Electric lamp and discharge devices – With luminescent solid or liquid material – Vacuum-type tube
Reexamination Certificate
1999-09-15
2002-09-10
Patel, Ashok (Department: 2879)
Electric lamp and discharge devices
With luminescent solid or liquid material
Vacuum-type tube
C313S496000, C313S311000, C313S309000, C313S336000, C313S351000
Reexamination Certificate
active
06448709
ABSTRACT:
FIELD OF THE INVENTION
The present invention generally relates to a field emission display (FED) device and a method for fabricating the device and more particularly, relates to a field emission display device with a diode structure and equipped with nanotube emitters as the electron emission source and a method for fabricating the device by a thick film printing technique.
BACKGROUND OF THE INVENTION
In recent years, flat panel display devices have been developed and widely used in electronic applications such as personal computers. One of the popularly used flat panel display device is an active matrix liquid crystal display which provides improved resolution. However, the liquid crystal display device has many inherent limitations that render it unsuitable for a number of applications. For instance, liquid crystal displays have numerous fabrication limitations including a slow deposition process for coating a glass panel with amorphous silicon, high manufacturing complexity and low yield for the fabrication process. Moreover, the liquid crystal display devices require a fluorescent backlight which draws high power while most of the light generated is wasted. A liquid crystal display image is also difficult to see under bright light conditions or at wide viewing angles which further limit its use in many applications.
Other flat panel display devices have been developed in recent years to replace the liquid crystal display panels. One of such devices is a field emission display device that overcomes some of the limitations of LCD and provides significant advantages over the traditional LCD devices. For instance, the field emission display devices have higher contrast ratio, larger viewing angle, higher maximum brightness, lower power consumption and a wider operating temperature range when compared to a conventional thin film transistor (TFT) liquid crystal display panel.
A most drastic difference between a FED and a LCD is that, unlike the LCD, FED produces its own light source utilizing colored phosphors. The FEDs do not require complicated, power-consuming backlights and filters and as a result, almost all the light generated by a FED is visible to the user. Furthermore, the FEDs do not require large arrays of thin film transistors, and thus, a major source of high cost and yield problems for active matrix LCDs is eliminated.
In a FED, electrons are emitted from a cathode and impinge on phosphors coated on the back of a transparent cover plate to produce an image. Such a cathodoluminescent process is known as one of the most efficient methods for generating light. Contrary to a conventional CRT device, each pixel or emission unit in a FED has its own electron source, i.e., typically an array of emitting microtips. A voltage difference existed between a cathode and a gate electrode which extracts electrons from the cathode and accelerates them toward the phosphor coating. The emission current, and thus the display brightness, is strongly dependent on the work function of the emitting material. To achieve the necessary efficiency of a FED, the cleanliness and uniformity of the emitter source material are very important.
In order for the electron to travel in a FED, most FEDs are evacuated to a low pressure such as 10
−7
torr in order to provide a log mean free path for the emitted electrons and to prevent contamination and deterioration of the microtips. The resolution of the display can be improved by using a focus grid to collimate electrons drawn from the microtips.
In the early development for field emission cathodes, a metal microtip emitter of molybdenum was utilized. In such a device, a silicon wafer is first oxidized to produce a thick silicon oxide layer and then a metallic gate layer is deposited on top of the oxide. The metallic gate layer is then patterned to form gate openings, while subsequent etching of the silicon oxide underneath the openings undercuts the gate and creates a well. A sacrificial material layer such as nickel is deposited to prevent deposition of nickel into the emitter well. Molybdenum is then deposited at normal incidence such that a cone with a sharp point grows inside the cavity until the opening closes thereabove. An emitter cone is left when the sacrificial layer of nickel is removed.
In an alternate design, silicon microtip emitters are produced by first conducting a thermal oxidation on silicon and then followed by patterning the oxide and selectively etching to form silicon tips. Further oxidation or etching protects the silicon and sharpens the point to provide a sacrificial layer. In another alternate design, the microtips are built onto a substrate of a desirable material such as glass, as an ideal substrate for large area flat panel displays. The microtips can be formed of conducting materials such as metals or doped semi-conducting materials. In this alternate design for a FED device, an interlayer that has controlled conductivity deposited between the cathode and the microtips is highly desirable. A proper resistivity of the interlayer enables the device to operate in a stable condition. In fabricating such FED devices, it is therefore desirable to deposit an amorphous silicon film which has electrical conductivity in an intermediate range between that of intrinsic amorphous silicon and n
+
doped amorphous silicon. The conductivity of the n
+
doped amorphous silicon can be controlled by adjusting the amount of phosphorous atoms contained in the film.
Generally, in the fabrication of a FED device, the device is contained in a cavity of very low pressure such that the emission of electrons is not impeded. For instance, a low pressure of 10
−7
torr is normally required. In order to prevent the collapse of two relatively large glass panels which form the FED device, spacers must be used to support and provide proper spacing between the two panels. For instance, in conventional FED devices, glass spheres or glass crosses have been used for maintaining such spacings in FED devices. Elongated spacers have also been used for such purpose.
Referring initially to
FIG. 1A
wherein an enlarged, cross-sectional view of a conventional field emission display device
10
is shown. The FED device
10
is formed by depositing a resistive layer
12
of typically an amorphous silicon base film on a glass substrate
14
. An insulating layer
16
of a dielectric material and a metallic gate layer
18
are then deposited and formed together to provide metallic microtips
20
and a cathode structure
22
is covered by the resistive layer
12
and thus, a resistive but somewhat conductive amorphous silicon layer
12
underlies a highly insulating layer
16
which is formed of a dielectric material such as SiO
2
. It is important to be able to control the resistivity of the amorphous silicon layer
12
such that it is not overly resistive but yet, it will act as a limiting resistor to prevent excessive current flow if one of the microtips
20
shorts to the metal layer
18
.
A completed FED structure
30
including anode
28
mounted on top of the structure
30
is shown in FIG.
1
B. It is to be noted, for simplicity reasons, the cathode layer
22
and the resistive layer
12
are shown as a single layer
22
for the cathode. The microtips
20
are formed to emit electrons
26
from the tips of the microtips
20
. The gate electrodes
18
are provided with a positive charge, while the anode
28
is provided with a higher positive charge. The anode
28
is formed by a glass plate
36
which is coated with phosphorous particles
32
. An intermittent conductive layer of indium-tin-oxide (ITO) layer
34
may also be utilized to further improve the brightness of the phosphorous layer when bombarded by electrons
26
. This is shown in a partial, enlarged cross-sectional view of FIG.
1
C. The total thickness of the FED device is only about 2 mm, with vacuum pulled inbetween the lower glass plate
14
and the upper glass plate
36
sealed by sidewall panels
38
(shown in FIG.
1
B).
The conventional FED devices formed by microtips
Chuang Feng-Yu
Wang Wen-Chun
Industrial Technology Research Institute
Patel Ashok
Tung & Associates
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