Electric lamp and discharge devices – With luminescent solid or liquid material – Vacuum-type tube
Reexamination Certificate
2002-12-20
2004-12-14
Patel, Nimeshkumar D. (Department: 2879)
Electric lamp and discharge devices
With luminescent solid or liquid material
Vacuum-type tube
C313S485000, C313S309000, C313S336000, C313S496000, C313S497000, C313S310000, C313S311000, C313S351000, C313S542000
Reexamination Certificate
active
06831403
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to an improvement in field emission display (FED) technology and, in particular, to a FED cathode assembly that substantially reduces or eliminates the occurrence of an adverse chemical reaction between a chromium gate electrode and an insulating (i.e., dielectric) oxide layer.
FIG. 1
illustrates a typical FED structure
10
, which includes a cathode assembly
9
and an anode assembly
8
separated from each other by spacers
25
. Cathode assembly
9
has a substrate or baseplate
12
with a base conductive layer
14
formed thereon, a resistive layer
15
(e.g., amorphous silicon) deposited on top of layer
14
, and a plurality of conical, cold cathode emitters
16
formed on layer
15
. Also formed on layer
15
is an electrically insulating (i.e., dielectric) layer
18
having a conductive layer located thereon, which forms gate electrode
20
. This electrode, which is typically formed from metal, functions as an extraction grid to control the emission of electrons from emitters
16
.
Anode assembly
8
has a transparent faceplate
22
, a transparent conductive layer
23
over faceplate
22
and a black matrix grille (not shown) formed over layer
23
to define pixel regions. A cathodoluminescent coating (i.e., phosphor)
24
is deposited on these defined regions. This assembly is positioned a predetermined distance from emitters
16
using spacers
25
. Typically, a vacuum exists between emitters
16
and anode
8
.
A power supply
26
is electrically coupled to conductive layer
23
, electrode
20
and conductive layer
14
for providing an electric field that causes emitters
16
to emit electrons and accelerate the electrons toward conductive layer
23
. A vacuum in the space between baseplate
12
and anode
22
provides a relatively clear path for electrons emitted from emitters
16
. The emitted electrons strike cathodoluminescent coating
24
, which emits light to form a video image on a display screen created by anode
8
.
FIG. 2
is a schematic diagram of a portion of the FED structure
10
shown in FIG.
1
. In operation, electrons flow from the conductive layer
14
to an emitter
16
through resistor
32
, which is formed by the resistive layer
15
. This resistive layer is current limiting. Even in the case of a short circuit between emitter
16
and electrode
20
, resistive layer
15
limits the flow of current, and thus the flow of electrons, through the circuit branch formed by conductive layer
14
, resistive layer
15
, and emitter
16
.
Referring again to
FIG. 2
, an electric potential placed on gate electrode
20
(which functions as an extraction grid) pulls an electron emission stream from emitter
16
. A second potential placed on layer
23
attracts the freed electrons, which accelerate toward this layer until they strike cathodoluminescent coating
24
. Specific examples of FEDs are disclosed in the following U.S. patents, each of which is hereby incorporated by reference in its entirety for all purposes: U.S. Pat. Nos. 3,671,798, 3,970,887, 4,940,916, 5,151,061, 5,162,704, 5,212,426, 5,283,500, and 5,359,256.
Successful FED operation depends upon, among other things, a dependable gate electrode that is capable of consistent and prolonged operation. The formation of conventional gate electrodes is well known and described, for example, in the following U.S. patents, each of which is hereby incorporated by reference in its entirety for all purposes: U.S. Pat. Nos. 5,186,670, 5,299,331, 5,259,799 and 5,372,973.
Chromium metal is considered an ideal gate electrode in field emission displays. Although the electrical conductivity of chromium (Cr) is less than aluminum and the noble metals, critical parameters such as chemical durability, adhesion to glass and nonreactivity with solutions such as “Piranha” (i.e., a 2:1 mixture of H
2
SO
4
and H
2
O
2
, commonly used to remove organic contamination and strip photoresist) and hydrofluoric acid (an aqueous solution of HF commonly used to etch SiO
2
) make chromium an attractive candidate for gate electrodes. In a conventional FED structure, such as shown in
FIG. 1
, electrodes formed from Cr layers (e.g., base conductive layer
14
and the conductive layer forming gate electrode
20
) are sputter deposited to a thickness of approximately 200 nm. An insulating layer of SiO
2
located between these layers (e.g., dielectric layer
18
) is deposited to a thickness of about 500 nm.
It has been observed that chromium used as a gate electrode (e.g., electrode
20
) adversely reacts with deposited silicon dioxide (SiO
2
; e.g., dielectric layer
18
) upon application of an electrical potential between the gate electrode and a base conductive layer (e.g., layer
14
), both in ambient and under vacuum conditions. Under ambient atmospheric pressure, the reaction occurs rapidly and results in a brown, bubbling reaction product at the surface of the chrome electrode. This reaction coincides with a rapid reduction in the breakdown voltage of the dielectric layer. Under vacuum conditions typical of an FED operating environment (i.e., about 1×10
−7
to 1×10
−8
Torr; referred to herein as “FED vacuum conditions”), no bubbling is observed on the chrome electrode, however, a gradual chemical transformation occurs at a site on the electrode where electrical contact is made with a probe tip (i.e., a standard tungsten probe tip commonly used for contacting structures during electrical measurements). Again, this reaction coincides with a gradual deterioration of the dielectric breakdown voltage.
Deterioration of dielectric breakdown voltage of a FED cathode assembly under FED vacuum conditions could lead to shorting between the Cr gate electrode and an associated base conductive layer, degradation in emission current of emitters (e.g., cold cathode emitters
16
), reduction in brightness of an associated FED display and eventual failure of the FED unit. Accordingly, the very reliability of a FED unit is jeopardized by this phenomena.
From the above, it is seen that a method and apparatus is desired for substantially reducing or eliminating the occurrence of an adverse chemical reaction between a chromium gate electrode and an insulating (i.e., dielectric) layer that coincides with a deterioration of dielectric breakdown voltage in a FED cathode assembly.
SUMMARY OF THE INVENTION
A FED cathode assembly and method for making same that substantially reduces or eliminates the occurrence of an adverse chemical reaction between a chromium gate electrode and an insulating (i.e., dielectric) layer is provided. In one embodiment, the invention provides a cathode assembly that includes a layer of insulating material, a buffer layer located over the insulating layer and a layer of chromium located over the buffer layer. In another embodiment, an FED is provided that includes a baseplate, a first layer of conductive material located over the baseplate, a layer of insulating material located over the first layer of conductive material, a buffer layer located over the insulating material and a second layer of conductive material located over the buffer layer. In both embodiments, the buffer layer may be formed from copper, aluminum, silicon nitride or silicon (e.g., amorphous, polycrystalline or microcrystalline).
In yet another embodiment, a method for forming a cathode assembly is provided that includes the steps of forming a layer of insulating material over a first layer of conductive material, forming a buffer layer over the insulating layer and forming a second layer of conductive material over the buffer layer.
A further understanding of the nature and advantages of the invention may be realized by reference to the remaining portions of the specification and the drawings. In the drawings, like reference numbers indicate identical or functionally similar elements.
REFERENCES:
patent: 3671798 (1972-06-01), Lees
patent: 3970887 (1976-07-01), Smith et al.
patent: 4940916 (1990-07-01), Borel et al.
patent: 5151061 (1992-09-01), Sandhu
patent: 5151168 (
Moradi Behnam
Raina Kanwal K.
Westphal Michael J.
Harper Holly
Micro)n Technology, Inc.
Patel Nimeshkumar D.
Wilmer Cutler Pickering Hale and Dorr LLP
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