Electric lamp or space discharge component or device manufacturi – Process – Electrode making
Reexamination Certificate
2000-11-29
2002-04-30
Ramsey, Kenneth J. (Department: 2879)
Electric lamp or space discharge component or device manufacturi
Process
Electrode making
C445S024000
Reexamination Certificate
active
06379210
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to electron emission devices. More specifically, this invention relates to the structure and manufacture of electron emissive elements used in flat panel displays.
2. Background Art
In a flat panel display, a matrix of electron emitters emit electrons that impinge on a transparent display panel coated with light emitting material such as phosphor. The principles of a flat panel display can be more clearly explained by referring to
FIGS. 1A
,
1
B, and
1
C (collectively FIG.
1
), which illustrate a flat panel display structure.
In
FIG. 1A
, backplate
120
is provided as a support to which electrically conductive emitter layer
113
is attached. Generally conical electron emitters
116
are formed on emitter layer
113
. In
FIG. 1B
, electron emitters
116
are formed within gate holes
115
B, under gate layer
115
A. Gate layer
115
A is separated from emitter layer
113
by dielectric layer
117
. Display panel
118
having light emissive layer
110
and anode layer
111
is situated above, and spaced vertically apart from, gate layer
115
A.
Portions of gate layer
115
A are provided with sufficiently greater voltage than emitter layer
113
and electron emitters
116
to enable layer
115
A to extract electrons from electron emitters
116
. Anode layer
111
is at a considerably greater voltage than emitter layer
113
or gate layer
116
. As a result, a large fraction of the electrons emitted from electron emitters
116
are attracted by anode layer
111
toward transparent panel
118
. With anode layer
111
being quite thin, the electrons pass through anode layer
111
and impinge on the phosphor coating
110
on panel
118
, causing light emissive layer
110
to emit light.
FIG. 1C
shows a cathode structure
100
for a flat panel display. Emitter layer
113
is divided into mutually insulated emitter rows
114
, while gate layer
115
A is divided into mutually insulated columns
184
. For a black and white display, the overlapping area of a row
114
and a column
184
(see
FIG. 1D
) represents a pixel, the smallest element of a picture. For a color display, several (normally three) overlapping row/column areas form a pixel. In order to cause a selected group of emitters
116
to emit electrons thereby to energize a pixel, an appropriate electric field must be created between electron emitters
116
and gate layer
115
A. In particular, a voltage must be applied between a selected row
114
and a selected column
184
to place that row
114
at a suitably greater potential than that column
184
, thereby causing electron emission from emitters
116
at that row/column intersection. When the voltage between the selected row
114
and the selected column
184
is below a non-zero threshold value, emitters
116
at the row/column intersection do not emit electrons, and the corresponding pixel is not excited.
Referring to
FIG. 1C
, a complete picture requires the scanning of every row and every column. In order to have the picture appear to be continuous to the human eye, the scanning must be performed at high speed. Thus the voltage between a specific row and column must change in a very short time.
The geometry of rows
114
and columns
184
together with the thickness H and dielectric constant of dielectric layer
117
determines the crossover capacitance between a row
114
and a column
184
. When thickness H is small, the crossover capacitance is large. This capacitance substantially slows down the activation of electron emitters
116
, resulting in poor display. Therefore, it is desirable that dielectric layer
117
be thick. When the thickness of dielectric layer
117
increases, the height of electron emitters
116
normally must also increase in order to bring their tips sufficiently close to gate layer
115
A to enable layer
115
A to extract electrons from them.
A thick dielectric layer also reduces the possibility of short circuiting. During display operation, undesirable conductive paths may be produced through dielectric layer
117
so as to short circuit emitter layer
113
and gate layer
115
A. As thickness H (
FIG. 1D
) of dielectric layer
117
increases, the likelihood of short circuiting gate layer
115
A to emitter layer
113
by creating such a conductive path decreases. Further, in
FIG. 1A
, hollow spaces
119
keep gate layer
115
A spaced apart from electron emitters
116
. Because gate holes
115
B are typically quite small, as little as
80
nm in diameter, a metal particle falling into hollow space
119
may cause short circuiting between gate layer
115
A and electron emitters
116
. With a thick dielectric layer
117
, hollow space
119
would have an elongated profile. A particle falling into hollow space
119
tends to rest within the hollow space and away from gate hole
115
B, and thus is less likely to cause short circuiting.
For conical electron emitters with a given aspect ratio (height to base diameter), larger gate holes
115
B are required in order to create higher conical electron emitters
116
. However, for fine quality picture, it is desirable to have more electron emitters per unit area. Thus it is desirable to have small gate holes. Small gate holes also give greater field strength at the emitters, resulting in lower applied voltage between rows and columns to achieve a given emission current. High aspect ratio cones allow a thick dielectric layer to be used, thus giving the advantages of reduced cross-over capacitance and greater short protection. Consequently, a higher aspect ratio is desirable for making a better cathode structure.
Certain materials such as nickel can be used to create electron emitters with a high aspect ratio. However, nickel does not have other properties desired for electron emitters. For example, nickel has poor chemical robustness. Also, nickel is easily oxidized. Oxidized nickel emitters have an increased extraction voltage and decreased emission stability.
Nickel has a relatively high work function. Work function is defined as the level of energy necessary to energize an electron to such a level that the electron is emitted from the material. A high work function means that a stronger electric field is required between the electron emitter
116
and corresponding column
184
of gate layer
115
A in order to energize the electrons. This stronger electric field translates to a greater column-to-row extraction voltage. A high column-to-row extraction voltage is undesirable because it results in high power consumption and more expensive circuitry.
It is therefore desirable to have electron emitters with a high aspect ratio, good chemical robustness and low work function.
GENERAL DISCLOSURE OF THE INVENTION
In accordance with the present invention, improved electron emitters are provided with high aspect ratios, good chemical robustness and low work function. Electron emitters are formed with electrically non-insulating material that allows deposition to a high aspect ratio at low deposition temperature. One candidate material for the electron emitters is nickel. Electron emitters so made are coated with surface material that has good chemical robustness and low work function. One candidate for the surface material is carbon. The emitter and surface materials may also be chosen for other desirable electrical or chemical properties. Work function of coated emitters is typically reduced by about 0.8 to 1.0 eV.
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Brandes George R.
Macaulay John M.
Spindt Christopher J.
Stanners Colin D.
Xu Xueping
Candescent Technologies Coporation
Meetin Ronald J.
Ramsey Kenneth J.
Skjerven Morrill & MacPherson LLP
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