Method for producing planar electron radiating device

A method for producing a planar type electron radiating device including a plurality of cathodes having pointed ends on a major surface of a substrate, and a gate electrode having holes in the vicinity of the cathodes, is disclosed. After a hole is formed in the gate electrode and a silicon oxide film for laying a part of the major surface of the substrate to outside, a chromium thin film as a tight metal bonding film is deposited on the bottom of the hole prior to formation of the cathode, and the cathode is formed on the chromium thin film to prevent the cathode from being detached during the production process.

BACKGROUND OF THE INVENTION 
This invention relates to a method for producing a planar type electron 
radiating device used for a flat panel display and, more particularly, to 
a flat type electron radiating device for radiating electrons from a 
plurality of pointed end cathodes. 
As an image display device to take the place of the currently employed CRT 
for a television receiver, investigations are presently conducted into a 
planar type image display device. Such planar type image display device 
may be exemplified by a liquid crystal display, an electroluminescence 
device and a plasma display panel. A field emission type image display 
device is also attracting attention in respect of display luminosity on 
the viewing screen surface. 
The field emission type image display device is now explained briefly. A 
number of conically-shaped cathodes of molybdenum etc. with a diameter of 
not more than 1.0 .mu.m, formed on a substrate by a semiconductor 
producing process, are used as radiation sources, and a plate-shaped gate 
electrode, provided with holes in register with the cathodes, are formed 
at the distal ends of the cathodes. The gate electrode is spaced apart 
from the distal ends of the cathodes and a high electrical voltage is 
applied across the gate electrode and the cathodes to produce field 
emission to extract an electron beam from the cathodes. This electron beam 
is irradiated on light emitting particles (phosphors) arranged on the back 
side of an anode to display a desired picture. Such field emission type 
image display device is described for example in U.S. Pat. No. 3,665,241, 
and the method for producing an electron radiating device, in which 
cathodes are formed on a substrate, is disclosed for example in JP Patent 
KOKOKU Publication No. 1-294336 (1989). 
The known methods for producing an electron radiating device by arraying a 
plurality of pointed-end cathodes employed in the field emission type 
image display device suffer from a drawback that the cathodes tend to be 
detached from the substrate. 
That is, with the known methods, an insulating film is formed on the 
substrate, and a gate electrode layer is formed in the insulating layer. 
The gate electrode layer and the insulating film are removed in regions 
thereof in which to form the cathodes, so that the major surface of the 
substrate is exposed at the bottom of the so-formed holes. A cathode 
electrode material is then deposited on the sidewall of the holes formed 
in the gate electrode layer by an oblique vacuum deposition technique to 
produce the pointed-end cathodes. 
However, if ultrasonic cleaning is carried out for peeling off any excess 
cathode electrode material, the cathodes deposited on the major surface of 
the substrate are detached to render it difficult to radiate the electron 
beam. 
OBJECT AND SUMMARY OF THE INVENTION 
It is a principal object of the present invention to provide a method for 
producing a planar type electron radiating device wherein the cathodes may 
be prevented from becoming detached during the production process to 
improve the production yield. 
In accordance with the present invention, there is provided a method for 
producing a planar type electrode planar device including a substrate on 
the major surface of which a plurality of cathodes with pointed ends are 
formed, and a gate electrode having a plurality of holes in the vicinity 
of said cathodes, said method comprising forming a tight metal bonding 
film on said major surface before forming said cathodes on said major 
surface. 
For the tight metal bonding film, chromium or a metal exhibiting tight 
bonding properties equivalent thereto may be employed. The tight metal 
bonding film is of a film thickness of an order of 500 .ANG. and 
preferably in the range of from 100 to 300 .ANG.. 
In accordance with the present invention, by forming the tight metal 
bonding film to a thin thickness on the major substrate surface, tight 
bonding between the major substrate surface and the cathodes is improved 
to prevent the cathodes from becoming detached from the major substrate 
surface. 
Other objects and advantages of the present invention will become apparent 
from the following description.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring first to FIG. 1, the construction of a planar type electron 
radiating device, produced by the method according to the present 
invention, is briefly explained. 
Referring to FIG. 1, showing the planar type electron radiating device, 
produced by the method of the present invention, an electrical voltage 
supplying layer 11 is formed on a substrate 10, which is a glass or 
silicon base late coated with e.g. a silicon oxide film. A silicon oxide 
film 12 is formed on the voltage supplying layer 11, and a plurality of 
holes 14 are formed above the voltage supplying layer 11 in regions in 
which to form cathodes 13. A gate electrode layer 15 formed on the silicon 
oxide layer 12 is also provided with holes 17 in register with the holes 
14. 
Each cathode 13 is in the form of a cone having a pointed end and is formed 
within the inside of each hole 14 from a material such as molybdenum or 
tungsten. A thin chromium film 16 is formed on the surface of the 
electrical voltage supplying layer 11 as a bonded metal film, before 
formation of the cathode 13, for suppressing detachment of the cathode 13. 
The cathodes are arrayed on the major surface of the substrate 11 in a 
two-dimensional matrix configuration and are each of a height of a 
submicron order. The chromium thin film 16 for improving tight bonding of 
the cathode 13 is of a thickness less than about 500 .ANG. and preferably 
of an order of 100 to 300 .ANG.. 
The substrate provided with an array of a number of cathodes 13 in this 
manner is faced by an anode electrode, not shown, and a phosphor layer 
provided on the anode electrode radiates light by an electron beam 
radiated from the cathodes 13 to effect image display. 
The method for producing the planar type electron radiating device 
according to the present invention is hereinafter explained step by step 
by referring to FIGS. 2 to 8. 
A silicon oxide film 22 as an interlayer insulating film is formed on the 
entire surface of a substrate 21. Although not shown, a cathode voltage 
supplying layer is formed on the substrate 21. The film thickness of the 
silicon oxide layer 22 is used as a measure of the distance between the 
gate electrode and the distal end of a cathode. A gate electrode layer 23 
is formed on the entire surface of the silicon oxide film 22, as shown in 
FIG. 2. The gate electrode layer 23 is formed of a metallic material, such 
as molybdenum or tungsten. 
After formation of the gate electrode layer 23, a resist layer 24 for 
patterning is coated on the entire surface of the layer 23. The resist 
layer 24 is developed by being selectively exposed to light. The resist 
pattern of the developed resist layer 24 is such a pattern in which 
regions thereof in which to form the cathodes are removed and the surface 
of the gate electrode layer 23 is exposed at the bottom of a so-formed 
opening 24. 
Etching is then performed, using the patterned resist layer 24 as a mask. 
By this etching, the gate electrode layer 23 is selectively removed after 
the pattern of the resist layer 24, and further the silicon oxide film 22 
is also selectively removed after the pattern of the resist layer 24 or 
the pattern of the gate electrode 23. After selective removal of the gate 
electrode layer 23 and the silicon oxide film 22, a hole 25 is formed in a 
region in which to form the cathode. The major surface of the substrate 21 
is exposed at a bottom 25b of the opening 25. The resist layer 24 used for 
the patterning is peeled off and removed. 
After the hole 25 is formed above the major surface of the substrate 21, a 
resist layer 27 is formed on the entire surface and developed by selective 
exposure to light after the same pattern as that of the resist layer 24 to 
form a window 27a. The opening 25 is located at the bottom of the window 
27a and the major surface of the substrate is exposed at a bottom 25b of 
the hole 25. Meanwhile, it is not absolutely necessary to remove the 
resist layer 24. After forming such resist layer 27, the thin chromium 
layer 26 as a tight metal bonding layer is formed to a thin thickness on 
the bottom 25b of the hole 25. This thin chromium layer 26, which is a 
film onto which the cathode now to be formed is bonded tightly, is 
deposited on the major surface of the substrate by e.g. electron beam 
deposition or evaporation by resistance heating. This chromium thin film 
26 is of an extremely thin thickness of e.g. 500 .ANG. or less and 
preferably of the order of 100 to 300 .ANG.. The chromium thin film 26 on 
the resist layer 27 is removed with peeling off of the resist layer 27, so 
that only the chromium thin film 26 deposited on the bottom 25b of the 
hole 25 is left. 
After the chromium thin film 26 for improving tight bonding is formed on 
the bottom of the hole 25, a layer of a first cathode electrode material 
28 is deposited on the entire substrate surface by oblique deposition. The 
oblique deposition is a technique of forming a film by deposition from a 
direction inclined at a predetermined angle relative to a rotational axis 
normal to the major substrate surface. The layer of the first cathode 
electrode material 28 is of e.g. molybdenum or tungsten and obliquely 
deposited on the gate electrode layer 23 to form a reverse taper in 
cross-section above the hole 25. At the same time that the layer of the 
first electrode material 28 is deposited on the gate electrode layer 23, a 
fraction 28a of the layer of the first cathode electrode material 28 is 
deposited on the bottom 25b of the hole 25. 
Then, as shown in FIG. 7, a layer of a second cathode electrode material 29 
of e.g. tungsten is formed on the entire substrate surface. At this time, 
the hole 25 is reduced in diameter by the layer of the first cathode 
electrode material 28, so that the layer of the second cathode electrode 
material 29 is deposited in a direction of reducing the diameter of the 
opening 25 to a narrower diameter 29d. The result is that a fraction of 
the layer of the second cathode electrode material 29 is deposited on the 
bottom 25b of the hole 25 in an area which becomes increasingly narrow 
with reduction in the size of the narrow opening 29d, so that ultimately a 
cathode 30 in the form of a cone having a pointed end is formed on the 
bottom 25b. 
Then, as shown in FIG. 8, the layer of the first cathode electrode material 
28 and the layer of the second electrode material 29 on the gate electrode 
layer 23 are removed by a lift-off method. Lift-off is carried out by 
ultrasonic washing using commonly used highly polar organic solvents, such 
as DMF (dimethylformamide) or acetone. The cathode 30 may be prevented 
from being detached because it is tightly bonded to the major substrate 
surface by the chromium thin film 26 as a tight metal bonding film. When 
producing the image display device later, the planar type electron 
radiating device is faced via a vacuum space by a front panel having an 
anode electrode and a phosphor layer stacked one on the other. 
With the planar type electron radiating device of the present invention, 
produced by the above process, the cathodes are positively affixed on the 
major substrate surface by the tight bonding metal film formed to a thin 
thickness on the major substrate surface. The result is that the cathodes 
may be prevented from becoming detached during the production process to 
improve the production yield. 
Although the chromium thin film is used in the present embodiment as the 
tight bonding metal film, it is also possible to use other thin metal 
films. Wet etching may also be used for forming the opening 25. Cathode 
electrode materials having equivalent properties to those shown in the 
above described embodiment may also be employed.