Electric lamp and discharge devices – With luminescent solid or liquid material – Vacuum-type tube
Reexamination Certificate
2002-01-18
2003-05-27
Wong, Don (Department: 2821)
Electric lamp and discharge devices
With luminescent solid or liquid material
Vacuum-type tube
C315S169300, C205S122000
Reexamination Certificate
active
06570321
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a thin film cathode, a process or method for manufacturing the thin film cathode and a display device, and more particularly, to a technique which can be effectively applied to a thin film cathode which has a three-layer structure of a base electrode, an insulator layer and a top electrode and wherein an anodic oxide is used as the insulator layer.
BACKGROUND ART
A thin film cathode is arranged, for example, so that a voltage is applied between top and base electrodes of a three-layer thin film structure of the top electrode, insulator layer and base electrode to emit electrons into a vacuum space from a surface of the top electrode.
A metal-insulator-metal (MIM) type and a metal-insulator-semiconductor (MIS) type thin film cathode, wherein a metal, insulator and metal layers are laminated and wherein a metal, insulator and semiconductor layers are laminated, respectively, are known as ones of such thin film cathodes.
The MIM type thin film cathode is described, e.g., in JP-A-7-65710.
FIG. 14
is a diagram for explaining the operational principle of a thin film cathode.
When a driving voltage Vd is applied from a driving voltage source to between a top electrode
13
and a base electrode
11
so that an electric field becomes about 1-10 MV/cm within a tunneling insulator
12
, electrons in the vicinity of a Fermi level in the base electrode
11
are transmitted through a barrier by a tunneling effect, injected into a conduction band of the tunneling insulator
12
and top electrode
13
to be formed as hot electrons.
Ones of these hot electrons having energies not smaller than a work function &phgr; of the top electrode
13
are emitted into a vacuum space
18
.
In this case, when a plurality of the top electrodes
13
and a plurality of the base electrodes
11
are provided in the form of lines so that these top and base electrode lines intersect with each other and thus a thin film cathode is provided in the form of a matrix; an electron beam can be generated from an arbitrary location. As a result, the thin film cathode can be used as an electron source of a display device or be applied as an electron source of an electron beam lithography system.
Electron emission has been observed conventionally from a metal-insulator-metal (MIM) type structure of gold (Au), aluminum oxide (referred to as Al
2
O
3
, hereinafter) and aluminum (referred to as Al, hereinafter) or the like.
In general, a high quality of tunneling insulator
12
for a thin film cathode is made of an anodic oxide.
In particular, a barrier type (no-porous) anodic oxide of Al or Al alloy is uniform in its thickness and can be formed in a defect-free insulator having a high breakdown voltage and a large surface area.
For this reason, this is most suitable as a method for forming an insulator in a thin film cathode mainly applied to a display device or the like.
However, the anodic oxidation method for forming an anodic oxide has a defect that, since this method is a wet oxidation method in an electrolyte, impurities tend to be easily introduced in the film.
FIG. 15
shows, in a model form, a method for forming an anodic oxide.
Anodic oxidation can be advanced by using the base electrode
11
as an anode in an electrolyte
21
and a mesh electrode
22
of platinum (Pt) or the like as a cathode and by applying an anodizing voltage Vox between them from an anodizing voltage source.
In an interface between the base electrode
11
of Al being anodically oxidized and the tunneling insulator
12
of Al
2
O
3
, a reaction of oxygen ions (O
2−
) supplied from the electrolyte
21
with the Al material of the base electrode causes oxidation to progress.
In an interface between the tunneling insulator
12
of Al
2
O
3
and the electrolyte
21
, further, a reaction of aluminum ions (Al
3+
) supplied from the base electrode (Al electrode)
11
with oxygen ions (O
2−
) supplied from the electrolyte
21
causes the Al
2
O
3
insulator to grow.
In this way, the growth of the Al
2
O
3
film as the tunneling insulator
12
takes place in the two interfaces. However, the growth in the interface between the base electrode
11
and tunneling insulator
12
occurs in an environment free of impurities other than Al and oxygen (O) and thus a relatively pure Al
2
O
3
film can grow. Meanwhile, in the interface between the tunneling insulator
12
and electrolyte
21
, electrolyte's anions
24
in the electrolyte
21
are incorporated into Al
2
O
3
to grow an Al
2
O
3
film containing lots of impurities.
Accordingly the tunneling insulator
12
has a double structure of a less-impurity inner layer
25
of an insulator provided inside of a surface position at the time of starting the anodic oxidation and a more-impurity outer layer
26
of the insulator provided outside thereof.
A ratio in film thickness between the inner and outer layers is determined by a transport number of aluminum ions (Al
3+
) and oxygen ions (O
2−
) during the anodic oxidation and by the type of the anodizing electrolyte.
In the case of an Al
2
O
3
barrier type anodic oxide prepared with use of an electrolyte of organic acid such as ammonium salt of tartaric acid or citric acid and with use of nonaqueous solvent such as ethylene glycol, a transport number of ammonium ions (Al
3+
) is 0.6 and a transport number of oxygen ions (O
2−
) is 0.4.
Therefore the film thickness ratio of the outer layer
25
to the insulator reaches 60% and the insulator's outer layer
25
contains carbon as impurities.
Similarly, when an aqueous solution of ammonium borate capable of forming the barrier type anodic oxide is used as the anodizing electrolyte, main impurities are boron, the transport number of aluminum ions (Al
3+
) is 0.4 and the transport number of oxygen ions (O
2−
) is 0.6.
Even in this case, the film thickness ratio of the outer layer
25
to the insulator is 40%.
FIG. 16
shows results when a mixture solution of aqueous tartaric acid ammonium and ethylene glycol is used, and when the composition of a tunneling insulator in a thin film cathode formed by an anodic oxidation method is analyzed and measured by a glow discharge spectroscopy.
The amount of carbon as impurities in the tunneling insulator is much in a region about 60% on its surface side, stepwise decreases in the tunneling insulator, and is less in a region about 40% inside thereof.
In this way, the double structure of the tunneling insulator
12
is clearly shown by analysis in a composition depth direction.
FIG. 17
shows a conduction band when such a thin film cathode is operated.
Electrons injected from the base electrode
11
by a tunnel phenomenon of Fowler-Nordheim run through the conduction band of the tunneling insulator
12
and reach the top electrode
13
.
At this time, electrons run through the insulator's outer layer
26
. However, since the insulator's outer layer
26
contains more impurities, the amount of structural fault becomes much and the amount of electron trap
27
becomes much.
As the number of electrons trapped in the insulator's outer layer
26
becomes large, an electric field within the tunneling insulator
12
becomes low on its base electrode
11
side and high on its top electrode
13
side as shown in FIG.
17
.
In this case, since the tunnel injection electric field is relaxed, a diode current decreases and thus an emission current also decreases.
Further, since an electric field becomes locally strong in the vicinity of an interface between the insulator's outer layer
26
and top electrode, this leads to destruction of the tunneling insulator
12
, thus reducing the reliability of a thin film cathode.
The present invention has been made for the purpose of solving the problems in the prior art. It is therefore an object of the present invention to provide a technique for a thin film cathode which can reduce electron trap in an outer layer having much impurity within an insulator formed by an anodic oxidation method to thereby prevent decrease of an
Ishizaka Akitoshi
Kusunoki Toshiaki
Okai Makoto
Sagawa Masakazu
Suzuki Mutsumi
Tran Chuc
Wong Don
LandOfFree
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