Imaging of nonplanar semiconductor-based structures using a scanning electron microscope

Abstract

The disclosure relates to a method and a system for imaging semiconductor-based structures by a scanning electron microscope that can provide improved image contrast and / or accuracy in identifying feature dimensions of semiconductor-based structures. In one implementation, the method comprises establishing first values of the SEM's beam parameters; a first adjustment stage to adjust the first values and obtain second values; a second adjustment stage to adjust second values and obtain third values; and controlling imaging components of the SEM using the third values.

Description

[0001] Description

[0002] IMAGING OF NONPLANAR SEMICONDUCTOR-BASED STRUCTURES USING A SCANNING ELECTRON MICROSCOPE

[0003] PRIORITY

[0004] This application claims priority of German application 102024 120 354.4 (filed July 18, 2024), the disclosure of which is incorporated herein by reference.

[0005] TECHNICAL FIELD

[0006] The disclosure relates to techniques for imaging semiconductor-based structures by a scanning electron microscope. In particular, techniques of determining and setting values of one or more beam parameters of a scanning electron microscope providing improved image quality for nonplanar semiconductor-based structures such as FinFETs are disclosed.

[0007] BACKGROUND

[0008] Current state-of-the-art semiconductor-based structures are built with minimum feature sizes or critical dimensions of down to about 5 nanometers; and semiconductor-based structures with smaller critical dimensions are being developed.

[0009] In order to obtain or ensure high quality semiconductor structures, features of such semiconductor-based structures are inspected. This may be part of process control of a fabrication process or part of end-of-line testing. Such inspection can be carried out by a scanning electron microscope, SEM. PCT / EP2023 / 025426, filed on October 10, 2023, describes a typical scanning electron microscope system, SEM, suitable for inspection of semiconductor-based structures.

[0010] To obtain high-quality images of semiconductor-based structures, values of one or more beam parameters of an SEM are determined and set. For instance, alignment of an SEM beam-shape is performed on test structures either on the inspected wafers (e.g. in the die cut frame) also including the semiconductor-based structures, on separate test samples, or onsite using the inspected structures. In the prior art techniques, optimize for the best contrast on the features of interest mainly the detection system of an SEM is optimized (energy range, acceptance angle).

[0011] WO 2023 / 099584 A1 aims at improving the performance of electron microscopes by automatically adjusting their focus and correcting distortions, known as astigmatism, which can blur images. It works by capturing multiple images of a sample at slightly different settings and analyzing them to determine the best adjustments.

[0012] US8766183 B2 uses a charged particle beam to scan and analyze samples. It includes a system that focuses the beam on the sample and can adjust based on the sample's height. The device has sensors that detect how the sample appears from different angles, which helps correct any visual distortions (called astigmatism) in the images produced.

[0013] US 2012 / 0138793 A1 describes a method and system for aligning a charged particle beam. The system uses two alignment coils to adjust the beam's direction by deflecting it in two different ways. To achieve proper alignment, the system collects multiple sets of image data while changing conditions like the beam's focus and the electric currents in the coils.

[0014] A specific type of SEM is a multi-beam SEM (MSEM). MSEM using multiple electron beams to image a sample in parallel. MSEM may be implemented via multiple beamlets where a beam is split by electron optics into sub-beams (beamlets). One example of electron optics can be a micro-optic lens such as the one disclosed in PCT / EP2023 / 025014, filed on January 18, 2023, describing a multi-beam generation unit with an array of micro-lenses or multi-pole elements of a multi-beam charged particle imaging system suitable for inspection of semiconductor-based structures. Therefore, PCT / EP2023 / 025014, filed on January 18, 2023 is hereby fully incorporated by reference. MSEMs offer a larger field of view compared to single-beam SEMs. This is achieved by contemporaneously scanning multiple beams; each beam scans a so-called sFOV (a single field of view); the sFOVs overlap and form an mFOV (multiple fields of view). Also, an MSEM offers the ability to set values of one or more beam parameters.

[0015] Currently SEM inspection and metrology tasks are performed mainly on two-dimensional (2D) structures using a stigmatic beam-shape. Modern semiconductor-based structures have nonplanar geometries (e.g. FinFET, GAA (gate-all-around) transistors, nanosheet transistors and alike) with dimension in the nanometre scale. Nonplanar semiconductorbased structures are often even more difficult to fabricate. Process control based on SEM inspection is therefore particularly important. Process control of modern integrated circuits involves investigating structure nested deep inside narrow trenches that may charge up with electron microscopes, both single beam and multibeam. For example, angular selective imaging of a source / drain semiconductor-based structure between narrow gates by an MSEM is described in Suzuki et. al. Proc. SPIE 101451 L (2017).

[0016] It has been observed that for images of nonplanar semiconductor-based structures the image quality of the SEM images when acquired using values of beam parameters that are determined in a conventional manner is insufficient. Particularly, it has been observed that other beam shapes than a conventional stigmatic beam may produce better imaging quality.

[0017] SUMMARY

[0018] In view of the above, there is a need to provide a method and a system for imaging semiconductor-based structures having nested structures. In particular, there is a need to provide techniques for imaging nonplanar semiconductor-based structures using an SEM or an MSEM.

[0019] This need is met by the features of the independent claims. The features of the dependent claims define additional embodiments.

[0020] Various techniques are based on the finding that imaging three-dimensional (3D) semiconductor-based structures requires careful setting of values of beam parameters - typically even more so if compared to imaging tasks for 2D semiconductor-based structures. In particular, charging effects may negatively affect the image quality and a respective compensation by appropriate setting of values of beam parameters may be helpful.

[0021] The disclosed techniques are used for mitigating various phenomena that occur particularly with nonplanar semiconductor-based structures such as FinFET structures. FinFET structures typically comprise protrusions (cross perpendicular linear structures) that are arranged on top of each other at different levels and have large relative differences in height when viewed from an inspection surface upstream an SEM beam. Additionally, certain parts of FinFETs form local capacities that can be recharged. It has been observed that with conventional settings of an SEM, for example, if a beam focus is set on randomly oriented structures in a plane, an optimum image quality cannot be achieved. In particular, when imaging a critical dimension (CD) of a feature of interest of the semiconductor-based structures (e.g., a trench critical dimension), certain properties of the nonplanar semiconductor-based structures may not be resolved using conventional techniques for determining and setting values of one of the parameters of an SEM.

[0022] It is an object of the disclosure to provide an improved imaging method and a system for imaging nonplanar semiconductor-based structures. In particular, it is an object of the disclosure to provide an improved imaging method and system for imaging at least one of the linear structures nested into a semiconductor-based structure.

[0023] For example, semiconductor-based structures may be FinFET structures having gates and fins, where fins may be referred to as “horizontal” structures and gates may be referred to as “vertical” structures.

[0024] Thus, the object of the invention is to provide an improved imaging method and a system for imaging at least one of the linear structures (horizontal or vertical) facilitating close to ideal imaging, preferably even for both, horizontal and vertical, at the same time.

[0025] One way of achieving better imaging when inspecting FinFET structures by an SEM includes controlling imaging components of the SEM (e.g., varying system settings such as autofocus and beam stigma providing beam astigmatism) to set beam parameters adapted to the FinFET geometry or more generally the 3D geometry of the nonplanar semiconductor-based structures. Tailored values of one or beam parameters, specifically adapted to the 3D geometry of the nonplanar semiconductor-based structures can be determined and set. For instance, an astigmatic beam may be used.

[0026] A stigmatic beam is typically focus at a single point in all imaging planes (planes of propagation) resulting in a circular cross-section perpendicular to the beam direction. An astigmatic beam typically has different focal points in different planes resulting in a noncircular cross-section perpendicular to the beam direction. Imaging of other geometries than FinFET (e.g., geometries comprising linear structures made of different materials nested into a semiconductor-based structure in a planar fashion) can also benefit from the method and the system of the current application, since the same or similar steps can be applied to mitigate spatial charge effects produced by a planar structure made of, e.g., different materials.

[0027] The embodiments of the current application are advantageously employing the multibeam capabilities of the SEM to mitigate the problems associated with the spatial charge effects. These are typically more pronounced in MSEM applications than in single beam SEMs.

[0028] In an aspect, a method for determining and setting values of one or more beam parameters of a scanning electron microscope, SEM, is provided. The method is adapted to imaging of nonplanar semiconductor-based structures. The one or more beam parameters comprise a working distance controlling beam focus, a stigX parameter controlling beam stigma in a first direction that is parallel to an outer surface of the semiconductor-based structures being imaged, and a stigY parameter controlling beam stigma in a second direction that is perpendicular to the first direction and parallel to the outer surface of the semiconductor-based structures. The method comprises establishing first values of the one or more beam parameters. The method further comprises performing a first adjustment stage, the first adjustment stage comprising: controlling one or more imaging components of the SEM to adjust the first values of the working distance to determine second values of the one or more beam parameters when a first feature of a test structure becomes visible or if a dimension of a second feature of the test structure is equal to or above a target value, wherein the test structure comprises a reference geometry and material for the nonplanar semiconductor-based structures to be imaged. The method further comprises, upon completing the first adjustment stage, performing a second adjustment stage, the second adjustment stage comprising: controlling the one or more imaging components of the SEM to incrementally adjust the second values of the one or more imaging parameters until a value of a contrast parameter in a region of interest reaches a threshold value, to thereby obtain third values of the one or more imaging parameters. The method further comprises controlling the one or more imaging components of the SEM to image the nonplanar semiconductor-based structures using the third values of the one or more beam parameters. According to some further aspects, a method for determining and setting values of one or more beam parameters of a scanning electron microscope, SEM, is provided. The method is adapted to imaging of nonplanar semiconductor-based structures. The one or more beam parameters comprise at least one of a working distance, controlling beam focus, an offset to the working distance, deltaZ, a stigX parameter controlling beam stigma in a first direction that is parallel to an outer surface of the semiconductor-based structures being imaged, a stigY parameter controlling beam stigma in a second direction that is perpendicular to the first direction and parallel to the outer surface of the semiconductorbased structures, a current applied at an objective lens of the SEM, OLCurrent, an offset current, deltaOLCurrent, applied at the objective lens, one or more calibration parameters comprising a current calibration parameter, deltaOLCurrentCalibration, applied to the objective lens and a working distance calibration parameter, deltaZCalibration. The method comprises pre-calibrating a stigmator, to obtain values of a first stigmator excitation parameter, E1 , and a second stigmator parameter, E2. Said pre-calibrating comprises setting first values of the one or more beam parameters comprising deltaZCalibration and deltaOLCurrentCalibration; performing autofocusing and autostigmation on a test structure by automatically adjusting first values of the one or more beam parameters comprising the working distance, the OLCurrent, stigX and stigY, applying deltaZCalibration to change the first value of the working distance to a second value and to obtain the value of the E1 parameter to be applied to a stigmator in order to readjust a line focus to be at the test structure; resetting the value second value of the working distance to the first value, and applying deltaOLCurrentCalibration to change the first value of the OLCurrent to a second value, and to obtain the value of the E2 parameter to be applied to the stigmator in order to readjust the line focus to be at the test structure. The method further comprises, after said pre-calibrating, performing autofocusing and autostigmation on a sample structure by automatically adjusting the first values of the one or more beam parameters comprising the working distance, OLCurrent, stigX and stigY until a first feature of the sample structure becomes visible. The method further comprises adjusting the first value of the working distance by deltaZ or adjusting the first value of OLCurrent by deltaOLCurrent until a trench at a region of interest on the sample structure becomes visible to obtain second values of the working distance and / or OLCurrent. The method further comprises calculating values of deltaStigX and deltaStigY parameters based on the values of E1 and E2, and a value of an angle selected according to structures of the sample structure. The method further comprises controlling the one or more imaging components of the SEM to image the nonplanar semiconductor-based structures using the values of deltaStigX and deltaStigY parameters. According to some further aspects, a method for determining and setting values of one or more beam parameters of a scanning electron microscope, SEM, is provided. The method is adapted to imaging of nonplanar semiconductor-based structures. The one or more beam parameters comprise a stigX parameter controlling beam stigma in a first direction that is parallel to an outer surface of the semiconductor-based structures being imaged, and a stigY parameter controlling beam stigma in a second direction that is perpendicular to the first direction and parallel to the outer surface of the semiconductor-based structures. The method comprises pre-calibrating one or more imaging components of the SEM comprising a stigmator. The pre-calibrating comprises determining a plurality of first values of stigX parameter and a plurality of first values of stigY parameter by imaging a test structure, wherein each of the plurality of first values of stigX parameter and each of the plurality of first values of stigY parameter is determined for a set of imaging parameters comprising electron energy and a beam current. The method further comprises, after determining the plurality of first values of stigX and the plurality of first values of stigY parameters, determining a plurality of a second values of stigX parameter and a plurality of second values of stigY parameter by imaging a sample structure, wherein the sample structure has a nonplanar geometry. The method further comprises calculating a plurality of stigX offset values and a plurality of stigY offset values, each stigX offset value being calculated as a difference between a first value of stigX and a second value of stigX, and each stigY offset value being calculated as a difference between a first value of stigY and a second value of stigY. The method further comprises performing autofocusing and autostigmation on the test structure to obtain third values of stigX and stigY parameters by changing values of the working distance, OLCurrent and stigX and stigY parameters. The method further comprises applying the stigX and stigY offset values to the stigmator to obtain fourth values of the one or more beam parameters for imaging the sample structure; and optionally, controlling the one or more imaging components of the SEM to incrementally adjust the fourth values until a value of a contrast parameter in a region of interest on the image of the test structure reaches a threshold value, to thereby obtain fifth values of the one or more beam parameters. The method further comprises controlling the one or more imaging components of the SEM to image the nonplanar semiconductor-based structures using at least the plurality of stigX offset values and the plurality of stigY offset values, and optionally, using the fourth or the fifth values of the one or more imaging parameters. In some examples, the test structure may be a silicone reference wafer having a planar geometry. According to some further aspects, a method for determining and setting values of one or more beam parameters of a scanning electron microscope, SEM, is provided. The method is adapted to imaging nonplanar semiconductor-based structures. The one or more beam parameters comprise a working distance controlling beam focus. The method comprises providing a sample structure comprising a first protrusion extending above a lower surface of the sample structure, the sample structure further comprising a second protrusion extending above the lower surface of the sample structure and above the first protrusion; determining a first value of the working distance, z1 , for focusing the beam along the an upper surface of the first protrusion, the first value of the working distance providing a first line focus along a first direction for imaging the nonplanar semiconductor-based structures at a first height, determining a second value of the working distance, z2, for focusing the beam along an upper surface of the second protrusion, the second value of the working distance providing a second line focus along a second direction perpendicular to the first direction and for imaging the nonplanar semiconductor-based structures at a second height greater than the first height; wherein the first line focus and the second line focus form a multi-line focus set at different heights in the direction parallel to the SEM beam; and controlling one or more imaging components of the SEM to image the nonplanar semiconductor-based structures using the first and second values of the working distance.

[0029] According to some further aspects, a method for determining and setting values of one or more beam parameters of a scanning electron microscope, SEM, for imaging of nonplanar semiconductor-based structures comprising a plurality of first protrusions and second protrusions is provided. The first protrusions are elongated along a first direction that is perpendicular to a second direction along which the second protrusions are elongated. The method comprises: setting the major axis orientation to be parallel to the first direction; and controlling one or more imaging components of the SEM to image the nonplanar semiconductor-based structures using the major axis orientation.

[0030] According to some further aspects, a method is provided. The method is used for determining and setting values of one or more beam parameters of a scanning electron microscope, SEM, for imaging of nonplanar semiconductor-based structures comprising one or more trenches. The one or more beam parameters comprise a numerical aperture defining beam divergence. The method comprises: receiving one or more target dimensions associated with the trench; determining at least one value of the numerical aperture based on the one or more target dimensions; and controlling one or more imaging components of the SEM to image the nonplanar semiconductor-based structures comprising the one or more trenches using the at least one value of the numerical aperture. In some examples, the one or more beam parameters may comprise a focus position along the beam axis; and the method may further comprise: iteratively adjusting a value of the focus position for the at least one value of the numerical aperture until a contrast value or a signal intensity in a region of interest, ROI; wherein controlling the one or more imaging components of the SEM to image the nonplanar semiconductor-based structures may further comprise using the at least one value of the focus position.

[0031] According to some further aspects, a computer system is provided. The computer system is adapted to determining and setting values of one or more beam parameters of a scanning electron microscope, SEM, for inspection of nonplanar semiconductor-based structures. The computer system comprises at least one computer processor configured to carry out method steps provided above. The computer system further comprises a computer interface communicatively coupling the SEM and the computer processor.

[0032] According to some further aspects, a computer program product is provided. The computer program product is adapted to determining and setting values of one or more beam parameters of a scanning electron microscope, SEM, for imaging nonplanar semiconductor-based structures. The computer program product comprises computer readable instructions, stored on an electronic storage medium, that when executed by a computer processor cause the computer processor to carry out method steps provided above.

[0033] It is to be understood that the features mentioned above and those yet to be explained below may be used not only in the respective combinations indicated, but also in other combinations or in isolation without departing from the scope of the invention.

[0034] BRIEF DESCRIPTION OF THE DRAWINGS

[0035] FIG. 1a schematically illustrates an example of spatial charge effects when a nonplanar semiconductor-based structure is imaged by a scanning electron microscope, SEM.

[0036] FIG. 1b schematically illustrates an example of optimizing resolution of fins (horizontal structures). FIG. 1c schematically illustrates an example of a deliberately introduced astigmatism for offsetting spatial charge effects induced by gates in upper levels as well as fins in lower levels of a nonplanar semiconductor-based structure.

[0037] FIG. 1d schematically illustrates an example of isotropic focus optimization.

[0038] FIG. 2 schematically illustrates a flow chart of a method 2000 according to various examples.

[0039] FIG. 3 schematically illustrates an example of optimizing contrast using multiple images with different beam-shapes.

[0040] FIG. 4a schematically illustrates an embodiment directed at determining a scanning region 4000a according to various examples.

[0041] FIG. 4b schematically illustrates an embodiment directed at applying an elliptical beam according to various examples.

[0042] FIG. 5 schematically illustrates technical advantages of a method according to various examples.

[0043] FIG. 6 is a schematic sectional view of an exemplary scanning electron microscope, SEM.

[0044] FIG. 7(a)-(c) schematically illustrate determining a numerical aperture of an SEM beam and / or a working distance for imaging different trenches or holes according to various examples.

[0045] FIG. 8(a)-(d) illustrate various examples of using the images obtained with the numerical apertures, such as determined in FIGs. 7, for measurement of trenches or holes.

[0046] DETAILED DESCRIPTION OF EMBODIMENTS

[0047] Some examples of the present disclosure generally provide for a plurality of circuits or other electrical devices. All references to the circuits and other electrical devices and the functionality provided by each are not intended to be limited to encompassing only what is illustrated and described herein. While particular labels may be assigned to the various circuits or other electrical devices disclosed, such labels are not intended to limit the scope of operation for the circuits and the other electrical devices. Such circuits and other electrical devices may be combined with each other and / or separated in any manner based on the particular type of electrical implementation that is desired. It is recognized that any circuit or other electrical device disclosed herein may include any number of microcontrollers, a graphics processor unit (GPU), integrated circuits, memory devices (e.g., FLASH, random access memory (RAM), read only memory (ROM), electrically programmable read only memory (EPROM), electrically erasable programmable read only memory (EEPROM), or other suitable variants thereof), and software which co-act with one another to perform operation(s) disclosed herein. In addition, any one or more of the electrical devices may be configured to execute a program code that is embodied in a non-transitory computer readable medium programmed to perform any number of the functions as disclosed.

[0048] In the following, embodiments of the invention will be described in detail with reference to the accompanying drawings. It is to be understood that the following description of embodiments is not to be taken in a limiting sense. The scope of the invention is not intended to be limited by the embodiments described hereinafter or by the drawings, which are taken to be illustrative only.

[0049] The drawings are to be regarded as being schematic representations and elements illustrated in the drawings are not necessarily shown to scale. Rather, the various elements are represented such that their function and general purpose become apparent to a person skilled in the art. Any connection or coupling between functional blocks, devices, components, or other physical or functional units shown in the drawings or described herein may also be implemented by an indirect connection or coupling. A coupling between components may also be established over a wireless connection. Functional blocks may be implemented in hardware, firmware, software, or a combination thereof.

[0050] Hereinafter, techniques of imaging semiconductor-based structures by a scanning electron microscope, SEM, are disclosed. Specifically, nonplanar semiconductor-based structures may be imaged.

[0051] The techniques described herein facilitate improved contrast of images obtained by the SEM according to the method of the current disclosure. Furthermore, the techniques described herein facilitate improved accuracy of measuring dimensions of features of interest of semiconductor-based structures. Various techniques are based on the finding that imaging nonplanar semiconductor-based structures may result in charging effects. Here, a local section of the nonplanar semiconductor-based structures is charged to another electric potential of compared to surrounding sections, impacting the primary and secondary electron beams in that area. Distortions can occur. The image quality is reduced.

[0052] FIG. 1a schematically illustrates aspects relating to charging effects when a nonplanar semiconductor-based structure is imaged by an SEM.

[0053] In particular, FIG. 1a illustrates a-stigmatic primary electron beam 3.i (impinging from an electron source along z-direction) focused at a nonplanar semiconductor based structure comprising wafer 7 having a planar surface 25 extending in the x-y plane. The semiconductor-based structure further comprises grooves between gates 703, insulating oxide layer 701 and gates 703 (present in some examples of a nonplanar structure or geometry). In some examples, fins 705 may be completely embedded in insulating oxide layer 701 with no electrical contact to bulk silicon wafer. In some examples, gates 703 may be formed by n+-doped conducting Polysilicon. In some examples, fins 705 may be formed by semiconducting weakly p-doped Polysilicon.

[0054] FIG. 1a further illustrates secondary electron trajectory 99 (99.1 and 99.2), extraction field 505, and cylindrical immersion field lenses 507.

[0055] In FIG. 1a, gates 703 are arranged along y direction and fins 705 are arranged along x direction at surface 25 (or nested into the surface). In this example, gates 703 and fins 705 protrude over surface 25, and thus, may be referred to as a plurality of first and second protrusions. In FIG. 1a, gates 703 have a height greater than fins 705. Gates 703 are arranged on top of fins 705, or nested into fins 705, in a linear fashion. In this example, gates 703 extend linearly along y direction while fins extend linearly along x direction, where x and y are perpendicular to each other and extend in a plane parallel to the surface of the wafer 7. Thus, a top view on surface 25 may amount to cross perpendicular lines along x and y representative of gates and fins (e.g., as illustrated by a layout in FIG. 5). The z direction is perpendicular to the plane defined by the x and y directions. Gates 703 and fins 705 protrude over surface 25 forming a nonplanar geometry. The 3D semiconductor-based structure comprising gates 703 and fins 705 on wafer 7 may be referred to as a nonplanar structure: The height variation (i.e., along z-axis) is significant if compared to their CDs along xy-directions.

[0056] In some examples, a depth difference between gates 703 and fins 705 in z-direction may be between 50nm - 100nm, or even more.

[0057] In some examples, gates (also referred to as vertical structures “V”) may be more prominent than fins (also referred to as horizontal “H” structures). This means that a cross-section in the XY-plane of the gates may be larger than a cross-section in the XY- plane of the fins. A cross section of the gates 703 may be, e.g., 25 nm x 100 nm. A crosssection of the fins 705 may be, e.g., 7 nm x 50 nm. Gates may be referred to as a first structure(s) and fins may be referred to as a second structure(s) of a nonplanar semiconductor-based structure.

[0058] The semiconductor-based structure may be associated with a test structure comprising a reference geometry representative of the geometry of the semiconductor-based structure. Such test structure may be nonfunctional, i.e., it may be prepared primarily or solely for imaging and process-control purposes. A test structure may comprise features representative of features of a sample structure. The semiconductor-based structure may also be a sample structure, i.e., a functional structure.

[0059] In FIG. 1a, interaction volume 707 of primary beam 3.i (or beamlets 3.1 in FIG. 6) with a sample structure serves as a source of secondary electrons producing the spatial charge that needs to be accounted for when imaging the sample structure.

[0060] In some examples, gates 703 may be isolated from fins 705 by an oxide layer SiO2 (layer 701 in FIG. 1) or SiNx-Layer (not shown in FIG. 1a).

[0061] Isolated conducting areas such as gates 703 may form capacities which charge up during irradiation with multiple beams of the SEM. During electron beam irradiation by the SEM, charges may stick to insulating surfaces such as to insulating oxide layer 701 thereby contributing to the spatial charge effects. The gate structures shown in FIG. 1a may charge up. For positive charge up, the secondary electrons originating from structures buried between the gates, the secondary electron may get attracted by the gates, possibly not reaching the detector. As a consequence, the signal stemming from the structures to be inspected is reduced, thus reducing throughput or even compromise the entire inspection or quality control of semiconductor-based structures during imaging by an SEM.

[0062] The charging effect make it challenging to image nonplanar semiconductor-based structures, particularly, concerning low kV-imaging of FinFET-Samples with multiple beams.

[0063] Field gradients of the extraction field may form cylindrical ..immersion" lenses 507 between gates 703. This effect can be amplified by charging effects within gates especially by multi-beam irradiation with high global current of beams.

[0064] This problem generally exists for topographies characterized by horizontal and vertical semiconductor-based structures (HV-structures). In the example of the FinFET, illustrated in FIG. 1a, ..vertical" (V) may relate to gates 703 elongated in y-direction and ..horizontal" (H) may related to fins 705 elongated in x-direction in FIG. 1a.

[0065] Typically, standard stigma and autofocus setting of multiple beams of an SEM at standard reference samples with planar surfaces and random oriented structures do not provide ideal imaging conditions at samples with the topography and structure orientation such as FinFETs illustrated by FIG. 1a (i.e., FinFETs comprising gates 703 and fins 705 on wafer 7).

[0066] It is an object of the current disclosure to mitigate the problems associated with the formation of the spatial charges for improving imaging of semiconductor-based structures such as FinFETs illustrated in FIG. 1a.

[0067] It has turned out that setting one or more beam parameters of the primary beams to certain values can improve imaging of semiconductor-based structures. The image quality can be improved. In some examples, a better imaging resolution of fins can be achieved at the expense of imaging quality of gates. In another example, better imaging quality of semiconductor-based structures can be similarly achieved for both structures (i.e., gates and fins). Thus, in other words, it is possible to tailor values of one of the parameters of the primary beams of an SEM to image a certain kind of semiconductor-based structure in an overall arrangement of semiconductor-based structures. It would also be possible to optimize beam parameters so that multiple different kinds of semiconductor-based structures in an overall arrangement of semiconductor-based structures can be imaged at similar quality.

[0068] As a general rule, such tailored determining and setting of values of one or more beam parameters can be implemented for a single-beam SEM. It would, however, also be possible to determine and set values of one or beam parameters for an MSEM. Here, it would be possible to collectively determine one and the same value for each of the multiple beams. It would also be possible to determine different values of the same beam parameter for different ones of the multiple beams of the MSEM.

[0069] Sometimes, appropriate hardware may be available to individually set values of a given beam parameter for each of multiple beams. For instance, a multi-aperture plate (MAP) with lenses for each primary beam may be available. For instance, beam stigma may be set for each of multiple beams (“multi-stigmator plate”). Then, it is possible to determine, individually for each of the multiple beams, the respective value of the beam parameter. On the other hand, sometimes, values of a certain beam parameter may not be individually set for each of multiple beams, even if a certain variation of the value is possible from beam to beam. For instance, it may be required to observe certain constraints in the variation of a value of a given beam parameter from beam to beam. I.e., it may be required to set values of a given beam parameter in a correlated manner for multiple beams of an MSEM.

[0070] One particular type of beam parameter that can be optimized is beam astigmatism. In other words, deliberately introducing beam astigmatism can improve the imaging quality (e.g., contrast and / or accuracy of a dimension of a feature of interest such as a critical dimension of a trench).

[0071] This deliberate astigmatism may be defined by one or more beam parameters, e.g., stigY and stigX parameters.

[0072] StigX parameter controls beam stigma in a first direction (e.g., along x-axis in FIG. 1a) that is parallel to an outer surface 25 of the semiconductor-based structures / wafer substrate being imaged. StigY parameter controls beam stigma in a second direction (e.g., along y-axis in FIG. 1a) that is perpendicular to the first direction (x) and parallel to the outer surface 25 of the semiconductor-based structures. The semiconductor-based structure in this example is a structure comprising gates 703 and fins 705 arranged on a wafer 7.

[0073] In various examples, the method and the system of the current application may be particularly suitable for imaging nonplanar semiconductor-based structures. Thus, various examples will be particularly explained in the context of nonplanar semiconductor-based structures. However, similar spatial charge effects can be seen in a planar geometry due to, for example, different materials forming horizontal and vertical lines on a planar surface in a way equivalent to gates and fins of the nonplanar geometries. Thus, the techniques disclosed herein can be equally applicable to imaging planar semiconductor-based structures.

[0074] FIG. 1b schematically illustrates an example of optimizing an image resolution for fins (horizontal structures) of FinFETs. In some examples, resolution of fins (horizontal structures) can be achieved at the expense of imaging quality of gates (vertical structures).

[0075] In this example, the nonplanar semiconductor-based structure may be associated with a topography having HV-structures (upper V-Gates 703, lower H-Fins 705) with a depth difference between upper gates and lower fins in a range of 50 nm to 100 nm. In this example, as in the previous examples, “V” may refer to vertical structures or protrusions (arranged along y-axis) and “H” may refer to horizontal structures or protrusions (arranged along x-axis). In this example, “H” structures are lower than “V” structures when viewed from surface 25 upstream beam 3.i of the SEM.

[0076] The example of FIG. 1b is based on the finding that charging of gates may result in a significant linear gradient forming cylindrical ..immersion" lenses. The ..immersion" lenses can be compensated for by applying an astigmatic beam shape for imaging or astigmatism described also in the context of FIG. 1a. The appropriate astigmatic beam shape compensating the ..immersion" lenses can be achieved by determining appropriate stigmator settings facilitating appropriate values of beam parameters (e.g., stigX and stigY parameters). Compensating for the ..immersion" lenses may include forming an el liptically shaped primary beam at a position 1002 above the “immersion” lenses. Then, at a position 1004, after passing the “immersion” lenses, beam astigmatism can be compensated, and high resolution can be achieved in imaging the fins. Thus, deliberately introduced beam astigmatism illustrated at 1002 in FIG. 1b can be “compensated” by the spatial charge produced in interaction volume 707 described in the context of FIG. 1a. Deliberately introduced astigmatism facilitates a fine focus, as illustrated at 1004 in FIG. 1b thereby improving imaging resolution of FinFET structures.

[0077] In some examples, field gradients can be strong, since distance between gates can be small. In some examples, kinetic energy of primary electrons can be low (e.g., below 0.5 keV or even below 0.3keV), thus “immersion” field associated with the “immersion” lenses can have relatively high impact on relatively slow primary electrons. Deliberately introduced astigmatism can offset said impact thereby improving imaging resolution.

[0078] FIG. 1c schematically illustrates an example of a deliberately introduced astigmatism for offsetting spatial charge effects induced by gates in upper levels as well as fins in lower levels of a nonplanar semiconductor-based structure. Using a beam shape of a primary beam as illustrated in FIG. 1c enables to optimize image quality for, both, the gates as well as the fins.

[0079] The semiconductor-based structures in FIG. 1c correspond to the semiconductor-based structures in FIG. 1b.

[0080] In reference implementations, stigmation and autofocusing, AF, algorithms are optimized for general objects with isotropic feature orientation. In the example of the HV-structures of different heights, optimization for HV-structures can generate the best imaging result with astigmatic beams, i.e. combined optimization of astigmatism (AS), and autofocusing, AF. Details with respect to such approaches of optimizing focus and astigmatism will be later on explained.

[0081] FIG. 1d schematically illustrates an example of focus optimization. FIG. 1d, left, illustrates the cross-sectional beam shape in the xy-plane of the primary beam 3 as a function of z- position. FIG. 1 d, right, illustrates the isotropic contrast Cl(z) as a function of z-position. In FIG. 1 d, the minimum beam width along x direction (line focus 76.2 along y direction) is observed at a working distance 76A.2 along z-direction; while the minimum beam width along y direction (line focus 76.1 along x direction) is observed at another working distance 76A.1. The z-positions / working distances 76A.1, 76A.2 are offset from a center plane 101 by an astigmatic difference AD / 2. Their distance 73 is also shown.

[0082] In this example, overall isotropic contrast Cl(z) is lower compared to directional contrast along x-direction and y-direction, Cy(z) and Cx(z), where Cx(z) illustrates contrast for structures extending in x direction, and Cy(z) illustrates contrast for structures extending in y direction. In this example, contrast loss over defocus is nearly parabolic close to focus.

[0083] By setting the working distance 76A.1 (76A.2) to a feature surface of a feature (another feature) extending in y direction (x direction), the respective imaging quality of the feature (another feature) can be optimized. In such case, the distance 73 corresponds to a height difference between the feature and the another feature.

[0084] Above, aspects with respect to shaping a primary beam of an SEM to obtain improved image quality have been disclosed. For this, values of one or more beam parameters are set. Next, techniques of determining such values of the beam parameters that allow for shaping the primary beam of the SEM are disclosed. Techniques are disclosed that enable to find best values of the beam parameters, optimizing the image quality.

[0085] FIG. 2 schematically illustrates a flow chart of a method 2000 according to various examples. The method is for determining values of one or more beam parameters.

[0086] The method 2000 comprises, at step S2002, establishing first values of SEM’s beam parameters; at step S2004, a first adjustment stage to adjust the first values and obtain second values; at step S2006, a second adjustment stage to adjust second values and obtain third values; and, at step S2008, controlling imaging components of the SEM using the third values.

[0087] The method 2000 is adapted for determining and setting values of one or more beam parameters of an SEM, for imaging of nonplanar semiconductor-based structures, the one or more beam parameters comprising a working distance controlling beam focus, a stigX parameter controlling beam stigma in a first direction that is parallel to an outer surface of the semiconductor-based structures being imaged, and a stigY parameter controlling beam stigma in a second direction that is perpendicular to the first direction and parallel to the outer surface of the semiconductor-based structures.

[0088] The first adjustment stage comprises controlling one or more imaging components of the SEM to adjust the first values of the working distance to determine second values of the one or more beam parameters when a first feature of a test structure becomes visible or if a dimension of a second feature of the test structure, e.g., calculated from an image of the test structure captured by the SEM, is equal to or above a target value, wherein the test structure comprises a reference geometry and material for the nonplanar semiconductorbased structures to be imaged.

[0089] The second adjustment stage comprises controlling the one or more imaging components of the SEM to incrementally adjust the second values of the one or more imaging parameters until a value of a contrast parameter in a region of interest reaches a threshold value, to thereby obtain third values of the one or more imaging parameters.

[0090] In some examples, the test structure may also be the nonplanar semiconductor-based structures themselves.

[0091] FIG. 3 schematically illustrates an example of optimizing contrast using multiple images with different beam-shapes.

[0092] In this example, images are recorded with different beam-shapes optimized for the visibility of different structures or features (e.g., gates, fins, trenches, nested linear structures, etc). These different images may be numerically combined as further described in the context of Embodiment 6. As a result, a higher contrast for the features of interest can be achieved compared to only a single scan.

[0093] For example, a different beam shape may be provided by a beam or subset of beams having a first astigmatism 3002 and a beam or subset of beams having a second astigmatism 3004.

[0094] According to various examples, a single scan with multiple beams may be carried out with subfields overlapping such that each of two subsets of beams cover the wafer surface, a first subset with first astigmatism, a second subset with second astigmatism. Image processing and image fusion may be applied after parallel image acquisition of two images with one scan.

[0095] Aggregating or fusing the two images for postprocessing can further improve the imaging since the two images can be obtained at different beam parameters allowing for averaging out the contrast or dimensions of features of interest from the two images.

[0096] FIG. 4a schematically illustrates an embodiment directed at determining a scanning region 4000a according to various examples The scanning or imaging may be implemented according to a scanning pattern comprising a plurality of scanning regions and excluded regions. As described above in connection with FIG. 1A, the area in between adjacent gates of a FinFET can charge up by imaging.

[0097] Avoiding charge up of gates may be achieved by imaging areas between the gates, using the fact that the gates structures are linear, much larger than a typical field of view (FOV) of an SEM and oriented along both scan directions, x and y.

[0098] According to various examples, a scanning area 4000A (i.e. , the region in which the primary beams are scanned in the xy-plane) may be restricted to an area to in-between adjacent gates 703, excluding the gates.

[0099] At Step 1 , a fast scan of the FOV may be performed. This is possible as the gates provide a strong SE signal in comparison to other structures (e.g., fins).

[0100] At Step 2, the y locations of the gate structures may be identified.

[0101] At Step 3, a scan pattern may be set up, wherein imaging only is performed in between the gates.

[0102] At Step 4, reduced imaging can be performed, in the sense that only the imaging area between the gates is imaged.

[0103] As an alternative, Step 1 and 2 could be replaced by identifying the y locations of the gate structures from layout design files indicative of the position of the gates in y-direction.

[0104] According to various examples, a pre-scan for identifying the gates can be, e.g., performed as the first frame (first image) of an entire set of frames (images). According to various examples, identification of gates can, e.g., be performed by integration of the pre-scan frame along the x-axis and applying a threshold. This can be performed within the scan electronics fast, e.g. after each line scan. In doing so, scan positions for the following frames (i.e., reduced imaging on a second image) can be defined for the next frame already. Moreover, if the gates are properly orientated in x direction, a simple line scan in y direction may be sufficient to identify the y positions of the gates.

[0105] As a result, from a first image (alternatively, from a line scan or a layout) scanning region 4000a can be identified. This scanning region provides a reduced imaging area on a second image to the area comprised between two consecutive gates thereby excluding imaging the gates for improving imaging of the fins.

[0106] FIG. 4b schematically illustrates an embodiment directed at applying an elliptical beam according to various examples.

[0107] Beams 3(a) and 3(b) in FIG. 4b are variations of primary beam 3.i in FIG. 1 or beamlets 3.1 in FIG. 6. Beam 3(b) has an elliptical shape and beam 3(a) has a circular shape.

[0108] The problem associated with spatial charging of gates 703 described above may be addressed by choosing the orientation of an elliptical focus and the size of the focal spot along the major axis of an ellipse.

[0109] As charging of the gates is related to the beam current density, the charging of the gates can be reduced by adjusting the focal spot shape. As an example, a suitable multipole lens in the column can be used to induce an astigmatic beam shape, i.e., setting StigX and StigY appropriately. The StigX and StigY parameters then can provide astigmatic beam such as, for example, illustrated in FIG. 3. By an appropriate refocusing, an elliptically shaped focal spot 3(b) can be produced at the sample surface (e.g., surface 25 in FIG. 1a).

[0110] By choosing the orientation of this ellipse (i.e., beam 3(b) in FIG. 4b) such that its major axis is parallel to the fins 705 (oriented along x-direction), the resolution (defect sensitivity) along the y-direction can be maintained while reducing the beam current density. The length of the major axis of beam 3(b) may be chosen to be longer than an inter-gate distance.

[0111] A further beneficial effect of such an elliptically shaped focal spot is that it may charge up several gates at once. This reduces the effects of the sample charging on the secondary electrons because the charging becomes more homogeneous. For example, elliptical focal spot of the beam 3(b) charges two gates simultaneously producing more homogeneous charging, while on the left-hand side of FIG. 4b the circular spot of the beam 3(a) can only charge one gate at a time producing inhomogeneous charging.

[0112] FIG. 5 schematically illustrates technical advantages of a method according to various examples.

[0113] In FIG. 5(a), a layout of a semiconductor-based structure is illustrated. The layout indicates a first structure (gates 703), a second structure (fins 705), and a dimension 5006 (distance between two gates). The semiconductor-based structure is oriented such that the gates 703 are arranged along the y axis and fins 705 are arranged along the x axis. Accordingly, FIG. 5(a) shows an x-y plane. A cross section of the semiconductor-based structure in y-z plane is further illustrated in FIG. 5(b). Figs. 5(c) and (d) illustrate an image of the semiconductor-based structure obtained by an SEM without and with optimisation of autofocus, AF, and astigmatism, AS, respectively.

[0114] Accuracy of measuring dimension 5006 may be identified by comparing the dimension obtained from the layout (FIG. 5, a) to the dimension obtained from an image (FIG. 5, c and 5, d). As can be seen from FIG. 5, optimisation provided by the method of the current application can improve accuracy of measuring dimensions of features of interest of semiconductor-based structures. Moreover, it increases the visibility of structures located in between the horizontal structures, thereby enabling inspection of these structures.

[0115] FIG. 6 is a schematic sectional view of an exemplary MSEM.

[0116] The method of the current disclosure may be used for or is adapted to a scanning electron microscope, SEM, such as the one described in the context of FIG. 6.

[0117] The one or more imaging components of the SEM referred to through the application may relate to objective lens 102, collimating lens 303, multi-aperture plates 304, 306, electrostatic or magnetic lenses 205.1 to 205.5, and / or alike. These components may be controlled by a computer processor such as a computer processor of control unit 800 in FIG. 6.

[0118] The schematic representation of FIG. 6 illustrates basic features and functions of multibeam charged-particle system 1 similar to the one disclosed in International Patent application PCT EP2023 / 025426, filed on October 10, 2023, which is hereby fully incorporated by reference.

[0119] System 1 in FIG. 6 comprises, among other components, an object irradiation unit 100; a detection unit 200; a control unit 800; charged particle beam spots 5 (i.e., beam focus) focused at a surface 25 of an object 7; objective lens, OL, 102 being one of the imaging components of SEM; field lens group 103 being one of the imaging components of the SEM; sample stage 500; a charged-particle multi-beam generator 300; primary electron beams 3 (3.1 , 3.2, 3.3); secondary electron beams 9 (9.1 , 9.2, 9.3); at least one collimating lens 303 being one of the imaging components of the SEM; a first multiaperture plate or filter plate 304 being one of the imaging component of the SEM; a second multi-aperture plate 306 being one of the imaging components of the SEM; electrostatic or magnetic lenses 205.1 to 205.5 each being one of the imaging components of SEM. FIG. 6 further illustrates a secondary charged-particle beam path 11 from a primary charged-particle beam path 13. The beams 3 and 9 are implemented by multiple beamlets 3.1 , 3.2, 3.3 and 9.1 , 9.2, 9.3, each beamlet formed by a part of the collimated primary charged particle beam 309 that is incident on the primary multi-beam forming unit 305. Thus, each primary charged particle beamlet (3.1, 3.2, 3.3) may be referred to as one of the plurality of primary charged particle beams (3). Similarly, each of beamlets 9.1, 9.2, 9.3 may be referred to as one of the plurality of beams 9.

[0120] Referring further to FIG. 6, some array elements, for example the plurality of primary charged particle beams, are identified by a reference number. Depending on the context, the same reference number may also identify a single element out or the array elements. Each primary charged particle beamlet (3.1 , 3.2, 3.3) is one of the plurality of primary charged particle beams (3).

[0121] It is to be noted that the symbols used in the figure have been chosen to symbolize their respective functionality. The type of system shown is that of a multibeam scanning electron microscope using a plurality of primary charged particle beams 3 for generating a plurality of primary charged particle beam spots 5 on a surface 25 of an object 7, such as a wafer or mask substrate located with a top surface 25 in an object plane 101 of an objective lens 102. For simplicity, only three primary charged particle beams 3.1 to 3.3 and three primary charged particle beam spots 5.1 to 5.3 are shown. The features and functions of multi-beam charged-particle system 1 can be implemented using electrons or other types of primary charged particles, such as ions, for example, Helium ions. Further details of the microscope system 1 are provided in International Patent application WO 2022 / 262970 A1 , filed on June 16, 2021 , which is hereby fully incorporated by reference.

[0122] The system 1 comprises an object irradiation unit 100 and a detection unit 200 and a secondary electron beam divider or beam splitter unit for separating the secondary charged-particle beam path 11 from the primary charged-particle beam path 13. The object irradiation unit 100 comprises a charged-particle multi-beam generator 300 for generating the plurality of primary charged-particle beams 3 and is adapted to focus the plurality of primary charged-particle beams 3 on the object plane 101, in which the surface 25 of an object or wafer 7 is positioned by a sample stage 500.

[0123] The primary beam generator 300 produces a plurality of primary charged particle beam spots in an intermediate image surface 321. The primary beam generator 300 comprises at least one source 301 of primary charged particles, for example electrons. The at least one primary charged particle source 301 emits a diverging primary charged particle beam, which is collimated by at least one collimating lens 303 to form a collimated or parallel primary charged particle beam 309. The collimating lens 303 usually includes one or more electrostatic or magnetic lenses, or by a combination of electrostatic and magnetic lenses. The collimated primary charged particle beam 309 is incident on the primary multi-beam forming unit 305. A multi-beam generating unit 305 is for example explained in US 2019 / 0259575, and in US 10,741 ,355 B1 , both hereby incorporated by reference. The multi-beam forming unit 305 basically comprises a first multi-aperture plate or filter plate 304 illuminated by the collimated primary charged particle beam 309. The first multiaperture plate or filter plate 304 comprises a plurality of apertures in a raster configuration for generating the plurality of primary charged particle beams 3, which are generated by transmission of the collimated primary charged particle beam 309 through the plurality of apertures. The multi-beam forming unit 305 comprises at least one further multi-aperture plate 306, which is located, with respect to the direction of movement of the electrons in beam 309, downstream of the first multi-aperture or filter plate 304. For example, a second multi-aperture plate 306 comprises for example four or eight of electrostatic elements for each of the plurality of apertures, for example to deflect each of the plurality of beams individually. The multi-beam forming unit 305 according to some embodiments is configured with a terminating multi-aperture plate 307. The multi-beam forming unit 305 is further configured with an adjacent electrostatic field lenses 308.1, which is in some examples combined in the multi-beam forming unit 305. Together with a second field lens 308.2, the plurality of primary charged particle beams 3 is focused in or in proximity of the intermediate image surface 321. The primary charged-particle source 301 and each of the active multi-aperture plates 306 are controlled by control unit 800.

[0124] The plurality of focus points of primary charged particle beams 3 passing the intermediate image surface 321 is imaged by field lens group 103 and objective lens 102 into the object plane 101, in which the surface 25 of the object 7 is positioned. A decelerating electrostatic field is generated between the objective lens 102 and the object surface 25 by application of a voltage to the object by the sample voltage supply. With the decelerating electrostatic field generated by the sample voltage supply, a landing energy of primary electrons is adjusted to for example below 1keV, below 500 eV, below 300eV or even less.

[0125] The object irradiation system 100 further comprises a collective multi-beam raster scanner 110 in proximity of a beam cross over 108 by which the plurality of charged particle beams 3 can be deflected in a direction perpendicular to the propagation direction of the charged particle beams. The propagation direction of the primary beams throughout the examples is in the positive z-direction. Objective lens 102 and collective multi-beam raster scanner 110 are centred at an optical axis (not shown) of the multi-beam charged-particle system 1 , which is perpendicular to wafer surface 25. The plurality of primary charged particle beams 3, forming the plurality of beam spots 5 arranged in a raster configuration, is scanned synchronously over the wafer surface 25.

[0126] In an example, the raster configuration of the focus spots 5 of the plurality of J primary charged particle 3 is a hexagonal raster (having a pitch of a hexagon) of about one hundred or more primary charged particle beams 3, for example J = 91, J = 100, or J approximately 300 or more beams. The primary beam spots 5 have a distance about 6pm to 45pm and a diameter of below 5nm, for example 3nm, 2nm or even below. In an example, the beam spot size is about 1.5nm, and the distance between two adjacent beam spots is 8pm (a pitch of the hexagon). At each scan position of each of the plurality of primary beam spots 5, a plurality of secondary electrons is generated, respectively, forming the plurality of secondary electron beams 9 in the same raster configuration as the primary beam spots 5. The intensity of secondary charged particle beams 9 generated at each beam spot 5 depends on the intensity of the impinging primary charged particle beam 3, illuminating the corresponding spot 5, the material composition and topography of the object 7 under the beam spot 5, and the charging condition of the sample at the beam spot 5. The plurality of secondary charged particle beams 9 are accelerated by the same electrostatic field between objective lens 102 and object surface 25, generated by voltage supply, and are collected by objective lens 102 and pass the first collective multi-beam raster scanner 110 in opposite direction to the primary beams 3. In other words, applying a voltage by the voltage supply can control one or more imaging components of the SEM (i.e. , providing required electrostatic field between objective lens 102 and object surface 25).

[0127] The plurality of secondary beams 9 can be deflected by the first collective multi-beam raster scanner 110. The plurality of secondary charged particle beams 9 can then be guided by a secondary electron beam divider or beam splitter unit to follow the secondary beam path 11 to the detection unit 200. The plurality of secondary electron beams 9 travels in the opposite direction from the primary charged particle beams 3, and the beam splitter unit can be configured to separate the secondary beam path 11 from the primary beam path 13 usually via magnetic fields or a combination of magnetic and electrostatic fields.

[0128] Detection unit 200 images the secondary electron beams 9 onto the image sensor 600 to form there a plurality of secondary charged particle image spots 15. The detector or image sensor 600 comprises a plurality of detector pixels or individual detectors. For each of the plurality of secondary charged particle beam spots 15, the intensity is detected separately, and the property of the object surface 25 is detected with high resolution for a large image patch of the object 7 with high throughput. For example, with a raster of 10 x 10 beams with 8pm pitch (i.e., the pitch of the hexagon), an image patch of approximately 88pm x 88pm is generated with one image scan with collective multibeam raster scanner 110, with an image resolution of for example 2nm or below. The image patch is sampled with half of the beam spot size, thus with a pixel number of 8000 pixels per image line for each beam, such that the image patch generated by 100 beams comprises 6.4 gigapixel. The digital image data is collected by control unit 800 configured to control the one or more imaging components of the SEM. Details of the digital image data collection and processing, using for example parallel processing, are described in international patent application WO 2020151904 A2 and in US-Patent US 9,536,702, which are hereby incorporated by reference.

[0129] Detection unit 200 further comprises at least a second collective raster scanner 222, which is connected to scanning and imaging control unit 860. Scanning control unit 860 is configured to compensate a residual difference in position of the plurality of focus points / focus spots 15 of the plurality of secondary electron beams 9, such that the positions of the plurality secondary electron focus spots 15 are kept constant at image sensor 600.

[0130] The detection unit 200 comprises further electrostatic or magnetic lenses 205.1 to 205.5 and a second cross over 21 of the plurality of secondary electron beams 9, in which a contrast aperture filter module 214 is located. The second cross over corresponds to a pupil plane 21 of the detection unit 200. In a pupil plane, a lateral coordinate with respect to the optical axis 2105 corresponds to a propagation angle of a secondary electron trajectory at the object plane 101. The propagation angle of a secondary electron trajectory is measured relative to the wafer surface normal, which corresponds to the optical axis 2105 of the detection unit 200.

[0131] The detection unit 200 further comprises at least a first multi-aperture corrector 216, with apertures and electrodes for individual influencing each of the plurality of secondary electron beams 9. The multi-aperture corrector 216 is arranged in proximity to an intermediate image plane 211, where the secondary electron beams are separated from each other.

[0132] The image sensor 600 is configured by an array of sensing areas in a pattern compatible to the raster arrangement of the secondary electron beams 9 focused by the projecting lenses 205 onto the image sensor 600. This allows for detection of each individual secondary electron beam independent from the other secondary electron beams incident on the image sensor 600. The image sensor 600 illustrated in FIG. 6 can be an electron sensitive detector array such as a CMOS or a CCD sensor. Such an electron sensitive detector array can comprise an electron to photon conversion unit, such as a scintillator element or an array of scintillator elements. In some embodiments, the image sensor 600 can be an electron to photon conversion unit or scintillator plate arranged in the focal plane of the plurality of secondary electron particle image spots 15. In such embodiments, the image sensor 600 can further comprise a relay optical system for imaging and guiding the photons generated by the electron to photon conversion unit at the secondary charged particle image spots 15 on dedicated photon detection elements, such as a plurality of photomultipliers or avalanche photodiodes (not shown). Such an image sensor is disclosed in US 9,536,702, which is cited above and incorporated by reference.

[0133] During an acquisition of an image patch by scanning the plurality of primary charged particle beams 3, the stage 500 is typically not moved, and after the acquisition of an image patch, the stage 500 is moved to the next image patch to be acquired. In some implementations, the stage 500 is continuously moved in a second direction while an image is acquired by scanning of the plurality of primary charged particle beams 3 with the collective multi-beam raster scanner 110 in a first direction. Stage movement and stage position is usually monitored and controlled by sensors known in the art, such as Laser interferometers, grating interferometers, confocal micro lens arrays, or similar.

[0134] During an image scan, the control unit 800 triggers the image sensor 600 to detect in predetermined time intervals a plurality of timely resolved intensity signals from the plurality of secondary electron beams 9, and the digital image of an image patch is accumulated and stitched together from all scan positions of the plurality of primary charged particle beams 3.

[0135] The control unit 800 of the multi-beam charged-particle system 1 further comprises: an imaging control module 810, configured to receive the data streams from the image sensor 600 and to generate a digital image (or as also referred to in the application, a first image, a second image, an image) of the surface of the sample 7 during operation; a secondary beam-path control module 820, configured to control the lenses 205 and other components of the detection unit 200; a primary beam-path control module 830, configured to control the elements of the object irradiation unit 100, including the charged- particle multi-beam generator 300; a stage control module 850, configured to control the stage positioning and alignment, and including control of the sample voltage supply unit 503; a scanning operation control module 860, configured to control a scanning operation by the first collective multi-beam raster scanner 110 and the second deflection system 222; a control operation processor unit 840, configured to execute inspection tasks of samples; and configured to control the modules 810, 820, 830, 850, 870, control unit 860 and a memory 880 for storing software, instructions and image data. The control operation processor unit 840 is further connected to an interface (not shown) for exchanging data, instructions, software or user interaction. The control unit 800 of the multi-beam charged-particle system 1 according to the disclosure further comprises a contrast control module 870, connected to the control operation processor unit 840. The contrast control module 870 is configured to receive instruction from the control operation processor unit 840 to control a contrast mechanism of the imaging of secondary electrons onto the image sensor 600. The contrast control module 870 is therefore connected to an aperture filter module 214 and configured to select an aperture filter 284 according to the selected contrast mechanism. For simplicity, only two different aperture filters 284a and 284b are shown, but there can be provided more than two different aperture filters 284. The aperture filters 284 can be mounted on an exchange mechanism, such as a rotary or linear moving mechanism 215 for placement of the selected aperture filter 284a in the common pupil position 21 of the plurality of secondary electron beams 9.

[0136] According to the techniques described herein, an MSEM can be used for feature inspection of semiconductor-based structures.

[0137] FIGs. 7(a)-(c) schematically illustrate determining a numerical aperture of an SEM beam and / or a working distance for imaging different trenches or holes according to various examples.

[0138] Trenches / holes with different depths show different brightnesses in SEM images because the secondary electron signal originating at the bottom is typically cut off depending on the aspect ratio of the trench / hole.

[0139] For example, Proceedings Volume 12496, Metrology, Inspection, and Process Control XXXVII; 124961V (2023), htps: / / doi.org / 10.1117 / 12.2656471 describes measuring trenches of different depth and the problems associated with signal originated at the bottom being cut off.

[0140] For example, in 3D NAND devices, holes with aspect ratios of approx. 1 :200 occur (see e.g. US 2021 / 0327770). If the hole depth is to be measured, the SE (scattering electron) signal is very weak and the sensitivity to the hole depth is therefore weak.

[0141] The trenches or holes may be formed by the first and / or second protrusions (gates and / or fins; or by alike structures) as described in the context of previous embodiments. In FinFET samples, for example, the fin features should be checked for bridging defects, i.e. whether the trenches between the fins are fully developed. Measuring the trench depth (e.g., formed by fins) with maximum sensitivity may be advantageous.

[0142] By choosing an appropriate value of a numerical aperture, NA, and the focus position of the primary beam, deeper holes (trenches) can appear even darker and shallower holes appear brighter. This increases the sensitivity of the measurement

[0143] In some examples, a numerical aperture (NA) value can be selected to match the trench depth, e.g. so that beam spot size on the trench floor is approximately equal to the trench diameter:

[0144] Spot_size_on_the_trench_floor = trench_diameter, (Equation 1).

[0145] Then, trenches (holes) that are deeper may appear darker.

[0146] Alternatively, in some other examples, an NA value may be selected so that the beam spot size on the trench floor is equal to double of the trench diameter (width):

[0147] Spot_size_on_the_trench_floor = 2*trench_diameter, (Equation 2).

[0148] Then deeper holes appear darker and shallower holes / trenches appear brighter.

[0149] For example, a formula for selecting a suitable NA value depending on the aspect ratio of the trenches can be determined as follows. The NA value can be set so that the beam spot size at a target value of the hole depth is equal to:

[0150] Spot_size_on_the_trench_floor = s*trench_diameter .(Equation 3), where the focus position is at the top edge of the trench. In some examples, trench_diameter may be equal to hole_diameter or trench_width.

[0151] For example, if s=2:

[0152] Spot_size_on_the_trench_floor = Spot_size_in_focus + 2*NA*trench_depth, (Equation 4); Spot_size_in_focus + 2*NA*trench_depth = s*trench_diameter, (Equation 5); 2*NA*trench_depth = s*trench_diameter - Spot_size_in_focus, (Equation 6); NA = (s* trench_diameter- Spot_size_in_focus”) I (2*trench_depth), (Equation 7).

[0153] If the spot size in focus is not taken into account compared to the trench / hole size, then NA value can be determined by Equation 8:

[0154] NA » s / 2 * (trench_diameter / trench_depth), (Equation 8).

[0155] In the example when s=2, the required NA corresponds exactly to the aspect ratio of the hole.

[0156] In some examples, the NA value in a multi-beam SEM can be adjusted using various parameters (landing energy, lens current / voltage, etc.). In this example, NA may be in an order of 5 to 30 mrad, depending on the adjustment algorithm and the selected landing energy (e.g., 300eV to 3000eV, with low landing energies the NA tends to be larger). According to the above calculation in the above equations, this corresponds to a hole aspect ratio of 1 :33 to 1 :200.

[0157] Thus, according to various examples, the one or more beam parameters may comprise a numerical aperture controlling beam divergence.

[0158] According to various examples, the one or more beam parameters may comprise a numerical aperture controlling beam divergence and a working distance controlling beam focus.

[0159] A schematic representation of semiconductor-based structure 7000 having deeper trenches 7002, 7006, and shallower trenches 7004, 7008 is illustrated in FIGs. 7 (a)-(c). An SEM beam 3 is focused e.g. on upper edge of semiconductor-based structure 7000. Deeper trenches produce less signal intensity 7010, 7018 than shallow trenches producing higher signal intensity 7012, 7020. An SEM beam 3 in FIG. 7(a) has a first numerical aperture value and a first focus position z1. An SEM beam in FIG. 7(b) has the first numerical aperture value and a second focus position value z2. The difference between z1 and z2 is indicated by dz. Thus, for the same numerical aperture value, the focus position along z direction can be adjusted to optimize the size of the beam spot to the size and the depth of trenches. The combination of the values of the numerical aperture and the working distance for the deeper trench 7002 in FIG. 7(a) results in a beam spot bigger than the size of trench. Thus, less secondary electrons (SE) are emitted from the bottom of the trench compared to the shallower trench 7004. Hence, the measured signal intensity at the detector 7010 is lower than the signal intensity for the shallower trench 7012. This effect is further amplified by the effect shown in FIG. 7(c): For the deeper trench 7014, some of the SE do not reach the detector measuring signal intensity 7018. For the shallower trench 7016, a greater portion of the SE can reach the detector measuring signal intensity 7020.

[0160] Choosing a combination of the values of the numerical aperture and the working distance suitable to the depth of the trenches 7002 and 7004, the difference in signal intensity can thus be amplified. The difference in signal intensity can for example be used to determine the difference in depth of the two trenches 7002 and 7004 or to detect sample defects.

[0161] Choosing a different working distance, the same values of the numerical aperture as in FIG. 7(a)result in a beam spot suitable for the size and the depth of the trenches 7006 and 7008 in FIG. 7(b). Again, the beam spot is bigger than the size of the trench for the deeper trench 7006 but it is smaller than the size of the trench for the shallower trench 7008.

[0162] Thus, the difference in measured signal intensity for trench 7006 compared to trench 7008 is amplified. The difference in signal intensity can for example be used to determine the difference in depth of the two trenches 7006 and 7008 or to detect sample defects.

[0163] Thus, by choosing a different focus position (working distance z) of the primary beam, the range of inspectable aspect ratios can be changed with the same available NA range. If the focus position is moved upwards (FIG. 7, b), deeper trench 7006 and shallower trench 7008 can be distinguished from one another with the same NA. In this case, deeper trench 7006 is not as deep as trench 7002 in FIG. 7 (a) though. Thus, in some examples, depending on the depth of the trenches, adjusting the beam parameters to the depth of the trench may further require adjusting the working distance in combination with adjusting the numerical aperture.

[0164] Conversely, even deeper holes or trenches can be measured for their depth by moving the focus position further downwards. For example, working distance z1 for deeper trench 7002 is more suitable than working distance z2, while working distance z2 is more suitable for measuring deeper trench 7006. Thus, the formula for selecting a suitable NA depending on the aspect ratio of the holes / trenches (Equation 7) can be generalized in Equation 10:

[0165] NA = (s*trench_diameter- spot_size_in_focus) I (2*(distance_focus_position_to_bottom_of_trench)), (Equation 10).

[0166] In an example, the available NA in an MSEM may be in an order of 5 to 30 mrad, depending on the adjustment algorithm and the selected landing energy (e.g., 300eV to 3000eV, with low landing energies the NA tends to be larger). According to the above calculation with Equation 7 and s=2, this corresponds to a hole aspect ratio of 1:33 to 1 :200. However, Equation 10 shows that this inspectable range of hole aspect ratios can be increased by variation of the focus position. As an example, if the focus position is chosen to be at a distance of trench_depth above the top edge of the trench, aspect ratios of approx, up to 1:17 can be inspected. Conversely, if the focus_position is chosen to be at a distance of trench_depth / 3 below the top edge of the trench, aspect ratios of down to 1 :300 can be inspected.

[0167] FIG. 8 illustrates various examples of using the images obtained with the numerical apertures, such as determined in FIGs. 7, for measurement of dimensions of trenches or holes.

[0168] According to various examples, the numerical aperture, and optionally, the working distance, determined as described in the context of FIG. 7, may be applied for measuring trenches or holes.

[0169] FIG. 8(a) illustrates a schematic representation of a semiconductor-based structure 8000 comprising two different trenches (holes), a deeper trench 8002 and a shallower trench 8004. In this example, the deeper trench is about twice as deep as the shallower trench 8004. Shallower trench 8004 is associated with a defect since fins of the semiconductorbased structure 8000 are not properly separated. Deeper trench 8002 is associated with a defect-free trench since fins of the semiconductor-based structure 8000 are properly separated.

[0170] FIG. 8(b) illustrates a simulated SEM image of the semiconductor-based structure 8000. Deeper trench 8002 in FIG. 8(a) appears on the simulated SEM image as trench 8010 and shallower trench 8004 as trench 812. FIG. 8(c) illustrates a signal intensity associated with the deeper and the shallower trenches in FIGs. 8(a)-(b). A deeper trench 8002 is associated with a minimum 8006 in signal intensity and a shallower trench 8004 is associated with a minimum signal intensity 8008. Minimum 8006 is lower than minimum 8008. Thus, the defect associated with the shallower trench 8004 can be identified from the comparison of the two minima. This example illustrates technical advantages of optimizing numerical aperture values following the principles described in the context of FIGs. 7 for detection of defects in semiconductor-based structures such as the ones associated with trenches or holes formed by fins.

[0171] FIG. 8(d) illustrates an example of defect sensitivity obtained from multiple SEM images, such as the one illustrated by FIG. 8(b), of the semiconductor-based structure 8000 for a plurality of numerical aperture values allowing for optimizing the aperture value based on the signal to noise ratio. Defect sensitivity (signal-to-noise) has an optimum for a certain NA > 0. In this example, NA values for the optimum in the signal-to-noise ratio is in the range between 6 and 8 mrad.

[0172] In view of the above, various ways of implementing the invention may be summarized in the following embodiments.

[0173] Embodiment 1

[0174] In this embodiment, a workflow for determining beam parameters optimal to image semiconductor-based structures may comprise exemplary steps 1 and 2. As a result of steps 1 and 2, the working distance and / or stigX or stigY parameter values may be determined to provide beam astigmatism and beam focus in accordance with a scenario as previously discussed in FIG. 1b, FIG. 1c, or FIG. 1d. Specifically, stigX or stigY parameter can provide beam astigmatism in x-y direction in FIGs. 1a-1d and 4a-4b, while the working distance can provide appropriate height or focus in z direction as illustrated in fig. 1c.

[0175] At Step 1, a coarse alignment (a first adjustment stage) with a large catch range may allow to align the beam-shape even when the structures of interest are not yet visible. Possible criteria are further described in Embodiment 1.2. Step 1 may implement S2004 in FIG. 2.

[0176] The optimization at this step may involve using all degrees of freedom (focus, stigX, stigY, higher order multipoles (e.g. hexapoles) for more complex shapes). Alternatively, the optimization may be simplified as described in the context of Embodiment 2 later on (precalibrate a stig-direction and transformation line defocus vs. stig-amplitude).

[0177] At Step 2 (e.g., implementing S2006 in FIG. 2), a fine alignment may be carried out (a second adjustment stage) via optimization of contrast / sharpness of structures of interest in a certain region of interest, ROI, determined via a layout of a semiconductor-based structure.

[0178] This embodiment may be summarized by Clauses 1 and 2. According to Clause 1 , a method for determining and setting values of one or more beam parameters of a scanning electron microscope, SEM, is provided. The method is adapted to imaging of nonplanar semiconductor-based structures. The one or more beam parameters comprise a working distance controlling beam focus, a stigX parameter controlling beam stigma in a first direction that is parallel to an outer surface of the semiconductor-based structures being imaged, and a stigY parameter controlling beam stigma in a second direction that is perpendicular to the first direction and parallel to the outer surface of the semiconductorbased structures. The method comprises establishing first values of the one or more beam parameters. The method further comprises performing a first adjustment stage, the first adjustment stage comprising: controlling one or more imaging components of the SEM to adjust the first values of the working distance to determine second values of the one or more beam parameters when a first feature of a test structure becomes visible or if a dimension of a second feature of the test structure, calculated from an image of the test structure captured by the SEM, is equal to or above a target value, wherein the test structure comprises a reference geometry and material for the nonplanar semiconductorbased structures to be imaged. The method further comprises, upon completing the first adjustment stage, performing a second adjustment stage, the second adjustment stage comprising: controlling the one or more imaging components of the SEM to incrementally adjust the second values of the one or more imaging parameters until a value of a contrast parameter in a region of interest reaches a threshold value, to thereby obtain third values of the one or more imaging parameters. The method further comprises controlling the one or more imaging components of the SEM to image the nonplanar semiconductor-based structures using the third values of the one or more beam parameters.

[0179] According to Clause 2 dependent on Clause 1 , one or more imaging components may comprise at least one higher order multipole lens. The at least one higher order multipole lens may be controlled, in the first adjustment stage, to adjust a setting until the first feature becomes visible. In some examples, the at least one higher order multipole lens may be used in the second adjustment stage.

[0180] In some examples, the setting may comprise one or more of a field strength, a field distribution, a pole alignment, a supply voltage, a supply current and / or one or more harmonic components of the electromagnetic field produced by the at least one higher order multipole lens.

[0181] In some examples, the at least one higher order multipole lens may be a hexapole component.

[0182] Embodiment 1.1 is a variation of Embodiment 1.

[0183] In this embodiment, a workflow / algorithm for determining beam parameters optimal to image semiconductor-based structures may comprise a possible optimization via an evolutionary / genetic algorithm. This algorithm may employ selection, recombination and mutation operators (e.g., inspired by nature) resulting in a higher likelihood of finding a global optimum than a local optimum. It may be advantageous to cover a large parameter space. The complexity of a process may be mapped to a fitness function / value.

[0184] Alternatively, a classical deterministic optimization algorithm may be used.

[0185] This embodiment may be summarized by Clause 3. According to Clause 3 dependent on Clause 1 or 2, the one or more imaging components may be adjusted in the first adjustment stage based on an algorithm. The algorithm may comprise an evolutionary algorithm or a deterministic algorithm based on a contrast function. The contrast function may comprise a plurality of contrast parameters.

[0186] In some examples, the evolutionary algorithm may comprise selecting, by a user, the first values of the one or more beam parameters as starting values and subsequently iteratively mutating the starting values to find a global optimum for the contrast function by obtaining a contrast value of each contrast parameter for each iteration of mutation and comparing contrast values between a previous and subsequent iterations of mutation. The deterministic algorithm may comprise iteratively moving in a direction of increase of the contrast function by obtaining a contrast parameter for each iteration and comparing contrast parameters between a previous and a subsequent iteration for determining the direction of increase.

[0187] Embodiment 1.2. is a variation of embodiment 1.

[0188] Embodiment 1.2 further describes optimization criteria directed at optimizing the beam of the SEM for a geometric parameter of the top surface layer.

[0189] For example, the geometric parameter may be an apparent trench / line CD optimized to a design value. Higher gates as compared to fins generally generate higher contrast. Accordingly, beam setting may be optimized to a lower surface layer. As a result, maximization of apparent line / trench CD may be achieved.

[0190] This embodiment may be summarized by Clause 4. According to Clause 4 dependent on any one of Clause 1-3, the second feature of the test structure may comprise a trench, and the dimension of the second feature of interest is a trench critical dimension, CD.

[0191] In some examples, the trench may be defined as a recession in an outer surface of the test structure being imaged facing the beam of the SEM.

[0192] Embodiment 2 is a variation of Embodiment 1.

[0193] This embodiment is directed at finding beam stigma settings via determination of beam focus. In other words, by determining the beam focus through pre-calibration, autofocus AF and autostigmation AS procedure, optimal parameter values for beam stigma can be obtained at the end of the procedure (i.e., at step 5 described below). The settings determined in this embodiment provide one possible set of settings allowing to achieve beam astigmatism such as the one described in the context of FIGs. 1 b-1 d. While other possible sets of settings (alternative or additional) may be determined by the method steps described in other embodiments. In this implementation, instead or in addition to the first stage in S2002 in FIG. 2, a recalibration procedure may be applied. After the recalibration the second stage in S2006 in FIG. 2 may follow.

[0194] This embodiment includes a pre-calibration. The pre-calibration may comprise measuring stigmator excitation E1 needed to shift a line focus by deltaZCalibration.

[0195] The line focus may refer to an elliptical focus as described in the context of FIGs. 1b, 3 and 4b or elliptical foci as described in the context of FIG. 1c. The deltaZCalibration in this case may shift the elliptical focus in z direction.

[0196] The pre-calibration may further comprise measuring stigmator excitation E2 needed to shift line focus by deltaOLCurrentCalibration. The pre-calibration may further comprise determining a typical direction given by an angle, alpha (polar angle in the x-y plane), which the stigmator needs to be adjusted in on sample of interest, e.g. a FinFET structure.

[0197] For example, the angle alpha may be determined from the orientation of the gates or fins on x-y plane in FIGs. 1 , 3-5. The angle, alpha, may translate the orientation of the stigmator to the orientation of the gates / fins on the sample surface.

[0198] After the pre-calibration steps, a workflow may be applied. The workflow may comprise, for example, Steps 1-4:

[0199] Step 1 may comprise performing autofocus AF / autostigmation AS on a test structure.

[0200] Step 2 may comprise moving to a sample region of interest, ROI, using z-stage or an objective lens, OL, current until a trench critical dimension, CD, will reach a threshold value (good), but an image may possibly be defocused (bad), or in other words, a contrast value obtained from the image may be below a threshold value.

[0201] Step 3 may comprise optimizing focus using z-stage (giving deltaZ value) and / or OL (giving deltaOLCurrent value). At this step, the trench CD value may be bad (i.e. , below a threshold value). Step 4 comprises computing the necessary delta of stigmator excitation from precalibrated values according to formulae: deltaStig = (-1)*(E1 / deltaZCalibration *deltaZ + E2 / deltaOLCurrentCalibration*deltaOLCurrent).

[0202] This allows to calculate deltaStigX according to the formula: deltaStigX = deltaStig * cos(alpha), and deltaStigY = deltaStig* sin(alpha), where:

[0203] E1 is a first stigmator excitation parameter needed to shift a line focus by deltaZCalibration;

[0204] E2 is a second stigmator excitation parameter needed to shift line focus by deltaOLCurrentCalibration; deltaZ is an offset to the working distance z; deltaOLCurrent is an offset current to offset the current OLCurrent applied at the objective lens; angle alpha is a polar angle as described above; deltaStigX and deltaStigY parameters provide offset for offsetting stigX and stigY parameters; stigX parameter controlling beam stigma in a first direction that is parallel to an outer surface of the semiconductor-based structures being imaged; stigY parameter controlling beam stigma in a second direction that is perpendicular to the first direction and parallel to the outer surface of the semiconductor-based structures.

[0205] Step 5 may comprise applying these values of deltaStigX and deltaStigY to the stigmator. At this step, trench CD will be good (about a threshold) and focus good (above a threshold).

[0206] If necessary, the above Step 2 may be repeated to achieve a better trench CD value (above another threshold) and / or focus.

[0207] This embodiment may be summarized by Clause 5. Clause 5 is in a way a simplified version of Clause 1 , in the sense of complexity of the optimization procedure. According to Clause 5, a method for determining and setting values of one or more beam parameters of a scanning electron microscope, SEM is provided. The method is adapted to imaging of nonplanar semiconductor-based structures. The one or more beam parameters comprise at least one of a working distance, controlling beam focus, an offset to the working distance, deltaZ, a stigX parameter controlling beam stigma in a first direction that is parallel to an outer surface of the semiconductor-based structures being imaged, a stigY parameter controlling beam stigma in a second direction that is perpendicular to the first direction and parallel to the outer surface of the semiconductor-based structures, a current applied at an objective lens, OLCurrent, an offset current, deltaOLCurrent, applied at the objective lens, one or more calibration parameters comprising a current calibration parameter, deltaOLCurrentCalibration, applied to the objective lens and a working distance calibration parameter, deltaZCalibration. The method comprises pre-calibrating a stigmator, to obtain a first stigmator excitation parameter, E1 , and a second stigmator parameter, E2. Said pre-calibrating comprises setting first values of the one or more beam parameters comprising deltaZCalibration and deltaOLCurrentCalibration; performing autofocusing and autostigmation on a test structure by automatically adjusting first values of the one or more beam parameters comprising the working distance, the OLCurrent, stigX and stigY, applying deltaZCalibration to change the first value of the working distance to a second value and obtain E1 parameter to be applied to a stigmator in order to readjust a line focus to be at the test structure; resetting the value second value of the working distance to the first value, and applying deltaOLCurrentCalibration to change the first value of the OLCurrent to a second value, and to obtain E2 parameter to be applied to the stigmator in order to readjust the line focus to be at the test structure. The method further comprises, after the pre-calibrating step, performing autofocusing and autostigmation on a sample structure by automatically adjusting the first values of the one or more beam parameters comprising the working distance, OLCurrent, stigX and stigY until a first feature of the sample structure becomes visible. The method further comprises adjusting the first value of the working distance by deltaZ or adjusting the first value of OLCurrent by deltaOLCurrent until a trench at a region of interest on the sample structure becomes visible to obtain second values of the working distance or OLCurrent. The method further comprises calculating values of deltaStigX and deltaStigY parameters based on the values of E1 and E2, and a value of an angle selected according to structures of the sample structure. The method further comprises controlling the one or more imaging components of the SEM to image the nonplanar semiconductor-based structures using the values of deltaStigX and deltaStigY parameters.

[0208] In some examples, the values of deltaStigX and deltaStigY parameters may be calculated according to formulae comprising: deltaStig = (-1)*(E1 / deltaZCalibration*deltaZ + E2 / deltaOLCurrentCalibration*deltaOLCurrent), deltaStigX = deltaStig * cos(alpha), and deltaStigY = deltaStig* sin(alpha), where deltaStigX and deltaStigY are offset values for the stigmator; and where angle, alpha, is chosen according to the structures on the sample structure of interest.

[0209] Embodiment 3 is directed at calibration of a stigmator offset.

[0210] In this embodiment, if settings are reproducible, stigmation on test structure can be performed (i.e. , a reference wafer) next to a sample. In particular, a planar 2D test structure can be used. Then the procedure can proceed to moving to the sample and finding stigmation settings by hand or an algorithm. The necessary stigmator offset for a set of illumination settings (landing energy, current, ...) may be stored, e.g., on an electronic medium. During application work, the method may involve a stigmation on test structure (reference wafer), storing calibration settings, loading the wafer for inspection. Afterwards, the procedure may proceed to adding the predetermined stigmation offset to the stigmator settings. The advantage of this procedure is fast determination of the values of the beam parameters.

[0211] Optionally, the method may further involve a "fine alignment" (e.g., a second adjustment stage as described in the context of Embodiment 1).

[0212] This embodiment may be summarized by Clause 6. Since this embodiment may be combined with features of Embodiment 1 , Clause 6 comprises optional features that are equivalent to features of Clause 1. According to Clause 6, a method for determining and setting values of one or more beam parameters of a scanning electron microscope, SEM, is provided. The method is adapted to imaging of nonplanar semiconductor-based structures. The one or more beam parameters comprise a stigX parameter controlling beam stigma in a first direction that is parallel to an outer surface of the semiconductorbased structures being imaged, and a stigY parameter controlling beam stigma in a second direction that is perpendicular to the first direction and parallel to the outer surface of the semiconductor-based structures. The method comprises pre-calibrating one or more imaging components of the SEM comprising a stigmator. The pre-calibrating comprises determining a plurality of first values of stigX parameter and a plurality of first values of stigY parameter by imaging a test structure, wherein the test structure comprises a silicone reference wafer comprising a planar geometry, wherein each of the plurality of first values of stigX parameter and each of the plurality of first values of stigY parameter is determined for a set of imaging parameters comprising electron energy and a beamcurrent. The method further comprises, after determining the plurality of first values of stigX and the plurality of first values of stigY parameters, determining a plurality of a second values of stigX parameter and a plurality of second values of stigY parameter by imaging a sample structure, wherein the sample structure comprises a nonplanar geometry and a semiconductor material. The method further comprises calculating a plurality of stigX offset values and a plurality of stigY offset values, each stigX offset value being calculated as a difference between a first value of stigX and a second value of stigX, and each stigY offset value being calculated as a difference between a first value of stigY and a second value of stigY. The method further comprises performing autofocusing and autostigmation on the test structure to obtain third values of stigX and stigY parameters by changing values of the working distance, OLCurrent and stigX and stigY parameters for fixed imaging parameters comprising electron energy and the beam current. The method further comprises applying the stigX and stigY offset values to the stigmator to obtain fourth values of the one or more beam parameters for imaging the sample structure; and optionally, controlling the one or more imaging components of the SEM to incrementally adjust the fourth values until a value of a contrast parameter in a region of interest on the image of the test structure reaches a threshold value, to thereby obtain fifth values of the one or more beam parameters. The method further comprises controlling the one or more imaging components of the SEM to image the nonplanar semiconductor-based structures using at least the plurality of stigX offset values and the plurality of stigY offset values, and optionally, using the fifth values of the one or more imaging parameters.

[0213] Embodiment 4.1 is directed at dedicated test structures for stigmator alignment / calibration.

[0214] A dedicated test structure may be provided on the sample die itself or as a separate sample (i.e., reference wafer). A dedicated test structure may refer to dedicated regions with test structures suitable for autostigmation.

[0215] An algorithm may involve reducing necessary stage moves. Test structures (on sample or on dedicated test chip) may be used with similar properties to the sample itself, but with its 2D layout being such that autostigmation algorithms may result in settings that match the structures of interest (i.e., use similar materials, similar 3D extent / geometry, possibly including charging effects). An example of a dedicated structure may be a sample with a relatively small feature of high contrast (e.g. gold Au on carbon C resolution sample) with top layer (e.g. customer structures, e.g. lines and spaces with high aspect ratio, HAR, trenches). This structure may be used for a calibration recipe / offsetting of the stigmator.

[0216] This embodiment may be summarized by the first alternative in Clause 7 directed at a test structure comprising a physical test structure. The second alternative in Clause 7 is described in more detail in Embodiment 4.2 and is directed at a computer representation of the physical test structure.

[0217] According to Clause 7, dependent on any one of Clause 1-6, the test structure for determining and setting values of the one or more beam parameters may further comprise a physical test structure or a computer representation of the physical test structure. The test structure can be suitable for autostigmation. The test structure may be a separate structure or integrated in the sample structure. The physical test structure may comprise a reference geometry representative of a geometry of candidate nonplanar semiconductorbased structures to be imaged, the reference geometry comprising one or more trenches characterised by an aspect ratio. The physical test structure may further comprise a reference material similar or identical to candidate nonplanar semiconductor-based structures to be imaged. The reference material may comprise gold layered on a carbonbased substrate. The computer representation of the physical test structure may comprise a computer representation of the reference geometry representative of the geometry of candidate nonplanar semiconductor-based structures to be imaged, the reference geometry comprising one or more spatial dimensions of nonplanar structures. The computer representation of the physical test structure may further comprise a computer representation of the reference material comprising one or more material properties. The computer representation of the physical test structure may comprise a computer representation of a primary SEM beam comprising a number of electrons and electron energies. The computer representation of the physical test structure may comprise a computer simulation of an interaction of the primary SEM beam with the reference geometry and the reference material according to a Monte Carlo algorithm. The computer simulation may be indicative of spatial charging and is used for determining optimal values of the one or more beam parameters for controlling the one or more imaging components of the SEM to image the nonplanar semiconductor-based structures.

[0218] For example, if a test structure comprises only vertical “V” lines / structures it is not suitable for autostigmation. This is because a 2-D optimization of the stigma is not possible in this case. A test structure may be referred to as “suitable for autostigmation” if it comprises perpendicular or horizontal “H” lines / structures such as illustrated by FIGs. 1a-1c, 4a-4b and 5. In some examples, a test structure may be referred to as “suitable for autostigmation” if it comprises “H” lines / structures in addition to “V” lines / structures.

[0219] Embodiment 4.2 is directed at simulated test structures for stigmator alignment / calibration.

[0220] Simulated test structures may be used as an alternative to physical test structures of Embodiment 4.1.

[0221] Using simulated test structures does not preclude using physical test structures in combination.

[0222] Obtaining simulated structures may comprise performing Monte Carlo simulations of the electron sample interaction on the real 3D sample geometry to determine the samples influence on the beam shape (e.g. by charging) as well as the optimal shape and divergence of the beam to image certain structures of interest. Thus, obtaining simulated structures may comprise setting optimization view CAD-Data and simulation of imaging, e.g. Monte-Carlo-simulation.

[0223] Simulated test structures obtained in Embodiment 4.2 may be referred to as a computer representation comprising properties of physical structures, as described in the context of Embodiment 4.1.

[0224] Thus, Embodiment 4.2 may be summarized by the second alternative of Clause 7 directed at the computer representation of the physical test structure. Clause 7 is described above in the context of Embodiment 4.1.

[0225] The dedicated test structures of Embodiment 4.1 and / or 4.2 may be used for any of the above embodiments 1-3 or other embodiments where alignment / calibration of a stigmator is involved or may be an advantageous option.

[0226] Embodiment 5 is directed at optimizing a beam shape for a 3D sample geometry. To optimize the beam shape for the 3D sample geometry, it may be beneficial - as shown in FIG. 1d - to set a line focus in one direction to one z-position, and another line focus to another z-position (e.g. top and bottom of trenches) via a deliberate astigmatism. As illustrated in FIG. 1c, one line focus having one astigmatism (elliptical focus at gates 703) may be set at z1 position, while another line focus having the same or different astigmatism (elliptical focus at fins 705) may be set at another position z2. Thus, z1 and z2 positions may be adapted to the positions of gates and fins.

[0227] Additionally or alternatively, beam astigmatism in x and y directions may be adapted to gates and / or fins. For example, there is one line focus in FIG. 1b, where astigmatism in x and y is adapted to better resolve the fins at one z position (i.e. , the beam is focused at fins). Each of the two line foci in FIG. 1c and 3 can be set to a different height z.

[0228] In this way, the focal spot size may be minimal in one direction in z-position z1 and minimal in another direction in z-position z2 at the same time. If sample structures of interest are oriented accordingly in z-position z1 and z-position z2, contrast of the structures can be maximized in both z-positions at once resulting in two planes of best focus / contrast.

[0229] This embodiment may be summarized by Clause 8. According to Clause 8, a method for determining and setting values of one or more beam parameters of a scanning electron microscope, SEM, is provided. The method is adapted to imaging nonplanar semiconductor-based structures. The one or more beam parameters comprise a working distance controlling beam focus. The method comprises providing a sample structure comprising a first protrusion extending above a bottommost surface of the sample structure when viewed upstream the beam of the SEM, the sample structure further comprising a second protrusion extending above the bottommost surface of the sample structure and above the first protrusion when viewed upstream the beam. The method further comprises determining a first value of the working distance, z1 , for focusing the beam along the topmost surface of the first protrusion, the first value of the working distance providing a first line focus for imaging the nonplanar semiconductor-based structures at a first height (when viewed upstream the beam). The method further comprises determining a second value of the working distance, z2, for focusing the beam along the topmost surface of the second protrusion, the second value of the working distance providing a second line focus for imaging the nonplanar semiconductor-based structures at a second height greater than the first height when viewed upstream the beam. The first line focus and the second line focus form a multi-line focus set at different heights when viewed upstream the beam. The method further comprises controlling one or more imaging components of the SEM to image the nonplanar semiconductor-based structures using the first and second values of the working distance.

[0230] In an example, the first and second protrusions may be gates 703 and fins 705 protruding above surface 25 as illustrated in figs. 1a-1c, where gates 703 are higher above the surface 25 than fins 705.

[0231] Embodiment 6 is directed at recording multiple images simultaneously and combining the images thereby improving ultimate contrast / accuracy of imaging. A corresponding example has been previously discussed in connection with FIG. 3.

[0232] This embodiment is based on the finding that different beam-shapes optimized for the visibility of different structures may be used to record multiple images simultaneously (featuring features of interest and other structures that are not investigated in the specific inspection or metrology task). These multiple images may be numerically combined. Various combination techniques are conceivable. A pixel-wise averaging would be a possibility for such combination. As a result, a higher contrast for the features of interest can be achieved compared to only a single image.

[0233] Accordingly, different images may be acquired using values of the one or more beam parameters optimized for imaging of different kinds of semiconductor-based structures. For instance, a first image may be optimized for imaging gates; while a second image may be optimized for imaging fins, the gates and fins forming a FinFET.

[0234] In some examples, a single scan with multiple beams may be carried out with subfields overlapping such that each of two subsets of beams cover the wafer surface, a first subset with first astigmatism, a second subset with second astigmatism. Image processing and image fusion may be applied after parallel image acquisition of multiple images of the wafer surface with one scan.

[0235] This embodiment may be summarized by Clause 9. According to Clause 9 dependent on any one of Clause 1-8, the method may further comprise applying one or more first values of the one or more beam parameters determined according to any one of the preceding Clauses for capturing a first image of at least two images of a nonplanar semiconductorbased structure, and, sequentially, applying one or more second values of the one or more beam parameters determined according to any one of the preceding Clauses for capturing a second image of the at least two images. The at least two images may be captured in a single scan by illuminating the nonplanar semiconductor-based structure with multiple beams comprising two subsets of beams. The method may further comprise combining the at least two images. Thereby a post-processed image of the nonplanar semiconductor-based structure is generated based on the aggregated images.

[0236] Embodiment 7 is directed at optimization of beam-shape in an MSEM.

[0237] More particularly, Embodiment 7 involves using any of the above-mentioned methods / workflows of embodiments 1-6 for determining values of one or more beam parameters for every beam individually; and optionally setting these values individually for each beam by means of a micro-optics lens array that is configured to individually shape each of multiple beams. For instance, a MAP may be available that enables to individually shape each of multiple beams traversing through a respective aperture of the MAP. Then, for each beam, a respective value of a given beam parameter can be set.

[0238] Sometimes, however, such MAP enabling individual / locally setting of beam parameter values for each of multiple beams may not be available. Rather, a MAP or respective beam-shaping optics may be available that globally / collectively shapes each of multiple beams. A global multipole is an example. Thus, given setting applied at such beamshaping optics may collectively act on each of multiple beams. Such coupled setting of values of a beam parameter may be taken into consideration when determining values of a beam parameter for multiple beams.

[0239] In particular in such a scenario, embodiment 7 may further involve optimizing for the average of all or a subset of beams and use global multipoles to set the beam-shape. The method of Embodiment 7 may further involve a single scan optimization comprising a parallel AS and AF with a single shot using multiple beams with different settings in parallel. A parameter (stigmator, focus, higher order) map imprinted on multi ple / all beams may be used. This can be either from non-uniform tool characteristics or artificially imprinted by non-uniform global or micro-optical array elements. The resulting mFOV (multiple fields of view) may become a representation of a parameter sweep. The method may further involve calculating optimum values from individual beams with different parameters and applying the optimal values to all beams. In a scenario in which multiple beam-specific values of the beam parameter are determined using different techniques disclosed herein, it would be possible to test out each of these multiple values. In other words, it would be possible to acquire respective image. For instance, if a micro-optics lens array is available that enables to individually and contemporaneously apply multiple beam-specific values to different beams that are scanned contemporaneously, it would then be possible to capture multiple images simultaneously using different beams to which different beam-specific values are applied. Each of these images can then be characterized, e.g., by determining a respective imagequality value and an optimum image-quality value can then be selected for further imaging of the semiconductor-based structures. For instance, an example image-quality characteristic is the contrast. Other image-quality metrics include distortion, blur, etc.

[0240] When an SEM comprises multiple beams, this variation of Embodiment 7 may be summarised in Clauses 11-14.

[0241] According to Clause 11 dependent on any one of Clause 1-9, the one or more imaging components of the SEM may further comprise micro-optic lenses. The one or more values of the one or more beam parameters may be applied to each beam individually.

[0242] For example, each beam in the context of Clause 11 may be implemented by a beamlet of a plurality of beamlets. Each beamlet may be formed by a collimated primary charged particle beam beam 309 incident on an imaging component (e.g., 305) that splits this beam 309 into multiple beams 3 (or beamlets 3.1, 3.2, 3.3, 3.i) as particularly illustrated in FIG. 6.

[0243] According to Clause 12 dependent on Clause 11 , the one or more imaging components of the SEM may further comprise global multipole lenses. The one or more values of the one or more beam parameters may be averaged and the average values may be applied to each beam individually or to a set of beams from the multiple beams.

[0244] According to Clause 13 dependent on Clause 11 , different values from the one or more values may be applied simultaneously to different beams from the multiple beams for controlling several imaging components of the SEM at the same time using different values of the one or more beam parameters. The method may further comprise capturing multiple images simultaneously with the different beams. The method may further comprise determining multiple contrast values, each contrast value being determined from each image from the multiple images. The method may further comprise applying values of the one or more beam parameters used to obtain an image with the highest contrast value to all beams from the multiple beams for controlling the one or more imaging components of the SEM to image the nonplanar semiconductor-based structures.

[0245] According to Clause 14 dependent on Clause 11 or 12, different values from the one or more values may be applied simultaneously to different beams from the multiple beams for controlling several imaging components of the SEM at the same time using different values of the one or more beam parameters. The method may further comprise capturing multiple images simultaneously with the different beams. The method may further comprise determining multiple contrast values, each contrast value being determined from each image from the multiple images. The method may further comprise applying values of the one or more beam parameters used to obtain an image with the highest contrast value to all beams from the multiple beams for controlling the one or more imaging components of the SEM to image the nonplanar semiconductor-based structures.

[0246] The skilled person would appreciate that although Embodiments 1-7 are described individually, this does not preclude from combining their features.

[0247] As may be inferred from the above, the concepts described in the context of FIGs. 1a-1d and 4-6 are applicable to all Embodiments 1-7. The concepts described in the context of FIG. 3 more particularly relate to Embodiment 6, however may also be combined with other embodiments.

[0248] Embodiment 8 is directed at determining and setting a scanning region as described in the context FIG. 4a.

[0249] The scanning region can be determined to include semiconductor-based structures of the first type; and may be determined to exclude semiconductor-based structures of a second type. For instance, semiconductor-based structures of the different types may have different heights above a wafer surface of the wafer on which they are formed. Alternatively or additionally, they can be made out of different materials.

[0250] Such a scanning region can be determined based on a reference image that captures both types of semiconductor-based structures. It would also be possible to rely on reference geometry data indicative of a relative arrangement of the semiconductor-based structures of the different types, e.g., mask layer data used for fabricating the semiconductor-based structures.

[0251] As may be inferred from the above, this embodiment may be combined with the features of any one of Embodiments 1-7.

[0252] Embodiment 8 may be summarized by Clause 15. According to Clause 15 dependent on any one of Clause 1-14, the method may further comprise, when the SEM has multiple beams, adjusting a pitch of a hexagon to be a multiple of a distance of a pitch of the one or more imaging components of the SEM comprising SEM gates. The method may further comprise controlling the one or more imaging components of the SEM to capture a first image of a sample structure comprising a plurality of first protrusions and second protrusions, wherein the first protrusions may be perpendicular to the second protrusions, and wherein the first protrusions may be higher than the second protrusions in a vertical direction that is perpendicular to an outer bottommost surface of a test structure being imaged facing a beam of the SEM, wherein the first protrusions may form gates and the second protrusions may form fins. The method may further comprise determining position of the gates on the first image, or alternatively to capturing the first image, determining position of the gates from a reference layout representative of the gates and the fins of the sample structure, or alternatively to capturing the first image, performing a line scan by the SEM and determining positions of the gates from the line. The method may further comprise restricting an imaging area for a second image to an area comprised between two consecutive gates for imaging the fins in the second image, thereby excluding the area where the gates are visible on the first image from the imaging area of the second image. The method may further comprise controlling the one or more imaging components of the SEM to image the restricted imaging area of the nonplanar semiconductor-based structures.

[0253] Thus, the multiple of the distance of the pitch of the one or more imaging components of the SEM may be the multiple of the distance of the pitch of the gates. An example of imaging components of the SEM comprising a hexagonal raster (having a pitch of a hexagon) is described in the context of FIG. 6.

[0254] Embodiment 9 is directed at determining orientation and dimensions of an elliptical focus as described in the context of FIG. 4b. This embodiment may be summarized by Clause 16. According to Clause 16, a method for determining and setting values of one or more beam parameters of a scanning electron microscope, SEM, is provided. The method is adapted to imaging of nonplanar semiconductor-based structures comprising gates and fins, the one or more beam parameters comprising a major axis orientation and a major axis length of an elliptical beam focus of the SEM. The method comprises setting the major axis orientation to be parallel to the fins of a nonplanar semiconductor-based structure. The method further comprises setting a value for the major axis length to be larger than a gap between two consecutive gates of the nonplanar semiconductor-based structure. The nonplanar semiconductor-based structure comprises a plurality of first protrusions and second protrusions, wherein the first protrusions are perpendicular to the second protrusions, and optionally, wherein the first protrusions are higher than the second protrusions in a vertical direction that is perpendicular to an outer bottommost surface of the test structure being imaged facing the beam of the SEM, and wherein the first protrusions form the gates and the second protrusions form the fins. The method further comprises controlling one or more imaging components of the SEM to image the nonplanar semiconductor-based structures using the major axis orientation and the value of the major axis length.

[0255] Embodiment 10 is directed at the use of the method of any one of previous embodiments 1-9 specifically for imaging of nonplanar semiconductor-based structures comprising a FinFET. For instance, the fins may be imaged. Alternatively or additionally, the gates may be imaged.

[0256] As will be appreciated, the fins form protrusions as well as the gates form protrusions. The protrusions formed by the fins are perpendicular to the protrusions formed by the gates. The two types of protrusions, formed by the fins and the gates, respectively, also have different heights above a wafer surface. The fins and the gates may be made of different materials.

[0257] This embodiment may be summarized by Clause 17. According to Clause 17, a use of the method of the current application is provided. The use comprises imaging of nonplanar semiconductor-based structures comprising a FinFET geometry. The use further comprises carrying out method steps of any one of Clause 1-16 for a test structure comprising the FinFET geometry defined by one or more gates and one or more fins. The one or more gates comprise a first protrusion extending from a bottommost surface of the test structure facing an inspection beam of the SEM. The one or more fins comprises a second protrusion extending from the bottommost surface facing an inspection beam of the SEM. The first protrusion has a height greater than a height of the second protrusion when viewed in the direction upstream the inspection beam from the bottommost surface.

[0258] Although Embodiment 10 is directed at nonplanar structures, as may be inferred from the above, the method specified in the previous embodiments may also be used for planar structures comprising horizontal “H” and vertical “V” lines that are in a way equivalent to fins and gates described in the context of figs. 1a-1c, 4a-4b and 5. A planar structure, e.g., comprised of different materials nested in a linear fashion forming a planar geometry, can also produce uneven spatial charges due the difference in material properties of the nested structures.

[0259] Embodiment 11 is directed at determining a numerical aperture of an SEM beam and / or a working distance for imaging different trenches or holes according to various examples, as described in the context of FIGs. 7(a)-(c). Embodiment 11 may be summarized by Clause 18.

[0260] According to Clause 18, a method is provided. The method is used for determining and setting values of one or more beam parameters of a scanning electron microscope, SEM, for imaging of nonplanar semiconductor-based structures comprising one or more trenches, the one or more beam parameters comprising a numerical aperture defining beam divergence. The method comprises: receiving one or more target dimensions associated with the trench; determining at least one value of the numerical aperture based on the one or more target dimensions, and controlling one or more imaging components of the SEM to image the nonplanar semiconductor-based structures comprising the one or more trenches using the at least one value of the numerical aperture.

[0261] In some examples, the one or more beam parameters may further comprise a focus position along the beam axis. The method may further comprise iteratively adjusting a value of the focus position for the at least one value of the numerical aperture until a contrast value or a signal intensity in a region of interest, ROI, on an image captured by the SEM reaches a target value, wherein the ROI comprises the one or more trenches.

[0262] Embodiment 12 is directed at use cases of the images obtained with the numerical apertures, such as determined in FIGs. 7. The features of Embodiment 12 are described in the context of FIG. 8. Embodiment 12 may be summarized by Clause 19. According to Clause 19, a use of the method of Clause 18 for detecting one or more defects in semiconductor-based structures is provided. The use comprises applying the at least one value of the numerical aperture, and optionally, the value of the focus position to capture one or more images of semiconductor-based structures by the SEM, wherein the semiconductor-based structures comprise one or more trenches; determining one or more intensity minima from the one or more images; and based on the one or more intensity minima, identifying one or more defects.

[0263] In some examples, the use may further comprise comparing the one or more intensity minima for identifying the one or more defects.

[0264] In some examples, the use may further comprise determining a height of one or more protrusions of the nonplanar semiconductor-based structures forming the trench and determining a depth of the trench.

[0265] In view of the above, general conclusions may be drawn summarized by the following examples.

[0266] According to various examples, one or more imaging components may comprise at least one higher order multipole lens. The at least one higher order multipole lens may be controlled, in the first adjustment stage, to adjust a setting until the first feature becomes visible. In some examples, the at least one higher order multipole lens may be used in the second adjustment stage.

[0267] According to various examples, the one or more imaging components may be adjusted in the first adjustment stage based on an algorithm. The algorithm may comprise an evolutionary algorithm or a deterministic algorithm based on a contrast function. The contrast function may comprise a plurality of contrast parameters.

[0268] According to various examples, the second feature of the test structure may comprise a trench, and the dimension of the second feature of interest may refer to a trench critical dimension, CD.

[0269] According to various examples, the test structure for determining and setting values of the one or more beam parameters may further comprise a physical test structure or a computer representation of the physical test structure. The test structure can be suitable for autostigmation. The test structure may be a separate structure or integrated in the sample structure. According to some examples, the physical test structure may comprise a reference geometry representative of a geometry of candidate nonplanar semiconductor-based structures to be imaged, the reference geometry comprising one or more trenches characterised by an aspect ratio. The physical test structure may further comprise a reference material similar or identical to candidate nonplanar semiconductorbased structures to be imaged. According to some examples, the reference material may comprise gold layered on a carbon based substrate. According to some examples, the computer representation of the physical test structure may comprise a computer representation of the reference geometry representative of the geometry of candidate nonplanar semiconductor-based structures to be imaged, the reference geometry comprising one or more spatial dimensions of nonplanar structures. According to some examples, the computer representation of the physical test structure may further comprise a computer representation of the reference material comprising one or more material properties. According to some examples, the computer representation of the physical test structure may comprise a computer representation of a primary SEM beam comprising a number of electrons and electron energies. According to some examples, the computer representation of the physical test structure may comprise a computer simulation of an interaction of the primary SEM beam with the reference geometry and the reference material according to a Monte Carlo algorithm. The computer simulation may be indicative of spatial charging and is used for determining optimal values of the one or more beam parameters for controlling the one or more imaging components of the SEM to image the nonplanar semiconductor-based structures.

[0270] According to various examples, the method may further comprise applying one or more first values of the one or more beam parameters determined previously for capturing a first image of at least two images of a nonplanar semiconductor-based structure, and, sequentially, applying one or more second values of the one or more beam parameters determined previously for capturing a second image of the at least two images. The at least two images may be captured in a single scan by illuminating the nonplanar semiconductor-based structure with multiple beams comprising two subsets of beams. According to some examples, the method may further comprise aggregating the at least two images, and generating a post-processed image of the nonplanar semiconductorbased structure based on the aggregated images. According to various examples, the one or more imaging components of the SEM may comprise one beam and global multipole lenses.

[0271] According to various examples, the one or more imaging components of the SEM may comprise multiple beams.

[0272] According to various examples, the one or more imaging components of the SEM may further comprise micro-optic lenses. According to some examples, the one or more values of the one or more beam parameters may be applied to each beam individually.

[0273] According to various examples, the one or more imaging components of the SEM may further comprise global multipole lenses. According to some examples, the one or more values of the one or more beam parameters may be averaged and the average values may be applied to each beam individually or to a set of beams from the multiple beams.

[0274] According to various examples, different values from the one or more values may be applied simultaneously to different beams from the multiple beams for controlling several imaging components of the SEM at the same time using different values of the one or more beam parameters. According to some examples, the method may further comprise capturing multiple images simultaneously with the different beams. According to some examples, the method may further comprise determining multiple contrast values, each contrast value being determined from each image from the multiple images. According to some examples, the method may further comprise applying values of the one or more beam parameters used to obtain an image with the highest contrast value to all beams from the multiple beams for controlling the one or more imaging components of the SEM to image the nonplanar semiconductor-based structures.

[0275] According to some examples, the method may further comprise, when the SEM has multiple beams, adjusting a pitch of a hexagon to be a multiple of a distance of a pitch of the one or more imaging components of the SEM comprising SEM gates. According to some examples, the method may further comprise controlling the one or more imaging components of the SEM to capture a first image of a sample structure comprising a plurality of first protrusions and second protrusions, wherein the first protrusions may be perpendicular to the second protrusions, and wherein the first protrusions may be higher than the second protrusions in a vertical direction that is perpendicular to an outer bottommost surface of a test structure being imaged facing a beam of the SEM, wherein the first protrusions may form gates and the second protrusions may form fins. According to some examples, the method may further comprise determining position of the gates on the first image, or alternatively to capturing the first image, determining position of the gates from a reference layout representative of the gates and the fins of the sample structure, or alternatively to capturing the first image, performing a line scan by the SEM and determining positions of the gates from the line. According to some examples, the method may further comprise restricting an imaging area for a second image to an area comprised between two consecutive gates for imaging the fins in the second image, thereby excluding the area where the gates are visible on the first image from the imaging area of the second image. According to some examples, the method may further comprise controlling the one or more imaging components of the SEM to image the restricted imaging area of the nonplanar semiconductor-based structures.

[0276] In some examples, the one or more beam parameters may further comprise a focus position along the beam axis. The method may further comprise iteratively adjusting a value of the focus position for the at least one value of the numerical aperture until a contrast value or a signal intensity in a region of interest, ROI, on an image captured by the SEM reaches a target value, wherein the ROI comprises the one or more trenches.

[0277] In some examples, the use may further comprise comparing the one or more intensity minima for identifying the one or more defects.

[0278] In some examples, the use may further comprise determining a height of one or more protrusions of the nonplanar semiconductor-based structures forming the trench and determining a depth of the trench.

[0279] Although the invention has been shown and described with respect to certain preferred embodiments, equivalents and modifications will occur to others skilled in the art upon the reading and understanding of the specification. The present invention includes all such equivalents and modifications and is limited only by the scope of the appended claims.

Claims

Claims1. A method for determining and setting values of one or more beam parameters of a scanning electron microscope, SEM, for imaging of nonplanar semiconductor-based structures, the one or more beam parameters comprising a working distance controlling beam focus, a stigX parameter controlling beam stigma in a first direction that is parallel to an outer surface of the semiconductor-based structures being imaged, and a stigY parameter controlling beam stigma in a second direction that is perpendicular to the first direction and parallel to the outer surface of the semiconductor-based structures; wherein the method comprises:- establishing (S2002) first values of the one or more beam parameters;- performing (S2004) a first adjustment stage, the first adjustment stage comprising: controlling one or more imaging components of the SEM to adjust the first values of the working distance to determine second values of the one or more beam parameters when a first feature of a test structure becomes visible or if a dimension of a second feature of the test structure is equal to or above a target value, wherein the test structure comprises a reference geometry and material for the nonplanar semiconductorbased structures to be imaged;- upon completing the first adjustment stage, performing (S2006) a second adjustment stage, the second adjustment stage comprising: controlling the one or more imaging components of the SEM to incrementally adjust the second values of the one or more imaging parameters until a value of a contrast parameter in a region of interest reaches a threshold value, to thereby obtain third values of the one or more imaging parameters; and- controlling (S2008) the one or more imaging components of the SEM to image the nonplanar semiconductor-based structures using the third values of the one or more beam parameters.

2. The method of claim 1 , wherein the one or more imaging components comprise at least one higher order multipole lens, and wherein the at least one higher order multipole lens is controlled, in the first adjustment stage, to adjust a setting until the first feature becomes visible.

3. The method of claim 1 or 2, wherein the one or more imaging components are adjusted in the first adjustment stage based on an algorithm, wherein the algorithm comprises an evolutionary algorithm or a deterministic algorithm based on a contrast function, the contrast function comprising a plurality of contrast parameters.

4. The method of any one of the preceding claims, wherein the second feature of the test structure comprises a trench, and the dimension of the second feature of interest is a trench critical dimension.

5. A method for determining and setting values of one or more beam parameters of a scanning electron microscope, SEM, for imaging of nonplanar semiconductor-based structures, the one or more beam parameters comprising at least one of a working distance, controlling beam focus, an offset to the working distance, deltaZ, a stigX parameter controlling beam stigma in a first direction that is parallel to an outer surface of the semiconductor-based structures being imaged, a stigY parameter controlling beam stigma in a second direction that is perpendicular to the first direction and parallel to the outer surface of the semiconductor-based structures, a current applied at an objective lens of the SEM, OLCurrent, an offset current, deltaOLCurrent, applied at the objective lens, one or more calibration parameters comprising a current calibration parameter, deltaOLCurrentCalibration, applied to the objective lens and a working distance calibration parameter, deltaZCalibration; wherein the method comprises:- pre-calibrating a stigmator, to obtain values of a first stigmator excitation parameter, E1 , and a second stigmator parameter, E2, wherein said pre-calibrating comprises:- setting first values of the one or more beam parameters comprising deltaZCalibration and deltaOLCurrentCalibration;- performing autofocusing and autostigmation on a test structure by automatically adjusting first values of the one or more beam parameters comprising the working distance, the OLCurrent, stigX, and stigY,- applying deltaZCalibration to change the first value of the working distance to a second value and to obtain the value of the E1 parameter to be applied to a stigmator in order to readjust a line focus to be at the test structure;- resetting the second value of the working distance to the first value, and applying deltaOLCurrentCalibration to change the first value of the OLCurrent to a second value, and to obtain the value of the E2 parameter to be applied to the stigmator in order to readjust the line focus to be at the test structure;- after said pre-calibrating, performing autofocusing and autostigmation on a sample structure by automatically adjusting the first values of the one or more beam parameters comprising the working distance, OLCurrent, stigX and stigY until a first feature of the sample structure becomes visible;- adjusting the first value of the working distance by deltaZ or adjusting the first value of OLCurrent by deltaOLCurrent until a trench at a region of interest on the sample structure becomes visible to obtain second values of the working distance and / or OLCurrent,- calculating values of deltaStigX and deltaStigY parameters based on the values of E1 and E2, and a value of an angle selected according to structures of the sample structure, where the values of deltaStigX and deltaStigY parameters provide offset values for offsetting the values of stigX and stigY parameters, and- controlling the one or more imaging components of the SEM to image the nonplanar semiconductor-based structures using the values of deltaStigX and deltaStigY parameters.

6. A method for determining and setting values of one or more beam parameters of a scanning electron microscope, SEM, for imaging of nonplanar semiconductor-based structures, the one or more beam parameters comprising a stigX parameter controlling beam stigma in a first direction that is parallel to an outer surface of the semiconductorbased structures being imaged, a stigY parameter controlling beam stigma in a second direction that is perpendicular to the first direction and parallel to the outer surface of the semiconductor-based structures, and a current applied at an objective lens of the SEM, OLCurrent; wherein the method comprises:- pre-calibrating one or more imaging components of the SEM comprising a stigmator, the pre-calibrating comprising:- determining a plurality of first values of stigX parameter and a plurality of first values of stigY parameter by imaging a test structure, wherein the test structure is a silicone reference wafer having a planar geometry, wherein each of the plurality of first values of stigX parameter and each of the plurality of first values of stigY parameter isdetermined for a set of imaging parameters comprising electron energy and a beam current;- after determining the plurality of first values of stigX and the plurality of first values of stigY parameters, determining a plurality of a second values of stigX parameter and a plurality of second values of stigY parameter by imaging a sample structure, wherein the sample structure has a nonplanar geometry;- calculating a plurality of stigX offset values and a plurality of stigY offset values, each stigX offset value being calculated as a difference between a first value of stigX and a second value of stigX, and each stigY offset value being calculated as a difference between a first value of stigY and a second value of stigY; and the method further comprises:- performing autofocusing and autostigmation on the test structure to obtain third values of stigX and stigY parameters by changing values of the working distance, OLCurrent and stigX and stigY parameters;- applying the stigX and stigY offset values to the stigmator to obtain fourth values of the one or more beam parameters for imaging the sample structure; and optionally, controlling the one or more imaging components of the SEM to incrementally adjust the fourth values until a value of a contrast parameter in a region of interest on the image of the test structure reaches a threshold value, to thereby obtain fifth values of the one or more beam parameters; and- controlling the one or more imaging components of the SEM to image the nonplanar semiconductor-based structures using at least the plurality of stigX offset values and the plurality of stigY offset values, and optionally, using the fourth or the fifth values of the one or more imaging parameters.

7. The method of any one of the preceding claims, wherein the test structure comprises a physical test structure or a computer representation of the physical test structure, wherein the test structure is suitable for autostigmation, wherein the test structure is a separate structure or integrated in the sample structure, and wherein the physical test structure comprises:- a reference geometry representative of a geometry of candidate nonplanar semiconductor-based structures to be imaged, the reference geometry comprising one or more trenches characterized by an aspect ratio, and / or- a reference material similar or identical to candidate nonplanar semiconductorbased structures to be imaged, optionally, wherein the reference material comprises gold layered on a carbon based substrate;wherein the computer representation of the physical test structure comprises:- a computer representation of the reference geometry representative of the geometry of candidate nonplanar semiconductor-based structures to be imaged, the reference geometry comprising one or more spatial dimensions of nonplanar structures,- a computer representation of the reference material comprising one or more material properties,- a computer representation of a primary SEM beam comprising a number of electrons and electron energies, and- a computer simulation of an interaction of the primary SEM beam with the reference geometry and the reference material according to a Monte Carlo algorithm, wherein the computer simulation is indicative of spatial charging and is used for determining optimal values of the one or more beam parameters for controlling the one or more imaging components of the SEM to image the nonplanar semiconductor-based structures.

8. A method for determining and setting values of one or more beam parameters of a scanning electron microscope, SEM, for imaging nonplanar semiconductor-based structures, the one or more beam parameters comprising a working distance controlling beam focus; wherein the method comprises:- providing a sample structure comprising a first protrusion extending above a lower surface of the sample structure, the sample structure further comprising a second protrusion extending above the lower surface of the sample structure and above the first protrusion;- determining a first value of the working distance, z1 , for focusing the beam along the an upper surface of the first protrusion, the first value of the working distance providing a first line focus along a first direction for imaging the nonplanar semiconductor-based structures at a first height,- determining a second value of the working distance, z2, for focusing the beam along an upper surface of the second protrusion, the second value of the working distance providing a second line focus along a second direction perpendicular to the first direction and for imaging the nonplanar semiconductor-based structures at a second height greater than the first height; wherein the first line focus and the second line focus form a multi-line focus set at different heights in the direction parallel to the SEM beam; and- controlling one or more imaging components of the SEM to image the nonplanar semiconductor-based structures using the first and second values of the working distance.

9. The method of any one of the preceding claims, further comprising:- applying one or more values of the one or more beam parameters determined according to any one of the preceding claims for capturing a first image of at least two images of a nonplanar semiconductor-based structure, and, sequentially, applying one or more further values of the one or more beam parameters determined according to any one of the preceding claims for capturing a second image of the at least two images, wherein the at least two images are optionally captured in a single scan by illuminating the nonplanar semiconductor-based structure with multiple beams comprising two subsets of beams,- combing the at least two images to thereby generate a post-processed image of the nonplanar semiconductor-based structure.

10. The method of any one of the preceding claims, wherein the one or more imaging components comprise a global multipole lens configured to collectively shape multiple beams.

11. The method of any one of the preceding claims, wherein the one or more imaging components of the SEM comprises a micro-optic lens array configured to individually shape each of multiple beams12. The method of claim 11, wherein the one or more values of the one or more beam parameters comprise multiple beam-specific values determined according to the preceding claims and applied to each beam individually by means of the micro-optic lens array.

13. The method of claim 10, wherein the one or more values of the one or more beam parameters determined according to the preceding claims are collectively applied to all of multiple beams.

14. The method of any one of claim 11 or 12, wherein different ones of the multiple beam-specific are applied simultaneously to different beams from the multiple beams; the method further comprising:- capturing multiple images simultaneously with the different beams to which different beam-specific values are applied,- determining multiple image-quality values, each image-quality value being determined based on a respective image from the multiple images, and- selecting an image-quality value from the multiple image-quality values for further imaging of the semiconductor-based structures.

15. The method of any one of the preceding claims, the semiconductor-based structure comprising a plurality of first protrusions and a plurality of second protrusions, the first protrusions being perpendicular to and higher than the second protrusions, the method further comprising:- restricting a scanning region to an area comprised between neighboring first protrusions, thereby excluding areas occupied by the first protrusions, and- controlling the one or more imaging components of the SEM to image based on scanning the restricted scanning region.

16. The method of claim 15, wherein restricting the scanning region comprises determining position of the first protrusions on a first image,17. The method of claim 15, wherein restricting the scanning region comprises determining position of the first protrusions from a reference layout, or performing a line scan by the SEM and determining positions of the first protrusions from the line scan.

18. The method of any one of claims 15-17, when the SEM has multiple beams, adjusting a pitch of a hexagon to be a multiple of a distance of a pitch of the one or more imaging components of the SEM comprising SEM gates,19. A method for determining and setting values of one or more beam parameters of a scanning electron microscope, SEM, for imaging of nonplanar semiconductor-based structures comprising a plurality of first protrusions and second protrusions, wherein the first protrusions are elongated along a first direction that is perpendicular to a second direction along which the second protrusions are elongated; wherein the method comprises:- setting the major axis orientation to be parallel to the first direction;- controlling one or more imaging components of the SEM to image the nonplanar semiconductor-based structures using the major axis orientation.

20. The method of claim 19, further comprising setting a value for the major axis length to be larger than a gap between two consecutive gates of the nonplanar semiconductorbased structure and controlling one or more imaging components of the SEM to image the nonplanar semiconductor-based structures using the major axis orientation and the value of the major axis length.

21. The method of claim 19 or 20, wherein the first protrusions are fins of a FinFET semiconductor-based structure, and wherein the second protrusions are gates of the FinFET semiconductor-based structure.

22. Use of the method of any one of claims 1-21 for imaging of nonplanar semiconductor-based structures comprising at least one of fins of a FinFET or gates of a FinFET.

23. A method for determining and setting values of one or more beam parameters of a scanning electron microscope, SEM, for imaging of nonplanar semiconductor-based structures comprising one or more trenches, the one or more beam parameters comprising a numerical aperture defining beam divergence; wherein the method comprises:- receiving one or more target dimensions associated with the trench,- determining at least one value of the numerical aperture based on the one or more target dimensions, and- controlling one or more imaging components of the SEM to image the nonplanar semiconductor-based structures comprising the one or more trenches using the at least one value of the numerical aperture.

24. The method of claim 23, wherein the one or more beam parameters further comprises a focus position along the beam axis, wherein the method further comprises:- iteratively adjusting a value of the focus position for the at least one value of the numerical aperture until a contrast value or a signal intensity in a region of interest, ROI,on an image captured by the SEM reaches a target value, wherein the ROI comprises the one or more trenches.

25. Use of the method of claim 23 or 24 for detecting one or more defects in semiconductor-based structures, the use comprising:- applying the at least one value of the numerical aperture, and optionally, the value of the focus position to capture one or more images of semiconductor-based structures by the SEM, wherein the semiconductor-based structures comprise one or more trenches;- determining one or more intensity minima from the one or more images; and- based on the one or more intensity minima, identifying one or more defects.

26. The use of claim 25 further comprising comparing the one or more intensity minima for identifying the one or more defects.

27. The use of claim 23 or 24 further comprising determining a height of one or more protrusions of the nonplanar semiconductor-based structures forming the trench and determining a depth of the trench.

28. A computer system for determining and setting values of one or more beam parameters of a scanning electron microscope, SEM, for inspection of nonplanar semiconductor-based structures, the computer system comprising:- at least one computer processor configured to carry out method steps according to any one of the preceding claims, and- a computer interface communicatively coupling the SEM and the computer processor.

29. A computer program product for determining and setting values of one or more beam parameters of a scanning electron microscope, SEM, for imaging nonplanar semiconductor-based structures, the computer program product comprising computer readable instructions, stored on an electronic storage medium, that when executed by a computer processor cause the computer processor to carry out method steps according to any one of claims 1-27.

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