Method of forming spacers for a flat display apparatus

A method disclosed herein for making a spacer 30 useful for maintaining a fixed spacing between the cathode 12 and anode 10 structures of a flat display. The method includes the steps of melting an end of a glass filament 40 held in the bore of a capillary 42, urging the melted end 46 against the surface 23 of the cathode structure 12 to form a bond thereon, and severing the filament 40 at a fixed distance h from the surface 23 to thereby form an upright spacer 30. The severing step may be accomplished by tilting or twisting the capillary 42 until the filament 40 is severed, or by cutting the filament 40 with a torch flame 54. The bonding process may be enhanced by preheating the cathode structure 12 and/or by subjecting the cathode structure 12 to ultrasonic vibration during bonding.

TECHNICAL FIELD OF THE INVENTION 
The present invention relates generally to flat panel displays and, more 
particularly, to a method of forming glass post spacers on a substrate for 
maintaining a fixed spacing between the emitter assembly and the display 
face of a substantially evacuated flat panel display. 
BACKGROUND OF THE INVENTION 
Advances in field emission display technology are disclosed in U.S. Pat. 
No. 3,755,704, "Field Emission Cathode Structures and Devices Utilizing 
Such Structures," issued 28 Aug. 1973, to C. A. Spindt et al; U.S. Pat. 
No. 4,940,916, "Electron Source with Micropoint Emissive Cathodes and 
Display Means by Cathodoluminescence Excited by Field Emission Using Said 
Source," issued 10 Jul. 1990 to Michel Borel et al; U.S. Pat. No. 
5,194,780, "Electron Source with Microtip Emissive Cathodes," issued 16 
Mar. 1993 to Robert Meyer; and U.S. Pat. No. 5,225,820, "Microtip 
Trichromatic Fluorescent Screen," issued 6 Jul. 1993, to Jean-Frederic 
Clerc. These patents are incorporated by reference into the present 
application. 
It is important in flat panel displays of the field emission cathode type 
that the electron emitting surface and the opposed display face be 
maintained insulated from one another at a relatively small but uniform 
distance throughout the full extent of the display face. There is a 
relatively high voltage differential, generally on the order of 200-5,000 
volts, between the emitting surface and the display face, and it is vital 
that electrical breakdown between these two surfaces be prevented. 
However, the spacing between the anode and cathode has to be small to 
assure that the desired thinness, high resolution and color purity are 
achieved. This spacing also has to be uniform for uniform resolution, 
brightness, to avoid display distortion, etc. Nonuniformity in spacing is 
much more likely to occur in a field emission cathode, matrix-addressed, 
flat vacuum-type display than in some other gas-filled display types, 
since there is typically also a high differential pressure on the opposite 
sides of the display face. Whereas the exposed side of such face may be at 
atmospheric pressure, a high vacuum of approximately 10.sup.-7 torr may be 
present between the emitting surface and the display face of the field 
emission flat panel display structure. 
In general, spacer arrangements of the prior art for field emission-type 
cathode flat panel displays may be divided into two categories: spacer 
structures which are formed as an integral part of either the emitting 
structure or the anode structure, and those which are separate from both 
of these structures, and which are placed between the two during final 
assembly. In the former category, U.S. Pat. No. 4,857,799, 
"Matrix-Addressed Flat Panel Display," issued 15 Aug. 1989, to C. A. 
Spindt et al., describes a spacer approach in which elongated, parallel 
legs are provided integrally connected with the display face plate 
interspersed between adjacent rows of pixels. Another approach, disclosed 
in U.S. Pat. No. 4,091,305, "Gas Panel Space Technology," issued 23 May 
1978, to N. M. Poley et al., for a gaseous discharge type of flat panel 
display, uses a metal to connect spacers, which metal is then coated with 
a dielectric layer. This approach is not conducive to being used in a 
field emission type arrangement, because of the high voltage differential 
necessary between the anode and cathode of such an arrangement. This high 
voltage can exceed the breakdown potential of the dielectric and result in 
the metal of the spacer posts causing an electrical short between the 
faceplate and the cathode emitting surface. 
Another approach in this category, disclosed in U.S. Pat. No. 4,422,731, 
"Display Unit With Half-Stud, Spacer, Connection Layer and Method of 
Manufacturing," issued 27 Dec. 1983, to J. P. Drogeut et al., is to 
provide interacting spacer parts on the display face and the cathode 
construction. U.S. Pat. No. 4,451,759, "Flat Viewing Screen With Spacers 
Between Support Plates and Method of Producing Same," issued 29 May 1984, 
to H. Heynisch, shows such an arrangement for a flat panel display in 
which metal pins on the face register with hollow cylinders projecting 
from the cathode. Finally, U.S. Pat. No. 5,063,327, "Filed Emission 
Cathode Based Flat Panel Display Having Polyimide Spacers," issued 5 Nov. 
1991, to I. Brodie et al., discloses polyimide spacers or pillars 
separating the emitting surface an the display face of a flat panel 
display. 
Many of these prior art approaches of the first-mentioned category have 
registration problems, and all of them add a level of complexity to the 
fabrication of the cathode and/or anode structure. 
In the latter category of prior art spacer arrangements, those which are 
separate from both the cathode structure and the anode structure, U.S. 
Pat. No. 4,183,125, "Method of Making an Insulator-Support for Luminescent 
Display Panels and the Like," issued 15 Jan. 1980, to R. L. Meyer et al., 
discloses a spacer comprising a stack of glass filaments, which are 
mutually bonded to form a unitary cellular latticework. 
In another prior art method of this latter category known to the 
applicants, uniform spacing between a field emission structure and an 
anode structure is provided by a multiplicity of glass spheres used as 
spacers between the cathode plate and the anode plate. These glass 
spheres, illustratively 200 microns in diameter, serve the dual purposes 
of providing voltage isolation between the plates, and also provide the 
standoff of the mechanical forces of vacuum on the two plates. The use of 
glass spheres as spacers provides a distinct advantage over the pillar 
structures of the prior art of the first-mentioned category cited above. 
This advantage is the relative invisibility of the glass spheres in the 
presence of an electron beam. 
The use of spheres as spacers presents a significant problem when their 
diameters are not exactly uniform, and one sphere, slightly larger than 
others in its vicinity, is burdened with an inordinate amount of pressure 
maintaining the spacing between the plates. In this situation, it is not 
uncommon for the sphere to be crushed, introducing loose glass fragments 
within the display device. 
U.S. Pat. No. 5,448,131, "Spacer for Flat Panel Display," issued 5 Sep. 
1995, to R. H. Taylor et al., discloses a spacer which comprises a 
comb-like structure having a plurality of elongated filaments joined to a 
support member, thereby providing ease of handing and assembly. The 
filaments, which may be glass, are positioned longitudinally in a single 
layer between the facing surfaces of the plates of a display. In an 
embodiment disclosed in the Taylor et al patent, the filaments are of 
nonuniform diameter such that they contact the facing surfaces only at the 
high spots, thereby reducing shadowing on the display surface. 
The attractiveness of the spacer arrangements of the latter category, those 
which are separate from both the cathode structure and the anode 
structure, begins to suffer as the spacing between the plates of the 
display is increased. Such an increase has been found to be necessary in 
order to allow a sufficiently high anode voltage for adequate display 
brightness. While a 200-micron spacing is appropriate for low brightness 
applications, spacings of one-half or even one millimeter may be required 
for higher brightness applications. Such a display would require one-half 
or one millimeter diameter glass sphere spacers, or filaments of that 
diameter, in the case of the Taylor et al patent, which size would clearly 
occult a noticeable portion of the display. 
In view of the above, it is easily understood that there exists a need for 
an apparatus for maintaining a uniform spacing between the emission 
surface and the anode of a field emission flat panel display device which 
is relatively simple to manufacture, and which is effective even where the 
spacing distance between the facing surfaces of the plates approaches and 
even exceeds the spacing between adjacent pixels on the display screen. 
SUMMARY OF THE INVENTION 
In accordance with the principles of the present invention, there is 
disclosed herein a method for making a spacer useful for maintaining a 
fixed spacing between two substantially parallel plates, the method 
comprising the steps of: providing a substrate; melting an end of a fiber 
being held in the bore of a capillary; urging the melted end against the 
substrate to form a bond thereon; and severing the fiber at a fixed 
distance from the substrate. 
In accordance with one embodiment of the present invention, the step of 
melting an end of the fiber comprises heating the end with a torch flame 
so as to form a ball at the end, and the step of urging the melted end 
against the substrate includes positioning the capillary so as to apply 
force on the ball against the substrate. In accordance with a preferred 
embodiment of the present invention, the step of urging the melted end 
against the substrate further includes ultrasonically agitating the 
substrate to assist formation of the bond. 
In accordance with one embodiment of the present invention, the step of 
severing the fiber at a fixed distance from the substrate includes 
positioning the capillary at the fixed distance from the substrate, and 
tilting the capillary until the fiber is severed. In accordance with 
another embodiment of the present invention, the step of severing the 
fiber at a fixed distance from the substrate includes positioning the 
capillary at the fixed distance from the substrate, and twisting the 
capillary until the fiber is severed. In accordance with still another 
embodiment of the present invention, the step of severing the fiber at a 
fixed distance from the substrate includes cutting the fiber with a torch 
flame. 
Further in accordance with the present invention there is disclosed a 
method for fabricating a flat display apparatus comprising the steps: of 
providing a substrate having a substantially planar surface; providing a 
display panel having a substantially planar face; providing a spacer 
element on one of the substantially planar surface and substantially 
planar face comprising the substeps of melting an end of a fiber being 
held in the bore of a capillary, urging the melted end against the one of 
the substantially planar surface and substantially planar face to form a 
bond thereon, and severing the fiber at a fixed distance from the one of 
the substantially planar surface and substantially planar face; repeating 
the previous step at various locations on the one of the substantially 
planar surface and substantially planar face; positioning the other of the 
substantially planar surface and the substantially planar face on the 
spacer elements; and sealing the substrate to the display panel.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring initially to FIG. 1, there is shown, in cross-sectional view, a 
portion of an illustrative field emission flat panel display device which 
includes spacers fabricated in accordance with the present invention. In 
this embodiment, the field emission display device comprises an anode 
portion having an electroluminescent phosphor coating facing a cathode 
portion, the phosphor coating being observed from the side opposite to its 
excitation. 
More specifically, the field emission display device of FIG. 1 comprises a 
cathodoluminescent anode 10 and a cathode 12. Cathode 12 comprises a 
plurality of electrically conductive microtips 14 formed on an 
electrically conductive coating 16, which is itself formed on an 
electrically insulating substrate 18. Coating 16 may be semiconducting or 
resistive instead of being conducting. 
A gate electrode comprises a coating of an electrically conductive material 
22 which is deposited on an insulating layer 20. Microtips 14 take the 
shape of cones which are formed within apertures through conductive layer 
22 and insulating layer 20. The thicknesses of gate electrode coating 22 
and insulating layer 20 are chosen in such a way that the apex of each 
microtip 14 is substantially level with the electrically conductive gate 
electrode coating 22. Conductive coating 22 may be in the form of a 
continuous coating across the surface of substrate 18; alternatively, it 
may comprise conductive bands across the surface of substrate 18. 
Conductive coating 22 forms a substantially planar surface 23 on cathode 
structure 12. 
Anode 10 comprises an electrically conductive film 28 deposited on a 
transparent planar support 26 which is positioned facing gate electrode 22 
and parallel thereto, the conductive film 28 being deposited on the 
surface of support 26 directly facing gate electrode 22. Conductive film 
28 may be in the form of a continuous coating across the surface of 
support 26 as shown in FIG. 1; alternatively, it may be in the form of 
electrically isolated stripes comprising three series of parallel 
conductive bands across the surface of support 26, as taught in U.S. Pat. 
No. 5,225,820, to Clerc. By way of example, a suitable material for use as 
conductive film 28 may be indium-tin-oxide (ITO), which is optically 
transparent and electrically conductive. 
Anode 10 also comprises a cathodoluminescent phosphor coating 24, deposited 
over conductive film 28 so as to be directly facing and immediately 
adjacent gate electrode 22. Phosphor coating 24 forms a substantially 
planar surface 25 on anode structure 10. In the Clerc patent, the 
conductive bands of each series are covered with a phosphor coating which 
luminesces in one of the three primary colors, red, blue and green. 
Anode 10 and cathode 12 are spaced apart from one another by a plurality of 
spacers 30 which are shown as columnar members. In a preferred embodiment, 
spacers 30 comprise rod-shaped glass filaments having enlarged base 
regions 31. The upper portions of spacers 30 have substantially circular 
cross sections and are of substantially equal length, although columnar 
members fabricated of other materials and having other cross-sectional 
configurations may be used. As an example, spacers 30 may be fabricated of 
a ceramic material or a metal., as well of glass. By way of illustration, 
the diameter of the upper portions of spacers 30 may range between 20 and 
500 .mu.meters, and the overall length of spacers 30 may range between 
0.05 and 10 millimeters. As used herein, the term "filament" means the 
individual fibers, threads, rods, strands, strings, posts, columns or 
canes which provide the spacing function between the opposed faces of 
anode structure 10 and cathode structure 12. 
The material from which spacers 30 are made must have the following 
qualities. It must have sufficient compressive strength to withstand the 
force exerted by anode structure 10 against cathode structure 12 in the 
presence of a vacuum. Second, it must be substantially free from 
outgassing when in a vacuum pressure of approximately 10.sup.-7 torr. This 
second quality practically dictates that the material of spacers 30 must 
be inorganic. Third, it may have to be electrically insulating, capable of 
withstanding a potential difference of up to approximately 5,000 volts in 
the application directed to its intended use as described herein. It must 
also be capable of withstanding the temperature at which anode structure 
10 and cathode structure 12 are sealed to one another, typically in the 
range of approximately 400.degree.-550.degree. C. Finally, the material of 
spacers 30 must be capable of undergoing the process of attachment to 
anode structure 10 (or cathode structure 12) which is described in 
subsequent paragraphs. In the present example, glass is considered the 
most advantageous material for use as spacers 30. 
Anode structure 10 and cathode structure 12 are sealed together at 
peripheral portions thereof by sealing material 32, illustratively 
comprising a glass frit rod which reflows at a temperature below the 
reflow temperature of spacers 30. The reflow temperature of sealing 
material 32 may be in the range of approximately 400.degree.-550.degree. 
C. 
A heating process, wherein sealing material 32 reflows to seal structure 10 
to structure 12, occurs in an environment of an inert gas, preferably 
argon. After the sealing process, the space 34 between anode structure 10 
and cathode structure 12 is evacuated to a pressure of approximately 
10.sup.-7 torr through an opening (not shown) in either emitter structure 
12 or anode structure 10. Alternatively, the sealing process may be 
conducted within a vacuum environment, obviating the need for separately 
evacuating the space between anode structure 10 and cathode structure 12. 
Referring now to FIGS. 2a through 2e, there is shown a sequence of steps 
for fabricating and assembling spacer elements 30 of the type shown in the 
field emission display device of FIG. 1. FIG. 2a illustrates a thin 
filament 40, illustratively made of glass having a diameter of between 20 
and 500 .mu.meters. Filament 40 is held in the bore of capillary 42 a 
short distance away from terminal end 41. Capillary 42 is shown in cross 
section, and the bore through which filament 40 extends is sized such that 
filament 40 can slide back and forth within it. 
Localized heat is applied to terminal end 41, softening or melting the 
material of filament 40 and resulting in the formation of a ball structure 
46, as shown in FIG. 2b. The source of the localized heat may 
illustratively be the flame of a hydrogen torch 44. In the present 
example, wherein filament 40 is glass, heating at a temperature of between 
400.degree. and 1000.degree. C. for between 1 and 1000 milliseconds is 
appropriate for adequate melting and formation of ball 46. 
Referring now to FIG. 2c, capillary 42 positions filament 40 substantially 
normal to surface 23 of cathode structure 12 and urges softened ball 46 
against a predetermined location on surface 23, deforming ball 46, and 
causing adhesion to surface 23. In the example shown, cathode structure 12 
is mounted on platform 36, which may provide heating and cooling to 
cathode structure 12 from heating/cooling device 50, and which further may 
provide ultrasonic vibration to cathode structure 12 from ultrasonic 
vibrator 52. It is recognized that the process of bonding softened ball 46 
to surface 23 may be enhanced by preheating cathode structure 12 to an 
elevated temperature, typically between 300.degree. and 600.degree. C. It 
is further recognized that the process of bonding softened ball 46 to 
surface 23 may be enhanced by subjecting cathode structure 12 to 
ultrasonic vibration during the bonding process, typically at a frequency 
of between 30 and 300 kHz. 
When the bond between ball 46 and surface 23 is firm, capillary 42 slides 
up filament 40 to a fixed height h above surface 23 of cathode structure 
12, as shown in FIG. 2d. In the present example, capillary 42 is caused to 
tilt to one side, as shown in FIG. 2e, severing filament 40 at a height h 
above surface 23, thereby forming upright spacer element 30, of the type 
shown in FIG. 1. The height h of spacer 30 is illustratively between 0.05 
and 10 millimeters. 
The process steps described in relation to FIGS. 2a through 2e are repeated 
at a multiplicity of locations over surface 23 of cathode structure 12, 
for as many spacers as are needed to maintain the proper spacing between 
cathode structure 12 and anode structure 10 (as shown in FIG. 1). The 
determination of the number of spacers 30 is set, at least, by the 
material strength of spacers 30 and by the atmospheric pressure load 
between anode structure 10 and cathode structure 12. 
The cycle time of the above process may be reduced by a final step of 
quickly lowering the temperature of cathode structure 12. Heating/cooling 
device 50 accomplishes this by cooling platform 36, which in turn quickly 
reduces the temperature of cathode structure 12. 
Finally, anode structure 10 is positioned such that its planar surface 25 
rests on the extended ends of spacers 30, and the two structures 10 and 12 
are sealed as described above and as shown in FIG. 1. 
While the process described in relation to FIGS. 2a through 2e teaches a 
technique of affixing spacer elements 30 to surface 23 of cathode 
structure 12, it should be recognized that essentially the same process 
can be followed by affixing spacer element 30 to surface 25 of anode 
structure 10, and subsequently positioning cathode structure 12 such that 
its planar surface 23 rests on the extended ends of filaments 30. In some 
instances, this latter process may be deemed preferable. 
FIG. 3 illustrates a first alternative to the step of severing filament 40 
as described in relation to FIG. 2e. In particular, when the bond between 
ball 46 and surface 23 is firm, capillary 42 slides up filament 40 to a 
fixed height h above surface 23 of cathode structure 12, and capillary 42 
is caused to twist about the axis of filament 40, as shown in FIG. 3, 
severing filament 40 at a height h above surface 23, thereby forming 
spacer element 30, of the type shown in FIG. 1. 
FIG. 4 illustrates a second alternative to the step of severing filament 40 
as described in relation to FIG. 2e. In this alternative, when the bond 
between ball 46 and surface 23 is firm, capillary 42 slides up filament 40 
and heat is applied to filament 40 at a fixed height h above surface 23 of 
cathode structure 12, as shown in FIG. 3, until filament 40 is severed at 
a height h above surface 23, thereby forming spacer element 30, of the 
type shown in FIG. 1. The source of the heat may typically be a finely 
directed flame from a hydrogen torch 54. 
A flat panel display device which includes the spacers disclosed herein, 
the method of forming and assembling the spacers disclosed herein, and the 
method of assembling a flat panel display device which includes the 
spacers disclosed herein, overcome many limitations and disadvantages of 
the prior art display devices and methods. The relatively simple process 
of forming spacers on one of the substrates in accordance with the present 
invention is a distinct improvement over the methods used to fabricate the 
latticework, pillar and leg structures of the prior art, and it is far 
easier to assemble than the prior art method involving the multiplicity of 
individual spheres. Furthermore, it permits spacing between display panels 
of as much as ten millimeters, without occulting a noticeable portion of 
the display. Hence, for the application to flat panel display devices 
envisioned here, the approach in accordance with the present invention 
provides significant advantages. 
While the principles of the present invention have been demonstrated with 
particular regard to the structures and methods disclosed herein, it will 
be recognized that various departures may be undertaken in the practice of 
the invention. The scope of the invention is not intended to be limited to 
the particular structures and methods disclosed herein, but should instead 
be gauged by the breadth of the claims which follow.