Electron gun and electron microscope

Abstract

An electron gun for an electron microscope or similar device includes a field emitter cathode having a field emitter projection extending from an output surface of a single crystal silicon substrate and an electrode configured to enhance electron emission from the tip of the field emitter projection to generate a primary electron beam. A continuous TiN layer is disposed directly on at least the tip of the field emitter projection utilizing a process that minimizes oxidation and defects in the TiN layer.

Description

[Technical field]

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS This application claims priority to a provisional patent application, filed on March 18, 2022, and assigned U.S. Application No. 63 / 321,112, the disclosure of which is incorporated herein by reference.

[0002] The present disclosure relates generally to electron guns suitable for use in scanning electron microscopes, electron beam lithography systems, and other systems suitable for reviewing and inspecting specimens (e.g., photomasks, reticles, and semiconductor wafers) under test, and more particularly to electron guns that utilize cooled electron field emitters and review and inspection systems that include these electron guns. [Background technology]

[0003] In the integrated circuit (IC) industry, as the size of defects and particles to be inspected continues to become smaller, potentially down to tens of nanometers (nm) or less, more sensitive inspection tools are required to detect them. These inspection tools operate at high speeds to inspect at least a large portion of the area of ​​a photomask, reticle, or wafer in a short period of time. For example, high speed inspections performed during IC manufacturing typically require less than an hour. More detailed high speed inspections performed for research and development (R&D) or troubleshooting may take up to several hours. High speed inspection tools can scan the sample surface using an incident energy beam with a pixel or spot size larger than the dimensions of the surface defect or particle of interest and monitor the amount of energy repelled from the sample surface. Thus, surface defects or unwanted particles are detected by subtle changes in the amount of energy repelled. High speed inspections are most often performed in manufacturing using inspection tools operated with an ultraviolet (UV) light source. More detailed high speed inspections performed in R&D may be performed using an electron gun.

[0004] After a defect or particle is found by high speed inspection, it is often necessary to create a higher resolution image and / or perform a material analysis to determine the origin or type of the particle or defect. This process is commonly referred to as review. Review is usually performed by a Scanning Electron Microscope (SEM). Review SEMs used in semiconductor manufacturing are typically required to review many thousands of potential defects or particles each day. Therefore, review SEMs are required to operate at speeds such that they can be operated to scan each defect / particle in as little as a few seconds.

[0005] Electron microscopes (e.g., SEMs) utilize an electron gun (i.e., electron source) to generate and direct an electron beam at a sample of interest. Thermionic sources typically include a tungsten or lanthanum hexaboride (LaB6) cathode. During thermionic emission, electrons boil off from the material surface if the electron thermal energy is high enough to overcome the potential barrier of the surface. Thermionic emitters typically require high temperatures for operation (e.g., above 1300K) and suffer from several drawbacks, including inefficient power consumption, wide energy spread, short lifetime, low current density, and limited brightness.

[0006] Currently, thermionic sources are the most common type of electron gun, but the demand for more efficient electron guns has prompted the development of Schottky emitters and electron field emitters.

[0007] Schottky emitters are generally made of a zirconium oxide (ZrO x The Schottky emitter is made of a tungsten wire coated with a layer of ZnO. This provides a much lower work function (up to 2.9 eV) than conventional thermionic emitters. The Schottky emitter achieves enhanced thermionic emission due to an effective potential barrier lowering caused by the image charge effect under an external electric field. However, thermally assisted Schottky emitters are difficult to fabricate at high temperatures (>1000 K) and in high vacuum (~10 -7Torr. Also, the high operating temperatures cause the electron emission energy spread to exceed the desired range. Thus, as IC feature sizes (i.e., target defect / particle sizes) decrease, Schottky emitters do not allow for the narrower energy spread, higher brightness, and higher current density electron guns that are desirable for semiconductor wafer and mask inspection, review, and lithography.

[0008] Electron field emitters (herein referred to as "field emitters") typically have a conical emitter tip (cathode), a circular gate aperture, and an anode located in a vacuum. In operation, under the application of an external electric field, a potential difference is created across the emitter cathode, gate, and anode. This results in a high electric field at the surface of the tip. Field emission occurs when an electric field is applied high enough to reduce the potential barrier at the tip-vacuum interface so that, at room temperature, electrons can tunnel through the barrier and travel through the gate aperture toward the more positively biased anode. Electrons in a field emitter are initially in internal thermodynamic equilibrium, and most of the emitted electrons escape by Fowler-Nordheim tunneling from electronic states close to the Fermi level of the emitter. In contrast, in a Schottky emitter, many electrons escape from states well above the Fermi level, over the top of the field-reduced barrier.

[0009] Early field emitter design efforts focused on the development of metallic field emitters. In particular, the Spindt-type molybdenum field emitter is perhaps the most common metallic field emitter. Molybdenum has a low resistivity (53.4 nΩm at 20°C) and a high melting point (2896 K). However, metallic emitters have several drawbacks, including non-uniformity due to metal deposition techniques and, more seriously, emission current degradation, mainly due to oxidation.

[0010] With the advent of modern semiconductor manufacturing techniques, semiconductor field emitters, especially silicon field emitters, have been investigated. Single crystal silicon is an attractive material for field emitters. Silicon crystals can be grown with high purity and few crystal defects. The electrical conductivity of silicon can be altered by doping and / or applying a voltage. More importantly, silicon has a well-developed technology base.

[0011] A typical prior art silicon field emitter structure is shown in FIG. 5. A silicon substrate 51 is typically n-doped. An optional gate layer 57 is attached to a dielectric layer 56, which may include one or more insulating layers, to form a cone-shaped emitter 54 on the silicon substrate 51. The optional gate layer 57 controls and extracts the emission current. A third electrode (i.e., anode, not shown) faces the gate layer 57 and is spaced from the cathode by a distance on the order of a few hundred microns. This is a typical triode silicon field emitter configuration. Note that the field emitter can be used as a diode if the gate layer 57 is eliminated. A bias voltage is applied across the emitter structure, resulting in quantum tunneling of electrons. A large electric field is created on the surface of the emitter tip, causing electrons to be emitted from the tip. Using this type of silicon field emitter to fabricate a cryogenic (i.e., room temperature emitting) electron gun for SEM tools for semiconductor inspection / renewal / lithography tools eliminates many of the problems associated with Schottky and thermionic emitters.

[0012] Silicon field emitters have shown promise in recent years, but are not yet commercially available. One serious problem with using silicon to form field emitters is that silicon is highly reactive, resulting in a thermal decomposition rate of about 10 -10Even at pressures of 10 Torr, contamination can occur within a few hours. Silicon surfaces are prone to the formation of native oxides. Even in vacuum, the silicon surface reacts with small amounts of oxygen and water in the vacuum, eventually forming native oxides. The interface between silicon and silicon dioxide has defects (dangling bonds) that increase the probability of electron recombination. Furthermore, the band gap of silicon dioxide is large (about 9 eV), and even thin oxides create an additional barrier higher than the work function that electrons must overcome to escape. For example, the thickness of native oxide on a smooth silicon surface is typically about 2 nm. In some circumstances, oxidation can also change the shape of the field emitter. These issues have led to low brightness and current, as well as poor stability, reliability, scalability, and uniformity, and have hindered the commercial use of silicon field emitters. Periodic flashing (i.e., temporary increase in tip temperature) is required to remove these contaminants. During that time, the system / instrument in which the low-temperature electron gun is realized is unavailable. During tip flashing, collection of higher resolution images and / or material analysis is stopped. This is problematic in the semiconductor industry, where inspection, review, and lithography tools are required to operate reliably and continuously for long periods without interruption. Therefore, most commercially available SEM tools used for semiconductor inspection, review, and lithography include electron guns that implement Schottky emitters.

[0013] Research efforts have expanded to explore surface treatments and coatings for field emitters to lower starting voltages, increase emission current density, reduce noise, and improve stability and performance. These treatments can include coating the emitter tip with refractory metals, silicides, carbides, diamond, etc. However, these coating materials are usually limited by manufacturing processes that produce smooth and uniform coating surfaces, and / or often suffer from oxide layers that form on the coating surface, creating additional energy barriers. For these reasons, low-temperature electron guns utilizing coated silicon field emitters have not yet been commercialized. [Prior art documents] [Patent documents]

[0014] [Patent Document 1] U.S. Patent No. 10943760 Summary of the Invention [Problem to be solved by the invention]

[0015] Therefore, there is a need for an electron gun and an electron microscope incorporating such an electron gun that overcomes some or all of the limitations of the prior art. [Means for solving the problem]

[0016] In a first embodiment, an electron gun is provided. The electron gun comprises a field emitter configured to generate a primary electron beam. The field emitter comprises a monocrystalline silicon substrate having opposed first and second surfaces and at least one integral field emitter projection. The field emitter projection has a fixed portion integrally connected to the silicon substrate and extending from the second surface to a tip. The field emitter further comprises a titanium nitride (TiN) layer contiguously disposed at least at the tip such that, in operation, electrons escaping from the monocrystalline silicon substrate through the tip of the field emitter projection to form the primary electron beam pass only through the TiN layer. The TiN layer comprises at least 75 atomic percent TiN.

[0017] In one example, the TiN layer comprises greater than 80 atomic percent TiN.

[0018] In one example, the single crystal silicon substrate is about 10 19 cm -3 In another example, the single crystal silicon substrate may be p-type doped at a doping level of less than about 10 15 cm -3 About 10 19 cm -3In yet another example, the single crystal silicon substrate may have a thickness greater than about 10 μm and may be n-type doped with a doping level between about 10 14 cm -3 It is p-doped at a doping level of less than 100 nm.

[0019] The electron gun can include a light source configured to illuminate the first surface of the single crystal silicon substrate, and the light source can include one of a laser diode and a light emitting diode configured such that an emission current of the primary electron beam is controllable in proportion to an amount of light transmitted from the light source into the single crystal silicon substrate.

[0020] The TiN layer may have a thickness in the range of 2 nm to 150 nm.

[0021] The portion of the TiN layer located within a 100 nm radius from the tip may contain less than 10 atomic percent oxygen.

[0022] The portion of the TiN layer located within a 100 nm radius from the tip may contain less than 10 atomic percent carbon.

[0023] The field emitter protrusions may include one of a cone, a pyramid, or a rounded whisker shape. The tip of the field emitter protrusion may have a lateral dimension of less than 300 nm. In one example, the tip of the field emitter protrusion has a lateral dimension of more than 10 nm.

[0024] The field emitter protrusions may include one of a cone, a pyramid, or a rounded whisker shape. The tips of the field emitter protrusions may have a diameter of less than 300 nm.

[0025] The field emitter may be configured to operate in a reverse bias mode in which a depletion layer is created adjacent the second surface in response to an applied electric field.

[0026] The field emitter may further comprise a first dielectric layer disposed on the second surface of the single crystal silicon substrate adjacent to the field emitter protrusion, a first conductive gate disposed on the first dielectric layer such that an edge of the first conductive gate is spaced a non-zero distance from the tip of the field emitter protrusion, a second dielectric layer disposed on the second surface of the first conductive gate, and a second concentrating gate disposed on the second dielectric layer such that an edge of the second conductive gate is spaced a non-zero distance from the tip of the field emitter protrusion. The thicknesses of the first and second dielectric layers are each between 500 nm and 3 μm, the first dielectric layer being ±300 nm relative to the height of the field emitter protrusion.

[0027] The electron gun may include a plurality of field emitter projections arranged in a two-dimensional periodic pattern. Each of the field emitter projections may have a corresponding anchor portion integrally connected to the silicon substrate and extending from the second surface to a tip. The TiN layer may be confined and disposed on at least a tip of each of the plurality of field emitter projections. In one example, the field emitter may further include a first dielectric layer disposed on the second surface of the single crystal silicon substrate adjacent to the plurality of field emitter projections, a first conductive gate disposed on the first dielectric layer such that an edge of the first conductive gate is spaced a non-zero distance from the tip of each of the plurality of field emitter projections, a second dielectric layer disposed on the second surface of the first conductive gate, and a second concentrating gate disposed on the second dielectric layer such that an edge of the second conductive gate is spaced a non-zero distance from the tip of each of the plurality of field emitter projections. The first and second dielectric layers each have a thickness of 500 nm to 3 μm, the first dielectric layer being ±200 nm relative to the nominal height of the plurality of field emitter protrusions.

[0028] In a second embodiment, a SEM is provided. The SEM includes an electron gun having a field emitter configured to generate a primary electron beam, electron optics configured to demagnify and focus the primary electron beam on a specimen, and a detector configured to detect at least one of backscattered electrons and secondary electrons from the specimen in response to the primary electron beam. The field emitter includes a monocrystalline silicon substrate having opposed first and second surfaces and having at least one integral field emitter protrusion. The field emitter protrusion has a stationary portion integrally connected to the silicon substrate and extending from the second surface to a tip. The field emitter further includes a titanium nitride (TiN) layer confinedly disposed at least at the tip such that, in operation, electrons escaping from the monocrystalline silicon substrate through the tip of the field emitter protrusion to form the primary electron beam pass only through the TiN layer. The TiN layer includes at least 75 atomic percent TiN. A portion of the TiN layer located within a 100 nm radius of the tip includes less than 10 atomic percent oxygen.

[0029] In a third embodiment, an apparatus is provided that includes an electron gun. The electron gun includes a field emitter configured to generate a primary electron beam. The field emitter includes a monocrystalline silicon substrate having opposed first and second surfaces and at least one integral field emitter projection. The field emitter projection has a fixed portion integrally connected to the silicon substrate and extending from the second surface to a tip. The field emitter further includes a titanium nitride (TiN) layer confinedly disposed at least at the tip such that, in operation, electrons escaping from the monocrystalline silicon substrate through the tip of the field emitter projection to form the primary electron beam pass only through the TiN layer. The TiN layer includes at least 75 atomic percent TiN. A portion of the TiN layer located within a 100 nm radius of the tip includes less than 10 atomic percent oxygen. The apparatus may include one of a scanning electron microscope (SEM) system or an electron beam lithography system. [Brief description of the drawings]

[0030] For a fuller understanding of the nature and objectives of the present disclosure, reference should be made to the following detailed description taken in conjunction with the accompanying drawings, which are listed below. [Figure 1] 1 illustrates an exemplary SEM incorporating embodiments of an electron gun, electron optics, backscattered electron detector, and secondary electron detector according to the present disclosure. [Diagram 2] FIG. 2 is a perspective view of an electron gun in a diode configuration with TiN coated silicon field emitter protrusions formed on a silicon substrate according to an exemplary embodiment of the present disclosure. [Figure 2A] FIG. 3 is a cross-sectional view showing the electron gun of FIG. 2. [Diagram 3] FIG. 13 is a cross-sectional side view of an electron gun in a triode configuration with a TiN coated silicon field emitter projection formed on a silicon substrate and a gate layer according to another embodiment of the present disclosure. [Figure 4A] FIG. 1 illustrates a cross-sectional side view of a multiple electron beam electron gun with an array of TiN coated silicon field emitter protrusions formed on a silicon substrate in a diode and triode configuration according to an embodiment of the present disclosure. [Figure 4B] FIG. 1 illustrates a cross-sectional side view of a multiple electron beam electron gun with an array of TiN coated silicon field emitter protrusions formed on a silicon substrate in a diode and triode configuration according to an embodiment of the present disclosure. [Diagram 5] FIG. 1 is a cross-sectional side view of a conventional electron gun with a silicon field emitter. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0031] Although the claimed subject matter is described in terms of certain embodiments, other embodiments, including embodiments that do not provide all of the advantages and features described herein, are within the scope of the present disclosure. Various structural, logical, process step, and electronic changes may be made without departing from the scope of the present disclosure. Accordingly, the scope of the present disclosure is defined solely by reference to the appended claims.

[0032] The embodiments disclosed herein relate to an electron gun for use in a SEM or other device / instrument. The electron gun comprises a field emitter (cathode) having one or more field emitter protrusions. The protrusion(s) are integrally connected to a single crystal silicon substrate such that they extend from an output (second) surface of the silicon substrate. The electron gun further comprises a substantially pure titanium nitride (TiN) layer (i.e., comprising or consisting of at least 75% TiN) hermetically disposed at least at the tip (free end) portion of the field emitter protrusions. The TiN layer is formed on the protrusion(s) such that, in operation, electrons escaping from the single crystal silicon substrate through the tip pass only through the TiN layer before the primary electron beam is formed. The integral connection of the field emitter protrusions to the silicon substrate (i.e., by etching portions of the single crystal silicon substrate such that the remaining unetched portions of the single crystal silicon form the protrusions) avoids the relatively high work function of the silicon substrate. This takes advantage of the field enhancement achieved by applying an external electric field to the substrate that is strong enough to reduce the potential barrier outside the tip. Thus, electrons can tunnel (i.e., by quantum-mechanical tunneling) through a reduced potential barrier. Furthermore, using a continuous, substantially pure TiN layer to hermetically seal the field emitter protrusion offers several advantages over conventional surface treatments and coatings. First, various techniques are known to form a substantially pure TiN layer as a pinhole-free coating that prevents native oxide formation on the output surface of the field emitter protrusion. Second, a substantially pure TiN layer has a smaller band gap than conventional coatings such as silicon dioxide, thereby increasing electron flow during operation. Third, TiN-coated silicon emitters provide thermally stable electron emission compared to uncoated silicon emitters.Thus, by producing an electron gun with a field emitter having a field emitter protrusion integrally formed on a single crystal silicon substrate and hermetically protected by a layer of nearly pure TiN, the embodiments disclosed herein facilitate the production of low-temperature electron guns that have the advantageous properties of silicon (e.g., absorption at wavelengths below 1 μm, high purity / low defect material, and long electron recombination times) while avoiding the negative aspects that have hindered widespread commercial production of low-temperature electron guns using coated silicon field emitters.

[0033] The electron gun of the embodiments disclosed herein utilizes the electric field enhancement by the field emitter protrusion to avoid the relatively high work function of silicon. A second electrode (i.e., anode) may be placed facing the field emitter cathode. The anode may be made of a metal such as tungsten, molybdenum, or stainless steel. A high voltage source, typically in the kilovolt range, may be used to create an external potential difference between the field emitter protrusion and the anode to create a strong electric field near the tip of the field emitter protrusion so that the electrons preferentially move toward the emitter tip. If a high potential difference between the field emitter protrusion and the anode (such as a voltage of about 1 kV or more) is used, the anode should be spaced about 50 μm from the field emitter protrusion. In one embodiment, the anode is placed at least 50 μm from the emitter tip with a positive voltage of at least 500 V relative to the field emitter protrusion. In another embodiment, the anode is placed near the emitter tip, such as a distance of about 2 μm or less, with a positive potential of less than 100 V relative to the emitter. Placing the anode closer to the emitter tip allows a sufficiently strong electric field to be generated at a lower voltage, which has the advantage of minimizing back-bombardment of the emitter by energetic ions that can shorten the emitter's life, but placing the anode closer to the emitter tip may require more precise alignment of the aperture in the anode to the emitter tip.

[0034] Field emission occurs when the applied electric field is large enough to reduce the potential barrier on the silicon / vacuum interface and electrons can tunnel through it (i.e., quantum mechanical tunneling). The electron density is determined by the local doping level and potential in the semiconductor, and the field emitter can be configured to operate in reverse bias mode, like a pn diode, where a depletion layer is created by the high electric field at the emitter surface. The interface between the vacuum and the TiN-coated, p-doped field emitter protrusion can form a pn junction, where the vacuum can be considered as an n-type medium. In this case, the conduction and valence bands bend downwards at the surface. If the electric field is sufficient to push the bottom of the conduction band below the Fermi energy level, there will be a large population of electrons at the apex of the emitter tip, and a current in the nA to μA range will be generated. In another embodiment, the electric field can be kept at a slightly lower level so that the bottom of the conduction band remains just above the Fermi level, and little or no emission current is generated. In this embodiment, light can be used to generate the electron-hole pairs. The electrons thus generated are strongly attracted to the tip, and many of them are emitted from the tip as an emission current. In this embodiment, the emission current can be controlled by controlling the level of light on or near the field emitter substrate. In yet another embodiment, a large portion of the silicon wafer can be n-doped. This allows a large amount of electrons to be available for forming the emission current. The emission current density can be estimated by a modified version of the Fowler-Nordheim theory, which takes into account the field enhancement factor due to the field emitter protrusion. Thus, by fabricating an electron emitter structure having both a TiN layer and a field emitter protrusion formed on a single crystal silicon substrate, the embodiments disclosed herein provide the attractive features of a field emitter (e.g., small emitter size, low power consumption, high brightness, high current density, high speed, and long emitter life) while providing the advantageous properties of silicon (e.g., high purity / low defect material, long electron recombination time, and advanced silicon-type manufacturing processes) while avoiding the downsides that have hindered widespread commercial production of silicon-type field emitter structures.

[0035] Additional layers and structures may further enhance the advantageous properties of the disclosed emitter structure. In one embodiment, one or several gate layers or control electrodes may be placed slightly lower than the height (i.e., from base to tip) of the field emitter protrusion. This further enhances the electric field at the emitter tip, providing fast and precise control of the emission current. The gate layer is typically formed on one or several insulating layers deposited on a substrate. If there are several gate layers, the spacers between these gate layers are the insulating layers. In another embodiment, one or several gate layers or control electrodes are placed slightly higher (100 to 200 nm) than the height (i.e., from base to tip) of the field emitter protrusion. This reduces current degradation, eliminates field relaxation, and maintains a high emission current when used with one or several focusing electrodes placed at least 400 nm lower than the gate layer. In yet another embodiment, multiple electron beam guns having multiple disclosed field emitter protrusions may be arranged in a two-dimensional periodic pattern (e.g., a field emitter array (FEA)) and positioned on the emitter surface to emit electrons in the presence of an electric field.

[0036] The electron gun disclosed herein may be incorporated into inspection, metrology, and review SEMs. Typically, the SEM includes an electron gun, electron optics, and a detector. The electron optics may be configured to demagnify and focus the primary electron beam on the specimen. The detector may be configured to detect backscattered and / or secondary electrons from the specimen. The electron gun generates a primary electron beam that is directed at the specimen. The electron gun has a TiN-coated silicon field emitter protrusion or a plurality of TiN-coated silicon field emitter protrusions. The electron gun may be configured to operate the silicon-type field emitter as a diode or triode. The electron optics demagnify and focus the primary electron beam on the specimen. The electron optics may further include a deflector capable of scanning the primary electron beam over an area of ​​the specimen. When the primary electron beam strikes the specimen, the specimen absorbs many of the electrons from the primary electron beam but scatters some of the electrons (backscattered electrons). Due to the absorbed energy, secondary electrons are emitted from the sample along with X-rays and Auger electrons. The secondary electrons can be collected by a secondary electron detector. The backscattered electrons can be collected by a backscattered electron detector. The X-rays can be collected by an X-ray detector.

[0037] The apparatus may incorporate an embodiment of the electron gun described herein. The apparatus may be an SEM with electron optics configured to demagnify and focus the primary electron beam on the sample. The apparatus may further include a detector configured to detect at least one of backscattered electrons from the sample and secondary electrons. In another embodiment, the apparatus may be an electron beam lithography system with electron optics configured to demagnify and focus the primary electron beam on the target. The apparatus may further include a modulator to modulate the intensity of the electron beam.

[0038] 1 shows an exemplary inspection, metrology, or review SEM system 100 configured to inspect or review a specimen 131, such as a semiconductor wafer, reticle, photomask, or other workpiece. SEM system 100 includes an electron gun 140, electron optics disposed in upper and lower stages 160 and 170 and configured to focus a primary electron beam 150 toward the specimen 131, one or more detectors disposed in lower stage 170 to detect electrons repelled from the specimen 131, and a computer 190.

[0039] The electron gun 140 includes a silicon field emitter (cathode) 141 disposed within a chamber 143 and an electrode 142. The field emitter 141 is configured in a diode or triode configuration and emits electrons that move through an applied electric field generated by the electrode 142 (which may include an anode). This forms a primary electron beam 150 having a desired beam energy, typically a beam current on the order of 1 nA to 10 μA, that exits the chamber 143 through an aperture 144. The electron gun 140 may further include a magnetic lens (not shown). The interior of the chamber 143 is maintained under high vacuum conditions during operation by a pump 145. This protects the field emitter 141 from contaminants and minimizes back bombardment by energetic ions. Suitable vacuum conditions may be maintained in the electron gun 140 while allowing the primary electron beam 150 to pass through the aperture 144 into the upper stage 160. The vacuum pump 145 may include at least one of an ion pump and a getter pump. Aperture 144 may have a diameter between about 100 μm and about 2 mm. Aperture 144 may function as both a beam limiting aperture and a differentially pumped aperture. In one embodiment, chamber 143, aperture 144, and vacuum pump 145 collectively pump field emitter cathode 141 to about 100 μm during operation of electron gun 140. -10 The vacuum chamber is configured to maintain a vacuum level of less than Torr.

[0040] 1, the field emitter cathode 141 generally comprises a single crystal silicon substrate 101 having at least one integrally connected field emitter protrusion 104. The silicon substrate 101 is essentially defect-free and has a thickness T1, measured between a flat (first) surface 102 and an opposing output (second) surface 103, in the range of between about 10 nm and about 800 μm. 19 cm -3 In another embodiment, the silicon substrate 101 is p-type doped at a doping level of less than about 10 15 From 10 19 cm -3The field emitter protrusion 104 is n-doped with a doping level between 100 μm and 100 μm. The field emitter protrusion 104 is integrally connected to the silicon substrate 101 and has a fixed base 105 extending from a surface 103 to a tip (free end) 106. In one example, the height H1 measured in a direction perpendicular to the surface 103 from the base 105 to the tip 106 is in the range of 0.5 μm to 5 μm, such as about 2 μm or less. In one embodiment, the field emitter protrusion 104 is fabricated by typical CMOS fabrication techniques, that is, by etching or otherwise removing portions of the single crystal material forming the substrate 101 (i.e., the field emitter protrusion 104 and the remainder of the substrate 101 are remnants of the same single crystal structure). For example, the field emitter protrusion 104 may be formed by depositing a silicon dioxide or silicon nitride mask by plasma enhanced chemical vapor deposition (PECVD) on the surface 103 of the substrate 101. The mask is then photolithographically patterned over a portion of surface 103 while exposing portions of surface 103 surrounding the masked portions. Portions of the single crystal material are then removed using a dry etching process (e.g., reactive ion etching (RIE), inductively coupled plasma (ICP) etching, or electron cyclotron resonance (ECR) etching), a wet etching process, or a combination of dry and wet etching. This leaves protrusions 104 surrounded by surface 103. In one embodiment, field emitter protrusions 104 may be formed in a variety of cantilever shapes, such as rounded whiskers (e.g., thin cylinders with rounded apexes), rounded cones, or pyramids.

[0041] In one example, a continuous substantially pure TiN layer 110 is hermetically disposed over at least the tip 106 of the field emitter projection 104 such that, in operation, electrons escaping from the substrate 101 through the tip 106 to form the primary electron beam 150 pass only through the continuous substantially pure TiN layer 110 (i.e., no other material / layer is formed on the TiN layer 110 adjacent to the tip 106). As used herein, the term "hermetically disposed" is defined as being formed substantially oxygen-free over at least the tip (free end) portion 106 of the field emitter projection 104 to form a vacuum-tight seal. As used herein, the term "continuous" with respect to the TiN layer 110 is defined to mean that the TiN layer 110 is uninterrupted over a designated portion of the substrate 101, including at least the area of ​​the tip 106 from which electrons are emitted. That is, most of the electrons emitted from the field emitter projection 104 are emitted from a region of the tip 106 having a radius of, for example, on the order of tens of nanometers, and this region is continuously (uninterrupted) covered by the TiN layer 110. In alternative embodiments, the TiN layer 110 may be continuous over a majority of the outer circumferential surface of the field emitter projection 104, including the tip 106, or may be continuous over a larger region including the entire outer circumferential surface of the field emitter projection 104, or over a portion or all of the output surface 103 surrounding the base 105 of the field emitter projection 104.

[0042] Various known techniques may be utilized to form the continuous substantially pure TiN layer 110. In one embodiment, the TiN layer 110 is formed by reactive magnetron sputtering with a steady power of 2 kW, a chamber pressure of 3.4 mTorr, an Ar / N2 flow rate of 10 / 16 sccm, and a rotating plate rotation speed of 20 rpm. In another embodiment, the TiN layer 110 is formed by a two-step rapid thermal nitridation process in an NH3 ambient where the titanium layer is thermally converted to a TiN / TiSi2 bilayer. The continuous substantially pure TiN layer 110 formed by these techniques avoids the problem of silicon oxidation by ensuring a hermetic sealing of the silicon surface against oxidation. The TiN layer 110 may contain various impurities, such as additional titanium atoms. Oxygen atoms may be present either at the TiN-Si interface, on the outer layer surface, or within the crystal lattice. For example, the ratio of nitrogen to titanium may vary at the interface of the TiN layer 110 with the substrate 101. The TiN layer 110 may contain a few atomic percent of oxygen (e.g., less than 10%, or less than 5%) remaining at the interface between the TiN layer 110 and the silicon surface of the protrusion 104. However, due to the hermetic sealing, this oxygen content does not increase significantly over time (e.g., over a period of one year). This low oxygen to silicon ratio means that there is no continuous silicon dioxide layer at the interface. Thus, electrons can easily escape from the silicon surface through the portion of the TiN layer 110 covering the tip 106. This low oxygen to silicon ratio also means that there is no continuous titanium oxide (TiO2) layer at the interface. Thus, electrons can easily escape from the TiN layer 110. Thus, the TiN layer 110 may include, consist of, or consist essentially of a 1:1 (titanium to nitrogen) TiN material. A substantially pure TiN layer 110 may include more than 75% 1:1 TiN material. For example, in other specific embodiments, the TiN content may include greater than 80% 1:1 TiN material, greater than 85% 1:1 TiN material, greater than 90% 1:1 TiN material, greater than 95% 1:1 TiN material, greater than 96% 1:1 TiN material, greater than 97% 1:1 TiN material, greater than 98% 1:1 TiN material, greater than 99% 1:1 TiN material. All percentages are atomic percentages.

[0043] The TiN layer 110 may contain less than 10% oxygen and / or less than 10% carbon. The percentages are atomic percent. The TiN layer 110 may contain less than 5%, 4%, 3%, 2%, or 1% oxygen. More than 10% oxygen in the TiN layer 110 may be problematic because TiO2 and SiO2 are dielectric materials that impede electron emission. The TiN layer 110 may contain less than 5%, 4%, 3%, 2%, or 1% carbon. These oxygen and carbon values ​​may be measured within a 100 nm radius from the tip. The oxygen and carbon values ​​may be higher than those listed herein and may be obtained outside of a 100 nm radius from the tip. The carbon percentage may be higher than the oxygen percentage in the TiN layer 110 and still provide reasonable performance.

[0044] By fabricating the electron gun 140 as described herein, a low-temperature electron gun with a coated silicon field emitter overcomes the limitations of the conventional approach. The integral connection of the field emitter protrusions 104 avoids the relatively high work function of the silicon substrate. It takes advantage of the electric field enhancement achieved by applying an external electric field EF to the substrate 101 that is strong enough to reduce the potential barrier outside the tip 106. Thus, electrons E can tunnel (i.e., by quantum mechanical tunneling) through the reduced potential barrier. The resulting electron emission current density can be estimated by a modified version of the Fowler-Nordheim theory. Furthermore, by forming a continuous, substantially pure TiN layer 110 to hermetically seal the tip 106, oxidation of the field emitter output surface is substantially and entirely prevented. Thus, the large barrier to electron emission caused by oxidation in the conventional approach is avoided. Thus, by fabricating an electron gun 140 having a field emitter cathode 141 having protrusions 104 integrally formed on a single crystal silicon substrate 101 and hermetically protected by a continuous layer of substantially pure TiN 110, a low-temperature electron gun can have the advantageous properties of silicon (i.e., absorption at wavelengths below 1 μm, high purity / low defect material, and long electron recombination times) while avoiding the drawbacks that have prevented widespread commercial production of low-temperature electron guns using coated silicon field emitters.

[0045] 1, the upper stage 160 of the SEM 100 includes one or more condenser lenses 165 that demagnify the primary electron beam 150 such that the incident portion of the primary electron beam 150 is contained primarily within a region of the upper surface of the sample 131 having a nominal diameter on the order of 20 nm or less. One or more optional deflectors 167 may be disposed on one or both sides of the condenser lens 165 to adjust (i.e., align, tilt, and / or shift) the primary electron beam 150.

[0046] The lower stage 170 includes a final lens 171 configured to focus the primary electron beam 150 on the sample 131 so that the incident electrons strike the sample 131 within a desired area, and includes various detectors 181, 182a, and 182b to receive electrons redirected from the sample 131. The lower stage 170 further includes one or more deflectors 172 that cooperate with the deflector 167 (if present) to scan the primary electron beam 150 across the area of ​​the sample 131. In one embodiment, the sample 131 is positioned on the stage 130 so that it can be easily moved relative to the electron gun 140 (i.e., to position different surface areas of the sample 131 under the electron column). The incident electrons of the primary electron beam 150 strike the surface of the sample 131 and are repelled or otherwise scattered in a direction determined by the surface features of the sample 131 that they strike. This produces secondary and backscattered electrons that travel away from the sample 131 (e.g., upward in FIG. 1). The secondary electrons are collected and accelerated by electrode 180 and directed to secondary electron detector 181 for detection. Backscattered electrons may be detected by backscattered electron detectors 182a and 182b. In one embodiment, each of electron detectors 181, 182a, and 182b is a solid-state device having at least one analog-to-digital converter. The converter converts an analog output signal generated in response to the number of electrons received into digital form and transmits it as one or more digital image data signals (i.e., image data signal ID1 generated by secondary electron detector 181 and / or signal ID2 generated by backscattered electron detector 182a or 182b) to computer 190. Computer 190 is configured to process image data signals ID1 and ID2 to generate an image 191 of the area of ​​sample 131 scanned by primary electron beam 150.

[0047] 1 illustrates the operation and configuration of electron gun 140 integrated within SEM 100, the electron guns disclosed herein may also be incorporated into other devices, such as electron beam lithography systems having electron optics configured to demagnify and focus a primary electron beam on a target, and a modulator to modulate the intensity of the electron beam.

[0048] FIG. 2 shows a partial perspective view of a cathode portion of an electron gun 200 including a field emitter protrusion 204 integrally connected to a substrate 201 according to an embodiment of the present disclosure. FIG. 2A is a cross-sectional view of the cathode portion of the electron gun 200 along line 2A-2A in FIG. 2. The electron gun 200 can be used in the SEM system 100 of FIG. 1. The electron gun 200 further includes an electrode, such as an anode (e.g., in the electron gun 140 in FIG. 1), facing the protrusion 204. The anode can be spaced apart from the tip 206 of the protrusion 204 by a distance between about one micron and several hundred microns. As in the previous embodiment, the field emitter cathode is formed on a silicon substrate 201 having an upwardly facing output surface 203 from which the field emitter protrusion 204 extends, and a continuous TiN layer 210 disposed on the output surface 203, generally covering the field emitter protrusion 204. As shown in FIG. 2, the field emitter protrusion 204 has a pyramidal shape that may be formed by anisotropic etching. It may have a tilt angle α close to 54.7° (see FIG. 2A), which corresponds to the intersection of the (100) and (111) planes of single crystal silicon. If a sharp field emitter tip 206 is desired, oxidation sharpening may be used, which may be performed at low to moderate temperatures (below about 950° C.) before forming the TiN layer 210. The apex of the sharpened tip 206 may include a substantially flat region with atomic length, such as a surface that is substantially parallel to the crystal plane of the silicon crystal that is substantially parallel to the (100) plane. In FIG. 2A, dimensions D1 and D2 indicate characteristic lateral dimensions of the base region 205 and the tip region 206, respectively. In one embodiment, the ratio of the lateral dimension D1 to the height H1 of the field emitter projection (see FIG. 3) may be in the range of about 1 to about 3, and the diameter (or other lateral dimension) D2 may be in the range of about 10 nm to about 300 nm. The anode (not shown) may be made of a metal such as tungsten, molybdenum, stainless steel, or other material. A voltage source may be used to create an external potential difference between the cathode and anode, which causes the electrons to move preferably toward the tip 206 of the field emitter projection 204. The electron gun 200 applies a strong electric field (about 100 Vμm) to the tip 206. -1to about 10 kVμm -1 ) electrons will likely be emitted from the substrate 201 through the output surface 203 of the field emitter 204, most likely near the tip 206 of the field emitter 204. As described above, the TiN layer 210 is formed using a process that hermetically seals the tip 206 against oxidation. In order for the TiN layer 210 to provide a good hermetic seal without forming a strong barrier to electron emission, the thickness T2 of the TiN layer 205 may be between about 2 nm and 150 nm. Even if a few percent of oxygen remains at the interface between the silicon substrate 201 and the TiN layer 210, no more oxygen can penetrate the TiN layer 210. Thus, no significant further oxidation may occur near the silicon / TiN interface. Because electron emission occurs from a small region near the apex of the tip 206, the quality of the TiN layer 210 near the tip 206 may be important. Thus, portions 210A of TiN layer 210 within radius R of tip 206 (shown in FIG. 2A), where the radius is about D2 or greater, can be formed without pinholes. In portions 210B of TiN layer 210, located outside radius R, a few pinholes or gaps within the coverage of TiN layer 210, or variations in the composition of TiN layer 210, can typically be tolerated without affecting emission.

[0049] The diameter D2 or other lateral dimension may be less than 300 nm. If the tip 206 is too obtuse, there may not be enough electric field enhancement for electron emission without the use of high voltages on the extraction electrodes, which may have other effects. In one example, the diameter D2 or other lateral dimension may be about 100 nm.

[0050] During operation of the cathode 200, substantially all electron emission occurs from a region of diameter D2 at the apex of the tip 206. Any pinholes in this region or in the region immediately adjacent to this region may result in non-uniform or unstable emission due to the different surface work function at the pinhole location compared to the region completely covered by the TiN layer 210. Thus, in one embodiment, there may be no pinholes within the emission region of the cathode 200.

[0051] In addition to being substantially pure TiN, being continuous in the region of the tip 206, and forming a hermetic seal as described above with reference to the TiN layer 110, in one embodiment, the TiN layer 210 may be formed to have a thickness T2 in the range of 2 nm to 150 nm, and may have a uniform thickness or may be formed with a different thickness on the field emitter protrusion 204 than on the other parts of the output surface 203. If the thickness T2 is less than 2 nm, the number of pinholes may increase, which may affect electron emission. If the thickness T2 is more than 150 nm, the diameter D2 may exceed about 300 nm, which may result in insufficient apex sharpness.

[0052] According to an embodiment of the present disclosure, the silicon substrate 201 is 19 cm -3 The silicon may include single crystal silicon (i.e., a single crystal of silicon) that is p-type doped at a doping level of less than about 0.005 Ω cm (i.e., a resistivity of about 0.005 Ω cm or greater). Minority carrier lifetimes and diffusion lengths decrease with increasing dopant concentration. Thus, when the silicon is thin, such as less than about 1 μm, the thickness of the silicon may be reduced by about 10 19 cm -3 On the other hand, when the silicon is thicker than about 1 μm, dopant concentrations of more than about 10 19 cm -3 For silicon thicknesses of more than a few microns, such as 10 μm or more, dopant concentrations of less than about 10 can be utilized to ensure longer carrier lifetimes and low dark currents. 14 cm -3A lower dopant concentration, such as less than about 500 nm, may be utilized. Since electrons are the minority carriers in p-type doped silicon, an embodiment utilizing a p-type silicon electron gun 200 includes an optional light source 220 configured to illuminate light 224 through the backside 202 to generate electron-hole pairs within the silicon substrate 201. The light source 220 may include a high intensity light source, such as a laser diode or a high brightness LED, and may have a bandwidth of about 20 nm or less. The light source 220 may be positioned behind the substrate 201 as shown to illuminate the backside 202, or may be positioned to illuminate the output surface 203 (i.e., positioned above the substrate 201 rather than below it as shown in FIG. 2A). When the light source 220 is positioned below the substrate 201 as shown, the light source 220 is configured to emit light at a wavelength that penetrates relatively deep into the silicon, such as a wavelength greater than about 500 nm. This generates electron-hole pairs away from the bottom surface 202 of the silicon substrate 201. If the light source 220 illuminates the field emitter protrusion 204 from above, a shorter wavelength, such as less than about 550 nm, may be utilized that penetrates shallower into the silicon, thereby generating electron-hole pairs relatively close to the tip 206 of the field emitter protrusion 204. In one embodiment, the light source 220 is configured such that the emission current of the primary electron beam generated by the electron gun 200 (see, e.g., beam 150 in FIG. 1 ) is controllable in proportion to the amount of light 224 emitted from the light source 220 onto the silicon substrate 201.

[0053] In another embodiment, the silicon is about 10 15 cm -3 For example, silicon can be n-doped with a dopant concentration of about 10 15 cm -3 From 10 19 cm -3The silicon may be n-doped with a dopant concentration of 0.1 to 0.5 n. The n-doped silicon has a large number of electrons available in the conduction band that can be drawn towards the tip 206 of the field emitter protrusion 204 to form an emission current. With n-doped silicon, the emission current can be controlled by adjusting the voltage applied to a gate electrode (such as the extractor and focussing electrode 142 of FIG. 1 or gate 307 of FIG. 3) or by adjusting the voltage difference between the anode and the field emitter cathode. If n-doped silicon is used, the optional light source 220 may be omitted. In either case, the field emitter protrusion 204 may be formed by the process described above for forming the field emitter protrusion 104.

[0054] According to another aspect of the disclosure, the field emitter cathode structure of the electron gun 200 may be configured to operate in reverse bias mode, like a pn diode where a depletion layer is created by the high electric field at the output surface 203. The interface between the vacuum and the TiN coated, p-type doped field emitter may form a pn junction where the vacuum is taken as an n-type medium. In this case, the conduction and valence bands bend downward at the surface. If the electric field is sufficient to lower the bottom of the conduction band below the Fermi energy level, electrons will be abundant at the apex of the tip 206 and a current on the order of 1 nA to 1 μA will be generated. If the applied electric field is high enough to reduce the potential barrier at the silicon / vacuum interface and electrons can tunnel through this barrier (quantum mechanical tunneling), field emission will occur. The emission current density can be estimated by a modified version of the Fowler-Nordheim theory, which takes into account the field enhancement factor due to the field emitter.

[0055] According to another aspect of the present disclosure, the TiN layer 210 comprises a continuous TiN film disposed directly on the output surface of the field emitter. As used herein, with respect to the interface between TiN and silicon, the phrase "directly on" refers to the TiN that may form at the Si / TiN interface. xBesides possible thin layers (i.e., several monolayers) including materials such as titanium silicide (x<1) and titanium silicide, there may be continuous intervening layers (e.g., oxides, or SiN x It is intended that no layer (or layer) is present. It is noted that the term "directly on" does not exclude the presence of a small amount of oxide between some portions of the TiN and the silicon. A TiN layer 210 is grown on the clean silicon by known techniques. This results in the TiN forming a pinhole-free coating on the field emitter protrusions 204 having a thickness T2 that is at least in the range of about 2 nm to 150 nm, including all ranges and values ​​in 0.1 nm increments therebetween. Other portions of the coating may include pinhole defects outside the coating on the field emitter protrusions 204, or the entire coating may be pinhole-free. In one example, pinhole defects outside the field emitter protrusions 204 are minimized.

[0056] For example, any native oxide may be removed from the silicon prior to the formation of the TiN layer 210, for example by wet cleaning followed by in-situ etching. The advantage of the TiN layer 210 is that when a pinhole-free coating is applied to a clean silicon surface, it prevents the formation of native oxide on the output surface of the field emitter. As mentioned above, silicon dioxide layers have a wide band gap and even a thin layer can prevent a large proportion of electrons from escaping from the silicon. Thus, the TiN layer 210 allows low energy electrons to escape from the silicon field emitter protrusion 204. The field emitter protrusion 204 formed on the silicon substrate 201 avoids the limitations of previous electron emitters. The pointed emitter also provides field enhancement and high emission current. Furthermore, in previous silicon devices, the absence of an oxide-free silicon layer at the time of coating did not prevent the formation of a silicon dioxide interfacial layer between the silicon and the low work function material. That is, without a pinhole-free impermeable protective layer on the silicon, oxygen will eventually migrate to the silicon surface and form an oxide layer. An advantage of using TiN to form layer 210 is that even a thin, pinhole-free TiN layer hermetically seals the silicon without oxygen penetration. Since the majority of emitted electrons occur in a small region near the apex of the field emitter protrusion, it is sufficient that there are no pinholes within a few hundred nanometers of the apex of the field emitter. Another advantage of TiN layer 210 is that the defect and interface trap density at the silicon / TiN interface is typically lower than at the silicon / silicon dioxide interface, resulting in higher emission currents.

[0057] FIG. 3 is a cross-sectional view of an electron gun 300 according to another embodiment. The electron gun 300 may be used in the SEM system 100 of FIG. 1. Similar to the structure described above in FIG. 2, the electron gun 300 includes a field emitter cathode formed by a field emitter protrusion 204 integrally connected to a silicon substrate 201 and extending upward from an output surface 203. The electron gun 300 includes a gate 307, or control electrode, disposed on one or more dielectric layers 306 such that an inner edge 307E of the conductive gate 307 is spaced a predetermined distance D3 from the tip 206 of the field emitter protrusion 204. To achieve high speed and precision control of the emission current, the thickness T3 of the dielectric layer 306 is controlled such that the height H2 of the gate 307 is similar to the height H1 of the field emitter tip 206 (i.e., measured relative to the output surface 203 located outside the outer periphery of the field emitter protrusion 204). Thus, the thickness T3 of the dielectric layer 306 is approximately the same as or less than the height H1 of the field emitter protrusion 204. The thickness T3 of the dielectric layer 306 may differ from the height H1 of the field emitter protrusion 204 by a few hundred nm or less (e.g., the height H2 is within ±300 nm of the height H1). For example, the thickness T3 may be selected so that the gate 307 is approximately the same height as the field emitter tip 206, approximately 200 nm higher, or approximately 200 nm lower. The dielectric layer 306 may include one or more dielectric materials, such as SiO2 or Si3N4. The dielectric layer 306 is disposed on the top surface of the silicon surrounding the field emitter protrusion 204, but does not necessarily cover the field emitter protrusion 204. In another example, the dielectric layer 306 partially covers the field emitter. The gate 307 and dielectric layer 306 may be fabricated by common CMOS fabrication techniques, such as using PVD, CVD, or ALD deposition methods. The gate 307 may comprise metal or polysilicon. The simplest emitter design uses a triode configuration with only one gate 307, although other configurations are possible. Typically, this gate 307 is formed on top of a dielectric layer 306, which may include one or several insulating layers deposited on the substrate 201. In more complex emitter designs, two or more gate layers (not shown) may be used.In this case, multiple dielectric layers are used as spacers between the gate layers. Note that the electron gun 300 is not limited to pyramidal or conical field emitters, but may include any shaped field emitter, such as rounded whisker shaped or other shaped field emitters. The embodiment of FIG. 3 may have modifications or different configurations with respect to FIG. 2.

[0058] 3, TiN layer 210 is shown covering only field emitter protrusions 204, however, TiN layer 205 may extend under dielectric layer 306. For example, TiN layer 210 may be formed on output surface 203 of substrate 201 and over field emitter protrusions 204 prior to formation of dielectric layer 306.

[0059] 4A and 4B are cross-sectional views of the disclosed electron gun according to an embodiment of the present disclosure, where additional layers and structures are utilized to further enhance the advantageous properties of the disclosed electron gun structure. It should be understood that the illustrated exemplary embodiments are not intended to be exhaustive, and electron guns including combinations of the additional layers and structures described below are possible. The embodiment of FIG. 4A and 4B may be similar to the embodiment of FIG. 2 and 3 in an array arrangement. Thus, TiN layer 410 may be similar to TiN layer 210 (described above). It should be noted that electron guns 400A and 400B are not limited to pyramidal or conical field emitter projections, but may include any shape of field emitter projections, such as rounded whisker shaped or other shaped field emitters. The embodiment of FIG. 4A and 4B may have modifications or different configurations with respect to FIG. 2 and 3.

[0060] 4A is a cross-sectional view of an electron gun 400A. The electron gun 400A includes a field emitter array (FEA) cathode formed on a silicon substrate 401 and an anode (not shown) facing the FEA cathode. The silicon substrate 401 has an upwardly facing output (top) surface 403 having a plurality of field emitter protrusions 404-1 and 404-2 arranged in a two-dimensional periodic pattern, and a continuous TiN layer 405 disposed on the output surface 403 covering the field emitter protrusions 404-1 and 404-2. The field emitter protrusions 404-1 and 404-2 have a pyramidal shape that may be formed by anisotropic etching. Each of the protrusions 404-1 and 404-2 has a base 405 that is generally aligned with the output surface 403, and a tip 406. Similar to the field emitter 204 described above, the field emitters 404-1 and 404-2 can be fabricated using common CMOS fabrication techniques. If a sharp field emitter tip 406 is desired, an oxide sharpening can be used, which can be performed at low to medium temperatures (less than about 950° C.) before forming the TiN layer 410. A second electrode (i.e., an anode, not shown) is placed facing the field emitter cathode. The anode can be made of metals such as tungsten, molybdenum, and stainless steel, among others. A voltage source can be used to create an external potential difference between the FEA cathode and the anode, which causes electrons to move preferably toward the emitter tip 406. When properly positioned, the electron gun 400A operates similarly to a typical low-temperature electron gun in that electrons are most likely to be emitted from the substrate 401 through the output surface 403 of the FEA, most likely near the tips 406 of the field emitters 404-1 and 404-2.

[0061] The spacing S between adjacent field emitters 404-1 and 404-2 is a parameter that affects the field emitter characteristics in the FEA cathode. A narrow spacing between emitters reduces the field enhancement factor due to screening effects, resulting in insufficient field penetration into individual emitters. Therefore, to minimize field shielding effects and optimize field emission current density, the distance between vertically arranged emitter protrusions 404-1 and 404-2, or emitter spacing S, can be quite large, ranging from tens of microns to centimeters. In one example, the spacing between field emitters can be 100 μm to 10 cm. All ranges and values ​​in 0.1 μm increments between these values ​​are included. For example, the spacing between field emitters can be 10 μm, 50 μm, 100 μm, 200 μm, or 500 μm. The spacing between emitters can be at least three times the nominal height H1 of emitter protrusions 404-1 and 404-2. Typically the spacing between the emitters is matched to the electron optics of the system in which they are housed, so a spacing between 100 μm and a few centimeters can be chosen.

[0062] 4B is a cross-sectional view of electron gun 400B. Similar to the structures described above, electron gun 400B includes a field emitter array (FEA) cathode formed on a silicon substrate 401 having an upwardly facing output (upper) surface 403 with a plurality of field emitter projections 404-1 and 404-2 arranged in a two-dimensional periodic pattern, and a continuous TiN layer 405 disposed on output surface 403 covering at least the tips 406 of field emitter projections 404-1 and 404-2. Electron gun 400B differs from electron gun 400A in that it includes a gate 427 (or control electrode) disposed at a height H2 similar to the nominal height H1 of the tips of field emitter projections 404-1 and 404-2 and attached to substrate 401 by dielectric 426. Gate 427 allows for fast and precise control of the emission current. A dielectric layer 426 is disposed on top of the silicon surrounding the field emitter projections 404-1 and 404-2, but does not cover any portion of the projections 404-1 and 404-2. In another example, the dielectric layer 426 partially covers one or both of the field emitter projections 404-1 and 404-2. The gate 427 and the dielectric layer 426 may be fabricated using common CMOS fabrication techniques, such as using PVD, CVD, or ALD deposition methods. The gate 427 may include metal or polysilicon. The simplest emitter design uses a triode configuration with only one gate layer, although other configurations are possible. The gate 427 is typically formed on top of an insulating layer (dielectric 426), which is typically an oxide layer deposited on a substrate. In more complex emitter designs (not shown) where the insulating layer forms a spacer between the gate layers, two or more gate layers (not shown) may be used. An inner edge 427E of gate 427 is spaced a predetermined distance D3 from tips 406 of field emitter projections 404-1 and 404-2, respectively.

[0063] Field emission from silicon field emitters can be described by the well-known Fowler-Nordheim tunneling. The local electric field at the emitter tip can be enhanced by a field enhancement factor compared to the applied field. The penetration of an external electric field into a semiconductor changes the carrier concentration in the near-surface region.

[0064] 10 7 Vcm -1 For high electrostatic bias fields such as , the conduction band of the p-type field emitter is depleted at the surface and a depletion region is created between the p-type interior and the n-type surface, where the Fermi level lies in the middle of the energy gap, resulting in a minimum concentration of electrons and holes in this region, similar to the case of a reverse-biased p-n junction.

[0065] When the cathode comprises n-type silicon, or when a cathode comprising p-type silicon is operated as a photocathode, the applied electrostatic field need only be strong enough to bend the conduction and valence bands at the tip, but not necessarily to bend the conduction band below the Fermi level. When such a field is applied, a few electrons are spontaneously generated, and most of the emission current arises from electrons injected from the n-type silicon into the region near the tip, or from electron-hole pairs generated by the absorption of light.

[0066] In conventional silicon field emitters, at least a thin oxide layer is present on the silicon surface. This oxide, even if only about 2 nm thick, is a large barrier to escaping electrons. The band gap of silicon dioxide is about 9 eV. Such a large band gap results in a local peak in the conduction band in the oxide that is several eV higher than the conduction band in silicon. The TiN layer on the surface of the disclosed field emitter prevents oxygen or water from reaching the silicon surface and prevents the growth of the oxide layer. Thus, an efficient electron gun is possible.

[0067] In one embodiment, the silicon field emitter is operated at a temperature close to room temperature, which minimizes the energy spread of the emitted electrons. In another embodiment, which is useful when a larger energy spread can be tolerated, the silicon field emitter can be operated at a higher temperature, such as a temperature between about 400 K and about 1000 K. This reduces contamination that sticks to the surface of the field emitter, allowing the silicon field emitter to operate in a less clean vacuum environment.

[0068] Various modifications of the described embodiments will be apparent to those skilled in the art, and the general principles defined herein may be applied to other embodiments. For example, additional electrodes may be placed near the electron emitter to control emission and / or focus the emitted electrons in a particular direction. It is expected that electron guns with silicon field emitters as disclosed herein will be particularly useful in various types of scanning electron microscopes and electron beam lithography systems, but it is also expected that these guns may be useful in other applications requiring high brightness and / or high current electron beams.

[0069] The electron guns and methods of manufacture described herein are not intended to be limited to the particular embodiments shown and described, but are to be accorded the widest scope consistent with the principles and novel features of the disclosure herein.

[0070] Although the present disclosure has been described with respect to one or more particular embodiments, it should be understood that other embodiments of the present disclosure may be practiced without departing from the scope of the present disclosure, and therefore, the present disclosure is to be deemed limited only by the appended claims and their reasonable interpretation.

Claims

1. 1. An electron gun comprising a field emitter configured to generate a primary electron beam, the field emitter comprising: a single crystal silicon substrate having opposed first and second surfaces and having at least one integral field emitter projection, the field emitter projection having an anchor portion integrally connected to the silicon substrate and extending from the second surface to a tip; a titanium nitride (TiN) layer disposed at least confined to the tip of the field emitter protrusion such that, in operation, electrons escaping from the single crystal silicon substrate through the tip of the field emitter protrusion to form the primary electron beam pass only through the TiN layer; Equipped with The electron gun, wherein the TiN layer comprises at least 75 atomic percent TiN.

2. 2. The electron gun of claim 1, wherein the TiN layer comprises greater than 80 atomic percent TiN.

3. The single crystal silicon substrate is about 10 １９ cm －３ 2. The electron gun of claim 1, wherein the electron gun is p-doped at a doping level of less than 100 nm.

4. The single crystal silicon substrate is about 10 １５ cm －３ From about 10 １９ cm －３ 2. The electron gun of claim 1, wherein the electron gun is n-doped with a doping level between .gtoreq..gtoreq..gtoreq..times ...

5. The single crystal silicon substrate has a thickness of more than about 10 μm and １４ cm －３ 2. The electron gun of claim 1, wherein the electron gun is p-doped at a doping level of less than 100 nm.

6. 2. The electron gun of claim 1, comprising a light source configured to illuminate the first surface of the single crystal silicon substrate, the light source comprising one of a laser diode and a light emitting diode configured such that an emission current of the primary electron beam is controllable in proportion to an amount of light transmitted from the light source into the single crystal silicon substrate.

7. The electron gun of claim 1 , wherein the TiN layer has a thickness in the range of 2 nm to 150 nm.

8. 2. The electron gun of claim 1, wherein the portion of the TiN layer located within a 100 nm radius of the tip contains less than 10 atomic percent oxygen.

9. 2. The electron gun of claim 1, wherein the portion of the TiN layer located within a 100 nm radius of the tip contains less than 10 atomic percent carbon.

10. 2. The electron gun of claim 1, wherein the field emitter protrusion comprises one of a cone, a pyramid, or a rounded whisker shape, and the tip of the field emitter protrusion has a lateral dimension of less than 300 nm.

11. 11. The electron gun of claim 10, wherein the tip of the field emitter protrusion has a lateral dimension greater than 10 nm.

12. 2. The electron gun of claim 1, wherein the field emitter protrusion comprises one of a cone, a pyramid, or a rounded whisker shape, and the tip of the field emitter protrusion has a diameter of less than 300 nm.

13. 2. The electron gun of claim 1, wherein the field emitter is configured to operate in a reverse bias mode in which a depletion layer is created adjacent the second surface in response to an applied electric field.

14. The field emitter is a first dielectric layer disposed on the second surface of the single crystal silicon substrate adjacent to the field emitter protrusion; a first conductive gate disposed on the first dielectric layer such that an edge of the first conductive gate is spaced a non-zero distance from the tip of the field emitter projection; a second dielectric layer disposed on the second surface of the first conductive gate; a second concentrating gate disposed on the second dielectric layer such that an edge of the second conductive gate is spaced a non-zero distance from the tip of the field emitter projection; Further equipped with 2. The electron gun of claim 1, wherein the thickness of each of the first and second dielectric layers is between 500 nm and 3 [mu]m, the first dielectric layer being within ±300 nm of the height of the field emitter protrusion.

15. 2. The electron gun of claim 1, further comprising a plurality of said field emitter projections arranged in a two-dimensional periodic pattern, each of said field emitter projections having one of said fixed portions integrally connected to said silicon substrate and extending from said second surface to a tip portion, and said TiN layer is confinedly disposed over at least said tip portion of each of said plurality of field emitter projections.

16. The field emitter is a first dielectric layer disposed on the second surface of the single crystal silicon substrate adjacent to the plurality of field emitter projections; a first conductive gate disposed on the first dielectric layer such that an edge of the first conductive gate is spaced a non-zero distance from the tip of each of the plurality of field emitter projections; a second dielectric layer disposed on the second surface of the first conductive gate; a second concentrating gate disposed on the second dielectric layer such that an edge of the second conductive gate is spaced a non-zero distance from the tip of each of the plurality of field emitter projections; Further equipped with 16. The electron gun of claim 15, wherein the thickness of each of the first and second dielectric layers is between 500 nm and 3 [mu]m, the first dielectric layer being ±200 nm relative to a nominal height of the plurality of field emitter protrusions.

17. A scanning electron microscope (SEM), an electron gun comprising a field emitter configured to generate a primary electron beam; an electron optical system configured to demagnify and focus the primary electron beam on a sample; a detector configured to detect at least one of backscattered electrons and secondary electrons from the sample in response to the primary electron beam; The field emitter comprises: a single crystal silicon substrate having opposed first and second surfaces and having at least one integral field emitter projection, the field emitter projection having an anchor portion integrally connected to the silicon substrate and extending from the second surface to a tip; a titanium nitride (TiN) layer disposed at least confined to the tip of the field emitter protrusion such that, in operation, electrons escaping from the single crystal silicon substrate through the tip of the field emitter protrusion to form the primary electron beam pass only through the TiN layer; Equipped with The TiN layer comprises at least 75 atomic percent TiN, and a portion of the TiN layer located within a 100 nm radius of the tip comprises less than 10 atomic percent oxygen. SEM.

18. 1. An apparatus comprising an electron gun, the electron gun comprising a field emitter configured to generate a primary electron beam, the field emitter comprising: a single crystal silicon substrate having opposed first and second surfaces and having at least one integral field emitter projection, the field emitter projection having an anchor portion integrally connected to the silicon substrate and extending from the second surface to a tip; a titanium nitride (TiN) layer disposed at least confined to the tip of the field emitter protrusion such that, in operation, electrons escaping from the single crystal silicon substrate through the tip of the field emitter protrusion to form the primary electron beam pass only through the TiN layer; Equipped with The TiN layer comprises at least 75 atomic percent TiN, and a portion of the TiN layer located within a 100 nm radius of the tip comprises less than 10 atomic percent oxygen.

19. 20. The apparatus of claim 18, wherein the apparatus comprises one of a scanning electron microscope (SEM) system or an electron beam lithography system.

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