Method of manufacturing a display apparatus

Abstract: An electron source emits electrons as a function of input signals. The electron source (74) comprises a substrate (1), a matrix of wires having m row wires (72) and n column wires (73) laid on the substrate (1) with an insulator layer interposed therebetween, and a plurality of surface-conduction electron-emitting devices (74) each having a pair of electrodes and a thin film including an electron emitting region and arranged between the electrodes. The electron-emitting devices are so arranged as to form a matrix with the electrodes connected to the respective row (72) and column wires (73). The electron source (74) further comprises selection means for selecting a row of the plurality of surface-conduction electron-emitting devices, and modulation means for generating modulation signals according to input signals and applying them to the surface-conduction electron-emitting devices selected by the selection means.

Description: This invention relates to a method of manufacturing a display apparatus. Thermal cathods and cold cathode electron sources are known two type of electron emitting devices, of which the latter include field-emission type (hereinafter referred to as FE type), metal/insulation layer/metal type (hereinafter referred to as MIM type) and surface-conduction electron emitting devices. Examples of FE type devices are proposed in W. P. Dyke & W. W. Dolan, "Field emission", Advance in Electron Physics, 8, 89 (1956), A. Spindt, "PHYSICAL Properties of thin-film field emission cathodes with molybdenum cones" J. Appl. Phys., 32, 646 (1961). At MIM type device is disclosed in C. A. Mead, "The tunnel-emission amplifier, J. Appl. Phys., 32, 646 (1961). A surface-conduction type electron-emitting device is proposed in M. I. Elinson, Radio Eng. Electron Phys., 10 (1965). A surface-conduction electron-emitting device utilizes the phenomenon that electrons are emitted out of a small thin film formed on a substrate when an electric current is forced to flow in parallel with the film surface. While Elison proposes the use of an SnO 2 thin film for a device of this type, the use of an Au thin film is proposed in [G. Dittmer: "Thin Solid Films", 9, 317 (1971)] whereas the use of an In 2 O 3 /SnO 2 and that of a carbon thin film are discussed respectively in [M. Hartwell and C. G. Fonstad: "IEEE Trans. ED Conf.", 519 (1975)] and [H. Araki et al.: "Vacuum", Vol. 26, No. 1, p. 22 (1983)]. Fig. 25 of the accompanying drawings schematically illustrates a surface-conduction electron-emitting device proposed by M. Hartwell. In Fig. 25, reference numerals 431 and 432 respectively denote an insulator substrate and an H-shaped metal oxide film for electron-emission formed thereon by sputtering. Reference numeral 433 denotes an electron-emitting region that becomes operational when electrified in a process generally referred to as "forming", which will be described hereinafter. The entire thin film including the electron-emitting region is designated by numeral 434 in Fig. 25. For a device as illustrated in Fig. 25, L1 is between 0.5 and 1mm and W is equal to 0.1mm. An electron-emitting region 433 is produced in a surface-conduction electron-emitting device normally by electrifying a thin film 432 for electron-emission on the device, a process generally referred to as "forming". More specifically, a DC voltage or a slowly rising voltage that rises, for instance, at a rate of 1V/min. is applied to the opposite ends of the thin film 432 for electron-emission to locally destroy or deform or structurally modify the thin film 432 for electron-emission to produce fissures in a part of the thin film, which constitute an electrically highly resistive electron-emitting region 433. Once the surface-conduction electron-emitting device is processed for forming, electrons will be emitted from those fissures and their neighboring areas when a voltage is applied to the thin film 434 including the electron-emitting region 433 to cause an electric current to flow through the device. Known surface-conduction electron-emitting devices are, however, accompanied by problems when they are put to practical use. The applicant of the present patent application who has been engaged in the technological field under consideration has already proposed a number of improvements to the existing technologies in order to solve some of the problems, which will be described in greater detail hereinafter. Surface-conduction electron-emitting devices are, on the other hand, advantageous in that they can be used in arrays in great numbers over a large area because they are structurally simple and hence can be manufactured at low cost in a simple way. In fact, many studies have been made to exploit this advantage and applications that have been proposed as a result of such studies include charged beam sources and electronic displays. A large number of surface-conduction electron-emitting devices can be arranged in an array to form a matrix of devices that operates as an electron source, where the devices of each row are wired and regularly arranged to produce columns. (See, for example, Japanese Patent Application Laid-open No. 64-31332 of the applicant of the present patent application.) As for image-forming apparatuses such as displays, although very flat displays comprising a liquid crystal panel in place of a CRT have gained polularity in recent years, such displays are not without problems. One of such problems is that a light source needs to be additionally incorporated into the display in order to illuminate the liquid crystal panel because liquid crystal does not emit light by itself. An emissive electronic display that is free from this problem can be realized by using a light source formed by arranging a large number of surface-conduction electron-emitting devices in combination with fluorescent bodies that are induced to selectively shed visible light by electrons emitted from the electron source. With such an arrangement, emissive display apparatus having a large display screen and enhanced display capabilities can be manufactured relatively easily at low cost. (See, for example, the United States Patent No. 5066883 of the applicant of the present patent application.) Incidentally, emissive display apparatus of the above identified category comprising an electron source formed by a large number of surface-conduction electron-emitting devices and fluorescent bodies can be operated by drive signals that are applied to the wires connecting the respective surface-conduction electron-emitting devices arranged in rows (row wires) and to the control electrodes arranged in the space separating the electron source and the fluorescent bodies along a direction perpendicular to the row wires (grids or column electrodes). (See, for example, Japanese Patent Application Laid-open No. 1-283749 of the applicant of the present patent application). There are, however, a number of difficulties that have to be overcome before such a display apparatus becomes commercially feasible. Some of the difficulties include the problem of accurately aligning individual surface-conduction electron-emitting devices and corresponding individual grids and that of securing a uniform distance between each grid and the corresponding surface-conduction electron emitting device, both of which are manufacture-related problems. In an attempt to solve these manufacture-related problems, there has been proposed an improved display apparatus of the category under consideration, in which the grids are formed into a layer and laid on the layer of the surface-conduction electron-emitting devices to produce a multilayer structure. (See, for example, Japanese Patent Application Laid-open No. 3-20941 of the applicant of the present patent Application.) Figs. 26 and 27 illustrate a known typical electronic display comprising conventional surface-conduction electron-emitting devices as disclosed Japanese Patent Publication No. 45-31615. Referring to Figs. 26 and 27, it comprises transversal current type electron-emitting bodies 442 connected in series, strip-shaped transparent electrodes 444 arranged perpendicularly to the electron-emitting bodies 442 to form a lattice therewith and a glass panel 443 provided with a number of small holes 443' and disposed between the electron-emitting bodies and the electrodes in such a manner that the holes are located on the respective crossings of the electron-emitting bodies and the electrodes. Each of the holes 443' contains gas hermetically sealed therein so that the display emits light by gaselectric discharge only at the crossings of those transversal current type electron-emitting bodies 442 that are currently discharging electrons and those transparent electrodes 444 to which an accelerating voltage E2 is currently being applied. While Japanese Patent Publication No. 43-31615 does not detailedly describe the transversal current type electron-emitting body, it may safely be presumed that it is a surface-conduction electron-emitting device because the materials (metal thin film, mesa film) and the structural features of the neck 442' described there exactly match their counterparts of a surface-conduction electron-emitting device. For the purpose of the present invention, the term "surface-conduction electron-emitting device" is used in the sense as defined in "The Thin Film Handbook". Now, some of the problems that have arisen with electronic displays comprising known surface-conduction electron-emitting devices will be discussed below. Three major problems have been pointed out for a display apparatus disclosed in the above cited Japanese Patent Publication No. 45-31615. (1) While the display apparatus is designed to operate for electric discharge as electrons emitted from the transversal current type electron-emitting bodies are accelerated and caused to collide with gas molecules, the pixels of the apparatus can glow by electric discharge with different levels of luminance and the luminance of a same pixel can fluctuate when the transversal current type electron-emitting bodies are energized to a same intensity. One of the possible reasons for this may be that the intensity of electric discharge of such an apparatus is heavily dependent on the state of the gas in the apparatus and not satisfactorily controllable, while another may be that the output level of a transversal current type electron-emitting body cannot necessarily be stabilized if the gas pressure is somewhere around 15mmHg as described in the Examples section of the cited patent document. Thus, the above described display apparatus is not able to provide any multiple-tone display and therefore can offer only a limited scope of use. (2) While the display apparatus can change the color for display by using a different type of gas, the use of various gases does not necessarily extend the scope of color display because the wavelength of visible light generated by electric discharge does not cover a wide range. Additionally, the optimum gas pressure used for the emission of light by electric discharge varies as a function of the type of gas involved. Thus, in order to achieve a color display by using a single panel, different gases must be sealed in the holes with varied gas pressures depending on the locations of the holes, making the manufacture of such an apparatus extremely difficult. If, for example, three laminated panels are used for a display apparatus to avoid this problem, it will become unrealistically heavy and the manufacturing cost will be prohibitive to produce such a heavy apparatus. (3) Since the display apparatus comprises a large number of components including the substrates of the transversal current type electron-emitting bodies, the strip-shaped transparent electrodes and the holes where gas is hermetically sealed, it is structurally very complicated and hence only a very small error margin is allowed for aligning the components. Additionally, since the threshold voltage used for the emission of light by electric discharge is as high as 35[V] as described in the cited document, each electric element used in the panel drive circuit is required to show a high withstand voltage. Thus, such a display apparatus will require a complicated process to follow before it is completed as well as a prohibitive manufacturing cost. It is mainly due to the above reasons that an electronic display of the above described type has not been able to find any practical applications in the field of television receiving set and other similar electronic apparatuses. On the other hand, the image-forming apparatuses proposed by the applicant of the present patent application and comprising an electron source formed by arranging a number of surface-conduction electron-emitting devices and a same number of fluorescent bodies juxtaposed therewith are not without problems. Firstly, in order to realize such an electron source, it is indispensable to arrange grids along a direction (column-directed wiring) perpendicular to the wires connecting the electron-emitting devices arranged in parallel (row-directed wiring) if the devices are selectively made to emit electrons. In this regard, no simple and easy process has been developed for manufacturing an electron source with which devices are selected for the emission of electrons and the level of electron emission is controllable. Secondly, in order for the fluorescent bodies of such an image-forming apparatus arranged in juxtaposition with the electron source to emit light at selected locations with a controlled level of luminance, a certain number of grids need indispensably be provided as in the case of the electron source. Again, no simple and easy process has been developed for manufacturing an image-forming apparatus comprising such fluorescent bodies, with which electron-emitting devices can be selected with out difficulty to cause them emit light at a controlled level according to incoming signals so that the fluorescent bodies may be made to glow at selected locations with a controlled level of luminance. Document EP-A-0 388 984 discloses an arrangement in which in an electron source, electron-emitting regions 36 are connected via device electrodes 35 to wires 34-a and 34-b, which are wires for driving electron-emitting devices. Wires 34-a and 34-b are formed in parallel. Control electrodes 32 intersect perpendicularly with the above mentioned wires, but are insulated from the wires and devices via an insulating substrate. In the electron source disclosed in this prior art document, electron beams are emitted corresponding to voltage pulses applied to the device wire electrodes. Furthermore, electron beams to be accelerated or suppressed are previously emitted. Thus, the modulation principle taught by this document resides in that a certain amount of electron beams are selected from previously generated electron beams corresponding to voltage pulses, which is to be understood in a sense of "filtering" an already generated electron beam. Document EP-A-0 354 750 teaches an image display apparatus having an electron source controlled by an X-Y matrix drive. In particular, a fixed voltage is applied to gate electrodes for electron beam emission. A portion of electron beams are accelerated by a high voltage applied to an anode to thereby collide a fluorescent body surface and emit light. However, modulation of electron beams are never disclosed or taught. Publication ELEKTRONIK, Vol. 41, No. 2, 21 January 1992, München, pages 48-50, 55, merely discloses an active matrix type liquid cristal display using TFT's. In particular, neither an electron source nor an application thereof is disclosed. Document EP-A-0 299 461 discloses an example of an electron-emitting device. In this publication it is proposed to adjust a drive voltage value at which electron emission is initiated by a variation of technological parameters during the production process. However, there is no disclosure or teaching in this document related to an electron source using the device or a modulation method of the same. Prior art document EP-A-0 479 450 discloses an electron source where row conductors 14 and column conductors 16 constitute an X-Y matrix, but in the intersections of the conductors, they are insulated from each other and electrons are emitted from metal cones 40 corresponding to the potential difference between them. Still further, documents EP-A-0 404 022, US-A-4 956 578 as well as EP-A-0 312 007 are not related to solving an object of preventing cross talk in connection with display apparatuses comprising an electron source. Finally, applicants earlier application EP-A-0 536 732, falling under the regulations of Art. 54(3) EPC, also includes a graph (Fig. 7) which shows a characteristic similar to a monotonously increasing one. This application discloses that ineffective currents can be reduced by controlling the area, particle size or distance between particles within the electron-emitting region, and also that the device is driven under 10 -4 to 10 -9 Torr. However, it does not disclose the specific If-Vf and Ie-Vf characteristics of the present invention, nor does disclose or teach that the monotonously increasing characteristic unchanging with time and regardless of voltage rising rate is obtained by baking under ultravacuum conditions. In view of the above identified problems, it is therefore an object of the invention to provide an improved method of manufacturing a display apparatus. According to the method of the invention, a display apparatus with an electron source can be manufactured at low cost because of its simple configuration and can be used in combination with a fluorescent material arranged vis-a-vis the electron source to produce a high quality image-forming apparatus capable of displaying images in color and in a multitude of tones. Another aim of the invention is to manufacture a display apparatus comprising an electron source and capable of displaying images with good gradation. A further aim of the invention is to manufacture a display apparatus comprising an electron source and an image display screen provided with pixels that are ingenuously so configured as to be free from crosstalks. According to the present invention, the above object is achieved by a method of manufacturing a display apparatus as defined in claim 1. Favorable refinements of the present invention are as set out in the dependent claims. The present invention as claimed is defined as a method of manufacturing an apparatus. The description of an apparatus and its use serves only to illustrate the method. Figs. 1A and 1B are schematic views illustrating the basic configuration of a plane type surface-conduction electron-emitting device that can be used for the purpose of the present invention. Figs. 2A through 2C are schematic views illustrating different steps of manufacturing a surface-conduction electron-emitting device, to be used for the purpose of the invention. Fig. 3 is a block diagram of a measuring system for determining the performance of a surface-conduction electron-emitting device, to be used for the purpose of the invention. Fig. 4 is a graph showing a voltage waveform to be used for forming a surface-conduction electron-emitting device, to be used for the purpose of the invention. Fig. 5 is a graph showing the relationship between the voltage applied to a surface-conduction electron-emitting device, to be used for the purpose of the invention and the current that flows therethrough as well as the relationship between the voltage and the emission current of the device. Fig. 6 is a schematic perspective view of a step type surface-conduction electron-emitting device that can be used for the purpose of the invention. Fig. 7 is a schematic plan view of an example of an electron source. Fig. 8 is a schematic perspective view of an example of an image-forming apparatus. Figs. 9A and 9B are schematic views illustrating two example types of fluorescent films that can be used. Fig. 10 is a schematic circuit diagram illustrating the method of driving fluorescent materials. Fig. 11 is an exploded and enlarged perspective view of an electron-emitting device and a face plate of an image-forming apparatus. Fig. 12 is a schematic view of a luminous spot that can be observed in a surface-conduction electron-emitting device. Fig. 13 is a schematic view of equipotential lines for illustrating a possible path of an electron beam in an image-forming apparatus comprising surface-conduction electron-emitting devices. Fig. 14 is a schematic plan view of a first embodiment of electron source, the source per se not being an embodiment of the claimed method. Fig. 15 is a schematic sectional view of the first embodiment of Fig. 14. Figs. 16A through 16D are schematic sectional views of the first embodiment, showing it in different manufacturing steps. Figs. 17E through 17H are schematic sectional views of the first embodiment, showing it in different manufacturing steps following that of Figs. 16A to 16D. Fig. 18 is a schematic plan view of a mask that can be used for the first embodiment. Fig. 19 is a graph similar to Fig. 5 but showing the voltage-current relationships for a specimen prepared for the purpose of comparison. Fig. 20 is a schematic sectional view of a second embodiment of electron source, the source per se not being an embodiment of the claimed method. Figs. 21A through 21F are schematic sectional views of the second embodiment of Fig. 14, showing it in different manufacturing steps. Fig. 22 is a schematic plan view of a third embodiment of electron source, the source per se not being an embodiment of the claimed method. Fig. 23 is a schematic sectional view of the third embodiment of Fig. 22. Figs. 24A through 24E are schematic sectional views of the third embodiment, showing it in different manufacturing steps. Fig. 25 is a schematic plan view of a known electron-emitting device. Fgis. 26 and 27 are schematic plan views of a known image-forming apparatus. Now, the present invention as claimed will be illustrated in greater detail by way of examples (only methods of manufacturing can be embodiments of the claimed invention.) Firstly, by referring to Japanese Patent Application Laid-open No. 2-56822, etc, of the applicant of the present patent application, some of the fundametal structural and functional features of an electro-emitting device, particularly of a surface-conduction electron-emitting device, that provides a basic unit of an electron source and an image-forming apparatus will be discussed along with a preferred method of manufacturing such a device. Some of the features of a surface-conduction electron-emitting device to be manufactured by use of the present invention include the following. 1) A thin film to be used for an electron-emitting region of a device is basically constituted of fine particles that are dispersed or obtained by sintering organic meatl before it is electrically treated by a process called "forming". 2) After the "forming" process, both the electron-emitting region and the remaining areas of the thin film including the electron-emitting region are also constituted of fine particles. There are two alternative profiles that can be taken for a surface-conduction electron-emitting device to be used for the purpose of the invention, a planar profile and a stepwise profile. Firstly, a plane type surface-conduction electron-emitting device will be described. Figs. 1A and 1B are a schematic plan view and a sectional view of a plane type surface-conduction electron-emitting device. As shown in Figs. 1A and 1B, the device comprises a substrate 1, a pair of electrodes 5 and 6 (referred to as device electrodes hereinafter) and a thin film 4 including an electron-emitting region 3. The substrate 1 is preferably a substrate such as a glass substrate made of quartz glass, glass containing Na and other impurities to a reduced level or soda lime glass, a multilayer glass substrate prepared by forming a SiO 2 layer on a piece of soda lime glass by sputtering or a ceramic substrate made of a ceramic material such as alumina. While the oppositely arranged device electrodes 5 and 6 may be made of any conductor material, preferred candicate materials include metals such as Ni, Cr, Au, Mo, W, Pt, Ti, Al, Cu and Pd, their alloys, printable conductor materials made of a metal or a metal oxide selected from Pd, Ag, RuO 2 , Pd-Ag and glass, transparent conductor materials such as In 2 O 3 -SnO 2 and semiconductor materials such as polysilicon. The distance L1 separating the electrodes is between hundreds angstroms and hundreds micrometers and determined as a function of various technical aspects of photolithography to be used for manufacturing the device, including the performance of the aligner and the etching method involved, and the voltage to be applied to the electrodes and the electric field strength designed for electron emission. Preferably it is between several micrometers and tens of several micrometers. The lengths W1 of the electrode 6 and the thickness of the device electrodes 5 and 6 may be determined on the basis of requirements involved in designing the device such as the resistances of the electrodes, the connections of the row and column wires, or X- and Y-wires as they are referred to hereinafter, and the arrangement of the plurality of electron-emitting devices, although the length of the electrode 6 is normally between several micrometers and several hundred micrometers and the thickness of the device electrodes 5 and 6 is typically between several hundred angstroms and several micrometers. The thin film 4 of the device that includes an electron-emitting region is partly laid on the device electrodes 5 and 6 as seen in Fig. 1B. Another possible alternative arrangement of the components of the device will be such that the area 2 of the thin film 4 for preparing an electron-emitting region is firstly laid on the substrate 1 and then the device electrodes 5 and 6 are oppositely arranged on the thin film. Still alternatively, it may be so arranged that all the areas of the thin film found between the oppositely arranged device electrodes 5 and 6 operates as an electron-emitting region. The thickness of the thin film 4 including the electron-emitting region is preferably between several angstroms and several thousand angstroms and most preferably between 1 nm and 50 nm (10 and 500 angstroms). It is determined as a function of the step coverage of the thin film 4 to the device electrodes 5 and 6, the resistance between the electron-emitting region 3 and the device electrodes 5 and 6, the mean size of the conductor particles of the electron-emitting region 3, the parameters for the forming operation that will be described later and other factors. The thin film 4 normally shows a resistance per unit surface area between 10 -3 and 10 -7 Ω/cm 2 . The thin film 4 including the electron-emitting section is made of fine particles of a material selected from metals such as Pd, Ru, Ag, Au, Ti, In, Cu, Cr, Fe, Zn, Sn, Ta, W and Pb, oxides such as PdO, SnO 2 , In 2 O 3 , PbO and Sb 2 O 3 , borides such as HfB 2 , ZrB 2 , LaB 6 , CeB 6 , YB 4 and GdB 4 , carbides such TiC, ZrC, HfC, TaC, SiC and WC, nitrides such as TiN, ZrN and HfN, semiconductors such as Si and Ge and carbon as well as other metals and metal compounds such as AgPd, NiCr, Pb and Sn. The term "a fine particle film" as used herein refers to a thin film constituted of a large number of fine particles that may be loosely dispersed, tightly arranged or mutually and randomly overlapping (to form an island structure under certain conditions). The electron-emitting region 3 is constituted of a large number of fine conductor particles with a mean particle size of preferably between several angstroms and hundreds of several angstroms and most preferably between 1nm and 50 nm (10 and 500 angstroms) and the thickness of the thin film 4 including the electron-emitting region is determined depending on a number of factors including the method selected for manufacturing the device and the parameters for the forming operation that will be described later. The material of the electron-emitting region 3 may be selected from all or part of the materials that can be used to prepared the thin film 4 including the electron-emitting region. While a number of different methods may be used for manufacturing an electron-emitting device comprising an electron-emitting region 3, Figs. 2A through 2C illustrate different steps of a specific method. In Figs. 2A through 2C, reference numeral 2 denotes a thin film to be used for an electron-emitting region and may typically be a fine particle film. Now, the method will be described below. 1) After a substrate 1 is thoroughly washed with detergent, pure water and organic solvent, a selected electrode material is deposited thereon at oppositely arranged locations by means of vacuum deposition, sputtering or some other appropriate technique and then processed by photolithography to produce a pair of device electrodes 5 and 6 (Fig. 2A). 2) An organic metal solution is applied to the surface of the substrate 1 as well as the device electrodes 5 and 6 on the substrate and let to dry to produce an organic metal thin film. The organic metal solution is a solution of an organic compound of a metal selected from Pd, Ru, Ag, Au, Ti, In, Cu, Cr, Fe, Zn, Sn, Ta, W and Pb as listed earlier. Thereafter, the formed organic metal thin film is heated for sintering and then subjected to a patterning operation, using a lift-off or etching technique, to produce a thin film 2 for preparing an electron-emitting region (Fig. 2B). While the organic metal thin film is prepared by applying an organic metal solution onto the substrate in the above description, such a film may also be formed by using a different technique such as vacuum deposition, sputtering, chemical vacuum deposition, distributed application, dipping or spinner. 3) Subsequently, the device electrodes 5 and 6 are subjected to a so-called forming operation, where a pulsed or rapidly increasing voltage is applied to them by a power source (not shown) to locally modify the structure of the thin film in an area that becomes an electron-emitting region 3 (Fig. 2C). More specifically, the thin film 2 is locally destroyed, deformed or structurally modified as it is electrified to become an electron-emitting section 3. As described above, the inventors of the present invention have proved through observation that the electron-emitting region 3 is constituted of fine conductor particles. Fig. 4 shows a graph illustrating the voltage waveform to be used for a forming operation. In Fig. 4, T1 and T2 respectively indicate the pulse width and the pulse interval of triangular pulsed voltage waves, T1 being between 1 microsecond and 10 milliseconds, T2 being between 10 microseconds and 100 milliseconds, the level of the peaks of the waves (peak voltage for forming) being e.g. between 4V and 10V. The forming operation is conducted for a time period between tens of several seconds to several minutes in a vacuum atmosphere. While a varying voltage in the form of triangular pulses is applied to the electrodes of an electron-emitting device in order to produce an electron-emitting region, it may not necessarily take a triangular form and rectangular waves or waves in some other form may alternatively be used. Likewise, other appropriate values may be selected for the pulse width, the pulse interval and the peak level to optimize the performance of the electron-emitting region to be produced depending on the intended resistance of the electron-emitting device. If the thin film for preparing the electron-emitting region of an electron-emitting device is formed by dispersing fine conductor particles, the above described forming process may be partly modified. Now, some of the functional features of a electron-emitting device and prepared in the above described manner will be described by referring to Figs. 3 and 5. Fig. 3 is a schematic block diagram of a measuring system for determining the performance of an electron-emitting device having a configuration as illustrated in Figs. 1A and 1B. In Fig. 3, an electron-emitting device comprising a substrate 1, a pair of device electrodes 5 and 6, a thin film 4 including an electron-emitting region 3 is placed in position in a measuring system comprising on its part a power source 31 for applying voltage Vf to the device (referred to as device voltage Vf hereinafter), an ammeter 30 for measuring the electric current running through the thin film 4 including the electron-emitting region and between the device electrodes 5 and 6, an anode 34 for capturing the emission current emitted from the electron-emitting region 3 of the device, a high voltage source 33 for applying a voltage to the anode 34 and another ammeter 32 for measuring the emission current Ie emitted from the electron-emitting region 3. When measuring the current If running through the device (referred to as device current hereinafter) and the emission current Ie, the device electrodes 5 and 6 are connected to the power source 31 and the ammeter 30, and the anode 34 connected to the power source 33 and the ammeter 32 is placed above the device. The electron-emitting device and the anode 34 are put into a vacuum chamber, which is provided with an exhaust pump, a vacuum gauge and other pieces of equipment necessary to operate a vacuum chamber so that the measuring operation can be conducted under a desired vacuum condition. Incidentally, the exhaust pump comprises an ordinary high vacuum system constituted of a turbo pump and a rotary pump and an ultra high vacuum system constituted of an ion pump. The entire vacuum chamber and the substrate of the electron-emitting device can be heated to approximately 200°C by a heater (not shown). A voltage between 1 KV and 10KV is applied to the anode, which is spaced apart from the electron-emitting device by distance H between 2mm and 8mm. As a result of intensive studies carried out on electron-emitting devices for the purpose of the present invention, the inventors of the present invention discovered critical functional features that paved the way to the present invention. Fig. 5 shows a graph schematically illustrating the relationship between the device voltage Vf, i.e. a drive voltage applied to the device electrodes, and the emission current Ie and the device current If typically observed by the measuring system of Fig. 3. Note that different units are arbitrarily selected for Ie and If in Fig. 5 in view of the fact that Ie has a magnitude by far smaller than that of If. As seen in Fig. 5, an electron-emitting device has three remarkable features in terms of emission current Ie, which will be described below. Firstly, an electron-emitting device shows a sudden and sharp increase in the emission current Ie when the voltage applied thereto exceeds a certain level (which is referred to as a threshold voltage hereinafter and indicated by Vth in Fig. 5), whereas the emission current Ie is practically unobservable when the applied voltage is found that the threshold value Vth. Differently stated, an electron-emitting device according to the invention is a non-linear device having a clear threshold voltage Vth to the emission current Ie. Secondly, since the emission current Ie is highly dependent on the device voltage Vf, the former can be effectively controlled by way of the latter. Thirdly, the emitted electric charge captured by the anode 34 is a function of the duration of time of applying the device voltage Vf. In other words, the amount of electric charge captured by the anode 34 can be effectively controlled by way of the time during which the device voltage Vf is applied. Because of the above remarkable features, an electron-emitting device may find a variety of applications. On the other hand, the device current If either rises monotoneously relative to the device voltage Vf (as shown by a solid line in Fig. 5, a characteristic referred to as MI, i.e. monotoneous increase, characteristic hereinafter) or varies to show a form specific to a voltage-controlled-negative-resistance (as shown by a broken line in Fig. 5, a characteristic referred to as VCNR characteristic hereinafter). The inventors of the present discovered that the either of the above features of the device current If appears depending on how the electron emitting device is actually manufactured. More specifically, the device current If of an electron-emitting device can take on a VCNR characteristic when the device is subjected to a forming operation in an ordinary vacuum system, although it can greatly vary depending on the vacuum degree and electric conditions of the measuring system during and after the forming operation, including the rate at which the voltage applied to the device is raised to obtain a particular current-voltage relationship for the device and the time during which the device is left in the vacuum chamber before the device is tested for its performance. Note that the emission current Ie always shows an MI characteristic. In view of the above described discoveries, the inventors of the present invention carried out an experiment where an electron-emitting device whose device current If had been showing a VCNR characteristic in an ordinary vacuum system was baked in an ultra high vacuum system at high temperature (e.g., 100°C for 15 hours) and found that after the baking operation both the device current If and the emission current Ie showed an MI feature if subjected to device voltage Vf. It should be noted that, while a monotoneously increasing device current If is observed on a device as disclosed in Japanese Patent Application Laid-open No. 1-279542 of the applicant of the present patent application when the device is subjected to a voltage rising at a relatively high rate after it is processed by a forming operation in an ordinary vacuum system, it is different from the emission current Ie and the device current If of an electron-emitting device manufactured according to the invention that monotoneously increase with the device voltage after it is processed in an ultra high vacuum system and therefore they may safely be assumed to be totally different from each other. Thus, the above described monotoneously increasing relationship between the current voltage Vf and the device current If and between the current voltage Vf and the emission current Ie of an electron-emitting device manufactured according to the invention may provide a wide area of application for the device in future. Now, a surface-conduction electron-emitting device having an alternative profile, or a step type electron-emitting device, will be described. Fig. 6 is a schematic perspective view of a step type surface-conduction electron-emitting device. As seen in Fig. 6, the device comprises a substrate 1, a pair of device electrodes 5 and 6, a thin film 4 including an electron-emitting region 3 and a step-forming section 67. Since the substrate 1, the device electrodes 5 and 6 and the thin film 4 including the electron-emitting region 3 are prepared from the materials same as those of their counterparts of a plane type electron-emitting device as described above, only the step-forming section 67 and the thin film 4 including the electron-emitting region 3 that characterize this device will be described in detail here. The step-forming section 67 is made of an insulator material such as SiO 2 and formed there by vacuum deposition, printing, sputtering or some other appropriate technique to a thickness between several hundred angstroms and tens of several micrometers, which is substantially equal to the distance L1 separating the electrodes of a plane type electron-emitting device described earlier, although it is determined as a function of the technique selected for forming the step-forming section, the voltage to be applied to the electrodes of the device and the electric field strength available for electron emission and preferably found between several thousand angstroms and several micrometers. As the thin film 4 including the electron-emitting region is formed after the device electrodes 5 and 6 and the step-forming section 67, it may preferably be laid on the device electrodes 5 and 6 and so shaped as to form suitable electrical connection with the device electrodes 5 and 6. The thickness of the thin film 4 including the electron-emitting region is a function of the method of preparing it and, in many cases, varies on the step-forming section and on the device electrodes 5 and 6. Normally, the thin film 4 is made less thick on the step-forming section than on the electrodes. The electron-emitting region 3 may be formed in any appropriate area of the thin film 4 other than the one in Fig. 6. While a surface-conduction electron-emitting device is described above in terms of its basic configuration and manufacturing method, such a device may be prepared with any other configuration and manufacturing method so long as it is provided with the above defined three features and appropriately used for an electron source or an image-forming apparatus and/or display apparatus. Now, an electron source and an image-forming apparatus utilizing such an electron-emitting device will be described. As described earlier, a surface-conduction electron-emitting device is provided with three remarkable features. Firstly, it shows a sudden and sharp increase in the emission current Ie when the voltage applied thereto exceeds a certain level (which is referred to as a threshold voltage hereinafter and indicated by Vth in Fig. 5), whereas the emission current Ie is practically unobservable when the applied voltage is found lower than the threshold value Vth. Differently stated, an electron-emitting device according to the invention is a non-linear device having a clear threshold voltage Vth to the emission current Ie. Secondly, since the emission current Ie is dependent on the device voltage Vf, the former can be effectively controlled by way of the latter. Thirdly, the emitted electric charge captured by the anode 34 is a function of the duration of time of applying the device voltage Vf. In other words, the amount of electric charge captured by the anode 34 can be effectively controlled by way of the time during which the device voltage Vf is applied. Consequently, electrons emitted from the surface-conduction electron-emitting device are controlled by the peak level and the width of the pulse of the pulse-shaped voltage applied to the oppositely arranged device electrodes under the threshold voltage, whereas practically no electrons are emitted beyond the threshold voltage. Thus, an apparatus comprising a large number of such surface-conduction electron-emitting devices can be controlled by controlling the pulse-shaped device voltage (pulse width, wave height, etc.) applied to each of the electron-emitting devices according to input signals. It should be noted that, while a number of different surface-conduction electron-emitting devices having the above identified three fundamental features may be conceivable, the most preferable ones are those manufactured as claimed, whose device curent If and emission current Ie monotoneously increase with reference to the device voltage Vf applied to the pair of device electrodes (showing the MI characteristic) as defined in the present claims. An electron source comprising substrate and a number of surface-conduction electron-emitting devices of the above described type typically operates in a manner as described below by referring to Fig. 7. In Fig. 7, 1 denotes a substrate and 73 and 74 respectively denote X- and Y-wires while 74 and 75 respectively designate a surface-conduction electron-emitting device and a connection. The surface-conduction electron-emitting device 74 may have a plannar or stepwise profile. The substrate 1 is a substrate such as a glass substrate as described earlier and its dimensions are determined as a function of its configuration, the number of devices arranged on the substrate 1 and, if it constitutes a part of a vacuum container for the electron source, the vacuum conditions of the container as well as other factors. There are a total of m X-wires 72 designated respectively as DX1, DX2, ..., DXm, which are typically made of a conductive metal and formed on the substrate 1 by vacuum deposition, printing or sputtering to show a desired pattern, although the material, the thickness and the width of the wires need to be so determined that a substantially as equal voltage as possible may be applied to all of the surface-conduction electron-emitting devices. On the other hand, there are a total of n Y-wires 73 designated respectively as DY1, DY2, ..., DYn, which are also typically made of a conductive metal and formed on the substrate 1 by vacuum deposition, printing or sputtering to show a desired pattern as in the case of X-wires 72, the material, the thickness and the width of the wires being so determined that a substantially as equal voltage as possible may be appiled to all of the surface-conduction electron-emitting devices. The m X-wires 72 are electrically insulated from the n Y-wires 73 by means of an insulator layer (not shown) laid therebetween, the X- and Y-wires forming a matrix. Both m and n are integers. The insulator layer (not shown) is typically made of SiO 2 and formed on the X-wires 72 carrying substrate 1 by vacuum deposition, printing or sputtering to show a desired contour, although the thickness, the material and the technique to be used for forming it need to be so selected that it may withstand the largest potential difference at the crossings of the X- and Y-wires. It may be so arranged that an insulator layer is found only on and near the crossings of the X- and Y-wires. With such an arrangement, a connection 75 and an X- or Y-wire may be electrically connected without using a contact hole. Each of the X- and Y-wires is led out to an external terminal. While n Y-wires 73 are laid on m X-wires 72 with an insulator layer interposed therebetween in the above description, m X-wires 72 may be conversely laid on n Y-wires 73 with an insulator layer inserted therebetween. The insulator layer may be used to form all or part of the step-forming sections of the step type surface-conduction electron-emitting devices constituting the electron source if such electron-emitting devices are used. The oppositely arranged device electrodes of the surface-conduction electron-emitting devices 74 are electrically connected to the respective X-wires 72 (DX1, DX2, ..., DXm) and Y-wires 73 (DY1, DY2, ..., DYn) by way of respective connections 75 that are also made of a conductor metal and formed by vacuum deposition, printing or sputtering. Either a same conductor material or totally or partly different conductor materials may be used for the m X-wires 72, n Y-wires 73, connections 73 and oppositely arranged device electrodes. Such materials may be appropriately selected from metals such as Ni, Cr, Au, Mo, W, Pt, Ti, Al, Cu and Pd, alloys of these metals, printing conductor materials constituted of a metal or a metal oxide such as Pd, Au, RuO 2 , Pd-Ag and glass and semiconductor materials such as polysilicon. As will be described in detail hereinafter, scan signal application means (not shown) is connected to the X-wires 72 for applying scan signals to the X-wires 72 in order to scan the rows of the surface-conduction electron-emitting devcie 74 according to input signals. On the other hand, modulation signal generation means (not shown) is connected to the Y-wires 73 for applying modulation signals to the Y-wires 73 in order to modulate the columns of the surface conduction electron-emitting device 74 according to input signals. A drive voltage is applied to each of the surface-conduction electron-emitting devices as the difference of the voltage of the scan signal and that of the modulation signal applied to the device. Now, an image-forming apparatus and/or display apparatus comprising an electron source having a configuration as described above will be described by referring to Figs. 8 and 9A and 9B, of which Fig. 8 schematically illustrates the configuration of the image-forming apparatus and Figs. 9A and 9B illustrate two types of fluorescent films that may be used for the apparatus. In Fig. 8, the apparatus comprises among others an electron source substrate 1, on which a number of electron-emitting devices are arranged, a rear plate 81 for securely holding the electron source substrate 1, a face plate 86 prepared by arranging a fluorescent film 84 and a metal back 85 on the inner surface of a glass substrate 83 and a support frame 82, casing 88 of the apparatus being formed by applying frit glass to the contact areas of the rear plate 81, the support frame 82 and the face plate 86 and burning them in ambident air or in a nitrogen atmosphere at 400 to 500°C for more than ten minutes to tightly bond them together. Note that reference numeral 74 in Fig. 8 denotes an electron-emitting region of the device of Figs. 1A and 1B and reference numerals 72 and 73 respectively designate X- and Y-wires connected to the pair of device electrodes of related surface-conduction electron-emitting devices. The wires connected to the device electrodes of a device may also be referred to as the device electrodes of that device hereinafter, if they are made of a material same as that of the proper electrodes. While the casing structure 88 is constituted of the face plate 86, the support frame 82 and the rear plate 81 in the above description, the rear plate 81 may be omitted from it if the substrate 1 has a sufficient strength because the rear plate 81 is simply a reinforcement for the substrate 1. If such is the case, the support frame 82 will be directly bonded to the substrate 1 so that the casing 88 will be constituted of the face plate 86, support frame 82 and the substrate 1. Figs. 9A and 9B show two types of fluorescent films that can be used for an image-forming apparatus. The fluorescent film 84 of Fig. 8 is constituted only of a number of fluorescent materials if the apparatus is designed as a monochrome display, whereas it is constituted of fluorescent materials 92 and a black conductor member 91 which is made of a black conductor material and may be called a black strip or black matrix depending on the shape and arrangement of the fluorescent materials. Such a black strip or black matrix is arranged in order to make the space for preventing color mixing of the fluorescent materials 92 for three primary colors and suppress any reduction in the contrast of the image on the face plate of the apparatus that can be given rise when external light is reflected by the surface of the face plate. While graphite is typically used for the black strip, any other materials may suitably be used so long as they are electrically conductive and show low transmissivity and reflectivity to light. The fluorescent material 83 are formed on the glass substrate 83 by printing or precipitation regardless if the apparatus is a monochrome or color display. A metal back 85 is normally arranged on the inner surface of the fluorescent film 84 because it reflects light directed to the inner surfaces of the fluorescent materials, operates as an electrode for applying a voltage to electron beams to accelerate their speed and protects the fluorescent materials from being damaged by negative ions that are generated inside the casing to collide with the fluorescent materials. After the fluorescent film is prepared and its inner surface is smoothed (in a process normally called "filming"), the metal back is formed thereon by depositing aluminum by means of vacuum deposition. A transparent electrode (not shown) may be formed on the outer surface of the fluorescent film 84 in order to raise the conductivity of the fluorescent film 84. Note that care should be taken to exactly align the fluorescent materials of each primary color and the respective corresponding electron-emitting devices before the components of the casing 88 are tightly bonded together. The casing 88 is evacuated by using an exhaust pipe (not shown) to produce a degree of vacuum of 1,33322-10 -4 Pa (10 -6 Torr) inside before it is hermetically sealed. At the same time, a voltage is applied to the oppositely arranged device electrodes of the electron-emitting devices by way of the external terminals Doxl through Doxm and Doyl through Doyn of the apparatus to carry out a forming operation and produce an electronemitting region in each of the devices, while the inside of the casing is held to a degree of vacuum of approximately 1,33322-10 -4 Pa (10 -6 Torr) by means of an ordinary vacuum system comprising a rotary pump or a turbo pump. However, in order for the surface-conduction electron-emitting devices to show an MI characteristic for the device current If and the emission current Ie, according to the invention, an additional process of baking them in a ultra high vacuum system comprising an ion pump at 80°C to 150°C for three to fifteen hours needs preferably to be carried out after the forming operation. A getter operation may be carried out on the casing 88 in order to ensure a high degree of vacuum for it after it is sealed. In this operation, a getter arranged at a given position (not shown) in the casing 88 is heated by resistance or high frequency heating to form a film by vapor deposition before the casing is hermetically sealed. The getter is normally made of a material containing Ba as a principal ingredient and the inside of the casing is held to a degree of vacuum between 1,33322-10 -3 Pa (1x10 -5 ) and 1,33322-10 -5 Pa (1x10 -7 Torr) because of the adsorption effect of the vapor deposited film. With an image-forming apparatus having a configuration as described above, images are displayed on the screen by applying a voltage to the electron-emitting devices via the external terminals Doxl through Doxm and Doyl through Doyn to cause them to emit electrons, applying a high voltage greater than several kilovolts to the metal back 85 or the transparent electrode (not shown) via a high voltage termianl Hv to accelerate the electrons in order to make them collide with the fluorescent film 84, which is consequently energized to emit light to produce images on the screen. While some of the structural and functional features of an image-forming apparatus manufactured according to the invention are described above, the materials and the configurations of the components of the apparatus are not limited to those described and other materials and configurations may alternatively be used whenever appropriate. Now, some recommendable drive methods for driving an electron source or an image-forming apparatus manufactured according to the invention will be described. According to a first drive method, said scan signal application means for applying scan signals is so designed as to apply a voltage V1[V] to wires selected from the m X-wires and another voltage V2[V] to the remaining X-wires so that the surface-conduction electron-emitting devices connected to the wires to which the voltage V1[V] is applied are selectively scanned. (V1[V] is not equal to V2[V].) On the other hand, said modulation signal generation means generates a pulse-shaped voltage having a given legnth for the n Y-wires and changes its peak level (referred to as Vm[V]) for each and every one of the n Y-wires according to the input signal for that Y-wire, which may be, for instance, a signal representing the brightness level of an incoming image signal, in order to modulate the brightness of the displayed image. More specifically, the absolute value of the drive voltage Vm-V1[V] applied to the selected N electron-emitting devices that are currently being scanned is modulated on the basis of the relationship between the Vf and Ie of the electron-emitting devices so that each and every electron beam may be emitted from any of the devices with a required intensity depending on the corresponding input signal, e.g., the brightness level of the corresponding incoming video signal. Meanwhile, the absolute value of the drive voltage Vm-V2[V] applied to the remaining electron-emitting devices that are currently not being scanned is so controlled as to never exceed a threshold voltage Vth predetermined for the electron-emitting devices. Thus, only the electron beams from the electron-emitting devices being scanned and hence having respective required intensities are output for a given period of time, whereas the remaining electron-emitting devices do not output any electron beams during that period. According to a second drive method, said scan signal application means for applying scan signals is so designed as to apply a voltage V3[V] to wires selected from the m X-wires and another voltage V4[V] to the remaining X-wires so that the surface-conduction electron-emitting devices connected to the wires to which the voltage V3[V] is applied are selectively scanned. (V3[V] is not equal to V4[V].) On the other hand, said modulation signal generation means generates a pulse-shaped voltage having a given peak level (referred to as Vp[V]) for the n Y-wires and changes the width of each pulse (referred to as Ps[S]) for each and every one of the n Y-wires as a function of the input signal for that Y-wire, which may be, for instance, a signal representing the brightness level of an incoming video signal, in order to modulate the brightness of the displayed image. More specifically, the absolute value of the drive voltage Vp-V3[V] applied to the selected N electron-emitting devices that are currently being scanned exceeds the absolute value of the predetermined threshold voltage Vth so that each and every electron may be emitted from any of the devices with a required electric charge depending on the corresponding input signal, e.g, the brightness level of the corresponding incoming image signal, by modulating the pulse width Pw[S] of each pulse individually. Meanwhile, the absolute value of the drive voltage Vm-V2[V] applied to the remaining electron-emitting devices that are currently not being scanned is so controlled as to never exceed a threshold voltage Vth predetermined for the electron-emitting devices. Thus, only the electrons emitted from the electron-emitting devices being scanned and hence having respective required electric charges are output, whereas the remaining electron-emitting devices do not output any electron beams. According to a third drive method, said scan signal application means for applying scan signals is so designed as to apply a voltage V5[V] to wires selected from the M X-wires and another voltage V6[V] to the remaining X-wires so that the surface-conduction electron-emitting devices connected to the wires to which the voltage V5[V] is applied are selectively scanned. (The difference between V5[V] and V6[V] needs to meet a certain condition.) On the other hand, said modulation signal generation means generates a pulse-shaped voltage for the N Y-wires and changes the timing of applying the pulse-shaped voltage or its peak level or both for each and every one of the N Y-wires as a function of the input signal to modulate the degree of brightness in the image being displayed. (Here, the timing of applying the pulse-shaped votlage means the pulse width or the phase of the pulse relative to the corresponding scan signal or both.) More specifically, the drive voltage applied to the selected N electron-emitting devices that are currently being scanned is a voltage pulse whose pulse width and peak value are modulated and it is so controlled that the electric charge of each electron emitted during the scanning period of each and every one of the electron-emitting devices has a quantity that matches the corresponding input signal, e.g., the brightness level the corresponding incoming video signal. Meanwhile, the drive voltage to the remaining electron-emitting devices that are currently not being scanned is so controlled as to never exceed a threshold voltage Vth predetermined for the electron-emitting devices. Thus, only the electron beams from the electron-emitting devices being scanned and hence having respective required intensities are output for the duration of the time scanning operation, whereas the remaining electron-emitting devices do not output any electron beams during that period. Incidentally, when an electron source or an image-forming apparatus manufactured according to the invention comprises surface-conduction electron-emitting devices that are provided with the above described fundamental feature that both the device current If and the emission current Ie of the device are substantially linearly proportional to the voltage applied thereto, no electron beams would be emitted from those devices that are not currently being scanned. Contrary to this, however, when the emission current Ie of such surface-conduction electron-emitting devices is monotoneously increasing to the voltage applied thereto but their device current If has a VCNR characteristic, electron beams may possibly be emitted from those electron-emitting devices that are not currently being scanned. This may be because, while the drive voltage Vm[V]-V2[V] is applied to the electron-emitting devices that are not currently being scanned, these device change their state so that somehow the drive voltage exceeds the threshold voltage level Vth. In the following, a divided drive method for driving an electron source or an image-forming apparatus will be described. Referring to Fig. 10, it shows an apparatus comprising electron-emitting device rows (X1, X2, ...) each having a plurality of electron-emitting devices A and modulation electrode columns (Y1, Y2, ...) arranged to form an X-Y matrix. Voltage Vf is applied to one of the electron-emitting device rows (X1, X2, ...) with a level sufficiently high for causing the devices of the row to emit electrons while a voltage is applied to one of the modulation electrode columns (Y1, Y2, ...) with a level that varies as a function of the input information signal to define an electron beam emission pattern for that electron-emitting device row as a function of the information signal. Then, this operation is repeated on a one-by-one basis for all the electron-emitting device rows to define an electron beam emission pattern for a frame and the operation of defining an electron beam emission pattern for a frame is repeated for a multitude of frames. Then, an image is formed for a frame by irradiating the image-forming member of the apparatus with beams in accordance with the defined electron beam emission pattern and this image forming operation is repeated for a multitude of frames. It should be noted for the above drive method that, when a voltage is applied to one of the modulation electrode columns (Y1, Y2, ...) with a level that varies as a function of the input information pattern, a cutoff voltage is applied to a modulation electrode (which may be, for instance, assumed to be Y2 here) to which an ON-state voltage is applied and its neighboring modulation electrodes (Y1, Y2) regardless of what information signal is given. Consequently, the modulation electrodes Y1 and Y3 are held to a constant voltage level. With such an arrangement, by applying a cutoff voltage, electron beams that are emitted and collide with the image-forming member are not adversely affected by the voltage applied to the neighboring modulation electrode columns. Additionally, any crosstalks among electron beams are effectively suppressed. In a preferred mode of carrying out the above described drive method, an information signal is fed to every n-th modulation electrode columns so that the signal input operation is carried out n+1 times while a cutoff signal is fed to the remaining modulation electrodes that are not give any information signal. Referring to Fig. 10, an input signal is fed to all the even number modulation electrode columns for the first time and then to all the odd number modulation electrode columns for the second time, whereas a cutoff signal is fed to all the odd number modulation electrode columns firstly and then to all the even number modulation electrode columns for the second time. Thus, voltage Vf that is required for electron emission is applied to electron-emitting device row X1, while an information signal given to the modulation electrode volumns (Y1, Y2, Y3, ...) is firstly 1) fed to modulation electrode columns Y1, Y3, Y5, ... while a cutoff signal is fed to modulation electrode columns Y2, Y4, Y6, ... and then secondly 2) fed to modulation electrode columns Y2, Y4, Y6, ... while a cutoff signal is fed to modulation electrode columns Y1, Y3, Y5, ... to define an electron beam emission pattern for row X1 according to the information signal. Then, this operation is repeated for all the electron-emitting device rows on a one-by-one basis to define an electron beam emission pattern for a frame. The operation of defining an electron beam emission pattern for a frame is repeated for a multitude of frames. Thereafter, an image is formed for a frame by irradiating the image-forming member of the apparatus with beams in accordance with the defined electron beam emission pattern and this image forming operation is repeated for a multitude of frames. In order to effectively irradiate the image-forming member of the apparatus with electron beams emitted from the electron source according to a defined electron emission pattern, an appropriate voltage must be applied to the image-forming member as a function of the level of the ON-state voltage and that of the cutoff voltage as well as the type of the electron-emitting devices involved. While an information signal (modulation signal) to be used for the purpose of the invention contains an ON-state signal which is a voltage signal for allowing irradiation of the image-forming member with electron beams beyond a given rate and a cutoff signal for blocking irradiation of the image-forming member with electron beams, it may additionally contain a voltage signal for varying the rate of electron beam irradiation of the image-forming member if images are to be formed with a multitude of tones. The ON-state signal and the cutoff signal are defined as a function of the type of the electron-emitting devices involved and the level of the voltage applied to the image-forming member. An electron source or an image-forming apparatus operated by the above drive method may comprise an image-forming member prepared by arranging red (R), green (G) and blue (B) fluorescent bodies. The divisor to be used for the drive method may be an appropriately selected integer other than two which is used for the arrangement of Fig. 10. While a cutoff signal is fed to the modulation electrodes adjacent to those where an input signal is fed in the above description, it should be noted that due to simultaneous driving of plural devices, the time allotted to each device is increased to ensure a sufficient emission of electrons if a cutoff signal is not used. In case of not feeding a cut off signal, the X 1 , X 2 , ... side can be divided for simultaneous driving, in place of the Y 1 , Y 2 , ... side. Now, preferred examples of electron source and image-forming apparatus will be described. Fig. 11 is an exploded and enlarged perspective view of a combination of an electron-emitting device and a face plate of an image-forming apparatus that comprises a plurality of surface-conduction electron-emitting devices as illustrated in Fig. 8, said view showing several tracks of electron beams emitted from the electron-emitting device. In Fig. 11, there is shown an surface-conduction electron-emitting device comprising a substrate 1, high and low potential device electrodes 5 and 6 arranged on the substrate 1 with a narrow gap 1, which is filled with a thin film to form an electron-emitting region 3. There is also shown a face plate 86 arranged vis-a-vis the substrate 1 of the electron-emitting device. Said face plate 86 comprises a glass plate 83, a metal back 85 and an image-forming member 84 (or a fluorescent material) and arranged above the substrate 1 with a distance H separating them from each other. When voltage Vf is applied to the device electrodes 5 and 6 by means of a device drive power source 10, electrons are emitted from the electron-emitting region 3 in the form of a beam and accelerated by acceleration voltage Va applied to the fluorescent material 84 via the metal back 7 by an electrode acceleration power source 11 until they collide with the fluorescent material 84 to cause the latter to luminesce and form a luminous spot 9 on the face plate 86. Fig. 12 is a schematic enlarged illustration of a luminous spot 9 observed by the inventors of the present invention in an apparatus shown in Fig. 11. It was found that, as seen in Fig. 12, a luminous spot of a fluorescent material is expanded to a certain extent both in the direction of voltage application of the device electrodes (X-direction) and in a direction perpendicular to it (Y-direction). While the reason why an electron beam is expanded to a certain extent before it collides with the image-forming member is not particularly clear, the inventors of the present invention believe on the basis of a number of experiments that it is possibly because electrons are scattered to a certain extent at the time when they are emitted from the electron-emitting region 3. The inventors of the present invention also believe that, of the electrons emitted in different directions, those that are directed to the high potential device electrode (in positive X-direction) get to the tip 18 of the luminous spot and those that are directed to the low potential device electrode (in negative X-direction) arrive at the tail 19 of the luminous spot to produce a certain width along X-direction. Since that the luminance of the luminous spot is low at the tail, it may be safely assumed that the electrons emitted toward the low potential device electrode are very small in number. It was also found by a number of experiments conducted by the inventors of the present invention that the luminous spot 9 is normally slightly deflected from the vertical axis of the electron-emitting region 3 into positive X-direction or toward the high potential device electrode 5. The inventors of the present invention believe this may be explained by that, as shown in Fig. 13 illustrating the potential distribution within a space above the surface-conduction electron-emitting device, the equipotential lines are not parallel with the surface of the image-forming member 85 near the electron-emitting region 3 and therefore electrons omitted from the region 3 and accelerated by the accelerating voltage Va fly away not only in Z-direction in Fig. 13 but also toward the high potential device electrode. Differently stated, the electrons emitted from an electron-emitting region 3 are inevitably deflected to a certain extent by the voltage Vf applied thereto for acceleration immediately after the emission. After looking into the size of the luminous spot 9 and the electrons deflected from the vertical axis of the electron-emitting region 3 into X-direction and other phenomena, the inventors of the present invention came to believe that the deviation of the front end of the luminous spot from the axis of the electron-emitting region (ΔX1 in Fig. 11) and that of the tail of the luminous spot from the axis of the electron-emitting region (ΔX2 in Fig. 11) can be expressed in terms of Va, Vf and H. When a target to which voltage Va(V) is applied is located above an electron source (in Z-direction) and separated by distance H and the space between the target and the electron source is filled with an evenly distributed electric field, the displacement in X-direction of an electron emitted from the electron source with an initial X-direction velocity of V (eV) and an initial Z-direction velocity of 0 is expressed by equation (1) below which is derived from the equation of motion. ΔX = 2H V Va Referring to Fig. 13, since it was discovered in a series of experiments conducted by the inventors of the present invention that, while the electric field is swerved near the electron-emitting region by the voltage applied to the device electrodes and therefore electrons are accelerated also in X-direction, the voltage applied to the image-forming member is sufficiently greater than the voltage normally applied to the electron-emitting device and consequently electrons are accelerated in X-direction only near the electron-emitting region and thereafter move in that direction at a substantially constant speed. Thus, the deviation in X-direction of the electron can be obtained by replacing V in equation (1) with a formula for expressing the X-direction velocity of an electron after it has been accelerated near the electron-emitting region. If the X-direction velocity component of an electron is C (eV) after it has been accelerated in X-direction near the electron-emitting region 3, C is a parameter that is to be modified by voltage Vf applied to the device. Thus, if C is expressed as a function of Vf, or C(Vf) (unit being eV) and the latter is used for equation (1), equation (2) below can be obtained for displacement ΔX0. ΔX0 = 2H (C(Vf)/Va) Equation (2) above expresses the displacement of an electron that is emitted from the electron-emitting region with an initial X-direction velocity of 0 and given an X-direction velocity of C (eV) near the electron-emitting region under the influence of voltage Vf applied to the device electrodes. In reality, the initial velocity of the electron has various directional components including the X-direction component. If the initial velocity has a quantity of v0 (eV), from equation (1) the largest and smallest displacements of an electron beam in X-direction will be expressed by equations (3) and (4) below respectively. ΔX1 = 2H√((C + v0)/Va) ΔX2 = 2H√((C - v0)/Va) Since v0 can also be assumed to be a parameter whose value changes depending on voltage Vf applied to the electron-emitting region and both C and v0 are functions of Vf, the following equations containing constants K2 and K3 can be obtained. √((C + v0)(Vf)) = K2 Vf and √((C - v0)(Vf)) = K3 Vf By modifying equations (3) and (4) and using the above formulas, equations (5) and (6) below can be produced. ΔX1 = K2 x 2H√(Vf/Va) ΔX2 = K3 x 2H√(Vf/Va) where H, Vf and Va are measurable quantities and so are ΔX1 and ΔX2. As a result of a number of experiments where the quantities of ΔX1 and ΔX2 are observed, varying the values of H, Vf and Va, the inventors of the present invention obtained the following values for K2 and K3. K2 = 1.25 ± 0.05 and K3 = 0.35 ± 0.05 The above values hold particularly true when accelerating electric field strength (Va/H) is not lower than 1kV/mm. From the above empirical achievements, the quantity (S1) of the voltage applied (in X-direction) to an electron in the electron beam spot on the image-forming member is expressed by a simple formula as shown below. S1 = ΔX1 - ΔX2. If K1 = K2 - K3, then equation (7) below is obtained from equations (5) and (6) above. S1 = K1 x 2H√(Vf/Va) where 0.8 ≦ K1 ≦ 1.0. As for the size of the electron beam spot in a direction perpendicular to the direction of the voltage applied to the electron-emitting region (Y-direction), while electrons are emitted with an initial velocity of v0 also in that direction, they would not be practically not accelerated in the direction at all. Thus, the displacement of the electron beam will be expressed by ΔY = 2H√(v0/Va) for both positive and negative Y-directions. From equations (3) and (4), √((ΔX1 2 - ΔX2 2 )/2) = 2H√(v0/Va) and, from equations (5) and (6), √((ΔX1 2 - ΔX2 2 )/2) = 2H√(Vf/Va) x √((K2 2 - K3 2 )/2) Using equations (9) and (10), then 2H√(v0/Va) = 2H√(Vf/Va) x √((K2 2 - K3 2 )/2) Thus, if √((K2 2 - K3 2 ) = K4 is assumed for the left side of equation (11), then the size of the electron beam spot on the image-forming member is expressed by equation (12) below for Y-direction, using L for the length of the electron-emitting region in that direction. S2 = L + 2ΔY = L + 2K4 x 2H√(Vf/Va) Since H, Vf, Va and L are measurable, the value of coefficient K4 can be determined by observing S2. Considering that K2 = 1.25 ± 0.05 and K3 = 0.35 ±0.05 and the definition of K4, a conclusion of 0.80 ≦ K4 ≦ 0.90 is finally drawn. This conclusion was backed by the results obtained in a series of experiments for determining the size of an electron beam spot in Y-direction. On the basis of the above equations, the inventors of the present invention went on the study of the behavior of electron beams emitted from a number of electron-emitting regions on the image-forming member. In a system illustrated in Fig. 11, emitted electrons get to the image-forming member to form an asymmetrical pattern there under the influence of a swerved electric field in the vicinity of the device electrodes (Fig. 13) and the edges of the electrodes as typically shown in Fig. 12. This phenomenon of a deformed electron beam spot and an asymmetrical pattern can give rise to a problem of degraded image resolution to such an extent that can render characters, if displayed, practically illegible and severely blur any moving images. The contour of an electron beam spot illustrated in Fig. 12 is asymmetrical relative to X-axis and the amount with which its tip or tail is displaced from the axis perpendicular to the electron-emitting region can be obtained by using equations (5) and (6) respectively. The inventors of the present invention discovered that a highly symmetrical luminous spot can be achieved when a plurality of electron-emitting regions provided between a higher potential electrode and a lower potential electrode, which surrounds the higher potential electrode and may be divided into a plurality of lower potential electrode pieces, are arranged with a distance D defined by equation (13) below for separating adjacent sections along the direction of voltage application and made to hit a same spot on the image-forming member. K2 x 2H√(Vf/Va) ≧ D/2 ≧ K3 x 2H√(Vf/Va) where K2 and K3 are constant and K2 = 1.25 ± 0.05 and K3 = 0.35 ± 0.05. As for a direction perpendicular to the direction of voltage application (Y-direction), electron-emitting regions may well be arranged with pitch P as defined by inequality (14) below if the electron beam spot formed by electrons emitted from those electron-emitting regions is required to show a high degree of continuity and if each of the electron-emitting regions has a length of L. P < L + 2K4 x 2H√(Vf/Va) where K4 = 0.80. If, to the contrary, the electron beam spot formed by electrons emitted from electron-emitting regions having a length of L is required to show discontinuity, they may well be arranged in Y-direction at pitch P that satisfies formula (15) below. P ≧ L + 2K5 x 2H√(Vf/Va) where K5 = 0.90. The concept of the present invention can be used for the manufacture of not only image-forming apparatuses but also for the manufacture of light sources that can replace the light emitting diodes of a conventional optical printer comprising a photosensing drum and light emitting diodes. Note that, if such is the case, not only linear electron beams but also two-dimensionally expanded flux of electron beams may be realized by selectively utilizing the m row'wires and n column wires of an electron source having a configuration as described earlier. Now, some preferable examples of such apparatus will be described below. (Embodiment 1) This example not being an embodiment of the claimed method is an electron source of an image-forming apparatus, which is realized by forming a number of plane type surface-conduction electron-emitting devices on respective insulator interlayers laid on substrates and using a same material or a material containing a same element for all the device electrodes, the X-wires, the Y-wires and the connections connecting the device electrodes and the wires of the apparatus. Fig. 14 shows a plan view of part of the embodiment of electron source. Fig. 15 illustrates a cross sectional view taken along line A-A' in Fig. 14. Figs. 16A through 17H illustrate different stops of manufacturing such an electron source. Note that same reference symbols are commonly used to respectively designate same components in Figs. 14 through 17H. More specifically, 1 denotes a substrate and 72 denotes an X-wire corresponding to DXm in Fig. 7 (also referred to as underwire) whereas 73 denotes a Y-wire that corresponds to DYn in Fig. 7. 4 denotes a thin film including an electron-emitting section and 5 and 6 denote respective device electrodes whereas 111 and 112 respectively denote an insulator interlayer and a contact hole to be used for electrically connecting the device electrode 5 and the underwire 72. This embodiment is prepared through the steps as illustrated in Figs. 16A through 17H and described below only for an electron-emitting device and related parts. Step a: A silicon oxide film is formed on a cleansed soda lime glass plate to a thickness of 0.5µm by sputtering to produce a substrate 1, on which a 5 nm (50Å) thick Cr layer and a 600 nm (6,000Å) thick Au layer are sequentially formed by vacuum deposition. Thereafter, photoresist (AZ 1370 available from HOECHST) is applied thereto by a spinner and baked. Then, the photoresist layer is exposed to light with a photomask arranged thereon and photochemically developed to produce a resist pattern for an underwire 72. Subsequently, the Au and Cr deposited layers is wet-etched, using the resist pattern as a mask to produce an underwire 72 (Fig. 16A). Step b: An insulator interlayer 111 of silicon oxide is formed to a thickness of 0.1µm by RF sputtering (Fig. 16B). Step c: A photoresist pattern 112 is formed on the silicon oxide film produced in step b and this insulator interlayer 111 is etched, using the photoresist pattern as a mask, to produce a contact hole 112 (Fig. 16C). RIE (Reactive Ion Etching) and CF 4 and H 2 gases are used for the etching operation in this step. Step d: Subsequently, another photoresist pattern is prepared (photoresist RD-2000N-41: available from Hitachi Chemical Co., Ltd.) for device electrodes 5 and 6 and an inter-electrode gap G and then a 5 nm (50Å) thich Ti film and a 100 nm (1,000Å) thick Ni film are sequentially formed by vacuum deposition. The photoresist pattern is dissolved in an organic solvent and the Ni and Ti deposit films are lift-off to produce device electrodes 5 and 6, which have a width W1 fo 300µm and separated from each other by a distance G of 3µm (Fig. 16D). Step e: Still another photoresist pattern is formed for an overwire 73 on the device electrodes 5 and 6 and then a 5 nm (50Å) thick Ti film and a 50 nm (500Å) thick Au film are sequentially formed by vacuum deposition. Unnecessary portions of these films are removed by lift-off to produce an overwire 73 having a desired pattern (Fig. 17E). Step f: Fig. 18 shows a plan view of part of a mask to be used in this step for forming a thin film 2, from which an electron-emitting section is made for an electron-omitting device. The mask has an opening for an inter-electrode gap and its neighboring areas. Using this mask, a 100 nm (1,000Å) thick Cr film 121 is formed by vapor deposition and subjected to a patterning operation. Then, organic Pd (ccp 4230 available from Okuno Pharmaceutical Co., Ltd.) is applied thereon by means of a spinner and heated at 300°C for 10 minutes for baking. (Fig. 17F). The formed thin fine particle film 2 which is made of fine particles of Pd as a main element and used for producing an electron-emitting section has a thickness of 10 nm (100Å) and a sheet resistance of 5x10 4 Ω/cm 2 . The term "a fine particle film" as used herein refers to a thin film constituted of a large number of fine particles that may be loosely dispersed, tightly arranged or mutually and randomly overlapping (to form an island structure under certain conditions). Step g: The Cr film 121 and the baked thin film 2 for an electron-emitting section are etched, using an acid etchant, to produce a desired pattern (Fig. 17G). Step h: A pattern is formed so that resist may be aFplied to all the surface areas except the contact hole 112 and, using this as a mask, a 5 nm (50Å) thick Ti film and a 50 nm (500Å) thick Au film are sequentially formed by vacuum deposition. Unnecessary portions of these films arc removed by lift-off and used to fill the contact hole 112 (Fig. 17H). Thus, an underwire 72, an insulator interlayer 111, an overwire 73, a pair of device electrodes 5 and 6 and a thin film 2 for an electron-emitting section are formed on an insulator substrate 1. Now, a display apparatus incorporating such an electron source will be described below by referring to Figs. 8, 9A and 9B. Firstly, the substrate 1 carrying thereon a large number of plane type surface-conduction electron-emitting devices is rigidly fitted onto a rear plate 81. Then, a face plate 86 (comprising a glass substrate 83 and a fluorescent film 84 and a metal back 85 arranged on the inner surface of the glass substrate 83) is arranged 5mm above the substrate 1 by way of a support frame 82 and frit glass is applied to the contact areas of the face place 82, the support frame and the rear plate 81 and burnt in ambient air atmosphere at 410°C for ten minutes to tightly bond them together (Fig. 8). The rear plate 81 is securely fitted to the substrate 1 also by means of frit glass. Note that reference numeral 74 in Fig. 8 denotes an electron-emitting region of the device of Fig. 1 and reference numerals 72 and 73 respectively designate X- and Y-wires connected to the pair of device electrodes of related surface-conduction type electron-emitting devices. The fluorescent film 84 is constituted only by fluorescent bodies if it is used for a monochrome display, whereas it comprises in this embodiment a number of stripe-shaped fluorescent bodies separated by black stripes of a popularly used black material containing graphite as a principal ingredient. The fluorescent stripes are formed on the glass substrate 83 by applying a fluorescent material in the form of slurry. An ordinary metal back 85 is arranged on the inner surface of the fluorescent film 84. It is prepared by smoothing the inner surface of the fluorescent film 84 (in an operation normally called "filming") and forming an A1 film thereon by vacuum deposition. While a transparent electrode (not shown) may be formed on the outer surface of the fluorescent film 84 in order to raise the conductivity of the fluorescent film 84, such a layer is not formed in this embodiment because the metal back 85 has a sufficiently high conductivity. Care should be taken to accurately align each set of color fluorescent bodies and an electron-emitting device, as a color display is involved, before the above listed components of the display apparatus are bonded together. The glass container prepared in a manner as described above and comprising a glass substrate 83 and other components is then evacuated by way of an exhaust pipe (not shown) and a vacuum pump to achieve a sufficient degree of vacuum in the container and then a voltage is applied to the device electrodes of the electron-emitting devices 74 by way of external terminals Doxl through Doxm and Doyl through Doyn to carry out a forming operation in order to produce an electron-emitting region out of the thin film for an electron-emitting region of each electron-emitting device. Fig. 4 shows the waveform of a pulse voltage to be used for a forming operation. In Fig. 4, T1 and T2 respectively indicate the pulse width and the distance separating adjacent pulses of a pulse voltage, which are respectively 1 millisecond and 10 milliseconds for this embodiment, while the peak level (peak voltage in the forming operation) of the voltage is 10V. The forming operation is conducted in a vacuum atmosphere of approximately 1,33322-10 -4 Pa (1x10 -6 Torr) for 60 seconds. The electron-emitting region prepared in a manner as described above contains fine particles made of palladium as a main element and having a mean particle size of 3 nm (30Å) that are dispersed throughout that section. Then, the exhaust pipe is heated by a gas burner until it is molten to hermetically seal the evacuated casing with a degree of vacuum of approximately 10 -6 . Finally, a getter operation is carried out by high frequency heating in order to maintain that degree of vacuum within the casing after it is sealed. An image-forming apparatus having a configuration as described above is operated by using signal generating means (not shown) and applying scan signals and modulation signals to the electron-emitting devices by way of the external terminals Dxl through Dxm and Dyl through Dyn to cause the electron-emitting devices to emit electrons. Meanwhile, 5kV is applied to the metal back 85 by way of high voltage terminal Hv to accelerate electron beams and cause them to collide with the fluorescent film 84, which by turn is energized to emit light to display intended images. In order to accurately understand the performance of a plane type surface-conduction electron-emitting device, an experiment was carried out, in which a sample of plane type surface-conduction electron-emitting device was prepared for comparison according to the same process as the electron-emitting device used in the above and tested for its properties by using a measuring apparatus provided with a normal vacuum system as shown in Fig. 3. Values same as those of a device according to the above were selected respectively for L1, W1, W2 and other variables shown in Fig. 1. For the test of the sample, the distance between the anode electrode and the electron-emitting device was 4mm and the anode voltage was 1kV, while the inside of the vacuum chamber of the gauging system was maintained to a degree of vacuum of 1.33322-10 -4 Pa (1x10 -6 Torr). The device voltage applied to the device was raised uniformly at a rate of approximately 1V/sec to increase monotoneously both device current If and electron emission current Ie. The device current If and the emission current Ie were measured while applying the device voltage to the device electrodes 5 and 6 of the sample for comparison to prove a current-voltage relationship illustrated in Fig. 5. (See Fig. 19). To the contrary, in a test using an electron-emitting device according to the above, the emission current Ie showed a rapid increase when the device voltage exceeded 8V and reached to 1.2µA when the device voltage was 14V, at which the device current If was 2.2mA so that an electron emission efficiency η (=Ie/Ifx100(%)) of 0.05% was obtained. Since a device changes its characteristics depending on the environmental factors including measuring and vacuum conditions, care was taken to carry out the experiment under same and constant conditions. (Embodiment 2) This example not being an embodiment of the claimed method is an electron source of an image-forming apparatus, which is realized by forming a number of step type surface-conduction electron-emitting devices on respective substrates and using a same material or a material containing a same element for all the device electrodes, the X-wires, the Y-wires and the connections connecting the device electrodes and the wires of the apparatus. This apparatus is characterized in that each electron-emitting device has an insulator interlayer which is laid between its X-wires and Y-wires and constitutes a raised section of the device. Since each electron-emitting device and related parts of the electron source have a plan view same as that of Fig. 14, it will not be described here any further. Fig. 20 shows a cross sectional view taken along line A-A' in Fig. 14. In Fig. 20, there are shown a substrate 1, an X-wire 72 (also referred to as overwire) that corresponds to Dxm in Fig. 7, a Y-wire 73 (also referred to as underwire) that corresponds to Dym in Fig. 7, a thin film 4 including an electron-emitting section, a pair of device electrodes 5 and 6 and an interlayer 111. This embodiment is prepared by following the steps described below and illustrated in Figs. 21A through 21F. Step a: A 500 nm (5,000Å) thick Pd layer is formed on a cleansed soda lime glass substrate and then photoresist (AZ 1370 available from HOECHST) is applied thereto by a spinner and baked. Then, the photoresist layer is exposed to light with a photomask arranged thereon and photochemically developed to produce a resist pattern for a Y-wire 73. Subsequently, the Pd film was etched to produce a Y-wire 73 and a device electrode 5 simultaneously (Fig. 21A). Step b: An insulator interlayer 111 of silicon oxide is formed to a thickness of 0.1µm by RF sputtering. Said interlayer is laid between an X-wire 72 and a Y-wire and serves as a raised section of the surface-conduction type standing electron-emitting device (Fig. 21B). Step c: A photoresist pattern 112 is formed on the silicon oxide film produced in step b for a step section 67 having a desired profile and an insulator interlayer 111 and then the insulator interlayer 111 is etched, using the photoresist pattern as a mask, to produce a raised section 67 with a desired profile and have the insulator interlayer 111 conform to the designed shape (Fig. 21C). RIE (Reactive Ion Etching) and CF 4 and H 2 gases are used for the etching operation in this step. Step d: Subsequently, another photoresist pattern is prepared (photoresist RD-2000N-41: available from Hitachi Chemical Co., Ltd.) for device electrodes 5 and 6 and a wire 75e and then a 100 nm (1,000Å) thick Pd is formed by vacuum deposition. The photoresist pattern is dissovled in an organic solvent and the Pd deposit film is lift-off to produce oppositely arranged device electrodes 5 and 6, which are separated by a distance equal to the thickness of the raised section 67 or 1.5µm. The device electrode shows a width W1 of 500µm. (Fig. 21D). Step e: Using a mask having an opening for the device electrodes 5 and 6 and their neighboring areas as in the case of Embodiment 1 above, a 100 nm (1,000Å) thick Cr film 121 is formed by vapor deposition and subsequently subjected to a patterning operation. Then, organic Pd (ccp 4230 available from Okuno Pharmaceutical Co., Ltd.) is applied thereon by means of a spinner and heated at 300°C for 10 minutes for baking. The formed thin fine particle film 2 which is made of fine particlas of Pd as a main element and used for producing an electron-emitting section has a thickness of 10 nm (100Å) and a sheet resistance of 5x10 4 Ω/cm 2 . Then, the Cr film 121 and the baked thin film 2 for an electron-emitting section are etched, using an acid etchant, to produce a desired pattern (Fig. 21E). Step f: An Ag-Pd conductor body is formed on the device electrode 6 to a thickness of approximately 10µm to form an X-wire 72 having a desired contour (Fig. 21F). Thus, an X-wire 72, an insulator interlayer 111, a Y-wire 73, a pair of device electrodes 5 and 6 and a thin film 2 for an electron-emitting section are formed on an insulator substrate 1. Then, a display apparatus incorporating such an electron source is formed in a manner similar to that of Embodiment 1. In order to accurately understand the performance of a step type surface-conduction electron-emitting device according to the above, an experiment was carried out, in which a sample of plane type surface-conduction electron-emitting device was prepared for comparison according to the same process as the electron-emitting device used in the above and tested for its properties by using a gauging apparatus provided with a normal vacuum system shown in Fig. 3 as in the case of Embodiment 1. Values same as those of a device according to the above were selected for the sample. The device current If and the emission current Ie were measured while applying the device voltage to the device electrodes 5 and 6 of the sample to obtain a current-voltage relationship illustrated in Fig. 5 (See Fig. 19). In test using an electron-emitting device according to the above, the emission current Ie showed a rapid increase when the device voltage exceeded 7.5V and reached to 1.2µA when the device voltage was 14V, at which the device current If was 2.2mA so that an electron emission efficiency (=Ie/If(%)) of 0.048% was obtained. An image-forming apparatus having a configuration as described above is operated by using signal generating means (not shown) and applying scan signals and modulation signals to the electron-emitting devices by way of the external termianls Dxl through Dxm and Dyl through Dyn to cause the electron-emitting devices to emit electrons. Meanwhile, 5kv is applied to the metal back 85 by way of high voltage terminal Hv to accelerate electron beams and cause them to collide with the fluorescent film 84, which by turn is energized to emit light to display intended images. (Embodiment 3) This example not being an embodiment of the claimed method is an electron source of an image-forming apparatus, which is realized by forming a number of plane type surface-conduction electron-emitting devices on respective substrates and insulator interlayers between respective X-wires and Y-wires, said insulator interlayers being found only on and near the crossings of the X- and Y-wires, connections for the X- and Y-wires and the corresponding device electrodes being electrically linked without using contact holes and arranged directly on the respective substrates. Fig. 22 shows a plan view of part of the embodiment of electron source. Fig. 23 illustrates a cross sectional view taken along line A-A' in Fig. 22. Note that same reference symbols are commonly used to respectively designate same components in Figs. 22 and 23. In Figs. 22 and 23, there are shown a substrate 1, an X-wire 72 (also referred to as overwire) that corresponds to Dmx in Fig. 7, a Y-wire 73 (also referred to as underwire) that corresponds to Dmy in Fig. 7, a thin film 4 including an electron-emitting region, a connection 76 and a pair of device electrodes 5 and 6. This embodiment is prepared by following the steps described below and illustrated in Figs. 24A through 24E. Step a: A silicon oxide film is formed on a cleansed Soda lime glass plate to a thickness of 0.5µm by sputtering to produce a substrate 1, on which a 5 nm (50Å) thick Cr layer and a 600 nm (6,000Å) thick Au layer are sequentially formed by vacuum deposition. Thereafter, photoresist (AZ 1370 available from HOECHST) is applied thereto by a spinner and baked. Then, the photoresist layer is exposed to light with a photomask arranged thereon and photochemically developed to produce a resist pattern for device electrodes 5 and 6, a connection 75 and a Y-wire 73. Subsequently, the Au and Cr deposit layer is wet-etched, using the resist pattern as a mask to produce device electrodes 5 and 6 (electrode width: 300µm, interelectrode distance: 2µm), a connection 75 and a Y-wire 73 simultaneously (Fig. 24A). Step b: An insulator interlayer 111 of silicon oxide to be arranged only on and near the crossing of a Y-wire 73 and an X-wire 72 is formed to a thickness of 0.1µm by RF sputtering (Fig. 24B). Step c: A photoresist pattern 112 for an insulator interlayer 111 to be arranged on and near the crossing of a Y-wire 73 and an X-wire 72 is formed on the silicon oxide film produced in Step b and the insulator interlayer 111 is etched, using the photoresist pattern as a mask, to produce an insulator interlayer 111 having a desired form (Fig. 24C). RIE (Reactive Ion Etching) and CF 4 and H 2 gases are used for the etching operation in this step. Step d: Subsequently, another photoresist pattern is prepared (photoresist RD-2000N-41: available from Hitachi Chemical Co., Ltd.) for an X-wire 72 and then Au was deposited thereon by vacuum deposition to a thickness of 500 nm (5,000Å). Thereafter, the photoresist pattern is dissolved in an organic solvent and the Au deposit film is lift-off to produce an X-wire 72 (Fig. 24D). Step e: Using a mask having an opening for the device electrodes 5 and 6 and their neighboring areas as in the case of Embodiment 1 above, a 100 nm (1,000Å) thick Cr film 121 is formed by vapor deposition and subsequently subjected to a patterning operation. Then, organic Pd (ccp 4230 available from Okuno Pharmaceutical Co., Ltd.) is applied thereon by means of a spinner and heated at 300°C for 10 minutes for backing. The formed thin fine particle film 2 which is made of fine particles of Pd as a main clement and used for producing an electron-emitting has a thickness of 7,5 nm (75Å) and a sheet resistance of 1x10 5 Ω/cm 2 . Then, the Cr film 121 and the baked thin film 2 for an electron-emitting region are etched, using an acid etchant, to produce a desired pattern (Fig. 24E). Thus, an underwire 72, an insulator interlayer 111, an overwire 72, a pair of device electrodes 5 and 6 and a thin film 2 for an electron-emitting region are formed on an insulator substrate 1. Then, a display apparatus incorporating such an electron source is formed in a manner similar to that of Embodiment 1. In order to accurately understand the performance of a plane type surface-conduction electron-emitting device according to the above, an experiment was carried out, in which a sample of plane type surface-conduction electron-emitting device was prepared for comparison according to the same process as the electron-emitting device used in the above and tested for its properties by using a gauging apparatus provided with a normal vacuum system shown in Fig. 3 as in the case of Embodiment 1. Values same as those of a device according to the above were selected for the sample. The device current If and the emission current Ie were measured while applying the device voltage to the device electrodes 5 and 6 of the sample to obtain a current-voltage relationship illustrated in Fig. 5. In a test using an electron-emitting device according to the above, the emission current Ie showed a rapid increase when the device voltage exceeded 7.0V and reached to 1.0µA when the device voltage was 14V, at which the device current If was 2.lmA so that an electron emission efficiency η (=Ie/If(%)) of 0.05% was obtained. An image-forming apparatus having a configuration as described above is operated by using signal generating means (not shown) and applying scan signals and modulation signals to the electron-emitting devices by way of the external terminals Dxl through Dxm and Dyl through Dyn to cause the electron-emitting devices to emit electrons. Meanwhile, a high voltage greater than several kV is applied to the metal back 85 by way of high voltage terminal Hv to accelerate electron beams and cause them to collide with the fluorescent film 84, which by turn is energized to emit light to display intended images. (Embodiment 4) This example not being an embodiment of the claimed method is an image-forming system comprising a pair of image-forming apparatuses as two units, for which electron sources are prepared by partly modifying the method of preparing an electron source of Embodiment 1 and to which the first and second drive methods are respectively applied. Otherwise, each unit of this embodiment has a configuration same as that of Embodiment 1 and hence can be manufactured in a way same as that of Embodiment 1. The forming operation and the operation of bonding together the face plate, the support frame and the rear plate to produce a casing for each unit are also same as their counterparts of Embodiment 1. It should be noted here, however, a pair of identical apparatuses are prepared at the same time for this embodiment. The casing of one of the prepared apparatuses is evacuated by means of an ordinary vacuum system to a degree of vacuum of approximately 1,33322-10 -4 Pa (10 -6 Torr) and then the exhaust pipe of the casing is heated and molten by a gas burner (not shown) to hermetically seal the casing. This apparatus is referred to herein as display panel A. On the other hand, the other apparatus is held by a pair of plate-shaped heat sources at the face and rear plates respectively and the entire apparatus was heated and baked at approximately 120°C for an hour. Then, the apparatus was evacuated by means of a super high vacuum system for ten hours while it is heated continuously. Subsequently, the exhaust pipe of the casing is heated and molten by a gas burner (not shown) to hermetically seal the casing. This apparatus is referred to herein as display panel B. Finally, both the display panels A and B are subjected to a getter process using a resistance heating technique in order to maintain an intended degree of vacuum after they are sealed. As described above, there are the following three features in basic characteristics of the electron emitting element of surface conduction type; first, the element produces the emission current Ie which is abruptly increases when an element voltage higher than a certain voltage (called a threshold voltage, Vth in Fig. 6), but which is little detected at a voltage lower than the threshold voltage Vth; namely, it is a non-linear element having the definite threshold voltage Vth with respect to the emission current Ie, secondly, the emission current Ie depends on the element voltage Vf and, therefore, it can be controlled with the element voltage Vf, and thirdly, emitted charges trapped by the anode electrode 34 depends on a period of time during which the element voltage Vf is applied; namely, an amount of charges trapped by the anode electrode 34 can be controlled with a period of time during which the elenent voltage Vf is applied, additionally, in the case according to the invention, both the element current If and the emission current Ie in the element has a monotonously increasing characteristic (called an MI characteristic) with respect to a voltage applied to a pair of element electrodes facing each other, electrons emitted from the electron emitting element of surface conduction type are controlled with the height and width of a pulse voltage applied between the element electrodes facing each other when the pulse voltage is higher than the threshold voltage. However, those electrons are little emitted when the pulse voltage is lower than the threshold voltage. Based on the above features, even for an array of numerous electron emitting elements, e.g., a device comprising plural electron emitting elements of surface conduction type which are each constituted by at least element electrodes and thin films inclusive of electron emitting regions and are arrayed in a matrix pattern on a base plate, the pairs of opposite element electrodes being respectively connected to m lines of row wirings and then n lines of column wirings laminated over the former wirings via insulating layers, a driving method which can select one of the electron emitting elements of surface conduction type and controlling an amount of electrons emitted therefrom in accordance with an input signal, by providing modulation means for producing a pulse having a height, a width, or a height and width depending on the input signal, and select means, which may be called scanning means, V for selecting the electron emitting element row successively one by one in accordance with the synch signal which is contained in the input signal. Thus, according to the novel construction and driving method based on the characteristics of an electron emitting element of surface conduction type, there is obtained a high-quality electron source which comprises numerous electron emitting elements of surface conduction type, and which can successively select the electron emitting elements and control an amount of emitted electrons in accordance with input signals by applying scan signals and modulation signals, both obtained from the input signals, to m lines of row wirings and n lines of column wirings one by one, respectively without using grid electrodes which have been essential in the prior art. Further, with the arrangement including pairs of opposite element electrodes in the electron emitting elements of surface conduction type, m lines of row wirings and n lines of column wirings, at least part of lines respectively connecting in parallel the pairs of opposite element electrodes in the electron emitting elements of surface conduction type the m lines of row wirings and the n lines of column wirings are partially or totally the same in their constituent elements. Therefore, particularly when a high temperature is applied during manufacture of the device, the problem of connecting between different kinds of metals is solved; hence the inexpensive and simple device structure can be provided with high reliability. Moreover, since insulating layers are present only in the vicinity of points where the m lines of row wirings and the n lines of column wirings cross each other, and a part or all of the insulating layers in the stepped portions of the vertical electron emitting elements of surface conduction type is manufactured by the same process, the manufacture method is simplified in such a point that the m lines of row wirings or the n lines of column wirings can be connected electrically to the elements without using contact holes. As a result, there can be provided an electron source and an image forming device which are inexpensive and simple in structure. According to another driving method, input signal dividing means for dividing input signals into plural groups of input signals is further provided, and plural rows (or columns) of the electron emitting elements of surface conduction type are selected and modulated in accordance with each group of divided plural input signals generated by the input signal dividing means, thereby providing a divided driving method. Therefore, a time allowed for each row (or column) of the electron emitting elements of surface conduction type can be increased; hence a driving IC and the electron emitting elements of surface conduction type can be designed with greater allowance. Further, according to that driving method, the row (or column) of the electron emitting elements adjacent to the row (or column) of the electron emitting elements being selected and modulated are maintained in a state under a constant potential applied. Therefore, no crosstalk occurs between electron beams emitted from the electron emitting elements on the image forming member to which the electron beams are irradiated. According to the electron source manufactured according to the present invention, since plural electron beams emitted from plural electron emitting portions in each electron emitting element of surface conduction type are superposed with each other, the electron beams can be controlled into a highly symmetrical shape on the electron irradiated surface. Also, by properly specifying the element array pitch in the Y-direction, it is possible to control superposition between the electron beams emitted from the electron emitting elements on the surface to which the electron beams are irradiated. As a result, there can be provided an electron source which can easily select those electron emitting elements from which electrons are to be emitted and also control an amount of the emitted electrons with a simple structure. The image forming device, e.g., the display, is a device for forming an image in accordance with input signals, the device comprising plural electron emitting elements of surface conduction type which are each constituted by at least element electrodes and thin films inclusive of electron emitting regions, are arrayed in a matrix pattern on a base plate corresponding to pixels making up an image, and the pairs of opposite element electrodes are respectively connected to m lines of row wirings and the n lines of column wirings laminated over the former wirings via insulating layers according to the input signal which is composed of synch signals and image signals, select means for selecting a desired row of the plural electron emitting elements of surface conduction type in accordance with the synch signals, and modulation means for producing modulation signals depending on the image signals and inputting the modulation signals to the electron emitting elements selected by the select means in accordance with the synch signals. Particularly, the image forming device includes fluorescent materials which are positioned in opposite relation to a base plate of the electron source and produce visible lights upon irradiation of electron beams. Preferably, the image forming device contains a vacuum therein and has such a feature that both the element current and the emission current in each electron emitting element of surface conduction type exhibits monotonously increasing characteristic (called an MI characteristic) with respect to a voltage applied to the pair of opposite element electrodes. Thus, according to the novel construction and driving method based on the characteristics of an electron emitting element of surface conduction type there is obtained a device which includes an electron source comprising numerous electron emitting elements of surface conduction type, which can successively select the electron emitting elements and control an amount of emitted electrons in accordance with input signals by applying scan signals and modulation signals, both obtained from the input signals, to m lines of row wirings and n lines of column wirings one by one, respectively, without using grid electrodes which have been essential in the prior art, and which can eliminate crosstalk between pixels, modulate display luminance with good control performance, and further enables display in finer gradations, making it possible to display a TV image with high quality, for example. Also, since the fluorescent materials are directly excited by the electron beams in a vacuum, those fluorescent substances in respective colors which are conventionally well known in the art of CRT and have superior luminescent characteristics, can be used as light emitting sources. It is therefore possible to easily realize color display and represent a large range of hues. Additionally, color display can be achieved just by separately coating the fluorescent materials respective colors, and the display panel can easily be manufactured. Since the voltages required for scan and modulation are small, electric circuits can easily be integrated. These advantages cooperatively make it possible to reduce a production cost and realize an extremely inexpensive display. As a result, there can be provided an image forming device such as a display which can emit lights with brightness selectively controlled and hence has high display quality. Further, with the arrangement including pairs of opposite element electrodes in the electron emitting elements of surface conduction type, m lines of row wirings and n lines of column wirings, at least part of lines respectively connecting in parallel the pairs of opposite element electrodes in the electron emitting elements of surface conduction type, the m lines of row wirings and the n lines of column wirings are partially or totally the same in their constituent members. The electron emitting elements of surface conduction type are formed on the base plate or the insulating layers. The insulating layers are present only in the vicinity of points where the m lines of row wirings and the n lines of column wirings cross each other, and a part or all of the insulating layers in the stepped portions of the vertical electron emitting elements of surface conduction type is of the same structure. Because of including the electron source having the above structural features, there can be provided an image forming device which is highly reliable, is inexpensive, and has a novel structure. According to another driving method adapted for the novel image forming device, input signal dividing means for dividing input signals into plural groups of input signal is further provided, and plural rows (or columns) of the electron emitting elements of surface conduction type are selected and modulated in accordance with each group of divided plural input signals generated by the input signal dividing means, thereby providing a divisional driving method. Therefore, a time allowed for each row (or column) of the electron emitting elements of surface conduction type can be increased; hence a driving IC and the electron emitting elements of surface conduction type can be designed with greater allowance. Further, according to that driving method, the row (or column) of the electron emitting elements adjacent to the row (or column) of the electron emitting elements being selected and modulated are maintained in a state under a constant potential applied. Therefore, no crosstalk occurs between electron beams emitted from the electron emitting elements on the image forming member. According to the image forming device, since plural electron beams emitted from plural electron emitting portions in each electron emitting element of surface conduction type are superposed with each other on the image forming member, a resulting luminescent bright spot can be controlled into a highly symmetrical shape. Also, by properly specifying the element array pitch in the Y-direction, it is possible to control superposition between the electron beams emitted from the electron emitting elements on the image forming member, with the result that a high-quality image corresponding to the input image can be presented. In addition, since the image forming device can use TV signals, signals from image input devices, image memories and computers, etc. as input signals, even a single unit can have functions of a display for TV broadcasting, a terminal for TV conferences, an image editor handling still and motion pictures, a computer terminal, an office automation terminal including a work processor, a game machine and so on; hence it can be applied to very wide industrial and domestic fields.

Claims: A method of manufacturing a display apparatus comprising the steps of producing an electron source adapted to emit electrons as a function of input signals by the steps of providing a substrate ( 1 ); laying on said substrate ( 1 ) a matrix of wires having m row wires ( 72 ) and n column wires ( 73 ) with an insulator layer interposed therebetween; and providing a plurality of surface-conduction electron-emitting devices ( 74 ) each having a pair of electrodes and a thin film including an electron-emitting region and arranged between the electrodes; arranging said plurality of surface-conduction electron-emitting devices ( 74 ) so as to form a matrix with one electrode of said pair of electrodes being connected to a respective row wire ( 72 ) and the other electrode being connected to a respective column wire ( 73 ); providing selection means connected to said m row wires (72), for selecting a row of the plurality of surface-conduction electron-emitting devices ( 74 ); and providing modulation means for generating modulation signals according to input signals, said modulation means applying said modulation signals to said surface-conduction electron-emitting devices ( 74 ) selected by said selection means such that an amount of electrons emitted therefrom is controlled, and wherein said manufacturing method includes the further step of enclosing the electron source in a casing, characterized by the step of baking each of said plurality of surface-conduction electron-emitting devices ( 74 ) constituting said electron source enclosed by said casing in an ultra high vacuum while the apparatus is heated, such that for each of said plurality of surface-conduction electron-emitting devices a characteristic ( Fig. 19 ) for a device current ( If ) and an electron emission current (Ie) both monotonously increasing as a function of the device voltage ( Vf ) applied thereto and exceeding a certain threshold voltage ( Vth ) is obtained. A method according to claim 1, wherein said step of baking is performed at a temperature of 80°C to 150°C. A method according to claim 1 or 2, wherein said step of baking is performed for a duration of 3 to 15 hours.

Inventors: ISONO AOJI, KANEKO TETSUYA, KASANUKI YUJI, KAWADE HISAAKI, MISHINA SHINYA, NAKAMURA NAOTO, NOMURA ICHIRO, OSADA YOSHIYUKI, SATO YASUE, SUZUKI HIDETOSHI, SUZUKI NORITAKE, TAKEDA TOSHIHIKO, TODOKORO YASUYUKI, TOSHIMA HIROAKI, YAMAGUCHI EIJI, YAMANOBE MASATO, ISONO, AOJI, KANEKO, TETSUYA, KASANUKI, YUJI, KAWADE, HISAAKI, MISHINA, SHINYA, NAKAMURA, NAOTO, NOMURA, ICHIRO, OSADA, YOSHIYUKI, SATO, YASUE, SUZUKI, HIDETOSHI, SUZUKI, NORITAKE, TAKEDA, TOSHIHIKO, TODOKORO, YASUYUKI, TOSHIMA, HIROAKI, YAMAGUCHI, EIJI, YAMANOBE, MASATO
Applicants: CANON KK, CANON KABUSHIKI KAISHA
Application year: 1993
Main classification: H01J31
IPC codes: H01J1, H01J31, G09G3