High voltage spacer for a flat panel display with specific cross section

The invention is a spacer method and apparatus for a high-voltage emissive vacuum flat panel display device. The display device has front and rear panels held in place by a supporting frame around the periphery of the panels. The back panel has an array of electron sources that accelerate electrons towards the front panel to bombard and excite a light-emissive material deposited thereon, thereby modulating the screen and displaying desired information patterns. A spacer according to the present invention can be described generally as having a body and N arms extending radially from the body, wherein N is at least three. The body and the N arms physically contact the front and back panels of the display to thereby separate the front panel from the back panel. The arms need not have the same length of extension from the body. Furthermore, the arms may or may not taper as they extend from the body. Such a spacer has a high compressive stress resistance, does not buckle easily, is easy to make invisible, is easy to fabricate and to assemble into the display device.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to flat panel display devices and, more 
particularly, to spacers for a high voltage flat panel display. 
2. Description of the Related Art 
An emissive vacuum flat-panel display typically has front and rear panels 
held in place by a supporting frame around the periphery of the panels. 
Examples of flat panel displays can be found in European Patent 
Application number 94301384.7, entitled "A Flat Panel Display", published 
Sep. 7, 1994, and in European Patent Application number 94306859.3, 
entitled "Focusing and Steering Electrodes for Electron Sources", 
published Mar. 29, 1995, both having the same inventor, Huei Pei Kuo. 
A thin layer of light-emissive material such as phosphor is typically 
deposited on the front panel of the display. The phosphor emits visible 
light when bombarded by energetic electrons. The back panel of the 
display, in turn, has an array of electron sources. The electron sources 
accelerate electrons towards the front panel to bombard and excite the 
phosphor material on the front panel. When appropriate signals are applied 
to the electron-source array and the phosphor screen, light emitted from 
the phosphor screen is modulated to display desired information patterns. 
If an array of field emission electron source is used as the electron 
sources, such a display is commonly known as a Field Emission Display 
("FED"). 
The front and back panels of a vacuum flat-panel display are typically made 
of transparent glass of a thickness of approximately 1 mm. The supporting 
frame around the periphery is used to maintain a distance of a few 
millimeters between the panels. The panels and the frame form a vacuum 
envelope which is evacuated to a vacuum better than approximately 
10.sup.-7 Torr. This causes the panels to be subjected to an atmospheric 
pressure of approximately 1 Kg/cm.sup.2. For a display with a screen 
dimension greater than approximately 1 cm by 1 cm, additional support 
structures, or spacers are required to prevent the panels from being 
distorted or destroyed under the atmospheric pressure. 
Basically, three types of spacer designs are used by the prior art: 
spheres, cylinders and walls. Although each design has one or more 
advantage, inherent to each design is one or more undesirable 
disadvantage. 
Spherical spacers are ideally shaped to withstand compressive stress; are 
easy to manufacture and displays using spherical spacers are easy to 
assemble. However, because the size of sphere equals the spacing between 
the panels, the sphere becomes visible when the panel spacing is greater 
than 200-300 .mu.m. 
Cylindrical post spacers have an excellent compressive strength. They also 
have a small lateral dimension, therefore the posts remain invisible for 
larger spacing between the panels than is possible in the case of 
spherical spacers. Cylindrical posts have poor buckling strength, however, 
and displays using cylindrical posts can be difficult to assemble. 
Finally, wall spacers have an excellent stiffness in the direction along 
wall and are easily fabricated. On the other hand, wall spacers have a 
poor buckling strength in the direction perpendicular to the wall; 
displays using wall spacers are difficult to assemble; and the wall 
spacers are difficult to make invisible. 
In a field emission display where high-voltage phosphor is used, the 
minimum spacing between the front and rear panel is approximately 1 mm for 
a 5 Kilovolt operation. The spherical spacer is too large and not suitable 
for this application. The cylindrical and wall spacers can be used, but 
suffer from the shortcomings listed above. 
Thus, it can be seen that the poor strength, visibility and assembly 
difficulty limits of current technology spacers for flat-panel display 
devices limits the use of these devices in many applications. 
Therefore, there is an unresolved need for a spacer for a high-voltage flat 
panel display that will significantly improve the strength, invisibility 
and assembly limits of current technology spacers. 
SUMMARY OF THE INVENTION 
The invention is a spacer method and apparatus for a high-voltage emissive 
vacuum flat panel display device. The display device has front and rear 
panels held in place by a supporting frame around the periphery of the 
panels. The back panel has an array of electron sources that accelerate 
electrons towards the front panel to bombard and excite a light-emissive 
material deposited thereon, thereby modulating the screen and displaying 
desired information patterns. 
A spacer according to the present invention can be described generally as 
having a body and N arms extending radially from the body, wherein N is at 
least three. The body and the N arms physically contact front and back 
panels of a flat-panel display to thereby separate the front panel from 
the back panel. The arms need not have the same length of extension from 
the body. Furthermore, the arms may or may not taper as they extend from 
the body. 
Such a spacer has a high compressive stress resistance, does not buckle 
easily, is easy to make invisible, is easy to fabricate and to assemble 
into the display device.

DETAILED DESCRIPTION OF THE INVENTION 
Embodiments of the invention are discussed below with reference to FIGS. 
1-7. Those skilled in the art will readily appreciate that the detailed 
description given herein with respect to these figures is for explanatory 
purposes, however, because the invention extends beyond these limited 
embodiments. 
FIG. 1 is a cut-away view of a flat-panel display device employing a spacer 
constructed according to the present invention. FIG. 1 shows an emissive 
vacuum flat-panel display 100 having respective front and rear panels 110 
and 120 held in place by a supporting frame 130 around the periphery of 
the panels. A thin layer of light-emissive material 140 such as phosphor 
is deposited on the front panel 110 of the display 100. The phosphor emits 
visible light when bombarded by energetic electrons. The back panel 120 of 
the display, in turn, has an array of electron sources 150. The electron 
sources accelerate electrons towards the front panel 110 to bombard and 
excite the phosphor material 140 on the front panel. When appropriate 
signals are applied to the electron-source array 110 and the phosphor 
screen 140, light emitted from the phosphor screen 140 is modulated to 
display desired information patterns. 
The panels 110 and 120 of vacuum flat-panel display 100 are typically made 
of transparent glass of a thickness of approximately 1 mm. The supporting 
frame 130 around the periphery is used to maintain a distance of a few 
millimeters between the panels. The panels and the frame form a vacuum 
envelope which is evacuated to a vacuum better than approximately 
10.sup.-7 Torr. This causes the panels 110 and 120 to be subjected to an 
atmospheric pressure of approximately 1 Kg/cm.sup.2. For a display 100 
with a screen dimension greater than approximately 1 cm by 1 cm, 
additional support structures, such as spacer 160, are required to prevent 
the panels 110 and 120 from being distorted or destroyed under the 
atmospheric pressure. 
In a field emission display 100 where high-voltage phosphor is used, the 
minimum spacing between the front panel 110 and the rear panel 120 is 
approximately 1 mm for a 5 Kilovolt operation. Thus, in FIG. 1 a side view 
of a cross-shaped spacer 160 embodiment constructed according to the 
present invention is shown. For one embodiment the height H of the spacer 
160 is 1 mm. Because spacer 160 physically contacts and separates panels 
110 and 120, it follows that the height of spacer 160 is the same as the 
desired separation between front panel 110 and rear panel 120. 
Spacer 160 can be made by drawing glass as is well known in the art of 
glass fiber manufacture. For one embodiment, spacer 160 is treated to make 
the spacer slightly conductive. Typically, as is known in the art, either 
a thin coating of conductive material is deposited on a spacer, or a 
conductive "doping" agent is included in the glass of the spacer. This 
slight conductivity permits electrons from electron source 150 to drain 
from the spacer 160. For an embodiment wherein spacer 160 is not made 
slightly conductive, electrons can accumulate on the spacer 160. These 
accumulated electrons can create charge that deflects the beam to distort 
the image being displayed on the screen. In the worst case, an electron 
accumulation can create a catastrophic discharge. 
FIG. 2 is a top view of the cross-shaped spacer 160 represented in FIG. 1 
wherein four arms 170 extend radially from a central body 180 to form a 
cross pin. For one cross pin embodiment, spacer 160 is a glass column 
wherein the end-to-end length L of the arms 170 is 0.3 mm and the width W 
of the arms 170 is 0.07 mm. For this embodiment, the height of the spacer 
160 is 1 mm. 
Cross pin spacers 160 have been fabricated according to this embodiment and 
the compressive strength of the spacer 160 has been tested. A typical 
experimental result is shown in FIG. 3. Thus, FIG. 3 is a column stress 
strain curve for an embodiment of the spacer 160 illustrated in FIGS. 1 
and 2. As can be seen in FIG. 3, it took more than 3.6 Kg, approximately 
9850 Kg/cm.sup.2, before the spacer 160 failed. The cross pins 160 failed, 
not from excessive compressive force, but by failure from the testing 
equipment. In the experiment the cross pin 160 under test was held by a 
pair of jaws, one on the top and one on the bottom of the spacer 160. In 
such a test, if the surfaces of the jaws are not perfectly parallel to 
each other, or if the cross pin 160 under test is not positioned 
perpendicular to the surface of each of the jaws, there is a tendency to 
eject the cross pin 160 from between the jaws when the applied pressure 
increases. 
Thus, the tested results indicate a lower limit on the compressive strength 
of the cross pin 160. If the tested value is taken as the strength of the 
cross pin 160, one spacer approximately every 1.27 cm is sufficient to 
support the panels 110. Therefore, the spacing between the spacers 160 
will be determined by the strength of the panel 110 and 120 glass rather 
than the compressive strength of the spacers 160. 
Prior testing with a round-post (i.e., cylindrical column) spacer having a 
0.1 mm diameter and a 1.0 mm height indicated that the round-post spacers 
failed by buckling at a force less than 0.4 Kg. Accordingly, it would 
require approximately 10 times more round posts to support the panels, 
than are required when using the cross pin spacers. Table 1 shows a 
comparison of the buckling strength between the round post and cross pin 
spacers. 
TABLE 1 
______________________________________ 
COMPRESSION 
FORCE 
AREA MOMENT AT BUCKLING 
AT BUCK- 
SHAPE CM.sup.2 CM.sup.4 MICRON LING KG 
______________________________________ 
Round 8.13 .times. 10.sup.-5 
0.53 .times. 10.sup.-9 
1.27 0.07 
Cross 38 .times. 10.sup.-5 
16.5 .times. 10.sup.-9 
8.4 2.2 
______________________________________ 
With the dimensions shown in Table 1, the buckling strength of the cross 
pin spacers is 30 times higher than the round post spacers. Moreover, the 
cross pin spacers were able to withstand a compression before buckling 
that was more than six times greater than that of the round pin. The 
improved compressibility of the cross pin spacers is significant because 
it is desirable to have supports that can withstand a fairly large amount 
of deflection before buckling. This is because in a practical display the 
tolerances between the panels are such that some supports will receive 
higher compressive forces than others. The improved compressibility of the 
cross pin spacers will permit the supports receiving the higher 
compressive forces to deflect rather than fail by buckling. 
The sizes of the arms 160 can be extended much beyond that shown. For 
example, to provide for ease of assembly, it is desirable to extend the 
length of the arms 170 to equal or greater than the height of the cross 
pin 160 to make assembly easier. 
Many alternate embodiments of spacer 160 are illustrated in FIGS. 4-8 
wherein each figure is selected to demonstrate a different parameter that 
can be varied when constructing a spacer according to the present 
invention. 
FIG. 4 is a top view of an alternate embodiment cross pin spacer wherein 
one set of arms 190 extend a greater radial distance from the body 210 
than another set of arms 200. 
FIG. 5 is a top view of an alternate embodiment cross pin spacer wherein 
the arms 220 taper out as they extend radially from the body 230. 
FIG. 6 is a top view of an alternate embodiment cross pin spacer wherein 
the arms 240 taper in as they extend radially from the body 250. 
FIG. 7 is a top view of an alternate embodiment spacer wherein three arms 
260 extend radially from a central body 270 to form a y-shaped pin. 
FIG. 8 is a top view of an alternate embodiment y-shaped pin spacer wherein 
three arms 280 taper in to a point as they extend radially from body 290. 
Thus, it can be seen that many possible embodiments of the spacer are 
possible. Generally, a spacer according to the present invention can be 
described as having a body and N arms extending radially from the body, 
wherein N is at least three. The body and the N arms physically contact 
front and back panels of a flat-panel display to thereby separate the 
front panel from the back panel. The arms need not have the same length of 
extension from the body. Furthermore, the arms may or may not taper as 
they extend from the body. 
The many features and advantages of the invention are apparent from the 
written description and thus it is intended by the appended claims to 
cover all such features and advantages of the invention. Further, because 
numerous modifications and changes will readily occur to those skilled in 
the art, it is not desired to limit the invention to the exact 
construction and operation as illustrated and described. Hence, all 
suitable modifications and equivalents may be resorted to as falling 
within the scope of the invention.