Patent Application
20060066215
The present application claims priority from Japanese application JP 2004-288391 filed on Sep. 30, 2004, the content of which is hereby incorporated by reference into this application.
1. Field of the Invention
The present invention relates to an image display device and a method for manufacturing the same, and particularly relates to an image display device also referred to as an emissive flat panel display using electron emitter arrays.
2. Description of the Background Art
An image display device (Field Emission Display: FED) using cathodes that are microscopic and can be integrated has been developed. Cathodes of such an image display device are categorized into field emission cathodes and hot electron emission cathodes. The former includes Spindt type cathodes, surface-conduction electron emission cathodes, carbon-nanotube cathodes, and the like. The latter includes thin-film cathodes of an MIM (Metal-Insulator-Metal) type comprised of a metal-insulator-metal lamination, an MIS (Metal-Insulator-Semiconductor) type comprised of a metal-insulator-semiconductor lamination, a metal-insulator-semiconductor-metal type, and the like.
For example, the MIM type has been disclosed in Patent Document 1. An MOS type (disclosed in Non-Patent Document 1 or the like) has been reported as the metal-insulator-semiconductor type. An HEED type (disclosed in Non-Patent Document 2 or the like), an EL type (disclosed in Non-Patent Document 3 or the like), a porous silicon type (disclosed in Non-Patent Document 4 or the like), etc. have been reported as the metal-insulator-semiconductor-metal type.
For example, an MIM type cathode is disclosed in Patent Document 2. The structure and operation of the MIM type cathode will be described below. That is, the MIM type cathode has a structure in which an insulator is inserted between an upper electrode and a base electrode. When a voltage is applied between the upper electrode and the base electrode, electrons near the Fermi level in the base electrode penetrate a barrier due to a tunneling phenomenon, so as to be injected into a conductive band of the insulator serving as an electron accelerator. Hot electrons formed thus flow into a conductive band of the upper electrode. Of the hot electrons, ones reaching the surface of the upper electrode with energy not smaller than a work function φ of the upper electrode are released to the vacuum.
Patent Document 1:
Patent Document 2:
Patent Document 3:
Non-Patent Document 1:
Non-Patent Document 2:
Non-Patent Document 3:
Non-Patent Document 4:
Such cathodes are arranged in a plurality of rows (for example, horizontally) and a plurality of columns (for example, vertically) so as to form a matrix. A large number of phosphors arrayed correspondingly to the cathodes respectively are disposed in the vacuum. Thus, an image display device can be configured. In order to perform image display in the image display device configured thus, a driving method called “one line at a time driving scheme” is adopted typically. This is a system in which, when 60 still images (60 frames) per second are displayed, each frame is displayed by scan line (horizontally). Accordingly, all the cathodes corresponding to the number of data lines on one and the same scan line are activated concurrently. A current flowing into the scan lines which are active can be obtained by multiplying, by the total number of scan lines, a current consumed by cathodes included in sub-pixels (sub-pixels constituting a color pixel for full color display). This scan line current leads to a voltage drop along the scan lines due to wiring resistance, so as to prevent uniform operation of the cathodes. Particularly in order to attain a large-size display device, the voltage drop caused by the wiring resistance of the scan lines becomes a large problem.
In order to solve the problem, it is necessary to reduce the wiring resistance of the scan lines. In the case of a thin-film cathode, it can be considered to reduce the resistance in a base electrode or an upper bus electrode (scan line) for supplying power to an upper electrode. However, when the thickness of the base electrode is increased to reduce the resistance, the irregularities of the wiring may be intense, the quality of an electron accelerator may deteriorate, or the upper bus electrode or the like may be disconnected easily. Thus, there occurs a problem in reliability. It is therefore preferable to use a method for reducing the resistance of the upper bus electrode so as to use the upper bus electrode as a scan line.
In order to reduce the wiring resistance of the upper bus electrode, it is effective to use a material low in resistivity and thick in film thickness. Copper Cu is the next lowest in resistivity to silver Ag. Cu is inexpensive and high in sputtering film formation rate. Thus, Cu is thickened easily. In addition, Cu can be formed into a thick film also by plating. Therefore, Cu is a material suitable to the upper bus electrode. However, Cu is oxidized easily. For example, when Cu is applied to an FED panel, Cu is oxidized easily in a high temperature frit sealing process. Therefore, the present inventors have considered that Cu is sandwiched in metal high in heat resistance and oxidation resistance from above and below so as to be prevented from being oxidized (Patent Document 3). When Cu is sandwiched in high-oxidation-resistance metal from above and below, a major part of Cu is prevented from being oxidized, but oxidation of the wiring side surface thereof cannot be prevented. It is desired to combine the upper bus electrode with a mechanism for separating the upper electrode from other upper electrodes by self-alignment. However, in Patent Document 3, an undercut portion formed out of Cu and the lower film is deformed due to the oxidization of the wiring side surface so that the pixel separation characteristic may deteriorate.
A first object of the present invention is to provide an image display device in which each upper bus electrode has a structure using a laminated wire made of metal high in heat resistance and oxidation resistance, and metal low in resistance and sandwiched in the metal high in heat resistance and oxidation resistance, so as to separate an upper electrode from upper electrodes by self-alignment, so that deformation of an undercut portion due to oxidization of a side surface of the low-resistance metal is suppressed to improve the self-alignment separation characteristic of the upper electrode.
In order to reduce the wiring resistance of the upper bus electrode, for example, it is effective to form a silver Ag or gold Au electrode by screen printing. Further, the upper bus electrode is required to have a structure for separating the upper electrode from other upper electrodes by self-alignment, and a function as a spacer electrode (a function of electrically connecting a spacer to the upper bus electrode) on which a spacer can be placed so that the spacer can be prevented from being charged, while a lower-layer wire or the like can be prevented from being mechanically damaged due to the atmospheric pressure applied onto the spacer. However, a complicated structure for attaining a pixel separation characteristic to separate the upper electrode from other upper electrodes by self-alignment cannot be made up by screen printing.
Patent Document 3 discloses a technique for laminating a thick-film wire on a thin-film wire formed by vacuum deposition or the like, by screen printing or the like using Ag or the like. In screen printing using paste of Ag, Au or the like, high-temperature heat treatment is performed in the condition that oxygen exists, for example, in the atmosphere in order to burn down a binder when the paste is sintered. As a result, the surface of the thin-film wire is oxidized so that the contact resistance between the thin-film wire and the thick-film wire is increased. Substantially the resistance of the thick-film wire cannot be reduced.
A second object of the present invention is to provide an image display device including low-resistance upper bus electrodes, wherein each upper bus electrode has a laminated structure made of a thin-film wire and a thick-film wire laminated on the thin-film wire by printing, while increase in contact resistance with the thick-film wire due to oxidization of the thin-film wire is suppressed when the laminated structure is produced.
In order to attain the first object, a manufacturing method is used as follows. Al or an Al alloy high in oxidation resistance is used as a low resistance material, and upper and lower electrodes are formed out of Cr, a Cr alloy, or the like, high in oxidation resistance and higher in standard electrode potential than Al. The Cr, Cr alloy or the like is selectively etched with respect to the Al or Al alloy, so that a lower-layer electrode of the Cr, Cr alloy or the like projects on one side, while the lower-layer electrode of the Cr, Cr alloy or the like forms an undercut on the other side with respect to the Al or Al alloy electrode. The metal material of the Cr, Cr alloy or the like higher in electrode potential is selectively etched with respect to the Al or Al alloy lower in electrode potential so as to form an undercut by wet etching. To this end, the film thickness of the upper layer of the Cr, Cr alloy or the like is made thicker than that of the lower layer. In addition, the exposed area of the Al or Al alloy not covered with the Cr, Cr alloy or the like of the upper layer is limited to control the local cell effect between the Al or Al alloy and the Cr, Cr alloy or the like. Thus, a proper undercut distance is secured.
In order to attain the second object, according to the present invention, the surface of a thin-film wire forming an upper bus electrode is coated with conductive oxide.
According to the aforementioned means for attaining the first object, it is possible to provide an image display device in which the deformation of the undercut portion can be suppressed to improve the self-alignment separation characteristic of the upper electrode.
According to the aforementioned means for attaining the second object, a low resistance upper bus electrode (scan electrode) can be produced without degrading the pixel separation characteristic even after high-temperature heat treatment in an oxygen containing atmosphere in a sealing process of the image display device. Accordingly, an image uniform in luminance within a display area can be obtained.
The best mode for carrying out the present invention will be described below in detail with reference to the drawings and in connection with embodiments. First, an example of an image display device according to the present invention will be described as an image display device using MIM type cathodes. However, the present invention is not limited to such MIM type cathodes. Not to say, the present invention is applicable to an image display device using various electron emission devices described in the chapter of the background art, in the same manner. Particularly the present invention is effective in hot electron emission cathodes or surface-conduction electron emission cathodes using thin electron emission electrodes and requiring low-resistance bus electrodes for releasing only a part of a device current into the vacuum.
In the cathode substrate 10, there are formed base electrodes 11, a metal film lower layer 16, a metal film intermediate layer 17, a metal film upper layer 18, protective insulators (field insulators) 14, other functional films which will be described later, etc. The base electrodes 11 constitute signal lines (data lines) connected to a data line driving circuit 50. The metal film lower layer 16, the metal film intermediate layer 17 and the metal film upper layer 18 form scan lines 21 connected to a scan line driving circuit 60 and disposed perpendicularly to the data lines. Each cathode (electron emission portion) is formed out of an upper electrode (not shown) connected to the upper bus electrode and laminated to the base electrode 11 through the insulator. Electrons are released from the portion of an insulator (tunneling insulator) 12 formed out of a thin layer portion of the insulator.
Referring to
The spacers 30 are disposed on the scan electrodes 21 constituted by the upper bus electrodes of the cathode substrate 10 so as to be hidden under the black matrix 120 of the phosphor substrate. The base electrodes 11 are connected to the data line driving circuit 50, and the scan electrodes 21 serving as the upper bus electrodes are connected to the scan line driving circuit 60.
In the cathode structure according to Embodiment 1, there is formed a laminated structure in which a low-resistance wire of Al or an Al alloy having heat resistance and oxidation resistance is sandwiched in Cr, a Cr alloy or the like having heat resistance and oxidation resistance. Accordingly, the upper bus electrode can be produced so that the upper bus electrode will not deteriorate even after the sealing process. Thus, a voltage drop due to the wiring resistance of the display device can be suppressed.
In each of the MIM type cathodes shown in
The material of the scan electrode 21 serving as the upper bus electrode is made of a three-layer lamination in which Al or an Al alloy high in oxidation resistance is sandwiched in Cr or a Cr alloy high in oxidation resistance from above and below. Due to the heat resistance and the oxidation resistance of Cr or the Cr alloy, damage on wiring can be avoided in the process or the like where the panel of the image display device is sealed at a high temperature. In addition, the request for reduction in resistance of wiring can be also satisfied when the wiring is thickened by use of the Al or Al alloy layer low in resistivity. For example, an Al—Nd alloy including 2 at % of Nd is used as the Al alloy, and a Cr alloy including at least 10 wt % of Cr is used as the Cr alloy. According to such a Cr alloy, a Cr2O3 barrier film formed in the surface prevents diffusion of oxygen to thereby prevent oxidization from progressing to the inside of the electrode even when heating treatment in the FED panel sealing process is performed in the atmosphere at 400° C. or higher as shown in
Next, an embodiment of the method for manufacturing the image display device according to the present invention will be described with reference to
After the film formation, the base electrode 11 having a stripe shape is formed by a patterning process and an etching process (
Next, a protective insulator 14 for limiting an electron emission portion and preventing electric field concentration on the edge of the base electrode 11, and an insulator 12 are formed. First, a portion which will be an electron emission portion on the base electrode 11 as shown in
Next, an interlayer film (interlayer insulator) 15, and a metal film serving as an upper bus electrode serving as a power feeder to the upper electrode 13 and a spacer electrode for disposing a spacer 30 are formed, for example, by a sputtering method or the like (
Here, pure Al is used for the metal film intermediate layer 17, and Cr is used for the metal film lower layer 16 and the metal film upper layer 18. The film thickness of pure Al is made as thick as possible in order to reduce the wiring resistance. The film thickness of the metal film upper layer is made not smaller than that of the metal film lower layer. Here, the metal film lower layer 16 is made 100 nm thick, the metal film intermediate layer 17 is made 4.5 μm thick, and the metal film upper layer 18 is made 200 nm thick.
Successively, the metal film upper layer 18 and the metal film intermediate layer 17 are formed into stripe shapes perpendicular to the base electrode 11 by two stages of patterning and etching. For example, wet etching with a cerium ammonium nitrate solution is used for etching Cr of the metal film upper layer 18, and wet etching with a mixed aqueous solution of phosphoric acid, acetic acid and nitric acid is used for etching of pure Al of the metal film intermediate layer 17 (
Successively, the metal film lower layer 16 is processed into a stripe shape perpendicular to the base electrode 11 by patterning and etching (
FIGS. 11 to 13 are schematic views for explaining the method for forming the upper bus electrode in more detail.
Successively, the interlayer insulator 15 is processed to open an electron emission portion. The electron emission portion is formed in a part of a perpendicular portion of a space surrounded by one base electrode 11 in the pixel and two upper bus electrodes perpendicular to the base electrode 11. For example, dry etching with an etching agent having CF4 or SF6 as its main component can be used for the etching (
Finally, a film of the upper electrode 13 is formed. For example, sputtering film formation is used as the method for forming the film. For example, a laminated film of Ir, Pt and Au is used as the upper electrode 13, and the film thickness is made 6 nm. In this event, the upper electrode 13 has a structure in which the upper electrode 13 is cut by the appentice structure in one of the two upper bus electrodes sandwiching the electron emission portion, while the upper electrode 13 is connected to the other upper bus electrode through the contact portion of the metal film lower layer 16 without disconnection so as to be supplied with power (
According to Embodiment 1, each upper electrode can be separated by self-alignment, while uneven luminance caused by a voltage drop can be suppressed even in a large size display device due to a laminated structure in which an Al layer of a metal film intermediate layer is sandwiched between a metal film upper layer and a metal film lower layer both made of Cr having heat resistance and oxidation resistance.
According to the present invention, a thick-film wire formed by a printing method such as screen printing is laminated to an upper bus electrode serving as a scan electrode as described above. In order to reduce the wiring resistance of the upper bus electrode, a thick-film wire is printed on a metal film upper layer of a Cr—Al—Cr multilayer structure of the upper bus electrode, in addition to the upper bus electrode with the Cr—Al—Cr multilayer structure by which adjacent pixels are separated by self-alignment as described in Embodiment 1. After the printing, it is necessary to provide a process for burning the thick film in an oxygen containing atmosphere at 400° C.-450° C. so as to burn down a binder etc. In this event, the surface of the Cr layer of the metal film upper layer is oxidized to provide a large contact resistance with the thick film formed on the Cr layer.
In order to reduce the wiring resistance of the upper bus electrode, it is effective to use a thick film material low in resistivity. For example, it is effective to form an electrode by screen printing with Ag paste or Au paste. Further, the upper bus electrode is required to have a structure for separating the upper electrode from other upper electrodes by self-alignment, and a function as a spacer electrode on which a spacer can be placed so that the spacer can be prevented from being charged, while the cathode can be prevented from being mechanically damaged due to the atmospheric pressure applied onto the spacer. However, a complicated structure for separating the upper electrode from other upper electrodes by self-alignment cannot be made up by screen printing.
Patent Document 3 discloses a technique for laminating a thick-film wire on a thin-film wire by printing using Ag or the like. In screen printing using paste of Ag, Au or the like, high-temperature heat treatment is performed in the condition that oxygen exists, for example, in the atmosphere in order to burn down a binder or the like when the paste is sintered. As a result, the surface of the thin-film wire is oxidized so that the contact resistance between the thin-film wire and the thick-film wire formed by screen printing may increase. Thus, there may occur a problem that the resistance cannot be reduced substantially.
Here, the process for manufacturing the cathode substrate according to Embodiment 2 will be described. As far as the metal film upper layer 18 of the upper bus electrode, the process is similar to that in Embodiment 1. That is, Cr is used for the metal film lower layer 16, Al is used for the metal film intermediate layer 17, and Cr is used for the metal film upper layer 18. A film of ITO is formed on Cr of the metal film upper layer 18 by sputtering.
Successively, the conductive oxide 19, the metal film upper layer 18 and the metal film intermediate layer 17 are processed into a stripe electrode perpendicular to the base electrode 11 by patterning and etching. ITO of the conductive oxide 19 is etched with a solution of oxalic acid. The metal film upper layer 18 and the metal film intermediate layer 17 are etched in the same manner as in Embodiment 1.
Successively, the metal film lower layer 16 is patterned and etched so that one stripe electrode perpendicular to the base electrode 11 is formed in one pixel. In this event, in the same manner as in Embodiment 1, one side of the stripe electrode is made to project over the metal film intermediate layer 17 so as to serve as a contact portion for securing connection with the upper electrode 13 in a subsequent process. On the other side of the stripe electrode, an undercut is formed using the metal film intermediate layer 17 as a mask, so as to form an appentice for separating the upper electrode 13 from the other upper electrodes 13 in a subsequent process. Thus, an upper bus electrode for feeding power to the upper electrode 13 can be formed.
Successively, the interlayer film 15 is processed to open an electron emission portion. The electron emission portion is formed in a part of a perpendicular portion of a space surrounded by one base electrode 11 of a sub-pixel and two stripe electrodes perpendicular to the base electrode 11. For example, dry etching with an etching agent having CF4 or SF6 as its main component is used for the etching for processing the interlayer film 15 in the same manner as in Embodiment 1.
Next, a film of the upper electrode 13 is formed. For example, sputtering is used as the method for forming the film. A laminated film of Ir, Pt and Au is used as the upper electrode 13, and the film thickness is made 6 nm by way of example. In this event, the upper electrode 13 has a structure in which the upper electrode 13 is cut by the undercut in one of the adjacent stripe-shaped scan electrodes, while the upper electrode 13 is connected to the other stripe-shaped scan electrode through the contact portion of the metal film lower layer 16 without disconnection so as to be supplied with power over the interlayer film 15 and across the insulator 12.
Finally, Ag paste is printed onto the upper electrode 13 by a screen printing method, so as to form a thick-film electrode 20. Ag paste can be formed to be about 10-20 μm thick. Accordingly, the wiring resistance can be reduced, and the pressure from the spacer can be absorbed. Further, due to the conductivity of the thick-film electrode 20, the spacer can be prevented from being charged. After the thick-film electrode 20 is dried, the thick-film electrode 20 is burnt in a high-temperature process at 400-450° C. for sealing the two substrates (cathode substrate and phosphor substrate) constituting the image display device. In this event, the surface of the upper bus electrode which is a thin film is made of ITO of the conductive oxide 19. Accordingly, even after the high temperature heat treatment in an oxidizing atmosphere such as the atmosphere, a failure in contact between the thin-film upper bus electrode and the Ag or Au thick-film electrode due to surface oxidization or the like can be prevented. Thus, a low-resistance scan line can be obtained.
The spacers 30 in
According to the configuration of Embodiment 2, the upper bus electrode is formed out of a laminated film of a thin-film electrode of conductive oxide having a structure for separating the upper electrode by self-alignment, and a thick-film electrode having a function of reducing the wiring resistance, a function of absorbing pressure from the spacer and a function of electrically connecting the spacer to thereby prevent the spacer from being charged. Accordingly, it is possible to obtain an image display device having thin-film cathodes in which a voltage drop in the scan wiring can be reduced, the lower layer wiring can be protected from mechanical damage from the spacers, and the spacers can be prevented from being charged.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2004-288391 | Sep 2004 | JP | national |
