WO2001008193A1 - Vacuum field-effect device and fabrication process therefor - Google Patents

Vacuum field-effect device and fabrication process therefor Download PDF

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Publication number
WO2001008193A1
WO2001008193A1 PCT/US2000/020230 US0020230W WO0108193A1 WO 2001008193 A1 WO2001008193 A1 WO 2001008193A1 US 0020230 W US0020230 W US 0020230W WO 0108193 A1 WO0108193 A1 WO 0108193A1
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WIPO (PCT)
Prior art keywords
vacuum
source
oxide
insulating
gate
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PCT/US2000/020230
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French (fr)
Inventor
Michael D. Potter
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Advanced Vision Technologies, Inc.
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Publication date
Application filed by Advanced Vision Technologies, Inc. filed Critical Advanced Vision Technologies, Inc.
Priority to CA002345629A priority Critical patent/CA2345629A1/en
Priority to JP2001512614A priority patent/JP2003505844A/en
Priority to EP00948950A priority patent/EP1116256A1/en
Priority to KR1020017003711A priority patent/KR20010075312A/en
Publication of WO2001008193A1 publication Critical patent/WO2001008193A1/en

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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
    • H01J1/00—Details of electrodes, of magnetic control means, of screens, or of the mounting or spacing thereof, common to two or more basic types of discharge tubes or lamps
    • H01J1/02—Main electrodes
    • H01J1/30—Cold cathodes, e.g. field-emissive cathode
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B82—NANOTECHNOLOGY
    • B82Y—SPECIFIC USES OR APPLICATIONS OF NANOSTRUCTURES; MEASUREMENT OR ANALYSIS OF NANOSTRUCTURES; MANUFACTURE OR TREATMENT OF NANOSTRUCTURES
    • B82Y30/00—Nanotechnology for materials or surface science, e.g. nanocomposites
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
    • C01B32/00—Carbon; Compounds thereof
    • C01B32/05—Preparation or purification of carbon not covered by groups C01B32/15, C01B32/20, C01B32/25, C01B32/30
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
    • H01J21/00—Vacuum tubes
    • H01J21/02—Tubes with a single discharge path
    • H01J21/06—Tubes with a single discharge path having electrostatic control means only
    • H01J21/10—Tubes with a single discharge path having electrostatic control means only with one or more immovable internal control electrodes, e.g. triode, pentode, octode
    • H01J21/105—Tubes with a single discharge path having electrostatic control means only with one or more immovable internal control electrodes, e.g. triode, pentode, octode with microengineered cathode and control electrodes, e.g. Spindt-type
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
    • H01J9/00—Apparatus or processes specially adapted for the manufacture, installation, removal, maintenance of electric discharge tubes, discharge lamps, or parts thereof; Recovery of material from discharge tubes or lamps
    • H01J9/02—Manufacture of electrodes or electrode systems
    • H01J9/022—Manufacture of electrodes or electrode systems of cold cathodes
    • H01J9/025—Manufacture of electrodes or electrode systems of cold cathodes of field emission cathodes
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B82—NANOTECHNOLOGY
    • B82Y—SPECIFIC USES OR APPLICATIONS OF NANOSTRUCTURES; MEASUREMENT OR ANALYSIS OF NANOSTRUCTURES; MANUFACTURE OR TREATMENT OF NANOSTRUCTURES
    • B82Y40/00—Manufacture or treatment of nanostructures

Definitions

  • This invention relates to microelectronic devices and more particularly to a vacuum-channel field-effect microelectronic device having a lateral field-emission source and preferably having insulated gate(s).
  • lateral emitter or “lateral field-emission source” are used interchangeably to refer to a field- emission source disposed parallel to a substrate.
  • horizontal and vertical are used herein to mean parallel and perpendicular to a substrate respectively, without implying any preferred orientation in space or any preferred orientation with respect to the earth's surface or with respect to the direction of a gravitational force.
  • insulating as in “insulating substrate” or “insulating layer,” is used in its common meaning and particularly for substances characterized by a resistivity greater than 10 s ⁇ - cm.
  • conductive refers to substances characterized by a resistivity less than or equal to 10 8 ⁇ - cm., i.e., including the resistivity range of both conductive and semiconductive substances.
  • An ultra-high-frequency vacuum-channel field-effect microelectronic device has a lateral field-emission source, a drain, and one or more insulated gates.
  • the insulated gate(s) is preferably disposed to extend in overlapping alignment with the emitting edge of the lateral field-emission source and with a portion of the vacuum channel region. If the gate(s) is omitted, the device performs as an ultra-high-speed diode.
  • a preferred fabrication process for the device uses a sacrificial material temporarily deposited in a trench for the vacuum channel region, which is covered with an insulating layer cover. An access hole in the cover allows removal of the sacrificial material.
  • the drain preferably acts also as a sealing plug, plugging the access hole and sealing the vacuum-channel region after the vacuum-channel region is evacuated.
  • FIG. 1 is a partially cut-away perspective view of an insulated-gate vacuum field-effect device made in accordance with the invention.
  • FIGS. 2a - 2 j are side-elevation cross-sectional views of the device in various stages of a preferred fabrication process.
  • FIG. 3 is a flowchart illustrating the steps of a preferred fabrication process performed in accordance with the invention.
  • VFED ultra-high-switching-speed vacuum field-effect device
  • the charge carrier source for the VFED is an electron emitter source operable via Fowler-Nordheim emission.
  • the channel region is a vacuum. Since there is no material in the channel region to scatter the electrons and the channel length is short, the electron transit time is very short. There is no vacuum path between the source and gate or between the drain and gate. Therefore, a relatively high drain potential can be maintained without causing electron emission from the gate.
  • the high drain potential together with the short vacuum channel leads to an electron transit time on the order of sub-picoseconds.
  • the electric permittivity preferably has a value greater than two.
  • the gain parameter, ( ⁇ I SVd/dVg
  • @ Id constant), can be large due to the strong influence of the gate on the channel current with a vacuum channel length in the range of about equal to or greater than 0.5 micrometer.
  • FIG. 1 (not to scale) is a partially cut-away perspective view of an insulated- gate vacuum-channel field-effect device 10 made in accordance with the invention.
  • Device 10 is made on an insulating substrate 20.
  • Source layer 60 (a lateral field- emission cold cathode with emitting tip 85) is parallel to substrate 20.
  • FIG. 1 and the cross-section drawings 2f — 2 j show emitting tip 85 schematically as having a rectangular shape, the actual shape of the emitting tip 85 can be a very sharp edge , i.e., an extremely small radius, as is known in the art of field-emission cathodes.
  • a drain 150 collects electrons emitted from emitting tip 85 of source 60 when a suitable bias voltage is applied to source 60 and drain 150.
  • Drain 150 is spaced apart laterally from emitting tip 85 of the source 60 by a spacing preferably between about one nanometer and about one millimeter.
  • Gates preferably bottom gate 40 and top gate 160, are disposed in at least partial alignment with emitting edge 85 of source 60 and extend to overlap a portion of a vacuum-channel region 120.
  • a conductive bottom- gate contact 155 extends down and makes ohmic electrical contact with bottom gate 40.
  • Contact 155 is also connected to top gate 160 in the embodiment shown in FIG. 1.
  • the use of a recess in substrate 20 for bottom gate 40 allows for planarization of gate 40 and therefore provides for precise control and uniformity of the thickness of insulating layer 50 deposited over gate 40 in a preferred fabrication process, described in detail below.
  • gate 40 may be disposed on the top surface of substrate 20, without a recess.
  • each gate and the vacuum channel region prevents any electrons emitted by the source from reaching either gate, each gate being completely separated from the vacuum channel region by its respective insulating layer (50 or combination of 70 with 100).
  • Each of these insulating layers also prevents any vacuum path between its corresponding gate and the drain 150, so there is no possibility of electron current flowing through the vacuum between either gate and the drain (for example, secondary electron current). It will be understood that this is also true for an IGVFED having only one gate instead of the two-gate preferred embodiment described and illustrated herein.
  • the conductive contact 155 that interconnects them is also fully insulated from vacuum-channel region 120 by the insulators 50, 70, and 100.
  • the dimensions of vacuum-channel region 120 are designed to prevent vacuum- channel region 120 from extending back to the region of conductive contact 155.
  • a conventional passivation layer (not shown) may be deposited over device 10 to protect the device and to prevent surface leakage currents.
  • Conventional via- openings may be formed and conventional terminal metallurgy (not shown) may be deposited to contact the conductive elements shown in FIG. 1.
  • one aspect of the invention is a vacuum field-effect device 10 having a source 60 comprising a lateral field-emitter with an emitting tip 85 for emitting electrons, having a conductive drain 150 spaced apart laterally from the emitting tip, having a vacuum channel region 120 extending at least between the emitting tip 85 of the source and the drain 150, having at least one gate 40 or 160, completely separated from the vacuum channel region by an insulating layer 50, 70, or 100 disposed between the gate and the vacuum channel region 120 to prevent any electrons emitted by the source from reaching the gate, and having terminals (e.g., 140) for applying a bias voltage between the drain and source and for applying a control signal to the gate.
  • terminals e.g. 140
  • the terminals may be integral with their respective electrodes, such as 150 and 160 in FIG. 1.
  • the device preferably has two electrically common gates 40 and 160, which may be connected by an integrated conductive gate contact 155.
  • the device is constructed on an insulating substrate 20, which may consist of an insulating film on a conductive or semiconductive substrate.
  • the new terahertz vacuum field-effect device is much simpler to fabricate than compound semiconductor or heteroj unction semiconductor devices. No semiconductor materials are used in the preferred embodiment.
  • the structure fabrication is, however, compatible with standard IC metallization, passivation, and interconnecting processing. Furthermore, the new device can be integrated with either variations of the preferred embodiment fabrication process or with other integrated- circuit fabrication processes.
  • An overall process for fabricating the vacuum field-effect device includes the steps of providing a suitably flat insulating substrate, forming a source by disposing a lateral field emitter parallel to the substrate, forming an emitting tip on the lateral field emitter of the source, providing a conductive drain spaced apart laterally from the emitting tip for receiving electrons, forming a first opening for a vacuum channel region at least between the emitting tip and the drain, disposing at least one gate in at least partial alignment with respect to the emitting tip and in at least partially overlapping alignment with the first opening, substantially covering the first opening to form a closed vacuum channel chamber, removing any gases from the first opening to provide a vacuum, and sealing the vacuum channel chamber.
  • the overall process may also include the step of disposing an insulating layer between the gate and the vacuum channel region to prevent any of the electrons emitted by the source from reaching the gate, the gate being completely separated from the vacuum channel region by the insulating layer. Terminals are added for applying a bias voltage between the source and drain and for applying a control signal to the gate.
  • the step of providing an insulating substrate may be accomplished by first providing a base substrate, where the base substrate may have any degree of conductivity or semiconductivity, and then depositing an insulating surface layer on the base substrate.
  • the base substrate may be a conductor, a semiconductor, or any substance characterized by a resistivity of less than about 10 8 ⁇ -cm., or an insulator differing in composition from the insulating layer deposited on it.
  • the base substrate may be a metal, silicon, germanium, III-V compounds (GaAs, AlGaAs, InP, GaN, etc.), conducting oxides (e.g., indium tin oxide, indium oxide, tin oxide, copper oxide, or zinc oxide), transition metal nitrides, or transition metal carbides.
  • FIGS. 2a - 2 j and 3 gives details of a particularly preferred fabrication process.
  • FIGS. 2a - 2 j are not drawn to scale. This description includes steps for providing two gates, but it will be recognized that a VFED device may be made with one or more gates and that the gates may be omitted to make a high-speed diode.
  • FIGS. 2a - 2j show a series of side-elevation cross-sectional views showing the results of specific steps of the preferred process.
  • FIG. 3 shows a flow chart representing the preferred fabrication process, in which steps are designated by reference numerals SI, ..., S21. For each of these steps, the act performed is listed in Table I (following page). 51 Provide substrate
  • a suitably flat insulating substrate 20 is provided.
  • Insulating substrate 20 may comprise any suitable insulating material such as glass, ceramic, glass ceramic, diamond, quartz, aluminum oxide, sapphire, silicon oxide, silicon nitride, aluminum nitride, nickel oxide, plastic, polymer, polyimide, parylene, polyethylene terephthalate, and mixtures and combinations thereof.
  • the flat insulating substrate 20 provided in step SI may be made by first providing a conductive base substrate, such as a silicon semiconductor wafer, and depositing a surface layer of a suitable insulating material on the conductive base substrate to form an insulating surface.
  • the insulating layer may be any of the insulating materials listed above, for example.
  • step S2 a trench 30 is formed in the surface of the insulating substrate (FIG. 2a).
  • step S3 trench 30 is filled with a conducting layer 40 and planarized (FIG. 2b) to form a first gate.
  • the planarization may be done by chemical-mechanical polishing (CMP).
  • conducting layer 40 Some suitable materials for conducting layer 40 are aluminum, copper, silver, gold, platinum, palladium, bismuth, conducting oxides, conducting nitrides, the refractory transition metals (titanium, vanadium, chromium, zirconium, niobium, molybdenum, hafnium, tantalum, and tungsten), the refractory transition metal carbides, the refractory transition metal nitrides, boron carbide, doped boron nitride, transition metal suicides, carbon in any of its conductive forms (e.g., doped diamond, graphite, amorphous carbon, fullerenes, nanotubes, or nanocoralline), silicon (N - type or P - type, polycrystalline, amorphous, or single-crystal), germanium, and mixtures, alloys, and combinations thereof.
  • the refractory transition metals titanium, vanadium, chromium, zirconium, niobi
  • Step S4 consists of depositing a first insulating layer 50 over the planarized surface (FIG. 2c).
  • First insulating layer 50 may comprise any suitable insulator, such as glass, glass ceramic, quartz, aluminum oxide, sapphire, silicon oxide, silicon nitride, barium strontium titanate, titanium oxide, samarium oxide, yttrium oxide, tantalum oxide, barium titanium oxide, barium tantalum oxide, lead titanium oxide, strontium titanium oxide, strontium (zirconium, titanium) oxide, aluminum nitride, polyimide, parylene, or mixtures and combinations thereof.
  • the electric permittivity ⁇ of first insulating layer 50 preferably has a value greater than two.
  • Second insulating layer 70 may comprise any suitable insulator, such as any of the materials used for first insulating layer 50 (glass, glass ceramic, quartz, aluminum oxide, sapphire, silicon oxide, silicon nitride, barium strontium titanate, titanium oxide, samarium oxide, yttrium oxide, tantalum oxide, barium titanium oxide, barium tantalum oxide, lead titanium oxide, strontium titanium oxide, strontium (zirconium, titanium) oxide, aluminum nitride, polyimide, parylene, or mixtures and combinations thereof).
  • insulating layers 50 and 70 consist of the same insulating material.
  • the electric permittivity ⁇ of second insulating layer 70 preferably has a value greater than two.
  • a second trench 80 for a vacuum channel region is formed (step S7, FIG. 2f) by etching at least through second insulating layer 70 and source layer 60, but not down as far as first gate layer 40.
  • Trench 80 may be formed by directional reactive ion etching. Forming this trench also etches source layer 60 to form emitting tip 85. If necessary, further etching, such as an isotropic wet etch or plasma etch may be used to further etch emitting tip 85. As is known in the art of field-emission cathodes, it is desirable to form emitting edge 85 with an extremely small radius, to have a very sharp knife-edge shape.
  • Suitable conductive materials for source layer 60 are aluminum, copper, silver, gold, platinum, palladium, bismuth, conducting oxides, conducting nitrides, the refractory transition metals (titanium, vanadium, chromium, zirconium, niobium, molybdenum, hafnium, tantalum, and tungsten), the refractory transition metal carbides, the refractory transition metal nitrides, boron carbide, doped boron nitride, transition metal suicides, carbon in any of its conductive forms (e.g., doped diamond, graphite, amorphous carbon, fullerenes, nanotubes, or nanocoralline), silicon (N - type or P - type, polycrystalhne, amorphous, or single-crystal), germanium, and mixtures, alloys
  • second trench 80 is filled with a sacrificial material 90 and planarized (FIG. 2g).
  • Sacrificial material 90 may be an inorganic material or an organic material such as parylene.
  • a third insulating layer 100 is deposited (step S9, FIG. 2h).
  • Third insulating layer 100 may comprise any suitable insulator, such as any of the materials used for first insulating layer 50 and second insulating layer 70.
  • Insulating layer 100 preferably consists of the same insulating material as insulating layers 50 and 70 and preferably has an electric permittivity ⁇ of greater than two.
  • an access hole 110 is opened through third insulating layer 100 down at least into sacrificial material 90 (FIG. 2i).
  • Access hole 110 is preferably made at or near the edge of trench 80 that is spaced farthest from emitting tip 85.
  • step Sll source via-opening 130 and a via-opening (not shown) for bottom gate 40 are formed.
  • the bottom gate contact 155 (shown in FIG. 1) uses this bottom gate via- opening, which is out of the plane of the cross-sections of FIGS. 2a - 2j.
  • steps S10 and Sll may be combined and performed simultaneously, as indicated in FIG. 3 by a bracket joining these two steps.
  • sacrificial material 90 is removed through access hole 110, e.g., by dissolving sacrificial material 90 with a suitable solvent and removing the solution through access hole 110.
  • the solvent may be acetone.
  • the sacrificial material 90 is silicon dioxide, it may be removed by wet chemical etching, e.g., with HF. For many sacrificial materials, the removal process may be done by oxygen plasma etching. Removing the sacrificial material leaves an empty vacuum channel region 120. The next few steps may be performed in a vacuum environment, with a vacuum pressure preferably less than or equal to about one torr, provided in step S13.
  • step S14 a conductive source contact 140 is deposited and patterned.
  • step S15 a conductive top gate 160 is deposited and patterned.
  • step S16 a conductive bottom gate contact 155 (shown in FIG. 1) is deposited and patterned.
  • step SI 7 a conductive drain 150 is deposited and patterned.
  • Suitable conductive materials for conductive top gate 160, conductive bottom gate contact 155, and conductive drain 150 are aluminum, copper, silver, gold, platinum, palladium, bismuth, conducting oxides, conducting nitrides, the refractory transition metals (titanium, vanadium, chromium, zirconium, niobium, molybdenum, hafnium, tantalum, and tungsten), the refractory transition metal carbides, the refractory transition metal nitrides, boron carbide, doped boron nitride, transition metal suicides, carbon in any of its conductive forms (e.g., doped diamond, graphite, amorphous carbon, fullerenes, nanotubes, or nanocoralline), silicon (N - type or P - type, polycrystalhne, amorphous, or single-crystal), germanium, and mixtures, alloys, and combinations thereof.
  • the refractory transition metals titanium,
  • step S18 access hole 110 is filled to seal the vacuum channel region 120.
  • This step SI 8 is preferably performed at a vacuum pressure of less than or equal to about one torr. When the vacuum channel region 120 is sealed, the channel region will be at vacuum.
  • Steps S14 - S18 are preferably all performed together simultaneously as a step SI 9, as indicated in Fig. 3 by a bracket.
  • access hole 110 also defines the pattern for the lower part of drain 150 (inside vacuum chamber channel region 120). The resultant device after performing steps S14 - S18 or the combined step S19 is shown in the cross-sectional view of FIG. 2j and in the partially cut-away perspective view of FIG. 1.
  • forming conductive drain 150 and sealing vacuum channel region 120 may be accomplished by the methods of U.S. Pat. No. 5,700,176 to Potter, the entire disclosure of which is incorporated herein by reference.
  • a passivation layer may be deposited (step S20) and via-openings formed and terminal metallurgy deposited (step S21).
  • the vacuum field-effect device of this invention may be made with very wide ranges of dimensions and of material characteristics such as electric permittivity of insulators.
  • the vacuum channel length may be made between about one nanometer and about one millimeter.
  • electric permittivities ⁇ , drain voltage values, tradeoffs with coupling capacitance, and enhancement vs. retardation mode of operation are wide ranges of electric permittivities ⁇ , drain voltage values, tradeoffs with coupling capacitance, and enhancement vs. retardation mode of operation.
  • the thicknesses of the insulating layers 50 and combination of 70 and 100 is preferably chosen to be between about one nanometer and about 1000 nanometers when the electric permittivity of the insulating layer is less than or equal to 20, and the spacing is preferably chosen to be between about 10 nanometers and about 5000 nanometers when the electric permittivity of the insulating layer is greater than 20.
  • the device disclosed herein is particularly useful for high bandwidth communication requirements. Such uses of the device include transmitting and receiving data at the chip level, and thus, the device is suitable for short-range intra- LAN communication, wired or wireless.
  • the device also inherently has high thermal tolerance and radiation resistance. It is therefore desirable for applications in harsh environments. These applications include sensor applications for fission or fusion reactors, borehole sensors, accelerator sensors and instrumentation, applications in satellites, deep space and extrate ⁇ estrial exploration vehicles, and many other similar applications.

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Abstract

An ultra-high-frequency vacuum-channel field-effect microelectronic device (VFED or IGVFED) has a lateral field-emission source (60), a drain (150), and one or more insulated gates (40, 160). The insulated gate(s) are preferably disposed to extend in overlapping alignment with the emitting edge (85) of the lateral field-emission source and with a portion of the vacuum-channel region (120). If the gate(s) are omitted, the device performs as an ultra-high speed diode. A preferred fabrication process for the device uses a sacrificial material temporarily deposited in a trench for the vacuum-channel region which is covered with an insulating cover. An access hole in the cover allows removal of the sacrificial material. As part of a preferred fabrication process, the drain preferably acts also as a sealing plug, plugging the access hole and sealing the vacuum-channel region after the vacuum-channel region is evacuated.

Description

Title of Invention
VACUUM FIELD-EFFECT DEVICE AND FABRICATION PROCESS THEREFOR
Description
Technical Field
This invention relates to microelectronic devices and more particularly to a vacuum-channel field-effect microelectronic device having a lateral field-emission source and preferably having insulated gate(s).
This application is related to U.S. provisional application Serial No.
60/145,570 filed July 26, 1999, U.S. patent applications Serial Nos. 09/276,198 (now U.S. Pat. No. 6,004,830), and 09/276,200, both filed March 25, 1999, and U.S. patent applications Serial Nos. 09/ 477,788 and 09/ 476,984, both filed December 13, 1999.
Nomenclature And Notations
Throughout this specification and the appended claims, the terms "lateral emitter" or "lateral field-emission source" are used interchangeably to refer to a field- emission source disposed parallel to a substrate. For clarity and simplicity of expression, the terms "horizontal" and "vertical" are used herein to mean parallel and perpendicular to a substrate respectively, without implying any preferred orientation in space or any preferred orientation with respect to the earth's surface or with respect to the direction of a gravitational force. The acronyms "VFED" and "IGVFED" are used, referring to "vacuum field-effect device" and "insulated-gate vacuum field- effect device" respectively. The term "insulating," as in "insulating substrate" or "insulating layer," is used in its common meaning and particularly for substances characterized by a resistivity greater than 10s Ω - cm. The term "conductive" refers to substances characterized by a resistivity less than or equal to 108 Ω - cm., i.e., including the resistivity range of both conductive and semiconductive substances. Background Art
K. R. Shoulders described some vacuum integrated circuits in "Microelectronics Using Electron-Beam- Activated Machining Techniques," in F. L. Alt (Ed.), "Advances in Computers," Vol. 2 (Academic Press, New York, 1961) pp. 135 - 197. An article, "Vacuum Integrated Circuits" by R. Greene et al. (Technical Digest of 1985 International Electron Devices Meeting [IEDM], IEEE, Piscataway, NJ, 1985, pp. 172 - 175), reviewed the physics and fabrication of vacuum electronic devices and showed a concept of a FET-like vacuum field emitter triode. The field- emission devices described by R. Greene et al. in that article required grid biases of about 100 Volts and anode voltages of about 200 - 500 Volts. An article, "A Vacuum Field Effect Transistor Using Silicon Field Emitter Arrays" by Gray et al. (Technical Digest of 1986 International Electron Devices Meeting [IEDM], IEEE, Piscataway, NJ, 1986, pp. 776 - 779), described a vacuum field-effect transistor-like device using silicon field-emitter arrays. Another article by R. Greene et al., "Vacuum Microelectronics, " (Technical Digest of 1989 International Electron Devices Meeting [IEDM], IEEE, Piscataway, NJ, 1989, pp. 89-15 - 89-19), described an integrally gridded field emitter array and an interdigitated silicon planar field emitter array vacuum FET.
An article by H. H. Busta et al., "Lateral Miniaturized Vacuum Device" (Technical Digest of 1989 International Electron Devices Meeting [IEDM], IEEE, Piscataway, NJ, 1989, pp. 89-533 - 89-536), described two types of lateral field emitter triodes: one type with triangular shaped metallic emitters, a collector electrode, and an extraction electrode, and the other type with a tungsten filament emitter anchored to the sidewall of a polycrystalline silicon layer, a collector electrode, and an extraction electrode.
An article by W. J. Orvis et al., "A Progress Report on the Livermore Miniature Vacuum Tube Project (Technical Digest of 1989 International Electron Devices Meeting [IEDM], IEEE, Piscataway, NJ, 1989, pp. 89-529 - 89-531), described methods of making miniature vacuum diodes and triodes with Spindt-type field emitters. The triodes of Orvis et al. had free-standing anodes and grids.
An article by J. E. Cronin et al., "Field Emission Triode Integrated-Circuit Construction Method," IBM Technical Disclosure Bulletin, Vol. 32, No. 5B (Oct. 1989) pp. 242-243, described a process for fabrication of field-emission triodes having field-emission tips self-aligned to a control grid.
An article by B. Goodman, "Return of the Vacuum Tube," Discover, March 1990, pp. 55 - 58, described progress and problems in development of vacuum microelectronics. An article by S. Kanemura et al., "Fabrication and Characterization of Lateral Field Emitter Triodes," IEEE Transactions on Electron Devices, Vol. 38, No. 10, Oct. 1991, pp. 2334 - 2336, described lateral field-emitter triodes having an array of 170 field-emitter tips with a 10-micrometer pitch, a columnar gate, and an anode.
An article by W. N. Carr et al., "Vacuum Microtriode Characteristics," J. Vac. Sci. Technol., Vol. A8 (4), July/Aug 1990, pp. 3581 - 3585, described pentode-like I-V characteristics simulated for lateral vacuum microelectronic devices with wedge- shaped field-emission cathodes.
An article by A. A. G. Driskill-Smith et al., "The 'Nanotriode:' A Nanoscale Field-Emission Tube," Applied Physics Letters, Vol. 75 No. 18 (Nov. 1, 1999), pp. 2845 - 2847, described a nanoscale electron tube with a field-emission cathode
(vertical metal "nanopillars" with radii of about one nanometer), an integrated anode, and a control gate, all within vertical and horizontal dimensions of about 100 nm. An article by P. Weiss, "Vacuum Tubes' New Image: Too Small to See," in Science News, Vol. 156 (Nov. 6, 1999), summarized the vacuum tube development of Driskill-Smith et al. and reports some comments of others skilled in this field. An article, "Vacuum Tubes Attempt a Comeback," in Physics Today, Dec. 1999, p. 9, summarized some advantages of the vertically oriented devices of the Driskill-Smith et al. article and some remaining problems with that device design. Many prior U.S. patents have described vacuum microelectronic devices (especially field-emission devices) and their fabrication processes, including 3,753,022 to Fraser, Jr.; 3,755,704 and 3,789,471 to Spindt et al.; 4,163,949 to Shelton; 4,578,614 to Gray et al.; 4,721,885 to Brodie; 4,827,177 to Lee; 4,983,878 to Lee et al; 5,007,873 to Goronkm et al.; 5,012,153 to Atkinson et al.; 5,070,282 to Epsztein; 5,079,476 to Kane; 5,112,436 to Bol; 5,126,287 to Jones; 5,136,764 to Vasquez; 5,144,191 to Jones et al.; 5,214,347 to Gray; 5,221,221 to Okaniwa; 5,245,247 and 5,267,884 to Hosogi; 5,268,648 to Calcatera; 5,270,258 and 5,367,181 to Yoshida; 5,394,006 to Liu; 5,493,177 to Muller et al.; and 5,834,790 and 5,925,975 to Suzuki.
A number of prior U.S. patents have described microelectronic device structures with lateral field-emission cathodes and fabrication processes for such structures, including Lee 4,827,177; Bol 5,112,436; Jones, et. al. 5,144,191; Gray 5,214,347; Cronin et al. 5,233,263, 5,308,439, 5,312,777, and 5,530,262; Xie et al. 5,528,099; Mandelman et al. 5,604,399, 5,629,580, 5,736,810, and 5,751,097; and Potter 5,616,061, 5,618,216, 5,628,663, 5,630,741, 5,644,188, 5,644,190, 5,647,998, 5,666,019, 5,669,802, 5,691,599, 5,700,176, 5,703,380, 5,811,929, 5,831,384, 5,850,123, 5,872,421, 5,920,148, 5,965,192, 6,004,830, 6,005,335, 6,015,324, 6,015,326, 6,017,257, 6,037,708, and 6,071,633. There is a continuing need for ultra-high-frequency electronic devices. At present, many of the needs for ultra-high frequency devices are met by semiconductor devices and integrated circuits. Since electron mobilities in semiconductor devices are reduced by carrier collisions with atoms of the crystal lattice, the potentially better high-frequency performance of miniature vacuum devices is attractive. Such vacuum devices, if made small enough and capable of operating at low-enough voltages, with sufficiently high and stable currents, will find a wide range of electronics applications, both digital and analog. Disclosure of Invention
An ultra-high-frequency vacuum-channel field-effect microelectronic device (VFED or IGVFED) has a lateral field-emission source, a drain, and one or more insulated gates. The insulated gate(s) is preferably disposed to extend in overlapping alignment with the emitting edge of the lateral field-emission source and with a portion of the vacuum channel region. If the gate(s) is omitted, the device performs as an ultra-high-speed diode. A preferred fabrication process for the device uses a sacrificial material temporarily deposited in a trench for the vacuum channel region, which is covered with an insulating layer cover. An access hole in the cover allows removal of the sacrificial material. As part of a preferred fabrication process, the drain preferably acts also as a sealing plug, plugging the access hole and sealing the vacuum-channel region after the vacuum-channel region is evacuated.
Brief Description of Drawings
FIG. 1 is a partially cut-away perspective view of an insulated-gate vacuum field-effect device made in accordance with the invention.
FIGS. 2a - 2 j are side-elevation cross-sectional views of the device in various stages of a preferred fabrication process.
FIG. 3 is a flowchart illustrating the steps of a preferred fabrication process performed in accordance with the invention.
Modes for Carrying Out the Invention
Disclosed herein is a new ultra-high-switching-speed vacuum field-effect device (VFED). The charge carrier source for the VFED is an electron emitter source operable via Fowler-Nordheim emission. The channel region is a vacuum. Since there is no material in the channel region to scatter the electrons and the channel length is short, the electron transit time is very short. There is no vacuum path between the source and gate or between the drain and gate. Therefore, a relatively high drain potential can be maintained without causing electron emission from the gate. The high drain potential together with the short vacuum channel leads to an electron transit time on the order of sub-picoseconds. Furthermore, taking into account the very small parasitic capacitance terms (sub-femtofarads/micrometer) of the new VFED, in-depth calculations predict switching speeds up to 10 terahertz for a 0.5 micrometer vacuum channel length. For a 0.1 micrometer vacuum channel length, the calculated speed of the device is nearly 30 terahertz.
For applications where it is desirable to minimize the output impedance, (rp = @ Vg = constant), a very short vacuum channel length will allow a significant change in drain current due to the drain's potential influence on the source field. Here, Vd is the drain voltage, Id is the drain current, and Vg is the gate voltage. Furthermore, a number of individual devices arranged to be electrically in parallel will reduce the effective output impedance without diminishing switching speed. The transconductance, (gm =
Figure imgf000007_0001
@ Vd = constant), can be high due to the effective close proximity of the gate to the source. The use of high dielectric-constant insulators can aid the gate influence; however, the consideration of increasing the gate to source parasitic capacitance should be taken into account. The electric permittivity preferably has a value greater than two. The gain parameter, ( μ = I SVd/dVg | @ Id = constant), can be large due to the strong influence of the gate on the channel current with a vacuum channel length in the range of about equal to or greater than 0.5 micrometer.
FIG. 1 (not to scale) is a partially cut-away perspective view of an insulated- gate vacuum-channel field-effect device 10 made in accordance with the invention. Device 10 is made on an insulating substrate 20. Source layer 60 (a lateral field- emission cold cathode with emitting tip 85) is parallel to substrate 20. While FIG. 1 and the cross-section drawings 2f — 2 j show emitting tip 85 schematically as having a rectangular shape, the actual shape of the emitting tip 85 can be a very sharp edge , i.e., an extremely small radius, as is known in the art of field-emission cathodes. A drain 150 collects electrons emitted from emitting tip 85 of source 60 when a suitable bias voltage is applied to source 60 and drain 150. Drain 150 is spaced apart laterally from emitting tip 85 of the source 60 by a spacing preferably between about one nanometer and about one millimeter. Gates, preferably bottom gate 40 and top gate 160, are disposed in at least partial alignment with emitting edge 85 of source 60 and extend to overlap a portion of a vacuum-channel region 120. A conductive bottom- gate contact 155 extends down and makes ohmic electrical contact with bottom gate 40. Contact 155 is also connected to top gate 160 in the embodiment shown in FIG. 1. The use of a recess in substrate 20 for bottom gate 40 allows for planarization of gate 40 and therefore provides for precise control and uniformity of the thickness of insulating layer 50 deposited over gate 40 in a preferred fabrication process, described in detail below. However, in other embodiments, gate 40 may be disposed on the top surface of substrate 20, without a recess.
An insulating layer between each gate and the vacuum channel region prevents any electrons emitted by the source from reaching either gate, each gate being completely separated from the vacuum channel region by its respective insulating layer (50 or combination of 70 with 100). Each of these insulating layers also prevents any vacuum path between its corresponding gate and the drain 150, so there is no possibility of electron current flowing through the vacuum between either gate and the drain (for example, secondary electron current). It will be understood that this is also true for an IGVFED having only one gate instead of the two-gate preferred embodiment described and illustrated herein. In the embodiment of FIG. 1, which has two gates, the conductive contact 155 that interconnects them is also fully insulated from vacuum-channel region 120 by the insulators 50, 70, and 100. As shown in FIG. 1, the dimensions of vacuum-channel region 120 are designed to prevent vacuum- channel region 120 from extending back to the region of conductive contact 155.
A conventional passivation layer (not shown) may be deposited over device 10 to protect the device and to prevent surface leakage currents. Conventional via- openings may be formed and conventional terminal metallurgy (not shown) may be deposited to contact the conductive elements shown in FIG. 1.
Thus, one aspect of the invention is a vacuum field-effect device 10 having a source 60 comprising a lateral field-emitter with an emitting tip 85 for emitting electrons, having a conductive drain 150 spaced apart laterally from the emitting tip, having a vacuum channel region 120 extending at least between the emitting tip 85 of the source and the drain 150, having at least one gate 40 or 160, completely separated from the vacuum channel region by an insulating layer 50, 70, or 100 disposed between the gate and the vacuum channel region 120 to prevent any electrons emitted by the source from reaching the gate, and having terminals (e.g., 140) for applying a bias voltage between the drain and source and for applying a control signal to the gate. The terminals may be integral with their respective electrodes, such as 150 and 160 in FIG. 1. The device preferably has two electrically common gates 40 and 160, which may be connected by an integrated conductive gate contact 155. The device is constructed on an insulating substrate 20, which may consist of an insulating film on a conductive or semiconductive substrate.
Fabrication Process
The new terahertz vacuum field-effect device (VFED) is much simpler to fabricate than compound semiconductor or heteroj unction semiconductor devices. No semiconductor materials are used in the preferred embodiment. The structure fabrication is, however, compatible with standard IC metallization, passivation, and interconnecting processing. Furthermore, the new device can be integrated with either variations of the preferred embodiment fabrication process or with other integrated- circuit fabrication processes.
An overall process for fabricating the vacuum field-effect device includes the steps of providing a suitably flat insulating substrate, forming a source by disposing a lateral field emitter parallel to the substrate, forming an emitting tip on the lateral field emitter of the source, providing a conductive drain spaced apart laterally from the emitting tip for receiving electrons, forming a first opening for a vacuum channel region at least between the emitting tip and the drain, disposing at least one gate in at least partial alignment with respect to the emitting tip and in at least partially overlapping alignment with the first opening, substantially covering the first opening to form a closed vacuum channel chamber, removing any gases from the first opening to provide a vacuum, and sealing the vacuum channel chamber. The overall process may also include the step of disposing an insulating layer between the gate and the vacuum channel region to prevent any of the electrons emitted by the source from reaching the gate, the gate being completely separated from the vacuum channel region by the insulating layer. Terminals are added for applying a bias voltage between the source and drain and for applying a control signal to the gate.
The step of providing an insulating substrate may be accomplished by first providing a base substrate, where the base substrate may have any degree of conductivity or semiconductivity, and then depositing an insulating surface layer on the base substrate. Thus the base substrate may be a conductor, a semiconductor, or any substance characterized by a resistivity of less than about 108 Ω-cm., or an insulator differing in composition from the insulating layer deposited on it. For example, the base substrate may be a metal, silicon, germanium, III-V compounds (GaAs, AlGaAs, InP, GaN, etc.), conducting oxides (e.g., indium tin oxide, indium oxide, tin oxide, copper oxide, or zinc oxide), transition metal nitrides, or transition metal carbides.
Within the framework of this overall fabrication process, many variations of particular materials and specific process methods and their sequence may be made. The following description, read in conjunction with FIGS. 2a - 2 j and 3, gives details of a particularly preferred fabrication process. FIGS. 2a - 2 j are not drawn to scale. This description includes steps for providing two gates, but it will be recognized that a VFED device may be made with one or more gates and that the gates may be omitted to make a high-speed diode.
FIGS. 2a - 2j show a series of side-elevation cross-sectional views showing the results of specific steps of the preferred process. FIG. 3 shows a flow chart representing the preferred fabrication process, in which steps are designated by reference numerals SI, ..., S21. For each of these steps, the act performed is listed in Table I (following page). 51 Provide substrate
52 Form a first trench
53 Fill first trench with conducting layer and planarize S4 Deposit first insulating layer
55 Deposit conductive material and pattern source
56 Deposit second insulating layer
57 Form second trench to define vacuum channel region
58 Fill second trench with sacrificial material and planarize S9 Deposit third insulating layer
S10 Form access hole through third insulating layer
SI 1 Form source via and bottom gate via openings
512 Remove sacrificial material
513 Provide vacuum environment SI 4 Deposit and pattern conductive source contact
515 Deposit and pattern conductive top gate
516 Deposit and pattern conductive bottom gate contact
517 Deposit and pattern conductive drain
518 S eal vacuum-channel region S19 (Combination of steps S14 through S18 performed together simultaneously)
520 If desired, deposit passivation
521 If desired, form via holes and terminal metallurgy
TABLE I. Process steps of FIG. 3
In step SI, a suitably flat insulating substrate 20 is provided. Insulating substrate 20 may comprise any suitable insulating material such as glass, ceramic, glass ceramic, diamond, quartz, aluminum oxide, sapphire, silicon oxide, silicon nitride, aluminum nitride, nickel oxide, plastic, polymer, polyimide, parylene, polyethylene terephthalate, and mixtures and combinations thereof. As mentioned above, the flat insulating substrate 20 provided in step SI may be made by first providing a conductive base substrate, such as a silicon semiconductor wafer, and depositing a surface layer of a suitable insulating material on the conductive base substrate to form an insulating surface. The insulating layer may be any of the insulating materials listed above, for example.
In step S2, a trench 30 is formed in the surface of the insulating substrate (FIG. 2a). In step S3, trench 30 is filled with a conducting layer 40 and planarized (FIG. 2b) to form a first gate. The planarization may be done by chemical-mechanical polishing (CMP). Some suitable materials for conducting layer 40 are aluminum, copper, silver, gold, platinum, palladium, bismuth, conducting oxides, conducting nitrides, the refractory transition metals (titanium, vanadium, chromium, zirconium, niobium, molybdenum, hafnium, tantalum, and tungsten), the refractory transition metal carbides, the refractory transition metal nitrides, boron carbide, doped boron nitride, transition metal suicides, carbon in any of its conductive forms (e.g., doped diamond, graphite, amorphous carbon, fullerenes, nanotubes, or nanocoralline), silicon (N - type or P - type, polycrystalline, amorphous, or single-crystal), germanium, and mixtures, alloys, and combinations thereof. The conductive material is chosen to be compatible in processing with the other materials of the device. Step S4 consists of depositing a first insulating layer 50 over the planarized surface (FIG. 2c). First insulating layer 50 may comprise any suitable insulator, such as glass, glass ceramic, quartz, aluminum oxide, sapphire, silicon oxide, silicon nitride, barium strontium titanate, titanium oxide, samarium oxide, yttrium oxide, tantalum oxide, barium titanium oxide, barium tantalum oxide, lead titanium oxide, strontium titanium oxide, strontium (zirconium, titanium) oxide, aluminum nitride, polyimide, parylene, or mixtures and combinations thereof. The electric permittivity ε of first insulating layer 50 preferably has a value greater than two.
In step S5, a conductive material is deposited and patterned to define a source layer 60 (FIG. 2d). In step S6, a second insulating layer 70 is deposited, covering the source layer 60 (FIG. 2e). Second insulating layer 70 may comprise any suitable insulator, such as any of the materials used for first insulating layer 50 (glass, glass ceramic, quartz, aluminum oxide, sapphire, silicon oxide, silicon nitride, barium strontium titanate, titanium oxide, samarium oxide, yttrium oxide, tantalum oxide, barium titanium oxide, barium tantalum oxide, lead titanium oxide, strontium titanium oxide, strontium (zirconium, titanium) oxide, aluminum nitride, polyimide, parylene, or mixtures and combinations thereof). However, it is preferable that insulating layers 50 and 70 consist of the same insulating material. The electric permittivity ε of second insulating layer 70 preferably has a value greater than two.
A second trench 80 for a vacuum channel region is formed (step S7, FIG. 2f) by etching at least through second insulating layer 70 and source layer 60, but not down as far as first gate layer 40. Trench 80 may be formed by directional reactive ion etching. Forming this trench also etches source layer 60 to form emitting tip 85. If necessary, further etching, such as an isotropic wet etch or plasma etch may be used to further etch emitting tip 85. As is known in the art of field-emission cathodes, it is desirable to form emitting edge 85 with an extremely small radius, to have a very sharp knife-edge shape. This is accomplished by depositing a very thin source layer 60 in step S5, and then by etching the thin layer's edge in step S7. Suitable conductive materials for source layer 60 are aluminum, copper, silver, gold, platinum, palladium, bismuth, conducting oxides, conducting nitrides, the refractory transition metals (titanium, vanadium, chromium, zirconium, niobium, molybdenum, hafnium, tantalum, and tungsten), the refractory transition metal carbides, the refractory transition metal nitrides, boron carbide, doped boron nitride, transition metal suicides, carbon in any of its conductive forms (e.g., doped diamond, graphite, amorphous carbon, fullerenes, nanotubes, or nanocoralline), silicon (N - type or P - type, polycrystalhne, amorphous, or single-crystal), germanium, and mixtures, alloys, and combinations thereof. As is well known in the art, it is preferable to use a material of low work function, at least at emitting edge 85 of source layer 60.
In step S8, second trench 80 is filled with a sacrificial material 90 and planarized (FIG. 2g). Sacrificial material 90 may be an inorganic material or an organic material such as parylene. A third insulating layer 100 is deposited (step S9, FIG. 2h). Third insulating layer 100 may comprise any suitable insulator, such as any of the materials used for first insulating layer 50 and second insulating layer 70. Insulating layer 100 preferably consists of the same insulating material as insulating layers 50 and 70 and preferably has an electric permittivity ε of greater than two. In step S10, an access hole 110 is opened through third insulating layer 100 down at least into sacrificial material 90 (FIG. 2i). Access hole 110 is preferably made at or near the edge of trench 80 that is spaced farthest from emitting tip 85. In step Sll, source via-opening 130 and a via-opening (not shown) for bottom gate 40 are formed. The bottom gate contact 155 (shown in FIG. 1) uses this bottom gate via- opening, which is out of the plane of the cross-sections of FIGS. 2a - 2j. Optionally, steps S10 and Sll may be combined and performed simultaneously, as indicated in FIG. 3 by a bracket joining these two steps. In step S12, sacrificial material 90 is removed through access hole 110, e.g., by dissolving sacrificial material 90 with a suitable solvent and removing the solution through access hole 110. For example, if the sacrificial material 90 is photoresist or wax, the solvent may be acetone. If the sacrificial material 90 is silicon dioxide, it may be removed by wet chemical etching, e.g., with HF. For many sacrificial materials, the removal process may be done by oxygen plasma etching. Removing the sacrificial material leaves an empty vacuum channel region 120. The next few steps may be performed in a vacuum environment, with a vacuum pressure preferably less than or equal to about one torr, provided in step S13.
In step S14, a conductive source contact 140 is deposited and patterned. In step S15, a conductive top gate 160 is deposited and patterned. In step S16, a conductive bottom gate contact 155 (shown in FIG. 1) is deposited and patterned. In step SI 7, a conductive drain 150 is deposited and patterned. Suitable conductive materials for conductive top gate 160, conductive bottom gate contact 155, and conductive drain 150 are aluminum, copper, silver, gold, platinum, palladium, bismuth, conducting oxides, conducting nitrides, the refractory transition metals (titanium, vanadium, chromium, zirconium, niobium, molybdenum, hafnium, tantalum, and tungsten), the refractory transition metal carbides, the refractory transition metal nitrides, boron carbide, doped boron nitride, transition metal suicides, carbon in any of its conductive forms (e.g., doped diamond, graphite, amorphous carbon, fullerenes, nanotubes, or nanocoralline), silicon (N - type or P - type, polycrystalhne, amorphous, or single-crystal), germanium, and mixtures, alloys, and combinations thereof.
In step S18, access hole 110 is filled to seal the vacuum channel region 120. This step SI 8 is preferably performed at a vacuum pressure of less than or equal to about one torr. When the vacuum channel region 120 is sealed, the channel region will be at vacuum. Steps S14 - S18 are preferably all performed together simultaneously as a step SI 9, as indicated in Fig. 3 by a bracket. In this preferred process, access hole 110 also defines the pattern for the lower part of drain 150 (inside vacuum chamber channel region 120). The resultant device after performing steps S14 - S18 or the combined step S19 is shown in the cross-sectional view of FIG. 2j and in the partially cut-away perspective view of FIG. 1. Alternatively to this process, forming conductive drain 150 and sealing vacuum channel region 120 may be accomplished by the methods of U.S. Pat. No. 5,700,176 to Potter, the entire disclosure of which is incorporated herein by reference. If desired, a passivation layer may be deposited (step S20) and via-openings formed and terminal metallurgy deposited (step S21). A person skilled in the art will recognize that simply by omitting those process steps for incorporating control gate elements 40 and 160, an ultra-high frequency diode structure is realized. If one of the control gate elements 40 and 160 is omitted, and the other included, the device will still operate as a triode.
The vacuum field-effect device of this invention may be made with very wide ranges of dimensions and of material characteristics such as electric permittivity of insulators. For example, depending on the application, the vacuum channel length may be made between about one nanometer and about one millimeter. There are wide ranges of electric permittivities ε, drain voltage values, tradeoffs with coupling capacitance, and enhancement vs. retardation mode of operation. The thicknesses of the insulating layers 50 and combination of 70 and 100 (i.e., the spacing of the gates from source layer 60) is preferably chosen to be between about one nanometer and about 1000 nanometers when the electric permittivity of the insulating layer is less than or equal to 20, and the spacing is preferably chosen to be between about 10 nanometers and about 5000 nanometers when the electric permittivity of the insulating layer is greater than 20.
Industrial Applicability
The device disclosed herein is particularly useful for high bandwidth communication requirements. Such uses of the device include transmitting and receiving data at the chip level, and thus, the device is suitable for short-range intra- LAN communication, wired or wireless. The device also inherently has high thermal tolerance and radiation resistance. It is therefore desirable for applications in harsh environments. These applications include sensor applications for fission or fusion reactors, borehole sensors, accelerator sensors and instrumentation, applications in satellites, deep space and extrateπestrial exploration vehicles, and many other similar applications.
Other embodiments of the invention to adapt it for various uses and conditions will be apparent to those skilled in the art from a consideration of this specification or from practice of the invention disclosed herein. For example, additional gate electrodes may be incorporated into the structures. For another example, the device may be fabricated on an insulating substrate comprising a suitable plastic or other polymer, which may be flexible and/or transparent, or conductive elements may be made of conductive polymers. Also, the order of the various fabrication process steps may be varied for some purposes, and some process steps may be omitted for fabrication of the simpler structures. It is intended that the specification and examples be considered as exemplary only, with the true scope and spirit of the invention being defined by the following claims.
What is claimed is:

Claims

1. A vacuum field-effect device, comprising: a) a source, said source comprising a lateral field-emitter, said lateral field emitter having an emitting tip for emitting electrons, said vacuum field-effect device being characterized by further comprising: b) a drain spaced apart laterally from said emitting tip of said lateral field emitter, said drain comprising a conductive electrode; c) a vacuum channel region disposed at least between said emitting tip of said lateral field emitter and said drain; d) at least one first gate, said at least one first gate comprising a conductive material being completely separated from said vacuum channel region by a first insulating layer disposed between said at least one first gate and said vacuum channel region to prevent any of said electrons emitted by said source from reaching said at least one first gate; and e) terminals for applying a bias voltage between said drain and said source and for applying a control signal to said at least one first gate.
2. The vacuum field-effect device of claim 1, further comprising an insulating substrate, said lateral field-emitter of said source being disposed parallel to said insulating substrate.
3. The vacuum field-effect device of claim 2, wherein said insulating substrate comprises a material selected from the list consisting of glass, ceramic, glass ceramic, diamond, quartz, aluminum oxide, sapphire, silicon oxide, silicon nitride, aluminum nitride, nickel oxide, plastic, polymer, polyimide, parylene, polyethylene terephthalate, and mixtures and combinations thereof.
4. The vacuum field-effect device of claim 1, wherein said first insulating layer comprises a material selected from the list consisting of glass, glass ceramic, quartz, aluminum oxide, sapphire, silicon oxide, silicon nitride, barium strontium titanate, titanium oxide, samarium oxide, yttrium oxide, tantalum oxide, barium titanium oxide, barium tantalum oxide, lead titanium oxide, strontium titanium oxide, strontium (zirconium, titanium) oxide, aluminum nitride, polyimide, parylene, and mixtures and combinations thereof.
5. The vacuum field-effect device of claim 1, wherein said first insulating layer comprises a material having an electric permittivity equal to or greater than two.
6. The vacuum field-effect device of claim 1, wherein said at least one gate is at least partially aligned with said emitting tip of said lateral field emitter and disposed to be in at least partially overlapping alignment with said vacuum channel region.
7. The vacuum field-effect device of claim 1, wherein said at least one gate comprises a conductive material selected from the list consisting of aluminum, copper, silver, gold, platinum, palladium, bismuth, conducting oxides, conducting nitrides, the refractory transition metals (titanium, vanadium, chromium, zirconium, niobium, molybdenum, hafnium, tantalum, and tungsten), the refractory transition metal carbides, the refractory transition metal nitrides, boron carbide, doped boron nitride, transition metal suicides, carbon in any of its conductive forms (doped diamond, graphite, amorphous carbon, fullerenes, nanotubes, or nanocoralline), silicon (N - type or P - type, polycrystalhne, amorphous, or single-crystal), germanium, and mixtures, alloys, and combinations thereof.
8. The vacuum field-effect device of claim 1, wherein said lateral field-emitter comprises a thin-film conductor disposed on said first insulating layer.
9. The vacuum field-effect device of claim 8, further comprising a second insulating layer disposed over said thin film conductor.
10. The vacuum field-effect device of claim 9, wherein said second insulating layer comprises a material selected from the list consisting of glass, glass ceramic, quartz, aluminum oxide, sapphire, silicon oxide, silicon nitride, barium strontium titanate, titanium oxide, samarium oxide, yttrium oxide, tantalum oxide, barium titanium oxide, barium tantalum oxide, lead titanium oxide, strontium titanium oxide, strontium (zirconium, titanium) oxide, aluminum nitride, polyimide, parylene, and mixtures and combinations thereof.
11. The vacuum field-effect device of claim 9, wherein said second insulating layer comprises a material having an electric permittivity equal to or greater than two.
12. The vacuum field-effect device of claim 9, wherein said first and second insulating layers consist of the same insulating material.
13. The vacuum field-effect device of claim 1, further comprising at least one second gate, said at least one second gate being completely separated from said vacuum channel region by a third insulating layer disposed between said at least one second gate and said vacuum channel region to prevent any of said electrons emitted by said source from reaching said at least one second gate, and further comprising terminals for applying a control signal to said at least one second gate.
14. The vacuum field-effect device of claim 13, wherein said at least one second gate is at least partially aligned with said emitting tip of said lateral field emitter and disposed to be in at least partially overlapping alignment with said vacuum channel region.
15. The vacuum field-effect device of claim 13, wherein said at least one second gate comprises a conductive material selected from the list consisting of aluminum, copper, silver, gold, platinum, palladium, bismuth, conducting oxides, conducting nitrides, the refractory transition metals (titanium, vanadium, chromium, zirconium, niobium, molybdenum, hafnium, tantalum, and tungsten), the refractory transition metal carbides, the refractory transition metal nitrides, boron carbide, doped boron nitride, transition metal suicides, carbon in any of its conductive forms (doped diamond, graphite, amorphous carbon, fullerenes, nanotubes, or nanocoralline), silicon (N - type or P - type, polycrystalhne, amorphous, or single-crystal), germanium, and mixtures, alloys, and combinations thereof.
16. The vacuum field-effect device of claim 13, wherein said at least one second gate is at least partially aligned with said at least one first gate.
17. The vacuum field-effect device of claim 13, wherein said at least one first gate and said at least one second gate are aligned one with the other and are disposed symmetrically with each other about a plane through said source.
18. The vacuum field-effect device of claim 13, wherein said at least one second gate is spaced vertically apart from said source by a first predetermined spacing.
19. The vacuum field-effect device of claim 18, wherein said first predetermined spacing is between about one nanometer and about 1000 nanometers when the electric permittivity of said second insulating layer is less than or equal to 20, and said first predetermined spacing is between about 10 nanometers and about 5000 nanometers when the electric permittivity of said second insulating layer is greater than 20.
20. The vacuum field-effect device of claim 13, wherein said at least one first gate is spaced vertically apart from said source by a second predetermined spacing.
21. The vacuum field-effect device of claim 20, wherein said second predetermined spacing is between about one nanometer and about 1000 nanometers when the electric permittivity of said first insulating layer is less than or equal to 20, and said second predetermined spacing is between about 10 nanometers and about 5000 nanometers when the electric permittivity of said first insulating layer is greater than 20.
22. The vacuum field-effect device of claim 20, wherein said at least one second gate is spaced vertically apart from said source by a spacing substantially equal to said second predetermined spacing.
23. The vacuum field-effect device of claim 13, wherein said first and second gates are electrically common, and said first and second control signals are common to said first and second gates.
24. The vacuum field-effect device of claim 1, wherein said source comprises a conductive material selected from the list consisting of aluminum, copper, silver, gold, platinum, palladium, bismuth, conducting oxides, conducting nitrides, the refractory transition metals (titanium, vanadium, chromium, zirconium, niobium, molybdenum, hafnium, tantalum, and tungsten), the refractory transition metal carbides, the refractory transition metal nitrides, boron carbide, doped boron nitride, transition metal suicides, carbon in any of its conductive forms (doped diamond, graphite, amorphous carbon, fullerenes, nanotubes, or nanocoralline), silicon (N - type or P - type, polycrystalline, amorphous, or single-crystal), germanium, and mixtures, alloys, and combinations thereof.
25. The vacuum field-effect device of claim 1, wherein said drain comprises a conductive material selected from the list consisting of aluminum, copper, silver, gold, platinum, palladium, bismuth, conducting oxides, conducting nitrides, the refractory transition metals (titanium, vanadium, chromium, zirconium, niobium, molybdenum, hafnium, tantalum, and tungsten), the refractory transition metal carbides, the refractory transition metal nitrides, boron carbide, doped boron nitride, transition metal suicides, carbon in any of its conductive forms (doped diamond, graphite, amorphous carbon, fullerenes, nanotubes, or nanocoralline), silicon (N - type or P - type, polycrystalline, amorphous, or single-crystal), germanium, and mixtures, alloys, and combinations thereof.
26. The vacuum field-effect device of claim 1, wherein said drain is spaced apart laterally from said emitting tip of said lateral field emitter by a spacing between about one nanometer and about one millimeter.
27. A vacuum field-effect device, comprising:
a) an insulating substrate;
b) a first insulating layer; c) a source, said source comprising a lateral field-emitter disposed parallel to said insulating substrate, said lateral field-emitter comprising a thin-film conductor disposed on said first insulating layer, and said lateral field emitter having an emitting tip for emitting electrons;
d) a conductive drain spaced apart laterally from said emitting tip of said lateral field emitter, said conductive drain being disposed substantially perpendicular to said insulating substrate; e) a vacuum channel region disposed at least between said emitting tip of said lateral field emitter and said conductive drain, whereby electrons may move unimpeded from said emitting tip of said lateral field emitter to said drain;
f) first and second gates, said first and second gates being completely separated from said vacuum channel region by second and third insulating layers respectively disposed between said first and second gates and said vacuum channel region to prevent any of said electrons emitted from said source from reaching either of said first and second gates; and g) terminals for applying a bias voltage between said drain and said source and for applying first and second control signals to said first and second gates respectively.
28. The vacuum field-effect device of claim 27, wherein said insulating substrate comprises a conductive base substrate covered with an insulating surface layer.
29. The vacuum field-effect device of claim 27, wherein said first and second gates are electrically common for applying said first and second control signals in common to said first and second gates.
30. An ultra-high frequency vacuum diode device, comprising:
a) an insulating substrate; b) a source, said source comprising a lateral field-emitter disposed parallel to said insulating substrate, said lateral field emitter having an emitting tip for emitting electrons; c) a drain spaced apart laterally from said emitting tip of said lateral field emitter, said drain comprising a conductive electrode;
d) a vacuum channel region disposed at least between said emitting tip of said lateral field emitter and said drain; and e) terminals for applying a voltage signal between said drain and said source.
31. An ultra-high frequency vacuum diode device, comprising: a) an insulating substrate; b) a first insulating layer; c) a source, said source comprising a lateral field-emitter disposed parallel to said insulating substrate, said lateral field-emitter comprising a thin-film conductor disposed on said first insulating layer, and said lateral field emitter having an emitting tip for emitting electrons; d) a conductive drain spaced apart laterally by a spacing between about one nanometer and about one millimeter from said emitting tip of said lateral field emitter, said conductive drain being disposed substantially perpendicular to said insulating substrate; e) a vacuum channel region disposed at least between said emitting tip of said lateral field emitter and said conductive drain, whereby electrons may move unimpeded from said emitting tip of said lateral field emitter to said drain; and
f) terminals for applying a voltage signal between said drain and said source for inducing a cuπent of said electrons to flow directly from said source to said conductive drain.
32. A process for fabricating a vacuum field-effect device, comprising the steps of:
a) providing an insulating substrate;
b) forming a source by disposing a lateral field emitter parallel to said substrate;
c) forming an emitting tip on said a lateral field emitter of said source; d) providing a conductive drain spaced apart laterally from said emitting tip of said lateral field emitter for receiving electrons; e) forming a first opening at least between said emitting tip of said lateral field emitter and said drain for a vacuum channel region; f) disposing at least one first gate in at least partial alignment with respect to said emitting tip and in at least partially overlapping alignment with said first opening;
g) substantially covering said first opening to form a closed vacuum channel chamber;
h) removing any gases from said first opening to provide a vacuum therein; and
i) sealing said vacuum channel chamber.
33. The vacuum field-effect device made by the process of claim 32.
34. The process of claim 32, further comprising the step of: j) disposing a first insulating layer between said at least one first gate and said vacuum channel region to prevent any of said electrons emitted by said source from reaching said at least one first gate, said at least one first gate being completely separated from said vacuum channel region by said first insulating layer.
35. The process of claim 32, further comprising the step of: k) forming terminals for applying a bias voltage between said drain and said source and for applying a control signal to said at least one first gate.
36. The process of claim 32, wherein the step (a) of providing an insulating substrate is performed by providing a base substrate, said base substrate having any degree of conductivity or semiconductivity, and depositing an insulating layer on said base substrate.
37. The process of claim 32, wherein the steps (c) of forming an emitting tip and (e) of forming a first opening are performed together and substantially simultaneously.
38. The process of claim 37, further comprising directionally etching at least through said lateral field emitter.
39. The process of claim 32, wherein the steps (d) of providing said conductive drain and (i) of sealing said vacuum channel chamber are performed together and substantially simultaneously.
40. A process for fabricating a vacuum field-effect device, comprising the steps of: a) providing an insulating substrate; b) forming a first trench in said insulating substrate; c) filling said first trench with a first conductive layer for providing a bottom gate and planarizing; d) depositing a first insulating layer over said insulating substrate and said first conductive layer for insulating said bottom gate; e) depositing a second conductive layer and patterning to form a source layer parallel to said substrate; f) depositing a second insulating layer over said source layer; g) forming a second trench for a vacuum channel region, while forming an emitting tip of said source layer, thereby completing formation of a lateral field emitter source; h) filling said second trench with a sacrificial material and planarizing; i) depositing a third insulating layer extending over said sacrificial material; j) forming an access hole through said third insulating layer to said sacrificial material; k) forming openings for a source via and for a bottom gate via; 1) removing said sacrificial material through said access hole; m) providing a vacuum environment; and n) depositing and patterning a top gate, a conductive source contact, a bottom gate contact, and a conductive drain, while sealing said vacuum channel region, whereby said top gate is insulated by said third insulating layer from said vacuum channel region.
41. The vacuum field-effect device made by the process of claim 40.
42. The process of claim 40, wherein the step (a) of providing an insulating substrate is performed by providing a base substrate, said base substrate having any degree of conductivity or semiconductivity, and depositing an insulating layer on said base substrate.
43. The process of claim 40, wherein said vacuum-environment-providing step (m) is performed by providing a vacuum pressure of less than or equal to about one torr.
44. The process of claim 40, wherein said vacuum-environment-providing step (m) and said depositing and patterning step (n) are performed together and substantially simultaneously.
45. The process of claim 40, wherein said insulating-substrate-providing step (a) comprises providing a substrate of an insulating material selected from the list consisting of glass, ceramic, glass ceramic, diamond, quartz, aluminum oxide, sapphire, silicon oxide, silicon nitride, aluminum nitride, nickel oxide, plastic, polymer, polyimide, parylene, polyethylene terephthalate, and mixtures and combinations thereof.
46. The process of claim 40, wherein said first-trench-forming step (b) comprises etching a recess into said insulating substrate.
47. The process of claim 46, wherein said first-trench- forming step (b) comprises directional etching with ions.
48. The process of claim 40, wherein said first-trench-filling step (c) comprises filling said first trench with a conductive material selected from the list consisting of aluminum, copper, silver, gold, platinum, palladium, bismuth, conducting oxides, conducting nitrides, the refractory transition metals (titanium, vanadium, chromium, zirconium, niobium, molybdenum, hafnium, tantalum, and tungsten), the refractory transition metal carbides, the refractory transition metal nitrides, boron carbide, doped boron nitride, transition metal suicides, carbon in any of its conductive forms (doped diamond, graphite, amorphous carbon, fullerenes, nanotubes, or nanocoralline), silicon (N - type or P - type, polycrystalline, amorphous, or single-crystal), germanium, and mixtures, alloys, and combinations thereof.
49. The process of claim 40, wherein said first-insulating-layer depositing step (d) comprises depositing an insulating material selected from the list consisting of glass, glass ceramic, quartz, aluminum oxide, sapphire, silicon oxide, silicon nitride, barium strontium titanate, titanium oxide, samarium oxide, yttrium oxide, tantalum oxide, barium titanium oxide, barium tantalum oxide, lead titanium oxide, strontium titanium oxide, strontium (zirconium, titanium) oxide, aluminum nitride, polyimide, parylene, and mixtures and combinations thereof.
50. The process of claim 40, wherein said first-insulating-layer depositing step (d) comprises depositing an insulating material having an electric permittivity equal to or greater than two.
51. The process of claim 40, wherein said second-conductive-layer depositing and patterning and source-layer-forming step (e) comprises depositing a conductive material selected from the list consisting of aluminum, copper, silver, gold, platinum, palladium, bismuth, conducting oxides, conducting nitrides, the refractory transition metals (titanium, vanadium, chromium, zirconium, niobium, molybdenum, hafnium, tantalum, and tungsten), the refractory transition metal carbides, the refractory transition metal nitrides, boron carbide, doped boron nitride, transition metal suicides, carbon in any of its conductive forms (doped diamond, graphite, amorphous carbon, fullerenes, nanotubes, or nanocoralline), silicon (N - type or P - type, polycrystalline, amorphous, or single-crystal), germanium, and mixtures, alloys, and combinations thereof.
52. The process of claim 40, wherein said second-insulating-layer-depositing step (f) comprises depositing an insulating material selected from the list consisting of glass, glass ceramic, quartz, aluminum oxide, sapphire, silicon oxide, silicon nitride, barium strontium titanate, titanium oxide, samarium oxide, yttrium oxide, tantalum oxide, barium titanium oxide, barium tantalum oxide, lead titanium oxide, strontium titanium oxide, strontium (zirconium, titanium) oxide, aluminum nitride, polyimide, parylene, and mixtures and combinations thereof.
53. The process of claim 40, wherein said second-insulating-layer-depositing step (f) comprises depositing an insulating material having an electric permittivity equal to or greater than two.
54. The process of claim 40, wherein said second-trench- forming step and emitter- tip-forming step (g) comprises directional etching with ions.
55. The process of claim 54, wherein said second-trench-forming and emitter-tip- forming step (g) further comprises plasma etching.
56. The process of claim 54, wherein said second-trench-forming and emitter-tip- forming step (g) further comprises wet etching.
57. The process of claim 40, wherein said second-trench-filling and planarizing step (h) comprises filling said second trench with an organic sacrificial material.
58. The process of claim 40, wherein said second- trench- filling and planarizing step (h) comprises filling said second trench with a sacrificial material selected from the list consisting of parylene, photoresist, wax, and silicon dioxide.
59. The process of claim 40, wherein said third-insulating-layer depositing step (i) comprises depositing an inorganic insulating material.
60. The process of claim 40, wherein said third-insulating-layer depositing step (i) comprises depositing an insulating material selected from the list consisting of glass, glass ceramic, quartz, aluminum oxide, sapphire, silicon oxide, silicon nitride, barium strontium titanate, titanium oxide, samarium oxide, yttrium oxide, tantalum oxide, barium titanium oxide, barium tantalum oxide, lead titanium oxide, strontium titanium oxide, strontium (zirconium, titanium) oxide, aluminum nitride, polyimide, parylene, and mixtures and combinations thereof.
61. The process of claim 40, wherein said third-insulating-layer depositing step (i) comprises depositing an insulating material having an electric permittivity equal to or greater than two.
62. The process of claim 40, wherein said access-hole-forming step (j) comprises reactive ion etching through said third insulating layer to at least said sacrificial material.
63. The process of claim 40, wherein said sacrificial-material-removing step (1) comprises oxygen plasma etching through said access hole.
64. The process of claim 40, wherein said sacrificial-material-removing step (1) comprises dissolving said sacrificial material with a solvent.
65. The process of claim 40, wherein said sacrificial-material-removing step (1) comprises wet chemical etching through said access hole.
66. The process of claim 40, wherein said vacuum-environment-providing step (m) comprises providing a vacuum pressure less than or equal to about one torr.
67. The process of claim 40, wherein said depositing and sealing step (n) comprises the steps of: o) depositing and patterning a top gate, p) depositing and patterning a conductive source contact, q) depositing and patterning a bottom gate contact, r) depositing and patterning a conductive drain, and
s) sealing said vacuum channel region.
68. The process of claim 40, wherein said depositing and sealing step (n) includes the step of: t) disposing said conductive drain spaced apart laterally from said emitting tip of said source layer.
69. The process of claim 40, further comprising the step of depositing a passivation layer over said device.
70. The process of claim 40, further comprising the steps of defining via-openings and depositing and patterning terminal metallurgy.
71. A process for fabricating an ultra-high frequency vacuum diode device, comprising the steps of: a) providing an insulating substrate; b) forming a source by disposing a lateral field emitter parallel to said substrate; c) forming an emitting tip on said a lateral field emitter of said source;
d) providing a conductive drain spaced apart laterally from said emitting tip of said lateral field emitter for receiving electrons; e) forming a first opening at least between said emitting tip of said lateral field emitter and said drain for a vacuum channel region; f) substantially covering said first opening to form a closed vacuum channel chamber;
g) evacuating said first opening; and h) sealing said vacuum channel chamber.
72. The process of claim 71, wherein the step (a) of providing an insulating substrate is performed by providing a base substrate, said base substrate having any degree of conductivity or semiconductivity, and depositing an insulating layer on said base substrate.
73. A process for fabricating an ultra-high frequency vacuum diode device, comprising the steps of:
a) providing an insulating substrate;
b) depositing a first insulating layer over said insulating substrate; c) depositing a first conductive layer and patterning to form a source layer parallel to said substrate; d) depositing a second insulating layer over said source layer; e) forming a trench for a vacuum channel region, while forming an emitting tip of said source layer, thereby completing formation of a lateral field emitter source; f) filling said trench with a sacrificial material and planarizing; g) depositing a third insulating layer extending over said sacrificial material; h) forming an access hole through said third insulating layer to said sacrificial material; i) forming an opening for a source via; j) removing said sacrificial material through said access hole; k) providing a vacuum environment; and
1) depositing and patterning a conductive source contact and a conductive drain, while sealing said vacuum channel region.
74. The vacuum field-effect device made by the process of claim 73.
75. The process of claim 73, wherein the step (a) of providing an insulating substrate is performed by providing a base substrate, said base substrate having any degree of conductivity or semiconductivity, and depositing an insulating layer on said base substrate.
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