Electric lamp and discharge devices: systems – Plural power supplies – Plural cathode and/or anode load device
Reexamination Certificate
2001-05-16
2004-03-30
Wong, Don (Department: 2821)
Electric lamp and discharge devices: systems
Plural power supplies
Plural cathode and/or anode load device
C313S309000, C313S499000
Reexamination Certificate
active
06713970
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to the field of flat display screens, and more specifically to so-called cathodoluminescent screens, an anode of which supports phosphor elements likely to be excited by electron bombarding. The present invention more specifically applies to screens of field-effect type, in which the electron bombarding canes from microtips supported by a screen cathode.
2. Discussion of the Related Art
FIG. 1
shows an example of a conventional structure of a flat color microtip screen of the type to which the present invention relates. Such a screen is essentially formed of a cathode
1
with microtips
2
and of a grid
3
provided with holes
4
corresponding to the locations of the microtips. Cathode
1
is placed opposite to a cathodoluminescent anode
5
, a substrate
6
of which, for example, made of glass, generally forms the screen surface.
The operating principle and a specific embodiment of a microtip screen are described, for example, in U.S. Pat. No. 4,940,916 of the Commissariat à
1
′ Energie Atomique.
Cathode
1
is generally arranged in columns and is formed, on a substrate
10
, for example, made of glass, of cathode conductors arranged in meshes from a conductive layer. Microtips
2
are generally made on a resistive layer
11
deposited on the cathode conductors and are arranged within the meshes defined by the cathode conductors.
FIG. 1
partially shows the inside of a mesh, without showing the cathode conductors. Cathode
1
is associated with grid
3
, comprising row conductors. Gate
3
is deposited on the cathode plate with an interposed insulating layer
12
. The intersection of a grid row and of a cathode column generally defines a pixel.
This device uses the electric field created between cathode
1
and grid
3
to extract electrons from microtips
2
. The electrons are then attracted by phosphor elements
7
of anode
5
, if said elements are properly biased. In the case of a color screen such as illustrated in
FIG. 1
, anode
5
is, for example, provided with alternate strips of phosphor elements
7
r,
7
g,
7
b
, corresponding to each of the colors (Red, Green, Blue). The strips may be separated from one another by an insulator
8
. The phosphor elements are deposited on electrodes
9
, for example, formed of corresponding strips of a conductive layer (transparent if the anode forms the screen surface), for example, indium and tin oxide (ITO). The sets of red, green, blue strips are for example alternately biased with respect to cathode
1
, so that the electrons extracted from the microtips
2
of a pixel of the cathode/grid are alternately directed to the phosphor elements
7
facing each of the colors.
In the case, not shown, of a monochrome screen, the anode supports phosphor elements of same color arranged in a single plane or in two sets of separately-biased alternate strips.
Other cathode-grid and anode structures than those described hereabove may be encountered. For example, the phosphor elements of the anode may be distributed in elementary patterns corresponding to the sizes of the screen pixels. The anode may further, while being formed of several sets of strips or of elementary patterns of phosphor elements, not be switched by sets of strips or patterns. All the strips or patterns then are at a same voltage, for example, by being supported by a conductive plane. The anode is then said to be “unswitched”, as opposed to switched anodes where the colors are alternately biased.
The anode strips or patterns supporting phosphor elements to be excited are biased under a voltage of several hundreds, or even a few thousands, of volts with respect to the cathode. In the case of a switched anode screen having several sets of strips, the other strips are at a zero voltage. The choice of the values of the biasing voltages is linked to the characteristics of the phosphor elements and of the emissive means on the cathode side.
For an electron emission by the cathode microtips, said cathode must be submitted, with respect to grid
3
, to a sufficient potential difference. Conventionally, under a potential difference on the order of 50 V between the cathode and the grid, there is no electron emission, and the maximum emission used corresponds to a potential difference on the order of 80 V. For example, the rows of grid
3
are sequentially biased to a voltage on the order of 80 V while the columns of cathode
1
are brought to respective voltages ranging between a maximum emission voltage and a no emission voltage (for example, respectively 0 and approximately 40 V). The brightness of all pixels in a row is thus determined (per color component if the anode includes several sets of strips selectively biased color per color).
FIG. 2
shows the equivalent electric diagram of a conventional pixel of a color microtip screen. It is arbitrarily assumed to be a pixel, but it should be noted that this same equivalent electric diagram corresponds to that of each emissive microtip. However, since the microtips are several thousands per screen pixel, the present description is simplified by referring to a pixel (or to a sub-pixel in the case where the grid rows are divided up per color).
The pixel microtips electrically form a current source
20
, a first terminal
21
of which is connected, via a resistor
22
symbolizing the resistive layer (
11
, FIG.
1
), to a terminal
23
of application of cathode voltage V
1
. The other terminal
24
of current source
20
corresponds to the tips of microtips
2
directed towards the anode symbolized by a plate
25
to which is applied a biasing voltage V
5
. The insulator (
12
,
FIG. 1
) between the grid and the cathode can be modeled by a capacitor
26
connecting terminal
21
of current source
20
to a grid row
28
, and thus to a terminal
27
of application of a biasing voltage V
3
of the grid row. Due to the holes (
4
,
FIG. 1
) made in the grid, grid row
28
is connected directly connected to the tip (current source
20
).
FIGS. 3A and 3B
schematically illustrate the meshing of the cathode conductors of a conventional microtip screen.
FIG. 3A
partially shows in top view a cathode plate of a flat screen, that is, a microtip cathode associated with a grid, and
FIG. 3B
is a cross-section view along line B-B′ of FIG.
3
A. For clarity, the limits between the different layers have been shown in top view, in a shifted way in
FIG. 3A
, to be made visible. It should however be noted that, except for the microtips, the edges of the different layers can be considered as being substantially vertical, their inclination being essentially due to the used deposition and etch techniques, the manufacturing of microtip screens using techniques currently used in integrated circuit manufacturing.
Several microtips
2
, for example, sixteen, are arranged in each mesh
31
defined by cathode conductors
32
. Although a reduced number of meshes has been shown for each pixel
33
defined by the intersection of a column
34
of cathode
1
with a line
35
of grid
3
, it should be noted that the microtips are generally several thousands per screen pixel.
Cathode
1
is generally formed of layers successively deposited on substrate
10
. A conductive layer, for example, formed of niobium, is deposited on substrate
10
. This layer is etched according to the pattern of columns
34
, each column comprising meshes
31
surrounded with cathode conductors
32
. A resistive layer
11
is then deposited on cathode conductors
32
. Resistive layer
11
, formed, for example, of phosphorus-doped amorphous silicon, has the purpose of protecting each microtip
2
against an excess current upon its starting. Such a resistive layer
11
aims at homogenizing the electron emission of the microtips
2
of a pixel of cathode
1
and thus at increasing its lifetime. The resistive layer may be etched according to the column pattern and/or at least partially opened above the cathode conductors. An insulating layer
12
, for example, made of silicon oxide (SiO
2
)
Pixtech S.A.
Tran Thuy Vinh
Wong Don
LandOfFree
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