Electric lamp or space discharge component or device manufacturi – Process – Electrode making
Reexamination Certificate
2002-05-16
2003-02-25
Ramsey, Kenneth J. (Department: 2879)
Electric lamp or space discharge component or device manufacturi
Process
Electrode making
C430S025000, C427S068000
Reexamination Certificate
active
06524154
ABSTRACT:
TECHNICAL FIELD
This invention relates in general to visual displays for electronic devices and in particular to improved focusing apparatus and techniques for field emission displays.
BACKGROUND OF THE INVENTION
FIG. 1
is a simplified cross-sectional view of a portion of a field emission display
10
including a faceplate
20
and a baseplate
21
, in accordance with the prior art.
FIG. 1
is not drawn to scale. The faceplate
20
includes a transparent viewing screen
22
, a transparent conductive layer
24
and a cathodoluminescent layer
26
. The transparent viewing screen
22
supports the layers
24
and
26
, acts as a viewing surface and as a wall for a hermetically sealed package formed between the viewing screen
22
and the baseplate
21
. The viewing screen
22
may be formed from glass. The transparent conductive layer
24
may be formed from indium tin oxide. The cathodoluminescent layer
26
may be segmented into localized portions. In a conventional monochrome display
10
, each localized portion of the cathodoluminescent layer
26
forms one pixel of the monochrome display
10
. Also, in a conventional color display
10
, each localized portion of the cathodoluminescent layer
26
forms a green, red or blue sub-pixel of the color display
10
. Materials useful as cathodoluminescent materials in the cathodoluminescent layer
26
include Y
2
O
3
:Eu (red, phosphor P-56), Y
3
(Al, Ga)
5
O
12
:Tb (green, phosphor P-53) and Y
2
(SiO
5
):Ce (blue, phosphor P-47) available from Osram Sylvania of Towanda Pa. or from Nichia of Japan.
The baseplate
21
includes emitters
30
formed on a planar surface of a substrate
32
, which may include semiconductor materials. The substrate
32
is coated with a dielectric layer
34
. In one embodiment, this is effected by deposition of silicon dioxide via a conventional TEOS process. The dielectric layer
34
is formed to have a thickness that is approximately equal to or just less than a height of the emitters
30
. This thickness is on the order of 0.4 microns, although greater or lesser thicknesses may be employed. A conductive extraction grid
38
is formed on the dielectric layer
34
. The extraction grid
38
may be formed, for example, as a thin layer of polysilicon. The radius of an opening
40
created in the extraction grid
38
, which is also approximately the separation of the extraction grid
38
from the tip of the emitter
30
, is about 0.4 microns, although larger or smaller openings
40
may also be employed.
In operation, the extraction grid
38
is biased to a voltage on the order of 100 volts, although higher or lower voltages may be used, while the substrate
32
is maintained at a voltage of about zero volts. Intense electrical fields between the emitter
30
and the extraction grid
38
cause field emission of electrons from the emitter
30
in response to the voltages impressed on the extraction grid
38
and emitter
30
.
A larger positive voltage, also known as an anode voltage V
A
, ranging up to as much as 5,000 volts or more but often 2,500 volts or less, is applied to the faceplate
20
via the transparent conductive layer
24
. The electrons emitted from the emitter
30
are accelerated to the faceplate
20
by the anode voltage V
A
and strike the cathodoluminescent layer
26
. This causes light emission in selected areas, i.e., those areas adjacent to where the emitters
30
are emitting electrons, and forms luminous images such as text, pictures and the like.
When the emitters
30
emit electrons, the resultant beam of electrons spreads as the electrons travel from the emitter
30
towards the faceplate
20
. When the electron emissions associated with a first localized portion of the cathodoluminescent layer
26
also impact on a second localized portion of the cathodoluminescent layer
26
, both the first and second localized portions of the cathodoluminescent layer
26
emit light. As a result, the first pixel or sub-pixel uniquely associated with the first localized portion of the cathodoluminescent layer
26
correctly turns on, and at least a portion of a second pixel or sub-pixel uniquely associated with the second localized portion of the cathodoluminescent layer
26
incorrectly turns on. In a color field emission display
10
, this can cause purple light to be emitted from a blue sub-pixel and a red sub-pixel together when only red light from the red sub-pixel was desired. This is problematic because it degrades the image formed on the faceplate
20
of the field emission display
10
.
In a monochrome field emission display
10
, color distortion does not occur, but the resolution of the image formed on the faceplate
20
is reduced by this spreading of the electron beams from the emitters
30
. This is exacerbated in either type of field emission display
10
as the resolution of the field emission display
10
is increased by crowding pixels or sub-pixels more closely together.
A second problem that may occur is that the entire emitted beam of electrons may travel at an angle to the path that they were intended to take, i.e., form a tilted beam of electrons. This may occur because of electrostatic effects involving interactions with other pixels. Alternatively, variations in shapes of tips of the emitters
30
or in extraction grid
38
geometry resulting from normal manufacturing variability may result in some electron beams being tilted relative to others. As a result, more than one pixel may be impacted by an electron beam intended to result in light emission from only a single pixel.
These problems may be referred to as bleedover. The likelihood of bleedover is increased by any misalignment between the localized portions of the cathodoluminescent layer
26
and their associated sets of emitters
30
. Additionally, as the current from any one of the emitters
30
is increased, the problem of bleedover increases.
In some applications, a small field emission display
10
is intended to be viewed through magnifying optics, such as lenses or magnifying reflectors. These applications require a high resolution field emission display
10
. High resolution field emission displays
10
use fewer emitters
30
per pixel or sub-pixel. This arises for several reasons, one of which is that a smaller pixel or sub-pixel subtends a smaller area in which the emitters
30
can be provided. As a result, each emitter
30
in a high resolution field emission display
10
has a greater influence on the light emitted from the pixel or sub-pixel associated with it. This increases the need to be able to control electron emissions and the spread of electron emissions from each emitter
30
.
In conventional field emission displays
10
, attempts have been made to alleviate bleedover in several ways. The anode voltage V
A
applied to the transparent conductive layer
24
of the conventional field emission display
10
is a relatively high voltage, such as 1,000 volts or more, so that the electrons emitted from the emitters
30
are strongly accelerated to the faceplate
20
. As a result, the electron emissions spread out less as they travel from the emitters
30
to the faceplate
20
. The gap between the faceplate
20
and the baseplate
21
of the conventional field emission display
10
is relatively small (ca. one thousandth of an inch or twenty-five microns per 100 volts of anode voltage V
A
), again reducing opportunity for spreading of the emitted electrons.
Some solutions that have been tried for reducing bleedover either increase the anode voltage V
A
applied to the transparent conductive layer
24
or decrease the spacing between the faceplate
20
and the baseplate
21
in order to reduce spreading of the electron emissions. However, it has been found that these are impractical solutions because the anode voltage V
A
applied between the transparent conductive layer
24
and the baseplate
21
may cause arcing when either of these solutions is attempted.
Another way in which bleedover is reduced in conventional field emission displays
10
is by spacing the localized portions of the cathodolumin
Browning Jimmy J.
Cathey David A.
Watkins Charles M.
Xia Zhongyi
Dorsey & Whitney LLP
Micro)n Technology, Inc.
Ramsey Kenneth J.
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