US20070000782A1

US20070000782A1 - Method for batch fabricating electron emission source of electrophoresis deposited carbon nanotubes

Info

Publication number: US20070000782A1
Application number: US11/168,487
Authority: US
Inventors: Kuei-Wen Cheng; Jin-Lung Tsai; Shie-Heng Lee; Yu-An Li; Chun-Yen Hsiao
Original assignee: Teco Electric and Machinery Co Ltd
Current assignee: Teco Electric and Machinery Co Ltd
Priority date: 2005-06-29
Filing date: 2005-06-29
Publication date: 2007-01-04

Abstract

A method for batch fabricating electron emission source of electrophoresis deposited carbon nanotubes includes preparing a semi-manufactured cathode structure and a metallic plate connected with electrophoresis electrodes. The cathode structure and the metallic plate that are parallel to each other are separated with a fixed distance and are rinsed in an electrophoresis solution. A processe is performed to remove the bubbles formed in the cathode structure, and to electrophoresis deposit the carbon nanotubes onto the cathode structure. After each deposition process is completed, the cathode structure is baked under a low temperature so as to remove the residual solution remained on the cathode structure. After a homogeneous carbon nanotubes layer is deposited to form the electron emission source, a sintering process is performed so as to transfer the chargers into conductive metallic oxide salts. Thereby, the electron conduction of the carbon nanotubes and the cathode electrode layer is enhanced.

Description

BACKGROUND OF THE INVENTION

The present invention relates generally to a method for batch fabricating electron emission source of electrophoresis deposited carbon nanotubes, and more particularly to a method that removes the bubbles formed in the cathode structure before the electrophoresis deposition, so as to uniformly deposit the carbon nanotube powder on the cathode structure, thereby forming an electron emission source.
Conventional triode field emission display includes an anode structure and a cathode structure. There is a spacer disposed between the anode structure and the cathode structure, thereby providing a space and a support for the vacuum region between the anode structure and the cathode structure. The anode structure includes an anode substrate, an anode conducting layer, and a phosphorus layer. The cathode structure includes a cathode substrate, a cathode electrode layer, an electron emission source layer, a dielectric layer and a gate layer. The gate layer is provided a voltage difference to induce the emission of electrons from the electron emission source layer. The conducting layer of the cathode structure provides a high voltage to accelerate the electron beam, such that the electron beam can have enough kinetic energy to impinge and excite the phosphorous layer on the anode structure, thereby emitting light. Accordingly, in order to maintain the movement of electrons in the field emission display, a vacuum apparatus is required to keep the vacuum degree of the display being below 10⁻⁵torr. Therefore, the electrons can have appropriate mean free paths. Meanwhile, the pollution and toxication of the electron emission source and the phosphorous layer should be prevented from happening. Furthermore, in order for the electrons to accumulate enough energy to impinge the phosphorous powder, a space is required between the two substrates. Thus, the electrons can be accelerated to impinge the phosphorous layer, thereby exciting the phosphorous layer and emitting light therefrom.
The electron emission source layer is composed of carbon nanotubes. Since carbon nanotubes, discovered by Iijima in 1991 (Nature, 354, 56 (1991)), comprises very good electronic properties, they can be used to build a variety of devices. The carbon nanotubes also has a very large aspect ratio, mostly larger than 500, and a very high rigidity of Young moduli larger than 1000 GPn. In addition, the tips or defects of the carbon nanotubes are of atomic scale. The properties described above are considered an ideal material for building electron field emitter, such as an electron emission source of a cathode structure of a field emission display. Since the carbon nanotubes comprise the physical properties described above, many manufacturing processes that pattern the carbon nanotubes on an electronic device can be developed, e.g. screen printing, or thin film processing.
However, the art of manufacturing the cathode structure employs carbon nanotubes as an electron emission material, which is fabricated on the cathode conducting layer. The manufacturing process can employ chemical vapor deposition process, or any kind of process that can pattern the photosensitive carbon nanotube solution on any pixel of the cathode conducting layer. Moreover, the cathode structure can also be manufactured by coating the carbon nanotubes solution while incorporating with a mask, or depositing the carbon nanotubes on the cathode conducting layer by an electrophoresis method. Nevertheless, according to the electron emission source structure of the triode field emission display described above, it is still difficult to lower the cost of manufacturing process and to fabricate cubic structures on each pixel of the cathode electrode structure. In particular, it is difficult to achieve high homogeneity of large size electron emission source.
Recently, a so-called electrophoresis deposition (EPD) technology has be developed and disclosed in the United States Patent Publication No. US2003/0102222A1 entitled “Deposition Method for Nanostructure Materials.” In this patent publication, the carbon nanotubes are formulated into alcoholic suspension solution. On the other hand, ionic salts of magnesium, lanthanum, yttrium, aluminum act as a charger to prepare electrophoresis solution. The cathode structure to be deposited is connected with one electrode of the electrophoresis solution. By providing a DC or AC voltage, an electric field is formed in the solution. The chargers in the solution are then dissolved into ions, so as to adhere onto the powder of carbon nanotubes. For this reason, the electric field forms an electrophoresis force to assist the carbon nanotubes depositing onto a certain electrode. In this manner, the carbon nanotubes are disposited on a patterned electrode. By using the so-called electrophoresis technology described above, the carbon nanotubes are easily deposited onto an electrode and can easily circumvent the limitations of the cathode structure of the triode field emission display. Therefore, this method is widely used in the application of cathode plate fabrication.
Although the electrophoresis deposition method has been widely adopted, part of the mechanism requires improvements. For example, the ions of chargers are deposited together on the electrode. In general, the anode ions react with the cathode ions (the OH cathode ions from the oxidated water molecule in the solution) on the surface of electrode to form salt hydroxide, and are deposited together with the carbon nanotubes. Since the electrophoresis process requires a repeated baking process to remove the unnecessary solvent and organic material, such metallic salt hydroxide will be converted into metallic salt oxide. Taking the conventional magnesium chloride as an example, the salts of magnesium oxide will be deposited on the electrode together with the carbon nanotubes, or on the surface of carbon nanotubes. Since the magnesium oxide is not a good conductor, although the weighted concentration thereof in the electrophoresis process is generally controlled to be below 0.1%, which does not affect the efficiency for generating electron beams from the carbon nanotubes electron emission layer, it does not enhance the efficiency, either.
Another conventional art is disclosed in the Taiwanese published patent no. 353188 entitled “Method for fabricating low energy electron excitation screen.” In this patent, a metallic salt is used to form conductive metallic oxide on the phosphorus layer after the electrophoresis process, so as to enhance the light emission property of the phosphorus powder. Accordingly, other metallic salts that are solvable in water or alcoholic solution are also employed to form conductive metallic salt oxide in the baking process after the electrophoresis deposition process, thereby enhancing the efficiency of electron production from the carbon nanotubes electron emission source.
However, the electrophoresis deposition process described above still contain restrictions to mass production and to the manufacturing of large size field emission display. One of the restrictions is in that most of the structures are embedded between the dielectric layers. It is easy to form bubbles between part of electron emission source pixels and the cavity of the dielectric layers, when the cubic structure is rinsed in the electrophoresis solution. The existence of bubbles will prevent the carbon nanotubes from depositing on the electrode layer to form electron emission source. The primary reason for this is that the 30 minutes electrophoresis deposition process is only performed once. Therefore, a homogeneous electron emission source can not be formed due to the existence of those bubbles. Consequently, the display quality of the manufactured field emission display is no good. On the other hand, since the electrophoresis deposition process is continuously performed only once for 30 minutes, it is easy to induce polarization on the cathode and anode plates, and the solution in the electrophoresis tank. This will lower the efficiency of electrophoresis deposition. In addition, the electrophoresis deposition rate of some regions will become non-uniform. This will also render the deposition inhomogeneity of carbon nanotubes. Furthermore, the polarization of electrodes will lower the stability of the electrophoresis solution, alter the charger, and further affect the adhesion and conduction of the carbon nonotubes.

BRIEF SUMMARY OF THE INVENTION

The present invention is to provide a method that employs the electrophoresis deposition technology to fabricate a electron emission source. The electrophoresis solution is prepared by using the carbon nanotubes powder made from arc discharge. A proper charger is also used to produce better ionic distribution effect, such that the carbon nanotubes are evenly distributed in the water or alcoholic electrophoresis solvent. This will enhance the homogeneity of electrophoresis deposition formed on the cathode structure. Further, the properly selected charger can form conductive metallic oxide salt after the electrophoresis deposition is baked, thereby enhancing the electron production rate of the carbon nanotubes electron emission source layer. Meanwhile, after a batch electrophoresis deposition processes is performed, the bubbles formed in the cathode structure can be removed. This allows a homogeneous electron emission source to form in the pixels of the cathode structure, so as to increase the display quality and to prevent the polarization phenomena from happening.
In order to achieve the above and other objectives, the method of the present invention for batch fabricating electron emission source of electrophoresis deposited carbon nanotubes includes the following steps. First, a semi-manufactured cathode structure is prepared. The semi-manufactured cathode structure and a metallic plate are connected with the electrophoresis electrodes. The cathode structure and the metallic plate that are parallel to each other are separated with a fixed distance and are rinsed in the electrophoresis solution, so as to perform the first electrophoresis deposition process. The anti-bubble device of the first electrophoresis tank is turned on, so as to remove the bubbles formed in the cathode structure. Then, the anti-bubble device is turned off. The cathode structure is kept rinsing in the solution for performing the electrophoresis deposition process. After the first deposition process is completed, the cathode structure is moved out from the solution and is baked under a low temperature, so as to remove the residual solution remained on the cathode structure. After the first electrophoresis deposition process is completed, the cathode structure is rinsed again in the second electrophoresis tank to perform the second electrophoresis deposition process. The anti-bubble device is also turned on to remove the bubbles formed in the cathode structure. Then, the anti-bubble device is turned off. The cathode structure is kept rinsing in the solution for performing the electrophoresis process. After the second deposition process is completed, the cathode structure is moved out from the solution and is baked under a low temperature, so as to remove the residual solution remained on the cathode structure. The electrophoresis deposition process described above is repeated until a homogeneous carbon nanotubes layer is deposited on the cathode electrode layer to form the electron emission source. The cathode structure is taken out again from the solution and baked under a low temperature, so as to remove the residual solution remained on the cathode structure., In addition, a sintering process is performed so as to re-oxidate the indium hydroxyl on the cathode electrode layer back to indium oxide. This will enhance the electron conduction of the carbon nanotubes and the cathode electrode layer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1(a) to FIG. 1(g) illustrates the method for fabricating a semi-manufactured cathode structure of the present invention.
FIG. 2 is a flow diagram illustrating the process for manufacturing the electron emission source of the cathode structure of the present invention.
FIG. 3 illustrates the combination of the cathode structure and the metallic plate.
FIG. 4(a) to FIG. 4(c) illustrates the electrophoresis deposition process after the cathode structure and the metallic plate are combined.
FIG. 5 illustrates the arc discharge method for manufacturing carbon nanotubes.

DETAILED DESCRIPTION OF THE INVENTION

In order to better understanding the features and technical contents of the present invention, the present invention is hereinafter described in detail by incorporating with the accompanying drawings. However, the accompanying drawings are only for the convenience of illustration and description, no limitation is intended thereto.
Referring to FIG. 1(a) to FIG. 1(g), a method for fabricating a semi-manufactured cathode structure of the present invention is illustrated. As shown, the present invention relates to a method for batch fabricating electron emission source of electrophoresis deposited carbon nanotubes. The method employs the electrophoresis technology with some proper choice of charger to deposit carbon nanotubes, thereby fabricating the electron emission source. During the electrophoresis deposition process, bubbles formed on the cathode structure are removed, so as to enhance the deposition homogeneity of carbon nanotubes on the surface of the cathode structure 10.
First, a cathode electrode layer 2 is formed on the surface of a glass substrate 1. A dielectric layer 3 is formed on the surface of the cathode electrode layer 2. A gate layer 4 is then formed on the surface of the dielectric layer 3. A sagged area 41 is formed by employing lithography technology on the gate layer 4, thereby exposing the dielectric layer 3. A protection layer is further formed on the gate layer 4. By employing etching technology, a sagged area 31 that exposes the cathode electrode 2 is formed on the dielectric layer 3. Afterwards, the protection layer 5 is peeled off. Another protection layer 6 is coated on the dielectric layer 3 and the gate layer 4. The semi-manufactured cathode structure is thus fabricated.
Referring to FIG. 2 to FIG. 4(c), the process for manufacturing the electron emission source of the present invention and the electrophoresis deposition process after the combination of the cathode structure and the metallic plate are illustrated. As shown, the deposition process of carbon nanotubes is performed after the semi-manufactured cathode structure is fabricated.
First, an electrophoresis solution is prepared using alcohol as the solvent. A 1% to 10% of pure water (preferably 5%) is added to the solution. The carbon nanutubes powder used in the electrophoresis solution is made from an arc discharge process. The carbon nanotubes are of a multi wall structure with an average length below 5 μm, and an average diameter below 100 nm. The weighted concentration is approximately 0.1% to 0.005% (preferably 0.02%). In addition, some chargers of weighted concentration 0.1% to 0.005% (preferably 0.01%) is added to the electrophoresis solution. These chargers are selected from many conductive metallic oxide salts, such as indium chloride and indium nitride, or other chargers such as tin. The prepared solution is then poured into the electrophoresis tank 7.
Second, the electrophoresis deposition process is performed, after the electrophoresis solution is prepared. The cathode electrode layer 2 of the field emission cathode structure 10 is connected with the cathode of the electrophoresis electrode 8 via a cathode conducting wire 101. The anode 82 of the electrophoresis electrode 8 is connected with the metallic plate. The metallic plate 9 described above can be made of platinum, titanium, or a screen plate.
After the second step is completed, the third step is performed which requires one or more independent electrophoresis tanks 7. Each electrophoresis tank 7 includes an anti-bubble device 71. The anti-bubble device 71 can be a blender rotor or an ultrasonic device. The prepared solution is poured into each electrophoresis tank 7. Meanwhile, one surface of the cathode structure 10 to be electrophoresis deposited is parallelly separated with the metallic plate 9 by a fixed distance. The cathode structure 10 and the metallic plate 9 are both disposed in the first electrophoresis tank 7 a, so as to perform the first electrophoresis deposition process (as shown in FIG. 4(a)). The anti-bubble device 71 of the first electrophoresis tank 7 a is turned on to stir the solution for 5 to 10 minutes, so as to remove the bubbles within the sagged area 31 of the cathode structure. The anti-bubble device 71 is then turned off. The cathode structure 10 is then kept in the solution for 5 to 10 minutes for performing the electrophoresis process. A power supply is used to provide a DC or pulse voltage to the anode and the cathode, thereby forming an electric field of 0.5 to 10 V/cm, preferably 2 V/cm. The carbon nanotubes are then electrophoresis deposited on the cathode electrode layer 2, thereby forming an electron emission source 21 (as shown in FIG. 5). The cathode structure 10 is then removed from the electrophoresis tank, and is baked under a low temperature of 80° C., so as to remove the residual alcoholic solution remained on the cathode structure 10. Now, the charger indium chloride and the electrolyte hydroxyl forms indium hydroxide.
After the first electrophoresis deposition process is completed, the baked cathode structure 10 is transferred and rinsed in the second electrophoresis tank 7 b, so as to perform the second electrophoresis deposition process (as shown in FIG. 4(b)). The anti-bubble device 71 of the second electrophoresis tank 7 b is turned on to stir the solution for 5 to 10 minutes, so as to remove the bubbles within the sagged area 31 of the cathode structure. The anti-bubble device 71 is then turned off. The cathode structure 10 is then kept in the solution for 5 to 10 minutes for performing the electrophoresis process. A power supply is used to provide a DC or pulse voltage to the anode and the cathode, thereby forming an electric field. The carbon nanotubes are then electrophoresis deposited on the cathode electrode layer 2, thereby forming an electron emission source 21 (as shown in FIG. 5). The cathode structure 10 is then removed from the electrophoresis tank, and is baked under a low temperature of 80° C., so as to remove the residual alcoholic solution remained on the cathode structure 10. Now, the charger indium chloride and the electrolyte hydroxyl forms indium hydroxide.
After the above two electrophoresis deposition processes are completed, the baked cathode structure 10 is transferred and rinsed in the third electrophoresis tank 7 c, so as to perform the third electrophoresis deposition process (as shown in FIG. 4(c)). The anti-bubble device 71 of the third electrophoresis tank 7 b is turned on to stir the solution for 5 to 10 minutes, so as to remove the bubbles within the sagged area 31 of the cathode structure. The anti-bubble device 71 is then turned off. The cathode structure 10 is then kept in the solution for 5 to 10 minutes for performing the electrophoresis process. A power supply is used to provide a DC or a pulse voltage to the anode and the cathode, thereby forming an electric field. The cathode structure 10 is then removed from the electrophoresis tank, and is baked under a low temperature of 80° C., so as to remove the residual alcoholic solution remained on the cathode structure 10. Now, the charger indium chloride and the electrolyte hydroxyl forms indium hydroxide.
After a plurality of electrophoresis deposition processes are performed, the carbon nanotubes are homogeneously deposited to the cathode electrode layer 2 and an electron emission source 21 (as shown in FIG. 5) is fabricated. Meanwhile, the polarization phenomena can be prevented from happening.
After the cathode structure 10 is batch fabricated under the electrophoresis deposition process, it is taken out from the electrophoresis tank and is baked under a low temperature of 80° C., so as to remove the residual alcoholic solution remained on the cathode structure 10. Later, the cathode structure 10 is sintered under 400° C., so as to remove the protective layer 6. The indium hydroxyl on the cathode electrode layer 2 will be re-oxidated to form indium oxide. Since indium oxide is conductive, the electron emission source 21 will include conductive indium oxide particles in addition to the carbon nanotubes. In this manner, the indium oxide charger can substitute the conventional magnesium salt charger, which provides only electrophoresis force. In addition, the electron transmission of the carbon nanotubes and the cathode ectrode layer is enhanced.
Further, the carbon nanotubes electrophoresis deposited to the electron emission source layer is easily adhered to the surface of the cathode electrode. Thus, a homogeneous carbon nanotubes layer is easily formed. The average thickness is controlled as being below 2 μm. Moreover, since the sintered carbon nanotubes are deposited together with the indium oxide salt, they will not be detached from the cathode structure 10.
Even further, the number of electrophoresis deposition steps is determined according to the quality of the fabricated cathode structure 10 of the electron emission source. One can increase or decrease the number of steps accordingly.
Even further, the employment of blender rotors or ultrasonic devices can fully remove the bubbles formed in each pixel hole. By dividing the continuous electrophoresis process into sequential steps, the polarization phenomena can be prevented from happening. Also, the mass production process can be performed in batch processes. The baking process in each batch can crystallize and deposit the chargers among the carbon nanotubes, which can enhance the adhesion and conduction of the carbon nanotubes. The rinsing time and the number of electrophoresis steps can be adjusted to conform to the continuous mass production process.
Since, any person having ordinary skill in the art may readily find various equivalent alterations or modifications in light of the features as disclosed above, it is appreciated that the scope of the present invention is defined in the following claims. Therefore, all such equivalent alterations or modifications without departing from the subject matter as set forth in the following claims is considered within the spirit and scope of the present invention.

Claims

1. A method for batch fabricating an electron emission source of electrophoresis deposited carbon nanotubes, the method comprising the steps of:

preparing a semi-manufactured cathode structure, performing an electrophoresis deposition to the semi-manufactured cathode structure by connecting the cathode structure and a metallic plate to an electrophoresis electrodes;

preparing an electrophoresis solution, pouring the prepared solution into a plurality of electrophoresis tanks, disposing parallelly separated cathode structure and metallic plate into the solution of first electrophoresis tank, turning on an anti-bubble device of the first electrophoresis tank to remove bubbles formed in the cathode structure, turning off the anti-bubble device, keeping the cathode structure in the solution for performing the electrophoresis deposition, removing the cathode structure from the solution, and baking the cathode structure with a low temperature so as to remove residual solution remained on the cathode structure;

rinsing the baked cathode structure into second electrophoresis tank for performing second electrophoresis deposition, turning on an anti-bubble device of the second electrophoresis tank to remove bubbles formed in the cathode structure, turning off the anti-bubble device, keeping the cathode structure in the solution for performing the electrophoresis deposition, removing the cathode structure from the solution, and baking the cathode structure with a low temperature so as to remove residual solution remained on the cathode structure, repeating this step as necessary until carbon nanotubes are homogeneously deposited on a cathode electrode layer and an electron emission source is formed; and

after the deposition process of the cathode structure is completed, baking the cathode structure with a low temperature so as to remove residual alcoholic solution on the cathode structure, next performing a sintering process for re-oxidating indium hydroxide on the cathode electrode layer back to indium oxide, thereby enhancing an electron conductivity of the carbon nanotubes and the cathode electron layer.

2. The method as recited in claim 1, wherein the step for preparing the semi-manufactured cathode structure comprises:

forming the cathode electrode layer on a glass substrate, forming a dielectric layer on the cathode electrode layer, forming a gate layer on the dielectric layer, and forming a sagged area on the gate layer by means of lithography technology to expose the cathode electrode layer;

forming a protection layer on the surface of the gate layer, etching the dielectric layer to form the sagged area exposing the cathode electrode, and performing peeling operation to the protection layer; and

forming another protection layer to cover the dielectric layer and the gate layer, thereby completing the manufacturing process of the semi-manufactured cathode structure.

3. The method as recited in claim 1, wherein the cathode electrode layer of the cathode structure is connected to the cathode of electrophoresis electrode, while an anode of the electrophoresis is connected to the metallic plate, the metallic plate being one of a platinum plate, a titanium plate and a screen plate.

4. The method of claim 1 wherein an electric field is provided by a power supply, which has an intensity of approximately 0.5˜10 V/cm.

5. The method as recited in claim 1, wherein the carbon nanotubes made from arc discharge are multi-wall carbon nanotubes comprising an average length below 5 μm, and an average diameter below 100 nm.

6. The method as recited in claim 1, wherein the solution contained in the electrophoresis tank comprises alcohol as solvent, pure water, carbon nanotube powder, and charger.

7. The method as recited in claim 6, wherein the pure water added to the solution is about 1%˜10%.

8. The method as recited in claim 6, wherein a weighted concentration of the carbon nanotube powder added to the solution is approximately 0.1%˜0.005%.

9. The method as recited in claim 6, wherein the weighted concentration of the charger added to the solution is approximately 0.1%˜0.005%.

10. The method as recited in claim 6, wherein the charger is selected from a conductive metallic salt oxide of indium chloride, indium nitride, and salt of tin after the electrophoresis process.

11. The method as recited in claim 1, wherein the anti-bubble device is a blender rotor or an ultrasonic device.

12. The method as recited in claim 1, wherein the anti-bubble device stirs the solution for 5 to 10 minutes so as to remove the bubbles in the cathode structure.

13. The method as recited in claim 1, wherein the cathode structure is kept in the solution for performing electrophoresis deposition for 5 to 10 minutes after the anti-bubble device is turned off.

14. The method as recited in claim 1, wherein a number of batch electrophoresis deposition processes can be increased or decreased according to the deposition quality of the fabricated electron emission source.

15. The method as recited in claim 1, wherein the baking temperature is about 80° C.

16. The method as recited in claim 15, wherein the baking process removes the residual solution remained on the cathode structure, the residual solution being alcohol.

17. The method as recited in claim 16, wherein during the low temperature baking process that removes the residual alcoholic solution, indium chloride charger and electrolyte hydroxyl forms indium hydroxide.

18. The method as recited in claim 1, wherein the sintering temperature is about 400° C.

19. The method as recited in claim 1, wherein a conductive indium oxide is deposited on the cathode electrode layer to enhance the electron conductivity between the carbon nanotubes and the cathode electrode layer.

20. The method as recited in claim 1, wherein the electrophoresis deposited carbon nanotubes of the electron emission source layer are easily adhered to the surface of the cathode electrode, whereby a homogeneous carbon nanotubes layer is easily formed, the average thickness of which is below 2 μm, and the sintered carbon nanotubes are deposited together with an indium oxide salt, which are not easily detached from the cathode structure.