Field-emission type electronic device

A field-emission electronic device works as a field-emission electron source. The field-emission electronic device comprises an anode electrode, a first insulating member disposed on the anode electrode, a cathode electrode disposed on the first insulating member, a second insulating member disposed on the anode electrode at a distance from the first insulating member, and a gate electrode disposed on the second insulating member. Therefore, the field-emission electronic device can be formed to make the distance between the electrodes smaller than that of the known field-emission electronic device. Concretely, the distances between the cathode electrode and the gate electrode and between the cathode electrode and the anode electrode are allowed to be reduced. This results in lowering a gate voltage and an anode voltage.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a field-emission type electronic device 
containing an electron source which is operated to emit electrons on the 
principle of field emission, and more particularly to a cold cathode 
provided in the field-emission type electronic source. 
2. Description of the Related Art 
In recent days, a remarkable progress has been made about a technique for 
manufacturing the field-emission type electronic device for emitting 
electrons in a high electric field in vacuum as a result of developing a 
fining technique utilized in the field of an integrated circuit or thin 
film deposition. In particular, a field-emission type cold cathode having 
a quite fine structure has been manufactured. This type of field-emission 
type cold cathode is the most fundamental electron-emission device 
included in the essential parts of a micro electronic tube or electron 
gun. 
The field-emission type electronic device or the field-emission type 
electron source containing a lot of electron-emission devices has been 
invented for an essential component for a micro triode or a thin display 
element, for example. The operation and the manufacturing method of the 
field-emission type electronic device or the field-emission type electron 
source have been known in the technical report: C. A. Spindt, et. of 
Stanford Research Institute, pp. 5248 to 5263, Vol. 47, December (1976) of 
Journal of Applied Physics. Further, they have been disclosed in U.S. Pat. 
Nos. 4,307,507 and 4,513,308 invented by H. F. Gray, et. 
Then, some of the related arts will be described later. 
In a conventional field-emission electron source, a substrate electrode is 
formed of monocrystalline silicon having low resistance in order to keep 
compatibility with a fining technique in the field of an integrated 
circuit or thin film deposition, lower the cost, and make it monolithic. 
On the substrate electrode, a lot of conical cold cathode chip are formed. 
Each cold cathode chip is made of the same monocrystalline silicon as the 
substrate electrode or a high melting point metal such as tungsten (W) or 
molybdenum (Mo). An insulating layer is formed on the substrate electrode 
around the cold cathode chip. On the insulating layer, a gate electrode is 
deposited. An anode electrode is provided to cover those cold cathode 
chips and the gate electrode as keeping vacuum space between the anode 
electrode and the side of the cold cathode chips and the gate electrode. 
In such a electron source, a voltage of about 100 to 200 V is applied as a 
gate voltage between each cold cathode chip and the gate electrode. The 
application results in causing a strong electric field of about 10.sup.7 
V/cm between each cold cathode chip and the gate electrode, thereby 
allowing each cold cathode chip to emit electrons on the field-emission 
principle. The anode voltage of 300 to 500 V applied to the anode 
electrode causes emitted electrons to reach the anode electrode. 
In the current techniques, the critical diameter of the conical cold 
cathode chip is about 1 .mu.m and the critical height thereof is about 1 
.mu.m. Further, it is practically impossible to avoid variable 
electron-emission characteristics in those chips caused by the variations 
of the cold cathode chips. To overcome the disadvantageous matter, the 
anode electrode is made of a transparent material and a fluorescent 
material is coated on the transparent anode electrode. A trial is now 
being made for a thin display unit using the cold cathode chips as 
electron-emission sources only. In a case that this type of field-emission 
electronic device applies to the thin display unit, it is unnecessary to 
accurately control the emitted electrons. Hence, 1000 or more 
electron-emission cold cathode chips, which are arranged per one pixel in 
an array manner, are driven in parallel for the purpose of averaging the 
variation of the electron-emission cold cathode chips and obtaining the 
necessary amount of emitted electrons. 
In a case that the field-emission cold cathode chips are used for a micro 
triode, the resulting triode may break the shortcomings and the limits 
entailed in the solid device such as a semiconductor device. The solid 
device has such a limit that the saturated traveling speed of electrons in 
the solid device is about c/1000 (c is a light speed). On the other hand, 
in the field-emission electronic device, the emitted electrons travel in 
vacuum. Hence, the traveling speed of the electrons may be faster than the 
traveling speed of the electrons in the solid device by one or more 
digits. Further, the field-emission electronic device is more endurable in 
high temperature and radioactive rays. For example, in a case that a 
voltage of 50 V is applied between the electrodes keeping a spacing of 1 
.mu.m therebetween, the traveling speed of electrons is 2.times.10.sup.8 
cm/s on average and the traveling time for a distance of 1 .mu.m is 0.5 
psec. 
The use of the triode having dimensions on sub-micron order, therefore, 
makes it possible to realize a super high-speed device having a response 
speed on tera-hertz level. 
In the known field-emission type electron source, a field-emission type 
cold cathode chip is formed like a conical form on a substrate electrode 
made of a metal or semiconductor material as mentioned above. An 
insulating layer is formed to cover the substrate electrode around the 
field-emission type cold cathode. On the insulating layer, a gate 
electrode is deposited. When a voltage is applied between the 
field-emission type cold cathode and the gate electrode, a high electric 
field takes place between the cold cathode and the gate electrode so that 
electrons can be emitted from the field-emission cold cathode on the basis 
of the field-emission principle. 
The field-emission cold cathode is made of silicon or metal such as 
tungsten (W) or molybdenum (Mo). Further trial is now being made for 
optimizing the form of the field-emission cold cathode in order to reduce 
an operating voltage on which electrons are emitted. 
In another conventional field-emission electron source, like the foregoing 
composition, a field-emission cold cathode is formed like a conical form 
on a substrate electrode. An insulating layer is formed on the substrate 
electrode around the field-emission cold cathode. On the insulating layer, 
a gate electrode is deposited. The substrate electrode is made of 
semiconductor or metal. Unlike the foregoing composition, the substrate 
electrode is projected like a pyramid at the site where the conical 
field-emission cold cathode is to be formed. On the pyramid portion, a 
coating layer is deposited. The coating layer is made of a material having 
a low work function such as cesium (Cs) or lanthanum hexabolaide (LAB6). 
It means that the pyramid portion of the substrate electrode and the 
coating layer deposited thereon compose the field-emission cold cathode. 
Next, the shortcomings of the conventional compositions will be described. 
For the known field-emission electric devices, the following shortcomings 
take place. Since the distance between the cold cathode chip served as a 
cathode electrode and the gate electrode is not made so small, it is 
necessary to apply a large voltage between the cathode electrode and the 
gate electrode for obtaining the necessary electric field to allowing the 
tip of the cold cathode chip to emit electrons. Further, since the 
distance between the cathode electrode and the anode electrode is made 
larger, it needs a considerable time to travel electrons between the 
cathode electrode and the anode electrode. 
The cold cathode chip has a cut-off frequency f.sub.T represented by the 
express ion: 
EQU f.sub.T =g.sub.m /(2.pi.C.sub.gc) 
wherein g.sub.m is a mutual conductance and C.sub.gc is a capacitance 
between the gate electrode and the cathode electrode. 
To realize a cold cathode chip enabling to operate at high speed, 
therefore, it is necessary to increase the mutual conductance g.sub.m but 
decrease the capacitance C.sub.gc. However, in the structure of the known 
field-emission electronic devices, the electron emission is made possible 
only at the tip of the cold cathode chip. Further, since it is difficult 
to make the spacing between the adjacent cold cathode chips small in light 
of the manufacturing technique, the area where electrons are emitted and 
the amount of emitted electrons are both small. Hence, it is difficult to 
increase the mutual conductance g.sub.m of the electronic device depending 
on the current density of the field emission. Further, the field-emission 
electronic devices has the structure where the gate electrode layer is 
opposed to the cathode electrode layer as keeping the insulating layer 
therebetween. The structure inevitably increases the value of the 
capacitance C.sub.gc between the gate electrode and the cathode electrode. 
In turn, for the first conventional field-emission electron source, in a 
case that the field-emission cold cathode is made of a high melting point 
metal such as tungsten (W), molybdenum (Mo) or titanium (Ti), those metals 
are thermally endurable and mechanically strong, but have so high work 
functions. For example, the work function of tungsten is about 4.3 eV and 
one of molybdenum is about 4.2 eV. They disadvantageously need high 
operating voltages. 
For the second known composition of a field-emission electron source as 
mentioned above, the work function of the coating layer is so low such as 
about 2.1 eV in case of using cesium (Cs) and about 2.7 eV in case of 
using lanthanum hexabolaid (LaB.sub.6). Hence, the operating voltage is 
made smaller. The difference of thermal expansion coefficient between the 
material of the coating layer and the material of the substrate electrode 
causes the resulting cold cathode to be thermally unstable and 
mechanically weak. Since the material of the coating layer is chemically 
active, a shortcoming takes place that the work function is subject to 
change. Additional, since the material of the coating layer such as 
selenium has a far larger than the substrate electrode made of metal or 
semiconductor, the electric conduction between both is made worse, so that 
the electron emission is difficult to take place. 
SUMMARY OF THE INVENTION 
It is therefore an object of the present invention to provide a 
field-emission electronic device which is capable of realizing high-speed 
operation. 
It is another object of the invention to provide a field-emission electron 
source which is physically stable and excellent in electric and mechanical 
characteristics and has a low work function. 
The first object of the invention can be achieved by a field-emission 
electronic device comprising an anode electrode, a first insulating member 
disposed on the anode electrode, a cathode electrode disposed on the first 
insulating member, a second insulating member disposed on the anode 
electrode at a distance from the first insulating member, and a gate 
electrode disposed on the second insulating member. 
A field-emission electronic device according to another aspect of the 
invention includes a substrate, a first insulating member disposed on the 
substrate, a cathode electrode disposed on the first insulating member, a 
second insulating member disposed on the substrate at a distance from the 
first insulating member, a gate electrode disposed on the second 
insulating member, and an anode electrode disposed between the first 
insulating member and the second insulating member and electrically 
connected with the substrate. 
The field-emission electronic device according to this invention is formed 
to make the distance between the electrodes smaller than that of the known 
field-emission electronic device. Concretely, the distances between the 
cathode electrode and the gate electrode and between the cathode electrode 
and the anode electrode are allowed to be reduced. This results in 
lowering a gate voltage and an anode voltage. In the structure of this 
invention as described above, the value of the capacitance between the 
cathode electrode and the gate electrode can be made smaller as compared 
to the known field-emission electronic device wherein the cathode 
electrode and the gate electrode are laminated with the insulating layer 
laid therebetween. In a case that the anode electrode is provided on the 
substrate located between the cathode electrode and the gate electrode, 
the values of capacitance caused between the cathode electrode and the 
anode electrode and between the gate electrode and the anode electrode can 
be reduced. 
For example, if a voltage of 20 V to 100 V is applied between the cathode 
electrode and the gate electrode, a strong electric field of about 
10.sup.7 V/cm takes place between the tip of the cathode electrode and the 
gate electrode in quick response to the application of the voltage. The 
cold cathode tip serves to emit electrons at its upper tip on the basis of 
the field-emission principle. 
In carrying out the second object, a field-emission electronic device 
comprises a substrate, and a field-emission cold cathode, which is formed 
of metallic carbide, metallic nitride, metallic oxide or metallic boride, 
disposed on and electrically connected with the substrate electrode, a 
composition ratio of carbon, nitrogen, oxygen or boron of the cathode 
being gradually increased from a bottom portion thereof adjacent to the 
substrate to a top portion thereof. 
The field-emission cold cathode is formed of metallic carbide, metallic 
nitride, metallic oxide or metallic boride. The work function of such a 
material is smaller than that of the metal used in the related art such as 
molybdenum (Mo) or titanium (Ti). This results in being able to reduce the 
operating voltage on which electrons are emitted. Moreover, the 
field-emission cold cathode may have a deposition structure wherein the 
composition ratio of carbon, nitrogen, oxygen or baron is progressively 
increased from the substrate (base of the conical form) to the tip (of the 
conical form). In the structure, since the electric resistance is 
continuously changed from the substrate electrode to the tip, the electric 
conductivity in the cold cathode is improved as compared to the structure 
where the hold cathode coating layer is directly deposited on the 
substrate electrode. As another advantage, it is possible to suppress the 
difference of a thermal expansion coefficient between the layers and 
improve the bonding strength between the cold cathode and the substrate 
electrode and the thermal stability of them, The present invention can 
thus offer a field-emission electron source which is physically stable and 
is excellent in electric and mechanical characteristics. 
Further objects and advantages of the present invention, will be apparent 
from the following description of the preferred embodiments of the 
invention as illustrated in the accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
In turn, the description will be directed to a field-emission electronic 
device according to a first embodiment of the invention. 
FIG. 1 is a perspective view showing the field-emission electronic device. 
FIG. 2 is a sectional view cut on the II--II line of FIG. 1. 
A field-emission electronic device employs as its substrate a 
high-resistance monocrystalline silicon substrate 4 such as a non-doped 
silicon substrate. On the silicon substrate 4, there is formed an anode 
electrode layer 3 made of molybdenum. On the anode electrode layer 3, a 
cathode electrode layer 1 is located with an insulating layer 5 laid 
therebetween and a gate electrode layer 2 is located with an insulating 
layer 6 laid therebetween. The cathode electrode layer 1 is opposed to the 
gate electrode with a groove 7 laid therebetween. The insulating layer 5 
and 6 are both made of silicon dioxide. The cathode electrode layer 1 and 
the gate electrode layer 2 are both made of molybdenum. The horizontal 
distance d1 between the cathode electrode layer 1 and the gate electrode 
layer 2 is set as 0.1 to 0.5 .mu.m. The thickness h1 of the insulating 
layer 5 is set as 0.2 to 1.0 .mu.m and the thickness h2 of the insulating 
layer 6 is set as 0. 1 to 0.5 .mu.m in a manner to keep a relation of 
h1&gt;h2. That is, the gate electrode layer 2 is provided between the anode 
electrode layer 3 and the cathode electrode layer 1. 
As shown in FIG. 1, two layers opposed to each other with the groove 7 laid 
therebetween are formed to have a sawtooth form. The cathode electrode 
layer 1 serves to emit electrons at the tip of the sawtooth. There are 
arranged a plurality of linear-array sawtooth portions each having a lot 
of electron emitters. The tip 1a of the cathode electrode layer 1 is made 
acute in a manner to be inclined toward the gate electrode layer 2. The 
acute tip 1a is projected from the insulating layer 5 toward the groove 7. 
Likewise, the tip 2a of the gate electrode 2 is projected from the 
insulating layer 6 toward the groove 7. 
As a material for each electrode layer, molybdenum is used. It is possible 
to use the conventional electrode materials such as chromium, tungsten, 
gold, silver, copper, aluminum. Any material may be used for the 
insulating layer if it has an insulating characteristic. 
In the field-emission electron device arranged as above, when a voltage of 
about 20 V to 100 V is applied between the cathode electrode 1 and the 
gate electrode 2, a strong electric field of about 10.sup.7 V/cm takes 
place between the tip of the cathode electrode 1 and the gate electrode 2, 
so that the cathode electrode 1 may emit electrons at its tip on the 
field-emission principle. The emitted electrons reach the anode electrode 
layer 3 to which a predetermined voltage has been applied. As such, the 
groove 7 is an electron-moving space For the electrons emitted From the 
acute tip 1a of the cathode electrode 1. The amount of electrons emitted 
from the cathode electrode 1 increases or decrease as the gate voltage 
changes. Since the change of the gate voltage appears as the change of the 
anode current, therefore, the field-emission electronic device operates as 
a triode device. 
As mentioned above, the distance between the electrodes is made to be 
changed from a known value of about 1 .mu.m to a smaller value. Hence, it 
is possible to obtain the intensity of an electric field required for 
field emission when a lower voltage is applied to the gate. Further, since 
a distance between the anode electrode and the cathode electrode, that is, 
a thickness h1 of the insulating layer 5 can be set as 0.2 to 1.0 .mu.m, 
it is possible to reduce the voltage applied to the anode and a time taken 
in moving electrons between the anode electrode and the cathode electrode. 
Moreover, in the field-emission electronic device according to this 
embodiment, as compared to the lamination of the cathode electrode and the 
gate electrode in the known structure, the overlapping area of the cathode 
electrode with the gate electrode can be reduced, resulting in making the 
capacitance between the cathode electrode and the gate electrode smaller. 
As such, the electronic device is capable of providing so large a cut-off 
frequency that it may operate at high speed. 
In turn, the description will be directed to a field-emission electronic 
device according to another embodiment of the invention as referring to 
FIGS. 3 to 11. Each structure shown in each figure corresponds to one 
embodiment. 
FIGS. 3 to 5 show a lamination composed of an insulating layer and a gate 
electrode layer, a lamination composed of an insulating layer and a 
cathode electrode layer, and a planar form of a groove spacing these 
layers from each other, respectively. FIG. 3 shows the same planar form of 
the lamination as that of the third embodiment. It has a structure where 
the mountains and the values of a sawtooth cathode electrode 11 engage 
with those of a sawtooth gate electrode 12. FIG. 4 shows the tip of the 
sawtooth cathode electrode 13 which is made more acute than that shown in 
FIG. 3. The gate electrode 14 is provided around each acute tip. In this 
structure, the electric field more effectively concentrates on the tip of 
the cathode electrode 13 through the form effect. It is therefore possible 
to reduce the gate voltage. However, the field emission takes place only 
at the tips. It results in inevitably making the field-emission area 
small. FIG. 5 shows a structure where the convexes and concaves of the 
cathode electrode are engaged with those of the gate electrode without 
using acute tips. As compared to the structures shown in FIGS. 3 and 4, 
the field concentration is disadvantageously made smaller, while the area 
for emitting electrons is advantageously made larger. 
The structure shown in FIG. 3 has an intermediate feature between the 
structure shown in FIG. 4 and that shown in FIG. 5. That is, it is 
possible to set the planar form of the cathode electrode or the gate 
electrode in a manner to suit to the requested feature. 
FIGS. 6 to 8 show other sectional forms of the tip of the cathode electrode 
layer in the groove served as an electron-moving space, respectively. The 
structure shown in FIG. 6 is the fundamental form. The tip 21a of the 
cathode electrode 21 is projected from the insulating layer 24 without 
changing the thickness of the tip 21a at the same level of the cathode 
electrode 21 on the insulating layer 24. This structure provides the tip 
of the cathode which is excellent in mechanical strength and is allowed to 
be manufactured by an easier process. The structure shown in FIG. 7 
provides the tip 31a of the cathode electrode 31 projected in a manner to 
be inclined toward the gate electrode 32. This structure is formed by 
considering the optimization of the distribution of an electric field 
around the tip of the cathode electrode 31 and the direction of electron 
emission based on the field emission. The structure shown in FIG. 8 is 
formed so that the tip 41a of the cathode electrode 41 is made acute 
toward the thickness of the cathode electrode. This structure offers at 
advantage that an electric field is concentrated around the tip 41a of the 
cathode electrode 41 through the form effect. The advantage makes it 
possible to lower the gate voltage. The structure shown in FIGS. 1 and 2 
is a combination of the structure shown in FIG. 7 and the structure shown 
in FIG. 8. 
As described above, the field-emission electronic device according to the 
present invention enables to freely take a form of an electron-emitting 
portion of the cathode electrode and orient the tip. Hence, the field 
concentration around the cathode tip can be effectively implemented, 
resulting in achieving the increase of an emitted current density based on 
the field emission. 
As shown in FIG. B, the field-emission electronic device may provide a 
conductive anode electrode substrate 53 having an integral combination of 
the substrate and the anode electrode. For the anode electrode substrate 
53, a low-resistance monocrystalline silicon substrate or a metal plate 
may be used. In a case that the anode electrode substrate 53 is made of 
monocrystalline silicon, an oxidized silicon layer formed by heat 
oxidation may be used for insulating layers 55 and 56 in light of the 
manufacturing process. The silicon dioxide layer obtained by thermally 
oxidizing monocrystalline silicon is more excellent in an insulating 
characteristic as compared to the layer formed by the vacuum evaporation, 
for example. Hence, it is suitable to the insulating layer. In addition, 
the silicon substrate is allowed to be monolithically integrated with 
another electronic component. This makes contribution to simplifying the 
manufacturing process. 
As a structure of another embodiment, as shown in FIG. 10, a beltlike 
(extending in the vertical direction on paper) anode electrode layer 83 is 
deposited on the surface of the silicon substrate 84 located on the bottom 
of a groove 67. As a structure of another embodiment, as shown in FIG. 11, 
a beltlike (extending in the vertical direction on paper) anode electrode 
layer 78 is buried in the silicon substrate 74 in a manner to expose its 
surface on the bottom of a groove 77. Herein, the substrate 74 employs a 
high-resistance monocrystalline silicon substrate such as a non-doped 
silicon substrate and the anode electrode 73 may be formed of an n-type 
low-resistance area by doping an n-type impurity such as phosphorus on the 
beltlike part of the substrate 74. The low-resistance area may be a p-type 
low-resistance area formed by doping an p-type impurity such as boron. In 
the structures shown in FIGS. 10 and 11, the area of the anode electrode 
layer occupying the substrate is made smaller. This makes it possible to 
reduce the overlapped area of the cathode electrode and the anode 
electrode (against the substrate surface) and the overlapped area of the 
gate electrode and the anode electrode. As such, it is possible to reduce 
the capacitances between the cathode electrode and the gate electrode, 
between the cathode electrode and the anode electrode and between the gate 
electrode and the anode electrode. This results in increasing a cut-off 
device f.sub.T of the device, thereby being able to operate the electronic 
device at high speed. 
turn, the description will be directed to a process for manufacturing a 
field-emission electronic device according to the first embodiment as 
referring to FIG. 12. 
The manufacturing method according to this embodiment is arranged to 
independently set each interval between the anode electrode and the gate 
electrode, between the gate electrode and the cathode electrode, or 
between the anode electrode and the gate electrode. Further, the method 
makes it possible to make the cathode electrode acute or orient the acute 
electrode in respective steps. In addition, the method needs just one 
transfer of a fine mask pattern to a resist. As such, it does not need to 
accurately position the mask pattern. 
The sections shown in FIGS. 12a to 12f show the respective manufacturing 
steps. As shown in FIG. 12a, an anode electrode metal layer 83 having a 
thickness of about 0.1 .mu.m is deposited on a substrate 84. An insulating 
layer 86a having a thickness of about 0.3 .mu.m is deposited on the layer 
83. Then, a gate electrode metal layer 82a having a thickness of about 0.1 
.mu.m is deposited on the insulating layer 86a. Further, a resist mask 88 
is formed on the layer 82a. The thickness of the insulating layer 86a 
corresponds to an interval between the anode electrode and the gate 
electrode. The electrode metal layers 83, 82a and the insulating layer 86a 
have been formed by the electron-beam evaporating technique. In place, the 
sputtering technique or the CVD technique may be used according to the 
used material. 
Next, along the mask 88, as shown in FIG. 12b, the gate electrode metal 
layer 82a is selectively etched for removal. Then, the gate electrode 
metal layer 82a is side-etched by a width shown by d81. The side-etched 
length d81 corresponds to a horizontal distance between the cathode 
electrode 81 and the gate electrode 82. Next, like the removal of the gate 
electrode metal layer 82a, the insulating layer 86a is etched for removal. 
As shown in FIG. 12c, an insulating layer 85a is formed by the vacuum 
evaporating technique using an electron beam. Herein, by moving the 
evaporating source or rotating the substrate 84 as shown by an arrow B, 
the angle of evaporating direction is relatively changed by several 
degrees (until 20). Then, the insulating layer 85a is evaporated toward 
the mask 88 in a manner to make its thickness somewhat thinner. With the 
evaporation, it is possible to set the direction of the tip of the cathode 
electrode. The thickness of the overall insulating layer 85a corresponds 
to an interval between the anode electrode and the cathode electrode. As 
shown in FIG. 12d, the cathode electrode metal layer 81 is formed by the 
electron-beam vacuum evaporating technique. By moving the evaporating 
source or rotating the substrate 84, as shown by an arrow C, the angle of 
the evaporating direction is allowed to be relatively changed from some to 
twenty degrees. The cathode electrode metal layer 81 is evaporated against 
the resist mask 88 in a manner to make the metal layer 81 more acute 
toward its thickness. 
Later, the mask 88, the insulating layer 85b deposited on the mask, and the 
cathode electrode material layer 81a deposited on the layer 85b are all 
removed. The resulting structure is as shown in FIG. 12e. Further, the 
insulating layers 85a and 86b are side-etched so that the acute tip of the 
cathode electrode 81 and the tip of the gate electrode 82 are allowed to 
be projected toward the groove 87. The resulting structure is as shown in 
FIG. 12f. This is an intended field-emission electronic device. 
With this manufacturing method, it is possible to manufacture the 
field-emission electronic device which provides a lower operating voltage 
and a high-speed operation. 
In turn, the description will be directed to a field-emission electronic 
device according to a second embodiment of the invention as referring to 
FIGS. 13 to 18. This field-emission electronic device relates to a 
field-emission electron source. 
FIG. 13 is a perspective view showing a field-emission electronic element 
according to the second embodiment of the invention. 
A substrate electrode 104 is formed of monocrystalline silicon having low 
resistance. On the substrate electrode 104, a lot of conical cold cathode 
chip 101 are formed. Each cold cathode chip is made of the same 
monocrystalline silicon as the substrate electrode 104 or a high melting 
point metal such as tungsten (W) or molybdenum (Mo). An insulating layer 
105 is formed on the substrate electrode 104 around the cold cathode chips 
101 On the insulating layer 105, a gate electrode 102 is deposited. An 
anode electrode 103 is provided to cover those cold cathode chips 101 and 
the gate electrode 102 with keeping a space surrounded by at least the 
anode electrode 103, the cold cathode chips 101 and the gate electrode 102 
to be vacuum. 
FIG. 14 is a sectional view showing an essential side portion of a 
field-emission cold cathode chip included in the second embodiment of FIG. 
13. FIG. 15 is a sectional view cut on the line XV--XV of FIG. 13 showing 
an essential portion of the field-emission electron source and dimensions 
of components included in the field-emission electron source. 
As shown in FIG. 14, the cold cathode 101 is composed of three kinds of 
layers 101a, 101b and 101c. A titanium layer 101a is deposited on a 
silicon substrate electrode 104. On the titanium layer 101a, a titanium 
and titanium carbide layer 101b having a plurality of layers is deposited 
on the titanium layer 101a. The layer 101b are made of titanium and 
titanium carbide and the mixing ratio of the materials at the upper 
portion of the layer is larger than the lower portion. That is, the upper 
layer of the titanium and titanium carbide layer 101b has a larger 
composition ratio of carbon than the lower layer. On the top of the 
titanium and titanium carbide layer 101b, a titanium carbide layer 101c is 
deposited. 
As shown in FIG. 15, on the substrate electrode 104 around the 
field-emission cold cathode 101, an insulating layer 105 is formed in a 
manner to surround the field-emission cold cathode. On the insulating 
layer 104, there is laminated a gate electrode 102. Above the gate 
electrode layer 102, an anode electrode 109 is disposed to cover the cold 
cathode 101 and the gate electrode 102. A space surrounded by the 
field-emission cold cathode 101, the gate electrode 102, the insulating 
layer 105, the substrate 104 and the anode electrode 103 is kept to be 
vacuum. 
The conical field-emission cold cathode 101 is formed in such a manner that 
its bottom diameter d is d=about 0.8 .mu.m and its height h is h=about 1 
.mu.m. The substrate electrode 104 has a thickness t1 of about 0.75 mm. 
The insulating layer 105 has a thickness t2 of about 0.75 .mu.m. The gate 
electrode 102 has a thickness t3 of about 0.5 .mu.m. The distance l 
between the anode electrode 103 and the substrate electrode 104 is l=about 
10 .mu.m. 
In the field-emission electron source according to the second embodiment 
mentioned above, the field-emission cold cathode is made of metallic 
carbide having a small work function. Hence, it operates on a smaller 
operating voltage as described later. The field-emission cold cathode 101 
has a lamination structure having the titanium carbide layer 101c, the 
titanium and titanium carbide layer 101b and the titanium layer 101a as 
shown in FIG. 14. As such, the difference of a thermal expansion 
coefficient between the titanium layer 101a and the substrate 104 is 
small. Likewise, the differences of the thermal expansion coefficient 
between the titanium carbide layer 101c and the titanium and titanium 
carbide layer 101b and between the titanium layer 101a and the titanium 
and titanium carbide layer 101b and between the titanium layers 101a are 
also small. It means that the lamination structure is sufficiently 
thermally stable and mechanically endurable. Moreover, since the electric 
resistance from the titanium layer 11a to the titanium carbide layer 101c 
located at the tip is continuously changed, the electric conductivity 
inside of the cold cathode is improved. 
Next, the description will be directed to a process for manufacturing the 
field-emission electron source according to the second embodiment of the 
invention as referring to FIGS. 16 and 17. 
FIG. 16 is a sectional side view showing a process for manufacturing the 
field-emission electron source shown in FIG. 15. FIG. 17 is an explanatory 
view showing a method for manufacturing the field-emission cold cathode in 
detail. 
At first, the top surface of the silicon substrate electrode 104 is subject 
to the thermal oxidation of about 1100.degree. C. The silicon substrate 
electrode 104 is conductive (0.01 .OMEGA..cm) and has a thickness of about 
0.75 mm. After the thermal oxidation, the insulating layer 105 made of 
silicon dioxide (SiO.sub.2) is formed to have a thickness of about 0.75 
.mu.m. Then, on the insulating layer 105, a layer corresponding to the 
gate electrode 102 is formed by the electron-beam evaporation or 
sputtering. The layer is made of molybdenum and has a thickness of about 
0.5 .mu.m. Next, a resist (not shown) is spin-coated on the gate electrode 
102 in a manner to have a thickness of about 1 .mu.m. Then, a spot pattern 
having a diameter of about 1 .mu.m is exposed by an electron beam. The 
exposed resist is developed by isopropyl alcohol so that a spot opening 
may be formed on the molybdenum layer. The spot opening has a diameter of 
about 1 .mu.m. Next, the molybdenum layer and the insulating layer under 
the spot opening are selectively etched so that a circular opening 106 
having a diameter of about 2 .mu.m may be formed on the substrate 
electrode 104. Next, after the resist is removed by an organic solvent, 
the etching is carried out by using hydrofluoric acid. Then, the layer 
made of molybdenum, which will become the gate electrode 102, is undercut 
so as to form the structure shown in FIG. 16a. In this embodiment, the 
molybdenum has been used for making the gate electrode 102. However, any 
metal may be used if it has the substantially same performance. Likewise, 
the silicon oxide has been used for making the insulating layer 105. 
However, any material may be used if it has the substantially same 
performance. 
Next, the structure shown in FIG. 16a is installed in a vacuum evaporating 
unit, in which the silicon substrate electrode 104 is rotated on the shaft 
of the circular opening 106. From an upper oblique location shown by an 
arrow A of FIG. 16b, aluminum is deposited on the gate electrode 102 in a 
manner that the diameter of the circular opening 106 may progressively 
become smaller from the lower to the upper. The resulting structure is 
that shown in FIG. 16b. 
Then, a material for the field-emission cold cathode is deposited on the 
substrate electrode 104 through the circular opening 106 by an electron 
beam evaporation so that the field-emission cold cathode 101 may be formed 
on the silicon substrate electrode 104. When the material is evaporated by 
an electron beam from the direction shown by an arrow B through the 
circular opening 106 as shown in FIG. 16c, a deposit layer 101a of the 
material is formed in a manner to gradually decrease the diameter of the 
circular opening 106 and finally close the circular opening 106. This is 
the conical field-emission cold cathode 101. By removing the aluminum 
layer 107 and the deposit layer 101a, the structure shown in FIG. 16d is 
formed. In this embodiment, about 5000 field-emission cold cathodes 101 
are formed with a spacing of about 10 .mu.m between the adjacent ones. 
In this embodiment, as shown in FIG. 17, for forming the field-emission 
cold cathode 101, a two-source evaporation is used. The evaporation has a 
metal evaporating source 120 of titanium (Ti) and a metallic carbide 
evaporating source 121 of titanium carbide (TiC). At first, with the metal 
evaporating source 120 only, the titanium layer is evaporated. Then, by 
adjusting an evaporating rate of the two evaporating sources 120 and 121, 
the titanium and titanium carbide layer is formed in a manner to 
continuously keep the carbon ratio higher from the bottom to the tip. 
Finally, with the metallic carbide evaporating source 121 only, the 
titanium carbide layer is formed on the top of the field-emission cold 
cathode 101. In place of the titanium evaporating source 120, it is 
possible to use a metal evaporating source for zirconium (Zr), molybdenum 
(Mo) and hafnium (Hf). In place of the metallic carbide evaporating source 
of titanium carbide (TiC), it is possible to use a metal evaporating 
source for metallic nitride, metallic oxide or metallic boride. 
FIG. 18 is a graph showing a relation between an operating voltage and a 
discharged current density, that is, emission current caused by field 
emission per one pixel in the field-emission electron source of this 
embodiment and the known field-emission electron source. Herein, the 
operating voltage is a voltage to be applied between the anode electrode 
and the substrate electrode. 
The curves indicated by symbols A1 to A3 represent the relations between 
the discharged current density and the operating voltage in this 
embodiment, and the curve of symbol A4 represents the relation of a 
conventional field-emission type electron source. The curves A1 to A4 
correspond to respective materials of the field-emission cold cathode, 
that is, zirconium carbide, titanium carbide, titanium nitride, and 
molybdenum. The curve indicated by a symbol A4 represents the relation of 
the known field-emission electron source. 
The relation indicated in FIG. 18 is obtained by applying a positive 
voltage V2 of 50 V between the substrate electrode 104 and the gate 
electrode 102 based on the voltage of the substrate electrode 104 and 
measuring the discharged current as changing the applied voltage V1 
(operating voltage) between the anode electrode 103 and the substrate 
electrode 104 (see FIG. 15). As is obvious from this figure, in the 
relation A4 of the conventional device, the threshold value of the 
operating voltage is about 300 V. On the other hand, in the relations A1 
to A3 of this embodiment, the threshold values are about 100 V to 150 V. 
The great reduction of the operating voltage results from the reduction of 
the work function of the field-emission cold cathode. 
In turn, the description will be directed to a field-emission electronic 
device according to a third embodiment of the invention as referring to 
FIGS. 19 and 20. 
FIG. 19 is a side sectional view showing an essential portion of the 
field-emission cold cathode 131 included in a field-emission electron 
source according to the third embodiment. FIG. 20 is a side sectional view 
showing a process for manufacturing the field-emission electron source. 
A silicon substrate electrode 130, as shown in FIG. 19, comprises a pyramid 
convex portion 130a. Though one convex portion 130a is shown in FIG. 19, 
in actual, the substrate 130 includes a number of convex portions 130a 
formed on the same surface thereof. On each convex portion 130a, a pyramid 
field-emission cold cathode 131 is formed. The form of the convex portion 
130a and the field-emission cold cathode 131 is not limited to a pyramid. 
It may be a conical or an edge-sawtooth form to be discussed with respect 
to a third embodiment. This edge-sawtooth form implements a larger surface 
area. 
The bottom of each field-emission cold cathode 131 is formed of a titanium 
layer 131a. On the titanium layer 131a, a titanium and titanium carbide 
layer 31b is formed in a manner to progressively increase a composition 
ratio of carbon. On the layer 131b, a titanium carbide layer 131c is 
formed. 
Next, the process for manufacturing a field-emission electron source shown 
in FIG. 19 will be described as referring to FIG. 20. At first, a 
conductive (0.01 .OMEGA..cm) silicon substrate electrode 130 is prepared. 
It needs to have a thickness of about 0.4 mm is prepared. The silicon 
substrate electrode is subject to heat oxidation at the temperature of 
about 1100.degree. C. so as to form silicon dioxide (SiO.sub.2) having a 
thickness of about 0.2 .mu.m. Next, a resist of about 1 .mu.m is coated on 
this layer. The resist is exposed by ultraviolet rays and is developed for 
forming a resist mask (not shown). The silicon dioxide layer is etched by 
a mixed liquid of hydrofluoric acid and ammonium fluoride so as to form a 
mask 132 of silicon oxide. Then, the resist is removed by an organic 
solvent. The resulting structure is shown in FIG. 20a. 
Next, the structure shown in FIG. 20a is etched by an etchant of a mixed 
liquid of hydrofluoric acid, nitric acid and acetic acid. The etching 
results in eroding the silicon substrate electrode 130 so that pyramid 
convex portions 130a are formed as shown in FIG. 20b. Next, the silicon 
oxide mask 132 is removed by a mixed liquid of hydrofluoric acid and 
ammonium fluoride. As shown in FIG. 20c, the pyramid convex portions 130a 
are left as a mother body of the field-emission cold cathode 131. This 
kind of pyramid convex portion 130a may be formed by anisotropic etching 
with an alkali mixed liquid containing potassium hydroxide and isopropyl 
alcohol or dry etching such as RIE (Reactive Ion Etching). 
A material of the field-emission cold cathode 131 is coated on the convex 
portion 130a by means of the sputtering. The layers composing the 
filed-emission cold cathode 131 are formed as shown in FIG. 20d. For a 
sputter target, some metal such as titanium is used. For a reactive gas, a 
mixed gas containing argon (Ar) and methane (CH.sub.4) is used. By the 
reactive sputtering, a thin film made of the titanium carbide is formed. 
By controlling a mixing ratio of the reactive gas, as shown in FIG. 19, at 
first, the titanium layer 131a is evaporated. Then, the titanium and 
titanium carbide layer 131b is formed in a manner to progressively 
increase a composition ratio of carbon from the bottom to the tip. At the 
top, the titanium carbide layer 131c is formed. For a sputter target, in 
place of titanium (Ti), zirconium (Zr), molybdenum (Mo) or hafnium (Hf). 
As a reactive gas, nitrogen or ammonium is used for nitride and oxygen is 
used for oxide. 
Next, as shown in FIG. 20e, an insulating layer 133 and a gate electrode 
134 are formed on the silicon substrate electrode 130 around the pyramid 
field-emission cold cathode 131. Then, after an anode electrode is formed 
(not shown), the process for manufacturing the field-emission electron 
source is terminated. 
The field-emission electron source according to the third embodiment is 
thus capable of reducing the operating voltage like the second embodiment. 
In manufacturing the field-emission cold cathode according to the second 
and third embodiments, it is possible to use a vapor growth method such as 
CVD or MOCVD or another filming method. The form of the field-emission 
cold cathode is not limited to the pyramid form. In actual, several forms 
can be realized by selecting the method. For example, with the vapor 
growth method, to form the titanium carbide layer, it is possible to 
employ a method for reacting titanium tetrachloride with methane. 
In turn, the description will be directed to a field-emission electronic 
device according to a fourth embodiment of the invention as referring to 
FIGS. 21 to 24. The electronic device also relates to a field-emission 
electron source. The electron source according to this embodiment has a 
cold cathode Formed unlike the pyramid. FIG. 21 is a plane view showing 
the Field-emission electron source according to the fourth embodiment. The 
field-emission electron source is constructed to have a sawtooth-edge cold 
cathode emitter 141 and a linear-edge gate 144 on a crystalline substrate 
149 in a manner to oppose the cold cathode emitter 141 to the gate 144. 
The cold cathode emitter 141 serves to emit electrons at its tip 142. FIG. 
22 is an expanded plane section showing a horizontal section of the 
field-emission cold cathode, in particular, an A portion. Toward the 
bottom of the emitter, there are formed a titanium (Ti) layer 141a and a 
mix layer 141b of a titanium and titanium nitride is formed on the 
titanium layer 141a. The mix layer 141b is composed of a plurality of 
layers having a different mixing ratio of a titanium and titanium nitride. 
The layer 141b is formed in a manner to increase a nitrogen density as it 
comes closer to the tip. A titanium nitride (TiN) layer 141c is formed on 
the surface of the mix layer 141b. 
FIG. 23 is a sectional view showing a vertical section of the 
field-emission cold cathode, in particular, an essential portion cut on 
the line XXIII--XXIII of FIG. 22. The structure formed in a manner to 
increase a nitrogen density as it comes closer to the side surface. 
In the sawtooth field-emission electron source of the third embodiment, in 
FIG. 21, a distance S1 between the tip 142 of the cold cathode emitter 141 
and the edge 145 of the gate 144 is 1 .mu.m. A distance S2 between the 
adjacent tips 142 of the cold cathode emitters is 5 .mu.m. A distance S3 
between the tip 142 and a mother body 143 of the cold cathode emitter is 5 
.mu.m. In FIG. 23, a thickness S4 of the emitter 141 is 0.5 m. 
Next, the process for manufacturing the field-emission electron source will 
be described as referring to FIG. 24, which is a vertical sectional view 
showing an essential portion cut on the line XXIV--XXIV of FIG. 21. 
At first, photo-etching is performed on the crystalline (SiO.sub.2) 
substrate 149 so as to pattern the substrate 149 to have a convex portion 
148 of a sawtooth, which will become a ground of the cold cathode emitter. 
The resulting structure is shown in FIG. 24a. 
Next, a titanium layer 140 is formed on the convex portion 148 by the 
sputtering. The layer 140 will become the cold cathode emitter. Further, 
the crystalline substrate is side-etched by a mixed liquid of hydrofluoric 
acid and ammonium Fluoride (BHF). The resulting structure is shown in FIG. 
24b. Next, with a mixture gas of argon (Ar) and ammonium (NH.sub.3), a 
titanium nitride (TiN) layer is formed on the surface of the thin film 
cold cathode 141. In the formation, as the flow rate of an argon gas to 
ammonium is being controlled, the mixture ratio of the gas is continuously 
changed in a manner to gradually increase a ratio of ammonium to the argon 
gas when the nitride reaction of the material for the cold cathode is 
carried out at a high temperature of about 500.degree. to 900.degree. C. 
The resulting structure is shown in FIG. 24c. In this case, as a sputter 
target, in place of titanium (Ti), zirconium (Zr) or molybdenum (Mo) may 
be used. As a reactive gas, nitrogen (N.sub.2) may be used in place of 
ammonium. 
By performing the photo-etching and evaporating the gate metal, the gate 
144 is formed. The resulting structure is shown in FIG. 24d. This is an 
end of the process for manufacturing the field-emission electron source. 
Many widely different embodiments of the present invention may be 
constructed without departing from the spirit and scope of the present 
invention. It should be understood that the present invention is not 
limited to the specific embodiments described in the specification, except 
as defined in the appended claims.