LATERAL FIELD EMITTER DEVICE AND METHOD OF MANUFACTURING SAME

Technical Field The invention relates to field emitters, and more particularly to lateral luminescent field emitter devices in flat panel displays.

Background Art Field enhancing structures such as conical tips, film edges, whiskers, etc. provide only a low current, high density electron beam. Such beams are not ideally suited for producing luminesence over a large area. Also these structures suffer from uniformity limitations. Triodes are not ideally suited to flat panel displays because they need precise alignment in two planes and the spacers are problematic. Furthermore, none of these

structures provides a broad area emission surface to direct electrons laterally to a luminescent anode located, as is desirable for a flat panel display.

Disclosure of Invention The present invention provides a means to direct electrons from a broad emission surface to a major portion of a surface of a laterally disposed anode. Also provided is a lateral field emitter device and a method for manufacturing such that the device has uniform field emission, has a large field enhancement factor, needs only a relatively small voltage for field emission to occur, and is suitable for a multi-color array of lateral, luminescent field emitter diodes for use in a full-color flat panel display using low voltage IC drivers. A feature of one of the embodiments is a method of manufacturing an entire array of lateral field emitter devices. The field emitter devices in the array may be of varied luminescent colors so that they may be used in a full-color, flat panel display. In order to make the array, the step of depositing the anode material is altered and repeated. The embodiments disclosed make use of thin film technology principles which have been known and studied for many years and published in several books. See, for example, Maissel and Glang, Handbook of Thin Film Technology. 1983 Reissue, McGraw-Hill Book Company. An advantage of some of the present embodiments is that they provide an efficient method of manufacturing a small gap between the anode and the cathode of a field emitter which does not need high resolution photoreduction. Another advantage is that they provide a method of manufacturing a field emitter which does not need accurately aligning elements of the device in two planes.

A still further advantage is a relatively low temperature method of manufacturing which can be performed without melting a glass substrate (melting point approximately 500 ^β C). A still further advantage is that they provide a low cost, high yield method for manufacturing an array of field emitter devices for large ^* area (10 inch diagonal or greater) flat panel displays on a single substrate using standard semiconductor technologies. These and other features and advantages of the disclosed embodiments will be further described and more readily apparent from a review of the detailed description which follows.

Brief Description of Drawings Fig. 1 shows a perspective view of a lateral field emitter device of the present invention having an anode and cathode disposed side by side. Fig. 2 shows a perspective view of a lateral field emitter device of the present invention having an interleaving, forked-shaped anode and cathode. Figs. 3A-3G show cross-sectional views of successive first stages of fabricating a field emitter device of the present invention. Figs. 4A-4D show cross-sectional views of successive second stages of fabricating a field emitter device of the present invention in accordance with a first embodiment. Figs. 5A-5F show cross-sectional views of successive second stages of fabricating a field emitter device of the present invention in accordance with a second embodiment. Figs. 6A-6C show enlarged top plan views of the field emitter device of the present invention with flat and serrated sidewalls defining the gap between the anode and the cathode. Figs. 7A-7C shown enlarged top plan views of a portion of the field emitter device of Figs. 6A-6C illustrating calculated lines of equal potential between the anode and the cathode.

Fig. 8 shows a cross-sectional view of the field emitter device of the present invention illustrating a stream of electrons and photons during operation. Fig. 9 shows an elevated perspective view of an array of the fork- shaped field emitter devices of Fig 2.

Best Mode for Carrying Out the Invention In the accompanying drawings similar elements are designated by the same reference numeral throughout the several views. Elements depicted are not always shown to scale. Referring to Fig. 1, there is shown a field emitter device 10. At the base of device 10 is a flat substrate 12. A portion of substrate 12 is covered by an electrically conductive layer 14. Disposed on conductive layer 14 is anode 16. Disposed on another portion of conductive layer 14 is electrically insulative layer 20, upon which is disposed cathode 22. Anode 16 has a flat side 24 which extends orthogonally upward from the top surface of substrate 12. Likewise, cathode 22 has a flat side 26 which extends orthogonally upward from the top surface of substrate 12. Side 24 of anode 16 is spaced from and parallel to corresponding side 26 of cathode 22. As a result, gap 30 extends above conductive layer 14 between anode side 24 and cathode side 26. As also seen, top surface 32 of anode 16 extends above top surface 34 of cathode 22, and bottom surface 36 of anode 16 extends below bottom surface 38 of cathode 22. Referring to Fig. 2, there is shown another embodiment of field emitter device 10 with an interleaving, fork-shaped anode and cathode configuration. In this embodiment, the length of anode legs 40, 42 and 44 exceeds the height of anode 16; likewise, the length of cathode legs 46 and 48 exceeds the height of cathode 22. Anode sides 24a, 24b, 24c, 24d, 24e, 24f, and 24g are spaced from and parallel to corresponding cathode sides 26a, 26b, 26c, 26d, 26e, 26f, and 26g. The corresponding anode sides 24a- 24g and cathode sides 26a-26g define gap 30 which is uniform and extends the entire length of anode sides 24a-24g.

Because of its lateral structure and the long uniform gap between the anode and the cathode, field emitter device 10 is well suited to provide pixel shapes for flat panel displays. The anode and cathode of Fig. 2 form a rectangle having 150 micrometer long sides 49a and 49b. Such a field emitter device has more than 50% luminescent surface area. The gap 30 may include only gases with ionization potentials of more than 10 volts - for example, air at normal atmospheric pressure. Accordingly, when an electrical potential of 10 volts is applied between the anode and cathode with such a gap the cathode emits electrons which will be imparted an energy level of 10 eV or less. Because the energy level imparted to the emitted electrons is below the ionization potential of the air there is no ionization of gases in the gap. Alternatively, if it is desired to operate the FED at a higher voltage to increase luminescence of the anode, the gap 30 may be evacuated to a pressure of 10 ^" °torr or less so that the gases in the gap have a density less than a predetermined critical density. The emission surface will then be preserved despite a small amount of ionization. Further details of the structure of field emitter device 10 will be described and depicted in the following methods for producing the lateral field emitter device.

First Embodiment Referring to Figs. 3A-3G, there are shown successive cross-sectional views depicting the first stages for partially fabricating field emitter device 10. These first stages are common to a first and second embodiment for fabricating device 10, with the final stages shown in Figs. 4A-4D and Figs. 5A-5F, respectively. With reference to Fig. 3A, substrate 12 is provided with conductive layer 14 thereon. Substrate 12 is preferably an insulator such as glass, silicon, or an appropriate metal, although other materials can be used provided they furnish a substantially flat, stable surface upon which a plurality of field effect devices can be fabricated. A continuous, 0.1

micrometer thick conductive layer 14 is disposed on the entire top surface of substrate 12 using thin or thick film deposition techniques. In the event luminescence from the bottom of device 10 is desired, substrate 12 and conductive layer 14 should each be transparent, in which case substrate 12 is preferably glass and conductive layer 14 is preferably indium tin oxide deposited by sputtering or evaporation. If, on the other hand, luminescence from the top of device 10 is desired then conductive layer 14 is preferably a reflective material such as aluminum. If only a single anode and cathode are desired then substrate 12 and conductive layer 14 may be the same conductive material. With reference to Fig. 3B, a several micrometer thick continuous layer of photoresist 50 is overlayed on conductive layer 14. Thinner photoresist masks are acceptable provided the film is continuous. With reference to Fig. 3C, photoresist 50 is patterned through standard lithographic techniques to form photoresist etch mask 52 containing a predetermined pattern of openings 54 thereby exposing portions 56 of conductive layer 14 while covering portions 58 of conductive layer 14. With reference to Fig. 3D, the exposed portions 56 (Fig. 3C) of conductive layer 14 are etched and removed thereby exposing the underlying portions 60 of substrate 12. A suitable dry or wet chemical etch can be used, as is conventional. With reference to Fig. 3E, photoresist etch mask 52 (Fig. 3D) is removed, such as by dissolving the mask in a solvent as is well known in the art, thereby exposing unetched portions 58 of conductive layer 14. With reference to Fig. 3F, anode material 62 is disposed on conductive layer 14. For instance, anode material 62 may be deposited as a layer 2 micrometers to 10 micrometers thick on the entire unetched portion 58 of conductive layer 14 without being deposited on the exposed portions 60 of substrate 12, for instance using thin or thick film techniques such as sputtering anode material 62 through a patterned metal mask (not shown). Anode material 62 may be a low energy conductive phosphor which emits

light upon bombardment by electrons, preferably with energy of approximately 400 electron-volts or less. (Herein, emitting light due to electron bombardment is referred to as "luminescing".) Suitable low energy phosphors include ZnO.Zn, ZnCd:Ag, and ZnS:Ag,Al. With reference to Fig. 3G, using the photolithographic method described in Figs. 3B and 3C above, a second etch mask, shown as photoresist etch mask 64, is overlayed on anode material 62. Etch mask 64 is patterned with openings 66 exposing portions 68 of anode material 62. For example, to obtain the fork-shaped anode of Fig. 2, mask 64 would be similarly patterned. It should be understood that the anode material and conductive layer are not normally etched and patterned in the same step since the conductive layer typically includes interconnecting portions outside the anode material. Furthermore, if a full-color flat panel display is desired then the steps in Figs. 3F and 3G can be performed in sequence three times to selectively pattern three different anode materials. This may include depositing a discontinuous layer of red, blue and green phosphor on different portions of the conductive layer. As mentioned above, Figs. 3A-3G provide the first stages for fabricating a field emitter device in accordance with a first and second embodiment. The second stages of the first embodiment are shown in Figs. 4A-4D; the second stages of the second embodiment are shown in Figs. 5A- 5F. The field emitter devices produced in the first and second embodiments have similar structures except the gap between the anode and the cathode in the second embodiment may be relatively smaller. With reference to Fig. 4A, the exposed portions 68 (Fig. 3G) of anode 62 are etched, preferably in wet chemicals, thereby exposing first portions 70 of conductive layer 14 directly beneath mask openings 66. In addition, the etch is a side-etching process that undercuts sides 24 of anode material 62 thereby forming anodes 16 beneath mask 64 and exposing second portions 74 conductive layer 14 beneath mask 64. Thus, second conductor otions 74 are adjacent to anode sides 24. Second conductor

portions 74 are preferably 0.8 micrometer to 1.2 micrometer in length and define the lateral location of gap 30 between anode 16 and cathode 22. With reference now to Fig. 4B, an electrically insulative film 80 is deposited on device 10, including the top surfaces of mask 64 and first portions 70 of conductive layer 14. However, insulative film 80 is not deposited on a substantial portion of anode sides 24 or second conductor portions 74, which remain "shielded" by mask 64. Preferably, essentially no portion of anode sides 24 or second conductor portions 74 are contacted by insulative film 80 and gap 30 remains substantially uniform. In addition, insulative film 80 should not provide step coverage or "bridges" between upper insulative film 82 on mask 64 and lower insulative film 84 on first conductor portions 70. Thus, insulative film portions 82 and 84 are spaced apart and separate from one another. As a result, sides 86 of lower insulative film 80 are spaced from anode sides 24 by gap 30. Preferably, insulative film 80 is approximately 1 micrometer thick and is deposited by a thin film physical vapor deposition technique such as sputtering or evaporation through a metal mask (not shown). Preferred materials for insulative film 80 include silicon dioxide (Siθ2) or silicon nitride (Si N^). With reference now to Fig. 4C, a layer of cathode material 90 is deposited on device 10, including the top surfaces of upper insulative film 82 and lower insulative film 84. As a result, cathodes 22 are disposed on lower insulative film 84. Cathode material 90 may be deposited by a thin film physical vapor deposition technique such as sputtering or evaporation through a metal mask (not shown). Suitable cathode materials include molybdenum, tungsten, diamond or cermet. The use of diamond as a cathode material is disclosed in U.S. patent nos. 5,199,918; 5,180,951; and 4,663,559. A diamond cathode material may be deposited, for instance, as disclosed in U.S. patent nos. 5,098,737 and 4,987,007. Preferably, the thickness of cathodes 22 is no more than approximately 10% of the thickness of lower insulative film 84, and the combined thickness of cathodes 22 and lower insulative film 84 is no more than 80% of the thickness of the anodes 16. Cathode material 90, like insulative film 80,

should not spread or provide step coverage so as to interfere with gap 30. That is, cathode material 90 is not deposited on a substantial portion of anode sides 24 or second conductor portions 74, which remain "shielded" by mask 64 and upper insulative layer 82. Preferably, essentially no portion of anode sides 24 or second conductor portions 74 are contacted by cathode material 90, cathode sides 26 and insulating film sides 86 are substantially aligned, and gap 30 remains substantially uniform. In addition, cathode material 90 should not spread or provide step coverage ("bridges") between the cathode material on mask 64 and the cathode material on first conductor portions 70. As is seen, upper and lower cathode material 92 and 94, respectively, are spaced and separate, and are deposited on upper and lower insulative film 82 and 84, respectively. In addition, sides 26 of cathodes 22 are spaced from anode sides 24 by gap 30. In this manner, a gap of approximately 1.0 micrometer is readily provided. With reference now to Fig. 4D, mask 64 is stripped and removed, for instance by dissolving mask 64 in a solvent. This "liftoff of mask 64 also removes upper insulative film 82 and upper cathode material 92 thereon. The completed field emitter device 10 thus includes cathodes 22 disposed on lower insulative layer 84 and laterally separated from anodes 16 by gap 30 extending from exposed portions of conductive layer 14 to top surface 34 of cathodes 22.

Second Embodiment Figs. 5A-5F show cross-sectional views of successive second stages of fabricating a field emitter device in accordance with a second embodiment. With reference now to Fig. 5A, the exposed portions 68 (Fig. 3G) of anode material 62 are etched away to form sides 24 of anodes 16 directly beneath openings 66. (Unlike the first embodiment, the etch need not and preferably does not undercut the sides of the anode material beneath the mask.) As a result, portions 70 of conductive layer 14 beneath mask openings 66 are exposed whereas the portions of layer 14 beneath mask 64

remain covered by anode material 62. Anisotropic dry etching is preferred to assure anode sides 24 correspond directly to mask openings 66. With reference now to Fig. 5B, a continuous layer of insulative film 80 is deposited over the entire device. (Unlike the first embodiment, insulative film 80 completely covers anode sides 24 and extends through openings 66.) As is seen, in the second embodiment insulative film 80 not only includes upper and lower insulative film 82 and 84, respectively, but also insulative film sidewalls 100 extending from conductive layer 14 to the top of mask 64. Thus, in sharp contrast to the first embodiment, insulative film 80 not only contacts anode sides 24 but completely covers anode sides 24. Therefore, in Fig. 5B insulative film 80 is preferably deposited by plasma enhanced chemical vapor deposition (as opposed to sputtering or evaporation as in Fig. 4B) to assure proper step coverage. With reference now to Fig. 5C, cathode material 90 is deposited on device 10. (Unlike the first embodiment, cathode material 90 does not correspond to openings 66 due to insulative film sidewalls 100 therein.) As may be seen, in the second embodiment cathode material 90 does not completely cover insulating film 80. That is, upper portion 102 of insulating film sidewalls 100 between upper cathode material 92 and lower cathode material 94 remains exposed. However, lower portion 104 of insulating film sidewalls 100 is sandwiched between anode sides 24 and cathode sides 26. Thus, in the second embodiment, lower insulating film sidewall portion 104 shall define gap 30. With reference now to Fig. 5D, upper insulating film sidewall portion 102 is removed, such as by wet chemical etching. Upper cathode material 92, however, covers and protects upper insulating film 82. Likewise, lower cathode material 94 covers and protects lower insulating film 84. Furthermore, little or none of lower insulating film sidewall portion 104 is removed in this step. With reference now to Fig. 5E, mask 64 is stripped and removed, for instance by dissolving the mask in a solvent. This liftoff step also removes layers 82 and 92 on mask 64. Such liftoff would be difficult or impossible if

upper insulating film sidewall 102 were to remain on device 10 since sidewall 102 would shield mask 64 from the etch as well as clamp mask 64 to device 10. With reference now to Fig. 5F, lower insulating film sidewall portion 104 is stripped and removed, such as by wet chemical etching, thereby creating gap 30 between anode sides 24 and cathode sides 26. It is understood that similar wet chemical etchants may be used in Figs. 5D and 5F. Preferably, cathode sides 26 and insulating film sides 86 are substantially aligned, and gap 30 remains substantially uniform extending from exposed portions of conductive layer 14 to top surface 34 of cathodes 22. Thus it may be seen that the completed field emitter device 10 fabricated in accordance with the first embodiment (Figs. 3A-3G and 4A- 4D) has a structure similar to the completed device fabricated in accordance with the second embodiment (Figs. 3A-3G and 5A-5F). However, the size of the gap 30 provided in the first embodiment (Fig. 4A) is determined by the depth of the undercutting beneath the mask, whereas the size of the gap 30 provided in the second embodiment (Fig. 5B) is determined by the thickness of vertical sidewalls 100 of insulative film 80. Therefore, the second embodiment may provide a smaller gap than the first embodiment, although the second embodiment uses more steps than the first embodiment to produce the finished field emitter device. It should therefore be appreciated that the above methods provide economical, high yielding manufacture of laterally disposed field emitter diodes which are well suited for use in flat panel displays. Multiple alignment steps are unnecessary. In addition, the small gap between the anode and cathode is provided without the need for precisely aligning the anode above the cathode. Figs. 6A-6C show enlarged top plan views of a portion of field emitter 10 in which anode sides 24 and cathode sides 26 assume various shapes. The shapes of sides 24 and 26 may be provided by appropriate patterning of mask 64 according to the step depicted in Fig. 3G. Figs. 7A-7C illustrate

calculated lines of equal electrical potential across a portion of gap 30 in Figs. 6A-6C, respectively. In Fig. 6A, there is shown an enlarged plan view of a portion of field emitter device 10 wherein anode sides 24 and cathode sides 26 are substantially flat (stripes). In Fig. 6B, anode sides 24 are flat but cathode sides 26 are serrated (wedge-shaped). In Fig. 6C, sides 24 and 26 are each serrated in a matching pattern. As is seen, gap 30 is substantially uniform in Figs. 6A and 6C, but is not substantially uniform in Fig. 6B. In Fig. 7A, equal potential lines 112 are uniformly spaced as is expected given the flat surfaces in Fig. 6A; however, in Fig. 7B and in Fig. 7C the equal potential lines 112 converge toward point 114, thereby increasing the concentration of the electric field along the serrated cathode sidewall 26 and increasing the field enhancement factor for device 10. An increased field enhancement factor may reduce or eliminate the need for a low work function cathode material thereby expanding the scope of cathode materials suitable. On the other hand, low work function materials such as diamond or cermets may provide suitable cathodes with flat surfaces. While an increased field enhancement factor is generally desirable, forming serrated sides 24 and/or 26 as compared to flat sides will typically need smaller photolithography resolution for mask 64. For example, photolithography resolution of 25 micrometers may be small enough for forming flat sides but too large for forming serrated sides. Referring now to Fig. 8, there is shown an enlarged cross-sectional view of device 10 during operation. As is seen, a stream of electrons "e" is emitted from cathode 22 across gap 30 to a major portion of surface 24 of anode 16. Furthermore, in this instance anode 16 is a phosphor, substrate 12 and conductive layer 14 are transparent, and a stream of photons "p" generated by anode 16 flows through conductive layer 14 and substrate 12, respectively, thereby producing luminescence from the bottom of device 10. Referring now to Fig. 9, there is shown an array 120 of the field emitter devices 10 having anodes 16 of varied phosphor materials, including ZnCd:Ag, a red phosphor 16a; ZnS:Ag,Al, a blue phosphor 16b;

and ZnO:Zn, a green phosphor 16c. Conductive lines 122 connect anodes 16 to bonding pads 124. Likewise conductive lines 126 connect cathodes 22 to bonding pads 128. Bonding pads 126 and 128 are adapted to be connected to other electronic components (e.g., low voltage IC drivers) in a flat panel display. Array 120 may itself define a single pixel which may be repeated in matrix organization to provide a full-color display. Since the present method does not need precise alignment in two planes to obtain a small gap between the anode and the cathode it is particularly well suited for producing large area flat panel displays. The present invention is therefore well adapted to carry out the objects and attain the ends and advantages mentioned, as well as others inherent therein.