Electric lamp and discharge devices: systems – Plural power supplies – Plural cathode and/or anode load device
Reexamination Certificate
2001-06-28
2003-01-28
Wong, Don (Department: 2821)
Electric lamp and discharge devices: systems
Plural power supplies
Plural cathode and/or anode load device
C315S169100, C315S169400, C313S496000, C313S497000, C345S074100, C345S075200
Reexamination Certificate
active
06512335
ABSTRACT:
FIELD OF THE INVENTION
The present invention pertains to the field of flat panel display screens. More specifically, the present invention relates to the field of flat panel field emission display screens.
BACKGROUND OF THE INVENTION
Flat panel field emission displays (FEDs), like standard cathode ray tube (CRT) displays, generate light by impinging high energy electrons on a picture element (pixel) of a phosphor screen. The excited phosphor then converts the electron energy into visible light. However, unlike conventional CRT displays which use a single or in some cases three electron beams to scan across the phosphor screen in a raster pattern, FEDs use stationary electron beams for each color element of each pixel. This requires the distance from the electron source to the screen to be very small compared to the distance required for the scanning electron beams of the conventional CRTs. In addition, FEDs consume far less power than CRTs. These factors make FEDs ideal for portable electronic products such as laptop computers, pagers, cell phones, pocket-TVs, personal digital assistants, and portable electronic games.
One problem associated with the FEDs is that the FED vacuum tubes may contain minute amounts of contaminants which can become attached to the surfaces of the electron-emissive elements, faceplates, gate electrodes, focus electrodes, (including dielectric layer and metal layer) and spacer walls. These contaminants may be knocked off when bombarded by electrons of sufficient energy. Thus, when an FED is switched on or switched off, there is a high probability that these contaminants may form small zones of high pressure within the FED vacuum tube.
In addition, electron emission from the emitter electrodes to the gate electrodes can cause both emitter and gate degradation. For instance, the gate is positive with respect to the emitter causing an attraction of electrons from the emitter electrodes to the gate electrodes. In addition, the presence of the high pressure facilitates electron emission from emitters to gate electrodes. The result is that some electrons may strike the gate electrodes rather than the display screen. This situation can lead to gate electrode degradation including overheating of the gate electrodes. The emission to the gate electrodes can also affect the voltage differential between the emitters and the gate electrodes. Electron emission from the emitter electrodes to the gate electrodes can also cause ions and other material debris to be released from the gate and thereby become attached to the emitter electrode. This can cause emitter degradation.
It is worth noting that electrons may also hit spacer walls and focus electrodes, causing non-uniform emitter degradation. Problems occur when electrons hit any surface except the anode, as these other surfaces are likely to be contaminated and outgas because they are not scrubbed by the electron beam during normal tube operation. In addition, as the electrons jump the gap between the electron-emissive elements and the gate electrode, a luminous discharge of current may also be observed. Severe damage to the delicate electron-emitters may also result. Naturally, this phenomenon, generally known as “arcing,” is highly undesirable.
Conventionally, one method of avoiding the arcing problem is by manually electron scrubbing the FED vacuum tubes to remove contaminant material. However, it is difficult to remove all contaminants with that method. Further, the process of manual scrubbing is time-consuming and labor intensive, unnecessarily increasing the fabrication cost of FED screens.
Cathode burn-in is a technique to condition a FED screen.
FIG. 1
is a cross section structural view of part of a flat panel FED unit
75
that utilizes a gated field emitter
40
with spacer walls
17
and a focus waffle
10
. The electron emitter
40
is electrically coupled to a cathode structure
60
. During the cathode burn-in process, the emitter current is gradually raised up to its normal full bright level and the anode
20
is generally held to a low voltage, e.g., 750 volts, which is just high enough for electrons emitted from the emitter
40
to escape the focus wells
11
, but low enough to minimize the occurrences of damaging arcs. Cathode burn-in is a conditioning step and is typically performed after the tube has been sealed but not yet run.
The process of cathode burn-in performs at least two functions which cannot be performed well at high anode voltage conditions. The first is that cathode burn-in techniques forward bias the emitters
40
and allow inevitable “blow-outs” (e.g., shorts) to occur under low anode voltage conditions where they are much less likely to cause damaging faceplate-cathode arcs described above. The second function performed by cathode burn-in techniques is that they perform an important out-gassing function by cracking and pumping non-getterable gases with emitted electrons. For instance, cathode burn-in techniques ionize gases thereby making them more reactive and therefore more likely to be captured by the getter or other parts of the display. Further, by ionizing the gases, they are driven into the faceplate and are thereby pumped. Typically, the tubes begin cathode burn-in with a high pressure (10
−4
Torr) of methane gas which is mostly pumped away by the end of cathode burn-in.
A serious problem with prior art cathode burn-in techniques stems from emitted electrons hitting the spacer walls
17
(
FIG. 1
) thereby causing display non-uniformities near the spacer walls. During cathode burn-in, the emitters near the spacer walls experience a different concentration of gas molecules and ion sputtering than those located away from the spacer walls. This is a result of the high gas pressure in the tube and the low voltage applied to the anode
20
. This difference causes display non-uniformities in the tube after cathode burn-in. These unwanted display non-uniformities in the tube may persist or emerge later in the life of the tube.
The low anode voltage applied during cathode burn-in allows the electron beam
14
to spread out more than during normal high anode voltage operation so many electrons
12
a
,
12
b
hit the spacer walls
17
. When hit by low energy electrons (e.g., <1000 ev), the spacers walls emit secondaries and charge. The charged walls
17
deflect the electrons causing a non-uniform current density into the faceplate
70
near the spacer walls
17
. This causes non-uniform faceplate outgassing which may effect nearby emitters. The spacer walls
17
also may outgas under electron bombardment. The secondary electrons from the spacer walls themselves
17
ionize gas molecules which sputter the cathode adjacent to the spacer wall more than those that are located further away. Typically, these anomalies near the spacer walls
17
cause the rows around the spacer walls (e.g., for spacers running in the direction of the rows) to emit more electrons than those further from the spacer walls by the end of cathode burn-in conditioning thereby causing display non-uniformities.
SUMMARY OF THE DISCLOSURE
Accordingly, it would be advantageous to reduce or eliminate the display non-uniformities described above while maintaining the benefits of cathode burn-in conditioning. Embodiments of the present invention are directed as alternatives to cathode burn-in conditioning which prevent these non-uniformities by collecting the emitted electrons at the cathode so that very few or none of the emitted electrons hit the spacer walls of the faceplate. According to the present invention, the electrons can be collected either by the metal layers of the cathode or by the focus waffle. By preventing electrons from hitting the spacer walls during cathode burn-in conditioning, the present invention reduces or eliminates the display non-uniformities described above while maintaining the benefits of cathode burn-in. These and other advantages of the present invention not specifically described above will become clear within discussions of the present invention herein.
Methods for perfo
Cummings William J.
Dunphy James C.
Pan Lawrence S.
Stanners Colin D.
Candescent Technologies Corporation
Vo Tuyet T.
Wagner , Murabito & Hao LLP
Wong Don
LandOfFree
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