Electric lamp and discharge devices – Discharge devices having a multipointed or serrated edge...
Reexamination Certificate
1999-09-20
2002-02-26
Patel, Vip (Department: 2879)
Electric lamp and discharge devices
Discharge devices having a multipointed or serrated edge...
C313S495000, C313S336000, C313S351000
Reexamination Certificate
active
06351059
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a field-emission type cold cathode, and in particular, to one having a gate electrode in the vicinity of the emitter which emits electrons. The present invention also relates to applications of such a field-emission type cold cathode.
This application is based on Patent Application No. Hei 10-266950 filed in Japan, the contents of which are incorporated herein by reference.
2. Description of the Related Art
The field-emission type cold cathode is a device comprising a sharp conical emitter and a gate electrode which is formed in the vicinity of the emitter and has a sub-micron-order aperture. In such a device, a high field is concentrated at the pointed end of the emitter, from which electrons are emitted in a vacuum. The emitted electrons are received by an anode electrode which is separately formed. Recently, such devices have become smaller based on the development of the fine manufacturing techniques, and such smaller devices are widely used as constituents of subminiature triode electron tubes or electron guns used in thin display devices.
In conventional field-emission type cold cathodes having a gate diameter of approximately 1 &mgr;m, a voltage of approximately 100 V is applied between the emitter and the gate electrode so as to emit electrons from the head point of the emitter. However, with an operating voltage of 100 V or more, the operating conditions are limited with respect to the power consumption, control circuits, or the like; thus, operation with a lower voltage has been required. An example method for satisfying such a requirement is to provide gate electrode with a fine aperture (i.e., with a very small diameter). However, making a fine aperture is accompanied with having a thinner thickness of the insulating film between the emitter and gate electrode, thereby degrading the withstand voltage. Therefore, a method for making a gate electrode of a fine diameter without degrading the withstand voltage has been required.
In addition, the emitter and the gate electrode are separately but closely arranged, so that a discharge may occur between the emitter and the gate electrode. If such a discharge produces an excess current flowing through the emitter or the gate electrode, the material of the electrode may melt and short-circuit breakdown may occur between the emitter and gate electrode.
In order to prevent such a failure, that is, to suppress an excess current due to the discharge, it is effective to provide another element for suppressing current, at the emitter or gate electrode. A typical known method is to form a resistor connected to the emitter. However, in this method, an area for forming a resistor is necessary, and the current-suppressing effect is also effective during the normal operation so that the operating voltage may rise. In these circumstances, a vertically-formed current control element having non-linear current/voltage characteristics has peen proposed.
Japanese Unexamined Application, First Publication, No. Hei 10-12128 discloses an example of a field-emission type cold cathode comprising such a vertically-formed current control element.
FIGS. 8A
to
8
F are sectional views for explaining the manufacturing processes in the first conventional example performed in turn.
As shown in
FIG. 8A
, masking film
14
consisting of an oxide film is formed on silicon substrate
1
at a thickness of 1 &mgr;m, and then the patterning of masking film
14
is performed using a resist or the like, so that the substrate
1
is exposed. The anisotropic etching using the masking film
14
as a mask (for the etching) is performed on the exposed substrate so that trench
4
of 10 &mgr;m depth is formed.
Next, a BPSG (boron-phosphorus silicate glass) film of 2 &mgr;m thickness is formed using the LPCVD (low pressure chemical vapor deposition) method, and the etchback process is performed until the BPSG film
5
is embedded within the trench
4
, as shown in FIG.
8
B.
Next, as shown in
FIG. 8C
, oxide film
6
is deposited at a thickness of approximately 400 nm by using the CVD method, and gate electrode film
7
is further deposited at a thickness of 200 nm by using a spattering method, so as to perform the patterning of the electrode having a desired shape.
Then, as shown in
FIG. 8D
, gate aperture
8
having an approximately 0.5 &mgr;m diameter is formed in an area where an emitter is provided later, by selectively etching the gate electrode
7
and oxide film
6
, and a sacrificial layer
9
such as an alumina film is deposited on the top face of the gate electrode
7
and on the side walls of the gate electrode
7
and oxide film
6
by performing the rotational vapour deposition from a slantwise direction. An emitter material such as molybdenum is then deposited from a vertical direction by the vapour deposition, so that emitter
10
a
and extra emitter material
10
b
are respectively formed on the substrate and the sacrificial layer.
Lastly, as shown in
FIG. 8F
, the sacrificial layer is etched using phosphoric acid or the like, so that the emitter material
10
b
is lifted off and a field-emission type cold cathode is obtained.
In the above conventional example, the portion surrounded by trench
4
functions as a discharge-current suppressing element having non-linear current/voltage characteristics, thereby preventing a short-circuit breakdown of the device.
FIG. 9
shows an example of a structure for further improving the operational characteristics. In this structure, a set of oxide film
6
and gate electrode
7
, having an aperture, is deposited on silicon substrate
1
. A sharp conical emitter
10
a
is formed in the gate aperture
8
, and the area of emitter
10
a
is surrounded by trench
4
(in substrate
1
) in which BPSG film
5
is embedded. In addition, an n type area, more specifically, n type diffusion layer
12
(to which n type impurity is doped with a higher concentration than that of substrate
1
) is provided in the top face of the substrate
1
.
That is, in this example, the n type diffusion layer
12
is added to the structure shown in FIG.
8
F. The structure shown in
FIG. 9
can reduce the contact resistance at the lower part of the emitter and prevent the current (path) from concentrating at the lower part of the emitter. That is, without the n type diffusion layer
12
, current concentrates at a local area when a discharge occurs, so that a voltage is applied to a local area due to the contact resistance between the emitter and the substrate, which causes a breakdown and reduces the effective length of the discharge-current suppressing element (provided in the area surrounded by the trench and having non-linear current/voltage characteristics). That is, the structure shown in
FIG. 9
prevents a high field from applying to both sides of the element having a shorter effective length; thus, degradation of the withstand voltage can be prevented.
The first problem related to the conventional technique is that a thermal (i.e., thermally oxidized) oxide film having better insulating capability cannot be used for forming the insulating film below the gate electrode, so that the finer the gate diameter, the thinner the insulating film is, thereby reducing the withstanding voltage between the emitter and gate electrode. This is because the height of the emitter is in proportion to the diameter of the gate aperture. For example, if the diameter of the gate aperture decreases from 0.8 &mgr;m to 0.4 &mgr;m, then the thickness of the oxide film also decreases from 0.4 &mgr;m to 0.2 &mgr;m.
The second problem related to the conventional technique is that when the oxide film between the emitter and gate electrode (being accompanied with a finer gate diameter) becomes thinner, the creeping distance along the side wall of the oxide film becomes shorter. Generally, a surface of the oxide film, exposed in a vacuum, between the emitter and gate electrode may include a path relating to the generation of surface states, accretion, discharge on the relevant
Hutchins, Wheeler & Dittmar
Patel Vip
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