GB2258756A - Sealing and making connection to electron tube devices. - Google Patents

Abstract

A method of assembly of evacuated electron tube devices, such as image intensifiers has external electrical contacts each connected to a conductive layer or electrode contained within the vacuum envelope of the device by means of a conductive film layer 28, 30, 32 or 34 on a predetermined portion of a surface of an electrically non- conductive component e.g. ceramics 4, 6 or 8 or windows 10 or 24 to form a conductive "feedthrough", applying a solder glass to both a predetermined portion of the conductive film layer and to a portion of the surface of a second of the electrically non-conductive components and connecting the two components by adhering the layers of solder glass under temperature and applied pressure, e.g. below 450 DEG C. The conductive film layer is formed of a paste formed from a mixture of fine particles of metal or alloy and fine particles of glass frit suspended in an organic medium which is heated to around 900 DEG C. Alternatively the two components may be connected using low melting point alloy in place of the solder glass, a foil washer or paste layer of the alloy being placed between two previously metallised ceramic surfaces and then heated sufficiently in a suitable atmosphere to solder the components together. <IMAGE>

Description

IMPROVEMENTS IN AND RELATING TO ELECTRON TUBE DEVICES This invention relates to a method of assembly of evacuated electron tube devices.

Electron tube devices commonly comprise, within an evacuated envelope, a sequence of accurately parallel or aligned and precisely spaced electrode surfaces, these surfaces usually being axi-symmetric about an axis perpendicular to the surfaces or end planes. It is normally necessary to provide an electrically conductive pathway, or "feedthrough", from the electrode surface within the evacuated envelope to the outside of the device. In such devices, achievement of the desired electrical and/or electro-optical performance of the device depends critically on the maintenance of the integrity of the vacuum within the envelope and of the accuracy of the electrode spacings and of their parallelism or alignment during the construction and processing of the tube.

One family of such devices is commonly known as image intensifiers or image converters, which are used either to intensify images initially near or below the visibility threshold to a level which the human eye can see, or to convert an image composed of electromagnetic radiation at or about the range of visible radiation wavelengths, including radiation wavelengths invisible to the human eye, to a corresponding image in the visible spectrum.

The evacuated electron tubes of such devices are commonly fabricated from, for example, a number of complex ceramic cylinders, optical glass windows, thin wall metal pressings, metal to ceramic brazed seals, glass to metal seals using a glass ceramic, argon arc metal welding and indium metal gasket seals.

When the finished tube of such a device is energized, electrical perturbations may be caused by resistive surface leakages, or by spurious electron emissions resulting from chemical micro-contamination of insulating surfaces or from adhered microparticulates. Such electrical perturbations degrade the electrical and/or electro-optical performance of the device. The number of separate preparation and assembly processes of conventional devices is large, and includes wet chemical and electro-chemical processes and heating of piece parts in furnaces with special atmospheres.

These processes increase the risk of micro-contamination of the insulating surfaces and of a change of shape or dimension leading to the distortion of the spacing and/or parallelism of the surfaces. Furthermore such assembly processes require very strict control at each stage of assembly in order to achieve an acceptable final yield.

A method of assembly of evacuated electron tube devices in accordance with the invention, wherein the device has external electrical contacts each connected to a conductive layer or electrode contained within the vacuum envelope of the device, comprises forming a conductive film layer on a predetermined portion of a surface of an electrically non-conductive component to form a conductive "feedthrough", the application of a solder glass to both a predetermined portion of the conductive film layer and to a portion of the surface of a second electrically non-conductive component and the connection of the two components by adhering the layers of solder glass under temperature and applied pressure.

The formation of the conductive film layer may be achieved by sputtering a metal in a suitable inert or reducing atmosphere, by vaporising a metal in vacuo, by pyrolytic decomposition of a paint containing a precious metal halide and a glass forming substance in an oxidising atmosphere, or preferably by heating a paste applied by any suitable method, where the paste is formed from a mixture of fine particles of metal or alloy and fine particles of glass frit suspended in an organic medium.

'7 To'heating of such a paste to form the conductive film layer may be carried out at a temperature around 900 OC, Preferably the heating is carried out at a temperature not greater than about 850 or. Processes carried out at temperatures substantially greater than 8500c risk distorting the shape of the components and/or damaging the electrode surfaces.

The connection of the two components could be carried out using a low melting point alloy in place of the solder glass. Such a method would require a foil washer or paste layer of the alloy to be placed between two previously metallised ceramic surfaces and then heated sufficiently in a suitable atmosphere so as to solder the components together. This process is difficult to carry out, necessitating close control of surface preparation, rate of heating and uniformity of heating in order to form a satisfactory vacuum-tight bond.

The temperature at which the two components are caused to adhere is preferably below 4500c.

Two devices may form sub-assemblies which may be connected together under a vacuum by the use of an indium gasket seal closure.

Preferably each successive process step is carried out at a lower temperature than the preceding step. This ensures that the result achieved by each process step is not significantly degraded during the assembly process.

Such a method has the following advantages: assembly may be carried out at temperatures low enough to prevent shape changes in accurately machined ceramic parts and to avoid damage to temperature-sensitive materials incorporated within the evacuated envelope; a reduction ir the number of separate piece parts and a consequent reduction in the number of assembly process steps; the elimination of wet chemical processes and thereby a reduction in the risk of micro-contamination of insulating surfaces, and a simplification of the assembly processes and a reduction in the number of potential vacuum leakage sites.The adhesion of two components together using a solder glass is an advantage over the known method of connection by means of metal shell pressings or flanges, as it reduces the capacitance effect between electrodes and also the stray capacitance to earth, which is an undesirable consequence of using such metal shell pressings or flanges. Assembled components may be tested for vacuum integrity by, for example, the helium mass spectrometry method; where the vacuum sealing is found to be faulty, it is possible to re-work the assembly process by reheating the components sufficiently under axial pressure to cause the solder glass to flow slightly so as to improve the vacuum seal. Such a method also enables the construction of a strong and rigid device which may be both smaller and lighter than devices manufactured using conventional methods.

The invention will now be described by way of example only, with reference to the accompanying drawing, in which Figure 1 is a cross-sectional view of a completed image intensifier tube shown in exaggerated thickness for clarity.

An image intensifier tube 2 is symmetric about an axis 50. Three annular ceramic components 4,6,8 provide a housing for a front optical glass window 10, a micro-channel plate 16 and a rear optical glass window 24 respectively. A photo-emissive surface 12 is applied to the rear surface of the front window 10, a cathodo-luminescent layer 25 is applied to the front surface of the rear window 24, and electrode surfaces 18,20 are applied to the front and rear respectively of the micro-channel plate 16, all by known methods.

The periphery of the front window 10 is coated with a conductive film layer 28 to form an electrical contact with the photo-emissive surface 12. Similarly, the periphery of the rear window 24 is coated with a conductive film layer 34. The cathodo-luminescent layer 25 and a portion of the conductive film layer 34 is then coated with a thin metal film 26 by a known method to form an electrical contact between conductive film layer 34 and metal film 26. Portions of the surfaces of ceramic components 4,8 are coated with further conductive film layers 30,32 respectively.

After firing in air at about 8500c to clean ceramics, the conductive film layers are formed by depositing by, for example, screen printing or by syringe or brush application according to the shape of the surface to be coated, a metal paste, which is composed of a mixture of fine particles of metal or alloy (usually, though not necessarily, a noble metal or alloy thereof) and fine particles of glass frit suspended in an organic medium, onto the surfaces to be coated. The metal paste is fired at approximately 1200c to remove organic solvent, and then slowly heated in air to about 8500c to bond the conductive film layer firmly to the ceramic component or window.

Once the organic medium has burnt away, and the temperature is raised to about 8500c, the glass frit melts, bonding the metal particles in contact with one another and to the base insulator while completely filling the interstices between metal particles to form a coherent glass matrix which is vacuum tight. During this process the metal particles sinter together where they touch, but do not melt. The maximum temperature of 8500c is well below that at which any movement or distortion of the ceramic takes place.

The coated periphery of the front window 10 is adhered to the corresponding interior surface of annular ceramic component 4 by a layer of solder glass 36 in a suitable jig under conditions of temperature and applied pressure. The solder glass is applied to both the mating surfaces as a paste of finely divided glass frit in a printing medium by any suitable application method.

the paste is dried, the printing medium burnt out and the glass frit fused at or about 4500c to form a smooth glaze surface. Subsequently the cold pre-glazed surfaces are brought into contact in a suitable jig under applied axial pressure and heated to a temperature at or about 4250c in a neutral or very slightly oxidising atmosphere to cause the layers of glaze to soften and flow slightly so as to fuse to form a continuous glass sandwich layer 36 between the two components 10,4. When cooled, this layer 36 forms a vacuum tight and mechanically strong bond between the components 10,4. The mechanical strength of the fixture is improved if the glasses forming the windows and the solder glass have a linear co-efficient of expansion which is slightly lower than that of the ceramic, thus keeping the glass components in compression.

The coated periphery of the rear window 24 is similarly adhered to the corresponding interior surface of annular ceramic component 8 by a layer of solder glass 40. An annular component 44 is sandwiched between components 24 and 8 and acts as a "gas getter" when the device is assembled. Component 44 suitably comprises a coated wire ring, which is discontinuous so that it may be mechanically sprung outwardly and to reduce the inductive effect when or if the device is subjected to a radio frequency oscillating magnetic field. The coating acts to absorb any contaminating trace elements or gases given off into the vacuum from the internal surfaces of the vacuum envelope and so maintain the purity of the vacuum.

Annular ceramic components 8 and 6 are adhered together by a layer of solder glass 38 by a similar process.

After vacuum processing, the two sub-assemblies are brought together while still under a vacuum, with the micro-channel plate 16 between them. Anrular electrically-conductive spacers 46,48 are interposed between the micro-channel plate 16 and ceramic components 4,8 respectively. These spacers act as shock absorbers, to insulate the micro-channel plate 16 from shock damage. Spacers 46,48 may be made from soft copper or a suitable resilient refractory metal, or may comprise a mesh of stainless steel, and are configured and dimensioned so as to hold micro-channel plate 16 securely yet with the possibility of some axial displacement to counter mechanical shock loading. The electrode surface layer 18 is in electrical contact, through conductive spacer 46, with conductive film layer 30, and electrode surface 20 is in electrical contact, through conductive spacer 48, with conductive film layer 32. The two sub-assemblies are adhered together by the use of an indium gasket seal closure 42 which is a technique well-known in the art.

The assembled tube has evacuated chambers 14 and 22, which are connected by the channels in micro-channel plate 16, and annular areas of conductive film layer 28,30,32,34 providing electrical contacts with photoemissive surface 12, electrode surfaces 18,20 and cathodo-luminescent layer 25 respectively, which surfaces are all contained within the vacuum envelope.

Claims (10)

1. A method of assembly of evacuated electron tube devices wherein the device has external electrical contacts each connected to a conductive layer or electrode contained within the vacuum envelope of the device, comprising forming a conductive film layer on a predetermined portion of a surface of an electrically non-conductive component to form a conductive "feedthrough", applying a solder glass to both a predetermined portion of the conductive film layer and to a portion of the surface of a second electrically non-conductive component and connecting the two components by adhering the layers of solder glass under temperature and applied pressure.

2. A method as claimed in Claim 1 wherein the conductive film layer is formed by spluttering a metal in a suitable inert or reducing atmosphere, by vaporising a metal in vacuo, or by pyrolytic decomposition of a paint containing a precious metal halide and a glass forming substance in an oxidising atmosphere.

3. A method as claimed in Claim 1 wherein the conductive film layer is formed by heating a paste applied by any suitable method, where the paste is formed from a mixture of fine particles of metal or alloy and fine particles of glass frit suspended in an organic medium.

4. A method as claimed in Claim 3 wherein the heating of the paste to form the conductive film layer is carried out at a temperature around 9000C.

5. A method as claimed in Claim 4 wherein the heating is carried out at a temperature not greater than about 8500C.

6. A method as claimed in any preceding Claim wherein the two components are connected using a low melting point alloy in place of the solder glass, a foil washer or paste layer of the alloy being placed between two previously metallised ceramic surfaces and then heated sufficiently in a suitable atmosphere so as to solder the components together.

7. A method as claimed in any preceding Claim wherein the temperature at which the two components are caused to adhere is below 4500C.

8. A method as claimed in any preceding Claim wherein two devices form sub-assemblies which are connected together under a vacuum by the use of an indium gasket seal closure.

9. A method as claimed in any preceding Claim wherein each successive process step is carried out at a lower temperature than the preceding step.

10. An image intensifier tube substantially as hereinbefore described and with reference to Figure 1.