Liquid metal ion source and method for providing a liquid metal ion source

Abstract

The invention relates to a method for producing an emitter structure (1, 14) for a liquid metal ion source, comprising the following steps: - providing a magnetic suspension (3) having a liquid metal and ferromagnetic particles located therein; - applying the magnetic suspension (3) to a substrate (2, 13); - arranging a magnetic field source (4, 12) with a magnetic pole parallel to the substrate such that the magnetic suspension (3) is shaped and forms a dome structure (31).

Description

[0001] Liquid metal ion source and method for providing a liquid metal ion source

[0002] Technical area

[0003] The invention relates to liquid metal ion sources for field emission drives or ion beam structuring systems and a method for producing an emitter structure for ion emission.

[0004] Technical background

[0005] Liquid metal ion sources for ion propulsion or ion beam structuring systems typically comprise a liquid metal reservoir and an emitter structure. During operation, the emitter structure is wetted by the liquid metal. This is ionized there and emitted toward a perforated cathode. The resulting fluidic vacuum in the fluid film on the emitter structure feeds liquid metal from the liquid metal reservoir.

[0006] As known, for example, from M. Tajmar, I. Vasiljevich, and W. Grienauer, "High current liquid metal ion source using porous tungsten multiemitters," Ultramicroscopy, vol. 111 , no. I, pp. 1-4, 2010, do : 10.1016 / j.ultramic.2010.09.005 or from PD Prewett and GLR Mair, ""Source Fabrication," in Focused Ion Beams from Liquid Metal Ion Sources", 1st ed., vol. I, no. 1 , Research Studies Press Ltd., 1991 , p. 21., the emitter structure can be needle-shaped or designed as a capillary, in single or multiple emitter configuration, and is set during operation with a positive electrical potential with respect to an opposite hole cathode or extractor electrode. Due to the small radius at the emitter tip, there is an increase in the electric field, which initially leads to the formation of a conical fluid extrusion, the so-called Taylor cone.This leads to a Taylor Cone, which has an even smaller tip radius and thus further increases the electric field. This field enhancement then leads to the extraction of metal ions from the liquid metal film on the emitter structure and subsequently to their ionization and acceleration through an exit opening in the apertured cathode. The metal ions are ejected in a focused ion beam and can be used for microstructuring surfaces and in field emission propulsion for orbit and attitude control of spacecraft.

[0007] The fabrication of the emitter structure currently relies on complex and cost-intensive manufacturing processes. Previous methods for producing emitter structures for liquid metal ion sources include, among other things, producing a porous needle structure using powder injection molding and electrochemical etching. The emitter structure thus produced is connected to a reservoir of liquid metal. To ensure that the emitter structure of the liquid metal ion source is wetted with the liquid metal, the assembly is subjected to a high-vacuum process and heat treatment, as described, for example, in P.D. Prewett and G.L.R. Mair, "Source Fabrication," in Focused Ion Beams from Liquid Metal Ion Sources, 1st ed., vol. I, no. 1, Research Studies Press Ltd., 1991, p. 21.

[0008] Due to the high-vacuum process required to wet the emitter structure with liquid metal, the manufacturing process for such conventional liquid metal ion sources is complex and costly. Furthermore, the production of the emitter structure in the manner described above, based on a porous needle structure, is error-prone and associated with high reject rates, as the emitter structures are highly sensitive to mechanical influences. The manufacturing process requires quality control of the emitter structure using a scanning electron microscope.

[0009] In addition, the porous needle structures of the emitter structure are often made of tungsten or other refractory metals that are soluble in indium, which is often used as a liquid metal. This reduces the lifetime of the liquid metal ion source produced in this way and leads to the gradual degradation of the needle geometry of the emitter structure and thus the emission properties during operation.

[0010] Such emitter structures are also sensitive to occasional high-voltage discharges, which release material from the emitter structure and thus lead to increased degradation.

[0011] As known from B. Nembo and W. Engel, “Method and System for Generating an Ion or Electron Beam,” AT 523319 AI, 2019, and N. Buldrini and L. Bettiol, “Advanced Electrical Propulsion Activities at FOTEC,” EPIC Workshop, Naples, 2023, needle structures with magnetic suspensions can be generated using ultrasonic waves. This avoids the problems of conventional, fragile and complex emitter structures. However, there are disadvantages for space propulsion. First, the needle generation and thus the needle geometry are undefined and difficult to reproduce, which affects the consistency of the emission characteristics. Second, the ultrasonic frequency generator requires a constant supply of electrical energy, which ultimately reduces the thrust-to-power ratio and the electrical efficiency of the space propulsion system.

[0012] The concept disclosed in US Pat. No. 10,330,090 B2 also simplifies the creation of emitter structures, but relies on ferrofluids as propellants. However, the creation of a true ferrofluid based on liquid metal has not yet been achieved, which prevents its application in field emission propulsion systems. The colloidal suspension of single-domain particles (approx. 10 nm) in a carrier fluid requires steric stabilization of the particles against sedimentation in gravitational fields or magnetic field gradients by coating them with surfactants, as described in R.E. Rosensweig, "Ferrohydrodynamics: Magnetic Fluids," Dover Publications Inc., 1997, pp. 33-72.

[0013] It is an object of the present invention to provide an improved liquid metal ion source which can be manufactured in a simpler manner and which avoids the disadvantages of previous emitter structures.

[0014] Disclosure of the invention

[0015] This object is achieved by the method for producing an emitter structure for a liquid metal ion source according to claim 1 and by an emitter structure for a liquid metal ion source and a liquid metal ion source according to the independent claims.

[0016] Further embodiments are specified in the dependent claims.

[0017] According to a first aspect, a method for producing an emitter structure for a liquid metal ion source is provided, comprising the following steps:

[0018] Providing a magnetic suspension containing a liquid metal and ferromagnetic particles therein;

[0019] - Applying the magnetic suspension to a substrate; - Arranging a magnetic field source with a magnetic pole parallel to the substrate so that the magnetic suspension is shaped and forms a dome structure.

[0020] The above method involves the use of a magnetic suspension and a magnetic field to produce an emitter structure.

[0021] The magnetic suspension is formed by ferromagnetic particles suspended in liquid metal. To ensure the ferromagnetic particles are thoroughly wetted, a flux can be used to remove an oxide layer from the particles during their production.

[0022] The magnetic suspension preferably has a mass fraction of ferromagnetic particles made of iron, cobalt, nickel, and / or gadolinium of 5% to 80%. The particles can have an average size of between 0.5 and 10 pm and, in particular, a uniformly or normally distributed mixture of sizes between 0.5 pm and 10 pm.

[0023] The liquid metal used can then include low-melting gallium alloys, such as Galinstan, alloys with cesium, mercury and indium, low-melting alkali alloys or low-melting bismuth alloys.

[0024] The melting temperature of the liquid metal is tied to the operating temperature of the permanent magnets used (magnetic field source). In the preferred case, neodymium-iron-boron permanent magnets (NdFeB) can be used because, firstly, they exhibit strong magnetic fields in a small volume and, secondly, they are electrically conductive. (Contacting) Although NdFeB magnets have a Curie temperature of 310 °C (above this temperature, the material is no longer ferromagnetic but paramagnetic), partial demagnetization of the material occurs at temperatures as low as 80 °C. Therefore, the operating temperature of NdFeB is 80 °C. In this material system, low-melting means a melting point below 80 °C.However, since NdFeB magnets can be further alloyed, thus increasing their operating temperature, and other materials such as samarium cobalt (SmCo) can also be used, low-melting refers to a temperature below the operating temperature of the material used in the permanent magnets used as a magnetic field source. The typical operating temperature of SmCo is 350°C. Therefore, all liquid metals with a melting point below 350°C can be classified as low-melting. These metals can therefore be supplemented by pure indium (157°C) and pure bismuth (270°C).

[0025] To produce the magnetic suspension, ferromagnetic particles in the micrometer range that are not colloidally suspended should be used, so that they exhibit a magnetorheological effect when exposed to magnetic fields. This represents an advantage over ferrofluids, which cannot achieve a magnetorheological effect and thus cannot solidify.

[0026] The magnetic suspension is applied to a surface of the substrate and exposed to a magnetic field source. For this purpose, the magnetic field source is positioned with its magnetic pole on a first surface of the substrate, which is opposite a second surface of the substrate to which the magnetic suspension is applied. By applying a magnetic field, a dome structure of solidified suspension forms above the magnetic pole due to a magnetorheological effect, which remains dimensionally stable. The emitter structure is thus formed by the solidified magnetic suspension, which simultaneously serves as the liquid metal reservoir for the liquid metal ion source. The solidification occurs due to the network-like contraction of the magnetic particles, so that the liquid metal is located in the pores that form between them.

[0027] Furthermore, the substrate can be coated with tungsten or other refractory metals to keep the solubility in the liquid metal as low as possible.

[0028] It can be provided that the base is formed with a particularly conical, pin-shaped or dome-like elevation which protrudes from the magnetic pole so that the dome structure of the magnetic suspension is formed thereon.

[0029] This dome structure of the magnetic suspension can already be used as an emitter structure if the apex of the dome structure is arranged opposite a hole cathode and an operating voltage is applied which causes a field increase at the tip or apex of the dome structure, which leads to the escape of metal ions of the liquid metal in the magnetic suspension of the dome structure.

[0030] The strengthening due to the magnetorheological effect is reproducible, and the composition of the magnetic suspension allows for adjustment of the ion emission characteristics. The magnetic field source can preferably comprise a permanent magnet, which is preferably conductive for applying the electrical potential if the substrate is directly formed by a magnetic pole of the permanent magnet. In particular, the permanent magnet should preferably have a remanence of at least 0.1 Tesla.

[0031] To apply the electrical potential during operation of the liquid metal ion source formed with the emitter structure, the substrate can also be conductive. The permanent magnet can then also be made of electrically non-conductive hard ferrite.

[0032] Alternatively, the magnetic field source can also be implemented in the form of an electromagnet.

[0033] By means of a former, which represents a negative mold, the magnetorheologically strengthened magnetic suspension can be formed into a tapered or tapered shape, in particular a conical shape, which has a more defined, i.e. further tapered tip, in order to enhance the effect of the field enhancement during operation.

[0034] Since points and edges should be avoided in high-voltage applications, the cone shape should be circular (a solid of revolution of a particularly linear function). Furthermore, the half-angle of the circular cone should not be less than 10°, as otherwise it would become too delicate and thus unstable. Solids of revolution of convex functions (e.g., parabolas) also promote stability, while solids of revolution of concave functions (e.g., natural logarithms) can lead to very delicate and therefore unstable structures.

[0035] Furthermore, a needle geometry of the magnetic suspension can be created by approaching a magnetic conductor to the apex or tip of the magnetic suspension and then removing the magnetic conductor.

[0036] For example, the cone tip can be tapered or sharpened by bringing the magnetic conductor closer. The magnetic flux density between the tip or vertex and the magnetic conductor, and the flux density gradient, are adjusted in such a way that the tip or vertex of the solidified magnetic suspension is forced into a needle shape by magnetic forces. The structure thus created remains stable in the presence of a magnetic field, even after the magnetic conductor is removed.

[0037] Furthermore, during the shaping process, the negative mold or the magnetic conductor can exert a mechanical vibration on the substrate and / or the strength of the magnetic field of the magnetic field source can be varied. Thus, the needle geometry can be further improved by varying the magnetic field, for example, by varying the distance of the magnetic pole of the permanent magnet from the substrate or by varying the magnetic field strength of the electromagnet as the magnetic conductor approaches, since the magnetic field strength determines the degree of solidification of the magnetic suspension and thus its deformability.

[0038] The possibility of electrical contact via the permanent magnet or via the substrate enables the production of liquid metal reservoirs from non-conductive materials such as plastic or ceramic.

[0039] Furthermore, with appropriate dimensioning of the magnetic and electrohydrodynamic forces, the magnetic suspension can flow to the tip of the emitter structure in order to regenerate it.

[0040] According to a further aspect, an emitter structure for a liquid metal ion source is provided, comprising: a substrate; a magnetic suspension having a liquid metal and ferromagnetic particles therein, which is applied to a substrate; a magnetic field source that applies a magnetic field to the magnetic suspension to magnetorheologically solidify the magnetic suspension so that it maintains a dome-like, tip-like, or needle-like structure.

[0041] Furthermore, the base can have a particularly conical, pin-shaped or dome-shaped elevation on which the magnetic suspension is magnetically held.

[0042] The magnetic field source may comprise a permanent magnet, an electromagnet or the like, wherein in particular the base is formed by a flat pole of the electrically conductive permanent magnet or by a conductive foil or plate.

[0043] According to a further aspect, a liquid metal ion source is provided, comprising: a liquid metal reservoir for receiving liquid metal with a perforated plate having at least one opening; the above at least one emitter structure,

[0044] - wherein the shaped magnetic suspension of the at least one emitter structure protrudes from the liquid metal reservoir through the corresponding at least one opening in the perforated plate, so that an edge of the opening and the shaped magnetic suspension adjoin one another, ie no gap remains.

[0045] The absence of a gap between the edge of the opening and the formed magnetic suspension ensures tightness against unwanted leakage of liquid metal. The afterflow of liquid metal is achieved by the contact of the liquid metal in the reservoir with the magnetic suspension of the dome structure.

[0046] Furthermore, a perforated cathode having at least one opening may be arranged opposite an associated vertex or an associated tip of the at least one shaped magnetic suspension, wherein an electrical voltage is applied between the perforated cathode and the substrate.

[0047] Brief description of the drawings

[0048] Embodiments are explained in more detail below with reference to the attached drawings. They show:

[0049] Figures 1a - 1e show the sequence of a method for producing an emitter structure for a liquid metal ion source; and

[0050] Figure 2 is a schematic representation of a field-effect ion drive with an emitter structure, manufactured according to the method of Figures 1a-1d. Description of embodiments

[0051] Figures 1a to 1e schematically show a process for producing an emitter structure 1 for a liquid metal ion source.

[0052] In step 1a of Figure 1a, a magnetic suspension 3 is applied to a conductive base 2, e.g. a diameter of the (circular) base of 2 mm with a layer thickness of between 0.5 and 3 mm and / or a volume of between 1 l and 10 pl.

[0053] The magnetic suspension preferably contains 50% by mass of ferromagnetic particles made of iron, cobalt, nickel, and / or gadolinium. The particles can have a size of, for example, 5 pm, and are suspended in a liquid metal made of a low-melting gallium alloy, such as Galinstan.

[0054] As shown in Figure 1b, the magnetic suspension can be subjected to a magnetic field from a magnetic field source 4. For this purpose, a pole 41 of a permanent magnet or electromagnet can be arranged on the side opposite the suspension 3, so that the suspension is subjected to a magnetic field. The strength of the magnetic field can be 0.1 Tesla or more and is preferably selected depending on the desired or to-be-achieved viscosity of the magnetic suspension. Alternatively, the base 2 can be omitted and the magnetic suspension can be applied directly to a pole 41 of the magnetic field source 4 designed as a permanent magnet. The permanent magnet can then preferably be designed to be electrically conductive or at least its pole 41 can have an electrically conductive surface.During operation, the magnetic suspension can be exposed to an electrical potential as an emitter structure via the electrically conductive base 2 or the electrically conductive permanent magnet 4. Alternatively, the emitter structure can also be contacted via the liquid metal or the liquid metal reservoir. Furthermore, the permanent magnet can also be designed with low conductivity, such as a hard ferrite.

[0055] By applying a magnetic field, preferably at a level greater than 0.5 Tesla, a dome structure 31 forms from solidified magnetic suspension 3. Solidification occurs due to a magnetorheological effect, which leads to a structured, net-like arrangement of the magnetic particles in the suspension 3. The resulting dome structure 31 is already suitable for use as an emitter structure in a liquid metal ion source, since the apex has a curvature that can cause a sufficient field enhancement when the operating voltage is applied.

[0056] To improve the tip of the dome structure 31, as shown in Figure 1c, a negative mold 5 can be used, with which the dome structure 31 can be brought into a conical structure 32 by placing it on the magnetic suspension 2, so that a tip of the solidified magnetic suspension is formed and is permanently maintained when the magnetic field is still applied.

[0057] By bringing a magnetic conductor 6 close to the tip of the conical structure 32, the magnetic flux density and its gradient in the region of the formed tip of the conical structure can increase, so that, as shown in Figure 1d, a portion of the magnetic suspension 33 migrates from the conical tip 32 to the magnetic conductor 6 and, due to the applied magnetic field, is forced into a needle geometry, down to very small diameters at the tapered needle tip. Upon removal of the magnetic conductor 6, as shown in Figure 1e, a portion of the magnetic suspension 33 is removed along with the magnetic conductor 6, thus forming the needle structure 33. This remains stable due to the applied magnetic field.

[0058] The formation of the needle structure 33 can be assisted by mechanical vibration or variation of the magnetic field of the permanent magnet or the electromagnet during the approach and subsequent removal of the magnetic conductor 6.

[0059] Figure 2 illustrates a liquid metal ion source 10 that utilizes an emitter structure 14 fabricated according to the method of Figures 1a to 1e. The liquid metal ion source 10 can be used as part of a field emission propulsion system or as an ion beam source for microstructuring components.

[0060] The liquid metal ion source 10 comprises a liquid metal reservoir 11, which may, for example, be cylindrical. The liquid metal reservoir 11 may be electrically conductive or non-electrically conductive, e.g., made of plastic or ceramic. The liquid metal reservoir 11 represents a closed container that is or can be filled with liquid metal 16. The liquid metal 16 used, which is accommodated in the liquid metal reservoir 11, preferably has the same composition as the liquid metal of the magnetic suspension 3.

[0061] A magnetic field source 12 in the form of a permanent magnet or an electromagnet is arranged inside the liquid metal reservoir 11. This source is connected to the emitter structure 14 via an electrically conductive base 13, so that a magnetic field is permanently applied to the magnetic suspension of the emitter structure 14. This maintains the shape of the emitter structure 14.

[0062] The assembly comprising the emitter structure 14, the base 13, and the magnetic field source 12 is arranged in the liquid metal reservoir 11 using a holder 17 such that a portion of the emitter structure is located within the liquid metal reservoir 11 and protrudes from the liquid metal reservoir 11 through an opening in a perforated plate 15. The perforated plate 15 represents part of the boundary of the liquid metal reservoir 11 and can, for example, close an axial end of the cylindrically shaped liquid metal reservoir 11.

[0063] The opening of the perforated plate 15 can be arranged centrally of the perforated plate 15 and dimensioned such that during operation of the liquid metal ion source 10 no gap remains between the emitter structure 14 and the edge of the perforated plate 15 and the liquid metal flows through the emitter structure.

[0064] Furthermore, a hole cathode 18 can be arranged opposite the emitter structure 14, so that ion emission can occur during operation when an electrical voltage is applied between the emitter structure 14 and the hole cathode 18.

Claims

Claims 1. A method for producing an emitter structure (1, 14) for a liquid metal ion source, comprising the following steps: Providing a magnetic suspension (3) comprising a liquid metal and ferromagnetic particles therein; - applying the magnetic suspension (3) to a base (2, 13); - Arranging a magnetic field source (4, 12) with a magnetic pole parallel to the base so that the magnetic suspension (3) is shaped and forms a dome structure (31).

2. Method according to claim 1, wherein the magnetic suspension (3) has a mass fraction of ferromagnetic particles, in particular with iron, cobalt, nickel and / or gadolinium, of 5% to 80%, wherein in particular the particles have an average size of between 0.5 and 10 pm and in particular a uniformly distributed or normally distributed mixture of sizes between 0.5 pm and 10 pm.

3. The method according to claim 1 or 2, wherein the liquid metal comprises a low-melting gallium alloy, an alloy with cesium, mercury and / or indium, a low-melting alkali alloy or a low-melting bismuth alloy.

4. Method according to one of claims 1 to 3, wherein the base (2, 13) is formed with a particularly conical, pin-shaped or dome-like elevation, which protrudes in particular centrally from the magnetic pole, so that the dome structure of the magnetic suspension (3) is formed thereon.

5. Method according to one of claims 1 to 4, wherein the magnetic suspension (3) is brought into a tapered or tapered shape, in particular a conical shape, by means of a negative mold (5) with an applied magnetic field.

6. Method according to one of claims 1 to 5, wherein by approaching a magnetic conductor (6) to the apex or tip of the magnetic Suspension (3) and subsequent removal of the magnetic conductor creates a needle geometry of the magnetic suspension (3).

7. Method according to one of claims 5 to 6, wherein during the shaping by the negative mold (5) or the magnetic conductor a mechanical vibration is exerted on the base (2, 13) and / or the strength of the magnetic field of the magnetic field source (4, 12) is varied.

8. An emitter structure (1, 14) for a liquid metal ion source, comprising: a base (2, 13); a magnetic suspension (3) with a liquid metal and ferromagnetic particles therein, which is applied to a base (2, 13); a magnetic field source (4, 12) which exerts a magnetic field on the magnetic suspension (3) in order to magnetorheologically solidify the magnetic suspension (3) so that it forms or maintains a dome-like, tip-like, or needle-like structure.

9. Emitter structure (1, 14) according to claim 8, wherein the base (2) has a particularly conical, pin-shaped or dome-shaped elevation on which the magnetic suspension (3) is magnetically held.

10. Emitter structure (1, 14) according to claim 8 or 9, wherein the magnetic field source (4) comprises a permanent magnet, in particular a neodymium-iron-boron magnet, or an electromagnet, wherein in particular the base is formed by a flat pole of the electrically conductive permanent magnet or by a conductive foil or plate.

11. A liquid metal ion source (10), comprising: a liquid metal reservoir (11) for receiving liquid metal (16) with a perforated plate (15) with at least one opening; at least one emitter structure (14) according to one of claims 8 to 10, - wherein the shaped magnetic suspension of the at least one emitter structure (14) protrudes from the liquid metal reservoir (11) through the corresponding at least one opening in the perforated plate (15), so that an edge of the opening and the shaped magnetic suspension (3) adjoin one another or no gap or opening remains.

12. Liquid metal ion source according to claim 11, wherein a perforated cathode having at least one opening is arranged opposite an associated vertex or tip of the at least one shaped magnetic suspension (3), wherein an electrical voltage is applied between the perforated cathode and the base (2).

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