Photoemission of charged-particle beams in multi-beam charged-particle microscopes

Abstract

Various examples of the disclosure pertain to a multi-beam charged-particle microscope such as a multi-beam scanning electron microscope, MSEM. For the generation of charged-particle beams, photoemission through a photocathode is employed.

Description

[0001] D E S C R I P T I O N

[0002] PHOTOEMISSION OF CHARGED-PARTICLE BEAMS IN MULTI-BEAM CHARGED- PARTICLE MICROSCOPES

[0003] TECHNICAL FIELD

[0004] Various examples of the disclosure generally relate to multi-beam charged-particle microscopes. Various examples specifically relate to using a photocathode illuminated by multiple laser beams as source of multiple charged-particle beams.

[0005] BACKGROUND

[0006] A scanning electron microscope (SEM) produces images by focusing a beam of electrons onto a sample surface. The electron beam including primary electrons is scanned across the sample, and the resulting signal, often generated by secondary electrons, is detected.

[0007] A Multi-Beam Scanning Electron Microscope (MSEM) utilizes multiple electron beams to contemporaneously scan and image a sample, allowing for higher throughput and faster data acquisition compared to traditional single-beam SEMs.

[0008] The current state-of-the-art in generating electron beams in MSEMs involves the use of ( / ) thermionic / field / thermal field emission, e.g., using tungsten tips, and ( / / ) photoemission through a photocathode. This disclosure is concerned with ( / / ) photoemission through a photocathode.

[0009] Photoemission includes absorption of photon in the photocathode. Each photocathode material has a characteristic energy barrier known as the work function. For an electron to be emitted from the surface, the energy of the incoming photon must be equal to or greater than this work function. The energy may match internal energy states of the photocathode material. If the photon's energy is sufficient, it may transfer its energy to an electron, which then gains enough kinetic energy to escape from the atomic lattice of the photocathode material and is emitted from the surface.

[0010] It has been observed that generation of electron beams using a photocathode illuminated by multiple laser beams can suffer from various drawbacks such as downtime for maintenance of the photocathode, aging effects of the photocathode reducing image quality, etc. Additionally, the electron optics in the MSEM cause distortions and aberrations in the electron beam. For instance, the electrons are emitted in circular areas on the photocathode, which are then distorted as they pass through electron optics. This distortion leads to aberrations, including (field) astigmatism, which degrades the resolution.

[0011] SUMMARY

[0012] Accordingly, a need exists for advanced techniques of generating charged-particle beams for multi-beam charged-particle microscopes using a photocathode. A need exists for techniques that overcome or mitigate at least some of the above-identified restrictions and drawbacks.

[0013] This need is met by the features of the independent claims. The features of the dependent claims define embodiments.

[0014] One aspect of the present disclosure relates to a multi-beam charged-particle microscope. The microscope comprises at least one laser. The microscope further comprises a vacuum chamber. The microscope further comprises a frame for arranging a photocathode in the vacuum chamber so that the photocathode emits multiple charged-particle beams when being illuminated by multiple laser beams provided by the at least one laser. The microscope further comprises one or more optical elements arranged along the multiple laser beams and configured to act upon each of the multiple laser beams.

[0015] A multi-beam charged-particle microscope may be understood as an instrument producing images by focusing multiple beams of charged particles onto a sample. The multiple charged-particle beams are scanned across the sample.

[0016] A laser may be understood as a device emitting high-intensity light that is preferably coherent in time and space and has a narrow wavelength spectrum. Photons emitted by the laser have a sufficient energy to trigger emission of charged particles from the photocathode. Depending on the photocathode material, different types of charged- particle beams may be generated, e.g., electron beams. However, the present disclosure is not limited to electron-based multi-beam charged-particle microscopes. Also, ion-based multi-beam charged-particle microscopes may be subject to the disclosure. A vacuum chamber may be understood as an enclosure evacuated to a pressure below atmospheric pressure. Thereby, the average free beam path without a scattering event can be increased for the charged-particle beams. A vacuum pump is coupled to the vacuum chamber to evacuate the vacuum chamber.

[0017] A photocathode may be understood as a material emitting electrons or other charged particles upon illumination with light. The photocathode may be a thin-film photocathode. For instance, a thickness of the photocathode may be less than 50 pm, or optionally less than 1 pm. For instance, tungsten or cesium or gold may be coated onto a substrate such as silicon or glass or sapphire glass. Cesium-based photocathodes can include compounds like cesium antimonide (Cs3Sb) or cesium telluride (CsTe). Another material used to convert photons to electrons is doped or undoped Gallium Arsenide (GaAs) or Indium Gallium Arsenide (InGaAas). The doping can affect the electron states. The coating is the optically active material where the conversion from photons to electrons takes place.

[0018] A frame for arranging a photocathode in the vacuum chamber may be understood as a support structure that engages the photocathode at one or more support positions, typically arranged along the circumference of the photocathode. For instance, the frame may be made from metal or plastic material. The frame may include one or more engagement features to engage the photocathode. Typically, the mechanical stability of the photocathode is relatively limited, due to the extended thin planar geometry of the photocathode. By using the frame, additional stability can be provided to the arrangement of the frame and the photocathode. The frame may releasably engage the photocathode. I.e. , it would be possible that the frame can be operated to retain or release the photocathode. For instance, for restoring the photocathode, the photocathode may be released from the frame. It would also be possible that each photocathode in the multi-beam charged-particle microscope is provided with a respective frame and that instead of releasing the photocathode from the frame the frame is moved together with the photocathode between multiple positions.

[0019] Optical elements may be understood as components influencing the properties of light, such as its direction, intensity, or shape. Examples include, mirrors, wavefrontshaping elements, pinholes, wegdes, shaped mirrors, freeform mirrors, etc.. An effect may be that the multiple laser beams can be conveniently shaped using the one or more optical elements, enabling the pre-compensation of aberrations otherwise appearing in the multiple charged-particle beams. This is because the shape of the laser spot defines the electron emission area, thereby determining the shape of the electron beam. By shaping the at least one laser beam, aberrations can be corrected, resulting in improved image quality and increased precision in the multi-beam charged-particle microscope.

[0020] As described above, multiple laser beams are used to illuminate the photocathode. Each of these laser beams may be associated with a respective optical element. Multiple optical elements may be arranged along the laser beams and configured to act upon each of the laser beams. Each of the multiple laser beams may be associated with a respective one of the multiple optical elements. Thus, different optical elements act on different laser beams. Each laser beam can generate a respective charged-particle beam.

[0021] An effect of this arrangement is that each laser beam can be individually shaped to compensate for beam-specific aberrations, allowing for improved resolution and reduced distortion in the final image.

[0022] In an example, one or more spatial light modulators (SLMs) may be used as optical elements to shape the laser beams.

[0023] SLMs may be implemented using liquid crystal on silicon (LCoS) displays, which include of a layer of liquid crystals on top of a silicon substrate. Another implementation is through the use of microelectromechanical systems (MEMS), which may, e.g., utilize an array of micro-mirrors that can be individually tilted to modulate light.

[0024] SLMs enable shaping a wavefront of each of multiple laser beams. The wavefront can be shaped by adjusting a phase of the incident laser light as a function of a position along the beam diameter. Thereby, it is possible to tailor the beam characteristics with respect to multiple degrees of freedom. For instance, a laser beam may be deflected, enlarged or constricted, the divergence may be adjusted, the beamwidth may be changed, etc.

[0025] The one or more SLMs may comprise multiple SLMs, each configured to act upon a respective laser beam. A control unit may be provided to control the multiple spatial light modulators, and this control unit may be configured to divert each of the multiple laser beams to adjust a pattern formed by the multiple laser beams on the photocathode.

[0026] An effect of this arrangement may be that individual virtual origins, i.e., virtual emission positions of the charged-particle beams, are adjusted by adjusting the pattern formed on the photocathode, which helps in correcting a field curvature aberration. This correction enables improved imaging performance and reduced distortion in the final image.

[0027] A field-curvature aberration may be understood as an aberration that can be described by a curved focal surface of a charged-particle beam. For instance, the field-curvature aberration can stem from a charged-particle beam being formed by multiple planar waves incident on a charged-particle lens at different angles. A fieldcurvature aberration may also arise from imperfections in the magnetic or electrostatic fields that shape the lensing effect.

[0028] In an example, the control unit may be configured to control the multiple spatial light modulators to divert each of the multiple laser beams based on a priori knowledge of a sample that is imaged by the multiple charged-particle beams. In other words, it would be possible to adjust the pattern formed by the multiple laser beams based on the a priori knowledge of the sample. For instance, certain samples may include repetitive structures, specifically periodic structures. Examples include semiconductor samples including repetitive semiconductor structures such as memory cells or transistors. Such repetitive semiconductor structures often are characterized by repetitive conductive and non-conductive regions. By adjusting the pattern based on a priori knowledge of a periodicity of repetitive structures of the sample, it is possible to avoid or mitigate local charge up of the sample. For instance, the time of the multiple electron beams on non-conductive regions of the sample may be minimized. By reducing local charge up, charge up induced distortions in the final image can be reduced.

[0029] Alternatively or additionally to adjusting the pattern formed by the multiple laser beams on the photocathode, a control unit may be provided that controls an angle of incidence of each of the multiple laser beams on a surface of the photocathode. Also, the angle of incidence of a laser beam enables to compensate for inhomogeneous fields observed in the aperture of a field lens, thereby enabling to compensate aberrations, e.g., field-curvature aberrations, experienced by the charged-particle beams. This correction enables improved imaging performance and reduced distortion in the final image.

[0030] In an example, a control unit may be provided. The control unit may be configured to control the multiple SLMs to activate or deactivate a footprint of each of the multiple laser beams on the photocathode. In other words, individual laser beams may be switched on or may be switched off. Thus, in turn, individual charged-particle beams may be switched on or may be switched off.

[0031] The number of charged-particle beams used for imaging can thereby be controlled. For instance, it would be possible to maintain a single laser spot and a single charged-particle beam, when temporarily operating in a high-resolution imaging mode. Thus, a multi-beam charged-particle microscope may be temporarily operated in a single-beam mode. For instance, that laser spot that generates the charged- particle beam that passes closest to a center axis of a charged-particle field lens may be retained. Since aberrations, e.g., field-curvature aberrations, typically have prominent effects primarily in the outer areas of the aperture of the charged-particle field lens, the charged-particle beams that pass through a center of the aperture are oftentimes less affected by the aberrations. In the high-resolution imaging mode, fewer charged-particle beams are used for imaging the sample; it would be possible to only use a single charged-particle beam for imaging the sample. It is then possible to tailor any compensation of aberrations to the particular position of that single charged-particle beam in the aperture of a field lens, thereby effectively compensating any aberration. On the other hand, since only a single charged-particle beam is used for imaging, the overall throughput is reduced. The sample area image per time unit is reduced. Thus, it is possible to dynamically switch between different imaging modes, e.g., a high-resolution imaging mode only including a few or only a single charged-particle beam on the one hand; and a high-throughput imaging mode using a larger number of charged-particle beams. By dynamically deactivating off- center charged-particle beams, it is possible to selectively activate a high-resolution imaging mode. On the other hand, by (re-)activating the of-center charged-particle beams, it is possible to selectively activate a high-throughput imaging mode, e.g., if a fast scan time is desired. In an example, a control unit may be configured to activate or deactivate the footprint of each of the multiple laser beams depending on at least one of a center distance of each of the multiple laser beams with respect to a center of a surface of the photocathode, or a center distance of each of the associated charged-particle beams with respect to a center of an aperture of a charged-particle field lens.

[0032] This arrangement enables adaptive control over multiple laser beams (individual laser beams may be switched off). When certain off-center charged-particle beams are switched off, this eliminates the need to compromise between multiple settings for multiple beams in a charged-particle field lens, thus achieving better resolution. However, this may come at the cost of reduced imaging speed, e.g., if too many laser beams are switched off.

[0033] In an example, a control unit may be configured to distort the footprint of each of the multiple laser beams depending on at least one of a center distance of each of the multiple laser beams with respect to a center of a surface of the photocathode, or a center distance of each of the associated charged-particle beams with respect to a center of an aperture of a charged-particle field lens.

[0034] By distorting the footprint of the multiple laser beams, a beam-specific astigmatism and other aberrations can be compensated. Beam-specific astigmatism can be introduced by aberrations of a field lens. In an example, a control unit may be configured to control multiple SLMs adjust the footprint of each of the multiple laser beams. Thereby, the emission area of the charged-particle beams can be adjusted. Additionally, the intensity can be varied to change the footprint and the emission current.

[0035] This arrangement may have an effect of enabling to pre-compensate for aging effects on the photocathode. The photocathode may experience different conversion efficiencies in various areas, depending on an ageing state of the various areas. By adjusting the intensity of each laser beam, this arrangement ensures consistent performance across the entire photocathode surface. Specifically, by increasing the intensity of laser beams impinging on aged areas of the photocathode with poor conversion efficiency, this arrangement counteracts the effects of aging. Above, some scenarios have been disclosed in which the one or more optical elements include one or more SLMs. SLMs are potentially complex and bulky, difficult to maintain and control. To mitigate this, other types of optical elements may be used.

[0036] In an example, the multi-beam charged-particle microscope may include one or more non-circular pin holes.

[0037] The use of pin holes may reduce the overall complexity of the system, making it easier to manufacture and maintain.

[0038] For instance, with a shaped backside pin hole (e.g., elliptic) aberrations (also higher order aberrations) can be pre-compensated. An astigmatisms can be compensated. A tailored astigmatism, e.g., for imaging elongated structures, may be imprinted. The pin hole shape is imprinted in the electron emission area. Adjusted sizes and / or shapes of the footprint of the multiple laser beams on the photocathode can be utilized to shift the virtual origins. All such effects can help in increasing an image quality of the acquired image.

[0039] In an example, the multi-beam charged-particle microscope may include one or more optical elements that may include at least one adjustable optical element. The microscope may further include a control unit configured to control the at least one adjustable optical element.

[0040] An adjustable optical element may be understood as an optical element that does not have fixed effects on the respective laser beam, but rather can be adjusted so that it has a variable effect on the laser beam. For instance, there may be provided an electrical actuator, e.g., a motor, that is controlled by the control unit to adjust the adjustable optical element. Thereby, depending on the particular setting, a different effect on the beam can be obtained.

[0041] For instance, the multi-beam charged-particle microscope may include multiple adjustable optical elements, wherein each laser beam is associated with at least one of the multiple adjustable optical elements. Different laser beams can be associated with different ones of the multiple adjustable optical elements. For instance, one or more adjustable optical elements may be arranged in each beam path of the multiple laser beams.

[0042] This arrangement of employing one or more adjustable optical elements has an effect of additionally enabling dynamic adaptation of laser beam properties in response to changes in the operating conditions or degradation of components over time. By controlling the adjustable optical element(s), this arrangement allows compensation for temporal drifts, ageing effects such as degradation of the photocathode, and other environmental factors that may affect the performance of the microscope. This adaptability ensures consistent and optimal operation of the multi-beam charged- particle microscope, even in the presence of disturbances or degradation of components.

[0043] In an example, a control unit may be configured to control the at least one adjustable optical element to shift and / or distort virtual origins of each of the multiple charged- particle beams. In this arrangement, the virtual origins may be arranged upstream of the photocathode along a virtual extension of the multiple charged-particle beams.

[0044] A virtual origin may be understood as an effective point source of a charged-particle beam, which may not necessarily coincide with the actual physical location of the photocathode.

[0045] This arrangement has an effect of enabling a compensation of aberrations of a charged-particle field lens by dynamically adjusting the virtual origins of the charged- particle beams.

[0046] In an example, the multi-beam charged-particle microscope may include at least one laser that is wavelength-tunable. The multi-beam charged-particle microscope may further include a control unit configured to estimate a position of virtual origins and / or aberrations of each of multiple charged-particle beams based on a wavelength of each of the at least one laser. Additionally, the control unit may be configured to shift an / or distort the virtual origins of each of the multiple charged-particle beams based on the estimated positions of the virtual origins and aberrations.

[0047] Here, a wavelength-tunable laser may be understood as a laser capable of emitting light at different wavelengths, which can be adjusted dynamically. A wavelength- tunable laser may be implemented using state-of-the-art methods, such as Optical Parametric Amplification (OPA), which is a nonlinear process that generates a tunable output beam by mixing a pump beam with a signal beam in a nonlinear crystal. Alternatively, TitaniurmSapphire (Ti:Sa) laser tuning or Excimer lasers can also be employed to minimize the energy spread of the multiple laser beams.

[0048] Specifically, Ti:Sa laser tuning utilizes a titanium-doped sapphire crystal as the gain medium, allowing for wide tunability and high peak power. Excimer lasers, on the other hand, are ultraviolet lasers that utilize an excited dimer, typically xenon chloride (XeCI), to produce a high-intensity beam with a narrow spectral linewidth.

[0049] This arrangement has an effect of enabling minimization of the energy spread of each charged-particle beam by adjusting the laser wavelength to match the internal electron states of the photoemitter material. Single or multi-photon processes can be considered. This minimized an energy spread of the emitted charged-particle beams. Having charged-particle beams of a relatively small energy bandwidth enables more accurate focusing of the charged-particle beams by reducing chromatic aberrations, which in turn increases the image quality.

[0050] By estimating the position of the virtual origins based on the laser wavelength and shifting them accordingly with respect to that estimated position by control of the at least one adjustable optical element, this arrangement allows for a precise control over the emission current and energy distribution of the charged-particle beams. A field-curvature aberration can be accurately pre-compensated. This results in an improved overall performance and accuracy of the microscope.

[0051] In an example, the multi-beam charged-particle microscope may include a control unit configured to control at least one adjustable optical element based on a closed control loop implemented during imaging. The control unit may dynamically adjust the at least one adjustable optical element in response to feedback from an imaging process.

[0052] Here, a closed control loop may be understood as a system where the output of a process is monitored and used to adjust the input to the process, thereby creating a continuous cycle of measurement and correction.

[0053] For instance, it would be possible to monitor one or more of the following characteristics during imaging: intensity of each charged-particle beam; pattern of charged-particle beams; shape of each charged-particle beam; inter-beam distance between adjacent charged-particle beams; one or more image properties (e.g., one or more image aberrations) of an image acquired using the one charged particle beams; etc.

[0054] This arrangement has an effect of enabling more accurate compensation of aberrations during imaging, allowing for dynamic correction of errors of the charged- particle beams in real-time. Ageing effects and other temporal drifts can be compensated continuously.

[0055] In an example, the multi-beam charged-particle microscope may include a control unit configured to control at least one adjustable optical element to compensate aberrations of a field lens acting upon multiple charged-particle beams.

[0056] In particular for MSEMs or other multi-beam multi-beam charged-particle microscopes, some of the charged-particle beams are arranged offset from a center of an aperture of a field lens. Thus, different ones of the charged-particle beams will experience different fields, depending on their location within the aperture of the field lens. By compensating for aberrations, it is possible to obtain a beam-specific aberration correction, even for multi-beam charged-particle microscopes.

[0057] One aspect of the present disclosure relates to a multi-beam charged-particle microscope. At least one laser is included in the multi-beam charged-particle microscope. A vacuum chamber is included in the multi-beam charged-particle microscope. A frame for arranging a photocathode in the vacuum chamber is included in the multi-beam charged-particle microscope, so that the photocathode emits multiple charged-particle beams when being illuminated by multiple laser beams. One or more preparation chambers are attached via one or more valves to the vacuum chamber, and each of the one or more preparation chambers is configured to execute a photocathode restoration process.

[0058] A preparation chamber may be understood as a separate enclosure where a photocathode can be restored through, e.g., a recovery injection process. For a closed valve, the vacuum level in the preparation chamber can vary from the vacuum level in the main vacuum chamber in which the photocathode is arranged. Each preparation chamber may have a separate vacuum pump. Thus, the vacuum level of the preparation chamber and the (main) vacuum chamber can be separately controlled.

[0059] A photocathode restoration process may include heating I thermal annealing of the photocathode. A photocathode restoration process may include exposure to gaseous chemicals. A photocathode restoration process may include ion bombardment. One effect of including one or more preparation chambers may be that it allows for the continuous imaging using a first photocathode in the vacuum chamber, even when a second photocathode is being restored in the preparation chamber.

[0060] A motorized mechanism to handle multiple photocathodes may be included in the multi-beam charged-particle microscope, said handling comprising moving each of the multiple photocathodes between the frame and each of the one or more preparation chambers. One advantage of including a motorized mechanism to handle multiple photocathodes is that it allows for the automated execution of a restoration process. Automated transfer between an imaging position and a restoration position is possible.

[0061] The one or more preparation chambers may include multiple preparation chambers. This enables to implement multiple recovery processes in parallel. Furthermore, it would be possible to store a photocathode in vacuum conditions for later use.

[0062] In an example, two preparation chambers are utilized. Photocathode 1 is initially used to generate the electron beam, which is then focused onto a sample using a lens system. Meanwhile, photocathode 2 is prepared in chamber 2 by applying a high-vacuum environment and performing cleaning or surface treatment procedures. Subsequently, photocathode 1 is moved to chamber 1 , allowing it to be recovered and prepared for future use through similar clean processes, such as degassing and re-coating. Concurrently, photocathode 2 is inserted into the electron column and utilized to generate a new electron beam, ensuring continuous operation of the system without downtime. This process increases the overall efficiency and productivity.

[0063] One aspect of the present disclosure relates to a multi-beam charged-particle microscope. At least one laser is included in the microscope. The at least one laser defines at least one laser light source. A vacuum chamber is included in the microscope. A frame for arranging a photocathode is provided in the vacuum chamber so that the photocathode emits multiple charged-particle beams when being illuminated by multiple laser beams. A motorized mechanism to move at least one of the at least one laser or the frame relatively to each other is included in the microscope. The at least one laser light source defines an origin of laser beams incident on the photocathode. For instance, it would be possible that at least one laser generates laser light that is then coupled into a respective optical fiber. For instance, a singlemode fiber or a multi-mode fiber may be used. Then, the laser light is guided within the optical fiber to a distal end of the optical fiber. The laser light is decoupled from the optical fiber at the distal end of the optical fiber. It would be possible to provide one or more lenses to collimate the laser beam. For instance, gradient index lenses may be used. This arrangement at the distal end of the optical fiber can then implement the laser-light source for the respective laser light. It would be possible to have multiple laser-light sources per laser. For instance, a single laser may provide laser light that is coupled to multiple optical fibers. Thereby, the number of lasers can be reduced while still generating multiple distinct charged-particle beams.

[0064] One advantage of this arrangement is that the motorized mechanism allows for adjustment of the position and orientation of the photocathode relatively to the incident laser light, enabling to adapt to changes in the quality of the charged-particle beam by shifting and / or rotating the photocathode to target an unused section along the photocathode surface. This results in a longer lifespan of the photocathode because it is exposed to incident laser light more evenly.

[0065] Various degrees of positional freedom provided by the motorized mechanism are conceivable. For instance, the motorized mechanism may be configured to translate and / or rotate the frame with respect to the at least one laser-beam source.

[0066] The motorized mechanism may be implemented using piezoelectric actuators. A linear screw drive or a belt drive may be used. Other implementation options are possible.

[0067] In an example, a multi-beam charged-particle microscope may further include a control unit that may be configured to monitor a degradation of a quality characteristic associated with multiple charged-particle beams. The control unit may also be configured to control a motorized mechanism to move at least one of a laser-beam source or a frame relatively to each other based on the monitoring of the degradation of the quality characteristic.

[0068] This arrangement provides an effect of in-life monitoring of degradation, which enables optimized use of the photocathode. By continuously monitoring the degradation of the quality characteristic, the control unit can adjust the motorized mechanism to compensate for any changes in a quality of the charged-particle beams, thereby maintaining performance and extending the lifespan of the photocathode.

[0069] Example quality characteristics may include, e.g., a cross-sectional shape of a charged-particle beam, and inter-beam distance between adjacent charged-particle beams (when using multiple charged-particle beams), a pattern formed by multiple charged-particle beams, and intensity of a charged-particle beam, quantum efficiency of the emission process, etc. It would also be possible that the quality of the charged- particle beams is not directly measured, but rather inferred. For instance, it would be possible to inspect an image acquired using scanning of the multiple charged-particle beams across the sample. Then, image aberrations can be determined and quantified and based on these image aberrations, it is possible to conclude on the quality characteristic associated with the multiple charged-particle beams. For instance, beam astigmatism of the one or charged-particle beams can be identified by a certain directional blur in images acquired using the multi-beam charged-particle microscope. Similarly, a defocus can be determined by an omnidirectional blur of certain structures on the sample, as depicted in the image.

[0070] For instance, such and other quality characteristics may be measured and / or inferred and compared against a predefined threshold on multiple predefined thresholds, to judge whether degradation is present or not.

[0071] One aspect of the present disclosure relates to a multi-beam charged-particle microscope that includes at least one wavelength-tunable laser, a vacuum chamber, and a frame for arranging a photocathode in the vacuum chamber so that the photocathode emits multiple charged-particle beams when being illuminated by multiple laser beams provided by the at least one laser.

[0072] Such techniques are based on the finding that the wavelength of the incident laser beams has an impact on the virtual emission positions. Shifting the virtual emission positions enables to compensate for a field-curvature aberration of charged-particle lens, e.g., an objective lens.

[0073] One aspect of the present disclosure relates to a charged-particle microscope that includes multiple lasers, a vacuum chamber, as well as a frame for arranging a photocathode in the vacuum chamber so that the photocathode emits multiple charged-particle beams when being illuminated by multiple laser beams provided by the multiple lasers.

[0074] Thereby, a multi-beam charged-particle microscope can be implemented using a photocathode as beam-source technology.

[0075] One aspect of the present disclosure relates to a multi-beam charged-particle microscope. The multi-beam charged-particle microscope comprises at least one laser. The multi-beam charged-particle microscope comprises a vacuum chamber. The multi-beam charged-particle microscope comprises a frame for arranging a photocathode in the vacuum chamber so that the photocathode emits multiple charged-particle beams when being illuminated by multiple laser beams provided by the at least one laser. The multi-beam charged-particle microscope comprises a cooling system coupled to the frame and configured to cool the photocathode.

[0076] An effect may be that a cooling system for the photocathode can stabilize the temperature and sustain the quality of each charged-particle beam. The cooling system helps to prevent overheating of the photocathode (due to high intensity laser light incident thereon), which can cause thermal noise, evaporation of the optically active material, and thus generally causing degradation of each charged-particle beam.

[0077] For example, the cooling system may include at least one of a Peltier element or a fluid heat sink.

[0078] The cooling system may be an adjustable cooling system. I.e. , a cooling power may be adjusted, e.g., by appropriate control signals provided by the control unit. A closed-loop control of the temperature can be implemented.

[0079] One aspect of the present disclosure relates to a multi-beam charged-particle microscope. The multi-beam charged-particle microscope comprises at least one laser. The multi-beam charged-particle microscope comprises a vacuum chamber. The multi-beam charged-particle microscope comprises a flexible frame for arranging a photocathode in the vacuum chamber so that the photocathode emits multiple charged-particle beams when being illuminated by multiple laser beams provided by the at least one laser. The multi-beam charged-particle microscope comprises one or more actuators coupled to distort the flexible frame, to thereby distort a shape of a surface of the photocathode.

[0080] By deforming the photoemitter surface of the photocathode, pointing variations can be introduced to the charged-particle beams, which mimic a multi-deflection microoptics placed after the emitter. An effect may be that field distortions of a field lens can be pre-compensated.

[0081] A flexible frame may be understood as a frame - e.g., as disclosed above - capable of being deformed or bent without breaking.

[0082] One or more actuators may be understood as devices capable of applying a force or motion to an object. The one or more actuators may include a micromanipulator and / or a piezo element.

[0083] It is to be understood that the features mentioned above and those yet to be explained below may be used not only in the respective combinations indicated, but also in other combinations or in isolation without departing from the scope of the invention.

[0084] BRIEF DESCRIPTION OF THE DRAWINGS

[0085] FIG. 1 schematically illustrates a multi-beam MSEM according to various examples.

[0086] FIG. 2 schematically illustrates a photoemission charged-particle beam source and specifically a laser beam path according to various examples.

[0087] FIG. 3 schematically illustrates a photoemission charged-particle beam source and specifically a laser beam path according to various examples.

[0088] FIG. 4 schematically illustrates a photoemission charged-particle beam source and specifically a laser beam path according to various examples.

[0089] FIG. 5 schematically illustrates a photoemission charged-particle beam source and specifically a laser beam path according to various examples.

[0090] FIG. 6 schematically illustrates a photoemission charged-particle beam source and specifically a laser beam path according to various examples.

[0091] FIG. 7 schematically illustrates an astigmatic aberration of a charged-particle beam due to use of electron optics according to various examples. FIG. 8 schematically illustrates pre-compensation of the astigmatic aberration of FIG.

[0092] 7 according to various examples.

[0093] FIG. 9 schematically illustrates various aberrations of charged-particle beams according to various examples.

[0094] FIG. 10 schematically illustrates a beam-specific aberration of charged-particle beams according to various examples.

[0095] FIG. 11 schematically illustrates a relative movement of a photocathode with respect to incident laser beams according to various examples.

[0096] FIG. 12 schematically illustrates an adjustment of a pattern of laser beams incident on a photocathode according to various examples.

[0097] FIG. 13 schematically illustrates a circular pinhole aperture used in a photoemission charged-particle source according to various examples.

[0098] FIG. 14 schematically illustrates an elliptical pinhole used in a photoemission charged-particle source according to various examples.

[0099] FIG. 15 schematically illustrates preparation chambers attached to an imaging vacuum chamber of a multi-beam charged-particle microscope according to various examples.

[0100] FIG. 16 schematically illustrates multiple lasers and multiple adjustable optical elements according to various examples.

[0101] FIG. 17 is a schematic top view of a photocathode retained by a frame according to various examples, wherein furthermore actuators are provided to distort the flexible frame, to thereby distort a shape of a surface of the photocathode.

[0102] FIG. 18 is a schematic side view corresponding to the top view of FIG. 17.

[0103] DETAILED DESCRIPTION

[0104] Some examples of the present disclosure generally provide for a plurality of circuits or other electrical devices. All references to the circuits and other electrical devices and the functionality provided by each are not intended to be limited to encompassing only what is illustrated and described herein. While particular labels may be assigned to the various circuits or other electrical devices disclosed, such labels are not intended to limit the scope of operation for the circuits and the other electrical devices. Such circuits and other electrical devices may be combined with each other and / or separated in any manner based on the particular type of electrical implementation that is desired. It is recognized that any circuit or other electrical device disclosed herein may include any number of microcontrollers, a graphics processor unit (GPU), integrated circuits, memory devices (e.g., FLASH, random access memory (RAM), read only memory (ROM), electrically programmable read only memory (EPROM), electrically erasable programmable read only memory (EEPROM), or other suitable variants thereof), and software which co-act with one another to perform operation(s) disclosed herein. In addition, any one or more of the electrical devices may be configured to execute a program code that is embodied in a non-transitory computer readable medium programmed to perform any number of the functions as disclosed.

[0105] In the following, embodiments of the invention will be described in detail with reference to the accompanying drawings. It is to be understood that the following description of embodiments is not to be taken in a limiting sense. The scope of the invention is not intended to be limited by the embodiments described hereinafter or by the drawings, which are taken to be illustrative only.

[0106] The drawings are to be regarded as being schematic representations and elements illustrated in the drawings are not necessarily shown to scale. Rather, the various elements are represented such that their function and general purpose become apparent to a person skilled in the art. Any connection or coupling between functional blocks, devices, components, or other physical or functional units shown in the drawings or described herein may also be implemented by an indirect connection or coupling. A coupling between components may also be established over a wireless connection. Functional blocks may be implemented in hardware, firmware, software, or a combination thereof.

[0107] Various techniques disclosed here in our related to imaging a sample using a multibeam charged-particle microscope. More specifically, the techniques relate to an MSEM.

[0108] The MSEM jointly scans multiple primary electron beams across a sample. The associated secondary electron beams are detected. Multiple images are formed, one for each pair of primary-secondary electron beam pairs. Each image has an associated FOV (sFOV). A composite image is then determined based on a stitching operating. The composite FOV corresponds to the aggregation of the FOVs. The composite FOV (mFOV) of the composite image captures the entire illuminated area of the sample. Illumination of the sample using the electron beams may cause a charge buildup.

[0109] Hereinafter, techniques will be primarily explained in the context of an MSEM. However, techniques disclosed herein can be equally applicable to other types of multi-beam charged-particle microscopes, e.g., multi-beam ion microscope.

[0110] Various disclosed techniques relate to the charged particle source. According to various examples, a photoemission photocathode is employed. Various disclosed techniques enable to generate high-quality electron beams using photoemission. Multiple laser beams are incident at distinct positions and / or angle of the photocathode, thereby generating multiple distinct charged-particle beams having a certain orientation and / or beam shape. Aberrations can be reduced. Ageing effects of the photocathode material can be mitigated.

[0111] FIG. 1 is a schematic illustration of an MSEM 1. Further information relating to such MSEMs and components used therein, such as, for instance, particle sources, multiaperture plate and lenses, can be obtained from the international patent applications WO 2005 / 024881 , WO 2007 / 028595, WO 2007 / 028596, WO 2011 / 124352 and WO 2007 / 060017 and the German patent applications having the publication numbers DE 10 2013 016 113 A1 and DE 10 2013 014 976 A1 , the disclosure of which in the full scope thereof is incorporated by reference in the present application.

[0112] The MSEM 1 uses a plurality of charged particle electron beams (also referred to as beamlet or simply beam) for imaging a sample 7. The MSEM 1 generates a plurality of J primary beams 3.1 , 3.2, 3.3 which strike the sample 7 to generate interaction products, e.g., secondary electrons, which emanate from the sample 7, form secondary beams 9.1 , 9.2, 9.3, and are subsequently detected.

[0113] Each one of the primary and secondary beams 3.1 , 3.2, 3.3, 9.1 , 9.2, 9.3 is formed and guided by a respective imaging subsystem of the MSEM 1. Each imaging subsystem is associated with a respective FOV. Images acquired by a respective imaging subsystem depict the respective FOV. The multiple FOVs are arranged in a spatial pattern to thereby define a composite FOV. The primary beams 3.1 , 3.2, 3.3 are formed by electrons which are incident on a surface of the sample 7 at a plurality of locations and generate a plurality of primary electron beam focus spots 5,1 5.2, 5.3 that are spatially separated from one another.

[0114] The sample 7 to be examined can be of any desired type, e.g., a semiconductor wafer or a semiconductor mask, and can comprise an arrangement of miniaturized elements.

[0115] The surface of the sample 7 is arranged in a sample plane 101 of an objective lens system 102 of a first particle optical unit 100 (also referred to as illumination system).

[0116] A diameter of the minimal beam spots or focus spots 5,1 5.2, 5.3 shaped in the sample plane 101 can be small. Exemplary values of this diameter are below ten nanometers, for example four nm or less. The focusing of the primary beams 3.1 , 3.2, 3.3 for shaping the focus spots 5,1 5.2, 5.3 is carried out by the objective lens system 102. In this case, the objective lens system 102 can comprise a magnetic immersion lens. Further examples of focusing means are described in the German patent DE 10 2020 125 534 B3, the entire content of which is herewith incorporated in the disclosure.

[0117] The number J of primary beams 3.1 , 3.2 and 3.3 (i.e. , the number of FOVs) may be five, 25, 90 to 100, or more (for sake of simplicity, only three primary beams 3.1 , 3.2 and 3.3 with corresponding focus points 5.1 , 5.2 and 5.3 are shown in FIG 1 ).

[0118] In practice, the number of beams J, and hence the number of FOVs, can be chosen to be significantly greater, such as, for example, J = 10 x 10, J = 20 x 30 or J = 100 x 100. The beams can furthermore be arranged in any shape, for instance hexagonal. Exemplary values of the pitch between the incidence locations and FOVs are 1 micrometer, 10 micrometers, or more, for example 40 micrometers.

[0119] The number of primary and secondary beams J defines the number of FOVs. Each imaging subsystem has a respective FOV. The respective FOV is defined by scanning the respective pair of primary and secondary beams (e.g., beams 3.1 and 9.1 ) over the sample 7 in the respective FOV.

[0120] The primary beams 3.1 , 3.2, 3.3 striking the sample 7 generate interaction products, e.g., secondary electrons, back-scattered electrons, which emanate from the surface of the sample 7, or primary particles that have experienced a reversal of movement for other reasons. A charge buildup may occur, depending on the charge dissipation properties of the sample 7. The interaction products emanating from the surface of the sample 7 are shaped by the objective lens system 102 to form the secondary beams 9.1 , 9.2, 9.3. Secondary electrons included in the secondary beams 9.1 , 9.2, 9.3 are used for imaging.

[0121] The MSEM 1 , at a detection side, provides a detection beam path for guiding the plurality of secondary beams 9.1 , 9.2, 9.3 to a secondary electron imaging system 200. The secondary electron imaging system 200 includes several electron-optical lenses 205.1 to 205.5 for directing the secondary beams 9.1 , 9.2, 9.3 towards a spatially resolving detector system 600.

[0122] The imaging with the secondary electron imaging system 200 is strongly magnifying such that both the pattern of the primary beams on the wafer surface and the size and shape of focal points of the primary beams are imaged in much magnified fashion. By way of example, a scale factor I magnification is between 100x and 300x such that one nm on the wafer surface is imaged enlarged to between 100 nm and 300 nm. In an example, an image field of a multi-beam device with for example 100 pm diameter is enlarged to approximately 30 mm.

[0123] The primary beams 3.1 , 3.2, 3.3 are generated, at the illumination side, in an imaging vacuum chamber 300 comprising a particle source 301 , at least one collimation lens 303, a multi-aperture arrangement 305 (which is generally optional) and a first field lens 331 and a second field lens 333. The particle source 301 generates at least one diverging particle beam 309, which is at least substantially collimated by the at least one collimation lens 303, and which illuminates the multi-aperture arrangement 305. The particle source 301 includes a photocathode. The photocathode may be held by a frame. The particle source 301 operates based on photoemission. For each primary beam 3.1 , 3.2, 3.2, a respective laser beam (not shown in FIG. 1 ) is incident on the photocathode.

[0124] The multi-aperture arrangement 305 includes a multi-aperture plate (MAP) 304 (also referred to as filter plate or multi-hole aperture plate), which has a plurality of J openings formed therein in a first raster arrangement. Particles of the illuminating particle beam 309 pass through the J apertures or openings of the MAP 304 and form the plurality J of primary beams 3.1 , 3.2, 3.3. Particles of the illuminating particle beam 309 which strike the first aperture plate 304 are absorbed by the latter and do not contribute to the formation of the primary beams 3.1 , 3.2, 3.3. A multi-aperture arrangement 305 sometimes has at least a further MAP 306, 310 that may include beam deflection means, for example a lens array, a stigmator array, or an array of deflection elements. Such beam deflection means may individually deflect each of the multiple primary beams 3.1 , 3.2, 3.3.

[0125] As a general rule, using a multi-aperture arrangement such as the multi-aperture arrangement 305 is optional. In some scenarios, the MSEM 1 may not include a multi-aperture arrangement in a beam path of the primary beams 3.1 , 3.2, 3.3. In particular, the use of a multi-aperture arrangement such as the multi-aperture arrangement 305 limits the degrees of freedom with which the pattern of primary beams 3.1 , 3.2, 3.3 can be adjusted. For instance, individual beam positions of the beam pattern may not be shifted relative to each other, because otherwise the beams cannot pass through the multi-aperture arrangement. On the other hand, various disclosed techniques benefit from the degrees of freedom offered by an extended photocathode surface across which the incident laser beams can be freely repositioned, thereby also resulting in a repositioning of the generated primary beams 3.1 , 3.2, 3.3. Accordingly, various techniques are based on the finding that it can be beneficial to tailor beam characteristics of the primary beams 3.1 , 3.2, 3.3 an optical domain rather than an electron domain. In other words, by using one or more adjustable optical elements for each of the incident laser beams, the properties of the laser beams can be adjusted, thereby also adjusting the properties of the primary electron beams 3.1 , 3.2, 3.3. The degrees of freedom available for adjusting the primary beams 3.1 , 3.2, 3.3 by adjusting the laser beams in the optical domain are significantly higher than reference implementations which use multi-aperture arrangement such as the multi-aperture arrangement 305.

[0126] A first field lens 331 and a second field lens 333 focus each of the primary beams 3.1 , 3.2, 3.3 in such a way that focal points are formed in an intermediate image surface 321. Alternatively, the beam foci and the intermediate image surface 321 can be virtual. The intermediate image surface 321 can be curved to pre-compensate a field-curvature aberration of the imaging system arranged downstream of the intermediate image surface 321 .

[0127] The at least one field lens 103 and the objective lens system 102 provide a first imaging particle optical unit for imaging the surface 321 , in which the beam foci are formed, onto the sample plane 101 such that a second pattern of focus spots 5,1 5.2, 5.3 of the primary beams 3.1 , 3.2, 3.3 is formed there. Typically, the surface 25 of the sample 7 is arranged in the sample plane 101 , and the focal spots 5,1 5.2, 5.3 are correspondingly formed on the object surface 25. The plurality of primary beams 3.1 ,

[0128] 3.2, 3.3 form a crossover point 108, in the vicinity of which a first deflection scanner 110 is arranged. The first deflection scanner 110 is used to deflect the plurality of primary beams 3.1 , 3.2, 3.3 collectively and synchronously such that the plurality of focus spots 5,1 5.2, 5.3 are scanned jointly and contemporaneously over the surface 25 of the sample 7. Raster scanning is implemented, thereby imaging the sample 7. The first deflection scanner 110 is driven by a scanning control unit 860 such that in an inspection mode of operation, a plurality of two-dimensional image data of the surface is acquired. Additionally, the MSEM 1 can include further static deflectors configured to adjust the position of the plurality of the primary beams 3.1 , 3.2, 3.3.

[0129] The objective lens system 102 and the projection lenses 205 provide a secondary electron imaging system 200 for imaging the sample plane 101 onto an imaging plane 225. The objective lens system 102 is thus a lens or a lens system that is part of both the first and the second particle optical unit, while the field lenses 103, 331 and 333 belong only to the first particle optical unit 100, and the projection lenses 205 belongs only to the secondary electron imaging system 200.

[0130] A beam divider 400 is arranged in the beam path of the first particle optical unit 100 between the field lens 103 and the objective lens system 102. The beam divider 400 is also part of the second optical unit in the beam path between the objective lens system 102 and the projection lenses 205.

[0131] The first deflection scanner 110 is arranged in a primary electron beam path or in a joint electron beam path. In the example shown in FIG. 1 , the secondary beams 9.1 ,

[0132] 9.2, 9.3 transmit during use the first deflection scanner 110 in opposite direction and the scanning movement of the secondary beams 9.1 , 9.2, 9.3 is partially compensated. The secondary electrons have typically a different kinetic energy compared to the primary electrons. Therefore, the scanning movement of the moving irradiation positions is only partially compensated. To fully compensate the scanning movement of the secondary beams 9.1 , 9.2, 9.3, the collective beam deflector 222 is arranged in the secondary electron beam path.

[0133] The secondary electron imaging system 200 includes the second, collective beam deflector 222 which is arranged in the vicinity of a crossover point of the secondary beams 9.1 , 9.2, 9.3. The second, collective beam deflector 222 is operated synchronously with the first deflection scanner 110 and compensates during use a beam deflection of the secondary beams 9.1 , 9.2, 9.3 such that spots 15 of the beams 9 remain at constant position on the imaging plane 225. Thereby, each secondary beam 9 is kept within the area of a set of detection elements, which is assigned to the individual secondary beam 9.

[0134] The secondary electron imaging system 200 includes electron-optical lenses 205.1 to 205.5 to adjust a focus plane of the secondary beams 9.1 , 9.2, 9.3. A defocus can be applied. The electron-optical lenses 205.1 to 205.5 can thus implement corrective elements to correct the focus plane. The electron-optical lenses 205.1 to 205.5 are shown as magneto-optical elements but are not limited to magneto-optical elements and can comprise also electro-static lens elements or stigmators. With the electron- optical lenses 205.1 to 205.5, the secondary beams 9.1 , 9.2, 9.3 can be focused into the imaging plane 225 of the secondary electron imaging system 200.

[0135] The secondary electron imaging system 200 can include a plurality of further corrective elements, for example at least one of a multi-aperture array element, a deflector or an exchangeable aperture stop. Together with the objective lens system 102, the lenses serve to focus the secondary beams 9.1 , 9.2, 9.3 on the spatially resolving detector system 600 and, in the process, allow to correct or compensate the magnification and rotation of the pattern of the secondary beams 9.1 , 9.2, 9.3 in the imaging plane 225. Thereby, the pattern of the plurality of secondary beams 9.1 , 9.2, 9.3 can stabilized. For example, a first and second magnetic lenses 205.4 and 205.5 (as further examples of corrective elements) are designed in reversed order to one another and have oppositely directed magnetic fields. A Larmor rotation of the secondary beams 9.1 , 9.2, 9.3 can be compensated by suitably applying control signals to (driving) the magnetic lenses 205.4 and 205.5. The secondary electron imaging system 200 - in the illustrated example - includes further corrective elements, specifically a multi-aperture plate 216.

[0136] The MSEM 1 furthermore is associated with a processing device 800 configured both for controlling the individual particle optical components of the multiple particle beam system and for evaluating and analyzing the signals obtained by the detector system 600. The processing device 800 can be separated from the MSEM 1 or can be part of the MSEM 1 . For example, the processing device 800 can be configured to acquire pairs of test images and then evaluate the test images to determine values of one or more imaging parameters. In this case, the control or processing device 800 can be constructed from a plurality of individual electronic computers or electronic components. By way of example, the processing device 800 includes a control processor 880, a control module 840 for the control of the electro-optical elements of the secondary electron imaging system 200, and a control module 830 for the control of the electro-optical elements of the primary beam generation unit. The processing device 800 is further connected to a control module 503 for supplying a voltage to the sample 7, said voltage also being referred to as extraction voltage. Thereby, during use, an extraction field is generated between the objective lens system 102 and the surface 25 of the sample 7. During use, the extraction field decelerates the primary electrons of the primary beams 3.1 , 3.2, 3.3 before the object surface 25 is reached and generates an additional focusing effect on the plurality of primary beams 3.1 , 3.2, 3.3. At the same time, the extraction field serves during use to accelerate the secondary particles out of the surface 25 of the sample 7. Further, the processing device 800 includes the scanning control unit 860 for the raster scanning.

[0137] The detector system 600 includes a plurality of sets of detection elements with one set of detection elements for each secondary beam 9, for providing strongly magnified images for each FOV. During use, each set of detection elements is configured to record the intensity signal of the assigned secondary beam 9. The plurality of intensity signals for the plurality of secondary beams 9.1 , 9.2, 9.3 is transferred to the image data acquisition unit 810, where the image data is processed and stored in memory 890. Accordingly, multiple images are acquired, one for each imaging subsystem. These multiple images (or an aggregated image determined based on images of respective sequences) can be combined to a composite image having a composite FOV.

[0138] FIG. 2 illustrates aspects with respect to the particle source 301 . FIG. 2 illustrates an implementation of the particle source 301 using photoemission of electrons from a photocathode 3150. In FIG. 2, the particle source 301 is configured for generating a single electron beam. However, similarly, the particle source 301 may be configured for generating multiple electron beams, e.g., the context of an MSEM; the techniques disclosed herein are related to an MSEM. Thus, in the particle source of an MSEM, the component illustrated in FIG. 2 may be duplicated, i.e. , all components illustrated in FIG. 2 may be available for each electron beam. A laser is connected to an optical fiber 3105, e.g., a single-mode fiber. Laser light exits the optical fiber 3105 (thereby defining a laser-beam source) and then passes through a collimator lens or lens array 3110. The optical fiber 3105, when implemented as a single-mode optical fiber, provides a Gaussian beam profile. It would also be possible to use a multi-mode fiber.

[0139] The particle source 301 also includes a polarizer 3115. The polarizer 3115 provides light with a well-defined linear polarization - which is helpful for operating the polarization-sensitive SLM 3125 arranged downstream along the light path.

[0140] The particle source 301 also includes a mirror 3120 for deflecting the laser beam towards the SLM 3125. In the example of FIG. 2, a reflective SLM 3125 is employed; in other examples, a transmissive SLM 3125 may be employed; in this scenario, the mirror 3120 may not be required.

[0141] The SLM 3125 may be operated in a “first order” operating mode, i.e. , a specific blazed grating is imposed and the first order diffraction maximum is used for illuminating the photocathode 3150. The zeroth order diffraction maximum is captured by a beam dump 3130.

[0142] The laser beam 3106 can be polarized downstream of the SLM 3125, using a further polarizer 3135. The laser beam can be passed through a lens such as a Fourier optics lens 3140, to generate the desired light intensity distribution 3151 on the surface of the photocathode 3150. The distribution 3151 is defined by the phase function imprinted by the SLM 3125. For this, the SLM 3125 is arranged in a front focal plane of the Fourier optics lens 3140. A stray light blocking element 3145 may be optionally helpful to reduce unwanted diffracted light. The stray light blocking element 3145 is arranged adjacent to the photocathode 3150.

[0143] By appropriately controlling the SLM, it is possible to change the distribution 3151 .The intensity of the laser beam incident on the photocathode 3150 can be adjusted, by diverting a portion of the light to the beam dump. It would also be possible to divert the entire laser beam to the beam dump 3130, thereby preventing laser light from reaching the photocathode 3150. Thereby, it is possible to deactivate the footprint of the respective laser beam on the photocathode 3150. Upon deactivating the footprint of a laser print on the photocathode, a charged-particle beam is not generated. Beyond such deactivation of a laser beam, other measures are possible where the laser beam is not deactivated, but rather manipulated. Some examples are provided next.

[0144] The spot size - i.e. , the footprint of the laser beam on the photocathode 3150 - can be tailored (e.g., enlarged or compressed), as can be seen from a comparison of FIG. 2 and FIG. 3. For instance, by imposing an aperture function to the SLM, meaning switching all pixels outside any given area to no phase delay, the size of the focus can be enlarged. Also, asymmetric distortions of the shape of the footprint would be possible. By adding a phase tilt (Z2 / 3 Zernike polynomial) to the blazed grating of the SLM 3125, the focus can be diverted laterally on the photocathode 3150, as can be seen from a comparison of FIG. 2 and FIG. 4. Lateral offsets along the surface of the photocathode 3150 can be obtained; the angle of incidence of the electron beams on the surface of the photocathode 3150 can be slightly adjusted. By adding a parabolic phase function (Z4 Zernike polynomial) to the blazed grating of the SLM 3125, the focus can be shifted axially along a center axis of the laser beam, see comparison of FIG. 2 and FIG. 5. If a phase function Z5 / Z6 is added, an astimatic focus (as will be later on explain in connection with FIG. 9, middle) can be generated. I.e., the footprint can be distorted. If a phase function Z7 / Z8 is added, coma (as will be later on explain in connection with FIG. 9, middle) will be added to the intensity distribution of the focus. I.e., the footprint can be distorted. All Zernike polynomials can be added with the desired amplitude. Limitations are given by the SLM pixels that can generate phase delays by ~8pi. As shown in FIG. 6, It can be useful to image the far field of the SLM 3125 onto the photocathode 3150 to have a better straylight reduction or for having more space available for any polarizer 3135, e.g., a CU polarizer or retarder. A magnification factor of -1 (FIG. 6 shows imaging optics 3250, 3251 for 1 :-1 imaging) can then be useful to obey Sine and Herschel condition simultaneously. The last one can be advantageous when moving the generated focus along the optical axis.

[0145] As will be appreciated from the above descriptions of FIG. 2, FIG. 3, FIG. 4, FIG. 5, and FIG. 6, the SLM can generate an arbitrary far field I focus form, that respects the diffraction limit by optimization with e.g. IFTA based methods. The focus form imprints in the electron emission area and can be tailored to pre-compensate electron optical aberrations. As a general rule, using the SLM, a laser beam can be distorted (the shape of the footprint can be changed), diverted (the center can be shifted), its intensity may be changed, e.g., reduced in intensity, and / or it may be deactivated (switched off) altogether.

[0146] FIG. 7 schematically illustrates a laser beam having a circular footprint 3205 impinging on the photocathode 3150. This generates an electron beam also having a circular footprint 3210. The electron beam then passes through electron optics 3215 that may include one or more of the collimation lens 303 or the field lens 333 or the field lens 103 or the objective lens 102. It may or may not include a multi-aperture arrangement such as the multi-aperture arrangement 305. Due to lens aberrations, this distorts the laser beam such that a noncircular footprint 3220 results. To mitigate this aberration, the SLM can be controlled to pre-distort the footprint of the laser beam. This is shown in FIG. 8. In FIG. 8, the footprint 3305 of the laser beam incident on the photocathode 3150 is noncircular due to a respective distortion achieved by the SLM; accordingly, also the footprint 3310 of the generated electron beam is noncircular. After passing through the electron optics 3215, a circular footprint 3320 is obtained. This may be desirable in certain scenarios.

[0147] FIG. 9 is an illustration of the footprint of multiple laser beams that form a pattern on the surface of the photocathode. FIG. 9, left illustrates circular footprints of the laser beams; FIG. 9, middle illustrates elliptical footprints for tangential astigmatism correction. Grayscale encodes the local light intensity. FIG. 9, right illustrates footprints for coma correction.

[0148] FIG. 10 schematically illustrates a pattern 3500 of footprints of laser beams incident on the photocathode 3150. As illustrated in FIG. 10, the footprints of different ones of the laser beams of the pattern 3500 have different shapes. This means that different laser beams can be distorted differently. In particular, it would be possible to distort the laser beams depending on their distance to a center axis 3501 . I.e. , a center distance can be considered. The center axis 3501 may be defined as a center of the photocathode 3510. Alternatively or additionally, the center axis 3501 may be defined as the particular axis at which a laser beam is generated that then passes along a center axis of a field lens in the electron optic 3215. Depending on the center distance (i.e., distance between center of beam and the center axis 3501 ), and depending on the aberrations of such field lens, a different strength and / or type of aberration is encountered by the electron beams. Thus, by considering this center distance when distorting the footprints of the laser beam, it is effectively possible to pre-compensate for the field-curvature aberration for each beam, i.e. , the precompensation may be executed beam-by-beam.

[0149] The amount of pre-compensation by beam-individual modification, e.g., of the position of the footprint (also referred to as “profile”) and / or the shape of the footprint, may be limited by the physical arrangement of a multi-aperture arrangement (cf. FIG. 1 : multi-aperture arrangement 305) in the beam path of the charged-particle beams. For instance, if the individual footprints of the laser beams are shifted with respect to each other above a threshold amount, this may result in certain charged-particle beams not being incident on the apertures of the multi-aperture arrangements (if present), anymore. Thus, a situation may result in which the desirable precompensation in optical domain is not accessible, due to the limitations imposed by the multi-aperture arrangement in the electron domain. In such scenario, it would be possible to switch off / deactivate those laser beams that generate electron beams that cannot be appropriately pre-compensated. For instance, in such a scenario, only those laser beams may remain activated that generate electron beams that suffer from relatively mild or minor aberrations. The remaining laser beams may be deactivated. Accordingly, it is possible to increase the image quality, by reducing the number of electron beams. At the same time, the number of contemporaneously scanned electron beams is reduced, so that the mFOV is reduced.

[0150] Above, scenarios have been discussed according to which laser beams or even individual laser beams are activated or deactivated, distorted, and / or deflected. Similarly, it would be possible to adjust the intensity of individual laser beams, e.g., between multiple finite values. In such a scenario, it would be possible to compensate for observed changes in the intensity of electron beams, e.g., measured at a detector system such as the detector system 600. For instance, a closed control loop could be implemented that stabilizes the amplitude / intensity of each electron beam in a pattern of electron beams.

[0151] Such closed control loop cannot only be implemented with respect to stabilization of the intensity of the electron beams but may be fully implemented with respect to stabilization of the pattern of multiple electron beams, e.g., considering the interbeam distance of adjacent electron beams, to give just one example. An astigmatism of each electron beam may be determined by inspecting images acquired for each sFOV. FIG. 11 illustrates a pattern of multiple incident laser beams (full circles) incident on the photocathode. A hexagonal pattern is implemented. A motorized mechanism may be used to rotate a frame that engages the photocathode, typically at its circumference, relative to the incident laser beams, i.e. , relative to the laser-beam source. This is illustrated in FIG. 11 , right side; the photocathode 3150 is rotated underneath the incident laser spots (illustrated by the square notch in the upper part of the photocathode 3150.)

[0152] This has the effect that the laser spots are incident at different areas of the photocathode 3150, before and after rotation. A similar effect can also be achieved by translation of the photocathode 3150, e.g., using a linear motor. Such techniques can be helpful to mitigate aging effects of the photocathode. Typically, aging of the photocathode is accelerated in those areas at which the laser beams are incident. This can be due to, e.g., heating, material deposition, etc. By rotating and / or translating the photocathode 3150 with respect to the incident laser spots, more uniform wear out of the photocathode 3150 can be achieved.

[0153] For instance, in various examples it would be possible that such relative movement of the incident laser beams with respect to the photocathode may be dynamically triggered. For instance, it would be possible that a control unit monitors a degradation of a quality characteristic associated with the one or charged-particle beams and controls the motorized mechanism to move the at least one of the laser-beam source or the frame relatively to each other based on said monitoring of the degradation of the quality characteristic. For instance, it would be possible to monitor a brightness of pixels in an image acquired using the multi-beam charged-particle microscope or it would be possible to monitor the brightness or current of the charged-particle beams themselves using a diagnostic detector. Then, upon the brightness / intensity degrading below a threshold, it would be possible to trigger a repositioning of the footprints of the laser beams on the surface of the photocathode.

[0154] It should be understood that alternatively or additionally to moving the photocathode 3150 with respect to the incident laser beams, it would be possible to move the incident laser beams with respect to the photocathode 3150. This can be achieved by a beam steering element, e.g., a SLM such as the SLM 3125 (cf. FIG. 4).

[0155] Alternatively or additionally, it would also be possible to use a motorized mechanism to move the laser-beam source, e.g., translate and / or tilt the distal ends of the optical fibers 3105.

[0156] FIG. 11 illustrates a scenario in which the relative arrangement of the laser spots with respect to each other is preserved. I.e. , the pattern is not changed. It would be possible that the pattern of the incident laser sports is changed, as illustrated in FIG. 12. The pattern of laser spots can be changed by beam steering, e.g., as discussed above in connection with FIG. 4 using the SLM or another beam-steering element. It would also be possible to move, using a motorized mechanism, the laser-beam sources relatively to each other.

[0157] An adjustment of the pattern of the laser spots incident on the photocathode 3150 is not only helpful for mitigating local aging effects of the photocathode material, but also helpful to tailor the quality of the imaging to the particular samples. In particular, certain samples - e.g., semiconductor samples - include periodically arranged structures, e.g., memory cells or transistors. It may happen that a periodicity of the structures of the sample matches a periodicity of the pattern of the charged particle beams. Under certain conditions, this may be undesired. An example would be that isolating and conductive regions are matched to a beam pitch of the electron beams. Thereby, surface charging can be amplified. Then, incident electron beams are distorted by surface charges on the sample. Thus, by setting the pattern (e.g., beam pitch) based on a-priori knowledge of the sample, specifically a-priori knowledge on spatial periodicities of the sample and / or isolating vs. charging areas of the sample, the overall image quality may be increased. Subsequent switching of the electron beams to not scan charging areas may further increases the image quality.

[0158] In addition to adjusting the pattern of the laser beams on the photocathode 3150, it would also be possible to adjust the positions of virtual origins of the electron beams. This enable telecentric electron beams at the sample. This can enable adjusting the inter-beam pitch of the electron beams at the sample. For instance, a shape of the mFOV can be adjusted.

[0159] Above, various examples have been disclosed in which an adjustable optical element is arranged in the beam paths of laser beams, wherein the adjustable optical element is implemented by a SLM. The implementation of an adjustable optical element by a SLM is only one of various possible examples. Other adjustable optical elements include, e.g., an adjustable mirror, and adjustable intensity filter element, e.g., operating based on polarization, a movable field stop, or a shape-adjustable pin hole. Adjustable optical elements may include field stops or pinholes that can be selectively introduced or removed from the beam path of the laser light.

[0160] It is not required in all scenarios that an adjustable optical element is used. In some scenarios, a nonadjustable optical element, i.e. , an optical element that has a fixed impact on the laser beam, may be used. A non-circular or specifically elliptical pin hole may be used.

[0161] For instance, a pinhole may be used. Such a scenario is illustrated in FIG. 13 and FIG. 14. In FIG. 13, a circular pinhole is used to reshape their footprint 3505 of a laser beam that is incident on the photocathode 3150. The footprint 3515 of the generated electronic beam, accordingly, also has a circular shape. The electron optics 3215 then introduce aberrations, resulting in a non-circular footprint 3520 of the electron beam, leading to reduced image quality. In the scenario of FIG. 14, instead of using a circular pinhole, and elliptic pinhole 3610 is used. This precompensates for the aberrations that are later on introduced by the electron optics 3215; so that the footprint 3620 of the electron beam has a reduced astigmatism.

[0162] FIG. 15 is a schematic view of parts of the MSEM 1 . FIG. 15 illustrates the multibeam source and parts of the vacuum chamber 300. FIG. 15 pertains to technologies for mitigating aging effects of photocathodes such as the photocathode 3150.

[0163] Illustrated is the vacuum chamber 300. Also illustrated are laser beam sources 3704, 3705, e.g., distal ends of optical fibers coupled to lasers. The respective laser beams pass 3106-1 , 3106-2 through a light optics arrangement 3701 that may include one or more passive optical elements and / or one or more active / adjustable optical elements, e.g., SLMs, lenses, mirrors, etc. For instance, the light optics arrangement 3701 may include multiple arrangement as shown in FIG. 2, one such arrangement for each laser beam.

[0164] Then, laser beams 3106-1 , 3106-2 are incident on a photocathode 3150 that is retained by a frame 3790 in the vacuum chamber 300. Furthermore, illustrated are the electron beams 3.1 , 3.2 that traverse electron optics 3798, 3799. The electron optics 3799 may include an MAP, i.e., defining multiple apertures for each of the multiple electron beams 3.1 , 3.2. In some scenarios, it is beneficial to not include an MAP. The electron optics 3798 includes a field lens, defining a joint aperture for all electron beams 3.1 , 3.2. The field lens 3798 generates an electrical or magnetic field that acts upon all electron beams 3.1 , 3.2. The field lens 3798 can have exhibit deviations of the field from a nominal shape. These deviations may act differently on different electron beams 3.1 , 3.2

[0165] To mitigate heat-induced aging of the photocathode 3150 that is retained by the frame 3790, a cooling element 3791 is arranged at the circumference of the photocathode 3150. The cooling element 3791 is configured to absorb heat that is introduced by the incident laser beams. The cooling element 3791 may be a Peltier element or a fluid heat sink holding a cooling fluid.

[0166] In addition to mitigating heat-induced aging of the photocathode 3150 by means of the cooling element 3791 , further means for mitigating aging of the photocathode 3150 are provisioned: Also illustrated are preparation chambers 3787, 3788. The preparation chambers 3787, 3788 are configured for executing a photocathode restoration process. For this, a motorized mechanism 3983, 3984 is provided in each preparation chamber 3787, 3788. Using the motorized mechanism 3983, 3984, it is possible to load the photocathode 3150 into one of the process chambers 3787, 3788. For this, a vacuum pump 3785 is coupled to the process chamber 3787 and a vacuum pump 3786 is coupled to the process chamber 3788. Using the vacuum pumps 3785, 3786, the vacuum level of each of the process chambers 3787, 3788 can be adjusted to approximately match the vacuum level of the vacuum chamber 300, defined by operation of the vacuum pump 3780. Then, a respective one of the load locks / valves 3783, 3784 can be opened and the respective motorized mechanism 3983, 3984can be advanced to engage the frame 3790 and the photocathode 3150 and move these elements into the respective process chamber 3787, 3778. Then, the respective load lock / valve 3783, 3784 can be closed again and the photocathode restoration process can commence. For this, each of the preparation chambers 3787, 3788 includes a respective heater 3981 , 3982 and a respective gas inlet 3781 , 3782. Process gases can be entered into the preparation chambers 3787, 3788 through the gas inlets 3781 , 3782. Thereby, a chemical cleaning can be implemented. Using the heaters 3981 , 3982, thermal annealing can be implemented.

[0167] While the photocathode restoration process is ongoing in one or more of the process chambers 3787, 3788, it is possible to continue imaging of a sample using another photocathode retained by another frame in the vacuum chamber 300. While in the scenario FIG. 15, two preparation chambers 3787, 3788 are attached to the vacuum chamber 300, as a general rule, it would be possible to use only a single preparation chamber or more than two preparation chambers.

[0168] By using one or more preparation chambers, it is possible to restore the quality of an aged photocathode. Various techniques are based on the finding that as aging of a photocathode progresses (e.g., due to incident laser beams), the internal electron states of the photocathode change. This is due to local changes in the material properties of the photo-active material. With changing electron states, the intensity of the generated electron beams changes (for fixed intensity of the incident laser beams). This is due to the narrowband linewidth of the laser light, possibly detuned from the electron states. To mitigate this, it would be possible to counteract local aging effects by relatively moving the positions of the laser spots on the surface of the photocathode, e.g., as previously explained in connection with FIG. 11 and FIG. 12. Another option is to use a wavelength tunable laser and to adjust for varying electron states by tuning the wavelength accordingly. Respective details are shown in connection with FIG. 16.

[0169] FIG. 16 illustrates an arrangement including three wavelength tunable lasers 4005, 4010, 4015. Each of the three wavelength-tunable lasers 4005, 4010, 4015 generates laser light that is coupled into respective three optical fibers 4011 , 4012, 4013, 4021 , 4022, 4023, 4031 , 4032, 4033. The laser light then is guided to a number of optical systems 4111 , 4112, 4113, 4121 , 4122, 4123, 4131 , 4132, 4133. Each of the optical systems may include one or more adjustable optical elements, e.g., a respective SLM. Each of the optical systems may be configured as shown in FIG. 2, i.e. , including one or more lenses such as collimating lenses, a spatial light modulator, etc. It would also be possible to use other types of adjustable optical elements, e.g., an adjustable pinhole.

[0170] By using multiple optical systems, it is possible to apply beam-specific precompensation of aberrations introduced in an electron domain. Each beam can be individually shaped, diverted, activated or deactivated, etc.

[0171] Upon traversing the respective optical system, each laser beams (dashed arrows) impinges on the photocathode 3150, thereby generating electron beams (dotted arrows). Note that some laser beams may be deactivated by appropriately setting the adjustable optical element (not shown in FIG. 16). By providing multiple wavelength-tunable lasers 4005, 4010, 4015, different ones of the laser beams can be generated at different wavelengths. Thereby, it becomes possible to tailor the wavelength of the laser light incident on different parts of the photocathode to the electron states observed at that particular region of the photocathode. Different regions of the photocathode can exhibit different electronic properties due to local aging effects. For instance, a closed control loop may stabilize the intensity of each electron beam by tuning the wavelength of the lasers 4005, 4010, 4015.

[0172] In various examples, it is further possible to compensate for shifts in the virtual origins depending on the wavelengths. In particular, the position of the virtual origins of each of one or charged-particle beams can be estimated based on a wavelength of each of the one or more lasers. Then, the virtual origins can be shifted with respect to that estimated position.

[0173] While FIG. 16 illustrates three optical arrangements per laser, other relationships would be possible, even a 1 :1 relationship.

[0174] FIG. 17 and FIG. 18 illustrate the photocathode 3150 that is attached to the frame 3790. Furthermore, there are provided actuators 4705 - e.g., micro-manipulators and / or piezo elements, e.g., bending piezoelectric actuators - configured to distort the frame 3790. For instance, the actuators 4705 can move up and down perpendicular to a surface of the photocathode 3150, thereby distorting the shape of the surface of the photocathode 3150. Thus, even while maintaining the same orientation of the incident laser beams (dashed arrows), the orientation of the electron beams generated at the photocathode 3150 (dotted arrows) can be varied, depending on the orientation of the surface of the photocathode 3150. The virtual origins of the electron beams are shifted. Thereby, a field-curvature aberration can be pre-compensated.

[0175] Summarizing, techniques have been disclosed to manipulate multiple laser beams using optical elements, e.g., adjustable optical elements. It has been shown to manipulate multiple laser beams using elliptic or free-form apertures.

[0176] Techniques of employing a wavelength-tunable laser or multiple wavelength-tunable lasers for generating laser lights that is incident on a photocathode for multiple generating charged-particle beams have been disclosed. It has been disclosed to use one or more preparation chambers for regeneration of the optically active material of a photocathode. The one or more preparation chambers can be attached to a vacuum chamber in which a frame is arranged that can hold a photocathode during imaging.

[0177] Techniques have been disclosed to move, e.g., translate and / or rotate, the photocathode. Thereby, local spatial aging effects can be offset or compensated.

[0178] It has been disclosed to apply a curvature to a surface of the photocathode, thereby tailoring the direction of the electron beams generated by the photocathode. The curvature of the surface of the photocathode can be adjusted by micro-manipulators that apply stress or strain to the photocathode.

[0179] Thermal control of the photocathode has been disclosed. For instance, cooling is possible, to remove heat induced by the incident multiple laser beams. It would also be possible to heat the photocathode, to avoid deposition of contaminants on a surface of the photocathode.

[0180] It has been disclosed to dynamically activate or deactivate individual electron beams by switching on and switching off individual laser beams. For instance, it would be possible to employ multiple modes, a high-throughput mode and a high-resolution mode. For instance, multiple electron beams can be jointly scanned across the sample to define and mFOV in the high-throughput mode; then, upon acquiring an initial image of the sample, the high-resolution mode may be activated and a single electron beam may be scanned across the sample. More generally, the beam pattern including beam pitch and unit cell shape as well as the number of electron beams can be adjusted. This can be done to match certain periodic structures of a sample, e.g., defining isolating and conductive regions.

[0181] Although the invention has been shown and described with respect to certain preferred embodiments, equivalents and modifications will occur to others skilled in the art upon the reading and understanding of the specification. The present invention includes all such equivalents and modifications and is limited only by the scope of the appended claims.

Claims

C L A I M S1. A multi-beam charged-particle microscope, comprising:- at least one laser (4005, 4010, 4015),- a vacuum chamber (300),- a frame (3790) for arranging a photocathode (3150) in the vacuum chamber (300) so that the photocathode emits multiple charged-particle beams when being illuminated by multiple laser beams (3106, 3106-1 , 3106,2) provided by the at least one laser, and- one or more optical elements (3125) arranged along the multiple laser beams and configured to act upon each of the multiple laser beams.

2. The multi-beam charged-particle microscope of claim 1 , wherein the one or more optical elements comprise multiple optical elements, wherein each of the multiple laser beams is associated with a respective one of the multiple optical elements.

3. The multi-beam charged-particle microscope of claim 1 or 2, wherein the one or more optical elements comprise one or more spatial light modulators.

4. The multi-beam charged-particle microscope of claim 3, wherein the one or more spatial light modulators comprise multiple spatial light modulators, wherein the multi-beam charged-particle microscope further comprises:- a control unit configured to control the multiple spatial light modulators to divert each of the multiple laser beams to adjust a pattern formed by the multiple laser beams on the photocathode and / or to adjust angles of incidence of each of the multiple laser beams on the photocathode.

5. The multi-beam charged-particle microscope of claim 4, wherein the control unit is configured to control the multiple spatial light modulators to divert each of the multiple laser beams based on a-priori knowledge of a sample imaged by the multiple charged-particle beams.

6. The multi-beam charged-particle microscope of any one of claims 3 to 5, wherein the one or more spatial light modulators comprise multiple spatial light modulators, wherein the multi-beam charged-particle microscope further comprises:- a control unit configured to control the multiple spatial light modulators to activate or deactivate or distort a footprint (3205, 3305) of each of the multiple laser beams on the photocathode.

7. The multi-beam charged-particle microscope of claim 6, wherein the control unit is configured to activate or deactivate and / or to distort the footprint of each of the multiple laser beams depending on at least one of a center distance of each of the multiple laser beams with respect to a center of a surface of the photocathode or a center distance of each of the associated multiple charged particle beams with respect to a center of an aperture of a charged-particle field lens of the charged-particle microscope.

8. The multi-beam charged-particle microscope of any one of claims 3 to 7, wherein the one or more spatial light modulators comprise multiple spatial light modulators, wherein the charged-particle microscope further comprises:- a control unit configured to control the multiple spatial light modulators to adjust an intensity of each of the multiple laser beams.

9. The multi-beam charged-particle microscope any one of the preceding claims, wherein the one or more optical elements comprise one or more non-circular pin holes.

10. The multi-beam charged-particle microscope of any one of the preceding claims, wherein the one or more optical elements comprise at least one adjustable optical element, wherein the multi-beam charged-particle microscope further comprises:- a control unit configured to control the at least one adjustable optical element.11 . The multi-beam charged-particle microscope of claim 10, wherein the control unit is configured to control the at least one adjustable optical element to shift virtual origins of each of the multiple charged-particle beams, the virtual origins being arranged upstream of the photocathode along a virtual extension of the multiple charged-particle beams.

12. The multi-beam charged-particle microscope of claim 11 , wherein the at least one laser is wavelength-tunable, wherein control unit is configured to estimate a position of the virtual origins of each of the multiple charged-particle beams based on a wavelength of each of the at least one laser, wherein the control unit is configured to shift the virtual origins of each of the multiple charged-particle beams based on the estimated positions of the virtual origins of each of the multiple charged-particle beams.

13. The multi-beam charged-particle microscope of claim 11 or 12, wherein the control unit is configured to control the at least one adjustable optical element based on a closed control loop implemented during imaging.

14. The multi-beam charged-particle microscope of any one of claims 10 to 13, wherein the control unit is configured to control the at least one adjustable optical element to compensate aberrations of a field lens acting upon the multiple charged-particle beams.

15. A multi-beam charged-particle microscope, comprising:- at least one laser,- a vacuum chamber,- a frame for arranging a photocathode in the vacuum chamber so that the photocathode emits multiple charged-particle beams when being illuminated by multiple laser beams provided by the at least one laser, and- one or more preparation chambers (3787, 3788) attached via one or more valves to the vacuum chamber and each of the one or more preparation chambers being configured to execute a photocathode restoration process, and- a motorized mechanism (3983, 3984) to handle multiple photocathodes, said handling comprising moving each of the multiple photocathodes between the frame and each of the one or more preparation chambers.

16. The multi-beam charged-particle microscope of claim 15, wherein the one or more preparation chambers comprise multiple preparation chambers.

17. A multi-beam charged-particle microscope, comprising:- at least one laser defining at least one laser- beam source,- a vacuum chamber,- a frame for arranging a photocathode in the vacuum chamber so that the photocathode emits multiple charged-particle beams when being illuminated by multiple laser beams provided by the at least one laser-beam source, and- a motorized mechanism to move at least one of the at least one laser-beam source or the frame relatively to each other.

18. The multi-beam charged-particle microscope of claim 17, wherein the motorized mechanism is configured to translate and / or rotate the frame with respect to the at least one laser-beam source.

19. The multi-beam charged-particle microscope of claim 17 or 18, wherein the charged-particle microscope further comprises:- a control unit configured to monitor a degradation of a quality characteristic associated with the multiple charged-particle beams and configured to control the motorized mechanism to move the at least one of the at least one laser-beam source or the frame relatively to each other based on said monitoring of the degradation of the quality characteristic.

20. A multi-beam charged-particle microscope, comprising:- at least one wavelength-tunable laser,- a vacuum chamber, and- a frame for arranging a photocathode in the vacuum chamber so that the photocathode emits multiple charged-particle beams when being illuminated by multiple laser beams provided by the at least one laser.

21. A multi-beam charged-particle microscope, comprising:- multiple laser -beam sources,- a vacuum chamber, and- a frame for arranging a photocathode in the vacuum chamber so that the photocathode emits multiple charged-particle beams when being illuminated by multiple laser beams provided by the multiple laser-beam sources.

22. A multi-beam charged-particle microscope, comprising:- at least one laser,- a vacuum chamber,- a frame for arranging a photocathode in the vacuum chamber so that the photocathode emits multiple charged-particle beams when being illuminated by multiple laser beams provided by the at least one laser, and- a cooling element (3791 ) coupled to the frame (3790) and configured to cool the photocathode.

23. The multi-beam charged-particle microscope of claim 22, wherein the cooling element comprises at least one of a Peltier element or a fluid heat sink.

24. A multi-beam charged-particle microscope, comprising:- at least one laser,- a vacuum chamber,- a flexible frame (3790) for arranging a photocathode in the vacuum chamber so that the photocathode emits multiple charged-particle beams when being illuminated by multiple laser beams provided by the at least one laser, and- one or more actuators (4705) arranged to distort the flexible frame, to thereby distort a shape of a surface of the photocathode.

25. The multi-beam charged-particle microscope of claim 24, wherein the one or more actuators comprise a micromanipulator and / or a piezo element.

Images