Electric lamp and discharge devices: systems – Plural power supplies – Plural cathode and/or anode load device
Reexamination Certificate
2000-09-14
2001-11-20
Wong, Don (Department: 2821)
Electric lamp and discharge devices: systems
Plural power supplies
Plural cathode and/or anode load device
C315S169100, C315S169400
Reexamination Certificate
active
06320324
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a thin-film electron source and a display device by using the same, having three-layer structure of metal-insulator-metal, for emitting electron into a vacuum therefrom.
BACKGROUND ART
A thin-film electron source has three-thin-layer structure, such as an upper electrode-an insulating layer-a lower electrode, and it emits electron into a vacuum from the surface of the upper electrode, by applying voltage between the upper electrode and the lower electrode, having such a polarity that the upper electrode comes to have a positive one. The operation principle of the thin-film electron source is shown in
FIG. 2. A
driving voltage
20
is applied across the upper electrode
13
and the lower electrode
11
, so as to establish the electric field of around 10 MV/cm within the insulating layer, then electron in the vicinity of the Fermi level within the lower electrode
11
penetrates through the barrier due to the Tunnel phenomenon of Fower-Nordheim, so as to be injected into conductive bands of the insulating layer
12
and the upper electrode
13
, thereby becoming hot electrons. Among those electrons, those having energy being equal to or greater than the work function &phgr; of the upper electrode are emitted into the vacuum
16
. Because of utilization of the Tunnel phenomenon, the film thickness of the insulating layer must be very thin, such as being about from 3 nm to 15 nm.
With such the thin-film electron source, it is possible to generate electron beam from any place where a matrix is formed by crossing or intersecting plural pieces of the upper electrodes and plural pieces of the lower electrodes, therefore it can be used as an electron source in a display device, or the like.
Heretofore, emission of electron was observed from MIM (Metal-Insulator-Metal) structure, such as Al—Al
2
O
3
—Au structure.
In particular, with the thin-film electron source, which uses Al as the lower electrode and anodized film (or film oxidized by anodic oxidization) as the insulating layer, since it is possible to form Al
2
O
3
insulating layer, being durable with high voltage and having a uniform thickness due to the anodic oxidization or anodization as well, then the electron emission of good quality can be obtained.
DISCLOSURE OF THE INVENTION
A problem to be dissolved in the display device using the thin-film electron source is that the life time thereof is insufficient. Concentration of electric field at an edge portion of the three-layer structure, diffusion of the material of the upper electrode into the insulating layer, and deterioration of the insulating layer due to accumulation of electrical charge upon an interface between the insulator layer and the upper electrode, etc., are the main reasons therefor. The concentration of electric field at the edge portion was already dissolved by introduction of a thick protection insulating layer at the edge portion. The diffusion of the material of the upper electrode into the insulating layer was already dissolved by applying the material having high sublimation enthalpy as the material of the upper electrode. On the other hand, the deterioration of the insulating layer may be suppressed by a method of reversing the polarity of a driving voltage alternatively, thereby to release the electric charges accumulated one after another, therefore the property of life time thereof was improved, however it is still desired that is improved furthermore.
An aspect on which a measure should be taken for that purpose is a fact that there is a large asymmetry in the characteristic curve between current and voltage (current-voltage characteristic curve) of the insulating layer. Namely, when voltage is applied to in such the polarity that the upper electrode comes to be a positive voltage (a forward direction), thereby to cause the emission of electron, current is likely to flow in the insulating layer easily, but on the contrary to this, when voltage is applied to in such the polarity that the lower electrode comes to be a positive voltage (a reverse direction), current is reluctant to flow in the insulating layer.
This is because, when applying the voltage in the reverse direction, sufficient electric field is not applied to the insulating layer in the vicinity of the interface between the upper electrode and the insulating layer. Due to this, it is impossible to release the electric charges accumulated in the vicinity of the interface of the insulating layer between the upper electrode, even if the voltage in the reverse direction is applied thereto. Accordingly, the difference between the accumulated electric charges and the released electric charges is accumulated onto the vicinity of the interface of the upper electrode, in proportion to the continuation of the driving thereof, thereby deteriorating the insulating layer.
One of the reasons of the asymmetry in the current-voltage characteristic curve of the insulating layer lies in ununiformity of the composition of the anodized film, in particular in the direction of thickness thereof. In
FIG. 3
is shown a process for forming the anodized film, in diagrammatic manner. The anodization (or the anodic oxidization) of electrode is proceeded within electrolysis liquid
21
, while applying formation voltage to a sheet electrode
22
of such as Pt, etc., as the cathode. Upon the interface of Al
2
O
3
between Al of the lower electrode
13
and the insulating layer
12
, during the anodization, oxygen ion
02
supplied from the electrolysis liquid and Al react on each other, thereby proceeding the oxidization. Also, upon the interface
25
of the insulating layer
12
between Au
2
O
3
and the electrolysis liquid
21
, aluminum ion Al
3+
supplied from the Al electrode is oxidized, therefore Al
2
O
3
grows up. In this manner, the growth of Al
2
O
3
film, coming to be the insulating layer
12
, occurs upon the two (2) interfaces, however since it grows up under the circumference of existing no impurity other than Al and O, pure A
1
2
O
3
glows up upon the interface
24
between the lower electrode and the insulating layer, on the other hand, upon the interface between the insulating layer and the electrolysis liquid, since anion
26
of an electrolyte within the electrolysis liquid
21
is attracted or drawn onto the interface, the insulating layer
17
grows up including the anion
26
as the impurity therein.
Because of the existence of the insulation layer
17
including the anion
26
as impurity therein, the voltage in the vicinity of the inter face between upper electrode and the insulation layer is suppressed when applying the voltage in the reverse direction, and it comes to be a reason that current is reluctant to flow in the reverse direction.
Other reason thereof lies in the difference of the structure, between the interfaces, one between the lower electrode and the insulation layer and the other between upper electrode and the insulation layer. The interface between the lower electrode and the insulating layer is formed through the anodization thereof, therefore it comes to have the structure of shifting from Al to A
1
2
O
3
, continuously. On the other hand, the interface between the upper electrode and the insulating layer is formed from a film of the material of upper electrode through deposition in vacuum, therefore the composition shifts abruptly therein. In such the case, it is reported that the asymmetry occurs, for example in the current-voltage property (Journal of Vacuum Science And Technology A, Volume 10 (1992), p.2992).
The above-mentioned is shown in the band structure when applying a forward voltage onto the conventional electron source of thin-film type and in the band structure when applying a reverse voltage onto it thereafter, in
FIG. 4
, collectively. When applying the forward voltage, a part of electrons injected into the insulating layers
12
and
17
from the lower electrode
11
through the Tunnel phenomenon is emitted into a vacuum (e
−
), while the remaining thereof comes to be electron
1
which flows i
Kusunoki Toshiaki
Suzuki Mutsumi
Hitachi , Ltd.
Mattingly Stanger & Malur, P.C.
Vu Jimmy T
Wong Don
LandOfFree
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