Electron source for generating an electron beam

Abstract

Electron source (2) for generating an electron beam (8) with a cathode (1) and an anode (4) arranged on the same silicon carbide substrate (5), wherein the cathode (1) has a graphene layer (12) arranged directly on the same substrate (5), characterized in that the electron source (2) has an acceleration section between the cathode and the anode to extract electrons from the cathode due to an electrical potential difference between the cathode and the anode and to accelerate them from the cathode towards the anode, wherein the potential difference is greater than 1 kV.

Description

Field of invention

[0001] The invention relates to an electron source for generating an electron beam. Furthermore, the invention relates to a method for generating an electron beam. Background of the invention

[0002] Electron sources are used, among other things, in electron microscopy and for the generation of X-rays. In electron microscopy, in particular, a point source with high brilliance and low energy dispersion is required; at the same time, high stability and lifetime under the least critical vacuum conditions are desirable.

[0003] Thermal emitters with low brilliance but simple construction are known from the prior art. Single-crystal tungsten or LaB6 field-effect emitters are also known; while they exhibit high brilliance, they require extremely low vacuum conditions due to ion bombardment and can only be operated stably for short periods. Finally, so-called Schottky emitters are known, which operate at high temperatures and represent a reasonable compromise between field-effect emitters and the more stable thermal emitters. However, Schottky emitters are expensive, generally require manual selection, and necessitate complex alignment during installation; moreover, their lifespan is limited to one to a few years.

[0004] A recent evaluation of the different source types is given in X. Shao, A. Srinivasan, WK Ang, A. Khursheed, A high-brightness large-diameter graphene coated point cathode field emission electron source, Nature communications, 9 (2018) 1288. This paper also discloses a field-effect emitter that has a cathode made of graphene-coated nickel.

[0005] The fabrication of known electron sources can be disadvantageously complex. The size of the installation space required by known electron sources can also be a disadvantage. In particular, it can be disadvantageous that the cathode and the means for extracting electrons from the cathode form discrete assemblies. Similarly, in known electron sources that include electron optics, post-acceleration, and / or electronic control, it can be disadvantageous that these assemblies are designed as discrete assemblies relative to the cathode assembly(s) and / or the means for extracting electrons.

[0006] From EP 3 252 796 A1, an arrangement is known in which a two-dimensional cold cathode, a two-dimensional grid, and a two-dimensional anode are arranged on a substrate. In one embodiment of the invention, the cathode, grid, and anode each consist of a first silicon carbide layer, a graphene layer, and a second silicon carbide layer.

[0007] CN 1 02 103 953 A discloses a cold cathode with a graphite layer. The graphite layer consists of a large number of graphene strips that are epitaxially grown onto a silicon carbide substrate in such a way that they are perpendicular to the substrate.

[0008] From US patent 2006 / 0145582A1, a field emitter is known that consists of a substrate on which a cathode and two gates are arranged. The substrate can be made of silicon carbide, and suitable materials for the cathode include titanium dioxide, molybdenum, chromium, gold, and platinum. The cathode features an emitter structure made of carbon nanotubes, nanorods, or nanowires.

[0009] Further state of the art can be found in the publications US 2018 / 0 374 669 A1, US 2013 / 0 299 773 A1, US 2019 / 0 198 279 A1 and RU 181 863 U1. The problem underlying the invention

[0010] The invention is based on the objective of providing an improved electron source for generating an electron beam. In particular, the invention aims to overcome disadvantages of electron sources known from the prior art. Inventive solution

[0011] The reference numerals in all claims have no limiting effect, but are intended only to improve their readability.

[0012] The problem is solved by an electron source having the features of claim 1. It is also solved by an electron source having the features of claim 2. Finally, the problem is also solved by an electron source having the features of claim 10. The electron source according to the invention has a cathode and an anode arranged on the same silicon carbide substrate. For the purposes of the present invention, the substrate of a component (for example, the cathode) of the electron source is a material on which the component (and possibly other components) is fixedly arranged either directly (in the sense that the component is in direct mechanical contact with the substrate) or indirectly (in the sense that the component is in mechanical contact with the substrate by means of another component or another material).

[0013] Silicon carbide according to the present invention may contain defects and / or be doped with foreign atoms. It may be single- or multi-crystalline.

[0014] This aspect of the invention can exploit the fact that silicon carbide exhibits good high-voltage strength. It can also take advantage of the availability of industrial-scale processes for manufacturing silicon carbide. Finally, with silicon carbide, it is advantageously possible to achieve a situation where, at electrical potential differences of several kilovolts, as required for an electron source, surface currents play only a minor role.

[0015] The invention advantageously enables the arrangement of several electron source components on a common silicon carbide substrate, which can result in higher integration. This simplifies the fabrication of the electron source according to the invention. Furthermore, miniaturization, i.e., a reduction of the installation space required for the electron source, is advantageously achievable. In particular, the invention enables the realization of an on-chip electron source.

[0016] The invention also advantageously enables one or more components of the electron source to be arranged with other electronic, mechanical, or mechatronic components on a common silicon carbide substrate, which can also result in a higher integration density. This simplifies the fabrication of devices comprising an electron source. Furthermore, miniaturization, i.e., a reduction in the installation space required for a device comprising an electron source, is advantageously achievable. In particular, the invention enables the realization of an on-chip device incorporating an electron source.

[0017] A graphene layer according to the present invention is a graphene monolayer or a graphene multilayer with at most 100 stacked graphene monolayers. A graphene monolayer is defined here as a single layer of carbon atoms arranged in a hexagonal lattice. A graphene monolayer according to the invention may be chemically modified—locally or completely—and may exhibit lattice distortions and / or defects and / or be doped with foreign atoms. A completely or partially oxidized graphene layer is also a graphene layer according to the present invention. However, the preferred graphene layer is not, or only partially, oxidized or chemically modified. A multilayer graphene layer according to the invention preferably comprises fewer than 50, more preferably fewer than 15, more preferably fewer than 4, and more preferably fewer than 3 monolayers.

[0018] This aspect of the invention can exploit the fact that graphene is among the materials with particularly high permissible current densities. The invention can also exploit the fact that graphene is exceptionally robust.

[0019] The invention is suitable for the monolithic fabrication of a miniaturized source of a high-energy, focused electron beam, including its use as an on-chip X-ray source. All components of the electron source according to the invention can be fabricated from or on a single silicon carbide chip. Preferred embodiments of the invention

[0020] Advantageous training and further education programs, which can be used individually or in combination, are the subject of the dependent claims and the following description.

[0021] Particularly preferably, the substrate consists predominantly, and especially preferably entirely, of silicon carbide. The preferred silicon carbide of the invention is one of the polytypes 4H-SiC and 6H-SiC.

[0022] The preferred cathode comprises a graphene layer; preferably, the cathode consists of one or more graphene layers. The preferred graphene layer(s) of the cathode is / are in contact with the silicon carbide of the substrate. Preferably, the graphene layer of the cathode is epitaxially intergrown with the silicon carbide, for example, by thermal decomposition. A suitable method for producing epitaxial graphene on silicon carbide is known from KV Emtsev, A. Bostwick, K. Horn, J. Jobst, GL Kellogg, L. Ley, JL McChesney, T. Ohta, SA Reshanov, J. Rohrl, E. Rotenberg, AK Schmid, D. Waldmann, HB Weber, T. Seyller, Towards wafer-size graphene layers by atmospheric pressure graphitization of silicon carbide, Nat Mater, 8 (2009) 203-207 and from Hertel, F. Kisslinger, J. Jobst, D. Waldmann, M. Krieger, HB Weber, Current annealing and electrical breakdown of epitaxial graphene, Applied Physics Letters, 98 (2011) 212109.The methods described there can also be applied analogously to 4H-SiC.

[0023] Epitaxial graphene, especially that on silicon carbide, can advantageously allow particularly high current densities and be exceptionally robust. This can be exploited in the invention to achieve high currents or to withstand an opposing, accelerated parasitic ion bombardment, thus enabling stable operation of the electron source according to the invention.

[0024] The electron source has an anode. It also preferably has an acceleration section between the cathode and the anode to extract electrons from the cathode and accelerate them towards the anode due to an electrical potential difference, also known as the extraction voltage, between the cathode and the anode. In a preferred embodiment of the invention, the potential difference is greater than 100 V (volts), preferably greater than 300 V, particularly preferably greater than 1 kV (kilovolt), particularly preferably greater than 9 kV, and particularly preferably greater than 15 kV. Preferably, the potential difference is less than 40 kV, particularly preferably less than 30 kV, and particularly preferably less than 25 kV, for example, 20 kV. This embodiment of the invention is particularly suitable for an electron source for generating X-rays.In a preferred embodiment of the invention, the potential difference is greater than 90 kV, preferably greater than 200 kV, and particularly preferably greater than 350 kV. Preferably, the potential difference is less than 1500 kV, and particularly preferably less than 800 kV, for example 400 kV. This embodiment of the invention is particularly suitable for generating high-energy electron beams.

[0025] The preferred anode comprises a graphene layer; preferably, the anode consists of one or more graphene layers. The preferred anode has a substrate comprising silicon carbide. The preferred graphene layer(s) of the anode is / are in contact with the silicon carbide of the substrate. Preferably, the graphene layer of the anode is epitaxially intergrown with the silicon carbide of the substrate, for example, by thermal decomposition.

[0026] The preferred anode is a hole anode in the sense that it has a channel which acts as the hole of the anode. Graphene layer(s) border the channel, preferably on opposite sides of the channel. The channel can be open or closed at the end facing away from the cathode. In a preferred embodiment, the hole anode comprises two graphene layers, which are particularly preferably located in a common plane. The edges of the graphene layers facing each other form the open-ended channel of the hole anode.

[0027] The preferred channel comprises a groove extending longitudinally in the substrate. The graphene layer(s) of the anode are preferably arranged, directly or indirectly, on the areas of the substrate adjacent to the channel.

[0028] The preferred substrate comprises or consists of silicon carbide. The preferred substrate is structured, doped, and / or metallized to dissipate unwanted portions of the electron beam or to counteract or even completely prevent static charging.

[0029] The cathode and anode are arranged on the same substrate, which contains or consists of silicon carbide. It is advantageous to exploit the fact that silicon carbide, especially in combination with epitaxial graphene, exhibits a high breakdown field strength and low leakage currents.

[0030] An additional structure can be provided between the cathode and anode, constructed according to the design of one of the anode embodiments described above. During operation, this structure can be maintained at a negative potential relative to the cathode, thus performing the function of a conventional Wehnelt cylinder.

[0031] In a preferred electron source, the graphene layer of the cathode is arranged such that electrons are emitted from an edge of the graphene layer and accelerated towards the anode. Preferably, the graphene layer is epitaxially positioned on a pedestal of the cathode substrate. The edge preferably faces the anode of the electron source.

[0032] A preferred graphene layer of the cathode is formed as a strip. The width of the preferred strip is less than 300 nm (nanometers), particularly preferably less than 100 nm, particularly preferably less than 30 nm, and particularly preferably less than 10 nm. Suitable strips and a suitable fabrication method are described, for example, in M. Sprinkle, M. Ruan, Y. Hu, J. Hankinson, M. Rubio-Roy, B. Zhang, X. Wu, C. Berger, WA de Heer, Scalable templated growth of graphene nanoribbons on SiC, Nature nanotechnology, 5 (2010) 727-731. This method can exploit the special characteristics of graphene growth on silicon carbide surfaces. Preferably, during operation of the electron source, a current flows along the longitudinal direction of the graphene strip. Electron emission occurs at the edge of a narrow side of the strip.The emitting edge is preferably less than 300 nm, more preferably less than 100 nm, more preferably less than 30 nm, and more preferably less than 10 nm wide. In one embodiment of the invention, the cathode comprises not just one, but two or more of the strips. The preferred strips are arranged parallel to each other.

[0033] The graphene of the preferred cathode graphene layer is chemically functionalized at the electron-beam-emitting edge of the graphene layer, for example, by carboxyl groups, fluorine, or oxidation with oxygen. This advantageously narrows the energy distribution of the electrons and / or improves the stability of the emission edge. In one embodiment of the invention, the cathode graphene layer is divided longitudinally along its emission edge into several sections, one or more of which are non-functionalized and another or more of which are functionalized, or one or more of which are functionalized differently than another or more of which. For example, a non-functionalized section can be bounded on both sides by functionalized, for example, oxidized, sections.With this embodiment of the invention, a narrowing of the emitting area of ​​the emitting edge can be achieved.

[0034] The preferred cathode comprises a graphene-graphene tunnel junction or a graphene-graphene nanobridge. A tunnel junction according to the present invention is formed by two opposing electrodes whose smallest distance is less than 1 nm in the region of the tunnel junction. A nanobridge according to the present invention is formed by two electrodes connected by a bridge, the bridge being less than 100 nm, particularly preferably less than 10 nm, and most preferably less than 1 nm wide at its narrowest point. The graphene-graphene tunnel junction or the graphene-graphene nanobridge is constructed and arranged such that electrons are emitted from the vicinity of the graphene-graphene tunnel junction or a graphene-graphene nanobridge.

[0035] Preferably, the electrons are emitted in the immediate vicinity of the graphene-graphene tunnel junction or the graphene-graphene nanobridge. This embodiment of the invention is based, among other things, on the finding that a spatially confined electron plasma (also referred to as nanoplasma) can form above the positive electrode in a graphene-graphene tunnel junction and a graphene-graphene nanobridge. It is therefore particularly preferred that the cathode emits electrons from this plasma.

[0036] The electron emission technique in the region of a graphene-graphene tunnel junction or a graphene-graphene nanobridge advantageously avoids the direct impact and damage of a parasitic, backward-directed ion bombardment on a solid-state tip. Instead, the ion bombardment can be achieved by grazing the cathode material. With a cathode according to this embodiment of the invention, a large dynamic range of possible emission currents, excellent long-term stability, and fast on / off times in the nanosecond range are also advantageously achievable.

[0037] According to the invention, the electrons emitted in the region of the graphene-graphene tunnel junction or the graphene-graphene nanobridge can be accelerated towards the anode. For this purpose, the preferred graphene-graphene tunnel junction or the preferred graphene-graphene nanobridge faces the anode.

[0038] In one embodiment of the invention, the cathode comprises one or more carbon nanotubes. Preferably, electrons are emitted from the carbon nanotube(s) to be accelerated towards the anode. The preferred nanotube(s) is or are arranged at the emitting edge of the graphene layer(s) of the cathode. For this purpose, it is preferred that the carbon nanotube(s) are synthesized at the edge of the graphene layer(s). A suitable method for this is known, for example, from JR Sanchez-Valencia, T. Dienel, O. Gröning, I. Shorubalko, A. Mueller, M. Jansen, K. Amsharov, P. Ruffieux, R. Fasel, Controlled synthesis of single-chirality carbon nanotubes, Nature, 512 (2014) 61. Advantageously, the carbon nanotube(s) can be used to geometrically narrow the emission site of the electrons at the cathode.

[0039] A carbon nanotube within the meaning of the present invention comprises both single-walled and multi-walled carbon nanotubes. A carbon nanotube can be open at both ends, open at one end, or closed at both ends. Carbon nanorolls, i.e., rolled-up single- or multi-layer graphene sheets, also fall under the definition of carbon nanotubes within the meaning of the present invention. A carbon nanotube according to the invention can be chemically modified—locally or completely—and may exhibit lattice distortions and / or defects and / or be doped with foreign atoms. A carbon nanotube that is completely or partially oxidized is also a carbon nanotube within the meaning of the present invention. However, the preferred carbon nanotube is not, or only partially, oxidized or chemically modified in any other way.The wall of a multi-walled carbon nanotube according to the invention preferably has fewer than 8, and particularly preferably fewer than 4 layers of carbon atoms arranged in a hexagonal lattice.

[0040] In a preferred embodiment of the invention, the end of the nanotube according to the invention pointing away from the cathode is semiconducting or metallic.

[0041] A preferred electron source comprises electron optics, for example, an electrostatic lens for focusing the electron beam. The electron optics preferably comprise a substrate having a graphene layer, particularly preferably epitaxially fused, and / or a metallization. Preferably, the substrate comprises or consists of silicon carbide. The preferred electron optics are arranged on the same substrate as the cathode and / or the anode.

[0042] The electron source according to the invention can incorporate a dielectric resonance structure to accelerate the electrons of the electron beam. Suitable resonance structures are known, for example, from RJ England, RJ Noble, K. Bane, DH Dowell, C.-K. Ng, JE Spencer, S. Tantawi, Z. Wu, RL Byer, E. Peralta, K. Soong, C.-M. Chang, B. Montazeri, SJ Wolf, B. Cowan, J. Dawson, W. Gai, P. Hommelhoff, Y.-C. Huang, C. Jing, C. McGuinness, RB Palmer, B. Naranjo, J. Rosenzweig, G. Travish, A. Mizrahi, L. Schachter, C. Sears, GR Werner, RB Yoder, Dielectric laser accelerators, Reviews of Modern Physics, 86 (2014) 1337-1389. A resonance structure can be used to further accelerate the electrons of the electron beam, for example to generate high-energy electrons.Suitable applications for such high-energy electron beams include microchip electron beam generation for medical applications, a compact particle accelerator, an on-chip synchrotron, and compact electron microscopy. The resonance structure preferably comprises a substrate that includes or consists of silicon carbide. The preferred resonance structure is arranged on the same substrate as the cathode and / or the anode and / or the electron optics.

[0043] In one embodiment, the electron source has a target that is arranged and configured such that the electron beam strikes the target to generate X-rays. Suitable targets include, in particular, metallic layers, for example, copper, nickel, or graphite. It is advantageously achieved that characteristic bremsstrahlung in the form of X-rays is generated over a small area at the point of impact of the electron beam on the target and radiates outwards. In one embodiment of the invention, the localized heat load on the target can be reduced by pulsed operation or by varying the point of impact of the electron beam over time. For example, the electron beam can be deflected electrostatically for this purpose using one or more deflection plates, for instance, to scan the target line by line.

[0044] The target preferably comprises a substrate, which most preferably includes or consists of silicon carbide. Most preferably, the target is arranged on the same substrate as the cathode and / or the anode and / or the electron optics. The good thermal conductivity of silicon carbide can be advantageously utilized in this context. The invention is particularly suitable for providing microfocus X-ray sources that require precise focusing of the electron beam. It is also suitable for miniaturized and cost-effective X-ray sources for novel applications, especially in the medical field, aerospace, and materials research.

[0045] A preferred electron source comprises power electronic components, for example, for driving the cathode, the anode, the electrostatic lens, and / or the dielectric resonance structure. Preferably, the power electronic components are arranged on the same substrate as the cathode, the anode, the electron optics, and / or the dielectric resonance structure. Suitable components are known, for example, from S. Hertel, D. Waldmann, J. Jobst, A. Albert, M. Albrecht, S. Reshanov, A. Schoner, M. Krieger, HB Weber, Tailoring the graphene / silicon carbide interface for monolithic wafer-scale electronics, Nature communications, 3 (2012), from S. Hertel, M. Krieger, HB Weber, Monolithic circuits with epitaxial graphene / silicon carbide transistors, Physica Status Solidi-Rapid Research Letters, 8 (2014) 688 - 691, from EP 2535937 B1 and from DE 102011016900 A1. Brief description of the drawings

[0046] Further advantageous embodiments are described in more detail below with reference to several exemplary embodiments shown in the drawings, to which, however, the invention is not limited.

[0047] It shows schematically: Fig. 1 the cathode of an electron source according to the invention; Fig. 2 an electron source according to the invention with the cathode made of Fig. 1, in which the cathode and anode are arranged on a common substrate; Fig. 3 the electron source Fig. 2, further developed such that the graphene layers of the hole anode consist of Fig. 2 two further graph layers are opposite each other; and Fig. 4 an electron source according to the invention with a target for generating X-ray radiation. Detailed description of embodiments of the invention

[0048] In the following description of preferred embodiments of the present invention, the same reference numerals denote identical or comparable components.

[0049] In Fig. Figure 1 shows an embodiment of a cathode 1 of the electron source 2 according to the invention. Electron emission occurs from an edge 3 of a graphene layer of the cathode 1, which is placed at a high negative potential. The edge 3 of the cathode 1 is located, as shown in Fig. 2 shown, facing the anode 4 of the electron source 2.

[0050] The electron source 2 shown in the figures is based on the material system epitaxial graphene on silicon carbide. The starting material is a silicon carbide substrate 5. Due to its material properties, commercially available high-purity semiinsulating 4H-SiC (HPSI) is particularly suitable. After wet-chemical cleaning of the semiconductor, the pedestal 9 for the emitting edge 3 is lithographically structured and etched by a dry etching process in a reactive ion etching (RIE) system. Following a further cleaning step, an epitaxial graphene layer 6 is then produced by thermal decomposition of the silicon carbide surface (silicon side), as described in the publication by KV Emtsev et al., Towards wafer-size graphene layers by atmospheric pressure graphitization of silicon carbide, Nat Mater, 8 (2009) 203-207, cited above.For this purpose, the sample is heated in an oven under an argon atmosphere at near atmospheric pressure for 30 minutes at approximately 1700°C. In a second lithography step, the graphene area 6 on the pedestal is defined and exposed by an oxygen plasma etching step. This removes excess graphene areas. The metal contact 7 to the graphene emitter is created in a final lithography step by vapor deposition of 5 nm titanium (Ti) and 50 nm gold (Au) and a subsequent lift-off process.

[0051] In an alternative embodiment of the cathode, not shown in the figures, it is spatially tapered by the use of epitaxially defined graphene strips. The graphene strips are only a few nanometers wide. During operation, the current flows along the graphene strips, and the electrons are emitted from the edge, which is only a few nanometers long.

[0052] The graphene strips can be produced using the method described in the aforementioned publication by M. Sprinkle et al., "Scalable templated growth of graphene nanoribbons on SiC," Nature nanotechnology, 5 (2010) 727-731. This involves first creating nanofacets on the (1-10n) crystal facet. This is achieved either, as suggested by Sprinkle et al., by RIE dry etching of a single step followed by an annealing step at approximately 1200°C, or by a step-bunching process at 1700°C for 30 minutes, with the sample placed in a silicon carbide ceramic container. In a second annealing step without a silicon carbide container, graphene strips are grown on the nanofacets at a slightly lower temperature (approximately 1400°C to 1500°C). This method exploits the fact that graphene growth is faster on these surfaces than on the silicon surface of the terraces. Further processing is carried out as described above.

[0053] The emitting edge(s) 3 of the cathode 1 shown in the figures or of the graphene strip cathode described above can be equipped with one or more carbon nanotubes. The method described in the aforementioned publication by JR Sanchez-Valencia et al., Controlled synthesis of single-chirality carbon nanotubes, Nature, 512 (2014) 61, is suitable for the fabrication of these nanotubes.

[0054] In another alternative embodiment of the cathode, not shown in the figures, it is designed as a graphene-graphene tunnel junction or graphene-graphene nanobridge. The inventors have found that a spatially confined electron plasma forms above the positive electrode of such a tunnel junction or nanobridge, which can be used to extract electrons. Due to its light emission, similar to blackbody radiation at 2000 K, a corresponding energy dispersion (approximately 200 meV) is to be expected.

[0055] To create the graphene-graphene tunnel contact, two graphene surfaces are fabricated on the pedestal using lithography and oxygen plasma etching. A thin graphene channel, approximately 50 nm wide, connects the two surfaces at the pedestal edge. Each graphene surface is contacted with a Ti / Au contact as described above. The thin graphene channel is then opened using an electroburning process, creating a gap approximately 1 to 3 nm wide, also known as a nanogap. In this electroburning process, a voltage ramp of approximately 0 to 100 V is applied to the two Ti / Au contacts in air. This causes the current through the graphene channel to increase, heating it and resulting in a localized "burn-through" of the channel. This is indicated by a drop in current, at which point the voltage ramp is immediately interrupted.By repeating the process, the size of the nanogap can be adjusted via the ohmic resistance.

[0056] During operation of the electron source 2, the electrons emitted from the cathode 1 of the electron beam 8 are accelerated towards a hole anode 4 by means of an extraction voltage. The simplest design of such a hole anode 4 is in Fig. Figure 2 illustrates this. To produce this anode, both the pedestal 9 for the cathode 1 and an anode bridge 10 with the interruption forming a channel 11 of the anode 4 are produced in the same RIE dry etching step. In the subsequent graphene structuring process (lithography and oxygen plasma etching), the graphene layer 12 is left on the anode bridge 10 and contacted by means of Ti / Au contacts (not shown); no additional process step is required for this. To improve the hole anode properties, the anode bridge 10 can also be doped. For this purpose, an implantation mask for the anode bridge 10 is defined lithographically before the graphene growth process, and the latter is doped by ion implantation, for example with nitrogen ions, such that good conductivity is achieved; a suitable nitrogen concentration is 10 19 cm -3The sample is then covered with a carbon cap, the implantation damage is healed at 1700°C for 30 minutes, and the doping is activated. The carbon cap is then removed by oxidation in oxygen at 800°C for 30 minutes, and the sample is purified again. Further preparation is carried out as described above.

[0057] To achieve a more accurate approximation of a conventional hole anode, an enclosed structure can be produced by capping the previously described structure. For this purpose, a second silicon carbide substrate 13 is structured such that a mirror-image anode bridge 14 is formed for the top side, as shown in Fig. Figure 3 illustrates this process. This structuring is carried out using the same process steps as previously described for the underside, including graphene growth. The two semiconductor wafers are then pressed and clamped together, glued, or joined by a wafer bonding process. This places the components of the electron source 2 inside the chip, providing additional protection against external influences. The electron beam 8 exits through the lateral opening of the hole anode.

[0058] In the same way as this hole anode, a second bridge with corresponding contacts can be produced (not shown in the figures), which takes over the function of a Wehnelt cylinder for adjusting the electron beam intensity.

[0059] Fig. 4 shows further training of the in the Fig.Figure 2 shows an electron source 2, which additionally features a target 15 for generating X-rays 16. In a further lithography step, an opening for the inclined target platform 17 is structured for fabrication. This process step is to be carried out before the structuring of the cathode platform 9 and the anode bridge 10. The inclined target surface is achieved either by an isotropic RIE dry etching step (i.e., at higher process pressure), by grayscale lithography, or by selective electrochemical etching. In the latter method, aluminum is implanted before graphene growth under oblique incidence; the electrochemical process selectively removes the aluminum-doped areas. Target metallization is achieved by vapor deposition of metallic layers, for example copper or nickel, through a lithographically defined mask followed by a lift-off process.The metal and layer thickness depend on the specific application. Typical layer thicknesses range from 10 nm to 1 µm. The subsequent process steps are carried out as described above.

Claims

[1] Electron source (2) for generating an electron beam (8) with a cathode (1) and an anode (4) arranged on the same substrate (5) made of silicon carbide, wherein the cathode (1) has a graphene layer (12) arranged directly on the same substrate (5), characterized by that the electron source (2) has an acceleration section between the cathode and the anode in order to extract electrons from the cathode due to an electrical potential difference between the cathode and the anode and to accelerate them from the cathode towards the anode, wherein the potential difference is greater than 1 kV. [2] Electron source (2) for generating an electron beam (8) with a cathode (1) and an anode (4) arranged on the same substrate (5) made of silicon carbide, wherein the cathode (1) has a graphene layer (12) arranged directly on the same substrate (5) and the cathode (1) has a graphene-graphene tunnel contact or a graphene-graphene nanobridge which is arranged so that electrons can be emitted from the surroundings of the graphene-graphene tunnel contact or the graphene-graphene nanobridge, characterized by that the electron source (2) has an acceleration section between the cathode and the anode in order to extract electrons from the cathode due to an electrical potential difference between the cathode and the anode and to accelerate them from the cathode towards the anode, wherein the potential difference is greater than 100 V. [3] Electron source (2) according to one of claims 1 and 2, characterized bythat the graphene layer (6) of the cathode is arranged such that electrons are emitted at an edge (3) of the graphene layer of the cathode (1) in order to be accelerated towards the anode (4). [4] Electron source (2) according to one of claims 1 to 3, characterized by that the graphene layer of the cathode (1) is formed as a strip. [5] Electron source (2) according to claim 1, characterized by that the cathode (1) has a graphene-graphene tunnel contact or a graphene-graphene nanobridge which is arranged such that electrons are emitted from the surroundings of the graphene-graphene tunnel contact or the graphene-graphene nanobridge in order to be accelerated towards the anode (4). [6] Electron source (2) according to one of claims 1 to 5, characterized by that the cathode (1) comprises a carbon nanotube. [7] Electron source (2) according to one of the preceding claims, characterized bythat it has at least one electrostatic lens for focusing the electron beam (8). [8] Electron source (2) according to one of the preceding claims, characterized by that it has a dielectric resonance structure to accelerate the electrons of the electron beam (8). [9] Electron source (2) according to one of the preceding claims, characterized by in that it comprises a target (15) which is arranged and designed such that the electron beam (8) strikes the target (15) in order to generate X-ray radiation (16). [10] Electron source (2) for generating an electron beam (8) with a cathode (1) and an anode (4) arranged on the same substrate (5) made of silicon carbide, wherein the cathode and / or the anode has a graphene layer (12) arranged directly on the same substrate (5), characterized bythat it comprises power electronic components arranged on the same substrate (5) as the cathode (1) and / or the anode (4).

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