Electric lamp or space discharge component or device manufacturi – Process – With assembly or disassembly
Reexamination Certificate
1998-07-02
2001-02-20
Ramsey, Kenneth J. (Department: 2879)
Electric lamp or space discharge component or device manufacturi
Process
With assembly or disassembly
C313S309000
Reexamination Certificate
active
06190223
ABSTRACT:
TECHNICAL FIELD
This invention relates to field emission devices, and more particularly to processes for creating gate and focus ring structures which are self-aligned to emitter tips using chemical mechanical planarization (CMP) and etching techniques.
BACKGROUND OF THE INVENTION
Flat panel displays have become increasingly important in appliances requiring lightweight portable screens. Currently, such screens generally use electroluminescent or liquid crystal technology. A relatively new technology is the field emission display which uses of a matrix-addressable array of cold cathode emission devices to excite cathodoluminescent material on a screen.
With reference to
FIG. 1
, a conventional field emission display includes a base plate
12
and a face plate
24
spaced from each other to define a sealed envelope
11
therebetween. The sealed envelope
11
may be evacuated as is conventional in field emission displays.
The base plate
12
may include a substrate
18
of silicon or some other material on which a conductive layer
20
is formed, the conductive layer
20
supporting a plurality of conical emitters
22
. Only one emitter
22
has been shown to simplify the discussion. An extraction grid
16
formed of a conducting material is positioned above the substrate
18
by a first insulating layer
26
of dielectric material. Each emitter
22
extends into a respective aperture
31
formed in the extraction grid
16
. A focus ring layer
14
is positioned over the extraction grid
16
. The focus ring layer
14
is also formed of a conductive material and is spaced from the extraction grid
16
by a second insulating layer
28
of a dielectric material. A plurality of apertures
33
are formed in the focus ring layer
14
, each aperture
33
aligned with a respective aperture
31
formed in the extraction grid
16
.
The face plate
24
includes a transparent substrate
38
coated with a transparent layer of conductive material
40
, such as iridium, forming an anode
36
. The anode
36
is, in turn, coated with a layer of cathodoluminescent material
42
.
In practice, the emitters
22
(which may be in sets of interconnected emitters) are arranged in columns while individual extraction grids
16
are arranged in rows. An individual emitter
22
can then be selected for electron emission by driving a column of emitters
22
to a relatively low voltage and driving an extraction grid
16
row to a relatively high voltage. Electrons
34
are emitted from the emitter
22
in the energized column of emitters
22
that intersects with the energized extraction grid
16
row.
A relatively high positive voltage on the order of 1000 volts is applied to the anode layer
40
. The strong positive voltage attracts the electrons
34
emitted by the emitter
22
so that they pass through the focus ring
14
and strike the cathodoluminescent layer
42
. The cathodoluminescent layer
42
then emits light which is visible through the transparent substrate
38
.
While the focus ring
50
nominally serves the function of collimating the electron beam
34
, the primary purpose of the focus ring layer
14
is to protect the underlying structure from electromagnetic radiation such as soft x-rays and ultraviolet radiation, thus serving as an opaque. Ultraviolet radiation and soft x-rays result from back-scattering from the emitted electrons
34
striking the cathodoluminescent layer
42
, resulting in some of the electromagnetic radiation being reflected back toward the back plate
12
from the face plate
24
.
The clarity, or resolution, of a field emission display is a function of a number of factors, including emitter tip sharpness, alignment and spacing of the gates, or grid openings
31
, which surround the emitter tips
22
, pixel size, as well, as cathode-to-gate and cathode-to-screen voltages. Another factor which affects image sharpness is the angle at which the emitted electrons
34
strike the phosphors
42
of the display screen
36
.
The distance that the emitted electrons
34
must travel from the base plate
12
to the face plate
24
is typically on the order of several hundred microns. The contrast and brightness of the display are optimized when the emitted electrons
34
impinge on the phosphors
42
located on the cathode luminescent screen
36
or face plate
24
, at a substantially 90° angle. However, the contrast and brightness of the display are not currently optimized due to the fact that the initial electron trajectories assumes substantially conical patterns having an apex angle of roughly 30°, which emanates from the emitter tip
22
. In addition, the space-charge affect results in coulombic repulsion among emitted electrons
34
, which lends to further dispersion within the electron beam
34
. Even though the focus rings
50
are normally maintained at ground, they will exert a force on the emitted electrons
34
. Since the focus rings
50
are spaced relatively above and outward of the gate structures
30
the force exerted will contribute to the dispersion of the emitted electrons
34
.
The current design and positioning of focus ring layer
14
causes several problems. The position of the focus ring
50
which is spaced relatively above the low potential anode or extraction grid
16
with respect to the cathode luminescent panel
36
tends to further disperse the emitted electron beam
34
. The current method of fabricating the base plate
12
of the field emission display device
10
requires one CMP step and three etching steps which increases the cost and time required to produce the field emission display. Further, the substantial gap between the extraction grid
16
and the focus ring
50
required by the existing design increases the likelihood of electromagnetic radiation leakage past the opaque.
SUMMARY OF THE INVENTION
The present invention overcomes the limitations of the prior art by providing a flat panel display structure having a focus ring which lies in substantially the same plane as the extraction grid. The base plate of the field emission display is manufactured by covering an emitter substrate having emitters tips with a dielectric insulating material to form a first insulating layer, depositing an extraction grid layer over the first insulating layer, etching the extraction grid layer to define a plurality of gate structures, depositing a second insulating layer over the etched structure, depositing a focus ring layer, and chemical-mechanical planarizing the resulting structure to an endpoint at which the emitter tips are at least partially exposed, thus defining self-aligned and in-plane gate and focus ring structures. The structure may then be optionally selectively wet etched to remove portions of the first and second insulating layers for further exposing the emitter tips.
The base plate includes a substrate, a cathode formed on the substrate having an emitter tip, a first insulating layer formed superadjacent the cathode, an extraction grid formed superadjacent the first insulating layer, the extraction grid having a distal surface with respect to the substrate, a focus ring formed superadjacent the extraction grid, the focus ring having a distal surface with respect to the substrate, the distal surface of the extraction grid and the distal surface of the focus ring being substantially planar proximate the emitter tip.
Placement of the focus ring in substantially the same plane as the extraction grid provides a number of benefits over the current design. In plane placement of the focus ring significantly reduces the dispersive effect the focus ring has on emitted electron beam. Use of the in-plane focus ring also permits the number of processing steps to be reduced from three etching steps and one CMP step, to either one or two etching steps and one CMP step, thereby saving substantial time and costs in the manufacturing process. One of the etching steps is performed before the CMP step. The optional second etching step is performed after the CMP step. The in-plane placement of the focus ring also permits a smaller spacing to be used betwee
Tjaden Kevin W.
Williams Terry N.
Dorsey and Whitney LLP
Micro)n Technology, Inc.
Ramsey Kenneth J.
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