Abstract:
A method of manufacturing a field emission device. The method having the steps of preparing a field emitter array having a plurality of electron emitting elements made of conductive material capable of emitting electrons upon application of an electric field, and impinging particle beams upon the plurality of electron emitting elements at the same time to mill a tip of each electron emitting element and form a sharp tip.

Description:
This application is based on Japanese patent application No. 9-241364 filed on Sep. 5, 1997, the entire contents of which are incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     a) Field of the Invention 
     The present invention relates to a method of manufacturing a field emission device, and more particularly to a method of manufacturing a field emission device having an emitter tip with a small radius of curvature and a small apex angle. 
     b) Description of the Related Art 
     A field emission element emits electrons from a sharp tip of an emitter (electric field emission emitter) by utilizing electric field concentration. For example, a flat panel display can be structured by using a field emitter array (FEA) having a number of emitters. Each emitter controls the luminance or the like of a corresponding pixel of the display. 
     A conventional manufacture method of a field emission device will be described. First, an emitter having a starting rough shape is made of emitter material. Thereafter, a focussed ion beam (FIB) is applied to the tip of the emitter to make a sharp emitter tip. This technique is disclosed In “Tip Surface Modification of Si Field Emitter Arrays” by M. Takai et al, Proceedings of the 2nd International Display Workshops, Oct. 19, 1995 at Hamamatsu City, Shizuoka, Japan. 
     This technique makes the emitter sharp by using FIB. Specifically, ions of material having a large atomic weight, such as Ga, are converged into a beam having a very small diameter by using an electronic lens or the like, and this beam is applied to an emitter. 
     The diameter of the beam is 20 to 240 nm which is smaller than the tip diameter of an emitter. In order to apply the beam to each emitter for a predetermined time, high precision alignment is required. With a manufacture method using FIB, the process time becomes long as the-number of emitters becomes large. 
     With the method using FIB, a lens system for passing a beam therethrough is required in order to form an effective beam, and also an alignment system is required to precisely align the tip of each emitter with the beam, because the beam diameter is small. Since the lens and alignment systems are required, the manufacture system becomes complicated and expensive. 
     With this method, since a beam is applied to each emitter after alignment, it takes a long time to sharpen the tips of all emitters. Furthermore, since the lens and alignment systems are required, the manufacture system becomes complicated and expensive. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a manufacture method of a field emission device capable of sharpening the tip of an emitter (electron emitting electrode) in a short time and with ease. 
     According to one aspect of the present invention, there is provided a method of manufacturing a field emission device, comprising the steps of: (a) preparing a field emitter array having a plurality of electron emitting elements made of conductive material capable of emitting electrons upon application of an electric field, and (b) impinging particle beams upon the plurality of electron emitting elements at the same time to mill a tip of each electron emitting element and form a sharp tip. 
     As a particle beam is impinged upon the electron emitting element, its tip is sharpened by milling. As the particle beams are impinged upon a plurality of electron emitting elements at the same time, the tips of the plurality of electron emitting elements can be sharpened at the same time. 
     Since the tips of the plurality of electron emitting element can be sharpened at the same time, the process time can be shortened. Since the particle beams are impinged upon the plurality of electron emitting elements, a high precision alignment system and a high precision lens system are not required to that a field emission device can be manufactured by a simple and inexpensive system. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1 to  4  are cross sectional views of a field emission device, illustrating the processes of manufacturing the field emission device according to a first embodiment of the invention. 
     FIG. 5 is a cross sectional view of a flat panel display using a field emission device. 
     FIGS. 6A to  6 H are cross sectional views of a field emission device, illustrating the processes of manufacturing the field emission device according to a second embodiment of the invention. 
     FIGS. 7A to  7 G are cross sectional views of a field emission device, illustrating the processes of manufacturing the field emission device according to a third embodiment of the invention. 
     FIGS. 8A to  8 G are cross sectional views of a field emission device, illustrating the processes of manufacturing the field emission device according to a fourth embodiment of the invention. 
     FIGS. 9A to  9 G are cross sectional views of a field emission device, illustrating the processes of manufacturing the field emission device according to a fifth embodiment of the invention. 
     FIGS. 10A to  10 F are cross sectional views illustrating the processes of manufacturing the field emission device shown in FIG.  1 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIGS. 1 to  4  are cross sectional views of a field emission device, illustrating the processes of manufacturing the field emission device according to the first embodiment of the invention. 
     As shown in FIG. 1, a substrate  1  is prepared which is formed with interlayer insulating films  2 , gate layers  3 , and emitters  4 . This substrate is formed, for example, by the following processes. 
     (1) As shown in FIG. 10A, on the whole surface of a substrate  1  made of, for example, Si, an interlayer insulating film  2  is deposited through chemical vapor deposition (CVD). The interlayer insulating film  2  is made of, for example, SiO x  (SiO 2 ). 
     (2) As shown in FIG. 10A, a gate layer  3  is deposited by CVD on the interlayer insulating film  2 . The gate layer  3  is made of, for example, polysilicon doped with P or B. The material of the gate layer  3  may be amorphous Si, WSi x , MoSi x , TaSi x , Al, Cu, or W. This film forming method may use sputtering instead of CVD. A sacrificial film may be deposited on the gate layer  3 . 
     (3) As shown in FIG. 10B, the gate layer  3  and interlayer insulating film  2  are locally removed by photolithography and anisotropic etching to form a plurality of holes (gate holes)  7  where emitters  4  are formed later. The substrate  1  is exposed on the bottom of each hole  7 . 
     (4) As shown in FIG. 10C, over the whole surface of the substrate, a sacrificial film  6  made of, for example, SiN x , is isotropically deposited thin by CVD. Next, the sacrificial film  6  on the flat top surfaces on the gate layers  3  is removed by anisotropic etching. As shown in FIG. 10D, the sacrificial film  6  on the side walls of the interlayer insulating film  2  and gate layer  3  becomes side spacers. This sacrificial film  6  corresponds to a part of the space formed between the emitter shown in FIG.  1  and the interlayer insulating film  2  and gate film  3 . 
     (5) As shown in FIG. 10E, an emitter layer  4  made of, for example, TiN x , is deposited by reactive sputtering, filling the hole (gate hole)  7  formed at the process (3) above. Instead of TiN x , the emitter layer  4  may be made of Mo, Cr, Ti, or W. Instead of sputtering, CVD may be used. 
     (6) The deposited emitter layer  4  is etched back to form an emitter  4  having a predetermined shape as shown in FIG.  10 F. 
     (7) The sacrificial film  6  on the side walls of the interlayer insulating film  2  and gate film  3  is etched and removed. With the above processes, the substrate shown in FIG. 1 is completed. 
     With the above processes, a plurality of emitters  4  are formed on the substrate  1 . One emitter  4  is formed in one gate hole. The tip (upper end portion) of the emitter  4  is cylindrical, and its top surface  11  Is flat. The emitter  4  is not limited to a cylindrical block shape, but it may take a rectangular block shape. 
     Next, as shown in FIG. 2, for example, Ar ions  5  are impinged upon the whole surface of the substrate. The substrate surface is milled with Ar ions. Ion milling is a process (physical sputtering) of making ionized atoms collide with an object to impart kinetic energy to the object and physically sputter the object. More specifically, ion milling is a process of making ionized atoms collide with a plurality of emitters  4  at the same time without substantially converging the ionized atoms. 
     For example, the ion milling is performed by using Ar ion gas at an acceleration energy of 700 eV and a current of 800 mA. If the emitter  4  is made of noble metal such as Au, Ag, Pt, Pd, and Cu, it is preferable that the ion beam incidence angle is 30 to 60 degrees relative to a substrate normal direction. The term “noble metal” used in this specification is intended to include Ag and Cu in addition to Au, Pt and Pd. When the ion beam is applied obliquely, it is necessary to rotate the substrate. If the emitter  4  is made of conductive material other than noble metal, such as Si, WSi x , Al and TiN x , it is preferable that the ion beam incidence angle is 0 degree relative to the substrate normal direction. 
     The flat top surface  11  of the emitter  4  is gradually removed by physical sputtering by Ar ions  5 , and at the same time the edge of the flat top surface  11  of the emitter  4  is gradually removed by the physical sputtering to change the right angle edge to a slanted surface  12 . 
     The flat top surface  13  of the gate film  3  is also gradually removed by the physical sputtering by Ar ions, and at the same time the edge of the flat top surface  13  of the gate film  3  is gradually removed by the physical sputtering to change the right angle edge to a slanted surface  14 , similar to the emitter  4 . 
     The slanted surfaces  12  and  14  are formed at the same time on the edges of the flat top surfaces of the plurality of emitters  4  and gate films  3 . The conditions of ion milling (ion beam) are preferably selected so that the slanted surfaces  12  and  14  are uniformly formed in the whole area of the substrate. 
     As shown in FIG. 3, as the substrate is further milled through the ion milling by Ar ions  5 , the flat surfaces  11  and  13  of the emitter  4  and gate film  3  are further removed by the physical sputtering to reduce the thicknesses thereof in a vertical direction, and at the same time the slanted surfaces of the emitter  4  and gate film  3  are further removed in the angle direction to increase the areas of the slanted surfaces  12  and  14  formed at the originally right angle edges. Next, this angle θ will be detailed. 
     This angle θ is an angle between the substrate normal direction and a line normal to the milled surface when Ar ions  5  are impinged upon the substrate in the vertical direction. In the following, it is assumed that the emitter  4  is made of material other than noble metal, such as Si, WSi x , Al, and TiN x . 
     As the ion beam incidence angle θ becomes large starting from 0°, the milling rate becomes fast. For example, the maximum milling rate is achieved in the angle θ range from 40° to 60°. As the surfaces of the emitter  4  and gate film  3  having the right angle edges are ion-milled at an ion beam incidence angle θ of 0°, the right angle edges are ion-milled at the slanted surface angle θ of 40° to 60° providing the maximum milling rate. The angle of the slanted surfaces  12  and  14  is therefore 90°−θ (θ=40°−60°) relative to the substrate normal direction. 
     As shown in FIG. 4, as the substrate is further ion-milled with Ar ions  5 , the slanted surfaces  12   a  and  12   b  as viewed in FIG. 4 at the originally right angle edges increase their areas further, and eventually intersect with each other at an apex. Since the emitter  4  is originally cylindrical, the tip of the emitter  4  becomes conical. A sharp edge  15  is therefore formed at the intersection of the slanted surfaces  12   a  and  12   b  as viewed in FIG.  4 . The slanted surface of the gate film  3  also increases its area further. 
     As described above, by impinging Ar ions upon the whole surface of the substrate, the tips of the plurality of emitters  4  can be sharpened at the same time in a short time, so that the manufacture throughput can be improved. 
     Since Ar ions  5  are impinged upon the whole surface of the substrate, an alignment system for applying an ion beam to a single emitter and a lens system for converging the ion beam are not necessary, so that the manufacture system can be made simple and inexpensive. 
     Milling may by performed by using ion particles other than Ar ion particles. Instead of the ion beam, a beam of particles not ionized may also be applied. 
     The shape of the emitter  4  before milling is not limited to only a cylindrical block shape, and its top surface is not necessarily required to be flat. The top surface of the emitter  4  before milling may be more or less sharp, and the above milling is performed to sharpen the tip further. 
     FIG. 5 is a cross sectional view of a flat panel display using a field emission device. 
     The field emission device manufactured by the embodiment method is used. Formed on a support substrate  61  made of insulating material, are a wiring layer  62  made of Al, Cu, or the like and a resistor layer  63  made of polysilicon or the like. On the resistor layer  63 , a number of emitter electrodes  64  whose tip has a small apex angle and a small radius of curvature are disposed to form a field emitter array (FEA). Each gate electrode  65  has a hole (gate hole) near at the tip of each emitter electrode  64  and a voltage can be applied independently to each gate electrode, although not illustrated in FIG. 5. A plurality of emitter electrodes  64  can also be independently applied with a voltage. 
     Facing an electron source including the emitter electrode  64  and gate electrode  65 , an opposing substrate is disposed including a transparent substrate  66  made of glass, quartz, or the like. The opposing substrate has a transparent electrode (anode electrode)  67  made of ITO or the like disposed under the transparent electrode  66  and a fluorescent member  68  disposed under the transparent electrode  67 . 
     The electron source and opposing substrate are joined together via a spacer  70  made of a glass substrate and coated with adhesive, with the distance between the transparent electrode  67  and emitter electrode  64  being maintained about 0.1 to 5 mm. The adhesive may be low melting point glass. 
     Instead of the spacer  70  of a glass substrate, a spacer  70  made of adhesive such as epoxy resin dispersed with glass beads may be used. 
     A getter member  71  is made of Ti, Al, Mg, or the like and prevents emitted gas from attaching again to the surface of the emitter electrode  64 . 
     An air exhaust pipe  69  is coupled to the opposing substrate. By using this air exhaust pipe  69 , the inside of the flat display panel is evacuated to about 10 −5  to 10 −9  Torr, and then the air exhaust pipe  69  is sealed by using a burner or the like. Thereafter, the anode electrode (transparent electrode)  67 , emitter electrode  64 , gate electrode  65  are wired to complete the flat panel display. 
     The flat display panel has a number of three-electrode elements (triodes). A three electrode element has the anode electrode (transparent substrate)  67 , emitter electrode  64  and gate electrode  65 . The gate electrodes  65  have a number of gate holes. Each emitter electrode  64  is formed in correspondence with each gate hole. 
     The anode electrode  67  is always maintained at a positive potential. The triodes are two-dimensionally disposed by emitter wirings and gate wirings, and the triode at the cross point of the voltage-applied emitter wiring and gate wiring is selected. 
     The emitter electrode  64  and gate electrode  65  of the selected triode are applied with negative and positive potentials, respectively, so that electrons are emitted from the emitter electrode  64  toward the anode electrode  67 . Electrons emitted from the emitter electrode  64  collide with the fluorescent member  68  which emits light. 
     FIGS. 6A to  6 H are cross sectional views of a field emission device, illustrating the processes of manufacturing the field emission device according to the second embodiment of the invention. 
     As shown in FIG. 6A, a first sacrificial film  20   b , a gate electrode  35   b , and a second sacrificial film  22  are deposited on a substrate  20   a  in this order from the bottom. The manufacture method thereof will be detailed hereinunder. 
     The substrate  20   a  is made of, for example, Si. SiN x  is deposited on the substrate  20   a  by CVD to form the first sacrificial film  20   b  having a thickness of 0.2 μm. For example, the sacrificial film  20   b  is formed at a substrate temperature of 800° C. by using NH 3 +SiH 4 +N 2  as source gas. 
     SiN x  may be deposited by low pressure CVD. In this case, the sacrificial film  20   b  is formed at a substrate temperature of 770° C. by using NH 3 +SiCl 2 H 2 +N 2  as source gas. Instead of low pressure CVD, plasma CVD, ECR-CVD, or sputtering may be used for depositing SiN x . 
     Next, polycrystalline Si is deposited on the first sacrificial film  20   b  by CVD to form the gate electrode  35   b  having a thickness of, for example, 0.2 μm. For example, the gate electrode  35   b  is formed at a substrate temperature of 625° C. by using SiH 4  diluted with He as source gas. Impurity ions such as P and B are diffused or implanted into the polycrystalline Si to lower the resistance of the gate electrode  35   b.    
     Next, SiN x  is deposited on the gate electrode  35   b  to form the second sacrificial film  22  having a thickness of, for example, 0.2 μm. The second sacrificial film  22  is formed under the same conditions as described above. 
     Next, a resist film  8  having a predetermined pattern is formed on the second sacrificial film  22 . By using the resist film  8  as a mask, the second sacrificial film  22 , gate electrode  35   b , and first sacrificial film  20   b  are selectively etched to form a recess  31  having generally a vertical side wall as shown In FIG.  6 B. The recess  31  has as its side wall the side walls of the first sacrificial film  20   c , gate electrode  35   c , and second sacrificial film  22   a , and as its bottom the surface of the substrate  20   a . For example, the recess  31  has a diameter of 0.5 μm and a depth of 0.6 μm. Thereafter, the resist film  8  is removed. 
     Next, as shown in FIG. 6C, SiO x  (SiO 2 ) is isotropically deposited on the surfaces of the second sacrificial film  22   a  and recess  31  by CVD to form a third sacrificial film  24   a  having a thickness of, for example, 0.10 μm. For example, the third sacrificial film  24   a  is formed at a substrate temperature of 400° C. by using TEOS and O 3  as source gas. 
     Next, as shown in FIG. 6D, polycrystalline Si is deposited over the whole surface of the substrate by low pressure CVD to form an emitter electrode layer  27  having a thickness of 0.15 μm or thicker. The emitter electrode layer  27  is formed under the same conditions described above. The emitter electrode layer  27  is filled in the recess over the substrate. 
     Instead of polycrystalline Si, the emitter electrode layer  27  may be formed by depositing amorphous Si, WSi x , TiN x , Al, Cu or the like by CVD. The emitter electrode layer  27  is not limited to a single layer film, but it may be a multi-layer film. For example, a multi-layer emitter electrode layer  27  may be formed by depositing TiN x  by sputtering and thereafter depositing W by CVD. 
     Next, the substrate  20   a  is removed by etching to expose the bottom surfaces of the first and third sacrificial films  20   c  and  24   a  as shown in FIG.  6 E. The silicon substrate  20   a  may be etched by using HF+HNO 3 +CH 3 COOH. 
     Next, the third sacrificial film  24   a  is partially etched and removed as shown in FIG. 6F to leave some part of the third sacrificial film  24   a  and expose a projection of the emitter electrode  27 . The third sacrificial film  24   a  made of SiO x  (SiO 2 ) may be etched by using HF+NH 4 F. 
     Next, ion milling is performed from the lower side to sharpen the tip of the emitter electrode  27   a  as shown in FIG.  6 G. The flat top surface edge of the emitter electrode is abraded and the flat top surface edge of the first sacrificial film  20   d  is also abraded. 
     For example, ion milling is performed at an acceleration energy of 700 eV and a current of 800 mA by using Ar ion gas. If the emitter electrode  27  is made of noble metal such as Au, Ag, Pt, Pd and Cu, the ion beam incidence angle is preferably 30 to 60 degrees relative to the substrate normal direction. In this case, an ion beam is impinged upon the substrate while the latter is rotated. If the emitter electrode  27  is made of material other than the noble metal, such as Si, WSi x , Al, and TiN x , the ion beam incidence angle is preferably near 0 degree relative to the substrate normal direction. 
     Next, the first sacrificial film  20   d  is etched and removed to expose the bottom surface of the gate electrode  35   c  as shown in FIG.  6 H. The first and second sacrificial films  20   d  and  22   d  are both made of SiN x . Therefore, while the first sacrificial film  20   d  is wholly etched and removed, the second sacrificial film  22   a  is partially etched to leave some portion of the second sacrificial film  22   b . SiN x  is etched by using HPO 3  heated to 170° C. 
     With the above processes, a two-electrode element having the emitter electrode  27   a  and gate electrode  35   c  is completed. In the first embodiment (FIG.  4 ), while the emitter electrode  4  is sharpened, the flat top surface edge of the gate electrode  3  is also rounded. In the second embodiment (FIG.  6 H), the gate electrode  35   c  is not abraded by ion milling so that the gate electrode  35   c  can easily take a desired shape and the gate diameter (gate hole diameter) can be made small. Therefore, a voltage applied to the gate electrode  35   c  to emit electrons from the emitter electrode  27   a  can be lowered. 
     It is sufficient if at least one of the second and third sacrificial films  22   b  and  24   b  is an insulating film. 
     FIGS. 7A to  7 G are cross sectional views of a field emission device, illustrating the processes of manufacturing the field emission device according to the third embodiment of the invention. 
     As shown in FIG. 7A, a first sacrificial film  20   c , a gate electrode  35   c , and a second sacrificial film  22   s  respectively having a predetermined pattern are formed on a substrate  20   a  by the same processes shown in FIGS. 6A and 6B. A recess  31  has as its side wall the side walls of the first sacrificial film  20   c , gate electrode  35   c , and second sacrificial film  22   a , and as its bottom the surface of the substrate  20   a.    
     Next, as shown in FIG. 7B, a third sacrificial film  24   a  is isotropically deposited on the surfaces of the second sacrificial film  22   a  and recess  31 , by the same process shown in FIG.  6 C. 
     Next, the whole surface of the third sacrificial film  24   a  is anisotropically etched (etch-back) to leave the third sacrificial film  24   c  only on the side wall of the recess  31  as a side spacer, as shown in FIG.  7 C. This etch-back is performed by anisotropic dry etching. For example, this etching is performed by using a magnetron RIE system at a reaction chamber pressure of 50 mTorr and a mixture gas of CHF 3 +CO 2 +Ar+He as etching gas. 
     Next, as shown in FIG. 7D, polycrystalline Si is deposited over the whole surface of the substrate by low pressure CVD to form an emitter electrode layer  27  having a thickness of 0.25 μm or thicker. The emitter electrode layer  27  is formed under the same conditions described above. The emitter electrode layer  27  is filled in the recess surrounded by the side spacer  24   c.    
     Instead of polycrystalline Si, the emitter electrode layer  27  may be formed by depositing amorphous Si, WSi x , TiN x , Al, Cu or the like by CVD. 
     Next, the substrate  20   a  and side spacer  24   c  are removed by etching to expose the bottom surfaces of a projection of the emitter electrode  27  and the first sacrificial film  20   c , shown in FIG.  7 E. The silicon substrate  20   a  may be etched by using HF+HNO 3 +CH 3 COOH. The side spacer  24   c  made of SiO x  (SiO 2 ) may be etched by HF+NH 4 F. 
     Next, ion milling is performed from the lower side to sharpen the tip of the emitter electrode  27   a  as shown in FIG.  7 F. The flat top surface edge of the emitter electrode  27   a  is abraded and the flat top surface edge of the first sacrificial film  20   d  is also abraded. 
     For example, ion milling is performed at an acceleration energy of 700 eV and a current of 800 mA by using Ar ion gas. If the emitter electrode  27  is made of noble metal such as Au, Ag, Pt, Pd and Cu, the ion beam incidence angle is preferably 30 to 60 degrees relative to the substrate normal direction. In this case, an ion beam is impinged upon the substrate while the latter is rotated. If the emitter electrode  27  is made of material other than the noble metal, such as Si, WSi x , Al, and TiN x , the ion beam incidence angle is preferably near 0 degree relative to the substrate normal direction. 
     Next, the first sacrificial film  20   d  is etched and removed to expose the bottom surface of the gate electrode  35   c  as shown in FIG.  7 G. The first sacrificial film  20   d  made of SiN x  is etched by HPO 3  heated to 170° C. 
     With the above processes, a two-electrode element having the emitter electrode  27   a  and gate electrode  35   c  is completed. In the first embodiment (FIG. 4) and second embodiment (FIG.  6 H), the projection of the emitter electrode is cylindrical and has generally the same diameter. In the third embodiment, the projection of the emitter electrode  27   a  becomes thinner toward its tip. Therefore, the emitter electrode layer  27  can be easily filled in the recess and the tip of the emitter electrode can be sharpened easily. 
     FIGS. 8A to  8 G are cross sectional views of a field emission device, illustrating the processes of manufacturing the field emission device according to the third embodiment of the invention. 
     As shown in FIG. 8A, a gate electrode  35   b  and a first sacrificial film  22  are deposited on a substrate  20   a  in this order from the bottom. The substrate  20   a  is made of, for example, Si. First, polycrystalline Si is deposited on the substrate  20   a  to form the gate electrode  35   b  having a thickness of, for example, 0.2 μm. For example, the gate electrode  35   b  is formed at a substrate temperature of 625° C. by using SiH 4  diluted with He as source gas. Impurity ions such as P and B are diffused or implanted into the polycrystalline Si to lower the resistance of the gate electrode  35   b.    
     Next, SiN x  is deposited on the gate electrode  35   b  to form the first sacrificial film  22  having a thickness of, for example, 0.25 μm. For example, the first sacrificial film  22  is formed at a substrate temperature of 800° C. by using NH 3 +SiH 4 +N 2  as source gas. 
     Next, a resist film  8  having a predetermined pattern is formed on the first sacrificial film  22 . By using the resist film  8  as a mask, the first sacrificial film  22  and gate electrode  35   b  are selectively etched to form a recess  31  having generally a vertical side wall as shown in FIG.  8 B. The recess  31  has as its side wall the side walls of the gate electrode  35   c  and second sacrificial film  22   a , and as its bottom the surface of the substrate  20   a . For example, the recess  31  has a diameter of 0.5 μm and a depth of 0.45 μm. Thereafter, the resist film  8  is removed. 
     Next, as shown in FIG. 8C, SiO x  (SiO 2 ) is isotropically deposited on the surfaces of the first sacrificial film  22   a  and recess  31  by CVD to form a second sacrificial film  24   a  having a thickness of, for example, 0.10 μm. For example, the second sacrificial film  24   a  is formed at a substrate temperature of 400° C. by using TEOS and O 3  as source gas. 
     Next, the whole surface of the second sacrificial film  24   a  is anisotropically etched (etch-back) to leave the second sacrificial film only on the side wall of the recess  31  and further etched (over-etch) to leave the second sacrificial film  24   c  on the lower side wall of the recess as a side spacer. With this etching, a recess  31  is formed on the substrate under the original recess  31  as shown in FIG.  8 D. This etching is performed by anisotropic dry etching. For example, this etching, is performed by using a magnetron RIE system at a reaction chamber pressure of 50 mTorr and a mixture gas of CHF 3 +CO 2 +Ar +He as etching gas. 
     Next, as shown in FIG. 8E, polycrystalline Si is deposited over the whole surface of the substrate by low pressure CVD to form an emitter electrode layer  27  having a thickness of 0.25 μm or thicker. The emitter electrode layer  27  is formed under the same conditions described above. The emitter electrode layer  27  is filled in the recess surrounded by the side spacer  24   c  and the substrate  20   e.    
     Instead of polycrystalline Si, the emitter electrode layer  27  may be formed by depositing amorphous Si, WSi x , TiN x , Al, Cu or the like by CVD. 
     Next, the substrate  20   e  and side spacer  24   c  are removed by etching to expose the bottom surfaces of a projection of the emitter electrode  27  and the gate electrode  35   c , as shown in FIG.  8 F. The silicon substrate  20   e  may be etched by using HF +HNO 3 +CH 3 COOH. The side spacer  24   c  made of SiO x  (SiO 2 ) may be etched by HF+NH 4 F. 
     Next, ion milling is performed from the lower side to sharpen the tip of the emitter electrode  27   a  as shown in FIG.  8 G. The flat top surface edge of the emitter electrode  27   a  is abraded and the flat top surface edge of the gate electrode  35   d  is also abraded. 
     For example, ion milling is performed at an acceleration energy of 700 eV and a current of 800 mA by using Ar ion gas. If the emitter electrode  27  is made of noble metal such as Au, Ag, Pt, Pd and Cu, the ion beam incidence angle is preferably 30 to 60 degrees relative to the substrate normal direction. In this case, an ion beam is impinged upon the substrate while the latter is rotated. If the emitter electrode  27  is made of material other than the noble metal, such as Si, WSi x , Al, and TiN x , the ion beam incidence angle is preferably near 0 degree relative to the substrate normal direction. 
     With the above processes, a two-electrode element having the emitter electrode  27   a  and gate electrode  35   d  is completed. In the fourth embodiment, the recess  20   e  is formed on the substrate  20   e  by over-etch at the etching process shown in FIG.  8 D. Therefore, the tip of the emitter electrode  27   a  can be projected in an electron emission direction (toward a lower side as viewed in FIG.  8 D. Therefore, a voltage applied to the gate electrode  35   d  to emit electrons from the emitter electrode  27   a  can be lowered. 
     FIGS. 9A to  9 G are cross sectional views of a field emission device, illustrating the processes of manufacturing the field emission device according to the fifth embodiment of the invention. 
     As shown in FIG. 9A, a first gate electrode  36   b , a second gate electrode  35   b , and a first sacrificial film  22  are deposited on a substrate  20   a  in this order from the bottom. The substrate  20   a  is made of, for example, Si. First, Ti is deposited by sputtering on the substrate  20   a  to form the first gate electrode  36   b  having a thickness of, for example, 0.1 μm. For example, a DC sputtering system is used and Ti is used as a target while Ar gas is introduced. 
     Instead of Ti, the first gate electrode  36   b  may be made of TiN x  or TiO x N y . TiN x  or TiO x N y  may be deposited by reactive sputtering using Ar+N 2  or Ar+N 2 +O 2  as source gas. 
     Polycrystalline Si is deposited by CVD on the first gate electrode  36   b  to form the second gate electrode  35   b  having a thickness of, for example, 0.15 μm. For example, the second gate electrode  35   b  is formed at a substrate temperature of 625° C. by using SiH 4  diluted with He as source gas. Impurity ions such as P and B are diffused or implanted into the polycrystalline Si to lower the resistance of the second gate electrode  35   b.    
     Next, SiN x  is deposited on the second gate electrode  35   b  to form the first sacrificial film  22  having a thickness of, for example, 0.2 μm. For example, the first sacrificial film  22  is formed at a substrate temperature of 800° C. by using NH 3 +SiH 4 +N 2  as source gas. 
     Next, a resist film  8  having a predetermined pattern is formed on the first sacrificial film  22 . By using the resist film  8  as a mask, the first sacrificial film  22 , second gate electrode  35   b , and first gate electrode  36   b  are selectively etched to form a recess  31  having generally a vertical side wall as shown in FIG.  9 B. The recess  31  has as its side wall the side walls of the first gate electrode  36   c , second gate electrode  35   c , and first sacrificial film  22   a , and as its bottom the surface of the substrate  20   a . For example, the recess  31  has a diameter of 0.5 μm and a depth of 0.45 μm. Thereafter, the resist film  8  is removed. 
     Next, as shown in FIG. 9C, SiO x  (SiO 2 ) is isotropically deposited on the surfaces of the first sacrificial film  22   a  and recess  31  by CVD to form a second sacrificial film  24   a  having a thickness of, for example, 0.10 μm. For example, the second sacrificial film  24   a  is formed at a substrate temperature of 400° C. by using TEOS and O 3  as source gas. 
     Next, the whole surface of the second sacrificial film  24   a  is anisotropically etched (etch-back) to leave the second sacrificial film  24   c  only on the side wall of the recess  31  as a side spacer as shown in FIG.  9 D. This etching is performed by anisotropic dry etching. For example, this etching is performed by using a magnetron RIE system at a reaction chamber pressure of 50 mTorr and a mixture gas of CHF 3 +CO 2 +Ar+He as etching gas. 
     Next, as shown in FIG. 9E, noble metal such as Au, Ag, Pt, Pd and Cu is deposited over the whole surface of the substrate to form an emitter electrode layer  27 . Noble metal can be deposited by coating independent and dispersive ultra-fine particles and baking it at 200 to 300° C. Noble metal may also be deposited by plating, sputtering, or vapor deposition. The emitter electrode layer  27  is filled in the recess surrounded by the side spacer  24   c.    
     Next, the substrate  20   a  and side spacer  24   c  are removed by etching to expose the bottom surfaces of a projection of the emitter electrode  27  and the first gate electrode  36   c , as shown in FIG.  9 F. The silicon substrate  20   a  may be etched by using HF+HNO 3 +CH 3 COOH. The side spacer  24   c  made of SiO x  (SiO 2 ) may be etched by HF+NH 4 F. 
     Next, ion milling is performed from the lower side to sharpen the tip of the emitter electrode  27   a  as shown in FIG.  9 G. The flat top surface edge of the emitter electrode  27   a  made of noble metal is abraded and the flat top surface edge of the first gate electrode  36   c  made of Ti is also abraded. 
     For example, ion milling is performed at an acceleration energy of 700 eV and a current of 800 mA by using Ar ion gas. Since the emitter electrode  27  is made of noble metal such as Au, Ag, Pt, Pd and Cu, the ion beam incidence angle is preferably 30 to 60 degrees relative to the substrate normal direction. 
     Ion milling under the above conditions abrades Au by 140 nm, Ag by 140 nm, Pt by 78 nm, Pd by 110 nm, and Ti by 30 nm. Noble metal is abraded more than Ti. 
     The flat top surface edge of the emitter electrode  27   a  made of noble metal is easy to be abraded, whereas the flat top surface edge is hard to be abraded. Therefore, while the tip of the emitter electrode  27   a  can be sharpened, the shape of the first gate electrode  36   c  is maintained nearly at it is. 
     If reactive ion milling added with O 2  gas is performed, the first gate electrode  36   c  made of Ti is oxidized and becomes TiO x  (TiO 2 ). A milling rate of TiO 2  is 10 nm/min which is one third that of Ti. The abrasion amount of the first gate electrode  36   c  can be made small. 
     With the above processes, a two-electrode element having the emitter electrode  27   a  and first and second gate electrodes  36   c  and  35   c  is completed. In the fifth embodiment, provision of the first gate electrode  36   c  made of Ti or the like having a low milling rate allows the tip of the emitter electrode  27   a  to be sharpened, while the gate electrode  36   c  is hardly abraded. By preventing the gate electrode  36   c  from being abraded, the gate diameter (gate hole diameter) can be prevented from being broadened. With a smaller gate electrode diameter, a voltage applied to the gate electrode  35   d  to emit electrons from the emitter electrode  27   a  can be lowered. 
     The second gate electrode  35   b  may not be formed at the process shown in FIG.  9 A. In this case, the first gate electrode  36   b  is formed slightly thick (e.g., 0.25 μm). 
     In the first to fifth embodiments described above, ions such as Ar ions collide with the whole substrate surface so that the tips of a plurality of emitters can be sharpened at the same time in a short time and the manufacture throughput can be improved. 
     Since ions such as Ar ions are impinged upon the whole surface of the substrate, an alignment system for impinging an ion beam upon a single emitter and a lens system for converging the ion beam are not necessary, so that the manufacture system can be made simple and inexpensive. 
     Emitters may be milled at the last manufacture process as in the first, fourth, and fifth embodiments. A sacrificial film may be removed after the milling process as in the second and third embodiments. If a field emitter array having a plurality of emitters is prepared and subjected to the milling process, other processes may be performed or not performed thereafter. 
     The present invention has been described in connection with the preferred embodiments. The invention is not limited only to the above embodiments. It Is apparent that various modifications, improvements, combinations, and the like can be made by those skilled in the art.