EP1670017A1

EP1670017A1 - Electron beam window, window assembly, and electron gun

Info

Publication number: EP1670017A1
Application number: EP04257542A
Authority: EP
Inventors: Alan David MBDA UK LIMITED Hart; Adam MBDA UK LIMITED Armitage; Hilary Jane MBDA UK LIMITED Harrop; David Alan MBDA UK LIMITED Brown
Original assignee: MBDA UK Ltd
Current assignee: MBDA UK Ltd
Priority date: 2004-12-03
Filing date: 2004-12-03
Publication date: 2006-06-14

Abstract

An electron beam window (10) is protected from damage by a high energy electron beam (15) by providing an electrically conductive path in the form of one or more conductive layers (26 and/or 27) earthed (18, 21) to a structure (12) of an electron beam gun (11). The electrically conductive layers (26 and/or 27) are adhered to their respective surfaces (24 and/or 25) and may comprise a conductive layer that is sufficiently thin that it will not significantly impede the passage of electrons through the window (10). Each layer (26 and/or 27) is preferably formed of aluminium but could also be formed from magnesium, titanium, platinum, gold, indium tin oxide, or silica doped to make it electrically conductive. Alternatively, the electron beam window (10) can be made conductive by incorporating an electrically conductive dopant in the diamond, or by ion implantation of at least one of the surfaces (24, 25).

Description

This invention relates to an improved electron beam window, an electron beam window assembly, and to an electron gun provided therewith.
It is desirable to transmit high energy electrons from an electron source in a high vacuum chamber, through an electron beam window, into a higher pressure environment. The electron beam window needs to be mechanically robust to withstand the force generated by the pressure difference between the high vacuum chamber and the higher pressure environment, whilst being thin enough to prevent too much attenuation of the electron beam.
The unique properties of chemical vapour deposition (CVD) diamond, particularly its relative transparency to electrons, its high strength, and its high thermal conductivity, make it an ideal material for forming an electron beam window.
However we have encountered a serious problem in the use of CVD diamond for forming electron beam windows required to transmit intense electron beams (50 to 150 KeV with beam currents up to 2A) as they punch holes through the CVD diamond thereby destroying the mechanical integrity of the electron beam window and depressurising the high vacuum chamber.
Our research has shown that a high energy electron beam causes local electrical charging of the electron beam window at the point of transmission and that, providing the electron beam energy is sufficiently high, this charge will increase until it attains a value that will generate an arc discharge through the thin diamond window. The power of an electron beam is KeV x A and the energy stored in such a charge is proportional to the square of the beam voltage.
The value of the destructive charge is proportional to the square of the beam voltage, and the beam current determines the rate at which this charge increases.
US 2002/0048344 and US 2002/0048345 recognise that a diamond window, used in an X-ray radiation device, can develop a charge which can deflect, decelerate, or stop the passage of an electron ray, and teach that this problem can be solved by doping the diamond window to make it conductive. This doping is achieved by doping the diamond with boron during the gas phase deposition. Such beam deviation arises because the electric field generated external to the window, by the charging of the window, is large in comparison with the accelerating voltage of the electron beam. These is no recognition of the problem of a diamond window being damaged by an arc discharge.
US 5,235,299 recognises that a foil window can be formed from diamond and can be melted, softened or ruptured due to overheating, and teaches that this problem can be solved by a structure which enables the window to be cooled. Again there is no suggestion that a diamond window can be damaged by an arc-discharge.
Whilst we have identified that the damage to CVD diamond windows, by the transmission of intense electron beams, is caused by an electrical charge that causes an arc-discharge through the window, the actual physical mechanism is not known. Window perforation results from high internal electrical fields generated within the diamond window, but such fields are unexpected since conventional mechanisms for charge generation would anticipate a uniform charge and thus no field across the thickness of the diamond layer. As diamond is a strong insulator, we believe that an intense electron beam may generate electron-hole pairs within the diamond, and that these electron-hole pairs are unable to move in a manner that would maintain charge neutrality. Alternatively the mechanism could be the capture of some electrons from the intense electron beam, these electrons then being trapped in local defects in the diamond and thereby becoming immobile.
Although we are particularly concerned with electron beam windows formed of CVD diamond, we believe that this invention could also be useful with any electron beam window formed from an appropriate insulating material, or a material having low electrical conductivity.
According to one aspect of the invention, an electron beam window is formed from an insulating material to define a first surface for receiving a high energy electron beam and a second surface for transmitting the electron beam, which is protected from perforation by an electrically conductive path that prevents the electron beam from establishing any significant electrical field across the thickness of the window.
The electrically conductive path may be provided by incorporating an electrically conductive dopant in the insulating material. In this case the electron beam window is preferably formed from diamond incorporating a dopant selected from the group comprising phosphorus, sulphur, nitrogen and boron. The dopant may be incorporated only in a stratum of the diamond forming at least one of said surfaces.
Alternatively, the electrically conductive path may be provided by ion implantation in at least one of said surfaces.
Alternatively, the electrically conductive path may be provided by an electrically conductive layer which is adhered to one of said faces, the layer being sufficiently thin that it will not significantly impede the passage of electrons through the window. In this case the electrically conductive layer may be adhered to both of said faces and the combined thickness of the layers is sufficiently thin that they will not significantly impede the passage of electrons through the window. In this event, the layers are preferably electrically connected. The layer thickness may be less than I micron and is preferably between 10 and 100 atoms.
Each layer may be formed from a material chosen from the group comprising indium tin oxide, magnesium, aluminium, titanium, platinum, gold and silica doped to make it electrically conductive, preferably magnesium, aluminium or titanium, more preferably aluminium. The layer may be formed by vacuum deposition.
According to another aspect of the invention, an electron beam window assembly is formed by attaching an electron beam window, having any of the features detailed above, over an aperture defined by an electrically conductive mounting plate such that the electron beam window covers the aperture, and an hermetic seal is positioned between the electron beam window and the plate. In the case where the electron beam window has a conductive layer adhered to it, the conductive layer is preferably connected to the electrically conductive mounting plate.
According to a further aspect of the invention, an electron gun, arranged to produce a high energy electron beam within a vacuum chamber and to direct the electron beam through an electron beam window into a region of higher pressure, has an electron beam window which is formed from an insulating material and is protected from perforation by an electrically conductive path that prevents the establishment of any potentially damaging electrical charge across the thickness of the electron beam window by the passage of the electron beam.
The invention will now be described, by way of example only, with reference to the accompanying drawings, in which:-

Figure 1 is a diagrammatic longitudinal section through an electron gun illustrating the position of its electron beam window;
Figure 2 is an enlarged section illustrating the mounting of one form of electron beam window to a support structure within the electron gun shown in Figure 1, and
Figure 3 is an enlarged section, similar to Figure 2, illustrating the mounting of another form electron beam window to a support structure within the electron gun shown in Figure 1.

With reference to Figure 1, an electron beam window 10 is positioned inside a typical electron gun 11 with its periphery supported by a structure 12 which connects a vacuum chamber 13 to a chamber 14 that is to receive an electron beam 15 from an electron beam generator 16. The vacuum chamber 13 is evacuated to generate a vacuum of typically 10^-6 mbar. The chamber 14 defines a region of higher pressure, as is well known in the art, the electron beam window 10 serving as a physical barrier to preserve the pressure difference between the chambers 13 and 14. Consequently, the electron beam window 10 must withstand a force equal to its cross sectional area multiplied by the pressure difference between chambers 13 and 14, this force being transmitted to the structure 12.
Figures 2 and 3 illustrate the mounting of the electron beam window 10 to the structure 12 in much greater detail. The structure 12 is a cast web of stainless steel formed with a cylindrical orifice 17 through which the electron beam will pass towards the electron beam window 10. An annular copper sealing gasket 18 is trapped between the electron beam window 10 and an annular edge 19 formed integral of the structure 12.
In order to withstand the force created by the differential pressure across the electron beam window 10, a bracket 20 is slidably mounted on an array of stainless steel bolts 21 and is urged against the electron beam window 10 by corresponding lock-nuts 22. The arrangement illustrated is diagrammatic and the actual mounting of the window would generally include a compliant member positioned between the bracket 20 and the electron beam window 10. The bracket 20 is formed with a central aperture 23 to allow free passage of the electron beam. As the heads of the bolts 21 are within the vacuum chamber 13, they are provided with respective copper sealing washers as shown. In both Figure 2 and Figure 3, the electron beam window 10 is formed from diamond by chemical vapour deposition as this is much less costly than using natural diamond. Various methods are known for the synthetic production and shaping of the diamond. For instance, US Patents 5,264,071 and 5,349,922 teach the production of monolithic diamond sheet by passing a mixture of hydrogen and a hydrocarbon at a high temperature over a cooled substrate on which diamond is deposited. The CVD diamond may be single crystal or polycrystalline, the latter being more available.
The electron beam window 10 comprises a cylindrical disk of polycrystalline diamond which has been ion beam etched to define a thinner pane, as shown, for the passage of the electron beam 15. The thickness of the pane is sufficient to withstand a predetermined pressure differential across it and is typically between 25 microns and 5 microns, and is preferably about 10 microns. In this manner, the electron beam window 10 defines a first surface 24 for receiving a high energy electron beam 15, and a second surface 25 for transmitting the electron beam into the higher pressure chamber 14.
As previously stated, the passage of the high energy electron beam 15 through the electron beam window 10 generates an electrical charge which will increase until it attains a value that generates an arc-discharge through the thin diamond window pane. This destructive electrical charge is proportional to the square of the beam voltage and increases at a rate dependant on the beam current. When the arc-discharge occurs, the electron beam window is physically damaged, such damage typically taking the form of a hole punched right through the electron beam window 10, surrounded by collateral crazing of the polycrystalline diamond.
This invention provides three alternative solutions to this problem, each of which comprises the provision of an electrically conductive path that prevents the electron beam from establishing any significant electrical field across the thickness of the electron beam window 10.
The first of these solutions is illustrated in Figure 2 in which the electrically conductive path is provided by an electrically conductive layer 26 which adheres to surface 24. It is, of course, important for the electrically conductive layer 26 to be sufficiently thin that it will not significantly impede the passage of electrons through the electron beam window 10. From Figure 2 it will be noted that the conductive layer 26 extends radially outwards to make good electrical contact with the copper sealing gasket 18 which now performs the additional function of earthing the conductive layer 26 to the structure 12.
The second solution is illustrated in Figure 3 which is identical to Figure 2 except in so far as a second conductive layer 27 is adhered to the second surface 25 of the electron beam window 10. This second conductive layer 27 extends radially to form an electrical contact with the bracket 20 which is earthed to the structure 12 by the bolts 21 and their copper washers. With this embodiment it is, of course, important for the combined thickness of the conductive layers 26 and 27 to be sufficiently thin that they will not significantly impede the passage of electrons through the electron beam window 10. If desired, the electrically conductive layers 26, 27 may be joined together by extending them around the peripheral edge of the electron beam window 10.
The electrically conductive layer 26, or the electrically conductive layers 26, 27, are preferably formed of aluminium, but may also be formed of magnesium, titanium, platinum, gold, indium tin oxide, and silica doped to make it electrically conductive. Each electrically conductive layer may be multilayered, for example, a titanium layer adhered to the window to enhance adhesion and electrical contact, and covered by an aluminium layer to provide the bulk of the electrical conductivity. We have found that a layer thickness of less than one micron is satisfactory and that the layer thickness should normally be between 10 and 100 atoms. Each layer may be adhered to its respective surface 24 or 25 by any process known in the art. With an aluminium layer, vacuum deposition or sputtering is convenient.
In general the material used for forming the electrically conductive layer 24 and or 25 should have a low attenuation coefficient for high energy electrons (that is ideally to have a low atomic number), a high electrical conductivity when less than than 1000nm thick, be capable of deposition on to diamond with a minimal amount of stress but with adequate adhesion, be realisable onto planar and non-planar diamond surfaces, and be compatible with mounting of the diamond electron beam window to a suitable frame or support 12 such that the layer 24 or 25 is electrically grounded.
The third solution is not illustrated but can readily be visualised with reference to the general structures illustrated in Figures 2 and 3. Instead of adhering a conductive layer 26 to the surface 24, or a conductive layer 27 to the surface 25, the diamond forming the electron beam window 10 can be treated to render it conductive.
One method of making CVD diamond conductive is to introduce a dopant, during growth of the diamond, to introduce an impurity that makes the diamond electrically conductive or semi-conductive. One such impurity is boron which, in the dopant range of 10¹⁵ to 10²¹ , makes CVD diamond electrically semi-conducting or conducting. The introduction of boron as a dopant is conveniently achieved by introducing 1,000 ppm of diborane (B₂H₆) into the process gases whilst the CVD diamond is being grown.
The advantage of imparting bulk conductivity to this CVD diamond, rather than relying on surface conductivity by the conductive layers 24, 25, is that a higher overall electrical conductivity can be achieved than is practicable by the low attentuation metallic layers. Furthermore, doping avoids the need to ensure good coating adhesion and also provides both chemical inertness and mechanical robustness. However, bulk electrical conductivity has the disadvantage, when compared with the application of electrically conductive layers 24, 25, in that the thermal conductivity is significantly reduced below that of the diamond.
Another option for rendering the electron beam window 10 conductive is to implant dopant atoms within the first micron of the surfaces 24 or 24 and 25 by ion implantation. Such ion implantation generally requires a subsequent annealing process to reduce lattice damage in the diamond and to activate the dopant.
Another approach is to use a composite electron beam window of which the bulk is pure CVD diamond which has the advantage of high thermal conductivity, but the surfaces 24 and 25 are made as an electrically conducting stratum either by ion implantation of carbon or boron atoms, or by boron doping. The main advantage of this solution is that the electron beam window retains high bulk thermal conductivity whilst having very robust, diamond-based electrically conducting surface. However, this solution has the disadvantage in that it is more complex to fabricate. Whilst growing CVD diamond, it is possible to make a first stratum electrically conducting by using a suitable dopant, then stopping the introduction of the dopant whilst the bulk of the CVD diamond is grown, and then by resuming doping for the last stratum of diamond growth.
The precise nature of the application, and the severity of window charging will of course dictate which of these solutions is used.
The electron beam window 10 is preferably part of an electron beam assembly (not illustrated) which incorporates an electrically conductive mounting plate defining an aperture, the electron beam window being positioned over this aperture, and an hermetic seal being formed between the electron beam window and the mounting plate. In use, this electron beam assembly would be positioned over the orifice 17 and secured to the structure 12 by any convenient means, such as the bolts 21 and lock-nuts 22. The mounting plate would, of course, need to be sealed to the structure 12 and this can be achieved by arranging a copper gasket between them. The hermetic seal must be electrically conducting and can take the form of a mechanical seal, an adhesive, solder, or brazing securing the window 10 to the mounting plate. In the case where the window has an electrically conductive layer 26 or 27, this layer is preferably formed over at least part of a surface of the mounting plate to provide electrical continuity.
This invention enables diamond electron beam windows to be used for transmitting electron beams having a significantly higher intensity than hitherto. The electron beam windows may be of any convenient shape or geometry, for instance curved or part of a spherical shell presenting a convex face towards the higher pressure domain. The electron beam window may incorporate supports and/or coolant means as are already known in the art.

Claims

An electron beam window, formed from an insulating material to define a first surface for receiving a high energy electron beam and a second surface for transmitting the electron beam, which is protected from perforation by an electrically conductive path that prevents the electron beam from establishing any significant electrical field across the thickness of the window.
An electron beam window, according to Claim 1, in which the electrically conductive path is provided by incorporating an electrically conductive dopant in the insulating material.
An electron beam window, according to Claim 2, in which the dopant is incorporated only in a stratum of the diamond forming at least one of said surfaces.
An electron beam window, according to Claim 1, in which the electrically conductive path is provided by ion implantation in at least one of said surfaces.
An electron bean window, according to Claim 1, in which the electrically conductive path is provided by an electrically conductive layer which is adhered to one of said faces, the layer being sufficiently thin that it will not significantly impede the passage of electrons through the window.
An electron beam window, according to Claim 5, in which the electrically conductive layer is adhered to both of said faces and the combined thickness of the layers is sufficiently thin that they will not significantly impede the passage of electrons through the window.
An electron beam window, according to Claim 5 or 6, in which each layer is formed from a material chosen from the group comprising indium tin oxide, aluminium, magnesium, titanium, platinum, gold and silica doped to make it electrically conductive.
An electron beam window assembly formed by attaching an electron beam window, according to any one of the preceding claims, over an aperture defined by an electrically conductive mounting plate such that the electron beam window covers the aperture, and an hermetic seal is positioned between the electron beam window and the plate.
An electron beam window assembly, according to Claim 8 as appendant to either of Claims 5 or 6 in which the electrically conductive layer is connected to the mounting plate.
An electron gun, arranged to produce a high energy electron beam within a vacuum chamber and to direct the electron beam through an electron beam window into a region of higher pressure, having an electron beam window which is formed from an insulating material and is protected from perforation by an electrically conductive path that prevents the establishment of any potentially damaging electrical charge across the thickness of the electron beam window by the passage of the electron beam.