Electric lamp or space discharge component or device manufacturi – Apparatus
Reexamination Certificate
1998-05-14
2002-01-15
Heinrich, Samuel M. (Department: 1725)
Electric lamp or space discharge component or device manufacturi
Apparatus
C250S325000, C427S551000
Reexamination Certificate
active
06338663
ABSTRACT:
TECHNICAL FIELD
This invention relates in general to field emission displays for electronic devices and, in particular, to improved cathodoluminescent layers for field emission displays.
BACKGROUND OF THE INVENTION
FIG. 1
is a simplified side cross-sectional view of a portion of a 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 forms a hermetically sealed package 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 pixels yielding different colors to provide a 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 surface of a substrate
32
, which may be a semiconductor such as silicon. Although the substrate
32
may be a semiconductor material other than silicon, or even an insulative material such as glass, it will hereinafter be assumed that the substrate
32
is silicon. The substrate
32
is coated with a dielectric layer
34
that is formed, in one embodiment, 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 may be 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, for example, 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 emitter
30
allow electrons to flow to the emitter
30
. Intense electrical fields between the emitter
30
and the extraction grid
38
then cause emission of electrons from the emitter
30
. A larger positive voltage, ranging up to as much as 5,000 volts or more but generally 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 adjacent to the emitters
30
, and forms luminous images such as text, pictures and the like.
When the emitted electrons strike the cathodoluminescent layer
26
, compounds in the cathodoluminescent layer
26
may be dissociated, causing outgassing of materials from the cathodoluminescent layer
26
. When the outgassed materials react with the emitters
30
, their work function may increase, reducing the emitted current density and in turn reducing display luminance. This can cause display performance to degrade below acceptable levels and also results in reduced useful life for displays
10
.
Residual gas analysis indicates that the dominant materials outgassed from some types of cathodoluminescent layers
26
include hydroxyl radicals. The hydroxyl radicals reacting with the emitters
30
leads to oxidation of the emitters
30
, and especially to oxidation of emitters
30
formed from silicon. Silicon emitters
30
are useful because they are readily formed and integrated with other electronic devices on the substrates
32
when the substrate is silicon. Electron emission is reduced when silicon emitters
30
oxidize. This leads to time-dependent and/or degraded performance of displays
10
.
In conventional cathode ray tubes (“CRTs”), some scrubbing of the cathodoluminescent screen is typically carried out after the tube is sealed using an electron gun of the type contained in a CRT. “Scrubbing,” as used here, means to expose the cathodoluminescent layers (e.g., cathodoluminescent layer
26
) to an electron beam until a predetermined charge per unit area has been delivered to the cathodoluminescent layer
26
. This scrubbing is carried out at a very low duty cycle and at a very low current density because the electron beam is rastered over the area of the cathodoluminescent screen. It is also carried out at the same current levels that the CRT is expected to support in normal operation, typically 100 microamperes/cm
2
or less. However, this approach will not work for scrubbing cathodoluminescent layers
26
for the displays
10
, in part because the emitters
30
in the displays
10
are poisoned by the chemical species evolving from the cathodoluminescent layer
26
in response to the scrubbing operation. Moreover, the cathodoluminescent layer
26
is typically much less than a millimeter away from the emitters
30
, i.e., the mean free path for any gaseous chemical species evolving from the cathodoluminescent layer
26
is much larger than the distance separating the cathodoluminescent layers
26
from the emitters
30
. In contrast, the electron gun used To scrub cathodoluminescent layers in a CRT are not adversely affected by this chemical species and electron guns are, as a rule of thumb, displaced from the cathodoluminescent screen by a distance approximately equal to the diagonal dimension of the CRT screen.
There is therefore a need for a technique to prevent evolution of oxygen-bearing compounds from cathodoluminescent screens in field emission display faceplates.
SUMMARY OF THE INVENTION
In accordance with one aspect of the invention, a low voltage, high current, large area cathode for electron scrubbing of cathodoluminescent layers is described. The electron scrubbing is particularly advantageous for use with cathodoluminescent screens of field emission displays having silicon emitters. The present invention includes an apparatus to irradiate a cathodoluminescent layer in a vacuum with an electron beam and a device to move the cathodoluminescent layer relative to the irradiating apparatus. The irradiation is stopped when a predetermined total Coulombic dose has been delivered to the cathodoluminescent layer. Significantly, the scrubbing results in a cathodoluminescent layer that does not outgas materials that are deleterious to performance of silicon emitters. This results in a more robust display and extended display life.
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A. Hoffman and P.J.K. Paterson, “Surface and Subsurface 1 keV Electron Stimulated Reduction of Sapphire Studied by Electron Spectroscopy,”Applied Surface Science 93:301-307, 1996.
A. Pfahnl, “Aging of Electronic Phosphors in Cathode Ray Tubes,”Advances in Electron Tube Technology, pp. 204-208, Sep. 1960.
D.B.M. Klaasen and D.M. de Leeuw, “Degradation of Phosphors Under Cathode-Ray Excitation,”Journal of Luminescence 37:21-28, 1987.
Peter A. Keller,The Cathode-Ray Tube: Technology, History, and Applications, Palisades Press, New York, New York, 1991, pp. 18-22.
J. Sebastian et al., “Electron-Stimulated
Dynka Danny
Watkins Charles M.
Dorsey & Whitney LLP
Heinrich Samuel M.
Micro)n Technology, Inc.
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