Patent Application
20100231118
This invention relates to a cathode body, a fluorescent tube comprising the cathode body, and a method of manufacturing the cathode body.
In general, a cold cathode fluorescent tube comprising a cathode body of the type is used in a light source of a backlight of a liquid crystal display device in a monitor, a liquid crystal television, or the like. The cold cathode fluorescent tube comprises a fluorescent tube member which is formed of a glass tube and which has an inner wall coated with a phosphor, and a pair of cold electrode members for emitting electrons. In the fluorescent tube member, a mixed gas, such as Hg—Ar, is confined.
Patent Document 1 proposes a cold cathode fluorescent tube comprising a cold cathode body having a cylindrical cup shape. Specifically, the cold cathode body of the cylindrical cup shape for emitting electrons comprises a cylindrical cup formed of nickel and an emitter layer having a boride of a rare earth element as a main constituent and formed on inner and outer wall surfaces of the cylindrical cup. In Patent Document 1, YB6, GdB6, LaB6, and CeB6 are exemplified as a boride of a rare earth element. The boride of a rare earth element is prepared into a fine powder slurry, applied to the inner and the outer wall surfaces of the cylindrical cup by flow coating, dried, and sintered to form the emitter layer.
On the other hand, Patent Document 2 discloses that a cold cathode body having a cylindrical cup shape is formed by mixing a material selected from La2O3, ThO2, and Y2O3 with another material having a high thermal conductivity, such as tungsten. The cold cathode body of the cylindrical cup shape disclosed in Patent Document 2 is formed by, for example, injection-molding, namely, MIM (Metal Injection Molding) of a tungsten alloy powder containing La2O3.
Further, Patent Document 3 discloses a discharge cathode device for use in a plasma display panel. The discharge cathode device comprises, on a glass substrate, an aluminum layer formed as a base electrode and a LaB6 layer formed on the aluminum layer. The aluminum layer is formed on the glass substrate kept at a preselected temperature by sputtering, vacuum vapor deposition, or ion plating while the LaB6 layer is formed on the aluminum layer by sputtering or the like.
Patent Document 1: JP-A-10-144255
Patent Document 2: WO2004/075242
Patent Document 3: JP-A-5-250994
In Patent Document 1, the emitter layer is formed by applying the slurry having the rare earth element as a main constituent onto the cylindrical cup formed of Ni (nickel), drying the slurry, and sintering the slurry.
Patent Document 1 discloses that the emitter layer is reduced in thickness on the side of an opening end of the cylindrical cup and increased in thickness on the side of an external extraction electrode. Generally, the cylindrical cup has an inner diameter of approximately 0.6 to 1.0 mm and a length of approximately 2 to 3 mm. Therefore, when the emitter layer is formed by the technique of applying, drying, and sintering the slurry, it is difficult to apply the slurry to a desirable thickness. Further, the emitter layer obtained by applying, drying, and sintering the slurry is insufficient in adhesion with Ni. In addition, it is difficult to completely remove an organic material, moisture, and oxygen contained in a binder. As a result, in Patent Document 1, it is difficult to obtain a high-intensity and long-life cold cathode body.
In Patent Document 2, pellets are obtained by mixing the tungsten alloy powder containing La2O3 with a resin, such as styrene, and injection-molded in a mold to form a cold cathode body having a cylindrical cup shape. By using a material, such as tungsten, having a high thermal conductivity, it is possible to improve thermal conduction in the cold cathode body and to achieve a long life of the cold cathode body. However, the cold cathode body is insufficient in electron emission characteristic. Therefore, in Patent Document 2, it is difficult to obtain a high-intensity and high-efficiency cold cathode body.
Patent Document 3 discloses that a discharge cathode pattern comprising the LaB6 layer and the aluminum layer is formed on the glass substrate by sputtering. However, the above-mentioned technique assumes that the aluminum layer and the LaB6 layer are formed on the glass substrate of a flat shape by sputtering. No disclosure is made about a technique of forming the layers by sputtering on the cold cathode body having the cylindrical cup shape which is not flat. Further, Patent Document 3 does not disclose that, on a material except the glass substrate, the LaB6 layer is formed with high adhesion without interposing the aluminum layer. Furthermore, Patent Document 3 does not point out improvement in electron emission efficiency of the cold cathode body having a cylindrical cup shape.
It is therefore one technical object of the present invention to provide a cathode body having a high intensity, a high efficiency, and a long life.
It is another technical object of the present invention to provide a method of manufacturing a cathode body having a high intensity, a high efficiency, and a long life.
It is still another technical object of the present invention to provide a manufacturing method suitable for a cathode body having a cylindrical cup shape.
In JP-A-2007-99778 and so on, the present inventors have previously proposed a magnetron sputtering apparatus which is capable of preventing local erosion of a target by moving a ring-shaped plasma region on the target with time and of increasing a film-forming rate by increasing a plasma density. The magnetron sputtering apparatus has a structure in which the target is disposed to face a substrate to be processed and a magnet member is arranged on a side opposite to the substrate with respect to the target.
Specifically, the magnet member of the magnetron sputtering apparatus mentioned above comprises a rotating magnet group comprising a plurality of plate magnets attached to a surface of a rotating shaft in a spiral arrangement, and a fixed outer circumferential frame magnet which is arranged at a periphery of the rotating magnet group in parallel with a target surface and which is magnetized in a direction perpendicular to the target. With this structure, by rotating the rotating magnet group, a magnetic field pattern formed on the target by the rotating magnet group and the fixed outer circumferential frame magnet is continuously moved in a direction of the rotating shaft. Consequently, a plasma region on the target can continuously be moved with time in the direction of the rotating shaft.
By using the magnetron sputtering apparatus mentioned above, it is possible to uniformly use the target over a long time and to improve the film-forming rate.
According to an experiment performed by the present inventors, it is found that the above-mentioned magnetron sputtering apparatus is applicable also to film formation of the cathode body having a cylindrical cup shape according to the present invention.
According to one aspect of the present invention, there is provided a cathode body characterized by comprising an electrode member having tungsten or molybdenum as a main constituent and containing at least one selected from a group consisting of La2O3, ThO2, and Y2O3, and a film of a boride of a rare earth element formed on a surface of the electrode member by sputtering.
According to the present invention, there is also provided a cathode body characterized by having a carbon nanofiber layer formed on a conductor substrate, and a film of a boride of a rare earth element formed on a surface of the carbon nanofiber layer by sputtering
According to the present invention, there is also provided a cathode body characterized by comprising an electrode member having tungsten, molybdenum, or silicon as a main constituent provided with micro pyramids formed on a surface thereof and provided with a film of a boride of a rare earth element formed on a surface of the micro pyramids by sputtering.
Preferably, a LaB6 film formed by sputtering is annealed in an inert gas atmosphere. In this event, a specific resistance of the LaB6 film can be decreased.
According to the present invention, use is made of the electrode member formed of a mixture of tungsten having a high thermal conductivity and the material having a high electron emission efficiency. Furthermore, the boride film having a high electron emission efficiency is formed on the electrode member by sputtering. As a consequence, the boride film having an excellent adhesion can be attached to the electrode member. Thus, it is possible to obtain a cathode body having a high intensity, a high efficiency, and a long life.
Further, according to the present invention, it is possible to obtain a boride film which is formed by sputtering and which has a high electron emission efficiency.
Hereinbelow, an embodiment of the present invention will be described with reference to the drawing.
The magnetron sputtering apparatus shown in
As seen from the target 1, the fixed outer circumferential frame magnet 4 has a structure surrounding the rotating magnet group 3 comprising the spiral plate magnet group and is, herein, magnetized so that a S pole is formed on a side faced to the target 2. The fixed outer circumferential frame magnet 4 and each plate magnet of the spiral plate magnet group are formed of a Nd—Fe—B sintered magnet.
Further, in a process chamber space 11 inside a processing chamber shown in the figure, a plasma shielding member 16 is provided and a cathode body manufacturing jig 19 is disposed. The space is depressurized and plasma gas is introduced therein.
The plasma shielding member 16 shown in the figure extends in an axial direction of the columnar rotary shaft 2 and defines a slit 18 for opening the target 1 to the cathode body manufacturing jig 19. A region which is not shielded by the plasma shielding member 16 (namely, a region opened to the target 1 by the slit 18) is a region where a magnetic field intensity is high and a high-density low-electron-temperature plasma is generated so that a cathode body disposed on the cathode body manufacturing jig 19 is free from charge-up damage and ion irradiation damage and where a film-forming rate is high. A remaining region except the above-mentioned region is shielded by the plasma shielding member 16 so that film formation free from damage can be carried out without substantially decreasing the film-forming rate.
The backing plate 6 is provided with a coolant passage 8 for a refrigerant to pass therethrough. Between the housing 7 and an outer wall 14 defining the processing chamber, an insulating material 9 is disposed. A feeder line 12 connected to the housing 7 is extracted to the outside through a cover 13. The feeder line 12 is connected to a DC power source, a RF power source, and a matching unit (not shown in the figure).
With the above-mentioned structure, the DC power source and the RF power source supply a plasma excitation power to the backing plate 6 and the target 1 through the matching unit, the feeder line 12, and the housing to excite plasma on a surface of the target. It is possible to excite plasma only by a DC power or only by a RF power. However, in view of film quality controllability and film-forming rate controllability, both of these powers are desirably applied. The RF power has a frequency which is normally selected from a range between several hundreds kHz and several hundreds MHz. In order to achieve a high-density and low-electron-temperature plasma, a high frequency is desirable. In the present embodiment, a frequency of 13.56 MHz is used.
As shown in
Referring to
Each of the supporting portions 32 of the cathode body manufacturing jig 19 comprises a receiving portion 321 defining an opening portion having a size adapted to receive the cylindrical electrode portion 301 of the cylindrical cup 30, a flange portion 322 defining a hole having a diameter smaller than that of the receiving portion 321, and a slope portion 323 connecting the receiving portion 321 and the flange portion 322. As shown in the figure, the cylindrical electrode portion 301 is inserted into and positioned in the supporting portion 32 of the cathode body manufacturing jig 19. Specifically, the lead portion 302 of the cylindrical electrode portion 301 passes through the flange portion 322 of the cathode body manufacturing jig 19 and an outer end of the cylindrical electrode portion 301 is brought into contact with the slope portion 323 of the cathode body manufacturing jig 19.
Herein, the cylindrical cup 30 shown in the figure is formed of tungsten (W) with 4% to 6% lanthanum oxide (La2O3) added thereto by volume ratio and comprises the cylindrical electrode portion 301 having an inner diameter of 1.4 mm, an outer diameter of 1.7 mm, and a length of 4.2 mm and the lead portion 302. The length may be shortened to, for example, approximately 1.0 mm. In this example, the cylindrical cup 30 is formed by mixing tungsten which is a fire-resistant metal having an excellent thermal conductivity with La2O3 having a work function as small as 2.8 to 4.2 eV. By using tungsten, heat generated in the cylindrical cup 30 can efficiently be discharged. By mixing lanthanum oxide having a small work function, electrons can be emitted from the cylindrical cup 30 itself also. Incidentally, as a high-thermal-conductivity metal for forming the cylindrical cup 30, molybdenum (Mo) may be used instead of tungsten.
Herein, a method of manufacturing the cylindrical cup 30 will be described in detail. First, a tungsten alloy powder containing 3% La2O3 by volume ratio was mixed with a resin powder. As the resin powder, styrene was used and a mixing ratio of the tungsten alloy powder and styrene was 0.5:1 by volume ratio. Next, a very small amount of Ni was added as a sintering agent to obtain pellets. Using the pellets thus obtained, metal injection molding (MIM) was performed in a mold having a cylindrical cup shape and at a temperature of 150° C. to form a molded product having a cup shape. The molded product thus formed was heated in a hydrogen atmosphere to be degreased. Thus, the cylindrical cup 30 was obtained.
The cylindrical cup 30 thus obtained was fixed to the cathode body manufacturing jig 19 illustrated in
Argon was introduced into the processing chamber 11 to reduce a pressure to approximately 20 mTorr (2.7 Pa). The cathode body manufacturing jig 19 was heated to a temperature of 300° C. and sputtering was performed.
Referring back to
In the example illustrated in the figure, the thick LaB6 film 341, the thin LaB6 film 342, and the bottom LaB6 film 343 have thicknesses of 300 nm, 60 nm, and 10 nm, respectively.
By an experiment conducted by the present inventors, it was confirmed that the cathode body having the above-mentioned LaB6 films could maintain a high efficiency and a high intensity over a long time.
For example, on a surface of a molybdenum electrode free from an additive, a LaB6 film was formed by sputtering using Ar plasma on the condition of DC power of 900 W, a temperature of 300° C. of a substrate 301 (namely, the jig 19), and a vacuum degree of 20 mTorr (2.7 Pa). Then, annealing was performed at a temperature of 800° C. Those electrodes thus obtained were used as a pair of cold cathodes and enclosed in a glass tube having a length of 300 mm and a diameter of 3 mm to form a cold cathode fluorescent tube. Then, a lamp current of 6 mA was applied to the cold cathode fluorescent tube and a lamp voltage was measured. As a result, the cold cathode fluorescent tube required the lamp voltage of 550 to 553 Vrms. As compared to a case where a cold cathode fluorescent lamp using an electrode with no LaB6 film required a lamp voltage of 566 Vrms, the lamp voltage was reduced by 13V to 16V. Thus, it was confirmed that an electric power necessary for light emission could be reduced and, therefore, a high-efficiency lamp was obtained.
As a condition for forming the LaB6 film by sputtering, it is preferable that a surface of an electrode material is first cleaned by plasma before film formation. For example, it is suitable to use Ar plasma at 90 mTorr (12 Pa) and RF power of 300 W. When a chamber during sputtering is kept at a pressure of around 20 mTorr (2.7 Pa) (with Ar plasma, an electron temperature of approximately 1.9 eV, an ion irradiation energy of approximately 10 eV), a specific resistance is minimized (approximately 200 μΩcm before annealing). At this time, a film-forming rate is 90 nm/minute. If a pressure is reduced to 10 mTorr (1.3 Pa), the film-forming rate is increased to 100 nm/minute or more and the specific resistance is increased only slightly. Accordingly, the pressure is preferably 5 to 35 mTorr (0.67 Pa to 4.7 Pa). If a substrate temperature (stage temperature) is increased, the specific resistance is further reduced. With Ar at 20 mTorr (2.7 Pa) and at a substrate temperature of 300° C., the specific resistance is approximately 175 μΩcm. Furthermore, by annealing after film formation, the specific resistance is further reduced. If annealing is performed at a temperature of 800° C. in high-purity Ar, the specific resistance is approximately 100 μΩcm. An annealing temperature is preferably 400° C. to 1000° C. An annealing time must be not less than 30 minutes. For example, the annealing time not more than 3 hours is sufficient. Preferably, annealing is carried out in an inert gas atmosphere.
Next, for the purpose of examining an optimum condition for film formation of the LaB6 film by sputtering, an experiment was carried out as follows. A SiO2 film having a thickness of 90 nm was formed on a Si substrate by thermal oxidation and a LaB6 film having a thickness of 80 nm was deposited thereon using the rotating magnet sputtering apparatus in
According to a result of the XRD measurement, it was found that the LaB6 film formed by sputtering using the rotating magnet sputtering apparatus exhibited extremely low intensities for (210), (200), and (110) crystal planes and an extremely high intensity for a (100) crystal plane and had an excellent film quality. As compared to a conventional film formation by sputtering in which a (100) intensity was low, the above-mentioned feature is said to be one of the characteristics of the present invention.
On the other hand,
In the above-mentioned embodiment, the cathode body for a cold cathode tube has been described. However, the present invention is also applicable to a fluorescence emitting apparatus of a surface-emitting type. Specifically, the present invention is effective when it is applied to the fluorescence emitting apparatus of a surface-emitting type which comprises a cathode substrate and an anode substrate faced to each other, a cathode electrode and an emitter formed on the cathode substrate, an anode electrode formed on the anode substrate, and a carbon nanotube, a carbon nanofiber, a graphite fiber, or the like used for the emitter. Specifically, by providing the emitter mentioned above with the LaB6 film according to the present invention, which is formed by sputtering using the rotating magnet sputtering apparatus, it is possible to construct a light-emitting apparatus having a high efficiency, a high intensity, and a long life.
Further, the present invention is also applicable to a cathode body for a hot cathode tube.
Specifically, a member having tungsten or tungsten with 2 to 4% La2O3 and Th2O3 added thereto and a LaB6 thin film formed on a surface thereof is used as the cathode body for a hot cathode fluorescent lamp.
By adhering a patterned nonreflecting plastic film to a surface of a tube of a fluorescent lamp using the above-mentioned cathode body, it is possible to improve an efficiency by 30 to 40% as compared to a conventional product.
Further, when the present invention is applied to the cathode body for the hot cathode tube, the cathode body may also be used for a bulb-type fluorescent lamp (fluorescent lamp usable with a socket for an incandescent lamp and adapted to be directly fitted thereto).
In this case, a distance between electrodes is shortened and voltage drop due to recombination of electrons and ions on a tube wall is suppressed. Therefore, a luminance efficiency becomes 2 to 2.5 times that of a conventional product.
As compared to a tube-type fluorescent lamp, the bulb-type fluorescent lamp has a smaller distance between electrodes. Presumably, an effect of the tube wall is small and an effect of an electrode material is more significantly reflected.
In the foregoing, the present invention has been described in connection with the W or the Mo electrode member containing at least one material selected from a group consisting of La2O3, ThO2, and Y2O3. However, an excellent effect is obtained also if the LaB6 film is formed by sputtering according to the present invention on a surface of a commonly-used cathode body having tungsten or molybdenum as a main constituent, or on a surface of a substrate formed of a different material.
Further, it is possible to obtain a more excellent cathode body by comprising a carbon nanofiber layer formed on a conductor substrate and a film of a boride of a rare earth element formed on a surface of the carbon nanofiber layer by sputtering according to the present invention. This is because the carbon nanofiber layer has a high electron emission effect since a number of very small sharp projections are formed on the surface thereof. Similarly, an excellent effect is obtained by forming a number of micro pyramids on a surface of an electrode member having tungsten, molybdenum, silicon, or the like as a main constituent and forming a film of a boride of a rare earth element by sputtering on a surface of the micro pyramids.
The present invention is applicable not only to a cold cathode body provided with a cylindrical cup but also to a hot cathode body provided with a filament and a surface-emitting-type fluorescence emitting apparatus having an emitter in a similar manner.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2007-239219 | Sep 2007 | JP | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/JP2008/066530 | 9/12/2008 | WO | 00 | 3/12/2010 |
