Method of fabricating a field-emission cold cathode

Abstract: In a field-emission cold cathode forming an emitter by the vacuum deposition method, contamination at the side surface of the insulating layer due to deposition of an emitter material during formation of the emitter is prevented. Thereby, deterioration of insulating resistancel and dielectric strength between gate and emitter can also be prevented. With an oblique vacuum deposition, a sacrificing layer 5 is formed to the entire area of the side surface within the cavity 4 and next an emitter is vacuum deposited and thereafter emitter material atoms are removed together with the protecting film.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a method of fabricating a cold cathoce which is used as an electron emission source and particularly to a method of fabricating a field-emission cold cathode for emitting electron from a sharpened tip end. Description of the Related Art A so-called Spindt type cold cathode is disclosed in Journal of Applied Physics, Vol. 39, No. 7, pp. 3504, 1968. This Spindt type cold cathode provides a higher current density than a hot cathode and is characterized in having small velocity distribution of electrons emitted. Moreover, in comparison with single field-emission emitter, this cold cathode provides a small current noise and operates with a voltage as low as several tens voltage to 200 V. Furthermore, this cold cathode operates under the vacuum condition of about 10 -8 Pa (10 -10 torr)in the electron microscope. However, in this case, it can be operated, based on the report, within the glass tube of 10 -4 -10 -6 Pa (10 -6 to 10 -8 torr) with a plurality of emitters. Fig. 5 shows a cross-section of the principal structure of the Spindt type cold cathode as the related art. A miniaturized conic emitter 102 in height of about 1 µm is formed on a conductive substrate 101 by the vacuum deposition method and a gate layer 103 and an insulating layer 104 are formed around the emitter 102. The substrate 101 and emitter 102 are electrically connected and a DC voltage of about 100 V is applied across the substrate 101 (and emitter 102) and the gate layer 103 (positive side). Since a distance between the substrate 101 and gate layer 103 is set approximately to 1 µm, an aperture diameter of the gate layer is as narrow as about 1 µm and the end point of the emitter 102 is shaprened, an intensive field is applied to the end point of the emitter 102. When the field becomes 2 to 5 x 10 7 V/cm or higher, the emitter 102 emits electrons from the end point providing a current of 0.1 to several 10 µA per emitter. Arrangement of a plurality of miniaturized cold cathodes having such a structure on a substrate 101 in the form of array will constitute a flat type cathode for emitting a large current. A method of fabricating the Spindt type cold cathode will be explained with reference to Fig. 6. An insulating layer 62 such as silicon dioxide (SiO 2 ) and a low resistance gate layer 63 which will become a gate electrode are formed on a conductive substrate 61 of silicon which also works as a cathode electrode (Fig. 6A). Next, the cavity 65 (Fig. 6B) patterned on the resist 64 by the photolithography technology, etc. is transferred to the gate layer 63 and insulating layer 62 by the etching method (Fig. 6C). Next, in view of forming a sacrificing layer 66 for layer lift-off on the gate layer 63 and at the edge of the cavity 65, the aluminum oxide is vacuum deposited from the oblique direction while the substrate 61 is being rotated (Fig. 6D). Thereafter, in order to form an emitter, an emitter material 67 such as molybdenum is vacuum deposited in vertical for the substrate (Fig. 6E). In this case, since the aperture of cavity is gradually narrowed with progress of vacuum deposition, a conic emitter 68 is formed on the bottom surface of cavity. Finally, the sacrifice layer 66 is etched to remove the unwanted film at the surface and to expose the emitter 68 (Fig. 6F). For the operation of the field-emission cold cathode, about 100 V is applied across the electrodes providing a distance of approximately 1 µm. Therefore, insulation characteristic between the gate layer and emitter is very important. If insulation between gate and emitter is poor, operation is not stable and operation life is also shortened. In the method of related arts, almost conic emitter electrode is formed in just upper direction by the vacuum deposition method, but all evaporated atoms are not deposited as the emitter electrode but a little fraction of emitter material is also deposited to the side surface of insulating layer within the cavity, thereby deteriorating the insulation characteristic between the gate layer and emitter. Moreover, a Japanese Unexamined Patent Laid-Open No. Hei 6-89651 discloses the art to form the emitter electrode with various materials by a sputtering method. In the sputtering method, however, the degree of vacuum is lower than that of the vacuum deposition method and scattering of vacuum deposition particles due to gas molecule gives higher influence. Thereby, deposition of the emitter material to the side surface of the insulating layer increases, deteriorating the insulation characteristic to a large extent. This influence particularly results in distinctive deterioration of the insulation characteristic and sometimes disables the operation itself for the cathode in the constitution where many emitters are arranged in parallel. A Japanese Unexamined Patent Laid-Open No. Hei 6-96664 discloses a method of fabricating Spindt type cold cathode. In this method, on the occasion of forming a sacrificing layer with the oblique vacuum deposition method as shown in Fig. 6D, only a part of the side surface of the insulating layer is covered with the sacrificing layer. Accordingly, when vacuum deposition is carried out thereafter, the emitter material is deposited on the greater part of the other side surface of the insulating layer and thus make it almost impossible to expect improvement in the insulation characteristic. A method of fabricating a field-emission cold cathode according to the pre-characterizing part of claim 1 is known from US-A-5,249,340. US-A-5,151,061 teaches a method to form self-aligned cathode emission tips which are used as a mold for forming metal pillar. In order to obtain a cone-shaped electrode this document shows a process to etch away the metal pillar. SUMMARY OF THE INVENTION To overcome the problems and disadvantages of the methods of the prior art the invention uses the features of claim 1. Further advantageous embodiments are shown in the sub claims. At the time of forming a sacrificing layer, while the substrate is rotated around the vertical axis, the sacrificing layer material is deposited at the angle of almost tan -1 (D g /(t g + t i )) from the rotating axis to the sacrificing layer material deposited at the side surface of the insulating layer within the cavity as the protecting film. Moreover, after the protection film is formed by the CVD method, the protection film deposited on the area of the substrate where the emitter electrode should be formed is removed, leaving the protection film only at the side surface of the insulating layer. Otherwise, it is also possible that a protection film is deposited by the vacuum deposition method or sputtering method and the film deposited to the side surface of the insulating layer in the cavity scattered on the occasion of removing the protection film, by the sputter etching method, deposited on the region of the substrate where the emitter electrode is to be formed is used as the protection film. Since the cold cathode may be formed without contamination of side surface of the insulating layer with a conductive emitter material, the insulation resistance between emitter and gate is not deteriorated and dielectric strength is also not affected. Thereby, a gate current during operation can be reduced and stable operation can be assured. Moreover, a cold cathode having matrix-arrayed emitters can operate stably with increase of an emission current. BRIEF DESCRIPTION OF THE DRAWINGS Other objects and advantages of the present invention will be apparent from the following detailed description of the presently preferred embodiments thereof, which description should be considered in conjunction with the accompanying drawings in which: Figs. 1A to 1D are diagrams for explaining the steps of manufacturing a field-emission cold cathode of the first embodiment of the present invention. Figs. 2A to 2C are diagrams for explaining the steps of fabricating a field-emission cold cathode of the second embodiment of the present invention. Figs. 3A to 3E are diagrams for explaining the steps of fabricating a field-emission cold cathode of the third embodiments of the present invention. Figs. 4A to 4C are diagrams for explaining the steps of fabricating a field-emission cold cathode of the fourth embodiment of the present invention. Fig. 5 is a cross-sectional view of the principal portion of the Spindt type cold cathode. Figs. 6A to 6F are diagrams for explaining the steps of fabricating the Spindt type cold cathode disclosed in the related art, Japanese Unexamined Patent Laid-Open No. Hei6-96664. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be explained in detail with reference to the accompanying drawings. Fig. 1 illustrates a constitution and processes of a field-emission cold cathode showing an embodiment of the present invention. As illustrated in Fig. 1A, an insulating layer 2 (thickness t i =about 0.8 µm) and a gate layer 3 (thickness t g = about 0.2 µm) are stacked on a silicon substrate 1 and a minute cavity 4 (diameter D g = about 1 µm) is formed on the gate layer 3 and insulating layer 2 by the photolithography and etching process. As the material of the insulating layer 2 and gate layer 3, silicon dioxide or tungste, for example, is used. Next, a sacrificing layer 5 is formed. In this case, while the substrate 1 is rotated around the axis perpendicular thereto, aluminum is vacuum deposited. In this process, the vacuum deposition is carried out in the incident angle of tan -1 (D g /(t g + t i )) (in this case, about 45 degrees from the rotating axis) so that aluminum is deposited to the entire part of the gate layer 3 and side surface of the insulating layer within the cavity 4 to cause the sacrificing layer 5 to work also as a protection film (Fig. 1B). Thereby, the aluminum layer formed is continuous to the side surface of the insulating layer in the cavity 4 from about the gate layer 3. Usually, diameter D g of the cavity 4 is about 0.2 to 2 µm and height of emitter (≒ t i + t g ) is set to 0.8 to 2 times the diameter D g . Therefore, the optimum tan -1 (D g /t g + t i )) is in the range of 25 to 50 degrees. Typically, the preferential angle is about 45 degrees. Thereafter, while the substrate 1 is rotated around the axis perpendicular thereto, molybdenum is vacuum deposited at normal incidence above the substrate 1 to form an emitter 7. During this process, emitter material particles 8 migrating due to scattering of residual gas in the vacuum condition are adhered to the sacrificing layer (protection film) 5 on the side surface of the insulating layer (Fig. 1C). Finally, the sacrificing layer 5 is dissolved by phosphoric acid to remove unwanted emitter material 6 and emitter material particles 8 in order to realize not-contaminated side surface of the insulating layer (Fig. 1D). As the emitter material, gold, platinum, rhodium can be used as well as molybdenum, while as the gate layer material, tungsten silicide, molybdenum, polycrystal silicon can be used as well as tungsten, as the insulating layer material, silicon nitride, etc. can be used as well as silicon dioxide, and as the sacrificing layer material, aluminum oxide, silicon nitride, nickel can be used as well as aluminum. Fig. 2 illustrates some of the processes of fabricating a field-emission cold cathode showing another embodiment of the present invention. In Fig. 2, the elements like those of Fig. 1 are designated by the like reference numerals. Moreover, material and size of each constitutional element are same as those in the first embodiment shown in Fig. 1. As shown in Fig. 2, an insulating layer 2, a gate layer 3 and sacrificing layer 9 of aluminum are stacked and a minute cavity 4 is formed to the sacrificing layer 9, gate layer 3 and insulating layer 2 (Fig. 2A). Subsequently, aluminum which will become a protection film material 10 is formed on the gate layer 3 and on the surface of cavity 4 by using a CVD method (Fig. 2B). Thereafter, the protection film 11 is left only at the side surface of the insulating layer 2, gate layer 3 and sacrificing layer 9 by performing anisotropic etching with the reactive ion etching (RIE) utilizing carbon tetrachloride gas to expose the bottom surface of the cavity 4 (Fig. 2C). Processes after formation of emitter are same as the first embodiment shown in Figs. 1C and 1D. In above explanation, aluminum is used as the material of sacrificing layer and protecting film, but aluminum oxide, silicon nitride or combination thereof can also be used additionally by replacing an introduced gas at the time of CVD or RIE. Fig. 3 illustrates a constitution and processes of a field-emission cold cathode showing an embodiment of the present invention. The processes up to formation of the cavity 4 are the same as those of the second embodiment of Fig. 2A. Subsequently, the side surface of the insulating layer is etched with fluoric acid to form the shape formed by eaves of the gate layer as shown in the figure (Fig. 3A). Thereafter, the upper and side surfaces and the bottom surface of the cavity 4 are coated with a positive resist 12 (Fig. 3B) and the resist 12 is left, as the protection film 13, only in the area where is shadowed at the time of exposure by the exposure and development from above the substrate (Fig. 3C). The processes up to separation of the sacrificing layer from formation of emitter (Fig. 3D) are the same as those of the first embodiment shown in Figs. 1C and 1D. Finally, the contamination-free side surface of the insulating layer can be realized by removing the protection film 13 by using the remover (Fig. 3E). Fig. 4 illustrates some of the processes of fabricating a field-emission cold cathode showing another embodiment of the present invention. The processes up to the etching for the side surface of the insulating layer are the same as those in the third embodiment. Moreover, the protection film material (aluminum) 14 is vacuum deposited in the vertical direction with respect to the substrate 1 (Fig. 4A). Thereafter, the sputter etching is performed using argon ion. The sputter etched protection material 14 at the bottom surface of the cavity 4 is removed and are then adhered to the side surface of the insulating layer as the protection film 15 (Fig. 4C). The processes after formation of emitter are the same as those of the first embodiment shown in Figs. 1C and 1D. As explained heretofore, the present invention can prevent deposition of emitter material to the side surface of the insulating layer to fabricate cold cathode without deterioration of the insulating characteristic. As a result, discharge and leak currents particularly generated when the emitters are matrix-arrayed can be reduced to increase an emission current and also improve the characteristic yield. Moreover, deterioration of insulating characteristic due to deposition can be prevented at the time of forming an emitter electrode by the sputtering method. Therefore, the range for selection of emitter material can easily be widened up to a high melting point compound which is difficult to be used to form a film by the vacuum deposition method. Although preferred embodiments of the present invention have been described and illustrated, it will be apparent to those skilled in the art that various modifications may be made without departing from the principles of the invention.

Claims: A method of fabricating a field-emission cold cathode comprising the steps of: forming an insulating layer (2) on a substrate (1) having a conductive surface; forming a conductive gate layer (3) on said insulating layer (2) ; forming a cavity (4) in said insulating layer (2) and said gate layer (3); forming a sacrificing layer (5, 9) on said gate layer (3) ; forming a protecting film (5, 11, 13, 15) on a cavity wall of said first insulating layer (2); forming an emitter electrode (7) on said substrate (1) within said cavity (4) by depositing emitter electrode material (6) ; removing said sacrificing layer (5, 9) together with said emitter electrode material deposited on said sacrificing layer (5, 9); and removing said protecting film (5, 11, 13, 15) ; characterized in that said emitter electrode (7) is a cone-shaped electrode and a bottom thereof is directly contacted onto said substrate (1), and emitter material particles (8) adhered to inner wall region of said protecting film (5, 11, 13, 15) extending from said substrate (1) to said gate layer (3) due to scattering of residual gas in a vacuum condition are removed by removing said protecting film (5, 11, 13, 15). A method of fabricating a field-emission cold cathode as claimed in claim 1, wherein said sacrificing layer is formed by using a vacuum deposition method while said substrate is rotated around the axis perpendicular thereto, and material of said sacrificing layer is deposited in the angle of about tan -1 (D g /(t g + t i )) from said axis when a diameter of said cavity is designated as D g , thicknesses of said gate layer and insulating layer respectively as t g and t i to form both of said sacrificing layer and said protecting film. A method of fabricating a field-emission cold cathode as claimed in claim 2, wherein said material of said sacrificing layer is continuously deposited up to the side surface of the insulating layer at the inside of the cavity to form said emitter electrode from the surface of the gate layer. A method of fabricating a field-emission cold cathode as claimed in claim 1, wherein after said protecting film is formed by CVD method, said protecting film deposited on an area of said substrate to form said emitter electrode is removed by using one of methods selected from sputter etching method and anisotropic dry etching method. A method of fabricating a field-emission cold cathode as claimed in claim 1, wherein said protecting film is formed by coating a positive photoresist. A method of fabricating a field-emission cold cathode as claimed in claim 1, wherein said protecting film is formed by providing material of said protecting film on a bottom of said cavity and sputtering said material. A method of fabricating a field-emission cold cathode as claimed in claim 2, wherein said angle is set in the range of 25 to 50 degrees.

Inventors: MATSUNO FUMIHIKO, SEKO NOBUYA, MATSUNO, FUMIHIKO, SEKO, NOBUYA, Matsuno, Fumihiko, c/o NEC Corporation, Seko, Nobuya, c/o NEC Corporation
Applicants: NIPPON ELECTRIC CO, NEC CORPORATION
Application year: 1996
Main classification: H01J9
IPC codes: H01J9