Flat display screen including resistive strips

An anode (5) for a flat display screen includes at least one group of phosphor strips (7) deposited over corresponding electrode strips (17) separated one from another by an insulating layer (8) etched out in front of the phosphor strips (7), and at least one conductor (21) interconnecting the electrode strips (17) of the group of phosphor strips (7). Each of the electrode strips (17) is formed by a resistive strip (18) for receiving one phosphor strip (7) and at least one biasing strip (19) which is parallel to and joins the interconnecting conductor (21). The biasing strip (19) has a low resistivity with respect to the resistivity of the associated resistive strip (18). The biasing strip (19) is parallel to, laterally borders, and is in contacting engagement with the resistive strip (18). The anode (5) eliminates the risk of electrical arcs between the anode (5) and gate (3) or between adjacent phosphor strips (7) of the anode (5), without impairing the brightness of the screen.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to anodes for flat display screens. It more 
particularly relates to the realization of connections of luminescent 
elements of an anode for color screens such as color screens including 
microtips. 
2. Discussion of the Related Art 
FIG. 1 represents the structure of a flat display screen with microtips of 
the type used according to the invention. 
Such microtip screens are mainly constituted by a cathode 1 including 
microtips 2 and by a gate 3 provided with holes 4 corresponding to the 
positions of the microtips 2. Cathode 1 is disposed so as to face a 
cathodoluminescent anode 5, formed on a glass substrate 6 that constitutes 
the screen surface. 
The operation and the detailed structure of an example of such a microtip 
screen are described in U.S. Pat. No. 4,940,916 assigned to Commissariat a 
l'Energie Atomique. 
The cathode 1 is disposed in columns and is constituted, onto a glass 
substrate 10, of cathode conductors arranged in meshes from a conductive 
layer. The microtips 2 are disposed onto a resistive layer 11 that is 
deposited onto the cathode conductors and are disposed inside meshes 
defined by the cathode conductors. FIG. 1 partially represents the inside 
of a mesh, without the cathode conductors. The cathode 1 is associated 
with the gate 3 which is arranged in rows. The intersection of a row of 
gate 3 with a column of cathode 1 defines a pixel. 
This device uses the electric field generated between the cathode 1 and 
gate 3 so that electrons are transferred from microtips 2 toward phosphor 
elements 7 of anode 5. In color screens, the anode 5 is provided with 
alternate phosphor strips 7r, 7b, 7g, each corresponding to a color (red, 
blue, green). The strips are separated one from the other by an insulating 
material 8. 
The phosphor elements 7 are deposited onto electrodes 9, which are 
constituted by corresponding strips of a transparent conductive layer such 
as indium and tin oxide (ITO). 
The groups of red, blue, green strips are alternatively biased with respect 
to cathode 1 so that the electrons extracted from the microtips 2 of one 
pixel of the cathode/gate are alternatively directed toward the facing 
phosphor elements 7 of each color. 
The control of the phosphor element 7 (the phosphor element 7g in FIG. 1) 
that should be bombarded by electrons from the microtips 2 of cathode 1 
requires to selectively control the biasing of the phosphor elements 7 of 
anode 5, for each color. 
FIG. 2 schematically illustrates an anode structure of a conventional color 
television screen. FIG. 2 partially represents a perspective view of an 
anode 5 fabricated according to known techniques. The anode electrode 
strips 9, deposited on substrate 6, are interconnected outside the useful 
area of the screen, for each color of phosphor elements, in order to be 
connected to a control device (not shown). Two interconnection paths 12 
and 13 of anode electrodes 9g and 9b, respectively, are achieved for two 
of the three colors of phosphor elements. An insulating layer 14 
(represented in dotted lines in FIG. 2) is deposited on the 
interconnection path 13. A third interconnection path 15 is connected, 
through conductors 16 deposited on the insulating layer 14, to the strips 
of anode electrodes 9r designed for the phosphor elements of the third 
color. 
Generally, the rows of gate 3 are sequentially biased at a voltage of 
approximately 80 volts whereas the phosphor strips (for example 7g in FIG. 
1) that must be excited are biased at a voltage of approximately 400 
volts, the other strips (for example 7r and 7b in FIG. 1) are at zero. The 
columns of cathode 1, whose potential determines for each row of gate 3 
the brightness of the pixel defined by the intersection of the cathode 
column and the gate row in the considered color, are brought to respective 
voltages ranging between a maximum emission potential and a zero-emission 
potential (for example, 0 and 30 volts respectively). 
The values of the biasing voltages are determined by the characteristics of 
the phosphor elements 7 and microtips 2. 
Conventionally, below a voltage difference of 50 volts between the cathode 
and the gate, no electron emission occurs, and the maximum emission used 
corresponds to a voltage difference of 80 volts. 
The voltage difference between the anode and the cathode depends on the 
inter-electrode gap. For increasing the brightness of the screen a maximum 
voltage difference is desired, which requires an inter-electrode gap as 
wide as possible. 
However, the structure of the inter-electrode gap, which includes spacers 
(not shown) that may generate shadow areas on the screen if they are 
over-sized, prevents this inter-electrode gap from being increased. 
Therefore, the inter-electrode gap of a conventional screen is 
approximately 0.2 mm. This makes it necessary to select an anode-cathode 
voltage which is critical as regards the formation of electric arcs. Thus 
destroying electric arcs can occur due to the slightest irregularity of 
the distance separating a microtip, or the gate layer, from the phosphor 
elements of the anode. Furthermore, such irregularities are unavoidable 
because of the small size of the components and the techniques used to 
fabricate the anode and the cathode-gate. 
On the side of the cathode, the resistive layer 11 limits the formation of 
destroying short-circuits between the microtips and the gate. 
However, on the anode side, electric arcs may occur between the gate 3 and 
the anode phosphor elements 7 which are biased so as to attract the 
electrons emitted by the microtips 2 (for example, the phosphors 7g in 
FIG. 1). Electric arcs can also occur between two adjacent phosphor strips 
(for example 7g and 7r in FIG. 1) due to the voltage difference between 
the two strips. 
SUMMARY OF THE INVENTION 
An object of the invention is to avoid the above drawbacks by providing an 
anode for a flat display screen which eliminates the risk for electric 
arcs to occur between the anode and the gate or between two adjacent 
phosphor strips of the anode, without impairing the brightness of the 
screen. 
To achieve this object, the present invention provides an anode for a flat 
display screen including at least a group of phosphor strips deposited 
over strips of corresponding electrodes separated one from the other by an 
insulating layer including holes facing the phosphor strips, and at least 
one conductor interconnecting the electrode strips of the group; each 
electrode strip being formed by a resistive strip for receiving one 
phosphor strip and at least one first biasing strip which is parallel 
thereto and joins this interconnection conductor, the biasing strip having 
a low resistivity with respect to the resistivity of the resistive strip 
associated therewith. 
According to an embodiment of the invention, each resistive strip is 
bordered by two parallel biasing strips, each biasing strip joining the 
interconnection conductor. 
According to an embodiment of the invention, the resistive strips are in a 
transparent and electrically conductive non-stoichiometric oxide, the 
resistivity of the resistive strips being determined by the oxygen ratio 
of the oxide. 
According to an embodiment of the invention, the resistive strips and the 
biasing strips are made of the same material whose resistivity is higher 
in a central portion designed to receive the phosphor strips than in 
lateral areas joining the interconnection conductor. 
According to embodiment of the invention, the insulating layer is used as a 
mask to increase the resistivity of the resistive strips through annealing 
in an oxygen atmosphere. 
According to an embodiment of the invention, the resistivity of the 
resistive strips is determined by the thickness of the strips. 
According to an embodiment of the invention, the insulating layer is used 
as an etching mask in a process for reducing the thickness of the 
resistive strips. 
According to an embodiment of the invention, the anode includes three 
groups of alternated resistive strips carrying phosphor elements, each 
corresponding to one color, and at least three interconnection conductors 
of the biasing strips associated with the resistive strips of the same 
color. 
According to an embodiment of the invention, all the resistive strips 
associated with the same interconnection path have the same resistivity. 
According to an embodiment of the present invention, the resistive strips 
are made of indium or tin oxide. 
The foregoing and other objects, features, aspects and advantages of the 
invention will become apparent from the following detailed description of 
the present invention when taken in conjunction with the accompanying 
drawings.

DETAILED DESCRIPTION 
FIG. 3 is a cross-sectional view of some phosphor strips of the anode of a 
flat display screen according to a first embodiment of the invention. 
A distinctive feature of the present invention is that the strips 17 of 
anode electrodes each includes a resistive strip 18 supporting phosphor 
elements 7 and at least one parallel biasing strip 19. Preferably, as 
represented in the figures, each resistive strip 18 is longitudinally 
bordered by two biasing strips 19. 
Thus, an anode according to the invention is formed, from a transparent 
substrate 6, for example made of glass, by parallel strips 18 made of an 
electrically conductive and transparent material, such as indium or tin 
oxide. Each strip 18 supports a corresponding phosphor strip 7. Each strip 
18 is bordered by two lateral highly conductive biasing strips 19, for 
example made of aluminum, copper or gold. For a color screen, these strips 
19 are connected at one of their ends to an interconnection path (not 
shown) of the phosphor strips 7 of the same color. 
A characteristic of the present invention is that the biasing strips 19 are 
achieved in such a manner that they have a low resistivity with respect to 
the resistivity of the material constituting the strips 18. Thus, the 
resistive strips 18 create a lateral access resistance toward each pixel 
of the screen. 
For this purpose, according to this first embodiment, the intrinsic 
properties of a transparent oxide layer are used. It can be, for example, 
a layer of indium oxide (In.sub.2 O.sub.x), tin oxide (SnO.sub.x) or 
indium and tin oxide (ITO). 
The thickness and oxygen ratio of the oxide layer are optimized to impart 
the desired resistance and transparency to each strip 18. 
Preferably, the oxide that is used is indium or tin oxide. The use of such 
an oxide is advantageous in that its resistivity is easily controllable to 
impart the desired resistance to the strip, because the resistivity of 
such a strip increases with the oxygen ratio. To increase the resistivity 
of indium or tin oxide, an annealing step in oxygen atmosphere is carried 
out at a temperature ranging from 300.degree. to 400.degree. C. 
A further advantage of an indium or tin oxide is that it has a better 
transparency than ITO. 
Preferably, as represented in FIG. 4, a transparent and electrically 
conductive oxide layer having a reduced thickness, is used to form the 
resistive strips 18'. 
FIGS. 5 and 6 illustrate two further embodiments of an anode according to 
the invention. According to these embodiments, all the resistive and 
biasing strips are made of a transparent and electrically conductive 
oxide. 
FIG. 5 is a cross-sectional view of some phosphor strips forming an anode 
of a flat display screen according to a third embodiment of the invention. 
The anode is formed of electrode strips 17' made of a transparent and 
electrically conductive oxide, whose central portion, having a high 
resistivity, acts as a resistive strip and is bordered by two lateral 
areas 19' having a minimum resistivity and acting as biasing strips. The 
difference in resistivity is obtained by an oxygen ratio that differs for 
the lateral areas 19' and the central area 18. For this purpose, strips 
17' are formed from an oxide layer, for example indium or tin, having a 
minimum resistivity. Then, the insulating layer 8, for example in silicon 
oxide, is deposited and etched out in front of the central areas 18 
designed to receive the phosphor strips 7. Layer 8 is then used as a mask 
to increase the resistivity of the central portions 18 by increasing their 
oxygen ratio, by annealing in an oven in an oxygen atmosphere at a 
temperature of approximately 400.degree. C. FIG. 6 is a cross-sectional 
view of some phosphor strips forming an anode of a flat display screen 
according to a fourth embodiment of the invention. 
In this embodiment, the anode is also formed by electrode strips 17' of 
transparent and electrically conductive oxide, whose central portion 18', 
having a high resistivity, acts as a resistive strip and is bordered by 
two lateral areas 19' having a minimum resistivity and acting as biasing 
strips. In contrast, in this case, the resistivity is identical for the 
central areas 18' and lateral areas 19' and preferably corresponds to a 
minimum resistivity. The high resistivity of the central areas 18' is 
obtained by imparting a small thickness to these areas. The insulating 
layer 8 is used as an etching mask for etching the central areas 18'. 
To improve the protection of the phosphor elements nearest to the biasing 
strips, it is possible, according to a fifth embodiment of the invention 
represented in FIG. 7, to provide for the insulating layer 8 to overlap 
the resistive strips. Thus, an intermediate resistive area 18" devoid of 
phosphor elements and protected by layer 8 is created between the biasing 
strips and the central areas 18'. Such an overlapping is, for example, 
achieved by positioning the mask used to define the resistive strips in 
relation with the mask used to etch layer 8. 
In FIG. 7, the biasing strips are metal strips, for example made of 
aluminum. Lateral areas 19' of oxide strips can also be used as biasing 
strips as for the embodiments represented in FIGS. 5 and 6. 
Of course, all the above described embodiments can be combined in a single 
electrode strip. 
Thus, for example, strips of transparent and electrically conductive oxide, 
which have a high resistivity in a central areas bordered by biasing 
strips, for example of aluminum, can be provided. These biasing strips are 
deposited on oxide lateral areas. The insulating layer, which covers the 
biasing strips and the lateral areas of conductive and transparent oxide, 
is still used as an etching mask and/or to increase the oxygen ratio. 
The electrical interconnection of the electrode strips 17, or 17', is 
illustrated in FIG. 8 which represents the electric equivalent diagram of 
a microtip color screen with an anode according to the invention. This 
electrical interconnection is similar to that disclosed with relation with 
FIG. 2, except that the interconnection paths 21 connect the biasing 
strips 19, or 19', and no longer directly the strips 18, or 18', which 
receive the phosphor elements 7. Thus, the addressing of an anode 
according to the invention can be conventionally achieved. 
During biasing of a predetermined gate row, each phosphor strip 7r, 7g or 
7b is individually protected against electric arcs by a resistance Ra in 
series between this strip and the interconnection path 21 with which it is 
associated. The value of resistance Ra formed by the resistive layer 18, 
or 18', is such that it limits the current in the electrode strip 17 or 
17' to a value selected to prevent destroying electric arcs from 
occurring, without causing an important drop of the anode voltage. 
Resistance Ra corresponds in fact to the lateral resistances formed by the 
resistive strips 18, or 18', between the phosphor elements 7 and the 
biasing strips 19, or 19'. 
FIG. 8 represents the microtips of cathode 1 in the form of one microtip 2 
for each pixel whereas, in practice there are several thousand microtips 
per screen pixel. Thus, a resistance Rk, which corresponds to the 
resistive layer 11 between the cathode conductors and the microtips, is 
formed. The resistance Rk homogenizes the electron emission of the 
microtips 2 and prevents electric short-circuits from occurring between 
the gate 3 and microtips 2. The resistance Ra formed by each resistive 
strip 18, or 18', is electrically connected in series to this resistance 
Rk for each pixel. 
It should understood that resistance Ra can be selected significantly 
higher than resistance Rk for a pixel without causing an important voltage 
drop in the resistive strips, because the biasing voltage (approximately 
400 volts) of the anode strips is generally higher than the difference in 
the gate-cathode potential on which resistance Rk intervenes. The value of 
resistance Rk is generally approximately 500 k.OMEGA. for a biasing 
voltage of the gate rows of approximately 80 volts and a biasing voltage 
Vk of the cathode columns ranging from 0 to 30 volts. 
By way of a specific example, for a typical current consumption of 10 .mu.A 
per pixel and for a 400-volt biasing voltage Va of strips 19, or 19', 
strips 18, or 18', having a resistivity of approximately 200 .OMEGA..cm 
can be used. Such strips that are formed with a thickness of approximately 
50 nm have a layer resistivity of approximately 40 .OMEGA. per square. For 
a pixel having a 300-.mu.m side, this value forms a global resistance Ra 
of approximately 2 M.OMEGA.. This enables to limit the voltage drop in the 
resistive strip to approximately 20 volts. Such a resistivity value 
prevents destroying electric arcs from occurring by limiting the current 
in each strip 19, or 19', to approximately 200 .mu.A, while maintaining 
the brightness of the screen. 
It will be understood that the addition of the resistances Ra does not 
impair the switching speed of the anode rows since the resistance of the 
biasing strips remains low (a few k.OMEGA.), the product of their 
resistance by the capacitance of the anode rows (a few nF) corresponds to 
a time constant much lower than the switching time of the anode (a few 
milliseconds). 
The current limitation, individually for each anode electrode strip, 
further prevents electric arcs from occurring between two adjacent strips 
which are at different potentials. 
A further advantage of the present invention is that resistance Ra is the 
same for all the pixels of the screen. Indeed, for a determined pixel, 
this resistance is independent of the distance separating this pixel from 
the interconnection path 21, provided that the resistivity of the biasing 
strips 19, or 19' is low. 
As is apparent to those skilled in the art, various modifications can be 
made to the above disclosed preferred embodiments. More particularly, each 
constituent described for the layers constituting the anode can be 
replaced with one or more constituting elements providing the same 
function. 
Furthermore, although the description refers to a color screen, the 
invention also applies to a mono-color screen having an anode including 
parallel phosphor strips. The invention also applies to a multicolor 
screen in which ranges, or sectors, covering several pixels are assigned 
to one color. The invention further applies to a color screen in which the 
anode strips are not switched but continuously biased. In this case, a 
single interconnection path is necessary; however, on the anode side, the 
pixels are partitioned into sub-pixels, each sub-pixel being assigned to 
one color and being disposed so as to face the corresponding anode strip.