Electric lamp and discharge devices – With luminescent solid or liquid material – Vacuum-type tube
Reexamination Certificate
1998-05-26
2001-12-04
Patel, Ashok (Department: 2879)
Electric lamp and discharge devices
With luminescent solid or liquid material
Vacuum-type tube
Reexamination Certificate
active
06326725
ABSTRACT:
TECHNICAL FIELD
This invention relates in general to visual displays for electronic devices and in particular to improved focusing electrodes and techniques for field emission displays.
BACKGROUND OF THE INVENTION
FIG. 1
is a simplified side 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
(A
1
, 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
that is preferably a semiconductor material such as silicon. 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. An opening
40
is created in the extraction grid
38
having a radius that is also approximately the separation of the extraction grid
38
from the tip of the emitter
30
. The radius of the opening
40
may be 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. Signals coupled to the emitters
30
allow electrons to flow to the emitter
30
. Intense electrical fields between the emitter
30
and the extraction grid
38
cause emission of electrons from the emitter
30
.
A larger positive voltage, ranging up to as much as 5,000 volts or more but usually 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 this voltage and strike the cathodoluminescent layer
26
. This causes light emission in selected areas, i.e., those areas opposite the emitters
30
, and forms luminous images such as text, pictures and the like.
Electrons emitted from each emitter
30
in a conventional field emission display
10
tend to spread out as the electrons travel from the emitter
30
to the cathodoluminescent layer
26
on the faceplate
20
. If the electron emission spreads out too far, it will impact on more than one localized portion of the cathodoluminescent layer
26
of the field emission display
10
. This phenomenon is known as “bleedover.” The likelihood that bleedover may occur is exacerbated by any misalignment between the localized portions of the cathodoluminescent layer
26
and their associated sets of emitters
30
.
When the electron emission from an emitter
30
associated with a first localized portion of the cathodoluminescent layer
26
also impacts 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 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. As a result, a degraded image is 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 bleedover. In conventional field emission displays
10
, bleedover is alleviated in several ways. A relatively high anode voltage V
a
may be applied to the transparent conductive layer
24
of the conventional field emission display
10
, 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
. A relatively small gap between the faceplate
20
and the baseplate
21
may be used, again reducing opportunity for spreading of the emitted electrons. However, it has been found that these are impractical solutions because too high a voltage applied between the transparent conductive layer
24
and the baseplate
21
, or too small a gap between the faceplate
20
and the baseplate
21
may cause arcing.
Another way in which bleedover is reduced in conventional field emission displays
10
is by spacing the localized portions of the cathodoluminescent layer
26
relatively far apart. This is possible because of the relatively low display resolution provided by conventional field emission displays
10
. As a result, the electron emissions impact on the correct localized portion of the cathodoluminescent layer
26
.
Another approach to controlling the spatial spread of electrons emitted from a group of the emitters
30
is to surround the area emitting the electrons with a focusing electrode (not illustrated in FIG.
1
). This allows increased control over the spatial distribution of the emitted electrons via control of the voltage applied to the focusing electrode, which in turn provides increased resolution for the resulting image. One such approach, where each focusing element serves many emitters, is described in U.S. Pat. No. 5,528,103, entitled “Field Emitter With Focusing Ridges Situated To Sides Of Gate”, issued to Spindt et al.
There are several disadvantages to these prior art approaches. In most prior art approaches, the focusing electrode is biased by a voltage source that is independent of other bias voltage sources associated with the emitter
30
. As a result, the use of a focusing electrode generally requires another bias voltage source to bias the focusing electrode. This, in turn, leads to problems due to variations in turn on voltage from one emitter
30
to another when a single bias voltage is applied for several focusing electrodes. When a group of emitters
30
are all affected by a single focusing electrode, some of the emitters
30
may exhibit a turn on voltage that differs from that exhibited by other emitters
30
. The effect that the focusing electrode has on the electrons emitted from each of these emitters
30
will differ. Additionally, some of the current through the emitter
30
will be collected by the focusing electrode. This complicates the relationship between the emitter current and light emission because some of the current through the emitter
30
is diver
Dorsey & Whitney LLP
Micro)n Technology, Inc.
Patel Ashok
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