Electron Amplifier Device

The present invention .relates to an electron amplifier using an electron emissive material, and in particular, but not exclusively, to such an amplifier implemented as a planar substrate having through holes, electrode layers on the opposing faces of the substrate, and the electron emissive material deposited on the electrode layers.

Discussion of the prior art

A variety of electrical devices make use of secondary electron emission, in which a free primary electron strikes a low work function emissive material causing the emission of multiple free secondary electrons. The effect is frequently used in combination with an accelerating electrical field such that the secondary electrons gain energy, strike another area of the emissive material and lead to the release of further secondary electrons in a cascade or avalanche .

Example technologies include photomultiplier tubes which may be used to detect single photons, and micro channel plates which are used in low light cameras and night vision goggles. The cascade of secondary electrons may be directed to a scintillating material to cause visible or ultraviolet light emission, or may be electrically detected using suitable electrodes and circuitry.

EP1004134 discloses a micro array of electron multiplier stacks with a pixel spacing of less than 0.5mm. Multiple dynode layers give rise to a high level of electron amplification. In each dynode layer magnesium oxide is used as an emissive material . Several dynode layers are required

to achieve an adequate level of amplification. The disclosed structure is therefore quite complex.

Kim et al . , "Secondary electron emission from magnesium oxide on multiwalled carbon nanotubes", Applied Physics Letters, VoI 81, no. 6, 2002, discusses an improved electron emissive material formed by covering vertically grown multiwalled carbon nanotubes with magnesium oxide. The nanotubes are grown by thermal chemical vapour deposition on a nickel coated silicon substrate. The magnesium oxide layer is deposited onto the nanotubes using electron beam deposition. Substantial increases in the secondary electron yield are observed, with values of at least 22000, compared with about 1000 for conventional porous MgO.

An electron amplifier using an electron emissive material incorporating carbon nanotubes is disclosed on US2003/0173884A1. A plurality of through holes are formed in a substrate. A resistive layer is deposited on the sidewalls of the holes. Carbon nanotubes are added to a sol-gel solution containing magnesium oxide, and the substrate is dipped in the solution to deposit an electron emissive layer on the resistive layer. After baking, electrode layers are formed on the upper and lower faces of the substrate .

The present invention seeks to address problems and limitations of the related prior art.

Summary of the invention

The present invention provides an electron amplifier device comprising: a substrate having first and second faces and an array of apertures, or vias, between the faces,- an electrically conductive electrode layer formed on at least one face of the substrate; and a secondary electron emission layer formed on the electrode layer.

Of course, in defining a layer to be formed on another layer or element, it is not intended to exclude the possibility of there being provided intervening layers of various kinds for various purposes.

Preferably, the secondary electron emission layer is partly or completely excluded from the walls of the apertures .

Each primary electron or primary radiation photon striking the emission layer generates a large number of secondary electrons which pass through the apertures for detection or to generate further secondary electrons.

Preferably the conductive layer and the emission layer are provided on both faces of the substrate . An electric field between the respective parts of the conductive layer on the two faces may then be used to accelerate secondary electrons passing through an aperture to strike the emission layer on the face beyond the aperture and generate further secondary electrons in the vicinity of the aperture. To set up the electric field it is necessary to ensure that the conductive layer is discontinuous through the apertures.

The emission layer preferably comprises carbon nanotubes coated with a low work function material such as porous magnesium oxide. However, a variety of other arrangements may be used to provide high secondary emission, for example by roughening the conductive layer directly before deposition of the low work function material, or by using a variety of different types of nanoparticles providing an appropriate scale of roughness.

The conducting layer may be constructed using a metallic adhesion layer formed from a material selected to adhere to the substrate, and a metallic seeding layer formed on the adhesion layer from a material selected for growth of

carbon nanotubes. In alternative embodiments, carbon nanotubes or other nanoparticles are applied to the substrate, for example by electrostatic deposition, screen printing, inkjet printing or sputtering.

The device may further comprise a suitable primary electron source to supply primary electrons to the emission layer, and a suitable secondary electron detector, such as a scintillation layer or electrode array with associated current detection circuitry.

The invention also provides a method of constructing an electron amplifier device from a planar substrate having first and second faces and a plurality of apertures through the substrate between the first and second faces, comprising: providing an electrically conductive layer on at least one face of the substrate; and depositing a secondary electron emission layer on the conductive layer.

If the conductive layer is provided on both faces then low angle deposition is preferably used to prevent interconnection through the apertures of the respective parts of the conductive layer on the first and second faces. An alternative construction method is to form the apertures after providing the conductive layer.

The conductive layer may comprise an adhesion layer and a seeding layer as discussed above, and all or parts or the conductive layer may be deposited using, for example, ion beam deposition or electron gun evaporation.

Brief description of the drawings

Embodiments of the invention will now be described, with reference ot the accompanying drawings, of which:

Figure 1 illustrates a portion of a device embodying the invention, showing a substrate with an array of through apertures;

Figure 2 is a cross section through one of the apertures shown in figure 1, showing an electron amplifier layer applied to the substrate;

Figure 3 shows a cross section through the amplifying layer of figure 2;

Figure 4 is an enlarged view of the section of figure 3, showing the growth of carbon nanotubes on a discontinuous seeding layer;

Figures 5 and 6 are enlarged views of alternative configurations for the amplifying layer;

Figure 7 shows angled deposition of the conductive layer of the amplifier layer, to avoid deposition throughout the aperture ; and

Figure 8 illustrates the device in use; and

Figure 9 shows a stack of two devices .

Description of preferred embodiments

Devices

Referring now to figure 1, there is shown in perspective view a portion of a device embodying the invention. A planar substrate 10 is formed of an insulating material, such as quartz, a glass, silica, insulated silicon, multi-layer PCB (rigid or flexible) , or a ceramic for example Al ₂ O ₃ .

Various flexible media may also be used, such as plastics, in the form of tapes or other configurations.

The substrate may typically be between about lOOμm and lmm thick and between 5mm and 500mm across, depending upon the application.

A large number of similar or identical apertures 12 are formed through the substrate, typically on a regular grid. The apertures may be straight sided, or may more preferably be tapered, and can have a variety of shapes including circular and hexagonal .

One or both of the major faces of the substrate are at least partially covered with an amplifying layer made up of an electrically conductive layer, upon which is provided a secondary electron emission layer incorporating a low work function material .

Figure 2 shows in more detail a cross section through one of the apertures 12. The amplifying layer 14,16 is present on each face of the substrate 12, but is not generally present within the aperture 12, with the exception of limited regions around the ends of the aperture which arise due to the techniques of construction discussed below.

The amplifying layer is shown in more detail in figure 3. The conductive layer 20 is adjacent to the substrate. The emission layer is then formed by preferential deposition of carbon nanotubes 24 onto the conductive layer 20, and deposit of a low work function emissive material 26 onto the carbon nanotubes 24. Suitable low work function materials are oxide and fluoride based materials such as MgO, CaF ₂ , MgF ₂ , SiO ₂ and La2θ ₃ .

The respective parts of the conductive layer on either face of the substrate do not connect within the aperture, so that the parts on either face are electrically isolated.

A single layer of metal could be used for the conductive layer 20. However, to ensure adequate adhesion to

the insulating substrate 10 and to also seed growth of carbon nanotubes, multiple metal layers may be used. This is illustrated in figure 4 in which an adhesion layer 28 is shown applied to the substrate, and a seeding layer 29 is applied to the adhesion layer. The carbon nanotubes then form on or in the seeding layer, preferentially to other areas of the device such as the exposed substrate inside the apertures .

The material of the adhesion layer 28 is selected to adhere to the substrate, and may be, for example, Mo, W, Ti or Cr. The thickness of the adhesion layer, or the combined conductive layer, may be chosen to provide sufficient electrical conductivity, for example being between about O.lμm and 5μm thick.

The material of the seeding layer 28 is chosen to seed carbon nanotube growth, and may be, for example, Cu, Ni, Au or Al . The seeding layer may be much thinner than the adhesion layer, for example less than 50nm thick, and is preferably discontinuous, as shown in figure 4.

A single material combining both the functions of the adhesion and seeding layers may be used.

Figure 5 illustrates an alternative way in which the parts of the amplifying layer 14, 16 may be arranged. The conductive layer 20 is deposited either in such a way that the surface of this layer forms with a rough surface, or the layer 20 is formed and then roughened. Suitable techniques for roughening include etching and abrading, and the techniques used for deposition of the layer may be adapted to encourage a suitable subsequent degree and scale of roughness .

Preferably, the scale of roughness of the surface of the conductive layer is about lOμm to lOOμm. The roughness

may be random, or may be regular, for example by etching to produce an array or ridges, peaks or points. Following formation of the rough surface conductive layer 20 the low work function emissive material 26 is deposited as previously described.

Figure 6 illustrates another alternative way in which the parts of the amplifying layer 14, 16 may be arranged. Onto the conductive layer 20, and optionally one or more further intermediate layers such as adhesion layer 30 are deposited preformed nanoparticles 27, to provide a rough surface on which to deposit the low work function emissive material 26. The skilled person will be aware of a variety of commercially available nanoparticles 27, especially highly irregular nanoparticles, which could be used such as irregular carbon black particles, as well as preformed carbon nanotubes .

Construction techniques

The apertures may be formed through the substrate in a variety of ways including by powder blasting, physical or chemical etching, and laser drilling, either before, during or after the other steps required to form the device.

If a flexible substrate is used then the apertures 12 may be vacuum formed, for example by drawing small regions of the substrate into corresponding apertures in an underlying die .

The conductive layer is preferably deposited at low angles of incidence, for example less than 30° from the plane of the substrate, to avoid deposition deep within each aperture, as illustrated in figure 7. The angle of incidence of deposition 32 will be constrained by the geometry of the apertures and the deposition technique used. Suitable

deposition techniques include ion-beam deposition and electron gun evaporation. Of course, if the apertures are formed after formation of the or parts of the conductive layer then the need to use angled deposition may be avoided, and the reliable electrical separation of the two faces of the substrate may more easily be achieved.

The carbon nanotubes may be grown by plasma CVD, thermal CVD, carbon arc deposition, RF/microwave plasma deposition and other well known methods. The seeding layer may be very thin and discontinuous, acting as a series of nucleation sites. The carbon nanotubes may either grow on top of or underneath these sites, pushing the seeding metal up.

If preformed nanoparticles are used to provide the rough surface then these may be deposited on or bonded to the underlying conductive layer or further adhesion layers by a variety of techniques including sputtering, electrostatic powder coating, screen printing and inkjet printing. In some embodiments the nanoparticles may be formed during the coating or deposition process, and in other embodiments they may be supplied in advance.

The low work function emission layer may be grown by sputtering, electron beam deposition and other known methods. This layer may be deposited all over the substrate, including throughout the apertures, because it will generally be an electrical insulator so will not provide an electrical connection between the faces of the substrate.

Operation

Operation of a device as discussed above is illustrated in figure 8. A driving voltage, typically 100V - 500V is applied between the conductive layer parts on the upper 40

and lower 42 faces of the substrate. An incident primary electron 44 or photon collides with the emission layer on the upper face 40 and a large number of much lower energy secondary electrons 46 are released by the collision. These are accelerated by the electric field resulting from the driving voltage. Many of these pass through the adjacent aperture 12 and collide with the emission layer on the lower face 42, releasing further secondary electrons 48.

The device may also include, or be arranged with a primary electron source 50 for generating primary electrons, and a secondary electron detector 52 for detecting or otherwise using the secondary electrons generated at the emission layer. By way of example, the primary electron source could be a photocathode from which electrons are ejected by incident X-rays, and the secondary electron detector could be an array of electrodes for separately detecting the secondary electrons emitted close to each aperture 12.

A gain of around 10 ⁷ secondary electrons for each incident primary electron or photon can be achieved with appropriate constructions.

Two or more devices may be stacked, for example as shown in figure 9 which illustrates a parallel spaced arrangement of a first device 60 and a second device 62. Each device can take any of the forms discussed above, arranged in sufficiently close proximity that secondary electrons emitted at the first device 60 travel to the second device 62 where they cause emission of further secondary electrons. In this way, the total amplification achieved is increased. To facilitate this effect, a driving voltage is provided between the two devices as well as

across each device. In the arrangement of figure 9 the apertures in each substrate are aligned.

In alternative embodiments the apertures of each device are offset from each other, and three or more stacked devices may be used. Moreover, each device may be provided with an emissive layer on either one or both faces of the substrate .

Applications

The device may be used in the construction of large area pixelated detectors. One particular application is in X-ray detectors for medical, security and other uses. Other applications include copying devices such as photocopiers, optical detector arrays for cameras, applications in high energy physics, transmission electron microscopes and so on.

The size of the substrate, the spacing and geometry of the apertures, the primary source and secondary detector, and other details of the device may be adapted in various ways .