multibeam particle microscope

Description

[0001] The invention relates to a multi-beam particle microscope.

[0002] Multibeam particle microscopes are known, for example, from WO 2012 / 041464 A1 and DE 10 2016 120 902 A1. In these microscopes, a plurality of primary particle beams from a bundle of primary particle beams are focused onto an object. The primary particle beams generate secondary electrons at the points where they impinge on the object. These secondary electrons are accelerated away from the object by an electric field applied to the object and formed into secondary particle beams, which are directed onto a detector array comprising a plurality of detectors.Here, each individual primary particle beam generates a secondary particle beam associated with the primary particle beam at the location of its impact on the object. The secondary particle beam strikes at least one detector of the detector array associated with it, so that by detecting the particle intensity striking this at least one detector, information about the location of the object at which the primary particle beam is directed can be obtained. The bundle of primary particle beams can be deflected by the multi-beam particle microscope, and the object can be displaced relative to the multi-beam particle microscope in order to systematically scan its surface with the primary particle beams and to obtain an electron microscopic image of the object from the detected secondary particle beam intensities.

[0003] A similar multi-beam particle beam microscope is disclosed in US 2010 / 0320382 A1. JP 2007-059111 A discloses a single-beam electron microscope designed to detect backscattered electrons emanating from the object at small starting angles. For this purpose, a plate with a hole or a mesh is brought into contact with the object.

[0004] It has been shown that with the well-known multi-beam particle microscope, certain structures on objects cannot be visualized as clearly and with as much contrast in the electron microscopic image as is the case with single-beam particle microscopes.

[0005] Accordingly, it is an object of the present invention to propose a multi-beam particle microscope which has extended possibilities for making structures on objects clearly and contrast-richly visible in the electron microscopic image.

[0006] This object is achieved by providing a multi-beam particle beam microscope having the features specified in the appended claim 1. Advantageous embodiments are specified in the appended dependent claims.

[0007] According to the invention, a multi-beam particle microscope comprises a multi-beam particle source configured to generate a plurality of primary particle beams; an objective lens through which beam paths of the plurality of primary particle beams pass and which is configured to direct the plurality of primary particle beams onto an object plane and to focus them there; a detector arrangement configured to detect intensities of a plurality of electron beams, wherein electrons of the electron beams are generated by particles of the primary particle beams on an object that can be arranged in the object plane.

[0008] During operation of the multi-beam particle microscope, each of the primary particle beams passes through an aperture of a multi-aperture plate and impinges on the object in a focused manner. At the point of impact with the object, the primary particle beam generates secondary electrons, which exit the object and are accelerated toward the multi-aperture plate. These secondary electrons pass through the aperture of the multi-aperture plate in a direction away from the object, forming an electron beam that passes through the objective lens and is guided to the detector array.

[0009] The secondary electrons generated by the primary particle beam typically have kinetic energies upon exiting the object's surface that are independent of the kinetic energy of the incident primary particles and are typically less than 50 eV. However, the primary particle beam also typically generates so-called backscattered electrons, which exit the object at the point of impact of the primary particle beam and have kinetic energies greater than 50 eV, sometimes reaching the kinetic energy of the primary particles. The inventors have recognized that, compared to the secondary electrons, these higher-energy electrons are guided to the detector array, where they generate detection signals, to a very small extent.However, with single-beam particle microscopes, it is possible to detect backscattered electrons separately and generate electron microscopic images from the backscattered electron detection signals, which clearly depict different materials in a specimen with high contrast. The inventors have determined that it would be desirable to have the ability to efficiently detect backscattered electrons in multi-beam particle microscopes as well.The inventors have further recognized that the electron beams which emanate from the object and are guided to the detector arrangement are essentially formed from secondary electrons, since these can be bundled into the individual electron beams due to their low energy when emerging from the surface of the object, whereas the much higher-energy backscattered electrons cannot be formed into individual electron beams which can be guided from the location of the primary particle beam impinging on the object to an element of the detector arrangement assigned to this location.

[0010] The multi-aperture plate of the multi-beam particle microscope therefore comprises a plurality of converters, wherein at least one converter of the plurality of converters is provided at each opening of the plurality of openings, wherein the converters are configured to convert backscattered electrons generated by the primary particle beams at the object, which have kinetic energies at the object that are greater than a predetermined first energy threshold, into electrons that have kinetic energies at the converter that are smaller than a predetermined second energy threshold and which provide electrons that form the electron beams detected by the detector arrangement.

[0011] This converts the backscattered electrons, with their comparatively high kinetic energy, close to the object and close to the point where they emerge from the object's surface into electrons with significantly lower kinetic energy. These lower kinetic energy electrons are created near the point where the primary particle beams impact the object. The resulting low-kinetic-energy electrons can then—like the secondary electrons generated at the point of impact of the primary particle beam—also be formed into electron beams and fed to the detector array for detection. There, these converted low-energy electrons generate detection signals proportional to the number of backscattered electrons generated at the point of impact of the primary particle beam.Thus, due to the provision of the converters on the multi-aperture plate, it is also possible to detect backscattered electrons with the multi-beam particle microscope.

[0012] According to exemplary embodiments, the converters each comprise a thin membrane oriented substantially parallel to the object plane. The thin membranes can define or delimit the apertures in the multi-aperture plate. The backscattered electrons generated by the object can impinge on the surface of the membrane facing the object plane and be converted by the latter into lower-energy electrons, which can at least partially emerge from the surface of the membrane facing away from the object plane and contribute to the electron beam that passes through the objective lens and is fed to the detector array.

[0013] According to exemplary embodiments, the thickness of the membranes is in a range from 0.5 nm to 50 nm and in particular in a range from 4 nm to 20 nm.

[0014] A high conversion rate of backscattered electrons into lower-energy electrons can be achieved, for example, when the membranes are made of a material made of heavy elements. According to exemplary embodiments, the membranes are made of a material consisting of atoms with atomic numbers greater than 40 on average. Examples of such materials are gold and others. However, membranes made of heavy elements should be sufficiently thin to avoid excessive absorption of the secondary electrons by the membrane. For example, a monolayer of gold can be used as the membrane. The thin layer of gold can, for example, be vapor-deposited onto a thicker, more stable carrier layer made of, for example, aluminum.

[0015] According to exemplary embodiments, the converters are each formed from a material that is provided on the inner walls of the openings of the multi-aperture plate. According to exemplary embodiments, this material is different from a material that is the structural material of the multi-aperture plate. The material that forms the converter is then applied, for example, as a layer to the inner walls of the openings of the multi-aperture plate.

[0016] According to exemplary embodiments, the material consists of atoms having atomic numbers that are on average greater than 40.

[0017] According to further exemplary embodiments, the openings of the multi-aperture plate have a conical shape such that the cross-section of an opening decreases in the direction away from the object plane. According to exemplary embodiments, the openings of the multi-aperture plate have a first cross-sectional area on their side facing the object plane and a second cross-sectional area on their side facing away from the object plane, wherein the first cross-sectional area is at least 1.5 times and in particular at least 2.0 times larger than the second cross-sectional area.

[0018] According to exemplary embodiments, the multi-beam particle microscope comprises a voltage supply system configured to supply an adjustable electrical potential to the object and the multi-aperture plate, respectively.

[0019] According to an exemplary embodiment herein, the voltage supply system is configured, in a first operating mode, to supply electrical potentials to a side of the multi-aperture plate facing the object and to the object such that the object is at a negative electrical potential relative to the multi-aperture plate, so that secondary electrons generated at the object pass through the openings of the multi-aperture plate and also provide electrons that form the electron beams detected by the detector arrangement; and wherein the voltage supply system is configured, in a second operating mode, to supply electrical potentials to the side of the multi-aperture plate facing the object and to the object such that the object is at a positive electrical potential relative to the multi-aperture plate, so that secondary electrons generated at the object do not pass through the openings of the multi-aperture plate.This makes it possible to switch between an operating mode in which secondary electrons contribute to the electron beams detected by the detector array and an operating mode in which secondary electrons do not contribute to the detected electrons, and thus only the backscattered electrons generated by the object contribute to the detected electrons. In this case, the electron microscopic image generated by the detected electrons is a so-called pure backscattered electron image.

[0020] According to exemplary embodiments, the multi-aperture plate comprises three mutually insulated conductive layers, wherein the voltage supply system is configured to supply adjustable electrical potentials to the three conductive layers, respectively.

[0021] The first energy threshold can be, for example, greater than 50 eV or greater than 200 eV. The second energy threshold can be, for example, less than 200 eV and, in particular, less than 100 eV. The number of primary particle beams can be, for example, greater than 7 or greater than 50.

[0022] According to exemplary embodiments, the multi-aperture plate is arranged at a distance from the object plane which is less than 200 µm, in particular less than 50 µm, in particular less than 30 µm, in particular less than 20 µm and in particular less than 10 µm.

[0023] According to exemplary embodiments, the multi-aperture plate is arranged at a distance from the object plane which is smaller than a smallest distance between the apertures of the multi-aperture plate.

[0024] According to further exemplary embodiments, a minimum distance between two adjacent primary particle beams at the multi-aperture plate is less than 50 µm, in particular less than 30 µm, in particular less than 20 µm and in particular less than 12 µm.

[0025] Embodiments of the invention are explained in more detail below with reference to the figures. Herein: Figure 1 is a schematic diagram to explain the operation of a multi-beam particle microscope according to an embodiment; Figure 2 is a schematic diagram of a part of the multi-beam particle microscope of the Figure 1 ; Figure 3 a schematic sectional view of a multi-aperture plate with converters of the multi-beam particle microscope of the Figure 1 according to a first embodiment; Figure 4 shows a schematic sectional view of a multi-aperture plate with converters of the multi-beam particle microscope of the Figure 1according to a second embodiment; and Figure 5 shows a schematic sectional view of a multi-aperture plate with converters of the multi-beam particle microscope of the Figure 1 according to a third embodiment.

[0026] Figure 1is a schematic representation of a multi-beam particle microscope that uses multiple particle beams. The multi-beam particle microscope generates multiple primary particle beams that strike an object under investigation, generating secondary particles there. These particles emanate from the object and are formed into secondary particle beams, which are subsequently detected. The multi-beam particle microscope 1 is of the scanning electron microscope (SEM) type, which uses electron beams 3 as primary particle beams that strike a surface of the object 7 at locations 5, generating multiple electron beam spots. The electrons of the primary particle beam that strike the object at these beam spots generate electrons as secondary particles, which are formed into secondary particle beams and detected.These electrons, created as secondary particles, are often classified into backscattered electrons and secondary electrons based on their kinetic energy. Generally, electrons with kinetic energies less than 50 eV are called secondary electrons, and electrons with kinetic energies greater than 50 eV are called backscattered electrons.

[0027] However, the primary particle beams can also be ion beams. These can also generate electrons as secondary particles upon impacting the object, which are then formed into secondary particle beams and detected. Here, too, electrons with kinetic energies less than 50 eV can be referred to as secondary electrons, and electrons with kinetic energies greater than 50 eV can be referred to as backscattered electrons.

[0028] The object 7 to be examined can be of any type and can include, for example, a semiconductor wafer, a biological sample, an array of miniaturized elements, or the like. The surface of the object 7 is arranged in an object plane 101 of an objective lens 102 of an objective lens system 100.

[0029] The enlarged section I1 of the Figure 1 shows a plan view of the object plane 101 with a regular rectangular field 103 of impact locations 5 of primary particle beams 3, which are formed in the plane 101. In Figure 1 The number of impact points is 25, arranged as a 5 x 5 array 103. The number 25 is a small number chosen for the sake of simplicity. In practice, the number of beams or impact points can be chosen to be significantly larger, such as 20 x 30, 100 x 100, and the like.

[0030] In the illustrated embodiment, the array 103 of impact locations 5 is a substantially regular rectangular array with a constant spacing p1 between adjacent impact locations. Exemplary values ​​of the spacing p1 are 50 µm, 30 µm, or 10 µm. However, it is also possible for the array 103 to have other symmetries, such as hexagonal symmetry.

[0031] The diameter of the beam spots formed in the object plane 101 can be small. Example values ​​of this diameter are 1 nm, 5 nm, 10 nm, and 30 nm. The focusing of the particle beams 3 to form the beam spots 5 is performed by the objective lens system 100.

[0032] The particles striking the object, which in the example explained here are electrons, in turn generate particles that are essentially electrons and that emanate from the surface of the object 7. The electrons emanating from the surface of the object 7 are formed into electron beams 9 by the objective lens system 100. The multi-beam particle microscope 1 provides a secondary beam path 12 to guide the plurality of electron beams 9 to a detection system 200. The detection system 200 comprises electron optics with a projection lens system 205 to direct the electron beams 9 onto an electron multi-detector 209.

[0033] Section I2 in Figure 1shows a plan view of a plane 211 in which individual detection areas are arranged, onto which the electron beams 9 impinge at locations 213. The impingement locations 213 form a second field 217 with a regular spacing p2 between the impingement locations. Exemplary values ​​of the spacing p2 are 10 µm, 100 µm, 200 µm, and 500 µm.

[0034] The primary particle beams 3 are generated by a multi-beam particle source 300, which comprises at least one electron source 301 with an electron emitter, at least one collimating lens 303, a multi-aperture array 305, and a field lens system 307. The electron source 301 generates a diverging electron beam 309 from electrons emitted by the electron emitter, which is collimated by the collimating lens 303 to form a beam 311 that illuminates the multi-aperture array 305.

[0035] Section I3 in Figure 1shows a top view of the multi-aperture arrangement 305. The multi-aperture arrangement 305 comprises a multi-aperture plate 313 having a plurality of openings or apertures 315 formed therein. Centers 317 of the openings 315 are arranged in a field 319, which corresponds to the field 103 formed by the beam spots 5 in the object plane 101. A distance p3 between the centers 317 of the apertures 315 can have exemplary values ​​of 5 µm, 100 µm, and 200 µm. The diameters D of the apertures 315 are smaller than the distance p3 between the centers of the apertures. Exemplary values ​​of the diameters D are 0.2 x p3, 0.4 x p3, and 0.8 x p3.

[0036] Electrons from the illuminating beam 311 pass through the apertures 315 and form electron beams 3 as primary particle beams. Electrons from the illuminating beam 311 that strike the plate 313 are intercepted by the plate and do not contribute to the formation of the electron beams 3.

[0037] The multi-aperture arrangement 305 focuses the electron beams 3 such that beam foci 323 are formed in a plane 325. The diameter of the foci 323 can be, for example, 2 nm, 10 nm, 100 nm, and 1 µm.

[0038] The field lens system 307, a beam splitter 400 and the objective lens 102 provide a first imaging particle optics and form a primary beam path 10 to image the plane 325, in which the foci 323 are formed, onto the object plane 101, so that a field 103 of impact locations 5 or beam spots is formed there on the surface of the object 7.

[0039] The objective lens 102, the beam splitter 400, and the projection lens system 205 provide a second imaging particle optics and form a secondary beam path 12 to image the object plane 101 onto the detection plane 211. The objective lens 102 is thus a lens that is part of both the primary beam path 10 and the secondary beam path 12, while the field lens system 307 belongs only to the primary beam path 10 and the projection lens system 205 belongs only to the secondary beam path 12. The objective lens 102 is thus penetrated by both the beam paths of the primary particle beams 3 and the beam paths of the (secondary) electron beams.

[0040] The beam splitter 400 is arranged in the primary beam path 10 between the multi-aperture arrangement 305 and the objective lens system 100. The beam splitter 400 is also part of the secondary beam path 12 and is arranged there between the objective lens system 100 and the detection system 200. The beam splitter 400 separates the beam paths of the primary particle beams 10 from the beam paths of the (secondary) electron beams.

[0041] Further information on such multi-beam particle microscopes and components used therein, such as particle sources, multi-aperture plates and lenses, can be obtained from the international patent applications WO 2005 / 024881 A2, WO 2007 / 028595 A2, WO 2007 / 028596 A1 and WO 2007 / 060017 A2 and the patent applications US 2015 / 0083911 A1, US 2015 / 0069235 A1, DE 10 2014 008 383 A1, US 6,946,655 B2 and DE 10 2016 120 902 A1.

[0042] The multi-beam particle microscope 1 further comprises a multi-aperture plate 11 arranged between the objective lens 102 and the object plane 101, which has a plurality of openings 37. In the Figure 1 In the embodiment explained, 25 openings are provided in the multi-aperture plate 11 such that, during operation of the multi-beam particle microscope 1, each of the openings 37 is penetrated by a primary particle beam 3 and can be penetrated by a (secondary) electron beam 9.

[0043] Figure 2shows a simplified schematic representation of the objective lens 102, which comprises a coil former 19, an upper pole piece 21, and a lower pole piece 22, so that a gap is formed between them, at which a magnetic field is generated that focuses the primary particle beams. The multi-aperture plate 11 is held and fastened to the lower pole piece 22 of the objective lens 102 via an electrically insulating holder 31. As an alternative to fastening the multi-aperture plate 11 to the objective lens 102, it is possible to provide an actuator 23 that can selectively remove the aperture plate from the area between the objective lens 102 and the object plane 101 or arrange it there. The multi-aperture plate 11, when arranged between the objective lens 102 and the object plane 101, is pressed against the holder 31, which acts as a spacer, by a suitable mechanism.The actuator 23 is controlled via a control line 25 by a controller 27 of the multi-beam particle microscope 1.

[0044] The controller 27 further comprises a voltage supply system for setting the particle emitter of the particle source 301 to a first electrical potential U1, setting the upper pole piece 21 of the objective lens 102 to a second electrical potential U2, setting the multi-aperture plate 11 to a third electrical potential U3, setting the object holder 17 with the object 7 arranged thereon to a fourth electrical potential U4, and setting the lower pole piece 22 of the objective lens 102 to a fifth electrical potential U5. Potential supply lines 29 are provided between these components and the controller 27 to supply the electrical potentials U1, U2, U3, U4, and U5 to the particle emitter of the particle source 301, the upper pole piece 21, the multi-aperture plate 11, the object holder 17, and the lower pole piece 22.

[0045] A magnetic iron circuit extending between the end of the upper pole piece 21 and the end of the lower pole piece 22 in the objective lens is separated by an electrical insulator 24, which allows the upper and lower pole pieces to be set to different electrical potentials U2 and U5, respectively. The insulator 24 is designed such that the two parts of the iron circuit adjacent to the insulator 24 geometrically overlap to achieve low resistance to the magnetic flux.

[0046] The difference between the potentials U1 and U4 determines the kinetic energy with which the particles of the primary particle beams 3 impact the object. This difference can, for example, take values ​​between 50 V and 3 kV.

[0047] An electrode (anode) can be arranged in the beam path of the primary particle beams 3 in the particle source 301 or downstream to accelerate the particles to a high kinetic energy, so that they quickly travel the distance to and through the objective lens 102. This electrode can also be set to the second electrical potential U2. The difference between U1 and U2 then determines the kinetic energy of the particles between the multi-beam particle source and the objective lens 102 and can, for example, be greater than 5 kV, greater than 15 kV, or greater than 25 kV. Between the upper pole piece 21, which is set to the second electrical potential U2, and the lower pole piece 22, which is set to the fifth electrical potential U5, there is an electric field that decelerates the primary particles on their path toward the object 7 and accelerates the electron beams 9 on their path toward the detector.Furthermore, the multi-aperture plate 11 is set to the third electrical potential U3.

[0048] Figure 3 is an enlarged cross-sectional view of the multi-aperture plate 11. The multi-aperture plate 11 comprises a supporting substrate 401, onto which a converter layer 403 is applied. Openings 37, through which the primary particle beams 3 pass on their way to the object 7, are provided in the converter layer. The openings 37 can be circular, for example, and have a diameter of, for example, 1 µm, 2 µm, or 5 µm.

[0049] The substrate 401 has through-holes 405 with a diameter that is significantly larger than the diameter of the recesses 37 in the converter layer 403 and which are arranged with respect to the recesses 37 in the converter layer 403 such that the openings 405 in the substrate 401 are partially covered by the converter layer. The converter layer 403 thus defines and delimits the openings 37 through which the primary particle beams 3 pass. Furthermore, there are regions of the converter layer 403 along the circumference of the openings 37 at which the converter layer 403 is not supported by the substrate 401.

[0050] The diameter of the recesses 37 in the converter layer 403 is matched to the beam diameter of the primary particle beams 3 such that the primary particles do not cause any significant background interference from the secondary electrons generated by them directly at the converter layer before they impact the object 7. For example, the beam diameter of the primary particle beams 3 is at most 70%, in particular at most 50%, and in particular at most 20% of the diameter of the recesses 37 in the converter layer 403.The tipping point of the beam deflection system for scanning the object 7 with the primary particle beams 3 should be so close to the plane of the converter layer 403 that even when scanning the object, the migration of the primary particle beams 3 in the plane of the converter layer 403 remains sufficiently small so that the primary particles do not cause any significant background interference from the secondary electrons generated by them directly at the converter layer before they hit the object 7.

[0051] The particles of the primary particle beam 3 striking object 7 generate electrons at the impact point 5 on the surface of object 7 through interaction of the particles with the material of object 7, which electrons emerge from the surface of object 7. These electrons are typically divided into two categories: secondary electrons and backscattered electrons. Upon exiting the surface of object 7, the backscattered electrons exhibit kinetic energies that are equal to or slightly smaller than the kinetic energy of the incident primary particles. The secondary electrons exhibit energies that are significantly smaller than the kinetic energy of the incident primary particles.

[0052] Both the backscattered electrons and the secondary electrons emerge from the surface of the object 7 over a wide solid angle range. In a conventional multi-beam particle microscope, an electric field exists between the objective lens and the object, which, on the one hand, slows the particles of the primary particle beams on their path toward the object and, on the other hand, accelerates the electrons emerging from the object away from the object. Since the secondary electrons emerging from the object have relatively small kinetic energies upon exiting the object, their kinetic energy, speed, and direction of motion are essentially determined by the electric field, which accelerates the electrons away from the object.In particular, the velocity components of the secondary electrons emerging from the object in the direction parallel to the surface of the object are small, so that after being accelerated by the electric field provided between the object and the objective lens, the secondary electrons move away from the object 7 essentially perpendicular to the object plane 101. Given a sufficient distance between the locations 5, trajectories of secondary electrons emerging from the object 7 at adjacent locations 5 therefore hardly overlap. This makes it possible to form electron beams 9 from the secondary electrons emerging from the object 7, which can be guided as separate beams in the secondary beam path 12 to the detector arrangement 211.

[0053] Since the backscattered electrons exhibit a relatively high kinetic energy upon exiting object 7, their kinetic energy, velocity, and direction of movement after acceleration in the electric field provided between the objective lens and object 7 are not primarily determined by this field but also significantly by their direction of movement upon exiting the surface of the object. If, for a given distance between adjacent impact locations 5 of the primary beams 3 on object 7, the components of the velocities of the backscattered electrons exiting the object oriented parallel to the object plane 101 are sufficiently large, the trajectories of backscattered electrons exiting object 7 at adjacent locations 5 can overlap.It is then not possible to guide backscattered electrons emanating from different impact points 5 through the secondary beam path 12 as separate beams to the detector array. This situation arises for a relatively large portion of the backscattered electrons due to their relatively high kinetic energy upon exiting the object, so that the efficiency with which backscattered electrons can be detected and assigned to the individual impact points 5 is very low. This means that with a conventional multi-beam particle microscope, contrasts between different materials in the object, which can be easily visualized with a single-beam particle microscope by detecting the backscattered electrons, are barely detectable.

[0054] The converter layer 403 provided on the multi-aperture plate 11 is configured to convert backscattered electrons into secondary electrons 409. For this purpose, backscattered electrons 407, which emerge from the object 7 at a location 5 in the plane 101, strike the side of the converter layer 403 facing the object 7, specifically in the regions where the converter layer 403 is not supported by the substrate 401. The backscattered electrons 407 generate secondary electrons in the converter layer 403, at least a portion of which 409 emerge from the converter layer 403 on the side facing away from the object 7. These converted secondary electrons 409 have a kinetic energy when they exit the converter layer 403, which is significantly lower than the kinetic energy of the backscattered electrons 407 when they exit the object 7.These secondary electrons 409 have, in particular, small velocity components parallel to the object plane 101 and are accelerated away from the object 7 in the electric field provided between the objective lens 102 and the object 7. They can be fed through the secondary optics 12 to the detector array 209 without the trajectories of secondary electrons 409 generated from backscattered electrons 407 emanating from various locations 5 on the surface of the object 7 overlapping. Thus, these secondary electrons 409 can form electron beams that are fed through the secondary optics 12 as separate beams to the detector array. In particular, these secondary electrons 409, together with the secondary electrons that emerge from the object 7 directly at the impact locations 5 and pass through the openings 37, can form electron beams whose intensities can be detected by the detector array.The converter layer 403 thus makes it possible to generate measurement signals which can be assigned to a specific impact location 5 of a primary electron beam 3 on the object 7 and which represent the intensity of the backscattered electrons 407 generated at this location 5.

[0055] It is possible to operate the multi-beam particle microscope 1 in two different operating modes. In a first of the two operating modes, the potential U3 applied to the multi-aperture plate 11 is greater than the potential U4 applied to the object 7. As a result, the secondary electrons emerging from the surface of the object 7 are accelerated toward the multi-aperture plate 11 and partially pass through the openings 37 in the converter layer 403, so that these secondary electrons contribute to the electron beams formed from the secondary electrons 409 formed from the converter layer 403 after conversion from backscattered electrons 407. In the first operating mode, both secondary electrons and backscattered electrons emerging from the object 7 are detected.

[0056] In a second of the two operating modes, the electrical potential U3 supplied to the multi-aperture plate 11 is smaller than the electrical potential U4 supplied to the object 7. This means that the secondary electrons with low kinetic energy emerging from the object cannot reach the multi-aperture plate 11 and, in particular, cannot pass through the openings 37 in the converter layer 403. The secondary electrons emerging from the object therefore do not generate detection signals generated by the detector array. Only backscattered electrons are detected in the second operating mode.

[0057] The converter layer 403 preferably consists of a material with a high atomic number. An example of a suitable material is gold. The thickness d of the converter layer is dimensioned such that backscattered electrons 407 striking the side of the converter layer 407 facing the object 7 result in a high number of secondary electrons 409 emerging from the side of the converter layer 403 facing away from the object 7. The optimal thickness determined in this way depends on the kinetic energy of the backscattered electrons 407 and is thus in turn dependent on the kinetic energy with which the primary particles strike the object. However, the thickness d of the converter layer 403 does not have to correspond to this optimal thickness, and it is possible to use a converter layer 403 of a given thickness d for a wide range of kinetic energies of the primary particles upon impacting the object.The thickness of the converter layer can, for example, be in a range of one atomic layer to 40 nm or 0.5 nm to 20 nm.

[0058] The substrate 401 is formed from a material that can fulfill the function of serving as a supporting structure for the converter layer 403. For example, the substrate 401 can be formed from a silicon single crystal. For this purpose, a plate made of a silicon single crystal, which forms the substrate 401 of the multi-aperture plate 11, can be coated with a material that forms the converter layer 403 of the multi-aperture plate 11, and the openings 405 in the silicon plate and the openings 37 in the converter layer 403 can be introduced into the silicon plate and the converter layer, respectively, using lithographic methods to manufacture the multi-aperture plate 11.

[0059] Figure 4 shows one of the Figure 3corresponding representation of a multi-aperture plate 11 which is configured to convert backscattered electrons into secondary electrons which form electron beams which are fed through the secondary optics 12 as separate beams to the detector arrangement 209.

[0060] In the Figure 4 In the example of a multi-aperture plate shown, the converter material is not applied as a layer 403 on a flat side of a substrate 401 of the multi-aperture plate 11, as in the example of Figure 3is the case, but it is attached as a layer to the inner walls of openings 405 in the substrate 401. Thus, the surfaces of the layers 411 of converter material form the openings 37 of the multi-aperture plate 11, through which the primary beams 3 pass. Backscattered electrons 407 emerging from the object 7 strike the converter layer 411 within the apertures 37 and are converted into secondary electrons 409, which emerge from the layer 411 into the opening 37. These secondary electrons 409 emerging from the converter layer are accelerated away from the object 7 by the electric field provided between the objective lens and the object 7 and form beams, which are fed to the detector arrangement 211 by the secondary optics 12. Thus, it is also possible with a multi-aperture plate 11 according to the Figure 4shown embodiment, it is possible to convert electrons 407 emerging from the object 7 into secondary electrons, which are detectable by the detector arrangement 209. The multi-aperture plate of the Figure 4 The embodiment shown can be operated in the two operating modes described above in connection with the Figure 3 These two operating modes allow the detection of backscattered electrons alone as well as backscattered electrons and secondary electrons, both of which emerge from object 7.

[0061] Reference number 407' in Figure 4denotes a backscattered electron emanating from the surface of the object 7 at a location 5 with a large velocity component parallel to the object plane 101. This backscattered electron 407' succeeds in hitting the converter layer 411 on the inner wall of the recess 37, which is associated with the impact location 5 of the primary beam 3, which is different from the location from which the backscattered electron 407 emanates. The backscattered electron 407 thus generates secondary electrons 409 in the "wrong" recess 37, and the resulting detection events are thus attributed to the wrong primary particle beam 3. This effect can be referred to as crosstalk between different particle beams.

[0062] Figure 5 shows a cross-section of a multi-aperture plate 11 according to another embodiment in which this crosstalk effect is reduced.

[0063] The Figure 5The multi-aperture plate 11 shown in cross section has a multi-layer structure in which the apertures 37 are arranged. The following layers are shown in the illustration of the Figure 5 from top to bottom, ie in the direction towards the object plane 101: an electrically conductive layer 411, a converter layer 413 which is electrically insulating, an electrically conductive layer 415, an electrically insulating layer 417 and an absorber layer 419 which is electrically conductive.

[0064] It may be advantageous if the electrically conductive layers 411, 415, or 419 are at a predetermined electrical potential, which is determined such that the primary beams 3 are not significantly influenced by electric fields arising between the electrically conductive layers 411, 415, and 419. Furthermore, it is possible for the electrically insulating converter layer 413 to have a sufficiently large residual resistance such that a current can flow through the layer, which current defines the potentials of the electrically conductive layers 411, 415, and 419 during operation.

[0065] The converter layer 413 is either formed entirely from a material which converts backscattered electrons 407 into secondary electrons 409 and which is exposed on the inner walls of the recess 37, or it is formed from a merely electrically insulating material in which layers of a converter material are applied to the inner walls of the recesses 37, as described above in connection with the Figure 4 shown embodiment was explained.

[0066] The absorber layer 419 consists of a material which generates very few secondary electrons per incident backscattered electron and in particular significantly fewer secondary electrons than the converter material.

[0067] It may be advantageous to design the layers such that during operation the absorber layer 419 has an electrical potential that is, for example, 10 V, 20 V or 50 V higher than the converter layer 413 in order to prevent secondary electrons generated at the absorber from reaching the converter and from there to the detector.

[0068] With the reference number 407 in Figure 5denotes a backscattered electron which emerges from a location 5 on the surface of the object 7 and strikes the inner wall of the aperture 37 associated with the location 5 from which the backscattered electron 407 originates. The backscattered electron 407 is converted at the inner wall of the recess 37 in the converter layer 413 into one or more secondary electrons 409, some of which emerge from the opening 37 on the side of the multi-aperture plate facing away from the object 7 and, as described above, can be detected by the detector arrangement in such a way that they can be assigned to the correct primary particle beam 3.

[0069] With the reference number 407' in Figure 5A backscattered electron 407 is shown, which strikes the inner wall of the adjacent opening 37 and thus the inner wall of the "false" opening, potentially generating crosstalk between different detected electron beams. However, the geometry of the openings 37, the thickness of the absorber layer 419, and the distance between the surface of the multi-aperture plate 11 or the absorber layer 419 facing the object 7 and the surface of the object 7 are selected such that the backscattered electrons 407' emanating from location 5 and striking the adjacent, "false" opening 37 only strike the inner surfaces of the opening 37, which lie in the region of the absorber layer 419. However, these incident backscattered electrons 407' are not converted, or are converted only to a very small extent, into secondary electrons, which are ultimately detected.

[0070] In the embodiment of the Figure 5The voltage supply system is configured to apply different and adjustable electrical potentials to the conductive layers 411, 415 and 419. In particular, the following operating modes are possible: The layer 419 arranged closest to the object 7 can be set to a greater or smaller electrical potential relative to the object 7 in order to achieve the Figure 3 described two operating modes, in which backscattered electrons and secondary electrons emanating from the object are detected together or only backscattered electrons are detected.

[0071] A potential difference can be applied between the conductive layers 411 and 415, which creates an electric field within the recesses 37 in the region of the converter layer 413, which field conducts the secondary electrons generated on the walls of the recess 37 in the Figure 5upwards and thus away from the object 7. This increases the probability that a secondary electron generated on the wall of the recess 37 also exits the opening 37 at the top of the multi-aperture plate 11 and is not absorbed within the opening.

[0072] The openings 37 can furthermore, as in Figure 5 shown, have a conical shape, so that the cross-sectional area of ​​the recesses 37 increases in the direction toward the object 7. Together with the provision of the absorber layer 419, it is thus possible to increase the solid angle at which the backscattered electrons 407 can emanate from the location 5 on the surface of the object 7 and be converted into secondary electrons 409 on the inner wall of the associated recess 37, without increasing the probability of crosstalk.

[0073] In this embodiment, the diameter of the recesses 37 is matched to the beam diameter of the primary particle beams 3 such that the primary particles, before impinging on the object 7, do not cause any significant background interference from the secondary electrons generated by them directly at the converter layer 413. For example, the beam diameter of the primary particle beams 3 is at most 70%, in particular at most 50%, and in particular at most 20% of the diameter of the recesses in the layers 411 or 413.The tipping point of the beam deflection system for scanning the object 7 with the primary particle beam bundles 3 should be so close to the plane of the substrate 401 of the multi-aperture plate that even when scanning the object, the migration of the primary particle beams 3 within the recesses 37 remains sufficiently small so that the primary particles do not cause any significant background interference from the secondary electrons generated by them directly on the converter layer 413 before they hit the object 7.

Claims

1. Multi-beam particle microscope, comprising: a multi-beam particle source (300) configured to create a plurality of primary particle beams (3); an objective lens (102) penetrated by beam paths of the plurality of primary particle beams (3) and configured to direct each of the plurality of primary particle beams (3) to an object plane (101) and to focus said primary particle beams there; and a detector arrangement (200) configured to detect intensities of a plurality of electron beams (9), electrons in the electron beams (9) being created by particles of the primary particle beams (3) at an object (7) that is arrangeable in the object plane (101); characterized by a multi-aperture plate (11) arranged between the objective lens (102) and the object plane (101) at a distance from said object plane (101) and having a plurality of openings (37), wherein different openings (37) of the multi-aperture plate (11) are penetrated by beam paths of different primary particle beams (3), and wherein the multi-aperture plate (11) comprises a plurality of converters (403; 411; 413), at least one converter of the plurality of converters being provided at each opening of the plurality of openings, the converters being configured to convert backscattered electrons that are created by the primary particle beams at the object and that have higher kinetic energy levels at the object than a predetermined first energy threshold value into electrons that have lower kinetic energy levels at the converter than a predetermined second energy threshold value and that provide electrons which form the electron beams that are detected by the detector arrangement.

2. Multi-beam particle microscope according to Claim 1, wherein the converters each comprise thin membranes (403) that are oriented substantially parallel to the object plane (101).

3. Multi-beam particle microscope according to Claim 2, wherein the backscattered electrons created at the object strike the surface of the membranes facing the object plane and the electrons resulting from the backscattered electrons emerge from the surface of the membranes facing away from the object plane.

4. Multi-beam particle microscope according to Claim 2 or 3, wherein the membranes have a thickness of 0.5 nm to 50 nm, more particularly 4 nm to 20 nm.

5. Multi-beam particle microscope according to any of Claims 1 to 4, wherein the openings in the multi-aperture plate have a first cross-sectional area on their side facing the object plane and a second cross-sectional area on their side facing away from the object plane, the first cross-sectional area being at least 1.5-times and, in particular, at least 2.0-times larger than the second cross-sectional area.

6. Multi-beam particle microscope according to any of Claims 1 to 5, further comprising a voltage supply system (27) configured to supply a respective adjustable electric potential to the object and to the multi-aperture plate; wherein in particular the voltage supply system is configured in a first mode of operation to supply electric potentials to a side of the multi-aperture plate facing the object and to the object, in such a way that the object is at a negative electric potential relative to the multi-aperture plate such that secondary electrons created at the object pass through the openings in the multi-aperture plate and likewise provide electrons which form the electron beams that are detected by the detector arrangement; and the voltage supply system is configured in a second mode of operation to supply electric potentials to the side of the multi-aperture plate facing the object and to the object, in such a way that the object is at a positive electric potential relative to the multi-aperture plate such that secondary electrons created at the object do not pass through the openings in the multi-aperture plate.

7. Multi-beam particle microscope according to Claim 6, wherein the multi-aperture plate comprises three conductive layers (411, 415, 419) that are insulated from one another, the voltage supply system being configured to supply respective adjustable electric potentials to the three conductive layers.

8. Multi-beam particle microscope according to any of Claims 1 to 7, further comprising a plurality of absorbers (405) which, as seen in the direction of the object plane (101), are arranged between mutually adjacent openings (37) in the multi-aperture plate (11) and which, as seen perpendicular to the object plane, are arranged between the object plane (101) and the converters (403; 411; 413).

9. Multi-beam particle microscope according to Claim 8, wherein the absorbers consist of a material consisting of atoms that have atomic numbers which, on average, are less than 40, in particular less than 20 and in particular less than 15.

10. Multi-beam particle microscope according to Claim 8 or 9, wherein the multi-beam particle microscope is configured in such a way that, during operation, the absorbers are positively charged relative to the converters.

11. Multi-beam particle microscope according to Claim 10, wherein, during operation, there is a potential difference of more than 10 V, in particular more than 20 V and in particular more than 50 V between the absorbers and the converters.

12. Multi-beam particle microscope according to any of Claims 1 to 11, wherein the first energy threshold value is greater than 50 eV and in particular greater than 200 eV; and / or wherein the second energy threshold value is less than 200 eV and in particular less than 100 eV.

13. Multi-beam particle microscope according to any of Claims 1 to 12, wherein the number of primary particle beams is greater than 7.

14. Multi-beam particle microscope according to any of Claims 1 to 13, wherein the distance is less than 200 µm, in particular less than 50 µm, in particular less than 30 µm, in particular less than 20 µm and in particular less than 10 µm.

15. Multi-beam particle microscope according to any of Claims 1 to 14, wherein a minimum distance between two mutually adjacent primary particle beams at the multi-aperture plate is less than 50 µm, in particular less than 30 µm, in particular less than 20 µm and in particular less than 12 µm.