Vacuum devices

Abstract: A vacuum valve device comprises a substrate (1) on which is formed an undoped silicon layer (3) from which a silicon dioxide layer (5) is grown. First, second and third electrode structures (7, 9, 11) are formed on the silicon dioxide layer by depositing a metallic layer and etching away unwanted portions of the layer. The first electrode structure (7) has a pointed end (8) and/or a sharp edge and/or is formed of low work function material so that, when a suitable voltage is applied between the first and third electrode structures, electrons are emitted from the first electrode structure due to a field emission process. Electrons therefore flow from the first to the third electrode structure substantially parallel to the substrate. The third electrode structure acts as a control electrode.

Description: This invention relates to field emission vacuum devices. In recent years there has been a resurgence of interest in vacuum devices as radiation hard alternatives to semiconductor devices. Known vacuum devices are however normally discrete, relatively large devices. US-A-3 748 522 discloses a thermionic (i.e. not field emission) device in which cathode, gate and anode electrodes are formed on a substrate in a substantially planar configuration so that electron flow from the cathode to the anode is along a path generally parallel to the substrate. The gate electrode lies in the path of the electron flow. GB-A-2 054 959 discloses a semiconductor cathode which emits electrons in a direction perpendicular to the plane of the cathode. An accelerating electrode is not coplanar with the cathode but spaced from it. US-A-3 678 325 discloses a tapered cathode body with metal ions implanted therein. An article in Japanese Journal of Applied Physics, Vol. 10, No. 6, June 1971, by C. Munakata, entitled "A Scanning Electron Microscopic Method of Obtaining Electric Field Distributions Using Solid-State Models", pages 781-785, discloses a model for a hairpin-type electron gun in which a tapered electrode formed on a substrate represents the filament of the electron gun. US-A-4 578 614 discloses a field emission device having a cathode tip which points away from the substrate. The initial emission is therefore perpendicular to the substrate. An article in Journal of the Electrochemical Society, Vol. 133, 1986 by C.Y. Lu and N.S. TSAI, entitled "Thermal Oxidation of Undoped LPCVD Polycystalline - Silicon Films", pages 446-447, discloses thermal growth of a silicon dioxide layer from an undoped silicon layer. It is an object of the present invention to provide a vacuum device which is of relatively small dimensions and is capable of integration. According to one aspect of the invention there is provided a field emission vacuum device, comprising a substrate; electron emission electrode means for field emission therefrom, control electrode means and electron collection electrode means, all being applied to the substrate in a substantially coplanar configuration and being located within an evacuated space, whereby electrons emitted by field emission from the emission electrode means flow to the collection electrode means along a path which is substantially parallel to the substrate; the control electrode means for regulating the flow from the electron emission electrode means to the electron collection electrode means lying substantially outside said path. According to another aspect of the invention, there is provided a process for forming a field emission vacuum device, comprising applying to a common substrate, as a substantially co-planar construction, electron emission electrode means for field emission therefrom, control electrode means and electron collection electrode means, for electron flow of electrons emitted by field emission from the emission electrode means to the collection electrode means along a path substantially parallel to the substrate, forming the control electrode means for regulating the flow from the electron emission electrode means to the electron collection electrode means to lie substantially outside said path. The emission electrode means, when negatively biased relative to the collection electrode means, acts as a source of electrons (a cathode) preferably by virtue of its having a lower threshold voltage for electron emission or by virtue of its having a larger electric field strength at its surface than the collection electrode means. The electrons are emitted from the cathode by an electric field induced process, whereby the device operates at ambient temperatures without requiring internal or external heat sources, as would be required for thermionic emission. The electrons are collected by the collection electrode means (an anode), which is biased positively with respect to the cathode, and since the anode is formed on the same substrate as the cathode, the electron motion is substantially parallel to the plane of the substrate. The device also includes one or more additional structures, substantially co-planar with the emission and collection electrode means, to act as control electrodes (i.e. grids) for modulating the cathode-anode current. Such control electrodes may operate by controlling the electric field at the cathode, thereby producing a large transconductance in the device, by virtue of the strong dependence of the emitted electron current on the field strength at the cathode. Embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, in which:- Figure 1 is a schematic pictorial view of a first device in accordance with the invention, the scales of the components being distorted in order to clarify the figure; Figure 2 is a cross section through the device of Figure 1 along the line II-II; Figure 3 is a cross section through a first modification of the device of Figure 1; Figure 4 is a cross section through a second modification of the device of Figure 1; Figure 5 is a schematic plan view of a two-electrode field emission device not being an embodiment of the invention but being useful for understanding the invention; Figure 6 is a schematic plan view of a second device in accordance with the invention; Figure 7 is a schematic plan view of a third device in accordance with the invention; Figure 8 is a schematic cross section through a fourth device in accordance with the invention, and Figure 9 is a schematic view of a fifth device in accordance with the invention. Referring first to Figures 1 and 2, the first device to be described comprises a sapphire base 1 on which is grown an undoped silicon layer 3. The free surface of the layer 3 carries a thermally-grown silicon dioxide layer 5 which is between 1 and 2µm thickness and is thereby able to withstand electric fields of 2 x 10⁸ volts/metre. The growth of this oxide layer preferably results in the complete oxidation of the layer 3. On this layer 5 there are formed three metallic electrode structures 7, 9, 11 constituting respectively the cathode, grid and anode of the device, as further explained below. The electrode structures are formed on the underlying silicon dioxide layer 5 by evaporation or sputtering of a metallic layer of a few tens of nanometres (a few hundred angstroms) to a few microns in thickness covering the layer 5. A lithographic technique is then used to etch through portions of the metallic layer selectively to produce the electrode shapes as shown in the figure. The cathode, grid and anode electrode structures 7, 9 and 11 respectively, thus formed are therefore coplanar. The whole device is then encapsulated, either as a single unit or with a number of similar devices formed on the same sapphire base, within a suitable evacuated enclosure (not shown). In use of the device, a voltage source (not shown) is connected across the cathode and anode electrode structures 7 and 11. Due to the high field gradients in the vicinity of the apex of the cathode electrode structure 7, that structure will have a lower electron emission threshold voltage than the anode electrode structure 11 and, for negative biases exceeding this threshold value, will emit electrons by an electron field emission process. The high electric field at the emission tip 8 of the cathode structure 7 is due to the thinness of the metal layer, the lithographic shaping in the plane of the layer, and its close proximity to the positively-biased grid 9 and/or anode 11 electrodes. Hence, the device may be made to operate as a rectifier, with a preferred direction of electron flow when the cathode is negative with respect to the anode structure. Suitable electrical biases may be applied to the grid electrode structure 9 in order to further modulate this electron flow. Non-linear characteristics suitable for digital switching applications may readily be achieved, and the operation of the device is particularly fast as its speed will not be limited by the velocity of sound, which normally limits the speed of operation of solid state devices. It will be appreciated that, whilst In the device described above the cathode electrode structure 7 and the anode electrode structure 11 are formed from the same metallic layer, the difference in electron emissivity between the cathode and anode electrode structures may be enhanced further by choosing materials of different thicknesses, layers of different shapes in the electrode plane or materials of different work functions for these two structures. Any inhomogeneity in the material composition of the cathode structure will further enhance the local field strength, thereby also increasing the electron emissivity of the cathode electrode structure. In particular, the electron emissivity of the cathode electrode structure may also be increased by the implantation of suitable dopant materials, resulting in increased electron emission from the implanted sites. One particularly suitable dopant material is carbon. It will be appreciated that in some devices in accordance with the invention a layer of material such as carbon may advantageously be carried on the surface of the cathode structure rather than implanted therein. Turning now to Figure 3, in order to reduce the danger of electronic short circuits through the silicon dioxide layer 5, it may be advantageous to etch through at least part of this layer between the cathode 7 and grid 9 electrode structures and between the grid 9 and anode 11 electrode structures to produce the supported electrode structures 7, 9, 11 as shown in this figure. Subsequent isotropic etching may be used to produce undercut electrode structures as shown in Figure 4. With modern lithographic techniques it is found that the above etching can be performed to produce devices of 1µm and less separation between the anode and cathode electrode structures, this resulting in switch-on voltages of 100 volts and less. Turning now to Figures 5, 6 and 7, it is clear that many alternative configurations are possible for devices in accordance with the invention. The embodiment of Figure 5 is not an embodiment of the invention. However, Figure 5 illustrates how a wide emission edge 12 of a cathode can be realized. Figure 5 shows a device in which a wide emission edge 12 of a cathode 13 allows a larger current flow than the cathode tip 8 of Figure 1. For operation as a diode device with an applied voltage of about 100v, the gap between the cathode 13 and the anode 11 should be approximately 1µm, but will be dependent upon both the work function of the cathode 13 and the thickness of the metal of the cathode. Generally such a cathode electrode structure would be formed of a lower work function material than that of the anode structure. Figure 6 shows a device configuration in which a cathode electrode structure 17 is of needle-like form, the grid electrode structure comprising two similar needle-like conductive patterns 19 and 21 and the anode electrode structure 11 being of rectangular form as before. Such a device configuration results in a particular sensitivity of the device characteristics to electric fields applied across the grid electrode structure. The same is true of a device configuration shown in Figure 7, in which a cathode electrode structure 25 is of "V" formation. In this configuration a grid electrode structure 27 is disposed round the tip of the "V" structure, so that particularly strong field gradients are present round the tip of the cathode 25. Such a disposition of the grid 27 should allow operation of the device with the grid biased negatively with respect to the cathode. In such a case, the anode 11 would have to be approximately 1µm from the tip of the cathode 25 in order to allow operation with a 100 volt potential difference between the anode 11 and the cathode 25. It will be appreciated that where the grid electrode structure is to be negatively biased, this electrode structure will generally be formed from a material of higher work function than that of the cathode structure In order to avoid electron emission from the grid electrode structure. Such devices will, of course, require a two stage metallisation process in order to deposit the required electrode structures. In addition, such a two stage metallisation will also be required to provide a thicker anode structure, which will again give assymmetric current/voltage characteristics as a result of lower geometric field enhancement at the anode. For particularly small devices requiring two-stage metallisation, a self-aligning metallisation process is desirable. Figure 8 shows a device in which an etched channel 23 is formed in a silicon dioxide layer 26, an initial metallisation of a low work function material 28 being followed by a metallisation of a high work function material 29 using the same masking structures. The upper metallised area within the channel 23 may be used as a grid electrode structure. Since the initial low work function layer 28 in the channel 23 is completely covered by the high work function layer 29, this grid electrode can be operated either positively or negatively with respect to the upper electrodes 30 and 31. It should be noted that the configuration of Figure 8 allows an operable device to be achieved with a close spacing of the cathode, anode and grid structures, irrespective of the number of metallisations. It is found that for devices of the general forms shown in Figures 1 to 8, reasonable operating voltages are possible for anode-cathode electrode structure separations of between 0.5 and 20µm, the grid electrode structure being biased between the cathode and anode voltages at separations of up to 5µm from the cathode electrode structure. More complex electrode structures are, of course, possible. Figure 9 shows a device in which a cathode electrode structure 32 is in the form of multiple undercut tips, and an anode electrode structure 33 is in the form of a rectangular strip, as before. A grid electrode structure 35 comprises a series of metallic pins 41 anchored to a doped stripe 37 in the underlying silicon 39. It will be appreciated that whilst in the devices described above the electrode structures are carried on a layer of silicon dioxide grown from a layer of silicon, which is in turn carried on a sapphire base, the electrode structures may be carried by any large band gap insulating substrate. The use of a sapphire base is particularly useful, however, as sapphire is a radiation hard material and is readily available with an epitaxial silicon layer, which can be oxidised to give an easily etchable substrate.

Claims: A field emission vacuum device, comprising a substrate (1); electron emission electrode means (7) for field emission therefrom, control electrode means (9) and electron collection electrode means (11), all being applied to the substrate in a substantially coplanar configuration and being located within an evacuated space, whereby electrons emitted by field emission from the emission electrode means flow to the collection electrode means along a path which is substantially parallel to the substrate; the control electrode means for regulating the flow from the electron emission electrode means (7) to the electron collection electrode means (11) lying substantially outside said path. A device as claimed in Claim 1, characterised in that the emission electrode means (7) has a lower work function than the collection electrode means (11), whereby electrons are preferentially emitted from the emission electrode means. A device as claimed in Claim 1, characterised in that the emission electrode means (7) has a thin edge (12) facing the collection electrode means (11) for enhancement of electron emission from the first electrode structure. A device as claimed in any preceding claim, characterised in that the emission electrode means (7) tapers in a direction towards the collection electrode means (11) for enhancement of electron emission from the emission electrode means. A device as claimed in Claim 1 or Claim 2, characterised in that the emission electrode means (7) includes an implanted dopant for enhancement of electron emission from the emission electrode means. A device as claimed in any preceding claim, characterised in that the emission electrode means (7) has a surface coating for enhancement of electron emission from the emission electrode means. A device according to Claim 1, characterised by a channel (23) in the substrate (26); a first portion of a first conductive layer (28) in the channel; and second and third portions of the first conductive layer on the substrate on opposite sides of the channel, said first, second and third portions being electrically isolated from each other and forming, respectively, the control electrode means, the emission electrode means and the collection electrode means. A device as claimed in Claim 7, characterised by first, second and third portions of a second conductive layer (29) deposited on said first, second and third portions, respectively, of said first layer (28), the material of said second conductive layer (29) having a higher work function than the material of said first conductive layer (28). A process for forming a field emission vacuum device, comprising applying to a common substrate (1),as a substantially co-planar construction, electron emission electrode means (7) for field emission therefrom, control electrode means (9) and electron collection electrode means (11), for electron flow of electrons emitted by field emission from the emission electrode means to the collection electrode means along a path substantially parallel to the substrate, forming the control electrode means for regulating the flow from the electron emission electrode means (7) to the electron collection electrode means (11) to lie substantially outside said path. A process as claimed in Claim 9, characterised by forming an insulating layer (5) on the substrate (1); forming a conductive layer over the insulating layer; and etching away one or more portions of the conductive layer to leave areas of the conductive layer forming the emission, control and collection electrode means (7,9,11) spaced from one another. A process as claimed in Claim 10, characterised in that an undoped silicon layer (3) is deposited on the substrate (1) and a silicon dioxide layer is thermally grown therefrom to form the insulating layer (5). A process as claimed in Claim 10 or Claim 11, characterised in that the conductive layer is formed by vacuum evaporation or sputtering of refractory metal, such as tungsten, molybdenum, or a material or combination or materials giving a low work function surface. A process as claimed in Claim 10 or Claim 11, characterised in that a dopant is implanted into the emission electrode means (7). A process as claimed in Claim 10, characterised in that portions of the insulating layer (5) between the emission and control electrode means (7,9) and between the control and collection electrode means (9,11) are etched away. A process as claimed in Claim 14, characterised in that following the etching away of the portions of the insulating layer (5), the insulating layer beneath the facing edges of the electrode structures is undercut by isotropic etching. A process as claimed in Claim 9, characterised by forming an insulating layer (26) on the substrate; etching a channel (23) into the insulating layer; depositing a first layer (28) of a low work function material over the insulating layer; and depositing a second layer (29) of a high work function material over the first layer; wherein the depth of the channel is sufficient such that the portion of the first and second layers within the channel is separated from the portions on either side of the channel, whereby the emission and collection electrode means (30,31) are formed on either side of the channel, and the control electrode means is formed within the channel.

Inventors: CADE NEIL ALEXANDER, LEE ROSEMARY ANN, CADE, NEIL ALEXANDER, LEE, ROSEMARY ANN
Applicants: GEN ELECTRIC CO PLC, THE GENERAL ELECTRIC COMPANY, P.L.C.
Application year: 1987
Main classification: H01J21
IPC codes: H01J21