Description THE FIELD EMISSION DEVICE

Technical Field

[1] The present invention relates to a field emission device, and more particularly, to a field emission device whose field emitters and electrode structure are not damaged by a high- voltage anode. Background Art

[2] Flat panel displays may be broadly classified either as emissive displays or non- emissive displays.

[3] The emissive displays include Plasma Display Panels (PDPs), Field Emission

Displays (FEDs) and so forth, and the non-emissive displays include Liquid Crystal Displays (LCDs) and so forth.

[4] The LCD has a number of advantageous features including light weight and low power consumption. However, it is a non-emissive display that does not spontaneously emit light but displays images by reflecting external incident light. For this reason, the LCD has difficulty in displaying an image in a dark place. To solve this problem, the LCD has a backlight unit disposed at the rear side of the display panel.

[5] In the conventional backlight unit, a Cold Cathode Fluorescent Lamp (CCFL) and a

Light Emitting Diode (LED) are usually employed as a line light source and a point light source, respectively.

[6] However, such a backlight unit has the following disadvantages. That is, in general, its manufacturing costs are high because its structure is complex, and it consumes excessive power when light is reflected and transmitted because the light source is disposed at the side of the backlight unit. In particular, it becomes more difficult to ensure uniform brightness as the LCD becomes larger.

[7] Therefore, in recent years, a field emission device having a flat emissive structure has been developed. Such a field emission device is advantageous in that it consumes less power and achieves more uniform brightness over a larger area than the backlight unit using the CCFL and so on. In addition, the field emission device allows localized or regional brightness to be adjusted, and unlike the CCFL, the field emission device can be pulse driven so that the manufacture of the LCD having a high contrast ratio and no after-images is made possible.

[8] A field emission type backlight unit is generally vacuum-packaged and includes a cathode substrate where field emitters are formed and an anode substrate where a fluorescent material is formed. The two substrates face each other and are spaced apart by a predetermined distance. Electrons emitted from the field emitter impact against the

fluorescent material of the anode substrate causing it to emit light, a phenomenon known as cathodoluminescence.

[9] FIGS. IA to 1C illustrate a conventional field emission device of a top-gate type tripolar structure, which includes a cathode substrate 110, a cathode 112 formed on the cathode substrate 110, a field emitter 114 formed on the cathode 112, a gate insulator 130 formed on the cathode substrate 110, a gate electrode 132 formed on the gate insulator 130, an anode substrate 120, an anode 122 formed on the anode substrate 120, and a fluorescent layer 124 formed on the anode 122.

[10] First, according to the field emission device using a thin film dielectric as the gate insulator 130 as shown in FIG. IA, the height of the gate insulator 130 is low so that the field emitter 114 can be formed using a printing method or the like after the gate electrode 132 is formed, and its surface can be easily processed. However, the field emitter 114 and the electrode structure may be damaged by arcs, and its manufacturing cost is high because a lithography process is required.

[11] In response to the above problems, a field emission device having a structure in which a thick dielectric or a dielectric mesh substrate is used as the gate insulator 130 so that the height of the gate insulator 130 is formed high was developed as shown in FIG. IB. The field emission device of FIG. IB is unlikely to be damaged by arcing so that a high voltage can be relatively easily applied thereto. On the other hand, the field emitter 114 has to be formed and surface processing has to be carried out before the gate structures 130 and 132 are formed. Also, electrons emitted from the field emitter 114 impact against the thick gate insulator 130 and cause charge to accumulate there. Consequently, the thick gate insulator 130 cannot be easily formed when a large-sized element is manufactured.

[12] Meanwhile, in order to overcome the drawbacks of electron beam spreading and difficulty in applying a high voltage to the anode in the structure of FIG. IA, a method was proposed for forming a metal grid 140 above the gate electrode 132, as shown in FIG. 1C. In such a structure, the gate electrode 132 acts to induce field emission from the field emitter 114, and the metal grid 140 acts to focus electron beams emitted from the field emitter 114 or protect the field emitter 114 from arcs resulting from a high- voltage anode. However, in the field emission device of FIG. 1C, the distance between the gate electrode 132 and the cathode 112 is short, so that an electrical short circuit may easily occur. Disclosure of Invention Technical Problem

[13] The present invention is directed to a field emission device which has a simple structure, can be easily manufactured, and whose electrode structure and field emitters

are not damaged by arcs resulting from a high- voltage anode.

[14] The present invention is also directed to a field emission device which facilitates local dimming and pulse driving. Technical Solution

[15] One aspect of the present invention provides a field emission device including: an anode substrate and a cathode substrate spaced apart by a predetermined distance and facing each other; an anode and a fluorescent layer formed on the anode substrate; a cathode formed on the cathode substrate; a plurality of field emitters spaced apart from each other by a predetermined distance on the cathode; and a metal gate substrate disposed between the anode substrate and the cathode substrate, inducing electron emission from the field emitters, and having a plurality of openings through which the emitted electrons pass.

[16] Here, the longitudinal section of the openings of the metal gate substrate can be a tapered type such that the diameter in the direction of the anode substrate is larger than the diameter in the direction of the cathode substrate, and the thickness of the metal gate substrate may be 0.05mm to lmm.

[17] A first spacer may be formed between the cathode substrate and the metal gate substrate, and a second spacer may be formed between the anode substrate and the metal gate substrate. Here, the length of the first spacer may be 0.5 to 3 times a diameter of a section facing the cathode substrate among the longitudinal sections of the openings.

[18] In addition, a plurality of cathodes may be disposed on the cathode substrate at a predetermined distance apart and electrically isolated from each other, and the amount of current flowing into each of the cathodes may be adjusted by means of Pulse Width Modulation (PWM) or Pulse Amplitude Modulation (PAM) through a semiconductor switching circuit while a voltage applied to the metal gate substrate is fixed.

Advantageous Effects

[19] As described above, according to the present invention, a metal gate substrate is interposed between an anode substrate and a cathode substrate to act as a gate electrode that induces electron emission. Thus, a gate electrode need not be separately formed, which simplifies the structure of a field emission device and makes it easier to manufacture.

[20] In addition, according to the present invention, a current flowing into each of electrically isolated cathodes can be adjusted, so that the local brightness of a field emission device can be adjusted. Brief Description of the Drawings

[21] FIGS. IA to 1C illustrate a conventional field emission device of a top-gate type

tripolar structure; [22] FIG. 2 schematically illustrates a field emission device according to an exemplary embodiment of the present invention;

[23] FIG. 3 illustrates trajectories of electron beams in the structure shown in FIG. 2;

[24] FIG. 4 schematically illustrates a field emission device according to another exemplary embodiment of the present invention;

[25] FIG. 5 illustrates trajectories of electron beams in the structure shown in FIG. 4;

[26] FIG. 6 is a perspective view of a field emission device having the structure shown in

FIG. 4; [27] FIG. 7 schematically illustrates a field emission device in which circular field emitters and circular openings are formed; [28] FIG. 8 schematically illustrates a field emission device in which orthogonal pairs of rectangular openings are arranged;

[29] FIG. 9 schematically illustrates a field emission device in which a plurality of electrically isolated cathodes are formed; and [30] FIG. 10 schematically illustrates an external driving circuit for adjusting a current flowing into each of the cathodes in the structure shown in FIG. 9. [31] * Description of Major Reference Numerals in the Figures

[32] 110: cathode substrate

[33] 112: cathode

[34] 114: field emitter

[35] 120: anode substrate

[36] 122: anode

[37] 124: fluorescent layer

[38] 130: gate insulator

[39] 132: gate electrode

[40] 140: metal grid

[41] 210: cathode substrate

[42] 212: cathode

[43] 214: field emitter

[44] 220: anode substrate

[45] 222: anode

[46] 224: fluorescent layer

[47] 232: metal gate substrate

[48] 234: opening

[49] 242: first spacer

[50] 244: second spacer

Mode for the Invention

[51] Hereinafter, exemplary embodiments of the present invention will be described in detail. However, the present invention is not limited to the exemplary embodiments disclosed below, but can be implemented in various types. Therefore, the present exemplary embodiments are provided for complete disclosure of the present invention and to fully inform the scope of the present invention to those ordinarily skilled in the art.

[52] FIG. 2 schematically illustrates a field emission device according to an exemplary embodiment of the present invention.

[53] Referring to FIG. 2, the field emission device according to an exemplary embodiment of the present invention includes a cathode substrate 210, a cathode 212 formed on the cathode substrate 210, a field emitter 214 formed on the cathode 212, an anode substrate 220, an anode 222 formed on the anode substrate 220, a fluorescent layer 224 formed on the anode 222, a metal gate substrate 232 in which a plurality of openings 234 are formed, and first and second spacers 242 and 244.

[54] The cathode substrate 210 and the anode substrate 220 face each other and keep a predetermined distance from each other by means of the first and second spacers 242 and 244 interposed therebetween.

[55] The first spacer 242 acts to maintain a predetermined distance between the metal gate substrate 232 and the cathode substrate 210, and the second spacer 244 acts to maintain a predetermined distance between the metal gate substrate 232 and the anode substrate 220. Here, the second spacer 244 is preferably formed on the same axis as the first spacer 242, to effectively transmit pressure between the anode substrate 220 and the cathode substrate 210.

[56] At least one field emitter 214, and preferably a plurality of field emitters 214, spaced apart from each other by a predetermined distance, is formed on the cathode 212. The field emitters 214 are preferably formed of a material that has superior electron emission characteristics, such as carbon nanotubes, carbon nanofibers, carbon-based synthetic materials, etc.

[57] The anode 222 is formed on the anode substrate 220 facing the cathode substrate

210, and a fluorescent layer 224 is applied on the anode 222. The anode 222 can be formed of a transparent conductive material such as ITO, IZO, ITZO, etc.

[58] The metal gate substrate 232 is disposed between the cathode substrate 210 and the anode substrate 220, acts as a gate electrode that induces electron emission from the field emitters 214, and is maintained at a predetermined distance from the cathode substrate 210 and from the anode substrate 220 by means of the first and second spacers 242 and 244.

[59] The metal gate substrate 232 preferably has a thickness of at least 0.05mm to lmm, in order to prevent the metal gate substrate 232 from being bent in a region where the first and second spacers 242 and 244 are not formed.

[60] A plurality of openings 234 are formed on the metal gate substrate 232, and these openings 234 are arranged to correspond to positions of the field emitters 214 to allow electron beams emitted from the field emitters 214 to easily move toward the anode 222

[61] Thus, the field emission device of the present invention is characterized in that a separate gate electrode is not required and the metal gate substrate 232 itself acts as a gate electrode. The effects of using the metal gate substrate 232 in the present invention, compared to the related art described above, are as follows.

[62] As described above, since the conventional field emission device shown in FIG. IA usually has an electrode structure in which a dielectric thin film or a dielectric substrate is used as an insulator, the electrode structure may be damaged due to arcs when a high voltage is applied, and the manufacturing cost may be high because of a lithography process.

[63] In addition, in the field emission device having the structure shown in FIG. 1C, the metal grid 140 is additionally formed in the presence of the gate electrode 132 inducing electron emission from the field emitter 114 and acts to focus electron beams emitted from the field emitter 114 or protect the field emitter 114 from the effects of the high-voltage anode. However, the distance between the gate electrode 132 and the cathode 112 is short, so that an electrical short circuit may easily occur between the gate electrode 132 and the cathode 112 in the conventional field emission device having the structure described above.

[64] However, in the field emission device of the present invention, a gate electrode is not separately formed, and the metal gate substrate 232 having openings 234 for transmitting electron beams emitted from the field emitters is used as the gate electrode, so that the electrode structure is not damaged due to arcs resulting from the high- voltage anode and a gate leakage current is insignificant compared to the related art.

[65] In addition, since no separate gate electrode is prepared, the overall structure is simplified, fewer manufacturing processes are required, and electrical short circuiting between the gate electrode and the cathode can be prevented.

[66] Meanwhile, when the thick metal gate substrate 232 is used as the gate electrode instead of a general thin film electrode, it is preferable to decrease the number of first and second spacers 242 and 244 as much as possible. The goal is to decrease electric charge buildup resulting from the electron beams emitted from the field emitter 214 as much as possible without complicating manufacture.

[67] Here, noise from oscillation of the metal gate substrate 232 due to pulse driving may be generated in a region where the first and second spacers 242 and 244 are not formed. Thus, it is preferable to set the distance between the metal gate substrate 232 and each of the first and second spacers 242 and 244 so that the noise is outside of the audible range.

[68] The number of first spacers 242 between the metal gate substrate 232 and the cathode substrate 210 may be larger than the number of second spacers 244 between the metal gate substrate 232 and the anode substrate 220, if necessary.

[69] Meanwhile, when the thick metal gate substrate 232 is used as the gate electrode instead of a general thin film electrode, electron beams emitted from the field emitters 214 may impact against the metal gate substrate 232 and leak midway failing to reach the anode 222, as illustrated in FIG. 3.

[70] Referring to FIG. 3, it can be seen that some electron beams 301 emitted from the field emitter 214 pass through the openings 234 while the other electron beams 302 impact against the metal gate substrate 232. This reduces luminous efficiency and problems such as heating up the metal gate substrate 232 enough to cause deformation, etc. can occur.

[71] In response to this problem, the openings 234 formed in the metal gate substrate 232 may be tapered, as shown in FIG. 4. That is, the diameter a of the anode side of the opening 234 may be made greater than the diameter b of the cathode side of the opening, so that electron beams emitted from the field emitters 214 can travel toward the anode 222 without impacting against the metal gate substrate 232.

[72] FIG. 5 illustrates electron beams emitted from the field emitters 214 propagating without impact by making the diameter a of the anode side of the opening 234 greater than the diameter b of the cathode side of the opening. As illustrated in Fig. 3, in the event that the diameter of the cathode of the opening and the diameter of the anode of the opening are the same, electron beams 302 impact against the metal gate substrate 232 and fail to reach the anode 222 when the opening 234 is not tapered. However, referring to Fig. 5, it can be seen that the electron beams 302 reach the anode 222 by means of the tapered shape of the opening 234.

[73] Here, the degree of tapering of the openings 234 may be determined in consideration of the thickness of the metal gate substrate 232 and the distance between openings 234 and so on.

[74] Meanwhile, the height of the first spacer 242 is preferably made equal to or greater than the diameter b of the cathode side of the openings 234 to protect the field emitter 214 from effects of the high- voltage anode and its resultant arcs.

[75] When the length of the first spacer 242 is greater than the diameter of the openings

234, a sufficient electric field is generated in the opening 234, so that the field emitter

214 can be protected from effects of the high- voltage anode and its resultant arcs. However, when the length of the first spacer 242 is too great, the electron beam trajectory spreads out and increases a gate leakage current or a gate voltage required for field emission and so on. Thus, it is preferable for the length of the first spacer 242 to be 0.5 to 3 times the diameter b of the cathode side of the opening 234. When the cross-sectional shape of the opening 234 is not circular but elliptical or rectangular, for example, the short diameter or the short side thereof may be used as a reference.

[76] Meanwhile, the metal gate substrate 232 is preferably thick enough to maintain its distance from the cathode substrate 210, even when the first spacer 242 between the metal gate substrate 232 and the cathode substrate 210 and the second spacer 244 between the metal gate substrate 232 and the anode substrate 220 are arranged at the same interval. When the openings 234 are formed to be smaller on the side of the anode substrate 220 as described above, even though the metal gate substrate 232 is thick, the amount of leakage current can be considerably reduced. FIG. 6 illustrates a field emission device according to an exemplary embodiment of the present invention whose openings 234 are formed to be smaller on the side of the anode substrate 220 as described above.

[77] Meanwhile, a shape that is more appropriate for the openings 234 is needed to yield more uniform fluorescent emission. The openings 234 formed in the metal gate substrate 232 of the present invention may have several shapes, including circles, squares, ellipses, rectangles, etc.

[78] For example, FIG. 7 illustrates a field emission device having circular field emitters

214 and circular cross-sectional openings 234, and FIG. 8 illustrates a field emission device having linear field emitters 214 and a repeated pattern of orthogonal pairs of rectangular openings 234. Although not shown in the drawings, a repeated pattern of orthogonal groups of three or more rectangular openings 234 may be employed, if necessary.

[79] In the field emission device having the structure shown in FIG. 8, the intensity of the electric field applied to the field emitters 214 is formed anisotropically corresponding to the shape of the openings 234, and is stronger toward the short side of the rectangular openings 234. Accordingly, electron beams are emitted from the field emitters 214 toward the short side of the rectangular openings 234. Meanwhile, since pairs of two rectangular openings 234 are repeatedly patterned to be orthogonal to each other, the electron beams are uniformly emitted to reach the anode 222, thereby allowing the fluorescent emission to be more uniform compared to the structure shown in FIG. 7.

[80] Meanwhile, in the case of the general field emission device or the field emission lamp allowing local dimming, the gate electrode in the sub-pixel or the specific block

region is electrically isolated from the adjacent sub-pixel or the block, so that a grayscale can be represented by inputting different electrical signals to adjust electron beam current emitted from the field emitters.

[81] However, in the case of the metal gate substrate 232 used as the gate electrode described above, the overall metal gate substrate 232 has the same potential so that different gate voltages per sub-pixel or block cannot be applied. Thus, a new driving method is required.

[82] To this end, gray scale representation of a specific portion and pulse driving can be implemented by adjusting the amount of current flowing into the cathode 212 while the voltage applied to the metal gate substrate 232 and the anode 222 is fixed in the present invention, which is described in more detail below.

[83] FIG. 9 illustrates the field emission device in which the cathodes 212 are electrically isolated per pixel or block to supply different currents to the field emitters 214 of each cathode 212 so that the electron beams can be adjusted to implement gray scale representation.

[84] Referring to FIG. 9, several cathodes 212 are electrically isolated from each other to form one block and a driving circuit for current control is connected to each block. At least one field emitter 214 is formed in each block, and the amount of electrons emitted from the field emitter 214 formed in each block can be adjusted by controlling the current applied to each block, so that the local brightness can be adjusted.

[85] In the structure shown in FIG. 9, the amount of current flowing into each cathode

212 can be adjusted using a semiconductor switching circuit 252 such as Thin Film Transistor (TFT) or a Metal Oxide Semiconductor Field Effect Transistor (MOSFET), as shown in FIG. 10. And the amount of electrons emitted from the field emitter 214 through each of the electrically isolated cathodes 212 can be adjusted by means of Pulse Width Modulation (PWM) or Pulse Amplitude Modulation (PAM). Here, the metal gate substrate 232 has the same potential at every part of the panel.

[86] As described above, the field emission device of the present invention uses the metal gate substrate 232 as the gate electrode, so that a separate gate electrode need not be formed. The field emission device thus has a simpler structure, and can be manufactured with fewer processes than the conventional field emission device. In addition, current flowing into the electrically isolated cathodes 212 can be adjusted to adjust local brightness.

[87] While the invention has been shown and described with reference to certain exemplary embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.