Electron microscope, electron source for electron microscope, and method of operating an electron microscope

Abstract

An electron microscope (100) is described, comprising an electron source (110) for generating an electron beam, a condenser lens (130) for collimating the electron beam downstream of the electron source, and an objective lens (140) for focusing the electron beam on a sample (16). The electron source comprises a cold field emitter with an emission tip (112), an extraction electrode (114) for extracting the electron beam (105) from the cold field emitter for propagation along an optical axis (A), the extraction electrode (114) having a first aperture (115) configured as a first beam stop aperture, a first cleaning device (121) for cleaning the emission tip (112) by heating the emission tip, and a second cleaning device (122) for cleaning the extraction electrode (114) by heating the extraction electrode. Also described is a method of operating such an electron microscope.

Description

[Technical field]

[0001] The embodiments described herein relate to electronic devices, particularly electron microscopes, especially scanning electron microscopes (SEMs), for inspection or imaging system applications, test system applications, lithography system applications or other similar applications. In particular, the embodiments described herein relate to electron microscopes with cold field emitters that provide high brightness electron beams for high resolution and high throughput applications. More particularly, high throughput wafer inspection SEMs are described. The embodiments described herein further relate to electron sources for electron microscopes and methods of operating electron microscopes. [Background technology]

[0002] Electron microscopes have many functions in multiple industrial sectors, including but not limited to inspection or imaging of semiconductor substrates, wafers and other samples, critical dimension measurement, defect investigation, exposure systems for lithography, detection devices and test systems. There is a great demand to structure, test, inspect and image samples on the micrometer and nanometer scale. Electron microscopes offer superior spatial resolution, e.g., superior spatial resolution compared to photon beams, enabling high-resolution imaging and inspection.

[0003] An electron microscope includes an electron source or "electron gun" that generates an electron beam that strikes a specimen. Various types of electron sources are known, including thermal field emitters, Schottky emitters, thermally assisted field emitters, and cold field emitters. A cold field emitter (CFE) includes an emission tip that is cold (=unheated) during operation, and emits electrons by applying a high electrostatic field between the emission tip and an extractor electrode. While thermal field emitters are typically capable of providing a high current electron beam, cold field emitters have the potential to provide a high brightness electron beam probe suitable for achieving high resolution.

[0004] However, the CFE is particularly sensitive with respect to contamination, and therefore it should be operated under very good vacuum conditions, in particular under ultra-high vacuum conditions, in an evacuated gun housing. Nevertheless, unwanted ions, ionized molecules or other contaminating particles may be present in the evacuated gun housing. For example, charged contaminating particles may be accelerated towards the emitter in such a way that the emitting tip may be mechanically deformed or otherwise negatively affected. Furthermore, accumulation of particles on the emitter surface or other surfaces of the electron source may introduce noise and other beam instabilities.

[0005] In particular, contaminant particles in this region of the electron gun can lead to an unstable or noisy electron beam, for example leading to fluctuating beam currents or variable beam cross sections.The vacuum conditions within the electron microscope, and in particular within the gun housing containing the CFE, are therefore crucial.

[0006] In view of the above, it would be beneficial to improve the beam stability of the electron beam in an electron microscope and to reduce the amount of contaminating particles in the gun housing. In particular, it would be beneficial to provide a compact electron microscope with a CFE electron gun that emits a high brightness electron beam with improved stability that may further increase achievable resolution and throughput. Additionally, it would be beneficial to provide a method of operating an electron microscope to provide a high brightness electron beam with improved beam stability. Summary of the Invention

[0007] In view of the above, there are provided an electron microscope, an electron source and a method of operating an electron microscope according to the independent claims. Further aspects, advantages and features are apparent from the dependent claims, the description and the accompanying drawings.

[0008] According to one aspect, an electron microscope is provided. The electron microscope includes an electron source, a condenser lens, and an objective lens. The electron source includes a cold field emitter (CFE) with an emitting tip, an extraction electrode for extracting an electron beam from the cold field emitter for propagation along an optical axis, the extraction electrode having a first aperture configured as a first beam limiting aperture, a first cleaning arrangement for cleaning the emitting tip by heating the emitting tip, and a second cleaning arrangement for cleaning the extraction electrode by heating the extraction electrode. The condenser lens is a lens for collimating the electron beam downstream of the electron source, and the objective lens is a lens for focusing the electron beam on a sample.

[0009] According to one aspect, there is provided an electron source for an electron microscope as described herein. The electron source includes a cold field emitter (CFE) with an emission tip, an extraction electrode for extracting an electron beam from the cold field emitter for propagation along an optical axis, a first cleaning device for cleaning the emission tip by heating the emission tip, and a second cleaning device for cleaning the extraction electrode by heating the extraction electrode. The electron source can be used in an electron microscope as described herein or another electron device that uses a high brightness electron gun.

[0010] According to another aspect, a method is provided for operating an electron microscope having an electron source with a cold field emitter, the method including, in a first cleaning mode, cleaning an emission tip of the cold field emitter by heating the emission tip, in a second cleaning mode, cleaning an extraction electrode of the electron source by heating the extraction electrode, and in an operating mode, extracting an electron beam from the cold field emitter for propagation along an optical axis, the electron beam being shaped by a first aperture that may be provided in the extraction electrode, collimating the electron beam with a condenser lens, and focusing the electron beam on a sample with an objective lens.

[0011] According to another aspect, a method of cleaning an electron source comprising a cold field emitter is provided, the method including cleaning an emission tip of the cold field emitter in a first cleaning mode by heating the emission tip, and cleaning an extraction electrode of the electron source in a second cleaning mode by heating the extraction electrode. After cleaning in the first and second cleaning modes, the electron source can be operated to generate an electron beam, for example to generate an electron beam in an electron microscope as described herein.

[0012] A cleaning controller may be provided for setting the electron microscope in a first cleaning mode, for example after a predetermined interval of operating the electron microscope, and / or for setting the electron microscope in a second cleaning mode, for example after flooding the gun housing with air or to improve beam stability.

[0013] The embodiments are also directed to apparatus for carrying out the disclosed methods, including apparatus parts for carrying out each described method feature. These method features may be carried out by hardware components, a computer programmed by appropriate software, any combination of the two, or any other means. Additionally, the embodiments are also directed to methods of manufacturing the described apparatus, methods of operating the described apparatus, and methods of inspecting or imaging a sample using the described electron microscope. The methods include method features for carrying out any function of the described apparatus.

[0014] In order that the above-listed features of the present disclosure may be understood in detail, a more detailed description thereof may be had by reference to the above briefly outlined embodiments, the accompanying drawings relating to embodiments of the present disclosure and described below. [Brief description of the drawings]

[0015] [Figure 1] FIG. 1 is a schematic cross-sectional view of an electron microscope with an electron source including a cold field emitter according to embodiments described herein. [Diagram 2] FIG. 1 is a schematic cross-sectional view of an electron microscope with an electron source including a cold field emitter according to embodiments described herein. [Diagram 3] 1 is a flow diagram illustrating a method of operating an electron microscope according to embodiments described herein. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0016] Reference will now be made in detail to various embodiments, one or more examples of which are illustrated in the figures. In the following description, like reference numerals refer to like components. Generally, only the differences with respect to individual embodiments will be described. Each example is provided for purposes of explanation and not for purposes of limitation. Furthermore, features illustrated or described as part of one embodiment can be used on or in conjunction with other embodiments to yield additional embodiments. It is intended that this description include such modifications and variations.

[0017] In an electron microscope, an electron beam is directed onto a sample placed on a sample stage. In particular, the electron beam is focused on the surface of the sample to be inspected. When the electrons strike the sample, signal particles are emitted, scattered, and / or reflected by the sample. The signal particles include in particular secondary electrons and / or backscattered electrons, in particular both secondary electrons (SE) and backscattered electrons (BSE). The signal electrons are detected by one or more electron detectors, and the corresponding respective detector signals may be processed or analyzed by a processor to inspect or image the sample. For example, an image of at least a portion of the sample may be generated based on the signal electrons, or the sample may be inspected to determine defects, to verify the quality of deposited structures, and / or to perform critical dimension (CD) measurements.

[0018] FIG. 1 is a schematic diagram of an electron microscope 100 according to an embodiment described herein. The electron microscope 100 includes an electron source 110 configured to generate an electron beam 105, e.g., an electron beam 105 that can be used for inspection or imaging applications. The electron microscope 100 further includes a condenser lens 130 configured to reduce the divergence of the electron beam (referred to herein as "collimation"), in particular to provide a slightly diverging, parallel or converging electron beam that propagates along an optical axis A toward an objective lens 140 for focusing on a sample 16. In particular, the combined action of the condenser lens 130 and the objective lens 140 may focus the electron beam 105 on a surface of the sample 16, which may be placed on a sample stage 18. The sample stage 18 may be movable.

[0019] According to the embodiment described herein, the electron source 110 comprises a cold field emitter (CFE) having an emission tip 112. The CFE is configured to emit an electron beam by cold field emission. Cold field emitters are particularly sensitive to contamination within the gun housing in which they are located, such that it is beneficial to provide an ultra-high vacuum within the gun housing. The gun housing containing the CFE is also referred to herein as a "first vacuum region 10a," which may be located upstream of one or more additional vacuum regions (e.g., second vacuum region 10b and third vacuum region 10c) that enable differential pumping.

[0020] In some embodiments, the cold field emitter (CFE) may have a tungsten tip. In some implementations, which may be combined with other embodiments, the emitting tip 112 is made of a crystal that is etched to a sharp tip, in particular a sharp tip with a final radius (tip radius) in the range of 10 nm to 500 nm, in particular 200 nm or less, especially 100 nm or less. The crystal may typically be a tungsten crystal, in particular a tungsten crystal oriented in a (3,1,0) crystal orientation along the optical axis A, in particular a tungsten single crystal. When the emitting tip has a sharp tip with a small radius, the crystal area from which electron emission occurs is reduced, which improves the brightness of the generated electron beam.

[0021] The electron source 110 further includes an extraction electrode 114 for extracting the electron beam 105 for propagation along the optical axis A. The extraction electrode 114 has a first aperture 115, which may be configured as a beam stop aperture. In particular, the first aperture 115 may have a size designed to pass electrons propagating near the optical axis A ("axial electrons") and block electrons further away from the optical axis A, in a manner that allows the formation of a beam cross section according to the size and shape of the first aperture 115.

[0022] In some embodiments, the first aperture 115 may be a circular aperture configured to generate a rotationally symmetric beam cross section of the electron beam 105. In some embodiments, which may be combined with other embodiments described herein, the first aperture 115 may have a diameter of 100 μm or less, in particular 50 μm or less, or even 20 μm or less. The first aperture 115 having a small dimension reduces the size of the electron beam propagating towards the extraction electrode 114, thus suppressing the loss of brightness due to electron-electron interactions.

[0023] During operation of the electron microscope, the extraction electrode 114 may be set at a positive potential with respect to the potential of the emission tip 112, for example with a potential difference between the emission tip 112 and the extraction electrode 114 in the range of several kilovolts (kV), for example 5 kV or more. This potential difference is large enough to generate an electric field at the surface of the emission tip 112 to cause cold field emission. The primary extraction mechanism for cold field emitters is tunneling through a surface potential barrier at the tip surface. This tunneling can be controlled by the extraction electric field of the extraction electrode.

[0024] In some embodiments, the distance between the emitting tip 112 and the extraction electrode 114 is 0.1 mm or more and 3 mm or less, particularly 1 mm or less. The small distance accelerates the emitted electrons rapidly towards the condenser lens 130 in such a way that the loss of brightness due to electron-electron interactions can be reduced.

[0025] The electron microscope 100 includes several mechanisms for improving vacuum conditions and reducing contamination in the first vacuum region 10a where the cold field emitters are located. The excellent vacuum conditions and reduced contamination in the gun housing improve the beam stability and brightness of the electron beam 105, which is particularly beneficial when a CFE is used. A high brightness electron beam is particularly beneficial in a high throughput EBI system.

[0026] The electron microscope 100 includes a first cleaning device 121 for cleaning the emission tip 112 of the CFE by heating the emission tip 112, and a second cleaning device 122 for cleaning the extraction electrode 114 by heating the extraction electrode 114.

[0027] The electron microscope 100 may switch to a first cleaning mode to clean the emission tip 112 using the first cleaning device 121 by heating the emission tip 112, in particular by heating it to a temperature of 1500° C. or more. The electron microscope 100 may switch to a second cleaning mode to clean the extraction electrode 114 using the second cleaning device 122 by heating the extraction electrode 114, in particular by heating it to a temperature of 500° C. or more. In some embodiments, the first cleaning device 121 may include a first heater, in particular a resistive heater, which may be in thermal contact with the emission tip 112 to heat the emission tip, in particular by allowing a current to flow through the first heater. By allowing a current to flow through the first heater, the first heater may be heated together with the emission tip 112 in thermal contact with the first heater. Alternatively or additionally, the second cleaning device 122 may include a second heater, in particular a heating wire 126 (also referred to herein as a "cleaning emitter" because it emits thermal electrons), which may be positioned in close proximity to the extraction electrode 114 to heat the extraction electrode 114, in particular to heat the extraction electrode 114 by allowing current to flow through the second heater.

[0028] Since electrons are emitted from a very small surface area of ​​the emission tip of an operating cold field emitter, the emission is very sensitive to single or even small numbers of contaminating atoms on the emission surface. Atoms that may adsorb to the emission surface may originate from surrounding surfaces such as the extraction electrode, where desorption may be stimulated by electrons of the electron beam striking the extraction electrode, for example, the area surrounding the first opening 115 of the extraction electrode. Thus, not only is a clean emission tip beneficial, but a clean extraction electrode is also beneficial.

[0029] The second cleaning device 122 may be operated by heating a heating wire 126 of the second cleaning device 122 arranged adjacent to the extraction electrode 114 in such a manner that electrons are thermally emitted by the heating wire, impinge on the surface of the extraction electrode, and heat the extraction electrode. The heating wire may be heated to a temperature of 1500° C. or more, in particular 2000° C. or more, so that the heating wire provides an intense thermal emission of electrons. These thermal electrons desorb molecules and atoms that may be present on the surface of the extraction electrode even under high vacuum conditions. In other words, the extraction electrode may be cleaned by electron stimulated desorption caused by the thermal electrons emitted by the heated heating wire. For example, the thermal electrons may be accelerated towards the extraction electrode by applying a corresponding respective potential difference between the extraction electrode and another electrode, for example between the extraction electrode and a supressor electrode and / or the emission tip. Additionally, thermoelectrons impinging on the extraction electrode may heat the extraction electrode in such a manner that the extraction electrode is also cleaned by thermal outgassing. In some embodiments, the second cleaning device 122 is configured to clean the extraction electrode by two cleaning mechanisms: (1) thermal outgassing and (2) electron stimulated desorption.

[0030] Further, optionally, a suppression electrode 113 may be arranged in the gun housing, for example partially between the emission tip 112 and the heating wire 126. In the second cleaning mode (i.e. during heating with the second cleaning device 122), the suppression electrode 113 may be set to a predetermined potential suitable for deflecting the electrons emitted by the heating wire 126 towards the extraction electrode 114 and / or away from the emission tip 112. This may reduce the risk of deformation of the emission tip 112 by the hot electrons of the second cleaning device 122 and / or may help to direct the hot electrons towards the area of ​​the extraction electrode to be cleaned, in particular to be cleaned by electron stimulated desorption.

[0031] In some embodiments, a voltage source 129 is provided to connect any one or more of the extraction electrode 114, the suppression electrode 113 and / or the emission tip 112 to a predetermined potential, for example during cleaning and / or operation.

[0032] In some embodiments, the heated wire 126 of the second cleaning device 122 may be located in close proximity to the extraction electrode 114, in particular at a distance of 2 mm or less, or even 1 mm or less, from the extraction electrode 114. In particular, the heated wire 126 may be located close to the area of ​​the extraction electrode 114 surrounding the first aperture 115, which area is typically impacted by electrons of the electron beam 105 during operation of the electron microscope.

[0033] In some embodiments, the second cleaning device 122 may include a heating wire or heating filament through which an electric current for heating can be sent. In particular, a first end of the heating wire 126 may be connected to a first output terminal of a current source, and a second end of the heating wire 126 may be connected to a second output terminal of the current source set to a different potential. The heating wire 126 or heating filament may at least partially surround the first opening 115 of the extraction electrode 114 (e.g., by a circumferential angle of 180° or more, or even 270° or more) in such a manner that the edge of the first opening 115 can be heated by the second cleaning device 122 in a targeted manner. For example, the heating wire 126 may extend in the shape of a ring or circle around the first opening 115.

[0034] In some embodiments, which may be combined with other embodiments described herein, the second heater, in particular the heating wire 126, of the second cleaning device 122 may comprise or be made of tungsten or tantalum, in particular tantalum. Tantalum provides particularly convincing cleaning results when used as a second heater for cleaning the extraction electrode, and tantalum is particularly suitable as a thermionic emitter in ultra-high vacuum environments. Thus, although not limited thereto, the embodiments disclosed herein typically use a tantalum heater in the second cleaning device 122 located in the immediate vicinity of the extraction electrode 114, in particular a tantalum heater in the form of a heating wire located in the immediate vicinity of the extraction electrode 114 and at least partially surrounding the first opening 115.

[0035] To heat the extraction electrode at least partially to a temperature of at least 500° C., in particular at least 600° C., in particular to a temperature in the range between 600° C. and 800° C., the electron microscope may further comprise a cleaning controller 128 configured to allow a current to flow through the second heater of the second cleaning device in the second cleaning mode. In particular, the area of ​​the extraction electrode 114 surrounding the first opening 115 may be heated by the second cleaning device. In a previous calibration, the current to be passed through the second heater to bring the temperature of the extraction electrode to 500° C. or more, in particular to 600° C. to 800° C., may be identified and stored. When switching to the second cleaning mode, the cleaning controller 128 may then pass the corresponding respective current through the second cleaning device 122. During this heating, the second heater itself, in particular the heating wire 126, may have a temperature of 1500° C. or more, in particular 2000° C. or more or even 2200° C. or more.

[0036] In some embodiments, which can be combined with other embodiments described herein, the first cleaning device 121 includes a heating filament 125 in thermal contact with the emission tip 112. The emission tip 112 may be coupled to the heating filament 125 or may be attached to the heating filament 125. In particular, the heating filament 125 may be a V-shaped heating filament and the emission tip 112 may be coupled to a bent portion of the V-shaped heating filament. The two ends of the V-shaped heating filament may be connected to two output terminals of a current source that can be set to different potentials to allow a current to flow through the V-shaped heating filament.

[0037] In some embodiments, the heating filament 125 is a tungsten filament and / or the emission tip 112 of the CFE coupled to the heating filament 125 is a tungsten tip.

[0038] When a current is passed through the heating filament 125, the heating filament 125 heats up together with the emission tip 112 which is in thermal contact with the heating filament 125. The first cleaning device 121 may be configured to heat the emission tip 112 to a temperature of 1500° C. or more, in particular 2000° C. or more, especially 2000 K or more, in the first cleaning mode.

[0039] Heating the emission tip 112 via the heating filament 125 can evaporate adsorbed molecules, which helps clean the emission tip 112 and provide a more stable electron beam emission. Additionally, heating the emission tip can shape the emission tip in a manner that can provide and / or maintain a sharp tip. Optionally, during heating of the emission tip in the first cleaning mode, the extraction electrode 114 can be set to a predetermined potential, which can prevent or reduce rounding or flattening of the emission tip during heating and / or can facilitate maintaining a sharp emission tip.

[0040] To heat the emission tip 112 to a temperature of at least 1500° C., in particular at least 2000° C., the electron microscope may include a cleaning controller 128 configured to enable a current to flow through the heating filament 125 of the first cleaning device 121 in the first cleaning mode. In a preceding calibration stage, a current to be passed through the heating filament 125 to achieve a temperature of the emission tip 112 of at least 2000° C. may be identified. When switching to the first cleaning mode, the cleaning controller 128 may then pass a corresponding respective current through the first cleaning device 121.

[0041] In some embodiments, one cleaning controller 128 may be provided to allow current to flow through the heating filament 125 to heat the emission tip in a first cleaning mode, and to allow current to flow through the heating wire 126 to heat the extraction electrode 114 in a second cleaning mode, as exemplarily shown in FIG. 1. In some embodiments, separate cleaning controllers may be connected to the first and second cleaning devices. During operation of the electron microscope, the emission tip 112 may be set to a predetermined potential relative to the extraction electrode 114 to allow cold field emission from the emission tip, for example by applying the same voltage to both ends of the V-shaped heating filament so that no current flows and therefore no heating of the emission tip occurs.

[0042] The first cleaning mode may also be referred to as a "flushing mode" since the emission tip is heated to a high temperature for a relatively short period of time to evaporate adsorbed particles and contamination and ensure a more stable electron beam. The cleaning controller 128 may be configured to set the electron microscope 100 in the first cleaning mode at regular intervals, for example, once an hour, before starting operation of the electron microscope and / or after a predefined period of operation if the electron microscope is in operation. By periodically switching to the first cleaning mode, a constantly clean and sharp emitter tip can be ensured.

[0043] Alternatively or additionally, the cleaning controller 128 may be configured to set the electron microscope in the second cleaning mode before operation of the electron microscope, after the gun housing has been ventilated or flooded with air, and / or during maintenance or service operations on the electron microscope, and / or if the electron beam exhibits undesirable instabilities. Thus, the interval between two first cleaning modes is typically shorter than the interval between two second cleaning modes.

[0044] In some embodiments, which can be combined with other embodiments described herein, the distance between the emitting tip 112 and the first aperture 115 of the extraction electrode 114 may be 5 mm or less, in particular 3 mm or less, in particular 1 mm or less, and / or 0.1 mm or more. Thus, the electrons emitted by the emitting tip 112 are accelerated very rapidly during the short propagation distance towards the extraction electrode, which reduces electron-electron interactions and improves the brightness of the electron beam.

[0045] The electron microscope 100 may include an acceleration section for accelerating the electron beam, for example to an electron energy of 5 keV or more, which is arranged upstream of and / or at least partially overlaps with the condenser lens 130. The electrons may be accelerated towards the extraction electrode 114, which is set at a positive potential with respect to the emission tip, and optionally, the electrons may be further accelerated towards an anode, which may be arranged downstream of the extraction electrode 114, for example between the extraction electrode and the condenser lens or inside the condenser lens (shown in FIG. 2). In some embodiments, the electrons are accelerated to an electron energy of 10 keV or more, 30 keV or more, or even 50 keV or more. A high electron energy in the column can reduce the negative effects of electron-electron interactions.

[0046] In some embodiments, the electron microscope 100 may include a deceleration section for decelerating the electron beam from an energy of 5 keV or more to a lower landing energy, which may be downstream of the objective lens 140 or may at least partially overlap the objective lens 140. For example, the electrons may be decelerated to a landing energy of 3 keV or less, in particular 2 keV or less, or even 1 keV or less, for example 800 eV or less. Electrons with a low landing energy are more suitable for interacting with sample structures in such a way that a low landing energy may improve the obtainable resolution. For example, a proxy electrode located near the sample stage may brake the electrons before they impinge on the sample, or the sample may be set to a braking potential.

[0047] Signal particles released from the sample 16 may be accelerated along the deceleration section towards the objective lens, and they may propagate through the objective lens towards an electronic detector (not shown in this figure).

[0048] The electron microscope may include a gun housing that is a first vacuum region 10a that can be evacuated using one or more vacuum pumps, in particular a first vacuum region 10a that can be evacuated to ultra-high vacuum using one or more vacuum pumps. The gun housing, which contains the electron source 110, is typically located upstream of the column of the electron microscope.

[0049] The electron microscope may use several so-called differentially pumped regions separated by corresponding respective differential pumping apertures to improve the vacuum conditions in the gun chamber. Differentially pumped regions may be understood as vacuum regions that can be separately evacuated by one or more corresponding respective vacuum pumps and separated by corresponding respective gas separation walls to improve the vacuum conditions of the most upstream vacuum region. The gas separation walls may be provided with differential pumping apertures, i.e. small openings for the electron beam, in such a way that the electron beam can propagate along the optical axis from the upstream differential pumping section into the downstream differential pumping section. The term "downstream" as used herein may be understood as downstream in the direction of propagation of the electron beam along the optical axis A.

[0050] In some embodiments, the first aperture 115 of the extraction electrode 114 may be arranged to function as a first differential pumping aperture, i.e., an aperture in a gas separation wall that allows differential pumping between the gun housing and a second vacuum region 10b downstream of the gun housing. When the first aperture 115 functions as both a beam-stop aperture (i.e., a beam-optical aperture) and a differential pumping aperture, a more compact electron microscope can be provided that facilitates good vacuum conditions in the gun housing 10a and thus good beam stability. As shown diagrammatically in FIG. 1, the extraction electrode 114 with the first aperture 115 may be part of a gas separation wall between the first vacuum region 10a and the second vacuum region 10b.

[0051] As shown diagrammatically in FIG. 1, the electron microscope 100 may include a second vacuum region 10b downstream of the gun housing, which may house a condenser lens 130.

[0052] In some embodiments, the electron microscope may further include a second beam aperture 132 between the condenser lens 130 and the objective lens 140. The condenser lens 130 may be configured to adjust the beam divergence of the electron beam, and thus the portion of the electron beam propagating through the second beam aperture 132. Thus, excitation of the condenser lens 130 may be used to adjust the beam current of the electron beam downstream of the second beam aperture 132.

[0053] Optionally, the second beam stop aperture 132 may be arranged to function as a second differential pumping aperture. In other words, the second beam stop aperture 132 may be arranged in a gas separation wall between the second vacuum region 10b and a third vacuum region 10c downstream of the second vacuum region 10b to allow differential pumping between said regions. The vacuum conditions in the gun housing can be further improved and contamination can be further reduced. For example, the second beam stop aperture 132 may have a diameter of 100 μm or less, in particular 50 μm or less, in particular 20 μm or less, or even 10 μm or less.

[0054] Thus, as a result of the above-mentioned differential pumping concept, the vacuum conditions in the first vacuum region 10a in which the cold cathode field emitter is placed can be further improved, and the first vacuum region can be subjected to a very low pressure, e.g. -11 Extremely low pressures of the order of millibars or less can be provided and maintained throughout the operation of the electron microscope. -6 Millibar or more than 10 -5 mbar and / or 10 -3 Pressures below millibars, especially 10 -3 millibars ~ 10 -6 The pressure can be maintained within the gun housing even though it may be significantly higher than the pressure, such as a pressure between millibars.

[0055] According to some embodiments described herein, both the first aperture 115 and the second beam aperture 132 are beam optical apertures. That is, during operation, both apertures affect the shape and / or dimensions of the electron beam 105. In addition, both the first aperture 115 and the second beam aperture 132 may be configured to function as pressure stage apertures. In other words, both apertures are not only positioned to improve the vacuum conditions in the gun housing 10a, but are also part of a beam optical system that affects the electron beam. Thus, the first aperture 115 and the second beam aperture 132 may also be referred to as "beam optical pressure stage apertures" or "beam defining pressure stage apertures."

[0056] In some embodiments, which can be combined with other embodiments described herein, the electron microscope further includes at least one third differential pumping aperture 133 between the second differential pumping aperture and the objective lens 140. In particular, the at least one third differential pumping aperture 133 may be arranged in a gas separation wall between the third vacuum region 10c and a fourth vacuum region 10d downstream of the third vacuum region 10c, which allows differential pumping from the gun housing 10a across the second and third vacuum regions to the fourth vacuum region 10d, in which the objective lens may be arranged. The vacuum conditions in the gun housing can be further improved. At least one or more beam optical components, such as a second condenser lens, an aberration corrector, a beam separator for separating signal electrons from the electron beam, and / or an electron detector for detecting the signal electrons, may be arranged in the third vacuum region 10c. The objective lens 140 may be located in the fourth vacuum region 10d (or alternatively the third vacuum region if a fourth vacuum region is not provided).

[0057] Each of the first vacuum region 10a, the second vacuum region 10b, the third vacuum region 10c and the fourth vacuum region 10d (if present) may be provided with an exhaust port 11 for attachment of a vacuum pump. The exhaust ports 11 may be configured to attach a vacuum pump, such as an ion getter pump, to the corresponding respective vacuum region.

[0058] In some embodiments, which can be combined with other embodiments described herein, the emission tip 112 is located in the first vacuum region 10a, and the condenser lens 130 is located in the second vacuum region 10b. An ion getter pump 13 and a non-evaporable getter (NEG) pump 14 may be provided to evacuate the first vacuum region 10a where the emission tip 112 is located. For example, the ion getter pump 13 and the non-evaporable getter pump may be attached to the exhaust port 11 of the first vacuum region 10a, or the ion getter pump may be located separately from the non-evaporable getter pump, for example, at another exhaust port of the first vacuum region 10a. The vacuum condition at the location of the emission tip can be further improved.

[0059] In some embodiments, the electron microscope is a scanning electron microscope (SEM). The electron microscope may include a scanning deflector 152, for example located near or within the objective lens 140. In particular, the electron microscope may be an electron beam inspection system (EBI system), and in particular an SEM for high throughput electron beam inspection, for example an SEM for high throughput electron beam inspection of wafers or other semiconductor substrates. More particularly, the electron microscope may be a High Throughput Wafer Inspection SEM.

[0060] According to the embodiments described herein, a high performance electron microscope with a CFE electron source is provided, which allows for the inspection of samples, particularly wafers and other semiconductor samples, with a high brightness electron beam at high resolution and high throughput. For example, wafers and other samples can be inspected quickly with high resolution. By providing and operating the first and second cleaning apparatus described herein, a high brightness of the electron beam can be provided and maintained due to improved vacuum conditions and reduced contamination. Furthermore, due to the excellent vacuum conditions in the gun housing, a high brightness is possible despite the compactness of the electron microscope. This is due to reduced electron-electron interactions.

[0061] According to another aspect described herein, an electron source 110 for a high performance electronic device is provided, the electron source including a cold field emission emitter with an emitting tip 112 and an extraction electrode 114 that can be cleaned by first and second cleaning apparatuses described herein, respectively.

[0062] 2 is a schematic cross-sectional view of an electron microscope 200 with an electron source 110 including a cold field emitter according to an embodiment described herein. The electron microscope 200 of FIG. 2 may include some or all of the features of the electron microscope 100 of FIG. 1, in a manner that may be referenced above, and the description of which will not be repeated here.

[0063] In particular, the electron microscope 200 includes a cold cathode field emission emitter having an emission tip 112 that can be cleaned by heating using a first cleaning device 121 in a first cleaning mode, and an extraction electrode 114 that can be cleaned by heating using a second cleaning device 122 in a second cleaning mode.

[0064] The first opening 115 of the extraction electrode 114 may function as a beam aperture for shaping the electron beam, and may optionally also function as a differential pumping aperture to enable differential pumping between the first vacuum region 10a and the second vacuum region 10b.

[0065] According to some embodiments, which can be combined with other embodiments described herein, the condenser lens 130 is a magnetic condenser lens. In particular, the magnetic condenser lens may include a first inner pole piece and a first outer pole piece, and a first axial distance (D1) between the emitter tip 112 and the first inner pole piece is greater than a second axial distance (D2) between the emitter tip 112 and the first outer pole piece. Such a magnetic lens, in which the outer pole piece projects further toward the electron source than the inner pole piece, has an axially extending gap between the pole pieces and is therefore sometimes referred to as an "axial gap lens." An axial gap magnetic lens may generate a magnetic field that extends beyond the axial gap into an area, i.e., axially beyond the outer pole piece toward the electron source. In other words, the axial gap condenser lens may be an immersion lens and may provide a magnetic interaction region that extends towards the electron source in such a way that the collimation effect of the condenser lens may act on the electron beam 105 near or inside the electron source 110. A more compact electron microscope may be provided and the negative effects of electron-electron interactions may be reduced.

[0066] In some embodiments, the first axial distance (D1) between the emission tip 112 and the first inner pole piece of the condenser lens is 20 mm or less, in particular 15 mm or less. In some embodiments, the second axial distance (D2) between the emission tip 112 and the condenser lens is 15 mm or less, in some embodiments 8 mm or less.

[0067] The acceleration section of this electron microscope for accelerating electrons to energies of 5 keV or more, particularly 10 keV or more, may partially overlap with the magnetic interaction region of the condenser lens, which shortens the overall beam propagation distance within the electron microscope.

[0068] According to some embodiments, the objective lens 140 is a magnetic objective lens having a second inner pole piece and a second outer pole piece, and a third axial distance (D3) between the second inner pole piece and the sample stage 18 is greater than a fourth axial distance (D4) between the second outer pole piece and the sample stage 18. In particular, the magnetic objective lens may be an axial gap lens in which the outer pole piece protrudes further toward the sample stage 18 than the inner pole piece such that an axial gap is formed between the end of the outer pole piece and the end of the inner pole piece. A magnetic interaction region provided by the magnetic objective lens may extend axially beyond the pole pieces of the magnetic objective lens towards the sample 16, which may be placed on the sample stage 18. This allows the objective lens to have a short focal length and to be placed close to the sample stage 18.

[0069] In some implementations, the distance between the objective lens 140 and the sample stage 18 (i.e. the fourth axial distance (D4)) may be 20 mm or less, in particular 10 mm or less, especially 5 mm or less. In particular, the focal length of the objective lens 140 may be 10 mm or less, or even 5 mm or less. In some embodiments, the third axial distance (D3) between the sample stage 18 and the second inner pole piece of the objective lens 140 is greater than the fourth axial distance (D4), in particular 30 mm or less, especially 10 mm or less.

[0070] In some embodiments, both the condenser lens 130 and the objective lens 140 may be axial gap lenses, and the condenser lens 130 and the objective lens 140 may be arranged symmetrically with respect to each other along the optical axis A. In particular, the condenser lens 130 may have an axial gap opening towards the electron source 110, and the objective lens 140 may have an axial gap opening towards the sample, and both of these lenses may be configured as immersion lenses facing in opposite directions. Using corresponding lens types as the condenser lens and the objective lens, a compact electron microscope suitable for providing a small beam probe on the sample and therefore good resolution can be obtained.

[0071] Details of the first cleaning device 121, the second cleaning device 122 and the differential pumping have been described with respect to the electron microscope 100 of FIG. 1 and will not be repeated here.

[0072] FIG. 3 shows a flow diagram of a method of operating an electron microscope according to embodiments described herein.

[0073] The electron microscope may have a gun housing containing an electron source comprising a cold field emitter, the gun housing providing a first vacuum region. A second vacuum region may be disposed downstream of the first vacuum region along the optical axis, and optionally a third or further additional vacuum region may be disposed downstream of the second vacuum region along the optical axis, the third or further vacuum regions being capable of being differentially pumped. The first and second vacuum regions may be separated by a first gas separation wall having a first differential pumping aperture formed therein, and the second and third vacuum regions may be separated by a second gas separation wall having a second differential pumping aperture formed therein.

[0074] The electron source of this electron microscope includes a cold field emitter with an emitting tip and an extraction electrode for extracting the electron beam from the cold field emitter for propagation along an optical axis A.

[0075] In boxes 310 and 320 of FIG. 3, the electron microscope is prepared for operation with two cleaning stages, for example before operating the electron microscope for the very first time or after flooding the interior of the electron microscope with air, for example during service operations or maintenance.

[0076] In box 310, the electron microscope is placed in a second cleaning mode in which the extraction electrode of the electron source is cleaned by heating the extraction electrode, in particular by heating it to a temperature of 500° C. or higher, and in particular by heating it to a temperature between 600° C. and 800° C. In particular, the area of ​​the extraction electrode surrounding the first aperture through which the electron beam propagates during operation is heated to a temperature between 600° C. and 800° C.

[0077] In the second cleaning mode, a current may be passed through a second heater disposed adjacent to the extraction electrode to heat the extraction electrode to a temperature higher than 500° C., in particular to a temperature between 600° C. and 800° C. This second heater may be a heating wire 126 disposed near the first aperture, which may optionally extend at least partially around the first aperture upstream of the extraction electrode. In some embodiments, the heating wire 126 may be a tantalum wire or tantalum filament.

[0078] The current to be applied in the second cleaning mode may be determined in a preceding calibration stage.

[0079] Optionally, in the second cleaning mode, the suppression electrode and / or the extraction electrode may be set to one or more predetermined potentials, which may help to direct the thermoelectrons emitted by the heating wire towards the extraction electrode and / or away from the emission tip.

[0080] In box 320, the electron microscope is set to a first cleaning mode in which the emission tip of the cold field emitter is cleaned by heating the emission tip, in particular by heating to a temperature of 1500° C. or higher, in particular by heating to a temperature of 2000° C. or higher, or even by heating to a temperature of 2000 K or higher.

[0081] In a first cleaning mode, a current may be applied to a heating filament, in particular a V-shaped heating filament, to which the emission tip is coupled, in order to heat the emission tip to a temperature higher than 2000° C. Particles adhering to the emission tip may be evaporated and the emission surface may be cleaned. The current applied in the first cleaning mode may be determined in a preceding calibration stage.

[0082] Optionally, in the first cleaning mode, the suppression electrode and / or extraction electrode may be set to one or more predetermined potentials, particularly a high voltage compared to the emission tip, which may facilitate maintaining a sharp emission tip.

[0083] After cleaning in the first and second cleaning modes, the electron microscope may be set in an operating mode, indicated by box 330. In the operating mode, an electron beam is extracted from the cold field emitter for propagation along the optical axis, and the electron beam is shaped by propagating through a first aperture, which may be provided in the extraction electrode. The electron beam is then collimated by a condenser lens downstream of the electron source, i.e. the divergence of the electron beam is reduced. In particular, the divergence of the electron beam may be adjusted by adjusting the excitation of the condenser lens. The collimated electron beam is then focused on the sample by means of an objective lens.

[0084] In the operating mode, the electrons of the electron beam may be accelerated to an energy of 5 keV or more, in particular 10 keV or more, in an acceleration section, which is arranged upstream of the condenser lens and / or at least partially overlaps with the condenser lens. For example, a first part of the acceleration section may extend between the emission tip and an extraction electrode, which may be set at a higher voltage compared to the emission tip. A second part of the acceleration section may extend downstream of the electron source, for example between the extraction electrode and an anode, which may be set at a higher voltage compared to the extraction electrode. The anode may be arranged close to the condenser lens or inside the condenser lens. The acceleration section may thus overlap with a magnetic interaction region provided by the condenser lens.

[0085] In the operating mode, the condenser lens may be used to collimate the electron beam. The condenser lens may be a magnetic lens having a first inner pole piece and a first outer pole piece, and a first axial distance between the emitting tip and the first inner pole piece may be greater than a second axial distance between the emitting tip and the first outer pole piece. In particular, the condenser lens may be an axial gap lens, i.e., the first outer pole piece of the condenser lens may protrude further toward the electron source than the first inner pole piece of the condenser lens.

[0086] In an operating mode, the electrons of the electron beam may be decelerated to an entrance energy of 3 keV or less, in particular 1 keV or less, in a deceleration section downstream of or at least partially overlapping the objective lens. For example, a potential difference may be applied between a first electrode arranged near or inside the objective lens and a proxy electrode arranged near or on the sample itself. The deceleration section may thus overlap a magnetic interaction region provided by the objective lens.

[0087] The electron beam may be focused onto the sample, and the generated signal electrons may be accelerated towards and through an objective lens, and the signal electrons may be detected by one or more electron detectors (not shown in this figure) to inspect the sample, e.g., to generate an image of the sample.

[0088] In some embodiments, which may be combined with other embodiments described herein, the emission tip is disposed in a first vacuum region and the condenser lens is disposed in a second vacuum region downstream of the first vacuum region. The first opening in the extraction electrode may function as a differential pumping aperture between the first vacuum region and the second vacuum region. The method may include differentially pumping the first vacuum region and the second vacuum region.

[0089] Optionally, a third vacuum region may be provided downstream of the second vacuum region, and a second differential pumping aperture may be provided in the gas separation wall between the second and third vacuum regions. The method may further include differentially pumping the first, second and third vacuum regions, and optionally at least one additional vacuum region downstream of the third vacuum region.

[0090] As shown diagrammatically by box 340 in Figure 3, after a predetermined time in the operation mode of box 330, for example after about one hour of operation, the microscope may switch back to a first cleaning mode, in which the emission tip may be cleaned in such a way as to ensure a stable electron beam. The microscope may switch back to operation in box 350.

[0091] In some embodiments, the method includes switching from the operation mode to the first cleaning mode after a predetermined period in the operation mode, for example after about one hour of operation. In particular, the electron microscope may automatically switch to the first cleaning mode after a predetermined interval of operation, for example after a predetermined interval of operation of more than one hour and less than or equal to three hours. Switching to the first cleaning mode after the predetermined interval of operation can enable a constantly stable high brightness electron beam in the operation mode.

[0092] The second cleaning mode may be performed less frequently, for example only after flooding the gun housing with air, and / or at predetermined service intervals which may be longer than one month, and / or if the electron beam exhibits undesirable instability or reduced brightness.

[0093] In particular, the following embodiments are described herein:

[0094] Embodiment 1: An electron microscope (100) comprising an electron source (110), the electron source (110) being a cold field emitter with an emission tip (112), an extraction electrode (114) for extracting an electron beam (105) from the cold field emitter for propagation along an optical axis (A), the extraction electrode (114) having a first aperture (115) configured as a first beam stop aperture, a first cleaning device (121) for cleaning the emission tip (112) by heating the emission tip, and a second cleaning device (122) for cleaning the extraction electrode (114) by heating the extraction electrode, the electron microscope further comprising a condenser lens (130) for collimating the electron beam downstream of the electron source, and an objective lens (140) for focusing the electron beam on a sample.

[0095] In some embodiments, the emitting tip is a tungsten tip, particularly a single crystal of tungsten with a (3,1,0) orientation.

[0096] Embodiment 2: An electron microscope as described in embodiment 1, wherein the first cleaning device (121) comprises a heating filament (125) in thermal contact with the emission tip, the emission tip being attached or coupled to the heating filament.

[0097] The first cleaning device may be a flash cleaning device configured to clean the tip by heating it, in particular by heating it at regular intervals, for example after a predefined operating time. The tip may be heated to a temperature higher than 1000° C., in particular higher than 2000° C.

[0098] In some embodiments, the heating filament is a V-shaped heating wire, with the emission tip being coupled to a bent portion of the V-shaped heating wire.

[0099] In some embodiments, the heating filament is a metal filament, in particular a tungsten filament, and the emission tip is a tungsten tip.

[0100] Embodiment 3: The electron microscope according to embodiment 1 or 2, wherein the second cleaning device comprises a second heater arranged adjacent to the extraction electrode (114), in particular a second heating wire (126) arranged adjacent to the extraction electrode (114). The second heater may be configured to be heated to a temperature of 1500° C. or more, in particular 2000° C. or more, in particular by allowing a current to flow through the second heater.

[0101] Embodiment 4: The electron microscope of embodiment 3, wherein the heating wire is positioned to at least partially surround the first opening (115) of the extraction electrode.

[0102] Embodiment 5: The electron microscope of embodiment 3 or 4, wherein the heating wire (126) comprises or is made of tantalum.

[0103] Embodiment 6: The electron microscope according to any of the embodiments 1 to 5, further comprising a cleaning controller (128) configured to allow current to flow through a heating filament (125) in thermal contact with the emission tip in a first cleaning mode to heat the emission tip to a temperature higher than 1500° C. Alternatively or additionally, the cleaning controller is configured to allow current to flow through a heating wire (126) of a second cleaning device in a second cleaning mode to at least partially heat the extraction electrode to a temperature higher than 500° C. and / or to cause electron stimulated desorption at a surface of the extraction electrode.

[0104] In particular, in the second cleaning mode, the area of ​​the extraction electrode surrounding the first opening is heated to a temperature of more than 500° C., in particular to cause thermal outgassing of the extraction electrode.

[0105] Embodiment 7: An electron microscope according to any one of embodiments 1 to 6, wherein the distance along the optical axis between the emission tip (112) and the first opening (115) of the extraction electrode (114) is 5 mm or less, in particular 1 mm or less.

[0106] Embodiment 8: An electron microscope described in any of embodiments 1 to 7, wherein the condenser lens (130) is a magnetic condenser lens having a first inner pole piece and a first outer pole piece, and a first axial distance (D1) between the emission tip and the first inner pole piece is greater than a second axial distance (D2) between the emission tip and the first outer pole piece.

[0107] In particular, the magnetic condenser lens may be an axial gap lens.

[0108] In some embodiments, the first axial distance (D1) between the tip and the first inner pole piece is 20 mm or less, in particular 15 mm or less, and in some embodiments, the second axial distance (D2) between the tip and the first inner pole piece is 15 mm or less, or even 8 mm or less.

[0109] Embodiment 9: An electron microscope described in any of embodiments 1 to 8, wherein the objective lens (140) is a magnetic objective lens having a second inner pole piece and a second outer pole piece, and a third axial distance between the second inner pole piece and the sample stage is greater than a fourth axial distance between the second outer pole piece and the sample stage.

[0110] In particular, this magnetic objective lens may be an axial gap lens.

[0111] In some embodiments, the magnetic condenser lens and the magnetic objective lens may be positioned approximately symmetrically with respect to each other along the optical axis.

[0112] Embodiment 10: An electron microscope as described in any one of claims 1 to 9, comprising an acceleration section for accelerating an electron beam to an energy of 5 keV or more, the acceleration section being upstream of a condenser lens or at least partially overlapping with the condenser lens, and / or a deceleration section for decelerating an electron beam from an energy of 5 keV or more to an incident energy of 3 keV or less, the deceleration section being downstream of an objective lens or at least partially overlapping with the objective lens.

[0113] Embodiment 11: The electron microscope according to any one of embodiments 1 to 10, wherein the first opening (115) is arranged so as to function as a first differential pumping aperture.

[0114] Embodiment 12: An electron microscope described in any of embodiments 1 to 11, further comprising a second beam aperture (132) between the condenser lens (130) and the objective lens (140), the second beam aperture (132) being arranged to function as a second differential exhaust aperture.

[0115] Embodiment 13: An electron microscope as described in embodiment 12, further comprising at least one third differential pumping aperture (133) between the second differential pumping aperture and the objective lens.

[0116] Embodiment 14: An electron microscope according to any of embodiments 1 to 13, wherein the emission tip (112) is disposed in the first vacuum region (10a), the condenser lens (130) is disposed in the second vacuum region (10b), and the electron microscope comprises an ion getter pump (13) and a non-evaporable getter pump (14) for evacuating the first vacuum region (10a).

[0117] Embodiment 15: An electron microscope according to any one of embodiments 1 to 14, further comprising a scanning deflector, the electron microscope being configured as a scanning electron microscope (SEM) for high throughput wafer inspection.

[0118] Embodiment 16: An electron source for an electron microscope according to any of the embodiments described herein.

[0119] Embodiment 17: A method of operating an electron microscope having an electron source with a cold cathode field emission emitter, comprising: in a first cleaning mode, cleaning an emission tip of the cold cathode field emission emitter by heating the emission tip; in a second cleaning mode, cleaning an extraction electrode of the electron source by heating the extraction electrode; and in an operating mode, extracting an electron beam from the cold cathode field emission emitter for propagation along an optical axis (A), the electron beam being shaped by a first aperture provided in the extraction electrode; collimating the electron beam using a condenser lens; and focusing the electron beam on a sample using an objective lens.

[0120] Embodiment 18: The method of embodiment 17, wherein in the first cleaning mode, an electric current is passed through a heating filament to which the emission tip is coupled in order to heat the emission tip to a temperature greater than 1500° C.

[0121] Embodiment 19: The method according to embodiment 17 or 18, wherein in the second cleaning mode, an electric current is passed through a second heater, in particular a heating wire (126) disposed adjacent to the extraction electrode, in order to heat the extraction electrode to a temperature higher than 500° C.

[0122] Embodiment 20: The method according to any of embodiments 17-19, wherein in a second cleaning mode, an electric current is passed through a heating wire disposed adjacent to the extraction electrode to thermally eject electrons from the heating wire for cleaning the extraction electrode by at least one of electron stimulated desorption and thermal gas emission. In some embodiments, the heating wire is heated to a temperature of 1500° C. or higher, in particular to a temperature of 2000° C. or higher.

[0123] Embodiment 21: A method as described in any of embodiments 17 to 20, comprising switching from the operating mode to the first cleaning mode after a predetermined period in the operating mode, in particular automatically switching to the first cleaning mode at a predetermined operating interval.

[0124] Embodiment 22: A method according to any of embodiments 17 to 21, wherein the emission tip is disposed in a first vacuum region, the condenser lens is disposed in a second vacuum region downstream of the first vacuum region, and the first opening functions as a differential pumping aperture between the first vacuum region and the second vacuum region, and the method includes differentially pumping the first vacuum region and the second vacuum region, and may include differentially pumping a third vacuum region disposed downstream of the second vacuum region through a second differential pumping aperture disposed between the second vacuum region and the third vacuum region.

[0125] Embodiment 23: The method of any of embodiments 17 to 22, further comprising, in an operating mode, one or more of: (i) accelerating electrons of the electron beam to an energy of 5 keV or more in an acceleration section, the acceleration section being upstream of or at least partially overlapping with a condenser lens; (ii) collimating the electron beam using a condenser lens having a first inner pole piece and a first outer pole piece, the first axial distance between the emission tip and the first inner pole piece being greater than a second axial distance between the emission tip and the first outer pole piece; and / or (iii) decelerating electrons of the electron beam to an entrance energy of 3 keV or less in a deceleration section, the deceleration section being downstream of or at least partially overlapping with the objective lens.

[0126] In some embodiments, the electrons of the electron beam are accelerated in the acceleration section to an energy of at least 10 keV, in particular at least 15 keV, especially at least 30 keV.

[0127] In some embodiments, the electrons of the electron beam are decelerated in the deceleration section to an entrance energy of 2 keV or less, in particular 1 keV or less.

[0128] It is to be understood that each of the claims set forth below may relate to one or more of the preceding claims, and that the present disclosure encompasses such embodiments comprising the features of any subset of the claims. While the above description is directed to embodiments, other and additional embodiments may be devised without departing from the basic scope, the scope of which is determined by the claims set forth below.

Claims

1. An electron source (110), a cold field emitter having an emitting tip (112); an extraction electrode (114) for extracting the electron beam (105) from the cold field emitter for propagation along an optical axis (A), the extraction electrode (114) having a first aperture (115) configured as a first beam stop aperture; a first cleaning device (121) for cleaning said discharge tip (112) by heating said discharge tip; and a second cleaning device (122) for cleaning the extraction electrode (114) by heating the extraction electrode; The electron source (110), a condenser lens (130) for collimating the electron beam downstream of the electron source; an objective lens (140) for focusing the electron beam onto a sample; An electron microscope (100).

2. 2. The electron microscope of claim 1, wherein the first cleaning device (121) comprises a heating filament (125) in thermal contact with the emission tip, the emission tip being attached or coupled to the heating filament.

3. 3. The electron microscope of claim 1, wherein the second cleaning device (122) comprises a heating wire (126) disposed adjacent to the extraction electrode, the heating wire (126) being configured to be heated to a temperature of 1500° C. or more.

4. 4. The electron microscope of claim 3, wherein the heating wire is positioned to at least partially surround the first opening (115) of the extraction electrode.

5. 5. The electron microscope of claim 3 or 4, wherein the heating wire (126) comprises or is made of tantalum.

6. A cleaning controller (128), comprising: configured to allow an electric current to flow through a heating filament (125) in thermal contact with the emission tip in a first cleaning mode to heat the emission tip to a temperature greater than 1500° C.; and / or configured to allow current to flow through a heating wire (126) of the second cleaning apparatus to at least one of heat the extraction electrode at least partially to a temperature greater than 500° C. and induce electron stimulated desorption at a surface of the extraction electrode in a second cleaning mode.

6. The electron microscope according to claim 1.

7. Electron microscope according to any one of the preceding claims, wherein the distance between the emission tip (112) and the first opening (115) of the extraction electrode (114) is 5 mm or less, in particular 1 mm or less.

8. The electron microscope of any one of claims 1 to 7, wherein the condenser lens (130) is a magnetic condenser lens having a first inner pole piece and a first outer pole piece, and a first axial distance (D1) between the emission tip and the first inner pole piece is greater than a second axial distance (D2) between the emission tip and the first outer pole piece.

9. The electron microscope of any one of claims 1 to 8, wherein the objective lens (140) is a magnetic objective lens having a second inner pole piece and a second outer pole piece, and a third axial distance between the second inner pole piece and a sample stage is greater than a fourth axial distance between the second outer pole piece and the sample stage.

10. an acceleration section for accelerating the electron beam to an energy of 5 keV or greater, the acceleration section being upstream of or at least partially overlapping the condenser lens; a deceleration section for decelerating the electron beam from the energy of 5 keV or more to an entrance energy of 2 keV or less, the deceleration section being downstream of or at least partially overlapping the objective lens; The electron microscope according to any one of claims 1 to 9, comprising:

11. 11. The electron microscope according to claim 1, wherein the first beam stop aperture is arranged so as to function as a first differential exhaust aperture.

12. The electron microscope of any one of claims 1 to 11, further comprising a second beam aperture (132) between the condenser lens (130) and the objective lens (140), the second beam aperture (132) being arranged to function as a second differential pumping aperture.

13. 13. The electron microscope of claim 1, wherein the emission tip (112) is arranged in a first vacuum region (10a), the condenser lens (130) is arranged in a second vacuum region (10b), and the electron microscope comprises an ion getter pump (13) and a non-evaporable getter pump (14) for evacuating the first vacuum region (10a).

14. 14. The electron microscope of claim 1, further comprising a scanning deflector, the electron microscope being configured as a scanning electron microscope (SEM) for high throughput wafer inspection.

15. 1. An electron source for an electron microscope, comprising: a cold field emitter having an emitting tip; an extraction electrode for extracting the electron beam from the cold field emitter for propagation along an optical axis; a first cleaning device for cleaning the discharge tip by heating the discharge tip; a second cleaning device for cleaning the extraction electrode by heating the extraction electrode; The electron source comprises:

16. 1. A method of operating an electron microscope having an electron source comprising a cold field emitter, comprising the steps of: in a first cleaning mode, cleaning the emission tip of the cold field emitter by heating the emission tip; in a second cleaning mode, cleaning an extraction electrode of the electron source by heating the extraction electrode; and In the operating mode, extracting an electron beam from the cold field emitter for propagation along an optical axis, the electron beam being shaped by a first aperture provided in the extraction electrode; collimating the electron beam with a condenser lens; and focusing the electron beam onto a sample using an objective lens; A method comprising:

17. 17. The method of claim 16, wherein in the first cleaning mode, an electric current is passed through a heating filament to which the emission tip is coupled to heat the emission tip to a temperature greater than 1500°C.

18. 18. The method according to claim 16 or 17, wherein in the second cleaning mode, an electric current is passed through a heating wire disposed adjacent to the extraction electrode to cause electrons to be thermally ejected from the heating wire for cleaning the extraction electrode by at least one or both of electron stimulated desorption and thermal gas emission.

19. A method according to any of claims 16 to 18, comprising switching from said operational mode to said first cleaning mode after a predefined period in said operational mode, in particular automatically switching to said first cleaning mode at predefined operational intervals.

20. the emission tip is disposed in a first vacuum region, the condenser lens is disposed in a second vacuum region downstream of the first vacuum region, the first aperture functions as a differential pumping aperture between the first vacuum region and the second vacuum region, and the method further comprises: and differentially pumping the first vacuum region and the second vacuum region, and may include differentially pumping a third vacuum region disposed downstream of the second vacuum region through a second differential pumping aperture disposed between the second vacuum region and the third vacuum region. The method according to any one of claims 16 to 19.

21. In the operating mode, accelerating electrons of the electron beam to an energy of 5 keV or greater in an acceleration section, the acceleration section being upstream of or at least partially overlapping the condenser lens; collimating the electron beam with the condenser lens having a first inner pole piece and a first outer pole piece, wherein a first axial distance between the emitting tip and the first inner pole piece is greater than a second axial distance between the emitting tip and the first outer pole piece; and decelerating the electrons of the electron beam in a deceleration section to an entrance energy of 3 keV or less, the deceleration section being downstream of the objective lens or at least partially overlapping the objective lens. The method of any of claims 16 to 20, further comprising:

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