NL2029294B1

NL2029294B1 - Multiple particle beam microscope and associated method with fast autofocus around an adjustable working distance

Info

Publication number: NL2029294B1
Application number: NL2029294A
Authority: NL
Inventors: Mueller Ingo; Zeidler Dirk; Schubert Stefan
Original assignee: Carl Zeiss Multisem Gmbh
Priority date: 2020-09-30
Filing date: 2021-09-30
Publication date: 2023-09-27
Also published as: NL2029294A; DE102020125534B3

Abstract

The invention relates to a multiple particle beam microscope and an associated method With a fast autofocus around an adjustable working distance. Proposed is a system having one or more fast autofocus correction lenses and] or further fast correction means for adapting, in high-frequency fashion, the focusing, the position, the landing angle and the rotation of individual particle beams upon incidence on a wafer surface during the wafer inspection. A fast autofocus correction lens can in particular be realized by electrostatic elements Which can be arranged at specially selected positions in the particle optical beam path. Fast autofocusing in the secondary path of the particle beam system can be implemented in analogous fashion.

Description

P131252NL00

Title: Multiple particle beam microscope and associated method with fast autofocus around an adjustable working distance

FIELD OF THE INVENTION

The invention relates to multiple particle beam microscopes for inspecting semiconductor wafers with HV structures.

PRIOR ART

With the continuous development of ever smaller and ever more complex microstructures such as semiconductor components, there 1s a need to develop and optimize planar production techniques and inspection systems for producing and inspecting small dimensions of the microstructures. By way of example, the development and production of the semiconductor components requires monitoring of the design of the test wafers, and the planar production techniques require a process optimization for a reliable production with a high throughput. Moreover, there have been recent demands for an analysis of semiconductor wafers for reverse engineering and for a customer-specific, individual configuration of semiconductor components. Therefore, there is a need for inspection means which can be used with a high throughput for examining the microstructures on wafers with high accuracy.

Typical silicon wafers used in the production of semiconductor components have diameters of up to 300 mm. Each wafer is subdivided into 30 to 60 repetitive regions ("dies") with a size of up to 800 mm?. A semiconductor apparatus comprises a plurality of semiconductor structures, which are produced in layers on a surface of the wafer by planar integration techniques. Semiconductor wafers typically have a plane surface on account of the production processes. The structure size of the integrated semiconductor structures in this case extends from a few pm to the critical dimensions (CD) of 5 nm, wherein the structure dimensions will become even smaller in the near future; in future, structure sizes or critical dimensions (CD) are expected to be less than 3 nm, for example 2 nm, or even under 1 nm. In the case of the aforementioned small structure sizes, defects of the size of the critical dimensions must be identified quickly in a very large area. For several applications, the specification requirement on the accuracy of a measurement provided by an inspection device is even higher, for example by a factor of two or one order of magnitude. By way of example, a width of a semiconductor feature must be measured with an accuracy of below 1 nm, for example 0.3 nm or even less, and a relative position of semiconductor structures must be determined with a superposition accuracy of below 1 nm, for example 0.3 nm or even less.

Therefore, it is a general object of the present invention to provide a multiple particle beam system that operates with charged particles and an associated method for operating same with a high throughput, which facilitates a highly precise measurement of semiconductor features with an accuracy of below 1 nm, below 0.3 nm or even 0.1 nm.

The MSEM, a multi-beam scanning electron microscope, is a relatively new development in the field of charged particle systems (charged particle microscopes,

CPMs). By way of example, a multi-beam scanning electron microscope is disclosed in US 7 244 949 B2 and in US 2019/0355544 Al. In a multi-beam electron microscope or MSEM, a sample is irradiated simultaneously with a multiplicity of individual electron beams, which are arranged in a field or grid. By way of example, 4 to 10 000 individual electron beams can be provided as primary radiation, with each individual electron beam being separated from an adjacent individual electron beam by a distance of 1 to 200 micrometers. By way of example, an MSEM has approximately 100 separated individual electron beams ("beamlets"), which for example are arranged in a hexagonal grid, with the individual electron beams being separated by a distance of approximately 10 um. The multiplicity of charged individual particle beams (primary beams) are focused on a surface of a sample to be examined by way of a common objective lens. By way of example, the sample can be a semiconductor wafer which is fastened to a wafer holder that is assembled on a movable stage. During the illumination of the wafer surface with the charged primary individual particle beams, interaction products, for example secondary electrons or backscattered electrons, emanate from the surface of the wafer. Their start points correspond to those locations on the sample on which the multiplicity of primary individual particle beams are focused in each case. The amount and the energy of the interaction products depends on the material composition and the topography of the wafer surface. The interaction products form a plurality of secondary individual particle beams (secondary beams), which are collected by the common objective lens and which are incident on a detector arranged in a detection plane as a result of a projection imaging system of the multi-beam inspection system. The detector comprises a plurality of detection regions, each of which comprises a plurality of detection pixels, and the detector captures an intensity distribution for each of the secondary individual particle beams. An image field of for example 100 pm x 100 um is obtained in the process.

The multi-beam electron microscope of the prior art comprises a sequence of electrostatic and magnetic elements. At least some of the electrostatic and magnetic elements are adjustable in order to adapt the focus position and the stigmation of the multiplicity of charged individual particle beams. The multi- beam system with charged particles of the prior art moreover comprises at least one crossover plane of the primary or the secondary charged individual particle beams. Moreover, the system of the prior art comprises detection systems to make the adjustment easier. The multi-beam particle microscope of the prior art comprises at least one beam deflector ("deflection scanner") for collective scanning of a region of the sample surface by means of the multiplicity of primary individual particle beams in order to obtain an image field of the sample surface. Further details regarding a multi-beam electron microscope and a method for operating the same are described in the German patent application with the application number 102020206739.2, filed on May 28, 2020, the disclosure of which is incorporated in full in this patent application by reference.

In the case of scanning electron microscopes for wafer inspection, it is desirable to keep the imaging conditions stable such that the imaging can be carried out with great reliability and high repeatability. The throughput depends on a plurality of parameters, for example the speed of the stage and of the realignment at new measurement sites, and the area measured per unit of capture time. The latter is determined, inter alia, by the dwell time on a pixel, the pixel size and the number of individual particle beams. Additionally, time-consuming image postprocessing may be required for a multi-beam electron microscope; by way of example, the signal generated from charged particles by the detection system of the multi-beam system must be digitally corrected before the image field from a plurality of image subfields or partial images is put together ("stitching").

Here, the grid positions of the individual particle beams on the sample surface can deviate from the ideal grid position in a plane arrangement. The resolution of the multi-beam electron microscope can be different for each of the individual particle beams and can depend on the individual position of the individual particle beam in the field of individual particle beams, and consequently can depend on the specific grid position of said individual particle beams.

Conventional systems of charged particle beam systems are stretched to their limits with increasing demands on resolution and throughput.

It is therefore an object of the invention to provide a multiple particle beam system which facilitates highly precise and high-resolution image recording with a high throughput.

One approach for improving precision and resolution lies in the use of a so-called autofocus. Here, while scanning the sample surface, the current relative focal position of the individual electron beams is ascertained continuously ("on-the-fly") in view of the sample surface/object plane and an appropriate correction of the relative focal position is undertaken. By way of example, the focusing of the individual particle beams is adapted for each image field. By way of example, this procedure is based on a model of the sample or the assumption that the sample properties do not change much from image field to image field such that prediction values for improved focusing can be ascertained by extrapolation or interpolation.

Nevertheless, the known autofocusing method is comparatively slow: This is because the relative focal position is optimized either by changing the working distance (WD) or by way of a different control of the objective lens. Here, a change in the working distance by displacing the height of the sample stage (so-called "z- stage") is only possible with a certain restricted precision and speed. Moreover, not every sample stage is displaceable in terms of its height. If there is changed control of the objective lens or of other magnetic lenses for the purposes of varying the 5 relative focal position, this adjustment is comparatively slow: The prior art uses magnetic objective lenses and, in particular, immersion lenses, the inductance of which is too high to allow for an even faster adaptation. In this case, too, the time for changing the excitation ranges between several ten and several hundred milliseconds. Moreover, the optics of multiple electron microscopes is far more complex than that of individual beam systems since meaningful recordings require the magnification in the object plane (coupled to the beam pitch of the individual particle beams in the object plane) and also the orientation, i.e., the rotation, of the array of individual electron beams (grid arrangement) to remain unchanged when updating the relative focal position. The same applies to the landing angle of the individual particle beams on the sample. As a rule, the aforementioned particle optical parameters (and optionally further parameters) cannot be set independently of one another by means of only a single lens. A change in the control of the magnetic objective lens is therefore accompanied by a changed control of other particle optical components in the primary path. Thus, changes in the excitation of other magnetic and electrostatic elements typically also become necessary, with the adjustment times for the magnetic lenses being limiting in terms of time and likewise ranging from several ten to several hundred milliseconds. Similar considerations apply to particle optical components in the secondary path and to the adjustment of the focal position for a precise detection.

Against the above-described background and the increasing demands on throughput/speed and on precise measurement of ever smaller structures, the existing systems are therefore in need of improvement. There are enormous demands, especially on the inspection of semiconductor wafers as well. Then, a surface of a semiconductor wafer that is very flat per se can commonly no longer be assumed to be precisely flat within the scope of the precision inspection. Very small variations in the wafer thickness and/or the longitudinal position of the wafer surface relative to the objective lens have an influence on the optimal focus and hence on the accuracy of the measurements. This applies in particular to the inspection of polished wafer surfaces with HV structures. Thus — even under the not entirely realistic assumption of a lack of system drift and the like — it is no longer sufficient to adjust the multiple electron microscope once at a predefined working point with an associated working distance. Instead, very small changes in the working distance must be corrected by an altered relative focal position. A further precondition applying here is that the magnification must remain unchanged. The orientation of the grid arrangement on the sample surface must be exactly observed since, in the case of semiconductor wafers with HV structures, imaging is always carried out exactly parallel or orthogonal to these structures.

Moreover, it 1s imperative to keep the landing angle precisely constant. And lastly, the optical unit in the secondary path must also be updated quickly and highly precisely in order to obtain excellent imaging.

US 2011/0272576 Al discloses a multi-beam particle microscope. The publication discloses a charge control on a sample in advance of a sample inspection.

DE 10 2004 055 149 Al discloses a lithography device and method for imaging a multiple particle beam onto a substrate. A height measurement system in the form of a laser displacement measurement system is disclosed for more accurate positioning of the multiple particle beam. Correction lenses are disclosed for dynamically correcting focus, image field size, and image field rotation. No telecentricity correction is provided.

US 9,922,796 B1 discloses a multi-column microscope used for inspection of inclined or tilted specimens. For individual focusing of the single particle beams on the specimen, these beams pass through an objective lens array so that each single particle beam can be individually focused on the specimen surface.

DESCRIPTION OF THE INVENTION

It is consequently an object of the present invention to provide an improved multiple particle beam system for inspecting semiconductor wafers with HV structures and an associated method for operating the same. This should operate quickly and highly precisely.

It is a further object of the invention to provide a multiple particle beam system for inspecting semiconductor wafers with HV structures and an associated method for operating the same, which allows for additional fast autofocusing of the system at a working point with a specified working distance. In this case, other particle optical parameters such as the magnification, the telecentricity and the rotation should be kept constant with great precision.

The object is achieved by the independent patent claims. Advantageous embodiments of the invention are evident from the dependent patent claims.

The present patent application claims the priorities of German patent application 10 2020 125 534.9 filed on September 30th, 2020, the disclosure of which in the full scope thereof is incorporated into the present patent application by reference.

According to a first aspect of the invention, the latter relates to a multiple particle beam system for semiconductor inspection, comprising the following: a multi-beam particle generator, which is configured to generate a first field of a multiplicity of charged first individual particle beams; a first particle optical unit with a first particle optical beam path, which is configured to image the generated first individual particle beams onto a wafer surface in the object plane such that the first particle beams strike the wafer surface at incidence locations, which form a second field; a detection system with a multiplicity of detection regions that form a third field; a second particle optical unit with a second particle optical beam path, which is configured to image second individual particle beams, which emanate from the incidence locations in the second field, onto the third field of the detection regions of the detection system; a magnetic and/or electrostatic objective lens, in particular a magnetic and/or electrostatic immersion lens, through which both the first and the second individual particle beams pass;

a beam switch, which is arranged in the first particle optical beam path between the multi-beam particle generator and the objective lens and which 1s arranged in the second particle optical beam path between the objective lens and the detection system; a sample stage for holding and/or positioning a wafer during the wafer inspection; an autofocus determining element, which is configured to generate data for determining actual autofocus data during the wafer inspection; a fast autofocus correction lens; and a controller; wherein the controller is configured for static or low-frequency adaptation of a focusing in order to control at least the objective lens and/or an actuator of the sample stage at a first working point with a first working distance in such a way that the first individual particle beams are focused on the wafer surface situated at the first working distance, wherein the controller is configured for high-frequency adaptation of the focusing in order to generate an autofocus correction lens control signal on the basis of the actual autofocus data at the first working point during the wafer inspection in order to control the fast autofocus correction lens during the wafer inspection at the first working point; wherein the first working point is furthermore defined by a landing angle of the first individual particle beams in the object plane and by a grid arrangement of the first individual particle beams in the object plane, and wherein the controller is furthermore configured to keep the landing angle and the grid arrangement substantially constant during the high-frequency adaptation at the first working point.

The charged particles can be, e.g., electrons, positrons, muons or ions or other charged particles. Preferably, the charged particles are electrons generated, e.g, using a thermal field emission source (TFE). However, other particle sources can also be used.

The number of first individual particle beams is able to be chosen variably in this case. However, it is advantageous if the number of particle beams is 3n (n-1)+1,

where n is any natural number. This allows a hexagonal grid arrangement of the detection regions. Other grid arrangements of the detection regions, e.g., in a square or rectangular grid, are likewise possible. By way of example, the number of first individual particle beams is more than 5, more than 60 or more than 100 individual particle beams.

The multi-beam particle generator can comprise a plurality of real particle sources, which each emit an individual particle beam or else each emit a plurality of individual particle beams. However, the multi-beam particle generator might also comprise a single particle source and, in the downstream particle optical beam path, a multi-aperture plate in combination with a multi-lens array and/or a multi- deflector array. Then, as a result of the multi-beam particle generator, the multiplicity of individual particle beams are generated and imaged onto an intermediate image plane. This intermediate image plane can be a real intermediate image plane or a virtual intermediate image plane. In both cases, the locations of the individual particle beams in the intermediate image can be considered to be virtual particle sources and can consequently be considered to be origins for the further particle optical imaging using the first particle optical beam path. The virtual particle sources in this intermediate image plane are consequently imaged onto the wafer surface or into the object plane and the wafer to be inspected can be scanned using the multiplicity of individual particle beams.

If the objective lens system comprises a magnetic objective lens, the latter can provide a weak or a strong magnetic field. According to a preferred embodiment of the invention, the objective lens is a magnetic immersion lens. Here, this can be a weak immersion lens or strong immersion lens. Magnetic immersion lenses can be realized, for example, by virtue of the bore hole in the lower (sample-facing) pole shoe of the lens having a larger diameter than the bore hole in the upper (sample- distant) pole shoe of the lens. In contrast to objective lenses which provide only a low magnetic field at the object, immersion lenses have the advantage of being able to achieve lower spherical and chromatic aberrations, and also the disadvantage of greater off-axis aberrations. In the magnetic field of the lens, the individual particle beams passing therethrough experience a Larmor rotation (both in the primary path and in the secondary path).

According to the invention, a sample stage for holding and/or positioning a wafer during the wafer inspection is provided. Here, it is possible for the sample stage to have a mechanism for adjusting the height (e.g., z-stage) in order to set a working distance. However, there might not be an option for adjusting the height. Then, the sample stage only serves to hold the wafer but not to position the latter in the z-direction. In both cases, it is possible but not mandatory for the sample stage to be movable along one axis (e.g., x-axis, y-axis) or in a plane (e.g., xy-plane).

Further, an autofocus determining element is provided, which is configured to generate data for ascertaining actual autofocus data during the wafer inspection.

In this case, the actual autofocus data describe the current focal position with respect to the wafer surface directly or indirectly. By way of example, the autofocus determining element can comprise or consist of an autofocus measuring element.

The data can then be measurement data. However, it is additionally or alternatively also possible for the data for ascertaining the actual autofocus data to be generated on the basis of a model. By way of example, this is possible if there is a sufficiently exact model of the wafer to be scanned.

In principle, autofocus measuring elements are known from the prior art and are described in US 9 530 613 B2 and in US 2017/0117114 Al, for example, the disclosures of which are included in this application in full by reference. By way of example, use can be made of a height sensor (z-sensor). In principle, to ascertain the focal position, the current focal position of the individual particle beams with respect to the wafer surface is deduced by means of a measurement (deduction of actual autofocus data). Ideally, all foci are located exactly on the wafer surface.

The focal position of an individual particle beam is defined in this case by the position of the beam waist of a beam.

US 9 530 613 B2 discloses the use of astigmatic auxiliary beams for setting or adjusting the focus. Depending on the focusing present, the known astigmatic (e.g.,

elliptic) beam profile changes during the imaging. This change allows conclusions to be drawn about the focus and hence about the necessary focus corrections for the stigmatic beams.

US 2017/0117114 Al discloses an "on-the-fly"-type autofocus. In the process, the current focal position of the individual particle beams is deduced during scanning of a sample surface from the data of an image field (measured intensities) and a continuous/"on-the-fly" adjustment of the focus is implemented for the subsequent image field. In particular, it is not necessary here to scan the same sample region multiple times. An object property is determined in each case, optionally indirectly, by the measurement. By way of example, this object property can be a height profile of the sample surface. Then, for the subsequent image recording, a predicted value for the height 1s ascertained from the ascertained height profile and another, better adapted focus position with respect to the sample surface is set.

The multiple particle beam system according to the invention comprises a controller. The controller is configured to control particle optical components in the first and/or in the second particle optical beam path. Preferably, but not necessarily, the controller is a central controller for the entire multiple particle beam system. The controller can have a one-part or multi-part embodiment and can be functionally subdivided.

The controller is configured for static or low-frequency adaptation of the focusing in order to control at least the objective lens and/or an actuator of the sample stage at a first working point with a first working distance, in such a way that the first individual particle beams are focused on the wafer surface situated at the first working distance, and it is configured for high-frequency adaptation in order to generate an autofocus correction lens control signal on the basis of the actual autofocus data at the first working point during the wafer inspection in order to control the at least one fast autofocus correction lens during the wafer inspection at the first working point. In this case, for the high-frequency adaptation, control of the objective lens is preferably not altered; a change in the excitation of the objective lens is regularly only implemented in the case of a static or low-frequency adaptation of the focal position. In this case, the objective lens comprises at least one magnetic and/or at least one electrostatic objective lens, i.e, the objective lens can be embodied in the form of a corresponding objective lens system.

Thus, the controller controls two different focal settings at one working point which — optionally in addition to other parameters — is defined by an associated working distance between the objective lens and the wafer surface. Firstly, it controls with a significant stroke the focusing by way of a control of the objective lens and optionally further lenses and/or by way of a control of an actuator for displacing the sample stage. These final controlling elements react comparatively slowly to the control signal; in this case, an adaptation typically requires several ten to several hundred milliseconds and is required, in particular, when a working point with the chosen working distance is honed in on for the first time, for example when a wafer is changed. By way of example, the stroke for changing the working distance can be +/-100, +/-200 um or +/-300 um.

According to the invention, the controller secondly also controls the focal setting by way of controlling the fast autofocus correction lens according to the invention.

This lens can have different embodiments, for example it can be embodied as a fast electrostatic lens. Various embodiment variants and possible positions of the autofocus correction lens in the beam path will still be described in more detail below. It 1s also possible to provide for a plurality of autofocus correction lenses and for these to be controlled individually. In any case, an autofocus correction lens can be used for a quick adjustment and it acts on the relative focal position of the individual particle beams, wherein this effect may be quite pronounced or less pronounced. It is also possible for the autofocus correction lens to also exert an effect on other particle optical parameters in addition to the effect on the focus. In this case, quick means that the excitation of the autofocus correction lens allows a high-frequency adaptation of the relative focal position; an adaptation time TA is in the range of ps, for example TA < 500 us, preferably TA < 100 us and/or TA < us. The stroke for changing the working distance is typically several um, for example +/-20 um, +/-15 um and/or +/-10 pm.

According to a preferred embodiment of the invention, an adaptation time TA for the high-frequency adaptation is shorter than the adaptation time TA for the low- frequency or static adaptation at least by a factor of 10, preferably at least by a factor of 100 or 1000. Moreover, a stroke for setting the working distance for the low-frequency or static adaptation can be greater than the stroke for the high- frequency adaptation at least by a factor of 5, preferably at least by a factor of 8 and/or 10.

In the two adjustment variants for the focus, it may be necessary to also update other particle optical components of the system. For these corrections, too, the controller can provide appropriate control signals. In the case of the low-frequency or static adaptation, the final controlling elements can likewise be slowly adjustable final controlling elements or they can be quickly adjustable final controlling elements. In this case, the limiting elements in terms of time are the magnetic lenses, which include for example magnetic field lenses and also the magnetic objective lens, and/or the time for displacing the sample stage in the z- direction. In the case of the high-frequency adaptation, it is necessary for the other final control elements to also be essentially quickly adjustable. Here, their respective adaptation times are preferably of the same order of the adaptation time of the fast autofocus correction lens. By way of example, they can be slower at most by a factor of 2. However, they can also be faster than the adaptation time of the fast autofocus correction lens. By way of example, the fast additional final controlling elements can be electrostatic lenses, electrostatic deflectors and/or electrostatic stigmators. Air coils with only a few turns can also be used as fast correctors.

According to a preferred embodiment of the invention, a second working point is defined at least by a second working distance between the objective lens and the wafer surface, wherein the second working distance differs from the first working distance of the first working point. Then, the controller is configured to carry out a low-frequency adaptation in the case of a change between the first working point and the second working point and control at least the magnetic objective lens and/or an actuator of the sample stage at the second working point such that the first individual particle beams are focused on the wafer surface situated at the second working distance. By way of example, a change in the working point is implemented when a wafer is changed; the thickness of the wafers can be different in this case. A wafer change is a comparatively slow procedure, and so a slow adaptation is sufficient in this case. However, it is also possible, for example, to alter the working point or the working distance because the inspection task has changed.

Preferably, the controller is configured to generate an autofocus correction lens control signal for high-frequency adaptations on the basis of the actual autofocus data at the second working point with the second working distance during the wafer inspection in order to control the fast autofocus correction lens during the wafer inspection at the second working point. Furthermore, all the statements already made above in conjunction with the first working point at the first working distance apply to setting the fast autofocus at the second working point with the second working distance.

According to the invention, the first and/or the second working point are furthermore defined by a landing angle of the first individual particle beams in the object plane and by a grid arrangement of the first individual particle beams in the object plane. The controller is then configured to keep the landing angle and the grid arrangement substantially constant during the high-frequency adaptation at the first and/or second working point. In this case, the term grid arrangement comprises the pitch between the individual particle beams in the object plane and the rotation of the individual particle beam arrangement; by way of example, the grid arrangement can be present in the form of the aforementioned hexagon image field. Thus, when the grid arrangement is kept constant, both the magnification, which is coupled to the pitch of the individual particle beams, and the orientation of the second field of points of incidence of the individual particle beams in the object plane are kept constant. Here, the magnification is preferably kept constant to approximately 50 ppm, 20 ppm, 10 ppm, 1 ppm or better (e.g., 50 nm, 20 nm, 10 nm, 1 nm or better in the case of a 100 um image field size). The maximum angle deviation from the desired landing angle on the wafer surface is no more than +/-0.1°, +/-0.01° or +/-0.005°.

According to a further preferred embodiment of the invention, the controller is configured to keep the landing angle and the grid arrangement substantially constant even during a change between the first working point and the second working point. Thus, this relates to keeping the aforementioned parameters constant even in the case of a low-frequency adaptation of the focus. Here, the magnification is preferably kept constant to approximately 50 ppm, 20 ppm, 10 ppm, 1 ppm or better (e.g., 50 nm, 20 nm, 10 nm, 1 nm or better in the case of 100 pm). The maximum angle deviation from the desired landing angle on the wafer surface is no more than +/-0.1°, +/-0.01° or +/-0.005°.

The final controlling elements for adapting particle optical parameters such as, e.g. landing angle and grid arrangement (position or magnification and rotation) and, in particular, keeping these constant can, in full or in part, be the same for the low-frequency adaptation as for the high-frequency adaptation. However, if these are the same final controlling elements in full or in part, these final controlling elements must necessarily also be suitable for high-frequency adaptation.

According to a preferred embodiment of the invention, the autofocus correction lens comprises an electrostatic lens or consists of an electrostatic lens. As a matter of principle, settings of electrostatic lenses can be altered substantially faster than settings of magnetic lenses, in which hysteresis effects, eddy currents and self- and mutual inductances prevent a fast adaptation. According to the invention, an electrostatic lens can be provided as a complete lens, for example as a tube lens.

However, it is also possible that only an additional component in the form of an additional electrode is provided as autofocus correction lens, the latter developing its electrostatic lens effect in conjunction with other components or surrounding voltages.

The fast autofocus correction lens can be arranged at various positions in the first particle optical beam path, which offer different advantages and disadvantages.

What needs to be taken into account is, firstly, the available installation space in the overall system but also, secondly, the effect of the autofocus correction lens on other particle optical parameters than the focus. As already mentioned at the outset, a lens in multiple particle beam systems normally does not act only on a single particle optical parameter; as a rule, the effects of particle optical components are not orthogonal to one another. The inventors have examined these relationships in more detail and have discovered that there are a few positions in the particle optical beam path of multiple particle beam systems which have special properties: Normally, a crossover point or a crossover plane is provided in the primary beam path of a multiple particle beam system according to the invention, where the individual particle beams are superposed or cross one another. This crossover plane is normally situated just upstream of the objective lens. Comprehensive calculations have shown that an additional lens on the crossover substantially acts on the focus of the first individual particle beams and af at all) only acts weakly on other particle optical parameters such as position, telecentricity or rotation. Consequently, it is generally advantageous to arrange the autofocus correction lens at the crossover or in the crossover plane of the first individual particle beams. However, in practice, the crossover is not a singular point but has a spatial extent, and so it is often only possible to attain an arrangement of the autofocus correction lens close to the crossover/close to the crossover plane. According to the invention, there are a number of options to this end:

According to a preferred embodiment of the invention, the autofocus correction lens is arranged in a beam tube extension, which projects into the objective lens from the direction of the upper pole shoe. In general, the individual particle beams are guided within a beam tube. The latter is evacuated. Here, the beam extension tube is precisely the region of the beam tube which protrudes a little into the magnetic objective lens from the upper pole shoe. The beam tube is at ground potential, and so the autofocus correction lens or an associated electrode can be arranged well within the beam tube extension.

According to a preferred embodiment of the invention, a beam deflection system is furthermore provided between the beam switch and the objective lens and it is configured to raster-scan the wafer surface with a scanning movement of the individual particle beams, wherein the autofocus correction lens is realized as an offset on the beam deflection system. Typically, a beam deflection system ("deflection scanner" or “scan deflector") is realized by two or more deflectors arranged in succession in the beam path. Now, the offset voltage is provided at all electrodes involved in the deflection. Here, the lens effect arises as a result of the superposition of the deflection field with an Einzel lens field. The embodiment described offers the advantage that no further changes are required in terms of the hardware of the system.

According to one embodiment of the invention, the multiple particle beam system furthermore comprises a beam deflection system between the beam switch and the objective lens, configured to raster-scan the wafer surface with a scanning movement of the individual particle beams, wherein the beam deflection system comprises an upper deflector and a lower deflector arranged in succession in the direction of the beam path and wherein the autofocus correction lens is arranged between the upper deflector and the lower deflector. This embodiment is also simple to realize since only small changes have to be undertaken in the hardware of existing systems.

According to one embodiment of the invention, the multiple particle beam system furthermore comprises a beam deflection system between the beam switch and the objective lens, configured to raster-scan the wafer surface with a scanning movement of the individual particle beams, wherein the beam deflection system comprises an upper deflector and a lower deflector arranged in succession in the direction of the beam path and wherein the autofocus correction lens is arranged between the lower deflector and the upper pole shoe of the magnetic objective lens.

The autofocus correction lens is also close to the crossover plane in this embodiment variant.

According to a preferred embodiment of the invention, the autofocus correction lens is arranged between the wafer surface and a lower pole shoe of the magnetic objective lens. Although this position is no longer in the vicinity of the crossover and the effect of the lens no longer extends only very predominantly on the focus, this embodiment offers the advantage that the autofocus correction lens only has small subsequent aberrations as it normally is the last lens directly in front of the wafer surface.

According to another preferred embodiment of the invention, the autofocus correction lens is arranged between the upper and the lower pole shoe of the magnetic objective lens. This embodiment is likewise advantageous in that it is realized far toward the bottom of the beam path (autofocus correction lens as penultimate lens), and so only small subsequent aberrations arise in this case too.

According to a preferred embodiment of the invention, the multiple particle beam system furthermore comprises a beam tube that is able to be evacuated and that substantially surrounds the first particle optical beam path from the multi-beam particle generator to the objective lens, wherein the beam tube has an interruption and wherein the autofocus correction lens is arranged within this interruption.

Here, the beam tube is substantially tight in the aforementioned region, i.e, embodied such that a vacuum or high vacuum can be generated therein. It can have different cross sections and/or else chambers along the beam path. Here, the interruption in which the autofocus correction lens is arranged is preferably the only interruption in the beam tube. Apart from the locations of the interruption where the autofocus correction lens is situated, the inner wall of the beam tube is at ground potential. Possible connecting points/contact points between vacuum chambers and the actual beam tube should not be considered to be interruptions in this context.

According to a preferred embodiment of the invention, the multiple particle beam system furthermore comprises a field lens system arranged in the first particle optical beam path between the multi-beam particle generator and the beam switch, wherein the interruption of the beam tube in which the autofocus correction lens is arranged is arranged between the field lens system and the beam switch. This embodiment offers comparatively large amounts of space for the arrangement of the autofocus correction lens.

According to a preferred embodiment of the invention, the beam switch comprises two magnet sectors and the interruption of the beam tube in which the autofocus correction lens is arranged is provided in the region of the beam switch between the two magnet sectors. This embodiment offers comparatively large amounts of space for the arrangement of the autofocus correction lens.

According to a preferred embodiment of the invention, the multiple particle beam system furthermore comprises a beam deflection system between the beam switch and the objective lens, configured to raster-scan the wafer surface with a scanning movement of the individual particle beams, wherein the interruption of the beam tube in which the autofocus correction lens is arranged is provided between the beam switch and the beam deflection system. This embodiment offers comparatively large amounts of space for the arrangement of the autofocus correction lens.

According to a preferred embodiment of the invention, the multiple particle beam system furthermore comprises a field lens system which is arranged in the first particle optical beam path between the multi-beam particle generator and the beam switch. This field lens system can comprise one or more lenses. It comprises at least one magnetic field lens. In this embodiment of the invention, the interruption of the beam tube in which the autofocus correction lens is arranged is arranged within the one magnetic field lens of the field lens system. Comparatively large amounts of installation space are also available in this position. However, the autofocus correction lens in this position acts on the focus, the position and the tilt of the individual particle beams. Equally, it is advantageous that a position and/or beam tilts can (also) be compensated in this embodiment.

According to a preferred embodiment of the invention, the multiple particle beam system furthermore comprises a beam tube that is able to be evacuated and that substantially surrounds the first particle optical beam path from the multi-beam particle generator to the objective lens. In this case, the autofocus correction lens 1s embodied as a tube lens and arranged within the beam tube. Thus, the beam tube has no interruption or perforation, simplifying the sealing/tightness of the beam tube. Once again, there are a plurality of ways of implementing this embodiment variant, four of which are specified below:

According to a preferred embodiment of the invention, the multiple particle beam system furthermore comprises a field lens system which is arranged in the first particle optical beam path between the multi-beam particle generator and the beam switch, wherein the autofocus correction lens is arranged within the beam tube between the field lens system and the beam switch. This embodiment offers comparatively large amounts of space for the arrangement of the autofocus correction lens.

According to a preferred embodiment of the invention, the beam switch has two magnet sectors and the autofocus correction lens is provided within the beam tube between the two magnet sectors. This embodiment offers comparatively large amounts of space for the arrangement of the autofocus correction lens.

According to a preferred embodiment of the invention, the multiple particle beam system furthermore comprises a beam deflection system between the beam switch and the objective lens, configured to raster-scan the wafer surface with a scanning movement of the individual particle beams, wherein the autofocus correction lens is provided within the beam tube between the beam switch and the beam deflection system. This embodiment offers comparatively large amounts of space for the arrangement of the autofocus correction lens.

According to a preferred embodiment of the invention, the multiple particle beam system furthermore comprises a field lens system which is arranged in the first particle optical beam path between the multi-beam particle generator and the beam switch, wherein the autofocus correction lens is arranged within the beam tube within a magnetic field lens. This embodiment offers comparatively large amounts of space for the arrangement of the autofocus correction lens. In this position, the autofocus correction lens acts on the position and the tilt of the individual particle beams in addition to the focus. This facilitates (possibly additional) corrections of position and landing angle of the first individual particle beams.

According to a further embodiment of the invention, the fast autofocus correction lens comprises a fast magnetic lens, in particular an air coil, or consists of a fast magnetic lens, in particular an air coil. Such an air coil only has comparatively little inductance and can therefore, to a certain extent, also be used as a fast autofocus correction lens. By way of example, such an air coil has several ten to several hundred turns, for example 10 <k < 500 and/or 10 <k <200 and/or 10 <k < 50 applies to the number k of turns and the following may apply to the adaptation times TA of the air coil: TA < 500 us, preferably TA < 100 ‚us and/or TA < 50 us. In any case, this applies if the air coil is arranged so that no magnetic material, or at best little magnetic material, 1s situated in the vicinity thereof.

According to a preferred embodiment of the invention, the multiple particle beam system furthermore comprises a beam tube that is able to be evacuated and that substantially surrounds the first particle optical beam path from the multi-beam particle generator to the objective lens, wherein the fast magnetic lens is arranged outside around the beam tube. Thus, the beam tube need not be perforated or interrupted in this case. Producing this embodiment variant is comparatively simple.

According to a preferred embodiment of the invention, the multiple particle beam system furthermore comprises a field lens system which is arranged in the first particle optical beam path between the multi-beam particle generator and the beam switch, wherein the fast magnetic lens is arranged around the beam tube between the field lens system and the beam switch. Thus, the beam tube need not be perforated or interrupted in this case. Producing this embodiment variant is comparatively simple.

According to a preferred embodiment of the invention, the beam switch has two magnet sectors and the fast magnetic lens is arranged around the beam tube between the two magnet sectors. Thus, the beam tube need not be perforated or interrupted in this case. Producing this embodiment variant is comparatively simple.

According to a preferred embodiment of the invention, the multiple particle beam system furthermore comprises a beam deflection system between the beam switch and the objective lens, configured to raster-scan the wafer surface with a scanning movement of the individual particle beams, wherein the fast magnetic lens is arranged around the beam tube between the beam switch and the beam deflection system. Thus, the beam tube need not be perforated or interrupted in this case.

Producing this embodiment variant is comparatively simple.

According to a preferred embodiment of the invention, the multiple particle beam system furthermore comprises a beam deflection system between the beam switch and the objective lens, configured to raster-scan the wafer surface with a scanning movement of the individual particle beams, wherein the beam deflection system comprises an upper deflector and a lower deflector arranged in succession in the direction of the beam path; and wherein the fast magnetic lens is arranged around the beam tube between the upper deflector and the lower deflector. Thus, the beam tube need not be perforated or interrupted in this case. Producing this embodiment variant is comparatively simple.

According to a preferred embodiment of the invention, the multiple particle beam system moreover comprises a fast telecentricity correction means, which is configured to substantially contribute to correcting a tangential or radial telecentricity error of the first individual particle beams in the second field, and the controller of the multiple particle beam system is set up to generate a telecentricity correction means control signal for high-frequency adaptations at the respective working point during the wafer inspection on the basis of the actual autofocus data in order to control the fast telecentricity correction means during the wafer inspection. As already explained above, a fast adaptation of other particle optical components is often also required within the scope of fast autofocusing in order to be able to keep other particle optical parameters constant.

One of these parameters is the telecentricity or the landing angle of first individual particle beams on the wafer surface (the terms telecentricity and landing angle are used synonymously in this patent application). Here, when applying an element provided for the telecentricity correction, it is also the case that this element does not necessarily only act on the telecentricity but once again interacts with other particle optical parameters on account of the non-orthogonality of the effects of the particle optical components. Therefore, within the scope of this patent application, the fast telecentricity correction means is defined as intended to act substantially — and hence not necessarily exclusively — on the telecentricity. Then, an essential effect relates to the telecentricity. Strictly speaking, it is also possible for a fast autofocus correction lens to (also) be a fast telecentricity correction means, and vice versa.

How the tangential telecentricity error and a rotation error, which are generated by an immersion lens as magnetic objective lens, arise is explained below: In a reference arrangement of the magnetic immersion lens with a first imaging scale and a first focal plane in the magnetic field of the magnetic immersion lens, a first grid arrangement with a first beam pitch or pitch of the first individual particle beams and with a first orientation is formed in the object plane. In the process, charged particles in the magnetic field of the magnetic immersion lens are steered onto helical trajectories. Reference is made to a magnetic immersion lens if the magnetic field of an objective lens extends up to the sample or the object, for example a semiconductor wafer. The grid arrangement of the beam foci in the object plane, in which a wafer is arranged for example, is also rotated as a result of the helical particle trajectories. To generate a first grid arrangement in the object plane with a desired, predefined orientation, the twist or rotation of the grid arrangement is usually held in reserve, for example by arranging a generating device of the grid arrangement (e.g., in the form of a multi-aperture plate as a constituent part of a multi-beam particle generator) in a predetermined, pre- rotated position, which counters the rotation due to the magnetic immersion lens.

First individual particle beams also receive a tangential velocity component which,

in the case of an immersion lens, leads to the individual particle beams no longer being incident on a sample in perpendicular fashion but in a manner tilted or inclined to a perpendicular of the sample surface in the tangential direction.

Particularly in the case of a multi-beam system, first individual particle beams have different tangential inclination angles, which increase with the distance from the optical axis of the magnetic immersion lens in the radial direction. This error is referred to as tangential telecentricity error. Usually, the tangential telecentricity error can be compensated by virtue of an appropriate tangential velocity component of the first individual particle beams being generated upstream of the magnetic immersion lens in targeted fashion, said tangential velocity component counteracting the tangential telecentricity error and compensating the latter at the wafer surface.

A change in the excitation of the magnetic immersion lens, a change in the relative focal position or a change in the imaging scale of the first grid arrangement of the multiplicity of first individual particle beams leads to unwanted, parasitic effects.

By way of example, a tangential and/or radial telecentricity error is generated by each of the aforementioned changes.

Each of the aforementioned changes alters the fraction of a revolution of the helical electron trajectories or the rotation angle of the rotation of the grid arrangement.

Consequently, a second grid arrangement of the multiplicity of primary electron beams is formed, which is rotated counter to the first grid arrangement. This rotation is unwanted and, according to the invention, compensated by means for changing the rotation of the grid arrangement.

According to a preferred embodiment of the invention, the telecentricity correction means comprises a first deflector array which is arranged in an intermediate image plane of the first particle optical beam path. By way of example, such a deflector array is known from DE 10 2018 202 421 B3 and WO 2019/243349 Al; the disclosure of both documents is incorporated into this patent application in full by reference. Here, a deflector array comprises a multiplicity of deflectors arranged in an array, wherein a group of individual particle beams passes through each of the deflectors during operation. Here, a group may also consist of only one individual particle beam.

According to a preferred embodiment of the invention, the multiple particle beam system moreover comprises a fast rotation correction means, which is configured to substantially contribute to correcting a rotation of the first individual particle beams in the second field, wherein the controller is set up, during the wafer inspection at the respective working point, to generate a rotation correction means control signal for high-frequency adaptations on the basis of the actual autofocus data in order to control the fast rotation correction means during the wafer inspection. The rotation correction means does not necessarily act only on the rotation but instead, once again, interacts with other particle optical parameters on account of the non-orthogonality of the effects of the particle optical components.

Therefore, within the scope of this patent application, the fast rotation correction means is defined as intended to act substantially — and hence not necessarily exclusively — on the rotation. Then, an essential effect relates to the rotation.

Strictly speaking, it is also possible for a fast autofocus correction lens to (also) be a fast rotation correction means, and vice versa.

According to a preferred embodiment of the invention, the rotation correction means comprises an air coil. By way of example, such an air coil has several ten to several hundred turns, for example 10 < k < 500 and/or 10 <k <200 and/or 10 <k < 50 applies to the number k of turns and the following may apply to adaptation times TA of the air core coil: TA <500 ps, preferably TA < 100 us and/or TA < 50 ps.

In any case, this applies if the air coil is arranged so that no magnetic material, or at best little magnetic material, is situated in the vicinity thereof.

According to a preferred embodiment of the invention, the rotation correction means comprises a second deflector array which, at a distance, is arranged directly upstream or downstream of the first deflector array which acts as fast telecentricity correction means. Thus, in this embodiment, a further deflector array is arranged, at a distance, upstream or downstream of the deflector array for telecentricity correction, said further deflector array causing a change in the focal position on the wafer surface as a result of deflecting individual beams and hence, in total, causing a rotation of the grid arrangement by way of an appropriate control. The openings of the respective downstream deflector array are formed correspondingly larger in this case and designed for a beam deflection of the preceding deflector array. Consequently, a compensation of rotation and telecentricity error is facilitated by way of two deflector arrays arranged in succession.

According to a preferred embodiment of the invention, the rotation correction means comprises a multi-lens array which is arranged, at a distance, directly upstream or downstream of the first deflector array which acts as a telecentricity correction means, in such a way that the first individual particle beams pass through the multi-lens array in off-axis fashion. Hence, a deflecting effect also arises in addition to a focusing effect. As a result of an offset of an individual particle beam in the tangential direction 1n relation to an axis of a microlens, the individual particle beam is deflected in the tangential direction. By way of example, the tangential beam offset can be set by an upstream deflector array or by a rotation of the multi-lens array with respect to the grid arrangement. A change in the tangential beam deflection can be generated by an active deflector array upstream of the multi-lens array or by a multi-lens array with a variable refractive power. Then, the deflection angle also changes with the change in the refractive power. The change in the refractive power can be compensated by a further electrostatic lens, which for example acts on all individual particle beams.

A further option lies in an active rotation of the multi-lens array through a few mrad. Since the deflection is amplified by the lens effect, a rotation angle for rotating the multi-lens array can be smaller than the rotation angle of the rotation of the grid arrangement.

According to a further preferred embodiment of the invention, the multi-beam particle generator comprises the fast rotation correction means and the rotation correction means is actively rotated by the rotation correction means control signal. By way of example, the multi-beam particle generator contains at least one deflector array or at least one multi-lens array. A twist of the grid arrangement can be brought about by appropriate active rotation of the entire multi-beam particle generator or the entire generating device of the grid arrangement or by active rotation of individual array components.

According to a preferred embodiment of the invention, the fast rotation correction means comprises a first magnetic field generating device for a first weak magnetic field and a second magnetic field generating device for a second weak magnetic field, wherein the first magnetic field generating device is only controlled for a rotation in a positive rotation direction and the second magnetic field generating device is only controlled for a rotation in a negative rotation direction by the controller by means of the rotation correction means control signal. Since a compensation of the twist or rotation of the grid arrangement must be very fast in conjunction with a fast autofocus, individual magnetic elements are unsuitable to this end. However, the inventors have discovered that a fast rotation of a grid arrangement together with a change in the focal position can be achieved using at least two magnetic elements by virtue of using each of the magnetic elements for rotating in one direction only. Hysteresis is avoided by two magnetic components which are each operated in one direction only and consequently a fast rotation of the grid arrangement in two rotation directions is possible. Both components can be reset in brief breaks between inspection tasks, for example while positioning the wafer from a first inspection site to a second inspection site. Thus, for example, an axial magnetic field for rotation in the positive direction can be combined with a magnetic immersion lens at the exit of the pencil of the primary beams from the generating device for rotation in the negative direction.

According to a preferred embodiment of the invention, the first and the second magnetic field have an axial configuration and are arranged in a converging or diverging pencil of the first individual particle beams in the first particle optical beam path. Such arrangements and the underlying physical effects are described, for example, in the German patent application with the application number 10 2020 123 567.4, which was not yet laid open at the time of this application and which was filed on September 9, 2020, the disclosure of which is incorporated in this application in full by reference.

According to a preferred embodiment of the invention, a maximum deviation of each individual particle beam from a desired landing position on the wafer surface 1s no more than 10 nm, 5 nm, 2 nm, 1 nm or 0.5 nm. This is an absolute maximum deviation — it applies to any direction on the wafer surface (which is planar or approximated as planar) and can be ensured, in particular, by means of one or more of the above-described means for telecentricity correction and/or rotation correction and/or position correction.

According to a preferred embodiment of the invention, the controller is configured to carry out the determination of the autofocus correction lens control signal and/or the rotation correction means control signal and/or the telecentricity correction means control signal on the basis of the actual autofocus data using an inverted sensitivity matrix which describes the influence of excitation changes of particle optical components on particle optical parameters that characterize the particle optical imaging at the respective working point. Such an inverted sensitivity matrix 1s described in the German patent application DE 10 2014 008 383 Al, the disclosure of which is incorporated in this patent application in full by reference.

The change of the effect of only one particle optical component in a multi-beam particle optical unit leads to a change in a plurality of parameters which characterize the particle optical imaging. However, in practice, it is desirable for settings of the particle optical unit to be changed such that only one parameter which characterizes the particle optical imaging changes as a result of the change in the setting while the remaining parameters remain unchanged. To this end, it is necessary to change the settings of effects of the plurality of particle optical components together. To determine which settings have to be changed to change only one parameter, and to determine how these changes have to be implemented, it is possible, for example, to determine the entries of a matrix A, which describes these setting changes, from m x n measurements. Here, n corresponds to the number of particle optical components and m corresponds to the number of parameters which characterize the particle optical imaging. After determining the entries, this matrix can then be inverted and it is possible to determine which excitation changes have to be undertaken on what particle optical components in order to change precisely one parameter which describes the particle optical imaging.

According to a preferred embodiment of the invention, the controller of the multiple particle beam system is furthermore configured for a static or low-frequency adaptation of a focusing in the second particle optical beam path in order, at the respective working point with the associated working distance, to control particle optical components in the second particle optical beam path in such a way that the second individual particle beams, which emanate from the wafer surface situated at the respective working distance, are focused on the detection regions in the third field. By way of example, the particle optical components which can be used for setting the focus and/or further particle optical parameters describing the particle optical imaging in the second particle optical beam path can be a projection lens system. The particle optical components and, in particular, the projection lens system can also comprise a magnetic lens or a plurality of magnetic lenses, the effect(s) of which can be adjusted comparatively slowly by the controller. Other and/or further magnetic and/or electrostatic lenses, deflectors and/or stigmators can also be controlled by the controller to set the focus and/or other parameters such as the magnification (pitch of the second individual particle beams in the detection plane, position), the rotation and/or the telecentricity at the respective working point with a specified working distance. It is possible for the control of some or all components to be implemented quickly and not slowly (at a low frequency); however, a fast control is not necessary in the secondary path for the basic adjustment at the first working point.

According to a preferred embodiment of the invention, the multiple particle beam system furthermore comprises a fast projection path correction means, which may have a multi-part embodiment and which is configured to undertake a high- frequency adaptation of the focus of the second individual particle beams, of the grid arrangement, of landing angles and/or of the contrast of the second individual particle beams upon incidence on the detection regions in the third field. Here, the controller is configured to generate a projection path control signal or a set of projection path control signals on the basis of the actual autofocus data during the wafer inspection at the respective working point in order to control the fast projection path correction means. The set of projection path control signals is generated, in particular, if the projection path correction means has a multi-part embodiment and the components thereof are controlled separately.

In particular, the high-frequency adaptations in the secondary path are necessary if the second individual particle beams, which emanate from the wafer surface, also pass through the fast autofocus correction lens. This is because the latter also has an influence on the trajectory of the second individual particle beams in this case. However, even if the second individual particle beams do not pass through the fast autofocus correction lens, a resetting of the focus and/or other parameters describing the particle optical imaging in the secondary path may be implemented or may become necessary in the secondary path. In the secondary path, it is normally desirable for the second individual particle beams to be incident on the detection regions in focused fashion and with predetermined landing angles, in particular in telecentric fashion, and with a predetermined grid arrangement (pitch of the incidence locations and orientation of the incidence locations in the third field). Therefore, a high-frequency adaptation of fast particle optical components is also advantageous in the secondary path. The manner of the adaptation can be implemented here substantially analogously to the procedure in the primary path. Here, too, the particle optical components described above in conjunction with the primary beams, or else other components, can be used to undertake — optionally after an appropriate orthogonalization — fast/high- frequency corrections in the beam profile of the second individual particle beams.

By way of example, a further fast autofocus correction lens can be arranged in the (pure) secondary beam path, 1.e., between the beam switch and the detection unit.

By way of example, this can be a fast electrostatic lens or a fast magnetic lens, in particular in the form of an air coil with only a few turns. By way of example, this second autofocus correction lens can be arranged in the region of a crossover plane in the secondary path. By way of example, such a crossover plane in the secondary path is arranged in the region of the projection lens system in the secondary path.

However, a different arrangement of the second autofocus correction lens in the secondary path is also possible. By way of example, the fast telecentricity correction means described in conjunction with the primary path can also be used in the secondary path; in said means, for example, a deflector array is arranged in an intermediate image plane in the secondary path. It is also possible, as described for the primary path, to use a rotation correction means which, for example in the form of a further deflector array, can be arranged directly upstream or downstream of the deflector array for correcting the telecentricity in the secondary path.

According to the embodiment described, the generation of the projection path control signals is based on the ascertained actual autofocus data for the first particle optical beam path. To this end, work can be carried out using empirical values/lookup tables, for example, which directly or indirectly assign to the actual autofocus data necessary corrections for the focus on the detector and/or for other parameters in the secondary path. The associated control signals/the set of control signals can be stored.

According to a further embodiment of the invention, the multiple particle beam system furthermore comprises a projection path measuring element for generating projection path measurement data for characterizing the particle optical imaging in the secondary path during the wafer inspection, wherein the multiple particle beam system furthermore has a fast projection path correction means, which may have a multi-part embodiment and which is configured to undertake a high- frequency adaptation of the focus of the second individual particle beams, of the grid arrangement, of landing angles and/or of the contrast of the second individual particle beams upon incidence on the detection regions in the third field, and wherein the controller is configured to generate a projection path control signal or a set of projection path control signals on the basis of the projection path measurement data during the wafer inspection at the respective working point in order to control the fast projection path correction means. Thus, the controller does not or not only use the actual autofocus data for the high-frequency/fast adaptation of the particle optical components in this embodiment variant of the invention; instead, the measurement data in the secondary path are used for the high- frequency adaptation. Fast measurement methods, which supply data for an adaptation "on-the-fly", are already known, in principle, from the prior art. Data for a high-frequency adaptation can be ascertained for example by way of evaluating images of a CCD camera, which are recorded in addition to the scanned images that are obtained by means of the detection regions in the third field. By means of known measuring methods, it is possible, in particular, to determine the current relative focal position, the landing angle and/or the grid arrangement in the third field upon incidence on the detection regions.

There may be a particular requirement on the second particle optical beam path in respect of the topography contrast: It is possible to provide a contrast aperture stop within a crossover plane in the second particle optical beam path. A ring- shaped stop can be used to filter the interaction products in accordance with their starting angle upon emergence from the wafer. Then, only those second individual particle beams that have left the wafer surface within a certain angular range can pass through the contrast aperture stop. The topography contrast can be increased by means of such a contrast aperture stop since the interaction products (e.g., secondary electrons) predominantly emerge at a greater inclination angle relative to the incident particles at edges of the wafer surface. Further information in respect of contrast settings and in respect of aperture stops can be gathered from

DE 10 2015 202 172 B4 and US 2019/0355544 A1, the disclosures of which are each incorporated in this application in full by reference. According to a preferred embodiment of the invention, a contrast aperture stop is arranged in the second particle optical beam path in a crossover plane, wherein the projection path correction means comprises a fast contrast correction means with at least one electrostatic deflector, at least one electrostatic lens and/or at least one electrostatic stigmator for influencing the particle optical beam path through the contrast aperture stop, and wherein the controller is configured to control the contrast correction means using a contrast correction control signal or a set of contrast correction control signals, in such a way that a contrast of the second individual particle beams is kept substantially constant upon incidence on the detection regions in the third field. What can be achieved by means of the electrostatic components of the fast contrast correction means is a high-frequency adaptation and, in particular, a constancy of the contrast. Here, the contrast correction control signal can be determined for example on the basis of the projection path measurement data of the secondary path and/or on the basis of the actual autofocus data of the primary path.

All the aforementioned explanations apply not only to fast autofocusing but also to fast auto-stigmation. By definition, focusing also comprises a stigmation within the scope of this application. In principle, a stigmation can be physically equated to focusing in only one direction or with different focusings in different directions.

The number of particle optical parameters describing the particle optical imaging is increased or doubled if a stigmation is taken into account: By way of example, two parameters for the focus and two parameters for the position, two parameters for the landing angle and two parameters for the rotation are required in each case.

In this context, reference is also made to fast multi-pole lenses which are described, for example, in the German patent application with application number 10 2020 107 738.6, filed on March 20, 2020, which has not yet been laid open; the disclosure of said patent application is incorporated in the present patent application in full by reference.

According to an example, a multiple particle beam system for semiconductor inspection, comprising the following: a multi-beam particle generator, which is configured to generate a first field of a multiplicity of charged first particle beams; a first particle optical unit with a first particle optical beam path, which is configured to image the generated individual particle beams onto a wafer surface in the object plane such that the first particle beams strike the wafer surface at incidence locations, which form a second field; a detection system with a multiplicity of detection regions that form a third field; a second particle optical unit with a second particle optical beam path, which is configured to image second individual particle beams, which emanate from the incidence locations in the second field, onto the third field of the detection regions of the detection system; a magnetic and/or electrostatic objective lens, in particular a magnetic and/or electrostatic immersion lens, through which both the first and the second individual particle beams pass;

a beam switch, which is arranged in the first particle optical beam path between the multi-beam particle generator and the objective lens and which 1s arranged in the second particle optical beam path between the objective lens and the detection system; a sample stage for holding and/or positioning a wafer during the wafer inspection; an autofocus determining element, which is configured to generate data for determining actual autofocus data during the wafer inspection; a fast autofocus correction lens; and a controller; wherein the controller is configured for static or low-frequency adaptation of a focusing in order to control at least the magnetic objective lens and/or an actuator of the sample stage at a first working point with a first working distance, in such a way that the first individual particle beams are focused on the wafer surface situated at the first working distance.

Thus, in this embodiment of the invention the controller is configured to set the focusing for a given first working point which is assigned a first working distance.

Thus, it is possible by means of the system to adjust the working point as described and then set the focusing.

According to an example, the controller is furthermore configured for high- frequency adaptation of the focusing in order to generate an autofocus correction lens control signal on the basis of the actual autofocus data at the first working point during the wafer inspection in order to control the fast autofocus correction lens during the wafer inspection at the first working point.

Otherwise, everything that was defined and/or described in conjunction with the first aspect of the invention also applies to the described example.

According to a second aspect of the invention, the latter relates to a method for operating a multiple particle beam system, in particular a multiple particle beam system as described in conjunction with the first aspect of the invention. All terms and definitions explained or introduced in conjunction with the first aspect of the invention also apply to the method according to the invention. The method for operating a multiple particle beam system includes the following steps: - generating measurement data at a first working point for a current focus on the wafer surface; - determining actual autofocus data on the basis of the measurement data; - determining an autofocus correction lens control signal on the basis of the actual autofocus data; and - controlling a fast autofocus correction lens system with a first fast autofocus correction lens and keeping the focus on the wafer surface constant in high- frequency fashion, wherein the landing angle and the grid arrangement of the first individual particle beams upon incidence on the wafer surface are likewise kept constant at the first working point.

According to a preferred embodiment of the invention, the fast autofocus correction lens comprises at least one electrostatic lens and/or consists of exactly one electrostatic lens. What was already stated in conjunction with the multiple particle beam system according to the invention applies in respect of the options for configuring the electrostatic lens and the placements thereof in the beam path.

According to another preferred embodiment of the invention, the fast autofocus correction lens comprises at least one fast magnetic lens, in particular an air coil, and/or consists of exactly one magnetic lens. What was already stated in conjunction with the multiple particle beam system according to the invention applies in respect of the options for configuring the magnetic lens and the placements thereof in the beam path.

To keep the grid arrangement on the wafer surface and the landing angle constant, it is possible — as described above in conjunction with the first aspect of the invention — to use a fast telecentricity correction means and/or a fast rotation correction means and/or a fast position correction means. The fast telecentricity correction means, the fast rotation correction means and/or the fast position correction means then forms/form the autofocus correction lens system together with the autofocus correction lens with optionally a multi-part embodiment.

According to a preferred embodiment of the invention, the method furthermore includes the following steps: - generating a telecentricity correction control signal on the basis of the actual autofocus data; and - controlling the fast telecentricity correction means.

According to a preferred embodiment of the invention, the method furthermore includes the following steps: - generating a rotation correction control signal on the basis of the actual autofocus data; and - controlling the fast rotation correction means.

According to a preferred embodiment of the invention, the method furthermore includes the following steps: - orthogonalizing effects of the particle optical components which are used for the correction or corrections of beam parameters.

According to a preferred embodiment of the invention, the method furthermore includes the following steps: - generating projection path measurement data for characterizing the particle optical imaging in the secondary path; - determining a projection path control signal or a set of projection path control signals on the basis of the projection path measurement data; and - controlling a fast projection path correction means, which may have a multi- part embodiment, by means of the projection path control signal or by means of the set of projection path control signals, wherein the focus, the grid arrangement and the landing angle of the second individual particle beams upon incidence in a detection plane are kept constant at the first working point.

Thus, when the focus is kept constant, the relative focal position is updated while the grid arrangement and the landing angle are kept constant.

According to a preferred embodiment of the invention, the method furthermore includes the following step: - controlling a fast contrast correction means by means of a contrast correction control signal or a set of contrast correction control signals and keeping the contrast constant in the detection plane.

By controlling the fast contrast correction means, it is also possible to influence the relative position of the crossover in the secondary path in a targeted fashion, in particular to keep the latter constant.

According to a further aspect of the invention, the latter relates to a computer program product having a program code for carrying out the method as described above in conjunction with the third and fourth aspect of the invention.

The above-described embodiments of the invention in accordance with the above- described aspects of the invention can be combined with one another in full or in part, provided that no technical contradictions arise as a result.

The invention will be understood even better with reference to the accompanying figures. In the figures: fig. 1: shows a multi-beam particle microscope in a schematic illustration; fig. 2: shows a schematic illustration of an excerpt of a controller of the multi-beam particle microscope with a fast autofocus correction lens; fig. 3: shows a schematic illustration of a larger excerpt of a controller of the multi-beam particle microscope with a fast autofocus correction lens; fig. 4: schematically shows a method for setting a fast autofocus by means of an autofocus correction lens; fig. 5: schematically shows a section through a multi-beam particle microscope in which the autofocus correction lens according to the invention can be arranged; fig. 6: schematically illustrates an embodiment of the invention with an autofocus correction lens;

fig. 7: schematically illustrates an embodiment of the invention with an autofocus correction lens; fig. 8: schematically illustrates an embodiment of the invention with an autofocus correction lens; fig. 9: schematically illustrates an embodiment of the invention with an autofocus correction lens; fig. 10: schematically illustrates an embodiment of the invention with an autofocus correction lens; fig. 11: schematically illustrates an embodiment of the invention with an autofocus correction lens; fig. 12: schematically illustrates further embodiments of the invention with an autofocus correction lens; fig. 13: schematically illustrates further embodiments of the invention with an autofocus correction lens; fig. 14: schematically illustrates an embodiment of the invention with an autofocus correction lens; fig. 15: schematically illustrates an embodiment of the invention with an autofocus correction lens; fig. 16: schematically illustrates an embodiment of the invention with an autofocus correction lens; fig. 17: schematically illustrates an embodiment of the invention with an autofocus correction lens;

Below, the same reference signs denote the same features, even if these are not explicitly mentioned in the text.

Figure 1 is a schematic illustration of a particle beam system 1 in the form of a multi-beam particle system 1, which uses a multiplicity of particle beams. The particle beam system 1 generates a multiplicity of particle beams which strike an object to be examined in order to generate there interaction products, e.g., secondary electrons, which emanate from the object and are subsequently detected.

The particle beam system 1 is of the scanning electron microscope (SEM) type, which uses a plurality of primary particle beams 3 which are incident on a surface of the object 7 at a plurality of locations 5 and generate there a plurality of electron beam spots, or spots, that are spatially separated from one another. The object 7 to be examined can be of any desired type, e.g., a semiconductor wafer, in particular a semiconductor wafer with HV structures (i.e., with horizontal and/or vertical structures), or a biological sample, and can comprise an arrangement of miniaturized elements or the like. The surface of the object 7 1s arranged in a first plane 101 (object plane) of an objective lens 102 of an objective lens system 100.

The enlarged excerpt I; in figure 1 shows a plan view of the object plane 101 having a regular rectangular field 103 of incidence locations 5 formed in the first plane 101. In figure 1, the number of incidence locations is 25, which form a 5 x 5 field 103. The number 25 of incidence locations is a number chosen for reasons of simplified illustration. In practice, the number of beams, and hence the number of incidence locations, can be chosen to be significantly greater, such as, for example, x 30, 100 x 100 and the like.

In the embodiment illustrated, the field 103 of incidence locations 5 is a substantially regular rectangular field having a constant pitch Pi between adjacent incidence locations. Exemplary values of the pitch P; are 1 micrometer, 20 10 micrometers and 40 micrometers. However, it is also possible for the field 103 to have other symmetries, such as a hexagonal symmetry, for example.

A diameter of the beam spots shaped in the first plane 101 can be small. Exemplary values of said diameter are 1 nanometer, 5 nanometers, 10 nanometers, 100 nanometers and 200 nanometers. The focusing of the particle beams 3 for shaping the beam spots 5 1s carried out by the objective lens system 100. In this case, the objective lens system can comprise a magnetic immersion lens, for example.

The primary particles striking the object generate interaction products, e.g., secondary electrons, back-scattered electrons or primary particles that have experienced a reversal of movement for other reasons, which emanate from the surface of the object 7 or from the first plane 101. The interaction products emanating from the surface of the object 7 are shaped by the objective lens 102 to form secondary particle beams 9. The particle beam system 1 provides a particle beam path 11 for guiding the multiplicity of secondary particle beams 9 to a detector system 200. The detector system 200 comprises a particle optical unit with a projection lens 205 for directing the secondary particle beams 9 at a particle multi-detector 209.

The excerpt I2 in figure 1 shows a plan view of the plane 211, in which individual detection regions of the particle multi-detector 209 on which the secondary particle beams 9 are incident at locations 213 are located. The incidence locations 213 lie in afield 217 with a regular pitch P2 with respect to one another. Exemplary values of the pitch P2 are 10 micrometers, 100 micrometers and 200 micrometers.

The primary particle beams 3 are generated in a beam generation apparatus 300 comprising at least one particle source 301 (e.g., an electron source), at least one collimation lens 303, a multi-aperture arrangement 305 and a field lens 307, or a field lens system made of a plurality of field lenses. The particle source 301 generates at least one diverging particle beam 309, which is collimated or at least substantially collimated by the at least one collimation lens 303 in order to shape a beam 311 which illuminates the multi-aperture arrangement 305.

The excerpt Is in figure 1 shows a plan view of the multi-aperture arrangement 305. The multi-aperture arrangement 305 comprises a multi-aperture plate 313, which has a plurality of openings or apertures 315 formed therein. Midpoints 317 of the openings 315 are arranged in a field 319 that is imaged onto the field 103 formed by the beam spots 5 in the object plane 101. A pitch P3; between the midpoints 317 of the apertures 315 can have exemplary values of 5 micrometers, 100 micrometers and 200 micrometers. The diameters D of the apertures 315 are smaller than the pitch P3 between the midpoints of the apertures. Exemplary values of the diameters D are 0.2 x P3, 0.4 x P3 and 0.8 x Py.

Particles of the illuminating particle beam 311 pass through the apertures 315 and form particle beams 3. Particles of the illuminating beam 311 which strike the plate 313 are absorbed by the latter and do not contribute to the formation of the particle beams 3.

On account of an applied electrostatic field, the multi-aperture arrangement 305 focuses each of the particle beams 3 in such a way that beam foci 323 are formed in a plane 325. Alternatively, the beam foci 323 can be virtual. A diameter of the beam foci 323 can be, for example, 10 nanometers, 100 nanometers and 1 micrometer.

The field lens 307 and the objective lens 102 provide a first imaging particle optical unit for imaging the plane 325, in which the beam foci 323 are formed, onto the first plane 101 such that a field 103 of incidence locations 5 or beam spots arises there. Should a surface of the object 7 be arranged in the first plane, the beam spots are correspondingly formed on the object surface.

The objective lens 102 and the projection lens arrangement 205 provide a second imaging particle optical unit for imaging the first plane 101 onto the detection plane 211. The objective lens 102 is thus a lens that is part of both the first and the second particle optical unit, while the field lens 307 belongs only to the first particle optical unit and the projection lens 205 belongs only to the second particle optical unit.

A beam switch 400 is arranged in the beam path of the first particle optical unit between the multi-aperture arrangement 305 and the objective lens system 100.

The beam switch 400 is also part of the second optical unit in the beam path between the objective lens system 100 and the detector system 200.

Further information relating to such multi-beam particle beam systems and components used therein, such as, for instance, particle sources, multi-aperture 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 Al and DE 10 2013 014 976 A1, the disclosure of which in the full scope thereof is incorporated by reference in the present application.

The multiple particle beam system furthermore comprises a computer system 10 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 multi-detector 209. In this case, the computer system 10 can be constructed from a plurality of individual computers or components. It can also control the fast autofocus correction lens according to the invention and the telecentricity correction means and/or the fast rotation correction means and/or further fast correction means (none of which are illustrated in figure 1).

Figure 2 shows a schematic illustration of an excerpt of a controller of the computer system 10 of the multi-beam particle microscope 1 with a fast autofocus correction lens 824. Specifically, the excerpt shows the controller 821 for the fast autofocus.

The controller 821 for the fast autofocus is set up to carry out high-frequency adaptations of the focusing at a working point during the wafer inspection. This means that adaptations of the focusing can be carried out very quickly, for example within a few microseconds. In addition to the overarching control system 821 (a part of the computer system 10 in this case), the following further components are provided for these fast adaptations: a measuring element 822, an autofocus algorithm 823 for processing the measurement data, and at least one final controlling element which is set in accordance with the processing of the measurement data. In the specific example, a final controlling element is provided by the autofocus correction lens 824. Additional fast final controlling elements, specifically a telecentricity correction means 825, a fast rotation correction means 826 and a fast position correction means 827 in this case, are likewise provided in this example. In this case, these additional final controlling elements can be formed by further fast autofocus correction lenses; however, they can also be configured differently to fast lenses. The measuring element 822 is configured to generate measurement data for determining actual autofocus data during the wafer inspection. In this case, the actual autofocus data describe the current position of the focus with respect to the wafer surface directly or indirectly. Instead of the autofocus measuring element 822, provision can also be made more generally of an autofocus determining element which generates data for determining actual autofocus data, for example on the basis of a model of a wafer.

In principle, autofocus measuring elements are known from the prior art.

Examples in this respect are the use of astigmatic auxiliary beams for setting the focus and height measurements at a sample surface (e.g., by means of a z-sensor). What is important is that the measuring member 822 or measuring members 822 also allow the determination of continuous, i.e., ongoing "on-the-fly" settings of the focus for each image field obtained in each case by means of the multiplicity of individual particle beams.

Now — depending on the measuring element 822 and manner of evaluation — the autofocus algorithm 823 is set up to generate actual autofocus data from the measurement data and to generate an autofocus correction lens control signal on the basis of the actual autofocus data in order to control the fast autofocus correction lens 824 in high-frequency fashion during the wafer inspection at a working point.

As a result, the relative focal position is adapted.

As already explained multiple times, the effects of particle optical components of a multiple particle beam system are normally not orthogonal to one another.

This means that a variation of an effect at only one particle optical component normally does not allow only a single parameter characterizing the particle optical imaging to be altered.

Instead, the system is more complex and changing a parameter of the particle optical imaging normally requires a variation of effects at a plurality of particle optical components.

In the specific case, this means that a readjustment/fine setting of the relative focal position is accompanied by the change in further particle optical parameters.

By way of example, these are the magnification (coupled to the beam pitch of the individual particle beams), the telecentricity and the rotation of the individual particle beams upon incidence on the sample or the wafer 7. However, a change in these additional parameters is undesired, and so these are also corrected and/or kept constant within the scope of the fast autofocus.

Thus, provision is made in exemplary fashion of a telecentricity correction means 825, a rotation correction means 826 and a position correction means 827. The fast telecentricity correction means is configured to substantially contribute to correcting a tangential or radial telecentricity error of the first individual particle beams 3 in the second field 103, and the fast autofocus controller 821 is set up to generate a telecentricity correction means control signal for high-frequency adaptations at the respective working point during the wafer inspection on the basis of the actual autofocus data in order to control the fast telecentricity correction means during the wafer inspection. By way of example, a first deflector array arranged in an intermediate image plane, for example in the intermediate image plane 325, of the first particle optical beam path can be used as telecentricity correction means. However, other embodiment variants are also possible.

To correct the rotation, specifically the unwanted rotation of the grid arrangement in the second field 101, provision is furthermore made of a fast rotation correction means 826, which is configured to substantially contribute to correcting a rotation of the first individual particle beams 3 in the second field 101. Here, the fast autofocus controller 821 is configured to generate a rotation correction means control signal for high-frequency adaptations on the basis of the actual autofocus data during the wafer inspection at the respective working point in order to control the fast rotation correction means 826 during the wafer inspection. By way of example, such a rotation correction means 826 can be realized as a second deflector array, which is arranged, at a distance, directly upstream or downstream of the first deflector array for the telecentricity correction. However, other embodiments are also possible, for example by means of a multi-lens array which is arranged, at a distance, directly upstream or downstream of the first deflector array and in such a way that the first individual particle beams 3 pass through the multi-lens array in off-axis fashion. Alternatively, the multi-beam particle generator 305 can comprise the fast rotation correction means 826 and the rotation correction means 826 can be actively rotated by the rotation correction means control signal. It is also possible to combine two magnetic field generating devices to one another for weak magnetic fields directed against one another and to use each of the magnetic fields only for a change of the rotation in a certain direction.

Figure 3 shows a schematic illustration of a larger excerpt of a controller of the computer system 10 of the multi-beam particle microscope 1 with a fast autofocus correction lens 824. Control units 810 for the primary path and 830 for the secondary path are illustrated in exemplary fashion. In this case, the controller of the computer system 10 can have further constituent parts to the ones shown in figure 3. In view of the present invention, a few important control elements should be discussed below. The controller 810 in the primary path comprises a controller 811 for setting the working point and the controller 821 for setting the fast autofocus. In this case, in particular, the controller 811 is configured for static or low-frequency adaptation of a focusing in order to control at least the magnetic objective lens and/or an actuator of the sample stage at a first working point with a first working distance, in such a way that the first individual particle beams are focused on the wafer surface situated at the first working distance. In addition to the focus, other parameters of the particle optical imaging are also set, for example the individual beam spacing (pitch), the magnification connected therewith, a rotation of the grid arrangement of the individual particle beams upon incidence on the wafer surface and the desired landing angle upon incidence on the wafer surface. Thus, the working point setting 811 comprises a slow autofocus and additional correction functions. For the setting itself, provision is made of a measuring element 812, an adjustment algorithm 813 and various final controlling elements 814. These final controlling elements 814 include, in particular, the magnetic and/or electrostatic objective lens 102 and, in the case of a height- adjustable sample stage, optionally an actuator of the sample stage as well. The final controlling elements 814 for setting the working point moreover comprise for example a field lens system 307 and the multi-beam particle generator 305.

Further particle optical elements in the first particle optical beam path can act as further final controlling elements 814; they can be magnetic and/or electrostatic lenses. A comparatively long stroke for changing the working distance can be generated by the means for setting the working point; by way of example, said stroke can be +/-300, 200, 100 um. An adaptation time to a selected working distance 1s comparatively long in this case; by way of example, it can lie in the range of several ten to several hundred milliseconds.

The controller 821 for fast autofocusing comprises the measuring element 822 (or, more generally, the autofocus determining element), an autofocus algorithm 823 and at least the autofocus correction lens 824; however, other correction means may also be provided, for example the above-described telecentricity correction means 825, the rotation correction means 826 and/or the position correction means 827. A high-frequency adaptation of the focus is possible by means of the controller 821 for the fast autofocus, typical adaptation times lie in the region of a few microseconds; by way of example, an adaptation time is TA < 500 ps, preferably

TA < 100 ps and/or TA < 50 us. The stroke for changing the relative focal position is typically several micrometers, for example +/-20 pm, +/-15 um and/or +/-10 um.

In this case, for example, an adaptation time TA for the high-frequency adaptation is shorter than the adaptation time TA for the low-frequency or static adaptation by means of the controller for setting the working point 811 at least by a factor of 10, preferably at least by a factor of 100 and/or 1000.

A change in the relative focal position or the position of the wafer surface may also entail a necessary resetting or readjustment of particle optical components in the secondary path. Accordingly, the controller 830 for controlling the secondary path is part of the controller of the computer system 10. The control elements in the secondary path can also be subdivided into low-frequency or static control elements 831 and high-frequency control elements 841 (corresponding to a second fast autofocus, for example). The slow working point setting is controlled by the controller 831; a measuring element 832, for example a CCD camera, a second adjustment algorithm 833 and a final controlling element 834 or a plurality of final controlling elements 834 are provided to this end. By way of example, these final controlling elements 834 include magnetic projection lenses 205, which are controlled in such a way that the foci of the second individual particle beams 9 are imaged exactly on the surface of the detection regions of the detection unit 209.

However, other final controlling elements can also be controlled by means of the controller 831 for setting the working point. The controller 841 controls the fast second autofocus in the secondary path: In this case, refocusing is carried out in the secondary path during the wafer inspection. Also, it is possible that further particle optical parameters such as position, telecentricity and rotation are likewise quickly readjusted. To this end, the controller 841 comprises a measuring element 842, a second autofocus algorithm 843 and fast projection path correction means 844, in particular electrostatic lenses, electrostatic deflectors, and/or electrostatic stigmators, in this embodiment. By way of example, a fast CCD camera is considered as measuring element 842, or else, e.g., means for measuring current around a contrast stop arranged in a crossover plane in the secondary path.

However, it is also possible to dispense with the measuring element 842 in the secondary path and instead work with a feedforward loop. Then, control signals for the fast projection path correction means 844 are determined by means of the second autofocus algorithm 843 on the basis of values/settings that were determined for the primary path, and the projection path correction means 844 are controlled accordingly. In this case, the autofocus algorithm 843 can comprise lookup tables. It is also possible to combine the two described variants with one another, i.e, additionally use a measuring element 842 and, for example, explicitly redetermine the settings of the final controlling elements/projection path correction means 844 for the secondary path only in the case of certain measured deviations from a reference value.

The controller of the computer system 10 with control elements 810 for controlling the primary path and 830 for controlling the secondary path now is set up, furthermore, in such a way that the controllers 810 and 830 are matched to one another in time, 1.e., synchronized, with their respective constituent parts. The electronics used for the control are likewise very fast but it is necessary to ensure that, for example for each image field (mFOV), settings of the particle optical components in the primary path and also in the secondary path that are as optimal as possible are guaranteed. Details in respect of realizing a fast control of particle optical components/in respect of fast electronics are known to a person skilled in the art and are also disclosed, for example, in the German patent application 102020209833.6, filed on August 5, 2020, the disclosure of which is incorporated in this patent application in full by reference.

Figure 4 schematically shows a method for setting a fast autofocus by means of an autofocus correction lens 824. It is assumed that a (slow) adjustment of the system at a first working point with an associated first working distance has already been implemented by means of adjusting the magnetic objective lens and/or by means of controlling an actuator for a sample stage; in the process, other parameters have also already been set in accordance with specifications for the working point (magnification, telecentricity, rotation).

In a method step S1, measurement data are generated for a current focus at the selected working point AP. A working point is defined at least by the working distance between the objective lens and the wafer surface; however, further parameters can also be used to define the working point. Examples thereof include the relative focal position, the position and the telecentricity or the landing angle of individual particle beams 3 on the wafer surface and the rotation of a grid arrangement of individual particle beams 3 upon incidence on the wafer surface.

An example is intended to be used below but should not be construed as restrictive for the invention. In a method step S2, actual autofocus data are determined on the basis of measurement data. These measurement data can be obtained using the above-described measuring elements 812 and the actual autofocus data can be determined therefrom by means of the adjustment algorithm 813. Thus, for example, the actual autofocus data specify whether over-focusing or under- focusing is present, or what the magnitude of the same is. However, it is also possible for the measurement data to form the actual autofocus data directly (identity map). After the actual autofocus data have been determined, control signals are generated in steps S3, S4 and S5 on the basis of the actual autofocus data: In step S3, an autofocus correction lens control signal is generated on the basis of the actual autofocus data. In step S4, a telecentricity correction means control signal is generated on the basis of actual autofocus data. In step S5, a rotation correction means control signal is generated on the basis of the actual autofocus data. In this case, adjusting the autofocus correction lens does not only alter the relative focal position but normally also alters the magnification (position, not illustrated), the telecentricity and/or the rotation of a grid arrangement of the individual particle beams. Within the scope of determining the control signals, an orthogonalization matrix or inverted sensitivity matrix 850 is used in the example shown; from this, it is possible to derive which particle optical components have to be excited differently by what absolute value in order to exactly set a particle optical parameter differently. As a result, there is preferably simultaneous control of the autofocus correction lens in step S6, control of the telecentricity correction means in step S7 and control of the rotation correction means in step S8, and optional control of further fast correction means.

Once these adjustments have been implemented for the primary path, the secondary path is updated in high-frequency fashion: This is feedforward in the example shown, while feedback is implemented in the primary path: Second measurement data for the current second relative autofocus position (detection plane) in the secondary path are generated in a method step S9. In addition or as an alternative thereto, it is possible to determine the current relative position of the crossover of the second individual particle beams in the secondary path. Second actual autofocus data for the secondary path are ascertained in method step S10.

In addition or as an alternative thereto, values can also be used for the secondary path which were already assigned to the actual autofocus data of the primary path in advance. Projection path correction means control signals are then determined on the basis of the second actual autofocus data in method step S11. Here, this may be a set of control signals. Preferably, a second orthogonalization matrix or second inverted sensitivity matrix 851 for the secondary path is used for the generation of the control signals. Then, fast projection path correction means are controlled in a method step S12 using the control signals. This preferably includes a fast second autofocus correction lens. Moreover, it is possible to control a fast telecentricity correction means (e.g., in the form of a deflector array in an intermediate image plane in the secondary path) and/or a fast rotation correction means (e.g., in the form of a second deflector array directly upstream or downstream of the deflector array for fast telecentricity correction in the secondary path) and/or further fast correction means, for example electrostatic lenses, electrostatic deflectors and/or electrostatic stigmators. It is also possible to control a fast contrast correction means. By way of example, a fast contrast correction means can be integrated in the projection lens system of the secondary path, for example as described in US 2019/0355544 Al, the disclosure of which is incorporated in this application in full by reference. Then, an image field is recorded in method step S13 using the settings of step S12. Then, measurement data for the current focus at the working point can be generated anew (method step S1). A corresponding procedure is carried out until the entire image recording process has been completed.

In one example, the first or second orthogonalization or inverted sensitivity matrix 850, 851 may depend on the working point setting according to the adjustment using the controllers 811 and 831. By way of example, a necessary dynamic correction for a tangential or radial telecentricity error parallel to a fine correction of a focus plane may depend on a few um of the working point or the coarse focus setting within the long-range focal range of several 100 um. In this case, the orthogonalization or inverted sensitivity matrices 850, 851 for a selected working point are chosen from a memory in which a plurality of orthogonalization or inverted sensitivity matrices 850, 851 are stored for different focus setting within the long-range focal range.

Figure 5 schematically shows a section through a multi-beam particle microscope 1 in which the autofocus correction lens 824 according to the invention can be arranged. The multiple particle beam system 1 initially comprises a particle source 301. In the shown example, this particle source 301 emits an individual particle beam with charged particles, e.g., electrons. In figure 5, particle beams or a particle optical beam path are illustrated schematically by the dashed line with reference sign 3. The individual particle beam initially passes through a condenser lens system 303 and subsequently strikes a multi-aperture arrangement 305. This multi-aperture arrangement 305, possibly with further particle optical components, serves as a multi-beam generator. The first particle beams emanating from the multi-aperture arrangement 305 then pass through a field lens or a field lens system 307 and subsequently enter a beam switch 400. This beam switch 400 comprises a beam tube arrangement 460, which has a Y-shaped embodiment and three limbs 461, 462 and 463 in the example shown. Here, in addition to two flat, interconnected structures for holding the magnetic sectors 410, 420, the beam switch 400 includes the magnetic sectors 410 and 420 which are contained in, or secured to, said structures. After passing through the beam switch 400, the first particle beams pass through a scan deflector 500 and, thereupon, a particle optical objective lens 102, before the first particle beams 3 are incident on an object 7, in this case a semiconductor wafer with HV structures. As a result of this incidence, secondary particles, e.g., secondary electrons, are released from the object 7. These secondary particles form second particle beams, which have assigned a second particle optical beam path 9. After emerging from the object 7, the second particle beams initially pass through the particle optical objective lens 102 and subsequently pass through the scan deflectors 500, before said second particle beams enter the beam switch 400. Subsequently, the second particle beams 9 emerge from the beam switch 400, pass through a projection lens system 205 (illustrated in much simplified fashion), pass through an electrostatic element 260 and then are incident on a particle optical detection unit 209 (in this case, the reference sign 260 denotes the so-called anti-scan, which compensates the otherwise occurring scanning movement of the secondary beams 9 upon incidence on the detection unit 209).

Situated within the beam switch 400, there 1s the beam tube arrangement 460, which also extends beyond the beam switch 400 in the example shown. Splitting the beam path within the beam switch 400 into the first particle optical beam path 3 and the second particle optical beam path 9 is implemented within the beam switch 400 with the aid of magnetic sectors 410, 420. In the example illustrated in figure 5, the beam tube arrangement 460 also continues outside of the beam switch 400. In this case, it extends, in particular, to the particle optical objective lens 102 or into the particle optical objective lens 102 (beam tube extension). The beam tube arrangement 460 expands into vacuum chambers 350, 355 and 250 in the region of the particle source 301, in the region of the multi-aperture arrangement 305, and in the region of the detector unit 209. At least in the region of the beam switch 400, the beam tube arrangement normally has a one-piece embodiment, 1.e., it has neither weld points or weld seams nor solder points or solder seams. The beam tube arrangement contains copper in the shown example; however, it could also include titanium or any other element or any other compound. Here, there is a high vacuum in the region of the beam tube arrangement 460 within the beam switch 400, preferably with a pressure of less than 10-5 mbar, in particular less than 10-7 mbar and/or 109 mbar. In the chambers 350, 355 and 250, already mentioned, there is a vacuum, preferably with respective pressures of less than 10-5 mbar, in particular less than 10-7 mbar and/or 10° mbar.

In the example shown, the objective lens 102 has an upper pole shoe 108 and a lower pole shoe 109. A winding 110 for generating a magnetic field is situated between the two pole shoes 108 and 109. Here, the upper pole shoe 108 and the lower pole shoe 109 can be electrically insulated from one another. In the example shown, the particle optical objective lens 102 is a single magnetic lens in the form of an immersion lens; however, the objective lens or the objective lens system can also comprise further magnetic lenses or electrostatic lenses.

Now, the fast autofocus correction lens 824 according to the invention can be integrated, in a plurality of configurations and at a plurality of positions, optionally with further fast correctors, in the multi-beam particle microscope 1 shown in figure 5. Depending on the position, the fast autofocus correction lens 824 acts more or less strongly on the focus of the individual particle beams 3 in this case; however, it may also act on other particle optical parameters such as the position, the landing angle and/or the rotation of the individual particle beams 3.

Additionally, a second or an additional or a plurality of further autofocus correction lens(es) can be integrated in the primary path and/or in the secondary path; optionally, further fast correction means can be provided in the primary path and/or in the secondary path.

Figure 6 schematically illustrates an embodiment of the invention with a fast autofocus correction lens 824. In this embodiment, the autofocus correction lens 824 is provided in the form of an additional electrode. By way of example, it can be embodied as a single aperture plate with a central opening, to which a voltage Uar is applied. Level and the sign of the voltage can be provided in this case by means of the controller 821 for the fast autofocus. This exemplary embodiment is advantageous in that the autofocus correction lens, as penultimate lens, 15 realized comparatively far down in the beam path. As a result, only small subsequent aberrations are generated. The greater the voltage Uar is in terms of absolute value, the more difficult it is to realize fast changes in the voltage from a technical point of view. The exemplary embodiment shown is therefore particularly well suitable if the sample voltage Usample applied to the sample 7 is not too high.

Figure 7 schematically illustrates a further embodiment of the invention with an autofocus correction lens 824. In the example shown, the autofocus correction lens 824 is arranged within the magnetic objective lens 102. Here, the autofocus correction lens 824 1s situated between the upper pole shoe 108 and the lower pole shoe 109 of the objective lens 102. In this case, a voltage U; is applied to the upper pole shoe 108 and a voltage Uz is applied to the lower pole shoe 109. These voltages can be comparatively high and are a few kilovolts, for example. The same can also apply to the voltage Uar that is able to be applied to the autofocus correction lens 824. In this case, too, the autofocus correction lens 824 can therefore be operated at a comparatively high voltage Uar. However, if the upper pole shoe 108 is at ground potential, the voltage Uar can be chosen to be comparatively small in terms of absolute value. In this embodiment, too, the autofocus correction lens 824 is arranged comparatively far down in the first particle optical beam path; it is the penultimate particle optical element in the example shown. This is once again advantageous in that possible subsequent aberrations are also small in this embodiment variant.

Figure 8 shows a further embodiment of the invention with a fast autofocus correction lens 824 in a schematic illustration. In this embodiment variant, the autofocus correction lens 824 is provided between the beam deflection system 500 and the upper pole shoe 108 of the magnetic objective lens 102. This is a quickly controllable electrode, to which the voltage Uar is applied, the value of the latter being adjustable by means of the controller 821 of the fast autofocus. This embodiment variant is advantageous in that the electrode 824 is substantially arranged within the crossover plane. Comprehensive calculations of the inventors in this respect have shown that the influence of the electrode 824 at this position is substantially directed at the focus. The other particle optical parameters such as position, landing angle and rotation remain substantially unchanged. Moreover, this embodiment is advantageous in that the effect in the crossover on all individual particle beams is identical. This makes it easier to precisely set the autofocus.

Figure 9 schematically illustrates a further embodiment of the invention with autofocus correction lens 824. In this case, too, the autofocus correction lens 824 is embodied as a fast electrostatic element or as a fast electrostatic lens. Starting from the upper pole shoe 108 of the objective lens 101, the beam tube extension 464 protrudes a little into the magnetic objective lens 101. This beam tube extension 464 is at ground potential — like the entire beam tube 460. Here, the autofocus correction lens 824 is arranged within the beam tube extension 464. An adjustable voltage Uar is applied, in turn, to the former by way of the controller 821. It can be comparatively low. In this case, the illustrated position of the autofocus correction lens 824 is situated close to the crossover plane.

Comprehensive calculations have shown that positioning the autofocus correction lens 824 at the crossover or in the vicinity of the crossover acts predominantly on the focus of the individual particle beams. Therefore, adaptations of further particle optical parameters such as position, landing angle and rotation are either not mandatory or, at least, smaller. This allows a faster readjustment of the remaining parameters or the correction elements can have a weaker design. This generates smaller subsequent aberrations.

Figure 10 schematically illustrates a further embodiment of the invention with a fast autofocus correction lens 824. In the exemplary embodiment shown, the autofocus correction lens 824 is provided as an offset for the scan deflector 500: In the illustrated example, the scan deflector 500 comprises an upper deflector 5004 and a lower deflector 500b. Here, in principle, the upper deflector 5004 and the lower deflector 500b can have the same design. By way of example, they can be embodied as a deflector plate pair, as a quadrupole element or as an octupole element. The voltage Uar is now applied to both the upper deflector 5004 and the lower deflector 500b as an offset. Once again, the corresponding control signal is provided by means of the controller 821 for the fast autofocus. This embodiment variant is advantageous in that the fast autofocus correction lens 824 1s once again arranged close to the crossover of the individual particle beams 3. In this case, too, an excitation of the autofocus correction lens 824 therefore acts substantially on the focus. Moreover, this realization does not require additional hardware: It is only necessary to apply the voltage Uar as an offset to the upper deflector 5004 and the lower deflector 500b.

Figure 11 shows a further embodiment of the invention with a fast electrostatic autofocus correction lens 824. In this embodiment, the fast autofocus correction lens 824 is provided as a ring electrode between the upper deflector 500a and the lower deflector 500b. In this case, it is also true that the autofocus correction lens 824 1s arranged relatively close to the crossover of the individual particle beams 3.

Therefore, the lens 824 mainly acts on the focus of the individual particle beams.

Moreover, changes to the hardware of the system 1 can be carried out comparatively easily. Instead of as a ring electrode, the fast autofocus correction lens 824 can also be embodied as an air coil around the beam tube 861 (not illustrated in figure 11).

Figure 12 shows further embodiments of the invention with a fast autofocus correction lens 824 in a schematic illustration. In these embodiments, the beam tube 460 is interrupted at the sites at which the autofocus correction lens 824 is provided. In the overall system 1, these positions offer comparatively large amounts of space, simplifying an integration of the autofocus correction lens 824 in the system overall. Specifically, three different positions at which the autofocus correction lens 824 can be arranged are illustrated in figure 12: According to a first example, the autofocus correction lens 8244 is situated above the beam switch 400 or above the magnetic sector 410 in the particle optical beam path. Expressed differently, the interruption of the beam tube 460, in which the autofocus correction lens 824a is arranged, is situated between the field lens system 307 (not illustrated in figure 12) and the beam switch 400. A second option lies in providing the interruption of the beam tube 460 between the two magnetic sectors 410 and 420 and arranging the autofocus correction lens 824b in this interruption. A third option lies in arranging the beam tube 460 between the beam switch 400 and the beam deflection system 500. Part of the inner wall of the beam tube 460 is therefore replaced by the autofocus correction lens 824a, 824b and/or 824c in these embodiment variants or is not — like the beam tube 460 — at ground potential.

Figure 13 shows further embodiments of the invention with fast autofocus correction lenses 824. The example illustrated in figure 13 differs from the example illustrated in figure 12 in that no interruption of the beam tube 460 is provided.

Instead, a tube lens 824a, 824b and 824c is integrated in the beam tube 460 in each case. This makes it easier to design the beam tube 460 in sealed fashion and to maintain the vacuum or high vacuum situated therein. In the case of the realization variant with the tube lenses, the voltage Uar is applied to the central electrode; the upper and the lower electrode are preferably at ground potential.

Alternatively, a fast magnetic lens, for example in the form of an air coil, can be arranged around the beam tube 460 at the sites shown. The latter only has a few turns k, for example 10 < k < 500 and/or 10 <k <200 and/or 10 < k < 50 applies.

Figure 14 shows a further embodiment of the invention with a fast autofocus correction lens 824, wherein the beam tube 460 is interrupted. The autofocus correction lens 824 is arranged within this interruption. Here, this interruption is situated within a magnetic field lens of the field lens system 307. This embodiment variant can be realized comparatively easily on account of the available installation space. Moreover, the beam tube 460 is at ground potential, which 1s why only a comparatively low voltage, as voltage Uar, needs to be applied to the autofocus correction lens 824 in order to influence the individual particle beams 3.

However, in this embodiment the autofocus correction lens acts both on the focus and on the position and the landing angle of the individual particle beams upon incidence on the wafer surface. Conversely, it is possible to use a position within the field lens 307 to correct a tilt of the beams and also the position of the beams.

Figure 15 shows a further embodiment of the invention with a fast autofocus correction lens 824. In comparison with the embodiment variant illustrated in figure 14, it is the case here that the beam tube 460 does not have an interruption.

Instead, a tube lens as a fast autofocus correction lens 824 is arranged within the beam tube 460. In this embodiment variant, too, a realization is comparatively simple if sufficient installation space is available. Conversely, it is once again the case that the autofocus correction lens 824 also acts on the position and the landing angle of the individual particle beams 3 in addition to the focus. It is therefore optionally advantageous to (also) correct the tilt of the individual particle beams and/or the position of the individual particle beams by way of the autofocus correction lens.

Figure 16 shows a further embodiment of the invention with a fast autofocus correction lens 824 in a schematic illustration. In this exemplary embodiment, the autofocus correction lens 824 is arranged in the vicinity of the intermediate image plane 325: Here, the autofocus correction lens 824 in this example is embodied as a combined lens with a first constituent part 824a and a second constituent part 824b. If these two constituent parts 824a and 824b are provided symmetrically with respect to the intermediate image plane 325, the effect of the combination is the same as if the autofocus correction lens 824 were arranged directly within the intermediate image plane 325. The advantage of this embodiment variant is that further particle optical components of the overall system 1 can be arranged in the intermediate image plane 325 itself. By way of example, positioning in the intermediate image plane 325 is expedient for a first multi-deflector array since this can implement a fast telecentricity correction for the first individual particle beams, as described above in the general part of the application. However, the autofocus correction lens 824 can alternatively also be embodied in one-part fashion (i.e., only with the constituent part 8244 or only with the constituent part 824b in the vicinity of the intermediate image plane 325. In a further alternative, the autofocus correction lens 824 can be arranged in one-part fashion (i.e, only with the constituent part 824a or only with the constituent part 824b as exactly within the intermediate image plane 325 as possible. Then, the autofocus correction lens 824 has a comparatively large effect on the telecentricity of the individual particle beams 3 passing therethrough, like in the case of the symmetric arrangement of the constituent parts 824a and 824b.

Figure 17 shows a further embodiment of the invention with a fast autofocus correction lens 824. In this embodiment, the fast autofocus correction lens 824 is integrated in the multi-aperture arrangement 305. In addition to a multi-aperture plate 313, which is used to generate individual beams, this multi-aperture arrangement 305 comprises further multi-aperture plates or multi-lens arrays and/or multi-deflector arrays (e.g., for individual focusing and/or stigmation of the individual particle beams; not illustrated in figure 17). In this sequence of the so- called micro-optical unit, the fast autofocus correction lens 824 can be provided in the form of a fast multi-Einzel lens arrangement. In this case, the multi-aperture plate 824a and the multi-aperture plate 824c are at ground potential. Situated therebetween is the multi-aperture plate 824b, to which the autofocus correction voltage Uar can be applied by means of the controller 821. An advantage of this embodiment of the invention is that, in principle, no change in the position and no tilt of the individual particle beams is caused; however, spherical aberrations in the autofocus correction lens 824 in the form of a multi-Einzel lens arrangement and manufacturing tolerances in the multi-aperture plate may be critical.

Additionally, a comparatively high voltage must currently be used as voltage Uar.

The illustrated embodiments can be combined with one another in full or in part, provided that no technical contradictions arise as a result. Incidentally, the illustrated embodiments should not be construed as constrictive for the invention.

Further examples relating to the invention are listed below. These examples can be combined with the embodiments of the invention claimed in the patent claims provided no technical contradictions arise as a result.

LIST OF REFERENCE SIGNS

1 Multi-beam particle microscope 3 Primary particle beams (individual particle beams) 5 Beam spots, incidence locations 7 Object 9 Secondary particle beams

Computer system, controller 100 Objective lens system 10 101 Object plane 102 Objective lens 103 Field 108 Upper pole shoe of the objective lens 109 Lower pole shoe of the objective lens 110 Winding 122 Lower pole shoe of the magnetic field compensation lens 200 Detector system 205 Projection lens 209 Particle multi-detector 211 Detection plane 213 Incidence locations 217 Field 250 Vacuum chamber 260 Scan deflector in the secondary path 300 Beam generation apparatus 301 Particle source 303 Condenser lens system 305 Multi-aperture arrangement 313 Multi-aperture plate 315 Openings in the multi-aperture plate 317 Midpoints of the openings 319 Field 307 Field lens system

309 Diverging particle beam 311 Iluminating particle beam 323 Beam foal 325 Intermediate image plane 350 Vacuum chamber

355 Vacuum chamber 400 Beam switch 410 Magnetic sector 420 Magnetic sector

460 Beam tube, beam tube arrangement 461 Limb of the beam tube 462 Limb of the beam tube 463 Limb of the beam tube 464 Beam tube extension

500 Scan deflector in the primary path 810 Controller for the primary path 811 Controller for the working point setting (slow) 812 Measuring element 813 Adjustment algorithm

814 Final controlling elements in the primary path 821 Controller for the fast autofocus in the primary path 822 Measuring element, autofocus determining element 823 Autofocus algorithm 824 Fast autofocus correction lens

825 Fast telecentricity correction means 826 Fast rotation correction means 827 Fast position correction means 831 Controller for the working point setting in the secondary path (slow) 832 Measuring element

833 Second adjustment algorithm (secondary path) 834 Final controlling elements in the secondary path 841 Controller for the second fast autofocus (secondary path) 842 Measuring element

843 Second autofocus algorithm (secondary path) 844 Fast projection path correction means 850 Orthogonalization matrix or inverted sensitivity matrix for the primary path 851 Orthogonalization matrix or inverted sensitivity matrix for the secondary path

S1 Generating measurement data for the current focus at the working point AP

S2 Determining actual autofocus data on the basis of measurement data

S3 Generating an autofocus correction lens control signal on the basis of actual autofocus data

S4 Generating a telecentricity correction means control signal on the basis of actual autofocus data

S5 Generating a rotation correction means control signal on the basis of actual autofocus data

S6 Controlling an autofocus correction lens

S7 Controlling telecentricity correction means

S8 Controlling rotation correction means

S9 Generating second measurement data for a second autofocus in the secondary path

S10 Determining second actual autofocus data on the basis of second measurement data

S11 Generating a projection path correction means control signal (set)

S12 Controlling projection path correction means including a second autofocus correction lens

S13 Recording an image field

Claims

Conclusions

A multiple particle beam system (1) for wafer inspection, comprising: a multiple particle beam generator configured to generate a first field (327) of a plurality of charged first individual particle beams (3); a first optical particle unit with a first optical particle beam path, which is arranged to image the generated first individual particle beams (3) on a wafer surface in the object plane (101) such that the first particle beams (3) contact the wafer surface at impact locations (5), which form a second field (103); a detection system (200) with a plurality of detection regions (215) which form a third field (217), a second optical particle unit with a second optical particle beam path, which is adapted to image second individual particle beams (9) originating are of the incidence locations (5) in the second field (103), in the third field (217) of the detection regions (215) of the detection system (200); a magnetic and/or electrostatic objective lens through which both the first (3) and the second individual particle beams (9) pass; a beam switch (400) disposed in the first optical particle beam path between the multi-particle beam generator and the objective lens (102) and disposed in the second optical particle beam path between the objective lens (102) and the detection system ( 200); a sampling stage for holding and/or positioning a wafer (7) during wafer inspection; an autofocus determining element (812), which is adapted to generate data for determining actual autofocus data during wafer inspection;

a fast autofocus correction lens (824); and a controller (10); wherein the controller (10) is adapted to control optical particle components in the first and/or second optical particle beam path, wherein the controller (10) is adapted for static or low frequency adjustment of a focus to at least the objective lens (102 ) and/or to control an actuator of the sampling stage at a first operating point with a first working distance, in such a way that the first individual particle beams (3) are focused on the wafer surface located at the first working distance, in which the controller ( 10) is arranged for high-frequency focus adjustment to generate an autofocus correction lens control signal based on the actual autofocus data at the first operating point during wafer inspection to control the high-speed autofocus correction lens (824) during wafer inspection at the first operating point; wherein the first operating point is further defined by a landing angle of the first individual particle beams (3) on the object plane (101) and by a grid arrangement of the first individual particle beams (3) in the object plane (101), and wherein the controller (10) is further designed to keep the landing angle and the grid arrangement essentially constant during the high-frequency adjustment at the first operating point.

The multi-particle beam system (1) according to claim 1, wherein an adjustment time TA for the high-frequency adjustment is shorter than the adjustment time TA for the low-frequency adjustment by at least a factor of 10, in particular at least by a factor of 100 or 1000.

3. The multi-particle beam system (1) according to any one of the preceding claims,

wherein a stroke for adjusting the working distance of the low-frequency or static adjustment is greater than the stroke for the high-frequency adjustment by at least a factor of 5, in particular by a factor of 8 and/or 10.

The multiple particle beam system (1) according to any one of the preceding claims, wherein a second working point is defined at least by a second working distance between the objective lens (102) and the wafer surface and wherein the second working distance is different from the first working distance of the first operating point, in which the controller (10) is arranged to perform a low-frequency adjustment in the event of a change between the first operating point and the second operating point and to control at least the magnetic objective lens (102) and/or an actuator of the sampling stage at the second working point such that the first individual particle beams (3) are focused on the wafer surface located at the second working distance.

The multi-particle beam system (1) according to the preceding claim, wherein the controller (10) is configured to generate an autofocus correction lens control signal for high frequency adjustments based on the actual autofocus data at the second operating point during wafer inspection to adjust the high-speed autofocus correction lens (824). to be arranged during the wafer inspection at the second operating point.

The multiple particle beam system (1) according to any one of the preceding claims, wherein the second operating point is further defined by a landing angle of the first individual particle beams (3) in the object plane

(101) and by a grid arrangement of the first individual particle beams (3) in the object plane (101), and wherein the controller (10) is further arranged to keep the landing angle and the grid arrangement substantially constant during the high-frequency adjustment on the second working point.

The multiple particle beam system (1) according to the preceding claim, wherein the controller (10) is arranged to keep the landing angle and the grid arrangement substantially constant even during a change between the first operating point and the second operating point.

The multi-particle beam system (1) according to any one of the preceding claims, wherein the autofocus correction lens (824) comprises a fast electrostatic lens.

The multiple particle beam system (1) according to claim 8, wherein the autofocus correction lens (824) is arranged in a crossover plane of the first individual particle beams (3).

The multi-particle beam system (1) according to claim 8, wherein the autofocus correction lens (824) is arranged between the wafer surface and a lower pole shoe (109) of the magnetic objective lens (102).

The multi-particle beam system (1) according to claim 8, wherein the autofocus correction lens (824) is arranged between the upper (108) and lower (109) pole shoe of the magnetic objective lens (102).

The multi-particle beam system (1) according to claim 8,

wherein the autofocus correction lens (824) is arranged in a beam tube extension (464) which extends into the objective lens (102) from the direction of the upper pole shoe (108).

The multi-particle beam system (1) according to claim 8, further comprising a beam deflection system (500) between the beam switch (400) and the objective lens (102) adapted to raster scan the wafer surface by means of a scanning movement of the first individual particle beams (3), in which the autofocus correction lens (824) is realized as an offset on the beam deflection system (500).

The multi-particle beam system (1) according to claim 8, further comprising a beam deflection system (500) between the beam switch (400) and the objective lens (102) adapted to raster scan the wafer surface by means of a scanning movement of the first individual particle beams (3); wherein the beam deflection system (500) includes an upper deflector (500a) and a lower deflector (500b) arranged sequentially in the direction of the beam path; and wherein the autofocus correction lens (824) is arranged between the upper deflector (50024) and the lower deflector (500b).

The multi-particle beam system (1) according to claim 8, further comprising a beam deflection system (500) between the beam switch (400) and the objective lens (102) adapted to raster scan the wafer surface by means of a scanning movement of the first individual particle beams (3); wherein the beam deflection system (500) includes an upper deflector (500a) and a lower deflector (500b) arranged sequentially in the direction of the beam path; and wherein the autofocus correction lens (824) is disposed between the lower deflector (500b) and an upper pole shoe (109) of the magnetic objective lens (102).

The multi-particle beam system (1) of claim 8, further comprising a beam tube (460) capable of being cleared in 1 second and substantially defining the first optical particle beam path from the multi-particle beam generator to the objective lens (102). wherein the beam tube (460) has an interruption and wherein the autofocus correction lens (824) is arranged in this interruption.

The multi-particle beam system (1) of claim 16, further comprising a field lens system (307) disposed in the first optical particle beam path between the multi-particle beam generator and the beam switch (400), through which the interruption of the beam tube (460) wherein the autofocus correction lens (824) is arranged between the field lens system (307) and the beam switch (400).

The multiple particle beam system (1) according to claim 16, wherein the beam switch (400) comprises two magnetic sectors (410, 420) and wherein the interruption of the beam tube (460) in which the autofocus correction lens (824) is arranged is provided in the region of the beam switch (400) between the two magnetic sectors (410, 420).

The multi-particle beam system (1) according to claim 16, further comprising a beam deflection system (500) between the beam switch (400) and the objective lens (102) adapted to raster scan the wafer surface by means of a scanning movement of the first individual particle beams (3),

wherein the interruption of the beam tube (460) in which the autofocus correction lens (824) is arranged is provided between the beam switch (400) and the beam deflection system (500).

The multi-particle beam system (1) of claim 16, further comprising a field lens system (307) disposed in the first optical particle beam path between the multi-particle beam generator and the beam switch (400), wherein the interruption of the beam tube (460) wherein the autofocus correction lens (824) is arranged within a magnetic field lens of the field lens system (307).

The multi-particle beam system (1) of claim 8, further comprising a beam tube (460) capable of being cleared in 1 second and substantially defining the first optical particle beam path from the multi-particle beam generator to the objective lens (102). encloses, in which the autofocus correction lens (824) is designed as a tube lens and is arranged in the beam tube (460).

The multi-particle beam system (1) of claim 21, further comprising a field lens system (307) disposed in the first optical particle beam path between the multi-particle beam generator and the beam switch (400), wherein the autofocus correction lens (824) is disposed between the field lens system (307) and the beam switch (400) in the beam tube (460).

The multiple particle beam system (1) according to claim 21, wherein the beam switch (400) includes two magnetic sectors (410, 420) and wherein the autofocus correction lens (824) is provided between the two magnetic sectors (410, 420) in the beam tube (460).

The multiple particle beam system (1) of claim 21, further comprising a beam deflection system (500) between the beam switch (400) and the objective lens (102) adapted to raster scan the wafer surface by means of a scanning movement of the first individual particle beams (3), in which the autofocus correction lens (824) is provided between the beam switch (400) and the beam deflection system (500) in the beam tube (460).

The multi-particle beam system (1) of claim 21, further comprising a field lens system (307) disposed in the first optical particle beam path between the multi-particle beam generator and the beam switch (400), wherein the autofocus correction lens (824) is disposed within in a magnetic field lens (307) in the beam tube (460).

The multi-particle beam system (1) according to any one of claims 1 to 7, wherein the high-speed autofocus correction lens (824) comprises a high-speed magnetic lens, in particular an air coil.

The multi-particle beam system (1) of claim 26, further comprising a beam tube (460) capable of being cleared and substantially defining the first optical particle beam path from the multi-particle beam generator to the objective lens (102). in which the fast magnetic lens is disposed outside around the beam tube (460).

The multi-particle beam system (1) of claim 27, further comprising a field lens system (307) disposed in the first optical particle beam path between the multi-particle beam generator and the beam switch (400), wherein the fast magnetic lens is disposed between the field lens system (307) and the beam switch (400) around the beam tube (460).

The multiple particle beam system (1) according to claim 27, wherein the beam switch (400) has two magnetic sectors (410, 420) and wherein the fast magnetic lens is arranged between the two magnetic sectors (410, 420) around the beam tube ( 460).

The multiple particle beam system (1) of claim 27, further comprising a beam deflection system (500) between the beam switch (400) and the objective lens (102) adapted to raster scan the wafer surface by means of a scanning movement of the first individual particle beams (3), in which the fast magnetic lens is arranged between the beam switch (400) and the beam deflection system (500) around the beam tube (460).

The multi-particle beam system (1) according to claim 27, further comprising a beam deflection system (500) between the beam switch (400) and the objective lens (102) adapted to raster scan the wafer surface by means of a scanning movement of the first individual particle beams (3), wherein the beam deflection system (500) includes an upper deflector (500a) and a lower deflector (500b) arranged successively in the direction of the beam path; and wherein the fast magnetic lens is disposed between the upper deflector (50023) and the lower deflector (500b) around the beam tube (460).

The multi-particle beam system (1) according to any one of the preceding claims, wherein the multi-particle beam system (1) further comprises a fast telecentricity correction means (825) adapted to substantially assist in correcting a tangential telecentricity error of the first individual particle beams (3) in the second field (103), and wherein the controller (10) is arranged to generate a telecentricity corrector control signal for high frequency adjustments at the respective operating point during wafer inspection on the basis of the actual autofocus data to make the fast telecentricity corrector ( 825) during wafer inspection.

The multiple particle beam system (1) of claim 32, wherein the telecentricity corrector (825) comprises a first deflector array disposed in an intermediate image plane (325) of the first optical particle beam path.

The multi-particle beam system (1) according to any one of the preceding claims, wherein the multi-particle beam system (1) further comprises a fast rotation correction means (826) adapted to substantially contribute to correcting a rotation of the first individual particle beams (3) in the second field (103), and wherein the controller (10) is arranged to generate a rotation corrector control signal for high frequency adjustments based on the actual autofocus data during wafer inspection at the respective operating point to control the fast rotation corrector (826) to be arranged during the wafer inspection.

The multi-particle beam system (1) according to the preceding claim, wherein the rotation correction means (826) comprises an air coil.

The multi-particle beam system (1) of claims 33 and 34, wherein the rotation correction means (826) comprises a second deflector array spaced directly upstream or downstream of the first deflector array.

The multi-particle beam system (1) according to claims 33 and 34, wherein the rotation correction means (826) comprises a multi-lens array spaced directly upstream or downstream of the first deflector array and in such a manner that the first individual particle beams (3) pass through the multi-lens array in an off-axis manner.

The multi-particle beam system (1) according to claim 34, wherein the multi-particle beam generator includes the fast rotation corrector (826) and the rotation corrector (826) is actively rotated by the rotation corrector control signal.

The multiple particle beam system (1) according to claim 34, wherein the fast rotation correction means (826) includes a first magnetic field generating device for generating a first weak magnetic field and a second magnetic field generating device for generating a second weak magnetic field, and wherein the first magnetic field generating device is controlled only for rotation in a positive rotation direction and the second magnetic field generating device is controlled only for rotation in a negative rotation direction by the controller ( 10) by means of the rotation corrector control signal.

The multiple particle beam system (1) according to claim 39, wherein the first and second magnetic fields have an axial configuration and are arranged in a converging or diverging part of the first individual particle beams (3) in the first optical particle beam path.

The multi-particle beam system (1) according to any one of the preceding claims, wherein a maximum deviation of each individual particle beam (3) from a desired landing position on the water surface is not more than 10 nm, in particular not more than 5 nm, 2 nm, 1 nm, or 0.5 nm.

The multi-particle beam system (1) according to any one of the preceding claims, wherein the controller (10) is arranged to perform the determination of the autofocus correction lens control signal and/or the rotation corrector control signal and/or the telecentricity corrector control signal based on the actual autofocus data usage creating an inverted sensitivity matrix describing the influence of control changes of optical particle components on optical particle parameters that characterize the optical particle imaging at the respective operating point.

The multi-particle beam system (1) according to any one of the preceding claims, wherein the controller (10) is arranged for a static or low-frequency adjustment of a focusing in the second optical particle beam path to, at the respective operating point with the associated working distance, to arrange optical particle components in the second optical particle beam path in such a way that the second individual particle beams (9), which originate from the wafer surface positioned at the respective working distance, are focused on the detection regions (215) in the third field ( 217).

The multi-particle beam system (1) according to any one of the preceding claims, wherein the multi-particle beam system (1) further comprises a fast projection path correction means (844), which may have a multi-part embodiment and which is adapted to undertake a high-frequency adjustment of the focus of the second individual particle beams (9), of the grid arrangement, of landing angles and/or of the contrast of the second individual particle beams (9) after incidence on the detection regions (215) in the third field (217), and in which the controller (10) is arranged to generate a projection path control signal or a set of projection path control signals based on the actual autofocus data during wafer inspection at the respective operating point to control the fast projection path correction means (844).

The multi-particle beam system (1) according to any one of the preceding claims, wherein the multi-particle beam system (1) further includes a projection path measurement element (842) to generate projection path measurement data for characterizing the optical particle imaging in the second path during wafer inspection , wherein the multiple particle beam system (1) further comprises a fast projection path correction means (844), which may have a multi-part embodiment and which is adapted to undertake a high frequency adjustment of the focus of the second individual particle beams (9), of the grid arrangement, of landing angles and/or of the contrast of the second individual particle beams (9) after incidence on the detection regions (215) in the third field (217), and in which the controller (10) is arranged to generate a projection path control signal or a set of projection path control signals. based on the projection path measurement data during wafer inspection at the respective operating point to control the fast projection path correction means (844).

46. The multiple particle beam system (1) according to any one of claims 44 and 45, wherein a contrast diaphragm stop is disposed in the second optical particle beam path in a crossover plane, wherein the projection path corrector (844) comprises a fast contrast corrector with at least one electrostatic deflector, at least one electrostatic lens and/or at least one electrostatic stigmator for influencing the optical particle beam path through the contrast diaphragm stop, and wherein the controller (10) is arranged to control the contrast correction agent with a contrast correction control signal or a set of contrast correction control signals, in such a way that a contrast of the second individual particle beams (9) is kept essentially constant during the incidence on the detection regions (215) in the third field.

The multi-particle beam system (1) according to any one of the preceding claims, comprising a further autofocus correction lens (824) or a plurality of further fast autofocus correction lenses (824).

A method of operating a multi-particle beam system (1) according to any one of claims 1 to 47, the method comprising the steps of: generating measurement data at a first operating point for a current focus on the wafer surface;

determining actual autofocus data based on the measurement data; determining an autofocus correction lens control signal based on the actual autofocus data; and controlling a fast autofocus correction lens system (824) and keeping the focus on the wafer surface constant in a high frequency manner, wherein the grid arrangement and landing angle of the first individual particle beams (3) after incidence on the wafer surface are similarly kept constant on the first working point.

The method of operating a multi-particle beam system (1) according to the preceding claim, wherein the high-speed autofocus correction lens system (824) includes an electrostatic lens.

50. The method of operating a multi-particle beam system (1) according to claim 48 or 49, wherein the high-speed autofocus correction lens system (824) includes a magnetic lens.

The method of operating a multi-particle beam system (1) according to any one of claims 48 to 50, further comprising the steps of: generating a telecentricity control signal based on the actual autofocus data; and controlling the fast telecentricity means (825).

The method of operating a multi-particle beam system (1) according to any one of claims 48 to 51, further comprising the steps of: generating a rotation correction control signal on the basis of the actual autofocus data; and controlling the fast rotation corrector (826).

The method of operating a multi-particle beam system (1) according to any one of claims 48 to 52, further comprising the steps of: orthogonalizing effects of the optical particle components used for the correction or corrections.

The method of operating a multi-particle beam system (1) according to any one of claims 48 to 53, further comprising the steps of: generating projection path measurement data for characterizing the optical particle imaging in the second path; determining a projection path control signal based on the projection path measurement data; and controlling the fast projection path correction means (844), which may have a multi-part embodiment, by means of the projection path control signal or by means of a set of projection path control signals, in which the focus, the grip arrangement and the landing angle of the second individual particle beams (9) after incidence on the detection surface (211) must be kept constant at the first operating point.

The method of operating a multi-particle beam system (1) according to any one of claims 48 to 54, controlling a fast contrast correction agent by means of a contrast correction control signal or a set of contrast correction control signals and keeping the contrast constant in the detection plane ( 211).

A computer program product comprising program code for carrying out the method as claimed in any one of claims 48 to 55.