cold cathode for field emission displays is described by C. A. Spindt, et. al. “Physical properties of thin film field emission cathode with molybdenum cones” in the Journal of Applied Physics, V47, No. 12. The described devices produce electrons by quantum mechanical tunneling from the emitter surface into a vacuum under the influence of a large electric field. The electron current generated by any emitter is described by the “Fowler-Nordheim” equation and is influenced by a number of factors including the work-function of the emitting surface and the physical shape and geometry of the emitter cone. These factors are caused by processing conditions and may result in a large degree of variability in emission current within a grouping of emitters.
Flat-panel, field-emission displays are typically addressed by electrical means. Referring to
FIG. 1
, a typical prior art display is illustrated with a voltage signal applied between a first conductive electrode
11
disposed on a non-conductive substrate
10
and a second conductive electrode
12
overlying the first conductive electrode. The first and second electrodes are electrically isolated from each other by means of electrically non-conductive dielectric layer
13
lying between the electrode layers. Emitter elements
14
disposed between the first and second electrodes are activated by differential signals applied between the electrodes
11
and
12
to cause electrons
15
to be emitted into a vacuum
20
. The electrons are drawn to a light-emitting element
31
by the application of a voltage to an anode element
32
.
Light emitting elements
31
, opposite the electron-emitting elements
14
, generate light responsive to bombardment by electrons emitted from the corresponding emitting element
14
. Typically, a large number of electron-emitting elements are associated with each light-emitting element, which defines a pixel. The light-emitting elements
31
are carried by transparent conductive anode viewing screen
32
located parallel to, and spaced from, the electron-emitting elements
14
. The anode viewing screen is typically formed on a transparent insulating substrate
30
. The light-emitting elements are typically viewed through substrate
30
and transparent anode
32
.
Since there can be a large variance in the emission current from emitter to emitter in any grouping or array of emitters, there can be a corresponding variation in the intensity of light emitted by the light-emitting elements. This variation in intensity causes degradation in picture quality.
In addition, current emission heats the emitter. The emission current increases as the temperature of the emitter increases causing a positive feedback condition which can result in the thermal destruction of the emitter and/or arcing of the emitter to the anode causing destruction of the display.
In order to limit current in any emitter and to normalize emission current within the grouping of emitters, Kane, (U.S. Pat. No. 5,142,184) describes the imposition of a series ballast resistor
16
between the first conductive electrode
11
and emitter element
14
. The teachings of Kane, and other prior art, describe various configurations of resistive elements interconnecting, and disposed between, the first conductive electrode and the electron-emitting element. Typical prior art field emission displays,
FIG. 2
, comprise a plurality of substantially parallel first conductive electrodes
11
disposed in a generally horizontal direction on a non-conductive substrate, and a like plurality of second conductive electrodes
12
disposed in a direction generally perpendicular to, and overlying, the first conductive electrodes. The first and second electrode arrays are electrically isolated from each other by means of the electrically non-conductive dielectric layer
13
lying between the layers containing the electrode arrays. Electron-emitting elements
14
are disposed along each first conductive electrode at the intersection of the first and second conductive electrodes. When electrical signals are applied between first and second conductive electrodes, electrons are emitted from the electron-emitting elements located at the intersection of the first and second electrodes. Each electrode is made to be as narrow in width as possible to provide a large number of electrodes per display width. The resistance
17
of the electrode must be small in order to provide electrical addressing signals from one end of the electrode to the other without attenuation and consequent loss of signal fidelity. Consequently, electrodes are typically constructed from opaque metallic materials such as aluminum. Typical resistivities of these materials may be in the range of 2-5 micro-ohm-cm.
Referring to
FIGS. 1 and 3
, the ballast resistors
16
in series with the electron-emitting element and the first conductive electrode must be of a high resistance to provide sufficient retardation of excess current in the emitter element. This resistance is typically 10
5
to 10
7
Ohms. Ballast resistors
16
are electrically in parallel with the intrinsic series resistance
17
of the first conductive electrode
11
. Ballast resistors must be significantly higher in resistance than the electrode resistance along the electrode from one grouping of emitters to any other grouping of emitters. This favorable ratio minimizes nonuniformity of electron emission as a function of distance along the electrode. This is shown schematically in
FIG. 3
to provide illustration. A disadvantage of this type of ballasting is that the resistance of resistor
16
is dependent on geometry.
FIG. 10
shows various construction errors encountered in the photoresist and etching processes used to define the placement and shape of elements
11
,
14
and
16
.
FIGS. 10
a,
10
b
and
10
c
show varying lengths of resistor
16
due to x and y deviations in photoresist mask alignment causing varying resistance from one display to another.
FIG. 10
d
shows an alignment error through rotation of the cones
14
to the resistors
16
and metal line
11
. This causes a high variation in ballast resistance and non-uniform emission.
FIG. 10
e
show an abnormally thin ballast resistor
16
at emitter
14
a
due to undercutting of photoresist, and
FIG. 10
f
shows a necking down of ballast resistor
16
at emitter
14
a
that can occur where a right angle is formed between two elements such as elements
11
and
16
.
A second prior art display is described in U.S. Pat. No. 5,646,479, a portion of which is shown in the cross-sectional view, FIG.
4
. This display is viewed so that the light emitting element
31
may be viewed through substrate
40
. This configuration has several advantages, including the possibility of a reflective electrode
52
. This is a technique well known in the art for increasing the amount of light emitted as a function of the electron beam current. This art describes a display comprising a transparent first electrode
41
on which is disposed and mounted a plurality of emitter cones
14
. A second transparent, conductive electrode
42
is located plane-parallel to the first conductive transparent electrode and separated by a layer of dielectric material
13
. An electrical signal applied between the first and second transparent conductive electrode causes electrons
15
to be emitted from the emitter cones
14
disposed on the first transparent conductive electrode. The resistivity of the transparent conductive electrode
41
is selected so as to provide electrical short protection in the event of such a failure at any emitter. However, as may be seen from the illustrative schematic representation,
FIG. 5
, the resistance
17
between the signal source and emitter cone
14
is a variable function of the distance between the cone and the location of the applied electrical signal
5
. This variability in
Haven Duane A.
Naugler, Jr. W. Edward
Display Research Laboratories, Inc.
Dorsey & Whitney LLP
Patel Ashok
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