Semiconductor cold photocathode device using electric fields to control electron affinity.

Abstract

The electron emitter comprises a tapered emitting tip having a base surface and an apex opposite the base surface, the emitting tip consisting essentially of a semiconductor material that is partially doped n-type and partially doped p-type, the base surface being doped either n-type or p-type, and the apex being doped the type opposite the base surface, thereby forming a p-n junction at a location between the base surface and the apex.

Description

[Technical Field]

[0001] Related Applications This application claims priority from U.S. Application No. 18 / 393,329, filed December 21, 2023, which claims priority from U.S. Provisional Application No. 63 / 434,882, filed December 22, 2022, the disclosure of which is incorporated herein by reference in its entirety. [Background technology]

[0002] Disclosed embodiments include electron beam emission devices for electron microscopy (EM) and electron beam assisted manufacturing techniques such as electron beam lithography (EBL) and electron beam induced deposition (EBID).

[0003] Electron emitters have been used in a variety of electrical devices, from vacuum tubes and cathode ray tubes (CRTs) to electron columns used in electron microscopes such as scanning and transmission electron microscopes. Most electron sources are made from tungsten. In some cases, they exist as heated tungsten hairpins, where heat is used to extract electrons from a material (thermionic emission, or thermionic emitters). In other cases, they exist as sharp tungsten tips, where a combination of heat and an electric field is used to extract electrons from a material (hot-field emission, or Schottky emitters). In these cases, the tungsten is coated with zirconium oxide (ZrO), a material that can increase extraction efficiency by lowering the work function of tungsten. Finally, in some cases, the emitter exists as a sharp tungsten tip, where only an electric field is used to extract electrons from a material (cold-field emission, or cold-field emitters). All of these emitters work well in continuous emission mode. However, various next-generation applications of electron beams in nanometrology and nanofabrication require the use of fast pulsed or modulated electron beams. Therefore, new electron sources are needed.

[0004] Applicants have previously employed the use of a pulsed laser to illuminate a tungsten tip with a beam to generate a pulsed electron beam (photoemission, photocathode). However, such methods have inherent limitations in generating photoexcitations in metallic materials, emitting only 1 electron per 10 photons. Therefore, an alternative solution is desirable.

[0005] Photocathode materials are often used in light detection devices such as photomultiplier tubes, where the active part of the device is kept under vacuum and electrons emitted from the surface are collected, giving a measurement of the number of incident photons. These devices are usually made from metal alloys or semiconductors, and to obtain efficient electron emission, methods to reduce the electron affinity are required, often consisting of applying specific coatings such as cesium to the material.

[0006] To provide an efficient semiconductor-based cold field electron emitter, Shaw et al. proposed an emitter with a graded semiconductor coating on the substrate, a method of reducing the inherent electron affinity of the semiconductor material. Further information can be found in U.S. Patent No. 5,773,920.

[0007] A semiconductor-based electron source developed according to the above-described optical sensor structure is shown schematically in Figure 1. The semiconductor plate 100 is coated with cesium-101. The cesium coating lowers the electron emission barrier below the excitation photon energy, increasing the emission probability. In other words, lowering the so-called electron affinity, i.e., the energy required to extract electrons to the vacuum energy level, is always beneficial for photoemission. Under these conditions, a laser beam 110 illuminating the backside of the plate causes photons to impinge on the backside of the plate 100, thereby causing electron emission from the frontside of the plate. Note that in this disclosure, "frontside" refers to the side from which electrons are emitted. More information on this technology can be found at https: / / photoelectronsoul.com / en / technology. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] U.S. Patent No. 5,773,920 [Non-patent literature]

[0009] [Non-Patent Document 1] https: / / photoelectronsoul.com / en / technology Summary of the Invention [Problem to be solved by the invention]

[0010] For this structure to function properly, the front surface of the plate must be activated to have negative electron affinity (NEA) and the vacuum level for electron extraction must be reduced. This can be achieved by coating the front surface with cesium, as shown by coating 101 in Figure 1. During operation, the cesium coating degrades, thereby reducing the quantum yield and creating two problems. First, to maintain the same electron beam current, the laser power must be constantly adjusted to account for the cesium degradation. This is explained in the "Continuous Test Results" published in 2022, available from the website mentioned above. Furthermore, because the cesium coating degrades during use, the surface must be recoated in situ to avoid prolonged machine downtime. This requires the electron beam source to incorporate sophisticated mechanisms for cesium recoating, making the electron beam source expensive, large, and complex, making it prone to failure and difficult to maintain.

[0011] Therefore, new electron sources are desirable that alleviate some of the drawbacks of the prior art. Specifically, new electron sources should be able to provide efficient light emission without relying on coatings such as ZrO or cesium.

[0012] The following summary of the disclosure is included to provide a basic understanding of some aspects and features of the invention. This summary is not an extensive overview of the invention, and as such, it is not intended to particularly identify key or critical elements of the invention or to delineate the scope of the invention. Its sole purpose is to present some concepts of the invention in a simplified form as a prelude to the more detailed description that is presented below. [Means for solving the problem]

[0013] The disclosed embodiments provide a solution that overcomes the shortcomings of prior art emitters. Because tungsten filament-based emitters are poorly suited to discontinuous emission, the disclosed embodiments provide an emitter that can also operate well in a rapidly pulsed mode. Conversely, because current semiconductor light emitters require periodic surface reactivation, the disclosed embodiments provide an emitter that does not require any surface activation. As a result, the disclosed embodiments provide an emitter that does not suffer from the problems of the prior art, such as the limited emitter lifetime, loss of efficiency over time, and the complexity of in-situ recoating.

[0014] To avoid the need for cesium coating for surface activation, the disclosed embodiments utilize the combined effect of p-type doping to bring the work function close to vacuum levels and a strong electric field applied to the semiconductor surface to precisely control the electron affinity. Applying a sufficient electric field to a semiconductor can significantly affect the semiconductor's electron affinity and, therefore, its suitability for high photoemission yields. This effect is unique to semiconductors because, unlike metals, the electric field can penetrate into the semiconductor and exert electrostatic attractive or repulsive forces on electrons within the material. This effect occurs at large field strengths, typically 10 MV / m or greater. The generation of the necessary electric field is achieved, at least in part, by the tip effect (also known as the lightning rod effect), as further explained below. The electron emission itself is stimulated by the application of laser light to the emitter surface, although the application of temperature can also be used to further control the electrical properties of the material. In this sense, the electron emitter is a cold photocathode device.

[0015] In the disclosed embodiments, the semiconductor emitter is formed into a sharp-pointed shape, for example, a conical or pyramidal shape with a sharp tip, typically with a radius of less than 10 microns (micrometers). In the disclosed embodiments, the apex region is doped p-type. In further embodiments, the base region is also doped p-type or n-type, forming a p-n junction between the base and apex. As the inventors point out, p-type doping brings the work function close to the vacuum level, but is insufficient for efficient electron emission. Therefore, an electric field must be applied. In the apex region, the sharp tip shape (lightning rod effect) enhances the strong electric field, which aids electron emission efficiency by lowering the barrier (Schottky effect), and there is also a slight tunneling effect. The electric field strength is set to reduce the electron affinity of the selected semiconductor material, but not to the extent that cold field emission occurs. The extra energy required for emission is provided by laser photons. Since the emitted current is proportional to the intensity of the laser light, temporal modulation or interruption of the electron beam can be achieved by appropriately changing the laser intensity. Thus, in the disclosed embodiments, at least three knobs are provided to control emission efficiency: doping level, tip geometry, and laser beam parameters. [Effects of the Invention]

[0016] As current generation emitters are poorly suited to discontinuous emission, the device can successfully replace some alternative technologies in a cheaper and more efficient manner. [Brief explanation of the drawings]

[0017] [Figure 1] 1 shows a semiconductor emitter according to the prior art; [Figure 2] 1A and 1B illustrate a semiconductor electron emitter located within a lens assembly according to an embodiment. [Figure 2A] FIG. 10 shows another embodiment of an electron emitter. [Figure 2B] FIG. 10 shows another embodiment of an electron emitter. [Figure 3A] 1A-1C illustrate a process for fabricating a semiconductor electron emitter according to the present embodiment. [Figure 3B] 1A-1C illustrate a process for fabricating a semiconductor electron emitter according to the present embodiment. [Figure 3C] 1A-1C illustrate a process for fabricating a semiconductor electron emitter according to the present embodiment. [Figure 3D] 1A-1C illustrate a process for fabricating a semiconductor electron emitter according to the present embodiment. [Figure 4] FIG. 1 is a schematic diagram showing elements of an electron-based microscope according to an embodiment. [Figure 5] 1 is a schematic diagram illustrating elements of a parallel electron beam direct write lithography tool according to an embodiment. [Figure 5A] 1A and 1B are diagrams illustrating an embodiment of a multi-beam electron source. DETAILED DESCRIPTION OF THE INVENTION

[0018] Other aspects and features of the present invention will become apparent from the following detailed description, taken in conjunction with the drawings, in which: It should be understood that the detailed description and drawings provide various non-limiting examples of various embodiments of the invention as defined by the appended claims.

[0019] The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments of the present invention and, together with the description, serve to explain and illustrate the principles of the invention. The drawings are intended to diagrammatically illustrate major features of exemplary embodiments. The drawings are not intended to depict every feature of an actual embodiment or the relative dimensions of the depicted elements, and are not drawn to scale.

[0020] Next, embodiments of the innovative electron emitter will be described with reference to the drawings. Different embodiments or combinations thereof can be used for different applications or to achieve different advantages. Depending on the results to be achieved, different features disclosed herein can be utilized singly or in combination with other features, partially or fully utilized, while balancing advantages with requirements and constraints. Therefore, while particular advantages will be emphasized with reference to different embodiments, they are not limited to the disclosed embodiments. That is, the features disclosed herein are not limited to the embodiments in which they are described, but may be "mixed and matched" with other features and incorporated into other embodiments.

[0021] The various embodiments disclosed below detail an innovative semiconductor-based cold cathode electron emitter. This emitter is designed to employ an electric field, enhanced by the tip effect, strong enough to cause a sufficient drop in electron affinity for photoemission, but without causing continuous emission. Rather, emission is initiated by the supply of photons from a light source, e.g., a laser. We refer to this operating state as "field-induced negative electron affinity" (field-induced NEA). In this way, by controlling the operation of the laser emission, electron emission from the light source can be controlled in either continuous, modulated, or pulsed modes. Notably, instead of conventional beam blanking, the laser beam can simply be deflected or turned off, which can be done in an abrupt and extremely fast manner, such that the transition from the emission on state to the emission off state is a step function, unlike the conventional slow transition achieved by electron beam blanking.

[0022] According to the inventors, the design of an emitter based on field-induced NEA can follow certain guidelines as follows:

[0023] The semiconductor material must be able to achieve field-induced NEA at relatively moderate electric fields to avoid significant tunneling through the vacuum barrier, yet strong enough to minimize space charge effects during electron pulse propagation in vacuum. Extraction fields of 1 to 2 V / nm are achievable with any Schottky cathode assembly and appear to be a good starting point, as they can lower the emitter affinity between approximately 1 and 1.5 eV. Therefore, candidate materials must exhibit electron affinities lower than 1.5 eV, exceeding the band gap of p-doped variants.

[0024] Laser performance has improved sufficiently that laser options are likely available at any wavelength, from sub-picosecond pulses to continuous wave (CW). However, laser wavelengths between 300 and 450 nm are desirable, not only to keep the cost of the driving light source low, but also because the performance and ease of use of optical components decreases as the UV wavelengths deepen.

[0025] Emitters are generally considered cold cathodes because no heating is required for electron emission. However, at room temperature, electron emitters tend to accumulate impurities via adsorption from the residual atmosphere. This causes a steady decrease in emitter current, and periodic heating (with the extraction voltage switched off) is required to induce impurity desorption. Therefore, emitters must be capable of operating at high temperatures to avoid this problem. If this is not possible, they must at least be capable of short flash heating. It is desirable to aim for an operating temperature several hundred degrees Celsius above room temperature, e.g., 500°C.

[0026] The advantages of a virtual electron source include the reduction of space charge effects that reduce brightness and image quality (continuous and pulsed operation), time resolution (pulsed operation), and a reduction in source size due to a higher beam brightness and coherence. For this, the emitter must replicate the geometry of a Schottky emitter close to the emission area. This ensures that high electric fields of up to several V / nm can be reached in the environment of the cathode assembly.

[0027] In some embodiments, the emitter implementation is designed for use with existing cathode assemblies as a drop-in replacement for current technology. This allows for the fairest performance comparison with existing technology and allows for technology transfer to existing devices. The geometry of the electron emitter, attached to a tungsten wire, necessitates a back contact in these embodiments. In such embodiments, materials with readily available bulk substrates are beneficial rather than heteroepitaxial templates. Conversely, in embodiments not designed as a drop-in replacement, this constraint may be alleviated by moving away from vertical device designs or by designing the heteroepitaxial stack on a substrate that can inject electrons from the back contact.

[0028] FIG. 2 shows an embodiment of a semiconductor electron emitter 200 disposed within a lens arrangement (typically electrostatic lenses) including a suppressor lens 220 and an extractor lens 230. The emitter is formed with a body 202, shown here as cylindrical but which can be any shape suitable for attachment to a light source, and a protruding emitting tip 205 having a tapered shape, shown here as conical but which can also be other shapes, such as a polyhedral (e.g., up to 100p facet) pyramidal shape. In this embodiment, depending on the material used and the method used to fabricate the emitting tip, the outer wall of the tip is at an angle φ of approximately 10 to 20 degrees relative to an axis of rotational symmetry 204 passing through the apex and base of the tip, and may typically be at an angle of 15 degrees, depending on the methodology used to form the tip. Specifically, the sharper the tip, or the smaller the angle φ, the stronger the resulting tip effect and the more efficient the electron emission. In general, the level of doping and the sharpness of the tip work in conjunction to improve electron emission efficiency. Thus, in theory, the higher the doping and the sharper the tip, the better the electron emission efficiency, all other variables being held constant. Importantly, in the disclosed embodiments, none of the emitter surfaces are coated with cesium.

[0029] The emitter 200 may be made of diamond, gallium nitride (GaN), silicon carbide (SiC) containing allotropic forms of 4H or 6H, or gallium phosphide (GaP). GaN is alloyed with In x Ga 1-x N and Al x Ga 1-x GaP can contain N, and the alloy In x Ga 1-x P and Al x Ga 1-x It can contain P or more complex alloys of Al, In, Ga, N, and P. The p-type doping level at the emitting tip, and particularly at the apex of the emitting tip, is designed to bring the work function of the semiconductor material close enough to the vacuum level to allow, in conjunction with the tip effect, the emission of an electron upon absorption of a photon.

[0030] In another embodiment, the body of the device 202 is doped n-type, while the base region 206 of the emitting tip is doped n-type, and only the apex region 208 of the emitting tip is doped p-type. In effect, a p-n junction is formed perpendicular to the axis of symmetry 204 at a location between the base and apex of the emitting tip. This structure allows the n-type region to act as a source of electrons for the p-type region, allowing electrons to be photo-extracted from the emitter without losing charge. Typically, electron sources such as metal tips suffer from spurious emissions that reduce the intensity of the electron beam. Here, doping the body n-type suppresses spurious emissions when the shaft is illuminated with a laser beam. Therefore, the emission is concentrated at the p-type apex, maintaining its intensity.

[0031] In either embodiment, the body 202 and the emitting tip 205 are integrally formed as a monolithic structure by growth or by etching the emitting tip 205. For example, if the emitter is constructed from GaN, the starting point can be an n-type doped substrate, and magnesium (Mg) dopants are incorporated during crystal growth to form a p-type apex region 208 of the emitting tip. In another example, the entire emitter can be made of p-type diamond, and the body 202 can be coated with a non-emissive coating 209 to avoid parasitic emission and force emission only from the apex of the tip. Conversely, the body can be formed from one semiconductor material and the tip from a different semiconductor material; in this case, the body can be p-type or n-type doped and the tip can be p-type doped. For example, the body can be made of an n-type semiconductor and the tip can be p-type diamond.

[0032] According to another embodiment, the emitter is made of doped diamond. Diamond exhibits high thermal conductivity, hole mobility, and breakdown field, making it advantageous for emitter fabrication. Diamond emitters can be fabricated using high-pressure, high-temperature (HPHT) and chemical vapor deposition (CVD) processes, using phosphorus dopants to form n-type doping and incorporating boron dopants during growth to achieve p-type conductivity. Unlike other embodiments in which GaN is used, diamond can be used as a p-type substrate, eliminating the need for a p-n junction in the device design. That is, the entire emitter 200 can be fabricated from a p-type diamond substrate without n-type doping. Incidentally, diamond emitting tips can be annealed in ultra-high vacuum (UHV) conditions up to 900°C to remove oxygen and other contaminants.

[0033] In either embodiment, emission generally occurs from the area illuminated by the laser beam. As a result, the intensity and resolution of the electron beam are affected by the size of the laser beam. Therefore, by making the emitter 200 from an n-type doped material and p-doping only the apex, emission is limited to the apex region because the n-type material will not emit electrons in response to laser irradiation, even if the laser beam illuminates a portion of the emitter outside the apex that is larger than the apex. Therefore, an emitter 200 with only the apex doped p-type can further increase the intensity and resolution of the emitted electron beam.

[0034] The body 202 is formed with a back contact 203 that allows connection to a base voltage V0. The base voltage V0 serves as a "virtual ground" or reference potential for all other voltage measurements. This virtual ground voltage may be different from earth / instrument chassis ground. The suppressor lens 220 is connected to a negative Vsupp, and the extractor lens 230 is connected to a positive Vext. These lenses are provided by way of example; other lens arrangements may be used, such as existing lens arrangements used in filament-type emitters. The lenses are configured to create an electric field near the apex of the emitting tip to induce a lower electron affinity. In the disclosed embodiment, the apex is p-doped to obtain a conduction band very close to the vacuum level, and tuning of the electron affinity can be achieved by appropriate adjustment of the structure, geometry, and doping levels of the n-type and p-type regions within the electron emitter. Also, in the disclosed embodiments, the apex is shaped to a diameter of less than 10 microns to reach local electric fields of greater than 10 MV / m to induce electron affinities of, for example, 1.5 eV or less, 1.0 eV or less, or even less than 0.0 eV, which latter case can be referred to as a field-induced NEA.

[0035] In the embodiment shown in Figure 2, a laser source 211 is used to emit a laser beam 210 with a photon current Ihv toward the tip apex. Each time a photon stream from the laser beam 210 strikes the apex 208, the interaction of the photons with the material causes electron emission. Therefore, electron emission can be controlled by controlling the laser beam 210. Because the laser beam can be pulsed at various speeds and periods, laser control can be used to turn emission on and off very sharply and precisely, thereby eliminating the need for an electron beam blanking mechanism. In fact, using on / off control of the laser beam allows for blanking speeds not possible with conventional electron beam blanking devices, and the transition between the on and off states is also abrupt, which is not possible with conventional beam blanking. Furthermore, eliminating the need for a beam blanking device allows for the entire electron column to be smaller, less expensive, more reliable, and more accurate.

[0036] Figure 2A shows an embodiment similar to that of Figure 2, except that the entire emitter 200 is made from p-type semiconductor material with no n-type or p-n junctions. Contacts are also formed in a ring around the sidewall of the body 202, leaving the backside 201 of the body transparent and exposed to backside laser illumination. The callout in Figure 2A is an SEM micrograph of a polyhedron tip made of GaN with a base radius of 20 microns.

[0037] The disclosed embodiments utilize various effects that enable highly controlled electron emission from electron emitters. These effects include the use of doped semiconductor materials to control the work function, where p-type doping effectively lowers the work function to near vacuum levels, but high enough to avoid spontaneous and spurious emission. The disclosed embodiments also utilize the tip effect to amplify the applied electric field, thereby increasing electron emission efficiency. The tip effect on the applied electric field enhances the electric field at the tip, reducing electron affinity. The amount of doping and the sharpness of the tip geometry are designed to lower the work function and enhance the applied electric field, so that application of photon energy generates a large current of electron emission, several orders of magnitude greater than that achievable with filament emitters.

[0038] The foregoing provides a tapered emitting tip 205 having a base surface 206 and an apex 208 opposite the base surface, the emitting tip 205 consisting essentially of a semiconductor material. In all embodiments, the apex is doped p-type, and in some embodiments, the entire emitting tip 205 is doped p-type. Alternatively, the semiconductor material can be partially doped n-type and partially doped p-type, with the base surface doped n-type and the apex doped p-type, thereby forming a p-n junction 207 between the base surface and the apex. The p-n junction 207 defines a plane passing perpendicular to the axis of symmetry 204 of the emitting tip, the axis of symmetry passing through the center points of the apex and the base.

[0039] The electron emitter may further include a body 202 formed of a doped semiconductor that serves as an electron source for the emitting tip, the emitting tip extending from one surface of the body. An ohmic contact 203 is formed on the sidewall of the body or on the second surface of the body opposite the first surface. The semiconductor material is selected from doped diamond, gallium nitride (GaN) that may be alloyed with aluminum (Al) and / or indium (In), silicon carbide (SiC) containing 4H or 6H allotropes, or gallium phosphide (GaP) that may be alloyed with Al or In. In this embodiment, the base surface is doped n-type, and the tip is doped p-type. In this embodiment, the tip is made of GaN doped with magnesium (Mg), while in another embodiment, the tip is made of diamond doped with boron to form a p-type. The emitting tip in this embodiment is conical, while in other embodiments the emitting tip is pyramidal, with the definition of pyramidal being extended from four to any number of sides, for example up to six for GaN-based systems.

[0040] FIG. 2B illustrates another embodiment in which the emitting tip 205 is attached to a filament 240, such as a tungsten filament. The emitting tip 205 may be constructed from a semiconductor material according to any of the embodiments disclosed herein. This embodiment is particularly useful as a drop-in solution for systems designed for filament emitters. The mounting base 250 is an insulator, such as a ceramic insulator, and has a connector 255 on one side and a contact pole 245 on the other side. Electrical connection between the connector 255 and the contact pole 245 is made through the mounting base 250. The filament 240 is attached to the contact pole 245 and can receive electrical current from it. The electrical current heats the filament and can transfer thermal energy to the emitting tip 205. Because the energy applied to the emitting tip is generally below the spontaneous emission level, emission occurs only when a photon from the laser beam 210 strikes the apex of the emitting tip 205. Advantageously, an electric field is applied to the emitting tip via an electrostatic lens, such as a suppressor lens 220 or an extractor lens 230 (see FIG. 2).

[0041] In this embodiment, two-photon lithography is used to fabricate one or more emitters on a single substrate. In this method, the substrate is coated with two-photon resist (TPR) and the objective lens of the lithography system is immersed in the TPR. In this sense, TPR is used as a fabrication material and a resolution-enhancing immersion medium because its refractive index is similar to immersion oil, around 1.5. The objective lens contacts the TPR and focuses a laser beam within the TPR without passing air. At each exposure point of the beam, a thin, parabolic volume, called a voxel, is polymerized. By scanning the laser focus in the horizontal plane and moving the stage away from the objective lens, structures can be fabricated layer by layer from the liquid resist volume, similar to the manufacturing techniques of stereolithography and 3D printing.

[0042] While the developed TPR is sufficient as an etching mask, due to the high aspect ratio required for fabricating the emitting tips, a two-mask approach is also used in this embodiment: a TPR mask followed by a hard mask, such as a SiO2 mask. Figures 3A-3D illustrate the process for forming an emitting tip according to one embodiment. In Figures 3A-3D, the dashed-dotted lines indicate the top surface of the layers prior to the process steps shown in the figures. In this embodiment, three emitting tips are formed simultaneously, but the same process can also be used to form one or hundreds of tips simultaneously.

[0043] In FIG. 3A, a semiconductor substrate 300, e.g., GaN, is first coated with a hard mask layer 305, e.g., a layer of SiO2, and then a TPR layer 310 is formed on the hard mask layer 305. In FIG. 3B, the TPR layer is exposed using a laser beam and developed to form a TPR mask structure 315. The TPR mask structure 315 is used as a mask to etch a hard mask structure 320 in the hard mask layer, as shown in FIG. 3C. In FIG. 3D, the hard mask structure 320 is used to etch an emitting tip 325 in the substrate 300. As shown in FIG. 3D, if the upper region of the substrate is already p-doped, the thickness of the hard mask 305 is designed so that some of the hard mask structure 320 remains upon completion of etching of the emitting tip 325. This ensures protection of the p-type apex. Conversely, etching can be performed to generate a sharp apex without consuming the entire hard mask structure 320, and the apex can then be doped p-type, for example, by a diffusion or implant process. The etching step of Figure 3D can also be performed by reactive ion etching using appropriate chemistries, e.g., Cl2 / Ar chemistry for GaN substrates, oxygen plasma for diamond substrates, fluorine chemistry for SiC substrates, and Cl2 / Ar chemistry for GaP substrates. Any of these substrates can be used with a SiO2 hard mask and etched using a fluorine chemistry, e.g., C4F6.

[0044] The present disclosure provides a method for fabricating an electron emitter, which includes obtaining an n-type or p-type doped semiconductor substrate, forming an emitting tip on one surface of the substrate, doping the top region of the emitting tip n-type or p-type opposite to the doping of the substrate, and forming an ohmic contact on the surface opposite the one surface of the substrate. In this method, the amount of dopant and the shape of the tip are designed to generate a local electric field at the tip of greater than 10 MV / m (megavolts per meter). In this method, the amount of dopant and the shape of the tip are engineered to lower the bandgap below spontaneous emission but still allow emission to occur upon absorption of photons from a laser source. In this embodiment, fabrication begins with a doped semiconductor substrate, and a tip is grown on one surface of the electron emitter, for example, using known epitaxial methods. During the growth of the emitting tip, the dopant is switched from one of the n-type and p-type dopants to the other of the n-type and p-type dopants. The substrate is then cut to form the electron emitter bodies, and contacts are formed on the surface of the body opposite the emitting tip, for example by a sputtering process, although the contacts may be formed before cutting the substrate.

[0045] In another embodiment, fabrication begins with a doped semiconductor substrate, and a tip is etched into one surface of the electron emitter, for example, using reactive ion etching. The apex of the tip is then doped with a dopant of opposite polarity to that of the substrate, for example, using diffusion or ion implantation. Alternatively, one surface of the substrate is first doped with a dopant of opposite polarity to that of the original dopant present in the substrate before processing, and the emitting tip is etched to a depth beyond the junction between the original and opposite dopants. The substrate is then cut to form the body of the electron emitter, and a contact is formed on the surface of the body opposite the emitting tip, for example, by a sputtering process. In this embodiment, the etching process is performed by forming a layer of hard mask material, such as silicon dioxide, on the substrate and then forming a layer of two-photon photoresist on the hard mask layer. The two-photon photoresist is exposed using an immersion or other lithography process, and then the two-photon photoresist is developed to form a photoresist mask. The hard mask layer is etched through the photoresist mask to form a patterned hard mask, and then the substrate is etched through the hard mask to form at least one emission tip on the mask surface of the substrate. The etching of the substrate is stopped before the entire patterned hard mask material is consumed. In this embodiment, the etching of the substrate is performed to simultaneously form multiple emission tips on the substrate.

[0046] FIG. 4 shows a cross-sectional schematic of the lower portion of a microscope according to this embodiment. As shown in FIG. 4, the microscope generally includes an electron column 41 housed within a vacuum vessel 10 and an imager 42 in an atmospheric environment. The particular integrated microscope shown in FIG. 4 is capable of generating electron beam images, light beam images, cathodoluminescence (CL) images, and CL spectroscopy images. However, the electron beam forming and scanning sections shown in FIG. 4 may or may not include light optics, and any other electron microscope can be used. With the microscope shown, the imaged CL emission correlates to the structure and quality of the sample's material at the nanoscale. CL data can reveal material stress, impurities, crystallographic defects, and subsurface defects that are invisible in other imaging modes. Importantly, CL imaging allows for nondestructive sample inspection.

[0047] The electron column includes an electron emitter 1, such as a semiconductor electron source disclosed herein, which emits electrons when irradiated with a laser beam (shown here as pulse 2). The emitted electrons are converted into an electron beam 9 by various particle-optical elements, such as an electromagnetic lens 5', an electromagnetic objective lens 5, and an aperture disk (also called an aperture) 6. Note that the aperture disk 6 can function as an electrostatic lens when a potential is applied, and can also function as a suppressor lens or an extractor lens. In FIG. 4, pulse 2 indicates that electron emission from the semiconductor electron source 1 occurs in pulses, but this is not necessarily the case. For example, emission can occur continuously due to application of a continuous laser beam or increasing application of an electric field or heat.

[0048] The purpose of the magnetic field from lens 5 is to generate a converging electron beam 9 that focuses on the surface of sample 7. In this example, electron beam 9 generated by electron emitter 1 propagates downward from the top of the figure. The width of the electron beam can be changed by a condenser device such as lens 5', making it possible to diverge, collimate, or converge.

[0049] The electromagnetic objective lens 5 can be configured to be rotationally symmetrical along its optical axis, which essentially coincides with the path of the electron beam 9. The electromagnetic objective lens 5 is designed to focus the electron beam emitted from the electron source 1 onto a focal plane. The position of the focal plane, more specifically its height above the sample, can be adjusted by changing the strength of the magnetic field passing through the electromagnetic objective lens 5, but the electromagnetic objective lens 5 is optimized so that the probe size is smallest when the focal plane is positioned approximately 5 mm below the center of the electromagnetic objective lens 5.

[0050] The lens 5 has a hollow structure along its optical axis, allowing the electron beam 9 to pass through. In the illustrated embodiment, the hollow (passageway or gap) is wide enough to allow light emitted from and reflected by the sample 7 to pass through without significant obstruction. However, this is not necessarily the case in electron microscopes that do not produce optical images. To maintain good electron-optical performance, it is preferable to keep the output aperture 13 of the electromagnetic objective lens 5 as small as possible, and therefore it is preferable to build the system so that the working distance is small.

[0051] As shown in the figure, an optical reflective objective is provided within the electromagnetic objective 5 to optically image the surface of the sample 7. In this example, a Schwarzschild reflective objective is used. A Schwarzschild reflective objective is a two-mirror reflective objective that is rotationally symmetric about the optical axis z, aberration-free, and infinity-corrected. In the context of geometric optics, an objective is said to be infinity-corrected if all light rays entering the objective parallel to the optical axis converge to the same focal point to form a diffraction-limited spot, or conversely, if all light rays emanating from the focal point and passing through the objective form a bundle of rays parallel to the optical axis, i.e., a collimated output beam. The electromagnetic objective 5 and the reflective objective may have the same focal plane.

[0052] As shown in FIG. 4, the reflective objective lens in the electromagnetic objective lens 5 includes a first mirror M1 (also called the primary mirror), which in this example is spherically concave, and a second mirror M2 (also called the secondary mirror), which in this example is spherically convex. The diameter of the first mirror M1 is larger than the diameter of the second mirror M2. The first mirror M1 is positioned above the second mirror M2 and is arranged to reflect light emitted from the sample 7 when the electron beam 9 strikes the surface of the sample 7 and direct the light toward the second mirror M2, which is arranged between the sample and the first mirror M1. The second mirror M2 is arranged to redirect the light along the optical axis of the electromagnetic objective lens, and a third mirror M3, which is planar in this example, is arranged to redirect the light beam 4 toward the output. In this example, the third mirror M3 is at a 45-degree angle with respect to the axis of the electron beam 9 and is used to direct the light out of the vacuum vessel 10. All three mirrors M1, M2, and M3 have openings or apertures along the electron beam path to prevent the electron beam from being obstructed.

[0053] The microscope shown in FIG. 4 includes a first electron beam deflector 17 (referred to as the first deflector element or simply as the first deflector 17) and a second electron beam deflector (referred to as the second deflector element or simply as the second deflector 15). The first deflector 17 is at least partially disposed within the opening of the second mirror M2, and the second deflector 15 is at least partially disposed within the opening of the first mirror M1. In other words, the first deflector 17 is disposed radially inward from the opening of the second mirror M2 and at least partially coincides with the axial direction of the opening. Meanwhile, the second deflector 15 is disposed radially inward from the opening of the first mirror M1 and at least partially coincides with the axial direction of the opening. The deflectors 15 and 17 are positioned so as not to interfere with the propagation of the electron beam 9 or the reflected light beam.

[0054] Light reflected by mirror M3 is focused by lens 22 onto imaging monochromator 43. In this example, two imaging elements are provided: a CCD camera 45 and a detector 46, such as an InGaAs detector or a PMT detector. If mirror 24 is a half mirror, both imaging elements can be operated simultaneously. Conversely, if mirror 24 is a flip mirror, only one imaging element can be operated at a time. With this configuration, detector 46 can be used to detect the light intensity of a specific wavelength, and the CCD camera can be used to simultaneously detect the light intensity of multiple wavelengths.

[0055] To generate a light image of the sample, light source 26 is operated to generate a light beam that is reflected by flip mirror 27 to lens 22 and then directed by mirror M3 to mirror M2 and mirror M1 onto the sample. This reflected light retraces the path to the CCD camera. In this mode of operation, a mirror arrangement comprising three mirrors M1, M2, and M3 is used to direct light from light source 26 to sample 7 and to collect and direct light reflected from sample 7 onto CCD detector 45.

[0056] In the embodiment of Figure 4, an electron detector 19 is provided for detecting secondary electrons emitted from the sample or backscattered electrons reflected by the sample. The detector signal can be used to generate a scanning electron microscope (SEM) image. Also in the embodiment of Figure 4, the sample holder 47 can be configured as a cryogenic stage to keep the sample cool and avoid noisy optical emissions.

[0057] According to the above disclosure, there is provided an electron-based microscope comprising: a sample holder (e.g., a stage); an electron emitter formed in a tip shape with an apex made of a semiconductor material and doped p-type; a laser source that irradiates the apex with a laser beam to cause electrons to be emitted from the electron emitter; an electron optical system configured to form an electron beam from the electrons emitted from the electron emitter and irradiate the electron beam onto a sample; and a sensor arranged to detect emissions from the sample.

[0058] As semiconductor design rules approach single-digit nanometer sizes, i.e., 10-nm nodes and below, alternatives to lithography are needed. One promising technology is electron-beam direct write, more specifically, multiple parallel electron-beam direct write. This technique uses multiple electron beams in parallel to write circuit designs directly into resist, thereby bypassing the entire mask-making process. Several proposals for direct-write tools using multiple electron beams have been submitted. For example, Esashi et al. proposed a massively parallel electron-beam direct-write system using an active-matrix nanocrystalline silicon electron-emitter array. See Microsystems & Nanoengineering (2015) 1, 15029; doi:10.1038 / micronano.2015.29. However, no systems using doped semiconductors with tip effects have been proposed. Furthermore, beam on / off control (blanking) remains a complex problem in such miniaturized systems.

[0059] FIG. 5 illustrates a direct-write device according to one embodiment. A substrate 500, e.g., a semiconductor substrate, has multiple electron-emitting tips 505 formed on one side (front side) thereof. The semiconductor substrate 500 is doped either n-type or p-type and may be made of, for example, doped silicon, doped diamond, gallium nitride (GaN), silicon carbide (SiC) containing 4H or 6H allotropes, or gallium phosphide (GaP). Each electron-emitting tip 505 is formed according to any of the embodiments disclosed herein and may be partially n-type doped and partially p-type doped, with the apex of each electron-emitting tip 505 being p-type doped. In this embodiment, ohmic contacts 503 are formed on the surface of the substrate 500 opposite the front side on which the electron emitters are provided, or alternatively, are patterned around the electron emitters. An electrostatic lens layer 507 is disposed proximate to the multiple electron emitters and may include multiple regulating lenses. Each of the conditioning lenses may include a suppressor lens, an extractor lens, an aperture lens, etc. The number of conditioning lenses is equal to the number of electron emitters, and each conditioning lens applies an electric field to a corresponding one of the electron emitters.

[0060] A laser source 511 is used to generate a laser beam 510, which can be split into multiple laser beams by an optional beam splitter 512. An optional laser beam scanner 513 (e.g., an electro-optic laser beam scanner or an acousto-optic laser beam scanner) can also be included in the optical path to direct each laser beam to a different electron-emitting tip 505 from the side or rear of each electron-emitting tip 505. As a result, controlling the laser beam(s) effectively functions as an electron beam blocker. Finally, an optional beam forming and scanning layer 517 is provided to focus the multiple electron beams and scan them over the surface of a workpiece 501 positioned on an XYZ stage 550. The scanning layer 517 can include magnetic lenses, electrostatic lenses, or both electrostatic and magnetic lenses.

[0061] FIG. 5A illustrates an embodiment of a multi-beam electron source that can be used in applications such as inspection microscopy and direct writing. Much of the structure of this embodiment is the same as that shown in FIG. 5 and will not be repeated. However, in this embodiment, the laser beam illuminates the electron-emitting tip 505 from the backside of the substrate 500. Therefore, contacts 503 are provided on the sidewall of the substrate. Conversely, contacts can be provided on the front or backside of the substrate 500, allowing the laser to reach the emitter array while still ensuring efficient electrical contact. In this case, patterned contacts are required. Furthermore, when the multi-beam electron source is used in microscopy applications, an optional sensor layer 518 can be provided to detect secondary and backscattered electrons emitted from the workpiece 501.

[0062] It should be understood that the processes and techniques described herein are not inherently related to any particular apparatus and may be implemented by any suitable combination of components. Moreover, various types of general-purpose apparatus may be used in accordance with the teachings described herein. While the present invention has been described with reference to specific examples, these examples are intended in all respects to be illustrative and not restrictive. Those skilled in the art will recognize that various combinations will be suitable for practicing the present invention.

[0063] Furthermore, other embodiments of the present invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. Various aspects and / or components of the described embodiments may be used alone or in any combination. It is intended that the specification and examples be considered as exemplary only, with a true scope and spirit of the invention being indicated by the following claims.

Claims

1. a tapered discharge tip having a base surface and an apex opposite the base surface; said emitting tip consisting essentially of a semiconductor material; the semiconductor material is doped p-type at least at the apex; The doped p-type level, in combination with the tapered emitting tip geometry, brings the work function of the semiconductor material close enough to a vacuum level to allow electrons to be emitted upon absorption of a photon, an electron emitter.

2. 2. The electron emitter of claim 1, wherein said base surface is n-type doped.

3. 2. The electron emitter of claim 1, further comprising a body formed of a doped semiconductor of the same type as the doping of said base surface, said emitting tip extending from one surface of said body.

4. 4. The electron emitter of claim 3, further comprising an ohmic contact formed on a second surface of said body opposite said one surface.

5. 5. The electron emitter of claim 4, wherein said body and said emitting tip are formed from a monolithic semiconductor material.

6. 5. The electron emitter of claim 4, wherein said body and said emitting tip are formed from p-type doped semiconductor material.

7. 2. The electron emitter of claim 1, wherein the semiconductor material is selected from materials having an electron affinity less than 1.5 eV across the bandgap.

8. 2. The electron emitter of claim 1, wherein the semiconductor material is selected from any of doped diamond, gallium nitride (GaN), silicon carbide (SiC) containing 4H or 6H allotropes, or gallium phosphide (GaP).

9. 2. The electron emitter of claim 1, wherein said base surface is n-type doped and said apex is p-type doped.

10. 10. The electron emitter of claim 1, wherein said emitting tip is made of GaN and said apex is doped with magnesium (Mg).

11. 10. The electron emitter of claim 1, wherein said emitting tip is made from p-type diamond doped with boron.

12. 2. The electron emitter of claim 1, wherein the emitting tip has either a conical or pyramidal shape with 4 to n faces, where n is less than 100.

13. 2. The electron emitter of claim 1, wherein the apex has a shape that allows a local electric field of greater than 10 MV / m to be reached.

14. 2. The electron emitter of claim 1, wherein the apex has a radius of less than 10 microns.

15. an electron emitter; A suppressor lens, and an extract lens, 1. An electron source comprising: an electron emitter having a tapered emitting tip having a base surface and an apex opposite the base surface, the emitting tip consisting essentially of a semiconductor material, the semiconductor material being p-type doped at the apex, and the base surface being doped either n-type or p-type.

16. 16. The electron source of claim 15, further comprising a laser source positioned to focus a laser beam at the apex of the emitting tip.

17. 17. The electron source of claim 16, wherein the laser source operates at a wavelength of 300 to 450 nm.

18. 16. The electron source of claim 15, wherein the apex has a shape that allows reaching a local electric field of more than 10 MV / m at an extraction voltage of 1 to 20 kV.

19. 20. The electron source of claim 18, wherein the apex has a radius of less than 10 micrometers.

20. 16. The electron source of claim 15, further comprising a body formed of a doped semiconductor having a polarity similar to that of the base surface, the emitting tip extending from a surface of the body.

21. a substrate made of a semiconductor material, doped with an n-type or p-type dopant, and having a sidewall, a first surface, and a second surface; a plurality of tapered emission tips formed on the first surface of the substrate, each having a base attached to the first surface and an apex, each doped p-type at the apex; and an ohmic contact formed on the sidewall or the second surface of the substrate.

22. 22. The apparatus of claim 21 , further comprising an electrostatic lens layer disposed proximate to the plurality of emitting tips, the electrostatic lens layer including a plurality of conditioning lenses, each of the conditioning lenses including a suppressor lens and an extractor lens.

23. 23. The apparatus of claim 22, further comprising a laser source that generates a laser beam that illuminates the plurality of emitting tips.

24. 24. The apparatus of claim 23, further comprising at least one of a beam splitter that splits the laser beam into multiple laser beams and an optical scanner that scans the laser beam or the multiple laser beams.

Images