WO2022084370A1

WO2022084370A1 - Apparatus for generating ionised gaseous or vapour material

Info

Publication number: WO2022084370A1
Application number: PCT/EP2021/079036
Authority: WO
Inventors: Gareth Andrew Taylor; John Peter Carr; Paul Farrar
Original assignee: Evince Technology Limited
Priority date: 2020-10-22
Filing date: 2021-10-20
Publication date: 2022-04-28
Also published as: TW202232545A; EP3989260A1

Abstract

An apparatus (2) for generating an ionised gaseous material or ionised vapour is disclosed. The apparatus comprises a substrate (4) of crystalline material and metallic electrical conductors (6) having first ends located at a first surface of the substrate and second ends (12) located within the substrate, wherein the conductors are adapted to be connected to a voltage supply to enable emission of electrons from the second ends into the substrate, and emission of electrons from a second surface of the substrate by means of application of an electric field to the conductors. Fluid flow channels (18) extend through the substrate to the second surface of the substrate to enable a gaseous material or vapour to be emitted from the second surface and to enable ionisation of at least some of the gaseous material or vapour by electrons emitted from the second surface. Electrodes (26) apply an electric field to electrons emitted from the second ends of the conductors.

Description

APPARATUS FOR GENERATING IONISED GASEOUS OR VAPOUR MATERIAL

The present invention relates to an apparatus for generating ionised gaseous material or vapour.

Generation of ionic gas species is utilised in a plurality of applications that include, but are not limited to, analysis of gas species using spectroscopic techniques, materials deposition, etching of materials, and propulsion. In all such cases, the lower the ambient pressure, the more difficult the generation of ionic gas species becomes.

A known method of generating ionised gas species consists of the provision of a discrete electron source, which may be thermionic or utilise field emission, and a geometric arrangement of electrodes and/or magnetic fields so as to create a constrained local electromagnetic environment that facilitates the creation of an avalanche condition that leads to the ionisation of the target gas species. However, because the initial source of the ionisation process (i.e. the electron source) is discrete, the efficiency of such sources becomes dependent on the effectiveness of the initial confinement method to propagate the initial avalanche process.

At high pressures, the avalanche process is relatively easy to propagate because of the proximity of the gas species to the electron source, which means that ionisation is generally fairly efficient, and the overall design can be optimised to reduce loss of ionisation. At lower pressures, however, typically for operation to the left-hand side of the Paschen curve shown in Figure 1, mean free path lengths mean that species interaction is reduced and ionisation is much less efficient. This makes sources designed to operate in this regime difficult and costly to make.

In order to improve efficiency of the ionisation process, Spindt-type field emission cold cathode sources have been proposed that provide a plurality of electron sources distributed across a wider area, to maximise initial ion generation. These can be coupled with externally applied electro-magnetic fields to further increase the utilisation of generated electrons to this purpose. However, electron sources of this type suffer from the drawback that the resulting ion species can be attracted to the high field region around the field emission structures, which can lead to gradual degradation of the sources due to sputter bombardment, and which can in turn cause contamination of the target ionised species.

US 9194379 discloses an ion thruster comprising a permeable substrate which is formed of arrays of carbon nanotubes (CNTs), W-nanorods, P-siC nanorods, Zn — O nanopencils, other nanowires, and/or other nano - materials, singly or in combination, to accomplish the production of positive ions from a propell ant gas by field i onization. In one embodiment, a permeable substrate is formed of an array of carbon nanotubes with open spaces or apertures between the carbon nanotubes. To accomplish high field enhancement factors (p’s) at the tips, carbon nanotubes are grown with selected optimum height and spacings. In one embodiment, Ps of over 2000 are utilized in order to establish the high electric local fields needed for field ionization while the field in the overall gap (V/d) remains below vacuum breakdown.

Such an approach has some disadvantages, for example the nanostructures suffer degradation from exposure to ion species, which is exacerbated by the geometric property of the structure whereby the very high electri c field at the point of field emissi on also serves to accelerate ions towards it causing significant damage and gradual erosion of the structure. As well as limiting the available lifetime of the source, the voltage that needs be applied to maintain a given electron beam current also needs to be adjusted with time, therefore increasing the complexity of the power supply design.

Another disadvantage that many nanostructures have is that, since they operate in a vacuum, the only effective heat removal mechanism for heat generated at the point of electron emission is by thermal conduction to the base of the structure in contact with the source substrate. This means that the emission tip of the nanostructure can be several hundred degrees centigrade higher than the ambient surroundings, which can exacerbate degradation mechanisms. Heat due to emission or ohmic heating caused by flow of electrical current within the nanostructure can also serve to increase the electrical resistance of the nanostructure thereby restricting the emitted electron current to orders of magnitude below what is theoretically possible. International patent application W02019/020588 discloses an arrangement in which a field emission structure is embedded within a diamond substrate. This arrangement may be formed, for example, using a metal catalyst to effect the creation of a hole feature that can be filled with a conductive material to create a region of high electric field sufficient to initiate field emission of electrons into diamond. The efficiency of the emitter structure may be further enhanced through the incorporation of additional layers and terminations so as to yield Fowler-Nordheim Tunnelling, as described in international patent application WO 2018/172029. The field emission tip is in this way protected from ion bombardment. This embedded structure may thus form a suitable basis for an ion source which overcomes many of the problems of Spindt or other exposed nanostructure field emitters while also being amenable to highly reproducible integrated circuit manufacturing techniques.

Preferred embodiments of the present invention seek to provide an improved apparatus for generating ionised gaseous material or vapour.

According to an aspect of the present disclosure, there is provided an apparatus for generating an ionised gaseous material or ionised vapour, the apparatus comprising: a substrate of crystalline material; at least one elongate electrical conductor having a first end located at a first surface of the substrate and a second end located within the substrate, wherein the conductor is adapted to be connected to a voltage supply to enable emission of electrons from said second end into the substrate, and emission of electrons from a second surface of the substrate by means of application of an electric field to the conductor; at least one aperture extending through the substrate to said second surface of the substrate to enable a gaseous material or vapour to be emitted from said second surface and to enable ionisation of at least some of the gaseous material or vapour by electrons emitted from said second surface; and at least one electrode for applying an electric field to electrons emitted from at least one said second end.

By providing at least one aperture extending through the substrate to said second surface of the substrate to enable a gaseous material to be emitted from said second surface and to enable ionisation of at least some of the gaseous material by electrons emitted from said second surface, this provides the advantage of enabling ionisation at lower pressures by enabling the gaseous material and the emitted electrons to be located in close proximity to each other. Also, this enables the efficiency of ionisation to be improved since a plurality of conductors and apertures can be distributed across the substrate. Furthermore, by providing at least one elongate electrical conductor adapted to emit electrons into the substrate, this provides the advantage of protecting the second end of the conductor from degradation by collision with ions, and enables heat to be conducted away from the second end of the conductor through the substrate.

At least one said electrode may be adapted to increase the emission of electrons from at least one said second end.

At least one said electrode may be adapted to apply an electric field to electrons adjacent at least one said first end.

By having at least one electrode adapted to apply an electric field to electrons adjacent at least one said second end, the voltage that needs to be applied to stimulate the emission of electrons is kept to a minimum and electric fields can be precisely controlled.

At least one said electrode may be adapted to accelerate electrons emitted from said second surface.

This provides the advantage of improving control of the resulting distribution of the emitted electrons from the second surface both in terms of energy distribution and scatter.

At least one said electrode may be adapted to electrically oppose movement of ionised gaseous material or vapour towards said second surface. This provides the further advantage of deterring ions generated in the proximity of the apparatus from travelling back towards the negatively biased elongate source of electrons and reducing their energy if they do, thereby minimising the probability of damage caused by their collision with the said source of electrons.

At least one said electrode may be encapsulated within the apparatus.

This provides the advantages of preventing the electrons emitted from a second end from flowing to a said electrode, thereby creating a leakage path, and ensures that the electric field between the said electrode and the elongate conductors is maintained completely within a solid dielectric ensuring that the maximum possible field is generated at the second end of the elongate conductor.

This also provides the advantage of reducing degradation by further protecting the electrodes from stray ions. The further advantage is provided that the encapsulated electrodes can by brought into closer proximity to the point of electron generation, whereby the magnitude of the applied voltages can be greatly reduced while still achieving the same electric field intensity. The electrodes can also provide a combined means of focusing the distribution of the emitted electrons, thereby improving subsequent ionisation efficiency. The advantage is also provided of reducing the overall structure to a single die produced using semiconductor fabrication techniques. Also, potential erosion of said electrodes by the action of a plasma during manufacture of the apparatus is minimised, and the overall efficiency of the source is improved by removing any electrical leakage path to the electrodes.

At least one said electrode may be encapsulated within said crystalline material.

At least one said electrode may be encapsulated within said crystalline material doped with nitrogen. The apparatus may further comprise temperature control means for controlling the temperature of the gaseous material or vapour.

This provides the advantage of enabling a wider range of materials to be ionised.

The crystalline material may be a group IV material.

The group IV material may be diamond.

According to another aspect of the present disclosure, there is provided a method of forming an apparatus according to any one of the preceding claims, the method comprising: forming at least one recess in a substrate of crystalline material, wherein the recess extends from a first surface of the substrate; forming a respective elongate electrical conductor in at least one said recess, wherein said conductor has a first end located at said first surface of the substrate and a second end located within the substrate, wherein the conductor is adapted to be connected to a voltage supply to enable emission of electrons from said second end into the substrate, and emission of electrons from a second surface of the substrate by means of application of an electric field to the conductor; forming at least one aperture extending through the substrate to said second surface of the substrate to enable a gaseous material to be emitted from said second surface and to enable ionisation of at least some of the gaseous material by electrons emitted from said second surface; and forming at least one electrode for applying an electric field to electrons emitted from at least one said second end. At least one said recess may be formed by means of catalytic etching.

At least one said aperture may be formed by means of catalytic etching.

The method may further comprise encapsulating at least one said electrode within the apparatus.

According to another aspect of the disclosure, there is provided a method of generating ionised gas or vapour using an apparatus as defined above, the method comprising: applying a voltage to at least one said elongate electrical conductor to enable emission of electrons from said second end of said conductor into the substrate, and emission of electrons from the second surface of the substrate; supplying a gaseous material or vapour to at least one said aperture through the substrate to enable the gaseous material or vapour to be emitted from said second surface and to enable ionisation of at least some of the gaseous material or vapour by electrons emitted from said second surface; and applying a control signal to at least one said electrode.

The method may further comprise controlling the temperature of the substrate.

Embodiments of the disclosure will now be described, by way of example only and not in any limitative sense, with reference to the accompanying drawings, in which:-

Figure 1 shows Paschen curves for various gaseous materials;

Figure 2 shows an apparatus for generating ionised gaseous material of a first embodiment;

Figure 3 is a view along the line III-III in Figure 2;

Figure 4 is an apparatus of a second embodiment; Figure 5 is an apparatus of a third embodiment; and

Figure 6 is an apparatus of a fourth embodiment.

Referring to Figure 2, an apparatus 2 for generating an ionised gaseous material comprises a substrate 4 of crystalline material in the form of as diamond in the example shown in Figure 2, and a controlled field emission electron source structure in the form of a plurality of elongate metallic electrical conductors 6 formed in elongate recesses 8 extending from a first surface 10 of the substrate 4 such that distal ends 12 of the conductors 6 are embedded in the substrate 4 near a second surface 14 of the substrate 4. The conductors 6 are connected to cathodes 16 formed on the first surface 10 of the substrate 4. Those skilled in the art will recognise that overall electrical configuration of the apparatus will feature one or more additional electrodes or structures, that may be a considerable distance from the apparatus, to ensure completion of the electrical circuit.

The apparatus 2 also has a microfluidic structure having apertures in the form of fluid flow channels 18 through which a target gas species 20 can pass. The fluid flow channels 18 extend through the substrate 4 from the first surface 10 of the substrate to the second surface 14. The fluid flow channels 18 have diameter typically 3 pm or less and the field emission conductors 6 have diameter 1 pm or more and aspect ratios of greater than 5: 1.

As shown in Figure 3, the conductors 6 and flow channels 18 can be distributed throughout the bulk of the substrate 4 in a regular array, which enables generation of free electrons 22 (Figure 2) over a larger area of the second surface 14 of the substrate 4, and enables the free electrons 22 to interact with flowing gas 20 so that both ionisation and overall directional flow move the resulting ionised gaseous species 24 into a larger area and away from the source of initial ionisation at the second surface 14 of the substrate 4.

The conductors 6 can be formed by using a lithographically patterned metal catalyst to effect a controlled etch of the diamond to form the recesses 8 of aspect ratios of at least 5: 1. A thin film (not shown) of metal such as nickel, cobalt or iron is deposited on the first surface 10 of the substrate 8 in selective small areas, and the diamond substrate 4 is then heated in a reducing atmosphere to around l,000°C to l,200°C such that the catalyst selectively attacks certain crystal planes of the diamond (if single crystal). By careful selection of the process temperature and reducing gas chemistry the catalyst can be tuned to preferentially react with a single crystal plane. For example, if the starting crystal plane of the diamond is { 100} then a hole feature can be created adjusting the temperature so that metal catalyst preferentially reacts with the { 111 } planes it is in contact with enabling an elongate square sided hole feature to be created whose sides are defined by the { 110} planes and the base of the hole by the { 111 } planes. Those skilled in the art will be able to apply surface treatments and metallise the elongate hole features using suitable deposition techniques in order to create one or more ordered field emission structures in the form of conductors 6.

The microfluidic channels 18 for gas flow through the substrate 4 can be created by means of similar use of a metal catalyst etch process as is used to form the recesses 8 for the conductors 6, but allowing the catalyst to completely etch through the diamond substrate 4 from the first surface 10 to the second surface 14, creating a generally square section channel if similar process conditions to those already described are used. Alternatively, those skilled in the art can utilise lithographic techniques combined with low pressure reactive ion etch processes to create microfluidic channels 18.

The cathodes 16 connected to the electrical conductors 6 at the first surface 10 of the substrate 4 enable connection of the conductors 6 to a voltage source for applying an electric field to the conductors 6. The cathode structure 16 may be encapsulated, which would provide the advantage of protecting the circuit path of the cathode 16 in the event that the species 20 to be ionised is capable of precipitating onto the cathode 16 surface and/or is capable of conducting electricity in the neutral state.

In operation, the species 20 to be ionised is passed through the flow channels 18 from the first surface 10 to the second surface 14. At the same time, an electrical bias is applied between the cathode 16 and emission control electrode 26 such that the resultant electric field between the elongate conductors 6 causes electrons to be emitted from the distal ends 12 of the conductors 6 into the substrate 4. Using additional control electrodes, not shown, positively biased with respect to cathode 16 electrons 22 are emitted from the substrate 4 through the surface 14 into the ambient atmosphere where they interact with the target species 20 resulting in the creation of ions 24.

A typical arrangement of the flow channels 18 and field emission features 6 is shown in Figure 3. This interdigitated approach has the advantage that a larger percentage of ions 24 are generated in the initial wave as a result of the higher number density of atoms or molecules of the target species 20 in proximity to the immediate source of electrons 22, i.e. the distal ends 12 of the conductors 6, which in turn leads to a larger overall ionisation efficiency.

Figure 4 shows an exemplar apparatus 102 of a second embodiment of the structure described in Figures 2 and 3, whereby a substrate 8, which incorporates the apparatus 2 is mounted in assembly that permits additional electrical biases to be applied by the use one or more external control grids as exemplified by 32 and 36 shown, which are electrically isolated using insulating spacers 34 and 38 respectively, to enhance the ionisation of the target species 20 to form ions 24. Those skilled in the art will appreciate that the design of such grids should result in the creation of a broadly uniform electric field between the grid and the apparatus 2 while simultaneously being of sufficient transparency to allow the target species 20 and resultant ions 24 to pass through unimpeded. The first external control grid 32 is used to attract and accelerate electrons produced by the apparatus 2 so ensure that they have sufficient energy to ionise the target species 20. A second optional additional external control grid 36 may be employed to subsequently accelerate ions 24 away from the apparatus 102. In the most typical case of the ions being positively charged the electrical biasing of this external control grid 36 will be negative with respect to external control grid 32, thereby reversing the direction of the electric field. Those skilled in the art will appreciate that depending on the nature of the target species 20 and the desired utilisation of the resultant ions 24, different regimes of electrical biasing and a plurality of additional electrodes 36 along with their spacing and shape may be employed. Referring to Figure 5, an apparatus 202 of a third embodiment is shown. The apparatus 202 shown in Figure 5 differs from the apparatus 102 of Figure 4 in that the first additional electrode 32 and the second additional electrode 36 are integrated into the diamond substrate structure along with the control gate 26 in the form of a multilayer control electrode structure 40. Since the gaseous material 20 to be ionised would generally be neutrally charged during its passage through the fluidic channels 18, the application of various biases to the control electrodes 40 should have no impact on gas flow through the structure 202.

This third embodiment offers several advantages over the second embodiment. First by bringing the additional control electrodes into closer proximity the point of electron generation 22 the magnitude of the applied voltages can be greatly reduced while still achieving the same electric field intensity. Secondly the control electrodes can provide a combined means of focusing the distribution of the emitted electrons improving subsequent ionisation efficiency. Thirdly this provides the advantage of reducing the overall structure to a single die produced using semiconductor fabrication techniques. Fourthly it eliminates potential erosion of the external gate electrodes by the action of the plasma impacting on the exposed grids. Fifthly, it increases the overall efficiency of the source by removing the electrical leakage path to the external electrodes.

The arrangement of Figure 5 also includes current suppression regions 44 of nitrogen doped diamond material for suppressing flow of electrons between the elongate electrical conductors 6 and the electrodes 26, 32, 36. The nitrogen doped layers of the current suppression regions are lightly doped (10¹⁶ to 10¹⁸ atoms per cm³) so, with some, but far fewer, vacancies electrons become trapped and hence electron mobility is inhibited, thereby enabling the nitrogen doped layers to acts as more effective insulators than undoped diamond.

Those skilled in the art will understand that such a structure can be achieved by using conventional semiconductor manufacturing processes whereby alternate lithographically patterned layers of a conductive metal such as copper, silver or aluminium and an electrically insulating material such a silicon dioxide or silicon nitride are deposited so as to achieve the desired structure. For applications where greater robustness is required the insulating material may instead be nitrogen doped diamond whose nitrogen concentration is in the range 10¹⁶ to 10¹⁸ atoms per cm’³ deposited using a plasma assisted chemical vapour deposition process (PA-CVD). In this case the choice of metals will be more restricted to ensure both adhesion to the diamond, compatibility with the typical 750C to 850C conditions encountered during the PA-CVD growth process and to ensure lattice compatibility so that the deposited nitrogen-doped diamond material is predominantly single crystal. Achieving this is likely to require the deposition of multiple metals to create each electrode, for example an initial layer of 30nm titanium which carbides with the diamond to ensure adhesion followed by lOOnm- 250nm of silver or copper to provide bulk electrical conduction and finally 5nm of iridium to provide close lattice constant matching to ensure heteroepitaxial single crystal growth.

In operation the control electrode 26 will still be biased with reference to the cathode 16 so as to stimulate and regulate the generation of electrons. The next control electrode 32 will be biased positively with respect to the control electrode 26 so as to provide a means of accelerating the electrons out of the diamond. The final control electrode 36 may be biased positively or negatively with respect to the control electrode 32. This has the advantage of providing better control of the resulting distribution of the emitted electrons 22 from the structure 202 in terms of energy distribution and scatter thereby providing more effective ionisation of the target species 20.

Referring to Figure 6, an apparatus 302 of a fourth embodiment is shown, which differs from the apparatus 202 of Figure 5 in that a thermal management system 42 is brought into proximity with the substrate 4 containing the ionisation source structure in order to effect heating or cooling of the substrate 4. This provides the further advantage of permitting a wider range of candidate species to be ionised, such as metal ions.

It will be appreciated by persons skilled in the art that the above embodiments have been described by way of example only, and not in any limitative sense, and that various alterations and modifications are possible, for example combining separate features of the embodiments of Figure 5 with those of Figure 4, without departure from the scope of the disclosure as defined by the appended claims.

Claims

1. An apparatus for generating an ionised gaseous material or ionised vapour, the apparatus comprising: a substrate of crystalline material; at least one elongate electrical conductor having a first end located at a first surface of the substrate and a second end located within the substrate, wherein the conductor is adapted to be connected to a voltage supply to enable emission of electrons from said second end into the substrate, and emission of electrons from a second surface of the substrate by means of application of an electric field to the conductor; at least one aperture extending through the substrate to said second surface of the substrate to enable a gaseous material or vapour to be emitted from said second surface and to enable ionisation of at least some of the gaseous material or vapour by electrons emitted from said second surface; and at least one electrode for applying an electric field to electrons emitted from at least one said second end.

2. An apparatus according to claim 1, wherein at least one said electrode is adapted to increase the emission of electrons from at least one said second end.

3. An apparatus according to claim 2, wherein at least one said electrode is adapted to apply an electric field to electrons adjacent at least one said first end.

4. An apparatus according to any one of the preceding claims, wherein at least one said electrode is adapted to accelerate electrons emitted from said second surface.

5. An apparatus according to any one of the preceding claims, wherein at least one said electrode is adapted to electrically oppose movement of ionised gaseous material or vapour towards said second surface.

6. An apparatus according to any one of the preceding claims, wherein at least one said electrode is encapsulated within the apparatus.

7. An apparatus according to claim 6, wherein at least one said electrode is encapsulated within said crystalline material.

8. An apparatus according to claim 7, wherein at least one said electrode is encapsulated within said crystalline material doped with nitrogen.

9. An apparatus according to any one of the preceding claims, further comprising temperature control means for controlling the temperature of the gaseous material or vapour.

10. An apparatus according to any one of the preceding claims, wherein the crystalline material is a group IV material.

11. An apparatus according to claim 10, wherein the group IV material is diamond.

12. A method of forming an apparatus according to any one of the preceding claims, the method comprising: forming at least one recess in a substrate of crystalline material, wherein the recess extends from a first surface of the substrate; forming a respective elongate electrical conductor in at least one said recess, wherein said conductor has a first end located at said first surface of the substrate and a second end located within the substrate, wherein the conductor is adapted to be connected to a voltage supply to enable emission of electrons from said second end into the substrate, and emission of electrons from a second surface of the substrate by means of application of an electric field to the conductor; forming at least one aperture extending through the substrate to said second surface of the substrate to enable a gaseous material to be emitted from said second surface and to enable ionisation of at least some of the gaseous material by electrons emitted from said second surface; and forming at least one electrode for applying an electric field to electrons emitted from at least one said second end.

13. A method according to claim 12, wherein at least one said recess is formed by means of catalytic etching.

14. A method according to claim 12 or 13, wherein at least one said aperture is formed by means of catalytic etching.

15. A method according to any one of claims 12 to 14, further comprising encapsulating at least one said electrode within the apparatus.

16. A method of generating ionised gas or vapour using an apparatus according to any one of claims 1 to 11, the method comprising: applying a voltage to at least one said elongate electrical conductor to enable emission of electrons from said second end of said conductor into the substrate, and emission of electrons from the second surface of the substrate; supplying a gaseous material or vapour to at least one said aperture through the substrate to enable the gaseous material or vapour to be emitted from said second surface and to enable ionisation of at least some of the gaseous material or vapour by electrons emitted from said second surface; and

16 applying a control signal to at least one said electrode.

17. A method according to claim 16, further comprising controlling the temperature of the substrate.

17