Method and system for combined characterization of structures etched in substrate

Abstract

The invention relates to a method (100) for characterizing structures etched in a substrate, such as a wafer, said method (100) comprising, for at least one structure etched in said substrate:-at least one imaging step (102), the imaging step comprises the steps of: capturing (104) at least one image of the top surface of the substrate with an imaging device located on the top side of the substrate, and # imgabs0 # measuring (106-110) first data relating to the structure from the at least one captured image; -at least one interferometric step (130) carried out with a low coherence interferometer located on said top side for measuring at least one depth data relating to the depth of said structure with a measurement beam located on said structure; wherein the method (100) further comprises a first adjustment step (120) for adjusting the measurement beam as a function of the first data. The invention also relates to a system for implementing such a method.

Description

Technical Field

[0001] The present invention relates to a method for characterizing structures etched in a substrate, in particular high aspect ratio HAR structures. The present invention also relates to a system for implementing such a method.

[0002] The field of the invention is that of inspecting and characterizing structures etched in a substrate, such as high aspect ratio structures such as through silicon vias TSVs or trenches etched in a substrate such as a wafer. Background Art

[0003] For example, in the semiconductor industry or the MEMs (i.e. “micro-electromechanical systems”) industry, many process steps involve etching structures, sometimes with high aspect ratio HAR, on a substrate such as silicon. Such HAR structures can be, for example, through-silicon vias (TSVs), which are typically blind vias used, for example, for interconnects in advanced packaging technologies, trenches with narrow widths and slender lengths, and more complex features. For example, they can be done by deep radiation ion etching (Deep RIE) or photolithography.

[0004] The aspect ratio of a structure is defined by the ratio between the depth and the lateral dimension, also referred to as the "critical dimension" (CD) of the structure. Some HAR structures may have aspect ratios greater than 5, 10, or 20, with lateral (critical) dimensions as narrow as 5µm or less. Some non-limiting examples include TSVs in the form of cylindrical holes etched in a silicon substrate with a diameter of 2µm to 3µm and a depth of 40µm to 50µm.

[0005] There is a need to measure and characterize these structures, in particular HAR structures, by measuring, for example, the depth of the structure. Document WO2007 / 042676 describes a technique for depth measurement of HAR structures during a deep RIE process. However, this technique has some limitations when measuring HAR structures with very small critical dimensions (e.g., 4 µm or less). There are also known techniques for characterizing dense arrays of HAR structures. These techniques cannot be used alone to characterize HAR structures.

[0006] It is an object of the present invention to overcome at least one of the disadvantages of the known art.

[0007] Another object of the present invention is to provide a solution for individually characterizing etched structures, in particular HAR structures.

[0008] Another object of the present invention is to provide a solution for characterizing etched structures with small critical dimensions, in particular HAR structures.

[0009] Another object of the present invention is to provide a solution for measuring the depth and critical dimension of etched structures, especially HAR structures.

[0010] Another object of the present invention is to provide a solution for characterizing structures, in particular HAR structures, more efficiently and / or faster. Summary of the invention

[0011] The present invention can achieve at least one of these objectives by a method for characterizing structures etched in a substrate (such as a wafer), in particular high aspect ratio HAR structures, for at least one structure etched in the substrate, the method comprising: - at least one imaging step comprising the following steps: capturing at least one image of a top surface of the substrate using an imaging device located on a top side of the substrate, and measuring first data associated with the structure from at least one captured image; - at least one interferometric measurement step, performed using a low-coherence interferometer located on said top side, for measuring at least one depth datum related to the depth of said structure using a measuring beam located on said structure; The method further comprises a first adjustment step of adjusting the measuring light beam according to the at least one first data.

[0012] Therefore, the present invention proposes to adjust the measuring beam used for measuring the depth of a structure according to first data previously measured for said structure from at least one image of said structure, such as the position and / or the diameter of the measuring beam. The first data represent valuable information about the structure, as will be explained later. Then, as an advantageous feature, the present invention proposes to use the first data, i.e. the information about the structure, to adjust the measuring beam used by the interferometer for measuring the depth data of the structure. Thus, the interferometric measurement step can be carried out faster and allows to measure the depth of the structure faster and more efficiently.

[0013] Furthermore, the first data on the structures enable adapting the measuring beam to a single structure, thus allowing individual characterization of the structures etched in the substrate, even for structures with small critical dimensions.

[0014] In this document, a "high aspect ratio structure" or "HAR structure" or "HARS" means a structure having an aspect ratio greater than or equal to 5, 10 or even 20.

[0015] For example, HAR structures can be through silicon vias (TSVs), trenches, and more complex features etched into a support such as a wafer. Some non-limiting examples include TSVs in the form of cylindrical holes etched into a silicon substrate with a diameter of 2 µm-3 µm and a depth of 40 µm-50 µm.

[0016] In this document, the "top side" or "top surface" of a substrate corresponds to the side or surface, respectively, of the substrate on which the HAR structure is etched.

[0017] According to some embodiments, the imaging step and the interferometric step can be performed by respective optical paths having common parts. In this case, the method according to the invention can further comprise synchronizing the imaging and interferometric steps by a synchronization unit so that the steps are performed sequentially and successively.

[0018] This method allows faster characterization of structures etched in a substrate and with a smaller and lower cost system than prior art systems. In fact, the imaging step and the interferometric measurement step can share common optical components and both measurements can be performed without the need to move the measurement system relative to the substrate between measurements.

[0019] But most importantly, synchronization makes it possible to perform the imaging step and the interferometric step in sequence, so that none of these steps interferes with the other. In other words, the interferometric step can be performed without interference caused by the imaging step. Likewise, the imaging step can be performed without interference that the interferometric step (in particular the interferometric beam) may cause to the imaging step. Therefore, the imaging step can perform more precise imaging, so that the first data can be determined in a more precise manner. Likewise, the interferometric step can perform more precise interferometric measurements, so that the depth data can be determined in a more precise manner.

[0020] The synchronization of the imaging and interferometry steps can be performed in different ways, either alone or in combination.

[0021] According to some embodiments, synchronization can be performed by controlling the position of a shutter located between the interferometer and the common part. The shutter can be moved between the following positions: - a closed position that blocks the passage of the measuring beam, and - Open position allowing the measurement beam to pass.

[0022] When the imaging step is performed, the shutter can be controlled to a closed position preventing the measurement beam from passing: this does not interfere with the imaging step. After the imaging step is performed, the shutter can be moved to an open position allowing the measurement beam to pass, so that interferometric measurements can be performed.

[0023] Alternatively or additionally, the synchronization can be performed by controlling the position of a mirror located between the interferometer and the common part. The mirror can be moved between the following positions: - a closed position that deflects the measuring beam away from the structure being inspected, and - directing the measuring beam towards the open position of the structure.

[0024] While the imaging step is performed, the mirror can be moved to the closed position: the measurement beam is deflected away from the HAR structure, or even away from the substrate. After the imaging step is performed, the mirror can be moved to the open position, deflecting the measurement beam towards the structure: interferometric measurements can be performed.

[0025] Alternatively or additionally, in addition to at least one of the previous synchronization options, synchronization can be performed by controlling the position of a filter (e.g. a neutral optical density filter or a spectral filter) located between the interferometer and the common part. The filter can be moved between the following positions: - a closed position or a strongly attenuated position for filtering the measuring beam, and - Open position that lets the measuring beam through.

[0026] When the imaging step is performed, the filter can be moved to a closed position: the measuring beam is filtered out or strongly attenuated and does not pass (at least significantly) towards the HAR structure. After the imaging step is performed, the filter can be moved to an open position, the measuring beam passes through the filter, so that the interferometric measurement step can be performed.

[0027] Alternatively or additionally, in addition to at least one of the previous synchronization options, synchronization can also be performed by controlling an attenuation value of an attenuation device located between the interferometer and the common optical path portion, attenuating the measuring beam during the imaging step and not attenuating the measuring beam otherwise.

[0028] Alternatively or additionally, in addition to at least one of the previous synchronization options, synchronization can also be performed by switching the interferometer light source on and off. Thus, the light source can be switched off during the imaging step and switched on during the interferometric step.

[0029] Alternatively or additionally, in addition to at least one of the previous synchronization options, synchronization is performed by triggering image acquisition by the imaging device relative to a pulsed light source of the interferometer, such that image acquisition is performed between pulses of said light source. In this case, the light source of the interferometer is a pulsed light source with a given pulse frequency or pulses triggered by the synchronization unit. The imaging device can be triggered between pulses, or control the pulses, or be triggered by the synchronization unit together with the pulsed light source, such that at least one image is captured when the pulsed light source does not emit light pulses.

[0030] In some embodiments, for at least one structure, first data related to the structure can be inferred / calculated from a single image captured by an imaging device. Alternatively or additionally, for at least one structure, first data related to the structure can be inferred from several images captured by an imaging device.

[0031] In some embodiments, for at least one structure, at least one image captured by the imaging device can relate only to said structure: in this case, the captured image only comprises information about said structure and no information about any other structure.

[0032] Alternatively or additionally, at least one image captured by the imaging device can relate to several, in particular all, structures of the substrate: in this case, the captured image comprises information about each of said structures, so that first data for each of said structures can be determined by processing said captured image. For example, before the first interferometric measurement step, an imaging step of capturing an image of the substrate can be performed. The captured image can comprise the entire top surface of the substrate. The captured image can then be processed to determine first data for each structure of the substrate.

[0033] In some embodiments, for at least one structure, the first data can include top-CD data related to a width of the structure, and the first adjusting step includes adjusting a diameter of the measuring beam according to the top-CD data.

[0034] Thus, the imaging step provides top-CD data which are then used in a first adjustment step to adjust the diameter of the measuring beam.

[0035] Therefore, the method according to the invention allows a faster and more accurate measurement of structure depths of a substrate, in particular when top-CD data of said structures are unknown and / or when the substrate comprises different structures with different top CDs.

[0036] In these embodiments, the method according to the invention proposes that, for at least one structure or HAR structure, the diameter of the measuring beam emitted by the interferometer on the top surface of the substrate is adjusted according to the top-CD of said structure previously measured during the imaging step. This allows a large part of the incident measuring beam to enter the structure, in particular the HAR structure, while only a small part is reflected at the top surface. Due to the rather small ratio of the diameter or top CD to the illumination wavelength (typically below 15), which leads to strong diffraction effects, and generally also due to the absorption properties of the materials involved (such as silicon), or more generally due to the conditions under which the light propagates into these structures, the losses within these structures are high, only a small part of the incident light is reflected from the bottom of the structure and coupled back to the interferometer. Thus, by using a large part of the incident power to compensate for the high losses in particular in the HAR structure, the sensitivity of the interferometric measurement is optimized and a more balanced interference signal is obtained.

[0037] The diameter of the measurement beam on the top surface can be adjusted, for example, to 0.8 to 2 times the smallest lateral dimension of the structure (which can be, for example, the diameter or cross-section of a TSV, or the width of a trench). The diameter of the measurement beam on the top surface can also be adjusted to between 0.3 or 0.5 and 5 times the smallest lateral dimension of the structure and still produce a usable signal.

[0038] For example, the diameter of the measuring beam can be defined as the part of the beam with 90% or 99% of the incident power. For a Gaussian beam, the diameter of the measuring beam on the top surface can also be defined, for example, as the part of the beam where the local intensity is above the peak intensity by 1 / e², corresponding to 86.5% of the power.

[0039] The diameter can also be adjusted so that at least 75%, 80% or 90% of the incident power is coupled into the structure.

[0040] For example, a measurement beam diameter size of 5 µm or 3 µm can be used to measure structures with a diameter of 3 μm, in particular HAR structures, in particular TSVs.

[0041] The optimal fraction of light focused into the structure depends primarily on the light losses inside the structure and therefore on the structure shape (e.g. its aspect ratio). Deeper and narrower HAR structures, such as TSVs or trenches, as well as HAR structures with strongly curved bottoms need to couple most of the incident power into them and therefore require a smaller beam diameter, whereas shallower or wider structures only need to couple a smaller fraction of the incident power into them. This is why an adaptable beam size is chosen on the top surface of the substrate.

[0042] Another advantage of adjusting the measurement beam size is that the measurement beam is limited to be sufficient to cover one structure, even for arrays of structures close to each other, which allows ensuring that structures can be individually characterized.

[0043] In some embodiments, the method according to the invention can comprise a second adjustment step for adjusting the diameter of the measuring beam as a function of at least one characteristic of the interference signal measured by the interferometer.

[0044] The second adjustment step can be performed during or after the first adjustment step.

[0045] The second adjustment step allows adjusting the diameter of the measuring beam, in particular during the measuring step, when the measured interference signal indicates that the size of the measuring beam is not suitable for the inspected structure and allows adjusting the measuring beam, in particular under practical conditions and in real time.

[0046] The at least one characteristic of the measured interference signal can include at least one of the following characteristics: - the visibility or modulation depth of the interference fringes, possibly relative to the average value, or the spectral modulation of the measured interference signal, and / or - the amplitude of the noise relative to the measured interference signal, and / or -The value of the depth data provided by the interferometric signal.

[0047] Therefore, if one of these characteristics does not have the expected value, or its value is not within the expected range, this indicates that the diameter of the measuring beam is not appropriate for the structure being inspected. The diameter of the measuring beam can be adjusted accordingly.

[0048] Alternatively or additionally, it may be possible to perform an optimization process, for example using a gradient-based algorithm, to find a diameter of the measurement beam for which at least one characteristic of the measured interference signal is optimized, or reaches a local minimum.

[0049] The adjustment of the measuring beam diameter on the top surface can be performed in different ways, either individually or in combination.

[0050] In some embodiments, the adjustment of the diameter of the measuring beam on the top surface can be performed by changing at least one optical element (such as a lens or a beam expander) placed between the interferometer and the top surface. The change of the optical element can be achieved in any way. For example, the change of the optical element can be achieved by moving or supporting the at least one optical element by a turntable.

[0051] Alternatively or additionally, the adjustment of the diameter of the measuring beam at the top surface can be performed by changing at least one focal length of an optical element (e.g. a lens or a zoom arrangement) placed between the interferometer and the top surface. The change of the focal length of the optical element can be achieved by, for example, modifying the shape or propagation properties of a deformable lens or metalens, or by changing the magnification of the optical element, or changing the position of components constituting the zoom arrangement of the optical element.

[0052] Alternatively or in addition to at least one of the previous options, the adjustment of the diameter measured on the top surface can be performed by changing the numerical aperture of the measuring beam on the top surface. The numerical aperture of the measuring beam can be changed by using a beam expander in the collimated beam path or using an aperture stop.

[0053] In some embodiments, for at least one structure, the first data can include position data relating to the position of the structure on the top surface. In this case, the first adjusting step can include adjusting the position of the measuring beam on the top surface according to the position data.

[0054] Thus, the imaging step provides data information related to the location of the structure by processing at least one image captured by the imaging device during the imaging step. The location of the structure on the substrate is then used to position the measuring beam faster and more accurately, so that the depth of the substrate is measured faster and more accurately than with known techniques.

[0055] In some embodiments, the method according to the invention can comprise a third adjustment step for adjusting the position of the measuring beam relative to the structure depending on at least one characteristic of the interference signal measured by the interferometer.

[0056] The third adjustment step can be performed during or after the first adjustment step.

[0057] The third adjustment step allows to precisely adjust the position of the measuring beam during the measuring step, in particular under real conditions and in real time, in order to optimize the measurement conditions of the depth data of the structure of interest.

[0058] The at least one characteristic of the measured interference signal can include at least one of the following characteristics: - the visibility or modulation depth of the interference fringes, possibly relative to the average value, or the spectral modulation of the measured interference signal, - the amplitude of the noise relative to the measured interference signal, -The value of the depth data provided by the interferometric signal.

[0059] Therefore, if one of these characteristics does not have the expected value, or its value is not within the expected range, this indicates that the position of the measuring beam relative to the structure is not optimal. The position of the measuring beam can be adjusted accordingly.

[0060] Alternatively or additionally, an optimization process can be performed, for example using a gradient-based algorithm, to find a position of the measuring beam for which at least one characteristic of the measured interference signal is optimized, or reaches a local minimum.

[0061] The adjustment of the position of the measuring beam on the top surface can be performed in different ways, either individually or in combination.

[0062] The position of the measuring beam can be adjusted by means of a positioning unit.The positioning unit can comprise a holder for the substrate and / or a holder for the interferometer.

[0063] The positioning unit enables the position of the measuring beam on the top surface to be adjusted by: - moving the interferometer relative to the substrate; and / or -Moving the substrate relative to the interferometer.

[0064] The present invention also relates to a system for characterizing structures etched in a substrate (e.g. a wafer), in particular high aspect ratio HAR structures, the system comprising: - an imaging arrangement comprising: an imaging device arranged on the top side of the substrate for capturing at least one image of the top surface of the substrate, and a processing unit for measuring first data related to a structure etched in the substrate from at least one captured image; and a low-coherence interferometer arranged on the top side for measuring at least one depth datum related to the depth of the structure with a measuring beam located on the structure; Characterized in that the system further comprises an adjustment unit for adjusting the measuring light beam according to the first data.

[0065] In some embodiments, for at least one structure, the first data can comprise top-CD data relating to a width of said structure. In this case, the adjusting unit can comprise means for adjusting a diameter of the measuring beam on a top surface.

[0066] Alternatively or additionally, as previously mentioned, the diameter of the measuring beam can be adjusted depending on at least one characteristic of an interference signal measured by the interferometer.

[0067] According to some non-limiting examples, the adjustment unit can include: - a turntable for changing at least one optical element, such as a lens or a beam expander, placed on the path of the measuring beam between the interferometer and the top surface; - an optical element, such as a lens or zoom lens, having an adjustable focal length, placed in the path of the measuring beam between the interferometer and the top surface; and / or - A beam expander, with an adjustable numerical aperture, is placed in the path of the measurement beam between the interferometer and the top surface.

[0068] Alternatively or additionally, for at least one structure, the first data can comprise position data relating to the position of the structure on the top surface. In this case, the adjustment unit can further comprise a positioning unit for adjusting the position of the measuring beam on the top surface.

[0069] Alternatively or additionally, as previously mentioned, the position of the measuring beam can be adjusted depending on at least one characteristic of an interference signal measured by the interferometer.

[0070] The positioning unit can include a holder for the substrate and / or a holder for the interferometer.

[0071] The positioning unit enables the position of the measuring beam on the top surface to be adjusted by: - moving the interferometer relative to the substrate; and / or -Moving the substrate relative to the interferometer.

[0072] In some embodiments, the respective optical paths of the imaging arrangement and the interferometer can have common parts.In this case, the system according to the invention can comprise a synchronization unit such that the imaging arrangement and the interferometer are used sequentially or one after the other.

[0073] In order to synchronize the imaging arrangement and the interferometer, the synchronization unit can comprise at least one of the following components: - a shutter located between the interferometer and the common part and movable between a first position blocking the passage of the measuring beam and a second position allowing the passage of said measuring beam; - a mirror located between the interferometer and the common part and movable between a first position deflecting the path of the measuring beam away from the structure and a second position deflecting said measuring beam towards said structure; A filter, such as a neutral density filter or a spectral filter, is located between the interferometer and the common part and is movable between a first position in which it filters the measuring beam to block it or strongly attenuate it and a second position in which it passes it.

[0074] an attenuation device, such as an optoelectronic attenuator, located between the interferometer and the common optical path portion, the attenuation value of which is controllable; - a controller that turns the interferometer light source on and off; - a controller controlling the imaging device and / or the light source to trigger the acquisition of one or more images by the imaging device relative to pulses of the pulsed light source of the interferometer such that image acquisition is performed between pulses of said light source.

[0075] In some embodiments, the imaging arrangement can include a light source and a camera optionally in combination with a microscope optical arrangement.

[0076] In some embodiments, the interferometer is coupled to an optical fiber that transmits the measurement beam and the reflected light. The optical fiber can be a single mode optical fiber.

[0077] The interferometer can be, for example, a time domain interferometer. In this case, the interferometer 410 comprises a broadband source (not shown) emitting polychromatic light. The interferometer 410 also comprises an optical delay line that allows the optical path difference between the two beams to be varied. When the optical path difference between the beams reflected at the top and bottom of the structure, respectively, or between these respective beams and a reference beam, is reproduced by the delay line, an interference burst or fringe can be observed on the photodetector, allowing this optical path difference to be measured. The depth of the structure is inferred from the optical path difference.

[0078] The interferometer can also be a spectral domain interferometer. In this case, the interferometer comprises a broadband source (not shown) emitting polychromatic light and a spectrometer for spectral analysis of the reflected light. The spectrum obtained by combining the light reflected from the top and bottom of the structure, respectively, on the spectrometer exhibits well-known patterns, such as modulation, analysis of which can provide the optical path difference between the superimposed beams. The depth of the structure is inferred from the optical path difference.

[0079] Interferometers can also include swept source interferometers, where a tunable laser emits monochromatic light whose optical frequency varies over time within a spectral range. The intensity of the reflected light is measured by a photodetector to reconstruct the spectrum, analysis of which also allows the measurement of the optical path difference between the reflected beams, and thus the depth of the structure.

[0080] For example, the interferometer may be similar to the interferometer described in publication number WO 2007 / 042676. BRIEF DESCRIPTION OF THE DRAWINGS AND DETAILED DESCRIPTION

[0081] Other advantages and features will become apparent by reviewing the detailed description of non-limiting embodiments and the accompanying drawings, in which: - Figure 1 and Figure 2 is a schematic diagram of a non-limiting example of a method according to the present invention; - Figure 3 is a schematic diagram of a non-limiting example of a structure etched in a substrate that may be characterized according to the present invention; and - Figures 4 to 6 is a schematic diagram of a non-limiting example of a system according to the present invention.

[0082] It should be understood that the embodiments described below are by no means limiting. In particular, it is conceivable to envisage variants of the invention comprising only a selection of the features described below (separated from the other features described), if the selected features are sufficient to confer a technical advantage or to distinguish the invention from the prior art. The selection includes at least one, preferably functional, feature without structural details or with only a portion of the structural details, as long as the portion of the structural details alone are sufficient to confer a technical advantage or to distinguish the invention from the prior art.

[0083] In the figures, elements common to several figures are given the same reference numerals.

[0084] Figure 1 is a schematic diagram of a non-limiting example of a method according to the invention.

[0085] Figure 1 The method 100 shown can be used to characterize structures etched in a substrate (eg, a wafer), particularly HAR structures. More specifically, Figure 1The method 100 can be used to measure the depth of a structure etched in a substrate.

[0086] The method 100 includes a step 102 of imaging a substrate.

[0087] The imaging step 102 includes a step 104 for capturing at least one image of the top surface of the substrate, for example by an imaging arrangement. The imaging arrangement can include a camera and a light source, optionally associated with a microscope, for imaging the top surface of the substrate. The captured image can include the entire top surface or only a portion of the top surface. The captured image can include only one structure etched in the substrate, or several structures, in particular all structures, of the structures etched in the substrate.

[0088] Then, at processing step 106, the captured image can be processed to identify first data associated with the at least one structure.

[0089] More specifically, the first data can include position data relative to the position of at least one structure visible in the captured image. In this case, the processing step 106 can include a processing step 108 for processing the image to determine the position data. Such processing can be done according to known image processing methods, for example using segmentation, pattern recognition or blob analysis algorithms.

[0090] Additionally or alternatively, the first data can include top-CD data associated with at least one structure visible on the captured image. In this case, the processing step 106 can include a processing step 110 for processing the captured image to determine the top-CD data. Image processing for determining the top-CD data can be performed according to known image processing methods. For example, once a structure is located and identified, its critical dimension can be obtained by using a pattern analysis algorithm and an imaging system calibrated in magnification or size.

[0091] In some embodiments, only one of the processing steps 108 and 110 can be performed. In some embodiments, both processing steps 108 and 110 can be performed. In the latter case, processing steps 108 and 110 can be performed during a single processing step.

[0092] The method 100 can comprise a first adjusting step 120 for adjusting the measuring beam according to the first data determined in the processing step 106 .

[0093] The adjusting step 120 can include an adjusting step 122 for adjusting the position of the measurement beam on the top surface according to the position data determined in step 108 so as to accurately position the measurement beam on the structure being characterized.

[0094] Alternatively, when the first data do not include position data, the position data can be a priori known data read from a memory. In fact, in some cases, because the position data is determined during the design or manufacture of the substrate, it is possible to provide position data of the structure etched in the substrate. In these cases, the position data can be stored in a memory and read during the method according to the invention. In these embodiments, the adjustment step 122 can be completed using the previously known position data read from the memory.

[0095] The position of the measuring beam on the top surface can be adjusted using a positioning unit. The positioning unit can include: - a translation stage for the interferometer and possible imaging arrangement, the translation stage being arranged to translate at least the interferometer relative to the plane of the substrate top surface, and / or - a translation stage for the substrate, the translation stage being arranged to move said substrate or a chuck holding the substrate relative to the interferometer and possible imaging arrangement.

[0096] Likewise, steps 108 and 122 are optional, so in some embodiments the method according to the invention may not include these steps, or may only include the adjustment step 122 performed using the position data read from the memory.

[0097] The first adjustment step 120 can include an adjustment step 124 for adjusting the diameter of the measuring beam on the substrate surface according to the top-CD data, in particular according to the top-CD data determined in step 110. The diameter of the measuring beam can be adjusted to ensure that the measuring beam enters the structure with sufficient power and reaches the bottom of the structure, in particular for HAR structures, to measure the depth of the structure.

[0098] Alternatively, when the first data do not include top-CD data, the top-CD data can be a priori known data read from a memory. In fact, in some cases, since the top-CD data are determined during the design or manufacture of the substrate, top-CD data of the structure etched in the substrate can be provided. In these cases, the top-CD data can be stored in a memory and read during the method according to the invention. In these embodiments, the adjustment step 124 can be performed using the previously known top-CD data read from the memory.

[0099] The adjustment of the diameter of the measuring beam on the top surface can be performed by changing at least one optical element (such as a lens or a beam expander) placed between the interferometer and the top surface. The change of the optical element can be achieved in any way. For example, the change of the optical element can be achieved by moving or supporting the at least one optical element by a turntable.

[0100] Alternatively or additionally, the adjustment of the diameter of the measuring beam at the top surface can be performed by changing at least one focal length of an optical element (e.g. a lens or a zoom arrangement) placed between the interferometer and the top surface. The change of the focal length of the optical element can be achieved, for example, by modifying the shape or propagation properties of a deformable lens or metalens, or by changing the magnification of the optical element, or by changing the position of components constituting the zoom arrangement of the optical element.

[0101] Alternatively or in addition to at least one of the previous options, the adjustment of the diameter measured on the top surface can be performed by changing the numerical aperture of the measuring beam on the top surface. The numerical aperture of the measuring beam can be changed by using a beam expander in the collimated beam path or using an aperture stop.

[0102] Likewise, steps 110 and 124 are optional, so in some embodiments the method according to the invention may not include these steps, or may only include the adjustment step 124 performed using the top-CD data read from the memory.

[0103] After the first adjustment step 120 , the method 100 comprises an interferometric measurement step 130 for at least one structure, during which depth data relating to the depth of said structure are measured by optical interferometry.

[0104] Interferometry is performed by a low coherence optical interferometer arranged on the top side of the substrate. Optical interferometry is a well known technique and will not be described in detail here. In short, a measuring beam of broadband light is sent to the structure. At least a portion of the measuring beam is reflected by the structure at the top or surface of the substrate and at the bottom of the structure, respectively. The reflected beams interfere with each other or with a reference beam emitted from the same source to obtain an interference signal, analysis of which provides depth data.

[0105] The measuring step 130 comprises at least one interferometric measurement 132 providing a measurement signal as described above. The measurement signal can then be processed to obtain depth data.

[0106] The method 100 can optionally include a second adjustment step 134, in particular during the measuring step 130, for adjusting the diameter of the measuring beam according to at least one characteristic of the interference signal measured by the interferometer. This second adjustment step 134 allows the diameter of the measuring beam to be adjusted in real time according to the measurement signal. For example, the diameter of the measuring beam can be adjusted according to the following factors: - the visibility or modulation depth of the interference fringes, possibly relative to the average value, or the spectral modulation of the measured interference signal; - the amplitude of the noise relative to the measured interference signal; and / or -The value of the depth data provided by the interferometric signal.

[0107] Therefore, if one of these characteristics does not have the expected value, or its value is not within the expected range of values, it indicates that the diameter of the measuring beam is not suitable for the structure being inspected. As described later, the diameter of the measuring beam can be adjusted accordingly. This process can be iteratively repeated using, for example, a gradient-based algorithm to find a diameter of the measuring beam for which at least one characteristic of the measured interference signal is optimized, or reaches a local minimum.

[0108] The method 100 can optionally include a third adjustment step 136, in particular during the measuring step 130, for adjusting the position of the measuring beam according to at least one characteristic of the interference signal measured by the interferometer. This third adjustment step 136 allows the position of the measuring beam to be adjusted in real time according to the measurement signal. For example, the position of the measuring beam can be adjusted according to the following factors: - the visibility or modulation depth of the interference fringes, possibly relative to an average value, or the spectral modulation of the measured interference signal; and / or - the amplitude of the noise relative to the measured interference signal; and / or -The value of the depth data provided by the interferometric signal.

[0109] Therefore, if one of these characteristics does not have an expected value, or its value is not within an expected range of values, this indicates that the position of the measuring beam relative to the structure is not optimal. As described later, the position of the measuring beam relative to the structure can be adjusted accordingly. This process can be iteratively repeated using, for example, a gradient-based algorithm to find a position of the measuring beam for which at least one characteristic of the measured interference signal is optimized, or reaches a local minimum.

[0110] The second adjustment step 134 and the third adjustment step 136 can be performed simultaneously or sequentially in any order. Alternatively, only one of the steps 134 and 136 can be performed, or neither of the steps 134 and 136 can be performed. After performing at least one of these steps, at least one further interferometric measurement step 132 can be performed. The interferometric measurement step 132 can thus be repeated until the interferometric measurement is successful.

[0111] Figure 2 is a schematic diagram of another non-limiting example of a method according to the present invention.

[0112] Figure 2 The method 200 shown can be used to characterize structures etched in a substrate (e.g., a wafer), particularly HAR structures. More specifically, Figure 2 The method 200 can be used to measure the depth of a structure etched in a substrate.

[0113] Method 200 includes referring to Figure 1 The steps of method 100 are described.

[0114] The method 200 further comprises a synchronization step 202, so that the capturing step 104 and the interferometric measurement step 132 are performed sequentially, so that the steps do not interfere with each other. In particular, the synchronization step allows preventing the measuring beam of the interferometer from reaching the top surface at least significantly when performing the image capturing step 104. This is particularly important when the image capturing step and the interferometric measurement step are performed by using corresponding optical paths having common parts.

[0115] This synchronization can be done in different ways. The synchronization step can be done in the following ways: -Control the position of the shutter located between the interferometer and the common part: o at a first position, during the image capturing step 104, blocking the measurement beam from passing, and o at a second position, allowing the measurement beam to pass, at least during the interferometric measurement step 132; -Control the position of the mirror located between the interferometer and the common part: o at a first position, during the image capturing step 104, directing the path of the measuring beam away from the top surface; and o in a second position, at least during the interferometric measurement step 132, deflecting the measuring beam towards the top surface; -Control the position of the optical density filter or spectral filter between the interferometer and the common part: o filtering or attenuating a majority of the measurement beam during the image capture step 104 at the first position; and o at the second position, passing the measuring beam at least during the interferometric measurement step 132; - controlling the attenuation value of the attenuation device located between the interferometer and the common part so that: o the measuring beam does not pass at least significantly towards the common portion during the image capturing step 104; and o the measuring beam passes at least during the interferometric measurement step 132; - switching the interferometer light source on and off so that the measuring beam is not emitted during the image capturing step and the measuring beam is emitted at least during the interferometric measurement step 132; - triggering the imaging arrangement to capture images with respect to pulses of a pulsed light source of the interferometer, or de-triggering, or synchronizing both triggering and de-triggering, so that image acquisition is performed between pulses of said pulsed source.

[0116] Therefore, in Figure 2In the method 200, the imaging arrangement captures an image of the top surface or at least the structure without being disturbed by a measuring beam emitted by the interferometer.

[0117] Figure 3 Non-limiting examples of the substrate are shown including structures etched in the substrate.

[0118] Figure 3 A substrate 302, such as a wafer, is shown including one or more structures. Figure 3 Only one structure 304 is shown in FIG. 3 . Of course, the substrate 302 can include more than one structure. The structure 304 can, for example, be a trench etched in the substrate 302 from a top surface 306 of the substrate 302. The substrate 302 also includes a bottom surface 308 opposite to the top surface 306.

[0119] The information to be measured on the etched structure 304 is the depth 310 and possibly the top-CD 312. In the example shown, the structure 304 can be, for example, a hole or a TSV, and the top-CD data 312 can represent the diameter at the top surface 306. The structure 304 can also be an elongated trench, and the top-CD 312 can be its smallest lateral dimension, or its width at the top surface 306.

[0120] Figure 4 is a schematic diagram of a non-limiting example of a system according to the present invention.

[0121] Figure 4 The system 400 shown can be used to characterize structures etched in a substrate (eg, a wafer), particularly HAR structures. More specifically, the system 400 can be used to measure the depth of structures etched in a substrate, particularly the substrate 302.

[0122] System 400 can be used to perform the method according to the present invention, in particular Figure 1 Method 100.

[0123] The system 400 includes a low coherence interferometer 410 placed on the side of the top surface 306 of the substrate 302. The low coherence interferometer 410 is used to characterize the structure of the substrate, in particular to measure at least one data related to the depth of the structure 304, and even more particularly to measure the thickness of the structure 304.

[0124] The interferometer 410 can be, for example, a time domain interferometer. In this case, the interferometer 410 comprises a broadband source (not shown) emitting polychromatic light. The interferometer 410 also comprises an optical delay line that allows the optical path difference between the two beams to be varied. When the optical path difference between the beams reflected at the top and bottom of the structure 304, respectively, or between these respective beams and a reference beam, is reproduced by the delay line, an interference burst or fringe can be observed on a photodetector, allowing such an optical path difference to be measured. The depth of the structure 304 is inferred from the optical path difference.

[0125] Interferometer 410 can also be a spectral domain interferometer. In this case, the interferometer includes a broadband source (not shown) emitting polychromatic light and a spectrometer for spectral analysis of the reflected light. The spectrum obtained by combining the light reflected by the top and bottom of structure 304, respectively, on the spectrometer exhibits well-known patterns, such as modulation, analysis of which can provide the optical path difference between the superimposed beams. The depth of structure 404 is inferred from the optical path difference.

[0126] Interferometer 410 can also include a swept source interferometer, in which a tunable laser emits monochromatic light whose optical frequency varies over time within a spectral range. The intensity of the reflected light is measured by a photodetector to reconstruct the spectrum, the analysis of which also allows the measurement of the optical path difference between the reflected beams, and therefore the depth of the structure 304.

[0127] The interferometer 410 can be similar to the interferometer described in WO 2007 / 042676, for example.

[0128] Optionally but preferably, the measuring beam emitted by the interferometer 410 and the reflected light received from the substrate 302 can be transmitted from the interferometer 410 to the interferometer using a single mode optical fiber 412 coupled to the interferometer 410 .

[0129] For measuring narrow structures, such as TSVs with a high aspect ratio (HAR) and a diameter less than 5 µm, the measurement light source can emit light, for example, in the visible spectral range, for example < 900 nm, for better penetration into the structure.

[0130] The system 400 further comprises an imaging arrangement for capturing at least one image of the substrate 302 , more specifically at least one image of the top surface 306 of the substrate 302 , and processing the at least one captured image for determining at least one first data associated with at least one HAR structure of the substrate 302 .

[0131] The imaging arrangement is arranged on the same side as the interferometer 410 , ie on the side of the top surface 306 of the substrate 302 .

[0132] exist Figure 4 In the non-limiting example shown, the imaging arrangement includes a camera 420 and an optional light source 422. The imaging arrangement can also include a tube lens 424 to image the top surface 306 of the substrate 302 on the camera 420.

[0133] The camera 420 is configured to capture / acquire at least one image of the top surface 306 of the substrate 302, which includes at least one structure etched in the substrate 302. The captured image is processed by standard image processing techniques performed by the processing module 426 to measure first data related to at least one structure, in particular at least one HAR structure. The processing module 426 can include a hardware module, such as a processor or chip, and / or a software module, such as a computer program.

[0134] For at least one structure or one HAR structure, the first data can include the position of the structure on the top surface 306. In this case, the position of the structure can be transmitted to the control unit to command / control the device for adjusting the position of the measuring beam on the top surface 306 of the substrate 302 so as to accurately position the measuring beam emitted by the interferometer 410 on the structure.

[0135] Alternatively, for at least one structure, the position of said structure can be a priori known data stored in the memory 428. This data can be read by the control unit in order to command / control the means for adjusting the position of the measuring beam on the top surface 306.

[0136] For at least one structure or one HAR structure, the first data can include the top-CD of said structure. In this case, the top-CD of the structure can be transmitted to the control unit to command / control the means for adjusting the diameter of the measuring beam emitted by the interferometer 410 .

[0137] Alternatively, for at least one structure, the top-CD of said structure can be a priori known data stored in the memory 428. This data can be read by the control unit in order to command / control the means for adjusting the diameter of the measuring beam on the top surface 306.

[0138] Optionally, the system 400 can further include means for adjusting the position of the interferometric measurement beam emitted by the interferometer 410 on the top surface 306 of the substrate 302 so as to adjust the position of the measurement beam to the position of the structure under inspection.

[0139] exist Figure 4 In the example shown, the position adjustment device includes a holder 430 (eg, a wafer chuck) associated with a displacement device 432 (eg, a translation stage and / or a rotation stage) to move and position the substrate 302 in the XY direction and optionally in the Z direction.

[0140] Optionally, the system 400 can further include means for adjusting the diameter of the interferometric measurement beam emitted by the interferometer 410 on the top surface 306 of the substrate 302, so as to adjust the diameter of the measurement beam to the structure under inspection.

[0141] exist Figure 4 In the example shown, the diameter of the measurement beam can be determined by the core of the optical fiber 412 or the mode field diameter of the light leaving the optical fiber and the magnification provided by the combination of the collimating lens 440 and the lens 442 (referred to as the front lens, such as a microscope objective lens). Therefore, at least one of these lenses 440-442 is changed to change the magnification and thus adjust the diameter of the measurement beam on the top surface 306.

[0142] The system 400 includes a turret 444 for changing at least one of the lenses 440 - 442 and / or varying the focal length of one of the lenses 440 - 442 to correspondingly change the magnification.

[0143] The system 400 can also include a beam expander (not shown) in the portion where the measurement beam is collimated, for example between the collimating lens 440 and the front lens 444. The beam expander modifies the beam diameter and thereby the numerical aperture NA of the beam focus. This ultimately defines the diameter of the measurement beam on the top surface 306.

[0144] The device 400 further comprises a control unit 450 .

[0145] The control unit 450 can be arranged to control means for adjusting the position of the measuring beam on the top surface 306 of the substrate in dependence on the position data. As described above, the position data can: - provided by the imaging arrangement, in particular by the processing module 426; or - Read from memory 428, such as a priori known data.

[0146] More specifically, the control unit 450 can be arranged to control the displacement device 432 for moving the holder 430 such that the substrate 302 moves relative to the measuring beam. Thus, the position of the measuring beam on the top surface 306 can be adjusted.

[0147] Alternatively or additionally, the control unit 450 can also be arranged to control means for adjusting the diameter of the measuring beam on the top surface 306 of the substrate in dependence on the top-CD data. As mentioned above, the top-CD data can: - provided by the imaging arrangement, in particular by the processing module 426; or - Reading from memory 428 in case the top-CD data is a priori known data.

[0148] More specifically, the control unit 450 is arranged to control a turntable 444 for changing at least one of the lenses 440-442 and / or changing the focal length of one of the lenses 440-442, thereby changing the magnification accordingly. Thus, the diameter of the measuring beam on the top surface 306 can be adjusted to obtain a desired diameter of the measuring beam on the top surface 306.

[0149] Alternatively or additionally, the measuring beam diameter and / or the measuring beam position on the top surface can also be adjusted depending on at least one characteristic of the signal measured by the interferometer.

[0150] Such an adjustment of the measuring beam diameter and / or position can be done during the interferometric measurement and allows adjusting the measuring beam under practical conditions and in real time, in particular when the measured interference signal indicates that the diameter, position, respectively, of the measuring beam is not suitable for the measured structure 404 .

[0151] At least one characteristic of the measurement signal that can be used to adjust the measurement beam diameter and / or position on the top surface 406 can be or can include at least one of the following characteristics: - the visibility or modulation depth of the interference fringes, possibly relative to the average value, or the spectral modulation of the measured interference signal; - the amplitude of the noise relative to the measured interference signal; and / or -The value of the depth data provided by the interferometric signal.

[0152] To this end, the interferometer 410 can be configured as, or can include (or can be coupled to) a module 452, which is configured to measure the value of the representation of the measured signal and compare the value with the expected value or the previous value in the iterative process. The comparison result can be transmitted to the control unit 450 for adjusting the beam diameter and / or the beam position on the top surface 306 as described above.

[0153] like Figure 4 As shown, the interferometer 410 and the imaging arrangement operate with respective optical paths having a common portion 460. In the example shown, this common portion 460 of the optical path starts at the level of a mirror 462, mirror 512: - reflecting the measuring beam from the interferometer 410 towards the substrate 302 and reflecting the reflected beam from the substrate towards the interferometer 410; - passing imaging light from light source 422 towards substrate 302 and passing reflected imaging light from substrate 302 towards camera 420; exist Figure 4 In the example of FIG. 4 , the common portion 460 of the optical path includes the front lens 442 .

[0154] Figure 5is a schematic diagram of another non-limiting example of a system according to the present invention.

[0155] Figure 5 The system 500 shown includes Figure 4 All components of system 400.

[0156] like Figure 5 and Figure 4 As shown, the interferometer 410 and the imaging arrangement operate using a common optical path portion 460. In the case where the interferometer beam is visible to the camera, the interferometer can degrade or even prevent proper imaging of the structure due to the uneven illumination produced by the interferometer, or even cause blurring of the camera 420. At the same time, the imaging light can interfere with the interferometric measurements performed using the interferometer 410. Therefore, it can be advantageous to synchronize the interferometer 410 and the imaging arrangement so that the interferometric measurements and image acquisition are performed sequentially or successively, and generally not simultaneously.

[0157] To this end, the system 500 includes a synchronization controller / unit 502, which can be a hardware unit such as a processor, a chip or even a computer, or a software module such as a computer program.

[0158] In the example shown, the synchronization controller 502 is configured to trigger the camera 420 between two pulses of a pulsed beam source (not shown) of the interferometer 410, or to trigger the pulsed light source between image acquisitions by the camera, or to command both. Thus, when the interferometer 410 is not emitting a measurement beam, the camera 420 captures or acquires at least one image.

[0159] Alternatively or additionally, the mirror 462 can be controllably movable, in particular rotated, and the synchronization controller 502 can be configured to command the position of the mirror 462. More specifically, the mirror 462 can be controllably rotated between: a first position, with the measurement beam deflected away from the structure or top surface 306 being characterized, and - a second position, directing the measuring beam towards the structure or top surface 306 .

[0160] When performing the imaging step, synchronization controller 502 commands mirror 462 to be in a first position: the measuring beam is deflected from structure 304 and an image can be captured by camera 420. After performing the imaging step, synchronization controller 502 commands mirror 462 to be in a second position: the measuring beam is directed towards structure 304: interferometry can be performed.

[0161] Alternatively or additionally, in the example shown, the system can include a controllable shutter 504 placed between the interferometer 410 and the mirror 462. The synchronization controller 502 can be configured to command the position of the controllable shutter 504. The controllable shutter 504 can be controlled between: - an "off" position, blocking the passage of the measuring beam from the interferometer 410, and An “open” position allowing the measurement beam from the interferometer 410 to pass.

[0162] When the imaging step is performed, the shutter 504 can be commanded into a closed position preventing the passage of the measuring beam. After the imaging step is performed, the shutter 504 can be commanded into an open position so that an interferometric measurement can be performed.

[0163] Alternatively or additionally, the system can include a movable filter 504, such as a neutral density or spectral filter, placed between the interferometer 410 and the mirror 462 in the example shown and mounted, for example, in a filter wheel. The synchronization controller 502 can be configured to command the position of the movable filter 504 within the filter wheel. The position of the filter 504 can be controlled between: - an "off" position, filtering the measurement beam from the interferometer 410, and An “open” position allowing the measurement beam from the interferometer 410 to pass.

[0164] When performing the imaging step, the movable filter 504 can be positioned into a closed position preventing the measurement beam from passing. After performing the imaging step, the movable filter 504 can be commanded into an open position allowing the measurement beam to pass, so that interferometric measurements can be performed.

[0165] Of course, other synchronization means can be used instead of referring to Figure 5 Synchronous device described or with Figure 5 The described synchronization device is combined with, for example, what is described above.

[0166] Figure 6 is a schematic diagram of another non-limiting example of a system according to the present invention.

[0167] Figure 6 The illustrated system 600 includes Figure 5 All components of system 500 except turntable 444.

[0168] In system 600, the diameter of the measurement beam is not adjusted.

[0169] The first data provided by the imaging arrangement, more specifically the processing module 426, is related to the position of the structure 304. The control unit 450 uses the position data to adjust the position of the measuring beam on the top surface and accurately place the measuring beam on the structure being characterized or the HAR structure.

[0170] However, the system 600 can optionally include a synchronization controller 502 that controls the camera 420 relative to the pulse source and / or the mirror 462 and / or the shutter 504 of the interferometer to achieve image capture and interferometry in sequence.

[0171] Of course, the present invention is not limited to the examples detailed above.

Claims

1. A method (100; 200) for characterizing a structure (304) etched in a substrate (302), such as a wafer, the method comprising, for at least one structure (304) etched in the substrate: - at least one imaging step (102) comprising the following steps: o capturing (104) at least one image of a top surface (306) of the substrate (302) using an imaging device (420; 422) located on the top side of the substrate (302), and o measuring (106-110) first data related to the structure (304) from at least one captured image; - at least one interferometric measurement step (130) performed with a low coherence interferometer (410) located on the top side for measuring at least one depth data related to the depth of the structure (304) with a measuring beam located on the structure (304); Therein, the method (100; 200) further comprises a first adjustment step (120) for adjusting the measuring light beam according to the at least one first data.

2. The method (200) according to the preceding claim, characterized in that The imaging step (102) and the interference measurement step (130) are performed through corresponding optical paths having a common part (460), and the method (200) also includes synchronizing (202) the imaging step and the interference measurement step (102, 130) by a synchronization unit (502), so that the steps are performed sequentially or one after the other.

3. The method (100; 200) according to any one of the preceding claims, characterized in that For at least one structure (304), the first data comprises top-CD data related to a width of the structure (304), and the first adjusting step (120) comprises adjusting a diameter of the measuring beam according to the top-CD data.

4. The method (100; 200) according to any of the preceding claims, further comprising a second adjustment step (134) for adjusting the diameter of the measuring beam depending on at least one characteristic of the interference signal measured by the interferometer (410).

5. The method (100; 200) according to any one of claims 3 or 4, characterized in that The diameter of the measurement beam on the top surface (306) is adjusted by varying: - at least one optical element (440, 442), such as a lens or a beam expander, placed between the interferometer (410) and the top surface (306); - at least one focal length of an optical element (440, 442), such as a lens or a zoom arrangement, placed between the interferometer (410) and the top surface (306); and / or - the numerical aperture of the measurement beam on the top surface (306).

6. The method (100; 200) according to any one of the preceding claims, characterized in that For at least one structure (304), the first data comprises position data relating to a position of the structure (304) on the top surface (306), and the first adjusting step (120) comprises adjusting a position of the measuring beam on the top surface (306) according to the position data.

7. The method (100; 200) according to any of the preceding claims, further comprising a third adjustment step (136) for adjusting the position of the measuring beam relative to the structure (304) depending on at least one characteristic of the interference signal measured by the interferometer (410).

8. The method (100, 200) according to any one of claims 6 or 7, characterized in that The position of the measuring beam on the top surface (306) is adjusted by: - moving the interferometer (410) relative to the substrate (302); and / or - moving the substrate (302) relative to the interferometer movement (410).

9. A system (400; ... 500; 600), the system (400; 500; 600) including: - an imaging arrangement comprising: o an imaging device (420) arranged on the top side of the substrate (302) for capturing at least one image of the top surface (306) of the substrate (302), and o a processing unit (426) for measuring first data related to a structure (304) etched in said substrate (302) from at least one captured image; - a low-coherence interferometer (410) arranged on the top side for measuring at least one depth data related to the depth of the structure (304) using a measuring beam located on the structure (304); Characterized in that the system further comprises an adjustment unit (440-444, 430-432, 450) for adjusting the measuring light beam according to the first data.

10. System (400; 500; 600) according to the preceding claim, characterized in that For at least one structure (304), the first data comprises top-CD data related to the width of the structure (304), the adjustment unit (440-444, 430-432, 450) comprising means (440-444, 450) for adjusting the diameter of the measuring beam on the top surface (306), for example: - a turntable (444) for changing at least one optical element (440, 442), such as a lens or a beam expander, placed on the path of the measuring beam between the interferometer (410) and the top surface (306); - an optical element, such as a lens or zoom lens, having an adjustable focal length, placed in the path of the measuring beam between the interferometer (410) and the top surface (306); and / or - a beam expander having an adjustable numerical aperture, placed in the path of the measuring beam between the interferometer (410) and the top surface (306).

11. The system (400; 500; 600) according to any one of claims 9 or 10, characterized in that For at least one structure (304), the first data includes position data related to the position of the structure (304) on the top surface (406), and the adjustment unit (440-444, 430-432, 450) further includes a positioning unit (430-432) for adjusting the position of the measuring beam on the top surface (306).

12. The system (500; 600) according to any one of claims 9 to 11, wherein the respective optical paths of the imaging arrangement and the interferometer (410) have a common part (460), the system (500; 600) further comprising a synchronization unit (502) so that the imaging arrangement and the interferometer (410) are used sequentially or one after the other.

13. System (500; 600) according to the preceding claim, characterized in that The synchronization unit includes at least one of the following components: - a shutter (504) located between the interferometer (410) and the common part (460) and movable between a first position blocking the passage of the measuring beam and a second position allowing the passage of the measuring beam; - a mirror (562) located between the interferometer (410) and the common part (460) and movable between a first position in which the path of the measuring beam is deflected away from the structure (304) and a second position in which the measuring beam is deflected towards the structure (304); - a filter, such as a neutral density filter or a spectral filter, located between the interferometer (410) and the common part (460) and movable between a first position filtering the measurement beam and a second position passing the measurement beam; - an attenuation device (504) located between the interferometer (410) and the common optical path portion (460), the attenuation value of the attenuation device being controllable; - a controller (502) which switches the interferometer light source on and off; - a controller (502) controlling the imaging device and / or the light source to trigger the imaging device to acquire one or more images relative to the pulses of the pulsed light source of the interferometer (410) so that image acquisition is performed between pulses of the source.

Images