Method and system for characterizing a structure through a substrate

Abstract

The present invention relates to a method (200, 300, 400) for characterizing a structure (104) etched in a substrate (102) such as a wafer. The method (200, 300) comprises the following steps: - illuminating (202) the bottom (105) of at least one structure (104) with an illumination beam emitted from a light source (130) having a wavelength adapted to pass through the substrate (102), - acquiring (204, 206) at least one image of the bottom (105) of the at least one structure (104) through the substrate (102) using an imaging device (120, 122, 124) disposed on the bottom side (108) of the substrate (102), - measuring (210) at least one data, called lateral data, regarding the lateral dimension of the bottom (105) of the at least one HAR structure (104) from the at least one acquired image. The present invention further relates to a system for implementing such a method.

Description

Technical Field

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

[0002] The field of the present invention is the field of inspection and characterization of high aspect ratio structures etched in a substrate such as a semiconductor wafer, for example, structures etched in a substrate such as silicon through vias (TSVs) or trenches.

Background Art

[0003] In the semiconductor industry, for example, the MEMS ("Micro-Electro-Mechanical Systems") industry, many processing steps are related to etched structures in a substrate such as silicon, sometimes having a high aspect ratio. Such structures having a high aspect ratio, hereinafter "HAR structures", can include, for example, silicon through vias (TSVs), which are usually blind holes, and are used, for example, for interconnections in advanced packaging technologies, trenches having a narrow width and an elongated length, and for more complex features. These can be realized, for example, by deep reactive ion etching (Deep RIE) or photolithography techniques.

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

[0005] For example, it is necessary to measure and characterize each structure by measuring the critical dimension at the bottom of the structure embedded in the substrate. This bottom CD may be different from the critical dimension at the top of the structure (top CD) and is therefore important information. Known techniques are described in "Spectral reflectometry for metrology of three-dimensional through-silicon vias" by Yi-Sha Ku, J. Micro / Nanolith. MEMS MOEMS 13(1)011209 (March 6, 2014), which enables measuring the depth profile of an array of TSVs by spectroscopic reflectometry. However, this method does not enable characterizing individual TSVs or other structures, but only enables characterizing high-density arrays of TSVs by using a light spot covering multiple TSVs. Furthermore, this method requires accurate modeling of the measurement signal, which is actually difficult to achieve. SUMMARY OF THE INVENTION

[0006] An object of the present invention is to overcome at least one of the drawbacks of the known techniques.

[0007] Another object of the present invention is to provide a solution for characterizing a structure, particularly a HAR structure, by measuring the critical dimension (bottom CD) at its bottom.

[0008] Another object of the present invention is to provide a solution for characterizing a structure, particularly a HAR structure, by measuring its depth.

[0009] Another object of the present invention is to provide a solution for characterizing a structure, particularly a HAR structure, individually.

[0010] Another object of the present invention is to provide a solution for characterizing structures with small critical dimensions, particularly HAR structures.

[0011] It is also an object of the present invention to provide a solution for more efficiently and / or more rapidly characterizing a structure, in particular a HAR structure.

[0012] At least one of these objects is achieved by a method for characterizing a structure etched in a substrate, such as a wafer, in particular a high aspect ratio (HAR) structure, the method comprising the following steps. - Illuminating at least one structure with an illumination beam emitted from a light source emitting light having a wavelength adapted to pass through the substrate. - Using an imaging device arranged on the bottom side of the substrate to acquire at least one image of the bottom of the at least one structure through the substrate. - Measuring at least one data, called lateral data, regarding the lateral dimension of the bottom of the at least one structure from the at least one acquired image.

[0013] The method according to the invention proposes to measure, at the bottom of these structures, lateral data relating to structures etched in a substrate, in particular HAR structures, by processing at least one image captured by this imaging device, thanks to the imaging device arranged on the bottom side of the substrate. The lateral data represents useful information when characterizing these structures. In particular, the lateral dimension at the bottom of a HAR structure may be different from its lateral dimension at the top. Thus, the bottom lateral dimension can advantageously complete the depth and / or top lateral dimension data obtained for an individual structure or one of the HAR structures.

[0014] The method according to the invention is implemented using a light source emitting light having at least one wavelength or wavelength range adapted to pass through the material of the substrate. Thereby, imaging of the bottom of the structure is performed through the substrate from the bottom side of the substrate. Then, lateral data of the bottom of the structure can be obtained from the acquired image by applying any known image processing method such as image segmentation, feature extraction, or identification or blob detection.

[0015] In this specification, the "high aspect ratio structure" or "HAR structure" refers to a structure with an aspect ratio of 5, or 10, or even 20 or more.

[0016] For example, a structure such as an HAR structure may be a silicon through via (TSV), a trench, and more complex features etched within a substrate such as a wafer.

[0017] In this specification, the terms "upper side" or "upper surface" of the substrate correspond to the side or surface where the structure is etched. The term "bottom side" corresponds to the opposite side of the substrate from the upper side. Further, the term "bottom of the structure" refers to the bottom of a via, trench, or any other etched structure, in other words, the deepest part of the structure within the substrate.

[0018] In this document, the "lateral dimension of at least one structure, or the bottom of the HAR structure" can refer to the following. - The diameter or interval at the bottom of a hole or via (TSV), - The width of the trench at its bottom, or - Any dimension within or between various pattern elements of the etched structure at its bottom. This lateral dimension, or critical dimension, is also referred to as the bottom CD.

[0019] The lateral data for a single structure or several structures may be derived from a single image captured by an imaging device. Alternatively or additionally, the lateral data for a single structure or several structures may be derived from several images captured by an imaging device.

[0020] At least one image captured by an imaging device may be related to only a single structure. In this case, the captured image contains only information about this single structure and no information about any other structure on the substrate.

[0021] Alternatively or additionally, at least one image captured by the imaging device may be related to some, in particular all, structures of the substrate, in which case the captured image contains information regarding each of said structures, so that it is possible to determine lateral data for each of said structures by processing the captured image.

[0022] According to one embodiment, the step of acquiring at least one image of the bottom of at least one structure may include, for example using an imaging device, scanning the at least one structure and / or the substrate under the at least one structure along a direction z corresponding to the depth direction of the structure.

[0023] Thanks to the scanning step, it is ensured that it is the bottom of the structure being imaged and not another part of the structure between the top and the bottom of that structure. The scanning step in particular makes it possible to examine and identify the image of the bottom among the images of the structure or other parts of the substrate.

[0024] According to one example, the step of scanning may include the following. - Moving an object plane optically conjugated to the image plane of the imaging device along the z direction, - Acquiring an image or a sequence of images of the object plane at different positions of the object plane along the z direction, - Identifying at least one image of the bottom of at least one structure acquired by the object plane located at the bottom of the at least one structure.

[0025] The scanning may start from the upper surface of the substrate and be carried out along the depth direction within the HAR structure.

[0026] The scanning may also be carried out within the substrate, starting from the bottom surface of the substrate and towards the bottom of the HAR structure.

[0027] The step of identifying at least one image of the bottom may include at least one of the following steps. - For example, to ensure that the deepest image of the structure corresponding to the bottom is identified, detecting the discontinuity in the sequence of images between at least one structure and the substrate below it, - For example, to identify the in-focus image of the bottom, measuring the sharpness or spatial frequency of the acquired images.

[0028] Of course, other image processing techniques may be used to identify the image of the bottom of the structure.

[0029] According to one embodiment, the method may further include the step of measuring at least one depth data regarding the depth of at least one structure.

[0030] The measurement of the depth data, in addition to the measurement of the lateral data of the bottom of the structure, enables a complete evaluation of the characteristics of the structure under consideration.

[0031] According to one embodiment, the step of acquiring at least one image of the bottom of at least one structure may be performed using the measured depth data.

[0032] In particular, the depth data can be used to position the object plane of the imaging device at or near the bottom of the structure. This makes it possible to limit the scanning range of the object plane, or even to avoid performing the scanning step by being able to directly position the object plane of the imaging device at the bottom of the structure. In this way, the object plane of the imaging device can be positioned at the bottom of the structure more quickly and efficiently.

[0033] According to some embodiments, the step of measuring at least one depth data regarding the depth of at least one structure may be performed by the operation of a low-coherence interferometer disposed above the substrate.

[0034] According to one embodiment, such an interferometer may operate in full field of view.

[0035] According to one embodiment, such an interferometer may have a point measurement beam. In that case, the interferometer can measure the depth of a single structure.

[0036] According to an example, at least one lateral data may include position data regarding the position of the bottom of at least one structure, and the method further includes adjusting the position of the measurement beam of the interferometer on the upper surface of the substrate according to the position data.

[0037] Therefore, the step of obtaining at least one image provides information regarding the position of the bottom of the structure, and thus the position of the structure itself, by processing at least one image obtained by the imaging device. Then, the position of the structure on the substrate is used for more rapid and accurate positioning of the measurement beam of the low-coherence interferometer. Therefore, the measurement of the depth of the structure is performed more rapidly and accurately compared to the prior art.

[0038] According to another example, or in addition thereto, at least one lateral data may include bottom CD data regarding the lateral dimension of the bottom of at least one structure, and the method further includes adjusting the diameter of the measurement beam according to the bottom CD data.

[0039] Therefore, the step of obtaining at least one image provides the bottom CD data used during the adjustment step for adjusting the diameter of the measurement beam.

[0040] Therefore, the method according to the present invention enables a more rapid and accurate measurement of the depth of the structure in the substrate when the bottom CD data of the structure is not known and / or when the substrate includes different structures having different bottom CDs.

[0041] When using a low coherence interferometer with a point measurement beam, such adjustment of the diameter of the measurement beam according to the structure, particularly the bottom CD of the HAR structure, enables most of the incident measurement beam to enter the structure while only a very small part of it is reflected from the upper surface. In fact, due to the fact that the ratio of the lateral dimension of the structure to the applied wavelength, which causes a strong diffraction effect, is quite small (usually less than 15), and also often due to the absorption characteristics of the materials involved, such as silicon, or more generally, due to the conditions of light propagation into these structures, there are many losses within these structures, and only a small part of the incident light is reflected from the bottom of the structure and coupled back to the interferometer.

[0042] Therefore, the sensitivity of the interferometric measurement is optimized by adjusting the diameter of the measurement beam such that most of the incident power is used to compensate for the many losses within the structure. This makes it possible to obtain a more stable interference signal, for example, with a higher contrast or a larger modulation amplitude.

[0043] The optimal portion of the interferometer light focused within the structure depends, among other things, on the light losses within the structure. These losses depend particularly on the lateral dimension or diameter (e.g., the upper CD and the bottom CD), the depth, and the shape of the structure (e.g., the shape of the bottom and the shape of the sidewalls). For example, a structure with a smaller diameter and a larger depth results in more losses and requires a larger portion of the power to be coupled into the structure. Therefore, the measurement beam size on the substrate can be adapted according to these constraints.

[0044] The diameter of the measurement beam on the upper surface can be adjusted or determined, for example, such that at least 75%, or 80%, or 90% of the incident power is coupled into the structure.

[0045] Another advantage of adjusting the size of the measurement beam is that it can ensure that even in an array of structures close to each other, the measurement beam is sufficiently restricted to cover only one structure, enabling the structures to be individually characterized.

[0046] The diameter of the measurement beam can be adjusted by changing the following. - At least one optical element such as a lens or a beam expander disposed between the interferometer and the substrate, - The focal length of at least one optical element such as a lens or a zoom device disposed between the interferometer and the substrate, and / or - The numerical aperture of the measurement beam.

[0047] The change of the optical element may be carried out by any means. For example, the change of the optical element may be carried out by the turret moving or supporting the at least one optical element.

[0048] The change of the focal length of the optical element may be carried out by changing the zoom ratio of the optical element or the position of the components of the zoom device constituting the optical element.

[0049] Alternatively, or in addition to at least one of the foregoing options, the adjustment of the diameter of the measurement spot on the upper surface may be carried out by changing the numerical aperture of the measurement beam on the upper surface. The numerical aperture of the measurement beam can be changed by using a beam expander in a collimated beam path or an aperture stop.

[0050] According to another aspect of the same invention, a system for characterizing a structure etched in a substrate such as a wafer, particularly a high aspect ratio (HAR) structure, is proposed, and this system comprises the following. - At least one light source for illuminating the structure, at least one light source emitting light of a wavelength adapted to pass through the substrate, - An imaging device disposed on the bottom side of the substrate for acquiring at least one image of the bottom of the structure through the substrate, and - Image processing means for measuring at least one data called lateral data regarding the lateral dimension of the bottom of the HAR structure from at least one acquired image.

[0051] The image processing means may provide at least one lateral data including, or consisting of, position data regarding the position of the bottom of at least one structure within the image or with respect to the substrate.

[0052] Alternatively or additionally, the image processing means may provide at least one lateral data including, or consisting of, bottom CD data regarding the lateral dimension of the bottom of at least one structure.

[0053] According to one embodiment, the system further comprises focus control means, which is configured as follows. - Move the object plane optically conjugate to the image plane of the imaging device along the z - direction, - Acquire images of the object plane at different positions of the object plane along the z - direction, and - Identify at least one image of the bottom of at least one structure obtained by the object plane located at the bottom of at least one structure.

[0054] In this way, the focus control means ensures that it is really the bottom of the structure that is imaged, rather than any other part of the structure.

[0055] In some embodiments, at least one light source may be arranged on the bottom side of the substrate.

[0056] In this case, the illumination of the bottom of at least one structure is performed by reflection.

[0057] Alternatively or additionally, at least one light source may be arranged on the upper side of the substrate.

[0058] In this case, the illumination of the bottom of at least one structure is performed by transmission.

[0059] For both lighting mechanisms, since at least one image must be acquired through the substrate, at least one light source shall have a wavelength capable of passing through this substrate.

[0060] As an example, in the case of a silicon substrate, the light source shall have an infrared spectrum with a wavelength longer than 900 nm or 1000 nm.

[0061] According to one embodiment, the imaging device can comprise a camera and optical imaging means.

[0062] The imaging device is preferably configured to image the bottom of at least one structure and not image the portion of the structure between its top and bottom. This is important because the dimensions of the structure can vary from the top to the bottom. Thus, the imaging device is preferably configured to image only a narrow slice of the bottom of at least one structure, so that the measured lateral data corresponds to that slice and not to the average data over a larger portion of the structure.

[0063] For example, the imaging device may be configured to have a depth of field less than 1 / 5 or 1 / 10 of the depth of the structure being measured, or the HAR structure during measurement. This condition can be applied, for example, to a HAR structure having a depth of about 50 μm, resulting in a depth of field (DoF) smaller than 5 μm to 10 μm.

[0064] According to one example, the imaging device can comprise a confocal imaging device.

[0065] Such a device enables imaging only a very thin layer of the structure.

[0066] According to another example, the imaging device may comprise an imaging device having a numerical aperture (NA) of 0.4 or more at the object.

[0067] In this way, the imaging device has a very narrow depth of field.

[0068] In the configuration of the present invention, the image at the bottom of the structure is acquired through a thick layer of the substrate. For some wafers, for example, the image may be acquired through a 725 μm layer of silicon. In that case, it may be subject to optical aberrations such as spherical aberration.

[0069] Therefore, the imaging device may comprise optical correction means configured to correct aberrations caused by the path of light through the substrate, such as spherical aberration.

[0070] The imaging device may in particular comprise an objective lens having a corrector for spherical aberration.

[0071] Such a corrector can be obtained by an optical lens arrangement that introduces a spherical aberration complementary to the spherical aberration due to a given substrate layer.

[0072] Advantageously, the system according to the invention may further comprise a low-coherence interferometer for measuring at least one depth data regarding the depth of the structure with a measurement beam.

[0073] The interferometer may be, for example, a time-domain interferometer. In that case, it comprises a broadband source that emits polychromatic light. It further comprises an optical delay line that enables the change of the optical path difference between two beams. When the optical path difference between the beams reflected at the top and bottom of the structure, or between each of these beams and a reference beam, is reproduced by the delay line, an interference burst or interference fringes can be observed on the photodetector, and this optical path difference can be measured. The depth of the structure is derived from the said optical path difference.

[0074] The interferometer may also be a spectral domain interferometer. In that case, it comprises a broadband source that emits polychromatic light and a spectrometer for spectral analysis of the reflected light. On the spectrometer, the spectrum obtained by combining the light reflected by the top and bottom of the structure respectively shows a well-known pattern such as modulation, and by analyzing it, the optical path difference between the overlapping beams can be obtained. The depth of the structure is derived from the said optical path difference.

[0075] The interferometer may also comprise a swept source interferometer having a wavelength tunable laser that emits monochromatic light having an optical frequency that varies temporally over a spectral range. The intensity of the reflected light is measured by a photodetector so as to reconstruct the spectrum, and its analysis also makes it possible to measure the optical path difference between the reflected beams and thereby the depth of the structure.

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

[0077] The interferometer may be similar to, for example, the interferometer described in the document published under the number WO 2007 / 042676. Other advantages and features will become apparent upon consideration of the detailed description of the non-limiting embodiments and the accompanying drawings.

Brief Description of the Drawings

[0078]

Figure 1

Figure 1a

Figure 2

Figure 3

Figure 4

Best Mode for Carrying Out the Invention

[0079] It should be fully understood that the embodiments described below are in no way limiting. In particular, it is possible to conceive of variant forms of the present invention that include only the selection of the features described below, separated from the other features described. Such a selection may be possible if this selection of features provides a technical advantage or is sufficient to distinguish the present invention with respect to the state of the prior art. Such a selection includes at least one, preferably functional, feature without structural details, or, if a part of the structural details alone provides a technical advantage or is sufficient to distinguish the present invention with respect to the prior art, such a selection includes at least one, preferably functional, feature together with only a part of the structural details.

[0080] In the figures, elements common to each figure can retain the same reference numerals.

[0081] FIG. 1 is an illustration of a non-limiting example of a system according to the present invention.

[0082] The system 100 shown in FIG. 1 can be used to characterize structures etched in a substrate such as a wafer, particularly HAR structures. More specifically, the system 100 can be used to measure lateral data at the bottom of a structure etched in a substrate.

[0083] As shown in FIG. 1, the system 100 is used to characterize structures etched in a substrate 102 such as a wafer. In FIG. 1, only one structure 104 is depicted for ease of understanding. Of course, the substrate may include two or more structures. The structure 104 may be, for example, a trench etched into the substrate 102 from the upper surface 106 of the substrate 102. The substrate 102 also has a bottom surface 108 opposite the upper surface.

[0084] FIG. 1a shows the information to be measured in the structure 104 etched in the substrate 102, which is namely the bottom CD 150 and optionally the depth 152. In the illustrated example, the structure 104 may be, for example, a hole or a TSV, and the lateral data or bottom CD data 150 may represent the inner diameter of the structure at its bottom 105. The structure 104 may also be an elongated trench, and the bottom CD 150 may be its minimum lateral dimension or its width at its bottom 105.

[0085] The system 100 comprises an imaging device for acquiring at least one image, more specifically at least one image of the bottom 105 of the structure 104 etched in the substrate 102, through the substrate 102, and image processing means for determining at least one first data regarding at least one structure of the substrate 102, for the at least one captured image.

[0086] The imaging device is arranged on the side of the bottom surface 108 of the substrate 102. As shown in the non-limiting example of FIG. 1, the imaging device comprises a camera 122, a back lens 120, and a tube lens 124 for imaging through the bottom surface 108 of the substrate 102 on the camera 122.

[0087] The imaging device is configured to have a very narrow depth of field to reliably image only a narrow slice of the bottom of the structure, such that the measured lateral data corresponds to that slice and not to the average data over a larger portion of the structure. For example, the imaging device uses a back lens 120 with a numerical aperture (NA) of 0.4 or more, such as a microscope objective lens with a magnification higher than x20.

[0088] The back lens 120 may also include correction means (not shown) configured to correct aberrations, particularly spherical aberration caused by the optical path of light passing through the substrate 102. Otherwise, the acquired image may have reduced resolution, particularly when the image is acquired through a thick substrate layer such as silicon of, for example, 725 μm. The correction means may include, for example, an optical device that introduces spherical aberration with an opposite effect.

[0089] The system 100 also includes a light source 130 disposed on the back or bottom side of the substrate 102. The wavelength emitted by the light source 130 is adapted to pass through the material of the substrate. For example, if the substrate is silicon, the light source shall emit light having a wavelength longer than 1 μm or 1.1 μm. The light source 130 illuminates the back side of the substrate 102 and thus one or more bottoms 105 of the structures 104 etched into the substrate 102 by reflection.

[0090] Alternatively or additionally, a light source 131 may also be disposed on the upper side of the substrate. In this case, the wavelength must also be adapted to pass through the material of the substrate. For example, if the substrate is silicon, the light source shall emit light having a wavelength longer than 1 μm or 1.1 μm. The light source 131 illuminates the upper surface 106 of the substrate 102 and thus one or more structures 104 etched into the substrate 102. Thus, these structures 104, particularly their bottoms 105, are illuminated by transmission through the structure itself and the surrounding substrate.

[0091] Camera 122 is configured to image / acquire at least one image of the bottom of at least one structure 104 etched within substrate 102. The acquired image is processed by standard image processing techniques executed by processing module 126 to measure lateral data regarding the lateral dimensions of the bottom 105 of at least one structure 104. Processing module 126 may be a hardware module such as a processor or chip, or a software module such as a computer program. The processing of the image may include, in particular, segmentation for identifying structures within the image and / or pattern detection.

[0092] Advantageously, system 100 further comprises a focus controller. The focus controller is preferably implemented in processing module 126. Alternatively, it may be implemented in a second separate processing module (not shown). The focus controller is used to ensure that when imaging the bottom 105 of structure 104, it is truly the bottom 105 of structure 104 that is being imaged and not another part of structure 104.

[0093] The focus controller is configured to move the object plane optically conjugate to the image plane of the imaging device (e.g., on the camera) along the z direction. To move the object plane, for example, the rear lens 120 may be moved along the z direction. Alternatively, the imaging device as a whole may be moved relative to substrate 102 along the z direction, or vice versa. The focus controller is further configured to control the acquisition of images of the object plane at different positions of the object plane along the z direction and to identify at least one image of the bottom 105 of structure 104, i.e., an image acquired at the object plane located at the bottom 105 of at least one structure 104.

[0094] The system 100 according to the embodiment shown in FIG. 1 optionally further comprises a low coherence interferometer 110 disposed on the side of the upper surface 106 of the substrate 102. The low coherence interferometer 110 is used to measure at least one data regarding the depth of the structure 104, particularly to measure the depth of the structure 104.

[0095] Accordingly, the low coherence interferometer 110 is disposed on the opposite side of the substrate with respect to the imaging device.

[0096] The interferometer 110 may be, for example, a time domain interferometer. In that case, it comprises a broadband source (not shown) that emits polychromatic light. It further comprises an optical delay line that enables changing the optical path difference between two beams. When the optical path difference between the beams reflected from the top and bottom of the structure 104, or between each of these beams and a reference beam, is reproduced by the delay line, interference bursts or fringes can be observed on the photodetector, and this optical path difference can be measured. The depth of the structure 104 is derived from the said optical path difference.

[0097] The interferometer may also be a spectral domain interferometer. In that case, it comprises a broadband source (not shown) that emits polychromatic light and a spectrometer for spectral analysis of the reflected light. On the spectrometer, the spectrum obtained by combining the light reflected by the top and bottom of the structure 104 shows a well-known pattern such as modulation, and by analyzing it, the optical path difference between the overlapping beams can be obtained. The depth of the structure 104 is derived from the said optical path difference.

[0098] The interferometer 110 may also comprise a swept source interferometer having a wavelength tunable laser that emits monochromatic light having an optical frequency that varies temporally over a spectral range. The intensity of the reflected light is measured by a photodetector so as to reconstruct the spectrum, and its analysis also enables measuring the optical path difference between the reflected beams and thereby the depth of the structure 104.

[0099] The interferometer 110 may be the same as, for example, that described in International Publication No. WO 2007 / 042676.

[0100] Optionally but preferably, the measurement beam emitted by the interferometer 110 and the reflected light received from the substrate 102 may be transmitted to and from the interferometer 110 using a single-mode fiber 112 coupled to the interferometer 110.

[0101] To measure narrow structures such as TSVs having a diameter smaller than 5 μm, the measurement light source may emit light in the visible spectrum range, for example, less than 900 nm, for better propagation into these structures, particularly high aspect ratio (HAR) structures.

[0102] The system 100 also includes a holder 114, such as a wafer chuck, coupled to a displacement means 116, such as a rotary stage, for moving and positioning the substrate 102 in the x-y directions and optionally in the z direction.

[0103] When implementing the interferometer 110, the system 100 also includes means for adjusting the diameter of the measurement beam on the upper surface 106 of the substrate emitted by the interferometer 110 to adjust the diameter of the measurement beam with respect to the structure under inspection.

[0104] In the example shown in FIG. 1, the diameter of the measurement beam can be determined by the core of the fiber 112 or the mode field diameter of the light exiting the fiber, and the magnification provided by the combination of the collimator lens 118 and the front lens 119. At least one of these lenses 118, 119 can be changed to change the magnification and thereby adjust the diameter of the measurement beam on the upper surface 106.

[0105] The adjusting means may include a turret (not shown) for changing at least one of the lenses 118, 119 and / or changing the focal length of one of the lenses 118, 119 and accordingly changing the magnification.

[0106] The adjustment means may also include a beam expander (not shown) at a portion where the measurement beam is collimated, such as between the collimator lens 118 and the front lens 119. The beam expander changes the beam diameter and thus the numerical aperture (NA) at which the beam is focused. Thereby, the diameter of the measurement beam on the upper surface 106 is ultimately determined.

[0107] System 100 also includes a control unit 132. The control unit 132 is configured to control means for adjusting the diameter of the measurement beam. The control unit 132 may control, for example, a turret that changes at least one of the lenses 118, 119. The control unit 132 is also configured to control the positioning of the measurement beam radiated by the interferometer 110 on the structure 104. In the embodiment shown in FIG. 1, the control unit 132 controls / commands the holder 114 and / or the stage 116.

[0108] For at least one structure, the lateral data may include the position of the bottom 105 of the structure 104 in the plane of the substrate 102. In this case, when using the interferometer 110, the position of the bottom 105 of the HAR structure 104 can be transmitted to the adjustment means to command / control the holder 114 and / or the stage 116 for the accurate positioning of the measurement beam radiated by the interferometer 110 with respect to the HAR structure 104.

[0109] Alternatively or additionally, for at least one HAR structure, the lateral data may include the bottom CD of the structure 104. In this case, when using the interferometer 110, the bottom CD of the structure 104 can be transmitted to the adjustment means to command / control the turret for adjusting the diameter of the measurement beam radiated by the interferometer 110.

[0110] System 100 can be implemented in the method according to the present invention to characterize structures etched in a substrate, particularly HAR structures.

[0111] Figure 2 illustrates a non - limiting example of the method according to the present invention. This method can be executed by the system according to the present invention, in particular the system 100 shown in Figure 1.

[0112] The method 200 shown in Figure 2 can be used to characterize structures etched in a substrate such as a wafer, in particular structures with a high aspect ratio, HAR. More specifically, using the method 100 of Figure 2, lateral data at the bottom of the structure etched in the substrate can be measured.

[0113] The method 200 includes a step 202 of illuminating at least one structure, in particular the bottom of at least one structure. The illumination can be carried out by a light source arranged on the upper side and / or the bottom side of the substrate, and the emitted light has a wavelength adapted to pass through the substrate.

[0114] After the illumination step 202, the method 100 includes a bottom - image acquisition step 204. During this step 204, at least one image of the bottom of at least one structure is captured by an imaging device. The captured image can include one or more or all of the bottoms of one or more or all of the structures etched in the substrate.

[0115] As described above, the imaging device comprises a camera and optical imaging means such as a microscope device.

[0116] The bottom - image acquisition step 204 includes an acquisition step 206 for acquiring one image or a sequence of images at different positions along the depth or z - direction. This is achieved by arranging an object plane optically conjugate to the image plane of the imaging device, such as the image plane of the camera, at different positions along the z - direction. For this purpose, the object plane is moved along the z - direction, for example, by moving a lens such as the back lens 120 in Figure 1, or by moving the imaging device relative to the substrate along the z - direction, or vice versa.

[0117] The bottom image acquisition step also includes an identification step 208, where an image of the bottom of at least one structure is identified from within the sequence of images, or, if only one image is acquired, it is verified whether it is appropriate. This identification can be achieved by known image processing techniques.

[0118] According to one embodiment, it includes the following steps. - Identify an image showing the structure from within the sequence of images, and distinguish it from the images acquired for the object plane located within the substrate below the structure, where the structure is out of focus. This can be done using image processing techniques such as image segmentation, feature extraction, or blob analysis on the local structures within the acquired images. - Identify the image within the sequence of images corresponding to the step at z between at least one structure and the substrate below it. - Find the image that can be considered to best represent the bottom of the structure, or the image acquired for the object plane that coincides with the bottom of the structure, and which (best) matches some quality criteria such as sharpness or content at high spatial frequencies, within the step region or the deepest image, among the images showing the structure.

[0119] Also, thanks to the small depth of field of the imaging system, it is certain that the image identified as such shows the bottom of the structure rather than another part of the structure. Of course, if there are several structures at different depths or a complex structure with several bottoms at different depths, several images corresponding to these different depths can be retained.

[0120] According to an example, the object plane moves through the substrate starting from the bottom surface of the substrate during step 206 until the bottom of the structure appears in the acquired image. The bottom surface of the substrate has the advantage of being an easily locatable surface in the image. Next, in step 208, the image showing the best representation of the bottom of the structure is identified as described above.

[0121] According to another example, during step 208, starting, for example, from the upper surface of the substrate, the object surface moves through the structure until the structure disappears from the image. Next, in step 208, the image that displays the best representation of the bottom of the structure is identified as described above.

[0122] According to another example, steps 206 and 208 are performed as previously described, but a fairly narrow z-scan range is positioned around the pre-identified bottom position of the structure. For this purpose, prior knowledge of the depth of the structure, and even rough knowledge, is used. That information may be, for example, the nominal or expected depth known by design. This can also be obtained, as will be described below, by measuring the depth of the structure using, for example, an interferometer. The depth information enables the position of the bottom of the structure to be specified relative to the upper surface of the substrate. Furthermore, the overall thickness of the substrate may also be used, which can also be obtained from design information or thickness measurements using, for example, an interferometer. In that case, the bottom of the structure can further specify its position relative to the bottom side of the substrate. This method enables faster and also more accurate measurements, especially by limiting the risk of false detection of the bottom.

[0123] According to yet another example, using the prior knowledge of the bottom position obtained as previously described, in step 206, one image is obtained directly at the estimated position of the bottom. Then it is verified as being valid in step 208 as described above by identifying the position of the structure and checking quality criteria. If the image does not meet some criteria, another image can be obtained, analyzed, and continued in the same way for another z in the vicinity until an image that meets the quality criteria is found. In that case, a scanning strategy based on the evaluation of some criteria of the neighboring images can be implemented to optimize the scanning procedure.

[0124] When at least one image of the bottom is captured, the image is processed in processing step 210 to obtain lateral data of the bottom of at least one structure. Processing step 210 can include identifying the region of the image corresponding to the bottom of the structure using generally known image processing methods such as image segmentation, feature extraction, or identification or blob detection, or using the information obtained in the previous step.

[0125] The lateral data may include, for example, information regarding the lateral dimensions of the structure at its bottom, or bottom CD information. The bottom CD information can be related to, for example, the diameter or interval of the bottom of a hole or via (TSV), or the width of the bottom of a trench, or any dimension at the bottom between various pattern elements of an etched structure. To obtain the bottom CD information, dimensional measurements are performed within the identified bottom image of the structure according to a measurement pattern adapted to the structure of interest.

[0126] If at least one captured image includes multiple bottoms, the image processing enables determination of lateral data for each of the bottoms.

[0127] FIG. 3 is an illustration of another non - limiting example of the method according to the present invention.

[0128] The method 300 shown in FIG. 3 can be used to measure or characterize structures etched within a substrate such as a wafer, or HAR structures. More specifically, using the method 300 of FIG. 3, the lateral data of the bottom of structures etched within the substrate, or HAR structures, as well as their depths, can be measured.

[0129] Method 300 includes an illumination step 202, a bottom image acquisition step 204, and an image processing step 210. The illumination step 202, the bottom image acquisition step 204, and the image processing step 210 are executed as described with reference to FIG. 2.

[0130] Method 300 further includes an interference measurement step 212 in which depth data regarding the depth of at least one structure is measured by an optical interference method.

[0131] The interference measurement is performed by a low-coherence optical interferometer disposed on the upper side of the substrate, such as that described with reference to the system of FIG. 1.

[0132] As described in connection with FIG. 1, the measurement step 212 results in an interference signal that can then be processed to obtain depth data.

[0133] Optionally, if not known by design, the method can also include a step of measuring the overall thickness of the substrate by the same interferometer or another interferometer.

[0134] The interference measurement step 212 can be performed before, after, or simultaneously with other steps of method 300 for obtaining lateral data at the bottom of the structure.

[0135] If the interference measurement step 212 is performed first, the position of the object plane of the imaging device can be aligned with the bottom of the structure in the bottom image acquisition step 204 using the depth data of the structure and optionally the thickness information of the substrate obtained in the measurement step 212. In particular, the focus controller 126 of the system of FIG. 1 can use the depth information of the structure provided by the measurement step 212 and optionally the thickness information of the substrate to position the object plane of the imaging device at the bottom of the structure or, at least, to limit the scan range or the number of images acquired in step 206 by scanning only a narrow vicinity around the bottom position previously identified as described in connection with FIG. 2.

[0136] FIG. 4 is an illustration of another non-limiting example of a method according to the present invention.

[0137] The method 400 shown in FIG. 4 can be used to characterize structures etched in a substrate, such as a wafer, particularly HAR structures. More specifically, using the method 400 of FIG. 4, lateral data at the bottom of the structures etched in the substrate, as well as their depths, can be measured.

[0138] Similar to methods 200 and 300, method 400 includes a step 202 of illuminating the structure and a step 204 of acquiring a bottom image. Method 400 also includes an interferometry step 212 of method 300.

[0139] Method 400 further includes an image processing step 230 that includes all steps of the image processing step 210 of methods 200 and 300. In particular, in step 216, bottom CD information is obtained from the lateral data in the same way as described for method 100.

[0140] The image processing step 230 may also optionally include a processing step 214 in which lateral data regarding position data is obtained. That position data corresponds to the position of the bottom of at least one structure visible in the image. This can be obtained by finding in the image the coordinates of the bottom of the structure, the coordinates of an index such as the center of gravity of the structure, etc.

[0141] In some embodiments, only processing step 216 may be executed. In some embodiments, both processing steps 214 and 216 may be executed. In the latter case, processing steps 214 and 216 may be executed during a single processing step.

[0142] Method 400 may optionally include an arbitrary step 218 for adjusting the position on the upper surface of the measurement beam of the interferometer according to the position data determined in step 214 so that the measurement beam is accurately positioned on the structure being characterized. The position on the upper surface of the measurement beam can be adjusted by moving the interferometer relative to the plane of the upper surface of the substrate and / or by moving the substrate relative to the interferometer.

[0143] As shown in FIG. 4, method 400 further includes a step 220 of adjusting the diameter of the measurement beam used for the interferometry according to the bottom CD data. The diameter of the measurement beam is adjusted, particularly for the HAR structure, to ensure that the measurement beam enters the structure and reaches the bottom of the structure in order to appropriately measure the depth of the structure.

[0144] Method 400 then includes the interferometry step 212 described in method 300 for measuring the depth of the structure. This step is performed when the measurement beam is appropriately adjusted in step 220 and optionally in step 218.

[0145] Of course, the present invention is not limited to the examples detailed above.

Claims

1. A method (200, 300, 400) for characterizing a structure (104) etched in a substrate (102) such as a wafer, the method (200, 300) comprising the following steps: - Illuminating at least one structure (104) with an illumination beam emitted from a light source (130, 131) having a wavelength adapted to pass through the substrate (102) (202), - Using an imaging device (120, 122, 124) disposed on the bottom side (108) of the substrate (102) to acquire at least one image of the bottom (105) of the at least one structure (104) through the substrate (102) (204), and - Measuring at least one data, called lateral data, regarding the lateral dimension of the bottom (105) of the at least one structure (104) from the at least one acquired image (210), The method (200, 300, 400) comprising the above steps.

2. The method (200, 300, 400) according to claim 1, characterized in that the step (204) of acquiring at least one image of the bottom (105) of the at least one structure (104) comprises a step (206) of scanning the at least one structure (104) and / or the substrate (102) under the at least one structure (104) along a direction z corresponding to the depth direction of the structure (104).

3. The step (204) of acquiring at least one image of the bottom comprises - Moving an object plane optically conjugate to the image plane of the imaging device (122) along the z direction (206), - Acquiring an image or a sequence of images of the object plane at different positions of the object plane along the z direction (206), and - Identifying at least one image of the bottom (105) of the at least one structure (104) acquired by the object plane located at the bottom (105) of the at least one structure (104) (208), The method (200, 300, 400) according to claim 2, characterized in that it comprises the above steps.

4. The step (208) of identifying at least one image of the bottom (105) comprises the following steps: - Detecting a break in a sequence of images between the at least one structure (104) and the substrate (102) thereunder, - Measuring the sharpness or spatial frequency of the acquired images, The method (200, 400) according to claim 3, characterized by comprising at least one of the following.

5. The method (300, 400) according to any one of claims 1 to 4, further comprising a step (212) of measuring at least one depth data regarding the depth of the at least one structure (104).

6. The method (300, 400) according to claim 5, wherein the step (204, 206) of obtaining at least one image of the bottom (105) of the at least one structure (104) is performed using the measured depth data.

7. The method according to claim 5 or 6, wherein the step (212) of measuring at least one depth data regarding the depth of the at least one structure (104) is performed using a low coherence interferometer (110) disposed on the upper side (106) of the substrate (102) and having a point measurement beam.

8. The at least one lateral data includes position data regarding the position of the bottom (105) of the at least one structure (104), and the method (300, 400) further comprises a step (218) of adjusting the position of the measurement beam on the upper surface (106) of the substrate (102) according to the position data. The method (400) according to claim 7, characterized in that it is included.

9. The at least one lateral data includes bottom CD data regarding the lateral dimension at the bottom (105) of the at least one structure (104), and the method (300, 400) further comprises a step (220) of adjusting the diameter of the measurement beam according to the bottom CD data. The method (300, 400) according to claim 7, characterized in that it is included.

10. A system (100) for evaluating the characteristics of a structure (104) etched in a substrate (102) such as a wafer, wherein the system (100) is - at least one light source (130) for illuminating the structure (104), the at least one light source (130, 131) emitting light of a wavelength adapted to pass through the substrate (102), - an imaging device (120, 122, 124) disposed on the bottom side (108) of the substrate (102) for obtaining at least one image of the bottom (105) of the structure (104) through the substrate (102), and - Image processing means (126) for measuring at least one data called lateral data regarding the lateral dimension of the bottom (105) of the structure (104) from the at least one acquired image A system comprising. **Claim 11** The system (100) further comprises focus control means (126), and the focus control means (126) - Move an object plane optically conjugate to the image plane of the imaging device (120, 122, 124) along the z direction, - Acquire images of the object plane at different positions of the object plane along the z direction, and - Identify at least one image of the bottom (105) of the at least one structure (104) acquired by the object plane located at the bottom (105) of the at least one structure (104), The system (100) according to claim 10, characterized in that it is configured as such. **Claim 12** The system (100) according to claim 10 or 11, characterized in that the at least one light source (130) is arranged on the bottom side (108) of the substrate (102). **Claim 13** The system (100) according to any one of claims 10 to 12, characterized in that the at least one light source is arranged on the upper side (106) of the substrate (102). **Claim 14** The system (100) according to any one of claims 10 to 13, characterized in that the imaging device (120, 122, 124) comprises a confocal imaging device. **Claim 15** The system (100) according to any one of claims 10 to 13, characterized in that the imaging device (120, 122, 124) comprises an imaging device having a numerical aperture of 0.4 or more for the object. **Claim 16** The system (100) according to any one of claims 10 to 15, characterized in that the imaging device (120, 122, 124) comprises an objective lens (120) having a spherical aberration corrector. **Claim 17** The system (100) according to any one of claims 10 to 15, further comprising a low coherence interferometer (110) for measuring at least one depth data regarding the depth of the structure (104) with a measurement beam.

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