Improved architecture of inspection apparatus for 3D tomography

Abstract

An architecture of a wafer inspection system (1) is provided with improved mechanical properties. The wafer inspection system (1) comprises a charged particle imaging system (40) and an optical imaging system (117) for wafer registration. Within the wafer inspection system (1), a wafer (8) can be rotated around an axis normal to the wafer surface by 360° (155.t) at any inspection position (6) of the wafer surface (55). The improvement is achieved by a reduction of an area of a footprint (167) of a vacuum enclosure (803). The vacuum enclosure (803) of reduced area or size is accommodating a wafer stage (155), allowing an inspection of a wafer surface (55) at any position and any rotation angle of the wafer (8).

Description

[0001] Title

[0002] Improved Architecture of Inspection apparatus for 3D tomography

[0003] Field

[0004] The present invention relates to an inspection apparatus for semiconductor objects within a semiconductor wafer, more particularly, to an apparatus and corresponding methods of operating the apparatus for performing 3D tomography at a wafer. The apparatus and corresponding method of operating the apparatus can be utilized for various inspection tasks, such as quantitative metrology, defect detection, process monitoring, or defect review of integrated circuits within semiconductor wafers.

[0005] Background

[0006] Semiconductor structures are amongst the finest man-made structures. Semiconductor manufacturing involves precise manipulation, e.g., lithography or etching, of materials such as silicon or oxide at very fine scales in the range of nm. A wafer made of a thin slice of silicon serves as the substrate for integrated microelectronic devices containing semiconductor structures built in and upon the wafer. The semiconductor structures are constructed layer by layer using repeated processing steps that involve for example chemical, physical, thermal, and optical processes. Dimensions, shapes and placements of the semiconductor structures and patterns are subject to several influences. For example, during the manufacturing of 3D-memory devices, the critical processes are currently etching and deposition. Other involved process steps such as the lithography exposure or implantation also can have an impact on the properties of the elements of the integrated circuits. Therefore, fabricated semiconductor structures suffer from rare and different imperfections. Devices for quantitative metrology, defect-detection or defect review are looking for these imperfections. These devices are not only required during wafer fabrication. As this fabrication process is complicated and highly non-linear, optimization of production process parameters is difficult. As a remedy, an iteration scheme called process window qualification (PWQ) can be applied. In each iteration, a test wafer is manufactured based on the currently best process parameters, with different dies of the wafer being exposed to different manufacturing conditions. By detecting and analyzing the test structures with devices for quantitative metrology and defect-detection, ideal manufacturing process parameters can be selected.

[0007] The aspect ratio and the number of layers of integrated circuits are constantly increasing, and the structures are growing into the 3rd(vertical) dimension. The current height of the memory stacks exceeds several tens of micrometers. In contrast, the minimum features size is becoming smaller. The minimum feature size or critical dimension is below 10nm, for example 7nm or 5nm, and is approaching feature sizes about and below 3nm in near future. While the complexity and dimensions of the semiconductor structures are growing into the 3rddimension, the lateral dimensions of integrated semiconductor structures are becoming smaller. Therefore, measuring the shape, dimensions and orientation of the features and patterns in three dimensions (3D) and their overlay with high precision becomes challenging. The lateral measurement resolution of charged particle systems is typically limited by the sampling raster of individual image points or dwell times per pixel on the sample, and the charged particle beam diameter. The sampling raster resolution can be set within the imaging system and can be adapted to the charged particle beam diameter on the sample. The typical raster resolution is 2nm or below, but the raster resolution limit can be reduced with no physical limitation. The charged particle beam diameter has a limited dimension, which depends on the charged particle beam operation conditions and lens. The beam resolution is limited by approximately half of the beam diameter. The lateral resolution can be below 2 nm, or for example even below 1 nm.

[0008] A common way to generate 3D tomographic data from semiconductor samples on nm scale is the so-called “slice and image” approach obtained for example by a dual beam device. A slice- and image approach is described in WO 20201244795 A1. According to examples of the WO 20201244795 A1 , a 3D volume inspection is obtained at an inspection sample extracted from a semiconductor wafer. In another example, the slice and image method is applied under a slanted angle into the surface of a semiconductor wafer, as described in WO 2021 1 180600 A1. According to this method, a 3D volume image of an inspection volume is obtained by slicing and imaging a plurality of cross-section surfaces within the inspection volume. For a precise measurement, a large number N of cross-section surfaces in the inspection volume is generated, with the number N exceeding 10, 100 or 1000 or even more image slices. For example, in a volume with a lateral dimension of 5pm and a slicing distance of 5nm, 1000 slices can be milled and imaged.

[0009] One exemplary task of semiconductor inspection is to determine a set of specific parameters of semiconductor objects such as high aspect ratio (HAR) - structures inside the inspection volume. Such parameters are for example a dimension, area, a shape, or other measurement parameters. With a typical sample of a plurality of HAR structures with a pitch of for example 70nm, about 5000 HAR structures are in one field of view, and a total sum of more than five million cross sections of HAR structures is generated.

[0010] The critical dimensions in single-digit nm regime require a high-resolution and very stable metrology system. During the measurements, any machine dynamics such as drifts and vibrations may deteriorate the measurement result and lead to unwanted degradation of the accuracy of the measurement task to be performed. Furthermore, wafers must be registered before placement of the point of interest or inspection sites under the charged particle system to ensure high accuracy alignment in the small field of view of a charged particle imaging system. For example, a wafer coordinate system is determined in reference to a machine coordinate system of the wafer inspection system. Furthermore, systems with dualbeam devices for forming cross-sections at a slanted angle into the wafer surface must be capable of selecting the orientation of the cross-section surface. Generally, operations using charged particle beams must be performed under vacuum conditions. It is a further task to reduce the time required for evacuation of a vacuum enclosure. Since space in a semiconductor manufacturing environment is expensive, it is an additional requirement to solve these tasks by minimizing the footprint of a wafer inspection system.

[0011] Wafer inspection systems utilizing the slice-and image approach typically comprise a dual beam system with a focused ion beam (FIB) column arranged under an angle GF to a surface of a wafer chuck. Thereby, cross-sections can be milled into a wafer surface under an angle GF, with the angle GF typically given between 20° to 50°, for example 36°. The angle GF is not limited thereto, and systems with even smaller angle GF, for example GF about 10°, have also been proposed. According to a task of the invention, a vacuum enclosure of a wafer inspection system utilizing the slice-and image approach is to be reduced. For example, WO20161 101978 A1 proposes a way of reducing large vacuum enclosure for SEM inspection of large flat panel displays. As example, a GEN 10 substrate has an area of more than nine square meters. WO20161 101978 A1 proposes a reduced vacuum space or footprint by using more than one scanning electron microscope (SEM) system. This is of course possible since the sizes of flat panels are significantly larger than diameters of SEM systems. SEMs typically have diameters of about 30cm, thus SEM systems have approximately the same diameter or are even larger compared to a 300mm wafer. Utilizing more than one SEM or dual beam device therefore is not reducing a footprint of a wafer-inspection system for wafer inspection of wafers with diameters of standard wafers of for example 200mm or 300mm. This is evident when looking at the first embodiment of US 8.324.594, which illustrates a pair of SEM systems arranged side by side, each having roughly a diameter of a wafer. Furthermore, the solution provided in WO20161 101978 A1 does not allow a rotation of the substrates. In wafer inspection systems utilizing the slice-and image approach, it is demanded to mill cross-sections into wafer surfaces at arbitrary orientation or rotation angles, i.e. under any rotation angle of the wafer at any position on a wafer surface. US 8.324.594 is provided with a rotation stage for rotating a wafer within a vacuum enclosure, but only for the reason to make any inspection position on a wafer accessible by a SEM within the limited vacuum enclosure. The mandatory rotation of a wafer provided in the solution of US 8.324.594 might be possible for surface inspection with a SEM system arranged perpendicular to the wafer surface, but not for a wafer inspection with a dual beam system with at least one charged particle beam column arranged at an angle with respect to the wafer surface. For example, a dual beam system comprises a FIB system at an angle GF with respect to the wafer surface. The structures and the vacuum enclosures provided in US 8.324.594 do not allow for an arbitrary, freely-selected orientation of a formation of crosssection surfaces at each potential inspection position on a surface of a wafer.

[0012] It is therefore an object of the invention to provide a wafer inspection apparatus with an improved arrangement for wafer registration. It is therefore an object of the invention to provide a vacuum enclosure for a wafer inspection apparatus comprising a dual beam system with at least one charged particle beam column arrange at an oblique angle with respect to a wafer surface. It is therefore an object of the invention to provide a wafer inspection apparatus allowing for milling cross sections at a slanted angle into a wafer surface under any rotation angle of the wafer. It is therefore an object of the invention to provide a wafer inspection apparatus allowing for milling cross sections at a slanted angle into a wafer surface at any position and under any rotation angle of the wafer. It is therefore an object of the invention to provide a wafer inspection apparatus with a vacuum enclosure allowing wafer rotation and wafer registration, wherein the vacuum enclosure has a reduced footprint or minimized volume. It is therefore an object of the invention to provide a wafer inspection apparatus with a vacuum enclosure allowing wafer rotation and wafer placement under a SEM column at any wafer position and a wafer registration, wherein the vacuum enclosure has a reduced footprint or minimized volume. It is therefore an object to improve an architecture of a wafer inspection apparatus to achieve reduced sensitivity to dynamic vibrations. It is therefore an object to improve an architecture of a wafer inspection apparatus and to enable a high flexibility of a volume inspection task with a reduced footprint or volume.

[0013] Summary

[0014] The objects of the invention are solved by the embodiments and examples below.

[0015] A wafer inspection system according to the embodiments and examples comprises a dual beam system, comprising a focused ion beam (FIB) column and a scanning electron microscope (SEM). Generally, a dual beam system comprising a first and a second charged particle beam column. Each charged particle beam column comprises an axis of symmetry. Both axes of symmetry are located in a predefined plane. Throughout the example, this predefined plane is identical to the y-z-plane, with the y-direction being the predefined direction of the mechanical arrangement of the dual beam system.

[0016] The wafer inspection system further comprises a wafer stage with a wafer holding surface or chuck. In an example, the scanning electron microscope (SEM) is arranged at an angle normal to the wafer holding surface or chuck. In an example, the focused ion beam (FIB) column is arranged at an angle GF to the wafer holding surface. Both optical axes of FIB column and SEM form an intersection point. With the wafer stage, each inspection position on a wafer surface can be positioned at the intersection position of the optical axes of FIB and SEM. With the wafer stage, a wafer can be rotated by 360° at each inspection position, thereby allowing a milling of cross-sections at every inspection position and any rotation angle of cross-section surfaces with respect to a normal to a wafer surface. The invention is, however, not limited to FIB columns arranged at angles GF. In an example, the scanning electron microscope (SEM) is arranged at an oblique angle with respect to the wafer surface, wherein the oblique angle is between 25° and 40°. In such examples, the FIB column can be arranged at an angle normal to the wafer surface. While cross-sections can be formed into the wafer normal to the wafer surface at any rotation angle of the scanning FIB beam, an observation of the cross sections by the SEM is limited to the fixed orientation direction of the SEM column. With the wafer stage, a wafer can be rotated by 360° at any inspection position, thereby allowing an observation of cross-sections at every inspection position and any rotation angle of cross-section surfaces

[0017] An improved wafer inspection system according to an example is comprising a wafer stage with at least one linear motion stage and a rotation stage. A wafer chuck for holding a wafer during use is arranged on the wafer stage. The wafer inspection system is further comprising a charged particle beam imaging system with an optical axis or line of sight. The wafer inspection system is further comprising a focused ion beam (FIB) column arranged at an angle to the charged particle beam imaging system. The focused ion beam (FIB) column is configured for ion-beam milling of cross-section surfaces approximately at an angle GF into the surface of a wafer hold by wafer chuck. The charged particle beam imaging system is configured for high-resolution observation or inspection of an inspection site on a wafer, for example it is configured for high-resolution image acquisition of a cross-section surface milled into the wafer surface. The wafer inspection system is further comprising at least one optical imaging system with an optical axis. The at least one optical imaging system is connected to an alignment control system, and both are configured for determining a wafer coordinate system and for registering the wafer coordinate system within a system coordinate system of the wafer inspection system. A distance between a first intersection point of the line of sight of the charged particle imaging system within an image plane and a second intersection point of the optical axis of the optical imaging system with the image plane is given by distance D. A maximum value of the diameters of the member selected from the group of members comprising the rotation stage, the wafer chuck, and the wafer is given by W. A reference area AREA1 is given by AREA1 = (D + 2W) x 2W. The improved wafer inspection system according to an example is further comprising a vacuum enclosure for enclosing the wafer stage, wherein a footprint of the vacuum enclosure has an area smaller than the reference area AREA1.

[0018] With the footprint of reduced footprint or area size, a dimension a vacuum enclosure is reduced. Within the reduced footprint of the vacuum enclosure, it is still possible to rotate a wafer at any inspection position by 360°. Thereby, it is possible to arrange any inspection position on a wafer surface at the intersection point of a dual beam system at any rotation angle with respect to the arrangement of the dual beam system.

[0019] With the footprint of reduced area size, a dimension a vacuum enclosure is reduced, and a vacuum enclosure can be provided with higher stiffness. With the smaller dimension, dynamic vibrations are reduced, and imaging performance is improved. The reduction in area is achieved by several examples.

[0020] According to a first example, the at least one optical imaging system is arranged at an angle with respect to the direction of a long extension of the vacuum enclosure. The first intersection point and the second intersection point form a first connection line, wherein the first connection line is arranged at an angle g with respect to a direction of a long extension of the footprint. The wafer inspection system according to the first example is configured for including a rotation of the wafer chuck during registration of the wafer coordinate system by the alignment control system. The vacuum enclosure has a footprint of AREA4 = [1 ,5W + D x cos a] x 2W, which is smaller than the reference AREA1 of the conventional arrangement.

[0021] In an example, the vacuum enclosure of the wafer inspection system comprises at least two recesses, each of them configured to accommodate a mounting pedestal. Thereby, a span or distance between mounting pedestal is reduced and a dynamic behavior of architecture of the wafer inspection system is further improved. According to a second example, the wafer inspection system is comprising a second optical imaging system arranged at an angle. The angle g is defined by a half angle between a first connection line through the first intersection point and the second intersection point and a second connection line through the first intersection point and a third intersection point of the optical axis of the second optical imaging system within the image plane. The wafer inspection system according to the second example has a footprint of a vacuum of AREA2 = [2W + D x cos(a)] x 2W, which is smaller than AREA1 of the conventional arrangement. In both the first and second example, the angle is given by the equation sin(g) = 0.5 x W / D. The angle g can for example be between 20° and 45°.

[0022] According to a third example, the wafer inspection system is comprising four optical imaging systems. Each of the four optical imaging systems is separated by an angle of 90° between each connection line through the first intersection point and a respective intersection point of the optical axis of each respective optical imaging system with the image plane. The vacuum enclosure requires a minimum footprint of AREA3 = [W + D x SQRT(2)]A2, which is smaller than AREA1 of the conventional arrangement for D <= W.

[0023] According to a fourth example, a vacuum enclosure comprises at least three protrusions configured for resting the vacuum enclosure on at least three mounting pedestals. A mounting pedestal can for example be arranged within a recess of the vacuum enclosure. For example, the vacuum enclosure further comprises four protrusions configured for resting the vacuum enclosure on four mounting pedestals. For example, two of the four mounting pedestals are arranged within recesses of the vacuum enclosure.

[0024] In an example, each of the protrusions is arranged at a system level H comprising the center of mass of components resting on the at least three mounting pedestals, the components comprising at least the vacuum enclosure, a frame or lid and a charged particle imaging system. Further components which are mounted to the frame and contributing to the mass resting on the mounting pedestals may be considered, such as optical imaging system of a focused ion beam system. In an example, the wafer inspection system is further comprising a focused ion beam (FIB) system with an FIB optical axis arranged at an inclined angle GF with respect to the image plane. The FIB optical axis is forming an intersection point with the line of sight.

[0025] In a further example, an improved architecture of a wafer inspection system is achieved by a wafer stage with at least one linear motion stage and a wafer chuck for holding during use a wafer. The at least one linear motion stage and the wafer chuck are mounted on top of at least one rotation stage. The wafer inspection system is further comprising a charged particle beam imaging system, with an optical axis or line of sight. The charged particle beam imaging system is configured for high-resolution inspection of an inspection site on a wafer hold by wafer chuck. The wafer inspection system is comprising at least two optical imaging systems, each with an optical axis. Each optical imaging system is connected to an alignment control system configured for determining a wafer coordinate system and for registering the wafer coordinate system within a system coordinate system of the wafer inspection system. The wafer inspection system is further comprising a vacuum enclosure for enclosing the wafer stage. A distance between a first intersection point of the line of sight of the charged particle imaging system within an image plane and a second intersection point of each optical axis of each optical imaging system with the image plane is given by distance D. A maximum value of the diameters of the member selected from the group of members comprising the wafer chuck and the wafer is given by W. The vacuum enclosure for enclosing the wafer stage has a footprint with a diameter LY6 < 2(D + W) according to a diameter of a conventional architecture with only one optical imaging system for wafer registration.

[0026] In a further example, an improved architecture of a wafer inspection system is comprising a wafer stage with at least one linear motion stage and a wafer chuck for holding during use a wafer. Both are mounted on top of a rotation stage. The wafer inspection system is comprising a charged particle beam imaging system with an optical axis or line of sight. The charged particle beam imaging system is configured for high-resolution inspection of an inspection site on a wafer hold by wafer chuck. The wafer inspection system is comprising a focused ion beam (FIB) system with an FIB optical axis arranged at an inclined angle GEF with respect to the line of sight, the FIB optical axis forming an intersection point with the line of sight, the focused ion beam (FIB) system being configured for milling cross-sections into a wafer at an angle GF with respect to the wafer surface. The angle GF is for example 20°, 25°, 30°, 35° or 40°. The rotation axis of the rotation stage comprises the intersection point. The wafer inspection system is further comprising a control unit configured for aligning an inspection site at the intersection point and for rotating the wafer chuck at a rotation axis without movement of the at least one linear motion stage. In an example, the wafer inspection system is further comprising a second rotation stage mounted between the at least one linear motion stage and the wafer chuck. With the arrangement, a wafer rotation and a change of an orientation of cross-sections within a wafer surface can freely be adjusted without movement of a linear motion axis. With the second rotation axis, placement for inspection and wafer registration can be achieved with a reduced number of linear motion axes, for example only one lateral motion axis and a second one for height adjustment.

[0027] With the arrangements and examples given above, the mechanical properties such as stiffness is improved and a reduced sensitivity to vibrations is achieved. Thereby, an imaging performance is improved, for example due to a reduction in imaging noise or jitter.

[0028] While the examples and embodiments are described at the examples of semiconductor wafers, it is understood that the invention is not limited to semiconductor wafers but can for example also be applied to general objects of disk shape such as reticles or masks for semiconductor fabrication. The invention described by examples and embodiments is not limited to the embodiments and examples but can be implemented by those skilled in the art by various combinations or modifications thereof. The present invention will be fully understood with reference to the following drawings:

[0029] Figure 1 shows an illustration of a wafer inspection or metrology system for 3D volume inspection with a dual beam device.

[0030] Figure 2 is an illustration of the slice-and image method of a volume inspection in a wafer.

[0031] Figure 3 illustrates an example of a cross section image, obtained by the slice-and image method

[0032] Figure 4 shows examples of architectures of a wafer inspection system

[0033] Figure 5 shows an example of an arrangement of stage axes of a wafer inspection systems according to an embodiment

[0034] Figure 6 shows an example of a required footprint for an arrangement of a dual beam system and an optical imaging system for wafer registration

[0035] Figure 7 illustrates a reduced footprint according to a first example

[0036] Figure 8 illustrates a reduced footprint according to a second example

[0037] Figure 9 illustrates a reduced footprint according to a third example

[0038] Figure 10 illustrates a further modification of the third example

[0039] Figure 11 illustrates further details of the third example

[0040] Figure 12 shows a comparison of footprint areas of several examples

[0041] Figure 13 illustrates an example with reversed order for axes of a wafer stage

[0042] Figure 14 illustrates an example with a wafer stage comprising two axes for rotation.

[0043] Throughout the figures and the description, same reference numbers are used to describe same features or components, and reference is also made to the respective first introduction of a feature or component. The coordinate system is selected that a wafer support surface 151 coincides with the XY-plane. The scanning frequency of the charged particle imaging beam is for example 50MHz or 80MHz or more, corresponding to typical dwell time at each individual pixel location of few ns, for example 10ns, 12.5ns, 20ns, 30ns or 50ns. In the disclosure, the term drift is used as describing any temporal position displacement on time scales between few nanoseconds and several hours, thus drift is used to cover a range of frequencies below one Hertz up to the several MHz, corresponding to the scanning frequency or even more. Such high-frequent drifts are also called dynamic vibrations or jitter and are for example introduced due to noise (e.g. acoustic noise, floor vibrations, noise of water cooling, or other machine internal vibration sources like vacuum pumps, colling ventilators or other).

[0044] For the investigation of 3D inspection volumes in semiconductor wafers, a slice and imaging method has been proposed, which is applicable to inspection of volumes inside a wafer. In an example, a 3D volume image is generated from an inspection volume inside a wafer by the so called “wedge-cut” approach or wedge-cut geometry, without the need of a removal of a sample piece from the wafer. The slice and image method is applied to an inspection volume with dimensions of few pm, for example with a lateral extension of 5pm to 10pm in wafers with diameters of 200mm or 300mm. The lateral extension can also be larger and reach up to 30 or 50 micrometers. A groove or edge is milled in the top surface of an integrated semiconductor wafer to make accessible a cross-section surface at an angle to the top surface. 3D volume images of inspection volumes are acquired at a limited number of inspection sites, for example representative sites of dies, for example at process control monitors (PCM), or at sites identified by other inspection tools. The slice and image method will destroy the wafer only locally, and other dies may still be used, or the wafer may still be used for further processing. The methods and inspection systems according to the 3D volume image generation are described in WO 2021 1 180600 A1 , which is fully incorporated herein by reference. An example of a wafer inspection system 1000 for 3D volume inspection is illustrated in Figure 1. The wafer inspection system 1000 is configured for a slice and imaging method under a wedge cut geometry with a dual beam device 1. For a wafer 8, several inspection sites, comprising inspection sites 6.1 and 6.2, are defined in a location map or inspection list generated from an inspection tool or from design information. The wafer 8 is placed on a wafer support surface 15 of a wafer holder / table or wafer chuck 151. The wafer chuck 151 is mounted on a stage 155 with actuators and position control.

[0045] Actuators and means for precision control for a wafer stage such as Laser interferometers or optical encoders are known in the art. A stage can comprise actuators and position control in up to six degrees of freedom. A control unit 16 is configured to control the wafer stage 155 and to adjust the wafer 8, such that the surface 55 of the wafer is within an image plane 101 and a first inspection site 6.1 is at the intersection point 43 of the dual-beam device 1. The dual beam device 1 is comprising a FIB column 50 with a FIB optical axis 48 and a charged particle beam (CPB) imaging system 40 with optical axis 42. The optical axis 42 is typically represented by a center line through an image field of the charged particle beam (CPB) imaging system 40, which is also referred to as line of sight 42. The optical axis or line of sight 42 is typically determined during a calibration of the charged particle beam (CPB) imaging system 40. The focused ion beam column 50 is arranged at an angle GF to the surface of the image plane 101 or the wafer support surface 15 of the wafer stage 155. Therefore, during use, the wafer surface 55 is arranged at a slant angle GF to the FIB axis 48. During use, the wafer surface 55, and more precisely, the area to be investigated 6.1 is arranged at the intersection point 43 of both optical axes of FIB and CPB imaging system. FIB axis 48 and line of sight 42 include an angle GFE, and the CPB imaging system axis forms an angle GE with the normal to the wafer support surface 15. In the coordinate system of figure 1, the normal to the wafer support surface 15 is given by the z-axis. The focused ion beam (FIB) 51 is generated by the FIB-column 50 and is impinging under angle GF on the surface 55 of the wafer 8. Slanted cross-section surfaces are milled into the wafer by ion beam milling at the inspection site 6.1 under approximately the slant angle GF. In the example of figure 1, the slant angle GF is approximately 30°. The actual slant angle of the slanted cross-section surface can deviate from the slant angle GF by up to 1° to 4° due to the beam divergency of the focused ion beam, for example a Gallium-lon beam. The FIB column 50 can for example be a Gallium FIB, or a FIB with a gas field ion source (GFIS) with other kinds of ion species, such as Xenon, Neon or Argon ions. The cross-section surface are formed in a predefined direction determined by the arrangement of the FIB system. In the example of figure 1 , the predefined direction is the y-direction. With the charged particle beam imaging system 40, inclined under angle GE to the normal to the wafer support surface 15, images of the milled cross-section surfaces are acquired. In the example of Figure 1 , the angle GE is about 15°. However, other arrangements are possible as well, for example with GE = GF, such that the CPB imaging system axis 42 is perpendicular to the FIB axis 48, or GE = 0°, such that the CPB imaging system axis 42 is perpendicular to the wafer support surface 15.

[0046] During imaging, a beam of charged particles 44 is scanned by a scanning unit of the charged particle beam imaging system 40 along a scan path over a cross-section surface of the wafer 8 at inspection site 6.1 , and secondary particles as well as scattered particles are generated. For example, secondary electron particle detector 17.1 collects at least some of the secondary particles and scattered particles and communicates the particle count with a control unit 19. Other detectors for other of interaction products may be present as well, for example in-lens detector 17.2 for collection of backscattered charged particles. Control unit 19 is in control of the charged particle beam imaging column 40, of FIB column 50 and connected to a stage control unit 16 to control the position of the wafer 8 mounted on the wafer support surface 15 via the wafer stage 155. Control unit 19 communicates with operation control unit 2, which triggers placement and alignment for example of inspection site 6.1 of the wafer 8 at the intersection point 43 via wafer stage movement and triggers repeatedly operations of FIB milling, image acquisition and stage movements.

[0047] Each new intersection surface is milled by the FIB beam 51 , and imaged by the charged particle imaging beam 44, which is for example a scanning electron beam or a Helium-lon- beam of a Helium ion microscope (HIM). In an example, the dual beam system comprises a first focused ion beam system 50 arranged at a first angle GF1 and a second focused ion column arranged at the second angle GF2. In an example, the wafer is rotated between milling at the first angle GF1 and the second angle GF2, while imaging is performed by the imaging charged particle beam column 40, which is for example arranged perpendicular to the wafer surface 55.

[0048] The dual beam system 1 further comprises a gas injection system (GIS) 79, with a gas nozzle connected via a valve (not shown) to at least one gas reservoir (not shown). Thereby, controlled amounts of precursor gases can be provided during milling or imaging, and for example metal coatings can be generated. For example, alignment marks or fiducials can be generated. For example, a Tungsten metal coating is generated by providing Tungsten Hexacarbonyl. The metal coating can be shaped by ion beam milling and alignment markers or fiducials are formed in proximity to an inspection site. Thereby, a precise registration and image alignment of the plurality of cross section images is enabled. With dedicated precursor gases, a milling operation by FIB 51 can be enhanced. For example, a homogeneity of a milling operation in compositions of different material can be improved and curtaining can be reduced. Compositions of materials in a semiconductor wafer can comprise Silicon, Silicon Dioxide, Silicon Nitride, Copper, Aluminum, Tungsten, or other materials. Preferred precursor gases are comprising at least one of Ammonia, Ammonium Hydroxide, Ammonium Carbamate, Bromine, Chlorine, Hydrazine, Hydrogen Peroxide, Hadacidin, Iodine, di-iodo- ethane, Isopropanol, Methy Difluoroacetate, Nitroethane, Nitroethanol, Nitrogen, Nitrogen Tetroxide, Nitrogen Trifluoride, Nitromethane, Nitropropane, Nitrobutane, Oxygen, Ozone, PMCPS, Tungsten Hexacarbonyl, Water, or Xenon Difluoride. Other gases are, however, are possible as well, for example methoxy acetylchloride, methyl acetate, methyl nitroacetate, ethyl acetate, ethyl nitroacetate, propyl acetate, propyl nitroacetate, nitro ethyl acetate, methyl methoxyacetate, and methoxy acetylchloride, Acetic acid or thiolacetic acid, Hexafluoroacetylacetone, silazane, trifluoroacetamide, dicobalt octacarbonyl, molybdenum hexacarbonyl, and combinations thereof.

[0049] Furthermore, dual beam system 1 further comprises a contact pin 81. Contact pin 81 is connected to a manipulator (not shown) for precise movement of the contact pin 81, for example under control of the charged particle beam 44 during an image acquisition. Thereby, structures present on the wafer surface can be contacted and electrically connected to control device 19.

[0050] Figure 2 illustrates the wedge cut geometry at the example of a 3D-memory stack. Figure 2a illustrates the situation, when the surface 52 is the most recently milled cross-section surface which was milled by FIB 51. The cross-section surface 52 is scanned for example by SEM beam 44, which is in the example of Figure 2a arranged at normal incidence to the wafer surface 55, and a high-resolution cross-section image slice is generated. The cross-section surfaces 53.1...53.N are subsequently milled with a FIB beam 51 at an angle GF of approximately 30° to the wafer surface 55, but other angles GF, for example between GF = 20° and GF = 60° are possible as well. The cross-section image slice comprises first crosssection image features, formed by intersections with high aspect ratio (HAR)-structures or vias (for example first cross-section image features of HAR-structures 4.1 , 4.2, and 4.3) and second cross-section image features formed by intersections with layers L.1 ... L.M, which comprise for example SiO2, SiN- or Tungsten lines. Some of the lines are also called “wordlines”. The HAR structures typically have diameters below 100nm, for example about 80nm, or for example 40nm. The cross-section image slices contain therefore first cross-section image features as intersections or cross-sections of the HAR structures at different depth (Z) at the respective XY-location. In case of vertical memory HAR structures of a cylindrical shape, the obtained first cross-sections image features are circular or elliptical structures at various depths determined by the locations of the structures on the sloped cross-section surface 52. The memory stack extends in the Z-direction perpendicular to the wafer surface 55. The thickness d or minimum distances d between two adjacent cross-section image slices is adjusted to values typically in the order of few nm, for example 30nm, 20nm, 10nm, 5nm, 4nm or even less. Once a layer of material of predetermined thickness d is removed with FIB, a next cross-section surface 53. i... 53. N is exposed and accessible for imaging with the charged particle imaging beam 44. During repeated milling and imaging, a plurality of cross sections is formed, and a plurality of cross section images are obtained, such that an inspection volume 160 is properly sampled and for example a 3D volume image can be generated. Through-out the description, the coordinate system of the wafer inspection system 1000 is selected such that the coincidence point 43 is at the origin of the machine coordinate system and the FIB axis 48 is arranged within an y-z-plane, perpendicular to the x-axis, such that a surface normal 59 to a cross section surface 52 is also in the yz-plane and perpendicular to the x-axis of the wafer inspection system 1000.

[0051] Figure 2b shows a slice-and image acquisition at a first inspection site 6.1 at a first milling direction 13.1 on a wafer surface 55. The system coordinate system is labelled by xS and yS in Figure 2b. A wafer coordinate system 23 (with coordinate xW and yW in Figure 2b) is first registered by inspection of known reference marks on the wafer surface 55 (more details are described below). The first inspection site 6.1 is located at coordinates (TX1, TY1) within the wafer coordinate system 23. By stage 155, the wafer 8 is moved and adjusted such that the inspection site 6.1 is at the intersection point 43. A first wedge is generated by FIB beam 51 arranged in the ys-z plane of the system coordinate system. The predefined or milling direction 13.1 projected onto the wafer surface 55 is therefore parallel to the yW-direction in wafer coordinate system. In an example shown in Figure 2c , a second inspection site 6.2 is located at wafer coordinates (TX2, TY2), and a milling direction is adjusted by rotation angle TR2. A corresponding lateral movement and rotation of a wafer stage 155 is computed and executed by control unit 19 and the wafer surface 55 is adjusted with the second inspection site 6.2 at the intersection point 43, corresponding to the origin of coordinate system (xS, yS). Milling is performed in the predefined or milling direction 13.2, which is rotated with respect to the wafer coordinate system (xW, yW) by rotation angle TR2. Typically, rotation angles TRi of each inspection site can be either 0°, 90°, 180° or 270°, but any other angle can be possible as well.

[0052] Figure 3 shows an example of a cross-section image slice 311 generated by the imaging charged particle beam 44, corresponding to the cross-section surface 52. The cross-section image slice 311 comprises an edge line 315 between the slanted cross-section and the surface 55 of the wafer at the edge coordinate y1. The edge line 315 is oriented perpendicular to milling direction 13. Right to the edge, the image slice 311 shows several cross-sections 307.1 ...307. S through the HAR structures which are intersected by the crosssection surface 52. In addition, the image slice 311 comprises cross-sections of several word lines 313.1 to 313.3 at different depths or z-positions.

[0053] Each digital image of each cross-section surface comprises first cross-section features of HAR channels and second cross-section features of word lines at different depths. The depth of the word lines 313.1 to 313.3 is constant over large areas of a wafer. In an example, the word lines 313.1 to 313.3 are used as reference for a determination of the depth coordinate of a cross-section image slice 311. With the word lines 313.1 to 313.3, a depth map Z(x,y) of the slanted cross-section surface 52 can be generated. In another example, the distance to the edge line 315 is used for computation of the depth map Z(x,y). Thereby, for each pixel with transversal coordinates [x,y] according to the scanning operation of the charged particle imaging system 40, a depth coordinate according to the depth map Z(x,y) can be computed and high precision volume measurements are possible with the slice-and image-method in wedge-cut geometry. Examples and further details of image registration and depth map computation are provided in WO 2021 / 180600 A1 , cited above and incorporated herein by reference. Further, after performing a segmentation and annotation of a cross-section image of a semiconductor object of interest, HAR channel cross sections are identified and properties of HAR channel cross sections are determined by machine learning methods. Examples are described in WO 2022 / 223229A1 and WO 2023 / 117238 A1 , which are hereby incorporated by reference.

[0054] Figure 4 illustrates some further details of an architecture of a wafer inspection system 1000. Same reference numbers as in figure 1 are used and reference is also made to the description above. For sake of simplicity, figure 4a shows some limited details of a dual beam device 1. An imaging charged particle beam column, for example a scanning electron microscope 40 is mounted on a frame 25. The sample stage 155 of this example comprises a first three-axis stage 155.xyz for placement of the sample 8 in x- and y-direction orthogonal to the z-axis, which here is parallel to the axis 42 of the scanning electron microscope 40. With the stage 155.xyz, distance of sample surface 55 is adjusted with respect to the image plane or focus position of the electron beam 44 of scanning electron microscope 40.

[0055] Typically, the xyz-stage 155. xyz has a rectangular shape. The sample or wafer 8 can further be rotated by a rotation stage 155. t mounted on top of the three axis-stage 155.xyz. A sample chuck 151 with the wafer support surface 15 is mounted on top of rotation stage 155. t. The rotation stage 155. t is shown in figure 4 only schematically and can for example have a circular shape with a diameter only slightly exceeding the diameter of a wafer 8. The sample, here wafer 8 is attracted by sample chuck 151 to the wafer support surface 15 for example via electrostatic forces. Other sample chuck 151 utilized vacuum force or clamping to hold the sample 8. Control unit 19 communicates the position coordinates and desired orientation of inspection site 6 with stage control unit 16, such that inspection site 6 is moved to and aligned at the focus position of electron beam 44 along the optical axis or line of sight 42 of scanning electron microscope 40 (see figure 1). While the stage 155 of figure 4 comprises a rotation stage 155. t for rotation around the z-axis, a stage 155 may comprise additional components for further rotation around x- and y-axis as well, configuring a six-axis motion stage 155. Furthermore, a stage 155 can comprise two motion axes for at least one of the movement directions of a stage. For example, a stage 155 can comprise for x- and y- axis a fast, long stroke movement stage, and a high precision, short stroke movement stage for precision alignment of the point of interest at the intersection point of the dual beam device.

[0056] Movement of the three-axis-stage 155.xyz is controlled via a measurement system 21, for example Laser interferometer 21 with Laser beam 27. As reference, Laser interferometer 21 is mounted to or rigidly connected to frame 25. For example, z-stage and rotation-stage 155. t is controlled by an encoder system like optical encoders. The structure of frame 25 is of complex shape with dimensions about 1m or more. Both, SEM 40 and frame 25 are engineered to have high Eigenfrequencies, for example Eigenfrequencies of more than 100Hz, for example about 300Hz. Thereby, the Laser interferometer 21 is in a mechanical reference to the charged particle beam system 40. During calibration, the position and orientation of the line of sight 42 and the relative position between line of sight 42 and a reference coordinate system of frame 25 is determined. For example, the line of sight 42 can be part of the reference coordinate system of frame 25.

[0057] The stage 155 is mounted on a stage support base 809 via stage bearings 811. Stage support base 809 is connected to the baseplate 807, which forms a rigid mounting support for both frame 25 and stage support base 809. Frame 25 is connected to the baseplate 807 via active damping system 813. The baseplate 807 is connected to the fab floor 801 via stiff mounting pedestals 831. In the example, the dual beam system 1, frame 25, and stage 155 are enclosed by vacuum housing 803. Vacuum pumps 805 and control units 2, 16, and 19 are installed in a separated support cabinet 61 , which is connected to vacuum enclosure 803 view a flexible bellow 63. Thereby, vibrations and drift of the dual beam system 1 are reduced to a minimum. The invention, however, is not limited to systems fully enclosed in vacuum enclosure 803. In another example, illustrated in Figure 4b, the vacuum enclosure is formed by support base 809 with a vessel-shaped form with a cover given by frame 25. Same reference numbers are used in figure 4b, and reference is made to the description of figure 4a.

[0058] The wafer inspection system 1000 of figures 4a and 4b further comprises a control unit 19. Control unit 19 is connected to operation control unit 2, which comprises a user interface with a user interface display and user command devices (not shown). Operation control unit 2 further comprises a processing engine and a memory for executing and storing instructions. Instructions comprise for example image processing instructions. Images from image acquisition and image processing are stored in a memory. Operation control unit 2 can be in communication with external control devices of network via an interface unit.

[0059] The wafer inspection system 1000 of figures 4a and 4b further comprises an optical imaging system 117, which is connected to an alignment control system 121. The optical imaging system 117 can be an optical microscope with for example 10X magnification, with an integrated illumination system and an image sensor configured for obtaining an image of a surface segment of the wafer surface 55. After a wafer is loaded and mounted to the wafer chuck 151 in the conventional manner, the position and orientation of the wafer 8 is known with limited accuracy of for example 100pm. With the optical imaging system 117 and the alignment control system 121 , the position of the wafer 8 is determined and the wafer coordinate system xW, yW is registered with reference to the frame 25 or system coordinate system xS, yS. For example, prefabricated reference marks on the wafer surface 55 are detected and their position is determined with high precision. For example, inspection sites 6 are given in a wafer coordinate system xW, yWwith respect to the prefabricated reference marks. After wafer registration, the position of inspection sites 6 is known within the coordinate system xS, yS of the wafer inspection system 1000.

[0060] In figure 4b, the image plane 101 of the charged particle imaging system 40 is indicated. The image plane 101 is parallel to the surface 55 of the wafer 8. Typically, the wafer surface 55 can be arranged by stage 155 within the image plane 101. It is to be noted, however, that the focus position of the charged particle imaging system 40 is not necessarily in the image plane 101 , since the cross-sections formed by ion beam 51 of ion beam column 50 are at an angle with the image plane 101. Typically, the charged particle imaging system 40 comprises a fast-focusing system configured to follow with the focus position of the charged particle imaging beam 44 the inclined cross-sections 52 and 53. The optical axis 119 of the optical imaging system 117 is typically perpendicular to the image plane 101. Thereby, an impact on the registration of the wafer 8 by displacement of the wafer surface 55 by stage 155 in a direction perpendicular to the image plane 101 (here the direction of the z-axis) is reduced. The charged particle imaging system 40 has a certain mechanical footprint, which does not allow an arrangement of the optical imaging system 117 very close to the line of sight 42 of the charged particle imaging system 40, and the optical imaging system 117 can only be arranged at a minimum distance D to the charged particle imaging system 40. According to the disclosure, a distance D is defined between the first intersection point of the line of sight 42 of the charged particle imaging system 40 with the image plane 101 and the second intersection point of the optical axis 119 of the optical imaging system 117 with the image plane 101. Typical values of D are D = 200mm, D = 250mm, or more, for example D = 300mm or D = 350mm. However, in some examples, D can also be smaller than 200mm, for example D = 180mm. In the example of figure 4, the line of sight 42 of the charged particle imaging system 40 is perpendicular to the image plane 101 (GE = 0°). However as illustrated in figure 1, this does not need to be the case and other angles GE are possible as well.

[0061] Figure 5 illustrates some further details of an architecture of a wafer inspection system 1000. Same reference numbers are used as in the figures 1 to 4, and reference is also made to the description thereof. The two axes of displacement 155.xy enable a complete coverage of a wafer surface 55, such that every position on a wafer surface 55 can be arranged at the intersection point 43. In addition, to allow for different orientations of milling directions 13 at an inspection site, rotation axis 155. t allows for a full wafer rotation. Therefore, vacuum enclosure (not shown in figure 5) is construed and formed to allow wafer movement over a large range LX x LY, with LX > 2W and LY > 2W, whereby W is either the diameter of the wafer 8, or the wafer chuck 151 , or the rotation stage 155. t, whichever is larger. Typically, the diameter of the mechanical structure of a charged particle imaging system 40 exceeds the diameter of a wafer. Typically, the wafer diameter W is 200mm or 300mm. A wafer chuck 151 typically slightly exceeds the diameter of a wafer by for example 10%. A rotation stage 155. t can further exceed the diameter of wafer chuck 151.

[0062] According to a first example, a wafer rotation is not allowed during wafer registration. For example, in a typical setup, a rotation by stage 155. t may introduce a small error in wafer coordinates, which would lead to a small error during the wafer registration. For wafer registration with optical imaging system 117, the stage 155 must allow an accurate positioning of at least the prefabricated reference marks on the wafer surface 55 at the optical axis 119 of the optical imaging system 117 and thereby maintain a high positioning accuracy of the several prefabricated registration marks. Given the mechanical constraints and limited by the physical diameter of charged particle imaging system 40, the optical axis 119 of the optical imaging system 117 can only be mounted to the frame 25 at a minimum distance D, while distance D is for example exceeding D > 150mm, for example D> 200mm, for example D = 225mm, or for example even more, even exceeding with D > W a wafer diameter. An example is illustrated in Figure 6, where D ~ 0.75W. A full wafer rotation by 360° around the intersection point 43 (the intersection of the line of sight 42 with the image plane 101) requires a footprint area encircled by circle 179.1. This circle 179.1 has a minimum diameter of 2 x W, with for example W = 300mm. A full registration of the wafer surface 55 by optical imaging system 117 requires a footprint area encircled by circle 179.2, which is centered at optical axis 119 of the optical imaging system 117, which is formed at a distance D from the intersection point 43. Depending on the distance D, the footprint area encircled by circle 179.1 for wafer rotation at any inspection position on the surface of a wafer, and footprint area encircled by circle 179.2 for wafer registration may overlap. The required footprint area 167.1 of a vacuum enclosure 803 without wafer rotation during registration is therefore given by

[0063] (eq. 1) AREA1 > LX1 x LY1 = (D + 2W) x 2W.

[0064] In the example illustrated in Figure 6, the footprint area 167.1 is determined by its rectangular shape. Typically, x-y-z-stage 155. xyz has a rectangular shape. With the rotation stage 155. t mounted on top of the x-y-z-stage 155. xyz, a footprint area 167.1 has a rectangular shape as well. However, with other geometries of a wafer stage 155, the footprint area 167.1 might be reduced by about 10% to 14% by rounding off the edges, thus forming a footprint area with rounded contour 169.

[0065] According to an embodiment, the required footprint area 167 is reduced. In a first example, the required footprint area 167 of a vacuum enclosure 803 is reduced by using two optical imaging systems 117.1 , 117.2, and the footprint area 167.2 is reduced in comparison to footprint area 167.1 as illustrated in figure 6. An example with a first and a second optical imaging system 117.1 and 117.2 is illustrated in Figure 7 at an example with D > W. Again, a full wafer rotation by 360° around the intersection point 43 (the intersection of the line of sight 42 with the image plane 101) requires a footprint area encircled by circle 179.1. This circle has a minimum diameter of 2 x W, with for example W = 300mm. Here, each first and a second optical imaging system 117.1 and 117.2 are arranged at distance D with respect to the intersection point 43, and at an angle g with respect to the y-axis. The distance D is given by the distance between a first intersection point 43 of the line of sight (42) of the charged particle imaging system (40) within an image plane (101) and a second intersection point of the optical axis (119) of the optical imaging system (117) with the image plane (101) (see also figure 4 or 5 and description thereof). The angle g is defined by the half angle between a first connection line through the first intersection point and the second intersection point and a second connection line through the first intersection point and a third intersection point of the optical axis (119) of the second optical imaging system (117.2) with the image plane (101).

[0066] The registration of pre-fabricated reference marks on the wafer surface 55 is now divided into two areas of the wafer surface 55, with an upper, first half 11a and a lower, second half 11b. Pre-fabricated reference marks on the first half 11a are imaged by the first optical imaging system 117.1 , and pre-fabricated reference marks on the second half 11b are imaged by the second optical imaging system 117.2. During imaging and registration, a wafer position may cover the areas illustrated by areas 179.2a and 179.2b. In an example, the angle g is determined according to

[0067] (eq. 2) Sin(g) = 0.5 x W / D

[0068] For example, for W = D, g = 30°. For example, with D = 0.8W, g = 38.7°. For example, with D =1.2W, g = 24.7°. The footprint area 167.2 is given by LX2 x LY2, with

[0069] (eq. 3) LY2 = 2 x W + D x cos g.

[0070] The required footprint area 167.2 is therefore given by

[0071] (eq. 4) AREA2 = [2W + D x cos g] x 2W.

[0072] For each distance D, AREA2 is smaller compared to AREA1. For illustration, reference is made to Figure 12 (see below). AREA2 can be for example at least 5% smaller than AREA1.

[0073] This may sound not much, but nevertheless enables a more compact mechanical layout and more stable architecture of the wafer inspection system.

[0074] For D < W, the required footprint area 167 can even further be reduced by adding two more optical imaging systems 117.3 and 117.4. Figure 8 illustrates an example. Here, D ~ 0.8W, and the four optical imaging systems 117.1 to 117.4 for wafer registration are arranged each separated by an angle 90° between each connection line through the first intersection point and a respective intersection point of the optical axis (119) of each respective optical imaging system (117.1, 117.2, 117.3,117.4) with the image plane (101). With each optical imaging systems 117.1 to 117.4, only prefabricated reference marks within one quarter of the wafer surface 55 is imaged and registered. With this configuration comprising four optical imaging systems 117.1 to 117.4 at distance D to the intersection point 43, which coincides with the origin of coordinate system x,y of the wafer inspection system. The required footprints area 167.3 is determined with

[0075] (eq. 5) LX3 = LY3 = W + 2D x cos (45°) according to

[0076] (eq. 6) AREA3 = [W + D x SQRT(2)]A2.

[0077] The resulting AREA3 of the required footprint 167.3 is illustrated in figure 12 below for different D. The example with four optical imaging systems 117.1 to 117.4 offers a smaller footprint 167.3 in the case where D <= W. The arrangement of four optical imaging systems 117.1 to 117.4 allows a reduction of reference AREA1 by more than 10%; for D « W, an even larger reduction by more that 20% is possible.

[0078] According to a second example, the wafer registration by optical imaging system 117 can be performed including a wafer rotation. Allowing a wafer rotation during wafer registration, the required footprint areas can even further be reduced. A wafer rotation during wafer registration is for example made possible by additional position monitoring of the wafer chuck 151 during rotation. In a conventional design, during rotation, a wafer chuck 151 may change its lateral position due to typical inaccuracies in a rotation stage. With a closed-loop monitoring system, however, a lateral position of a wafer chuck 151 can be controlled with high precision during rotation. Examples of closed-loop monitoring systems and improved designs of wafer stages are described in patent application DE 102024203800.8, filed on April 23, 2024, which is incorporated here within in its full disclosure.

[0079] An example is illustrated in Figure 9. Figure 9 illustrates an example with a single optical imaging system 117 at distance D to the coordinate center and at an angle g with respect to the y-axis. The footprint 167.4 comprises a direction of a long extension, which is parallel to the y-axis of the coordinate system selected in this disclosure. In other words, the first intersection point of the line of sight (42) of the charged particle imaging system (40) within an image plane (101) and the second intersection point of the optical axis (119) at least one optical imaging system (117, 117.1) form a first connection line, wherein the first connection line is arranged at an angle g with respect to a direction of a long extension of the footprint (167.4), which corresponding to the y-axis. Again, a full wafer rotation by 360° around the intersection point 43 (the intersection of the line of sight 42 with the image plane 101) requires a footprint area encircled by circle 179.1. This circle 179.1 has a minimum diameter of 2 x W, with for example W = 300mm.

[0080] During registration, the wafer covers area 179.2. The angle g can be determined according to equation (1) given above. The required footprint 167.4 covers AREA4 with LY4 given by

[0081] (eq. 7) LY4 = 1.5W + D * cos (g).

[0082] AREA4 is thus given by

[0083] (eq. 8) AREA4 = [1 ,5W + D x cos g] x 2W. AREA4 is illustrated for comparison in Figure 12 (see below). The minimum AREA4 with the optical imaging system for registration arranged at an angle g with respect to a long extension of the vacuum enclosure is smaller than AREA1 for any ratio of D to W by about 20%.

[0084] With a reduction of the footprint 167.1 by realizing one of the above-described machine layouts with reduced footprints 167.2 to 167.4, the overall size of the vacuum chamber is reduced. The reduced size is resulting in increased Eigenfrequencies of the mechanical structured and by that in an increased stiffness of the architecture of a wafer inspection system 1000.

[0085] The required footprint 167.4 can even be further reduced by limiting the vacuum space to movement areas 179.1 for slice-and image acquisition at investigation sites at any position of a wafer surface 55 under any rotation angle of the wafer and 179.2 for wafer registration. An example is illustrated in Figure 10. In the example, the footprint 167.5 of the vacuum chamber 803 of a wafer inspection system 1000 enables an improved active isolation of the vacuum chamber 803 without increasing the overall footprint of the wafer inspection system 1000. The footprint 167.5 of the vacuum chamber 803 is configured for a full wafer rotation by 360° around the intersection point 43 (the intersection of the line of sight 42 with the image plane 101). The footprint 167.5 comprises a footprint area encircled by circle 179.1. This circle 179.1 has a minimum diameter of 2 x W, with for example W = 300mm. The footprint 167.5 further comprises area 179.2 for wafer registration. Depending on the distance D, the footprint area encircled by circle 179.1 for wafer rotation at any inspection position on the surface of a wafer, and footprint area 179.2 for wafer registration may overlap. The configuration according to the example of figure 10 can be of advantage for a mechanical layout of the wafer inspection system 1000. As depicted in Fig 4b, isolation system 813 is typically mounted underneath the vacuum chamber 803, thereby not increasing the footprint of the system. This may result in an inverted pendulum mode, typically observed in such architectures. By reducing the size of vacuum chamber as described in Figure 11, the mounting pedestals with active isolation system 845.1 to 845.4 are arranged side-by and adjacent to the vacuum chamber 803, while keeping the overall machine footprint constant compared to conventional architectures as depicted in Fig. 4b. Doing so, the working point of the mounting with isolation system 845.1 to 845.4 are arranged at a reduced lateral distance and can be aligned to center of gravity in vertical (Z) direction. Such an arrangement allows a reduced footprint and is resulting in an improved overall dynamic behavior. Wafer exchange is possible for example through wafer exchange port direction 91. FIB system 50 can be mounted opposite to the wafer exchange port direction 91 , thus allowing for a balanced design of masses. For example, there is still sufficient space for gas injection systems 79 or other systems. In the example of figure 11, mounting pedestals 845.1 to 845.4 can be either part of frame 25 or part of support base 809. The vacuum chamber 830 comprises at least two recesses 851.1 , 851.2, each of them configured to accommodate a mounting pedestal 845.1 or 845.2. An example is illustrated in figure 11b. The vacuum chamber 803 comprises at least two protrusions 857 (only two of four shown, 857.2 and 857.3). The vacuum chamber 803 is resting via the protrusions 857 on the mounting pedestals 845.1 to 845.4 with active isolation systems 813 (only two shown). The mounting pedestals 845.2 and its active isolation system 813 are arranged within the recess 851.2. In this example, mounting pedestals 845 rest on the fab floor 801 and are not connected. In another example, the mounting pedestals 845 are jointly connected to a rigid mounting base or baseplate 807 (see figure 4a). The four protrusions 857 are approximately at the system level H of the center of mass of the vacuum enclosure 803, the frame or lid 25, the charged particle imaging system 40 and the optional focused ion beam system 50 (not shown in figure 11b). Thereby, the inverted pendulum mode is suppressed. While the figure 11a and 11b illustrate an example with four protrusions 857 and four mounting pedestals 845.1 to 845.4, generally other configurations are possible as well, comprising at least three protrusions 857 and three mounting pedestals 845.1 to 845.3. The arrangement of protrusions configured for resting the vacuum enclosure 803 at system level H on mounting pedestals 845.1 to 845.4 is not limited to the example illustrated in figure 11 but can be applied to each vacuum enclosure 803. It is understood that the system level H approximately comprises the center of mass of the components resting on the at least three mounting pedestals 845.1 to 845.4.

[0086] Figure 12 illustrates a comparison of the improvement achieved by the embodiments. Figure 12 illustrates the normalized area of the different examples over normalized distance D. Distance D represents the minimum distance D between the optical axis 119 of an optical imaging system 117 for wafer registration and the optical axis or a charged particle beam system 40 for imaging. W is approximately given by the diameter of the wafer chuck 151 or rotation stage 155. t, and typically exceeds the wafer diameter of for example 10% to allow some additional room for wafer chuck 151 or components of the rotation stage 155. t. AREA1 is the reference area illustrated in Figure 6. AREA1 at D / W = 1 is used as a reference for computing normalized footprint areas. For all areas, the rectangular areas are used and any reduction in area by rounding off the edges is not considered. Figure 12 illustrates that with two optical imaging systems, the required footprint area (AREA2) can be reduced for all ratios D / W. Figure 12 illustrates that with four optical imaging systems, a reduction of the required footprint area (AREA3) is achieved for ratios D / W < 1. Figure 12 illustrates that with wafer rotation during wafer registration, a reduction of the required footprint area (AREA4) is achieved for ratios all D / W. It should be noted that any reduction in area of the footprint of the vacuum enclosure is useful. It is not necessary to fully exploit the area limits of AREA2 or AREA3 or AREA4 illustrated in Figure 12 and the description above, but any reduction in area compared to the reference AREA1 improves the mechanical stability of a wafer inspection system. With the solutions provided above, for example an arrangement of an optical imaging system for registration at an angle g with respect to a long extension of the vacuum enclosure, or with arrangement of two or four optical imaging system for registration, or with the recesses of the vacuum enclosure described in the example of figures 10 and 11, a reduction of the area of a vacuum enclosure of at least 5%, for example 10% or even more, for example 20%, can be achieved.

[0087] The examples illustrated above illustrate examples of a wafer inspection system 1000 comprising a wafer stage 155 comprising, mounted on a support base 809, three axes of a stage 155.xy and 155.z, with a rotation stage 155. t mounted above the three axes. In this configuration, rotation around any point on the sample surface (“comp-centric rotation”) can only be realized by moving at least two axes (rotation + X- / Y-movement) if the point of interest is not located perfectly in the middle of the wafer. According to an embodiment, a wafer stage 155 comprises a rotation stage 155t connected to the support base 809, with the three axes of stage 155.xy and 155.z mounted above or on top of the rotation stage 155. t. An example is illustrated in Figure 13. Figure 13a illustrates the dual beam system 1 comprising charged particle imaging system 40 and focused ion beam system 50, mounted to frame 25. With the system according to figure 13a, once an inspection site 6 is positioned at the optical axis 42 of the charged particle imaging beam 40 by x-y-z movement, and with the optical axis 42 coincident with the rotation axis of rotation stage 155. t, the inspection site 6 can be freely rotated in the field of view in the charged particle imaging system 40 and a milling direction can quickly be changed, without additional X- / Y- movements for “comp centric rotation”. A wafer inspection system 1000 according to the example is therefore comprising a wafer stage 155 with at least one linear motion stage 155.x, 155. y, 155.z, and a wafer chuck 151 for holding during use a wafer 8. The at least one linear motion stage 155.x, 155. y, 155.z, and the wafer chuck 151 are mounted on top of a rotation stage 155. t. The optical axis or line of sight 42 of the charged particle beam imaging system 40 and the FIB optical axis 48 are forming an intersection point 43. The wafer inspection system 1000 is adjusted such that the rotation axis of the rotation stage 155. t (here the z-axis) comprises the intersection point 43.

[0088] Figure 13b illustrates the required footprint area with two optical imaging systems 117.1 and

[0089] 117.2 for wafer registration. In this example, a circular footprint 167.6 is reduced from 2(D+W) to a diameter LY6 > 2D + W by using two optical imaging systems 117.1 and 117.2. Thus, by arrangement of at least two optical imaging systems 117.1 and 117.2, a diameter LY6 is smaller than 2(D+W). A further example is illustrated in figure 14a. Here, wafer stage 155 comprises a second rotation stage 155.t2, with stages for x-y-z-movement mounted on a first rotation stage 155. t1. The first rotation stage 155. t1 is mounted to support base 809. The second rotation stage 155.t2 is mounted between stages for x-y-z-movement 155.xyz and wafer chuck 151. Thereby, a diameter of a footprint area 167.7 is reduced to LY7 > D + 1.5W (see Figure 14b). Further, this configuration enables an axis layout of only one linear axis with only half of the stroke length for horizontal (x- or y-) movement. Figure 14c illustrates an example, in which one linear motion stage 155y for lateral movement in y direction is mounted on the first rotation stage 155. t1. Combining rotation of both rotary axis with the linear movement of the one linear motion stage 155y in between first and second rotation stages 155. t1 and 155. t2, every point of the sample can be placed under the SEM, resulting in lower overall stack height and payloads.

[0090] A wafer inspection system 1000 according to an embodiment comprises at least one optical imaging system for wafer registration. A wafer inspection system 1000 comprises a vacuum enclosure which allows a free rotation of the wafer by 360° at any inspection position on the surface of the wafer. Thereby, within the wafer inspection system, a wafer can be rotated around a normal to the wafer surface by 360° at any inspection position on the wafer surface. A wafer inspection system 1000 according to some embodiments comprises a dual beam system and a stage capable for rotating the wafer and moving the wafer in lateral direction. Thereby, a wafer inspection system 1000 according to embodiments allows a formation or observation of cross sections under any rotation angle of the wafer at any position on the surface of the wafer. To accommodate the full wafer rotation, a minimum diameter of the vacuum enclosure is therefore exceeding a diameter W multiplied by two, with W being the diameter of wafer, wafer chuck or rotation stage, whatever is larger. According to the examples of the embodiments, a footprint area 167 of a vacuum enclosure 803 of a wafer inspection system 1000 is reduced, and thereby, also a volume of a vacuum enclosure is reduced. By reducing the footprint area, the mechanical architecture of the wafer inspection system 1000 can be designed with higher stiffness, for example by a reduced distance between mounting pedestal 845. The mounting pedestals 845 can be integral part of either a metrology frame 25 or a support base 809 with the shape of a vessel (see for example Figure 4a and 4b, 11a and 11b).

[0091] The method and wafer inspection system 1000 can be used for quantitative metrology, but can also be used for defect detection, process monitoring, defect review, and inspection of integrated circuits within semiconductor wafers. The disclosure is described by following examples:

[0092] Clause 1: A wafer inspection system (1000), comprising

[0093] - a wafer stage (155) with at least one linear motion stage (155.x, 155, y, 155.z), and a rotation stage (155. t), and a wafer chuck (151) for holding during use a wafer (8),

[0094] - a charged particle beam imaging system (40), with an optical axis or line of sight (42), the charged particle beam imaging system (40) being configured for high-resolution inspection of an inspection site (6) on a wafer (8) hold by wafer chuck (151);

[0095] - at least one optical imaging system (117, 117.1) with an optical axis (119), the at least one optical imaging system (117) being connected to an alignment control system (121), configured for determining a wafer coordinate system (23) and for registering the wafer coordinate system (23) within a system coordinate system (29) of the wafer inspection system (1000);

[0096] - a vacuum enclosure (803) for enclosing the wafer stage (155), wherein

[0097] - a distance between a first intersection point of the line of sight (42) of the charged particle imaging system (40) within an image plane (101) and a second intersection point of the optical axis (119) of the optical imaging system (117) with the image plane (101) is given by distance D; - a maximum value of the diameters of the member selected from the group of members comprising the rotation stage (155. t), the wafer chuck (151), and the wafer (8) is given by W;

[0098] - a reference area AREA1 is given by AREA1 = (D + 2W) x 2W; wherein a footprint (167.2, 167.3, 167.4, 167.5) of the vacuum enclosure (803) for enclosing the wafer stage (155) has an area smaller than the reference area AREA1 , wherein within the vacuum enclosure (803), the wafer (8) can be rotated by the rotation stage (155. t) around the first intersection point by 360° at any inspection site (6) on the wafer (8).

[0099] Clause 2: The wafer inspection system (1000) of clause 1, wherein the first intersection point and the second intersection point form a first connection line, wherein the first connection line is arranged at an angle g with respect to a direction of a long extension of the footprint (167.2, 167.3, 167.4, 167.5).

[0100] Clause 3: The wafer inspection system (1000) of clause 1 or 2, further configured for including a rotation of the wafer chuck (151) during registration of the wafer coordinate system (23) by the alignment control system (121), wherein the vacuum enclosure (803) has a footprint (167.4) of AREA4 = [1 ,5W + D x cos a] x 2W.

[0101] Clause 4: The wafer inspection system (1000) of clause 1 , further comprising a second optical imaging system (117.2), wherein an angle g is defined by a half angle between a first connection line through the first intersection point and the second intersection point and a second connection line through the first intersection point and a third intersection point of the optical axis (119) of the second optical imaging system (117.2) with the image plane (101). Clause 5: The wafer inspection system (1000) of clause 4, wherein the vacuum enclosure (803) has a footprint (167.2) of AREA2 = [2W + D x cos(a)] x 2W.

[0102] Clause 6: The wafer inspection system (1000) according to any of the clauses 2 to 5, wherein the angle is given by the equation sin(a) = 0.5 x W / D.

[0103] Clause 7: The wafer inspection system (1000) of clause 1 or 2, wherein D <= W, further comprising four optical imaging systems (117.1) to (117.4).

[0104] Clause 8: The wafer inspection system (1000) of clause 7, wherein the vacuum enclosure

[0105] (803) has a footprint (167.3) of AREA3 = [W + D x SQRT(2)]A2. Clause 9: The wafer inspection system (1000) of clause 7 or 8, wherein each of the four optical imaging systems (117.1) to (117.4) is separated by an angle of 90° between each connection line through the first intersection point and a respective intersection point of the optical axis (119) of each respective optical imaging system (117.1, 117.2, 117.3,117.4) with the image plane (101).

[0106] Clause 10: The wafer inspection system (1000) of clause 3, wherein the vacuum enclosure (803) comprises at least two recesses (851.1 , 851.2), each of them configured to accommodate a mounting pedestal (845.1 or 845.2).

[0107] Clause 11 : The wafer inspection system (1000) according to any of the clauses 1 to 10, wherein the vacuum enclosure (803) further comprises at least three protrusions (857, 857.1, 857.2, 857.3, 857.4) configured for resting the vacuum enclosure (803) on at least three mounting pedestals (845.1 , 845.2, 845.3, 845.4).

[0108] Clause 12: The wafer inspection system (1000) of clause 11, wherein the vacuum enclosure (803) further comprises four protrusions (857, 857.1 , 857.2, 857.3, 857.4) configured for resting the vacuum enclosure (803) on four mounting pedestals (845.1, 845.2, 845.3, 845.4). Clause 13: The wafer inspection system (1000) of clause 11 or 12, wherein each of the protrusions (857, 857.1, 857.2, 857.3, 857.4) is arranged at a system level H comprising the center of mass of components resting on the at least three mounting pedestals (845.1) to (845.4), the components comprising at least the vacuum enclosure (803), a frame or lid (25) and the charged particle imaging system (40).

[0109] Clause 14: The wafer inspection system (1000) according to any of the clauses 1 to 13, further comprising a focused ion beam (FIB) system (50) with an FIB optical axis (48) arranged at an inclined angle GF with respect to the image plane (101), the FIB optical axis (48) forming an intersection point (43) with the line of sight (42).

[0110] Clause 15: A wafer inspection system (1000), comprising

[0111] - a wafer stage (155) with at least one linear motion stage (155.x, 155, y, 155.z), and a wafer chuck (151) for holding during use a wafer (8), mounted on top of at least one rotation stage

[0112] (155. t, 155. t1), - a charged particle beam imaging system (40), with an optical axis or line of sight (42), the charged particle beam imaging system (40) being configured for high-resolution inspection of an inspection site (6) on a wafer (8) hold by wafer chuck (151);

[0113] - at least two optical imaging systems (117.1, 117.2), each with an optical axis (119), the at least two optical imaging systems (117.1 , 117.2) being connected to an alignment control system (121), configured for determining a wafer coordinate system (23) and for registering the wafer coordinate system (23) within a system coordinate system (29) of the wafer inspection system (1000);

[0114] - a vacuum enclosure (803) for enclosing the wafer stage (155), wherein

[0115] - a distance between a first intersection point of the line of sight (42) of the charged particle imaging system (40) within an image plane (101) and a second intersection point of each optical axis (119) of each optical imaging system (117.1, 117.2) with the image plane (101) is given by distance D;

[0116] - a maximum value of the diameters of the member selected from the group of members comprising the wafer chuck (151) and the wafer (8) is given by W;

[0117] - wherein the vacuum enclosure (803) for enclosing the wafer stage (155) has a footprint

[0118] (167.6) with a diameter LY6 < 2(D + W).

[0119] Clause 16: A wafer inspection system (1000), comprising

[0120] - a wafer stage (155) with at least one linear motion stage (155.x, 155, y, 155.z), and a wafer chuck (151) for holding during use a wafer (8), mounted on top of a rotation stage (155. t, 155. t1),

[0121] - a charged particle beam imaging system (40), with an optical axis or line of sight (42), the charged particle beam imaging system (40) being configured for high-resolution inspection of an inspection site (6) on a wafer (8) hold by wafer chuck (151);

[0122] - a focused ion beam (FIB) system (50) with an FIB optical axis (48) arranged at an inclined angle GEF with respect to the line of sight (42), the FIB optical axis (48) forming an intersection point (43) with the line of sight (42), the focused ion beam (FIB) system (50) being configured for milling cross-sections (52,53) into a wafer (8) at an angle GF with respect to the wafer surface (55), wherein the rotation axis of the rotation stage (155. t, 155. t1) comprises the intersection point (43).

[0123] Clause 17: The wafer inspection system (1000) according clause 16, comprising a control unit (19) configured for aligning an inspection site (6, 6.1) at the intersection point (43) and for rotating the wafer chuck (151) at a rotation axis without movement of the at least one linear motion stage (155.x, 155, y, 155.z).

[0124] Clause 18: The wafer inspection system (1000) according clause 16 or 17, further comprising a second rotation stage (155.t2) mounted between the at least one linear motion stage (155.x, 155,y, 155.z) and the wafer chuck (151).

[0125] Clause 19: A wafer inspection system (1000), comprising

[0126] - a wafer stage (155) with at least one linear motion stage (155.x, 155, y, 155.z), and a rotation stage (155. t), and a wafer chuck (151) for holding during use a wafer (8),

[0127] - a charged particle beam imaging system (40), with an optical axis or line of sight (42), the charged particle beam imaging system (40) being configured for high-resolution inspection of an inspection site (6) on a wafer (8) hold by wafer chuck (151);

[0128] - at least one optical imaging system (117, 117.1) with an optical axis (119), the at least one optical imaging system (117) being connected to an alignment control system (121), configured for determining a wafer coordinate system (23) and for registering the wafer coordinate system (23) within a system coordinate system (29) of the wafer inspection system (1000);

[0129] - a vacuum enclosure (803) for enclosing the wafer stage (155), wherein

[0130] - a distance between a first intersection point of the line of sight (42) of the charged particle imaging system (40) within an image plane (101) and a second intersection point of the optical axis (119) of the optical imaging system (117) with the image plane (101) is given by distance D; - a maximum value of the diameters of the member selected from the group of members comprising the rotation stage (155. t), the wafer chuck (151), and the wafer (8) is given by W;

[0131] - a reference area AREA1 is given by AREA1 = (D + 2W) x 2W; wherein an area of a footprint (167.2, 167.3, 167.4, 167.5) of the vacuum enclosure (803) for enclosing the wafer stage (155) has a reduced size at least 5% compared to the reference area AREA1.

[0132] Clause 20: A wafer inspection system (1000), comprising

[0133] - a wafer stage (155) with at least one linear motion stage (155.x, 155, y, 155.z), and a rotation stage (155. t), and a wafer chuck (151) for holding during use a wafer (8),

[0134] - a charged particle beam imaging system (40), with an optical axis or line of sight (42), the charged particle beam imaging system (40) being configured for high-resolution inspection of an inspection site (6) on a wafer (8) hold by wafer chuck (151);

[0135] - a vacuum enclosure (803) for enclosing the wafer stage (155), wherein

[0136] - wherein the vacuum enclosure (803) for enclosing the wafer stage (155) comprises at least two recesses (851.1 , 851.2), each for accommodating a mounting pedestals (845.1, 845.2, 845.3, 845.4).

[0137] The disclosure is however not limited to these clauses or examples, and combinations and variations are possible as well and included in the invention.

[0138] A list of reference numbers is provided:

[0139] 1 Dual Beam system

[0140] 2 Operation Control Unit

[0141] 6 inspection site

[0142] 8 wafer

[0143] 11 accessible wafer area

[0144] 13 milling direction

[0145] 15 wafer support surface

[0146] 16 stage control unit 17 Electron detector

[0147] 19 Control Unit

[0148] 21 measurement system

[0149] 23 wafer coordinate system

[0150] 25 Metrology Frame

[0151] 27 position measurement

[0152] 40 charged particle beam (CPB) imaging system

[0153] 42 Optical Axis of imaging system or line of sight

[0154] 43 Intersection point

[0155] 44 Imaging charged particle beam

[0156] 48 Fib Optical Axis

[0157] 50 FIB column

[0158] 51 focused ion beam

[0159] 52 cross section surface

[0160] 53 cross section surface

[0161] 55 wafer top surface

[0162] 59 cross section surface normal

[0163] 61 support cabinet

[0164] 63 flexible bellows

[0165] 79 Gas Injection system

[0166] 81 contact pin

[0167] 91 wafer exchange port

[0168] 117 optical imaging system

[0169] 119 optical axis of imaging system

[0170] 121 alignment control system

[0171] 151 wafer chuck

[0172] 155 wafer stage

[0173] 160 inspection volume 167 footprint area of vacuum enclosure

[0174] 169 footprint area with rounded contour

[0175] 179 area covered by wafer movement

[0176] 307 measured cross section image of HAR structure

[0177] 311 cross section image slice

[0178] 313 word lines

[0179] 315 edge with surface

[0180] 801 Fab floor

[0181] 803 vacuum enclosure

[0182] 805 vacuum pumps

[0183] 807 rigid mounting base ("Baseplate")

[0184] 809 support base

[0185] 811 stage bearings

[0186] 813 active damping system

[0187] 831 mounting pedestal

[0188] 845 mounting pedestals

[0189] 851 recess

[0190] 857 protrusion

[0191] 1000 wafer inspection system

Claims

CLAIMS1. A wafer inspection system (1000), comprising- a wafer stage (155) with at least one linear motion stage (155.x, 155, y, 155.z), and a rotation stage (155. t), and a wafer chuck (151) for holding during use a wafer (8),- a charged particle beam imaging system (40), with an optical axis or line of sight (42), the charged particle beam imaging system (40) being configured for high-resolution inspection of an inspection site (6) on a wafer (8) hold by wafer chuck (151);- at least one optical imaging system (117, 117.1) with an optical axis (119), the at least one optical imaging system (117) being connected to an alignment control system (121), configured for determining a wafer coordinate system (23) and for registering the wafer coordinate system (23) within a system coordinate system (29) of the wafer inspection system (1000);- a vacuum enclosure (803) for enclosing the wafer stage (155), wherein- a distance between a first intersection point of the line of sight (42) of the charged particle imaging system (40) within an image plane (101) and a second intersection point of the optical axis (119) of the optical imaging system (117) with the image plane (101) is given by distance D;- a maximum value of the diameters of the member selected from the group of members comprising the rotation stage (155. t), the wafer chuck (151), and the wafer (8) is given by W;- a reference area AREA1 is given by AREA1 = (D + 2W) x 2W; wherein a footprint (167.2, 167.3, 167.4, 167.5) of the vacuum enclosure (803) for enclosing the wafer stage (155) has an area smaller than the reference area AREA1 , wherein within the vacuum enclosure (803), the wafer (8) can be rotated by the rotation stage (155. t) around the first intersection point by 360° at any inspection site (6) on the wafer (8).

2. The wafer inspection system (1000) of claim 1 , wherein the first intersection point and the second intersection point form a first connection line, wherein the first connection line is arranged at an angle g with respect to a direction of a long extension of the footprint (167.2, 167.3, 167.4, 167.5).

3. The wafer inspection system (1000) of claim 1 or 2, further configured for including a rotation of the wafer chuck (151) during registration of the wafer coordinate system (23) by the alignment control system (121), wherein the vacuum enclosure (803) has a footprint (167.4) of AREA4 = [1.5W + D x cos a] x 2W.

4. The wafer inspection system (1000) of claim 1 , further comprising a second optical imaging system (117.2), wherein an angle g is defined by a half angle between a first connection line through the first intersection point and the second intersection point and a second connection line through the first intersection point and a third intersection point of the optical axis (119) of the second optical imaging system (117.2) with the image plane (101).

5. The wafer inspection system (1000) of claim 4, wherein the vacuum enclosure (803) has a footprint (167.2) of AREA2 = [2W + D x cos(a)] x 2W.

6. The wafer inspection system (1000) according to any of the claims 2 to 5, wherein the angle is given by the equation sin(a) = 0.5 x W / D.

7. The wafer inspection system (1000) of claim 1 or 2, wherein D <= W, further comprising four optical imaging systems (117.1) to (117.4).

8. The wafer inspection system (1000) of claim 7, wherein the vacuum enclosure (803) has a footprint (167.3) of AREA3 = [W + D x SQRT(2)]A2.

9. The wafer inspection system (1000) of claim 7 or 8, wherein each of the four optical imaging systems (117.1) to (117.4) is separated by an angle of 90° between each connection line through the first intersection point and a respective intersection point of the optical axis (119) of each respective optical imaging system (117.1 , 117.2, 117.3,117.4) with the image plane (101).

10. The wafer inspection system (1000) of claim 3, wherein the vacuum enclosure (803) comprises at least two recesses (851.1 , 851.2), each of them configured to accommodate a mounting pedestal (845.1 or 845.2).

11. The wafer inspection system (1000) according to any of the claims 1 to 10, wherein the vacuum enclosure (803) further comprises at least three protrusions (857, 857.1, 857.2, 857.3, 857.4) configured for resting the vacuum enclosure (803) on at least three mounting pedestals (845.1 , 845.2, 845.3, 845.4).

12. The wafer inspection system (1000) of claim 11, wherein the vacuum enclosure (803) further comprises four protrusions (857, 857.1, 857.2, 857.3, 857.4) configured for resting the vacuum enclosure (803) on four mounting pedestals (845.1 , 845.2, 845.3, 845.4).

13. The wafer inspection system (1000) of claim 11 or 12, wherein each of the protrusions (857, 857.1, 857.2, 857.3, 857.4) is arranged at a system level H comprising the center of mass of components resting on the at least three mounting pedestals (845.1) to (845.4), the components comprising at least the vacuum enclosure (803), a frame or lid (25) and the charged particle imaging system (40).

14. The wafer inspection system (1000) according to any of the claims 1 to 13, further comprising a focused ion beam (FIB) system (50) with an FIB optical axis (48) arranged at aninclined angle GF with respect to the image plane (101), the FIB optical axis (48) forming an intersection point (43) with the line of sight (42).

15. A wafer inspection system (1000), comprising- a wafer stage (155) with at least one linear motion stage (155.x, 155, y, 155.z), and a wafer chuck (151) for holding during use a wafer (8), mounted on top of at least one rotation stage (155. t, 155. t1),- a charged particle beam imaging system (40), with an optical axis or line of sight (42), the charged particle beam imaging system (40) being configured for high-resolution inspection of an inspection site (6) on a wafer (8) hold by wafer chuck (151);- at least two optical imaging systems (117.1, 117.2), each with an optical axis (119), the at least two optical imaging systems (117.1 , 117.2) being connected to an alignment control system (121), configured for determining a wafer coordinate system (23) and for registering the wafer coordinate system (23) within a system coordinate system (29) of the wafer inspection system (1000);- a vacuum enclosure (803) for enclosing the wafer stage (155), wherein- a distance between a first intersection point of the line of sight (42) of the charged particle imaging system (40) within an image plane (101) and a second intersection point of each optical axis (119) of each optical imaging system (117.1, 117.2) with the image plane (101) is given by distance D;- a maximum value of the diameters of the member selected from the group of members comprising the wafer chuck (151) and the wafer (8) is given by W;- wherein the vacuum enclosure (803) for enclosing the wafer stage (155) has a footprint (167.6) with a diameter LY6 < 2(D + W).

Images