Active solid-state devices (e.g. – transistors – solid-state diode – Field effect device – Having insulated electrode
Reexamination Certificate
2001-11-28
2004-11-16
Nelms, David (Department: 2818)
Active solid-state devices (e.g., transistors, solid-state diode
Field effect device
Having insulated electrode
C257S198000, C257S588000, C257S592000
Reexamination Certificate
active
06818941
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a thin-film electron emitter having an electrode-insulator-electrode or an electrode-semiconductor-insulator-electrode stacked structure for emitting electrons into a vacuum, a display device using the same, and an applied machine such as an electron-beam lithography apparatus.
BACKGROUND ART
A thin-film electron emitter according to the present invention is an electron emission element using hot electrons generated by applying a high electric field to an insulator. As a typical example, an MIM (Metal-Insulator-Metal) electron emitter constructed by a thin film having a three-layer structure of the top electrode-insulator-base electrode will be explained. This applies voltage between the top electrode and the base electrode to emit electrons from the surface of the top electrode. The MIM electron emitter is disclosed in, for example, Japanese Laid-Open Patent Publication No. Hei 7-65710.
FIG. 2
shows the operating principle of an MIM electron emitter which is a typical example of a thin-film electron emitter. A driving voltage
20
is applied between a top electrode
11
and a base electrode
13
so that the electric field in an insulator
12
is above 1 to 10 MV/cm. Electrons near the Fermi level in the base electrode
13
pass through the barrier by tunneling phenomena and are injected into the conduction band of the insulator
12
and the top, electrode
11
so as to become hot electrons. Part of these hot electrons is scattered by the interaction thereof with a solid in the insulator
12
and the top electrode
11
and loses energy. As a result, at time of reaching the interface between the top electrode
11
and a vacuum
10
, there are hot electrons having various energies. Of these hot electrons, the hot electrons having an energy above the work function &phgr; of the top electrode
11
are emitted into the vacuum
10
. Other hot electrons are flowed into the top electrode
11
. An electric current flowing from the base electrode
13
to the top electrode
11
is called a diode current Id, and an electric current emitted into the vacuum
10
is called an emission current Ie. An electron emission efficiency Ie/Id is about 1/10
3
to 1/10
5
. Electron emission by this principle in, for example, an An-Al
2
O
3
-Al structure is observed. The electron emitter has excellent electron emitter properties in that even when the surface of the top electrode
11
is contaminated due to attachment of ambient gas to change the work function &phgr;, this will not greatly affect the electron emission characteristic, and is expected as a new electron emitter.
DISCLOSURE OF THE INVENTION
As described above, the electron emission efficiency Ie/Id of a thin-film electron emitter is typically low and about 1/10
3
to 1/10
5
. In order to obtain a desired emission current Ie, the diode current Id must be increased. There is the problem that the capacity of a bus line feeding an electric current to the electron emitter must be high, and the high output electric current of a driving circuit is needed. In particular, when plural thin-film electron emitters are arranged in two dimensions for use, plural electron emitters are connected to one bus line, therefore, the high electric current is a significant problem. Further, in order to flow a large amount of the diode current, a higher electric field must be applied to the insulator. This can shorten the operating life of the thin-film electron emitter.
An object of the present invention is to increase the electron emission efficiency of a thin-film electron emitter. The reason why the electron emission efficiency is low is that, as described in connection with
FIG. 2
, hot electrons are scattered in the insulator and the top electrode and lose energy. The percentage of contribution of the insulator and the top electrode to scattering depends on various conditions of the thin-film electron emitter structuring material and the film thickness of the insulator and the top electrode. Although not generally evaluated, in any case, it is certain that the scattering in the top electrode considerably contributes to the electron emission efficiency. When using a top electrode material with a low occurrence probability of hot electron scattering, the electron emission efficiency is enhanced. We have found from various studies that the degree of hot electron scattering is associated with density of states of an electrode material. More specifically, as the density of states near the Fermi level of a material is low, the probability of hot electron scattering is decreased, and the electron emission efficiency of a thin-film electron emitter using the same is enhanced.
This will be explained as follows. The hot electron scattering in a solid is governed chiefly by electron—electron scattering.
FIG. 3
is a diagram schematically showing the energy states of electrons in a metal before and after scattering. An electronic state of a hot electron before scattering is indicated by 1, and its energy is indicated by E
1
, interacting with an electron in a state
2
.
The state below Fermi level E
F
is occupied by electrons. States
3
and
4
of two electrons after scattering can only be states above the E
F
. Providing energy references with respect to the Fermi level, the law of energy conservation gives:
E
1
+E
2
=E
3
+E
4
>0
In other words, the hot electron of the energy E
1
can interact with only a valence electron in the range of 0 to −E
1
, and can only be a state in the range of 0 to E
1
after scattering. The hot electron scattering probability is in almost proportion to the number of densities of states D(E) in this range.
In order to,actually measure a difference in electron scattering degrees between metal materials, we have made samples in which the top electrode
11
of an MIM electron emitter is structured by a double-layer film of M—Au (M=Au, Pt, Ir, Mo, or W) to measure electron emission efficiencies. As shown in
FIG. 4
, the electron emission efficiencies are increased in the order of W, Mo, Ir, Pt and Au.
FIG. 5
shows densities of states of these metal materials. The number of densities of states existing in the range of −7 eV to +7 eV is decreased in the order of W, Mo, Ir, Pt and Au. In this way, we have found the above-mentioned association of the hot electron scattering degree with the density of states of the materials.
A top electrode material with a low occurrence probability of hot electron scattering is found to have a low density of states near the Fermi level. In other words, it is understood that a material having a wide-bandgap near the Fermi level is particularly desirable.
An example in which n
+
−Si frequently used as an electrode material in a semiconductor process is employed for the top electrode of a thin-film electron emitter is reported in Journal of Vacuum Science and Technologies B, Vol. 11, pp. 429 to 432. In this document, this structure cannot obtain a sufficient emission efficiency. This shows that the Si bandgap (1.1 eV) is not enough to reduce the electron scattering probability.
With the above-mentioned studies, we have found the following materials as materials optimal for the top electrode material of a thin-film electron emitter. In other words, they have a bandgap wider than that of Si. The top electrode which flows a diode current must have low resistance.
As such materials, there are particularly conductive oxides. Among these, a group of materials called a transparent conductive film have a bandgap above about 3 eV to prevent light absorption and are suitable for the top electrode of a thin-film electron emitter since the resistivity is about 1×10
−4
to 8×10
−4
&OHgr;cm, the resistivity is below 10
−3
&OHgr;cm, and the conductivity is high. More specifically, typical examples include tin oxide, Sb-doped tin oxide, Sn-doped indium oxide (ITO, Indium Tin Oxide film), zinc oxide, and Cd
2
SnO
4
.
Examples of other conductive oxides include non-doped indium oxide, F-doped i
Ishizaka Akitoshi
Kusunoki Toshiaki
Okai Makoto
Sagawa Masakazu
Suzuki Mutsumi
A. Marquez, Esq. Juan Carlos
Fisher Esq. Stanley P.
Hitachi , Ltd.
Nelms David
Nguyen Dao H.
LandOfFree
Thin film electron emitter, display device using the same... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Thin film electron emitter, display device using the same..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Thin film electron emitter, display device using the same... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3284308