Field emission display having an improved emitter structure

A field emission display (FED) is provided. The FED has an emitter structure where the emitter, a conductor and a cathode electrode are so arranged to produce a certain electric field about the emitter. The electric field about the emitter causes the electron beam emitted from the emitter to have improved focus and have less dispersion. This causes the electron beam to hit the intended pixel without exciting phosphor layers in neighboring pixels, thus improving image quality.

BACKGROUND OF THE INVENTION

This application claims the priority of Korean Patent Application Nos. 2003-84963 and 2004-35534, filed on Nov. 27, 2003 and May 19, 2004, respectively, in the Korean Intellectual Property Office, the disclosures of which are incorporated herein in their entirety by reference.

1. Field of the Invention

The present invention relates to a field emission display and, more particularly, to a field emission display having an emitter structure that improves focusing characteristics of electron beams, thus improving image quality.

2. Description of the Related Art

Display devices, which account for one of the most important parts of conventional data transmitting media, have been used in personal computers and television receivers. The display devices include cathode ray tubes (CRTs), which use high-speed heat electron emission, and flat panel displays, such as a liquid crystal display (LCD), a plasma display panel (PDP), and a field emission display (FED), which have been rapidly developing in recent years.

Of those flat panel displays, an FED is a display device that enables an emitter arranged at regular intervals on a cathode electrode to emit electrons by applying a strong electric field to the emitter to radiate light by colliding the electrons with a fluorescent material coated on the surface of an anode electrode. Since the FED forms and displays images thereon by using the emitter as an electron source, the quality of the images may vary considerably depending on the material and structure of the emitters.

Early FEDs use a spindt-type metallic tip (or a micro tip) formed of molybdenum (Mo) as an emitter. In order to arrange such metallic tip-type emitter in an FED, however, an ultramicroscopic hole should be formed, and molybdenum should be evenly deposited on the entire surface of a screen, which requires the use of difficult techniques and expensive equipment and thus results in an increase in manufacturing costs. Therefore, there is a clear limit in manufacturing a wide screen FED.

In the industry of FEDs, research on methods of forming a flat emitter of an FED, which can emit sufficient amounts of electrons even at a low driving voltage and, eventually, can simplify processes of manufacturing the FED, is under way. Current trends in the FED industry show that carbon-based materials, for example, graphite, diamond, diamond-like carbon (DLC), fulleren (C60), or carbon nano-tubes (CNTs), are suitable for the manufacture of a flat emitter and the CNTs, in particular, are considered most desirable because they can successfully emit electrons even at a low driving voltage.

In order to have an FED display images of good quality, the electron beam emanating from the emitter must be focused and must not disperse too much so that only the phosphor layer in the intended pixel and not phosphor in neighboring pixels are impacted by the electron beam. Therefore, what is needed is an FED with superior image quality brought on by an improved design of the emitter so that the electron beam emanating from the emitter is focused and does not disperse too much so that the electron beam hits phosphor in the desired pixel and not phosphor in neighboring, unintended pixels.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide an improved FED.

It is also an object of the present invention to provide a design for an FED that improves image quality by better controlling the amount of dispersion of electron beams emanating from an emitter.

It is also an object of the present invention to provide an FED and an emitter design that improves the focusing characteristics of electron beams emanating from the emitter.

It is still an object of the present invention to provide an improved design for an emitter in an FED that results in an improved image quality.

These and other objects can be achieved by an improved field emission display (FED) design. The FED includes a first substrate, a cathode electrode formed on the first substrate, a conductive layer formed on the cathode electrode to have a first aperture, through which the cathode electrode is partially exposed, an insulation layer formed on the conductive layer to have a second aperture, which is connected to the first aperture, a gate electrode formed on the insulation layer to have a third aperture, which is connected to the second aperture, emitters formed on the cathode electrode exposed through the first aperture, the emitters being disposed a predetermined distance apart from each other at either side of the first aperture, and a second substrate disposed to face the first substrate with a predetermined distance therebetween, the second substrate, having an anode electrode and a fluorescent layer formed thereon.

A cavity may be formed in the cathode electrode between the emitters so that the first substrate can be exposed therethrough. The first, second, and third apertures and the cavity may be rectangles extending in a longitudinal direction of the cathode electrode. The widths of the third and second apertures may be larger than the width of the first aperture, and the width of the cavity is smaller than the width of the first aperture. The predetermined distance between the emitters may be smaller than the width of the first aperture, and the width of the cavity may be smaller than the distance between the emitters. The width of the third aperture may be the same as the width of the second aperture. The width of the third aperture may be larger than the width of the second aperture.

Conductive layers may be formed at both sides of the cathode electrode and may extend in the longitudinal direction of the cathode electrode, and the first aperture may be formed between the conductive layers. Conductive layers may be formed at both sides of the cathode electrode to have a predetermined length, and the first aperture may be formed between the conductive layers. The conductive layer may be formed on the cathode electrode to surround the first aperture. The conductive layer may include an insulation material layer formed to cover a top surface and side surfaces of the cathode electrode and a metal layer formed on the insulation material layer. A plurality of first apertures, a plurality of second apertures, and a plurality of third apertures may be formed for each pixel, and the emitters may be formed in each of the plurality of first apertures. The emitters may be formed of a carbon-based material. The emitters may be formed of carbon nano-tubes.

According to another aspect of the present invention, there is provided a field emission display (FED). The FED includes a first substrate, a cathode electrode formed on the first substrate, a conductive layer formed on the cathode electrode to have a first circular aperture, through which the cathode electrode is partially exposed, an insulation layer formed on the conductive layer to have a second circular aperture, which is connected to the first circular aperture, a gate electrode formed on the insulation layer to have a third circular aperture, which is connected to the second circular aperture, an emitter formed as a ring on the cathode electrode exposed through the first circular aperture, the emitter being disposed along an inner circumference of the first circular aperture, and a second substrate disposed to face the first substrate with a predetermined distance therebetween, the second substrate, on which an anode electrode and a fluorescent layer having a predetermined pattern are formed.

A cavity may be formed in the cathode electrode in the emitter to be circular so that the first substrate can be exposed therethrough. A plurality of first circular apertures, a plurality of second circular apertures, and a plurality of third circular apertures may be formed for each pixel, and the emitter may be formed in each of the plurality of first circular apertures.

According to another aspect of the present invention, there is provided a field emission display (FED). The FED includes a first substrate, a cathode electrode formed on the first substrate, an insulation material layer formed on the cathode electrode, a conductive layer formed on the insulation material layer, a first aperture formed through the insulation material layer and the conductive layer so that the cathode electrode can be partially exposed therethrough, an insulation layer formed on the conductive layer to have a second aperture, which is connected to the first aperture, a gate electrode formed on the insulation layer to have a third aperture, which is connected to the second aperture, emitters formed on the cathode electrode exposed through the first aperture, the emitters being disposed at both sides of the first aperture so that they can be a predetermined distance apart from each other, and a second substrate disposed to face the first substrate with a predetermined distance therebetween, the second substrate, on which an anode electrode and a fluorescent layer having a predetermined pattern are formed. The conductive layer may be insulated from the cathode electrode by the insulation material layer.

According to another aspect of the present invention, there is provided a field emission display (FED). The FED includes a first substrate, a cathode electrode formed on the first substrate, an insulation material layer formed on the cathode electrode, a conductive layer formed on the insulation material layer, a first circular aperture formed through the insulation material layer and the conductive layer so that the cathode electrode can be partially exposed therethrough, an insulation layer formed on the conductive layer to have a second circular aperture, which is connected to the first circular aperture, a gate electrode formed on the insulation layer to have a third circular aperture, which is connected to the second circular aperture, an emitter formed as a ring on the cathode electrode exposed through the first circular aperture, the emitter being disposed along an inner circumference of the first circular aperture, and a second substrate disposed to face the first substrate with a predetermined distance therebetween, the second substrate, on which an anode electrode and a fluorescent layer having a predetermined pattern are formed. The conductive layer may be insulated from the cathode electrode by the insulation material layer.

DETAILED DESCRIPTION OF THE INVENTION

Turning now to the figures,FIGS. 1A and 1Bare a cross-sectional view and a plan view, respectively, of an FED90. Referring toFIGS. 1A and 1B, the FED90has a triode structure made of a cathode electrode12, an anode electrode22, and a gate electrode14. The cathode electrode12and the gate electrode14are formed on a rear substrate11, and the anode electrode22is formed at the bottom of a front substrate21. A fluorescent layer23is formed of R, G, and B fluorescent materials, and a black matrix24is formed on the bottom surface of the anode electrode22so as to improve contrast. The rear substrate11and the front substrate21are a predetermined distance apart from each other. The predetermined distance between the rear substrate11and the front substrate21is maintained by a spacer31disposed between the rear substrate11and the front substrate21. When manufacturing the FED90, the cathode electrode12is formed on the rear substrate11, an insulation layer13and the gate electrode14, both perforated by minute apertures15, are deposited on the rear substrate11, and an emitter16is formed in each of the apertures15on top of the cathode electrode12.

The FED90ofFIGS. 1A and 1B, however, may lack good color purity and general picture quality for the following reasons. Most of the electrons emitted from the emitter16come from edges of the emitter16. The electrons are converted into an electron beam, and the electron beam proceeds to the fluorescent layer23. However, when proceeding to the fluorescent layer23, the electron beam may disperse due to a voltage of several to dozens of volts applied to the gate electrode14, in which case, the electron beam illuminates not only a fluorescent material of a desired pixel but also fluorescent materials of other pixels adjacent to the desired pixel.

In order to minimize the tendency of the electron beam emitted from the emitter to disperse toward the fluorescent layer23, a plurality of emitters, each having a smaller area than the emitter16corresponding to one pixel, can be disposed on the cathode electrode12in each of the apertures15. In this case, however, there is a clear limit as to the number of emitters that can be satisfactorily formed for each pixel having a predetermined size, the entire area of the emitter16for illuminating a fluorescent material of one pixel decreases, and an electron beam is not focused sufficiently.

In order to prevent an electron beam from dispersing when proceeding to a fluorescent layer, another FEDs respectively having structures, which are illustrated inFIGS. 2A and 2B, can be considered. The FEDs92and93ofFIGS. 2A and 2Brespectively each include an additional electrode disposed near a gate electrode to enhance the focusing characteristics of electron beams.

More specifically, in the FED92ofFIG. 2A, a focusing electrode54, which is ring-shaped, is disposed around a gate electrode53. In the FED93ofFIG. 2B, a double gate structure having a lower gate electrode63and an upper gate electrode64is provided to focus electron beams. However, the FEDs ofFIGS. 2A and 2Bhave a relatively complicated structure. In addition, the structure of the FEDs92and93ofFIG. 2Aor2B, in which an emitter52or62, which is a metallic micro-tip, is formed on a cathode electrode51or61, has not yet been proven satisfactorily fruitful when it comes to its application to an FED having a flat emitter.

In the meantime, U.S. Pat. No. 5,552,659 Macaulay et al. discloses an electron emitter that reduces electron emission divergence by imposing restrictions on a ratio between the thickness of a non-insulation layer formed on a substrate where the electron emitter is formed and the thickness of a dielectric layer and a ratio between the diameter of a hole formed through the non-insulation layer, the dielectric layer, and a gate layer formed on the dielectric layer and the thickness of the non-insulation layer. However, it is very difficult to manufacture the electron emitter because the electron emitter has a very complicated structure in which a plurality of holes are formed to correspond to each pixel, and a plurality of electron emitters are formed in each of the holes. In addition, there are spatial restrictions in manufacturing the electron emitter. Therefore, there is a limit in maximizing the number and area of emitters corresponding to each pixel, and the lifetime of the emitters may be shortened when driving the emitters for a long time.

Turning now toFIGS. 3 and 4,FIGS. 3 and 4are a cross-sectional view and a plan view, respectively, of a field emission display (FED)100according to a first embodiment of the present invention. Referring toFIGS. 3 and 4, the FED100includes two substrates, i.e., a first substrate110, which is also referred to as a rear substrate, and a second substrate120, which is also referred to as a front substrate. The rear substrate110and the front substrate120are formed so that they can be separated from each other by a predetermined distance. A spacer130is disposed between the rear substrate110and the front substrate120so that the predetermined distance therebetween can be maintained. The rear and front substrates110and120are typically formed of glass substrates.

A structure that can emit electrons is formed on the rear substrate110, and a structure that can realize images using the emitted electrons is formed on the front substrate120. More specifically, a plurality of cathode electrodes111are arranged on the rear substrate110at regular intervals in a predetermined pattern, for example, as stripes. The cathode electrodes111are formed by depositing a conductive metallic material or a transparent conductive material, such as indium tin oxide (ITO), on the rear substrate110to a thickness of, for example, several hundreds to several thousands of Å and patterning the deposited conductive metallic material or transparent conductive material as stripes. The material of the cathode electrodes111may be determined depending on how emitters115are formed, which will be described in greater detail later.

Cavities111a, having a width Wc are preferably formed in the cathode electrodes111and perforate cathode electrodes111so that the rear substrate110can be exposed therethrough. Each of the cavities111ais disposed between emitters115. It is within the scope of the invention not to have any cavities formed perforating the cathode electrode111. Also, it is within the scope of the invention to have more than one cavity per pixel, as will be discussed inFIGS. 9 and 10. For the FED100ofFIG. 3, there will be a one-to-one correspondence between the cavities111aperforating the cathode electrode111and the pixels125. In addition, the cavities111amay be formed, in consideration of the shape of their respective pixels125, as rectangles extending longer in the longitudinal (or +/−y) direction of the cathode electrodes111, i.e., rather than in the latitudinal (+/−x) direction.

A conductive layer112is formed on each of the cathode electrodes111so as to be electrically connected to each of the cathode electrodes111. The conductive layer112may be formed to a thickness of about 2-5 μm by coating a conductive paste on each of the cathode electrodes111using a screen printing method and plasticizing the conductive paste at a predetermined temperature. First apertures112ahaving width W1, through which the cathode electrodes111are partially exposed, are formed in and perforate the conductive layer112. The first apertures112amay be formed as rectangles that extend longer in the longitudinal direction of the cathode electrodes111(i.e., the Y direction) than in the latitudinal direction of the cathode electrodes111(i.e., the X direction) so that first aperture112acan correspond to one of the pixels125. In a case where the cavities111aare formed in the cathode electrodes111, as described above, the first apertures112aare formed to have a width W1, which is larger than a width Wcof the cavities111a, so that they can be connected to their respective cavities111a.

An insulation layer113is formed on the conductive layer112. The insulation layer113is formed on the entire surface of the rear substrate110so that not only the top surface of the conductive layer112but also the rear substrate110exposed between the cathode electrodes111can be covered with the insulation layer113, as shown inFIG. 3. The insulation layer113may be formed to a thickness of about 10-20 μm by coating a paste-type insulating material on the rear substrate110using a screen printing method and plasticizing the insulating material at a predetermined temperature. Second apertures113ahaving width W2are formed in the insulating layer113to perforate the insulating layer113so that they can be connected to their respective first apertures112a. The second apertures113amay be formed as rectangles that extend longer in the longitudinal direction of the cathode electrodes111(i.e., the Y direction) rather than in the latitudinal direction (i.e., the X direction) so that the second apertures113acan form a one-to-one correspondence with the pixels125. In addition, the second apertures113aare formed to have a width W2, which is larger than the width W1of the first apertures112a. Accordingly, the conductive layer112is partially exposed through the second apertures113a.

A plurality of gate electrodes114are formed on the insulation layer113at regular intervals in a predetermined pattern, for example, as stripes. The gate electrodes114extend in a direction perpendicular to the longitudinal direction of the cathode electrodes111(the Y direction), i.e., in the X direction. The gate electrodes114may be formed by depositing a conductive metal, e.g., chrome (Cr), on the insulation layer113using a sputtering method and patterning the conductive metal into stripes. Third apertures114ahaving width W3, which are connected to their respective second apertures113a, are each formed in and perforate the gate electrodes114. The third apertures114ahave the same shape as the second apertures113a. The third apertures114amay have a width W3, which is the same as the width W2of the second apertures113aas inFIG. 3or a width greater than W2as inFIG. 6.

The emitters115are formed on each of the exposed portions of the cathode electrodes111exposed through the first apertures112a. The emitters115are formed to have a smaller thickness than the conductive layer112and are formed to be flat on the cathode electrodes111. The emitters115emit electrons when affected by an electric field generated by voltage applied between the cathode electrodes111and the gate electrodes114. In the present invention, the emitters115are formed of a carbon-based material, for example, graphite, diamond, diamond-like carbon (DLC), fulleren (C60), or carbon nano-tubes (CNTs). Preferably, the emitters115are formed of CNTs, in particular, so that they can smoothly emit electrons even at a low driving voltage.

In the present embodiment ofFIGS. 3 and 4, the emitters115are disposed at either side of each of the first apertures112aso that they are a predetermined distance apart from each other. For example, two emitters115may be disposed in a first aperture112ain contact with side surfaces of exposed portions of the conductive layer112. The emitters115may be formed as parallel bars extending in the longitudinal direction of the first apertures112a(i.e., the Y direction). Accordingly, the emitters115have a larger area than the emitters ofFIGS. 1A,1B,2A,2B and Macaulay '659, and thus can guarantee a longer lifetime than those ofFIGS. 1A,1B,2A,2B and Macaulay '659 when driven for a long time. In addition, in a case where the cavity111ais formed between the emitters115, as described above, a distance between the emitters115is smaller than the width W1of each of the first apertures112abut larger than the width Wcof each of the cavities111a.

The emitters115may be formed in various manners. For example, in a first method, the emitters115may be formed by coating a photosensitive CNT paste on the top surface of the rear substrate110, applying ultraviolet (UV) rays to the bottom surface of the rear substrate110to selectively expose the photosensitive CNT paste, and developing the photosensitive CNT paste. In this case, the cathode electrodes111should be formed of a transparent conductive material, i.e., ITO, and the conductive layer112and the insulation layer113should be formed of an opaque material.

Alternatively, in a second method, the emitters115may be formed in the following manner. A catalyst metal layer of Ni or Fe is formed on the top surface of each of the cathode electrodes111exposed through the first aperture112a, and CNTs are vertically grown from the surface of the catalyst metal layer by supplying a carbon-based gas, such as CH4, C2H2, or CO2, to the catalyst metal layer. Still alternatively, in a third method, the emitters115may be formed by depositing photoresist in the first aperture112a, patterning the photoresist so that the photoresist can remain only on predetermined portions of the top surfaces of the cathode electrodes111where the emitters115are to be formed, coating a CNT paste on the remaining photoresist, and heating the rear substrate110to a predetermined temperature to enable the CNT paste to thermally react to the remaining photoresist. The second and third methods of forming the emitters115are free from the restriction of the first method of forming the emitters115as to the materials of the cathode electrodes111, the conductive layer112and the insulation layer113.

Turning now toFIGS. 5A,5B and5C,FIGS. 5A,5B, and5C illustrate three examples of the conductive layer112formed on one of the cathode electrodes111. Referring toFIG. 5A, conductive layers112are respectively formed at both sides of a cathode electrode111to extend in the longitudinal (+/−y) direction of the cathode electrode111, in which case, a first aperture112ais formed between the conductive layers112. Emitters115are formed between the conductive layers112to have a predetermined length in the longitudinal (+/−y) direction of the conductive layers112and contact side surfaces of the conductive layers112. A cavity111ais formed in the cathode electrode111between the emitters115to have the same length as the emitters115.

Referring toFIG. 5B, conductive layers112are formed at either side of a cathode electrode111to have a predetermined length, and a first aperture112ais formed therebetween. In the case ofFIG. 5B, the conductive layers112are illustrated as having the same length as emitters115.

Referring toFIG. 5C, a conductive layer112is formed in the form of a closed polygon on a cathode electrode111so as to completely surround a first aperture112a. All of the four sidewalls of a first aperture112aare defined by the conductive layer112. Accordingly, emitters115are completely surrounded by the conductive layer112.

Referring now toFIGS. 3 and 4, the structure formed on the front or second substrate120will now be discussed. An anode electrode121is formed on the bottom surface of the front substrate120, which faces the top surface of the rear substrate110, and fluorescent layers122are formed of R, G, and B fluorescent materials on the anode electrode121. The anode electrode121is formed of a transparent conductive material, such as ITO, so that visible rays emitted from the fluorescent layers122can pass therethrough. The fluorescent layers122are formed to extend in the longitudinal direction parallel to the cathode electrodes111, i.e., in the Y direction.

Black matrices123may be formed among the fluorescent layers122on the bottom surface of the front substrate120so as to improve contrast. A metallic thin layer124may be formed on the fluorescent layers122and on the black matrices123. The metallic thin layer124is formed of aluminium to have such a small thickness (e.g., several hundreds of Å) so that electrons emitted from the emitters115can easily pass therethrough. The R, G, and B fluorescent materials of the fluorescent layers122emit visible rays when excited by electron beams emitted from the emitters115, and the visible rays emitted from the R, G, and B materials of the fluorescent layers122are reflected by the metallic thin layer124. Thus, the amount of visible light radiated from the entire FED increases, and eventually, the brightness of the entire FED increases as well. In a case where the metallic thin layer124is formed on the front substrate120, the anode electrode121may not necessarily be formed because the metallic thin layer124can serve as a conductive layer, i.e., an anode electrode, when voltage is applied thereto.

The rear substrate110and the front substrate120are located a predetermined distance apart from each other so that the emitters115can face the fluorescent layers122. The rear substrate110and the front substrate120are bonded to each other by applying a sealing material (not shown) around them. As described above, the spacer130is disposed between the rear substrate110and the front substrate120so as to maintain the predetermined distance between the rear substrate110and the front substrate120.

The operation of the FED according to the preferred embodiment of the present invention will now be described. When predetermined voltages are applied to the cathode electrodes111, the gate electrodes114, and the anode electrode121, an electric field is formed among them so that electrons are emitted from the emitters115. At this time, a voltage of zero to minus dozens of volts, a voltage of several to dozens of volts, and a voltage of hundreds to thousands of volts are applied to the cathode electrodes111, the gate electrodes114, and the anode electrodes121, respectively. The conductive layer112is in contact with the top surface of the cathode electrodes111, and thus the same voltage applied to the cathode electrodes111is applied to the conductive layer112. The emitted electrons are converted into electron beams, and the electron beams are led to the fluorescent layers122so that they can eventually collide with the fluorescent layers122. As a result, the R, G, and B fluorescent materials of the fluorescent layers122are excited and emit visible rays.

As described above, since the emitters115are disposed at either side of each of the first apertures112a, electron beams, which are formed of electrons emitted from the emitters115, are focused rather than to be widely dispersed. In addition, since the conductive layer112is disposed at either side of the emitters115, the electron beams can be efficiently focused due to an electric field formed by the conductive layer112.

Moreover, the cavity111amay be formed in each of the cathode electrodes111so that the emitters115can be surrounded by equipotential lines of an electric field formed around the emitters115. Due to the electric field, current density increases, and a peak in the current density is precisely located in each of the pixels125of the fluorescent layers122. It is possible to more efficiently focus electron beams by adjusting the width Wcof the cavity111a.

As described above, color purity of an image can be enhanced by improving the focusing of electron beams emitted from the emitters115, and the brightness of the image can be enhanced by precisely placing a peak in current density in each of the pixels125. Therefore, it is possible to realize an image with high picture quality. Advantages of the FED according to the preferred embodiment of the present invention will be described in greater detail later with reference toFIGS. 11A through 13C.

Turning now toFIG. 6,FIG. 6is a cross-sectional view of one variation of an FED according to the first embodiment of the present invention. Referring toFIG. 6, FED106is similar to FED100inFIG. 3except that the width W3of third aperture114ais larger and thus not equal to the width W2of second aperture113a. By forming the third apertures114ato have a larger width W3than the width W2of the second apertures113a, a distance between the cathode electrodes111and their respective gate electrodes114can be lengthened, and thus, the voltage withstanding characteristics of the FED according to the first embodiment of the present invention can be improved.

Turning now toFIG. 7,FIG. 7illustrates yet another FED107according to the present invention, FED107being another variant of FED100ofFIG. 3. Referring toFIG. 7, the FED107includes a conductive layer112′ that may include an insulation material layer1121formed on each of the cathode electrodes111and a metal layer1122formed to cover the top surface and side surfaces of the insulation material layer1121, so that the metal layer1122is electrically connected to the cathode electrodes111so as to serve basic functions of the conductive layer112′. More specifically, the conductive layer112′ may be formed by forming the insulation material layer1121on each of the cathode electrodes111and forming the metal layer1122on the insulation material layer1121through a deposition, sputtering, or plating method. The metal layer1122can serve as a passivation layer that protects the conductive layer112′ from an etchant when forming the second apertures113ain the insulation layer113using the etchant. Therefore, it is possible to prevent damage to the conductive layer112′ caused by the etchant that is used to make the second apertures113a. More specifically, the conductive layer112ofFIG. 6may be damaged by the etchant because it is formed of a conductive paste. However, the conductive layer112′ ofFIG. 7is not aversely affected by the etchant because its surface is formed of the metal layer1122.

Turning now toFIG. 8,FIG. 8illustrates yet another variant to FED100ofFIG. 3. Referring to FED108inFIG. 8, an insulation material layer1123is formed on the cathode electrodes111, and a conductive layer112″ is formed on the top surface of the insulation material layer1123so that the conductive layer112″ can be disposed as much apart from the cathode electrodes111as the thickness of the insulation material layer1123and can be electrically isolated from the cathode electrodes111by the insulation material layer1123. Unlike FED107, conductive layer112″ in FED108does not include the insulation material1123. Therefore, unlike FED107ofFIG. 7, conductive layer112″ is not electrically connected to the cathode electrode111. In this case, the conductive layer112″ may be connected to a different power source from a power source connected to the cathode electrodes111, and thus a different voltage from a voltage applied to the cathode electrodes111can be applied to the conductive layer112″. Therefore, it is possible to maximize the electron beam-focusing effect of the conductive layer112″ by controlling the voltage applied to the conductive layer112″ independently of the voltage applied to the cathode electrodes111. Accordingly, the conductive layer112″ can serve as an independent electrode, i.e., a focusing electrode.

The conductive layer112″ may be formed by forming the insulation material layer1123on the cathode electrodes111and depositing a conductive metallic material on the top surface of the insulation material layer1123through a sputtering or plating method. Since the conductive layer112″ is formed of a metallic material rather than to be formed of a conductive paste, the conductive layer112″ can be prevented from being damaged by an etchant used in an etching process for forming the second apertures113ain the insulation layer113.

The rest of the elements of the FED108ofFIG. 8are the same as their respective counterparts of the FED100ofFIG. 3except that the first apertures112aare formed in the insulation material layer1123and in the conductive layer112″ at regular intervals and the emitters115disposed in each of the first apertures112aare formed in contact with side surfaces of the insulation material layer1123exposed through each of the first apertures112a. In the FED108ofFIG. 8, a longitudinal end of the conductive layer112″ may be electrically connected to each of the cathode electrodes111, in which case, the same voltage can be applied to the conductive layer112″ and the cathode electrodes111.

FIG. 9is a plan view of an FED200according to a second embodiment of the present invention. The FED according to the second embodiment of the present invention has the same cross-sectional structure as the FED according to the first embodiment of the present invention, and thus a cross-sectional view of the FED according to the second embodiment of the present invention will not be presented.

Referring toFIG. 9, in each pixel225, a plurality of first apertures212a, for example, two first apertures212aare formed in a conductive layer212, two second aperture213aare formed in an insulation layer213, and two third apertures214a, are formed in a gate electrode214. Emitters215are formed in each of the first apertures212a. Unlike FED100ofFIG. 3, there is now more than one set of apertures for each pixel in FED200. The emitters215, like the emitters115in the first embodiment of the present invention, are formed on a cathode electrode211and exposed through the first aperture212a. In addition, the emitters215are disposed at either side of each of the first apertures212aso that they are at a predetermined distance apart from each other. A plurality of cavities211a, for example, two cavities211a, may be formed in the cathode electrode211corresponding to each pixel225.

Other elements of the FED200according to the second embodiment of the present invention are the same as their respective counterparts of the FED100according to the first embodiment of the present invention, and thus their descriptions will be omitted. The variations of the FED according to the first embodiment of the present invention, shown inFIGS. 6,7, and8, may also be applied to the FED200according to the second embodiment of the present invention.

FIGS. 10A and 10Bare a plan views of an FED300according to a third embodiment of the present invention.FIG. 10Afocusses on a single emitter andFIG. 10Bshows how may circular emitter structures correspond to a single pixel325. The FED300according to the third embodiment of the present invention has the same cross-sectional structure as the FED100according to the first embodiment of the present invention, and thus a cross-sectional view of the FED300according to the third embodiment of the present invention will not be presented.

Referring toFIG. 10A, a first aperture312aformed in a conductive layer312, a second aperture313aformed in an insulation layer313, and a third aperture314aformed in a gate electrode314are all circular in shape instead of rectangular as in the first embodiment. An inner diameter D3of the third aperture314aand an inner diameter D2of the second aperture313aare larger than an inner diameter D, of the first aperture312a. In addition, the inner diameter D3of the third aperture314may be the same as the inner diameter D2of the second aperture313a.

An emitter315, which is ring-shaped, is formed on a cathode electrode311exposed through the first aperture312aalong an inner circumference of the first aperture312a. An inner diameter DEof the emitter315is smaller than the inner diameter D1of the first aperture312a. The emitter315, like the emitters115in the first embodiment of the present invention, may be formed of a carbon-based material, e.g., CNTs.

In the third embodiment of the present invention, like in the first embodiment of the present invention, a cavity311a, which is circular, may be formed to perforate the cathode electrode311. The cavity311ais disposed inside the emitter315. Therefore, an inner diameter DC of the cavity311ais smaller than the inner diameter D, of the first aperture312aand the inner diameter DE of the emitter315.

In the third embodiment of the present invention as illustrated inFIG. 10B, a plurality of first apertures312a, a plurality of second apertures313a, and a plurality of third apertures may be provided for each pixel325, in which case, the emitter315is formed in each of the plurality of first apertures312a. The rest of the elements of the FED300according to the third embodiment of the present invention are the same as their respective counterparts of the FED100according to the first embodiment of the present invention, and thus their descriptions will be omitted.

The variations of the FED according to the first embodiment of the present invention, shown inFIGS. 6,7, and8, may also be applied to the FED according to the third embodiment of the present invention. In other words, the inner diameter D3of the third aperture314aformed in a gate electrode314may be larger than the inner diameter D2of the second aperture313aformed in the insulation layer313, and the conductive layer312may include an insulation material layer formed on the cathode electrode311and a metal layer formed on the insulation material layer. In addition, the conductive layer312may be formed on the top surface of the insulation material layer, which is formed on the cathode electrode311.

It is to be appreciated that features from various embodiments and from various variations of embodiments may be mixed and matched to form an FED within the scope of the present invention. The aperture sizes may be rectangular, circular, have a one-to-one correspondence with the pixels or have a many-to-one correspondence with the pixels, the relative sizes of the apertures may vary and the presence or absence of a cavity are all within the scope of the present invention.

Empirical simulation results of an FED according to a preferred embodiment of the present invention and the FEDs ofFIGS. 1A and 1Bwill now be described in the following paragraphs. In electron beam emission simulations, the FED90ofFIGS. 1A and 1Band the FED100according to the first embodiment of the present invention, shown inFIG. 3, were respectively selected for an empirical comparison. More specifically, the FEDs according to the first through third embodiments of the present invention have almost the same cross-sectional structure and thus have almost the same electron beam emission characteristics, and thus, the FEDs ofFIGS. 3,6,7, and8were selected as exemplary embodiments of the present invention for the electron beam emission simulations. Therefore, the FEDs according to the first embodiment and their variations were empirically tested and test results for the FEDs200and300according to the second and third embodiments are not shown as they are essentially the same as that of the first embodiment.

Before the simulations, design dimensions of the FED's tested were fixed. For example, screens of the FED90ofFIGS. 1A and 1Band the FEDs according to the first embodiment of the present invention were each set to have an RGB trio pitch of about 0.69 mm in a case where they were designed to have an aspect ratio of 16:9, a diagonal line length of 38 inches, and a horizontal resolution of 1280 lines so as to realize high definition (HD)-level picture quality. In this case, in the FED according to the first embodiment of the present invention, an insulation layer113is preferably set to have a height of 10-20 μm, a conductive layer112is preferably set to have a height of 2-5 μm, first apertures112aformed in the conductive layer112are preferably set to have a width W1of 60-80 μm, second apertures113aformed in the insulation layer113are preferably set to have a width W2of 70-90 μm, third apertures114aformed in gate electrodes114are preferably set to have a width W3of 70-95 μm, and cavities formed in cathode electrodes111are preferably set to have a width Wcof 10-30 μm. However, the above-mentioned elements of the FED according to the first embodiment of the present invention may have different measurements from those set forth herein, depending on the size, aspect ratio, and resolution of the screen of the FED according to the first embodiment of the present invention.

FIGS. 11A through 11Cillustrate electron beam emission simulation results of the FED90ofFIGS. 1A and 1B. Referring toFIG. 11A, an electron beam emitted from an emitter16of the FED90disperses widely toward fluorescent layers23of the FED90. The vertical axis inFIG. 11Brepresents current density. Referring toFIG. 11B, peaks in the current density are located near the edges of a pixel, rather than the center of the pixel, because most electrons are emitted from the edges of the emitters16, as described above. If a central portion of the pixel has a low current density, fluorescent materials of the pixel cannot be sufficiently excited, thereby decreasing the brightness of an image displayed on the screen of the FED90. Particularly, in a case where emitters are not exactly arranged where they are supposed to be arranged, or in a case where front21and rear11substrates of the FED90are not precisely aligned with each other when bonding them together, peaks in current density are likely to be located near the edges of each pixel of the FED90, which results in a considerable decrease in color purity. Referring toFIG. 11C, the spot of an electron beam arriving at a fluorescent layer of the FED undesirably encroaches upon another pixel. In short, the FED90ofFIGS. 1A and 1Bmay end up in low color purity and low picture quality.

FIGS. 12A through 12Cillustrate electron beam emission simulation empirical results of the FED100according to the first embodiment of the present invention as shown inFIG. 3, modified for the case where there is no cavity111aperforating cathode electrode111(hereinafter referred to as modified FED100). Referring toFIG. 12A, electron beam emitted from emitters115that are respectively arranged at both sides of a first aperture112aof this modified FED100according to the first embodiment of the present invention are more focused and less dispersed than the electron beams of FED90ofFIGS. 1A and 1B. This improvement in the electron beam of the modified FED100is caused by the electric field formed by the conductive layer112. Referring toFIG. 12B, peaks in current density are generally located in a central portion of a pixel, unlike the empirical results of FED90illustrated inFIG. 11B.

Accordingly, as shown inFIG. 12C, the size of the spot of an electron beam arriving at a fluorescent layer is much smaller in this modified FED100than in FED90, and thus it is possible to solve the problem of the FEDs ofFIGS. 1A,1B,2A,2B and Macauley '659 that an electron beam aimed at one pixel encroaches upon another pixel as well. Even though current density is generally lower in the electron beam of modified FED100than in FED90, color purity of an image is higher for modified FED100than for FED90because the focusing characteristics of electron beams emitted from the emitters115of the modified FED100according to the first embodiment of the present invention are considerably improved, compared to FED90ofFIGS. 1A and 1B. In addition, since peaks in the current density are located in a central portion of each pixel for modified FED100, the brightness of an image displayed on the screen of the modified FED100according to the first embodiment of the present invention can be compensated for.

Turning toFIGS. 13A,13B and13C,FIGS. 13A through 13Cillustrate electron beam emission simulation empirical results of the FED100according to the first embodiment of the present invention, shown inFIG. 3, in a case where there is a one-to-one correspondence between cavities111aperforating cathode electrode111and pixels125.

Referring toFIG. 13A, due to the cavity11aformed in each cathode electrode111of the FED100ofFIG. 3, an electric field is formed around the emitters115so that the emitters115can be surrounded by equipotential lines of the electric field. Due to the electric field, electron beams emitted from the emitters115that are respectively disposed at both sides of a first aperture112acan be efficiently focused proceeding toward fluorescent layers122.

Referring toFIG. 13B, a peak in current density is precisely located in a central portion of a pixel. Accordingly, as shown inFIG. 13C, the size of the spot of an electron beam arriving at a fluorescent layer122is much smaller in a case where a cavity111ais formed in each cathode electrode111of the FED100according to the first embodiment of the present invention than in a case where no cavity111ais formed in each cathode electrode111of the corresponding modified FED100. In addition, current density is higher in a case where a cavity111ais formed in each cathode electrode111of the FED100according to the first embodiment of the present invention than in a case where no cavity111ais formed in each cathode electrode111of the corresponding modified FED100as well as the FEDs ofFIGS. 1A,1B,2A and2B. Therefore, by forming a cavity111ain each cathode electrode111of an FED, it is possible to enhance the focusing characteristics of electron beams, increase current density, place a peak in the current density in a central portion of each pixel of the FED, and eventually improve the color purity and brightness of the FED.

Turning now toFIGS. 14A,14B and14C,FIGS. 14A through 14Cillustrate electron beam emission simulation empirical results of the FED100according to the first embodiment of the present invention, shown inFIG. 3, in a case where the width Wc of the cavity111aformed in each cathode electrode111of the corresponding FED has been changed so that it is larger than the FEDs whose results are shown inFIGS. 13A,13B and13C.

Referring toFIG. 14A, an electric field is formed around the emitters115so that the emitters115can be better surrounded by equipotential lines of the electric field than in FIG.12A. Referring toFIG. 14B, a peak in current density is precisely located in a central portion of a pixel. Accordingly, as shown inFIG. 14C, the size of the spot of an electron beam arriving at a fluorescent layer122is much smaller than inFIG. 13C. In addition, the current density is also much higher inFIG. 14Cthan inFIG. 13C. Therefore, by adjusting the width Wc of a cavity111aformed in each cathode electrode111of FED100, it is possible to considerably increase current density, efficiently focus electron beams, and eventually realize high quality images.

FIGS. 15A,15B, and15C are diagrams illustrating empirical results of electron beam emission simulation results of the FED107ofFIG. 7. Referring toFIG. 15A, due to a conductive layer112′, which is formed of an insulation material layer1121and a metal layer1122, and a cavity111a, which is formed in a cathode electrode111, an electric field is formed around emitters115so that the emitters115can be surrounded by equipotential lines of the electric field. Accordingly, electron beams emitted from the emitters115can be efficiently focused. Therefore, as shown inFIG. 15B, peaks in current density are precisely located in their respective pixels. In addition, as shown inFIG. 15C, the size of a spot of an electron beam on a fluorescent layer122is very small. As described above, the FED107ofFIG. 7can have the same effects as the FED100of FIG.

FIGS. 16A and 16Bare diagrams illustrating electron beam emission simulation results of the FED108ofFIG. 8. Referring toFIGS. 16A and 16B, the FED108ofFIG. 8, in which a conductive layer112″ is formed on the top surface of an insulation material layer1123so that it can be insulated from a cathode electrode111, has the same effects as the FEDs100and107ofFIGS. 3 and 7. The FED108ofFIG. 8can focus electron beams more efficiently than the FEDs100and107ofFIGS. 3 and 7by adjusting a voltage applied to the conductive layer.

As described above, the FEDs according to the present invention can improve the focusing characteristics of electron beams emitted from emitters resulting in increased color purity of images and thus realize high quality images. In addition, the FED according to the present invention can improve the brightness of images by precisely placing a peak in current density in each pixel.