Vacuum envelope having niobium oxide gate electrode structure

A vacuum envelope with a built-in electron source capable of preventing peeling of gate electrodes, to thereby enhance reliability in operation over a long period of time and reducing a manufacturing cost thereof. The gate electrodes each are made of niobium oxide or niobium nitride. Nb for the gate electrode is previously oxidized or nitrided to prevent progress of oxidation of the gate electrode due to release of oxygen from lead oxide contained in a seal material during heating for sealing, resulting in expansion of the gate electrode. An insulating layer is formed on a cathode substrate and the gate electrodes are formed on the insulating layer. Then, the seal material is applied onto the insulating layer so as to cover a part of each of the gate electrodes, so that the cathode substrate may be sealedly joined to an anode substrate.

BACKGROUND OF THE INVENTION 
This invention relates to a vacuum envelope having an electron source 
incorporated therein, and more particularly to a vacuum envelope with a 
built-in electron source which is suitable for use for a field emission 
device (hereinafter also referred to as "FED") and a method for 
manufacturing the same. 
Recently, vacuum microelectronics provided by semiconductor fine-processing 
techniques have come to notice in the art, which are constructed in such a 
manner that a cold cathode is incorporated in a vacuum envelope made of 
glass or the like and vacuum microstructures of a size as small as microns 
are integrated. The vacuum microelectronics have been applied to research 
and development of a vacuum envelope with a built-in electron source for 
an active element, various sensors for detecting magnetism or the like, an 
image pickup device, an electron beam unit for lithography, a thin-type 
flat panel display unit or the like. 
The thin-type flat panel display unit is constructed so as to arrange a 
plurality of fine cold cathodes for every picture cell. The fine cold 
cathodes which have been proposed in the past include those constructed of 
a field emission cathode, an MIM type electron emission element, a surface 
conduction type electron emission element, a PN junction type electron 
emission element and the like. Of the fine cold cathodes proposed, the 
most typical one is an FED including field emission cathodes, as disclosed 
in Nikkei Electronics, No. 654 (Jan. 29, 1996), pp 89-98. Field emission 
is a phenomenon that when an electric field set to be about 10.sup.9 V/m 
is applied to a surface of a metal material or that of a semiconductor 
material, a tunnel effect occurs to permit electrons to pass through a 
barrier, resulting in the electrons being discharged to a vacuum even at a 
normal temperature. Such electron field cathodes include an electron 
emission cathode of the Spindt type by way of example. 
The MIM type electron emission element is constructed of a lamination 
structure wherein a metal layer, a thin insulating layer and a thin metal 
layer are laminated on each other in order. The MIM type electron emission 
element thus constructed is operated in a manner to apply a voltage 
between both metal layers to cause electrons to be emitted from the thin 
metal layer. The electron emission element of the surface conduction type 
is so constructed that two electrodes and a high-resistance thin film 
layer are formed on an insulating layer, wherein a voltage is applied 
between both electrodes to cause electrons to be emitted from the 
high-resistance thin film layer arranged between both electrodes. The PN 
junction type electron emission element is constructed so as to utilize 
avalanche breakdown. Alternatively, it may be constructed so as to apply a 
voltage to a PN junction in a forward direction to cause electrons 
injected into a P-layer to be emitted from a surface of the P-layer. 
Referring now to FIG. 6, a basic structure of the Spindt type FED is 
illustrated. In FIG. 6, reference numeral 1 designates a cathode 
substrate, 2 is an insulating layer, 51 is cathode electrodes, 52 is gate 
electrodes, 53 is apertures, 54 is an anode substrate, 55 is an anode 
electrode, A is an anode lead-out wiring, C1 to Cn are cathode lead-out 
wirings, and G1 to Gm are gate lead-out wirings. 
The cathode electrodes 51 are arranged in a stripe-like manner on the 
cathode substrate 1 and then the insulating layer 2 is deposited all over 
the cathode substrate 1 including the cathode electrodes 51. Then, the 
gate electrodes 52 are formed in a stripe-like manner on the insulating 
layer 2 while extending in a direction perpendicular to the cathode 
electrodes 51 and in parallel to each other. A plurality of the apertures 
53 are formed at each of intersections between the cathode electrodes 51 
and the gate electrodes 52 in a manner to commonly pass through the gate 
electrode 52 and the insulating layer 2 below the gate electrode 52. The 
apertures 53 each are formed therein with an emitter 57 of a conical shape 
while being arranged on the cathode electrode 51, as described hereinafter 
with reference to FIG. 8. A resistive layer may be often formed between 
the cathode electrodes 51 and the insulating layer 2. 
The anode electrode 55 is arranged on a lower surface of the anode 
substrate 54 made of a transparent glass material or the like. The anode 
electrode 55 is formed on a lower surface thereof with a phosphor layer 
(not shown). The Spindt type FED also includes a drive circuit (not shown) 
which functions to apply an anode voltage through the anode lead-out 
wiring A to the anode electrode 55, feed an image signal through the 
cathode lead-out wirings C1 to Cn to the cathode electrodes 51 and feed a 
drive signal through the gate lead-out wirings G1 to Gm to the gate 
electrodes 52. 
In the Spindt type FED thus constructed, the gate electrodes 52 are scanned 
in order and the cathode electrodes 51 are fed with an image signal while 
keeping an anode voltage applied to the anode electrode 55, to thereby 
permit the emitters arranged in the apertures 53 to emit electrons, which 
are impinged on the phosphor arranged on the anode electrode 55, resulting 
in luminescence of the phosphor. 
FIG. 7 is a schematic plan view of the Spindt type FED. In FIG. 7, 
reference numeral 4 designates a seal material and 56 is insulating studs. 
A plurality of such insulating studs 56 are vertically arranged on the 
insulating layer 2 to hold the cathode substrate 1 and anode substrate 54 
spaced from each other at a predetermined interval therebetween while 
ensuring that both substrates withstand an atmospheric pressure applied 
thereto, with the insulating studs 56 being interposed therebetween. Then, 
the seal material 4 of a low melting point such as seal glass (frit glass) 
or the like which is arranged between both substrates 1 and 54 is heated 
to sealedly join both substrates 1 and 54 to each other, to thereby 
provide an envelope, which is then evacuated to a high vacuum. 
The cathode substrate 1 and anode substrate 54 are superposed on each other 
while being deviated from each other in an oblique direction and while 
being spaced from each other at a predetermined interval and are 
hermetically joined to each other by means of the seal material 4. In FIG. 
7, the seal material 4 is arranged somewhat inside an outer contour of a 
superposed area between both substrates 1 and 54 and in a predetermined 
width. Actually, the seal material 4 is arranged so as to extend to the 
contour or a vicinity to the contour. 
A region in the thus-formed envelope on which the anode electrode 55 is 
arranged functions as an image display region. In a left-side region of 
the cathode substrate 1 positioned outside the seal material 4 in FIG. 7, 
terminal sections of the cathode electrodes 51 are led out to form the 
cathode lead-out wirings C. Likewise, the gate electrodes 52 have terminal 
sections led out in an upper region of the cathode substrate 1 positioned 
outside the seal member 4 in FIG. 7, resulting in the gate lead-out 
wirings G. Further, in a right-side region of the anode substrate 54 
positioned outside the seal material 4, the anode lead-out wirings A are 
formed so as to extend from the anode electrode 55. The cathode substrate 
1 and anode substrate 54 are arranged so as to be spaced from each other 
at a reduced interval while being opposite to each other, so that it is 
physically substantially impossible to carry out both connection of the 
cathode substrate 1 to the drive circuit and that of the anode substrate 
54 thereto at the same position. Thus, the respective lead-out wirings are 
formed so as to extend in directions different from each other as 
described above. 
Expansion of the above-described monochrome FED permits a color FED of the 
primary colors to be realized, although it is not shown in FIG. 7 for the 
sake of brevity. More specifically, in this instance, a plurality of such 
anode electrodes 55 are arranged in a stripe-like manner in correspondence 
to luminous colors of phosphors for the primary colors and connected to a 
plurality of anode lead-out wirings different from each other, 
respectively. 
Such a conventional Spindt type FED as described above may be constructed 
in such a manner as shown in FIG. 8 by way of example, which corresponds 
to a fragmentary sectional view taken along one of the gate electrodes in 
FIG. 7. In FIG. 8, reference numeral 57 designates emitters. The cathode 
substrate 1 made of glass or the like is provided thereon with the cathode 
electrodes 51, which are made of metal and arranged so as to extend in a 
direction perpendicular to the sheet of FIG. 8. Then, the insulating layer 
2 formed of a silicon dioxide (SiO.sub.2) film or the like is deposited 
all over the cathode substrate 1 to cover the cathode electrodes 51. The 
insulating layer 2 is formed into a thickness of about 1 .mu.m. 
Then, the gate electrodes 52 are formed into a thickness of about 0.2 .mu.m 
on the insulating layer 2 so as to extend in a direction perpendicular to 
the cathode electrodes 51. The apertures 53 formed so as to commonly 
extend through the gate electrode 52 and insulating layer 2 each have the 
emitter 57 of a conical shape arranged therein. The emitters 57 each are 
made of metal such as molybdenum or the like and formed on the cathode 
electrode 51. The emitters 57 each are exposed at a distal end thereof 
through the aperture 53 while being directed toward the anode electrode 
55. 
The emitters 57 are arranged at pitches of 10 .mu.m or less, so that tens 
of thousands to hundreds of thousands of such emitters may be arranged on 
one such cathode substrate 1. Also, the emitters 57 may be so arranged 
that a distance between the gate electrode 52 and the distal end of the 
emitter 57 is set to be as small as less than a micron, so that 
application of a voltage as low as tens of volts between the gate 
electrodes 52 and the emitters 57 permits electrons to be field-emitted 
from the emitters 57. Thus, in the conventional Spindt type FED, the 
cathode electrodes 51, emitters 57 and gate electrodes 52 cooperate with 
each other to provide an electrode source. Thus, when a positive voltage 
is kept applied to the anode electrode 55 shown in FIG. 6, the anode 
electrode 55 captures electrons emitted from the emitters 57, so that the 
phosphor provided on the anode electrode 55 emits light. 
As described above with reference to FIG. 6, the cathode substrate 1 and 
anode substrate 54 are so arranged that an interval therebetween is kept 
at, for example, 0.2 mm, resulting in the envelope being provided and the 
insulating studs 56 and seal material 4 are arranged between the 
substrates 1 and 54 to form a high vacuum in the envelope. The gate 
electrodes 52 are required to be arranged both inside and outside the 
sealed portion of the envelope defined by the seal material 4, so that the 
seal material 4 is caused to be contacted with the gate electrodes 52. 
FIGS. 9(A) and 9(B) each are a sectional view taken along line A--A of FIG. 
8 showing a construction employed for solving the above-described problem 
of the FED shown in FIG. 8; wherein FIG. 9(A) shows the gate electrode in 
an ideal state and FIG. 9(B) shows it after a heat treatment. 
In FIGS. 9(A) and 9(B), the gate electrodes 52 are conventionally made of 
niobium (Nb). In general, Nb is increased in adhesion to glass or the 
like, to thereby be readily used as compared with tungsten, so that a 
pattern of the gate electrodes 52 may be formed by dry etching. In this 
connection, the gate electrodes made of Nb should be inherently kept 
adhered onto the insulating layer 2 even after the heat treatment; 
however, heating of the gate electrodes 52 to a temperature of about 
500.degree. C. causes the gate electrodes of the lead-out sections to be 
oxidized by the frit glass seal material used for hermetically sealing the 
envelope as shown in FIG. 9(B). This causes the gate electrodes 52 to be 
peeled from the insulating layer 2, resulting in the seal material 4 
entering therebetween, leading to formation of a gap 58. The gap 58 thus 
formed causes a slow leak phenomenon which causes a vacuum in the envelope 
to be gradually reduced over a long period of time. Also, the oxidation 
leads to an increase in resistance of the gate electrodes 52 or breakage 
thereof, leading to a failure in conduction thereof. 
FIG. 10 shows another manner in which the conventional Spindt type FED may 
be constructed. In the example of FIG. 10, the gate electrodes 52 each are 
formed on a portion thereof contacted with the seal material 4 with a 
protective film 61 for protecting the gate electrode 52. The protective 
film 61 is formed of silicon dioxide (SiO.sub.2) into a thickness of about 
1 to 2 .mu.m. Such arrangement of the protective film 61 effectively 
prevents peeling of the gate electrode 52 from the insulating layer 2. 
Unfortunately, pattern-formation of the protective film 61 on only the 
portion of the gate electrode 52 on which the seal material 4 is formed 
leads to an increase in the number of steps in manufacturing of the FED 
and complication of a manufacturing process thereof. More particularly, 
steps for formation of the protective film 61 and for patterning thereof 
are additionally required. 
SUMMARY OF THE INVENTION 
Accordingly, it is an object of the present invention to provide a vacuum 
envelope with a built-in electron source which is capable of exhibiting 
increased reliability in operation over a long period of time. 
It is another object of the present invention to provide a vacuum envelope 
with a built-in electron source which is capable of facilitating 
manufacturing thereof. 
It is a further object of the present invention to provide a vacuum 
envelope with a built-in electron source which is capable of reducing a 
manufacturing cost thereof. 
In accordance with one aspect of the present invention, a vacuum envelope 
with a built-in electron source is provided. The vacuum envelope includes 
a cathode substrate formed thereon with an electron source and sealedly 
joined to another substrate by welding by means of a seal material to form 
a sealed space between the substrates. The electron source includes gate 
electrodes which are led out through a welded portion between the 
substrates. The sealed space is evacuated to a vacuum atmosphere. The gate 
electrodes each have at least a portion which passes through the welded 
portion made of one of niobium oxide and niobium nitride. 
In accordance with this aspect of the present invention, a vacuum envelope 
with a built-in electron source includes a cathode substrate formed 
thereon with an electron source and sealedly joined to another substrate 
by welding by means of a seal material to form a sealed space between the 
substrates. The electron source includes gate electrodes, which are led 
out through a welded portion between the substrates. The sealed space is 
evacuated to a vacuum atmosphere. The gate electrodes each have at least a 
surface which is contacted with the seal material made of one of niobium 
oxide and niobium nitride. 
In accordance with another aspect of the present invention, a method for 
manufacturing a vacuum envelope with a built-in electron source is 
provided. The method comprises the steps of sealedly joining a cathode 
substrate formed thereon with an electron source to another substrate by 
welding by means of a seal material to form a sealed space between the 
substrates, leading out gate electrodes of said electron source through a 
welded portion between the substrates, and evacuating the sealed space to 
a vacuum atmosphere. The gate electrodes each are made by forming a 
niobium film, subjecting the niobium film to patterning and subjecting at 
least a surface of at least a portion of the niobium film contacted with 
the seal material to an oxidizing or nitriding treatment. 
In accordance with this aspect of the present invention, a method for 
manufacturing a vacuum envelope with a built-in electron source is also 
provided. The method comprises the steps of sealedly joining a cathode 
substrate formed thereon with an electron source to another substrate by 
welding by means of a seal material to form a sealed space between the 
substrates, leading out gate electrodes of the electron source through a 
welded portion between the substrates, and evacuating the sealed space to 
a vacuum atmosphere. The gate electrodes each are made by forming a 
niobium film, subjecting at least a surface of at least a portion of the 
niobium film contacted with the seal material to an oxidizing or nitriding 
treatment and subjecting the niobium film to patterning.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Now, a vacuum envelope with a built-in electron source according to the 
present invention will be described hereinafter with reference to FIGS. 1 
to 5. 
Referring first to FIG. 1, a first embodiment of a vacuum envelope with a 
built-in electron source according to the present invention is 
illustrated. FIG. 1 is a sectional view similar to FIGS. 9(A) and 9(B) 
described above, wherein reference numeral 3 designates gate electrodes. 
The inventors made an analysis of the peeled Nb film shown in FIG. 9(B) and 
as a result, it was found that peeling of the gate electrode 52 shown in 
FIG. 9(B) is essentially caused due to cubic expansion of the gate 
electrode 52 by oxidation of Nb rather than a difference in thermal 
expansion between the gate electrode 52 and the seal material 4. The seal 
material 4 contains in addition to silicon dioxide (SiO.sub.2), lead oxide 
and lead titanate (PbTiO.sub.2) in order to reduce a melting point thereof 
or adjust a thermal expansion coefficient thereof. 
In FIG. 9(B), it would be considered that when a heat treatment is carried 
out in order to join the cathode substrate and anode substrate to each 
other, oxygen contained in the lead oxide moves toward the gate electrode 
52 on an interface of contact between the cathode substrate and the anode 
substrate, to thereby bond to Nb of the gate electrode to form niobium 
oxide (Nb--O), which is subject to cubic expansion in the heat treatment, 
resulting in the gap 58 being formed. A seal material decreased in content 
of lead oxide is commercially available. However, it is increased in 
melting point, resulting in being unsuitable for use for the FED. Also, 
when an additive like lead oxide which releases oxygen when it is subject 
to a heat treatment is contained in the seal material, a like disadvantage 
is caused. 
The vacuum envelope of the illustrated embodiment is so constructed that 
the gate electrodes 3 each are made of niobium oxide (Nb--O) or niobium 
nitride (NbN), unlike the first example of the conventional vacuum 
envelope described above with reference to FIG. 8. Nb contained in a 
material for the gate electrode 3 is previously oxidized or nitrided, to 
thereby prevent progress of oxidation of the material due to oxygen 
released from lead oxide or the like contained in a seal material 4 during 
heating of the seal material for formation of an envelope, to thereby 
eliminate expansion of the gate electrode 3 by oxidation of Nb, resulting 
in preventing peeling of the gate electrode 3 and a failure in conduction 
thereof. The gate electrodes 3 may be formed into a thickness of 0.2 to 
0.4 .mu.m. An insulating layer 2 is formed on a cathode substrate 1 and 
then the seal material 4 is arranged on the insulating layer 2 including a 
portion thereof on which the gate electrodes 3 are provided. The seal 
member 4 permits an anode substrate (not shown) formed in substantially 
the same manner as the anode substrate 54 shown in FIGS. 6 and 7 to be 
sealedly joined to the cathode substrate 1. 
Niobium nitride exhibits increased resistance to chemicals as compared with 
niobium oxide. Resistance of the niobium nitride to chemicals is equal to 
or above that of niobium. Although niobium oxide and niobium nitride are 
increased in resistivity as compared with niobium, increased resistivity 
of the niobium oxide or niobium nitride does not adversely affect use of 
niobium nitride for the gate electrode 3 because it is not required to 
flow a large amount of current through the gate electrode 3. 
The remaining part of the illustrated embodiment may be constructed in 
substantially the same manner as the Spindt type FED described above with 
reference to FIGS. 6 and 7 and has substantially the same sectional 
construction taken along one gate electrode as the first example of the 
prior art shown in FIG. 8. More specifically, the cathode substrate 1 on 
which an electron source including cathode electrodes formed with emitters 
of a conical shape, the gate electrodes 3 and the like is arranged is 
joined to the anode substrate including an anode electrode and a phosphor 
by welding using the seal material 4 while being kept spaced from the 
anode substrate at a predetermined interval. Also, the gate electrodes 3 
are led out of a welded portion between the cathode substrate 1 and the 
anode substrate and the envelope is evacuated to a high vacuum, resulting 
in a vacuum envelope which has the electron source incorporated therein 
being provided. 
In the illustrated embodiment, it is not required to form the whole gate 
electrode of a non-oxidizable material such as niobium oxide, niobium 
nitride or the like. It is merely required that at least a portion of the 
gate electrode extending through the welded portion between both 
substrates is made of the non-oxidizable material. 
Referring now to FIG. 2, manufacturing of the vacuum envelope of the 
illustrated embodiment is illustrated by way of example. Reference numeral 
11 designates Nb films. The gate electrodes 3 shown in FIG. 1 may be 
manufactured by plasma ashing. The Nb films 11 are formed on the 
insulating film 2, followed by patterning. Then, when the niobium oxide 
film is to be formed, oxygen is activated in plasma, resulting in O.sub.2 
ashing for forced oxidation being carried out. When niobium nitride film 
is to be formed, N.sub.2 ashing is likewise carried out. 
Referring now to FIG. 3, a second example of manufacturing of the vacuum 
envelope of the illustrated embodiment is illustrated by way of example. 
In FIG. 3, reference numeral 21 designates a niobium oxide film or niobium 
nitride film. In the example, the gate electrode 3 shown in FIG. 1 may be 
formed by reactive sputtering. Formation of the niobium oxide film on the 
insulating layer 2 is carried out by reactive sputtering using O.sub.2 and 
likewise that of the niobium nitride film thereon is carried out by 
reactive sputtering using N.sub.2. Then, patterning is carried out, 
resulting in the gate electrodes 3 shown in FIG. 1 being formed. 
Such formation of the niobium oxide film or niobium nitride may be carried 
out by any other suitable means such as, for example, reactive deposition. 
Alternatively, the oxidation or nitriding may be carried out by reactive 
ion etching (RIE). In general, etching is essentially used for removal of 
a material. However, when etching is used under reactive conditions while 
reducing an output of a unit for etching. Also, the oxidation or nitriding 
may be executed by chemical vapor deposition (CVD). 
Referring now to FIG. 4, a second embodiment of a vacuum envelope with a 
built-in electron source according to the present invention is 
illustrated. In FIG. 4, reference numeral 31 designates gate electrodes. 
In a vacuum envelope of the illustrated embodiment, a portion of each of 
the gate electrodes 31 contacted with a seal material 4 is made of niobium 
which was subject to oxidation or nitriding. Niobium oxide or niobium 
nitride is increased in resistivity, however, a decrease in thickness of 
the oxide or nitride restrains an increase in resistivity. It is not 
required to subject the whole gate electrode to a surface treatment for 
anti-oxidation. It is merely required to carry out the surface treatment 
on only a portion of the gate electrode 31 contacted with the seal 
material 4. The remaining part of the second embodiment may be constructed 
in substantially the same manner as the first embodiment described above. 
The vacuum envelope of the second embodiment thus constructed may be 
manufactured in such a manner as shown in FIG. 5. In FIG. 5, reference 
numeral 41 designates a niobium (Nb) film and 42 is an niobium oxide film 
or niobium nitride film. First, the Nb film 41 is formed on an insulating 
layer 2. When the niobium oxide film is to be formed, reactive sputtering 
using O.sub.2 is carried out; whereas formation of the niobium nitride 
film is executed by reactive sputtering using N.sub.2. This results in the 
niobium oxide film or niobium nitride film 42 being formed. Then, 
patterning is carried out, so that the gate electrodes 31 shown in FIG. 4 
may be formed. 
A thickness of the film prior to formation of the niobium oxide film or 
niobium nitride film 42 is between 0.2 .mu.m and 0.4 .mu.m. A thickness of 
the niobium oxide film 42 is required to be 50 .ANG. or more. Natural 
oxidation likewise contributes to formation of the niobium oxide film 42. 
However, it restrains a thickness of the film 42 to a level below 50 
.ANG., leading to a failure to prevent such peeling of the gate electrode 
as described above. 
The embodiments described above with reference to FIGS. 1 and 4 each do not 
require such formation of any protective film and patterning as described 
above with reference to FIG. 10, to thereby reduce both the number of 
steps in manufacturing of the vacuum envelope and a manufacturing cost 
thereof. Although formation of the niobium oxide film or niobium nitride 
film requires time, manufacturing of the vacuum envelope of each of the 
embodiments generally reduces time required therefor. Further, the 
embodiments each eliminate a necessity of a mask for patterning of the 
protective film, leading to a reduction in manufacturing cost. 
The above description has not been made on reaction between the anode 
electrode 55 and the seal material 4 shown in FIGS. 6 and 7. The anode 
electrode 55 is generally made of indium-tin oxide (ITO). Thus, the 
material is an oxide, resulting in free from the above-described problem 
encountered with Nb for the gate electrode 52. However, it is impossible 
to apply dry etching to ITO, therefore, ITO is not practicable for the 
gate electrode 52. 
Also, the insulating layer 2 is interposed between the cathode electrodes 
51 and the seal member 4, so that the cathode electrodes 51 are free from 
the above-described problem. Use of aluminum for the gate electrode 52 
permits the problem to be eliminated. However, aluminum is used for a 
lift-off layer during formation of a conical emitter, therefore, it is 
impossible to use it for the gate electrode. Thus, niobium oxide or 
niobium nitride is suitable for use for the gate electrode 52. 
The above description has been made on the Spindt type field emission 
cathode. However, a field emission cathode of any other suitable type or 
such a fine cold cathode element as described above may be suitably used 
in the present invention so long as it exhibits heat resistance sufficient 
to withstand heating for sealing by the seal material. Also, the present 
invention may be effectively applied to a vacuum envelope with a built-in 
electron source for an active element, a sensor, an electron beam unit or 
the like other than such a display device as described above. Also, the 
problem due to use of Nb for the gate electrode 52 has been described, 
however, peeling of the gate electrode may be prevented in substantially 
the same manner as described above also when a material which expands due 
to oxidation thereof like Nb may be used for the gate electrode 52. 
As can be seen from the foregoing, the present invention effectively 
prevents peeling of the gate electrode, a failure in conduction thereof, 
breakage thereof and the like, to thereby permit the vacuum envelope to 
exhibit satisfactory reliability in operation over a long period of time 
while eliminating a necessity of arrangement of such a protective film as 
employed in the prior art. Thus, the present invention simplifies 
manufacturing of the vacuum envelope, eliminates a deterioration in 
process margin and prevents a reduction in yields. 
While preferred embodiments of the invention have been described with a 
certain degree of particularity with reference to the accompanying 
drawings, obvious modifications and variations 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.