Abstract:
A method sequentially performs electrophoresis depositing carbon nanotube of field emission display. Only one cathode strip is subjected to electrical field at one time during electrophoresis deposition. Therefore, the electrophoresis deposition is confined to local area. A cathode plate includes a plurality of cathode strips and the cathode strips sequentially have potential difference with respect to the anode strips, whereby only one electrical field is present for one pixel at one time and carbon nanotube is formed at that pixel. The cathode strips are sequentially applied with voltage for global electrophoresis deposition.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to a method for electrophoresis depositing carbon nanotube on cathode strip for a field emission display, especially to a method for electrophoresis depositing carbon nanotube with certain powders such as glass powder or conductive powder.  
         [0003]     2. Description of Prior Art  
         [0004]     The field emission display uses cathode electron emitter to generate electron by electrical field. The emitted electron excites phosphor on anode plate for illumination. The field emission display has compact size and flexible viewable area. The field emission display does not have view angle problem encountered in LCD.  
         [0005]     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 conducting layer, an electron emission layer, a dielectric layer and a gate layer. The gate layer provides a voltage difference to induce the emission of electrons from the electron emission 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 −5  torr. 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 predetermined gap is required between the two substrates. Consequently, the electrons can be accelerated to impinge the phosphorous layer, thereby exciting the phosphorous layer and emitting light therefrom.  
         [0006]     The electron emission layer is composed of carbon nanotubes. Since carbon nanotubes, proposed by Iijima in 1991 (Nature, 354, 56 (1991)), possess very good electronic properties that 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, a variety of manufacturing process can be developed, e.g. screen printing, or thin film processing.  
         [0007]     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 (CVD) 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. However, it is still difficult to fabricate carbon nanotube in the cathode electrode in each pixel by above-mentioned processes. Especially for large-size FED display.  
         [0008]     Recently, an electrophoresis deposition process is proposed, for example, US pre-grant publication No. 2003/0102222 discloses an electrophoresis deposition process. An alcohol suspension for carbon nanotube is prepared and charger such as Mg, La, Y and Al is used to form an electrophoresis solution. The cathode electrode substrate to be deposited is connected to an electrode in the electrophoresis solution. A DC or AC voltage is applied to provide electrical field in the electrophoresis solution. The charger is dissolved in the electrophoresis solution and attached to the carbon nanotube powder. The electrical field will facilitate the carbon nanotube powder to deposit on an electrode. This electrophoresis deposition process can easily deposit the carbon nanotube on the electrode layer without the limit of forming triode field emission display on electrode. Therefore, the electrophoresis deposition process is extensively used on the fabrication of cathode plate.  
         [0009]     Moreover, the applicant of the present invention had also proposed a pulse electrophoresis deposition process to enhance uniformity of carbon nanotube. The deposition amount in unitary area is enhanced and the process can be used for aqueous solution. However, the current pulse electrophoresis deposition process still have following problems.  
         [0010]     1. The area electrophoresis is difficult for electrophoresis solution with complicated carbon nanotube suspension. Some particles are added to the electrophoresis solution to enhance the adhesion ability of carbon nanotube and the effect of manufacturing electron emission source. The carbon nanotube suspension is sensitive to electrical field distribution, and to concentration and thickness of the display. This problem is more serous for large-size display.  
         [0011]     2. The pulse electrophoresis deposition process has good effect for aqueous solution. However, the property of solution is critical to some carbon nanotube. For example, some non-aqueous solution such as alcohol solution has good property for most carbon nanotube. However, the pulse electrophoresis deposition process uses larger current and has burning risk for alcohol solution.  
         [0012]     3. The impedance distribution of cathode strip depends on distribution variation of strip length. Therefore, the impedance variation is serious, especially for large-size display. The end of the cathode strip close to power source encounters larger current and has greater deposition concentration. The electrophoresis deposition is not uniform.  
       SUMMARY OF THE INVENTION  
       [0013]     The present invention is to provide a method for sequentially electrophoresis depositing carbon nanotube of field emission display. In prior art electrophoresis depositing process for large-size anode/cathode plate, the current is large and the deposition is spares. Therefore, the electrophoresis deposition is not uniform for solution with complicated composition. In the present invention, the electrophoresis deposition is localized to one single cathode strip at one time. The complicated particles in the solution is deposited on the single cathode strip, and the remaining cathode strips are conducted successively and individually for global electrophoresis deposition.  
         [0014]     Accordingly, the present invention provides a method for sequentially electrophoresis depositing carbon nanotube of field emission display. The anode ends of a power source are connected to anode strips of an anode plate. The cathode ends of the power source are connected to one input ends of a plurality of controllers. The output ends of the controllers are connected to a plurality of cathode strips of a cathode plate. A signal generator is connected to another input ends of the controllers.  
         [0015]     An electrophoresis tank is provided with electrophoresis solution therein and the anode plate and the cathode plate are placed parallel in the electrophoresis tank. The voltages from anode ends of the power source is output to the anode strips. The signal generator sends pulse voltage signal to one of the controllers such that one of the cathode strip is conducted while the remaining cathode strips are not conducted, whereby only one electrical field is present for one pixel at one time and carbon nanotube is formed at that pixel. The next cathode strip is conducted successively and the remaining cathode strips are non-conducted to fabricate carbon nanotube electron emission source in sequential manner. 
     
    
     BRIEF DESCRIPTION OF DRAWING  
       [0016]     The features of the invention believed to be novel are set forth with particularity in the appended claims. The invention itself however may be best understood by reference to the following detailed description of the invention, which describes certain exemplary embodiments of the invention, taken in conjunction with the accompanying drawings in which:  
         [0017]      FIG. 1  shows a schematic diagram of the anode plate and cathode plate according to a preferred embodiment of the present invention.  
         [0018]      FIG. 2  shows the schematic diagram of connection of the anode plate and cathode plate to the electrophoresis deposition equipment.  
         [0019]      FIG. 3  shows the schematic diagram of connection of the anode plate and cathode plate to the electrophoresis deposition equipment during fabrication.  
         [0020]      FIG. 4  shows a simplified schematic diagram of connection of the anode plate and cathode plate to the electrophoresis deposition equipment. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0021]     With reference to  FIGS. 1 and 2 , in the method for electrophoresis depositing carbon nanotube on cathode strip for a field emission display according to the present invention, sequential electrophoresis deposition localizes current to a single pixel to fabricate carbon nanotube electron emission source. Therefore, the peak current can be reduced and the method can be applied to manufacture of large display.  
         [0022]     According to the method of the present invention, a cathode plate  1  is prepared with a plurality of cathode strips  11  (such as 32 cathode strips). The cathode strips  11  are already formed with gate and semi-finished sacrifice layer. The sacrifice layer is used to prevent unwanted deposition (such as gate, dielectric) on the non-electrophoresis deposition area. The sacrifice layer is removed after electrophoresis deposition process.  
         [0023]     Moreover, an anode plate  2  is prepared and the anode plate  2  is formed by platinum, titanium plate or screen-printing plate.  
         [0024]     A power source  3  is connected to the anode plate  2  by anode ends  31  thereof and is connected to input ends of controllers  4  by cathode ends  32  thereof The controller  4  is connected to the cathode strips  11  by output ends thereof.  
         [0025]     Another input end of the controller  4  is connected to a signal generator  5  to complete the connection for electrophoresis depositing. The signal generator  5  provides sequential signal for the cathode strips  11 . The controller  4  controls a conducting and an un-conducting state for the cathode strips  11  and can be realized by signal amplifier or switch. The signal amplifier decides to amplify or not to amplify the output signal from the signal generator  5 . A potential difference is present between the cathode strip  11  and the anode plate to provide an electrical field. Therefore, carbon nanotube electron emission source can be fabricated on a single cathode strip  11 .  
         [0026]     The above-mentioned switch is a timing switch and conducts a predetermined time period such that the signal generated by the signal generator  5  is applied to one cathode strip  11 . A potential difference is present between the cathode strip  11  and the anode plate to provide an electrical field. Therefore, carbon nanotube electron emission source can be fabricated on a single cathode strip  11 . When the predetermined time period is elapsed, the switch is turned off and a next conduction period is provided for a next single cathode strip  11  for fabricating carbon nanotube electron emission source successively.  
         [0027]     With reference to  FIGS. 3 and 4 , after the connection for the cathode plate  1 , the anode plate  2 , the scanning power source  3 , the signal amplifier  4  and the signal generator  5  is completed, an electrophoresis solution is prepared for the electrophoresis tank  6 . Alcohol is used for solution and carbon nanotube is used for electron emission source and manufactured by arc discharge. The carbon nanotube has average length below 5 μm and average diameter below 100 nm. The carbon nanotube has multiple wall, the carbon nanotube has an additive concentration of 0.1%-0.005% (preferably 0.02%). The charger uses metal salt is conductive after electrophoresis, for example, the metal salt is one of InCl and indium nitride or other salt with tin. The charger is with 0.1-0.005% weight concentration and glass powder with at 5% weight concentration to enhance adhesion. Preferably the charger is with 0.01% weight concentration.  
         [0028]     The cathode plate  1  and the anode plate  2  are placed in the electrophoresis tank  6  with 3-5 cm separation therebetween. The power source  3  provides a DC or a DC pulse voltage to the anode strip with 120V or 100-300V and with pulse frequency of 250 Hz. The signal  5  sends a continuous square-wave signal to the controller  4  acting as a signal amplifier. The controller  4  amplifies the continuous square-wave signal and sends the amplified continuous square-wave signal to the first one of the cathode strips  11 , while the remaining cathode strips  11  are not conducted. Therefore, an electrical field is established between the first cathode strip  11  and the first anode strip  21  due to a potential difference. A carbon nanotube can be fabricated on the position to be deposited with electron emission source on the first cathode strip  11 . The remaining cathode strips  11  are conducted one by one and other cathode strips  11  are not conducted. In this manner, the electron emission source can be fabricated. The duty cycle for the cathode strips  11  are 1/32 (frequency 32 Hz) or higher frequency provided that the electrophoresis deposition time period is 1 second. The electrophoresis deposition is 10 minutes and an electron emission source with 5-10 um thickness can be formed by one electrophoresis deposition operation.  
         [0029]     Alternatively, the signal generator  5  generates a signal to a plurality of signal amplifiers, where one of the signal amplifiers does not provide signal amplification. Therefore, the first cathode strip  11  is in low level while other cathode strips  11  are in high level, which level is the same as that of anode strips  21 . An electrical field is present in the first cathode strip  11  and the anode plate  2  such that carbon nanotube will be formed on the first cathode strip  11  and can be formed on other cathode strips  11  successively.  
         [0030]     When the controller  4  is timing switch, the signal generator  5  generates a continuous square wave signal to a plurality of timing switches, where the first timing switch is turned on and the remaining timing switches are turned off. Thereof, the first cathode strip  11  is conducted and an electrical field is present in the first cathode strip  11  and the anode plate  2 . A carbon nanotube will be formed on the first cathode strip  11 . When the electrophoresis deposition is performed, the first timing switch counts the deposition time. After a predetermined time period is over, the first timing switch is turned off and the second timing switch is turned on, while other timing switches are turned off. In this manner, the carbon nanotube will be formed on the remaining cathode strip  11  successively.  
         [0031]     To sum up, the scanning-matrix type electrophoresis deposition method according to the present invention has following advantages:  
         [0032]     1. The electrophoresis deposition method can be used for solution with complicated composition. The distribution is good and various particles can be effectively deposited.  
         [0033]     2. The electrical field intensity can be increased for a unitary electrophoresis deposition area.  
         [0034]     3. The cost and electrical current consumption can be reduced for large-size display.  
         [0035]     Although the present invention has been described with reference to the preferred embodiment thereof, it will be understood that the invention is not limited to the details thereof. Various substitutions and modifications have suggested in the foregoing description, and other will occur to those of ordinary skill in the art. Therefore, all such substitutions and modifications are intended to be embraced within the scope of the invention as defined in the appended claims.