Patent Grant
7025892
1. Field of Use
This invention relates to methods of forming gated filament cathode structures for a field emission display, and more particularly, to methods using particle tracking to define gate apertures, spacers as etch masks to form filaments between top planes and bottom planes of associated gate structures, and using the gate structures to define where the filament tip will be.
2. Description of the Related Art
Field emission displays include a faceplate, a backplate and connecting walls around the periphery of the faceplate and backplate, forming a sealed vacuum envelope. In some field emission displays, the envelope is held at vacuum pressure, which can be about 1×10−7 torr or less. The interior surface of the faceplate is coated with light emissive elements, such as phosphor or phosphor patterns, which define an active region of the display. Field emission cathodes, such as cones and filaments, are located adjacent to the backplate. Application of an appropriate voltage at the extraction electrode releases electrons which are accelerated toward the phosphors on the faceplate. The accelerated electrons strike their targeted phosphors, causing the phosphors to emit light seen by the viewer at the exterior of the faceplate. Emitted electrons for each of the sets of emitters are intended to strike only certain targeted phosphors.
Uniform illumination each phosphor is desirable if not necessary and requires, (i) a uniform phosphor, and (ii) provide a uniform number of electrons to each phosphor, thus requiring uniform electron (emitter) sources.
A variety of methods for forming field emitters are known.
U.S. Pat. No. 3,655,241 discloses fabricating field emitters using a screen with arrays of circular or square openings that is placed above a substrate electrode. A deposition is performed simultaneously from two sources. One of the sources consists of an emitter-forming metal, such as molybdenum, and atoms are deposited in a direction perpendicular to the substrate electrode. The other source consists of a closure material, such as a molybdenum-alumina composite. Atoms of the closure material are caused to impinge on the screen at a small angle to the substrate. The closure material progressively closes the openings in the screen. Thus the emitter-forming metal is deposited in the shape of cones or pyramids, depending on whether the screen openings are circular or square.
Another method of creating field emitters is disclosed in U.S. Pat. No. 5,164,632. Part of an aluminum plate is anodically oxidized to create a thin alumina layer having pores that extend nearly all the way through the alumina. An electrolytic technique is used to fill the pores with gold for the field emitters. An address line is formed over the filled pores along the alumina side of the structure, after which the remaining aluminum and part of the adjoining alumina are removed along the opposite side of the structure to re-expose the gold in the pores. Part of the re-exposed gold is removed during an ion-milling process utilized to sharpen the field emitters. Gold is then evaporatively deposited onto the alumina and partly into the pores to form the gate electrode.
Field emitters are fabricated in U.S. Pat. No. 5,150,192 by creating openings partway through a substrate by etching through a mask formed on the bottom of the substrate. Metal is deposited along the walls of the openings and along the lower substrate surface. A portion of the thickness of the substrate is removed along the upper surface. A gate electrode is then formed by a deposition/planarization procedure. Cavities are provided along the upper substrate surface after which the hollow metal portions in the openings are sharpened to complete the field emitter structures.
However, large area field emission displays require a relatively strong substrate for supporting the field emitters extending across the large emitter area. The requisite substrate thickness is typically several hundred microns to 10 mm or more.
The fabrication methods in U.S. Pat. Nos. 5,164,632 and 5,150,192 make it very difficult to attach the field emitters to the substrates of thickness required for large area displays.
In U.S. Pat. No. 4,940,916, a gated area field emitter consists of cones formed on a highly resistive layer that overlies a highly conductive layer situated on an electrically insulating supporting structure. For a thickness of 0.1 to 1microns, the highly resistive layer has a resistivity of 104 to 105 ohm-cm. The resistive layer limits the currents through the electron-emissive cones so as to protect the field emitter from breakdown and short circuits.
It is desirable to have uniformity of emission from the cathodes. A field emission cathode relies on there being a very strong electric field at the surface of a filament or generally on the surface of the cathode. Creation of the strong field is dependent on, (i) the sharpness of the cathode tip and (ii) the proximity of the extraction electrode (gate) and the cathode. Application of the voltage between these two electrodes produces the strong electric field. Emission nonuniformity is related to the nonuniformity in the relative positions of the emitter tip and the gate. Emission nonuniformity can also result from differences in the sharpness of the emitting tips.
Busta, “Vacuum Microelectronics-1992,” J. Micromech. Microeng., Vol. 2, 1992 pp. 43-74 provides a general review of field-emission devices. Among other things, Busta discusses Utsumi, “Keynote Address, Vacuum Microelectronics: What's New and Exciting,” IEEE Trans. Elect. Dev., Oct. 1990, pp. 2276-2283, who suggests that a filament with a rounded end is the best shape for a field emitter. Also of interest is Fischer et al., “Production and Use of Nuclear Tracks: Imprinting Structure on Solids,” Rev. Mod. Phys., Oct. 1983, pp. 907-948, which deals with the use of charged-particle tracks in manufacturing field emitters according to a replica technique.
A well collimated source of evaporant, as taught in U.S. Pat. No. 3,655,241, is necessary in order to obtain uniformity of cone or filament formation across the entire field emission display. In order to maintain a collimated source, the majority of evaporant is deposited on interior surfaces of the evaporation equipment. The combination of the expensive of the evaporation equipment, and the wastage of evaporant, is undesirable for commercial manufacturing and is compounded as the size of the display increases.
It would be desirable to provide a commercial manufacturing process suitable for large area field emission displays. The commercial viability of this process is due to (i) use of electroplating, in combination with a self alignment method that accommodates for nonuniformities and (ii) the use of spacers as both etch masks and as a part of a mold for plating filament structures.
Accordingly, it is an object of the invention to provide a commercial manufacturing process for forming filaments in a large field emission display.
Another object of the invention is to provide a commercial manufacturing process for forming filaments in a large field emission display using electroplating in combination with self alignment in order to accommodate for nonuniformities.
Still a further object of the invention is to provide a method for forming filaments in a field emission display which uses spacers as an etch mask and as part of the mold for plating the filament structures.
A further object of the invention is to provide a method of making filament structures in a field emission display using particle tracking to define gate apertures, and spacer technology, along with reactive ion etching, to define the pore where the filament is formed.
Another object of the invention is to provide a method for forming filaments in a field emission display which uses the gate to define where the filament tip will be.
Another object of the invention is to provide a method of forming filaments for a field emission display that uses the gate to define the geometry of the filament tip.
These and other objects of the invention are achieved in a method for creating gated filament structures in a field emission display. A multi-layer structure is provided that includes a substrate, an insulating layer and a metal gate layer positioned on at least a portion of a top surface of the insulating layer. For purposes of definition, an insulating substrate is, (i) a conductive or semi-conductive substrate with an insulating layer on a top surface of the substrate, (ii) a conductive or semi-conductive substrate with patterned insulating regions on a top surface of the substrate or (iii) an insulating substrate. A plurality of patterned gates are provided and define a plurality of gate apertures on the top surface of the insulating layer. The patterned gates can be part of the initial multi-layer structure, or formed thereafter. A plurality of spacers are formed in the gate apertures at edges of the patterned gates on the top surface of the insulating layer. The spacers are used as masks for etching the insulating layer and forming a plurality of pores in the insulating layer. The pores are plated with a filament material that extends from the pores, into the gate apertures, and creates a plurality of filaments. The spacers are then removed. Further, the multi-layer structure can include a conductivity layer on at least a portion of a top surface of the substrate.
In another embodiment of the invention, the multi-layer structure further includes, a metal row electrode positioned on the insulating substrate, a resistive layer at least partially positioned on a top surface of the metal row electrode, with the insulating layer positioned on a top surface of the resistive layer, and a tracking resist layer positioned on a top surface of the metal gate layer.
In a further embodiment, a method of creating a plurality of pores in an insulating layer of a field emission display is provided. A multi-layer structure is provided that includes a substrate, an insulating layer and a metal gate layer positioned on at least a portion of a top surface of the insulating layer. A plurality of patterned gates are also provided to define a plurality of gate apertures on the top of the insulating layer. A plurality of spacers are formed in the gate apertures at edges of the patterned gates on the top surface of the insulating layer. The spacers are utilized as masks for etching the insulating layer and forming a plurality of pores in the insulating layer.
Available options for forming the gate apertures include irradiation of the multi-layer structure with energetic charged particles to produce a plurality of tracks in the tracking layer. A plurality of patterned gates are formed which define a plurality of gate apertures on the top of the insulating layer. A plurality of spacers are formed in the gate apertures at edges of the patterned gates on the top surface of the insulating layer. The spacers are used as masks for reactive ion etching the insulating layer through the spacing over the insulating layer to the resistive layer, and form a plurality of insulating layer pores. The insulating pores are filled with a filament material by plating up from the resistive layer. The filaments extend from the insulating pores into the gate apertures. The spacers are removed. Additionally, a portion of the insulating layer that is adjacent to the filaments can also be removed.
As an alternative for creating the insulating pores is to track the pores, followed by etching.
Another method of creating a plurality of tracking resist apertures in the tracking resist layer is to etch the particle tracks. The metal gate layer is then etched to form a plurality of patterned gates defining a plurality of gate apertures on the top of the insulating layer. The plurality of spacers are formed by applying a conformal layer on a top of the patterned gates and into the gate apertures, followed by anisotropic etch to form the spacers.
FIG. 5(a) is a cross-sectional view of the structure of
FIG. 5(b) is a cross-sectional view of the structure of FIG. 5(a) when conforming layer 32 is anisotropicaly etched and material is removed. The anisotropic etching step removes the material, thus forming a spacer at a step.
For purposes of this disclosure, a large area field emission display is defined as having at least a 6 inch diagonal screen, more preferably at least an 8 inch diagonal screen, yet more preferably at least a 10 inch diagonal screen, and still more preferably at least a 12 inch diagonal screen.
The ratio of length to maximum diameter of a filament is at least 2, and normally at least 3. The length-to-maximum-diameter ratio is preferably 5 or more.
A gated filament structure 10 is created, as illustrated in
Referring now to
Suitable materials for the multi-layer structure include the following:
Multi-layer structure of
The other methods include but are not limited to conventional lithography, such as photolithography, x-ray lithography and electron beam lithography.
When charged particles are used, they impinge on tracking resist layer 24 in a direction that is substantially perpendicular to a flat lower surface of substrate 12, and therefore are generally perpendicular to tracking resist layer 24. The charged particles pass through tracking resist layer 24 in a straight path creating a continuous damage zone along the path. Particle tracks are randomly distributed across the multi-layer structure with a well defined average spacing. The track density can be as much as 1011 tracks/cm2. A typical value is 108 tracks/cm2, which yields an average track spacing of 1 micron.
In one embodiment, a charged particle accelerator forms a well collimated beam of ions which are used to form tracks. The ion beam is scanned uniformly across tracking resist layer 24. A preferred charged particle species is ionized Xe with an energy typically in the range of about 4 MeV to 16 MeV. Alternatively, charged particle tracks can be created from a collimated source of nuclear fission particles produced, for example, by the radioactive element Californium 252.
Once the particle tracks have been formed, a chemical etch, including but not limited to KOH or NaOH, etches and can over-etch the track formed in tracking resist layer 24 (FIG. 3). Instead of forming a cylindrical pore etched along the track, it is widened to open up an aperture 26 in tracking resist layer 24 that is conical with a generally trapezoidal cross-section. Aperture 26 has a diameter of about 50 to 1000 nm, such as by way of example 200 nm, at gate layer 20. Tracking resist layer 24 is used as a mask to etch gate layer 20 to produce, in one embodiment a 200 nm diameter gate hole 28 (FIG. 4). The etching can be reactive ion etching such as Cl2 for Cr and SF6 for Mo. The depth of reactive ion etching into insulating layer 18 is minimized. A variety of mechanisms are available to ensure that the reactive ion etching stops at insulating layer 18 including but not limited to, monitoring the process and stopping it at the appropriate time, the use of feedback devices, such as sensors, and use of a selective etch. Excess tracking resist 24 material is stripped away, leaving a patterned gate 30 on the top of insulating layer 18.
Referring now to FIG. 5(a), a conformal layer 32 is applied on top of patterned gates 30 and gate apertures 28. Suitable materials for conformal layer 32 include but are not limited to silicon nitride, amorphous or small grained polycrystalline Si, and SiO2. Methods for applying conformal layer include but are not limited to CVD.
As shown in FIG. 5(b) when conforming layer 32 is anisotropicaly etched material is removed. Material is removed from conformal layer 32 at surfaces which are parallel to a plane 33 defined by insulating substrate 12, e g., surface 35 is not etched. The anisotropic etching step removes the material, thus forming a spacer 36 at a step 34.
It is seen in
As shown in
Following reactive ion etching, it may be desirable to apply a chemical treatment on insulating pore 40 to remove the polymer. Suitable chemical treatments include but are not limited to, a plasma of CF4 with O2, or commercially available polymer strippers used in the semiconductor industry well known to those skilled in the art. Thereafter, an electrochemical cell is used, such as shown in FIG. 7.
Referring now to
Spacers 36 are subsequently removed with a removal process, including but not limited to selective plasma etching and wet etching. Thereafter, insulating layer 16 adjacent to filament 22 can be removed with an isotropic plasma or wet chemical (dilute HF) etch. The amount of insulating layer 18 removed is almost down to resistive layer 16.
Alternatively, insulating layer 18 is not removed (FIG. 9).
The use of spacers 36 along with reactive ion etching defines insulating pores 40 which are used to create filaments 22. An alternative process is to use tracking of the insulating layer 18 and chemical etching along the particle tracks.
With reference once again to
Filaments 22 can have a variety of geometries such as flat topped cylinders, rounded top cylinders, sharp cones and the like, which can be created by polishing/etching.
If there are nonuniformities in the thickness of insulating layer 18, or nonuniformities in plating, another embodiment of the invention, illustrated in
The initial multi-layer structure is illustrated in FIG. 10 and includes a substrate 12, a metal row electrode 14 positioned on a top surface of substrate 12, a resistive layer 16 on a top surface of metal row electrode 14, an insulating layer 18 on a top surface of resistive layer 16, a metal gate layer 20 positioned on a top surface of insulating layer 18, a gate encapsulation layer 42 positioned on a top surface of metal gate layer 20 and optionally a tracking resist layer 24 positioned on a top surface of gate encapsulation layer 42. It will be appreciated that tracking resist layer 24 need not be included in this embodiment. The appropriate choice of material for gate encapsulation layer 42 may permit gate encapsulation layer 42 to be used also as the tracking resist layer. The only differences between the multilayer structure in the two embodiments is the inclusion of gate encapsulation layer 42, with or without tracking resist layer 24. Gate encapsulation layer 42 provides two functions, (i) it encapsulates patterned gate 30 and (ii) allows for the formation of taller spacers 36, permitting plating filament 22 above patterned gate 30.
Particle tracking is utilized, as practiced in the first embodiment, and tracking resist layer 24 is etched (FIG. 11). A reactive ion etch through gate encapsulation layer 42 and gate layer 20 is performed (FIG. 12), creating gate hole 28 and patterned gate 30. Tracking resist layer 24 need not be included if gate encapsulation layer 42 can be tracked, etched and used as a resist for patterning gate 30. It will be appreciated that the same methods employed in the embodiment illustrated in
Tracking resist layer 24, if included, is removed and a spacer conformal layer 32 is formed over gate layer 20 and into gate hole 28 (FIG. 13). With the proper selection of materials for gate encapsulation layer 42 and spacer conformal layer 32, gate layer 20 is completely insulated; therefore eliminating concerns regarding controlling voltage on patterned gate 30 to ensure that plating will not occur on patterned gate 30.
With the anisotropic etching of spacer conformal layer 32, the resulting spacers 36 have a height equal to the height of gate layer 20 plus encapsulation layer 42 (FIG. 14).
Insulating pore 40 is formed (
Referring now to
Conformal layer 32 and spacers 36 are removed, leaving a filament 22 that extends beyond patterned gate 30. (FIG. 19). Patterned gate 30 can be used to electro-polish filament 22 with the circuitry illustrated in FIG. 19. Thus, patterned gate 30 is used to define the point where a tip 44 of filament 22 will be (FIG. 20). Patterned gate 30 serves as the cathode for the electro-polishing. A suitable electrolyte is well known to those skilled in the art. This essentially pinches off filament 22 so that excess material becomes free and can be washed away. The remaining filament 22 has a tip 44 geometry that is sharp.
Tip 44 of filament 22 is now located at the position of patterned gate 30.
Filament 22 and filament tip 44 are positioned in gate aperture 28 to establish a relative position for filament tip 44 with its associated gate aperture 28. Referring now to
Metal gate layer 20 has an average thickness “s” and a top metal gate planar surface 20(a) that is substantially parallel to a bottom metal gate planar surface 20(b). Metal gate layer 20 includes a plurality of pores 40 extending through metal gate 30. Each pore 40 has an average width “r” along a bottom planar surface of the aperture. Each pore defines a midpoint plane 46 positioned parallel to and equally distant from top metal gate planar surface 20(a) and bottom metal gate planar surface 20(b). A plurality of filaments 22 each have a filament tip 44 which terminates at a point “A” and a filament axis 48 that extends along a length of the filament through filament tip 44. At the intersection of filament axis 48 and midpoint plane 46, a point “O” is defined. A majority of all filament tips 44 of the display have a length “L” between each filament tip 44 at point A and point O along filament axis 48, where,
Preferably, at least 75% of all filament tips 44 have this relationship between point A and point O, more particularly, it is at least 90%.
The majority of filament tips 44 of the display can have, (i) point A above top metal gate layer planar surface 20(a), (ii) point A between top metal gate layer planar surface 20(a) and bottom metal gate layer planar surface 20(b), or (iii) point A below bottom metal gate layer planar surface 20(b).
With the method of the present invention every insulating pore 40 is overplated and vertical self-alignment is utilized. Patterned gate 30 is used to do the polishing/etching. With the inclusion of gate encapsulation layer 42 filament 22 is plated above patterned gate 30. Additionally, there may be more plating at the edges of the field emission display than in the middle. This can occur because of (i) current crowding effects and (ii) electrolytic depletion effects. As long as the plating is above patterned gate 30 in all places two advantages are achieved, (i) a tolerance on thickness uniformity of deposited insulating layer 18 is provided, and (ii) a high tolerance for the uniformity of plating is possible.
The result is the creation of filaments 22 for the field emission display and the position of each filament 22 is the same within each pore 40 (vertical alignment). Polished filament tips 44 can be created. Further, cones can be formed, as well as filaments using electroless deposition and selective deposition processes well known to those skilled in the art.
In another embodiment, the gate can be patterned and used as a mask to completely etch the insulating layer. The conformal layer is then deposited into the created pore. This can lead to complete encapsulation of the gate, making plating easier. Excess material formed on a bottom of the pore is removed with a suitable method including but not limited to plasma or wet etch. The pore is then overplated. Conformal layer is subsequently substantially removed chemically, and the desired filament tip is then electrochemically etched to created the desired geometry.
The foregoing description of preferred embodiments of the present invention has been provided for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Obviously, many modifications and variations will be apparent to practitioners skilled in this art. The embodiments were chosen and described in order to best explain the principles of the invention and its practical application, thereby enabling others skilled in the art to understand the invention for various embodiments and with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the following claims and their equivalents.
This application is a continuation-in-part of U.S. patent application Ser. No. 08/260,150 entitled “Field-Emitter Fabrication Using Charged-Particle Tracks, And Associated Field-Emission Devices” by Spindt et al., filed Jun. 15, 1994, now U.S. Pat. No. 5,541,957 which is a continuation-in-part of U.S. patent application Ser. No. 08/158,102 entitled “Field-Emitter Fabrication Using Charged-Particle Tracks, And Associated Field-Emission Devices” by Spindt et al., filed Nov. 24, 1993, now U.S. Pat. No. 5,559,389 which is a continuation-in-part of U.S. patent application Ser. No. 08/118,490 entitled “Structure And Fabrication Of Filamentary Field-Emission Device Including Self-Aligned Gate” by Macaulay et al., filed Sep. 8, 1993, now U.S. Pat. No. 5,462,467 all of which are incorporated herein by reference. This application is related to co-pending U.S. patent application entitled “Gated Filament Structures For A Field Emission Display” filed by Macaulay et al., Ser. No. 09/141,697, and co-pending U.S. application entitled “Method For Creating Filament Structures For Field Emission Devices” filed by Macaulay et al., Ser. No. 08/383,408, both filed on the same date as the present application.
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| Number | Date | Country | |
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| Child | 08383409 | US | |
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| Child | 08158102 | US |
