Electric lamp and discharge devices – With gas or vapor – Having particular electrode structure
Reexamination Certificate
2000-09-25
2002-09-17
Patel, Vip (Department: 2879)
Electric lamp and discharge devices
With gas or vapor
Having particular electrode structure
C313S491000
Reexamination Certificate
active
06452335
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a cold-cathode electron gun serving as an electron source for an apparatus such as a microwave tube as an application of an electron beam and, more particularly, to an electron gun mounted with a field emission type cold cathode with a focusing electrode as a cathode.
The structure of a conventional electron gun mounted with a field emission type cold cathode with a focusing electrode (to be referred to as a cold cathode hereinafter) will be briefly described with reference to
FIGS. 4
,
5
, and
6
A to
6
C.
As shown in
FIG. 4
, in a conventional electron gun
31
, a conical (trumpet-shaped) Wehnelt electrode
34
with a flange is formed on an electron emission surface
33
of a cold cathode
32
, and an emitter electrode
35
with a substantially T-shaped section is formed on the lower surface of the cold cathode
32
on a side opposite to the electron emission surface
33
.
The Wehnelt electrode
34
is held as it is fixed with its periphery to a cylindrical support (not shown) arranged around it. The emitter electrode
35
is supported by an emitter electrode support (not shown) through a spring
36
. The emitter (not shown) of the cold cathode
32
is connected to an external power supply through the emitter electrode
35
and the emitter electrode support.
The cold cathode
32
is urged by the emitter electrode support and the spring
36
against the central portion of the Wehnelt electrode
34
. In other words, the cold cathode
32
is supported as it is sandwiched between the Wehnelt electrode
34
and emitter electrode
35
.
The Wehnelt electrode
34
controls the direction of the flow of electrons (electron flow) emitted by the cold cathode
32
, and focuses the electron flow. The Wehnelt electrode
34
has an opening
37
formed at its center, and a conical portion
38
formed by bending its portion around the opening
37
conically toward the cold cathode
32
. The opening
37
of the Wehnelt electrode
34
passes the electron flow through it, and the distal end of the conical portion
38
is in contact with the cold cathode
32
. That portion of the cold cathode
32
which is surrounded by the distal end of the conical portion
38
forms the electron emission surface
33
.
The cold cathode
32
has a plurality of emitters
40
formed on the electron emission surface
33
as the surface of the central portion of a substrate
39
, and a gate electrode
41
and focusing electrode
42
surrounding the emitters
40
, as shown in
FIGS. 5 and 6A
. The gate electrode
41
is formed on the substrate
39
through a first insulating film
51
. The focusing electrode
42
is formed on the gate electrode
41
through a second insulating film
52
. Each of the focusing electrode
42
, gate electrode
41
, and first and second insulating films
51
and
52
is a thin film with a thickness of several &mgr;m or less. Gate electrode interconnections
45
for connecting the gate electrode
41
and gate electrode power supply pads
46
on the periphery of the cold cathode to each other are formed under the focusing electrode
42
through the second insulating film
52
.
The emitters
40
formed on the cold cathode
32
emit electrons from their sharp distal ends. The gate electrode
41
generates a strong electric field near the emitters
40
to cause the emitters
40
to emit electrons. The gate electrode
41
is connected to an external power supply through the gate electrode interconnections
45
and gate electrode power supply pads
46
, and receives power from it. The focusing electrode
42
is connected to another external power supply through the Wehnelt electrode
34
, and forms an electric field that focuses the electron flow emitted from the emitters
40
.
The gate electrode power supply pads
46
and the external power supply are connected to each other in a space defined between the upper surface of the Wehnelt electrode
34
and the upper surface of the cold cathode
32
by welding bonding wires
43
to the gate electrode power supply pads
46
.
The cold cathode
32
operates on the principle of extracting electrons by concentrating a high-voltage electric field (2 to 5×107 V/cm) to the distal ends of the emitters
40
. In order to decrease the operating voltage of the cold cathode
32
, the distance between the emitters
40
and gate electrode
41
is preferably as small as possible. The emitters
40
and gate electrode
41
can be designed and manufactured to be close to each other at a distance of as small as on the order of &mgr;m by utilizing a thin film process widely employed in the semiconductor field.
The focusing electrode
42
is usually arranged on the gate electrode
41
through the second insulating film
52
with a thickness of about several &mgr;m by considering matching with the thin film process described above, although it depends on the design conditions.
In order to apply predetermined voltages to the gate electrode
41
and focusing electrode
42
of the cold cathode
32
, terminals to be connected to the corresponding external power supplies must extend from the respective electrodes
41
and
42
. Since the focusing electrode
42
is exposed to the surface, the Wehnelt electrode
34
is urged against it from the surface, so that the focusing electrode
42
comes into contact with the corresponding terminal. The underlying gate electrode
41
is connected to the external power supply at a position outside the opening
37
of the Wehnelt electrode
34
in order to maintain the axial symmetry of the electric field in the opening
37
of the Wehnelt electrode
34
.
More specifically, the gate electrode interconnections
45
for connecting the gate electrode
41
of the cold cathode
32
to the gate electrode power supply pads
46
serving as the terminals to be connected to the external power supply to each other extend under the focusing electrode
42
from a central emitter area
47
to reach the gate electrode power supply pads
46
formed on the periphery of the cold cathode
32
. The gate electrode interconnections
45
and focusing electrode
42
are separated from each other by the second insulating film
52
with a thickness of several &mgr;m or less, so that they are insulated from each other.
In the conventional electron gun
31
, as shown in
FIG. 6B
, a contact portion where the Wehnelt electrode
34
is in contact with the focusing electrode
42
extends immediately above the gate electrode interconnections
45
. The focusing electrode
42
immediately above the gate electrode interconnections
45
naturally projects from its other portions where the gate electrode interconnections
45
are not present, by a length corresponding to the thickness (t &mgr;m) of the gate electrode interconnections
45
. Thus, when the conventional Wehnelt electrode
34
with a flat contact surface is brought into contact with the focusing electrode
42
, an excessive stress readily acts on the focusing electrode
42
and second insulating film
52
at the projecting portions.
The second insulating film
52
must have a predetermined thickness near the emitters
40
in order to satisfy the focusing characteristics. Accordingly, even if portions of the second insulating film
52
other than near the emitters
40
are to be made thick, it cannot actually have a thickness greatly exceeding several &mgr;m. Hence, as shown by a portion P of
FIG. 6C
, immediately above the gate electrode interconnections
45
and between the focusing electrode
42
and gate electrode interconnections
45
, an excessive stress can cause cracking or the like in the second insulating film
52
with a thickness of several &mgr;m or less, thus readily destroying it. As a result, the electrical reliability between the focusing electrode
42
and gate electrode interconnections
45
degrades.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a cold-cathode electron gun in which the electrical reliability between the focusing electrode and gate electrode is improved while holding the axial sy
Hayes & Soloway P.C.
Patel Vip
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