1. Field of the Invention
The present invention relates to a method of manufacturing a field emitter for use in field emission displays. More specifically, the invention relates to a method of manufacturing a carbon nanotube field emitter on electrodes of a field emitter substrate by electrophoretic deposition.
2. Description of the Related Art
The use of field emitters as electron emitters for field emission displays is expected to increase dramatically in future generation flat displays. Field emitters emit electrons by creation of a strong electric field around the electrons. The current emission density of electrons is proportional to the intensity of the electric field produced around the field emitter, while the intensity of the electric field is influenced by the geometrical shape of the field emitter. Field emitters, which act as electron emitters for field emission displays, are usually formed in a cone shape with a sharp tip.
FIG. 1 is a sectional view of a conventional field emitter with a tip cone manufactured by a spindt technique. The spindt technique has been widely used. The conventional field emitter comprises a cathode 12 on a glass substrate 11, a sharp tip 15 for emitting electrons that is arranged on the cathode 12, a dielectric film 13 patterned to surround the tip 15, and a gate 14 formed on the dielectric film 13 with an opening 14a above the tip 15. The opening 14a allows for electron emission. In a field emission display, a plurality of cathodes is arranged in strips on a glass substrate.
A method of manufacturing a conventional field emitter for field emission displays, such as the type depicted in FIG. 1, by the spindt technique is set forth below.
FIGS. 2 through 6 are sectional views of successive stages of the method of manufacturing the conventional field emitter of FIG. 1. Referring now to FIG. 2, an electrode 12 is formed in a strip on the glass substrate 11, and followed by the formation of the dielectric film 13 and a gate layer 14xe2x80x2. Next, as depicted in FIG. 3, a photoresist mask 16 is formed by photolithography on the gate layer 14xe2x80x2 and a gate 14 having an opening 14a. After removing the photoresist mask 16, the gate 14 is used as an etching mask to etch a hole 13a in the dielectric film 13.
Next, as shown in FIG. 5, after depositing a sacrificial layer 17 on the gate 14, the structure is spun to grow a tip 15 that has a high melting point material by electron beam deposition. Finally, the sacrificial layer 17 and a by-product layer 15a, which was deposited during the tip deposition, are removed by etching. This etching results in the field emitter as shown in FIG. 1.
The above-described method of manufacture results in several problems. For example, the lifetime of the tip in a field emitter such as that shown in FIG. 1 is shortened due to two factors: (1) ionized gases used for deposition and (2) presence of a non-uniform electric field distribution during operation. One possible alteration for this problem is to lower the driving voltage of a field emitter. This is accomplished by using a material having a low work function, such as molybdenum (Mo). This possible correction, however, does not fully solve the initial problem because the use of molybdenum as a material for the emitter tip does not provide a satisfactory lifetime of the tip as described above. In an attempt to overcome this drawback, diamond and carbonic substances have been deposited to form emitter tips at high temperatures. This technique, however, when performed at high temperatures results in non-uniform coating properties of the product. Moreover, the use of diamond causes a problem in that a large area of the emitter cannot be coated.
In order to solve at least some of the aforementioned problems, it is a feature of the present invention to provide a method of manufacturing a carbon nanotube field emitter by electrophoretic deposition. In this method of manufacturing, emitter tips are formed of an ultra fine carbon nanotube having a low work function. This lowers a driving voltage of electrodes, and the electrophoresis deposition at low temperatures avoids deterioration by ionization of residual gases during operation. Accordingly, the life of the emitters is elongated.
This feature of the present invention is provided by a method of manufacturing a carbon nanotube field emitter by electrophoresis on a field emitter substrate. This feature is further provided by cathodes arranged in strips on a substrate, a dielectric film having holes over the cathodes, and metal gates having openings located over the holes of the dielectric film. The method of manufacturing involves: first, loading an electrode plate and the field emitter substrate, which are spaced apart from one another, into an electrophoresis bath containing a carbon nanotube suspension for the electrophoresis; second, applying a predetermined bias voltage from a power supply between the electrode plate and the cathodes of the field emitter substrate to deposit, at room temperature, carbon nanotube particles on the surface of the electrodes exposed through the holes of the dielectric film; and third, drawing the field emitter substrate, on which the carbon nanotube particles have been deposited, out of the electrophoresis bath, and heating the field emitter substrate with carbon nanotube tips at a predetermined temperature.
In a preferred embodiment, carbon nanotube particles having a length of 0.1 to 1 micrometer are screened by field-flow fractionation for preparation of the carbon nanotube suspension used in the first step. Further, the carbon nanotube suspension, used in the first step, contains a surfactant selected from the group consisting of Tritron X-100, AOT and nitrates consisting of Mg(OH)2, Al(OH)3 and LA(OH)3. Additionally, the carbon nanotube suspension is sonicated during the electrophoresis. During the second step, it is preferable that the bias voltage, which is applied between the electrode plate and the cathodes of the field emitter substrate, is in the range of 1 to 1000 volts. In addition, it is preferable that the bias voltage is applied for a time period between 1 second and 10 minutes. Further, it is preferable that, during the second step, the carbon nanotube particles are deposited to a thickness of 0.01 to 0.5 micrometer. Finally, during the third step, it is preferable that the heating is performed at a temperature between 150xc2x0 to 500xc2x0 C.