Electric lamp and discharge devices – With luminescent solid or liquid material – Vacuum-type tube
Utility Patent
1998-05-06
2001-01-02
Patel, Ashok (Department: 2879)
Electric lamp and discharge devices
With luminescent solid or liquid material
Vacuum-type tube
C313S496000, C313S497000
Utility Patent
active
06169358
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to insulative spacers provided between parallel plates between which there is an electric potential. The insulative spacers of the invention may reduce the likelihood of surface electron flashover between the parallel plates.
BACKGROUND OF THE INVENTION
Electron beam emitting arrays are known. Presently, such arrays are being provided in the form of microminiature field emitters, which are known in the microelectronics art. These microminiature field emitters are finding widespread use as electron sources in microelectronic devices. For example, field emitters may be used as electron sources in flat panel displays for use in aviation, automobiles, workstations, laptops, head wearable displays, heads up displays, outdoor signage, or practically any application for a screen which conveys information through light emission. Field emitters, as well as other types of electron beam arrays, may also be used in non-display applications such as power supplies, printers, and X-ray sensors.
Referring to
FIG. 1
, the cross-section of a parallel plate type electron beam emission device
10
is shown. The device includes a lower plate
100
, a spacer structure
200
, and an upper plate
300
. The lower plate
100
may comprise a substrate
110
and a conductive element
120
. The lower plate
100
may include additional elements in the interior of the device
10
, which are useful for emitting electrons in the direction of the upper plate
300
. The upper plate
300
may comprise a substrate
310
and a conductive element
320
. The upper and lower plates may be connected along their respective outer edge regions with the spacer structure
200
. The spacer structure
200
may itself comprise an insulator frame or ring
210
bonded to the upper and lower plates with an upper glass frit
220
and a lower glass frit
230
, respectively.
In order to achieve a beam of electrons, from the lower plate
100
to the upper plate
300
, of a predetermined velocity, the upper conductive element
320
may be maintained at a high positive voltage relative to the source of electrons located on the lower plate
100
. Thus the upper conductive element
320
may also be referred to as an anode. If the device
10
is a display, the anode
320
may be replaced by a thin transparent conductive layer. A layer of phosphor (not shown) may be provided on the interior region of the plate
300
over the anode
320
. Electrons attracted to the anode
320
strike the phosphors, causing them to luminesce, and light emitted through the top side
312
of the support
310
may be viewed as part of an image, text, etc.
In order to operate the device
10
, the space between the lower plate
100
and the upper plate
300
should be evacuated. Typically, this space may be of the order of 0.5 to 5 millimeters. To maintain the vacuum between the upper and lower plates, they are sealed to one another along their respective edges by the spacer structure
200
. After being sealed, the space between the two plates,
100
and
300
, may be evacuated of air or gas and sealed off from the outside atmosphere.
Because the materials within the device
10
(such as phosphors) are very likely to outgas over time, a getter (not shown) may be provided within the evacuated space or in communication therewith. The getter is a substance which may absorb gas molecules that come in contact with it as a result of outgassing from materials within the device.
It is imperative to the operation of the device to capture as many of the outgassed gas molecules as possible. The reason being that these gas molecules may become ionized as a result of being bombarded by the electrons in the device. If the gas pressure is high enough, there will be a growth in the ionization leading to a gas-discharge (breakdown flashover) between the anode
320
and the elements of the lower plate
100
. In devices in which the potential between the anode
320
and the lower plate
100
is in the range of thousands of volts, such flashover may be catastrophic to the device
10
. Even if the flashover is not initially catastrophic, flashover may result in vaporization of materials within the device, resulting in the production of additional gas molecules therein, and sowing the seeds for a future flashover. The flashover problem is particularly noticeable during the bum-in of new displays. Burn-in is carried out by operating a display at anode voltages well above those that would be experienced by the display during normal operation. It is at this time that displays are particularly susceptible to flashover.
Any residual outgassing as a result of electron bombardment of phosphor, and the gates to a smaller extent, would result in the gas molecules flowing towards the sidewalls (spacer and frit) and the getter material. The occurrence of gas flow towards the getter is referred to as getter pumping. Because the absorption of gas molecules on the sidewalls is slow, the density of gas molecules close to the wall tends to be high on a short time scale. If the product (p)(d) of the local gas pressure (p) in the vicinity of the walls and the distance (d) between the anode and the gate is sufficient for a pachen breakdown, then a cumulative ionization leading to a gas discharge (flashover) will occur between the anode and the gate. The flashover between the anode and the gate can trigger a flashover between that gate and corresponding emitters. For this reason most flashovers take place close to the sidewalls in a field emission display.
Prior to the present invention, adequate flashover control has been virtually nonexistent. The primary method of combating flashover has been to reduce the operating potential between the anode
320
and the elements of the lower plate
100
. By decreasing the potential to levels of only a few hundred volts, the occurrence of flashover may be reduced, although it is far from eliminated. This reduction in the potential, however, has serious repercussions on the longevity of the display. In a display, most phosphor degradation occurs in proportion to the total number of electrons having struck the phosphor, and not in proportion to the total power or light output of the phosphor. High anode voltages, in excess of 5,000 volts, permit aluminized phosphor films to be used, which increase efficiency and reduce the rate of fixed patterns being burned into the phosphor screen.
Ise, U.S. Pat. No. 5,448,133 (issued Sep. 5, 1995) for a Flat Panel Field Emission Display Device with a Reflective Layer, touts the advantages of reducing the potential between the anode and cathode in a Field Emitter Display (FED). Ise states that a reduction of the operating voltage of a FED will reduce power consumption, which reduces battery size, and enables portability. Ise states that presently the low end threshold for anode to cathode potential is about 400 volts. Ise reports operation of his FED at as low as 100 volts of cathode to anode potential.
Reduction of the lower plate to anode potential, as suggested by Ise, may reduce FED lifespan. Lifespan may be reduced because the luminous efficiency of the FED phosphors depends on the coulomb charge per unit volume applied to the phosphors over a period of time. The application of charge to the phosphors seems to dislocate activators from their sites in the phosphor host lattice, and thus decreases the activator excitation efficiency (by increasing the vacancy density). A phosphor layer of certain thickness, if operated by high voltage and low current, tends to have low values of coulomb per unit volume due to the increased penetration depth of the charge delivering electrons. On the other hand, if the same layer is operated with low voltage and high current (maintaining the same power) the coulomb per unit volume increases due to the decreased penetration of the electrons (charge concentration at the surface of the layer). Increased coulomb per unit volume resulting from low voltage operation is more detrimental to the activators than high voltage operation over a given tim
Jones Gary W.
Zimmerman Steven M.
Collier Shannon Scott PLLC
eMagin Corporation
Guharay Karabi
Patel Ashok
Yohannan David R.
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