GB2204991A

GB2204991A - Vacuum electronic device

Info

Publication number: GB2204991A
Application number: GB08711652A
Authority: GB
Inventors: Neil Alexander Cade; Rosemary Ann Lee
Original assignee: General Electric Company PLC
Current assignee: General Electric Company PLC
Priority date: 1987-05-18
Filing date: 1987-05-18
Publication date: 1988-11-23
Anticipated expiration: 2007-05-18
Also published as: GB8711652D0; GB2204991B

Abstract

A cold cathode for a vacuum electronic device is formed by depositing a coating of an organic material, such as 22-tricosenoic acid, on the emission surface of a cathode body. The presence of the organic coating, in use of the cathode, enhances electron emission from the emission surface. The coating is preferably deposited by the Langmuir-Blodgett technique. The deposited coating may be polymerised by electron bombardment, either from an outside electron beam source or from the cathode itself.

Description

VACUUM ELECTRONIC DEVICES This invention relates to vacuum electronic devices, and particWlarly to a method for form'n c2+ e coatings for sud devices.

There has been a recent upsurge in interest in small vacuum electronic devices, particularly such devices in which the emission of electrons is obtained by field emission or photo emission from a cold cathode.

In order to achieve efficient electron emission, there is a need to provide cold cathodes which exhibit a low work function and which are not readily poisoned by gases remaining in imperfectly evacuated housings containing the cathodes.

It is an object of the present invention to provide a method of forming improved cathodes for such vacuum electronic devices.

According to the invention there is provided a method of forming a cold cathode of a vacuum electronic-device, which method comprises depositing on the emission surface of a cathode body a coating of an organic material.

Preferably the coating is deposited by the Langmuir-Blodgett film deposition technique, and preferably comprises a layer or layers of 22-tricosenoic acid. After deposition the layer or layers are preferably polymerised by electron beam bombardment to increase the saturation of the polymer.

An embodiment of the invention will now be described, by way of example, with reference to the accompanying drawing, in which: Figure 1 is a schematic pictorial view of apparatus for coating a cathode using the Langmuir-Blodgett film deposition technique: Figure 2 is a pictorial view of a cathode sample; and Figure 3 is a schematic view of apparatus used in testing samples of cathodes coated in accordance with the invention.

Referring to Figure 1, an example of an apparatus for depositing a Langmuir-Blodgett (LB) film on a substrate 1 comprises a trough 2 containing high-purity water, a member 3 which is slidable over the water surface under the control of a weight or of a more sophisticated driving system 4, and means 5 to pass the substrate successively downwards and upwards through the water surface for a desired number of passes.

A layer of monomolecular thickne U: trial to be deposited is firstly spread over the water surface by dissolving the material in an organic solvent and dispensing the solution, drop by drop, on to the surface. The solvent evaporates, leaving a loosely-packed monomolecular layer of the material on the water surface. The surface area of the layer is then reduced, by moving the member 3, until the adjacent molecules touch. The required amount of surface area reduction is determined by monitoring the surface tension of the water. When the molecules are closely packed, the surface tension begins to decrease. Surface area reduction beyond that point will cause the monomolecular layer to break up.If the substrate is then passed downwards through the water surface, a monomolecular layer of the material will be deposited on the substrate and will subsequently harden, forming an LB film. Successive monomolecular layers can be deposited, one over the other, by successive passes of the substrate through the water surface.

In putting the present invention into effect, a monomolecular layer of a suitable organic material, preferably 22-tricosenoic acid as a trichloroethane solution, is deposited on the water surface. The substrate 1, which comprises a cathode body formed of, for example, silver-plated copper, or copper with or without a low work function, and/or a refractory metal coating, is passed through the water surface a predetermined number of times. The surface coating is then polymerised, preferably by electron bombardment, either from an external electron beam source (not shown) or by electrons generated by the cathode itself when under the influence of an intense field, to produce a saturated organic film. The coating thickness may be in the range of, for example, 3nm to 1) m, the accuracy being limited only by the molecular length scale.

The organic polymer coating has negative electron affinity with respect to the vacuum, and also protects the metal of the cathode from at least some oxidative attack effects. Silver rapidly forms sulphide in the presence of air, and this formation is retarded by the LB coating. Due to the negative electron affinity, the coating does not trap negative charge, so there is no consequential decrease in electric field at the surface of the cathode when in operation. On tne other hand, positive charge might be trapped by the coating, which would lead to an increase in the electric field at the cathode, and hence enhanced electron emission.

In order to test the value of the method according to the invention, cathode samples 6 (Fig. 2) were prepared by depositing a number of layers of 22-tricosenoic acid on to a clean 14 mm diameter copper substrate coated with a layer of evaporated silver of approximately 80 nm thickness. The 22-tricosenoic acid was spread as a 10-3M 1,1,1-trichloroethane solution on to a pure water subphase (Elgastab spectrum) and was compressed to a surface pressure of 35mN/m. Multilayer samples were prepared by lowering (at O.lmn/s) and raising (at 0.05mm/s) the silvered substrate the required number of times. Four samples, each with two different thicknesses of film, were prepared as indicated in Table 1 below.

Following preparation, the cathode samples were stored under vacuum for at least 24 hours. Referring to Figure 3, electrical measurements were made in a UHV chamber 7 with the samples mounted in turn in a test module. This module comprised a stainless steel support 9 for holding the sample, the support being mounted on glass support insulators 10. A transparent, tin oxide coated,-glass anode plate 11 was mounted on a stainless steel support 12, also carried by the insulators 10, to provide a cathode/anode gap of approximately 0.3mm. By applying a gradually increasing voltage from a power supply 13 across this cathode/anode gap, the total gap current could be externally monitored using a series electrometer 14 protected by a current limiting series resistance 15. A device 16 protected the system against over-voltage.Simultaneous spatial resolution of emission centres on the cathode sample was obtained from anode phosphorescence. The image produced by currents as low as 1ALA at voltages of around 5kV could be'seen by the unaided eye through a sealed port 17 in the wall of the chamber 7, and the image was recorded photographically by a camera 18.

The following Table 1 indicates, for each sample, the number of LB films deposited on the sample, the total film thickness, the voltage at which electron emission sites were visible, the number of such sites and the average emission site density over the sample area.

TABLE 1 Sample No. of Film Voltage No. of Average Site LB Layers Thicknesses (kV) Sites Density mm-2 (nm > A O 0 7.0 0 0 3 9.1 25 0.20 B 7 21.1 5.5 3 0.06 11 33.2 16 0.20 C 15 45.3 5.4 6 0.08 19 57.4 23 0.43 D 23 69.5 3 0.05 27 81.6 6.5 8 0.10 It will be seen that an uncoated silver-plated copper cathode operated at 7kV did not produce any electron emission, whereas identical cathode bodies coated with the organic material produced substantial emission, even at lower applied voltages. From the results it appears that there was an optiumum total film thickness, for generation of stable emission, at around 50nm.

Although the samples described above comprised silver-plated copper substrates coated with 22-tricosenoic films by the LB technique, it is believed that cathodes of other lower work function materials coated with other organic materials, in particular other amphiphilic materials with different molecular size and/or substituents, deposited either by the LB technique or otherwise, may exhibit improved cold emission characteristics as compared with uncoated cathodes. The shape of the cathode shown in Figures 2 and 3 was convenient for test purposes, but it will be realised that cathodes of any desired shape may be coated by the method in accordance with the invention.

Claims

1. A method of forming a cold cathode of a vacuum electronic device, comprising depositing on the emission surface of a cathode body a coating of an organic material 2. A method as claimed in Claim 1, wherein the coating is deposited by the Langmuir-Blodgett film depositing technique.

3. A method as claimed in Claim 1 or Claim 2, wherein the coating comprises one or more layers of 22-tricosenoic acid.

4. A method as claimed in any preceding claim, wherein after deposition, the layer or layers are polymerised.

5. A method as claimed in Claim 4, wherein the polymerisation is effected by electron bombardment of the layers or layers.

6. A method as claimed in Claim 5, wherein the electron bombardment is effected by an electron beam source external to the cathode body.

7. A method as claimed in Claim 5, wherein the electron bombardment is effected by electrons emitted by the cathode body under the influence of an electric field applied thereto.

8. A method of forming a cold cathode of a vacuum electronic device, substantially as hereinbefore described with reference to the accompanying drawing.

9. A cold cathode of a vacuum electronic device, formed by a method as claimed in any preceding claim.

CLAIMS 1. A method of forming a cold cathode for a vacuum electronic device, comprising depositing on an emission surface of a cathode body a coating of an organic material whereby, in use of the cathode, the presence of the coating of organic material on the emission surface enhances emission from said surface.

2. A method as claimed in Claim 1, wherein the coating is deposited by a Langmuir-Blodgett film deposition technique.

4. A method as claimed in any preceding claim, wherein after deposition, the coating is polymerised.

5. A method as claimed in Claim 4, wherein the polymerisation is effected by electron bombardment of the coating.

8. A method as claimed in any preceding claim, wherein the emission surface comprises a layer of silver.

9. A method of forming a cold cathode of a vacuum electronic device, substantially as hereinbefore described with reference to the accompanying drawing.

10. A cold cathode for a vacuum electronic device, comprising a cathode body having an emission surface; and a coating of an organic material deposited on the emission surface to enhance electron emission from said surface.