Patent Publication Number: US-2006012282-A1

Title: Dielectric device

Description:
BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      This invention relates to a dielectric device using dielectric properties and electrical-mechanical conversion properties.  
      2. Description of the Related Art  
      In the related art, a type of dielectric device using ferroelectric ceramics is known. The dielectric device using this ferroelectric ceramics can usually be obtained by forming a laminated structure, by patterning an electrode on a ferroelectric ceramic thin plate formed by the screen printing method or the green sheet method, or joining to another metal or a ceramic thin plate.  
      Here, the screen printing method is a method of obtaining a ceramic thin plate used as the base of a dielectric device by forming a film on a predetermined substrate from a slurry containing a ceramic powder dispersed in an organic binder by screen printing, and sintering this film at a high temperature of 900° C. or higher. The green sheet method is a method of obtaining a ceramic thin plate by forming a thick film of predetermined thickness from the aforesaid slurry, drying to obtain a green sheet, and sintering at a high temperature in the same way as above after hole-punching. In recent years, this kind of dielectric device has been used as an electron emitter, for example in field emission displays (FEDs) or electron beam sources in electron microscopes.  
      The electron emitter is operated in a predetermined vacuum level, and when a predetermined electric field is applied to the electron emission portions (hereafter, emitters), electrons are emitted from the emitters. When this electron emitter is used in an FED, plural electron emitters are arranged in two dimensions, and plural fluorescent bodies are respectively disposed at a predetermined interval from the electron emitters so as to correspond to the plural electron emitters. By selectively driving one of these plural electron emitters arranged in two dimensions at any desired position, electrons can be emitted from an electron emitter in any desired position, fluorescence can be emitted from a phosphor in any desired position due to collision between the emitted electrons and the phosphor, and a desired display can thus be obtained.  
      Some early examples of this electron emitter are given for example in the following Patent References 1 to 5. These electron emitters do not use the aforesaid dielectric device, but instead have an emitter which includes minuscule conductive electrodes with sharp tips. When a predetermined drive voltage is applied between a reference electrode opposite to this emitter and the emitter, electrons are emitted from the tips of the emitter. Therefore, to form this minuscule conductive electrode, microfabrication by etching, forming or the like is required. Also, to emit a predetermined electron amount into a predetermined vacuum level from the tips of the aforesaid conductive electrodes, a sufficiently high voltage is required as the drive voltage, and an expensive drive element which can supply a high voltage, such as an IC for driving this electron emitter, is required.  
      As stated above, the problem of the electron emitter using a conductive electrode as an emitter has been that the production costs of not only the electron emitter itself but also the device to which the electron emitter is applied increase. In recent years, therefore, an electron emitter using the aforesaid dielectric device, i.e., an electron emitter wherein the emitter is comprised of a dielectric, has been designed, as disclosed for example in the following Patent References 6 and 7. General knowledge on the electron emission in the case of using a dielectric as the emitter is disclosed in the following Non-Patent References 1 to 3.  
      The electron emitter disclosed in Patent References 6 and 7 is configured so as to: cover a part of the upper surface of an emitter including a dielectric with a cathode electrode; and dispose an anode electrode at a position on or below the lower surface of the emitter or a position apart from the cathode electrode at a prescribed interval on or above the upper surface of the emitter. That is, the electron emitter is configured so that the exposed surface portion, where neither a cathode electrode nor an anode electrode is formed, of the emitter exists on the upper surface side of the emitter in the vicinity of the outer edge of the cathode electrode Then as the first step, voltage is applied between the cathode electrode and the anode electrode so that the cathode electrode has a higher potential, and the electric field formed by the applied voltage makes the emitter (the exposed portion in particular) get into a prescribed polarized state. Next as the second step, voltage is applied between the cathode electrode and the anode electrode so that the cathode electrode has a lower potential. At this time, primary electrons are emitted from the outer edge of the cathode electrode, the polarization of the emitter is reversed, the primary electrons collide with the exposed portion of the emitter where the polarization has been reversed, and thereby secondary electrons are emitted from the emitter (the exposed portion in particular). The secondary electrons fly toward a prescribed direction caused by a prescribed electric field applied from outside and thereby electrons are emitted from the electron emitter. 
      [Patent Reference 1] JP-A No. 311533/1989     [Patent Reference 2] JP-A No. 147131/1995     [Patent Reference 3] JP-A No. 285801/2000     [Patent Reference 4] JP-B No. 20944/1971     [Patent Reference 5] JP-B No. 26125/1969     [Patent Reference 6] JP-A No. 146365/2004     [Patent Reference 7] JP-A No. 172087/2004     [Non-Patent Reference 1] Yasuoka and Ishii “Pulsed Electron Source using Ferroelectric Cathode,”  J. Appl. Phys.,  Vol. 68, No. 5, pp. 546-550, 1999     [Non-Patent Reference 2] V. F. Puchkarev, G. A. Mesyats “On the mechanism of emission from the ferroelectric ceramic cathode,”  J. Appl. Phys.,  Vol. 78, No. 9, 1 Nov., 1995, pp. 5633-5637     [Non-Patent Reference 3] H. Riege “Electron emission ferroelectrics—a review,”  Nucl. Instr. and Mech.,  A340, pp. 80-89, 1994    

     SUMMARY OF THE INVENTION  
      In the dielectric device of the related art, as described above, a ceramic thin plate simply formed by the screen printing method or the green sheet method is used, so the following problems occurred.  
      First, the ceramic thin plate obtained by only the screen printing method or green sheet method is obtained by simply drying and sintering a slurry in which a ceramic powder is dispersed, so many types of ceramic particles having a broad particle size distribution from fine particles to coarse particles are present in the ceramic thin plate. In a dielectric device (in particular, an electronic device) such as the aforesaid electron emitter or surface acoustic wave (SAW) device wherein the physical properties in the vicinity of the surface of the ceramic thin plate have a large effect on performance, it is difficult to obtain a desired surface state and obtain better characteristics. Hence, in the related art, no consideration is given to the matching of the application of the dielectric device and its required characteristics to a suitable structure of the ceramic thin plate.  
      In the screen printing method or green sheet method, an organic binder from a slurry film or green sheet is decomposed/vaporized, and ceramic crystals are grown at a high temperature to cause adjacent ceramic particles to fuse together. By this process, the ceramic thin plate which forms the base of these dielectric devices can be obtained, but voids between adjacent ceramic particles cannot be completely eliminated, and it is difficult to obtain a high filling density of the ceramic material in the ceramic thin plate. Specifically, taking the case where there are no voids as the theoretical density, the density achieved is of the order of 70 to 80% of the theoretical density. Therefore, if a dielectric device is used as an electronic device employing dielectric properties such as the aforesaid electron emitter, a high electric field could not be applied.  
      Since sintering is performed at a high temperature of 900° C. or higher, the ceramic layer is damaged by differences of thermal expansion coefficient with the substrate underneath the ceramic layer, a material with low heat resistance such as glass cannot be used as the substrate, and in particular, it is difficult to increase screen size when applied to an FED.  
      Further, in the electron emitter disclosed in Patent References 6, 7 using the aforesaid related art dielectric device (hereafter, referred to simply as “related art electron emitter”), when electrons are emitted from a cathode electrode toward an emitter, electrons are emitted at the portion, on the surface of the cathode electrode, where lines of electric force concentrate and electric field intensity increases (here, the fact that lines of electric force concentrate on the surface of an electrode which is a conductor and thereby the electric field intensity at the portion where lines of electric force concentrate increases as stated above is hereunder referred to simply as “electric field concentration” and the portion where the electric field concentration occurs is hereunder referred to simply as “electric field concentrated portion”).  
      Here, an example of a conventional electron emitter is schematically shown in  FIG. 22 . In a conventional electron emitter  200 , an upper electrode  204  is formed on the upper surface of an emitter  202  and a lower electrode  206  on the lower surface thereof. The upper electrode  204  is formed on the emitter  202  in close contact therewith. In this case, the electric field concentrated portion is limited to the outer edge of the upper electrode  204  where the upper electrode  204 , the emitter  202  and a vacuum intersect with each other, namely the triple junction. In the case of the conventional electron emitter  200 , since the number of the electron emissive sites is limited as stated above, there have been such certain limitations as represented by the fact that drive voltage can only be increased up to the extent of not causing the dielectric breakdown of the emitter  202  even though an increase of the number of emitted electrons is tried.  
      It is therefore an object of the present invention, which is conceived in view of the problems inherent in the aforesaid related art, to provide a dielectric device of higher performance. It is a further object of the present invention to provide a dielectric device wherein the electron emission amount of an electron emitter can be increased by applying the invention to the electron emitter.  
      In order to achieve the aforesaid objects, the dielectric layer forming the dielectric device according to the present invention includes plural first dielectric particles and plural second dielectric particles having a smaller particle size than that of the first the electric particles, and the second dielectric particles are disposed in spaces surrounded by the adjacent plural first dielectric particles. In this arrangement, the second dielectric particles are interposed in the gaps between the plural first dielectric particles in the dielectric layer, so the filling density of the dielectric material in the dielectric layer can be increased. Therefore, the physical properties of the dielectric material constituting the dielectric layer can be utilized with less diminished in any way, and a higher performance dielectric device can be provided. In particular, the surface state (surface roughness, density) of the dielectric layer can be easily controlled by the filling degree of the second dielectric particles in the upper surface of the dielectric layer. Herein, if the first dielectric particles and second dielectric particles are of an identical material, a more uniform dielectric layer can be obtained.  
      The first dielectric particles are preferably coarse particles having an average particle size of micron order (at least 1 μm or more), and the particle size distribution may be fairly wide. The second dielectric particles are preferably fine particles whose average particle size is of submicron order with a narrow particle size distribution. In this context, the average particle size is a computed value obtained by image analysis of the cross-section. For example, each particle can be approximated by a circle having an identical area to that of the particle, the diameter of this circle is taken as the particle size, and the particle size distribution is computed in terms of area.  
      In this invention, in addition to the aforesaid construction, the first electric particles and/or second dielectric particles forming the dielectric layer are preferably formed by an aerosol deposition method from powder particles of a dielectric. In the aerosol deposition method, the powder is dispersed in a gas by vibration or the like to form a smoke-like state (aerosol), this aerosol is transported to a film-forming chamber under a predetermined reduced pressure, and is then sprayed onto a predetermined substrate via a nozzle to form a dense, crystallized solid film. According to this aerosol deposition method, a dielectric layer with few voids (e.g., about 90 to 95% of the theoretical density) and high filling density is obtained, so the physical properties required of the dielectric layer can be stably and definitively obtained.  
      In the present invention, the dielectric layer may be formed also by impregnating an aggregation of the first dielectric particles in the form of a layer with a dispersion of the second dielectric particles or a solution of a precursor, and evaporating a dispersion medium or solvent from the dispersion or solution (hereafter, this method will be referred to as “sol impregnation method”). In this way, a dielectric layer having few voids and high filling density identical to those obtained by the aerosol deposition, may be obtained by a very simple process. In this case, the first dielectric particles may form a layer by the screen printing or green sheet method of the prior art, or by the aerosol deposition method. In other words, the first dielectric particles have a so-called “powder origin” due to the screen printing, green sheet or aerosol deposition method, whereas the second dielectric particles have a “sol origin” due to the sol impregnation method. By packing the spaces between the particles of powder origin with particles of sol origin, the filling density of the dielectric material in the emitter can be increased.  
      Here, when the dielectric layer is prepared using the sol impregnation method, the aggregation of the first dielectric particles preferably contains coarse particles having a particle size of 1 μm or more, and fine particles having a particle size of 1 μm or less. In this case, the second dielectric particles have a smaller particle size than the aforesaid fine particles. In this arrangement, the filling density of the dielectric material in the emitter can be further increased.  
      It is a feature of the invention that, in addition to the aforesaid arrangement, the dielectric layer contains a metal which is mixed therewith. Specifically, metal fine particles and/or a metal film may be interposed between adjacent dielectric particles in the dielectric layer. Alternatively, the aforesaid metal objects such as the metal particulates may be interposed between the dielectric particles and a substrate at the interface between the dielectric layer and a predetermined substrate supporting the dielectric layer.  
      By filling the voids in the dielectric layer by a metal in this way, the dielectric constant of the dielectric layer can be increased. Also, by making the metal function as a binder using the ductility of the metal, the film of the dielectric layer can be formed satisfactorily.  
      In this case, the metal is preferably implanted so that it is dispersed (dispersed is discontinuously) throughout the dielectric layer. Here, the term “discontinuously” represents the state wherein gaps exist among metallic fine particles and/or metal films adjacent to each other along the thickness direction so as not to yield electric conductivity over the thickness direction of the dielectric layer (namely from the upper surface to the lower surface of the dielectric layer).  
      As a result, in addition to a higher dielectric constant and improved film-forming properties, satisfactory properties (dielectric properties such as electrostatic/electron emission properties and electrical-mechanical conversion properties such as piezoelectric/electrostriction properties) in the dielectric layer are obtained.  
      The present invention moreover includes an emitter having the aforesaid dielectric layer, and first and second electrodes formed on or above the emitter to which a drive voltage is applied to discharge electrons from the emitter&#39;s upper surface.  
      According to this arrangement, the emitter is a dielectric layer including plural first dielectric particles, and plural second dielectric particles having a smaller particle size than that of the first dielectric particles, the second dielectric particles being disposed in the spaces surrounding the adjacent, plural first dielectric particles. When a predetermined drive voltage is applied between the first electrode and second electrode, a predetermined electric field is applied to the emitter having the aforesaid structure, and electrons are emitted from the upper surface of the emitter. If the emitter is an emitter having the aforesaid structure, due to the relatively coarse first particles, the basic electron emission properties required of the emitter can be obtained, while at the same time due to the second dielectric particles, the filling density of the emitter as a whole can be increased. Further, if the emitter is an emitter having the aforesaid structure, and the upper surface of the dielectric layer is the upper surface of the emitter of the electron emitter, the physical properties (e.g., surface roughness, relative dielectric constant and D-E hysteresis properties) obtained are such that more electrons can be emitted when a predetermined electric field is applied.  
      Herein, the dielectric layer is preferably subjected to heat treatment at a lower temperature than the sintering temperature of the ceramics. Due to this heat treatment, the relative dielectric constant of the dielectric layer formed by the aerosol deposition method and/or the sol impregnation method can be increased, so a high-performance electronic device to which a higher electric field can be applied, can be provided. Further, when the first electrode or second electrode forming process includes a step for heating to the aforesaid temperature, the electrode-forming step may conveniently be combined with the heat treatment step to increase the dielectric constant. The aforesaid temperature is more preferably in the region of 500° C. Due to this, the aforesaid dielectric device can be formed even on an economical glass substrate which does not have such a high heat resistance temperature, so when it is applied to an electron emitter of an FED, large screens can be manufactured at low cost.  
      In the present invention, the first electrode is provided on the upper surface of the emitter, and air gaps are provided between a surface opposite to the upper surface of the emitter and the upper surface of the emitter in an edge of the first electrode. By adopting this configuration, the aforementioned triple junction is formed at a portion different from the tip of the edge of the first electrode (including the outer edge of the first electrode and, in the case where the first electrode has an opening as described later, the inner edge at the opening) and hence the electric field concentrated portion is formed also at the tip of the edge of the first electrode in addition to the aforementioned triple junction. Thereby the number of the electric field concentrated portions serving as electron emission sites increases more than ever and the amount of the emitted electrons at the electron emitter can be increased.  
      Here, it is preferable to dispose the second electrode to the lower surface side, namely on or above (below) the lower surface opposite with the upper surface of the emitter. Thereby the direction of the electric field applied to the emitter is identical to the thickness direction of the dielectric layer composing the emitter and only the first electrode is formed and disposed on or above the upper surface of the emitter. As a result, the area occupied by the electron emitter on a plan view reduces in comparison with the configuration wherein both the first and second electrodes are formed and disposed on the same surface of the emitter, and thus a higher degree of the integration of electron emitters becomes possible. In the case where electron emitters using the dielectric devices are applied to an FED in particular, the higher resolution of a display can easily be attained.  
      Further in this case, since an air gap (gap) is formed between the surface of the edge portion of the first electrode opposite to the upper surface of the emitter and the upper surface of the emitter, the electrostatic capacity of the virtual condenser formed between the surface of the edge portion of the first electrode opposite to the upper surface of the emitter and the upper surface of the emitter becomes smaller than that in the case. where such a gap is not formed. As a result, most part of the drive voltage is substantially applied to the gap portion, and thus the field intensity at the edge of the first electrode increases and the amount of emitted electrons also increases in comparison with a conventional electron emitter not having such a gap as stated above.  
      Furthermore, it is preferable that the first electrode is provided with plural openings that expose the upper surface of the emitter toward the exterior of the dielectric device. Thereby triple junctions and the tips at the edge of the first electrode serving as electric field concentrated portions are formed also at the inner edges of the plural openings in addition to the outer edge of the first electrode. As a result, the number of the electric field concentrated portions serving as electron emission sites further increases and thus the amount of emitted electrons at the electron emitter can also be increased. In addition, since the electric field concentrated portions serving as electron emission sites are formed at the outer edge and the plural openings located inside the outer edge in the first electrode, uniform electron emission having less deviation and dispersion in the region occupied by the first electrode can be obtained.  
      Here, it is particularly preferable that the electron emitter, to which a dielectric device is applied, according to the present invention functions as follows: as the first step, electrons are emitted (supplied) from the first electrode to the emitter by applying such a drive voltage that the potential of the first electrode is lower than that of the emitter, namely electrons are accumulated on the emitter (the emitter is electrified); and as the second step, the electrons already accumulated on the emitter are emitted by applying such a drive voltage that the potential of the first electrode is higher than that of the emitter. This sort of functions can be carried out in the following manner for example.  
      As the drive voltage applied between the first and second electrodes, for example the voltage which is applied as pulse voltage or alternating voltage relative to a prescribed reference potential (for example, 0 V) is used.  
      Firstly, at the first step, a drive voltage is applied between the first and second electrodes so that the potential of the first electrode is lower than that of the reference potential and the potential of the second electrode is higher than that of the reference potential. Then, by the electric field caused by the drive voltage, the polarization of the emitter is directed toward such a direction that positive electric charge appears on the upper surface of the emitter, the electric field concentration occurs at the aforementioned electric field concentrated portions, and electrons are supplied from the first electrode to the emitter. Thereby electrons are attracted toward the positive electric charge appearing on the upper surface of the emitter and thereby accumulate at the portion, on the upper surface of the emitter, corresponding to an opening of the first electrode. That is, the portion, on the upper surface of the emitter, corresponding to the opening at the first electrode is electrified. At this time, the first electrode serves as the supply source of the electrons.  
      Next, at the second step, the drive voltage changes drastically, and the drive voltage is applied between the first and second electrodes so that the potential of the first electrode is higher than that of the reference potential and the potential of the second electrode is lower than that of the reference potential. Then, by the electric field caused by the drive voltage, the polarization direction of the emitter is reversed and negative electric charge appears on the upper surface of the emitter. Thereby, at the first step, the electrons attached to the portion, on the upper surface of the emitter, corresponding to the opening of the first electrode undergo electrostatic repulsive force caused by the polarization reversal and thereby fly from the upper surface of the emitter, and the flown electrons are emitted outside through the opening.  
      According to such functions, the amount of electrostatic charge at the emitter can be controlled relatively easily at the first stage and hence controllability that is stable and allows a large amount of electrons to be emitted can be secured. In particular, the configuration formed by disposing the first electrode having an opening on the upper surface of the emitter and the second electrode on the lower surface of the emitter is the most suitable as the configuration of an electron emitter for the functions.  
      Further, an opening, of the first electrode, secluded from the upper surface of the emitter can function as a gate electrode or a focusing electron lens for the electrons emitted from the upper surface of the emitter and hence the linearity of the emitted electrons can also be improved.  
      Furthermore, a feature of the present invention is that the edge of the first electrode has such a shape as to concentrate the lines of electric force. Such a shape of an edge as to concentrate the lines of electric force can be implemented, for example: either by disposing a portion having an acute-angled shape on the inner wall surface of the edge in sectional side view; or by attaching to the inner wall surface protrusions or conductive fine particles the size of which is equal to or smaller than the thickness of the first electrode. Otherwise, it can also be implemented by forming the inner wall surface of the edge into a hyperbolic shape (particularly a hyperbolic shape configured so that both the upper end and lower end portions of the edge portion in a sectional side view have acute angles). Otherwise, such a shape of the first electrode as to concentrate the lines of electric force at an edge can also be implemented by various means in addition to the above measures. Thereby the degree of the electric field concentration at the tips of the edges of the first electrode (the outer edge of the first electrode and the inner edge of an opening) rises and the amount of the electrons emitted from the tips to the emitter can be increased.  
      In addition, a feature of the present invention is that the first electrode is the aggregation of conductive particles having a shape extending in the longitudinal direction in a sectional side view and the conductive particles are arranged so that the longitudinal direction is parallel with the upper surface of the emitter. Thereby it is possible to easily realize such a shape as stated above having an air gap between the face of the edge portion of the first electrode, opposite to the upper surface of the emitter and the upper surface of the emitter (hereunder referred to simply as “overhanging shape” occasionally).  
      Here, as the conductive particles, composing the first electrode, having a shape extending in the longitudinal direction in a sectional side view, various shapes of particles can be adopted, for example: particles of a scale-like shape, a disc-like shape, a coiled spring-like shape or a hollow cylindrical shape; or particles of, in a sectional side view, a rod-like shape, an acicular shape, a hemispherical shape, an elliptical shape or a semi-elliptical shape. Then the conductive particles are arranged in plurality on the upper surface of the emitter so that the longitudinal direction thereof is along the upper surface of the emitter. In this case, the longitudinal direction is not necessarily parallel precisely with the upper surface of the emitter and it is generally acceptable if the conductive particles are arranged on the upper surface of the emitter in the state of “lying” to the extent of forming the gap or overhanging shape causing such functions as described earlier. For example, it is preferable that an angle between the longitudinal direction of the conductive particles and the upper surface of the emitter in a sectional side view is about 30 degrees or less.  
      Further, in the case where the first electrode has an opening, it is preferable that the opening is formed by the outer edges of plural conductive particles. That is, merely by arranging the conductive particles in plurality on the upper surface of the emitter by coating or another means, a space surrounded by the outer edges of the adjacent conductive particles in a plan view is formed and thus the opening having the aforementioned overhanging shape can easily be formed.  
      Furthermore, it is preferable that the first electrode is formed by arranging the primary particles of the conductive particles and/or the secondary particles formed by assembling the primary particles in plurality along the upper surface of the emitter and the length of the primary or secondary particles in the longitudinal direction in a sectional side view is larger than the average grain size of the first dielectric particles at the upper surface of the emitter. That is, the dielectric layer composing the emitter is generally a polycrystalline material and concavities are likely to form at the crystal grain boundaries or the junctions of the powder particles. Hence, as long as the concavities are utilized, merely by arranging the primary or secondary particles of the conductive particles in plurality on the upper surface of the emitter, the aforementioned overhanging shape can easily be formed.  
      Still further, it is preferable that the first electrode is comprised of graphite. Here, the graphite powder is conductive particles of a shape having relatively sharp edges, such as a scale-like shape, a flake-like shape or the like. In other words, it has a shape extending in the longitudinal direction in a sectional side view. Hence, by composing the first electrode of the graphite powder, it is possible to easily form gaps (air gaps) between the emitter and the edges of the first electrode, overhanging shape at the edges of the first electrode, and a shape allowing the lines of electric force to concentrate at the edges of the first electrode, as stated above.  
      Still further, it is preferable that the first electrode further contains conductive fine particles. In addition to that, it is preferable that the conductive fine particles are deposited also on the upper surface of the emitter. Thereby, since the fine particles exist on the upper surface of the first electrode like protrusions, the fine particles can serve as electric field concentrated portions by the effect of the protrusions and hence the sites of electron emission can further be increased.  
      In addition, it is more preferable that the fine particles are deposited also on the upper surface of the emitter corresponding to the edges of the first electrode. Thereby fine float electrodes including the fine particles are formed on the emitter composed of a dielectric. The float electrodes are suitable for abundantly accumulating electrons emitted from the first electrode to the emitter and can further increase the amount of the emitted electrons at the electron emitter. Consequently, by forming the float electrodes with the fine particles, it becomes possible to form the float electrodes on the upper surface of the emitter through a simple process, for example, a process of coating the upper surface with the fine particles together with the material composing the first electrode when the first electrode is formed on the upper surface of the emitter.  
      Furthermore, it is preferable that the fine particles are comprised of silver. Thereby it becomes possible to produce the first electrode containing conductive fine particles easily at a low cost. In particular, when graphite is used as the first electrode and a heating process in an atmosphere containing an oxygen gas is included in the process of forming the first electrode, the graphite around fine silver particles is oxidized and eroded during the heating process. Thereby an edge of the first electrode is likely to have a sharp tip or an opening is likely to be formed by the perforation in the interior of the electrode. As a consequence, the electric field concentrated portions further increase and a more preferable electrode shape can be obtained.  
      Here, the first electrode stated above can be formed on the emitter by the following method.  
      That is, the first electrode is formed by: preparing paste produced by dispersing in a dispersing medium conductive particles having a shape extending in the longitudinal direction in a sectional side view; forming a film including the paste on the upper surface of the emitter; and heating the film at a temperature lower than the sintering temperature of ceramics (a preferable temperature is around 500° C.).  
      Thereby (by properly adjusting the viscosity and; compounding ratio of the paste), it becomes possible to put conductive particles in the state of “lying” as stated earlier by the effects of the self-weight, the surface energy and others of the conductive particles after the aforementioned film forming and before the heating of the film is finished and to easily produce a preferable electron emitter having gaps (air gaps) between the emitter and the openings of the first electrode and overhanging shapes at the openings of the first electrode. Further, it is possible to combine the process of forming the first electrode and the process of heat-treating the emitter as stated above.  
      Further, in the above production method, it is preferable to disperse conductive fine particles in the dispersing medium when paste is prepared. Thereby it becomes possible to easily produce an electron emitter having a larger number of electric field concentrated portions and an increased amount of emitted electrons as stated above.  
      As described above, according to the dielectric device of the present invention, a dielectric device having higher performance can be provided. Further, by applying this dielectric device to an electron emitter, the electron emission amount of the electron emitter can be increased. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a sectional view showing an electron emitter, excluding some portions, according to the first embodiment;  
       FIG. 2  is an enlarged sectional view showing the main part of the above electron emitter;  
       FIG. 3  is a plan view showing an example of the shape of an opening formed at an upper electrode;  
       FIG. 4  is a diagram showing a voltage wave form of drive voltage applied to the above electron emitter;  
       FIG. 5A  to  5 C include explanatory views showing the aspect of the behavior of the above electron emitter;  
       FIG. 6A  to  6 C include other explanatory views showing the aspect of the behavior of the above electron emitter;  
       FIG. 7  is an equivalent circuit diagram explaining the influence of forming gaps between an upper electrode and an emitter on the electric field between the upper electrode and a lower electrode;  
       FIG. 8  is another equivalent circuit diagram explaining the influence of forming gaps between the upper electrode and the emitter on the electric field between the upper electrode and the lower electrode;  
       FIG. 9  is a schematic configuration diagram explaining a method of forming the emitter;  
       FIG. 10  is a schematic configuration diagram explaining an another method of forming the emitter;  
       FIG. 11  is a sectional view showing an example of a modified emitter;  
       FIG. 12  is a configuration diagram showing an outline of a display to which the above electron emitter is applied;  
       FIG. 13  is a sectional view showing an electron emitter, excluding some portions, according to the second embodiment;  
       FIG. 14  is a sectional view showing an electron emitter, excluding some portions, according to the third embodiment;  
       FIG. 15  is a sectional view showing an electron emitter, excluding some portions, according to the forth embodiment;  
       FIG. 16  is a sectional view showing an example of a modified overhanging shape at the upper electrode of the electron emitter according to the first to third embodiments;  
       FIG. 17  is a sectional view showing an example of still another modified overhanging shape at the upper electrode;  
       FIG. 18  is a sectional view showing an example of still another modified overhanging shape at the upper electrode;  
       FIG. 19  is a sectional view showing an example of still another modified overhanging shape at the upper electrode;  
       FIG. 20  is a sectional view showing an example of a modification provided with a float electrode in the electron emitter according to the first to third embodiments;  
       FIG. 21  is a view showing an example of a modified opening shape in the electron emitter according to the first to third embodiments; and  
       FIG. 22  is a cross-sectional view showing a related art electron emitter with some parts omitted. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
      Preferred embodiments of a dielectric device according to the present invention are hereunder explained referring to drawings. The present embodiments show examples wherein a dielectric device according to the present invention is applied to an electron emission device used as an electron beam source in various devices, using electron beams, such as a display including an FED, an electron beam irradiation device, a light source, an alternative of an LED, electronic parts manufacturing apparatus, and an electronic circuit part.  
      Firstly, an electron emitter as a dielectric device according to the first embodiment is explained on the basis of FIGS.  1  to  8 .  FIG. 1  is an enlarged sectional side view of an electron emitter  10 A according to the present embodiment. The electron emitters  10 A are two-dimensionally formed in large numbers on a glass substrate  11  and one of them is shown in  FIG. 1 . The electron emitter  10 A is provided with: a tabular emitter  12 ; an upper electrode  14  as a first electrode formed on or above the upper surface  12   a  of the emitter  12  and equipped with openings  20 ; and a lower electrode  16  as a second electrode formed on the glass substrate  11  and disposed so as to have contact with the lower surface  12   b  as the second surface of the emitter  12 .  
      The emitter  12  includes a polycrystalline body of a dielectric material, including plural first particles  12   e  and plural second particles  12   f.  The first particles  12   e  are relatively coarse particles having an average particle size of 1 μm or more, and the second dielectric particles  12   f  are of an identical material to the first dielectric particles  12   e,  but they are fine particles of submicron order with a sharp particle size distribution. The second dielectric particles  12   f  are filled in spaces surrounding the plural first dielectric particles  12   e.    
      Herein, the upper surface of the upper layer  12   c  forms the upper surface  12   a  of the emitter  12  which has the role of emitting electrons when a predetermined electric field is applied. If the particle size is too small, crystal grain boundary and other defects increase and the relative dielectric constant falls, so the applied field strength decreases which is undesirable. Hence, the amount of coarse particles having a particle size of micron order is preferably as large as possible. Therefore, the particle size distribution of the dielectric particles (including the plural first particles  12   e  and plural second particles  12   f ) constituting the emitter  12  preferably contains 30% or more particles having a particle size of 1 μm or more, more preferably contains 50% or more particles having a particle size of 1 μm or more, and still more preferably contains 50% or more particles having a particle size of 2 μm or more. Here, the above particle sizes and proportions are the values calculated by subjecting a section to image analysis and, for example, the particle size of each particle is determined by regarding the diameter of a circle which has the same area as the relevant particle as the particle size and the proportion is determined as the value obtained by calculating the distribution of the particle sizes through the area standard.  
      The thickness of the emitter  12  is determined in relation to a drive voltage so that the polarization of the emitter  12  is reversed and field intensity is applied up to the extent of not causing the dielectric breakdown when a drive voltage is applied between the upper electrode  14  and the lower electrode  16 . For example, assuming that the dielectric breakdown voltage of the emitter  12  is 10 kV/mm or more, when an applied drive voltage is 100 V, the necessary thickness of the emitter  12  is 10 μm or more theoretically, but it is preferable to set the thickness of the emitter  12  at about 20 μm in consideration of the allowance enough to avoid the dielectric breakdown.  
      The upper electrode  14  is formed so that the thickness thereof is in the range from 0.1 to 20 μm and has plural openings  20  through which the upper surface  12   a  of the emitter  12  is exposed to the exterior. As shown in  FIG. 2 , the upper electrode  14  is composed of many conductive particles  15  (for example graphite) of a scale-like shape. That is, the upper electrode  14  is formed so that the many conductive particles  15  are disposed in a “lying” state wherein the longitudinal direction of the conductive particles  15  is along the upper surface  12   a  of the emitter  12  in a sectional side view. More specifically, the conductive particles  15  are disposed on the upper surface  12   a  of the emitter  12  so that the angle between the longitudinal direction of the conductive particles  15  and the upper surface  12   a  (smooth virtual surface formed by averaging the unevenness in the join parts between adjacent first dielectric particles  12   e  or crystal grain boundaries) of the emitter  12  is 30 degrees or less in a sectional side view. In the present embodiment, the conductive particles  15  are comprised of the particle which size of the primary particle (the longest length thereof in the longitudinal direction in a sectional side view) is larger than the particle size of the dielectric composing the emitter  12 . In the example of  FIG. 1 , if the average particle size of the dielectric particles  12   e  in the emitter  12  is in the vicinity of 1 to 2 μm as described above, the size of the first order particles of the electrically conductive particles  15  may be approximately 3 μm or more.  
      As it is obvious from  FIGS. 1 and 2 , the many conductive particles  15  composing one upper electrode  14  maintain the conductivity by overlapping each other. Then the openings  20  are formed by the outer edges  15   a  of the many conductive particles  15  in a plan view. In other words, the apertures  20   a  of the openings  20  are the spaces surrounded by the outer edges  15   a  of the plural conductive particles  15 .  
      The lower electrode  16  is composed of a metal thin film and a desirable thickness thereof is 20 μm or less, more desirably 5 μm or less. Then, a pulse generator  18  to apply drive voltage Va between the upper electrode  14  and the lower electrode  16  is connected to the upper electrode  14  and the lower electrode  16 .  
      The electron emitter  10 A is actuated in a prescribed vacuum atmosphere and, for example, a desirable vacuum level in the atmosphere (in particular the space above the upper surface  12   a  of the emitter  12  in  FIG. 1 ) is in the range from 10 2  to 10 −6  Pa, more desirably 10 −3  to 10 −5  Pa. Then the electron emitter  10 A is configured so as to accumulate the electrons supplied from the upper electrode  14  on the upper surface  12   a  of the emitter  12  corresponding to the openings  20  and thereafter emit the electrons accumulated on the upper surface  12   a  to the outer atmosphere (the upper side in  FIG. 1 ) through the openings  20 .  
      As described above, the emitter  12  is a polycrystalline body including the plural dielectric particles  12   e  and second dielectric particles  12   f,  so on the upper surface  12   a  of the emitter  12 , microscopic unevenness are formed in the join parts between the adjacent dielectric particles  12   e  or crystal grain boundaries, and as shown in  FIG. 3 , a concavity  24  is formed on the upper surface  12   a  of the emitter  12  (in  FIG. 3  and subsequent figures, the shapes of the emitter  12  and its upper surface  12   a  are simplified). Then the openings  20  of the upper electrode  14  are formed at the portions corresponding to the concavities  24 .  FIG. 1  shows an example of the case where one opening  20  is formed for one concavity  24 , but there are some cases where one opening  20  is formed for plural concavities  24 .  
      Further, as shown in  FIG. 3 , an opening  20  is composed of the aperture  20   a  surrounded by the inner edge of the opening  20  and the periphery  26  which is the surroundings of the aperture  20   a.  Then in the upper electrode  14 , the face  26   a,  of the periphery  26  of the opening  20 , opposite to the emitter  12  is isolated from the emitter  12 . In other words, in the upper electrode  14 , a gap  28  is formed between the face  26   a,  of the periphery  26  of the opening  20 , opposite to the emitter  12  and the emitter  12 , and the periphery  26  of the opening  20  in the upper electrode  14  is formed into the shape of an overhang (hence in the explanations below, the term “the periphery  26  of the opening  20  in the upper electrode  14 ” is described as “the overhang  26  of the upper electrode  14 ,” and the term “the face  26   a,  of the periphery  26  of the opening  20  in the upper electrode  14 , opposite to the emitter  12 ” is described as “the lower face  26   a  of the overhang  26  in the upper electrode  14 ”). Then as it is obvious from  FIGS. 2 and 3 , the overhang  26  is composed of the edges  15   a  of the plural conductive particles  15 .  
      Here, in the present embodiment, the maximum angle θ between the upper surface  12   a  (the face in the vicinity of the top of the convex portion of a jog) of the emitter  12  and the lower face  26   a  of the overhang  26  in the upper electrode  14  is set so as to satisfy the expression 1°≦θ≦60°. Also, the maximum distance d in the vertical direction between the upper surface  12   a  of the emitter  12  and the lower face  26   a  of the overhang  26  in the upper electrode  14  is set so as to satisfy the expression 0 μm&lt;d≦10 μm.  
      Then a triple junction (a triple point where an upper electrode  14 , an emitter  12 , and vacuum have contact with each other)  26   c  is formed at a portion where the upper surface of the emitter  12 , the upper electrode  14  and the medium surrounding the electron emitter  10 A (for example vacuum) contact. Then the triple junction  26   c  is the place (electric field concentrated portion) where the lines of electric force concentrate (electric force concentration) when drive voltage Va is applied between the upper electrode  14  and the lower electrode  16 . Here, “the place where the lines of electric force concentrate” means the place where the lines of electric force originated from the lower electrode  16  at equal intervals concentrate when the lines of electric force are drawn assuming that the upper electrode  14 , the emitter  12  and the lower electrode  16  are the flat plates extending infinitely in a sectional side view. The state of the accumulation of the lines of electric force (electric force concentration) can easily be simulated by the numerical analysis using the finite element method.  
      Further, in the present embodiment, the opening  20  is formed so that the inner edge  26   b  of the opening  20  serves as an electric field concentrated portion. More specifically, the overhang  26  of the opening  20  is formed so as to sharply protrude at an acute angle toward the inner edge  26   b  which is the tip of the overhang  26  (so as to reduce the thickness gradually) in a sectional side view. The upper electrode  14  having an opening  20  of such a shape can be formed by a simple method, namely, as stated above, by disposing the conductive particles  15  having a shape extending in the longitudinal direction in a sectional side view in the state of “lying” so that the longitudinal direction of the conductive particles  15  is along the upper surface  12   a  of the emitter  12  in a sectional side view. Note that, the electric field concentrated portion at the inner edge  26   b  of the opening  20  and the triple junction  26   c  as stated above are also formed at positions corresponding to the outer edges  21  at the outer periphery of the upper electrode  14  (refer to  FIG. 1 ).  
      Here, each of the openings  20  is formed so that it has the aperture  20   a  the diameter of which, the diameter being represented by the diameter of a virtual circle having the same area as the aperture  20   a  in a plan view, is in the range from 0.1 μm to 20 μm in average. The reasons are as follows.  
      As shown in  FIG. 3 , the portions, of the emitter  12 , where polarization is reversed or changed in accordance with the drive voltage Va applied between the upper electrode  14  and the lower electrode  16  (refer to  FIG. 1 ) are the portions immediately under the portions (first portions)  40  where the upper electrode  14  is formed and the portions (second portions)  42  corresponding to the regions from the inner edge (inner periphery) of the opening  20  toward the inner direction of the opening  20 . In particular, the region of electron emission at the second portions  42  varies in accordance with the level of the drive voltage Va and the degree of the electric field concentration at the portions. Then when the average diameter of the aperture  20   a  is within the range from 0.1 μm to 20 μm in the present embodiment, a sufficient amount of electrons emitted at the opening  20  can be secured and electrons can be emitted efficiently. In other words, when the average diameter of the aperture  20   a  is less than 0.1 μm, the area of the second portions  42  which is the main region contributing to the accumulation and emission of electrons supplied from the upper electrode  14  becomes small and thus the amount of the emitted electrons decreases. In contrast, when the average diameter of the aperture  20   a  exceeds 20 μm, the proportion (share) of the area of the second portions  42  to the area of the portion, of the emitter  12 , exposed through the opening  20  decreases and thus the efficiency of the electron emission lowers.  
      Next, the principle of electron emission in an electron emitter  10 A is explained on the basis of FIGS.  4  to  6 . In the present embodiment, as shown in  FIG. 4 , the drive voltage Va applied between the upper electrode  14  and the lower electrode  16  takes the shape of a rectangular wave the cycle of which is T 1 +T 2  so that, with the reference voltage being 0 V, at the first step of the duration T 1 , the drive voltage is V 2  which means that the potential of the upper electrode  14  is lower than that of the lower electrode  16  (negative voltage) and, at the succeeding second step of the duration T 2 , the drive voltage is V 1  which means that the potential of the upper electrode  14  is higher than that of the lower electrode  16  (positive voltage).  
      The principle is further explained on the assumption that, in the initial state, the emitter  12  is polarized in one direction and for example the negative pole of a dipole is in the state of being directed to the upper surface  12   a  of the emitter  12  (refer to  FIG. 5A ).  
      Firstly, in the initial state wherein the reference voltage is applied, as shown in  FIG. 5A , since the negative pole of a dipole is in the state of being directed to the upper surface  12   a  of the emitter  12 , electrons are in the state of scarcely accumulated at the upper surface  12   a  of the emitter  12 .  
      Thereafter, when the negative voltage V 2  is applied, the polarization is reversed (refer to  FIG. 5B ). By the polarization reversal, electric field concentration is caused at the inner edge  26   b  and the triple junction  26   c  serving as the electric field concentrated portions, electrons are emitted (supplied) from the electric field concentrated portions at the upper electrode  14  to the upper surface  12   a  of the emitter  12 , and electrons are accumulated, for example, at the portion, of the upper surface  12   a,  exposed through the opening  20  of the upper electrode  14  and the portion in the vicinity of the overhang  26  of the upper electrode  14  (refer to  FIG. 5C ). That means the upper surface  12   a  is electrified. The electrification can last until a certain saturated state appears due to the value of electric surface resistance of the emitter  12  and the amount of the electrification can be controlled by the time during which control voltage is applied. In this way, the upper electrode  14  (the electric field concentrated portions in particular) serves as the supply source of electrons to the emitter  12  (the upper surface  12   a ).  
      Thereafter, when the negative voltage V 2  returns to the reference voltage again as shown in  FIG. 6A  and then positive voltage V 1  is applied, the polarization is reversed again (refer to  FIG. 6B ) and the electrons accumulated on the upper surface  12   a  are emitted outside through the aperture  20   a  by the coulomb repulsive force caused by the negative pole of the dipole (refer to  FIG. 6C ).  
      Here, also at the outer edge of the outer periphery, of the upper electrode  14 , where no opening  20  is formed, electrons are discharged in the same way as described above.  
      Further, in the present embodiment, as shown in  FIG. 7 , in an electrical behavior, a condenser C 1  caused by the emitter  12  and an integrated condenser including plural condensers Ca caused by the gaps  28  are formed between the upper electrode  14  and the lower electrode  16 . That is, the plural condensers Ca caused by the gaps  28  are integrated as one condenser C 2  formed by connecting each other in parallel and, in terms of an equivalent circuit, that takes the form of serially connecting the condenser C 1  caused by the emitter  12  to the integrated condenser C 2 .  
      In actual operation, the condenser C 1  caused by the emitter  12  is not serially connected to the integrated condenser C 2  as it is and the components of the serially connected condensers vary in accordance with the number of the formed openings  20  of the upper electrode  14 , the overall formed area and others.  
      Here, as shown in  FIG. 8 , it is attempted to calculate the capacitance on the assumption that for example 25% of the condenser C 1  caused by the emitter  12  is serially connected to the integrated condenser C 2 . Firstly, the relative dielectric constant is one since the portions of the gaps  28  are vacuum. Then the maximum length d of the gaps  28  is set at 0.1 μm, the area S of the portion of one gap  28  is set at 1 μm×1 μm, and the number of the gaps  28  is set at 10,000 pieces. Further, setting the relative dielectric constant of the emitter  12  at 2,000, the thickness of the emitter  12  at 20 μm, and the area of the opposing faces of the upper electrode  14  and the lower electrode  16  at 200 μm×200 μm, the capacitance of the integrated condenser C 2  is 0.885 pF and the capacitance of the condenser C 1  caused by the emitter  12  is 35.4 pF. Then, when the part, of the condenser C 1  caused by the emitter  12 , serially connected to the integrated condenser C 2 , is assumed to be 25% of the total, the capacitance of the portion connected in series (the capacitance including the capacitance of the integrated condenser C 2 ) is 0.805 pF and the remaining capacitance is 26.6 pF.  
      Since the portion connected in series and the remaining portion are connected to each other in parallel, the overall capacitance is 27.5 pF. The capacitance corresponds to 78% of the capacitance 35.4 pF of the condenser C 1  caused by the emitter  12 . It means that the overall capacitance is smaller than the capacitance of the condenser C 1  caused by the emitter  12 .  
      In this way, with regard to the integrated capacitance of the condensers Ca caused by the plural gaps  28 , the capacitance of the condensers Ca caused by the gaps  28  is relatively small and thus most part of the applied voltage Va is applied to the gaps  28  and a high output of electron emission can be secured at the gaps  28  on account of the voltage divided from the condenser C 1  caused by the emitter  12 .  
      Further, since the integrated condenser C 2  is serially connected to the condenser C 1  caused by the emitter  12 , the overall capacitance is smaller than the capacitance of the condenser C 1  caused by the emitter  12 . As a result, preferable effects such as a high electron emission output and a low overall electric power consumption can be obtained.  
      Next, referring to  FIG. 9  and  FIG. 10 , the method of manufacturing the electron emitter  10 A of this embodiment will be described paying special attention to the method of forming the emitter  12 .  
       FIG. 9  is a schematic view showing an aerosol deposition device wherein the emitter  12  is formed by forming films of the first dielectric particles  12   e  and second dielectric particles  12   f  by the aerosol deposition method.  
      The aerosol deposition equipment  60  is provided with a film forming chamber  70  and a first aerosol feeder  80 . The film forming chamber  70  is provided with: a vacuum chamber  71  the interior of which is maintained to a prescribed vacuum level; an X-Y-Z-θ stage  72  that holds a glass substrate  11  in the vacuum chamber  71  and allows the glass substrate  11  to move in an arbitrary direction; a first nozzle  73  fixed in the vacuum chamber  71  in order to spray aerosol on the glass substrate  11  held on the X-Y-Z-θ stage  72 ; and a vacuum pump  76  to maintain the interior of the vacuum chamber  71  to a prescribed vacuum level.  
      The vacuum level in the interior of the vacuum chamber  71  is set at around 50 to 1,000 Pa with the vacuum pump  76 . The first nozzle  73  has an opening in the shape of a slit 10 mm×0.4 mm in size and ejects through the opening aerosol supplied from the first aerosol feeder  80  toward the glass substrate  11  in the vacuum chamber  71  having the above vacuum level. The glass substrate  11  is transferred in an arbitrary direction by the X-Y-Z-θ stage  72  and thereby, while moving relative to the first nozzle  73 , aerosol is sprayed.  
      The first aerosol feeder  80  is provided with: a first aerosolizing chamber  82  to store material powder  81 ; a compressed gas supply source  83  to store a carrier gas used for mixing it with the material powder  81  and generating aerosol in the first aerosolizing chamber  82 ; a compressed gas feed pipe  84  to feed the carrier gas from the compressed gas supply source  83  to the first aerosolizing chamber  82 ; a vibration agitator  85  to impose vibration on the first aerosolizing chamber  82  in order to mix the material powder  81  with the carrier gas and aerosolize them in the first aerosolizing chamber  82 ; an aerosol feed pipe  86  to feed aerosol from the first aerosolizing chamber  82  to the first nozzle  73 ; and a control valve  87  to control the amount of aerosol ejected from the first nozzle  73  to the glass substrate  11  by adjusting the flow rate of the aerosol in the aerosol feed pipe  86 .  
      The material powder  81  is a mixture of a relatively coarse dielectric powder having an average particle size of 1.5 to 5 μm (nominal value by the maker; measured by laser diffractometry or with a Coulter Multisizer (a registered trademark)), and fine dielectric particles having an average particle size of 1 μm (nominal value by the maker; the same as above).  
      This material powder  81  is aerosolized by undergoing vibration from the vibration agitator  85  and thereby being mixed with a carrier gas violently in the first aerosolizing chamber  82 . Since the aerosol behaves like a fluid, in the state of opening the control valve  87 , the aerosol flows toward the vacuum chamber  71  due to the pressure difference between the first aerosolizing chamber  82  and the vacuum chamber  71  and is ejected toward the glass substrate  11  at a high speed through the first nozzle  73 . Here, as the carrier gas stored in the compressed gas supply source  83 , besides compressed air, an inert gas such as a nitrogen gas or a noble gas including a helium gas, an argon gas or the like can be used. Then by opening the control valve  87  and ejecting the aerosol of the material powder  81  toward the glass substrate  11 , the emitter  12  is formed on the glass substrate  11  (more precisely on the lower electrode  16 ).  
       FIG. 10  is a configuration diagram showing the outline of another example of the aerosol deposition equipment in the case of forming an emitter  12  by forming films of the first dielectric particles  12   e  and second dielectric particles  12   f  by the aerosol deposition method. Here, the constituent components having the same operations and functions as the aerosol deposition equipment  60  in  FIG. 9  are marked with the same reference numerals and the explanations thereof are omitted.  
      The aerosol deposition equipment  160  is provided with a film forming chamber  70 , a first aerosol feeder  80 , and a second aerosol feeder  90 . The film forming chamber  70  is provided with, besides the aforementioned first nozzle  73 , a second nozzle  74  to eject aerosol supplied from the second aerosol feeder toward the glass substrate  11  in the film forming chamber  70 . The first aerosol feeder  80  is the same as the case of  FIG. 9  except that material powder  181  is used. The second aerosol feeder  90  also has the same construction as the first aerosol feeder  80  except that material powder  191  is used and is provided with a second aerosolizing chamber  92 , a compressed gas supply source  93 , a compressed gas feed pipe  94 , a vibration agitator  95 , an aerosol feed pipe  96 , and a control valve  97 . The second nozzle  74  has an opening in the shape of a slit 5 mm×−0.3 mm in size.  
      The material powder  181  is a dielectric in the form of relatively coarse powder 1.5 to 5 μm in average particle size (nominal value by a maker; the same as above) and the material powder  191  is a dielectric in the form of fine powder 1 μm or smaller in average particle size (nominal value by a maker; the same as above). Then by opening one or both of the control valve  87  and control valve  97  as required, the emitter  12  filled with the second dielectric particles  12   f  packed in the spaces surrounding the first dielectric particles  12   e,  is obtained on the glass substrate  11  (more precisely, on the lower electrode  16 ).  
      Note that, though situations differ in accordance with the ejection conditions of aerosol, mechanical properties of the material powder  81 ,  181  and  191 , and the like, since those particles of the material powder  81 ,  181  and  191  are ejected toward the glass substrate  11  and undergo impulsive force when they collide with the glass substrate  11  and the like, the shapes of the particles of the material powder  81 ,  181  and  191  are generally different from the shapes of the first dielectric particles  12   e  and the second dielectric particles  12   f  composing the emitter  12  formed on the glass substrate  11 .  
      Further, the emitter  12  can be formed also by the sol impregnation method. In the case of using the sol impregnation method, firstly, a dielectric thin plate of the first dielectric particles  12   e  is first prepared. This dielectric thin plate is formed by pasting dielectric powder with a disperse medium such as an organic binder, forming the paste into prescribed shape and thickness by screen printing or the like, and thereafter decomposing and transpiring the disperse medium at the temperature in the range from 400° C. to 500° C. The dielectric powder used to form this dielectric thin plate is a relatively coarse dielectric powder of average particle size 1.5 to 5 μm (nominal value by the maker; the same as above), or a mixture of this dielectric powder with a fine dielectric powder of average particle size 1 μm (nominal value by the maker; the same as above) or less. Then fluid dispersion (hereunder referred to as “sol solution”) of the second dielectric particles  12   f  is dripped on the dielectric thin plate and the sol solution is impregnated into the dielectric thin plate. Thereafter, after excessive sol solution is removed with a spin coater, the dielectric layer is dried at the temperature in the range from 100° C. to 200° C. and then subjected to heat treatment at the temperature in the range from 400° C. to 500° C. The impregnation of sol solution, drying and heat treatment are generally repeated several times. Then finally, by applying heat treatment at a temperature of 700° C. or lower, the emitter  12  is formed. Here, in the above processes, as the dielectric layer before the sol solution is dripped, a green sheet or even a film formed by the aerosol deposition method can also be used.  
      In this way, the emitter  12  formed by the aerosol deposition method or the sol impregnation method is formed as a very dense dielectric thin film having a high filling density, and adequate dielectric properties are obtained without sintering at 900° C. or higher. Therefore, film-forming can be performed at the low temperature of 700° C., 600° C. or lower. The upper surface  12   a  of the emitter  12  can also be formed in any desired surface state (surface roughness) depending on the film-forming conditions of the aerosol deposition method or the sol impregnation method.  
      As the dielectric material composing the emitter  12 , a dielectric of a comparatively high relative dielectric constant, for example 1,000 or more, can preferably be adopted. As such a dielectric, adopted can be, besides barium titanate: a substance such as lead zirconate, lead magnesium niobate, lead nickel niobate, lead zinc niobate, lead manganese niobate, lead magnesium tantalate, lead nickel tantalate, lead antimony stannate, lead titanate, lead magnesium tungstate, or lead cobalt niobate; ceramics containing arbitrary combination thereof; a substance mainly containing a chemical compound composed of those substances by 50 wt. % or more; or a substance produced by further adding to the above ceramics oxides of lanthanum, calcium, strontium, molybdenum, tungsten, barium, niobium, zinc, nickel, manganese, or the like, the combination thereof, or other chemical compounds in an appropriate manner.  
      For example, in the case of binary system nPMN-mPT (n and m are mole fractions) including lead magnesium niobate (PMN) and lead titanate (PT), by increasing the mole fraction of PMN, the Curie point can be lowered and thus the relative dielectric constant at room temperature can be increased. In particular, the relative dielectric constant is preferably 3,000 or more in the case of n=0.85 to 1.0 and m=1.0−n. For example, a relative dielectric constant of 15,000 can be obtained at room temperature in the case of n=0.91 and m=0.09, and even 20,000 at room temperature in the case of n=0.95 and m=0.05.  
      Next, in the case of ternary system including lead magnesium niobate (PMN), lead titanate (PT) and lead zirconate (PZ), in addition to the increase of the mole fraction of PMN, to obtain a composition close to the morphotropic phase boundary (MPB) between tetragonal system and pseudo-cubic system or between tetragonal system and rhombohedral-system contributes preferably to the increase of a relative dielectric constant. For example, particularly preferably, the relative dielectric constant is 5,500 in the case of PMN:PT:PZ=0.375:0.375:0.25 and the relative dielectric constant is 4,500 in the case of PMN:PT:PZ=0.5:0.375:0.125.  
      Further, it is preferable to improve a relative dielectric constant by further mixing metal such as platinum with such a dielectric in the range allowing the insulativity to be secured. In this case, it is preferable, for example, to mix platinum by about 20% in weight (about 17% in volume) with a dielectric.  
      More specifically, for example, metal fine particles (for example about 0.01 to 1 μm) having a particle size larger than that of the dielectric particles  12   f  and smaller than that of the dielectric particles  12   e,  are mixed with the material powder  81 . Alternatively, the metal fine particles may be mixed with the material powder  181  and/or  191 . Due to this, the structure shown in  FIG. 1  can be easily implemented.  
      That is, the particle size of the dielectric particles (hereafter referred to as “coarse particles”) contained in the material powder  81  or  181  constituting the dielectric particles  12   e  is much larger than that of the particle size of the dielectric particles (hereafter referred to as “fine particles”) contained in the material powder  81  or  191  constituting the dielectric particles  12   f.  Hence the kinetic energy when the coarse particles are sprayed from the nozzle  73  is much larger than the kinetic energy of the fine particles sprayed from the nozzle  73  or  74 . Therefore, in some sizes and material quality of the coarse particles, there are cases where almost all of the coarse particles are flicked on the glass substrate  11  (on the lower electrode  16 ) or the layer (hereafter, “film-forming layer”) of the dielectric particles  12   e,    12   f  formed on the glass substrate  11 . In those cases, it is very difficult to form the film with these coarse particles, and there is a possibility that the coarse particles will damage the surface of the glass substrate  11 /surface of the lower electrode  16 , or the film-forming layer. To cope with the problem, by mixing the aforesaid metal fine particles with the material powder  81 , and using the ductility of the metal (causing the metal to function as a bond), the film-forming layer can be formed satisfactorily on the glass substrate  11 .  
      Here, as metallic fine particles, besides the aforementioned platinum, used can be fine particles of noble metal such as gold, silver or the like, base metal such as nickel, copper, iron or the like, or alloy such as silver-palladium, platinum-rhodium, brass or the like. In the case of alloy, the capability of forming a layer improves by properly adjusting the composition and thus regulating ductility, and therefore it is preferable to use alloy fine particles as the metal fine particles.  
      Further, a preferable mix rate is 0.01 to 20%, yet preferably 0.05 to 10% in volume.  
       FIG. 11  is an enlarged cross-sectional view of an emitter  12 ′ with which a metal has been mixed. As shown in  FIG. 11 , metal portions  12   k  are dispersed (the dispersion state is such that the particles are discontinuous) between adjacent dielectric particles  12   e,    12   f,  and between the emitter  12 ′ and lower electrode  16  in the emitter  12 ′.  
      By taking above measures, the metal portions  12   k  function as a binder between adjacent dielectric particles  12   e  and  12   f  in the emitter  12 ′ and also between the emitter  12 ′ and the lower electrode  16 , and thereby the capability of forming the upper layer  12   c  and the lower layer  12   d  improves.  
      Further, by filling the gaps between adjacent dielectric particles  12   e  and  12   f  with the metal portions  12   k,  the dielectric constant of the emitter  12 ′ increases and it becomes possible to realize good dielectric performances at the emitter  12 ′.  
      Further, in order not to electrically connect the upper electrode  14  (refer to  FIG. 1 ) to the lower electrode  16  (so that the emitter  12 ′ does not exhibit electrical conductivity in the thickness direction) through plural metal portions  12   k,  at least one gap is formed appropriately between adjacent plural metal portions  12   k  in the thickness direction of the emitter  12 ′ (in the vertical direction in the figure). Thereby good dielectric properties (including piezoelectric and electrostrictive properties, electrification and electron emission properties, and the like) can stably be obtained at the emitter  12 ′.  
      Further, a piezoelectric and electrostrictive layer, an antiferroelectric layer, or the like can be used as the emitter  12  as stated above and, when a piezoelectric and electrostrictive layer is used as the emitter  12 , as such a piezoelectric and electrostrictive layer, adopted can be, for example: a substance such as lead zirconate, lead magnesium niobate, lead nickel niobate, lead zinc niobate, lead manganese niobate, lead magnesium tantalate, lead nickel tantalate, lead antimony stannate, lead titanate, barium titanate, lead magnesium tungstate, or lead cobalt niobate; or ceramics containing the arbitrary combination thereof.  
      It goes without saying that a substance containing those chemical components by 50% or more in weight as the main component can also be used. Among the above ceramics, ceramics containing lead zirconate are most frequently used as a constituent material of a piezoelectric and electrostrictive layer composing the emitter  12 .  
      Further, in the case of composing a piezoelectric and electrostrictive layer of ceramics, used may also be a ceramics produced by further adding to it: oxides of lanthanum, calcium, strontium, molybdenum, tungsten, barium, niobium, zinc, nickel, manganese, or the like; combination of some of them; or other chemical compounds in an appropriate manner. Furthermore, ceramics produced by adding SiO 2 , CeO 2 , Pb 5 Ge 3 O 11 , or combination of some of them to the above ceramics may also be used. More specifically, a material produced by adding SiO 2  by 0.2 wt. %, CeO 2  by 0.1 wt. %, or Pb 5 Ge 3 O 11  by 1 to 2 wt. % to a PT-PZ-PMN system piezoelectric material may preferably be used. For example, it is preferable to use ceramics containing a component including lead magnesium niobate, lead zirconate, and lead titanate as the main component and further containing lanthanum and strontium. A piezoelectric and electrostrictive layer may be either dense or porous and, if porous, a preferable porosity is 40% or less.  
      In the case of using an antiferroelectric layer as the emitter  12 , as the antiferroelectric layer, desirable is: a substance containing lead zirconate as the main component; a substance containing a component including lead zirconate and lead stannate as the main component; a substance produced by further adding lanthanum oxide to lead zirconate; or a substance produced by adding lead zirconate and lead niobate to a component including lead zirconate and lead stannate. Further, the antiferroelectric layer may be porous and, if porous, a preferable porosity is 30% or less.  
      Further, in the case of using strontium tantalate bismuthate (SrBi 2 Ta 2 O 9 ) as the emitter  12 , polarization reversal fatigue is preferably small. Such a material having small polarization reversal fatigue is a laminar ferrodielectric chemical compound and expressed by the general expression (BiO 2 ) 2+ (A m−1 B m O 3m+1 ) 2− . Here, the ions of the metal A are Ca 2+ , Sr 2+ , Ba 2+ , Pb 2+ , Bi 3+ , La 3+ , etc. and the ions of the metal B are Ti 4+ , Ta 5+ , Nb 5+ , etc. Further, it is also possible to render semiconducting capability by further adding an additive to piezoelectric ceramics of a barium titanate system, a lead zirconate system, or a PZT system. In this case, it is possible to allow an uneven electric field distribution to form in an emitter  12  and thus to concentrate electric fields in the vicinity of the interface with the upper electrode  14  that contributes to electron emission.  
      Furthermore, it is possible to lower the sintering temperature by mixing for example a glass component such as lead borosilicate glass or another low melting chemical compound (for example bismuth oxide or the like) with piezoelectric, electrostrictive and antiferroelectric ceramics.  
      Yet further, in the case of composing an emitter  12  of piezoelectric, electrostrictive and antiferroelectric ceramics, the shape thereof may be any of a solid sheet, a laminated sheet, or a shape formed by laminating or bonding one of them on a support substrate.  
      In addition, by using a non-lead type material as the emitter  12  and thus raising the melting point or the transpiration temperature of the emitter  12 , the emitter  12  becomes unlikely to be damaged by the collision of electrons or ions.  
      As the conductive particles  15  composing the upper electrode  14  of the present embodiment, scale-like powder such as scale-like graphite powder or metallic powder, or acicular or rod-like powder such as carbon nanotube is preferably used. As a method for forming the upper electrode  14 , used is the method including the processes of: dispersing the scale-like powder (while using a disperser if necessary) into an organic solvent (a binder) such as ethyl cellulose and thus forming paste; applying the paste by spin coating, screen printing, dipping, spraying, or the like and thus forming a thick film of the paste on the upper surface  12   a  of the emitter  12 ; and then subjecting the thick paste film to heat treatment. In the case of forming the thick film, when the viscosity of the paste is adjusted to about 100,000 to 200,000 cps so as to be suitable for the thick film forming, a preferable film thickness after printing is considered to be about 1 to 25 μm, more preferably about 3 to 15 μm. If the film thickness is excessive, the size of an opening  20  is too small and inversely, if the film thickness is too thin, the electrical conductivity in an upper electrode  14  cannot be secured. Then after the thick film in the above thickness range is formed, by applying heat treatment, the formed film to be the upper electrode  14  on the emitter  12  is transformed into only the electrode material along with the decomposition of the binder and simultaneously plural openings  20  are formed. As a result, without specific patterning treatment such as masking treatment being applied, plural openings  20  and overhangs  26  are formed in the upper electrode  14  as shown in  FIG. 1  and other figures. Here, an inert gas, such as nitrogen, atmosphere is preferably used as the atmosphere during sintering (in particular when a carbonaceous material is used) but atmospheric air or an oxygen atmosphere (including in the decompressed state) can also be used if the compounding ratio of the conductive particles  15  in the paste is properly adjusted.  
      When the upper electrode  14  is formed, as stated above, it is preferable to add also conductive fine particles  19 . As such conductive fine particles  19 , besides metallic fine particles, carbonaceous fine particles such as spheroidized graphite powder, carbon black, or the like can be used. When conductive fine particles  19  are added, in addition to metallic fine powder classified into a prescribed particle size range, a substance that can be finally transformed into conductive fine particles by sintering can also be used, for example resinate or the like may be used.  
      When a carbonaceous material is used as the conductive particles  15  (particularly in an oxygen atmosphere such as the atmospheric air or the like), the temperature at the heat treatment of the upper electrode  14  is preferably 500° C. or lower and further, when conductive fine particles  19  are added, it is necessary to select a temperature at which the conductive fine particles  19  do not aggregation or cause grain growth in excess of a prescribed particle size.  
      Meanwhile, as the lower electrode  16 , a material having electrical conductivity such as metal is used and the lower electrode  16  is composed of platinum, molybdenum, tungsten, or the like. Further, the lower electrode  16  is: composed of a conductor showing resistance to a high-temperature oxidizing atmosphere, such as pure metal, alloy, mixture of insulative ceramics and pure metal, mixture of insulative ceramics and alloy or the like; or preferably composed of high-melting noble metal such as platinum, iridium, palladium, rhodium, molybdenum or the like, a substance mainly composed of alloy such as silver-palladium, silver-platinum, platinum-palladium or the like, or a cermet material composed of platinum and ceramics. More preferably, the lower electrode  16  is composed of platinum only or a material mainly composed of platinum type alloy. Furthermore, as the lower electrode  16 , a material of a carbon or graphite type may also be used. Here, the proportion of the ceramic material added to the electrode material is preferably 5 to 30% in volume. It goes without saying that the same material as used for the aforementioned upper electrode may also be used. Then when the lower electrode  16  is made of the aforementioned metal or carbonaceous material, the aforementioned thick film forming method is preferably used.  
      As described in detail above, in the electron emitter  10 A according to this embodiment, the emitter  12  includes the relatively coarse first dielectric particles  12   e  of micron order, and the fine second dielectric particles  12   f  of submicron order, and the second dielectric particles  12   f  are disposed so that they fill the spaces surrounding the plural first dielectric particles  12   e.  Therefore, by suitably adjusting the filling degree of the second dielectric particles  12   f  in the spaces surrounding the first dielectric particles  12   e,  the physical/electrical properties required of the emitter  12  can be set appropriately without difficulty.  
      Also, microscopic unevenness are formed by the joins and crystal grain boundaries of the adjacent plural dielectric particles  12   e,    12   f  in the upper surface  12   a  of the emitter  12 . If these microscopic unevenness are used, an overhanging shape such as that of the aforesaid overhang  26  can easily be formed. In particular, merely by arranging the conductive particles  15  having a shape extending in the longitudinal direction in a sectional side view on the upper surface  12   a  of the emitter  12 , the aforementioned overhangs  26  are formed easily and abundantly. Also, due to the microscopic unevenness in the upper surface  12   a  of the emitter  12 , the surface area of the upper surface  12   a  which has the electron emission function in the emitter  12  is enlarged, and the electron emission amount can be increased. Also, the second dielectric particles  12   f  are of an identical material to that of the first dielectric particles  12   e,  so there is no difference of surface energy between the two in the joins, the first dielectric particles  12   e  and second dielectric particles  12   f  are highly compatible in the contact parts, and the filling density of the dielectric material in the emitter  12  can easily be increased.  
      If the emitter including the dielectric is formed into a film and filled by the aerosol deposition method and/or the sol impregnation method, a high dielectric thin layer of high dielectric constant with few defects and a high filling ratio can be obtained by a relatively low temperature process. Therefore, the electron emitter  10 A can be formed onto a relatively economical substrate with a relatively low heat resistance such as a glass substrate, so a large size screen for an FED can be obtained at low cost. The physical properties of the emitter  12  can be adjusted as desired by the film-forming conditions of the aerosol deposition method or the sol impregnation method. Further, by performing heat treatment at 500° C. or less when the upper electrode  14  is formed, the dielectric constant of the emitter  12  increases and the electron emission performance improves.  
      Further, in an electron emitter  10 A according to the present embodiment, by forming a overhang  26  at the upper electrode  14 , a triple junction  26   c  is formed at a site other than the inner edge  26   b  of the upper electrode. Further, an opening  20  has such a shape as to allow the inner edge  26   b  of the opening  20  to serve as an electric field concentrated portion. Hence it is possible to further increase the number of the electric field concentrated portions than in the case where the overhangs  26  are not formed. In particular, since a overhang  26  in the present embodiment is formed so that it protrudes at an acute angle toward the inner edge  26   b  that is the tip of the overhang  26 , the degree of electric field concentration is higher than the case where the shape of the inner edge  26   b  is a right angle or an obtuse angle and it becomes possible to increase the amount of the electrons accumulated on the upper surface  12   a  of the emitter  12 .  
      Furthermore, in the upper surface  12   a  of the emitter  12 , the maximum angle θ between the upper surface  12   a  (the face in the vicinity of the top of the convex portion of a jog) of the emitter  12  and the lower face  26   a  of a overhang  26  in the upper electrode  14  is set so as to satisfy the expression 1°≦θ≦60° and the maximum distance d in the vertical direction between the upper surface  12   a  of the emitter  12  and the lower face  26   a  of the overhang  26  in the upper electrode  14  is set so as to satisfy the expression 0 μm&lt;d≦10 μm, and by the configuration it becomes possible to increase the degree of the electric field concentration at a gap  28  and thus to increase the amount of the electrons accumulated on the upper surface  12   a  of the emitter  12 .  
      Further, by forming the aforementioned overhang  26 , a gap  28  is formed between the lower face  26   a,  opposite to the emitter  12 , of the overhang  26  at the opening  20  in the upper electrode  14  and the emitter  12 , most part of a drive voltage Va is substantially applied to the gap  28  by the influence of the electrostatic capacity of the virtual condenser at the gap  28 , and thus the electric field at the opening  20  is intensified. Thereby it becomes possible to decrease the absolute value of the drive voltage Va required for obtaining the same field intensity at the opening  20 .  
      Further, since the overhang  26  of the upper electrode  14  functions as a focus electron lens or a gate electrode (control electrode), the linearity of the emitted electrons can be improved. This is advantageous to the reduction of cross talk in the case of arraying many electron emitters  10 A and using those, for example, as the electron sources of a display.  
      In addition, as stated above, the openings  20  are formed numerously in a region of the upper electrode  14  in a plan view, and the electric field concentrated portions at the inner edges  26   b  of the openings  20  and the triple junctions  26   c  are also formed numerously in a region of the upper electrode  14  in a plan view (they are formed numerously also at the positions corresponding to the outer edge  21  at the outer periphery (refer to  FIG. 1 ) of the upper electrode  14 ). Thereby electrons are emitted uniformly without deviation in a region of the upper electrode  14  in a plan view.  
      Next, a display  100  using electron emitters  10 A according to the present embodiment is explained on the basis of  FIG. 12 .  
      In the display  100 , as shown in  FIG. 12 : a transparent plate  130  made of, for example, glass or acrylic is disposed above the upper electrode  14 ; a collector electrode  132  composed of, for example, a transparent electrode is disposed on the bottom surface (the face opposite to the upper electrode  14 ) of the transparent plate  130 ; and the collector electrode  132  is coated with a phosphor  134 . Here, a bias voltage source  136  (collector voltage Vc) is connected to the collector electrode  132  via a resistance. Further, an electron emitter  10 A is disposed in a vacuum atmosphere as stated above. The vacuum level in the atmosphere is preferably in the range from 10 2  to 10 −6  Pa, and more preferably in the range from 10 −3  to 10 −5  Pa.  
      The reason why such a range is selected is that, if the vacuum level is low, there arise the following risks; (1) since the gas molecules are abundant in the space, plasma tends to be generated and, if plasma is generated in too large quantity, a large number of positive ions collide with the upper electrode  14 , resulting in the increase of damages, and (2) the emitted electrons undesirably collide with gas molecules before they reach the collector electrode  132  and the excitation of the phosphor  134  by the electrons fully accelerated by the collector voltage Vc is insufficiently secured.  
      On the other hand, if the vacuum level is high, though electrons are likely to be emitted from the points where electric fields concentrate, the problem is that the sizes of the supports of the structure and the sealing system for vacuum increase and thus that is disadvantageous for downsizing.  
      Next, an electron emitter  10 B according to the second embodiment is explained referring to  FIG. 13 . The electron emitter  10 B according to the second embodiment has almost the same configuration as the aforementioned electron emitter  10 A according to the first embodiment, but the conductive particles  15  composing the upper electrode  14  exist on the upper surface  12   a  of the emitter  12  not only as the primary particles  15   b  but also as the secondary particles  15   c.  Then the specific feature here is that the length of the secondary particles  15   c  in the longitudinal direction in a sectional side view is larger than the crystal grain size of a polycrystalline material composing the emitter  12 . The electron emitter  10 B according to the second embodiment has also the same functions and effects as shown by the aforementioned electron emitter  10 A according to the first embodiment.  
      Further, an electron emitter  10 C according to the third embodiment is explained referring to  FIG. 14 . The electron emitter  10 C according to the third embodiment has almost the same configuration as the aforementioned electron emitter  10 A or  10 B according to the first or second embodiment, but the upper electrode  14  is composed of, in addition to the same conductive particles  15  as above, the conductive fine particles  19 . It is preferable that the size of such conductive fine particles  19  is nearly equal to or smaller than the thickness (the width thereof in the direction perpendicular to the longitudinal direction in a sectional side view) of the primary particles of the conductive particles  15 . For example, when the thickness of the conductive particles  15  is about 2 μm, the average particle size of the conductive fine particles  19  is preferably 1 μm or less, and more preferably 0.5 μm or less. Thereby the electrical conductivity among the conductive particles  15  can easily be secured in the same upper electrode  14 .  
      Here, it is preferable that the conductive fine particles  19  are exposed on the top surface of the upper electrode  14 , in particular at the overhangs  26  as shown in  FIG. 14 . Thereby the conductive fine particles  19  exist like protrusions on the top surface of the upper electrode  14 , thus the conductive fine particles  19  also serve as the electric field concentrated portions by the effect of the protruded shape, and hence the number of the sites of supplying electrons to the upper surface  12   a  of the emitter  12  can further be increased. Further preferably, the conductive fine particles  19  are also deposited on the upper surface  12   a  of the emitter  12  corresponding to the openings  20 . Thereby fine float electrodes including the conductive fine particles  19  are also disposed on the emitter  12  composed of a dielectric material. The float electrodes are suitable for accumulating electrons emitted from the upper electrode  14  toward the emitter  12  in large quantity and can further increase the amount of emitted electrons in the electron emitter. For that reason, by composing the float electrodes of the conductive fine particles  19 , as it is stated later, the float electrodes can be formed on the upper surface  12   a  of the emitter  12  through a simple process of, for example, mixing the conductive particles  15  with the conductive fine particles  19  and applying the mixture on the upper surface  12   a  of the emitter  12  when the upper electrode  14  is formed on the upper surface  12   a  of the emitter  12 .  
      Further, when a carbonaceous material (for example graphite) is used as the conductive particles  15  and silver is used as the conductive fine particles  19  and then heating treatment is applied at the time of the forming of the upper electrode  14 , during the heat treatment, since the graphite or the like in the vicinity of the silver fine particles is eroded by oxidation, the outer edge of the upper electrode  14  is likely to take the shape having a sharp edge or to have openings caused by perforation inside the electrode. Thereby the electric field concentrated portions further increase and it becomes possible to obtain a more suitable electrode configuration.  
      An electron emitter  10 D according to a fourth embodiment will now be described referring to  FIG. 15 . The electron emitter  10 D according to the fourth embodiment is substantially identical to those of the aforesaid embodiments, except that there is further provided a surface layer  12   g  (first layer) forming the upper surface  12   a ′ of the emitter  12 ″ on the dielectric layers of the dielectric particles  12   e,    12   f  forming the emitter  12  in the electron emitters  10 A to  10 C ( FIG. 1 ,  FIG. 13 ,  FIG. 14 ) according to the first to third embodiments.  
      The surface layer  12   g  includes plural dielectric particles  12   g   1 . The dielectric particles  12   g   1  include a dielectric material prepared beforehand by sintering or the like. For example, the dielectric particles  12   g   1  are of a piezoelectric/electrostrictive ceramics thin plate material which is sintered so that it has predetermined dielectric properties (dielectric properties such as for example piezoelectric/electrostrictive properties or electron emission properties). Specifically, the dielectric particles  12   g   1  are obtained by for example breaking and/or cutting the previously sintered piezoelectric/electrostrictive ceramics thin plate to a predetermined size.  
      The size of this dielectric element  12   g  can be suitably selected according to the surface roughness and flatness of the upper surface  12   a ′ of the dielectric layer due to the dielectric particles  12   e,    12   f.  In order to obtain satisfactory adhesion properties between the upper surface  12   a ′ and surface layer  12   g  and satisfactory dielectric properties (as few voids as possible) in the emitter  12 ″ even if the surface roughness and flatness of the upper surface  12   a ′ is not so good, the thickness is preferably approximately in the range from 10 to 20 μm, and the particle size (dimensions in the longitudinal direction i.e., the direction perpendicular to the thickness when viewed in lateral cross-section of the emitter  12 ″) is preferably in the range from 20 to 200 μm. The particle size of the dielectric particle  12   g  is a value obtained by analyzing the scanning electron microscope (SEM) image in an upper plan view and computation, e.g., the value obtained by approximating the particle to a circle having an equivalent area to that of the particle, taking the diameter of this circle as the particle size and computing the particle size distribution in terms of the area.  
      This surface layer  12   g  is joined to and formed in one piece with the upper surface  12   a ′, for example by sintering or the like. Specifically, films of dielectric layers of the dielectric particles  12   e,    12   f  are formed on the glass substrate  11 , plural dielectric particles  12   g   1  are laid over the upper surface  12   a ′ of these dielectric layer films, heat treatment is performed to form a one-piece construction, and the emitter  12 ″ is thereby obtained.  
      Herein, if the dielectric layers of the dielectric particles  12   e,    12   f  are formed by for example the above-described aerosol deposition method, an additive including glass or metal fine particles is mixed with the material powder and sprayed on the glass substrate  11 , the additive functions as a binder, so the emitter  12 ″ can be formed at a low heat treatment temperature (if metal fine particles are mixed in, as shown in  FIG. 11 , the metal portions  12   k  are dispersed between the adjacent dielectric particles  12   e  and  12   f  or at the interface between the dielectric particles  12   e  and lower electrode  16 , and the metal portions  12   k  may also be dispersed between the upper surface  12   a ′ and surface layer  12   g  in  FIG. 15 ).  
      In the electron emitter  10 D according to the embodiment having the aforesaid construction, predetermined dielectric properties can definitively be implemented on the upper surface  12   a ′ of the emitter  12 ″, so very good electron emission properties can be obtained. Specifically, the method of forming the dielectric layers of the dielectric particles  12   e,    12   f  has a considerable electrical/mechanical influence on the dielectric layers, so the dielectric properties in the vicinity of the upper surface  12   a ′ of the dielectric layers may be less than those expected. Hence, in this embodiment, the dielectric layers of the dielectric particles  12   e,    12   f  function as an insulating film of high dielectric constant. By forming the upper surface  12   a  of the emitter  12 ″ by the surface of the surface layer  12   g  having predetermined dielectric properties which is fixed to the insulating film, an electron emitter having predetermined properties (dielectric element) can easily be manufactured.  
      When the upper surface  12   a ′ 0  of the dielectric surface due to the dielectric particles  12   e,    12   f  is formed very smooth and flat, the surface layer  12   g  may be formed from one dielectric thin plate instead of an aggregation of the plural dielectric particles  12   g   1 .  
      Hereinabove, a typical embodiment of the present invention has been described, but it will be understood that the invention is not to be construed as being limited in any way thereby, and various modifications may be made without departing from the scope and spirit thereof.  
      For example, in the above embodiments, the upper electrode  14  and the lower electrode  16  are formed on the different surfaces (the upper surface and the lower surface) of the emitter  12 , but they may be formed on an identical surface (the upper surface).  
      The emitter  12 ′ containing metal ( FIG. 11 ) can be used in any of the aforesaid embodiments. Here, the metal may be mixed with the material powder  81  and the like by the aforesaid aerosol deposition method, or any desired method such as vapor deposition or coating may be used.  
      Further, with regard to the shape of an opening  20  serving as an electric field concentrated portion at the inner edge, in addition to the shapes explained in the above embodiments, various shapes may be adopted. For example, as shown in  FIG. 16 , such a shape that the lower surface  26   a  of the overhang  26  is nearly horizontally flat and the overhang  26  itself gradually tapers toward the inner edge  26   b  thereof may be adopted. Further, as shown in  FIG. 17 , the overhang  26  may be formed so that the overhang  26  itself gradually declines toward the concavity  24  of the emitter  12 . Furthermore, as shown in  FIG. 18 , the lower face  26   a  of the overhang  26  may form an inclined plane directed upward toward the inner edge  26   b  thereof in a sectional side view. Furthermore, as shown in  FIG. 19 , the surface of the inner wall of the opening  20  may be formed into a hyperbolic shape. Further, as shown in  FIG. 20 , a floating electrode  50  may be formed at a portion, on the upper surface  12   a  of the emitter  12 , corresponding to the opening  20 .  
      Further, when an opening  20  having such a shape as to serve as an electric field concentrated portion at the inner edge thereof is adopted as stated above, the upper surface  12   a  of the emitter  12  formed by the aerosol deposition method and/or the sol impregnation method may be smooth like a mirror surface. Thereby, by selecting the shape of the opening  20 , it becomes possible to secure a high packing density and a high dielectric constant of the emitter  12  while increasing the number of the electric field concentrated portions, and hence to increase the field intensity at the electric field concentrated portions and also increase the amount of the emitted electrons more than ever.  
      In the sol impregnation method of the aforesaid embodiment, the dielectric fine particles are dispersed in an organic solvent, but a precursor solution of dielectric fine particles which precipitates the dielectric fine particles due to heating or the action of a reaction initiator or the like, may also be used.  
      Furthermore, in the production method employed in the above embodiments, the opening  20  of the upper electrode  14  can be formed merely by forming a thick film by controlling the viscosity and compounding ratio of paste and the film thickness even though specific masking or the like is not applied, but, as shown in  FIG. 21 , the opening  20  may be formed by applying masking or the like so that a hole  32  of a specific shape is formed. In this case, the shape of the hole  32  is a perfect circle macroscopically but a deformed shape microscopically due to the influence of the shape of the conductive particles  15 , and the hole  32  can exhibit the functions and effects in increasing the number of the sites where electrons are supplied to the emitter  12 .