Optical modulation electron source

Abstract

An optical modulation electron source modulates the emission current of an electron beam emitted from a silicon-based field emitter using a photon beam source. The cathode of the field emitter includes a protrusion fabricated on a silicon substrate and having an emission tip covered by a coating. An extractor generates an electric field that attracts free electrons toward the emission tip for emission as part of the electron beam. The photon beam source generates a photon beam including photons having an energy greater than the energy band gap of silicon and includes optics that direct the photon beam onto the emission tip, where each absorbed photon generates a photoelectron that combines with the free electrons to enhance the emission current of the electron beam. A controller modulates the emission current by controlling the intensity of the photon beam applied to the emission tip. A monitor measures the electron beam and provides feedback to the controller.

Description

[0001] Related Applications

[0002] This application claims priority to U.S. Provisional Patent Application No. 62 / 906,095, filed September 26, 2019, and entitled “ELECTRON SOURCE,” which is incorporated herein by reference. TECHNICAL FIELD

[0003] The present disclosure relates generally to systems / devices that utilize electron sources (e.g., scanning electron microscopes, electron beam lithography systems, and x-ray sources). In particular, the present disclosure relates to electron sources suitable for use in various semiconductor inspection, metrology, and review systems suitable for reviewing and / or inspecting photomasks, reticles, and semiconductor wafers. BACKGROUND

[0004] The integrated circuit industry requires inspection tools with ever-higher sensitivity to detect ever-smaller defects and particles, which can be tens of nanometers (nm) or smaller in size. These inspection tools must operate at high speed to inspect a large fraction or even 100% of the area of a photomask, reticle, or wafer in a short period of time. For example, the inspection time can be hours or less for inspection during production, or up to several hours for R&D or troubleshooting. To inspect so quickly, the inspection tools use a pixel or spot size that is larger than the size of the defect or particle of interest, and only detect small signal changes caused by the defect or particle. High-speed inspection is most commonly performed using inspection tools that operate with UV light, which are most commonly used in production. Inspection in R&D can be performed with UV light or with electrons.

[0005] Once a defect or particle has been found by high-speed inspection, it is usually necessary to make a higher-resolution image and / or perform material analysis to determine the origin or type of the particle or defect. This process is commonly referred to as review. Review is commonly performed with a scanning electron microscope (SEM). Review SEMs used in semiconductor manufacturing typically need to review thousands of potential defects or particles per day, and each target can have at most a few seconds for review.

[0006] Electron microscopes, including inspection SEMs, utilize an electron source (also referred to as an electron emitter or electron gun) to generate an accelerated electron beam for illuminating a target specimen, thereby facilitating inspection of potential defects on the surface of the target specimen. Electron sources can be divided into two broad groups: thermionic sources that emit electrons that are thermally (high temperature) emitted and field emission sources that utilize high voltage to emit electrons. Thermionic sources are the most commonly used commercial electron emitters and are typically made of tungsten or lanthanum hexaboride (LaB6). In thermionic emission, electrons boil off the surface of a material when the electron thermal energy is high enough to overcome the surface potential barrier. Thermionic emitters typically require high temperatures (>1300 K for LaB6 and >2500 K for tungsten) to operate and have several drawbacks, such as inefficient power consumption, wide energy spread, short lifetime, low current density, and limited brightness. The need for more efficient electron sources has driven the research and development of Schottky emitters and cold electron sources, such as electron field emitters.

[0007] In Schottky emitters, thermionic emission is enhanced by a reduction in the effective potential barrier due to the image charge effect under an applied external electric field. Schottky emitters are typically made of tungsten wire with a tip coated with a layer of zirconium oxide (ZrOX), which exhibits a much lower work function (-2.9 eV). Thermally assisted Schottky emitters require operation at high temperatures (>1000 °K) and high vacuum (-10 -9 millibar) and have a wider electron emission energy spread than desired due to the high operating temperature. Semiconductor wafer and mask inspection, review, and lithography desire electron sources with lower energy spread, higher brightness (luminance), and higher current density compared to Schottky emitters because it would enable faster and more cost-effective inspection, review, and lithography.

[0008] Cold electron sources, particularly electron field emitters, have been used in field emission displays, gas ionizers, x-ray sources, electron beam lithography and electron microscopes, among other applications. Cold electron sources utilize a field emitter cathode that is not electrically heated by a filament (i.e., the cathode emits more electrons than can be supplied by thermionic emission alone), but does not necessarily operate at cryogenic temperatures; it is typically heated to an operating temperature above room temperature by the electron current of the field emission (i.e., the electrons emitted from the cathode). A typical field emitter is composed of an emitter cathode having a conical-shaped tip and a circular gate aperture (extractor) disposed in a low pressure (vacuum) environment. Under an applied external field, a potential difference is established across the emitter cathode and the gate, resulting in a high electric field at the surface of the tip. Field emission occurs when the applied electric field is high enough to lower the potential barrier on the tip-vacuum interface, such that electrons can tunnel this barrier (i.e., quantum mechanical tunneling) at an operating temperature near room temperature. Electrons emitted from the tip in this manner travel toward a positive field source (e.g., an anode) that is biased at a more positive potential than the gate. The emission current density can be estimated by a modified version of the Fowler-Nordheim theory, which takes into account a field enhancement factor due to the field emitter.

[0009] Field emitters have lower energy spread than Schottky and thermionic emitters because they can operate near room temperature, and can have higher brightness and electron current than thermionic emitters. However, in practical use, the output current of field emitters is less stable because contaminants can easily adhere to the tip of the emitter and change (increase or decrease) its work function, which will change (decrease or increase) the brightness and current and also change the shape of the area from which electrons are emitted. Periodic flashing (i.e., temporarily raising the tip temperature) is needed to remove the contaminants. During the flashing process, the instrument is not available for operation, which requires a considerable amount of time (i.e., tens of seconds to minutes) to heat the tip and then allow it to cool and stabilize. In the semiconductor industry, instruments are required to operate continuously and stably for a long time without interruption, so Schottky emitters are generally preferred over cold field emitters.

[0010] Early efforts have been directed to developing metal field emitters. Spindt-type molybdenum field emitters are particularly well known as metal field emitters because molybdenum has a low resistivity (53.4 nΩ-m at 20°C) and a high melting point (2896 K). However, metal emitters suffer from several drawbacks, such as lack of uniformity due to metal deposition techniques, and more seriously, emission current degradation primarily due to oxidation.

[0011] With the advent of modern semiconductor manufacturing technology, research has been conducted on semiconductor field emitters, especially silicon field emitters, because silicon field emitters have emerged as a replacement for metal nanoparticle tips (see, for example, PD Keathley, A. Sell, WP Putnam, S. Guerrera, L. Velásquez-García, and FX Katner). (See "Strong-field photoemission from silicon field emitter arrays" in Ann. Phys. 525, pp. 144–150, 2013). Silicon has practical advantages in its use for fabricating large-scale field emitter structures. Single-crystal silicon is an attractive material for field emitters. Silicon crystals can be grown with extremely high purity and very few crystal defects. The conductivity of silicon can be altered by doping and / or applying voltage. More importantly, silicon has a well-developed technological foundation (i.e., silicon field emitters can be fabricated using standard CMOS manufacturing techniques).

[0012] Even though silicon field emitters have shown promise in recent years, they are not yet commercially available. A serious problem with using silicon to form field emitters is that silicon is quite reactive, even at around 10... -10 Even under millibar pressure, contamination can occur within hours. Silicon readily forms native oxides on its surface. Even in a vacuum, native oxides will eventually form due to the small amount of oxygen present and the reaction of water with the silicon surface. The interface between silicon and silica has defects (attributed to dangling bonds), where electron recombination is highly probable. Furthermore, silica has a large band gap (approximately 9 eV), creating an additional barrier above the work function that electrons must overcome (even if the oxide is extremely thin). For example, native oxide on a smooth silicon surface is typically about 2 nm thick. In some cases, oxidation can also alter the shape of the field emitter. These aforementioned problems lead to low brightness and current, poor emission stability, lack of reliability, scalability, and uniformity, and have hindered the commercial use of silicon field emitters.

[0013] Research efforts have extended to finding surface treatments and coatings for field emitters to improve their performance for lower turn-on voltage, higher emission current density, lower noise, and improved stability. These treatments can include coating the emitter tips with refractory metals, silicides, carbides, and diamond, among others. However, these coating materials are often limited by the manufacturing process in forming a smooth and uniform coating surface, and / or are often affected by an oxide layer formed on the coating surface, creating an additional energy barrier. For these reasons, coated silicon field emitters have not yet become practical as cold electron sources.

[0014] Recently, photon-assisted electron emission has been investigated. Photon energy from a light source, such as a lamp and a laser, can further enhance electron emission, which can result in high beam current, high brightness, time-constant intensity, and low FWHM (full width at half maximum) of the electron kinetic energy distribution in the beam. In addition, modulation of the light beam can be used to modulate the resulting electron beam. To date, most research has focused on laser-driven emission from metal nanotips, such as gold or tungsten (see, e.g., M. R. Bionta, S. J. Weber, I. Blum, J. Mauchain, B. Chatel, and B. Chalopin, “Wavelength and shape dependent strong-field photoemission from silver nanotips,” New J. Phys. 18, 103010, 2016).

[0015] Accordingly, there is a need for an electron source that overcomes some or all of the limitations of the prior art. In particular, there is a need to provide the promising aspects of silicon-based field emitters (i.e., small emitter size, low power consumption, high brightness, high current density, high speed, and long emitter lifetime), while avoiding at least some of the negative aspects that have previously prevented widespread commercial use of silicon-based field emitter structures. SUMMARY

[0016] The present invention relates to an optical modulation electron source that utilizes a beam of photons to control the emission current of an electron beam emitted from a field emitter having a silicon type field emitter cathode. The field emitter cathode includes a p-type doped or n-type doped silicon substrate that is etched or otherwise treated on an output (first) surface to provide an integrated protrusion having an emission tip. The field emitter also includes at least one electrode (e.g., zero or more than zero of an extractor and suppressor, a gate / control electrode, and a focusing electrode) disposed adjacent to the field emitter cathode and including an aperture. In some embodiments, the extractor is positioned at a height that is within about ±300 nm of the height of the emission tip, and is maintained at a positive voltage between about 30 V and about 200 V relative to the field emitter cathode, where electrons in the doped silicon substrate are attracted toward the emission tip by an applied electric field generated between the field emitter cathode and the extractor. In some embodiments, one or more additional electrodes are operatively arranged downstream of the emission tip such that emitted electrons (i.e., electrons having sufficient energy to overcome a potential barrier at the emission tip surface / interface by quantum mechanical tunneling) are focused or otherwise formed into an electron beam that passes through an aperture of these additional electrodes. According to aspects of the invention, a beam of photons source (e.g., a laser or other optical source, an optional modulator, and an optional optical system) is configured to generate the beam of photons having an energy that is greater than the bandgap of silicon (i.e., having photons with a wavelength that is shorter than about 1 pm), and to direct the photons onto the field emitter cathode adjacent to (i.e., directly above and / or near) the emission tip, where at least some of the photons are absorbed by associated silicon atoms of the field emitter cathode and thus enhance (i.e., via photon-assisted field emission) the emission current of the electron beam by increasing the number of electrons in the conduction band of the field emitter cathode. That is, the photons absorbed by the silicon generate electron-hole pairs according to the photoelectric effect, thereby creating additional free electrons (i.e., photoelectrons) in the field emitter cathode near the emission tip. These photoelectrons combine with (i.e., are added to) the free electrons generated by the applied electric field, thereby enhancing the emission current of the electron beam by increasing the total number of free electrons in the conduction band of the field emitter (i.e., compared to the number of free electrons that would be present without the beam of photons). According to another aspect of the invention, the electron source also includes a control circuit configured to modulate the amount of emission current of the electron beam generated by the field emitter cathode by controlling (e.g., selectively increasing or decreasing) the intensity of the beam of photons transmitted from the beam of photons source to the field emitter cathode. For example, to increase (or decrease) the emission current, the control circuit controls the beam of photons source to increase (or decrease) the intensity of the beam of photons (e.g., via a corresponding beam of photons source control signal), thereby increasing (or decreasing) the number of electrons emitted from the field emitter cathode.With this arrangement, the present invention provides an electron source that has the beneficial qualities of silicon (i.e., high purity / low defect material, long electron recombination time, and well-established silicon-based fabrication processes), and the attractive features of field emitters (i.e., small emitter size, low power consumption, high brightness, high current, high speed, and long emitter lifetime), while avoiding at least some of the negative aspects that have previously prevented the widespread commercial use of silicon-based field emitter structures.

[0017] In some embodiments, one or more oxidation resistant coatings are disposed on at least a portion of the field emitter cathode, such that the coating completely and continuously covers at least the emission tip, thereby reducing or eliminating problems associated with oxide formation on conventional silicon-based field emitters. In preferred embodiments, problems associated with conventional silicon-based field emitters are further reduced by fabricating the field emitter cathode and the coating using well-established semiconductor fabrication (e.g., standard CMOS) techniques. In one embodiment, the silicon substrate is single-crystal (monocrystalline) silicon that is essentially defect-free, having a thickness in the range of about 10 nm to several hundred μιη. Silicon dioxide or silicon nitride masking material is deposited on the top / output (first) surface by PECVD and then patterned using photolithography. Dry etching (e.g., RIE, ICP, and ECR), wet etching, or a combination of dry and wet etching is then used to form the protrusions, which can take various shapes, such as a circular whisker (cylindrical post with a circular tip), a circular cone, or a pyramid, where in each case the emission tip of the protrusion has a lateral dimension in the range of 1 nm to 50 nm. The protective layer is then formed completely over the emission tip portion of at least the protrusions using standard CMOS deposition techniques. In one embodiment, the coating comprises an oxidation resistant material, such as substantially pure boron, a boride (e.g., lanthanum hexaboride), or a carbide (e.g., silicon carbide, hafnium carbide, or boron carbide), and has a thickness of between 1 nm and 10 nm. In other embodiments, the coating comprises another material, such as boron nitride, titanium nitride, tantalum nitride, or a metal silicide. Fabricating the integrated field emitter protrusions and the coating using standard CMOS processes facilitates reliable formation of the coating such that it continuously covers the emission tip and surrounding portions of the protrusions that can form the output surface of the emitter, thereby enabling the field emitter cathode to circumvent the relatively wide bandgap and low conductivity of silicon dioxide by taking advantage of the field enhancement due to the field emitter comprising a coating to minimize or prevent oxidation of the underlying silicon material.

[0018] In one embodiment, the photon beam source includes a light source (e.g., a laser or other illumination device) configured to generate photons of the photon beam that are formed at wavelengths in the visible or UV wavelength range (i.e., between 250 nm and 700 nm), whereby a large fraction of the photons in the photon beam are absorbed by silicon atoms positioned proximate to the surface of the emission tip (i.e., within about 1 pm thereof), thereby further enhancing the number of electrons available for emission from the field emitter cathode. In some embodiments, the photon beam source also includes a focusing apparatus (e.g., focusing optics including lenses and / or mirrors) and optionally a light modulation device disposed in the path of the photon beam between the light source and the field emitter cathode. The focusing optics (e.g., one or more optical elements such as lenses, mirrors, or combinations thereof) effectively define the path of the photon beam by directing and focusing photons emitted from the light source so that at least some of the photons impinge on the emitter protrusion. In one embodiment, the focusing apparatus includes a curved (e.g., parabolic) mirror configured (i.e., positioned and shaped) to direct the photon beam through an aperture formed in at least one of the electrodes of the field emitter (i.e., so that the photon beam is redirected from the light source to the field emitter cathode substantially along the same path but opposite direction of the electron beam emitted from the field emitter cathode). The light modulation device (e.g., an electro-optical modulator or an acousto-optical modulator) is disposed in the path of the photon beam and is used to modulate the photon beam, e.g., by blocking a portion of the emitted photon flux according to an applied light modulator control signal.

[0019] As mentioned above, the control circuit (controller) is configured to modulate the electron beam emitted from the silicon-type field emitter cathode by controlling the photon beam source to adjust (e.g., increase or decrease) the photon beam intensity according to a selected modulation scheme. In preferred embodiments, the electron source further includes a monitor (i.e., a measurement device or sensor) configured to sense or otherwise measure the emission current of the electron beam at every instant during operation, and generate a corresponding emission current measurement and transmit the value to the control circuit. In one embodiment, the monitor is disposed proximate to the electron beam (i.e., downstream of the field emitter cathode, e.g., proximate to the aperture of a focusing electrode or on an anode). In some modulation schemes, the control circuit utilizes the emission current measurement as a feedback signal to adjust or maintain the electron beam current. For example, when the modulation scheme involves maintaining the electron beam at a particular electron beam current, variations in the emission current measurement can reflect inherent current fluctuations of the field emitter, e.g., due to changes in the work function of the emitter as oxides form on the field emitter surface. When such inherent current fluctuations occur, the control circuit detects the corresponding variations in the emission current measurement, and effects a corrective adjustment to the photon beam intensity via one or more photon beam source control signals. For example, when the emission current measurement indicates that the electron beam current is deviating (increasing or decreasing) from a target level, the control circuit compensates by controlling the light source or light modulation device to effect a corresponding adjustment (decrease and / or increase) in the photon beam intensity until the emission current measurement indicates that the emission current of the electron beam has returned to the target level. By repeating this process, each time a deviation is detected, the control circuit utilizes the photon beam source to maintain the emission current at a substantially constant level (i.e., the target level). Other modulation schemes implemented by the control circuit can include, for example, adjusting the emission current to different values at different times (e.g., periodically switching between a higher current level for high speed scanning and a lower current level for low speed scanning), or gating the electron beam (i.e., turning the electron beam on / off). In the high / low or on / off modulation schemes, by providing a switchable (adjustable) light source that is operably controlled by the control circuit, the electron source facilitates modulation of the emission current of the electron beam so that it keeps pace with the switched photon beam intensity.

[0020] In a particular embodiment, the electron source is configured to implement an on / off modulation scheme, in which modulation of the electron beam involves bi-stable thixotropizing the photon beam source to switch electron emission from the field emitter cathode between a minimum / zero (off) current level and a selected non-zero (on) current level. To achieve zero (or near zero) electron emission when the photon beam is off, the field emitter cathode is fabricated on a p-doped silicon substrate, and the coating is a p-type material (e.g., boron), and the electrode is configured to generate an electric field (i.e., between the field emitter cathode and the electrode / anode) at the emission tip that is maintained just below the field strength at which emission occurs (i.e., the field causes the conduction band of the emitter cathode near the emission tip to be maintained just above the Fermi level of the cathode so as to prevent or minimize electron emission from the field emitter cathode). Specifically, the electric field is generated such that during a first time period, the conduction band near the emission tip of the field emitter cathode is maintained at a voltage level above the Fermi level of the field emitter cathode, thereby minimizing electron emission from the emission tip when the photon beam is off or minimized. In contrast, during a second time period (i.e., when the photon beam source is actuated), at least some photons are absorbed by the associated silicon atoms of the field emitter cathode, thereby generating photoelectrons in the conduction band that are strongly attracted toward the emission tip, with many of these photoelectrons being emitted from the emission tip to produce photon-assisted field emission of the electron beam from the emission tip at a relatively high (second) current. Thus, this embodiment provides a current source that can be rapidly switched between zero (or substantially zero) electron current and non-zero electron current entirely via turning on / off the photon beam source (i.e., without needing to change the electric field applied by the extractor), thereby greatly simplifying the switching process. Of course, the on / off modulation scheme can be further enhanced via utilizing a monitor (as described above) to ensure that the non-zero electron current is maintained at a desired level. In contrast, an electron source implementing the high / low modulation scheme can utilize other configurations (e.g., the cathode silicon is n-doped so that there are abundant electrons available to form an emission current even in the absence of a photon beam) by eliminating the need for zero current emission.

[0021] According to various alternative embodiments of the present disclosure, various additional layers and structures are used to further enhance the advantageous qualities of the emitter structures in the disclosed electronic sources. In some embodiments, the extractor is formed on a dielectric (electrically insulating) layer that is disposed on the top / output surface of the substrate and is patterned to protrude around the field emitter protrusions (i.e., such that the extractor is separated from the top / output surface of the substrate by the dielectric layer). In one embodiment, the dielectric layer is formed to have a thickness that is less than the height of the emission tips above the substrate surface, where the nominal height of the extractor that is subsequently formed can be reliably positioned within the desired ±300 nm range of the emission tip height. In some embodiments, one or several gate layers or control electrodes are formed over the extractor to further control the electric field at the emission tips and enable fast and precise control of the emission current. When several gate layers are implemented, insulating layers are used as spacers between each gate layer. In yet another embodiment, a multi-electron beam source (e.g., a field emitter array, FEA) includes a silicon field emitter cathode having a plurality of field emitter protrusions arranged in a two-dimensional periodic pattern on an output surface of the emitter, where each field emitter protrusion is configured to emit electrons in the presence of an electric field and receive a beam of photons in the manner described above.

[0022] By using field emitters formed on the single-crystalline silicon substrate, a first layer disposed on top of the field emitters, and a beam of photons directed toward the field emitters, the present disclosure provides the beneficial qualities of silicon (i.e., high purity / low defect material, long electron recombination time, and a well-established silicon-based manufacturing process) and enables the attractive features of field emitters (i.e., small emitter size, low power consumption, high brightness, high current, high speed, and long emitter lifetime) while avoiding at least some of the negative aspects that have previously prevented the widespread commercial use of silicon-based field emitter structures. In particular, the stability of the electron emission from such emitters can be improved by using light to control the emission current.

[0023] According to embodiments of the present disclosure, the electron sources disclosed herein are incorporated into inspection, metrology, and review scanning electron microscopes (SEMs). The SEM includes an electron source, electron optics, and a detector. The electron optics can be configured to de-magnify and focus a primary electron beam on a sample and the detector can be configured to detect at least one of backscattered electrons and secondary electrons from the sample. The electron source generates a primary electron beam that is directed toward a sample. The electron optics de-magnify and focus the primary electron beam onto the sample. The electron optics also include a deflector that can scan the primary electron beam across an area of the sample. When the primary electron beam impacts the sample, the sample absorbs many of the electrons from the primary electron beam, but scatters some of the electrons (backscattered electrons). The absorbed energy causes secondary electrons to be emitted from the sample along with X-rays and Auger electrons. The secondary electrons are collected by a secondary electron detector. The backscattered electrons can be collected by a backscattered electron detector. BRIEF DESCRIPTION OF DRAWINGS

[0024] The present disclosure is illustrated by way of example, and not by way of limitation, in the figures of the accompanying drawings in which:

[0025] Figure 1 is a simplified perspective view showing an exemplary light modulating electron source according to exemplary embodiments of the present invention;

[0026] Figure 2 is a cross-sectional side view illustrating a coated silicon field emitter cathode formed on a silicon substrate according to exemplary embodiments of the present invention;

[0027] Figure 3 is a block / circuit diagram illustrating a simplified electron source according to alternative embodiments of the present disclosure;

[0028] Figure 4 is a block / circuit diagram illustrating a partially simplified electron source according to another alternative embodiment of the present disclosure;

[0029] Figure 5A and 5B is a cross-sectional side view illustrating a field emitter cathode during operation of an electron source implementing an on / off modulation scheme according to another embodiment of the present invention;

[0030] Figure 6 is a cross-sectional side view illustrating a plurality of field emitter cathodes formed on a silicon substrate according to another exemplary embodiment of the present invention;

[0031] Figure 7 illustrates an electron source according to another exemplary embodiment of the present invention;

[0032] Figure 8 An exemplary SEM incorporating embodiments of electron sources, electron optics, backscattered electron detectors, and secondary electron detectors according to practical embodiments of the present disclosure. DETAILED DESCRIPTION

[0033] Although the claimed subject matter will be described in terms of certain embodiments, other embodiments (including embodiments that do not provide all of the benefits and features set forth herein) are also within the scope of the present disclosure. Various structural, logical, process steps, and electronic changes can be made without departing from the scope of the present disclosure. Accordingly, the appended claims define the scope of the disclosure.

[0034] The following description is presented to enable any person skilled in the art to make and use the disclosure as provided in the context of a particular application and its requirements. As used herein, directional terms such as "top," "bottom," "upper," "lower," "up," "down," "over," "under," and the like are used for description purposes in reference to the illustrations where the disclosure is presented. Those skilled in the art will appreciate that various modifications can be made to the preferred embodiments, and the general principles defined herein can be applied to other embodiments. Thus, the present disclosure is not intended to be limited to the embodiments presented, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

[0035] Figure 1 An exemplary light modulated electron source 100 that generates an electron beam 112 according to an exemplary embodiment of the present disclosure is shown. The electron source 100 generally includes a silicon-based field emitter 110 that includes a field emitter cathode 120, an extractor (electrode) 130, a photon beam source 140, and control circuitry 150. The particular shapes and proportions of the structures representing the various elements depicted in the figures (e.g., the field emitter cathode 120 and the extractor 130 in Figure 1 The particular shapes and proportions of the structures representing the various elements depicted in the figures (e.g., the field emitter cathode 120 and the extractor 130 in

[0036] The field emitter cathode 120 is fabricated on a silicon substrate 121 and includes an integral emitter protrusion 124. The silicon substrate 121 includes a p-type or n-type doped silicon layer having a top / output (first) surface 122 and an opposite bottom (second) surface 123. The emitter protrusion 124, which is formed / fabricated by removing silicon material from the silicon substrate 121, includes a base 125 that is integrally connected to the top surface 122, a body portion 126 that extends away from the top surface 122, and an emission tip 127 disposed at a distal end of the body portion 126.

[0037] An extractor (electrode) 130 is fixedly disposed adjacent to the field emitter cathode 120 and is configured to generate an electric field E for attracting free electrons (e.g., electron 113-1) in the silicon substrate 121 toward the emission tip 127. The extractor 130 includes an electrically conductive structure 132 that is maintained at a positive voltage (e.g., as by a voltage source V DC indication) to generate the electric field E during operation of the source 100. When a given free electron 113-1 reaches the emission tip 127 with sufficient energy to overcome the potential barrier at the emission tip surface / interface, it passes through the surface / interface (e.g., via quantum mechanical tunneling) and exits the field emitter cathode 120 (from which it is emitted) to become an emitted electron 113-3. When emitted electrons 113-3 are produced at a sufficient rate, an electron beam 112 is created, where the emission current Ie of the electron beam 112 is defined by the number of emitted electrons 113-3 per unit time. For illustrative purposes, the extractor 130 is depicted as having a ring-shaped electrically conductive structure 132 that defines a central aperture 134 and is arranged such that the inner edge of the central aperture 134 surrounds a portion of the emission path or protrusion 124 of the electron beam 112. That is, the extractor 130 can be positioned downstream of (i.e., farther from the top surface 122 than) the emission tip 127 (e.g., as depicted in FIG. 1A), such that the path of the electron beam 112, depicted by the dotted arrow in FIG. 1A, passes through the aperture 134. Conversely, when the extractor 130 is positioned at an equal distance from the top surface 122 and the emission tip 127 or closer to the top surface 122 than the emission tip 127 (e.g., as depicted in FIG. 1B, as discussed below), then the emission tip 127 can be approximately coplanar with the aperture 134 or can protrude through the aperture 134; in this case, the free electrons 113-1 and 113-2 pass through the aperture 134 while still contained within the protrusion 124 (i.e., before exiting the emission tip 127 and becoming emitted electrons 113-3). Figure 1 Figure 1 Conversely, when the extractor 130 is positioned at an equal distance from the top surface 122 and the emission tip 127 or closer to the top surface 122 than the emission tip 127 (e.g., as depicted in FIG. 1B, as discussed below), then the emission tip 127 can be approximately coplanar with the aperture 134 or can protrude through the aperture 134; in this case, the free electrons 113-1 and 113-2 pass through the aperture 134 while still contained within the protrusion 124 (i.e., before exiting the emission tip 127 and becoming emitted electrons 113-3). Figure 2

[0038] The photon beam source 140 is configured to generate and direct a photon beam 142 having an intensity Ip onto the field emitter cathode 120. In accordance with aspects of the present disclosure, the photon beam 142 includes photons 143 having an energy greater than the energy bandgap of silicon (i.e., such that the photons 143 forming the photon beam 142 have a wavelength λp shorter than about 1 μιη), where at least some of the photons 143 are absorbed by associated silicon atoms of the field emitter cathode 120. The photon beam 142 is directed (e.g., using optical elements described below) onto a surface portion of the field emitter cathode 120 adjacent to (i.e., directly above and / or near) the emission tip 127, where at least some of the photons 143 are absorbed by associated silicon atoms of the field emitter cathode 120. As in Figure 1 ​​The instructions state that each photon 143 absorbed by the field emitter cathode 120 generates an electron-hole pair EHP according to the photoelectric effect, thereby releasing a photon-electron 113-2 in the field emitter cathode 120 and increasing the number of free electrons in the conduction band of the field emitter, thereby increasing the number of emitted electrons and causing a corresponding increase in the emission current Ie of the electron beam 112. That is, in the absence of the photon beam 142, the number of free electrons 113-1 in the conduction band of the field emitter is relatively low, and the electron beam 112 has a relatively low emission current Ie. When generated with a sufficient intensity Ip, the photon beam 142 promotes photon-assisted field emission (i.e., generating the electron beam 112 to enhance / increase the emission current Ie) by increasing the number of free electrons in the conduction band of the field emitter (i.e., by supplementing the free electrons 113-1 in the conduction band with photoelectrons 113-2).

[0039] According to another aspect of the invention, the control circuit 150 is configured to modulate the emission current Ie of the electron beam 112 by controlling the operation of the photon beam source 140 in a manner that adjusts the intensity Ip of the photon beam 142. In some embodiments, the control circuit 150 is a processor, a field-programmable device, or an application-specific integrated circuit manufactured and configured according to known techniques, such that it is configured to implement a selected electron beam modulation scheme, generate one or more photon beam source control signals PBSC according to the selected modulation scheme, and transmit the photon beam source control signals PBSC to the photon beam source 140. Various modulation schemes are described below, one of which involves periodically switching the emission current Ie of the electron beam 112 between high and low current levels. In this example, in order to increase the emission current Ie from a low current level to a high current level, the control circuit 150 generates / transmits a photon beam source control signal PBSC having a first value, which causes the photon beam source 140 to increase the intensity Ip of the photon beam 142, thereby correspondingly increasing the emission current Ie in the manner described above. Conversely, in order to reduce the emission current Ie from a high current level to a low current level, the control circuit 150 generates a photon beam source control signal PBSC with a second value, which causes the photon beam source 140 to reduce the photon beam intensity Ip.

[0040] Figure 2 A field emitter 110A of an electron source according to an exemplary embodiment is illustrated in a cross-sectional view. Note the features of the field emitter 110 (such as...) Figure 1 The field emitter 110A, which is essentially the same as the one shown, is identified using the same component symbols, which means that the reference... Figure 1 and 2 The details of the corresponding features mentioned in both apply to both field emitters 110 and 110A. The suffix "A" at the end of the component symbol indicates that the relevant features may be different. See below for reference. Figures 3 to 8This convention is also used in other embodiments described where different suffix letters indicate one or more differences from previously described embodiments.

[0041] A field emitter cathode 120A is formed in a silicon substrate 121 having a field emitter protrusion 124 disposed on an upwardly facing output (top) surface 122. In preferred embodiments, the silicon substrate 121 is essentially defect free single crystal silicon (i.e., a single crystal of silicon) having a thickness Ts in the range of about 100 nm to a few hundred μιη. In some embodiments, the silicon substrate 121 is p-type doped with a doping less than about 10 19 cm -3 . Since minority carrier lifetime and diffusion length decrease as the dopant concentration increases, a dopant concentration higher than about 10 19 cm -3 may be used when the silicon is extremely thin (e.g., when the substrate thickness Ts is thinner than about 1 μιη), while a dopant concentration less than about 10 19 cm -3 may be preferred when the substrate thickness Ts is greater than about 1 μιη. For silicon thicker than a few microns (e.g., a thickness Ts of 10 μιη or more), a much lower dopant concentration (e.g., less than about 10 14 cm -3 may be preferred to ensure long carrier lifetime and low dark current. In alternative embodiments, the silicon can be n-type doped with a dopant concentration of about 10 15 cm -3 or more. For example, the silicon can be n-type doped with a dopant concentration between about 10 15 cm -3 and 10 19 cm -3 . Silicon with n-type doping has more available electrons in the conduction band than a p-type doped substrate, which can be attracted toward the emission tip 127 to form the desired emission current. In both n-type and p-type doped substrates, the number of electrons in the conduction band can be increased by illuminating the field emitter with light as described below. When large modulation is desired, a p-type doped substrate can be preferred because when the photon beam intensity is zero or low, the number of electrons in the conduction band will be very low, and when the photon beam intensity is increased, the relative increase in the number of electrons in the conduction band will be large. When high electron emission current is desired due to a large number of electrons in the conduction band, an n-type doped substrate can be preferred. In this case, the relative range of current adjustment that can be achieved can be less than for p-type doped silicon, but the range of current control will generally be sufficient to reduce or eliminate noise in the electron emission.

[0042] In one embodiment, the field emitter protrusion 124 is formed by anisotropic etching to have a pyramidal shape (e.g., as shown in FIG. 1A) or a conical shape (e.g., as shown in FIG. 1B), as indicated in Figure 1 In one embodiment, the field emitter protrusion 124 is formed by anisotropic etching to have a pyramidal shape (e.g., as shown in FIG. 1A) or a conical shape (e.g., as shown in FIG. 1B), as indicated in Figure 2 In one embodiment, the field emitter protrusion 124 is formed by anisotropic etching to have a pyramidal shape (e.g., as shown in FIG. 1A) or a conical shape (e.g., as shown in FIG. 1B), as indicated in

[0043] In some embodiments, the field emitter protrusion 124 can be formed using a non-pyramidal shape, such as a circular whisker (cylindrical post with a circular tip) or a cone (not shown). A circular whisker or a cone with a half-angle of about zero degrees provides higher field enhancement than a cone or pyramid with a half-angle much greater than zero. However, a circular whisker is a poorer thermal conductor than a cone or pyramid of similar height. Thus, there is generally a tradeoff between field enhancement and thermal stability.

[0044] The field emitter cathode 120A includes an oxidation-resistant coating 129 that completely and continuously covers (i.e., forms a complete layer without openings or holes) at least the emission tip 127. In Figure 2 In the exemplary embodiment of FIG. 1A, the coating 129 is depicted as a continuous layer disposed over the entire protrusion 124 such that all surfaces of the body portion 126 and the emission tip 127 are completely and continuously covered by the coating 129. In other embodiments (not shown), the coating 129 can be selectively formed only over the distal end of the protrusion 124 (i.e., over the entire emission tip 127, with at least a portion of the body portion 126 located proximate to the distal end of the protrusion 124). In yet other embodiments (e.g., such as shown in FIG. 1B), the coating 129 can be selectively formed only over the body portion 126 (i.e., over the entire body portion 126, with at least a portion of the emission tip 127 located proximate to the distal end of the protrusion 124). Figure 3In the depicted embodiment, the coating 129 can extend over the top / output surface 122. In an exemplary embodiment, the continuous anti-oxidation coating 129 comprises one of substantially pure boron, silicon carbide, a boride material (e.g., lanthanum hexaboride), a carbide material (e.g., hafnium carbide), a metal silicide (e.g., titanium silicide), a semi-metal nitride (e.g., titanium nitride or tantalum nitride), and a semi-conductive nitride (e.g., boron nitride). Forming the coating 129 such that it continuously covers the emission tip 127 means that the coating 129 hermetically seals the emission tip 127 against oxidation. To ensure that the coating 129 provides a good hermetic seal without forming a strong barrier to electron emission, the coating 129 is formed to have a thickness Tci between about 1 nm and 10 nm. All native oxides can be removed from the silicon surface prior to depositing the coating 129, for example, by performing a wet clean followed by an in-situ thermal hydrogen clean. Details of a method for coating silicon with boron can be found in Sarubbi et al., “Chemical vapor deposition of a-boron layers on silicon for controlled nanometer-deep p+-n junction formation,” J. Electronic Materials, Vol. 39, pp. 162-173, 2010, and U.S. Patents 10,133,181 and 10,748,730 to Chuang et al., all of which are incorporated by reference herein.Details of methods of coating silicon emitters with silicon carbide can be found in M. Nagao et al., “Damageless vacuum sealing of Si field emitters with CHF3plasma treatment,” J. Vac. Sci. Technol. B, Vol. 19, No. 3, May / June 2001, pp. 920-924, J. Liu et al., “Modification of Si field emitter surfaces by chemical conversion to SiC,” J. Vac. Sci. Technol. B, Vol. 12, No. 2, March / April 1994, pp. 717-721, and U.S. Published Patent Application 2020 / 0118783 (Attorney Docket No. KLA-076 (P5403)) entitled “Electron Gun and Electron Microscope” and filed September 11, 2019 by Chuang et al. All such documents are incorporated herein by reference. Other materials can be coated onto silicon as appropriate by CVD, plasma-enhanced CVD, or PVD processes.

[0045] According to the presently preferred embodiment, field emitters 120A are fabricated using standard CMOS fabrication processes. For example, substrate 121 can be prepared for etching using silicon dioxide or silicon nitride as a masking material deposited by PECVD, and the masking material can then be patterned using photolithography. Dry etching (e.g., RIE, ICP, or ECR), wet etching, or a combination of both dry and wet etching can then be used to form emitter protrusions 124. When a sharp field emitter tip is desired, oxidation trimming can be used prior to forming coating 129, which is typically performed at low to moderate temperatures (e.g., less than about 950°C). Coating 129 can then be deposited over the exposed portions of emitter protrusions 124 using standard CMOS deposition processes.

[0046] Referring again to Figure 2The field emitter cathode 120A includes an extractor electrode (or gate electrode) 130A, which is maintained at a height close to the emitter tip 127 and attached to the substrate 121 via a dielectric layer 128, and configured to facilitate rapid and precise control of the emitter current Ie. Preferably, the nominal height H2 of the extractor 130A above the top surface 122 is approximately ±300 nm relative to the base-to-tip height H1 of the emitter tip 127. Therefore, the dielectric layer 128 is approximately equal to or less than the height of the emitter tip 127. Preferably, the thickness T of the dielectric layer 128 is... Dabout 100 nm and 1 μηι. The dielectric layer 128 can comprise one or more dielectric materials, such as Si02or Si3N4. The dielectric layer 128 is disposed on the top surface 122 around the field emitter protrusion 124A, but does not necessarily cover the field emitter protrusion 124. In another example, the dielectric layer 128 partially covers the field emitter protrusion 124. In a preferred embodiment, the extractor 130A and the dielectric layer 128 can be fabricated by standard CMOS fabrication techniques, for example using PVD, CVD or ALD deposition methods. The extractor 130A can comprise a metal or polysilicon. The simplest emitter design is a triode configuration in which only one gate electrode configured as the extractor 130A is used, but other configurations are possible. The extractor 130A is typically formed on the top surface of a dielectric layer 128 (which comprises one or several insulating layers) deposited on the substrate 121. Two or more gate layers (not shown) can be utilized in more complex emitter designs, in which multiple dielectric layers are used as spacers between sequentially formed gate layers. In this embodiment, each gate electrode will have a different function. Only one electrode will act as an extractor, while another gate (or gates) can be used, for example, to control the divergence angle of the electron beam. Placing the extractor 130A close to the emission tip 127 allows for the generation of a strong enough electric field from a low voltage (for example a voltage between about 30 V and about 200 V), which has the advantage of minimizing back bombardment of the emitter by high-energy ions that can shorten the lifetime of the emitter. Since the extractor 130A is placed close to the emission tip 127, the aperture 134A defined in the extractor 130A should be precisely aligned with the emission tip 127. For example, the center of the aperture 134A can be aligned with the emission tip 127 within about 5% or less of the diameter of the aperture 134A.More details of field emitter cathode structures comprising one or more gates (electrodes) and methods of fabricating such structures can be found in H. Shimawaki et al., "Laser-induced Electron Emission from p-type Silicon Emitters," Technical Digest, 27th International Vacuum Nanoelectronics Conference, pp. 2-27, A. Koike et al., "Field Emitter Equipped with a Suppressor to Control Emission Angle," IEEE Electron Device Letters, vol. 34, no. 5, May 2013, pp. 704-706, and M. Nagao et al., "Fabrication of a Field Emitter Array with a Built-in Einzel Lens," Japanese Journal of Applied Physics, vol. 48, 2009, 06FK02. All of these documents are incorporated herein by reference.

[0047] Figure 3 An electron source 100B is illustrated according to another embodiment of the present disclosure. The electron source 100B includes an enclosure 111 containing a field emitter 110B, a photon beam source 140B, and a control circuit 150B.

[0048] The enclosure 111 is formed around a low pressure chamber 114 and includes a vacuum pump 115 (e.g., an ion pump or a getter pump) configured to maintain the chamber 114 at a vacuum condition (i.e., lower pressure) higher than the vacuum condition of the outside enclosure 111. The field emitter 110B is disposed in the chamber 114 and includes an emitter cathode 120B, an extractor 130B-1 having an aperture 134B-1, and an anode 130B-2 having an aperture 134B-2. The field emitter cathode 120B is formed on a silicon substrate 121 having a field emitter protrusion 124 disposed on an output (top) surface 122, and a coating 129 is formed at least over the surface of the protrusion 124. The extractor 130B-1 is positioned above the top surface 122 and is configured to operate as described above with reference to the field emitter 110A. The anode 130B-2 is disposed at an offset distance Dta of at least 1 mm from the emission tip 127 and is maintained at a positive voltage V Figure 2 The anode 130B-2 is disposed at an offset distance Dta of at least 1 mm from the emission tip 127 and is maintained at a positive voltage V DC2The electron source 100B can further include one or more additional optional electrodes 130B-3 with associated apertures 134B-3. The optional electrodes 130B-3 can include one or more focusing electrodes, one or more deflectors, aberration compensators, and / or beam blankers. The additional electrodes 130B-3 are depicted as positioned upstream of the anode 130B-2 (i.e., closer to the field emitter cathode 120B), but can be placed upstream or downstream of the anode 130B-2 as necessary to achieve the desired functionality and performance. The extractor 130B-1, the anode 130B-2, and the optional electrodes 130B-3 are configured such that the emitted electrons 113-3 form an electron beam 112B that passes through the apertures 134B-1, 134B-2, and 134B-3 and exits the chamber 114 via a small opening 116 that is sized such that it restricts gas diffusion into the chamber 114 and enables the pump 115 to maintain the chamber 114 at a desired low pressure. In some embodiments, the electron source 100B includes a magnetic lens (not shown) for focusing and / or deflecting the electron beam 112.

[0049] The photon beam source 140B includes a light source (e.g., a laser or other illumination device) 141B, an optional focusing apparatus 144B, and an optional light modulation device 145B. In this configuration, the light source 141B and the light modulation device 145B are depicted as being disposed outside the housing 111, and the focusing apparatus 144B is depicted inside the chamber 114. In other embodiments, one or more elements of the focusing apparatus 144B can be positioned outside the housing 111, or one or both of the light source 141B and the light modulation device 145B can be mounted inside the chamber 114. The light source 141B generates a photon beam that illuminates the field emitter cathode 120B. For descriptive purposes, the photon beam is identified in three segments that define a photon beam path, including a first photon beam segment 142-1 that extends from the light source 141B to the light modulation device 145B, a second photon beam segment 142-2 that extends from the light modulation device 145B through the window 117 (or other light-transmissive feature in the wall of the housing 111) to the optical device 144B, and a third / terminal photon beam segment 142-3 that extends from the optical device 144B to the field emitter cathode 120B. In one embodiment, the light source 141B is configured to generate photons 143 at a wavelength in the visible or UV wavelength range (i.e., between 250 nm and 700 nm), with a significant fraction of the photons 143 directed along the terminal beam portion 142-3 onto the field emitter cathode 120B being absorbed by silicon atoms positioned proximate to the surface of the emission tip 127, thereby further enhancing the generation of photoelectrons for a given photon beam intensity Ip. The focusing apparatus 144B is implemented by one or more optical elements (e.g., lenses and / or mirrors) and associated positioning structures collectively configured to form a focusing optic that focuses and / or directs the photons 143 generated by the light source 141B so that the terminal photon beam segment 142-3 is directed onto the field emitter cathode 120B. The focusing apparatus 144B can be omitted when all necessary focusing and directing functions are performed by the light source 141B. The light modulation (LM) device 145B (e.g., an electro-optical modulator or an acousto-optical modulator) is configured to modulate the photon beam (i.e., so that the terminal photon beam segment 142-3 has a selected intensity Ip3) by, for example, selectively not blocking or diverting a photon flux transmitted between the light source 141B and the field emitter cathode 120B in response to a light modulation control signal LMC, blocking or diverting some or all of the photon flux transmitted between the light source 141B and the field emitter cathode 120B in response to the light modulation control signal LMC. For example, when the light modulation control signal LMC has a “deactivated” value, the light modulation device 145B is configured to pass substantially all of the photons 143 to the field emitter cathode 120B (i.e., the intensity Ip1 of the beam segment 142-1 is substantially equal to the intensity Ip2 of the beam segment 142-2, which is in turn substantially equal to the intensity Ip3 of the beam segment 142-3).In contrast, when the light modulation control signal LMC has a "fully active" value, the light modulation device 145B is configured to block substantially all of the photons 143 from reaching the field emitter cathode 120B (i.e., the intensities Ip2and Ip3are substantially zero). When the light modulation control signal LMC has a "partially active" value, the light modulation device 145B is configured to block a corresponding fraction of the photons 143 generated by the light source 141B (i.e., the intensity Ip2is lower than the intensity Ip1by an amount determined by the partially active level of the light modulation device 145B). The light modulation device 145B can be omitted when all necessary light modulation functions are performed by the light source 141B in the manner described below. One advantage of light modulation using electro- or acousto-optical modulators is that such modulators can modulate at frequencies of several hundred MHz or several GHz, allowing fast modulation or switching of the electron beam. In contrast, electrical modulation of one or more electrodes can be effectively limited to several MHz or tens of MHz because the large capacitance of the electrodes would require very high drive currents in order to change their voltage rapidly.

[0050] The control circuit 150B is configured to modulate the emission current Ie of the electron beam 112B emitted from the electron source 100B by controlling the intensity Ip3of the terminal photon beam segment 142-3, which is in turn achieved by controlling at least one of the light source 141B (e.g., using the light source control signal LSC) and the light modulation device 145B (e.g., using the light modulation control signal LMC). In one embodiment, the control circuit 150B controls the photon beam intensity Ip3by controlling the number of photons flux passing through the modulator 145B in the manner described above. For example, the control circuit 150B generates and transmits a light modulator control signal LMC having a value (e.g., a voltage level) corresponding to a desired intensity level (e.g., such that the fraction of photons blocked by the modulator 145B is proportional to the voltage level of the light modulator control signal LMC). In other embodiments, the light source 141B is configured to generate a first photon beam segment 142-1 having an intensity Ip1that can be adjusted between an off (zero) and a fully (maximum) intensity level in response to the light source control signal LSC, the control circuit 150B controls the photon beam intensity Ip3by varying the operating power level of the light source 141B (e.g., the control circuit 150B causes the light source 141B to increase or decrease the intensity Ip3by increasing / decreasing the voltage level of the light source control signal LSC).

[0051] The electron source 100B includes at least one monitor 160B-1 and / or 160B-2 operably disposed to measure the emission current Ie of the electron beam 112B. An exemplary current monitor 160B-1 is electrically connected to the anode 130B-2, where the exemplary monitor 160B-1 is operably disposed to measure the portion of the emission current Ie that impinges on the anode 130B-2 and does not pass through the aperture 134B-2. The anode 130B-2 can be configured such that the aperture 134B-2 functions as a beam stop aperture that limits the angular spread and diameter of the electron beam 112B exiting the electron source. In this case, a large fraction (e.g., greater than 50%) of the current emitted by the field emitter 110B can be stopped by the anode 130B-2. Thus, while the current monitored by 160B-1 is not the entire emission current, it is strongly related to the total emission current and can be used to control and stabilize the total emission current. Alternatively or additionally, a current monitor 160B-2 monitors the emission current from the field emitter cathode 120B. In other embodiments, monitors can be connected to other electrodes (e.g., electrode 130B-3, or a beam stop electrode disposed downstream of the opening 116). The monitors 160B-1 and 160B-2 are operably configured (using known techniques) to generate a current measurement 165B determined by the measured amount of the emission current Ie. The control circuit 150B is configured to receive and utilize the emission current measurement 165B as a feedback signal to control the light source 141B or the light modulation device 145B to adjust the intensity Ip3 of the photon beam end segment 142-3 according to a selected modulation scheme (discussed below) in the manner described above.

[0052] As mentioned above, the control circuit (controller) 150B is configured to modulate the electron beam 112B via the control of the photon beam source 140B according to a selected modulation scheme (method). According to one such modulation scheme, the control circuit 150B utilizes the emission current measurement 165B as a feedback signal to maintain or adjust the electron beam 112B to maintain a selected target emission current level. To implement this modulation scheme using the electron source 100B, the control circuit 150B is configured to continuously monitor the emission current Ie via the emission current measurement 165B and adjust the photon beam source control signal (e.g., the light source control signal LSC or the light modulation control signal LMC) to cause a corrective change in the intensity Ip3 of the terminal photon beam segment 142-3. As mentioned above, the emission current Iemay experience inherent current fluctuations, for example due to a change in the emitter work function caused by contamination on the emission tip 127. In one exemplary embodiment, when a decrease in the level / value of the emission current measurement 165B indicates that such a change has occurred, the control circuit 150B then utilizes the emission current measurement 165B to modify the light source control signal LSC to a value (e.g., voltage level) that causes the light source 141B to increase the light source's output power, thereby increasing the intensity Ip1 of the photon beam segment 142-1 emitted from the light source 141B, which in turn increases the emission current Ie of the electron beam 112B. In another exemplary embodiment, the control circuit 150B utilizes the decreased emission current measurement 165B to modify the light modulation control signal LMC to a value that causes the light modulation device 145B to allow a greater portion of the photon beam segment 142-1 to pass, increasing the intensity Ip2. In both cases, the increased photon beam intensity is communicated to the field emitter cathode 120B, resulting in a corresponding increase in the emission current Ie of the electron beam 112B. The process of repeatedly monitoring the resulting changes in the emission current measurement 165B and transmitting subsequent control signal changes to the photon beam source 140B continues until the emission current Ie of the electron beam 112B has returned to the target level.

[0053] Figure 4 A portion of an electron source 100C according to another embodiment is depicted. The electron source 100C is similar to the electron source 100B Figure 3 ) in that the electron source 100C includes a field emitter 110C disposed in a low pressure chamber 114 and further includes a photon beam source (not shown) and a control circuit (not shown) configured to operate in the manner described above with reference to Figure 3 the electron source 100A. Similar to the field emitter 110B Figure 3), field emitter 110C includes a cathode 120C with a protrusion 124, an extractor 130C-1 disposed adjacent to an emission tip 127, an anode 130C-2 positioned a distance Dta from the emission tip 127, and a pair of monitors 160C-1 and 160C-2 that generate an emission current measurement 165C in the manner described above. Electron source 100C differs from electron source 100B in that anode 130C-2 is mounted to a lower wall of enclosure 111 such that aperture 134B-2 does not require a separate differential pumping aperture (i.e., Figure 3 the opening 116 shown in FIG. 1). That is, aperture 134C-1 serves both as a differential pumping aperture and to transmit electron beam 112C out of enclosure 111. To facilitate maintaining anode 130C-2 at a different potential than enclosure 111, anode 130C-2 is attached to the lower wall of enclosure 111 via an insulator 137 that also serves to maintain a vacuum seal that enables low pressure chamber 114 to be maintained at a desired low pressure. Similar to the electron source 100B described with reference to FIG. 1, electron source 100C can optionally be further configured to include additional electrodes or magnetic lenses (not shown). Figure 3

[0054] Figure 5A and 5B depicts an electron source 100D according to another exemplary embodiment in which electron beam 112D is modulated according to a high / low or on / off modulation scheme. Similar to the previous embodiments, field emitter cathode 120D is formed on a silicon substrate 121D and includes a coating 129D that continuously covers an emission tip 127D of an integrated protrusion 124D, an anode (electrode) 130D is maintained at a positive voltage level by a voltage source V DC3 that generates an electric field E C at the emission tip 127D, and a photon beam source 140D is configured to generate a photon beam 142D directed onto protrusion 124D in the vicinity of emission tip 127D according to a control signal received from a control circuit 150D (not shown). In this embodiment, electron source 100D is periodically switched between two operating states corresponding to two different target levels of electron beam emission current, this occurring during two consecutive different time periods depicted in FIGS. 2A and 2B, respectively. Specifically, Figure 5A and 5B depicts electron source 100D in a first operating state during a first time period TO when electron beam 112D is generated at a relatively low (first) emission current level Ie0, and Figure 5A depicts electron source 100D during a second time period T1 when electron beam 112D is generated at a relatively high (second) emission current Ie1 that is greater / larger than emission current Ie0. Figure 5B depicts electron source 100D during a second time period T1 when electron beam 112D is generated at a relatively high (second) emission current Ie1 that is greater / larger than emission current Ie0. Figure 5A and​5B The electron source 100D is specifically depicted during implementation of an on / off modulation scheme via indication of a relatively low emission current Ie0 of zero or substantially zero during a first time period To. When the electron source 100D is configured to implement a high / low modulation scheme, the relatively low emission current Ie0 can be substantially greater than zero, but less than the emission current Iel.

[0055] To implement Figure 5A and 5B the high / low or on / off modulation scheme depicted in Figure 5A , the control circuit 150D is configured to modulate the electron beam 112D via control of the photon beam source 140D such that the photon beam 142D is periodically switched between a relatively low (first) intensity (e.g., Ip0~0, as Figure 5B indicated) and a relatively high (second) intensity (e.g., Ip1»0, as Figure 5A indicated). Specifically, Figure 5B the control circuit 150D generates and transmits a first photon beam source control signal (e.g., PBSC=0) to the photon beam source 140D during a time period To, and Figure 5A the control circuit 150D generates / transmits a different (second) photon beam source control signal (e.g., PBSC=l) to the photon beam source 140D during a time period T1. As indicated in Figure 5B , in response to the first photon beam source control signal (PBSC=0), the photon beam source 140D generates the photon beam 142D at a relatively low intensity Ip0such that a relatively small amount (e.g., zero) of photons are transmitted onto the emitter cathode 120D, thereby generating a relatively small amount (e.g., zero or minimal) of photo-assisted field emission, with the electron beam 112D being generated by the emitter cathode 120D at a relatively low emission current Ie0. In other words, if zero photons are transmitted to the emitter cathode 120D during the time period To, then the emitter cathode 120D generates the electron beam 112C with the emission current Ie0 being relatively low, provided by the extraction field E C . In contrast, as indicated in , in response to the second photon beam source control signal (PBSC=l), the photon beam source 140D generates the photon beam 142D at a relatively high intensity Ip1such that a relatively large amount of photons are transmitted onto the emitter cathode 120D during the second time period T1, with the emitter cathode 120D generating current using the extraction field E C provided by the photon beam 142D in combination with the energy / electrons generated by the photon beam 142D, which in turn causes the electron beam 112D to have a relatively higher emission Iel during the time T1 (i.e., compared to the emission current Ie0 during the time To).

[0056] Figure 5A In one embodiment,5B The depicted modulation scheme is used to clear contaminants from the tip 127D. In a conventional cold field emitter, the tip temperature can be raised to cause contaminants to evaporate (flash off) from the tip. This process is slow because the tip needs several seconds to heat up, and then the tip needs to be maintained at a higher temperature for a long enough time for the contaminants to leave the tip, which can take several milliseconds or seconds, and then it is necessary to allow the tip to cool, which can take several seconds to several minutes. In addition, due to thermal expansion and contraction, the mechanical dimensions of the tip and the structure supporting it change with temperature. Because the changes in dimensions and tip position change the emission current, it is usually necessary to allow extra time for the tip emission to stabilize after flash off. In one embodiment of the present invention, a high current is used to drive contaminants away from the tip 127D. When the current is high (e.g., 1 μA or several μA), the electrode can frequently collide with contaminants, and can knock them off. This can occur in a time period of less than several microseconds or as short as a few nanoseconds. After this short time period, the photon beam intensity can be reduced to a lower value and normal system operation can be resumed. Since the time needed for cleaning is extremely short, the impact on system performance can be minimal or zero. For example, when no data is collected in a short time, the high current for cleaning can be implemented at the end of a scan line or scan frame, so that cleaning does not cause the time to acquire an image to change. The high current (e.g., a current greater than 1 μA) can damage the tip 127D if used for a long time, because the tip 127D can become very hot and can deform. But if the high current is present for only a few microseconds or less, the heating effect will be minimal, and will not cause tip damage.

[0057] According to a particular embodiment in which the electron source 100D is configured to implement an on / off modulation scheme, the scheme takes advantage of the following features. First, the emitter cathode 120D is fabricated on a p-type doped substrate 121D, and the coating 129D is a p-type material (e.g., boron), and the anode 130D is configured to create an electric field E Csuch that the potential barrier height (conduction band) of the field emitter cathode 120D is maintained at a level higher than the Fermi level of the field emitter cathode 120D at the emission tip 127D, thereby minimizing or eliminating electron emission from the emission tip 127D during the first time period TO (i.e., when the intensity Ip0of the photon beam 142D is minimized, the emission current Ie0of the electron beam 112D is zero or almost zero). In a semiconductor, where the electron concentration is determined by the local doping and potential, the field emitter can be configured to operate in a reverse-bias mode as in a p-n diode, where a depletion layer is created due to the high electric field at the emitter surface. The interface between the vacuum and the p-doped field emitter can form a p-n junction, where the vacuum is considered as an n-type medium. In this case, the conduction and valence bands will bend down at the surface. If the electric field is sufficient to make the lowest level of the conduction band lower than the Fermi level, there will be an abundance of electrons at the tip's apex and a current of about nA to mA is generated. According to the on / off modulation scheme shown in Figure 5A and 5B the electric field E C is maintained just above the Fermi level, such that no electrons are emitted when the photon beam 142D is omitted (e.g., when the photon beam source 140D is turned off, as indicated in Figure 5A . As indicated in Figure 5B , during the time period T1, the electric field E C is also maintained at this level, but in this case, when an incident photon 143 having an energy level / wavelength λp generates an electron-hole pair EHP, the photoelectron 113-2 generated will be strongly attracted towards the emission tip 127D, and many such photoelectrons will be emitted to provide an electron beam 112D having a desired emission current Ie1.

[0058] Figure 6 A silicon field emitter 110E of a field emitter array (FEA) multi-electron beam source according to yet another alternative embodiment is illustrated in cross-sectional view. Similar to the above with reference to Figure 2The described embodiment (field emitter 110E), extractor (electrode) 130E is fixedly disposed on the emitter cathode 120E via a dielectric layer 128E formed on the top surface 122E of the silicon substrate 121E. In this case, the emitter cathode 120E includes a plurality of field emitter protrusions (e.g., protrusions 124E-1 and 124E-2) arranged in a two-dimensional periodic pattern and integrally connected to the top (output) surface 122 of the silicon substrate 121E, respectively. The field emitter protrusions 124E-1 and 124E-2 are disposed in openings formed in the dielectric layer 128E, and form the extractor 130E, such that the apertures 134E-1 and 134E-2 surround the emitter protrusions 124E-1 and 124E-2, respectively. During operation, each emitter protrusion 124E-1 and 124E-2 is subjected to an electric field applied from the extractor (gate) 130E and receives photons from a photon source (not shown) for the purpose of modulating the electron beams emitted from the emitter tips 127E-1 and 127E-2, respectively, using the described method. Although the extractor 130E is depicted as a single continuous layer, it can be divided into segments corresponding to individual emitter protrusions 124E-1, 124E-2, etc., such that separate extraction voltages can be applied to each emitter segment to allow individual control of the electric field applied to each emitter tip. Other materials and dimensions of the field emitter cathode 100E can be similar to those of the field emitter cathode 110A described above. Figure 2 The materials and dimensions configuration of the illustrated field emitter cathode 110A, and will not be described separately here.

[0059] Figure 7A simplified electron source 100F is illustrated in accordance with another exemplary practical embodiment of the present application. Similar to the previous embodiments, the field emitter 110F includes a cathode 120F and an extractor 130F-1 operating as described above. In this embodiment, the photon beam source 140F includes a light source 141F configured to generate at least one photon beam 142F directed by a focusing device (optics) 144F onto the field emitter 110F for the purpose of modulating (controlling) the electron beam 112F in accordance with any of the modulation schemes mentioned above. The electron beam 112F can include beam portions 112F-1, 112F-2 and 112F-3. An electrode 130F-2 is placed at a distance from the surface of the field emitter for creating an acceleration field between the surface of the field emitter 110F and an anode 130F with a high voltage source, for example an acceleration voltage between about 1 kV and about 10 kV. The anode 130F-2 has an aperture 134F-2 which can be configured as a beam stop allowing passage of portions of the electron beam 112F. In other words, electrons originating from the field emitter 110F are directed through the aperture 134F-2 in the anode 130F-2 to form at least a portion of the electron beam 112F of the outgoing electron source 100F. In one embodiment, portions of the electron beam 112F impinge on the anode 130F-1 and a monitor 160F is operably configured to sense or otherwise measure the emission current of the electron beam 112F at every instant during operation via contact with the anode 130F-1 and generate and transmit corresponding emission current measurements 165F to a control circuit 150F. The control circuit 150F is configured to modulate the electron beam 112F by controlling the photon beam source 140F to adjust (e.g., increase or decrease) the intensity of the photon beam 142F in accordance with changes in the emission current measurements 165F and a selected modulation scheme.

[0060] Reference is made to Figure 7downstream (i.e., along the path of the electron beam 112F) of the anode 130F-2 and includes an aperture 134F-3 through which the end portion 112F-3 of the electron beam passes. The electrode 130F-3 can be configured to act as a beam stop, i.e., the aperture 134F-3 can be small enough to limit the angular range of the electron beam portion 112F-3 that passes through it. Alternatively, the aperture 134F-3 can be large enough to pass substantially all of the electron beam portion 112F-2 received from the anode 130F-2. In one embodiment, the electrode 130F-3 is made of metal (e.g., aluminum or silver) and includes a curved (e.g., parabolic) mirror 149 (which, together with the collimating lens 146, forms part of the focusing device 140F). The electrode 130F-3 is positioned and arranged relative to the light source 141F and the field emitter 110F such that the photon beam 142F is redirected and focused by the curved mirror 149 onto the output surface of the field emitter 110F (e.g., the emission tip of the field emitter 110F is placed at the focal point of the parabola formed by the curved mirror 149, and the incoming beam from the optics 146 is substantially collimated). A high voltage source can be used to bias the electrode 130F-3 relative to the field emitter cathode 120F and the anode 130F such that the electrons are accelerated after passing through the anode 130F-2. In an alternative embodiment, the anode 130F-2 can be omitted and the controller 150F can monitor the current flowing in the electrode (beam stop) 130F-2. In another embodiment, the optics used to direct and focus the photon beam 142F are separate from any electrodes and aperture stops. Other structures and elements that can be used in SEM systems are omitted for brevity.

[0061] During operation of the source 100F, the controller 150F modulates the emission current of the electron beam 112F via controlling the intensity of the photon beam 142F generated by the light source 141F according to a selected modulation scheme and directed onto the emitter cathode 1 10F via the parabolic mirror 149. The light source 141F can include a laser diode, a pulsed laser, a continuous wave (CW) laser, or a lamp. The wavelength of the photons generated by the light source 141F can be in the visible or UV wavelength range. The photon beam 142F can be collimated by a focusing device 144F (e.g., a collimating lens 146) that is also used to focus / direct the photon beam 142 onto the field emitter cathode 1 10F via the parabolic mirror 149 such that the focused photon beam passes through the aperture 136F of the anode 130F. The photons of the photon beam 142F assist in releasing electrons from the surface of the field emitter cathode 1 10F in the manner described above. The intensity of the photon beam 142F can be adjusted by the controller 150F via controlling the light source 141F (e.g., using a light source control signal LSC) or via controlling the optical modulator 145F (e.g., using a light modulator control signal LMC). When high frequency (e.g., frequencies of about 100 MHz or higher) control of electron emission is desired, the light modulator 145F is preferably an electro-optical modulator. The combination of lenses and mirrors forming the focusing device 140F illustrates one example of a photon beam source that can be configured to control electron emission from a field emitter. Other combinations of lenses, flat and / or curved mirrors can be configured to direct and focus a photon beam onto a field emitter cathode.

[0062] Figure 8 An exemplary inspection, metrology, or review scanning electron microscope (SEM) system (apparatus) 200 is described that is configured to inspect, measure, or review a specimen 231, such as a semiconductor wafer, reticle, or photomask. The SEM system 200 generally includes an electron column that includes an electron gun (electron source) 100, an upper column 241, and a lower column 242 that focuses and directs a primary electron beam 112 onto the specimen 231.

[0063] The electron gun 100 is configured in accordance with any of the embodiments set forth above to include a field emitter 110 (which includes a field emitter cathode 120 and an extractor 130), one or more electrodes 132, a photon beam source 140, and a controller (control circuit) 150. As set forth above, the field emitter cathode 120 includes a silicon substrate 121 with an integrated emitter protrusion 124 having an emission tip 127 disposed at a distal end thereof, and the extractor 130 is configured to generate an extraction (electric) field E at the emission tip 127 and form an electron beam 112 from the emission tip 127 through an aperture 134 and into an upper column 241. The photon beam (PB) source 140 and the controller 150 are used to facilitate photon-assisted field emission of the primary electron beam 112 from the electron gun 100 as described above. That is, the controller 150 monitors the primary electron beam 112, optionally using any of the methods discussed above, and controls the PB source 140 via a photon beam source control signal PBSC such that the photon beam applies a photon beam 142 to the emitter cathode 120 with an intensity Ip that causes the emitter cathode 120 to emit the primary electron beam 112 by virtue of an emission le received from the computer system 280 (as described below). The electrodes 132 can include one or more focusing electrodes, one or more deflectors, aberration compensators, and / or beam blankers. One of the electrodes 132 is configured as an anode to accelerate the electrons in the electron beam 112 after they have been extracted from the emission tip 127. The anode can be configured to operate as a beam stop. Alternatively or additionally, a beam stop can be included in the upper column 241.

[0064] The upper column 241 includes electron optics including one or more condenser lenses 207 that reduce the primary electron beam 112 to produce a small spot of impinging electrons on the underlying sample 231. One or more deflection electrodes (DEFs) 205 can be placed on either side of the condenser lenses 207. The upper column 241 can further include other electron optics, such as aberration compensators or beam blankers. The lower column 242 includes electron optics including a final lens 210 for focusing the primary electron beam 112 to a small spot on the sample 231. The lower column 242 also includes one or more deflection electrodes 209 that work in combination with the deflection electrodes 205 (if present) to scan the primary electron beam across the area of the sample 231. The sample 231 is placed on a stage 230 to facilitate movement of different regions of the sample 231 under the electron column. When the primary electron beam 112 impinges the sample 231, secondary electrons and backscattered electrons are emitted from the sample 231. The secondary electrons can be collected by an electrode 220 and accelerated and directed to a secondary electron detector 221. The backscattered electrons can be detected by backscattered electron detectors, such as the detectors shown at 222a and 222b.

[0065] The computer system 280 controls the electron source 100 and the electron optics of the upper column 241 and the lower column 242, and receives data from at least one of the secondary electron detector 221, the backscatter electron detector 222a, and the backscatter electron detector 222b. The computer system 280 also controls the stage 230. The computer system 280 causes the scanning electron microscope to perform inspection, measurement, or review of one or more areas on the specimen 231 using data from one or more detectors. The computer 280 directs the controller 150 to generate a selected electron beam current in the manner set forth above.

[0066] While described as integrated in a SEM, the electron sources disclosed herein can also be incorporated into other devices, such as an electron beam lithography system or an X-ray source. An electron beam lithography system can include electron optics configured to shrink and focus a primary electron beam on a target and a modulator for modulating the intensity of the electron beam. An X-ray source can include electron optics configured to direct an electron beam to an anode configured to emit X-rays.

[0067] Those of skill in the art will appreciate modifications to the described embodiments, and the general principles defined herein can be applied to other embodiments. For example, additional electrodes can be placed close to the electron emitter to control emission and / or to focus and direct emitted electrons in particular directions. While it is contemplated that the light-modulated electron sources disclosed herein will be particularly useful in various types of scanning electron microscopes and electron beam lithography systems, it is also envisioned that such sources can be useful in other applications where stable, high-radiation, and / or high-current electron beams are needed, such as in high-brightness X-ray generators.

[0068] The electron sources and methods described herein are not intended to be limited to the embodiments shown and described, but rather are to be accorded the broadest scope consistent with the principles and novel features disclosed herein. Accordingly, the disclosure is to be understood as being limited only by the appended claims and their reasonable interpretations.

Claims

1. An optical modulation electron source comprising: a field emitter cathode including: a silicon substrate having opposing first and second surfaces; and an emitter protrusion having a base integrally connected to the silicon substrate, a body portion extending from the first surface, and an emission tip disposed at a distal end of the body portion; an electrode fixedly positioned adjacent to the field emitter cathode and configured to generate an electric field that attracts free electrons in the silicon substrate toward the emission tip; a photon beam source configured to generate a photon beam including photons having a wavelength shorter than about 1 pm, and configured to direct the photon beam onto the emitter protrusion such that at least some of the photons are absorbed by the field emitter cathode; a control circuit configured to modulate an emission current of an electron beam including electrons emitted from the emission tip by controlling an intensity of the photon beam transmitted from the photon beam source and received by the field emitter cathode; and and a dielectric layer disposed on the first surface adjacent to the field emitter protrusion, wherein the electrode includes: an extractor disposed on the dielectric layer and maintained at a positive voltage between about 30 V and about 200 V relative to the field emitter cathode; and an anode disposed at an offset distance of at least 1 mm from the emission tip and maintained at a positive voltage of at least 500 V relative to the field emitter cathode, and wherein the control circuit is configured to control the photon beam source such that the photon beam is generated at a first intensity during a first time period and at a second intensity during a second time period, the second intensity being higher than the first intensity.

2. The electron source of claim 1, further comprising a dielectric layer, wherein an extractor is disposed on the dielectric layer such that the dielectric layer is disposed between the extractor and the substrate, wherein a thickness of the dielectric layer is less than a height of the emission tip above the first surface, wherein a difference between a nominal height of the extractor above the first surface and the height of the emission tip above the first surface is about ± 300 nm, and wherein the extractor is maintained at a positive voltage between about 30 V and about 200 V relative to the field emitter cathode, wherein the emission tip has a lateral dimension in a range of 1 nm to 50 nm, wherein the electron source further comprises an oxidation resistant coating disposed on the silicon substrate such that it completely and continuously covers the emission tip, and wherein the oxidation resistant coating includes one of boron, a boride, and a carbide and has a thickness between 1 nm and 10 nm.

3. The electron source of claim 1, wherein the photon beam source includes a light source configured to generate the photons having a wavelength in a range from 250 nm to 700 nm.

4. The electron source of claim 3, wherein the photon beam source further includes at least one of a focusing device and a light modulation device disposed in a path of the photon beam between the light source and the field emitter cathode.

5. The electron source of claim 4, wherein the focusing device comprises at least one mirror configured to direct the photon beam through an aperture formed in the electrode.

6. The electron source of claim 4, wherein the control circuit is configured to control the intensity of the photon beam by controlling at least one of the light source and the light modulation device.

7. The electron source of claim 6, further comprising at least one monitor configured to measure the emission current of the electron beam and generate a corresponding emission current measurement, wherein the control circuit is further configured to generate a control signal from the emission current measurement, and wherein at least one of the light source and the light modulation device is controlled by the control signal.

8. The electron source of claim 1, wherein the silicon substrate comprises p-type doped silicon, and wherein the electrode comprises an anode configured to generate an electric field at the emission tip such that a potential barrier of the field emitter cathode is maintained above a Fermi level of the field emitter cathode at the emission tip, and wherein the control circuit is configured to control the photon beam source such that the first intensity of the photon beam is minimized, thereby minimizing electron emission from an emission tip during the first time period, and such that the second intensity is higher than the first intensity during the second time period, wherein a sufficient number of the photons are absorbed by the field emitter cathode to generate a photon-assisted field emission of the electron beam from the emission tip.

9. The electron source of claim 1, wherein the field emitter cathode comprises a plurality of field emitter protrusions integrally connected to the first surface of the silicon substrate and arranged in a two-dimensional periodic pattern.

10. A method for modulating an electron beam emitted from a silicon-type field emitter cathode, the method comprising: generating an electric field at an emission tip of the field emitter cathode such that a conduction band at the emission tip is maintained above a Fermi level of the field emitter cathode; and controlling a photon beam source configured to generate a photon beam comprising photons having a wavelength shorter than about 1 pm and configured to direct the photon beam toward the field emitter cathode, wherein controlling the photon beam source comprises generating the photon beam at a first intensity during a first time period and generating the photon beam at a second intensity during a second time period, the second intensity being higher than the first intensity, such that during the first time period, electron emission from an emission tip is minimized, and such that during the second time period, a sufficient number of the photons are absorbed by the field emitter cathode to generate photoelectrons in the conduction band, thereby generating a photon-assisted field emission of the electron beam from the emission tip.

11. The method of claim 10, wherein generating the photon beam comprises generating the photons having a wavelength in a range from 250 nm to 700 nm.

12. The method of claim 10, wherein controlling the photon beam comprises controlling at least one of a light source and a light modulation device.

13. The method of claim 12, further comprising measuring an emission current of the electron beam and generating a corresponding emission current measurement, wherein controlling at least one of the light source and the light modulation device comprises generating a control signal from the emission current measurement and transmitting the control signal to one of the light source and the light modulation device.

14. A device comprising an electron source configured to generate a primary electron beam, wherein the electron source comprises: a field emitter cathode comprising: a silicon substrate having opposing first and second surfaces; and an emitter protrusion having a base integrally connected to the silicon substrate, a body portion extending from the first surface, and an emission tip disposed at a distal end of the body portion; an electrode having an aperture and maintained at a positive voltage relative to the field emitter cathode, the electrode configured such that electrons emitted from the emission tip form an electron beam that passes through the aperture; a photon beam source configured to generate a photon beam comprising photons having a wavelength shorter than about 1 pm and directed onto the emitter protrusion such that at least some of the photons are absorbed by the field emitter cathode; a control circuit configured to modulate an emission current of the electrons by controlling an intensity of the photon beam transmitted from the photon beam source and received by the field emitter cathode; and and a dielectric layer disposed on the first surface adjacent to the field emitter protrusion, wherein the electrode comprises: an extractor disposed on the dielectric layer and maintained at a positive voltage between about 30 V and about 200 V relative to the field emitter cathode; and an anode disposed at an offset distance of at least 1 mm from the emission tip and maintained at a positive voltage of at least 500 V relative to the field emitter cathode, and wherein the device further comprises electron optics configured to direct the primary electron beam to a sample, and wherein the control circuit is configured to control the photon beam source such that the photon beam is generated at a first intensity during a first time period and at a second intensity during a second time period, the second intensity being higher than the first intensity.

15. The device of claim 14, wherein the device is a scanning electron microscope (SEM), wherein the electron optics are configured to demagnify and focus the primary electron beam onto the sample, and wherein the device further comprises a detector for detecting at least one of backscattered electrons and secondary electrons from the sample.

16. The device of claim 14, wherein the device is an electron beam lithography system, wherein the electron optics are configured to demagnify and focus the primary electron beam onto the sample.

17. The device of claim 14, wherein the device is an X-ray source, and wherein the electron optics are configured to direct the primary electron beam to an anode.

18. A light modulating electron source comprising: a field emitter cathode comprising: a silicon substrate having opposing first and second surfaces; ​ and an emitter protrusion having a base integrally connected to the silicon substrate, a body portion extending from the first surface, and an emission tip disposed at a distal end of the body portion; a dielectric layer disposed on the first top surface adjacent to the emitter protrusion, an extractor disposed on the dielectric layer and fixedly positioned adjacent to the field emitter cathode, the extractor configured to generate an electric field that attracts electrons in the silicon substrate toward the emission tip; an anode disposed at an offset distance of at least 1 mm from the emission tip and having an aperture, the anode configured such that electrons emitted from the emission tip form an electron beam that passes through the aperture; a photon beam source configured to generate photons comprising a wavelength shorter than about 1 pm and configured to direct the photon beam onto the emitter protrusion such that at least some of the photons are absorbed by the field emitter cathode; and and a control circuit configured to modulate an emission current of the electron beam by controlling an intensity of the photon beam transmitted from the photon beam source and received by the field emitter cathode, wherein a difference between a nominal height of the extractor above the first surface and a height of the emission tip above the first surface is about ±300 nm, wherein the extractor is maintained at a first positive voltage relative to the field emitter cathode, and wherein the anode is maintained at a second positive voltage relative to the field emitter cathode, the second positive voltage being greater than the first positive voltage, and wherein the control circuit is configured to control the photon beam source such that the photon beam is generated at a first intensity during a first time period and at a second intensity during a second time period, the second intensity being higher than the first intensity.

19. The electron source of claim 18, wherein the emission tip has a lateral dimension in a range of 1 nm to 50 nm, wherein the electron source further comprises an oxidation-resistant coating disposed on the silicon substrate such that it completely and continuously covers the emission tip, and wherein the oxidation-resistant coating comprises one of boron, a boride, and a carbide and has a thickness between 1 nm and 10 nm.

20. The electron source of claim 19, wherein the extractor is maintained at at least 30 V relative to the field emitter cathode, and the anode is maintained at at least 500 V relative to the field emitter cathode.

21. The electron source of claim 18, wherein the photon beam source comprises a light source configured to generate the photons having a wavelength greater than 250 nm.

22. The electron source of claim 21, wherein the photon beam source further comprises at least one of a focusing device and a light modulation device disposed in a path of the photon beam between the light source and the field emitter cathode.

23. The electron source of claim 22, wherein the focusing device comprises at least one mirror configured to direct the photon beam through an aperture formed in the extractor.

24. The electron source of claim 22, wherein the control circuit is configured to control the intensity of the photon beam by controlling at least one of the light source and the light modulation device.

25. The electron source of claim 24, further comprising at least one monitor configured to measure the emission current of the electron beam and generate a corresponding emission current measurement, wherein the control circuit is further configured to generate a control signal from the emission current measurement, and wherein at least one of the light source and the light modulation device is controlled by the control signal.

26. The electron source of claim 18, wherein the field emitter cathode comprises a plurality of field emitter protrusions integrally connected to the first surface of the silicon substrate and arranged in a two-dimensional periodic pattern.

27. A light-modulated electron source, comprising: a field emitter cathode including a silicon substrate having opposing first and second surfaces; and an emitter protrusion having a base integrally connected to the silicon substrate, a body portion extending from the first surface, and an emission tip disposed at a distal end of the body portion; a dielectric layer disposed on the first top surface adjacent to the emitter protrusion, an extractor disposed on the dielectric layer and fixedly positioned adjacent to the field emitter cathode, the extractor configured to generate an electric field that attracts electrons in the silicon substrate toward the emission tip; an anode disposed at an offset distance of at least 1 mm from the emission tip and having an aperture, the anode configured such that electrons emitted from the emission tip form an electron beam that passes through the aperture; a photon beam source configured to generate photons including having a wavelength shorter than about 1 pm and configured to direct the photon beam onto the emitter protrusion such that at least some of the photons are absorbed by the field emitter cathode; and a control circuit configured to modulate an emission current of the electron beam by controlling an intensity of the photon beam transmitted from the photon beam source and received by the field emitter cathode, wherein the control circuit is configured to control the photon beam source such that the photon beam is generated at a first intensity during a first time period and at a second intensity during a second time period, the second intensity being higher than the first intensity, wherein the silicon substrate comprises p-type doped silicon, and wherein the anode is further configured to generate an electric field at the emission tip such that a potential barrier of the field emitter cathode is maintained above a Fermi level of the field emitter cathode at the emission tip, and wherein the control circuit is configured to control the photon beam source such that the first intensity of the photon beam is minimized, thereby minimizing electron emission from the emission tip during the first time period, and such that the second intensity is higher than the first intensity during the second time period, wherein a sufficient number of the photons are absorbed by the field emitter cathode to generate a photon-assisted field emission of the electron beam from the emission tip. ​ 28. An apparatus comprising an electron source configured to generate an electron beam, wherein the electron source comprises: a field emitter cathode comprising: a silicon substrate having opposing first and second surfaces; and an emitter protrusion having a base integrally connected to the silicon substrate, a body portion extending from the first surface, and an emission tip disposed at a distal end of the body portion; an extractor having a first aperture and configured to generate and apply an electric field on the field emitter cathode such that electrons emitted from the emission tip form the electron beam and such that the electron beam passes through the first aperture; an anode having a second aperture and disposed at an offset distance of at least 1 mm from the emission tip, the anode configured such that the electron beam passes through the second aperture; a photon beam source configured to generate photons comprising wavelengths shorter than about 1 pm and configured to direct the photon beam onto the emitter protrusion such that at least some of the photons are absorbed by the field emitter cathode; and a control circuit configured to modulate an emission current of the electron beam by controlling an intensity of the photon beam transmitted from the photon beam source and received by the field emitter cathode, wherein the apparatus further comprises electron optics configured to direct a primary electron beam generated by the electron source to a sample, wherein a difference between a nominal height of the extractor above the first surface and a height of the emission tip above the first surface is about ± 300 nm, wherein the extractor is maintained at a first positive voltage relative to the field emitter cathode, and wherein the anode is maintained at a second positive voltage relative to the field emitter cathode, the second positive voltage being greater than the first positive voltage, and wherein the control circuit is configured to control the photon beam source such that the photon beam is generated at a first intensity during a first time period and at a second intensity during a second time period, the second intensity being higher than the first intensity.

29. The apparatus of claim 28, wherein the apparatus is a scanning electron microscope (SEM), wherein the electron optics are configured to demagnify and focus the primary electron beam onto the sample, and wherein the apparatus further comprises a detector for detecting at least one of backscattered electrons and secondary electrons from the sample.

30. The apparatus of claim 28, wherein the apparatus is an electron beam lithography system, wherein the electron optics are configured to demagnify and focus the primary electron beam onto the sample.

31. The apparatus of claim 28, wherein the apparatus is an X-ray source, and wherein the electron optics are configured to direct the primary electron beam to an anode.

32. The apparatus of claim 28, further comprising a dielectric layer disposed on the first surface adjacent to the emitter protrusion, wherein the extractor is disposed on the dielectric layer such that the dielectric layer is disposed between the extractor and the substrate, ​ wherein the emission tip has a lateral dimension in a range of 1 nm to 50 nm, wherein the electron source further comprises an oxidation-resistant coating disposed on the silicon substrate such that it completely and continuously covers the emission tip, and wherein the oxidation-resistant coating comprises one of boron, a boride, and a carbide and has a thickness of between 1 nm and 10 nm.

33. The apparatus of claim 28, wherein the photon beam source comprises a light source configured to generate the photons with a wavelength greater than 250 nm.

34. The apparatus of claim 33, wherein the photon beam source further comprises at least one of a focusing device and a light modulation device disposed in a path of the photon beam between the light source and the field emitter cathode.

35. The apparatus of claim 28, wherein the field emitter cathode comprises a plurality of field emitter protrusions integrally connected to the first surface of the silicon substrate and arranged in a two-dimensional periodic pattern.

36. An apparatus comprising an electron source configured to generate an electron beam, wherein the electron source comprises: a field emitter cathode comprising a silicon substrate having opposing first and second surfaces; and an emitter protrusion having a base integrally connected to the silicon substrate, a body portion extending from the first surface, and an emission tip disposed at a distal end of the body portion; an extractor having a first aperture and configured to generate and apply an electric field on the field emitter cathode such that electrons emitted from the emission tip form the electron beam and such that the electron beam passes through the first aperture; an anode having a second aperture and disposed at an offset distance of at least 1 mm from the emission tip, the anode configured such that the electron beam passes through the second aperture; a photon beam source configured to generate photons comprising a wavelength shorter than about 1 pm and configured to direct the photon beam onto the emitter protrusion such that at least some of the photons are absorbed by the field emitter cathode; and a control circuit configured to modulate an emission current of the electron beam by controlling an intensity of the photon beam transmitted from the photon beam source and received by the field emitter cathode, wherein the apparatus further comprises electron optics configured to direct a primary electron beam generated by the electron source to a sample, and wherein the extractor is maintained at at least 30 V relative to the field emitter cathode, and the anode is held at at least 500 V relative to the field emitter cathode, and wherein the control circuit is configured to control the photon beam source such that the photon beam is generated at a first intensity during a first time period and at a second intensity during a second time period, the second intensity being higher than the first intensity. ​

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