WO1997042645A1

WO1997042645A1 - Field emission triode, a device based thereon, and a method for its fabrication

Info

Publication number: WO1997042645A1
Application number: PCT/RU1997/000133
Authority: WO
Inventors: Evgeny Invievich Givargizov; Nikolai Nikolaevich Chubun; Alla Nikolaevna Stepanova; Viktor Vladimirovich Zhirnov
Original assignee: Evgeny Invievich Givargizov; Nikolai Nikolaevich Chubun; Alla Nikolaevna Stepanova; Viktor Vladimirovich Zhirnov
Priority date: 1996-05-08
Filing date: 1997-04-30
Publication date: 1997-11-13
Also published as: JP2001501768A; RU2118011C1; KR20000010835A

Abstract

A field-electron-emission triode is based on silicon emitters prepared from an oriented whisker array. The whiskers were epitaxially grown on a (111) oriented single-crystalline silicon substrate by crystallization from vapor phase according to the vapor-liquid-solid mechanism. The same epitaxial process is used for preparation of columnar spacers that separate a gate (extracting) electrode from the substrate/cathode (spacers-1), and an anode from the cathode (spacers-2). Different diameters and heights of the tip emitters and of the columnar spacers are formed owing to different metal solvents that are used for initiation/localization of the oriented whisker growth. The semiconductor nature of the emitters makes it possible to use their high series resistance for uniformization of the field-electron-emission from large-area multiple-tip arrays. An additional uniformization of the emission is ensured owing to coating of the emitters with substances that decrease work function of the emitters.

Description

FIELD EMISSION TRIODE, A DEVICE BASED THEREON, AND A METHOD FOR ITS FABRICATION

Field of the invention

The present invention relates to devices that use the effect to emit electrons out of a solid into vacuum, to devices of vacuum microelectronics and, more particularly, to field emission cathodes including those with diamond coating, as well as to devices based on field electron emission such as field- emission displays, to microwave devices, etc.

Prior Art

Cathodes for field-electron-emission electronics and vacuum microelectronics are, as a rule, regular arrays of tip emitters formed by means of photolithography, etching, evaporation through a mask, etc.

It is known a field-electron-emision cathode prepared on a single- crystalline silicon substrate by controlled growing of whiskers on the same substrate with subsequent sharpening of the emitters and with coating of their apices by diamond or diamond-like carbon as materials that increase the emission of the cathode [1]. Such a cathode does not contain any gate ("extracting") electrode. This supposes that field-emission currents are controlled by application of the voltage to a remote electrode, for example, to an anode. This means that the device works in a diode mode. In such a cathode, the controlling voltage should be at the level 200 V or higher, while many applications need in cathodes that are able to work at controlling voltages 20-30 V compatiblle with modern microcircuits. It is known a field-electron-emission triode that contains tip emitters, a gate electrode made on a single-crystalline silicon substrate, and an anode, the tip emitters being separated from the gate electrode and from the anode by insulating spacers [2]. The gate electrode is there spaced from the emitters at a small (about 1 micrometer) distance; this increases the input capacity of the device. In such a triode, the gate electrode is separated from the emitters by a dielectrics that increases the capacity several times. The high capacity limits application of such a triode in microwave electronics. In addition, for stable field emission, vacuum in such devices should be as high as I O'⁸ Torr or better; this makes critical demands to the technology of fabrication of such devices.

It is known a display based on the diode-mode device [1]. The working voltages needed for such a display (200 V or higher) increase demands to the mechanical strength of the phosphor deposited onto the anode. This decreases the reliabilty of the display. In addition, other components and microcircuits (drivers) used in the display must be of high-voltage mode; this limits possibility to use standard integrated citcuits in such displays.

It is known a matrix-addressed display that contains a tip-emitter cathode arranged at single-crystalline silicon substrate, an anode, and a gate electrode spaced between the cathode and the anode [2]. In such a display, the distance between the gate electrode and the anode is small (about 50 micrometers). This makes it difficult to evacuate the anode-to-cathode space up to necessary ultrahigh vacuume. This is of special problem in the display where gases evolving from the phosphor can poison the cathode.

There are specific problems in the fabrication of the field-emission triodes and displays. One of them relates to the creation of spacers that separate the anode from the cathode, and the anode from the gate electrode. In the diode- made display described in the patent [1], the spacers are installed manually; this, of course, is a drawback of the technology. In the patent [2], the fabrication technology includes procedures for preparation of the tip emitters, of the spacers, of the gate electrode, and of the anode basing on an expensive submicron technology. The lower the gating voltages, the more complicated is the technology.

The drawbacks and limitations are eliminated in this invention. Summary of the invention

It is proposed a field-electron-emission triode that contains tip emitters made on an single-crystaline silicon substrate, a gate (extracting) electrode, and an anode, the tip emitters from the gate electrode, as well as the tip emitters from the anode, being separated by insulating spacers. The tip emitters are made from silicon whiskers grown epitaxially on the silicon substrate; the gate electrode is made as a separate body resting on the spacers. The spacers, at least those between the emitters and the gate electrode, are implemented as common, monolithic bodies with the substrate and coated by an insulating layer. The emitters and the spacers have different heights and different cross- sections, the emitters having a minimal height and a minimal cross-section, whereas the spacers separating the emitters from the anode having a maximal height and a maximal cross-section.

The gate electrode is made as a perforated dielectric plate with openings that are covered by a conducting grid. The gate electrode is resting on the spacers by the dielectric plate or by the conducting grid. The spacers have a shape of conical, cylindrical, prismatical, truncated-conical, or truncated-pyramidal columns. The spacers are distributed between the tip emitters or emitter groups. In a case of multiple-tip triode, the emitters can function as ballast series resistors. Ends (apices) of the tip emitters have coatings of a material (for example, of diamond or a diamond-like substance) that increases the electron emission. The task of the invention is also implemented by a design of a device

(display) that contains a matrix field-emission cathode with tips arranged on the single-crystalline silicon substrate, the anode, and the gate electrode placed between the cathode and the anode. The conducting grid of the gate electrode is sectioned as electrically-isolated buses, the anode is made of a transparent material, coated by a conductive layer and a phospor, the anode and/or the cathode being sectioned as electrically isolated conductive buses perpendicular to the buses of the extracting electrode. The device can be made on the basis of the above-described triode where the emitters, individual or groupped ones, are placed on the cathode buses, and the openings of the gate electrode are arranged against the emitters.

A method for fabrication of the triode is here proposed, too. The method includes formation of the tip-emitter cathode, of the gate electrode, of the anode, of spacers between the cathode and the anode and between the cathode and the gate electrode. The tip emitters and the spacers are created by growing of the whiskers from the vapor phase according to the vapor-liquid-solid mechanism with subsequent their sharpening and coating of the spacers with isolating layer. The gate electrode is made by evaporation of tungsten or molybdenum onto an aluminum foil. Sectioning and perforating of the electrode are performed by means of photolithography and plasma etching and, then, a through anodic oxidation of the aluminum foil is implemented. At first, the spacers are created and, then, the tip emitters are done. At the creation of the spacers and of the emitters by growing of whiskers, different metallic solvents of silicon, namely gold, platinum, or nickel are used. After the whisker growing, operations of their chemical sharpening, simultaneously with the removal of the solidified globules of the alloys of silicon with the solvents, formed on apices of the whiskers as a result of the vapor-liquid-solid growth mechanism, are made. At the creation of the emitters, a projection lithography is used.

A brief description of the drawings

The invention is illustrated by the following drawings.

Fig. 1. A scheme of the cathode with the gate (extracting) electrode that is supported by the spacers. Fig. 2. A scheme of the field-electron-emission triode with the spacers that support the anode.

Fig. 3. A scheme of the triode with an additional insulating layer between the emitters and the gate electrode.

Fig. 4. A scheme of another version of the field-electron-emission triode. Fig. 5. Schemes of the field-emission displays.

Fig. 6. A scheme of a fragment of the gate electrode. Fig. 7. A scheme of the triode with tip emitters against grid-covered openings in the gate electrode.

Fig. 8. An arrangement scheme of the components on a fragment of the display (a projection onto the cathode).

Best version for realization of the invention

A cathode with the gate (extracting) electrode is shown in fig. 1. This component of the field-electron-emission triode contains tip emitters 1, prepared from whiskers grown epitaxially on the single-crystalline silicon substrate 2. Spacers 3, coated by a dielectric (insulating) layer 4 (let's name them as spacers- 1), are grown, again epitaxially, on the same substrate. The emitters and the spacers form a monolithic single-crystalline unit with the substrate. The gate electrode 5, implemented on a dielectric plate 6, is resting on the spacers. The plate is coated, from the emitter side, by a metallic layer 7. In the plate, openings are made, the openings being covered by a conductive grid 8 that continues the metallic layer 7. The emitters and the spacers-1 have different heights so that the distance from the grid 8 to the emitters 1 is at least several micrometers.

A triode that contains, in addition to the cathode with the gate electrode, also an anode 9, resting on the substrate 2 via spacers 10 (let's name them as spacers-2) coated by the insulating layer 1 1 , is shown in fig. 2. The spacers are prepared, again, from whiskers epitaxially grown on the silicon substrate and form a monolithic unit with the substrate. Their height exceeds, as a rule, significantly the height of the spacers-1. A version of the field-electron-emission triode, where the conducting grid

8 and the metallic layer 7 of the gate electrode 5 are arranged from the anode side, is shown in fig. 3. In such a version of the triode, the isolation between the gate electrode and the emitters is enhanced, or it is no more necessity in coating of the spacers-1 by an insulating layer. One more version of the field-electron-emission cathode, where the anode is arranged directly on the gate electrode 5, is shown in fig. 4. Here, the anode is isolated from the grid 8 by a dielectric plate 6. In fig. 5a and 5b are shown two versions of displays based on the above- described cathodes. These displays contain, in addition to the cathode element with the gate electrode and spacers-1 and spacers-2 , also an anode implemented of a transparent material (for example, a glass) 12 coated with a layer of the transparent conductor 13 (usualy, this is a complex indium-tin oxide) and a phosphor 14.

The above-described spacers-1 and spacers-2 are grown by chemical vapor deposition (CVD) of whiskers according to the vapor-liquid-solid mechanism with subsequent removal, from the apices of the whiskers, of solidified globules of alloys of silicon with the metallic solvents. The removal is implemented by chemical etching. Accordingly, the spacers have a shape of conical, truncated-conical, or truncated-pyramidal columns with some curvature radius at their apices, ln order to decrease an electrical capacity of the contacts of the spacers with the electrodes, the radius must be as small as possible. The tip emitters are also grown from the vapor phase according to the vapor-liquid-solid mechanism. Owing to this technology, their geometric parameters (height, diameter, radius of curvature) are controllable. In particular, the height of the emitters is tens of micrometers, their diameter is l to 5 micrometers, and the radius of curvature is about 10 nanometers. The resistivity of the emitter material (silicon) is 10 to 100 Ohm.cm. Accordingly, the electrical resistance of an individual emitter is IO⁶ to IO⁷ Ohm. Such a resistance in the field-emission circuit implements a function of the ballast resistor. This levels the currents of the individual emitters in their massives and, in such a way, uniformity of the emission on large areas is ensured. Apices of the emitters are coated with a material that is characterized by a low electron work function, for example, with diamond or diamond-like substance. Owing to this, the field-electron-emission at relatively low voltages is ensured, ln addition, this decreases the spread of the field-electron-emission currents from different emitters, i.e., ensures an emission uniformity in the emitter massives.

A fragment of the gate electrode, grids 8 and tentative contact areas 15 of the gate electrode with the spacers-1 are shown in fig. 6. Here are also shown openings 16 for passings of the spacers-2. The both kinds of spacers are arranged in areas between groups of emitters positioned against the grids of the gate electrode, the spacers-1 and spacers-2 having different periods. The density of the spacers-2 per an area unit is smaller than that of the spacers-1 ; however, their diameter is significantly larger. For the aim of addressation, the groups of the emitters are arranged on linear areas of electrically-isolated conducting cathode buses along one of the coordinates. Another coordinate, perpendicular to it, is arranged on the gate electrode or on the anode.

A fragment of the gate electrode with the emitters against the grid-covered openings in the electrode is shown in fig. 7. Spacers-1 (designated as 17) are indicated as dashed hexagons. Also by dashed lines are delineated the buses at the cathode on which groups of emitters are arranged. The perpendicular buses of the gate electrode are not shown.

In fig. 8, an arrangement of components on a fragment of the display (a projection on the cathode) is shown; here, groups of emitters are placed on the cathode buses 18, against to the openings in the gate electrode. Hexagons 17 represent spacers-1. A spacer-2 is shown as a relatively large hexagon 19.

In the fabrication technology of the field-electron-emission cathode (and of the corresponding display) of such a design the following factors play a decisive role: (a) a special design and a special technology for preparation of the gate electrode;

(b) a dependence of the growth rate of whiskers that serve as a basis for the preparation of the tip emitters and of the both kinds of the spacers;

(c) possibilities of projection photolithography. The gate electrode represents a plate of aluminum oxide, 20 micrometers in thickness, perforated with the spacing of openings 300 micrometers. Each opening with the diameter 200 micrometers is covered by a tungsten grid (see figs. 6 to 8). The gate electrode is prepared on a technological glass substrate on which, at first, a tungsten film, 2 micrometers in thickness, is deposited. By means of photolithography and plasmochemical etching, a pattern of the grid is made on the tungsten film. Then, an aluminum layer is deposited on the tungsten film and, by means of photolithography, and of liquid etching, a pattern of the perforation and a through anodic oxidation of the aluminum down to the tungsten film is performed. Finally, the technological glass substrate is dissolved, and a free grid is formed. The openings of 150-micrometers-in-diameter for the spacers-2 are symmetrically made, with the spacing between the (relatively- small) openings.

The photomasks used for the perforation of the gate electrode have marks for fitting with photomasks used afterwards for fabrication of the spacers and of the emitters.

Before [3,4], it was established that: (1) growth rate of whiskers according to the vapor-liquid-solid mechanism depends on their diameter, the rate decreasing at diameters lower than 1 micrometer and larger than 10 micrometers;

(2) the growth rate depends on the kind of metal solvent; for example, in the series Au:Pt:Ni it is minimal with gold and maximal with nickel.

Example 1

Below, a procedure for preparation of the spacers and of the emitters is described.

(a) On thermally-oxidized silicon wafer (the thickness of the silicon dioxide is 0.5 to 0.7 micrometers), by means of photolithography and evaporation, a regular square array of platinum particles, 20 micrometers in diameter, 1 micrometer in thickness, is created at distances 300 micrometers between them, centro-symmetrically relative to the openings in the gate electrode, except of the areas intended for the openings of the spacers-2 (see above). The platinum particles are used then for creation of the spacers-1.

(b) The same wafer is, afterwards, coated by a pyrolithical oxide, 0.5-1 micrometer in thickness. By means of a repeatedly-fitted photolithography, 100- micrometer-diameter openings are opened, a nickel film, 0.5 micrometer in thickness, is evaporated through the openings and, then, the film is electrolithically thickened up to 5 micrometers in thickness. The nickel particles are used for creation of spacers-2 . (c) The same wafers is, again, coated by the pyrolithical oxide, 0.5-1 micrometer in thicknness, and, by the repeatedly-fitted photolithography, against the openings in the gate electrode, at circular areas 100 micrometers in diameter, are created regularly, at distances 10 micrometers one from another, openings of 4 micrometers in diameter. The openings are then deepened down to 0.5 micrometers inside the substrates. In the hollows, 0.2-thick gold film is evaporated. From all the surface, except of the bottom of the hollows, the gold then removed, for example, by mechanical wiping. Finally, by a solution of the fluoric acid, the oxide formed is removed from all the surfaces. The gold particles remained are then used for creation of the emitters.

(d) Whiskers are grown on the substrate by the vapor-liquid-solid mechanism at I000°C, in a flow reactor, in the hydrogen-silicon tetrachloride gas mixture at 5% chloride concentration. Duration of the crystallization process is chosen so that the whiskers, intended for subsequent transformation of them into the tip emitters and into the spacers, have different height: minimal - for emitters, and maximal - for the spacers-2, in accordance with the publication [3] .

(e) Globules of the alloys of silicon with the metallic solvents/catalysts solidified on the apices of the whiskers are removed by a treatment in the solution that etches silicon with a slow rate. During this procedure, simultaneously, a sharpening of the whiskers occurs with the formation of the spacers and emitters.

(f) After the preliminary sharpening, all the tips are oxidized with formation of the oxide up to 1 micrometer in thickness.

(g) The oxide from the tip emitters is removed by a dosed-out coating of the cathode surface by a solution that contains the fluoric acid. At such a procedure, apices of all the spacers remain coated by the oxide.

(i) The tip emitters are coated by diamond in a hot-filament chemical vapor deposition [5] . During this coating, all the spacers are coated by diamond as well. However, this procedure is not critical for the spacers because the diamond, being undoped, conserves its isolated properties.

(k) Gate electrode, prepared according to the above-described technology, is placed on the spacers-1 folowing to the scheme of fig. 2. At this arrangement, spacers-2 are passing through the above-indicated openings with the diameter 150 micrometers, opening-to-opening distances 1.5 mm.

A fragment of such a device is shown in figs. 5, 6, and 7.

(1) Aftewards, the anode is arranged on the spacers-2. (m) The prepared structure is installed in a vacuum-tight chamber having electrical contacts to the cathode, anode, and the gate electrode, is pumped and germetized.

Example 2 The same procedure, as in the Example l is made, however, after the stage (b), spacers are grown while, for the preparation of the tip emitters, a projection lithography is used. By the lithography, the gold particles are created at the bottom of the hollows, while the gold is removed from other areas of the oxide with the fluoric acid.

Example 3

In order to fabricate the display, mutually perpendicular systems of conductive buses are created: one at the gate elecrode, and another at the anode or at the cathode. In the case if the buses are created at the cathode (as it is shown in figs. 7 and 8), the conductive buses have a width, for example, 400 micrometers and the intervals between them are 200 micrometers. The spacers are created at the bare intervals whereas the emitters are done at the bus areas, and the groups of the emitters are created at areas 100 micrometers in diameter against the openings in the gate electrode, the distances between individual emitters in the groups being 10 micrometers. The spacers can have a hexagonal cross-section, as it is inherent in the silicon whiskers grown, as here, on the single-crystalline substrate of the orientation (1 1 1). However, they can have also other shapes (e.g., circular or trianle ones). Example 4

A display, according to the scheme depicted in fig. 5a, is fabricated with the emitters 50 micrometers in height, the radius of curvature at their apices 10 nanometers, and with the spacers- 1 having height 60 micrometers. At the clearance "emitter-grid" 10 micrometers, the capacity in such a display, as compared with the Spindt cathode, is decreased 10 times. If take into account the fact that, between the cathode and the gate electrode any layer with a dielectric constant about 5 units, as in the Spindt cathode, is absent, the real decrease of the capacity is about 50. Taking into account that, at the standard sharpening, the emission currents sufficient for displays are obtained at the voltage field l V/um, we have the necessary working voltage at 10 V.

References

1. E.I.Givargizov, V.V.Zhirnov, A.N.Stepanova, L.N.Obolenskaya, Field Emission Cathode and A Device Based Thereon, EP 0 726 589 Al , HO U 1/30,

(1996)

2. C.A.Spindt and C.E.Holland, Matrix addressed flat panel display, US Pat. 5.015.912, Cl. 313/495 (1991). 3. E.I.Givargizov, Fundamental aspects of VLS growth, J. Crystal Growth, 31, 20-30 (1975).

4. E.I.Givargizov, Growth of whiskers by the vapor-liquid-solid mechanism, in Current Topics in Materials Science, Ed. E.Kaldis, vol. 1 (North-Holland, Amsterdam), pp. 79-145 (1978). 5. E.I.Givargizov, V.V.Zhirnov, A.V. Kuznetsov, and P.S.Plekhanov, Growth of diamond particles on sharpened silicon tips, Mater. Lett. 18, 61-63 (1993).

Claims

1. A field-electron-emission triode that contains tip emitters, a gate

(extracting) electrode made on single-crystalline silicon substrate, an anode, the tip emitters being separated from the gate electrode and from the anode by insulating spacers, wherein the emitters are implemented of silicon whiskers grown epitaxially on the single-crystalline silicon substrate, the gate electrode is implemented as a separate detail resting on the spacers, the spacers, at least those arranged between the emitters and the gate electrode, are implemented as a monolithic single-crystalline unit (body) with the substrate and coated by an innsulating layer, the emitters and the spacers are different in their heights and cross-section, the emitters having a minimal height and cross-section, while the spacers between the emitters and the anode having a maximal height and cross- section.

2. The field-electron-emission triode according to the claim I wherein the gate electrode is implemented as a perforated dielectric plate with openings that are covered by a conductive grid.

3. The field-electron-emission triode according to the claim 2 wherein the gate electrode is resting on the spacers by its dielectric side.

4. The field-electron-emission triode according to the claim 2 wherein the gate electrode is resting on the spacers by its conducting- grid side.

5. The field-electron-emission triode according to the claims 3 or 4 wherein the spacers have a shape of conical, cylindrical, prismatical, truncated-conical, or truncated-pyramidal columns.

6. The field-electron-emission triode according to the claim 5 wherein the spacers are arranged between the tip emitters or between their groups.

7. The field-electron-emission triode according to the claim 6 wherein, in the case of multitip triode, the tip emitters implement a function of ballast resistors.

8. The field-electron-emission triode according to the claim 7 wherein apices of the tip emitters are coated by a material that enhances the electron emission.

9. The field-electron-emission triode according to the claim 8 wherein the apices are coated by diamond ar diamond-like material.

10. A device for optical imaging of information (display) that contains a matrix field-electron-emission cathode with tip emitters placed on a single- crystalline silicon substrate, an anode, and a gate electrode placed between the cathode and the anode wherein a conducting grid of the gate electrode is sectioned as electrically-isolated buses, the anode is prepeared of a transparent material and coated by a transparent conducting film and a phosphor, the anode and/or the cathode is sectioned as electrically-isolated buses perpendicular to the buses of the gate electrode.

1 1. The device according to the claims 1 or 2 or 9 or 10 wherein the emitters, single or groupped, are arranged along the cathode buses.

12. The device according to the claims 1 to 1 1 wherein the openings of the gate electrode are arranged against the emitters.

13. A method for fabrication of the field-electron-emission triode that includes operations for formation of tip-emitter cathode, gate electrode, anode, and insulating spacers between the cathode and the anode wherein additional spacers between the emitters and the gate electrode are formed, the tip emitters and the spacers are implemented by growing of whiskers from vapor phase according to the vapor-liquid-solid mechanism with subsequent their sharpening and by coating of the spacers by an isolating layer, the gate electrode is implemented by evaporation of tungsten or molybdenum on an aluminum foil, by sectioning and perforating of the electrode by means of photolithography and plasmochemical etching with subsequent through anodic oxidation of the aluminum.

14. The method according to the claim 13 wherein, firstly, the spacers are made and, then, the tip emitters are done.

15. The method according to the claims 13 or 14 wherein different metallic solvents are used for the growing of whiskers intended for creation of the tip emitters and of the spacers.

16. The method according to the claim 15 wherein gold is used for fabrication of the tip emitters, platinum is used for fabrication of the spacers separating the emitters from the gate electrode, and nickel is used for fabrication of the spacers separating the cathode from the anode.

17. The method according to the claim 14 wherein a projection lithography is used at creation of tip emitters.

18. The method according to the claims 14 or 15 wherein, after the growing of the whiskers, the operation of their sharpening is performed with simultaneous removal of the solidified globules of the alloy of silicon with the solvent (catalyst) that were formed at apices of the whiskers during the crystallization.