Electric lamp and discharge devices – With luminescent solid or liquid material – Vacuum-type tube
Reexamination Certificate
2000-02-23
2004-11-09
Cariaso, Alan (Department: 2875)
Electric lamp and discharge devices
With luminescent solid or liquid material
Vacuum-type tube
C313S497000, C313S336000
Reexamination Certificate
active
06815885
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a flat display screen anode having phosphors excited by electrons, for example, of microtip type. It more specifically relates to the biasing of phosphor elements of an anode provided with phosphor elements of different colors biased per color, for example, alternate strips of phosphor elements organized in combs.
2. Discussion of the Related Art
FIG. 1
very schematically shows the structure of a flat microtip screen of the type to which the present invention relates. This screen is comprised of two plates. A first plate
1
, currently called the cathode plate, is arranged to face a second plate
2
, currently called the anode plate. The two plates are spaced apart from each other by spacers
3
regularly distributed in the screen surface, and a vacuum is created in the area defined by the two plates and a peripheral sealing joint
4
.
Cathode plate
1
includes electron generation elements and pixel selection elements (not shown) that may be organized in different ways, for example, as described in U.S. Pat. No. 4,940,916 of the Commissariat à l'Energie Atomique in the case of microtip screens. Anode plate
2
is, in the case of a color screen, provided with alternate strips of phosphor elements, each strip corresponding to a color (red, green, blue).
FIGS. 2A and 2B
very schematically show a front view and a cross-section view of a portion of an anode plate. In
FIG. 2B
, the surface corresponding to the internal screen surface faces up. The anode includes, for example, alternate strips
4
R,
4
G,
4
B of respectively red, green, blue phosphor elements. As illustrated in
FIG. 2B
, the strips of phosphor elements are arranged on corresponding conductive strips
5
R,
5
G,
5
B generally organized in combs, all strips
5
R being interconnected, as well as all strips
5
G and all strips
5
B. In certain cases, the phosphor elements are distributed in elementary patterns, each of which generally corresponds to a pixel (in fact, a sub-pixel of each color for a trichromatic screen). The “pixelized” phosphor elements can then still be addressed by biasing electrodes in conductive strips (
5
G,
5
B and
5
R) such as described in relation with
FIGS. 2A and 2B
, but a specific mask is used to deposit the phosphor elements.
Two great categories of flat screens can be distinguished according to whether the user looks at the screen from the anode side or from the cathode side. In the first case, the light emitted by the phosphor elements propagates through the anode plate (downwards in FIG.
2
B). The material of conductive strips
5
R,
5
G,
5
B then is transparent, currently indium and tin oxide (ITO). In the second case, transparent electrodes
5
R,
5
B,
5
G are replaced with opaque and preferably reflective electrodes, so that the largest possible part of the light emitted by phosphor elements
4
R,
4
G,
4
B is sent back to the cathode once the phosphors have been excited by an electron bombardment. Electron generating plate
1
then is at least partially transparent and the observation is performed through this cathode plate.
In a color screen (or in a monochrome screen formed of two alternate sets of strips of phosphor elements of same color), the sets of strips (for example, blue, red, green) are often alternately positively biased with respect to cathode
1
, so that the electrons extracted from the emissive elements (for example, the microtips) of a pixel of the cathode are alternately directed towards phosphor elements
4
R,
4
G,
4
B facing each of the colors.
The selection control of the phosphor that is to be bombarded by the electrons imposes controlling, respectively, the biasing of the phosphor elements of the anode, color by color. Generally, the strips
5
R,
5
G,
5
B supporting phosphor elements to be excited are biased under a voltage of several hundreds of volts with respect to the cathode, the other strips being at a zero potential. The choice of the values of the biasing potentials is linked to the characteristics of the phosphor elements and of the emissive means.
In some cases, the anode may, while being formed of several sets of phosphor elements or the like, not be switched by sets of strips. All strips are then biased to a same potential, at least for the duration of a display frame. The anode is then said to be unswitched.
The potential difference between the anode and the cathode is essentially due to the inter-electrode distance, that is, to the thickness of the internal space. A maximum potential difference is desired for reasons of display brightness, which results in searching the greatest possible inter-electrode distance. However, the structure of the inter-electrode space, that includes spacers
3
likely to create dark areas in the screen if their size is too large, prevents from increasing this inter-electrode distance.
The necessary trade off leads to choosing an anode-cathode voltage value that is critical from the point of view of electric arc formation. Destructive electric arcs can then be caused by the smallest dimensional irregularity of the distance separating an emissive means of the cathode from the phosphor elements of the anode. Such irregularities are, moreover, inevitable given the small dimensions and the techniques used to form the anode and the cathode.
On the cathode side, a resistive layer is provided in the case of microtip screens to receive the microtips and thus limit the formation of destructive short-circuits between the microtips and a control grid associated with the cathode.
Conversely, on the anode side, arcs may occur not only between the cathode plate and those of the anode phosphor elements that are biased to attract electrons emitted by the microtips, but also between two neighboring strips of phosphor elements, due to the potential difference between the two strips. In the case of a monochrome screen where the anode is formed of a conductive plane supporting phosphor elements of same color or in the case of an anode (color or monochrome) with several unswitched strips, the risk of arcs only exists between the anode and the cathode.
To limit the occurrence of such lateral arcs, it is currently provided to arrange, between anode strips
5
B,
5
R,
5
G, interstitial strips
7
made of an insulating material (generally silicon oxide).
However, in practice, the efficiency of such insulating strips is limited for several reasons.
First, these strips are inoperative with respect to the forming of electric arcs between the anode and the cathode.
Further, and although this does not necessarily appear in
FIGS. 2A and 2B
in which the scales have not been respected, phosphor elements
4
R,
4
G,
4
B significantly extend beyond the interstitial strips. Indeed, the thickness of the strips of phosphor elements is generally on the order of some ten &mgr;m and the forming of silicon oxide insulating strips of such a thickness is, in practice, incompatible with the technologies used for manufacturing the anodes, so that the thickness of strips
7
generally is on the order of 1 to 2 &mgr;m, their width being on the order of 10 to 20 &mgr;m.
Further, during the deposition of the phosphor elements through a deposition mask, a slight misalignment of the mask may occur, so that a portion of conductive strips
5
R,
5
G,
5
B or of the insulated areas becomes accessible once the screen is completed and thus favors the forming of arcs.
A first known solution to attempt to reduce the forming of arcs between the anode and the cathode is to provide, at the end of each conductive strip
5
R,
5
G,
5
B, a resistor between the power supply line and the strip. As soon as a strong current appears in the strip, the resistor causes a voltage drop. As a result, the potential difference between the conductive strip and the cathode decreases and has the overvoltage generating the arc disappear.
A disadvantage of such a solution is that it does not protect from the forming of a lateral electric arc, that is, an arc between two neighboring strip
Bancal Bernard
Olivier Pascal
Cariaso Alan
Duane Morris LLP
Lee Guiyoung
Pixtech S.A.
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