Electric lamp and discharge devices – With luminescent solid or liquid material – Vacuum-type tube
Reexamination Certificate
2001-02-26
2003-09-02
O'Shea, Sandra (Department: 2875)
Electric lamp and discharge devices
With luminescent solid or liquid material
Vacuum-type tube
C445S050000, C445S046000, C445S024000, C445S049000, C445S051000, C445S025000
Reexamination Certificate
active
06614169
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a display device using a thin film cathode for emitting electrons into a vacuum from an electron emitting portion and a process for fabricating the display device.
BACKGROUND OF THE INVENTION
A thin film cathode has, basically, a three-thin-film structure of a top electrode, an insulator (or semiconductor layer or the like), and a base electrode. By applying a voltage between the top and base electrodes, electrons are emitted from the surface of the top electrode into a vacuum. For example, there are an MIM (Metal-Insulator-Metal) type, an MIS (Metal-Insulator-Semiconductor) type, and the like.
The MIM type thin film cathode is described in, for example, Japanese Unexamined Patent Publication No. 7-65710 by the inventors of the present invention.
FIG. 2
shows the operating principles of the disclosed technique. When a driving voltage Vd is applied between a top electrode
13
and a base electrode
11
to set the electric field in an insulator
12
to about 1 to 10 MV/cm, electrons around the Fermi level in the base electrode
11
pass through a barrier by the tunnel effect, are injected to a conduction band of the insulator
12
and the top electrode
13
, and become hot electrons. The hot electrons are scattered in the insulator
12
and the top electrode
13
, thereby losing their energy. A part of the hot electrons having energy of a work function &phgr; of the top electrode
13
or more is emitted to a vacuum
20
.
When a plurality of top electrodes
13
and a plurality of base electrodes
11
are arranged orthogonal to each other in a matrix, a thin film cathode can generate an electron beam from an arbitrary position. Consequently, the thin film cathode can be used as a cathode of a display device or the like. Electron emission has been observed from an MIM (Metal-Insulator-Metal) structure made of Au—Al
2
O
3
—Al and the like.
In the case of applying the thin film cathode to a display device or the like, it is desirable to use a thin film cathode having a high electron emission efficiency, that is, a high ratio of emission current to an injection current (diode current). The higher the electron emission efficiency becomes, the more the brightness of the display device improves and, at the same brightness, the power consumption of the display device decreases.
In order to increase the electron emission efficiency of the thin film cathode, it is effective to reduce the thickness of the top electrode
13
as much as possible to reduce energy loss due to the scatter of the hot electrons in the top electrode
13
.
For example, Japanese Unexamined Patent Publication No. H2-121227 discloses a technique of forming a thin portion and a thick portion in an electron emitting portion by vacuum evaporation, sputtering, or selective etching using a photoresist.
In the top electrode
13
formed by a conventional thin film forming apparatus, however, when the thickness of the top electrode
13
is reduced too much, the electrode film grows in island shapes on an insulating film, sheet resistance of the electrode sharply increases, and a voltage drop occurs in the surface of the electron emitting portion. Consequently, a problem such that an effective driving voltage Vd cannot be applied to the thin film cathode occurs.
Methods for solving the problem include, for example, a method proposed in Japanese Unexamined Patent Publication No. H2-172127 in which an inclined portion is formed in a thick top electrode so as to expose the surface of a lower insulator and electrons are emitted from a thin portion at the lower inclined portion, and a method proposed in Japanese Unexamined Patent Publication No. H3-55738 in which an opening is formed in a thick top electrode so as to expose a lower insulator and electrons are emitted from the opening.
In the methods, however, it is difficult to assure a metal thin film portion as a key of electron emission with high reproducibility, so that improvement in the electron emission efficiency is limited.
Meanwhile, the inventors of the present invention have proposed a method disclosed in Japanese Patent Application No. 11-191423 in which a pixel is formed by a plurality of thin electron emitting portions to reduce the area of each electron emitting portion, and a thick bus electrode is formed around the electron emitting portion, thereby preventing a voltage drop. The method is preferable since the above-described resistance can be reduced and the metal thin film of the top electrode can be formed thinly independent of the bus electrode. However, when the size of each electron emitting portion and that of the bus electrode are reduced to increase the electron emission efficiency and improve the brightness of the display device, it is feared that required accuracy of alignment increases, and the ratio of the electron emitting portion, that is, the aperture ratio becomes lower. A drastic solving method of further improving the electron emission efficiency and brightness is therefore desired, which is realized by enlarging the area of the electron emitting portion in the thin film cathode as large as possible within the range of the pixel pitch of a display device, for example, to about 50 &mgr;m per side which is about the dot pitch of a high precision display device also in a flat panel of a large screen.
The inventors of the present invention also have proposed the technique in Japanese Unexamined Patent Publication No. H8-180794 aiming at high-efficient electron emission, in which fine dots having a height of 20 nm or lower are formed on a top electrode and the external electric field is concentrated onto the dots to thereby decrease the effective work function of the top electrode. It is, however, difficult to obtain a thin film cathode having high electron emission efficiency of 1 to 2% or higher with high reproducibility.
SUMMARY OF THE INVENTION
An object of the invention is to improve electron emission efficiency of a thin film cathode and to provide a display device with accordingly improved brightness.
More particularly, an object of the invention is to provide a display device with improved brightness, realized by enabling an effective driving voltage Vd to be applied to a thinned film cathode and irradiating phosphors with electrons emitted through a flat thin film electrode, and a process of fabricating the display device.
Aspects of the present invention disclosed in the specification will be briefly described as follows.
The invention has been achieved by paying attention that the mean free path of hot electrons largely depend on materials used for a thin film electrode in a display device using a thin film cathode for emitting electrons through the thin film electrode into pressure-reduced atmosphere.
Specifically, the electron emission efficiencies of various materials used for a top electrode were analyzed, and it was found for the first time from the analysis that due to a very short mean free path, which is about 0.5 to 5 nm, of a hot electron in the top electrode until the hot electron is scattered, the electron emission efficiency deteriorates. The invention has been made on the basis of an idea that the top electrode has to be made thinner than a conventional one. The object is achieved by positively providing the top electrode with a flat thin film part which is thinner than 5 nm, and irradiating the surface of a phosphor disposed so as to face the top electrode with electrons emitted through the flat thin film part to a vacuum.
A display device according to the invention is constructed in such a manner that a top electrode having a common flat thin film part and a plurality of island parts is disposed apart from a base electrode, a bus electrode for power supply connected to the flat thin film part is provided, phosphors are disposed over the top electrode and the bus electrode and, by applying a voltage between the base electrode and the bus electrode, the phosphors are irradiated with the electrons emitted through the flat thin film part into a vacuum.
With such a configu
Kusunoki Toshiaki
Sagawa Masakazu
Suzuki Mutsumi
Hitachi , Ltd.
Krishnan Sumati
Mattingly Stanger & Malur, P.C.
O'Shea Sandra
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