Electric lamp and discharge devices – Electrode and shield structures – Point source cathodes
Reexamination Certificate
1999-03-05
2001-10-02
Patel, Nimeshkumar D. (Department: 2879)
Electric lamp and discharge devices
Electrode and shield structures
Point source cathodes
Reexamination Certificate
active
06297586
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a cold-cathode power switching device of field emission type, and more particularly to a cold-cathode array substrate which has a plurality of cold-cathode modules of field emission type and which is suitable for use in a large-current, high-voltage power switching device, and also to a method of manufacturing a cold-cathode power switching device of field emission type.
Hitherto, cold-cathode devices of field emission type have been developed for use mainly in displays. A cold-cathode device of this type is used in a display, as a source of electron beams for illuminating the fluorescent screen of the display. Therefore, the current supplied to the device is small, and the voltage applied thereto is only 1 kV or less.
In recent years, it has been proposed that the cold-cathode device of field emission type be used as a power switching device. A power switching device needs to operate for current ranging from a few tens of amperes to several thousands of amperes and for voltages ranging from a few kilovolts to several hundreds of kilovolts.
To allow passage of a large current, a cold-cathode device of field emission type must have a relatively large device area. In order to facilitate emission of electrons, however, it is desirable that the cone-shaped cathode of the cold-cathode device have an extremely sharp tip. It is very difficult for the cold-cathode device, which is manufactured by micro-structure process, not only to have a large device area but also to achieve uniform electron emission over the large device area. This is why the cold-cathode device of field emission type, hitherto made, is disadvantageous in terms of reliability and manufacturing yield.
When a high voltage of 1 kV or more is applied to the conventional cold-cathode device of field emission type, discharge takes place at an uneven part of the device, possibly resulting in malfunction or voltage breakdown of the cold-cathode device. Such an uneven part is formed, particularly in a cold-cathode device which is an active device having gate electrodes for supplying control signals. To prevent discharge, it is necessary to arrange the gate electrodes and gate wiring such an uneven part is not formed in the cold-cathode device.
There is a demand for a large-current, high-voltage cold-cathode power device of field emission type. To meet the demand, a multi-module device may be used which has a number of cold-cathode modules. The multi-module device needs complex gate wiring to connect the gate electrodes of the many cold-cathode modules. The complex gate wiring is likely to form an uneven part in the multi-module device, at which discharge may occur.
It is extremely difficult to use a multi-module cold-cathode device of field emission type as a power switching device. Thus, the demand for a large-current, high-voltage cold-cathode power device of field emission type has not been satisfied.
Cold-cathode devices of field emission type, which have a cold-cathode array substrate, may have high-speed response, good anti-radiation property and high heat resistance and which may operate for large current and high voltages. Researches have, therefore, been made of the cold-cathode devices with a cold-cathode array substrate, which have these advantageous features.
Research and development of a cold-cathode device of field-emission type was started by K. R. Shoulders et al. at Stanford Research Institute (SRI), who proposed a tunnel effect vacuum triode in their thesis “Microelectronics using electron-beam-activated machining techniques,” Advances in Computers, Voltage. 2, pp. 135-293, 1961. This field of art came to attract attention of many researchers when C. A. Spindt of SRI published a report on cold cathodes having a thin film (see J. Appl. Phys. 39, p. 3504, 1968).
A cold-cathode device of field emission type comprises an emitter electrode, an anode electrode, a cone-shaped emitter, and a gate electrode. When a high voltage is applied between the emitter electrode and the anode electrode, the emitter emits electrons, whereby main current flows. The main current is controlled by supplying a control signal to the gate.
The cone-shaped emitter is a miniature metal emitter. How the miniature metal emitter is made, along with the gate, by so-called “Spindt method” will be explained, with reference to
FIGS. 1A
to
1
C. The Spindt method is most widely used at present. In this method, rotational grazing vapor deposition and aluminum (Al) sacrifice layer etching are performed.
As shown in
FIG. 1A
, a gate insulating film
6
is formed on a silicon substrate
1
a
. A gate layer
4
a
, which is a thin metal film, is formed on the gate insulating film
6
. The gate insulating film
6
is etched by using the gate layer
4
a
as a mask. An opening is thereby made in the gate insulating film
6
.
Next, as shown in
FIG. 1B
, Al is deposited on the gate layer
4
a
by effecting rotational grazing vapor-deposition at a small grazing angle &phgr;. An Al sacrifice layer
31
is thereby formed on the gate insulating film
6
. Since the grazing angle is small as shown in
FIG. 1B
, Al is deposited on the gate layer
4
a
only, not on the silicon substrate
1
a
at all.
Then, as shown in
FIG. 1C
, molybdenum (Mo) vapor is applied in vertical direction onto the silicon substrate
1
a
through the opening made in the film
6
. Mo is thereby deposited on the substrate
1
a
, forming an emitter
26
. The emitter
26
is shaped like an acute cone, because the opening made in the Al sacrifice layer
31
gradually narrows as the deposition of Mo proceeds on the Al sacrifice layer
31
.
The method of forming a miniature metal emitter, which Gray et al. has proposed, will be described with reference to
FIGS. 2A
to
2
C.
First, as shown in
FIG. 2A
, an SiO
2
etching mask
32
is formed and patterned on a silicon substrate
1
a
. As shown in
FIG. 2B
, anisotropic wet etching solution is applied, thereby etching the silicon substrate
1
a
along the crystal plane. The silicon substrate
1
a
is thereby etched at its upper surface, except that part which is located beneath the SiO
2
etching mask
32
. As the anisotropic etching further proceeds, that part of the substrate
1
a
assumes a shape like a pyramid, and the SiO
2
etching mask
32
is removed from the silicon substrate
1
a
. As a result, a pyramid-shaped miniature silicon emitter
1
b
is formed, which protrudes from the silicon substrate
1
a.
Next, a gate insulating film
6
is deposited on the silicon substrate
1
a
, and a gate layer
4
a
is deposited on the gate insulating film
6
. As shown in
FIG. 2C
, an opening is made in that part of the gate layer
4
a
which is located above the miniature silicon emitter
1
b
. Selective etching is performed on the gate insulating film
6
by using the gate layer
4
a
as a mask. An opening is thereby formed in the gate insulating film
6
exposing the miniature silicon emitter
1
b.
The Spindt method and the Gray et al. method, described above, include microstructure process. It is therefore very difficult to form a number of miniature emitters on the silicon substrate, with a sufficiently high yield. No practical assembling methods that can arrange many cold-cathode tips in the form of an array.
A number of cold-cathode tips may be formed on a cold-cathode array substrate for use in a power device by these method. If any one of the miniature emitters is short-circuited with the gate layer, however, the cold-cathode array substrate will become useless in its entirety. This reduces the manufacturing yield of the cold-cathode array substrate.
The cold-cathode array substrate has projections protruding from its periphery. Like the miniature emitters, the projections are likely to emit electrons. If the projections emit electrons, a leakage current is generated, eventually degrading the voltage resistance of the power device comprising the cold-cathode array substrate. It should be noted that the gate layer cannot control the leakage current.
The silicon substrate
1
Nakayama Kazuya
Ono Tomio
Sakai Tadashi
Sakuma Naoshi
Hopper Todd Reed
Kabushiki Kaisha Toshiba
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Patel Nimeshkumar D.
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