Method of producing low voltage field emission cathode structure

A method of making a low voltage field device utilizing a preferentially etched unidirectionally solidified composite as the substrate. In the process, the composite is etched so that the electrically conducting rod-like or fiber phase protrudes above the matrix phase. The tip of the exposed fiber phase may be processed further to provide a rounded or needle-like geometry. Next, a layer of insulating material is deposited in a direction approximately parallel to the axes of the fibers to cause the formation of cone-like deposits of insulating material on the fiber tips which shadow the deposit on the matrix around the fibers and produce conical holes in the layer of insulating material about the fibers. Then, an electrically conductive film is deposited in approximately the same direction to produce on the insulating layer a cellular grid having openings corresponding in number and distribution to the fiber sites. Lastly, the cones of insulating material are removed from the fibers.

It is well known that electron emission can be stimulated from a variety of 
sharp pointed conductive materials by a high electric field. Low voltage, 
high electric field emitting arrays and the methods of producing such 
devices are disclosed, for example, in U.S. Pat. No. 3,812,559 in the name 
of Spindt et al and issued on May 28, 1978, U.S. Pat. No. 3,789,471 issued 
in the name of Spindt et al on Feb. 5, 1974, and U.S. Pat. No. 3,755,704 
issued in the name of Spindt et al., on Aug. 28, 1978 all assigned to the 
Stanford Research Institute. These devices utilize individual needle-like 
points vapor deposited on a silicon electrode. The major disadvantage of 
the Stanford Research Institute device is the formation of the field 
emitting tip from a vapor deposition process resulting in an amorphous or 
polycrystalline material. In contrast to the Stanford Research Institute 
device, the procedure disclosed here processes single crystal emitters 
that are formed and exposed prior to vapor deposition of a thin extractor 
grid. 
BRIEF SUMMARY OF THE INVENTION 
Accordingly, it is an object of this invention to provide an improved 
method of producing low voltage field emission devices. 
Briefly in accordance with the invention there is provided a method of 
making a low voltage field emission device including the step of etching 
an oxide-metal composite to a desired length to expose the metal fibers. 
The etching step can produce cylindrical tipped or pointed needle-like 
fibers or the tip geometry may be alterd so as to be hemispherical by ion 
milling at this stage of the process. Next, a layer of insulating material 
is deposited in a direction approximately parallel to the axes of the 
fibers to cause the formation of inverse-truncated cones of insulating 
material on the fibers and holes in the layer of insulating material about 
the fibers. Then, a metal film layer is deposited in the same direction to 
produce on the insulating layer cellular grid having openings 
corresponding in number and distribution of the fiber sites. The cones of 
insulating material are removed from the fibers. 
The method of making a low voltage field emission device utilizes in one 
embodiment single crystal tungsten fibers as the emitters. Since tungsten 
is the most refractory, highest melting point, and lowest vapor pressure 
metal known, the emitters are resistant to the failures associated with 
localized field emitters tip heating and subsequent vaporization. The 
utilization of this fabrication process with the unidirectionally 
solidified composites generates an emitter structure with an excess of 
10.sup.6 emitters per cm.sup.2. Hence this LVFE structure provides 
redundancy as well as reducing the current carrying need of the individual 
emitters to achieve current densities competitive with other structures. 
The formation of vapor deposits on protrusions from a substrate in the 
shape of inverse cones is a unique and fundamental step in this process. 
The growth of the cones appears to be a newly discovered material property 
and the cone angles are dependent on the composition of the deposited 
layer. During deposition, the cones generate self-aligned holes in the 
surrounding film due to shadowing by the expanding cone and the 
reproducibility of the LVFE structures is unparalleled compared to prior 
art methods of generating similar structures. Lastly, a variety of 
fabrication steps can be used in conjunction with the vapor deposition to 
yield different emitter and accelerator geometries which may prove 
beneficial for a variety of high electric field applications. For example, 
if the cones are removed at an intermediate stage of deposition and the 
deposition is then continued, new cones will expand from the protrusions 
while at the same time the holes in the surrounding film will contract 
toward the protrusions by the same mechanism that causes the cones on the 
protrusions to expand. When the new cone expands beyond the contracting 
hole in the surrounding film, shadowing of the surrounding film again 
occurs and the holes again expand due to shadowing by the cones. This 
process of removing cones at some intermediate stage of deposition and 
then continuing deposition is referred to as multiple deposition and has 
been used to vary the hole diameter independent of the thickness of the 
deposited film. It should be noted, that film composition may be changed 
at any time to provide for an extractor or accelerator electrode and that 
multiple conducting electrodes may be deposited to provide for electron 
control.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring now to the drawings, wherein like reference numerals designate 
identical or corresponding parts and more particularly to FIG. 1 thereof, 
there is shown a low voltage field emission device in accordance with the 
invention. The structure includes a matrix 14 in which a large number of 
needle-like conducting electrodes 15 called emitters are distributed with 
a high packing density. A surface called the accelerator or extractor 17 
is the electrode used to produce the field. It consists of a conducting 
film supported by the electric insulator 21 normal to the axes of the 
emitters 15. Holes 19 extending through the accelerator 17 into the 
electric insulator 21 are provided to expose the tip of an emitter 15 at 
each hole location. Upon application of a potential between the emitters 
15 and the accelerator 17 surface, an electric field is established 
between the tips of the emitters and the accelerator which is of polarity 
to cause electrons to be emitted from the emitter tips through the holes 
19 in the accelerator. The field emission device has a simple structure. 
The rim of the hole in the accelerator is positioned at an extremely short 
distance from the tip of the emitter. As a result of this, a strong 
electric field can be generated with a comparatively low voltage 
difference. 
FIGS. 2 and 3 show successively steps in the manufacture of the low voltage 
field emission device. In this case also a specific embodiment is 
described, in which, for example, variations are possible in the material 
choice and the treatments to be carried out. FIG. 2 shows an oxide-metal 
composite consisting of an oxide matrix 14 containing a plurality of 
unidirectionally aligned metallic fibers 15. Free standing emitters 15 are 
formed by etching the oxide matrix 14 to a desired depth. The composite 
can be fabricated by well-known prior art techniques. One fabrication 
approach which can be utilized is described in detail in the publication 
"Report No. 6: Melt Grown Oxide-Metal Composites" from the School of 
Ceramic Engineering, Georgia Institute of Technology, A. T. Chapman, 
Project Director (December 1973) hereby incorporated by reference, 
detailing fabrication of a melt grown oxide-metal composite consisting of 
about 10.sup.7 parallel metal fibers in each square centimeter of an oxide 
matrix. Preferred materials are single crystal W or Mo for the fibers, and 
UO.sub.2 for the oxide matrix, but other well-known materials can be 
utilized. The composite is grown in an induction furnace from a mix of 
oxide and of metal powders. Auxiliary heating brings the oxide-metal 
sample ingot close to the melting point. Induction heating melts a zone in 
the interior of the ingot but does not melt the outside of the ingot. The 
outer unmelted zone of the ingot acts as a crucible to contain the melt. 
Unidirectional solidification of the molten internal zone is accomplished 
by moving the zone up through the ingot. During solidification the metal 
precipitates to form small (&lt;1.mu.m diameter) fibers regularly arrayed and 
aligned in the oxide matrix. 
Next, the unidirectional composite is processed to produce metal conductors 
protruding above the matrix. For the system UO.sub.2 -W, etches are 
available that dissolve the UO.sub.2 matrix without dissolving the W which 
produces W fibers with cylindrical tips above the matrix. There are also 
etchs which dissolve the UO.sub.2 matrix and slowly attack the W fibers. 
This produces W fibers with pointed tips above the UO.sub.2 matrix. The 
tip shape can also be altered by ion milling the exposed fibers. Ion 
milling of exposed cylindrical tipped fibers produces a variety of tip 
geometries from cylindrical tips with rounded corners to hemispherical 
tips to pointed tips. 
After formation of the emitters 15, the support structure for the 
accelerator 17 is produced. Namely, an insulating layer 21 made of 
SiO.sub.2 film or Al.sub.2 O.sub.3 film is deposited at normal incidence 
on the oxide matrix 14, that is, roughly parallel to the axes of the 
fibers 15, by the well-known vapor deposition method. The insulating layer 
21 forms deposits on the electrodes in the shape of inverse truncated 
cones 23 having cone angles of from 30 to 90 degrees. The cones 23 in turn 
act as masks for annular regions which are concentric with the electrodes, 
so that during the deposition process, each electrode stands free within a 
gradually expanding opening 19 in the insulating layer 21. When the 
insulating layer reaches a desired thickness the deposition is terminated. 
The accelerator 17 is then formed by depositing a conducting film such as 
Mo on the insulating layer 21 at normal incidence thereto so as to produce 
a cellular grid whose openings correspond in number and distribution to 
the emitter 15 sites. The unit thus formed is shown in FIG. 3. 
Next, the structure is utlrasonically vibrated in a liquid such as water, 
which satisfactorily removes the cones 23 from the emitters 15. 
Alternatively, the cones may be removed by chemically attacking the 
insulator portion of the cones 23. Following cone removal, the structure 
is cleaned by etching in a variety of acids depending on the composition 
of the insulating and conducting layers. 
The structure illustrated and thus far described was tested electrically 
with the following results. For a structure utilizing W emitters and an Mo 
accelerator, current densities of 1 ampere per cm.sup.2 were achieved when 
a pulsed potential of 200 volts was applied between the emitters and the 
accelerator surface. If the emitters are conservatively operated at 10 
microamperes per emitter, a current density of 100 ampere per cm.sup.2 
should be obtained. 
In an alternate embodiment, as described previously, multiple depositions 
of insulating layers and removal of the cones at intermediate periods can 
be used to control the diameter of the holes 1 surrounding the individual 
emitters 15. 
Obviously, numerous additional modifications and variations of the present 
invention are possible in light of the above teachings. It is therefore to 
be understood that within the scope of the appended claims, the invention 
may be practiced otherwise than as specifically described herein.