Field emission microcathode array devices

Abstract: A field-emission microcathode array device (Fig. 7) includes an array of cone-shaped electrodes (2), each of which electrodes projects from a main face of a substrate (6) of the device, and also includes a gate electrode portion (30) arranged so as to be opposed to but spaced from the said main face and formed with apertures (3) that are in register respectively with the said electrodes (2). The production of the said apertures (3) in the said gate electrode portion (30) is controlled so that apertures (3a, 3b, 3c) of three different diameters are formed at preselected different respective locations. Another field-emission microcathode device (Fig. 10) includes an elongate electrode (4), which has a sharp linear edge (40), and the gate electrode portion (30) is formed with an aperture (5) that surrounds the said linear edge (40). In this case, the production of the aperture (5) is controlled so that its width varies along the length of the said edge (40) in a preselected manner. Such production techniques can improve the yield because the electrodes still emit electrons even if fabrication errors occur in the formation of the electrodes and apertures.

Description: The present invention relates to field emission microcathode array devices for use, for example, in vacuum microdevices such as very small microwave vacuum tubes and display elements. An example of one of said devices is given in US-A-4 908 539. Figures 1(A) and 1(B) of the accompanying drawings illustrate a structure of a field emission microcathode, Fig. 1(A) being a perspective view and Fig. 1(B) being a sectional view. In the figures, a substrate 1' is made of, for example, a semiconductor. A cone 2' serving as an emitter is formed on the substrate 1'. A tip 20' of the cone 2' is surrounded by a gate electrode 30. The substrate 1' is separated from the gate electrode 30 by a gate insulation film (not shown). A gate opening 3 is formed around the tip 20' of the cone 2'. Operational characteristics of this field emission microcathode are mainly determined by the radius Rg of the gate opening 3, the height Ht of the cone 2', and the thickness Hg of the gate insulation film. The semiconductor substrate 1' serves as a cathode electrode. This substrate may be made of insulation material and a cathode electrode made of a conductive film may be disposed between the substrate and the cone. Usually, these elements are made several micrometers or smaller in size by photolithography which is known in the field of semiconductor ICs. When a voltage is applied with the cone 2' being negative and the gate electrode 30 positive, the tip 20' of the cone 2' emits electrons. Namely, the cone 2' acts as a field emission microcathode. Although the example of Figs. 1(A) and 1(B) involves only one emitter cone, a plurality of cones may be arranged in an array on a single substrate. Figures 2(A) and 2(B) are examples of such a field emission microcathode array device for use in a display, Fig. 2(A) being a sectional view showing part of the display and Fig. 2(B) a diagram for explaining a method of driving the display. In the figures, the field emission microcathode array device 50' comprises many field emission microcathodes (electrodes) formed on a substrate 1'. The microcathodes may be arranged two-dimensionally, or in longitudinal and lateral rows to form an X-Y matrix on the substrate 1'. The field emission microcathode array device itself is already known. It may be made in sizes and pitches disclosed by the present inventors (Institute of Electronics, Information and Communication Engineers of Japan, Autumn National Convention, 1990, SC-8-2, 5-28-2). Opposite the field emission microcathode array 50' device, there is arranged a transparent substrate 10 made of, for example, glass. Anodes 12 are formed on the lower face of the substrate 10. Each of the anodes 12 is made of an ITO (In 2 O 3 -SnO 2 ) film having a thickness of 200 to 300 nm and an area of 100 x 100 µm. A pitch between the adjacent anodes 12 is about 30µm. On each of the anodes 12, a fluorescent dot 11 smaller than the anode 12 is disposed. The dot 11 is made of, for example, a ZnO:Zn film having a thickness of 2µm. Each dot 11 forms a pixel. The substrates 1' and 10 are spaced apart from each other by a distance of about 200 µm, to form a display panel 100. The display panel 100 is driven by a control circuit (an anode selection circuit) 200 shown in Fig. 2(B). The anode selection circuit 200 is connected to the anodes 12. A gate power source 260 applies a gate voltage so that the cones 2' simultaneously emit electrons, which are specifically attracted by a specific one of the anodes 12 that are selected by the anode selection circuit 200. The electrons attracted by the specific anode permit the fluorescent dot 11 on the anode 12 in question to emit light. In this way, the anode selection circuit 200 properly selects an optional anode 12, to which a positive potential is applied to allow the fluorescent dot 11 on the anode 12 in question to emit light, thus driving the display. Figures 3(A) to 3(C) show a previously-considered arrangement of a field emission microcathode array device, where Fig. 3(A) is a perspective view, Fig. 3(B) a partially enlarged view, and Fig. 3(C) a sectional view along a line X-X of Fig. 3(A). This device may be considered to include an array of electrodes, each of which electrodes projects from a main face of a substrate of the device, and also to include a gate electrode portion arranged so as to be opposed to but spaced from the said main face and formed with apertures that are in register respectively with the said electrodes. In the figures, the substrate 1 is made of glass. A cathode 6 is formed on the substrate 1, and an insulation film 7 is formed on the cathode 6. Many cones (electrodes) 2 are two-dimensionally formed in the insulation film 7. A gate electrode 30 having gate openings 3 is laminated such that each opening 3 surrounds a tip 20 of a corresponding cone 2, to thereby form a field emission microcathode array device 50'. In this example, the cones 2 are two-dimensionally arranged over the substrate 1. They may be arranged in longitudinal and lateral rows to form an X-Y matrix for each pixel (IEEE Trans. on Electron Device, Vol. 36, p. 225, 1989). The cone-shaped electrodes (microcathodes), each having a diameter of several micrometers, of the array device 50' may be arranged at intervals of several micrometers, so that several hundreds of microcathodes can be arranged for each pixel to form an area of about 100 x 100 µm. This produces a bright screen and provides good redundancy against unevenness in brightness caused by differences in the characteristics of individual microcathodes. To fabricate the device of Figs. 3(A) to 3(C), the substrate 1 is a glass plate of, for example, 1.1 mm thickness. The cathode 6 made of, for example, a Ta film having a thickness of 100 nm is formed by sputtering. The insulation film 7 made of, for example, an SiO 2 film of 1000 nm thickness is disposed over the cathode 6. On the cathode 6, there is formed the gate electrode 30 with a film of Cr, Ta, or Mo having a thickness of about 150 nm by a known method. The openings 3 are formed on the gate electrode film 30, and holes for cones are formed on the insulation film 7. Thereafter, Mo, for example, is obliquely deposited on the cathode 6 exposed at the bottoms of the holes, thereby forming cones 2 (J. Appl. Phys., Vol. 39, p. 3504, 1968). The centre of each opening 3 of the gate electrode 30 must correctly agree with the centre of the tip of a corresponding cone 2 according to a previously-considered fabrication method. What is important is the distance between the gate electrode and the tip of the cone. If the distance satisfies certain criteria, a sufficient emission current will be obtained. If the distance is not within the criteria, the emission current will be impractically low. Namely, the diameter of each gate opening or the distance between the tip of the cone and the gate electrode must be strictly controlled. Figure 4 explains this issue. When the diameter of the opening 3 of the gate electrode 30 is properly set, the emission current is desirably high. If the optimum condition is missed even slightly, the emission current becomes impractically low. Figure 5 shows a relationship between a gate voltage Vg and an emission current Ie for three different values of the gate opening diameter. In the figure, an ordinate represents the discharge current Ie, and an abscissa the gate voltage Vg. A curve (1) represents the characteristics of a field emission cathode with a middle-sized gate opening 3b, a curve (2) represents the characteristics of a field emission cathode with a small-sized gate opening 3, and a curve (3) represents the characteristics of a field emission cathode with a large-sized gate opening 3a. An optimum radius of the gate opening is Rgo. If the actual size of any gate opening is larger or smaller than the optimum size, it produces a very small emission current. Namely, a sufficient emission current will not be obtained if the radius of the gate opening is different from the optimum value. Accordingly, the area and shape of each opening of the gate electrode in the field emission microcathode array device must be strictly controlled during fabrication by precise designing and process control. Even under such strict control, the diameter of openings of the gate electrode may fluctuate for various reasons. In this case, the production costs of the microcathode array device may increase and the production yield may decrease. Accordingly, it is desirable to provide a field emission microcathode array device that is free from the above problems associated with the above-discussed production techniques, sufficiently demonstrates specified characteristics, and can be efficiently fabricated with a high production yield at a low cost. According to a first aspect of the present invention there is provided a field emission microcathode array device including an array of electrodes, each of which electrodes projects from a main face of a substrate of the device, and also including a gate electrode portion arranged so as to be opposed to but spaced from the said main face and formed with apertures that are in register respectively with the said electrodes; characterised in that the said apertures formed at preselected different respective locations differ from one another dimensionally in a preselected manner. In one embodiment the electrodes are in the form of cones, each having a base on the said main face and a sharp tip surrounded by one of the said apertures for emitting electrons by field emission when the device is in use; and the apertures are circular and are formed with at least two different diameters. In this embodiment, the gate electrode apertures that greatly influence electron beam emission characteristics are prepared in, for example, three sizes (large, middle, and small) and are intermingled. When such apertures having different sizes are intermingled, the large-sized gate apertures will have an optimum radius for field emission purposes if each aperture is inadvertently made with a reduced radius due to fabrication errors. On the other hand, if each aperture is inadvertently made with an increased radius due to fabrication errors, the small-sized gate apertures will have the optimum radius. According to a second aspect of the present invention there is provided a field emission microcathode device including an elongate electrode, which projects from a main face of a substrate of the device and has a sharp linear edge, and also including a gate electrode portion arranged so as to be opposed to but spaced from the said main face and formed with an aperture that surrounds the said linear edge of the said electrode; characterised in that the width of the aperture varies along the length of the said edge in a preselected manner. According to a third aspect of the present invention there is provided a field-emission microcathode array device, including a plurality of elongate electrodes arrayed over a main face of a substrate of the device, each electrode projecting from the said main face and having a sharp linear edge, and also including a gate electrode portion arranged so as to be opposed to but spaced from the said main face and being formed with a plurality of such apertures each surrounding the linear edge of a corresponding one of the electrodes, characterized in that the width of each aperture varies along the length of the edge it surrounds in a preselected manner. According to a fourth aspect of the present invention there is provided a method of producing a device embodying the aforesaid first aspect of the invention, including the steps of: forming a predetermined masking pattern on the said substrate, which pattern includes, at the said preselected different respective locations, aperture-defining portions that differ from one another dimensionally in a predetermined manner; etching exposed parts of the substrate to form a recessed substrate surface providing the said main face and having the said electrodes projecting therefrom at the said locations; forming a spacing layer on the said recessed substrate surface; forming an electrically-conductive layer on the spacing layer, the said aperture-defining portions protruding through the said electrically-conductive layer at the said locations; and etching away the said aperture-defining portions to form the said gate electrode portion in the said electrically-conductive layer, so that the said apertures that differ from one another dimensionally in a preselected manner are formed at the said preselected different respective locations. In all the above aspects of the invention, even if the actual sizes of some apertures deviate slightly from the intended optimum value for field emission purposes due to manufacturing errors, cones having apertures of the optimum radius, or optimum width portions of the elongate electrode as the case may be, made self-selectively emit electron beams. These arrangements can therefore ensure stable electron emission over a large area or along a long line in, for example, a display unit. Reference will now be made, by way of example, to the accompanying drawings, in which: Figs. 1(A) and 1(B) are respective perspective and sectional views of a field emission microcathode; Figs. 2(A) and 2(B) are respective sectional and general views showing a display including a field emission microcathode array device; Figs. 3(A) to 3(C) are views showing a previously-considered field emission microcathode array device; Figs. 4 and 5 are graphs illustrating relationships between the diameter of a gate electrode, an emission current and a gate voltage in a previously-considered field emission microcathode device; Figs. 6(1) to 6(6) are views showing steps in a process for fabricating a field emission microcathode array device; Fig. 7 is a view showing parts of a field emission microcathode array device according to a first embodiment of the present invention; Fig. 8 is a view showing a previously-considered structure of an emitter element of a field emission microcathode array device not embodying the present invention; Fig. 9 is a view showing an example of an arrangement of a plurality of the Fig. 8 emitters forming an array; Fig. 10 is a view showing parts of a further field emission microcathode array device according to a second embodiment of the invention; Fig. 11 is a view showing parts of an optical system of an optical printer; Fig. 12 is a view showing how a field emission microcathode array device embodying the present invention may be included in an optical printer; Fig. 13 is a view showing in more detail parts of the printer of Fig. 12; Fig. 14 is a diagram illustrating circuitry for driving the printer of Fig. 13; Fig. 15 is a view showing parts of another field emission microcathode array device suitable for use in a printer; Fig. 16 is a view showing parts of a further field emission microcathode array device suitable for use in a printer; Fig. 17 is a view showing parts of yet another printer including a field emission microcathode array device embodying the invention; and Fig. 18 is a diagram illustrating circuitry for driving the printer of Fig. 17. A field emission microcathode array device embodying the present invention may be made as described hereinafter. Figures 6(1) to 6(6) show examples of fabrication processes. These processes form a cold cathode cone by isotropic etching of a silicon substrate (Mat. Res. Soc. Symp., Vol. 76, p. 25, 1987). In Fig. 6(1), an SiO 2 film 500 of uniform thickness is formed on a silicon substrate 1 by thermal oxidation. In Fig. 6(2), the SiO 2 film 500 is etched by photolithography into a predetermined shape and size to form an SiO 2 mask pattern 500'. In Fig. 6(3), only silicon of the substrate is isotropically etched in a mixture of HF and HNO 3 , to form a cone 2 serving as an emitter under the SiO 2 mask pattern 500'. In Fig. 6(4), SiO 2 is deposited or sputtered over the processed substrate, to form an SiO 2 film 510 such that a space is formed around the cone 2. In Fig. 6 (5), a gate electrode film 310 made of, for example, Mo is uniformly formed. At this time, at least part of the side faces of the SiO 2 mask pattern 500' is exposed. In Fig. 6 (6), selective etching with HF is carried out to remove all of the SiO 2 mask pattern 500' and part of the SiO 2 film 510. As a result, an opening 3 is formed, and the cone 2 is exposed in the space. This completes the formation of a field emission microcathode on the silicon substrate. Although the above explanations relate to a single cathode, an array of cathodes can be formed on a substrate by employing a proper mask and photolithography technique. In the fabrication processes mentioned above, a positional relationship between each cone 2 and a corresponding opening 3 formed on the gate electrode 30 are very important. The tip of the cone 2 must agree with the centre of the opening 3. One problem in achieving such agreement is that the diameter or the width of a circular or rectangular gate electrode opening may fluctuate depending on fabrication conditions. This fluctuation is unavoidable even with strict designing. If the diameter of each opening 3 of the gate electrode 30 fluctuates, a required emission current may not be obtained. Referring again to Fig. 5, operational characteristics of the field emission microcathode are determined by the radius Rg of the gate electrode opening 3, the height Ht of the cone 2, and the thickness Hg of the gate insulation film. In the figure, an ordinate represents an emission current Ie, and an abscissa a gate voltage Vg. A curve (1) in Fig. 5 represents a typical example with the diameter of the opening 3 being equal to a required value (i.e. 2Rgo). When a voltage is applied and increased with the cone 2 being negative and the gate 30 positive, the top 20 of the cone 2 suddenly emits electrons at a certain threshold voltage. At an operational gate voltage of Vgo, an operational emission current of Ieo is obtained. If the diameter of any opening 3 of the gate electrode is larger than the required value as in the case of a curve (3) in Fig. 5, or smaller as in the case of a curve (2), an emission current obtained from the same gate voltage decreases significantly to an unacceptably low level. When fabricating the cones 2 for an array device of the kind shown in Fig. 3(A), the above problem may not be unduly serious when the number of cones 2 is small, because the height Ht of the cone 2 and the diameter 2Rg of the gate electrode opening 3 are each several micrometers or smaller. However, when forming many cones in a large area, or preparing a long linear edge, the above problem may arise in the processes of deposition, exposure, etching, etc. If the size of the gate electrode opening is larger or smaller than the optimum value, an emission current will be very small. Namely, a sufficient emission current is not obtained if the diameter of the gate electrode opening deviates from the optimum value. As a result, the production yield of field emission microcathode array devices having required characteristics deteriorates. A field emission microcathode array device according to a first embodiment of the invention comprises a substrate 1 on which cones 2 each having a sharp tip are formed, and gate electrode openings 3 each surrounding the tip 20 of a corresponding cone 2. The tip 20 of each cone 2 emits electron beams because of field emission. The gate electrode openings 3 have different sizes and are intermingled over the substrate. Another field emission microcathode array device, according to a second embodiment of the invention comprises a substrate on which an elongate electrode (wedge) 4 having a sharp blade-like edge (linear edge) 40 is formed, and a groove-like gate electrode opening 5 surrounding the edge 40. The blade-like edge 40 emits electron beams because of field emission. The width of the gate electrode opening 5 varies along the edge 40. A plurality of such field emission cathodes may be arranged in an array on the substrate. Figure 7 shows the first embodiment. For simplicity, this figure simply shows an arrangement of tips 20 of cones and gate electrode openings 3 that form a field emission microcathode array device 50a. The openings 3 have three sizes. Namely, they are classified into large-sized openings 3a, middle-sized openings 3b, and small-sized openings 3c that cyclically appear. This arrangement may be fabricated according to, for example, the processes explained with reference to Figs. 6 (1) to 6 (6). The sizes and intervals of the openings 3 are selected according to requirements. This embodiment positively forms the openings 3 having different sizes, which are selected based on a required size. It is preferable to prepare at least three opening sizes above and below the required size. It is possible to prepare more than three sizes. The openings 3 having different sizes may be randomly distributed or somewhat regularly arranged on the gate electrode 30. Even if some openings with one of the three sizes deviate from the required value, other openings with another size may agree with the required value, so that the field emission microcathode array as a whole will not be useless or rejected. Although this method may reduce the number of normally operating microcathodes to one third, this disadvantage can be cancelled out by its cost-saving effect. Figure 8 shows a previously-considered emitter structure for use in a field emission microcathode array device not embodying the present invention. Unlike the Fig. 1 emitter, which is conical, the Fig. 8 emitter (electrode) 4 is elongate and has a blade-like edge 40 which linearly emits electrons. Accordingly, a gate electrode opening 5 is shaped into a long thin groove having a width of 2Rg. This structure may be used for emitting a linear beam. Figure 9 shows one arrangement of gate electrode openings in a field emission microcathode array device having such emitters. For simplicity, this figure simply shows the blade-like edges 40 and gate electrode openings 5 of the field emission microcathode array device 50'b. Each electrode is the same as shown in Fig. 8. This example emits electron beams in a wide area. The example of Fig. 9 can suffer from the same problem as that explained with reference to Fig. 5. The second embodiment of the present invention, shown in Fig. 10, is intended to address this problem in the case of elongate electrodes. Figure 10 schematically shows edge blades 40 and gate electrode openings 5 used in a field emission microcathode array device 50b according to the second embodiment of the invention. According to the Fig. 10 embodiment, the width of each opening 5 is tapered along the length of the blade-like edge 40 of the corresponding electrode. At optimum width portions of the opening, electron beams are self-selectively emitted. Generally, by making the width of each opening 5 irregular along the length of the emitter edge 40, it can be ensured that optimum width portions of each opening 5 will self-selectively emit electrons. This is true for every electrode so that electron beams are stably emitted from a large area. The Fig. 10 embodiment relates to an array of emitter edges. The present invention is also applicable in another embodiment to a single long linear field emission cathode. As explained above, embodiments of the invention effectively provide large - middle -, and small-sized gate electrode openings 3 (5) and distribute them over the substrate. Even if the sizes of the openings fluctuate because of fabrication errors, some cones 2 or wedges 4 with their gate openings having an optimum spacing (optimum radius Rgo) may self-selectively emit electron beams. In this way, the embodiments can stably emit electron beams from a wide area or along a long line. Next, a printer employing a field emission microcathode array device embodying the present invention will be explained. Non-impact printers such as laser printers using optical line beams are in wide use these days. The laser printers require a device for guiding a light beam to many positions. Methods of guiding a light beam to many positions include a light beam scanning method and an optical array method. The optical array method arranges many light emitting elements such as laser diodes for corresponding optical points such as printing dots, respectively. The optical array method contributes to high-speed low-noise printing. The light beam scanning method scans an object with a light beam by rotating a light deflecting element such as a rotary polygon mirror and a hologram disk. This method is most widely used because it provides high resolution and a wide scanning angle. An example of an optical printer employing the light beam scanning method and a hologram will be roughly explained with reference to Fig. 11. A light source 610 such as a semiconductor laser emits a laser beam, which is converged by a convergent lens 604 such as a hologram lens into a predetermined diameter. At the same time, aberration of the beam is corrected. The beam is then made incident on a hologram 602 formed on a hologram disk 601. The hologram disk 601 is rotated by a motor 603. According to the rotation of the hologram disk 601, the incident beam is deflected by the hologram 602 in different directions. Accordingly, an outgoing beam 605 scans the surface of a photoconductor drum 300. Other devices such as a charger, developing unit, and sheet feeding mechanism necessary for forming the electrostatic recording optical printer are not shown in Fig. 11 for the sake of simplicity. On the other hand, the conventional optical array method for optical printers is inferior in brightness, resolution, and cost. The light beam scanning method mentioned above must employ a precision motor and fine rotation control mechanism for rotary elements such as the rotary polygon mirror and hologram disk, to meet high-quality printing requirements. This may increase the size and cost of the apparatus. These problems can be solved in an optical printer (Fig. 12) at least comprising a field emission cathode type optical head 100 including a fluorescent dot array and field emission microcathodes for emitting electron beams towards the fluorescent dot array, a control circuit 200 for turning on and off the optical head 100, and a photoconductor drum 300 having a photoconductor 301 on which a latent image is formed by the optical head 100 as it is turned on and off. The optical head 100 includes a field emission microcathode array device, having either cone-type or edge-type field emission microcathodes (electrodes), embodying the present invention. The use in the optical head 100 of a field emission microcathode array device embodying the present invention, together with the fluorescent elements, makes the optical printer compact, and provides low power consumption, a high degree of brightness, and a stable operation with no mechanically moving parts. Next, examples of optical printers employing a field emission microcathode array device of the cone- or edge-type, embodying the invention, will be explained. Although not shown specifically in the related Figures 12 to 18, the field emission microcathode array devices 50 used in the optical heads of these examples have intermingled gate electrode openings 3 (5) of different sizes, to further improve the efficiency of the printer. Figure 12 is a view showing an essential part of an optical printer employing such a device embodying the present invention. Numeral 100 denotes a field emission cathode type optical head, a 150 an array of lenses such as equal magnification erect lenses, 300 a photoconductor drum, and 301 a photoconductor. The optical head 100 comprises a fluorescent dot array (not shown) and a field emission microcathode array device (not shown) for emitting electron beams to the fluorescent dot array. The optical head 100 is turned on and off by a control circuit (not shown), and the lens array 150 forms a latent image on the photoconductor 301 such as a ZnO:Zn film coated around the photoconductor drum 300. Other devices such as a charger, developing unit, and sheet feeding mechanism necessary for the optical printer are not shown in the figure for the sake of simplicity, because these devices do not directly relate to the present invention. Figure 13 shows generally the application to a printer of a field emission microcathode array device embodying the present invention. Numeral 10 denotes a transparent substrate such as a glass substrate, and 12 denotes anodes formed on the transparent substrate 10. Each of the anodes 12 is made of, for example, an ITO (In 2 O 3 -SnO 2 ) film having a thickness of 200 to 300 nm and a size of about 50 µm. The anodes 12 correspond to printing dots and are arranged at pitches of about 70µm. On each of the anodes 12, there is arranged a fluorescent dot 11, which is smaller than the anode 12 and made of a ZnO:Zn film having a thickness of 2 µm. Numeral 50 denotes the field emission microcathode array device including its substrate 1. At predetermined dimensions and pitches, the array device 50 is fabricated according to, for example, a method disclosed by the present inventors (Institute of Electronics, Information and Communication Engineers of Japan, Autumn National Convention, 1990, SC-8-2, 5-28-2). The substrates 10 and 1 are spaced apart from each other by a distance of about 200 µm, to form a field emission cathode type head 100. This head is arranged as shown in Fig. 12 and assembled with a control circuit, charger, developing unit, sheet feeding mechanism, etc., to form an optical printer. Figure 14 shows circuitry for driving the device of Fig. 13. Numeral 30 denotes a gate electrode and 200 a control circuit for turning on and off the field emission cathode type optical head 100. In the Fig. 14 circuitry the control circuit 200 is a gate selection circuit. Numeral 250 denotes an anode power source, and 260 a gate power source. The control circuit 200 selectively applies a gate voltage provided by the gate power source 260 to a specific cone 2 whose tip 20 then emits electrons. The electrons are attracted by an anode 12 corresponding to the specific cone 2, the anode 12 being energized to positive potential by the anode power source 250. Accordingly, a fluorescent dot 11 formed on the anode 12 emits light. In this way, the control circuit 200 may properly select a gate 30 to which a gate voltage is applied, to thereby emit light from an optional fluorescent dot 11. In this device, each cone 2 serves as an emitter. With the diameter of each opening 3 being 2µm and a pitch between the tips 20 of the cones 4µm, electron beams are selectively emitted when a selecting gate voltage Vg of 80V and an anode voltage Va of 100 V are applied. The head, together with the control circuit 200, can provide a high performance optical printer that achieves greater brightness than a printer employing conventional optical accessing methods. Figure 15 is a schematic view showing parts of a printer having a field emission microcathode array device 50 embodying the invention that is arranged orthogonally to a fluorescent dot 11, so that electron beams may be emitted toward the fluorescent dot 11 from the side thereof. This construction improves light emission efficiency because the electron beams are not attenuated by the fluorescent dot 11. Figure 16 is a schematic view showing parts of a printer in which a fluorescent dot 11 and a field emission microcathode array device 50 embodying the invention are formed on the same plane. This arrangement improves light emission efficiently and is easy to fabricate because the two elements are formed on the same plane. The arrangement of Fig. 16 improves production yield and decreases cost. Figure 17 is a schematic view showing parts of another printer including a field emission microcathode array device embodying the invention. The same reference numerals as those used for the previous figures represent like parts. A field emission microcathode array 50 can be made very small by IC technology. For example, the tip of a cone 2 may have a size of about several micrometers. On the other hand, the size of a fluorescent dot 11 corresponding to a printing dot has a size of several tens to hundreds of micrometers. it is possible, therefore, to arrange many cones 2 for each fluorescent dot 11, as shown in the figure. This arrangement can increase the number of electron beams for irradiating each fluorescent dot 11 and improve the redundancy and reliability of the printer as a whole. Figure 18 shows circuitry for driving the device of Fig. 17. The circuitry differs from the driving circuitry of Fig. 14 in that a control circuit 200 serves not as a gate selection circuit but as an anode selection circuit. A gate voltage applied by a gate power source 260 causes electrons to be simultaneously emitted. The electrons are attracted by a specific anode 12 selected by the control circuit 200. The electrons then permit a fluorescent dot 11 on the anode 12 to emit light. The anode 12, to which positive potential is applied, is properly selected by the control circuit 200, so that light may be emitted from a required fluorescent dot 11. This device can provide a printer with greater performance and brightness compared with the conventional optical accessing methods. All of the above embodiments have been presented as examples, and the present invention is not limited to these embodiments except as defined in the appended claims. Other materials, processes, configurations may be employed to put the invention into effect. As described above, a field-emission microcathode array device embodying the invention includes a gate electrode having openings of different sizes to expand the operation margin. An optical printer may advantageously include a field-emission cathode type optical head that has field-emission microcathodes and fluorescent dots to serve as a light source of the printer, so as to make the printer compact, and provide low power consumption, a high degree of brightness, and a stable operation with no mechanically moving parts. A field emission microcathode array device of the cone or edge-type embodying the present invention can serve to enhance these advantages of the optical head, simplify the structure, stabilise the performance and lower the cost of such a printer.

Claims: A field emission microcathode array device including an array (4) of electrodes (2), each of which electrodes projects from a main face of a substrate (6) of the device, and also including a gate electrode portion (30) arranged so as to be opposed to but spaced from the said main face and formed with apertures (3) that are in register respectively with the said electrodes (2); characterised in that the said apertures (3a, 3b, 3c) formed at preselected different respective locations differ from one another dimensionally in a preselected manner. A device as claimed in claim 1, wherein: the electrodes (2) are in the form of cones, each having a base on the said main face and a sharp tip surrounded by one of the said apertures (3) for emitting electrons by field emission when the device is in use; and the apertures (3a, 3b, 3c) are circular and are formed with at least two different diameters. A device as claimed in claim 2, wherein the said apertures (3a, 3b, 3c) have at least three different diameter values, at least one diameter value being smaller than a predetermined optimum diameter value (2Rgo) for field emission, and at least one other diameter value being greater than that predetermined optimum diameter value. A field emission microcathode device including an elongate electrode (4), which projects from a main face of a substrate (6) of the device and has a sharp linear edge (4), and also including a gate electrode portion (30) arranged so as to be opposed to but spaced from the said main face and formed with an aperture (5) that surrounds the said linear edge (40) of the said electrode (4); characterised in that the width of the aperture (15) varies along the length of the said edge (40) in a preselected manner. A field emission microcathode array device, including a plurality of elongate electrodes (4) arrayed over a main face of a substrate (6) of the device, each electrode projecting from the said main face and having a sharp linear edge (40), and also including a gate electrode portion (30) arranged so as to be opposed to but spaced from the said main face and being formed with a plurality of such apertures (5) each surrounding the linear edge (40) of a corresponding one of the electrodes (4), characterized in that the width of each aperture (5) varies along the length of the edge (40) it surrounds in a preselected manner. A device as claimed in claim 4 or 5, wherein the or each aperture (5) tapers in width from one end to the other of the said edge (40) it surrounds. A method of producing a device as claimed in claim 1 including the steps of: forming a predetermined masking pattern on the said substrate, which pattern includes, at the said preselected different respective locations, aperture-defining portions (500') that differ from one another dimensionally in a predetermined manner; etching exposed parts of the substrate to form a recessed substrate surface providing the said main face and having the said electrodes (2) projecting therefrom at the said locations; forming a spacing layer (510) on the said recessed substrate surface; forming an electrically-conductive layer (310) on the spacing layer (510), the said aperture-defining portions (500') protruding through the said electrically-conductive layer at the said locations; and etching away the said aperture-defining portions (500') to form the said gate electrode portion (30) in the said electrically-conductive layer (310), so that the said apertures (3a, 3b, 3c) that differ from one another dimensionally in a preselected manner are formed at the said preselected different respective locations. A method as claimed in claim 7, wherein: the electrodes (2) are in the form of cones, each having a base on the said main face and a sharp tip surrounded by one of the said apertures (3) for emitting electrons by field emission when the device is in use; the apertures (3a, 3b, 3c) are circular; and the production of the apertures is controlled so that apertures of at least two different diameters are formed. A method as claimed in claim 8, wherein the production of the apertures is controlled to produce apertures (3a, 3b, 3c) having at least three different diameter values, at least one diameter value being smaller than a predetermined optimum diameter value (2Rgo) for field emission, and at least one other diameter value being greater than that predetermined optimum diameter value. A method of producing a device as claimed in claim 4 or 5, characterised in that the production of the or each said aperture (5) in the said gate electrode portion (30) is controlled so that the width of the aperture varies along the length of the said edge (40) it surrounds in a preselected manner. A printer comprising a photoconductor drum (300), having a photoconductor layer (301) on which a latent image can be formed by projecting light onto the layer, and also comprising an optical head (100) which faces the said photoconductor drum (300) when the printer is in use and which includes an array of fluorescent dots (11) and also includes a field emission microcathode array device, or a field emission microcathode device, arranged for selectively emitting an electron beam towards any one of the fluorescent dots (11) so as to cause the dot concerned to emit light onto the said photoconductor layer (301). A printer as claimed in claim 11, wherein the said field emission microcathode array device or the said field emission microcathode device, as the case may be, is a device as claimed in any one of claims 1 to 6.

Inventors: BETSUI KEIICHI, FUKUTA SHIN YA, INOUE HIROSHI, BETSUI, KEIICHI, FUKUTA, SHIN'YA, INOUE, HIROSHI
Applicants: FUJITSU LTD, FUJITSU LIMITED
Application year: 1992
Main classification: H01J9
IPC codes: H01J9, H01J31