WO2025223848A1 - Inspection apparatus for 3D tomography with improved stand-still performance - Google Patents

Abstract

A dual beam device for three-dimensional volume image generation of semiconductor objects within a wafer and a method of operating the dual beam device are provided. The dual beam device comprises a closed loop monitoring system between a focus point of a charge-particle beam imaging system and an inspection site on a wafer mounted on a wafer stage. Thereby, drifts between a charge-particle beam imaging system and the wafer stage are mitigated.

Description

Title

Inspection apparatus for 3D tomography with improved stand-still performance

Field The present invention relates to an inspection apparatus for semiconductor objects within a semiconductor wafer, more particularly, to an apparatus and corresponding method of operating the apparatus for performing 3D tomography at a wafer. The apparatus 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.

Background

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 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 repeated chemical, mechanical, 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 defectdetection, the best manufacturing process parameters can be selected. In this way, production process parameters can be tweaked towards optimality. Afterwards, a highly accurate quality control process and device for the metrology semiconductor structures in wafers is required.

Fabricated semiconductor structures are fabricated by determined processes and are therefore based on prior knowledge. The semiconductor structures are manufactured in a sequence of layers being parallel to a surface of a substrate. For example, in a logic type sample, metal lines are running parallel in metal layers or HAR (high aspect ratio) structures and metal vias run perpendicular to the metal layers. The angle between metal lines in different layers is either 0° or 90°. On the other hand, for VNAND type structures it is known that their cross-sections are circular on average. Furthermore, a semiconductor wafer has a diameter of 300 mm and consist of a plurality of several sites, so called dies, each comprising at least one integrated circuit pattern such as for example for a memory chip or for a processor chip. During fabrication, semiconductor wafers run through 1000 or more process steps, and within the semiconductor wafer, about 100 and more parallel layers are formed, comprising the transistor layers, the layers of the middle of the line, and the interconnect layers and, in memory devices, a plurality of 3D arrays of memory cells.

The aspect ratio and the number of layers of integrated circuits constantly increases and the structures are growing into 3^rd (vertical) dimension. The current height of the memory stacks is exceeding a dozen 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 3^rd dimension, 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, for example even below 1 nm.

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 2020 / 244795 A1. According to the method of the WO 2020 / 244795 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 / 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 100 or even more image slices. For example, in a volume with a lateral dimension of 5pm and a slicing distance of 5nm, 1000 slices are milled and imaged. 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. 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.

Generally, semiconductors comprise many repetitive three-dimensional structures. During the manufacturing process or a process development, some selected physical or geometrical parameters of a representative plurality of the three-dimensional structures have to be measured with high accuracy and high throughput. For monitoring the manufacturing, an inspection volume is defined, comprising the representative plurality the three-dimensional structures. This inspection volume is then analyzed for example by a slice and image approach, leading to a 3D volume image of the inspection volume with high resolution. From the large number of image slices, a three-dimensional volume image is derived with high accuracy. The critical dimensions in single-digit nm regime require a high-resolution metrology method. Therefore, the measurement time to generate a 3D volume image is quite long, and a measurement of a 3D volume image with N = 1000 cross-sections can require up to several hours, for example 24 hours or even more. During this long measurement time, machine drifts and vibrations may deteriorate the measurement result and lead to unwanted degradation of the accuracy of the measurement task to be performed. Specifically, vibrations of the sample under investigation lead to reduced image contrast.

The above-mentioned problems are of particular importance when a so-called stacked stage comprising a rotation stage is used for holding a wafer placed on a chuck. A stacked stage is for example a multi-axis wafer stage allowing for a movement in an X,Y plane (XY-stage), in height direction (Z-Stage) and allowing for a rotational movement T (rotation stage). In principle, each motion axis of the stages requires a degree of freedom in movement and does therefore not allow a stiff mechanical connection. Especially the rotational axis may introduce some uncertainty of position of the wafer chuck. Therefore, even if a relative position between the charged particle beam imaging system and the XY-stage is known with high precision, this is not necessarily the case for a relative position between the charged particle beam imaging system on the one hand and the chuck or other parts of the stage, in particular the rotation stage, on the other hand.

It is therefore a task of the invention to improve a wafer inspection apparatus and to enable a high-resolution measurement during long measurement times of a volume inspection task. It is an object of the invention to provide a wafer inspection apparatus configured for 3D volume image generation with high contrast even in case of dynamic vibrations or drifts of a wafer inspection system, in particular when the wafer inspection system comprises a stacked stage with at least an XY-stage and a rotational stage. It is an object of the invention to provide a wafer inspection system configured to execute a method for an improved 3D inspection including a mitigation of drift and vibration effects during 3D volume image generation.

US 2020/0234911 A1 discloses a wafer stage with an XY-stage and a Z-stage, but not with a rotational stage. Particular problems arising when using a rotational stage are therefore not addressed in the cited reference. In more detail, US 2020/0234911 A1 discloses to use interferometer sensors in a position control system in order to properly align a substrate supported on an object table with respect to an electron optics system. Additionally, a sensor for measuring a height position of the substrate is disclosed. Apparently, it is assumed in the cited reference that the entire stacked wafer stage is stiff and that there are no relative movements between different stages because of vibrations that might negatively influence an imaging accuracy.

Summary

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

The present patent application claims the priority of German patent application No. 10 2024 203 800.8 filed on 23 April 2024, the disclosure of which in the full scope thereof is incorporated in the present patent application by reference.

In a first embodiment of the disclosure, an improved wafer inspection system is provided. The improved wafer inspection system is comprising charged particle beam imaging system. The charged particle beam imaging system is forming an optical axis or line of sight. The charged particle beam imaging system is rigidly attached to a frame for supporting the charged particle beam imaging system. The improved wafer inspection system is further comprising a wafer chuck for receiving and holding during use a wafer. The wafer chuck is attached to a wafer stage comprising an XY-stage for translating or lateral movement and a rotation stage attached to the XY-stage for rotating the wafer chuck. The improved wafer inspection system is comprising a measurement system for determining a relative position of the XY-stage with respect to the frame and for positioning and aligning an inspection site of the wafer at the line of sight. With the use of the stage and the measurement system, an inspection site of the wafer is initially aligned at the line of sight of the charged particle beam imaging system. The improved wafer inspection system is further comprising at least one distance sensor for forming a closed control loop for monitoring changes in a relative distance between the frame and the inspection site on the wafer. The at least one distance sensor is arranged for measuring a change in a distance between two members selected from the group of distances comprising a distance between the frame and the wafer, a distance between the XY-stage and the wafer chuck, and a distance between the frame and the wafer chuck. Thereby, a relative distance between the frame and the inspection site on the wafer is monitored during image acquisition. The measurement system for determining a relative position of the XY-stage with respect to the frame and for positioning and aligning an inspection site of the wafer is typically not sufficient to measure and control the position of the inspection site at the line of sight. Even with precise control of the XY-stage, the wafer chuck may be subject to unknown jitter movements. With the at least one distance sensor arranged for measuring a change in a distance between two members mentioned above, a direct monitoring of any unwanted jitter of the wafer chuck is monitored during inspection, including during long-measurement times of 3D volume inspections.

In an example, the wafer inspection is comprising at least one first distance sensor for directly measuring a change in a distance between the frame and the wafer. Thereby, a closed monitoring and control loop between an inspection site on a surface of a wafer and a line of sight of a charged particle beam (CPB) imaging system is achieved. The wafer inspection system of this example further comprises a focused ion beam column and a control unit configured for forming a reference marker on the wafer. For example, the control unit is configured for forming a reference marker as a reflection element for reflecting a Laser beam of one first distance sensor. In an example, the at least one first distance sensor is arranged at an angle normal to a FIB optical axis of the focused ion beam column and wherein the control unit is configured for forming with FIB the reference marker as a reflection mirror perpendicular to a Laser beam of the at least one first distance sensor. Other reference markers are possible as well, for example retro-reflective diffraction gratings in Littrow-configuration (Littrow-mounted gratings).

In an example, the wafer chuck comprises at least one first reflection mirror surface and the wafer inspection system further comprising at least one second distance sensor mounted to the XY-stage for measuring a change in a distance between the XY-stage and the wafer chuck. In an example, the wafer chuck comprises at least one second reflection mirror surface, further comprising at least one third distance sensor configured for measuring a change in a distance between the frame and the wafer chuck. In an example, a wafer chuck comprises a first and a second reflection mirror. In an example, the first reflection mirror surface or the second reflection mirror surface are formed as a cylindrical or conical mirror surfaces surrounding the wafer chuck. In an example, the wafer inspection system comprises at least one third distance sensor. The third distance sensor is mounted on a drive configured to adjust a position or direction of a Laser beam of the at least one third distance sensor to normal incidence and reflection by the second reflection mirror surface. In an example, the wafer inspection system is further comprising at least one tilting mirror mounted on a drive configured to adjust a direction of a Laser beam of the at least one third distance sensor to normal incidence and reflection by the second reflection mirror surface. Thereby, a closed monitoring and control loop between an inspection site on a surface of a wafer and a line of sight of a charged particle beam (CPB) imaging system is achieved.

According to a second embodiment, a method of operating a wafer inspection system is provided. The method of operating a wafer inspection system is comprising adjusting an inspection site on a surface of a wafer at a line of sight of an imaging charged particle beam system with a wafer stage by using a measurement system between a frame and an XY- stage of the wafer stage and acquiring an image of a segment of a surface of the wafer. The method is further comprising - during acquiring of the image - monitoring a change of a relative position of the inspection site on the surface of the wafer with respect to the line of sight with at least one distance sensor for measuring a change in a distance between two members selected from the group of distances comprising a distance between a frame and the wafer, a distance between the XY-stage and a wafer chuck, and a distance between a frame and a wafer chuck. In an example, the method is further comprising compensating a change of the relative position by at least one of triggering a compensating movement of the wafer stage or triggering a compensating deflection of the imaging charged particle beam by a deflection scanner of the imaging charged particle beam system. In an example, the method is further comprising recording a residual change of a relative position of the surface of the wafer synchronized with the image acquisition.

In an example, the method of operating a wafer inspection system is further comprising forming at least two reference markers adjacent to the inspection site on the surface of the wafer by ion beam milling with a focused ion beam column. In an example, each reference marker is formed by ion beam milling as a mirror surface for retro-reflecting a Laser beam of a distance sensor.

In an example, the method of operating a wafer inspection system is further comprising moving and/or rotating a distance sensor with a drive such that a laser beam of a distance sensor is oriented normal to a second reflection mirror surface of the wafer chuck.

According to an example, a method of operating a wafer inspection system is comprising a step of adjusting an inspection site on a surface of a wafer at a line of sight of an imaging charged particle beam system by a wafer stage by using a measurement system between a frame and an xy-stage of the wafer stage. The method of operating a wafer inspection system is further comprising forming at least two reference markers adjacent to the inspection site on the surface of the wafer and monitoring a change of a relative position of the surface of the wafer by measuring a distance to each reference marker with at least one distance sensor. Each distance sensor is for example mounted to the frame. Thereby, a closed loop for monitoring and control of a position between an inspection site on a surface of a wafer and a line of sight of a charged particle beam (CPB) imaging system is achieved.

With the distance sensors according to the examples and embodiments, a relative change of a position between a line of sight of a charged particle imaging beam system and an inspection site on the surface of a wafer can be measured and monitored with single digit nm-precision, or with precision about 1 nm or below 1 nm. Thereby, very precise measurements of semiconductor features or mask features or the like are possible, as required for example for a measurement of a critical dimension or an overlay accuracy. With the distance sensors and the methods according to the examples and embodiments, a high accuracy and fidelity of an acquired image can be achieved even during long measurement times exceeding one second, including the long measurement times for 3D volume image acquisition of up to several hours.

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 even more fully understood with reference to the following drawings: Fig. 1 shows an illustration of a wafer inspection or metrology system for 3D volume inspection with a dual beam device.

Fig. 2 is an illustration of the slice-and image method of a volume inspection in a wafer.

Fig. 3 illustrates an example of a cross section image, obtained by the slice-and image method

Figs. 4a, b show a wafer inspection system according to prior art

Figs. 5a-d show examples of wafer inspection systems according to some embodiments

Fig. 6 shows an example of wafer inspection systems according to the first embodiment

Figs. 7a-d illustrate an example of a method of operating of wafer inspection systems according to the first embodiment

Fig. 8 shows an example of wafer inspection systems according to the first embodiment

Fig. 9 shows an example of wafer inspection systems according to the second embodiment

Fig. 10 shows an example of wafer inspection systems according to the third embodiment

Figs. 11 a-e show examples of wafer inspection systems according to the third embodiment

Figs. 12a-c show examples of wafer inspection systems according to a combination of embodiments

Fig. 13 illustrates a drift corrected pixel raster of a digital image

Fig. 14a-b shows a result of an inspection

Throughout the figures and the description, same reference numbers are used to describe same features or components. The coordinate system is selected that a wafer support surface 15 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). A 3D inspection of an inspection volume within a wafer or a wafer sample may require several hours of slicing and imaging. During such time scales, noise levels may change and drifts as well as jitter may deteriorate a measurement.

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 V-shaped 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 / 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. The wafer support surface 15 is mounted on a stage 155 with actuators and position control. Actuators and means for precision control for a wafer stage such as Laser interferometers or optical encoders are known in the art. A control unit 16 is configured to control the wafer stage 155 and to adjust an inspection site 6.1 of the wafer 8 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 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 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 or Argon ions. 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.

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.

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, and 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.

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.

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.

Figure 2 illustrates the wedge cut geometry at the example of a 3D-memory stack. Figure 2 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 2 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 maximum number M of layers is typically more than fifty, for example more than one hundred or even more than two hundred. The HAR-structures and layers extend throughout the inspection volume 160 in the wafer 8 but may comprise gaps. 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 of size LX x LY x LZ is properly sampled and for example a 3D volume image can be generated. Thereby, the damage to the wafer is limited to the inspection volume 160 plus a damaged volume in y-direction of length LYO. With an inspection depth LZ about 10pm, the additional damage volume in y-direction is typically limited to below 20pm.

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. 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 cross-section surface 52. In addition, the image slice 311 comprises crosssections of several word lines 313.1 to 313.3 at different depths or z-positions.

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 PCT/EP2022/082590, which are hereby incorporated by reference.

Figure 4 illustrates some further details of a wafer inspection system 1000. Same reference numbers as in figure 1 are used and reference is also made to the description above. Figure 4a shows some details of a typical charged particle beam device. An imaging charged particle beam column, for example a scanning electron microscope 40 is mounted on a frame 25. The sample stage 155 comprises a first stage 155.xy 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 (for axis 42, see figure 1). With the second stage 155.z, 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. During use, electrons of electron beam 44 are generated by electron source 31. The focus position of electron beam 44 is generated by charged particle beam lens 33, which is controlled by control unit 19. Within the image plane, the electron beam 44 is scanned by scanning deflector 29, which is controlled by control unit 19. Secondary electron detector 17.1 and backscattered electron detector 17.2 communicate the secondary or backscattered electron count during scanning to control unit 19. Thereby, an image of a surface of the sample 8 at the inspection site 6 is obtained. The sample or wafer 8 can further be rotated by a third or rotation stage 155. t mounted on to of the second or z-stage 155.z. A sample chuck 151 with the wafer support surface 15 is mounted on top of rotation stage 155. t. 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).

Control unit 19 is connected to operation control unit 2, which comprises a user interface 205 with a user interface display 400 and user command devices 401. Operation control unit 2 further comprises a processing engine 201 and a memory 203 for executing and storing instructions. Instructions comprise for example image processing instructions. Images from image acquisition and image processing are stored in memory 219. Operation control unit 2 can be in communication with external control devices of network via an interface unit 231.

Figure 4b illustrates a detail of a charged particle beam device according to the prior art. The scanning electron microscope (SEM) 40 is mounted via a rigid connection to frame 25.

Lateral stage movement by x-y-stage 155. xy 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. The structure of frame 25 is of complex shape with dimensions about 1 m. 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 791 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. Wafer and mask metrology tools usually consist of a multi-axis wafer stage 155 (linear movement in X,Y, Z-stage and rotational movement T). Each individual axis can be controlled using an encoder system like optical encoders. However, the stacking and mounting of stage components and especially the rotation stage 155.T does not allow a direct measurement of position of the wafer 8 mounted on the wafer chuck 151. The setup of systems of the prior art cannot measure at wafer chuck level because the wafer is laterally moved up to 300 mm (size of wafer) and rotated up to 360° under the SEM 40. Each motion axis of the stage requires a degree of freedom in movement and does therefore not allow a stiff mechanical connection. Especially the rotational axis may introduce some uncertainty of position of the wafer chuck 151. Via the frame 25, there is a mechanical reference 791 between SEM 40 and the measuring point of interferometer 21, and with interferometer 21, there is a reference 793 to the x-y-stage 155.xy, but there is no reference to inspection site 6 at wafer 8 and SEM column 40. In other words, the control circuit of the charged particle beam systems of figure 4b is not closed but has a metrology gap 789. Any noise or drift induced into the charged particle beam system consisting of frame 25, SEM 40 and multiaxis wafer stage 155 will thus cause an unknown displacement between line of sight or optical axis 42 of SEM 40 and inspection site 6 located within wafer coordinate system. Image acquisition with a charged particle beam system 40 is therefore subject to vibration noise (sometimes referred to as jitter), drift or deformation. Vibration noise can for example arise due to floor or acoustic vibrations. Drift or deformation can arise due to thermal drifts, material degradation or aging, and changes in external fields, for example electromagnetic fields or gravity field. In addition, (mechanical) vibration of the SEM column 40 (e.g., introduced by acoustic noise or floor vibrations) may arise during image acquisition.

Mechanical vibration of wafer is especially of concern during slice and image acquisition of large inspection volumes, requiring stand-still performance of wafer stage 155 with nm- precision or even sub-nanometer precision over long periods of time, for example time periods exceeding several hours. In high resolution imaging (nanometer and sub-nanometer regimes), vibrations of the wafer sample under investigation and SEM column are directly impacting image quality such as resolution, image fidelity or overlay accuracy.

Any movement between scanning electron-beam 44 and inspection site 6 deteriorates image quality. For sub-nanometer resolution requirements, active jitter suppression becomes mandatory because of external excitations such as floor vibrations and acoustic noise. Further, internal excitations due to for example water cooling or thermal stress may not completely be avoided. According to the embodiments, effective active jitter suppression is enabled by closed-loop jitter measurements. According to the embodiments, the significant jitter contribution of the stacked stage 155 including rotary axis is reduced. According to the embodiments, a closed loop control of inspection site 6 with respect to the line of sight 42 of scanning electron beam system 40 is provided.

According to a first embodiment, a closed loop control 781 of inspection site 6 with respect the line of sight 42 of scanning electron beam system 40 is provided by a direct reference 797 between wafer coordinate system of wafer 8 and frame 25. A functional equivalence is illustrated in Figure 5a. According to a second embodiment, the closed control loop 781 between line of sight 42 of scanning electron beam system 40 and inspection site 6 is achieved by a direct reference 795 from x-y-stage 155.xy to the wafer chuck 151. A functional equivalence according to the second embodiment is illustrated in Figure 5b.

According to a third embodiment, the closed control loop 781 between line of sight 42 of scanning electron beam system 40 and inspection site 6 is achieved by a reference 799 between frame 25 and the wafer chuck 151. A functional equivalence according to the third embodiment is illustrated in Figure 5c. The embodiments can also be combined, an example of a combination of the first and second embodiment is illustrated in Figure 5d. With any of the embodiments or combinations thereof, a closed control loop 781 is provided and the control gap 789 of the prior art is avoided. Thereby, a sub-nm closed-loop monitoring of the position of the inspection site 6 with respect to the line of sight 42 and corresponding control is enabled. With each of the examples according to the embodiments provided below, at least one dedicated distance sensor is provided to close the measurement or control gap 789 in the metrology loop. Thereby, jitter of the rotary axis and chuck jitter, which cannot be neglected for sub-nanometer applications, is taken into account. For example, the vibration measurement is combined with position sensors for X, Y and Z.

Figure 6 illustrates an example according to the first embodiment. With at least two first distance sensors 701.1 and 701.2, for example with Laser interferometers with Laser beams 703.1 and 703.2, a distance of the wafer surface 55 with reference to the support or metrology frame 25 is measured at least at two positions, or, in at least two independent directions, respectively. For position measurement, reference markers 706.1 and 706.2 are provided on the wafer surface 55. According to the first embodiment, a wafer inspection system 1000 is configured to form at least two reference markers 706.1 and 706.2 on a surface of a wafer 55 for Laser-distance measurement. Laser beams 703.1 and 703.2 are inclined to the optical axis or line of sight 42 by angles J1 and J2, such that with each first distance sensor 701.1 or 701.2, a lateral coordinate x or y of the wafer surface 55 with respect to line of sight 42 can be measured via the corresponding reference marker 706.1 or 706.2. Both first distance sensors 701.1 and 701.2 are connected to control unit 19 for closed loop control.

Figure 7a illustrates an example of a formation of a reference marker 706. With stage 151 (see figure 6), the wafer 8 is moved such that the position of the first reference marker 706.1 is at the intersection point 43 of the dual beam device 1. With focused ion beam 51 of FIB 50, a wedge cut is generated into the wafer surface 55. Wedge cut is formed such that the surface generated by milling with the focused ion beam is normal to the Laser beam axis 707. In this example, reference marker 706.1 is formed as a mirror surface. Optionally, the milled mirror surface 706.1 can be covered by a reflective coating. The dual beam system 1 therefore comprises a gas injection system 79 for providing a gas, for example a Tungsten metal coating is generated by providing Tungsten Hexacarbonyl.

In an example, the angles J1 and J2 of the laser beam axes 707 of distance sensors 701.1 and 701.2 are arranged to be normal to the angle of the optical axis 48 (see figure 1) of the FIB beam 51. Thereby, the mirror surfaces of reference markers 706.1 and 706.2 are fabricated by ion beam milling normal to the Laser beam axis 707. Reference markers 706 are not limited to mirror surfaces but can for example also be formed as diffraction gratings with grating frequency adjusted such that a diffraction order of reflected Laser light is parallel to an incident Laser beam 703.

After formation of the reference marker 706.1 adjacent to the inspection site 6, wafer 8 is moved by stage 155 to align the inspection site 6 at the line of sight 42. During performing the 3D inspection of the inspection volume at inspection site 6 by slicing with FIB 50 and scanning electron imaging with electron beam column 40, first distance sensors 701.1 and

701.2 measure the distance to reference markers 706.1 and 706.2 (see figure 7b). An example of the arrangement of the first distance sensors 701.1 and 701.2 and corresponding reference markers 706.1 and 706.2 on the top surface 55 of a wafer 8 is illustrated in figure 7c. First distance sensors 701.1 and 701.2 and corresponding reference markers 706.1 and

706.2 are arranged at 90° with respect to each other. With first distance sensor 701.1 and corresponding reference marker 706.1, a displacement in y-direction is measured. With first distance sensor 701.2 and corresponding reference marker 706.2, a displacement in x- direction is measured. The displacement measurement signals are received by control unit 19. Control unit 19 is configured to trigger via stage control unit 16 a compensating movement of wafer stage 155. The absolute position can still be controlled by stage interferometer 21. However, relative movements of the wafer 8 mounted on wafer chuck 151 can be measured by first distance sensors 701.1 and 701.2 and corresponding reference markers 706.1 and 706.2 with high accuracy, for example with an accuracy of below 1nm, for example 0.3nm or even less.

Figure 7d illustrates a method of operation corresponding to the first embodiment. In a first step I, the wafer inspection system 1000 determines or receives from a command file or user interaction an inspection site 6 within a wafer coordinate system. Step I can further comprise a calibration of the line of sight (42) of the imaging charged particle beam column 40 within the frame 25, to which the imaging charged particle beam column 40 is rigidly attached to.

In step S1 , the at least two reference markers 706.1 and 706.2 are formed at the wafer surface 55. First, a distance and orientation of each reference marker 706.1 or 706.2 with respect to the inspection site 6 is determined. For the determination, several parameters can be utilized, comprising parameters selected from a group of parameters including

- orientation and angle J1 , J2 of Laser beam axes 707.1 and 707.2 of the first distance sensors 701.1 and 701.2,

- the lateral position of the first distance sensors 701.1 and 701.2 with respect to the line of sight 42, and

- the focus distance of the wafer surface 55 with respect to a reference plane of the dual beam system 1.

These parameters are for example machine parameters determined according to the mechanical setup or determined during a calibration of the system. The focus distance can be selected according to an inspection task to be performed at an inspection site.

In step S2, wafer stage 155 is moved such that the inspection site 6 of wafer surface 55 is aligned with the line of sight 42 of the charged particle imaging beam system 40 and the reference markers 706.1 and 706.2 are at positions normal to the Laser beam 703.1 and 703.2. While the absolute positions might be aligned and adjusted by other laser interferometers and encoders of stage 155, the reference position of the inspection site 6 with respect to line of sight 42 is then determined by first distance sensors 701.1 and 701.2 and any deviation of the reference position or any relative movement of the wafer 8 is directly measured and communicated with control unit 19.

In step S3, the image acquisition of inspection volume at inspection site 6 is performed, for example by repeated slicing or milling for FIB- column 50 and imaging with scanning electron beam column 40. Image acquisition of the inspection volume can take several minutes or even several hours. The method is however not limited to 3D-volume image acquisition, but also applicable to single image acquisition of two-dimensional images. During image acquisition of image slices within the inspection volume, first distance sensors 701.1 and 701.2 communicate deviation of the reference position or any relative movement of the wafer 8 to control unit 19. In an example, control unit 19 triggers compensatory movement of wafer stage 155. In an example, control unit 19 triggers compensatory scanning offsets of the primary electron beam 44 by scanning deflectors 29. In an example, control unit 19 writes deviation vectors corresponding to dwell points of the scanning electron beam 44 to a memory for later compensation of deviations during in image post-processing. The means for compensating deviations can also be combined, for example based on a dynamic behavior of the deviations. In an example, deviation signals from first distance sensors 701.1 and 701.2 are analyzed by control unit 19 and separated in at least two different frequency regimes. For example, long-term or slow deviations such as thermal drift can be compensated by stage movements. Mid-frequency deviations can be compensated by scanning deflectors 29. High-frequent deviations can be compensated by image postprocessing.

Typically, Eigenfrequencies of frame (25) or xy-stage 155.xy are in the range on 100Hz. Therefore, dynamic control by for example stage movements is performed up to frequencies about a fraction of the Eigenfrequency of the stage 155, for example about 1/5 of the lowest Eigenfrequency. For example, the jitter or deviation of the reference position or any relative movement of wafer 8 with respect to line of sight 42 is analyzed and divided into different frequency regimes with a first frequency regime of jitter below 20Hz, a second frequency regime of jitter between 20Hz and 100kHz, and a third frequency regime of jitter above 100kHz. Jitter within the first frequency regime is compensated by stage movements, Jitter of the second frequency regime is compensated by scanning deflector 29, and jitter of the third frequency range is recorded and compensated for in a later digital image postprocessing.

In step S4, cross-section images obtained by scanning electron beam column 40 are postprocessed. For example, pixel coordinates of the cross-section images of slanted cross section surfaces 52, 53 are interpolated to a cartesian 3D raster of a volume image. In an example, deviations measured and stored during step S3 are considered during the interpolation. Interpolation of voxel coordinates of a three-dimensional volume image typically requires interpolation from image pixels at pixel coordinates within two different cross section images. Deviation vectors of pixel coordinates of two different cross section images can be very different.

The first embodiment is not limited to distance sensor 701.1 and 701.2 arranged under an inclined angle with respect to the wafer surface 55. In another example, a wafer inspection system 1000 further comprises additional distance sensor oriented normal to a wafer surface 55. Figure 8 illustrates an example with two additional distance sensor 701.5 and 701.7. While the inclined first distance sensors 701.1 and 701.2 are inclined to the line of sight 42 and arranged in proximity to the scanning electron beam column 40, such that a reference marker 706 can be arrange at a small distance of for example 1mm or less to an inspection site 6, additional normal distance sensor 701.5 and 701.7 can be arranged at larger distance on the wafer surface 55 with for example 300mm in diameter, and a deviation in tilt angle of the wafer 8 can be monitored with high precision. 1

The solution examples of the first embodiment allow a direct closed-loop monitoring of the position of the inspection site 6 with respect to a line of sight 42. However, solution examples of the first embodiment require a modification of the wafer 8. In a second embodiment, such modification is eliminated. An example according to the second embodiment is shown in Figure 9. Same reference numbers as in figures above are used and reference is also made to the description thereof. The dual beam system 1 comprises a wafer stage 155 with a x-y-stage 155.xy and a z-stage 155.z mounted on top of each other. X-y-movement of the wafer stage 155 is monitored and controlled by laser interferometer 21 in combination with stage control unit 16. The stage 155 further comprises a rotation stage 155. t and a wafer chuck 151 mounter on top of the rotation stage 155. t. Thereby a compucentric rotation of the wafer 8 can be performed. In the example of the second embodiment, second distance sensors 721.1 and 721.2 are mechanically mounted to the x-y-stage 155.xy or z-stage 155.z. The wafer chuck 151 further comprises reflection mirror surfaces 725.1 and 725.2. Reflection mirror surfaces 725.1 and 725.2 and second distance sensors 721.1 and 721.2 are arranged that Laser beams 723.1 and 723.2 are reflected in between. In an example, the wafer chuck 151 comprises a single conical or cylindrical mirror surface 725, serving a reflecting mirror for each of the at least two second distance sensors 721.1 and 721.2. Thereby, the position of the wafer chuck 151 on top of the rotation stage 155. t can be monitored with respect to the x- y-stage 155. xy. Thereby, a closed loop monitoring 781 of the position of the inspection site 6 on the wafer surface 8 relative to a line of sight 42 of the scanning electron beam system 40 is enabled and any effect of noise or vibration within the stage 155 is compensated in equivalence to the method steps S3 and S4 described above. The second embodiment relies on the high-precision measurement loop between frame 25 and within the stage 155 to monitor any movement of the wafer chuck 8 relative to the line of sight 42. In an example, the wafer inspection system 1000 comprises a frame 25 of high stiffness with Eigenfrequencies of about 100Hz or more. Lowest Eigenfrequencies above 100Hz are possible by proper material selection and design. While the second distance sensors 721.1 and 721.2 may not allow an absolute positioning of the wafer surface 55, second distance sensors 721.1 and 721.2 allow a relative monitoring of deviations from a position calibrated at an inspection site

6.

Within the second embodiment, second distance sensors 721.1 and 721.2 are mounted and rigidly connected within the stage 155. Driving power and signal from second distance sensors 721.1 and 721.2 can be provided by cable, which is for example moved in accordance with the wafer stage 155 by a mechanical robot. In a third example, additional cables are omitted. Figure 10 shows an example according to the third embodiment. Same reference numbers as in figures above are used and reference is also made to the description thereof. The dual beam system 1 comprises a wafer stage 155 with a x-y-stage 155.xy and a z-stage 155.z mounted on top of each other. X-y-movement of the wafer stage 155 is monitored and controlled by laser interferometer 21 in combination with stage control unit 16. In the example of the third embodiment, third distance sensors 741 are mechanically mounted to a drive 755, which is rigidly connected to frame 25. The wafer chuck 151 further comprises reflection mirror surface 747. For monitoring, third distance sensor 741 is adjusted by drive 755 in a position where it receives a reflected laser light of laser 743, reflected by the reflection mirror surface 747 of the wafer chuck 151. Thereby, a closed loop monitoring 781 of the position of the inspection site 6 on the wafer surface 8 relative to a line of sight 42 of the scanning electron beam system 40 is enabled and any effect of noise or vibration within the stage 155 is compensated in equivalence to the method steps S3 and S4 described above. It is to be considered that the wafer chuck 151 is mounted on the rotation stage 155.t and moved by x-y-stage 155.xy in lateral direction. Figure 11a illustrates a first example of the closed loop monitoring system 781 of the position of the wafer chuck 151. The Wafer stage 155 is aligned in an off-axis position such that the inspection site 6 is adjusted at line of sight 42 (here normal to x-y-plane at coordinates x = y = 0). The system comprises two third distance sensors 741.1 and 741.2 mounted to first and second drive 755.1 and 755.2. The wafer surface may be rotated by rotation stage 155. t. Wafer chuck 151 comprises a cylindrical form, forming a cylindrical mirror surface 747 in the circumference. Third distance sensors 741.1 and 741.2 are laterally moved along movement directions

757.1, 757.2 by first and second drive 755.1 and 755.2 such that laser beams 743.1 and

743.2 are normal to the cylindrical mirror surface 747. The third distance sensors 741.1 and

741.2 are aligned by first and second drive 755.1 and 755.2 in accordance with the position of the inspection site 6, such that, during a wafer inspection at an inspection site 6, any movement deviation of the wafer chuck 151 can directly be monitored with reference to the frame 25. Figure 11b illustrates another example of the closed loop monitoring system 781. Instead of a linear movement in accordance with the position of the inspection site 6 on a wafer surface 55, third distance sensors 741.1 and 741.2 are rotated by first and second drive 755.1 and 755.2 such that laser beams 743.1 and 743.2 are normal to the cylindrical mirror surface 747. Figure 11c illustrates another example of the closed loop monitoring system 781. Here, third distance sensors 741.1 and 741.2 are rigidly connected to frame 25 and laser beams 743.1 and 743.2 are deflected by tilting mirrors 749.1 and 749.2 such that laser beams 743.1 and 743.2 are normal to the cylindrical mirror surface 747. Figure 11 d illustrates another example of the closed loop monitoring system 781. Where linear movement 757.1 and 757.2 are combined with rotation movement 757.3 and 757.4.

Thereby, the circumferential shape of reflection mirror surface 747 can be of polygonal shape with a plurality of six, eight or more plane mirror facets. Linear movement 757.1 and 757.2 and rotation movement 757.3 and 757.4 by drives 755.1 to 755.4 are determined and configured such that laser beams 743.1 and 743.2 are normal to the plane facets of mirror surface 747. In an example illustrated in figure 11e, within this configuration also two parallel third distance sensors 741.1a and 741.1b can be applied and a deviation in rotation angle of wafer chuck 151 can be monitored.

Examples with solutions according to the fist or third embodiment provide short loops for metrology and monitoring of the inspection side 6 with respect to the line of sight 42, while examples according to the second embodiment typically has a longer measurement loop and might require higher mechanical stiffness of frame 25 and stacked stage 155. Examples with solutions according to the fist or third embodiment therefore support higher control bandwidths compared to examples according to the second embodiment. Examples according to the second embodiment can, however, be of reduced complexity. Different examples of the embodiments can also be combined, and various modifications are possible. With different closed loop monitoring system, error sources might be distinguished and separated and, for example, compensated and addressed separately. Figure 12a illustrates an example of a combination of examples of the second and third embodiments. Here, second distance sensors 721.1 and 721.2 are provided to monitor relative movement between x-y-stage 155.xy and wafer chuck 151 according to the second embodiment. Furthermore, at least a third distance sensor 741 mounted to a drive 755 is provided to further monitor relative movement between the wafer chuck 151 and the frame 25. Thereby, redundancy is generated, and further influences of noise or jitter can be eliminated.

Furthermore, it can be distinguished between different influences of noise or jitter, and elimination of the individual influences is possible.

Figure 12b illustrates a further example of a combination of examples of the second and third embodiments. Here, a cylindrical mirror surface 747 of the wafer chuck 151 is provided. In addition, second distance sensor 721 is mounted via a pedestal to x-y-stage 155.xy or z- stage 155.z with a direction normal to the cylindrical mirror surface 747. Figure 12c illustrates a further example of a combination of examples of the first, second and third embodiments. With such a combination, a closed-loop monitoring is enabled for wafer 8, where reference markers 706 can be applied, as well as with samples, where such reference markers 706 can not easily be applied, for example small wafer samples or the like.

Figure 13 shows a scanning raster of a scanning electron beam device 40, comprising a plurality of L scanning lines with ideal dwell points. According to the embodiments, deviations or position displacement vector components dx, dy can be monitored with high precision. At least a low frequency-contribution of deviations can be compensated, wherein the frequency cut-off is limited by the control circuit. Control circuits with bandwidths up to more than 100 kHz are possible, for example even bandwidths of several MHz. Residual position displacement vector components dx, dy of the final 2D pixel coordinate map correspond to the real coordinates of the dwell points on a cross section surface are compensated by postprocessing.

Figure 14 shows a typical result of a wafer inspection task. In Figure 14a, a trajectory of center coordinates of a HAR channel is shown. Each horizontal line corresponds to one contour of a feature 307, measured at a depth z inside an inspection volume of a wafer. Thereby, a HAR channel can be analyzed and for example an average tilt angle y of average channel trajectory 363 through an inspection volume is determined. Figure 14b illustrates a distribution of measured radius r of HAR structures of a plurality of wafer samples. The radius r shows a significant drift over wafer samples, which can be an indicator for a process drift during the manufacturing process of wafer. With the embodiments and examples of the disclosure, an accuracy of such a measurement is improved.

A wafer inspection system 1000 according to the embodiments comprises at least one closed metrology and control loop 781. According to the examples of the embodiments, a closed control loop 781 is provided for monitoring and controlling a position of a wafer within the wafer inspection system 1000. The closed control loop is achieved by a rigid attachment of a charged particle beam imaging system to a solid frame and at least one measurement system between a wafer chuck or wafer and the frame. The at one measurement system comprises at least one distance sensor for measuring a change in a distance between two members selected from the group of distances comprising a distance between the frame and the wafer, a distance between a XY-stage and the wafer chuck, and a distance between the frame and the wafer chuck. With the closed control loop 781 , a relative position of a line of sight 42 of an imaging charged particle beam system 40 with respect to an inspection site 6 of a wafer is determined with high precision. Thereby, deviations of the position (such as jitter) of an inspection site 6 with respect to a line of sight 42 of a scanning electron beam system 40 can be at least partially compensated. For example, jitter is analyzed and separated into at least two different frequency regimes. A first frequency regime is comprising the low frequencies of for example up to 20Hz, constituting a fraction of about 1/5 of lowest Eigenfrequency of for example the stage with Eigenfrequency of for example about 100Hz. A second frequency regime is comprising mid-range frequencies of 20Hz to about 100 kHz or more as a fraction of the scanning frequency of about for example 50MHz or more. A third frequency regime is comprising higher frequencies. With the improved wafer inspection system 1000 according to the embodiments, measurements of features within charged particle beam images can be performed with unprecedented high precision. The monitoring can be performed with high bandwidth, thus allowing for closed control loops with high bandwidth. Control loops include active compensation via wafer stage or deflection scanner, or implementation of countermeasures such as active damping or active vibration compensation. Knowledge of the residual displacements further enable correction in post-processing (e.g., image pixel correction).

With the method according to the embodiments, an unprecedented high accuracy of the measurement result can be achieved, with a measurement error below 1nm, below 0.5nm, below 0.3nm, or even less. Small drift or vibration errors, which are hard to be compensated by for example a deflection scanner, can be recorded and corrected during image postprocessing. For example, within a stack of image slices from a 3D-inspection volume, 3D-pixel interpolation from the plurality of two-dimensional cross section images can considered actual position coordinates of image pixels. Thereby, with a method according to an embodiment, a three-dimensional volume image of higher accuracy is computed from the plurality of two-dimensional cross section images. Thereby, with the systems according to an embodiment, a three-dimensional volume image of higher accuracy is acquired and/or computed from the plurality of two-dimensional cross section images of lower sampling rate. With the wafer inspection system 1000 according to the embodiments, a deterioration of the image quality induced by jitter, drift or environmental disturbances is reduced.

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.

A list of reference numbers is provided:

1 Dual Beam system

2 Operation Control Unit

4 cross sections of first structures

6 inspection site

8 wafer

15 wafer support surface

16 stage control unit

17 Electron detector

19 Control Unit

21 laser interferometer

23 Environment Sensor

25 Metrology Frame

27 position measurement

29 scanning deflector

31 electron source

33 objective lens

40 charged particle beam (CPB) imaging system

42 Optical Axis of imaging system or line of sight 43 Intersection point

44 Imaging charged particle beam

48 Fib Optical Axis

50 FIB column

51 focused ion beam

52 cross section surface

53 cross section surface

55 wafer top surface

79 Gas Injection system

81 contact pin

151 wafer chuck

153 stage support

155 wafer stage

157 roation bearing

159 linear bearing

160 inspection volume

201 processing engine

203 memory

205 User interface

219 memory

231 Interface unit

307 measured cross section image of HAR structure

311 cross section image slice

313 word lines

315 edge with surface

363 average HAR channel trajectory

400 user interface display

401 user command devices 701 distance sensor

703 laser beam

706 reference marker

707 Laser beam axis

721 distance sensor

725 reflection mirror surface

741 distance sensor

743 laser beam

747 reflection mirror surface

749 tilting mirrors

755 drive

757 movement direction

781 closed control loop

789 metrology gap

791 reference between charged particle beam column and stage interferometer

793 stage reference

795 reference between x-y-stage and wafer chuck

797 reference between wafer and frame

799 reference between frame and wafer chuck

1000 Wafer inspection system

Claims

1. A wafer inspection system (1000), comprising:

- charged particle beam imaging system (40), the charged particle beam imaging system (40) forming an optical axis or line of sight (42),

- a wafer chuck (151) for holding during use a wafer (8),

- a wafer stage (155) comprising an XY-stage (155.xy) and a rotation stage (155. t) attached to the XY-stage (155. xy) for translating and rotating the wafer chuck (151),

- a frame (25) for supporting the charged particle beam imaging system (40), wherein the charged particle beam imaging system (40) is rigidly attached to the frame (25),

- a measurement system (21) for determining a relative position of the XY-stage (155.xy) with respect to the frame (25) and for positioning and aligning an inspection site (6) of the wafer (8) at the line of sight (42),

- at least one distance sensor (701 , 721 , 741) for forming a closed control loop (781) for monitoring changes in a relative distance between the frame (25) and the inspection site (6) on the wafer (8).

2. The wafer inspection system (1000) of claim 1 , wherein the at least one distance sensor (701 , 721, 741) is arranged for measuring a change in a distance between two members selected from the group of distances comprising

- a distance between the frame (25) and the wafer (8),

- a distance between the XY-stage (155. xy) and the wafer chuck (151),

- a distance between the frame (25) and the wafer chuck (151).

3. The wafer inspection system (1000) of claim 1 or 2, comprising at least one first distance sensor (701, 701.1, 701.2) for measuring a change in a distance between the frame (25) and the wafer (8), wherein the wafer inspection system (1000) further comprises a focused ion beam column (50) and a control unit (19) configured for forming a reference marker (706, 706.1 , 706.2) on the wafer (8).

4. The wafer inspection system (1000) of claim 3, wherein the control unit (19) is configured for forming each reference marker (706, 706.1 , 706.2) as a reflection element for reflecting a Laser beam (703.1 , 703.2) of one first distance sensor (701 , 701.1 , 701.2).

5. The wafer inspection system (1000) of claim 4, wherein the at least one first distance sensor (701 , 701.1 , 701 .2) is arranged at an angle normal to a Fl B optical axis (48) of the focused ion beam column (50) and wherein the control unit (19) is configured for forming with FIB (50) the reference marker (706, 706.1 , 706.2) as a reflection mirror perpendicular to a Laser beam (703.1 , 703.2) of the at least one first distance sensor (701 , 701.1 , 701.2).

6. The wafer inspection system (1000) of any of the claims 1 to 5, wherein the wafer chuck (151) comprises at least one first reflection mirror surface (725, 725.1 , 725.2), further comprising at least one second distance sensor (721 , 721.1 , 721.2) mounted to the XY-stage (155.xy) for measuring a change in a distance between the XY-stage (155. xy) and the wafer chuck (151).

7. The wafer inspection system (1000) of any of the claims 1 to 6, wherein the wafer chuck (151) comprises at least one second reflection mirror surface (747), further comprising at least one third distance sensor (741 , 741.1 , 741.2) configured for measuring a change in a distance between the frame (25) and the wafer chuck (151).

8. The wafer inspection system (1000) of any of the claims 6 or 7, wherein the first reflection mirror surface (725, 725.1 , 725.2) or second reflection mirror surface (747) is formed as a cylindrical or conical mirror surfaces surrounding the wafer chuck (151).

9. The wafer inspection system (1000) of claim 7 or 8, wherein the at least one third distance sensor (741 , 741 .1 , 741.2) is mounted on a drive (755, 755.1 , 755.2, 755.3, 755.4) configured to adjust a position or direction of a Laser beam (743, 743.1 , 743.2) of the at least one third distance sensor (741 , 741.1 , 741.2) to normal incidence and reflection by the second reflection mirror surface (747).

10. The wafer inspection system (1000) of claim 7 or 8, further comprising at least one tilting mirror (749.1 , 749.2) mounted on a drive (755, 755.1 , 755.2, 755.3, 755.4) configured to adjust a direction of a Laser beam (743, 743.1 , 743.2) of the at least one third distance sensor (741 , 741.1 , 741.2) to normal incidence and reflection by the second reflection mirror surface (747).

11. A method of operating a wafer inspection system (1000), comprising:

- adjusting an inspection site (6) on a surface (55) of a wafer (8) at a line of sight (42) of an imaging charged particle beam system (40) by a wafer stage (155) by using a measurement system (21) between a frame (25) and an XY-stage (155. xy),

- acquiring an image of a segment of a surface (55) of the wafer (8), wherein, during acquiring of the image, performing

- monitoring a change of a relative position of the surface (55) of the wafer (8) with respect to the line of sight (42) with at least one distance sensor (701 , 721 , 741) for measuring a change in a distance between two members selected from the group of distances comprising

- a distance between a frame (25) and the wafer (8),

- a distance between the XY-stage (155.xy) and a wafer chuck (151),

- a distance between a frame (25) and a wafer chuck (151).

12. The method according to claim 11 , further comprising compensating a change of the relative position by at least one of triggering a compensating movement of the wafer stage (155) or triggering a compensating deflection of the imaging charged particle beam (44) by a deflection scanner (29) of the imaging charged particle beam system (40).

13. The method according to claims 11 or 12, further comprising recording a residual change of a relative position of the surface (55) of the wafer (8) synchronized with the image acquisition.

14. The method of operating a wafer inspection system (1000) according to any of the claims 11 to 13, further comprising forming at least two reference markers (706.1 ,

706.2) adjacent to the inspection site (6) on the surface (55) of the wafer (8) by using a focused ion beam column (50).

15. The method of operating a wafer inspection system (1000) according to claim 14, wherein each reference marker (706.1, 706.2) is formed as a mirror surface for retro- reflecting a Laser beam (703, 703.1, 703.2) of a distance sensor (701, 701.1 , 701.3).

16. The method of operating a wafer inspection system (1000) according to any of the claims 11 to 13, further comprising moving and/or rotating a distance sensor (741 , 741.1, 741.2) with a drive (755, 755.1 , 755.2) such that a laser beam (743, 743.1,

743.2) of the distance sensor (741, 741.1, 741.2) is oriented normal to a second reflection mirror surface (747) of the wafer chuck (151).

17. A method of operating a wafer inspection system (1000), comprising:

- adjusting an inspection site (6) on a surface (55) of a wafer (8) at a line of sight (42) of an imaging charged particle beam system (40) by a wafer stage (155) by using a measurement system (21) between a frame (25) and an xy-stage (155.xy),

- forming at least two reference markers (706.1, 706.2) adjacent to the inspection site (6) on the surface (55) of the wafer (8),

- monitoring a change of a relative position of the surface (55) of the wafer (8) by measuring a distance to each reference marker (706.1 , 706.2) with at least one distance sensor (701, 701.1, 701.3).