Field emission microcathode arrays

Abstract: A field emission microcathode array comprises a substrate (1) over which a plurality of cones (2) each having a sharp tip (20) are formed and a gate electrode (30) having a plurality of openings each surrounding the tip (20) of a corresponding cone, the cones (2) being grouped into small blocks (4) each including several cones (2) and having a gate electrode portion (33). The gate electrode portions (33) of the small blocks (4) are independent of one another, each of the gate electrode portions (33) being connected to a power source through a lead electrode (31). In such a field emission microcathode array if a cone (2) of one small block (4) is locally short-circuited to its gate electrode portion (33), the lead electrode (31) is destroyed so that the other small blocks (4) can continue to operate normally.

Description: The present invention relates to field emission microcathode arrays for use, for example, in vacuum microdevices such as very small microwave vacuum tubes and display elements. 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 forming 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 50' comprises many field emission microcathodes 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 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', 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 30 µm. 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 2 µm. 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 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). A device of this kind is disclosed, for example, in US 4908539 and may be considered to include a plurality of electrodes, each of which electrodes projects from a main face of a substrate of the device; a gate electrode 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 electrodes; and power supply means for causing a predetermined potential difference to exist between the electrodes and the said gate electrode when the device is in use. In Figures 3(A) to (C) 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. The gate electrode 30 having gate openings (apertures) 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 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 microcathodes each having a diameter of several micrometers, of the array 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. If the tip 20 of the cone 2 is short-circuited to the electrode 30 due to conductive dust or broken chips of the cone, a critical problem may arise, preventing the emission of electrons for a corresponding pixel or over a display screen as a whole. It is desirable to solve or ameliorate this problem. Figure 4 shows examples of defects that can occur in a field emission microcathode array. If the tip of a cone formed between the cathode 6 and the gate electrode 30 is broken as in the case of a cone 2', no electron will be emitted to drive the emitter in question. If the shape of a cone is deformed as in the case of a cone 2", the cone will be short-circuited to the gate electrode 30 to equalize the potential of the gate and emitter. This causes all emitters to malfunction and causes an excessive current to flow through the gate electrode 30, thereby possibly destroying the array as a whole. An embodiment of the invention can provide a field emission microcathode array that assists in achieving continuous operation of the array as a whole even if a cone is locally short-circuited to a gate electrode. According to a first aspect of the present invention the electrodes include first and second arrays of the electrodes, and the said gate electrode includes a first gate electrode portion whose apertures are in register respectively with the electrodes of the first array and a second gate electrode portion whose apertures are in register respectively with the electrodes of the said second array ; and the said power supply means are connected between the electrodes of the first array and the said first gate electrode portion by way of a first fusible link, and are also connected between the electrodes of the second array and the said second gate electrode portion by way of a second fusible link. In such a device the electrodes (field emission microcathodes) for each pixel area are grouped into separate arrays or small blocks. Each of the arrays (small blocks) has a lead electrode (fusible link) for conducting electricity to the gate electrode portion of the array concerned. The lead electrode 31 serves as a fuse. If one of the electrodes in any one of the small blocks is short-circuited to the gate electrode portion the failure to produce electron emission will be limited to the small block in question. In this way, dividing the electrodes into small blocks corresponding to the different pixel areas increases redundancy and improves the reliability of, for example, a display unit employing a field emission mircrocathode array device embodying the invention. According to a second aspect of the present invention the electrodes include first and second arrays of the electrodes, and the said gate electrode includes a first gate electrode portion whose apertures are in register respectively with the electrodes of the first array and a second gate electrode portion whose apertures are in register respectively with the electrodes of the said second array; and the said first and second gate electrode portions themselves, or first and second current paths that respectively connect those portions to the said power supply means, have a high resistance; and the device has a wiring portion connected to the said power supply means and extending outside the areas where the said first and second gate electrode portions are provided, the said wiring portion being made of a material having an electrical resistance lower than that of the material from which the said first and second gate electrode portions are made. One type (hereinafter "cone-type") of field emission microcathode array device embodying the present invention comprises a substrate over which a plurality of cone-shaped electrodes each having a sharp tip are formed, and a gate electrode having a plurality of openings each surrounding the tip of a corresponding cone. The tip of each cone emits electron beams because of field emission. 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 field emission microcathode array; Figs. 3(A) to 3(C) are views showing a previously-considered field emission microcathode array; Fig. 4 is a sectional view showing parts of a previously-considered field emission microcathode array for illustrating problems thereof; Figs. 5 and 6 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; Figs. 7(A) to 7(C) are views showing a field emission microcathode array device embodying the aforesaid first aspect of the invention; Figs. 8(A) and 8(B) are views showing another such array device embodying the first aspect of the invention; Figs. 9(A) and 9(B) are views showing a field emission microcathode array device embodying the second aspect of the invention; Fig. 10 is a view showing parts of a field emission microcathode array device not embodying the present invention, for comparison with Fig. 9; Fig. 11(1) to 11(6) are views showing steps in a process suitable for fabricating a field emission microcathode device embodying the present invention; Fig. 12 is a view showing parts of a field emission microcathode array device not embodying the present invention; Fig. 13 is a view showing an example of structure of an emitter element suitable for use in a field emission microcathode array device; Fig. 14 is a view showing an example of an arrangement of a plurality of the Fig. 13 emitters forming an array; Fig. 15 is a view showing parts of a further field emission microcathode array device not embodying the invention; Fig. 16 is a view showing parts of an optical system of an optical printer; Fig. 17 is a view showing how a field emission microcathode array device may be included in an optical printer; Fig. 18 is a view showing in more detail parts of the printer of Fig. 17; Fig. 19 is a diagram illustrating circuitry for driving the printer of Fig. 18; Fig. 20 is a view showing parts of another field emission microcathode array device suitable for use in a printer; Fig. 21 is a view showing parts of a further field emission microcathode array device suitable for use in a printer; Fig. 22 is a view showing parts of yet another field emission microcathode device suitable for use ina printer; and Fig. 23 is a diagram illustrating circuitry for driving the printer of Fig. 22. Figures 7(A) to 7(C) show an embodiment according to the first aspect of the invention, in which Fig. 7(A) is a plan view showing one pixel area 5, Fig. 7(B) a sectional view along a line A-A of Fig. 7(A), and Fig. 7(C) a sectional view along a line B-B of Fig. 7(A). In the figures, numeral 4 denotes a small block (array) including a plurality of field emission microcathodes (electrodes), 5 a pixel area, and 31 a lead electrode (fusible link) for connecting, in each small block 4, an inner portion (a subelectrode portion) 33 to an outer portion (a main electrode portion) 34 of a gate electrode 30. Namely, the lead electrode 31 connects the subelectrode portion 33 (gate electrode portion) of a corresponding small block 4 to the main electrode portion 34 outside the small block 4. The same reference numerals as those shown in previous drawings represent like parts, and therefore, their explanations will not be repeated. In Figs. 7(A) to 7(C), substrate 1 is a glass plate of, for example, 1.1 mm thickness. A cathode 6 made of, for example, a Ta film having a thickness of 100 nm is formed by sputtering. An 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. 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). In Fig. 7(A), the pixel area 5 spreads for 100 x 100 µm and involves 9 x 9 = 81 cones 2, i.e., field emission microcathodes arranged in a matrix. The 81 cones are grouped into nine small blocks 4 each involving 3 x 3 = 9 cones. Each of the small blocks 4 is surrounded by a groove 32 formed on the gate electrode 30. The width of the groove 32 may be about 5 µm. At part of the groove 32, the lead electrode 31 having a width of 2 to 3 µm is formed to electrically connect the subelectrode and main electrode portions 33 and 34 to each other, the electrode portions 33 and 34 being located inside and outside the small block 4, respectively. If one small block 4 whose coordinates (X, Y) are (2, 1) has a short circuit between one of its field emission microcathodes (2) and its subelectrode portion 33, a current flowing through the subelectrode portion 33 collects at the lead electrode 31, which will be heated to a high temperature and fused off. As a result, the short-circuit failure is confined in the small block 4 (2, 1), and the remaining eight small blocks 4 continue to normally display the pixel area 5, thereby greatly improving the reliability of the display unit as a whole. According to the embodiment, the gate electrode 30 may be formed of a Cr film having a thickness of 150 nm, and the lead electrode 31 may be formed by photoetching the Cr film to a width of 2 µm. In this case, when the field emission microcathode array is subjected to a gate voltage of 80 V and a normal gate current of 1 µA and if any one of the cones 2 is short-circuited in any one small block 4, a large current of about 10 mA flows to fuse the lead electrode 31 of the small block 4 in question in a very short time. Figures 8(A) and 8(B) show another embodiment according to the first aspect of the invention, in which Fig. 8(A) is a plan view showing one small block 4, and Fig. 8(B) a sectional view along a line A-A of Fig. 8(A). According to the embodiment of Figs. 7(A) to 7(C), the lead electrode 31 is made of the same metal film as that of the gate electrode 30 including the subelectrode portions 33 and main electrode portion 34. According to the embodiment of Figs. 8(A) and 8(B), the small block 4 is completely surrounded and isolated by a groove 32 formed on a gate electrode 30. At part of the groove 32, a separate lead electrode 31 having a width of 5 to 10 µm is formed to connect the inside and outside portions of the gate electrode film 30 of the small block 4 to each other. Since the gate electrode 30 is made of high-melting metal such as Cr, Ta, and Mo, the width of the lead electrode 31 must be relatively narrow, for example 2 µm, when it is made of such a metal. To avoid this, this embodiment completely surrounds the small block 4 with the groove 32 formed on the gate electrode 30, and forms the relatively wide lead electrode 31 at part of the groove 32, with a low-melting conductor such as solder. In this case, the width of the lead electrode 31 may be expanded to 5 to 10 µm, and the lead electrode 31 may be designed to fuse at any desired current value. As explained above, each small block 4 of the field emission microcathode array has the lead electrode 31 for supplying a current to the gate electrode film 30. The lead electrode 31 serves as a fuse, and even if any one of the cones 2 in the small block 4 is short-circuited to the subelectrode portion 33, the failure of electron emission is confined in the small block 4 in question. In this way, dividing each pixel area into many small blocks 4 can greatly improve the redundancy and reliability of, for example, a display unit employing such a field emission microcathode array embodying the invention. In the embodiments described above, the independent subelectrode portion 33 is formed in each small block 4 of the cones 2. More precisely, the gate electrode 30 comprises the main electrode portion 34 connected to a proper power source and the subelectrode portions 33 disposed in the main electrode portion 34 and each having openings surrounding the tips of the cones, respectively. Each of the subelectrode portions 33 is isolated from the main electrode portion 34 by the groove 32. At part of the groove 32, however, the main electrode portion 34 and the subelectrode portion 33 are electrically connected to each other through the lead electrode 31 made of electrically conductive material. In an embodiment of the second aspect of the invention, instead of arranging a separate subelectrode portion (gate electrode portion) 33 for each of the small blocks (arrays) 4, a gate electrode film 41 (Fig. 9) made or high resistance material may be formed for each of the small blocks 4. More precissely, the gate electrode portions 41-1 etc disposed around the cones 2 axe made of high resistance material, and wiring 42 made of low resistance material is arranged around the gate electrode portions 41-1 etc. If any one cone 2 is short-circuited to its gate electrode portion 41, the small block (array) containing the short-circuited electrode portion 41 may stop operating because the cone 2 and gate electrode portion concerned are set to equal potential. The other small blocks (array), however, operate normally because the gate electrode portions 41 of the other small blocks are connected to the wiring 42 made of low resistance material. This arrangement, therefore, improves the redundancy and reliability of, for example, a display unit employing such a field emission microcathode array embodying the invention. This will be explained in detail with reference to Figs. 9(A) and 9(B), in which Fig. 9(A) is a plan view showing one pixel area 5, and Fig. 9(B) a sectional view along a line A-A of Fig. 9(A). The same reference numerals as those shown in previous figures represent like parts, and therefore, their explanations will not be repeated. In Figs. 9(A) and 9(B), numeral 4 is a small block (nine small blocks are arranged for each pixel area 50 involving a plurality of field emission microcathodes (electrodes), 5 the pixel area, and 40 a gate electrode. According to this embodiment, the gate electrode 40 for exciting electrons comprises a high resistance film 41 made of high resistance material and a low resistance film 42 made of low resistance material. The high resistance film 41 is disposed around the cones 2 in each small block 4, and the low resistance film 42 is disposed around the high-resistance film 41. The low resistance film 42 is electrically connected to the high resistance film 41 and serves as wiring for setting the high resistance film 41 at a predetermined potential. Numeral 43 denotes emitter wiring electrically connected to the cones 2. As mentioned before, the conventional field emission microcathode array causes the critical problem that, if a gate electrode is short-circuited to a cone, electron emission for a whole pixel is stopped. Figure 10 shows one method of solving this problem. In the figure, a high resistance film 9 serving as emitter wiring is arranged under cones 2. According to this arrangement, it is necessary to supply a large current to the emitter electrode 9 connected to the cones 2, to emit electrons from the cones 2. In other words, the emitter wiring 9 should preferably have low resistance. Arranging the high resistance film 9 under the cones 2 may solve the problem that occurs when the gate electrode is short-circuited to the cones, but it causes another problem of inferior electron emission from the cones 2. To form a large display, it is necessary to reduce the resistance of the emitter wiring. The arrangement of Fig. 10 is, however, contrary to this requirement of reducing the resistance of the emitter wiring. The embodiment of Fig. 9 avoids this other problem. In the figure, the high resistance film 41 is formed on the gate electrode 40 side. If any one of the cones 2 is short-circuited to the gate electrode 40 in, for example, a small block 4-1 of the field emission microcathode array, the cone 2 in question and the gate electrode 40 will be electrically connected, and the emitter wiring 43 of the small block 4-1 will be subjected to a voltage drop and disabled because the gate electrode 40 corresponding to the small block 4-1 is made of a high resistance film 41-1. The cones 2 of the other small blocks 4, however, are not affected by this failure and are able to continue to operate, thereby realizing high failure redundancy. Under normal operation with no short circuit occurring, substantially no current flows to the gate electrode 40 so that no voltage drop occurs even with the high resistance film 41 serving as the gate electrode 40, and therefore, power consumption will never increase. Namely, the function of the field emission microcathode array is not affected by the high resistance film. The above embodiments are only examples and do not limit the present invention. In other examples according to the invention, other materials, processes, and/or dimensions may be employed. The above embodiments relate to lead electrode arrangements for supplying power to a gate electrode film provided for each of the small blocks in a field emission microcathode array. In embodiments of the first aspect of the invention the lead electrode serves as a fuse, so that,even if a cone is short-circuited to the gate electrode, the failure of electron emission will be confined in the small block in question. In this way, dividing each pixel area into small blocks can improve the redundancy and reliability of a display unit employing the field emission microcathode arrav of the invention. In embodiments of the second aspect of the invention the gate electrode may be made of high resistance material. In this case, when a cone is short-circuited to the gate electrode, a small block containing the short-circuited gate electrode may be disabled because the cone and gate electrode are set to equal potential. Gate electrodes of the other small blocks, however, operate normally because they are connected to low resistance material wiring. This arrangement can also improve the redundancy and reliability of a display unit employing the field emission microcathode array of the invention. The field emission microcathode array according to the present invention may be made as described hereinafter. Figures 11(1) to 11(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. 11(1), an SiO 2 film 500 of uniform thickness is formed on a silicon substrate 1 by thermal oxidation. In Fig. 11(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. 11(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. 11(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. 11(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. 11(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. What is important is the distance between the gate electrode and the top 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 5 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. 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. An optimum radius of the gate opening is Rgo. If the actual size of any gate is larger or smaller than the optimum gate, it produces a very small emission current. Namely, a sufficient emission current will not be obtained if the radius of the gate opening is outside of the optimum value. Figure 6 shows a relationship between a gate voltage Vg and an emission current Ie with a change in the diameter of the gate opening being a parameter. In the figure, an ordinate represents the discharge current Ie, and an abscissa the gate voltage Vg. A curve (1) in Fig. 6 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 tip 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 indicated with a curve (3) of Fig. 6, or smaller as indicated with 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, 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 arrays having required characteristics deteriorates. Accordingly, the area and shape of each opening of the gate electrode in the field emission microcathode array 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 may increase and the production yield may decrease. Accordingly, it is desirable to provide a field emission microcathode array 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. More precisely, as described hereinbefore, the cone-type of field emission microcathode device comprises a substrate 1 on which cone-shaped electrodes 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. In this type of device the gate electrode openings 3 preferably have different sizes and are intermingled over the substrate to solve the above-mentioned problem. Another type of field emission microcathode device comprises a substrate on which elongate electrodes 4 each having a sharp edge 40 are formed, and groove-like gate electrode openings 5 surrounding each edge 40. The blade-like edge 40 emits electron beams because of field emission. In such a device the width of the gate electrode opening 5 preferably varies along the edge 4 to solve the above-mentioned problem. A plurality of the field emission microcathodes may be arranged in an array on the substrate. Figure 12 shows an arrangement of tips 20 of cones and gate electrode openings 3 that form a field emission microcathode array 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. 11(1) to 11(6). The sizes and intervals of the openings 3 are selected according to requirements. This fabrication process 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 disavantage can be written off by its cost saving effect. Figure 13 shows another structure of a field emission microcathode. In Fig. 1, each emitter (electrode) is conical, while in Fig. 13 an emitter (electrode) 4 is elongate and has a blade 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 14 shows one arrangement of gate electrode openings of a field emission microcathode array having such emitters. This figure simply shows edge blades 40 and gate electrode openings 5 that form a field emission microcathode array 50'b. Details of each cathode are the same as those of Fig. 13. This example emits electron beams in a wide area. The example of Fig. 14 can suffer from the same problem as that explained with reference to Fig. 6. To address this problem the width of the openings 5 may be made irregular along the blade 40 of the emitter edge (instead of constant as in Figs. 13 and 14). Optimum width portions of the opening 5 may self-selectively emit electrons. This is true for every edge so that electron beams are stably emitted from a large area. Figure 15 shows one example of such width variation intended to address the Fig. 6 problem. In this example, the width of an opening 5 is tapered along a blade 40 of each edge. At optimum width portions of the opening, electron beams are self-selectively emitted. The above width variation relates to an array of edge-form emitters. As explained above the arrangements shown in Figs. 12 and 15 effectively provide large-, middle-, and small-sized gate electrode openings 3 (5) and distribute them over the substrate. Even if the size of the openings fluctuate because of fabrication errors, some cones 2 or edges 4 with their gate openings having an optimum radius of Rgo may self-selectively emit electron beams. In this way, these arrangements can stably emit electron beams from a wide area or along a long line. Next, a printer employing a field emission microcathode array device will be explained. It will be understood that, although the field emission microcathode devices described hereinafter with reference to Figures 17 to 23 do not specifically embody the present invention, the figures concerned and the associated description will be of assistance in explaining how devices embodying the invention can be applied to such a printer. 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 an 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. 16. 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. 16 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. 17) at least comprising a field emission cathode type optical head 100 including a fluorescent dot array and field emission microcathodes for emitting electron beams toward 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 as it is turned on and off. The optical head 100 is formed of a field emission microcathode array device, having either cone-type or edge-type electrodes (field-emission microcathodes), which may be (although not specifically shown as such) a device embodying the present invention as described with reference to the preceding Figures. The optical head 100 including the field emission microcathodes and 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. These advantages are strengthened when a field emission microcathode array device embodying the present invention is used to constitute the optical head 100. For example, although not shown, the field emission cathode type head 100 may employ a field emission microcathode array device 50 described with reference to Figs. 12 or 15 having intermingled gate electrode openings 3 (5) of different sizes, to further improve the efficiency of the printer. Next, examples of optical printers employing a field emission microcathode array device of the cone- or edge-type will be explained. Figure 17 is a view showing an essential part of an optical printer employing such a device. Numeral 100 denotes a field emission cathode type optical head, 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 18 shows generally the application to a printer of such a field emission microcathode array device. Numeral 10 denotes a transparent substrate such as a glass substrate, and 12 denates 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. 17 and assembled with a control circuit, charger, developing unit, sheet feeding mechanism, etc., to form an optical printer. Figure 19 shows circuitry for driving the device of Fig. 18 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. 19 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 2 µm and a pitch between the tips 20 of the cones 4 µm, electron beams are selectively emitted when a selecting gate voltage Vg of 80 V 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 20 is a schematic view showing a field emission microcathode array 50 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 and is suitable for use in a printer of the above-described kind. Figure 21 is a schematic view showing a modified construction suitable for use in such a printer. A fluorescent dot 11 and a field emission microcathode array 50 are formed on the same plane. This modification improves light emission efficiency and is easy to fabricate because the two elements are formed on the same plane. The modification improves production yield and decreases cost. Figure 22 is a schematic view showing another example of the use of a field emission microcathode array device in a printer. The same reference numerals as those used for the previous embodiments 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 23 shows circuitry for driving the device of Fig. 22 The circuitry differs from the driving circuitry of Fig. 19 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 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 of the present invention 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 hereinbefore embodiments of the invention can provide an improved field emission microcathode array device involving emitter cones for emitting electrons. The emitter cones are grouped into small blocks so that, even if an electrode-to-electrode short circuit occurs in any one of the small blocks, the failure will be confined to the small block in question, thereby improving the reliability of the array as a whole. Generally a field emission cathode type optical head including field emission microcathodes and fluorescent dots can serve as a light source of an optical printer and makes the printer compact, and can provide low power consumption, a high degree of brightness, and a stable operation with no mechanically moving parts. The field emission microcathode array device embodying the present invention can serve to enhance these advantages of the optical head, simplify the structure, stabilize the performance and lower the cost of such a printer.

Claims: A field-emission microcathode array device including: a plurality of electrodes (2), each of which electrodes projects from a main face of a substrate (6) of the device; a gate electrode (30, 33) 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 electrodes (2); and power supply means (260) for causing a predetermined potential difference to exist between the electrodes (2) and the said gate electrode (30, 33) when the device is in use; characterised in that the electrodes (2) include first and second arrays (4) of the electrodes, and the said gate electrode (30, 33) includes a first gate electrode portion (33) whose apertures (3) are in register respectively with the electrodes of the first array (4) and a second gate electrode portion (33) whose apertures (3) are in register respectively with the electrodes of the said second array (4); and in that the said power supply means are connected between the electrodes of the first array and the said first gate electrode portion (33) by way of a first fusible link (31), and are also connected between the electrodes of the second array and the said second gate electrode portion (33) by way of a second fusible link (31). A device as claimed in claim 1, wherein the said gate electrode further includes a main electrode portion (30) connected to the said power supply means (260) and extending outside the areas where the said first and second gate electrode portions (33) are provided, each gate electrode portion (33) being isolated from the said main electrode portion (30) by a gap (32) extending around the gate electrode portion concerned between it and the said main electrode portion; the said first and second fusible links (31) connecting the main electrode portion (30) to the said first and second gate electrode portions (33) respectively. A device as claimed in claim 1 or 2, wherein the said main electrode portion (30) and the said first and second gate electrode portions (33) are formed on a common flat supporting layer (7). A device as claimed in claim 2, wherein the said first and second fusible links (31), the said main electrode portion (30) and the said first and second gate electrode portions (33) are all made of the same electrically-conductive material. A device as claimed in claim 4, wherein the said first and second fusible links (31) are each constituted by a narrow portion of the said electrically-conductive material bridging the said gap (32). A device as claimed in claim 1, 2 or 3, wherein the said first and second fusible links (31) are each made of an electrically-conductive material having a low melting temperature. A field-emission microcathode array device including: a plurality of electrodes (2), each of which electrodes projects from a main face of a substrate (43) of the device; a gate electrode (41, 42) 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 electrodes (2); and power supply means (260) for causing a predetermined potential difference to exist between the electrodes (2) and the said gate electrode (41, 42) when the device is in use; characterised in that the electrodes (2) include first and second arrays (4) of the electrodes, and the said gate electrode (41, 42) includes a first gate electrode portion (41) whose apertures (3) are in register respectively with the electrodes of the first array (4) and a second gate electrode portion (41) whose apertures are in register respectively with the electrodes of the said second array (4); and in that the said first and second gate electrode portions (33) themselves, or first and second current paths that respectively connect those portions to the said power supply means (260), have a high resistance; and in that the device has a wiring portion (42) connected to the said power supply means (260) and extending outside the areas where the said first and second gate electrode portions (41) are provided, the said wiring portion (42) being made of a material having an electrical resistance lower than that of the material from which the said first and second gate electrode portions (41) are made. A device as claimed in any preceding claim, wherein the electrodes (2) of the said plurality 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) . 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 as claimed in any preceding claim 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).

Inventors: BETSUI KEIICHI, FUKUTA SHIN YA, INOUE HIROSHI, BETSUI, KEIICHI, C/O FUJITSU LIMITED, FUKUTA, SHIN'YA, INOUE, HIROSHI, Fukuta, Shin'ya, c/o Fujitsu Limited, Inoue, Hiroshi, c/o Fujitsu Limited
Applicants: FUJITSU LTD, FUJITSU LIMITED
Application year: 1992
Main classification: H01J1
IPC codes: H01J3, H01J31, H01J1