Large-scale stable field emitters for high-current applications

Abstract

The present invention relates to large area field emission devices based on the addition of macroscopic, microscopic and nanoscale field enhancement features and designed forced current sharing matrix layers to achieve stable high current density long life field emission devices. The present invention relates to a wide range of field emission sources and is not limited to a specific field emission technology. The present invention is described as an X-ray electron source, but can be applied to any application requiring a high current density electron source.

Description

[0001] (Priority Document)

[0002] This application claims priority from Australian Provisional Patent Application No. 2018901153 filed on April 6, 2018, entitled “LARGE SCALE STABLE FIELD EMITTER FOR HIGH CURRENT APPLICATIONS”, the entire contents of which are incorporated herein by reference. Technical Field

[0003] The present disclosure relates to field emission devices. In a specific form, the present disclosure relates to large-scale stable field emitters and methods of fabrication for high current applications. Background Art

[0004] Nanostructured field emission devices are well documented and have been extensively studied for over 60 years. These nanostructured field emission devices are defined as individual single point sources of electron emission and include at least carbon nanotubes (CNTs), carbon nanowires, tungsten nanowires, silicon pillars, silicon pyramids, and nanostructured diamond. Studies have shown that the current density of these devices is related to the field enhancement originating from the nanostructure of the device and the conductive properties of the device. Theoretical and actual electron current densities for single (i.e., isolated) field emission devices are well documented; and for many materials, the capabilities and limitations of individual emitters are well understood and mathematically modeled. Optimizing new and improved single nanostructured field emission devices continues to be an active area of ​​research.

[0005] Nanostructured electron emitting devices rely on quantum electron tunneling to transport electron currents, and these devices are typically grown in plasmas. Due to their size, emission physics, and manufacturing processes, each individual device is inherently unique. Due to this uniqueness, there will be some inherent variability in the specific emission performance of any device. Therefore, the emission model of any nanostructured electron emitter is based on a probability distribution of the general performance of the device. In a large distribution of individual emitters, each emitter behaves differently, and this will cause some emitters to be under greater stress while other emitters do not contribute fully or partially to the electron current. Stable large-area emitters require that all emitters participate within a narrow range of each other. As the area of ​​the emitter increases, the variations between individual emitters become more pronounced.

[0006] A single nanostructured field emission device cannot deliver enough current for many potential applications that require a high current electron source. Therefore, to increase the total (macroscopic) electron current, multiple individual nanostructured field emission devices are used together as a common electron current source. Research on nanostructured field emission devices has focused almost entirely on individual devices to increase the current density of a single device. To date, the literature describes methods for creating electron current sources based on multiple individual field emission devices by extending the performance of a single emitter to a wider range of emitters. Existing literature describes degradation of field enhancement, current density per unit area, emitter uniformity, emitter mechanical bonding, and emitter thermal confinement; all in the context of a single field emission device. The literature teaches us that optimizing a single field emission device, and replicating this optimization as narrowly as possible, is the key to producing a stable, high current source.

[0007] The literature on nanostructured field emission devices has long predicted widespread use of field emission devices as sources of electron current based on extending the principles of a single device to a distribution of devices. That is, research has focused on optimizing a single field emission device and then attempting to distribute these devices over a wider area. However, large-scale nanostructured field emission devices have so far failed to achieve the expected success. Attempts to design large-area electron emitters capable of delivering larger electron currents have mostly failed to repeatedly deliver stable, uniform electron emission across the emitter over long periods of operation. The limited successes have been difficult to replicate due to the increased size of the emitter and the resulting inability to generate sufficient electron current for the target application.

[0008] So far, only a single type of large-scale field emission device has been demonstrated to be able to transmit stable, uniform electron emission over a large area electron emitter. For example, US6277318 describes a method for manufacturing a device by depositing carbon nanotubes with a high aspect ratio onto a substrate. US655096 describes a carbon nanotube (CNT) field emitter for generating X-rays by using these devices. This relies on the use of a single nanostructured device carbon nanotube that is optimally bonded to an electrode and distributed in a semi-optimal manner to ensure uniform field enhancement on the carbon nanotube distribution. This narrowly defined method and device has been the subject of a wide range of academic journal articles and patents describing the use of such electron emitters in a range of applications including X-ray tubes and flat panel displays. Although widely published, this method and device has been shown to be effective only when a single combination of carbon nanotubes, electrodes, and bonding matrices is used. In addition to the bonding properties of the matrix and the purity of the carbon nanotubes, the rationality of this single combination is not described in the literature. These researchers have also focused on methods to improve the uniformity of the carbon CNTs or ways to improve the uniformity of deposition, such as by aligning at least 50% of the CNTs in the same direction, for example by applying an electric field as described in US 6630772. However, despite extensive research, it has not been possible to substantially improve upon the initial performance of these devices or to construct working devices with other materials.

[0009]

[0006] It would therefore be desirable to provide methods for making large-scale stable field emitters for high current applications as well as practical large-scale stable field emitters, or at least to provide useful alternatives to current methods and field emission devices. Summary of the invention

[0010] The present disclosure relates to methods for creating large-area, stable electron emitters for large currents (i.e., >1 mA) based on optimization of a single field emission device and a large distribution of related field emission devices. Note that the method is independent of any particular version of nanostructured field emission devices (i.e., not limited to carbon nanotubes). In one embodiment, a method for making a field emission electron source device includes:

[0011] forming a current sharing matrix layer electrically connected to the substrate, wherein the current sharing matrix is ​​bonded to the substrate, deposited on the substrate, formed directly on the substrate, or formed directly in the substrate;

[0012] An emitter surface is formed consisting of multiple nanoscale field emitters electrically connected to a matrix sharing layer, wherein the emitting surface is formed by obtaining multiple nanoscale field emitters and bonding them to a current sharing matrix layer or directly forming multiple nanoscale field emitters on the current sharing matrix layer.

[0013] The current sharing matrix includes multiple series resistance paths between multiple individual nanoscale field emitters and the substrate to force current sharing between multiple individual nanoscale field emitters. In addition, multiple individual nanoscale field emitters, matrix materials, and substrates are configured to form multiple structures so that the structures form electric field enhancement features at multiple scales from macroscale to nanoscale, wherein the structures are fractal, geometrically repeated (e.g., designed or intentionally formed) and / or randomly distributed. This optimizes the field enhancement over the entire large area of ​​the emitter surface, rather than optimizing a single field emission device. In addition, the current sharing matrix creates a passive resistance sharing network and is composed of a material or combination of materials with sufficient resistance to force current sharing between nanostructure field emission devices. This prevents a subset of field emission devices from being subjected to excessive stress and ensures that the current is stable over the entire emitter surface.

[0014] Additionally, sharing the resistance allows the current extraction of the individual emitters to reach the safe operating limit to be more closely matched. Without sharing, the derating of the average individual emitter operating current needs to be more stringent to allow for a wide spread of operating currents (due to inherent device variability) and is determined by the emitter with the strongest emission reaching the safe operating limit first. The method and associated devices allow a wider range of devices to be used in the same structure. This means higher repeatability and makes it easier to manufacture and scale up to larger areas and helps the devices have long lifetimes.

[0015] Similarly, in one embodiment, a field emission electron source device is provided, the device comprising:

[0016] Electrodes integrated into or bonded to a substrate;

[0017] an emitter surface composed of multiple individual nanoscale field emitters;

[0018] A current-sharing matrix layer matrix material is positioned between a plurality of individual nanoscale field emitters and a substrate or incorporated into the substrate.

[0019] In the device, a current sharing matrix layer includes a plurality of series resistance paths between a plurality of individual nanoscale field emitters and an electrode to force current sharing between the plurality of individual nanoscale field emitters, and the plurality of individual nanoscale field emitters, the current sharing matrix material and the substrate are configured to form a plurality of structures such that the structures form electric field enhancement features at a plurality of scales from macroscale to nanoscale, wherein the structures are fractal, geometrically repeated and / or randomly distributed. In use, the field emission electron source device emits electrons in a spatially uniform distribution across an emitter area of ​​an emitter surface having an area of ​​at least 1 square millimeter, and transmits stable electron emission at a current density greater than or equal to one milliampere per square millimeter.

[0020] In one or more embodiments of the method or device, the plurality of structures comprises macro-scale features having dimensions between 100 μm and 10 mm and is created by processing, forming or configuring a current sharing matrix material and / or substrate to include structures in a repeating geometric pattern that includes field emission.

[0021] In one or more embodiments of the method or device, the plurality of structures include micro-scale features created by surface treatment of the current sharing matrix layer and / or substrate to create a fractal or random structured three-dimensional surface with dimensions ranging from a few microns to tens of microns.

[0022] In one or more embodiments of the method or device, the plurality of structures include micro-scale features, and the micro-features include a matrix material and / or substrate formed into a three-dimensional surface having a fractal or random structure with a size of 1 to tens of microns.

[0023] In one or more embodiments of the method or device, the plurality of structures comprises micro-scale features, and the method further comprises creating the micro-scale features by patterning placement of a plurality of individual nanoscale field emitters on the matrix material and the substrate.

[0024] In one or more embodiments of the method or device, the plurality of structures include nano features that are either intrinsic features of the plurality of individual nanoscale field emitters or are created when the plurality of individual nanoscale field emitters are joined to a current sharing matrix material. In one form, the nano features are created by using a nanostructured material having fractal properties. In another form, the nanostructured material includes high aspect ratio features.

[0025] In one or more embodiments of the method or device, each structure adds an incremental electric field enhancement.

[0026] In one or more embodiments of the method or device, the resistive path is formed by using a high resistance material. In one form, the resistive path is formed by using a semiconductor material having resistive properties. In one form, the resistive path is formed by using a metal ceramic material having resistive properties.

[0027] In one or more embodiments of the method, the substrate is an electrode substrate, and the method further includes forming an electrical connection to the electrode substrate.

[0028] In one or more embodiments of the method or device, the method further includes bonding the electrode to the substrate and forming an electrical connection with the electrode.

[0029] In one or more embodiments of the device, the current sharing matrix layer is formed of a matrix material electromechanically bonded to a plurality of individual nanoscale field emitters. In one form, the current sharing matrix layer is formed of a matrix material electromechanically bonded to a substrate electrode. In one form, the current sharing matrix layer is formed within the substrate and bonded to the electrode.

[0030] In one or more embodiments, the device further includes a secondary electric field extraction electrode and control electronics for switching the field emission on and off.

[0031] The field emission electron source device can be incorporated into a range of products and systems. In one embodiment, an X-ray field emission device is provided, the X-ray field emission device comprising:

[0032] a cathode comprising one or more field emission electron source devices according to the embodiments described herein;

[0033] Electric field extraction electrode;

[0034] control electronics for turning field emission of one or more field emission electron source devices on and off;

[0035] an anode comprising a target material configured to produce X-ray emissions when struck by electrons generated by one or more field emission electron source devices; and

[0036] A vacuum chamber including a window that allows the X-rays generated from the anode to pass through.

[0037] Embodiments will now be described in further detail with reference to the accompanying drawings. It should be understood that the particularity of the accompanying drawings does not supersede the generality of the preceding description of the disclosure.

[0038] Throughout the specification and the claims that follow, unless the context requires otherwise, the words “comprise” and “comprising” and variations such as “including” and “having”, will be understood to imply the inclusion of stated features or groups of features but not the exclusion of any other features or groups of features.

[0039] The reference to any prior art in this specification is not, and should not be taken as, any form of suggestion that such prior art forms part of the general knowledge.

[0040] Those skilled in the art will appreciate that the use of the present disclosure is not limited to the specific one or more applications described. The present disclosure is not limited in its preferred embodiments to the specific elements and / or features described or illustrated herein. It should be understood that the present disclosure is not limited to the disclosed one or more embodiments, but can be rearranged, modified and replaced many times without departing from the scope as described and defined in the following claims. BRIEF DESCRIPTION OF THE DRAWINGS

[0041] Embodiments of the present disclosure are discussed with reference to the accompanying drawings, in which:

[0042] Figure 1 is a flow chart of a method for manufacturing a field emission electron source device according to an embodiment;

[0043] Figure 2 is a graph of emission device performance of three field emission devices according to an embodiment, the three field emission devices being of the same size and having different degrees of nano and micro field enhancement;

[0044] Figure 3A is a schematic diagram of field emission enhancement by fractal geometry according to an embodiment;

[0045] Figure 3B is a series of photographs of the surface of a field emission device according to an embodiment taken at different scales;

[0046] Figure 3C are photographs of the surface of another field emission device according to the embodiment taken at different scales;

[0047] Figure 4 is a schematic diagram of a field emission electron source device according to an embodiment;

[0048] Figure 5 is a schematic circuit diagram showing fractal field enhancement and distributed resistance according to an embodiment;

[0049] Figure 6 is a graph of emission device performance for four field emission devices, the four field emission devices being of equal size and having different resistance levels in a matrix layer of the device;

[0050] Figure 7 is a graph of emission device lifetime testing of two field emission devices, the two field emission devices being of equal size and tested at the same current density, having different resistance levels in the matrix layer of the devices; and

[0051] Figure 8 According to an embodiment, Figure 4 A schematic diagram of an X-ray field emission device is shown.

[0052] In the following description, like reference numerals designate like or corresponding parts throughout the drawings. DETAILED DESCRIPTION

[0053] Embodiments of methods and basic design principles for creating large-area electron emitters that deliver large, stable, uniform currents based on nanostructure field emission devices are described herein. The methods extend the prior art related to nanostructure field emission devices by focusing on a large distribution of devices rather than optimizing a single device. The methods illustrate, but are independent of, specific field emission performance of a single nanostructure field emission device. The present disclosure describes embodiments of optimally distributing a single nanostructure field emission device based on overall emitter stability and uniform current distribution rather than optimizing current density of a single device. The present disclosure further describes methods of electrically linking electrodes to a distribution of single nanostructure field emission devices to ensure a stable, uniform electron current across the entire distribution of emission devices.

[0054] Figure 1 1 is a flow chart of a method 100 for manufacturing a field emission electron source device according to an embodiment. The method includes a step 110 of forming a current sharing matrix layer electrically connected to a substrate, wherein the current sharing matrix is ​​bonded to the substrate, deposited on the substrate, formed directly on the substrate, or formed directly in the substrate. The method also includes a step 120 of forming an emitter surface composed of a plurality of nanoscale field emitters electrically connected to the matrix sharing layer, wherein the emission surface is formed by obtaining a plurality of nanoscale field emitters and bonding them to the current sharing matrix layer or forming a plurality of nanoscale field emitters directly on the current sharing matrix layer. Electrons are emitted by an emission region of the emission surface. This may be the entire emission surface or a smaller portion of the emission surface. For example, the emission surface may be divided into a plurality of emission regions, or some of the surface may be shielded to prevent emission (e.g., around the edges of the surface).

[0055] Embodiments of the method can solve problems caused by the inherent variability of nanostructured field emission devices. The device includes multiple structures so that the structure forms electric field enhancement features at multiple scales from macroscale to nanoscale. These structures may be fractal, geometrically repeated (e.g., features specially designed or manufactured with regular shapes and spacing) and / or randomly distributed (specifically, a combination of the three), and are used to enhance the electric field at successively smaller levels to create field enhancement regions, thereby enabling a wider range of individual field emission devices in the region to participate. The variability between individual field emission devices is solved by forcing current sharing between devices via a current sharing matrix layer. This forced current sharing fights the natural tendency of the best device to dominate the current contribution and burn itself out prematurely. The resistive paths of the current sharing matrix and multiple structures of different scales work together to enable the construction of devices that emit electrons with a spatially uniform distribution across an emitter area having an emitter surface of an area of ​​at least 1 square millimeter and at a current density greater than or equal to one milliampere per square millimeter.

[0056] The electric field strength can be defined based on the voltage potential difference between the two electrodes. If the electrodes are perfectly parallel, the electric field forms a consistent potential gradient between the plates that is proportional to the distance between the plates and the voltage difference. Features on the plates force the electric field gradient to be concentrated around the features. As the features become sharper, the concentration of the electric field increases. Field emission devices produce electric field concentrations high enough to force electron emission through quantum tunneling; the field concentration threshold for electron emission through tunneling is called the Fermi level. Nanostructured field emission devices produce small, sharp features with low Fermi levels; the low Fermi level enables the construction of stable electron emitters from these nanostructured devices.

[0057] Conventional field emission studies assume that nanostructured devices exist in an ideal equipotential electric field and that field enhancement occurs only due to the structure of the device. For example, carbon nanotubes (CNTs) can be 200nm to 20 microns in length and 0.4nm to 100nm in diameter, and typically have a high aspect ratio (i.e., large length compared to width), which creates very high field concentration for field emission. The only other electric field consideration is that the field enhancement of the device degrades due to the proximity of neighboring devices and space charge accumulation. Field enhancement unrelated to nanostructured devices that degrade the performance of individual devices is considered in the literature, for example, on a microscopic scale. Field enhancement unrelated to nanostructured devices creates high-intensity regions and low-intensity regions; only nanostructured devices with sufficient field strength in the high-intensity region will emit electrons. If the devices are uniformly distributed across the entire area, as is generally proposed in the current literature, this uneven electric field strength reduces the current density of the overall distribution of the device.

[0058] Current field emission literature teaches that large current emitters should be constructed by optimizing individual field emission devices and replicating them over a large area. In this configuration, the best large area emitters have an assumed uniform electric field over the emitter surface, and the only field enhancements come from specific nanostructured devices. In theory, this allows all devices to contribute to the total current, thereby maximizing the total current density of the emitter.

[0059] However, nanostructured devices have inherent variations; therefore, in a uniform electric field, electron emission is non-uniform across a large area of ​​a distributed device. Currently, the only solution to this non-uniformity is to reduce the differences between individual nanostructured devices.

[0060] The theoretically optimal large area and high current nanostructure field emitters consist of a distribution of identical high current density devices that are perfectly spaced so as not to interfere with each other. If the spacing is too close, the individual devices become suboptimal due to degradation of the field enhancement on each device. If the spacing is too far, the total current per unit area decreases because fewer devices can fit into the area. If the devices are not uniform, it is impossible to achieve the optimal spacing between emitters. The inherent variation of nanostructure field emission devices makes it impossible to achieve a theoretically optimal distribution over a large area. As the variation between devices increases, the spacing can drive the emitters to become significantly suboptimal; the use of ideal spacing increases the stress on individual high-prefabricated devices, and hot spot instabilities occur.

[0061] The field emission device and the method for manufacturing such a device described herein are developed based on such a recognition that, instead of trying to optimize the performance (i.e., current density) of a single field emitter and then strictly controlling the manufacturing and placement, a stable large-scale emitter device can be manufactured by forming electric field enhancement features of a series of scales from macroscopic to microscopic. Although this design is suboptimal in terms of field emission from a single device, the field emitter is distributed across the surface in combination with a current sharing matrix layer and provides field enhancements of different scales so that the total current density of the emitter can be optimized (or at least significantly improved) as a whole. The additional level of field enhancement allows the activation of a specific device to become a function of the unique characteristics of the nanostructured device and the device position. By controlling the additional field enhancement level, the total current density of the emitter is controlled, and the emitter is not susceptible to changes in the nanostructured device. In addition, if the device in the lowest field strength area fails, the emitter with a slightly higher field strength level can be activated to restore the lost current. In this way, multiple levels of redundancy and stability can be built into a single large area structure.

[0062] exist Figure 2 The advantages of increasing the field enhancement level are demonstrated in Figure 2 , the emission performance of three different large area devices is compared; each of the devices has the same emission area. Device 1 has a limited degree of additional field enhancement characteristics at the nano and micro scales. Device 2 has a moderate degree of additional field enhancement characteristics at the nano and micro scales. Device 3 has a significant degree of additional field characteristic enhancement at the nano and micro scales. Each of the devices achieves the same peak current density relative to the turn-on field strength at different field strengths. Each of the devices has a current density-field strength curve with two different slopes; in the first part of the slope 201, different areas of the emitter are turned on based on changes in local field strength, and, in the second part of the slope 202, the emitter emits uniformly across the entire device; all three devices are at about 0.4mA / mm 2This transition is reached. Device 1 requires the highest field strength to reach the point where all emitters emit uniformly because it has limited additional field enhancement to lower the threshold; this shows the effect of small changes in a single field emitter over a large area. Devices 1 and 2 show the beginning of a third change 203 in the slope at large current densities; this change in slope indicates that the maximum current is being drawn from some of the emitters and the device is no longer uniform. Device 3 has not yet reached the maximum current density because the additional field enhancement feature increases the number of active field emitters over a large area.

[0063] In illustrating enhancement of field emission by fractal geometry according to an embodiment Figure 3A , the disclosed device is schematically shown. The electric field 300 is enhanced by a plurality of consecutive layers 302, 304, and 306 having geometric features. Each feature enhances the local electric field in proportion to the feature size. Figure 3A 300 represents the electric field density around the device, which has macro features 302 (size 100 μm to 1 mm), micro features 304 (size 1 μm to 10 μm), and nano features 306 (less than 1 μm) of a single field emission device. Each feature size provides a small incremental electric field enhancement. In the field gradient around the device Figure 3A This is shown in FIG. 3 (from black at low field to white at high field). It can be seen that as additional features are added, the field strength increases (compare the field area around macrofeature 302 to the field area around nanofeature 306). As more features are added, the less optimized the individual field emission devices need to be, and / or, since the individual devices are placed in an already enhanced electric field, a lower global voltage can be used to deliver the same performance.

[0064] Multiple levels of field enhancement enable a wider range of field emission devices to contribute to the total electron current; however, the current contributed by each device varies significantly due to the local field enhancement and Fermi level of the individual devices. The present disclosure uses resistive paths linking a common conductive substrate electrode to the individual devices to ensure uniform current distribution (i.e., current sharing) across the entire emitter surface (emitting area).

[0065] Figure 4is a schematic diagram of a field emission electron source device 1 according to an embodiment, and includes a substrate 10, a current sharing matrix layer 20, and an emitter surface formed by a plurality of individual nanoscale field emitters 30. In the present embodiment, the electrode 12 is bonded to the base of the substrate 10, and the electric field extraction electrode 50 is located near the surface field emission source device and the device emits electrons 60 in use. In other embodiments, the current sharing matrix layer 20 is embedded in or formed in the electrode substrate 10. In some embodiments, the entire substrate is also formed as the current sharing matrix layer 20 (i.e., the substrate 10 is the current sharing matrix layer 20). Similarly, in other embodiments, the electrode 12 may be integrated with the substrate 10 (substrate electrode) instead of being bonded to the substrate 10. The current sharing matrix layer 20 includes a plurality of series resistance paths.

[0066] Figure 5 is a schematic circuit diagram showing fractal field enhancement and distributed resistance, and these resistance paths are shown as a series of resistor dividers coupled to a voltage source tree. The voltage source tree is the multiple levels of field enhancement required to start field emission, where the single global voltage difference between a single nano field emission device 30 and a secondary extraction electrode (extraction gate) 50 is successively decomposed into a series of voltage sources 42 (macro), 44 (micro), 46 (nanometer) slightly higher than the Fermi level of the device distribution. The distribution of devices and their electrical connection to the substrate electrode is a network of series resistors. When a source voltage greater than the Fermi level plus the field enhancement tree voltage is applied between the substrate electrode and the secondary extraction electrode, current will flow through the resistor and the field emission device. If the nano field emission device has a low resistance and is directly coupled to the substrate electrode, the current will flow unevenly to the device with the smallest resistance. Adding a controlled voltage divider network between the substrate electrode and the individual devices forces the current to be more evenly distributed to each individual device.

[0067] exist Figure 5, a single field emission device 30 represents a resistance of about 1.1Ω, which reflects a typical nano field emission device, and, for the best field emission devices (such as carbon nanotubes) that act as conductors, the resistance can be even lower. The conductive electrode 12 is represented as a 10mΩ resistor connected to a voltage source greater than or equal to the overall field enhancement potential difference. The electrode is bonded to the substrate 10, and the current sharing matrix layer 20 is bonded to the substrate. As shown by the presence of the 1kΩ lateral resistor 16, the resistance of the substrate material 10 is represented as a 100mΩ resistor and promotes the flow of current through the substrate. Current sharing occurs due to the large resistor 24 in the middle of the resistor network (current sharing matrix layer 20). These resistors 24 are much larger than any other resistor in the path between the electrode 12 and the single device 30, and are represented as 1 megohm resistors. The structure of the material promotes the flow of current through the material, and this is shown by the lateral resistor 26 with a value of about 100 megohms. These large resistors 24 dominate the current flow and thus enable uniform current sharing despite the variation in resistance of the individual field emission devices 30 .

[0068] High resistance materials are used to form the current sharing matrix layer 20 and Figure 5 The resistive path shown. The material can also directly bond a single field emission device to an electrode, or be a layer within a more complex matrix that provides electromechanical bonding. In some embodiments, the resistive path is formed by using a high resistance material. In some embodiments, the resistive path is formed by using a semiconductor material with resistive properties or using a metal ceramic material with resistive properties. Examples of materials that can be used for this purpose include silicon dioxide mixed with titanium oxide, zinc oxide, manganese titanium oxide / nitride, aluminum nitride or boron nitride; or a doped silicon layer. Multiple levels of field enhancement can be designed into the resistive material so that the material forms both macroscopic field enhancement and microscopic field enhancement and a resistive path; the material can also be deposited on top of an electrode with macroscopic and microscopic field enhancement features so that the material only provides a resistive path; the material can also be deposited on top of an electrode that contains features and other features added to the material. In this embodiment, the substrate 10 and the current sharing matrix layer 20 are shown as separate layers, but in other embodiments, they can be provided as an integrated layer 18. In these embodiments, the substrate can be processed to form a current sharing matrix layer, or a doping gradient can be used to form a layer from a low resistance area (i.e., Figure 2 The transition from the substrate 10 in the substrate to the high resistance region (ie, the current sharing matrix or resistive layer 20).

[0069] Forced current sharing regulates the current through each individual field emission device. The inherent variability of field emission devices results in variations in the current through the device when operated without forced current sharing. When applied to large area emitters, the variability of the current results in hot and cold spots appearing across the emitter. Forced current sharing suppresses the appearance of hot and cold spots in situations where the electric field enhancement is sufficient to produce field emission. The uniform distribution of field enhancement using multiple levels of enhancement combined with current sharing enables spatially uniform electron emission over a large area.

[0070] exist Figure 6 The advantages of enforcing current sharing by increasing the resistance in the emitter matrix layer are shown in Figure 6 In Figure 1, the emitter performance of four different large-area devices is compared; each of the devices has the same emitter area. Device 1 has a low resistance in the matrix, device 2 has a medium resistance in the matrix layer, device 3 has a high resistance in the matrix layer, and device 4 has no resistance in the matrix layer; therefore, device 1 has limited forced current sharing, device 2 has some forced current sharing, device 3 has a large forced current sharing, and device 4 has no forced current sharing. As the emitter matrix resistance increases, the current density increases much faster relative to the field strength; this relationship indicates that as the current density increases, the current sharing forces more individual field emitters to share the total current drawn from the device. In device 4, the matrix acts as a nearly perfect conductor with no resistance, and in this device, only the field emission sources that are turned on at the onset of field emission provide electrons; as shown by the very low slope in device 4, only this subset of sources increases current as the electric field strength increases, and no other sources turn on.

[0071] The methods disclosed herein enable the production of stable electron emitters with long lifetimes. Field emission devices degrade rapidly when the device is subjected to excessive stress and becomes chemically or thermally unstable. An ideal field emission device has a high aspect ratio to lower the Fermi level. However, this aspect ratio means that the heat accumulated in the center of the device must be transferred to the ends of the device to escape. The heat will be transferred to the active end; if the heat at the emission point becomes high enough, the device will begin to thermally emit electrons in addition to the quantum tunneling effect of the electrons. The thermally emitted electrons originate from the breaking of the atomic bond structure of the device; when the device thermally emits, it tears itself apart. The only way to prevent thermal emission in a field emission device is to keep the current through the device below a safe limit for a given amount of emitter on-time. This safety limit is unique to each individual device, but a general limit can be defined based on the general characteristics of the device (such as length, width, and atomic structure).

[0072] Without designed current sharing, nanostructured field emission devices emit current proportional to the nanofield strength seen by each device. If one device has a lower turn-on threshold than its neighbor, it will begin emitting current before its neighbor. As the electric field increases, the device with the lower turn-on threshold will continue to increase its current until it begins to thermally emit and degrade. Thermal emission is a slower process than field emission, so when a thermal emitting device reaches electrostatic equilibrium with its neighbors, it will continue to thermally emit until it cools. Therefore, a device that begins emitting earlier than its neighbors may eventually have a higher turn-on threshold due to thermal degradation. This process will go back and forth between devices until the entire area becomes degraded. In the prior art, the only way to prevent this from happening is to limit the current density of the entire emitter. Without current sharing, the derating of the average single emitter operating current needs to be more stringent to allow for a wide spread of operating currents and is effectively determined by the emitter with the strongest emission that reaches the safe operating limit first. Engineered sharing resistances in the structure allow the current extraction of individual emitters to reach the safe operating limit, thereby matching more closely. This enables a wider range of devices to be used in the same structure, which has advantages such as higher reproducibility, easier scalability to larger areas, easier manufacturing and longer lifetime.

[0073] The method disclosed herein ensures stable current over long life in two ways: first, current sharing reduces the risk of a group of field emission devices starting thermal emission, and second, multiple levels of field enhancement provide additional devices to replace devices that burn out prematurely.

[0074] Current sharing distributes the current evenly between devices and accounts for variations in the Fermi level between individual devices. By sharing the current between a large number of devices, the current stress on each device is reduced and higher overall emitter current density can be maintained without reducing emitter lifetime.

[0075] Multiple levels of field enhancement means more individual devices can be placed in the same area. Multiple levels shield the potential degradation effects of having multiple devices close together, and only the device with the lowest Fermi voltage turns on. If the turned-on device burns out, the global electric field increases. Increasing the global electric field turns on previously inactive devices. By placing devices on multiple levels, the emitter has a built-in level of redundancy, extending the lifetime of the emitter and maintaining the same spatially uniform current.

[0076] exist Figure 7The advantage in terms of device lifetime is demonstrated in Figure 2, which compares two nearly identical devices, both of the same size, with the same field emitters, the same field enhancement characteristics, but with different levels of matrix layer resistance. Both emitters were tested over a long period of time at the same current density to investigate the degradation of long term use on field emission devices. Both devices showed some performance degradation, as evidenced by the increase in the electric field strength required to deliver the same current density. However, device 1, with the lower matrix layer resistance, had more than twice the absolute degradation of device 2 and a degradation rate 2.5 times that of device 2. The increased degradation rate of device 1 is due to the reduced current sharing in device 1 compared to device 2. Reduced current sharing means that more current is drawn from fewer field emitters, and these field emitters are stressed and degrade faster.

[0077] As described above, the device is manufactured or constructed in a manner to form multiple structures so that the structures form electric field enhancement features at multiple scales from macroscale to nanoscale. Depending on the construction method, the structure includes one or more of fractal, geometrically repeating and / or randomly distributed structures.

[0078] In one embodiment, the plurality of structures include macro-scale structures having a size between 100 μm and 10 mm. The macro-features may be created by processing, forming or constructing a current sharing matrix material and / or substrate. In one embodiment, the characteristic shapes are repeating geometric patterns of field emission. These may be constructed by an elaborate design or manufacturing process that creates a structure and repeats the structure across a surface, or the structure may be created across a surface such as by masking and etching techniques or other conventional techniques such as photolithography.

[0079] In some embodiments, micro-scale features are created by surface treatment of a current sharing matrix layer and / or substrate to create a three-dimensional surface with a fractal or random structure of a few microns to tens of microns in size. In some embodiments, the micro-scale features include a matrix material and / or substrate formed into a three-dimensional surface with a fractal or random structure of 1 to tens of microns in size. In some embodiments, the micro-scale features are created or formed by patterning the placement of multiple individual nanoscale field emitters on the matrix material and substrate.

[0080] In some embodiments, the nanofeatures are either intrinsic features of the plurality of individual nanoscale field emitters or are created when the plurality of individual nanoscale field emitters are joined with a current sharing matrix material. In some embodiments, the nanofeatures are created by using nanostructured materials having fractal properties. In some embodiments, the nanostructured materials include high aspect ratio features.

[0081] Suitable nanostructured materials can be formed from the following materials: nanostructured carbon materials (i.e., carbon nanotubes, diamond or vertical graphene), ceramics (e.g., metal or non-metal oxides, such as aluminum oxide, silicon dioxide, iron oxide and copper oxide; metal or non-metal nitrides, such as silicon nitride and titanium nitride; metal or non-metal carbides, such as silicon carbide; metal or non-metal borides, such as titanium boride); metal or non-metal sulfides, such as cadmium sulfide and zinc sulfide; metal silicides, such as magnesium silicide, calcium silicide and iron silicide; and semiconductor materials (e.g., diamond, germanium, selenium, arsenic, silicon, tellurium, gallium arsenide, gallium antimonide, gallium phosphide, aluminum antimonide, indium antimonide, indium tin oxide, zinc antimonide, indium phosphide, aluminum gallium arsenide, zinc telluride and combinations thereof), tungsten nanowires, gold nanowires and other metal nanowires.

[0082] Figure 3B and 3C Further illustrating the presence of features at various scales, from macro to nano scale, these figures represent a series of scanning electron microscope (SEM) images of the surface of a field emission device constructed according to the methods described herein.

[0083] exist Figure 3B In the figure, image 311 represents a relatively low resolution image at 384 times resolution, the white line represents a 400 micron scale, and image 313 is a 2000 times resolution image, the white line represents a 50 micron scale. Image 314 is a 4000 times resolution image, the white line represents a 30 micron scale. Image 312 is a 7269 times resolution image, the white line represents a 20 micron scale. Image 315 is a 16000 times resolution image, the white line represents a 5 micron scale. Image 316 is a 50000 times resolution image, the white line represents a 3 micron scale, showing carbon nanotubes on an inverted peak (matrix sharing layer) structure. These images show the range of structures that form the device surface over a range of scales. In this embodiment, the matrix sharing material has resistive properties to force it to span 100mm 2 The surface shares the current with tens of thousands of individual carbon nanotube field emitters.

[0084] exist Figure 3C , image 317 represents a relatively low resolution image of a patterned device where the macro and micro features are formed into a uniform repeating pattern rather than a fractal random pattern. Image 318 is a 9000x resolution image with the white line representing the 8 micron scale. Figure 318 represents a high resolution image of a single macro to micro feature on a patterned device; the feature shown in 318 is covered by randomly distributed nanostructured field enhancement features. In this example, the macro and micro features are silicon pillars covered with diamond field emitters.

[0085] The large-scale stable field emitters described here can be used in a range of devices and systems. One application is Figure 8The X-ray field emission device 100 shown. The device includes a cathode including one or more field emission electron source devices 1 as described herein, an electric field extraction electrode 50, and control electronics 210 for turning on and off the one or more field emission electron source devices. The focusing electrode 170 can also be used to focus the electron beam 160. The anode 180 including a target material configured to generate X-ray emissions 190 when impacted by the electrons 160 generated by the one or more field emission electron source devices 1 is located in a vacuum chamber 200 including a window 220 for allowing the X-rays generated from the anode to pass.

[0086] Large-scale stable field emitters and methods of fabrication for high current applications are described herein. Embodiments of these devices may have an emission area of ​​at least 1 square millimeter and may be capable of providing stable electron emission at current densities greater than or equal to 1 milliampere per square millimeter. Design principles and methods for fabricating large-area electron emitters that deliver large, stable, uniform currents based on nanostructured field emission devices are described herein. Embodiments of the methods address the inherent variability of nanostructured field emission devices and describe the design criteria and performance required to provide uniform currents for many inherently unique nanostructured field emission devices. Embodiments of the methods described herein enable fabrication of a wider range of emitters and enable scalable processes for generating larger devices.

Claims

1. A method for manufacturing a field emission electron source device, comprising: forming a current sharing matrix layer electrically connected to a substrate, wherein the current sharing matrix layer is bonded to the substrate, deposited on the substrate, or formed directly in the substrate; forming an emitter surface composed of a plurality of nanoscale field emitters electrically connected to a matrix sharing layer, wherein the emitter surface is formed by obtaining a plurality of nanoscale field emitters and bonding to a current sharing matrix layer or forming a plurality of nanoscale field emitters directly on the current sharing matrix layer; in, The current sharing matrix layer includes a plurality of series resistance paths between the plurality of nanoscale field emitters and the substrate to force current sharing among the plurality of nanoscale field emitters, and, the plurality of nanoscale field emitters, the current-sharing matrix layer, and the substrate are configured to form a plurality of structures such that the structures form electric field enhancement features at a plurality of scales from a macroscale to a nanoscale, wherein the structures at the macroscale and at the nanoscale are fractal, geometrically repeating, and / or randomly distributed, and the nanoscale features are either intrinsic features of the plurality of nanoscale field emitters or are created when the plurality of nanoscale field emitters are joined to the current-sharing matrix layer; Such that, in use, the field emission electron source device emits electrons in a spatially uniform distribution across an emitter region of the emitter surface, wherein the emitter region has an area of ​​at least 1 square millimeter and emits electrons at a current density greater than or equal to one milliampere per square millimeter.

2. The method of claim 1, wherein: The plurality of structures include macro-scale structures having dimensions between 100 μm and 10 mm and are created by processing, forming or configuring the current sharing matrix layer and / or the substrate to include structures in a repeating geometric pattern that includes field emission.

3. The method of claim 1, wherein: The plurality of structures include microstructures created by surface treatment of the current sharing matrix layer and / or the substrate to create a three-dimensional surface with a fractal or random structure having a size of several micrometers to tens of micrometers.

4. The method of claim 1, wherein: The plurality of structures include a microstructure, and the microstructure includes the current sharing matrix layer and / or the substrate formed into a three-dimensional surface having a fractal or random structure with a size of 1 to tens of micrometers.

5. The method of claim 1, wherein: The plurality of structures include microstructures, and the method further includes creating the microstructures by patterning placement of the plurality of nanoscale field emitters on the matrix layer and substrate.

6. The method of claim 5, wherein: Nano features are created by using nanostructured materials with fractal properties.

7. The method of claim 6, wherein: Nanostructured materials include high aspect ratio features.

8. The method of claim 1, wherein: Each structure adds incremental electric field enhancement.

9. The method of claim 1, wherein: The resistive path is formed by using a high resistance material.

10. The method of claim 1, wherein: The resistance path is formed by using a semiconductor material having resistance properties.

11. The method of claim 1, wherein: The resistive path is formed by using a metal ceramic material having resistive properties.

12. The method of claim 1, wherein: The substrate is an electrode substrate, and the method further includes forming an electrical connection to the electrode substrate.

13. The method of claim 1, wherein: The method also includes bonding an electrode to the substrate and forming an electrical connection with the electrode.

14. A field emission electron source device, comprising: Electrodes integrated into or bonded to a substrate; an emitter surface composed of multiple nanoscale field emitters; a current-sharing matrix layer between the plurality of nanoscale field emitters and the substrate or incorporated into the substrate; in, The current sharing matrix layer includes a plurality of series resistance paths between the plurality of nanoscale field emitters and the electrode to force current sharing among the plurality of nanoscale field emitters, and The plurality of nanoscale field emitters, the current-sharing matrix layer, and the substrate are configured to form a plurality of structures such that the structures form electric field enhancement features at a plurality of scales from a macroscale to a nanoscale, wherein the structures at the macroscale and at the nanoscale are fractal, geometrically repeating, and / or randomly distributed, and the nanoscale features are either intrinsic to the plurality of nanoscale field emitters or are created when the plurality of nanoscale field emitters are joined to the current-sharing matrix layer; and In use, the field emission electron source device emits electrons in a spatially uniform distribution across an emitter region of the emitter surface, wherein the emitter region has an area of ​​at least 1 square millimeter and transmits stable electron emission at a current density greater than or equal to one milliampere per square millimeter.

15. The field emission electron source device according to claim 14, wherein: The current sharing matrix layer is formed of a matrix material that is electromechanically coupled to the plurality of nanoscale field emitters.

16. The field emission electron source device according to claim 14, wherein: The current sharing matrix layer is formed of a matrix material that is electromechanically bonded to the substrate electrode.

17. The field emission electron source device according to claim 14, wherein: The current sharing matrix layer is formed in the substrate and is bonded to the electrode.

18. The field emission electron source device according to claim 14, wherein: The plurality of structures comprises macro-scale features having dimensions between 100 μm and 10 mm and comprises a repeating geometric pattern of field emission.

19. The field emission electron source device according to claim 14, wherein: The plurality of structures include a microstructure, and the microstructure is a three-dimensional surface of a fractal or random structure having a size of several micrometers to tens of micrometers.

20. The field emission electron source device according to claim 14, wherein: The plurality of structures include a microstructure, and the microstructure includes the current sharing matrix layer and / or the substrate formed into a three-dimensional surface having a fractal or random structure with a size of 1 to tens of micrometers.

21. The field emission electron source device according to claim 14, wherein: Nanofeatures are nanostructured materials with fractal properties.

22. The field emission electron source device according to claim 21, wherein: The nanostructured material includes high aspect ratio features.

23. The field emission electron source device according to claim 14, wherein: Each structure adds incremental electric field enhancement.

24. The field emission electron source device according to claim 14, wherein: The current sharing matrix layer is made of high resistance material.

25. The field emission electron source device according to claim 14, wherein: The resistance path is formed by using a semiconductor material having resistance properties.

26. The field emission electron source device according to claim 14, wherein: The resistive path is formed by using a metal ceramic material having resistive properties.

27. The field emission electron source device of claim 14, further comprising a secondary electric field extraction electrode and control electronics for turning field emission on and off.

28. An X-ray field emission device, comprising: A cathode comprising one or more field emission electron source devices, wherein each of the one or more field emission electron source devices comprises: Electrodes integrated into or bonded to a substrate; an emitter surface composed of multiple nanoscale field emitters; a current-sharing matrix layer between the plurality of nanoscale field emitters and the substrate or incorporated into the substrate; in, The current sharing matrix layer includes a plurality of series resistance paths between the plurality of nanoscale field emitters and the electrode to force current sharing among the plurality of nanoscale field emitters, and The plurality of nanoscale field emitters, the current-sharing matrix layer, and the substrate are configured to form a plurality of structures such that the structures form electric field enhancement features at a plurality of scales from a macroscale to a nanoscale, wherein the structures at the macroscale and at the nanoscale are fractal, geometrically repeating, and / or randomly distributed, and the nanoscale features are either intrinsic to the plurality of nanoscale field emitters or are created when the plurality of nanoscale field emitters are joined to the current-sharing matrix layer; and In use, the field emission electron source device emits electrons in a spatially uniform distribution across an emitter region of the emitter surface, wherein the emitter region has an area of ​​at least 1 square millimeter and delivers stable electron emission at a current density greater than or equal to one milliampere per square millimeter; Electric field extraction electrode; control electronics for switching field emission of the one or more field emission electron source devices on and off; an anode comprising a target material configured to produce X-ray emission when struck by electrons generated by the one or more field emission electron source devices; and A vacuum chamber includes a window that allows the X-rays generated from the anode to pass through.

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