GB2030354A - Photoemissive Image Tube Manufacture - Google Patents

Abstract

In manufacturing a photoemissive image tube (e.g. proximity-focussed channel multiplier type intensifier, or camera tube) the cathode section and/or anode section are processed in isolation from each other within respective temporary chambers (Figs. 4, 5) including temporary chamber forming members 19, 36. In subsequent manufacture, following storage if desired, one cathode section module and an anode section module are selected, mounted within a vacuum chamber, the temporary chambers, opened such as by tearing (Figs. 6, 8 not shown) using hydraulic ramp to expose the photocathode and anode sections and the two sections are brought together and sealed, such as by pressure sealing tools 52, 53 (Fig. 7, not shown) for a Cu-Cu cold seal. Getter 54 is activated as the temporary seal is destroyed. Figs. 9, 10 (not shown) include a magnetically operated hinged cover (71) for the processed p/c, and the temporary seals may be opened by cutting rather than tearing (Fig. 11, not shown). Further details of the seals are given. <IMAGE>

Description

SPECIFICATION Improvements in or Relating to the Manufacture of Photoemissive Image Tubes This invention relates to the manufacture of photoemissive image tubes and in particular to the manufacture of such tubes wherein it is desirable to process the cathode thereof in isolation from the anode region thereof.

Examples of such tubes include proximity focused devices such as the proximity diode and the proximity focused channel intensifier or wafer tube (usually terms 2nd generation wafer tube if S25 photocathode is incorporated or 3rd generation wafer tube if a solid state photocathode such as GaAs (O) Cs or Si(O) Cs is incorporated), and, photoemissive devices where it is desirable to separate the anode section of the tube from the photocathode section of the tube during photocathode processing, e.g. the EBIC vidicon with silicon target where attack of the silicon target by the alkali metals used in S25 photocathode processing is to be avoided.

An example of a typical image intensifier, in this case an electrostatically-focused single-stage image intensifier is illustrated in Figure 1 of the accompanying drawing.

Referring to Figure 1 the intensifier has an input window 1, which may be of plain glass but in this case is a fibre optic input window, upon which a photocathode layer 2 is formed, this photocathode layer emitting photoelectrons when an input photon signal represented by the arrow 3 interacts with it.

At the other end of the tube an output window 4, again in this case a fibre-optic window, is provided on the inner surface of which a phosphor screen 5 is deposited.

Within the evacuated envelope formed by intermediate envelope section 6 and windows 1 and 4, are provided focusing electrodes as represented at 7 and 8. These act to focus the photoelectron beam from photocathode layer 2 onto the phosphor screen 5 under the influence of a high positive electric potential, typically from 6 to 1 5 kV, created between the input window and the output window 4.

Whilst not illustrated, where the tube is a channel intensifier tube a microchannel plate is interposed between the photocathode and the phosphor screen and operates as an electron amplifier stage.

A typical known method of manufacturing a non-proximity focused image intensifier as illustrated in Figure 1 will now be described.

The input window 1 and the output window 4, carrying the phosphor screen 5, are sealed to the intermediate envelope section 6 by argon arc welding, prior to evacuation. A side arm or arms (not shown) is or are connected to a vacuum pump and the envelope is then evacuated.

Materials for photocathode processing (e.g.

antimony, caesium, potassium sodium) are then introduced by means of side tubes (not shown) in the envelope. The distance between the photocathode and phosphor screen is greater than the photocathode diameter so that the evaporation distance of the antimony constituent of the photocathode layer is sufficient so that a uniformly thick photocathode layer results.

After photocathode processing is completed, the tube is sealed off from the vacuum pump by the closure of the side arm or arms by glass sealing or copper tube pinch sealing as appropriate.

Examples of a proximity diode image intensifier and a double-proximity microchannel plate image intensifier are illustrated in Figures 2 and 3 respectively.

Referring to Figure 2, in this case the photocathode 9 is closely spaced to the anode element, i.e. typically 2 mm from phosphor screen 10, and evaporation of the photocathode constituents cannot be effected if the tube is to be made by a method like that described in connection with Figure 1. The same is true of the intensifier shown in Figure 3 which is similar to that of Figure 2 (and like references are used for like parts) except for the interpositioning of a wafer channel plate 11 between photocathode 9 and fluorescent screen 1 0. In this last case the separation between photocathode 9 and fluorescent screen 10 is typically 0.5 mm.

In manufacturing a tube as described with reference to Figures 2 or 3 it is common practice therefore to use a 'vacuum transfer' method of Tube manufacture. In this the photocathode section of the tube and the anode section of the tube are mounted separated and spaced apart inside a larger vacuum chamber. The arrangement is such that there is room for the introduction of the various photocathode processing elements and evaporators. After photocathode processing is completed, the two tube sections are brought together and sealed within the larger vacuum chamber. The latter is then released to air and the finished intensifier tube extracted.

Typical methods of sealing the tube sections inside the vacuum chamber involve the use of (a) indium gaskets in vee grooves, (b) indium bismuth alloy gaskets, (c) copper to copper cold welded annular seals.

Compared to the method described in connection with Figure 1 , the vacuum transfer method as described above in relation to Figures 2 and 3 has a number of disadvantages.

Manufacture is more costly due to longer processing times (typical photocathode process time 6-8 hours) and need for additional vacuum transfer vacuum chambers. Photocathode processing is more difficult as the photocathode window is inside the vacuum chamber and therefore heating and cooling cycles are longer and more difficult to control, especially as the transfer chambers are typically bulky structures of stainless steel. Furthermore, with indium or indium bismuth seals, spitting of the molten indium or indium bismuth during vacuum bakeout and photocathode processing is a problem, whilst if a copper to copper annular seal is used the incorporation of bulky hardened steel press tools within the environment of tube processing chamber gives added outgassing surfaces and heating/cooling problems.

One object of the present invention is to provide an improved method of manufacturing a photoemissive image tube in which the above difficulties are mitigated. Whilst of particular application to proximity devices in which the cathode and anode structures are closely spaced such as to render difficult or impossible the processing of the cathode within the tube envelope in the presence of the anode, the invention may also be applied to the manufacture of tubes in which the cathode and anode are not so closely spaced.

According to one aspect of this invention a method of manufacturing a photoemissive image tube having a cathode section and an anode section includes a step of enclosing at least one of said sections in isolation from the other within a temporary chamber formed with a temporary chamber-forming member, processing said enclosed section as necessary to activate the same whilst within said temporary chamber to form a module consisting of a formed activated cathode or anode within a sealed temporary chamber.

Preferably both the cathode section and the anode section of the tube are enclosed and processed in isolation from each other as aforesaid to form cathode section modules and anode section modules respectively.

In subsequent stages of manufacture, after a period of storage if desired, one cathode section module and an anode section module are selected and mounted within a vacuum chamber said temporary chamber-forming members are removed whilst in said vacuum to expose said photocathode and anode sections and said photocathode and anode sections are brought together and sealed to form said photoemissive image tube.

According to a feature of this invention a cathode section or anode section module is provided comprising a formed activated cathode or anode within a sealed temporary chamber.

Preferably, in the case of each module the temporary chamber-forming member includes a flange which is united with a flange provided on the photocathode or anode section (as the case may be) by a temporary seal which is broken during final assembly within said vacuum chamber into which a photocathode section module and an anode section module are inserted.

Preferably the flange on said photocathode or anode section to which said flange on said temporary chamber-forming member is sealed also constitutes a flange to which the corresponding flange on said anode or photocathode section respectively is sealed during said final assembly.

Preferably said temporary seal is a copper to copper seal and means are provided within said vacuum chamber into which a photocathode section module and an anode section module are inserted for final assembly for tearing or cutting said temporary seal in order to release the respective temporary chamber-forming member.

Preferably the final sealing of the corresponding flanges on a photocathode section and an anode section is by a cold pressure weld.

Preferably the vacuum chamber within which a photocathode section module and an anode section module are inserted for final assembly includes a housing into either end of which a respective module is inserted with its temporary chamber within said cylinder and two rams are provided operative axially to approach said housing-in the first step to destroy said temporary seals and means are provided for subsequently moving said housing laterally (with respect to the axis of the housing) to remove said housing complete with said temporary chamber-forming members from between said rams and thereafter said rams are moved together again in order to permit tools carried by said rams to effect cold pressure welding together of the corresponding flanges on said photocathode section and said anode section.

Preferably the temporary chamber of a photocathode section module includes a getter source which is activated as the temporary seal of said module is destroyed.

Preferably again where said temporary seal of a module is copper to copper and the processing of the photocathode or anode enclosed therein involves baking preferably said temporary copper seal is surrounded by a non-oxidising atmosphere during such baking.

According to a modification of this invention a hinger cover for the processed photocathode surface of a photocathode section within its temporary chamber is provided said hinged cover being retained by said temporary chamberforming member and held over said processed photocathode surface during storage. Preferably the means for holding said cover is magnetic means said cover falling away from said photocathode when said magnetic means is rendered inoperative or removed as final assembly is required.

The invention is illustrated in and further described with reference to Figures 4 to 11 of the accompanying drawings in which Figure 4 illustrates a cathode module and Figure 5 an anode module provided in accordance with the present invention, Figures 6 to 8 illustrate various stages in the method of manufacturing a photoemissive image tube in accordance with the present invention, Figures 9 and 10 illustrate a modification and Figure 11 illustrates a further modification.

In all Figures, like references are used for like parts.

Referring to Figure 4, the input fibre optic window or face plate of a wafer channel in.tensifier tube is shown at 12. The eventual light input side of window 12 is shown at 13. Sealed to the rim 14 of the window 12 is a metal flange 1 5 which is sealed to a copper flange 1 6. Flange 16 is a permanent part of the tube structure, being in turn sealed to a rigid support flange 1 7.

Forming a cover for the output base 1 8 of the window 12 on which a photocathode is to be formed is a temporary chamber-forming member 19. The member 19 is dome-like and has a rim 20 which is sealed by ceramic or glass "washers" 21 and 21' to which is sealed a Nilo K flange 22. The outermost ceramic or glass "washer" 21 is also sealed to a Nilo K flange 23.

Within member 19 are all the necessary facilities for tube activation i.e. antimony evaporator 24 and alkali channels 25. The member 1 9 is also formed with a degassing pumping stem 26 and a stem 27 for connecting to an appendage ion pump.

The member 19 is sealed to cover the surface 18 of the window 12 by means of a continuous copper to copper seal 28 between flange 1 7 attached to copper flange 1 5 and hence to the frame 14 of the window 12 and flange 23 sealed to the glass "washer" 21' of the member 19.

With the chamber formed by the sealing in place of the chamber-forming member 19 over the surface 18 of the window 12, the photocathode is formed in customary fashion and when processing is complete stems 26 and 27 are sealed off as represented in dashed outline at 26' and 27' respectively.

In this form the module containing a processed photocathode structure is stored.

Referring to Figure 5 the anode assembly of the tube consists of a fibre optic twister 29 mounted in sealed fashion via intermediate metal flanges to a short ceramic cylinder 30. Mounted across ceramic cylinder 30 is a microchannel plate 31. Sealed to the exposed rim 32 of the cylinder 30 is a copper flange 33 which is in turn sealed to Nilo K flange 34. It should be noted at this stage that in the final assembly of a wafer channel intensifier tube the flange 1 6 of Figure 4 will be sealed to the flange 33 in the region 35.

Flange 33 therefore forms a permanent part of the tube structure.

Forming a cover for the anode section is a temporary chamber-forming member 36. The member 36 includes all the necessary facilities for anode section activation that is to say a channel plate electron scrubbing source 37 a degassing pumping stem 38 and a stem for connection to an appendage ion pump 39.

The member 36 is sealed to cover the anode section of the tube by means of a continuous copper to copper seal 40 between a flange 41 sealed to the rim of the chamber 36 and the flange 34 sealed to the copper flange 33 which is sealed to the rim 32 of the ceramic cylinder 30.

With the temporary chamber formed by temporary chamber-forming member 36 sealed over the anode section, anode section activation is carried out in customary fashion and when processing is completed stems 38 and 39 are sealed off as represented in dashed outline at 38' and 39' respectively.

In this form the module containing a processed anode section structure is stored.

Thus the photocathode section and the anode section of the wafer channel intensifier tube are processed separately. In order to form in one tube a photocathode may be selected from a plurality of photocathodes stored in module form for uniting with a given anode section stored in module form. Having chosen a photocathode module and an anode section module in essence the two modules are included in a vacuum chamber their respective temporary chamberforming members 19 and 36 are removed by destroying the copper to copper seals 28 or 40 (e.g. by tearing or cutting) and the photocathode and anode sections united by bringing together the flanges 1 6 and 33 and sealing these one to another by means, for example, of a copper cold welded seal.

Referring now to Figure 6 this illustrates the method of removing the temporary chamber forming members 19 and 36 of Figures 4 and 5 and sealing the photocathode and anode sections together. The photocathode module here referenced 43 illustrated in detail in Figure 4 is shown mounted within a support housing 44 and at one end thereof. Mounted within support housing 44 and at the other end thereof is the anode section module here referenced 45. At either end of support housing 44 is a hydraulic ram 46 and 47 respectively.The hydraulic rams 46 and 47 are provided with recesses at 48 and 49 respectively and the support cylinder 44 is likewise provided with recesses at 50 and 51 such that axial movement of the rams outward from the support cylinder 44 acts to tear the copper to copper seal 28 of the photocathode module 43 and the copper to copper seal 40 of the anode section module 44 thereby releasing the members 1 9 and 36 respectively. (During tearing of the copper to copper seal the housing 44 is prevented from moving in a direction along the axis of the rams 46 and 47 by the support rod shown in Figure 8).

With the copper to copper seals 28 and 40 destroyed the hydraulic rams 46 and 47 together with the photocathode section and anode section are withdrawn and the support housing 44 bearing the now redundant members 1 9 and 36 is moved in a sideways direction from out between the rams 46 and 47.

The situation after removal of the support cylinder 44 bearing the redundant members 19 and 36 is shown in Figure 7. The rams 46 and 47 are then brought together to unite flanges 1 6 (of the photocathode section) and 33 (of the anode section). In order to effect such uniting of the permanent flanges the rams 46 and 47 are provided with pressure sealing tools 52 and 53 which under a force of approximately 5 tons across the rams effect the required copper to copper cold welded seal in the region 35 as shown in Figure 5.

It will be noted that in Figure 4 the module is shown to include a getter source 54. This getter source 54 is fired before the copper to copper seal 28 is cut in order to absorb any gases released during the destruction of the seal and thus prevent any attack of the photocathode surface by released gases.

Referring now to Figure 8 this shows in greater detail and to a smaller scale the mechanism by means of which the temporary members 19 and 36 are removed and like references are used for like parts in Figures 6 and 7.

Housing 44 is mounted on a support 60 which is moveable in a vertical direction (as viewed) so that support housing 44 may be moved upwards from the position shown from out between the hydraulic rams 46 and 47. Support 60 moves through part of the wall 61 of a chamber which includes an evacuated space 62. This last mentioned chamber also includes wall portions 63 and 64 through which hydraulic rams 46 and 47 move respectively. The internal vacuum 62 is maintained by means of bellows 55 in respect of support 60, 66 in respect of ram 46 and 67 in respect of ram 47. Port 68 is connected to a high vacuum pump (not shown).

The operation of the apparatus shown in Figure 8 will it is believed be self-explanatory having regard to the description already given with reference to Figures 6 and 7. With modules 43 and 45 in position hydraulic rams 46 and 47 are operated to move away from each other thus tearing the copper to copper seals 28 and 40 (not separately represented in Figure 8). Hydraulic rams 46 and 47 are then withdrawn further.

Support 60 moves upwards to carry support housing 44 out from between the rams 46 and 47 and leaving the cathode and anode sections in position within recesses 69 and 70 respectively.

The hydraulic rams 46 and 47 are then brought together again until the sealing tools 52 and 53 act to unite flanges 16 of the photocathode section and 33 of the anode section utilising a pressure of 5 to 10 tons across the rams.

As has already been mentioned as destruction of the copper to copper seals 28 and 40 takes place so the getter 54-in the photocathode module is activated.

The gas pressure and the type of gases present within the vacuum chamber of Figure 8 are chosen as far as possible to ensure negligible loss of photocathode sensitivity by gas attack on the photocathode surface. As will be appreciated CO2 H20 and 02 are particularly to be avoided.

In order to ensure a low gas pressure and the presence of less harmful residual gases the chamber of Figure 8 is pre-baked before the insertion of the photocathode and anode section modules and flushed with decontamimating gases prior to or during the introduction of the two modules.

During the processing of the photocathode and anode section in their individual modules as part of the normal treatment baking is involved. During such baking the copper to copper seals 28 and 40 should be surrounded by non-oxidising atmosphere in order to prevent oxidation of the copper.

Referring to Figures 9 and 10 these illustrate a modification of the photocathode section module shown in detail in Figure 4 and like references are used in Figures 9 and 10 denote like parts in Figure 4. As will be seen the arrangement shown in Figures 9 and 10 is identical to that of Figure 4 except for the provision of a flanged cover plate 71 which is hinged to a support 72 carried by flange 22. Flange 22 is, it will be recalled, fixed to the chamber-forming member 19.

Flange cover plate 71 when attracted by means of magnet 73 forms a temporary protective cover for the photocathode when this is formed upon surface 18. Normally during storage of the module 43 the cover plate 71 is not in position over the photocathode surface 1 8.

When the photocathode module 43 is required to be mounted in the housing 44 in readiness for removal of the temporary member 19 the flanged cover plate 71 is manoeuvred into position (i.e. by action of gravity on tilting) over the photocathode formed on surface 18 and held in position by magnet 73. When photocathode module 43 and housing 44 is in position in the vacuum system of Figure 8 the situation is as shown. Magnet 73 is attached to member 74 (which may be a magnet) which in turn is attached to rod 75. Bellows 76 attached as shown to rod 75 and ram 46 serves to maintain vacuum within the chamber whilst permitting movement of the rod 75 as required.

When the temporary chamber 19 is removed and carried away with the housing 44, as has already been described with reference to Figures 6, 7 and 8 and prior to final sealing of the flanges 16 and 33, the temporary protective cover plate 71 is allowed to fall away from the photocathode section 18 by moving the magnet 73 in a direction away from the cover plate 71.

Movement is effected via the rod 75. It should be noted that in a preferred arrangement of the system of Figure 8 the photocathode surface 1 8 and the cover plate 71 would be situated horizontally so that on moving the target 73 upwards in a vertical direction the cover plate would fall vertically downwards without causing damage i.e. scratching to the photocathode surface 1 8. In this preferred arrangement a further member attached to the vacuum chamber (shown 77 in Figure 8) would be necessary to catch plate 71 after it had fallen away from photocathode 1 8 so that damage to the anode half mounted in ram 47 by the plate 71 did not occur. The further attached plate 77 complete with cover plate 71 would then be swung out of the path of the rams 46 and 47 prior to their being brought together for sealing of the flanges 1 6 and 33.

Referring to Figure 11, this illustrates an alternative method of opening a photocathode or anode module, in this case a photocathode module. Instead of temporary seals being torn to remove the temporary chamber-forming members, the removal process involves a cutting action. The housing 44 is provided as a thrust cylinder and the hydraulic rams 46 and 47 are provided with cutting edges such as that shown at 78 in the case of ram 46. These cutting edges act to cut the copper flanges such as 16 in the case of a photocathode module (33 in the case of an anode module) against a Nilo K flange 79 forming part of the module.

Claims (18)

Claims

1. A method of manufacturing a photoemissive image tube having a cathode section and an anode section including a step of enclosing at least one of said sections in isolation from the other within a temporary chamber, formed with a temporary chamber-forming member, processing said enclosed section as necessary to activate the same whilst within said temporary chamber to form a module consisting of a formed activated cathode or anode within a sealed temporary chamber.

2. A method as claimed in claim 1 and wherein both the cathode section and the anode section of the tube are enclosed and processed in isolation from each other as aforesaid to form cathode section modules and anode section modules respectively.

3. A method as claimed in claim 2 and wherein in subsequent stages of manufacture, after a period of storage if desired, one cathode section module and an anode section module are selected and mounted within a vacuum chamber said temporary chamber-forming members are removed whilst in said vacuum to expose said photocathode and anode sections and said photocathode and anode sections are brought together and sealed to form said photoemissive image tube.

4. A cathode section or an anode section module formed by the method as claimed in claim 1 and comprising a formed activated cathode or anode within a sealed temporary chamber

5. A method or module as claimed in any of the above claims and wherein in the case of each module the temporary chamber-forming member includes a flange which is united with a flange provided on the photocathode or anode section (as the case may be) by a temporary seal which is broken during final assembly within said vacuum chamber into which a photocathode section module and an anode section module are inserted.

6. A method or a module as claimed in claim 5 and wherein the flange on said photocathode or anode section to which said flange on said temporary chamber-forming member is sealed also constitutes a flange to which the corresponding flange on said anode or photocathode section respectively is sealed during said final assembly.

7. A module as claimed in claim 5 or 6 and wherein said temporary seal is a copper to copper seal.

8. A method as claimed in claim 5 or 6 and wherein said temporary seal is a copper to copper seal and means are provided within said vacuum chamber into which a photocathode section module and an anode section module are inserted for final assembly for treating or cutting said temporary seal in order to release the respective temporary chamber-forming member.

9. A method as claimed in claim 7 or 8 wherein the final sealing of the corresponding flanges on a photocathode section and an anode section is by a cold pressure weld.

10. A method as claimed in any of claims 1 to 6 or claim 8 or 9 and wherein the vacuum chamber within which a photocathode section module and an anode section module are inserted for final assembly includes a housing into either end of which a respective module is inserted with its temporary chamber within said cylinder and two rams are provided operative axially to approach said housing in the first step to destroy said temporary seals and means are provided for subsequently moving said housing laterally (with respect to the axis of the housing) to remove said housing complete with said temporary chamberforming members from between said rams and thereafter said rams are moved together again in order to permit tools carried by said rams to effect cold pressure welding together of the corresponding flanges on said photocathode section and said anode section.

11. A method or a module as claimed in any of the above claims and wherein the temporary chamber of a photocathode section module includes a getter source which is activated as the temporary seal of said module is destroyed.

12. A method as claimed in any of claims 1 to 6 or claims 8 to 10 wherein said temporary seal of a module is copper to copper and the processing of the photocathode or anode enclosed therein involves baking and wherein said temporary copper seal is surrounded by a nonoxidising atmosphere during such baking.

13. A method or a module as claimed in any of the above claims and wherein a hinger cover for the processed photocathode surface of a photocathode section within its temporary chamber is provided said hinged cover being retained by said temporary chamber-forming member and held over said processed photocathode surface during storage.

14. A method or a module as claimed in claim 13 and wherein the means for holding said cover is magnetic means said cover falling away from said photocathode when said magnetic means is rendered inoperative or removed as final assembly is required.

1 5. A method of manufacturing a photoemissive image tube having a cathode section and an anode section substantially as herein described with reference to Figures 4 to 8 of the accompanying drawings.

1 6. A method as claimed in claim 1 5 but modified substantially as herein described with reference to Figure 11 of the accompanying drawings.

1 7. An anode section or a cathode section module comprising a formed activated cathode or anode within a sealed temporary chamber substantially as herein described with reference to Figure 4 or Figure 5 respectively of the accompanying drawings.

18. An anode section or a cathode section module comprising a formed activated cathode or anode within a sealed temporary chamber substantially as herein described with reference to Figures 9 or 10 respectively of the accompanying drawings.