Method and system for characterizing structure through substrate
By using a light source and imaging equipment to acquire structural images at different depths on the substrate, the challenge of characterizing high aspect ratio structures was solved, and the consistency of the etching process and the electrical performance of the interconnects were improved.
Patent Information
- Application Number
- CN202480052917.8
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Priority Date
- 2023-08-21
- Filing Date
- 2024-08-02
- Publication Date
- 2026-03-31
AI Technical Summary
Existing technologies struggle to effectively measure and characterize the cross-sectional variation and sidewall tilt along the depth direction of high aspect ratio structures (such as TSVs or trenches), leading to inconsistencies in the etching process and issues with the resistivity of the interconnects.
A light source is used to emit an illumination beam suitable for transmission through the substrate. Images of the structure are acquired at different longitudinal positions using an imaging device. Lateral data is measured and longitudinal shape is determined. Image processing techniques are used to identify and analyze the lateral and longitudinal features of the structure.
It enables efficient and rapid characterization of cross-sectional changes and sidewall inclination of high aspect ratio structures, improving the consistency of etching processes and the electrical performance of interconnects.
Smart Images
Figure CN121773309A_ABST
Abstract
Description
Technical Field
[0001] This invention relates to a method for characterizing structures, particularly high aspect ratio structures etched in a substrate. The invention also relates to a system for carrying out this method.
[0002] The field of this invention is the detection and characterization of structures etched in a substrate, such as high aspect ratio structures etched in a substrate (e.g., a semiconductor wafer), such as through-silicon vias (TSVs) or trenches. Background Technology
[0003] In the semiconductor industry or MEMS (“microelectromechanical systems”) industry, many process steps involve etching structures, sometimes with a high aspect ratio, into substrates such as silicon.
[0004] Such structures with high aspect ratios (hereinafter referred to as "HAR structures") may include, for example, through-silicon vias (TSVs) (which are typically blind vias used for interconnects in advanced packaging technologies, for example), trenches with narrow widths and long lengths, and more complex features. They can be achieved using techniques such as deep radiation ion etching (Deep RIE) or photolithography.
[0005] The aspect ratio of a HAR structure is defined by the ratio of its depth to its lateral dimension, also known as the "critical dimension" (CD). Some HAR structures may have an aspect ratio greater than 10 or 20, with a lateral (critical) dimension as narrow as 5 µm or less. Some non-limiting examples include TSVs in the form of cylindrical vias etched into a silicon substrate with a diameter of 2–3 µm and a depth of 30–50 µm.
[0006] For narrow TSVs with high aspect ratios, such as TSVs with a diameter of 3 µm and a depth of 40–50 µm, or TSVs with a diameter of 2 µm and a depth of 30–40 µm, the etching process can cause the diameter or cross-section of the TSV to change with depth. Most commonly, the cross-section decreases with depth along the walls and / or near the bottom, resulting in the diameter at the bottom of the TSV being smaller than that at the top.
[0007] In subsequent process steps, TSVs are typically filled with metal to achieve interconnects, and the substrate is ground and polished from the back side until the metal at the bottom of the TSV is exposed. If the diameter of the TSV decreases with depth, it can result in interconnects having excessively high resistivity.
[0008] Therefore, it is necessary to measure the profile of the TSV or other high AR structure along its height to verify whether the etching process is satisfactory. Summary of the Invention
[0009] The purpose of this invention is to overcome at least one drawback of known technologies.
[0010] The purpose of this invention is to provide an apparatus and method for characterizing the cross-sectional variation of a high aspect ratio structure (such as a TSV or trench) with its depth.
[0011] Another object of the present invention is to provide an apparatus and method for measuring the inclination or cone angle of the sidewalls along a high aspect ratio structure.
[0012] Another object of the present invention is to provide an apparatus and method for characterizing isolated or single HAR structures, as well as for characterizing HAR structures individually.
[0013] Another object of the present invention is to provide a solution for characterizing structures, particularly HAR structures with small critical dimensions.
[0014] Another object of the present invention is to provide a solution for more efficient and / or faster characterization of structures, particularly HAR structures.
[0015] At least one of these objectives is achieved by a method for characterizing a structure etched in a first surface of a substrate (e.g., a wafer), the structure extending into the substrate in a longitudinal direction z, the method being carried out by a characterization system including a light source and an imaging device, wherein the light source is configured to emit an illumination beam with a wavelength suitable for transmission through the substrate, and the imaging device is positioned facing a second surface of the substrate opposite the first surface, the method comprising the following steps:
[0016] - Illuminate at least one structure with a beam of light.
[0017] - The object plane of the imaging device is then positioned at at least two different longitudinal locations;
[0018] - Acquire at least one image of the structure at each longitudinal position, the images being acquired through a substrate;
[0019] - Measure at least one lateral data related to the lateral dimension of the structure, based on at least one image acquired at each of the said longitudinal positions; and
[0020] - Determine at least one longitudinal data point related to the longitudinal shape of the structure based on transverse data from at least two longitudinal locations.
[0021] The method according to the invention proposes measuring longitudinal data related to structures, particularly HAR structures etched in a substrate, thanks to an imaging device positioned on the substrate opposite one side of the supporting structure, and by processing at least two images captured by the imaging device at different depths of the structure. Longitudinal data represents valuable information in characterizing these structures. In particular, longitudinal data can be used to assess the evolution of the structure's diameter, cross-section, or width along its depth. The lateral dimensions of the structure can indeed vary with depth, especially near the bottom of the structure. Longitudinal data is particularly useful for checking the consistency of the etching process of the structure.
[0022] The method according to the invention is implemented using a light source that emits light having at least one wavelength or wavelength range suitable for transmission through the material of the substrate. Therefore, imaging of the structure is achieved through the substrate from the side opposite to the side supporting the structure. Lateral data of the structure can then be obtained from the acquired image by applying any known image processing method (e.g., image segmentation, feature extraction, edge detection, or spot detection). It can also be obtained by comparison with a known reference image, possibly using deep learning-based techniques.
[0023] In this document, "high aspect ratio structure" or "HAR structure" refers to a structure with an aspect ratio greater than or equal to 5, or 10, or even 20.
[0024] For example, structures (such as HAR structures) can be TSVs, trenches, and more complex features etched into a substrate (such as a wafer).
[0025] In this document, "substrate" can refer to any substrate of any shape and material used in the semiconductor or MEMS industry at any stage of the process in which a HAR structure exists. Examples of such substrates include circular wafers, rectangular or square panels, composite substrates, dies, etc. The first surface of the substrate corresponds to one or more sides etched with one or more structures. It is also referred to below as "top," "top side," or "top surface." The second surface corresponds to the side of the substrate opposite the first surface. It is also referred to as "bottom," "bottom side," or "bottom surface." Furthermore, the term "bottom of structure" refers to the bottom of a via, trench, or any other etched or recessed structure, or in other words, the deepest part of the structure in the substrate.
[0026] The structure measured by this invention extends along the longitudinal direction z into the depth of the substrate. Therefore, the longitudinal direction may, for example, correspond to a direction perpendicular to the first surface.
[0027] In this document, "at least one lateral dimension of a structure" may refer to:
[0028] - The diameter or cross-section of a hole or through-hole (TSV).
[0029] - The width of the trench, or
[0030] - Any size within or between various pattern elements of the etched structure.
[0031] Lateral data related to the structure can be derived from a single image captured by the imaging device for a given longitudinal position. Alternatively or additionally, lateral data related to the structure can be derived from several images captured by the imaging device for the given longitudinal position.
[0032] Similarly, lateral data associated with multiple structures can be captured sequentially or simultaneously, for example, if they are imaged in the same field of view of the imaging device.
[0033] According to an embodiment, the step of determining at least one longitudinal data may include the step of determining the cone angle of at least one sidewall of at least one structure.
[0034] Alternatively or additionally, the step of determining at least one longitudinal data may include the step of determining the profile of the at least one structure in a longitudinal plane.
[0035] Therefore, the expression "at least one longitudinal shape of the structure" can refer to the cone angle of one or more sidewalls of the structure. In this case, it is assumed that at least in the measured structural portion, one or more walls are straight. To measure this cone angle, it is necessary to obtain transverse data at at least two longitudinal positions spaced at known distances, but not...
[0036] It is essential to know the positions of these two longitudinal positions relative to the bottom of the first surface or structure.
[0037] It can also refer to a profile, particularly the longitudinal cross-sectional profile of a structure or a portion thereof. This is especially useful when one or more walls of the structure are curved or non-straight, and is particularly useful for verifying the profile shape near the bottom of the structure, where etching processes may be less efficient.
[0038] According to an embodiment, the method of the present invention may further include the step of positioning the object plane of the imaging device at the bottom of the at least one structure.
[0039] Alternatively or additionally, the method of the present invention may also include the step of positioning the object plane of the imaging device at the top of the at least one structure.
[0040] In particular, for contour measurements, it may be necessary to know the start and / or end points of the structure.
[0041] According to an embodiment, the method may further include:
[0042] - Move the object plane along the z-direction;
[0043] - Obtain images or image sequences of the object plane at different positions along the z-direction on the object plane; and
[0044] - Identify at least one image of the bottom or top of the at least one structure, which is acquired when the object plane is positioned at the bottom or top of the at least one structure.
[0045] This scanning phase, in particular, allows for the searching and identification of images of the bottom or top from images of other parts of the structure or substrate.
[0046] Scanning can be performed along the depth direction within the HAR structure, for example, starting from the top surface of the substrate.
[0047] Scanning can also be performed within the substrate, for example, starting from the bottom surface of the substrate toward the bottom of the HAR structure.
[0048] The step of identifying at least one image at the bottom or top includes at least one of the following steps:
[0049] - Detect the transition of the image sequence between the at least one structure and its environment.
[0050] - Measure the sharpness or local spatial frequency of the acquired image.
[0051] Of course, other image processing techniques can also be used to identify images of the bottom or top of the structure.
[0052] According to an embodiment, the step of measuring at least one lateral data may include identifying at least one of the following in an image of the structure: a region corresponding to the interior of the structure, the location of its walls, and the transition between the structure and the substrate.
[0053] A structure is defined as an empty space within a substrate (interior) surrounded by walls. This can be achieved using well-known image processing techniques, such as grayscale-based segmentation, pattern detection for identifying structures in images, size / shape measurement, edge detection, blob analysis, etc.
[0054] According to another aspect of the same invention, a system for characterizing a structure etched in a first surface of a substrate (e.g., a wafer), the structure extending into the substrate along a longitudinal direction z, the system comprising:
[0055] - At least one light source for illuminating the structure, the at least one light source emitting light with a wavelength adapted to transmit light through the substrate;
[0056] - Imaging device, positioned with a second surface facing the substrate opposite to the first surface;
[0057] - A focus controller for subsequently positioning the object plane of the imaging device at at least two distinct longitudinal positions, and acquiring at least one image of the structure through the substrate at each of the longitudinal positions using the imaging device; and
[0058] - Image processing equipment, used for:
[0059] • Measure at least one lateral data related to the lateral dimension of at least one structure, based on at least one image acquired at each of the said longitudinal positions; and
[0060] • Determine at least one longitudinal data point related to the longitudinal shape of at least one structure based on transverse data from at least two longitudinal locations.
[0061] In some embodiments, at least one light source may be positioned on a second surface facing the substrate, corresponding to the bottom side.
[0062] In this case, the illumination of the structure is accomplished through reflection.
[0063] Alternatively or additionally, the at least one light source may be positioned facing the first surface of the substrate, corresponding to the top side.
[0064] In this case, the structure is illuminated through transmission.
[0065] For both illumination schemes, at least one image must be acquired through a substrate, therefore at least one light source should have a wavelength that can be transmitted through the substrate.
[0066] For example, in the case of a silicon substrate, the light source should have an infrared spectrum with a wavelength greater than 900 nm or 1000 nm.
[0067] According to an embodiment, the imaging device may include a camera and an optical imaging lens. Of course, the camera must be sensitive to wavelengths transmitted through the substrate.
[0068] At least one light source for transmission or reflection illumination may be located in a bright field configuration, with its illumination optical axis sufficiently close to the optical axis of the imaging system, such that transmitted light or specular reflection light on the top or bottom surface of the substrate is collected by the camera.
[0069] Alternatively, at least one light source for transmitted or reflected illumination can be located in a dark field configuration, with its illumination optical axis tilted sufficiently relative to the optical axis of the imaging system so that only light scattered by the structure is collected by the camera.
[0070] Imaging equipment is advantageously configured to image precise portions of a structure, such as the bottom, or the portion between the top and bottom of the structure. This is important because the dimensions of the structure may vary from top to bottom. Therefore, imaging equipment is advantageously configured to image only narrow slices of the structure at a defined depth or longitudinal location, such that the measured lateral data correspond to that slice, rather than average data corresponding to a larger portion of the structure.
[0071] For example, the imaging device can be configured to have a depth of field less than 1 / 5 or 1 / 10 of the depth of the structure being measured. For example, this standard can be applied to HAR structures with a depth of approximately 50 µm, resulting in a depth of field (DoF) of less than 5 µm to 10 µm.
[0072] According to the example, the imaging device may include a confocal imaging device.
[0073] This device allows imaging of only a very thin layer of the structure.
[0074] According to another example, the imaging device may include an imaging apparatus with a numerical aperture at the object equal to or greater than 0.4.
[0075] Therefore, the imaging device has a very narrow depth of field.
[0076] In the configuration of this invention, the image of the structural slice is acquired through a thick layer of the substrate. For some wafers, for example, the image may be acquired through a 775 µm silicon layer. In this case, the image may be affected by optical aberrations, such as spherical aberration.
[0077] Therefore, the imaging device may include an optical compensation device configured to compensate for aberrations (e.g., spherical aberration) caused by the path of light through the substrate.
[0078] Imaging devices may include, in particular, objectives with spherical aberration compensators.
[0079] Such a compensator can be obtained through an optical lens device that introduces or compensates for spherical aberration that is opposite to or inversely proportional to the spherical aberration caused by a given layer of the substrate.
[0080] Description of Drawings and Embodiments
[0081] Other advantages and features will become apparent upon review of the detailed description and accompanying drawings, which are by no means limiting, in that:
[0082] - Figure 1 This is a schematic representation of a non-limiting example of a system according to the present invention;
[0083] - Figure 2a and 2bThis is a schematic representation of an example of a HAR structure in a substrate that can be characterized using the methods and systems according to the present invention;
[0084] - Figure 3 This is a schematic representation of an image of a HAR structure acquired through the system of the present invention; and
[0085] - Figure 4-5 This is a schematic representation of a non-limiting example of the method according to the present invention.
[0086] It should be fully understood that the embodiments described below are by no means limiting. In particular, variations of the invention may be contemplated that include only selections of the features described below, isolated from other described features, if such selection is sufficient to provide a technical advantage or to distinguish the invention from the prior art. Such selections include at least one preferably functional feature without structural details, or only those structural details are included if such partial structural details are sufficient to provide a technical advantage or to distinguish the invention from the prior art.
[0087] In the accompanying drawings, elements shared by multiple figures may retain the same reference numerals.
[0088] Figure 1 This is a schematic representation of a non-limiting example of a system according to the present invention.
[0089] Figure 1 The system 100, as shown in the diagram, can be used to characterize structures etched in a substrate (e.g., a wafer), particularly HAR structures. More specifically, the system 100 can be used to measure lateral data of structures etched in a substrate.
[0090] like Figure 1 As shown, system 100 is used to characterize structures etched in substrate 102 (e.g., a wafer). Figure 1 For ease of understanding, only one structure 104 is shown in the diagram. Of course, the substrate may include more than one structure. Structure 104 may be, for example, a trench etched into the substrate 102 from the top surface 106 of the substrate 102.
[0091] Figure 2aThe diagram illustrates the measurable information of a structure 104 etched in a substrate 102 according to a first example 101. Structure 104 includes a top 140 and a bottom 147, and is characterized by its depth 154 and its width 150 at the bottom and its width 152 at the top. In the illustrated example, structure 104 can be, for example, a hole or a TSV. The measurable lateral data related to the lateral dimensions of the structure can correspond to the cross-section or inner diameter 141, 142, 143 of the hole or TSV 104 at different depths. Structure 104 can also be an elongated trench, in which case the lateral data can correspond to the cross-section, width, or lateral dimension of the trench at different depths. Based on the different measurements 141, 142, 143 of the lateral data, longitudinal data related to the longitudinal shape of structure 104 can be determined. Figure 2a In the example shown, the longitudinal data corresponds, for example, to the cone angle 156 of the wall of structure 104.
[0092] Figure 2b This describes the measurable information of a structure etched into a substrate according to the second example 103. Structure 104 can be, for example, a hole or a TSV. The measurable lateral data related to the lateral dimensions of the structure can correspond to the inner diameter cross-sections 144, 145, and 146 of the hole or TSV 104 at different depths. Structure 104 can also be an elongated trench, in which case the lateral data can correspond to the cross-section, width, or lateral dimension of the trench at different depths. Figure 2b In the example shown, the longitudinal data is determined based on different lateral data, such as the outline or shape corresponding to structure 104. This outline or shape can be obtained along the entire depth of the structure, or only along its sub-sections (e.g., the bottom).
[0093] refer to Figure 1 The system 100 includes an imaging device for acquiring at least one image of a structure 104 etched in the substrate 102 through the substrate 102, and an image processing device for acquiring the at least one image to determine at least one piece of data associated with the structure 104.
[0094] The imaging device is positioned on one side of the bottom surface 108 of the substrate 102. For example... Figure 1 As shown in the non-limiting example, the imaging device includes a camera 122, a rear-side lens 120, and a tube mirror 124 to image on the camera 122 through the bottom surface 108 of the substrate 102. The camera 122 may be, for example, a CCD or CMOS camera.
[0095] The imaging device is configured to have a very narrow depth of field to ensure that only a narrow slice of the structure is imaged at the desired depth, such that the measured lateral data corresponds to that slice, rather than average data for a larger portion of the structure. Therefore, the imaging device uses a back-side lens 120 with a numerical aperture (NA) of 0.4 or higher. Examples of such a back-side lens 120 that can be used in the device of the present invention are microscope objectives with a magnification of x20 and NA=0.45, or microscope objectives with a magnification of x50 and NA=0.65.
[0096] The rear-side lens 120 also includes a compensation device (not shown) configured to compensate for aberrations, particularly spherical aberration caused by the path of light through the substrate 102. Otherwise, the acquired image may have reduced resolution, especially if the image is acquired through a thick substrate layer (e.g., 775 µm of silicon). The compensation device may include, for example, optical devices that introduce spherical aberration with the opposite effect.
[0097] System 100 also includes a light source 130 disposed on the back or bottom side of substrate 102. The light source 130 emits one or more wavelengths suitable for transmission through the material of the substrate. For example, if the substrate is silicon, the light source should emit light with wavelengths exceeding 900 nm or 1000 nm. The light source 130 illuminates the back side of substrate 102, thereby reflecting light onto the structure 104 etched in substrate 102.
[0098] Alternatively or additionally, the light source 131 may also be arranged on the top side of the substrate. In this case, the wavelength must also be suitable for transmission through the material of the substrate. For example, if the substrate is silicon, the light source should emit light with a wavelength greater than 900 nm or 1000 nm. The light source 131 illuminates the top surface 106 of the substrate 102, thereby illuminating one or more structures 104 etched on the substrate 102. Thus, these structures 104 are illuminated in a transmissive manner, transmitted through the structure itself and the surrounding substrate. However, in this configuration, the image may be more difficult to interpret, especially for structures whose lateral dimensions decrease with depth.
[0099] System 100 also includes a positioning device configured to position an object plane optically conjugate to the image plane of the imaging device (e.g., the image plane of a camera) at different depth positions on structure 102, i.e., different positions along the z-direction. The positioning device may include a lens (e.g., a rear-side lens 120) adapted to move along the z-direction. Alternatively, the positioning device may include a displacement device adapted to move the imaging device relative to the substrate, or the substrate relative to the imaging device, along the z-direction. A focus controller (which may be implemented using a processing module) is used to control the positioning device.
[0100] System 100 also includes a clamp 114 (e.g., a wafer chuck) associated with displacement device 116 (e.g., a rotary table) to move and position substrate 102 in the xy and optionally z directions.
[0101] Therefore, system 100 is configured to capture / acquire at least one image of the structure 104 etched in the substrate 102 at a given location or depth along the structure on camera 122. This location is determined by the position of the object plane, which is optically conjugate to the image plane of the camera. Due to the very small depth of field of the optics, the image essentially corresponds to a slice of the structure. Figure 3 Examples of such images are shown, acquired using a light source 130 positioned on the back or bottom side of substrate 102. Structure 104 is an empty volume within the substrate, reflecting almost no light to the camera due to its geometry. Therefore, it appears as a darker area 162 in the image, compared to the surrounding substrate which reflects light (e.g., from the top surface). Thanks to the small depth of field of the imaging system, its cross-section or diameter corresponds to the cross-section or diameter of the structure where the object plane is located (e.g., ...). Figure 2b Positions 144, 145, or 146 in the text. Note that... Figure 3 This only represents one structure in the field of view, but depending on the substrate layout, there may be multiple structures in that field of view. In this case, the system of the present invention can acquire and process them simultaneously.
[0102] The image acquired using the camera is then processed by processing module 126 to measure lateral data related to the lateral dimensions of structure 104. Processing module 126 can be a hardware module (e.g., a processor or chip) or a software module (e.g., a computer program). This processing primarily involves identifying the location of the wall 160 of the structure, which can be defined as the transition between the interior 162 of the structure and the surrounding substrate. This can be achieved using well-known image processing techniques, such as segmentation based on grayscale analysis, and / or pattern detection for identifying structures in an image. Once the structure is identified or segmented, lateral data can then be obtained using any size / shape measurement technique or blob analysis.
[0103] System 100 can be implemented in the method according to the invention to characterize structures etched in a substrate, particularly HAR structures.
[0104] Figure 4 This is a schematic representation of a non-limiting example of a method according to the invention. The method can be implemented using a system according to the invention (particularly such as...). Figure 1 The system 100 shown is used to execute this.
[0105] Figure 4 The method 200 shown can be used to characterize structures etched in a substrate (e.g., a wafer), particularly high aspect ratio (HAR) structures. More specifically, Figure 4Method 200 can be used to measure longitudinal data of structures etched in a substrate.
[0106] Method 200 includes step 202 of irradiating at least one structure etched in a substrate. Irradiation can be achieved by a light source positioned on the bottom side and / or possibly the top side of the substrate, the light source emitting light having a wavelength suitable for transmission through the substrate.
[0107] Following illumination step 202, method 200 includes a positioning step 204, in which the object plane of the imaging device is positioned at at least two different depth positions z1, z2 along structure 104. For this purpose, the object plane is moved along the z-direction, for example by moving the lens (e.g., ...). Figure 1 The back-side lens 120, or the imaging device and the substrate can be moved relative to each other along the z-direction. At least two depth positions z1, z2 can, for example, correspond to Figure 2a At least two of the positions 141, 142, and 143 along structure 104 shown.
[0108] Once the object plane of the imaging device is positioned at the depth location, the method includes an image acquisition step 206 performed for each of the depth locations z1 and z2. Thus, for each of the depth locations z1 and z2, the imaging device captures at least one image of the structure through the substrate.
[0109] These images are further processed in processing stage 208.
[0110] To obtain lateral data of the structure at at least two depth locations, processing stage 208 includes a lateral data measurement step 210. This step 210 may include identifying regions in the image corresponding to the interior and / or walls of the structure and measuring them, as previously described.
[0111] The lateral data obtained in this way includes information related to the lateral dimensions of the structure, such as the diameter or cross-section of holes or through-holes (TSVs), or the width of trenches.
[0112] Therefore, the lateral data or dimensions at each depth position when the image was acquired were obtained.
[0113] Following the lateral data measurement step 210, the processing phase 208 of method 200 includes the longitudinal data determination step 212.
[0114] The longitudinal data contains information about the longitudinal dimensions or longitudinal shape of the structure. The longitudinal data is determined based on the lateral data of at least two depth positions z1 and z2 of the structure measured in the previous step 210.
[0115] according to Figure 4In the illustrated embodiment, the sought longitudinal data corresponds to the taper angle of at least one sidewall of the structure along the entirety or a portion of the structure. In this case, it can be assumed or approximated that the walls of the structure are straight. The taper angle can then be readily obtained from at least two measurements of the cross-section or diameter at two different depth locations along the structure. This taper angle is valuable information for monitoring the etching process. It also has the advantage that it is not necessary to know precisely the position of the depth location used relative to the top surface of the substrate or the bottom of the structure, only the distance z between these depth locations. d This is easier to obtain.
[0116] Figure 5 This is a schematic representation of another non-limiting example of the method according to the invention.
[0117] Figure 5 The method 300 shown can be used to measure or characterize structures etched in a substrate (e.g., a wafer), such as HAR structures. More specifically, Figure 5 Method 300 can be used to measure longitudinal data of structures etched in a substrate (especially HAR structures).
[0118] Method 300 includes an illumination step 202, a positioning step 204, an image acquisition step 206, and an image processing stage 208, as shown in the reference. Figure 3 As stated above.
[0119] according to Figure 5 In the illustrated embodiment, method 300 further includes a bottom image acquisition step 203, the purpose of which is to locate the bottom of the structure using the imaging system 100. The objective may be to accurately measure lateral data at the bottom of the structure and / or to locate the bottom in depth, for example, for measuring the depth of the structure or as a reference for depth location.
[0120] During bottom image acquisition step 203, at least one image of the bottom of the structure is captured by an imaging device. Bottom image acquisition step 203 includes acquisition step 205 for acquiring an image or sequence of images of the bottom of the structure when the object plane is positioned at or around the bottom of the structure at different z-positions. For this purpose, the object plane is moved along the z-direction, for example by moving the lens (e.g., ...). Figure 1 (The back-side lens 120), or the imaging device and the substrate are moved relative to each other in the z-direction.
[0121] The bottom image acquisition step 203 also includes an identification step 207, wherein the image of the bottom of the structure is identified in the acquired image sequence, or, if only one image is acquired, that image is verified. This identification can be performed using known image processing techniques.
[0122] According to an embodiment, it includes the following steps:
[0123] - Identify images displaying the structure within an image sequence to distinguish them from images acquired for an object plane located on the substrate beneath the structure (in which the structure is out of focus). This can be accomplished using image processing techniques such as image segmentation, feature extraction, or blob analysis to locate the structure in the acquired images. Alternatively, it can be done by comparison with known reference images, possibly using deep learning-based techniques.
[0124] - Identify images in an image sequence corresponding to the transition in the z-direction between the at least one structure and its underlying substrate;
[0125] - Find an image that shows the structure in the transition region and that may (best) match certain quality criteria, such as sharpness or high-frequency spatial frequency content. This image can be considered the best representation of the bottom of the structure, or an image obtained when the object plane coincides with the bottom of the structure.
[0126] Also thanks to the shallow depth of field of the imaging system, it can be determined that the image identified in this way shows the bottom of the structure, rather than the other parts of the structure.
[0127] According to the example, during step 205, the object plane moves across the substrate from the bottom surface until the bottom of the structure appears in the acquired image. The advantage of the bottom surface of the substrate is that it is easily found in the image. Then, in step 207, as described above, the image that best represents the bottom of the structure is identified.
[0128] According to another example, during step 205, the object plane moves through the structure, for example, starting from the top surface of the substrate, until the structure disappears from the image. Then, in step 207, as described above, an image showing the best representation of the bottom of the structure is identified.
[0129] According to another example, a narrower z-scan range can be used, positioned around a pre-identified bottom location of the structure. For this, prior knowledge (even if approximate) of the structure's depth is used. This information can be, for example, a nominal or expected depth (known from the design). It can also be obtained by measuring the structure's depth using, for example, an interferometer. The depth information allows the bottom of the structure to be located relative to the top surface of the substrate. Furthermore, the total thickness of the substrate can also be used, which can also be derived from design information or thickness measurements (e.g., using an interferometer). In this case, the bottom of the structure can also be located relative to the bottom side of the substrate. This approach allows for faster and more accurate measurements, especially by limiting the risk of erroneous bottom detection.
[0130] In yet another example, using the prior knowledge of the bottom position obtained as described above, an image is acquired directly at the estimated bottom position in step 205. Then, in step 207, the image is validated by locating the structure and checking quality criteria as previously described. If the image does not meet certain criteria, another image can be acquired at another z-position in the neighborhood and analyzed, and so on, until an image matching the quality criteria is found. In this case, a scanning strategy based on the evolution of certain criteria from images in the neighborhood can be implemented to optimize the scanning process.
[0131] according to Figure 5 The method 300 of the embodiment then continues as referenced Figure 4 Method 200 details the positioning step 204 and the image acquisition step 206. For example, starting from the bottom of the structure, images of the structure are acquired at different depth locations. The depth locations may, for example, correspond to... Figure 2b The positions 144, 145, and 146 along structure 104 are shown.
[0132] Once at least one image of the structure has been captured at its bottom and multiple depth locations, these images are processed in processing stage 208'. Processing stage 208' includes steps similar to those described in... Figure 4 Lateral data measurement step 210 of method 200 in the embodiment.
[0133] according to Figure 5 In the illustrated embodiment, processing stage 208' includes a longitudinal data determination step 214, according to which longitudinal data corresponding to the profile of a structure or a portion thereof is determined. This profile is obtained directly from the transverse data obtained at each z-position. This allows, for example, measuring the profile of a structure with non-straight or curved walls, or more accurately measuring the longitudinal shape of the structure (overall or, for example, a sub-section near the bottom).
[0134] According to some embodiments, method 300 can be performed without bottom image acquisition step 203, which is optional. A reference for the depth position z can be found in other ways, such as by locating the top surface of the substrate using imaging system 100, or by using prior knowledge. At step 214, the profile can also be obtained at least in a portion of the structure by using several lateral data points obtained at several z positions where only the relative positions are known.
[0135] Of course, the present invention is not limited to the detailed examples described above.
Claims
1. A method (200, 300) for characterizing a structure (104) etched in a first surface (140) of a substrate (102), for example a wafer, the structure (104) extending into the substrate (102) along a longitudinal direction z, the method (200, 300) being implemented by a characterization system (100) comprising a light source (130, 131) configured to emit an illumination beam of a wavelength adapted to be transmitted through the substrate (102), and an imaging device positioned facing a second surface (147) of the substrate (102) opposite the first surface (140), the method (200, 300) comprising the steps of: - illuminating (202) at least one structure (104) using the illumination beam, - subsequently positioning (204) an object plane of the imaging device at at least two different longitudinal positions; - acquiring (206) at least one image of the structure (104) at each of the longitudinal positions, the image being acquired through the substrate (102); - measuring (210) at least one lateral data related to a lateral dimension of the structure (104) from the at least one image acquired at each of the longitudinal positions; and - determining (212) at least one longitudinal data related to a longitudinal shape of the structure from the lateral data of the at least two longitudinal positions.
2. The method (200, 300) according to claim 1, characterized by The step of determining (212) at least one longitudinal data comprises a step of determining a taper angle of at least one sidewall (160) of the at least one structure (104).
3. The method (200, 300) according to claim 1 or 2, characterized by, The step of determining (212) at least one longitudinal data comprises a step of determining (214) a profile of the at least one structure (104) in a longitudinal plane.
4. The method (200, 300) according to claim 3, characterized by The method further comprises a step of positioning the object plane of the imaging device at a bottom (147) of the at least one structure (104).
5. The method (200, 300) according to claim 3 or 4, characterized by, The method further comprises a step of positioning the object plane of the imaging device at a top (140) of the at least one structure (104).
6. The method (200, 300) according to claim 4 or 5, characterized by, The method further comprises: - moving the object plane along the z direction; - acquiring (205) an image or a sequence of images of the object plane at different positions of the object plane along the z direction; and - identifying (207) at least one image of the bottom or top of the at least one structure acquired when the object plane is positioned at the bottom or top of the at least one structure.
7. The method (200, 300) according to the preceding claim, characterized by, The step of identifying (207) at least one image of the bottom (147) or top (140) comprises at least one of the following steps: - detecting a transition of the sequence of images between the at least one structure (104) and an environment of the structure, - measuring a sharpness or a local spatial frequency of the acquired image.
8. The method (200, 300) according to any of the preceding claims, characterized by, The step of measuring (210) at least one lateral data comprises a step of identifying in said at least one image of the structure (104) at least one of the following: a region corresponding to an interior (162) of said structure (104), a position of a wall (160) thereof, a transition between said structure (104) and said substrate (102).
9. A system (100) for characterizing a structure (104) etched in a first surface (140) of a substrate (102), for example a wafer, the structure (104) extending into the substrate (102) along a longitudinal direction z, the system (100) comprising: - at least one light source (130, 131) for illuminating the structure (104), the at least one light source (130, 131) emitting light at a wavelength adapted to be transmitted through the substrate (102); - an imaging device positioned facing a second surface (147) of the substrate (104) opposite the first surface (102); - a focus controller for subsequently positioning an object plane of the imaging device at at least two different longitudinal positions and acquiring at least one image of the structure (104) at each of the longitudinal positions using the imaging device through the substrate (102); and; - an image processing device for: • measuring, from the at least one image acquired at each of the longitudinal positions, at least one lateral data related to a lateral dimension of the at least one structure; and • determining, from the lateral data of the at least two longitudinal positions, at least one longitudinal data related to a longitudinal shape of the at least one structure.
10. The system (100) according to claim 9, characterized in that The at least one light source (130) is positioned facing the second surface (147) of the substrate (102).
11. The system (100) according to claim 9 or 10, characterized in that The at least one light source (131) is positioned facing the first surface (140) of the substrate (102).
12. The system (100) according to any one of claims 9 to 11, characterized in that The imaging device comprises a camera (122) and an optical imaging lens (120, 124).
13. The system (100) according to any one of claims 9 to 12, characterized in that The imaging device comprises a confocal imaging arrangement.
14. The system (100) according to any one of claims 9 to 13, characterized in that The imaging device comprises an imaging arrangement having a numerical aperture equal to or higher than 0.4 at the object.
15. The system (100) according to any one of claims 9 to 14, characterized in that, The imaging device comprises an objective (120) with a spherical aberration compensator.