Field emission cathodes

Abstract: Structures and methods of manufacture for field emission cathodes having cathode tips of minute size, whereby a block formed of pairs of substrates each having a patterned thin layer of cathode material sandwiched therebetween is sliced into a plurality of sections, to obtain array substrates each having an array of exposed regions of cathode material. A metal layer for constituting electron extraction electrodes and corresponding extraction apertures is formed over these exposed regions and appropriately shaped, after first forming mask layer portions upon the exposed cathode material regions.

Description: BACKGROUND OF THE INVENTION Field of Applicable Technology The present invention relates to structures for field emission cathodes of microtip configuration, functioning by cold-cathode electron emission, which can be formed as high-density arrays for use in such applications as matrixed flat panel display devices. Prior Art Technology When a field emission cathode is utilized as an electron source in a vacuum electronic device, it is necessary to generate an electric field strength of approximately 10⁶ volts/cm in order to achieve electron emission. However if such a field emission cathode is formed with a tip which has a radius of curvature of less than 10 µm, i.e. is formed with a sharply pointed tip, then the electrical field that is generated as a result of applying a voltage between that field emission cathode and a corresponding electron emission electrode in a vacuum will be concentrated at the tip of the cathode. As a result, cold-cathode electron emission can be achieved with a low level of drive voltage. In the following, an element formed as a combination of such a sharply pointed cathode member and an electron extraction electrode having an extraction aperture within which the tip of the cathode member is positioned, will be referred to as a field emission cathode. The microtip cathode member itself will be referred to simply as a cathode element. Such a field emission cathode has the following advantages, in addition to low-voltage operation: (1) A high level of current density is achieved. (2) Since it is not necessary to heat the cathode, the power consumption is very low. (3) The field emission cathode can be used as a point electron source. In the prior art, such field emission cathodes have been utilized, arranged in high element-density arrays, for example to implement a flat panel fluorescent display. This is described in the publication "Displays", P.37, January 1987. Prior art methods of manufacture of such field emission cathodes will be described in the following. One method is shown in Figs, 1A and lB. Here, an electrically conductive layer 102, an electrically insulating layer 103 and an electrically conductive layer 104 are successively deposited on an electrically insulating substrate 101, and an array of cavities 105 are formed in these superposed layers by using appropriate masks during the deposition process. Rotational evaporative deposition is then performed to deposit a suitable cathode material 106, with this rotational deposition being simultaneously executed both in a vertical direction towards the substrate and obliquely to the substrate. This results in portions 107 being formed at the upper openings of the cavities 105, and gradually closing these openings, while at the same time pyramid-shaped portions 108 of the cathode material become formed upon the electrically conductive layer 102 within each cavity 105. Lastly, as shown in Fig. 1(b), the portions 107 are removed. This method is described in the Journal of Applied Physics, Vol 39, P. 3504, 1968. A similar method for producing a thin-film field emission cathode having a molybdenum cone as a cathode is described in the Journal of Applied Physics, Vol 47, P. 5248, 1976. Another prior art method will be described referring to Figs. 2(a) to 2(f). With this method, a plurality of rectangular substrates 121 formed of an electrically insulating material are first prepared, then a film of cathode material is formed upon one face of each substrate 121. A plurality of the resultant cathode material-formed substrates 123 are then successively stacked together in a multilayer manner as shown in Fig. 2(a). The resultant multilayer block is then machined on its faces to obtain a multilayer substrate block 124. Next, as shown in Fig. 2(b), a metal layer 125 is formed by evaporative deposition upon a major face of this block 124, then as shown in Fig. 2(c), elongated slots 126, each having a length which is almost equal to the width of the block 124, are formed in the metal layer 125 by photo-etching. THese slots extend through the layer 125, to expose respective regions of the cathode material 122. The slots 126 serve as extraction electrode apertures. The cathode material-formed substrates 123 are then mutually separated, and as shown in Fig. 2(d), etching is performed on the cathode material 122 of each cathode material-formed substrates 123, to form a pattern of sharply pointed triangular portions 127. Appropriate chemical erosion is then selectively applied to the substrate 121 of each of the cathode material-formed substrates 123, to remove specific portions of the substrate 121, such that portions adjacent to each tip of a cathode material-formed substrates 123 is removed while in addition a portion of the substrate 121 adjacent to each extraction electrode aperture 126 is also removed. The cavities 128 are thereby formed in each cathode material-formed substrates 123, as shown in Fig. 2(e). The cathode material-formed substrates 123 are then once more successively stacked together in the same arrangement as that prior to being separated, and are mutually attached, to thereby form an array of field emission cathodes This method is described in Japanese Patent Laid-open No. 54-17551. However with the first of the above prior art methods, since it is necessary to execute rotational evaporative deposition of the cathode material both in a direction vertically above the cavities within which the microtip cathode elements are formed and also in an oblique direction, the manufacturing process is difficult. In the case of the second of the above prior art methods, in order to attain a high precision of aligning the electron extraction aperture 126 and the cathode regions 122, it is necessary to achieve a very high accuracy for the thickness of the substrate 121 and the film thickness of the cathode material thin film 122. In addition, it is necessary to position the sections of the multi-layer substrate block 124, when the block is finally re-assembled, in the respective mutual positions which the various sections had prior to being separated. However it is very difficult to achieve sufficient accuracy. SUMMARY OF THE INVENTION It is the objective of the present invention to provide a field emission cathode according to claims 1 and 6 whereby a high concentration of electric field can be easily achieved, and whereby the electron extraction efficiency can be high, and moreover whereby the withstanding voltage between a microtip cathode and a extraction electrode can be made high, while also providing high reliability. This object is achieved with a field emission cathode having the features of either claim 1 or claim 6. In one manufacturing process for a field emission cathode according to the present invention, elohgated parallel stripes of a layer of cathode material are formed on at least one electrically insulating substrate, another substrate is superposed on and attached to the first substrate, to sandwich the cathode material between the substrates, then the resultant block is sliced such as to obtain a plurality of blocks each having an array of exposed regions of cathode material on at least one face thereof. These exposed regions can then each be shaped to form a sharply pointed tip. Since the original cathode material layer can of course be made extremely thin and accurately formed, it becomes possible to form microtip cathodes having tips which are of extremely small size, with a high manufacturing yield. According to the invention, it is arranged that each strip of cathode material layer is enclosed within a layer of electrically insulating material, when sandwiched within such a superposed-layer block. After slicing, the resultant array substrate can be processed such as to leave a small portion of each cathode material layer portion protruding above the insulating material, as a microtip. Again, the dimensions of the cathode tip can be made extremely minute. More specifically, one embodiment of a field emission cathode structure according to the present invention is given in claim 1; another embodiment in claim 6 One method of manufacture of field effect cathodes according to the present invention comprises successive steps of: (a) forming a first metal layer upon a face of a first electrically insulating substrate; (b) forming a layer of cathode material upon the first metal layer; (c) forming a second metal layer upon the cathode material layer; (d) superposing a second electrically insulating substrate upon the face of the first substrate, to sandwich the cathode material layer between the first and second electrically insulating substrates, and mutually attaching the first and second electrically insulating substrates to obtain a superimposed substrate block; (e) slicing the superimposed substrate block in at least one plane which is perpendicular to the substrate face to thereby obtain at least one array substrate having on at least one face thereof; at least one exposed region of the cathode material layer enclosed by the metal layers (f) selectively forming a mask layer to cover only the exposed region on one face of the array substrate; (g) forming a third metal layer upon an upper surface of the mask layer and upon a region of the array substrate surrounding the exposed region; and (h) executing processing to remove the mask layer together with the third metal layer portions formed thereon, to thereby form at least one aperture functioning as an electron extraction aperture in the third metal layer surrounding the exposed region; (i) removing the first and second metal layers of the exposed region to a predetermined depth; and (j) forming a layer of electrically insulating material upon surfaces of the first and second metal layers within the exposed region. Another method of manufacture of field effect cathodes according to the present invention comprises successive steps of: (a) forming a first electrically insulating layer upon a face of a first electrically insulating substrate; (b) forming a layer of cathode material upon the first metal layer; (c) forming a second electrically insulating layer upon the cathode material layer; (d) superposing a second electrically insulating substrate upon the face of the first substrate, to sandwich the cathode material and electrically insulating layer layers between the first and second electrically insulating substrates, and mutually attaching the first and second electrically insulating substrates to obtain a superimposed substrate block; (e) slicing the superimposed substrate block in at least one plane which is perpendicular to the substrate face to thereby obtain at least one array substrate having, on at least one face thereof, at least one exposed region of the cathode material layer enclosed by the insulating layers ; (f) selectively forming a mask layer to cover only the exposed region on one face of the array substrate; (g) forming a metal layer upon an upper surface of the mask layer and upon a region of the array substrate surrounding the exposed region; and (h) executing processing to remove the mask layer together with the metal layer formed thereon, to thereby leave a portion of the metal layer to function as an electron extraction electrode and to form at least one aperture functioning as an electron extraction aperture in the metal layer surrounding the exposed region; and (i) removing the first and second insulating layers of the exposed region to a predetermined depth, to leave one end of the cathode material layer protruding above a surface of the insulating layers. BRIEF DESCRIPTION OF THE DRAWINGS Figs. 1(a) to 1(b) and 2(a) to 2(f) are diagrams for illustrating steps of manufacture of arrays of field emission cathodes according to the prior art; Fig. 3 is a partial cross-sectional view of an embodiment of a field emission cathode according to the present invention; Figs. 4(a) to (e) are oblique views to illustrate a method of manufacture for the embodiment of Fig. 3; Figs. 4(f) to (k) are cross-sectional views taken along the line II-II in Fig. 4(e); Figs. 5(a) to (d) are partial plan views showing three examples of patterns for a cathode material layer; Figs. 6(a) to (d) are plan views of Figs. 4(a) to (d); Fig. 7 is a partial oblique view of an example of a flat panel display unit which incorporates an array of field effect cathodes according to the present invention; Fig. 8 is a partial cross-sectional view of another embodiment of a field emission cathode according to the present invention; Fig. 9(a) through (f) are oblique views to illustrate a method of manufacture for the embodiment of Fig. 8; Figs. 9(g) to (k) are cross-sectional views taken along the line II-II in Fig. 9(f); Figs. 10(a) and 10(b) show a further example of a method of manufacture for the embodiment of Fig. 8, where Fig. 10(a) is a partial view in plan of a corresponding 1-dimensional array portion, and Fig. 10(b) is a partial view in plan showing the array of Fig. 10(a) with electron extraction electrodes removed; and Fig. 11(a) is a plan view of a 1-dimensional array, and Fig. 11(b) is a plan view showing the array of Fig. 11(a) with electron extraction electrodes removed. DESCRIPTION OF PREFERRED EMBODIMENTS Fig. 3 is a partial cross-sectional view of an embodiment of a field emission cathode according to the present invention. In Fig. 3, between two opposing vertical (as viewed in the drawing) faces of electrically insulating substrates 31 formed of a material such as glass or ceramic is formed a layer 32 of a metal such as.Al, or Ta, with a layer of electrically insulating material 33 vertically superposed thereon as shown. In the center of these layers 32 and 33 is formed a portion of a layer of cathode material 34 (formed of a material such as W, Mo, TiC, SiC, ZrC, or LaB₆) extending through the layers 32 and 33, elongated in a direction parallel to the aforementioned opposing substrate faces. The configuration of such a field emission cathode can be clearly understood from Fig. 7, which is an oblique view of a field emission cathode array used in a flat panel display unit. The upper surface of the insulating layer 33 is made lower than an upper surface of the substrates 31. The top surface of the cathode material layer portion 34 extends above the insulating layer 33, to be at substantially the same height as the upper surface of the substrates 31. The thickness of the portion 34 (as measured in a direction extending between the aforementioned vertical faces of the substrate 31) is made approximately 1·10⁻⁸ to 1·10⁻⁶m (100 Å to 1 µm.) The upper face of the substrates 31 has a patterned metal layer 35 formed thereon, constituting an electron extraction electrode for the field emission cathode. This metal layer is formed of a material such as Mo or Ta. If necessary, a patterned electrically conductive layer 36 can be formed on the face of the substrates 31 opposite to that on which the metal layer 35 is formed, with the layer 36 being in electrical contact with the cathode material 34. With this embodiment, due to the fact that the dimensions of the tip of the cathode material layer 34 can be made extremely small, a high concentration of electric field can be easily achieved. Thus highly effective extraction of electrons through the electron extraction aperture 37 can be obtained, even with only a low level of voltage being applied between the cathode material layer 34 and the electron extraction electrode 35. Furthermore, due to the fact that a gap and also the insulating layer 33 are disposed between the cathode material layer 34 and the metal layer 35, a high value of withstanding voltage between these, so that high reliability is attained. A method of manufacture for this embodiment will be described in the following. Figs. 4(a) to (k) show steps in this method. Figs. 4(a) to (e) are partial oblique views illustrating manufacturing steps. Figs. 4(f) to (k) are partial cross-sectional views taken along line II - II in Fig. 4(e), showing remaining steps in the manufacturing process. Figs. 6(a) to (d) are partial plan cross-sectional views corresponding to the steps of Fig. 4(a) to (d). The manufacturing process is as follows. Firstly, as shown in Fig. 4(a) and 6(a), an electrically insulating substrate 31 is formed from a material such as glass or alumina, and machined to a sufficient degree of flatness on surfaces thereof. Next as shown in Fig, 4(b), 6(b), a pattern of mutually parallel stripe portions of a first metal layer 32a (formed of a metal which can be readily oxidized to form an electrically insulating layer thereon, such as Al or Ta) are formed to a predetermined thickness (for example 0.5 to 1·10⁻⁶m (µm), on one face of the substrate 31. This stripe pattern of the first metal layer 32a is formed by a process such as evaporative deposition through a mask, or forming a metal layer over the entire surface of the substrate 31 by evaporative deposition or sputtering deposition, then executing photo-etching of the metal layer to form the stripe pattern. It should be noted that the embodiment is not limited to the use of such a stripe pattern for the first metal layer 32a, and that it would be equally possible to use some other suitable pattern, e.g. a grid pattern or a tooth pattern, etc, as shown in Figs. 5(a) and 5(b). The pattern is selected in accordance with specific requirements. As shown in Fig. 5(a), the grid pattern of cathode material 2' mentioned above consists of vertically extending (as seen in the drawing) narrow stripe portions 2a of the cathode material and horizontally extending frame (i.e. wide stripe) portions 2b, with the portions 2a and 2b mutually intersecting such that a set of short stripe portions 2'a extend horizontally at fixed spacings between each pair of the frame portions 2b. The grid pattern 2' of cathode material can be considered to consist of successive repetitions in the vertical direction (as seen in Fig. 5(a)) of a unit pattern, consisting of such a set of short stripe portions 2'a disposed at fixed spacings between a pair of the frame portions 2b. It is also possible to use other types of pattern for the cathode material layer 2′. For example as shown in Fig. 5(b), a tooth-shaped pattern can be formed, or as shown in Fig. 5(c) a pattern of parallel elongated stripes may be utilized. Alternatively as shown in Fig. 5(d), a "broken-line" pattern can be used. With the tooth-shaped pattern of Fig. 5(b), elongated narrow stripe portions 2'a are disposed mutually parallel at fixed spacings, while a wide frame portion 2b mutually links these stripe portions 2'a along the lower ends of these portions 2'a. WIth the stripe pattern of Fig. 5(c), a set of elongated stripe portions 2'a are disposed mutually parallel at fixed spacings. With the "broken-line" pattern of Fig. 5(d), unit patterns are successively formed each consisting of a set of short stripe portions 2'a which are arrayed at fixed spacings. The overall grid pattern of cathode material 2' consists of a plurality of these unit patterns, extending successively along the axial direction of the stripes, with the unit patterns being disposed at fixed spacings. Next, as shown in Fig. 4(c), 6(c), a layer of cathode material 34 consisting of a substance such as W, Mo, Ti C, Si C, is formed over each of the stripe portions of the first metal layer 32a, by a process such as mask evaporative deposition or CVD to a predetermined thickness (e.g. 1·10⁻⁸ to 1·10⁻⁶m (100 Å to 1 µm). The width of each cathode material layer 34 on each stripe portion of the first metal layer 32a is made identical to or slighly less than the width of the first metal layer 32a stripe. Next, as shown in Fig. 4(d), 6(d), stripe portions of a second metal layer 32b each of identical width to the stripes formed of the first metal layer 32a are respectively formed on each of the cathode material layer 34 stripes. The second metal layer 32b consists of the same material as the first metal layer 32a. A composite substrate 38 is thereby formed. A plurality of these composite substrates 38 are manufactured, and are then successively stacked together and mutually attached to form a single superposed-substrate block 39 as shown in Fig. 4(e). This superposition is executed such that each of the tri-layer combinations of a first metal layer 32a, cathode material layer 34 and second metal layer 32b is sandwiched between two of the substrates 31. In this superposing operation, surfaces that are brought into contact are made to mutually adhere, by utilizing a deposited adhesive material, or by thermal adhesion using a low melting-point glass frit, or by using a thermally resistant adhesive material. The substrates are thereby formed into a strongly solid block 39, which ensures that sufficient strength will be obtained in array substrates 40 that are produced as described hereinafter. Next, the block 39 is sliced along the lines A, B, C shown in Fig. 4(e), such as to transversely cut through the stripe portions of cathode material layer 34, perpendicular to the direction of elongation of these stripe portions. The resultant sections formed from the block 39 are then mechanically polished to thereby obtain the array substrates 40, one of which is shown in partial cross-sectional view in Fig. 4(f). This array substrate 40 has an array of cathode material layer 34 portions, which defines the field emission cathode array pattern, with exposed regions of these cathode material layer 34 portions appearing on each of opposing faces of the substrate. Each of these cathode material layer 34 portions is enclosed between metal layer 32a and 32b portions. Next as shown in Fig. 4(g), a pattern of a metal layer 41 is formed as a mask pattern on the array substrate 40, with respective portions of the metal layer 41 covering only the exposed regions of the cathode material layer 34 and metal layers 32a, 32b on one side of the substrate 40, to a predetermined thickness. Alternatively, if the substrate 31 is formed of an optically transparent material, a pattern of photoresist can be utilized to form this mask. In that case, a layer of photoresist is first coated over one face of the array substrate 40, then the opposite face of the substrate 40 is illuminated with ultra-violet radiation, and the portions of the photoresist that have been exposed to the radiation then developed and removed, to leave mask portions corresponding to the metal layer portions 41 of Fig. 4(g). After the mask portions have thus been formed, then as shown in Fig. 4(h), a patterned metal layer 35 consisting of a material such as W, Mo or Ta is formed by a process such as vacuum evaporative deposition over the mask portions 41 and the surrounding substrated surface, from a direction oriented vertically with respect to the substrate main faces. The mask portions 41 are then removed by etching using an appropriate etching material, to thereby also remove the metal layer 35 portions which have been formed thereon, and so form the electron extraction apertures 37. Further patterning of the metal layer 35 may be executed at this time, to appropriately mutually separate the electron extraction electrodes of different field emission cathodes, so that these electron extraction electrodes can be used as mutually independent modulation electrodes. Alternatively, the metal layer 35 may be deposited in step 4(h) in the form of a suitable pattern for interconnecting the electron extraction electrodes of specific field emission cathodes (e.g. as a parallel stripe pattern) for example as indicated in Fig. 7. Next, as shown in Fig. 4(j), the metal layers 32a, 32b which surround each cathode material layer 34 portion within an electron extraction aperture 37 are subjected to processing such as chemical etching, to be removed to a predetermined depth, for example to a depth of 1·10⁻⁸ to 5·10⁻⁶m (100 Å to 5µm), leaving the upper part of the corresponding cathode material layer 34 portion protruding above the metal layer portions by a predetermined length. It is necessary to select the material used for the metal layer 35 and for the cathode material layer 34 such that these materials will not be corroded during this etching process. Next, as shown in Fig. 4(k), the exposed surfaces of the metal layers 32a, 32b which have been etched in step 4(j) are subjected to processing such as anodic oxidation to form an electrically insulating layer thereon, formed of an oxide. The metal layers 32a, 32b are each preferably formed of Al or Ta, to enable this oxidation processing. If necessary, e.g. if it is required to mutually interconnect specific ones of the cathode material layer 34 portions, an electrically insulating layer can be formed of the array substrate 40 on the opposite face to that having the electron extraction electrodes formed, suitably patterned to achieve the desired interconnections. As shown in Fig. 7, an array of field effect cathodes produced as described above can be combined with a transparent substrate having a layer of photo-emissive material 14 formed on an inner face thereof, to form a flat panel display. With the above method of manufacture, simply by transversely slicing across a multi-substrate block formed of plural superposed electrically insulating substrates having patterned layers formed thereon as described above, an array substrate can be obtained upon which exposed surfaces of the cathode material are exposed, arranged in a desired array configuration. Furthermore as a result of selectively forming the mask portions 41 over respective ones of these exposed regions of cathode material and subsequently removing the mask material, the electron extraction apertures for the field emission cathodes are formed very simply, as a result of the removal of metal layer portions which lie upon the mask portions. This method enables accurate alignment of the electron extraction apertures 37 with the respective cathode material 34 portions, by a simple manufacturing process. With the method of manufacture described above, the first metal layer 32a is formed in a predetermined pattern. However it would be equally possible to form the metal layer 32a over an entire face of the substrate 31, and to then form a predetermined pattern of cathode material layer 34 upon the first metal layer 32a, and to then form the second metal layer 32b over the entire area. In addition, the method of manufacture given above has been described for the case of a 2-dimensional array being produced. However it would be equally possible to form a one-dimensional array. This can be done by forming a multi-substrate block in which it is arranged that each patterned cathode material layer is sandwiched between two electrically insulating substrates, i.e. by superposing a substrate which does not have a cathode material layer upon a substrate which has a cathode material layer, or by combining two substrates each having a patterned cathode material layer, such that the matching regions of the cathode material are brought into contact. In addition, it would be possible to form a 2-dimensional array by combining a plurality of such one-dimensional arrays. It should be noted that the above embodiment is not limited to forming point arrays of elements, but could also be applied to forming line arrays, or forming unit elements. With the above embodiment of a field emission cathode, the shape of the tip of cathode element is determined by the thickness of a layer of cathode material, so that the tip can be made extremely small. This enables a high concentration of electric field to be attained, so that the electron extraction efficiency is high. In addition, a gap and an electrically insulating layer are formed between the cathode element formed of the cathode material and the electron extraction electrode, so that there is a high value of withstanding voltage between these. Thus, high reliability is attained. Furthermore with the method of manufacture described above for that field emission cathode, the electron extraction aperture is formed by removal of a mask layer that has been formed over an array of exposed regions of the cathode material, with a metal layer that has been formed over the mask layer being also thereby removed. With this method, the manufacturing yield can be easily made high, and accurate alignment of the electron extraction apertures with the respective cathode material portions can easily be attained. Fig. 8 shows a partial cross-sectional view of another embodiment of a field emission cathode according to the present invention. In this embodiment, a layer 41 of an electrically insulating material such as Al₂O₃, SiO₂, or Si₃N₄ is formed between mutually opposing faces of electrically insulating substrates 31 formed of a material such as glass or ceramic. A layer of cathode material 34 (formed of a material such as W, Mo, TiC, SiC, ZrC, or LaB₆) is disposed centrally between the aforementioned opposing substrate faces, within the layer 41, elongated in a direction parallel to these opposing substrate faces. The upper surface of the insulating layer 33 is made lower than an upper surface of the substrates 31. The top surface of the cathode material layer portion 34 extends above the insulating layer 33, to be at substantially co-planar with the upper surface of the substrates 31. The thickness of the cathode material portion 34 (as measured in a direction perpendicular to the aforementioned opposing faces of the substrates 31) is made approximately 1·10⁻⁸ to 2·10⁻⁶m (100 Å to 2 µm). The upper face of the substrates 21 has a metal layer 35 formed thereon, to be used in forming an electron extraction electrode for the field emission cathode. This metal layer is formed of a material such as W, Mo or Ta. If a plurality of field emission cathodes as shown in Fig. 8 are to form an array, then a patterned electrically conductive layer 36 can be formed on the opposite face of the substrate 31 to that on which the metal layer 35 is formed, with the layer 36 being in electrical contact with the cathode material 34. With this embodiment, due to the fact that the dimensions of the tip of the cathode material layer 34 are determined by a film thickness, the tip size can be made extremely small, so that a high concentration of electric field can be easily achieved. Thus, effective extraction of electrons through the electron extraction aperture 37 can be obtained with only a low level of voltage being applied between the cathode material layer 34 and the electron extraction electrode 35. Furthermore, a gap and also the insulating layer 33 are disposed between the cathode material layer 34 and the metal layer 35, so that a high value of withstanding voltage between these is achieved, thereby ensuring high reliability. A method of manufacture for this embodiment will be described in the following. Figs. 9(a) to (k) show steps in this method. Figs. 9(a) to (f) are partial oblique views illustrating manufacturing steps. Figs. 9(g) to (k) are partial cross-sectional views showing further steps in the process, taken along line II - II in Fig. 9(f). As shown in Fig. 9(a), an electrically insulating substrate 31 is first prepared, formed of a material such as glass or alumina ceramic, and has surfaces thereof polished to a sufficient degree of flatness. Next, as shown in Fig. 9(b), a first insulating layer 41a is formed over substantially one entire face of the substrate 31. The first insulating layer 41a is formed of a material such as Al₂O₃, SiO₂, or Si₃N₄, and is formed to a predetermined thickness (e.g. 0.5 to 5·10⁻⁶m (µm)), by a process such as sputtering deposition or CVD. Next, as shown in Fig. 9(c), a patterned layer of a cathode material 34 is formed over the first insulating layer 41a, by a process such as sputtering deposition or CVD, to a predetermined thickness (e.g. 1·10⁻⁸ to 2·10⁻⁶m (100 Å to 2 µm)). In this example the cathode material layer 34 is patterned into parallel stripes, and is formed of a material such as W, Mo, TiC, SiC or ZrC. It should be noted that this embodiment is not limited to the use of a stripe pattern for the cathode material layer 34, and that it would be equally possible to use a grid pattern, a toothed pattern, etc, in accordance with requirements, and also to select the dimensions of the pattern in accordance with these requirements. The patterned cathode material layer 34 can be deposited by evaporative deposition through a mask, or by forming a layer of cathode material over the entire surface of the first insulating layer 41a by evaporative deposition or sputtering, then executing photo-etching. Next, as shown in Fig. 9(d), a second insulating layer 41b (consisting of the same material as the first insulating layer 41a) is formed over the cathode material layer 34 by a process such as sputtering or CVD. This second insulating layer 41b covers substantially the same area as the first insulating layer 41a, and has a thickness of approximately 0.5 to 5·10⁻⁶m (µm). A composite substrate 42 is thereby completed. A plurality of these composite substrates 42 are manufactured, then as shown in Fig. 9(e) these are successively superposed to form a solid multi-substrate block 44, such that each set of three layers 41a, 34 and 41b is sandwiched between two of the substrates 31. The composite substrates 42 of this block are mutually attached at attachment sections 43, by welding or by means of adhesive material such as low melting point frit glass, or by a heat-resistant adhesive material. The attachment sections 43 can be placed at various positions, in accordance with specific requirements. Next, as shown in Fig. 9(e), the block 44 is sliced along the lines A, B, C, ..... such as to transversely cut through the stripe portions of cathode material layer 34, perpendicular to the direction of elongation of these stripe portions. The resultant sections formed from the block 44 are then mechanically polished to thereby obtain the array substrates 45, one of which is shown in oblique view in Fig. 9(g). This array substrate 45 has an array of cathode material layer 34 portions, which defines the field emission cathode array pattern, with exposed regions of these cathode material layer 34 portions appearing on each of opposing faces of the substrate. Each of these cathode material layer 34 portions is enclosed between insulating layer 32a and 32b portions, Next, as shown in Fig. 9(g), a patterned mask layer 46 is selectively formed upon one side of the substrate 45, this mask layer consisting of a metal layer having a predetermined thickness, deposited by the usual electroplating process. The mask layer 46 is patterned such as to cover the exposed regions of the insulating layers 41a, 41b and the cathode material layer 34, and also to cover portions of the surface of the substrate 31 which are in the form of elongated strip regions which extend between the insulating layer 41a, 41b portions. Alternatively, if the substrate 31 is formed of an optically transparent material, a pattern of photoresist can be utilized to form the mask layer 46. In that case, a layer of photoresist is first coated over one face of the array substrate 45, then the opposite face of the substrate 45 is illuminated with ultra-violet radiation, and the portions of the photoresist that have been exposed to the radiation then developed and removed, to leave mask portions (46) corresponding to the metal layer portions (46) of Fig. 9(g). After the mask portions have thus been formed, then as shown in Fig. 9(h), an electrically conductive layer 35 for forming electron extraction electrodes, consisting of a material such as W, Mo or Ta is formed by a process such as vacuum evaporative deposition, sputtering deposition, or CVD over the mask portions 46 and the surrounding substrate surface. The mask portions 46 are then removed by etching using an appropriate etching material, to thereby at the same time remove the electrically conductive layer 35 portions which have been formed thereon, and so form electron extraction apertures 37. Next, as shown in Fig. 9(j), part of the insulating layers 41a, 41b which surround each cathode material layer 34 portion within an electron extraction aperture 37 are subjected to processing such as chemical etching, to be removed to a predetermined depth, for example to a depth of 1·10⁻⁸ to 5·10⁻⁶m (100 Å to 5 µm), leaving the upper part of the corresponding cathode material layer 34 portion protruding above the insulating layer portions by a predetermined length. It is necessary to select the material used for the metal layer 35 and for the cathode material layer 34 such that these materials will not be corroded during this etching process. For example if the insulating layers 41a, 41b each consist of Al₂O₃ or Si₃N₄, then phosphoric acid is a suitable etching medium. If on the other hand each of the insulating layers 41a, 41b is formed of SiO₂, then fluoric acid is a suitable etching medium. Suitable materials for the electron extraction electrode 35 and cathode material 34 are W, Mo, etc. If it is required to mutually interconnect specific ones of the cathode material layer 34 portions, an electrically insulating layer can be formed on the opposite face of the array substrate 45 to that having the electron extraction electrodes formed, suitably patterned to achieve the desired interconnections. A field emission cathode array formed by the above method of manufacture is suitable for combining with a transparent substrate having a layer of photo-emissive material 14 formed on an inner face thereof, to form a flat panel display. With the above method of manufacture, simply by transversely slicing across a multi-substrate block 44 formed of plural successively superposed substrates having patterned layers formed thereon as described above, an array substrate 45 can be obtained upon which exposed surfaces of the cathode material 34 are arranged in a desired array configuration. Furthermore as a result of selectively forming the mask portions 46 over respective ones of these exposed regions of cathode material and subsequently removing the mask material, the electron extraction apertures for the field emission cathodes can be formed by removal of electrically conductive layer portions which lie upon the mask portions. Thus, this method also enables accurate alignment of the electron extraction apertures 37 with the respective cathode material 34 portions, by a simple manufacturing process. Figs. 10(a) and 10(b) are diagrams for describing another method of manufacturing for the field emission cathode embodiment of Fig. 8. Fig. 10(a) is a plan view showing a one-dimensional array, while Fig. 10(b) is a plan view showing the one-dimensional array of Fig. 10(a) with an electron extraction electrode removed. With this embodiment, as shown in Fig. 10(a), 10(b), insulating layers 41a and 41b are formed as respective patterns of stripes which are wider than respective stripe-shaped layer portions of cathode material 34, rather than being formed as continuous layers as in the previous embodiment (as indicated in Figs. 9(b), 9(d)). Attachment sections 43 are provided between these stripe pattern portions, to mutually attach successive substrates to obtain a superposed-substrate block, as for the multi-substrate block 44 shown in Fig. 9(e). Apart from the above points, the remainder of this method of manufacture is identical to that of Figs. 9(a) to (d) described above. A cross-sectional view taking along line III - III in Fig. 10(a) corresponds to Fig. 8. Fig. 11(a) and (b) are plan views for illustrating another method of manufacture for the embodiment of Fig. 8, Fig. 11(a) shows a portion of an array substrate manufactured by this method, while Fig. 11(b) shows the array substrate of Fig. 11(a) without a metal layer for electron extraction electrodes. With this embodiment, the cathode material 34 is formed as a continous layer, between opposing continuous layers of insulating layer (41a, 41b), rather than being formed as a plurality of stripe layer portions as in the previous embodiment (as indicated in Fig. 9(c)). Apart from the above points, the remainder of this method of manufacture is identical to that of Figs. 9(a) to (d) described above. Structures and methods of manufacture for field emission cathodes having cathode tips of minute size, whereby a block formed of pairs of substrates each having a patterned thin layer of cathode material sandwiched therebetween is sliced into a plurality of sections, to obtain array substrates each having an array of exposed regions of cathode material. A metal layer for constituting electron extraction electrodes and corresponding extraction apertures is formed over these exposed regions and appropriately shaped, after first forming mask layer portions upon the exposed cathode material regions.

Claims: A field emission cathode comprising a pair of electrically insulating substrates (31) having at least respective upper faces thereof aligned in a common plane and with a gap formed between opposing side faces thereof, a first metal layer (32) formed within said gap, extending between said side faces, a layer of electrically insulating material (33) formed within said gap, extending between said side faces and in contact with the surface of said first metal layer (32) nearest said common plane and having a surface thereof recessed below said common plane, a layer of cathode material (34) formed extending substantially parallel to said side faces and positioned centrally between said side faces, extending within said first metal layer (32) and said layer of electrically insulating material (33), with one end of said layer of cathode material (34) protruding from said recessed surface of said electrically insulating layer (33),and a second metal layer (35) formed on said upper faces of said substrates (31) extending to said gap, to function as an electron extraction electrode. A field emission cathode according to claim 1, wherein the thickness of said layer of cathode material (34), as measured in a direction perpendicular to said side faces, is in a range of 1*10⁻⁸ m (100 A) to 1*10⁻⁶ m (1 µm). A field emission cathode according to claim 1, wherein said second metal layer (35) is formed of a material which is resistant to corrosion by predetermined etching liquids. A field emission cathode according to claim 1, wherein said first metal layer (32) is formed of a metal selected from a group which consists of Al and Ta. A field emission cathode according to claim 1, wherein said cathode material is selected from a group of materials which consists of W, Mo, TiC, SiC, ZrC, and LaB₆. A field emission cathode comprising a pair of electrically insulating substrates (31) having at least respective upper faces thereof aligned in a common plane and with a gap formed between opposing side faces thereof, a layer of electrically insulating material (41) formed within said gap, extending between said side faces, and having a surface thereof recessed below said common plane, a layer of cathode material (34) formed extending substantially parallel to said side faces and positioned centrally between said side faces, extending within said electrically insulating layer (41), with one end of said layer of cathode material (34) protruding from said recessed surface of said electrically insulating layer (41), and a metal layer (35) formed on said upper faces of said substrates (31) extending to said gap, to function as an electron extraction electrode. A field emission cathode according to claim 6, wherein the thickness of said layer of cathode material (34), as measured in a direction perpendicular to said side faces, is in a range of 1*10⁻⁸ m (100 A) to 1*10⁻⁶ m (1 µm). A field emission cathode according to claim 6, wherein said cathode material is selected from a group of materials which consists of W, Mo, TiC, SiC, ZrC, and LaB₆.

Inventors: KANAKO AKIRA, KANNO TORU, TOMII KAORU, KANEKO, AKIRA, KANNO, TORU, TOMII, KAORU, KANAKO, AKIRA
Applicants: MATSUSHITA ELECTRIC IND CO LTD, MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 1989
Main classification: H01J9
IPC codes: H01J1, H01J9