Measurement method and associated measurement device

Abstract

Disclosed is a substrate comprising at least one target, the target comprising: a reference substructure in a top layer; and a buried substructure in a buried layer below the top layer. The buried substructure includes a non-constant pitch, such as a zone plate, in at least one direction that causes a reflected wave reflected from the substructure to be focused at least partially toward a focus as a result of a measurement of the target.

Description

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims priority to U.S. application 63 / 445,412, filed on February 14, 2023, the entire contents of which are incorporated herein by reference. Technical Field

[0003] The present invention relates to a metrology method and apparatus which can be used, for example, to determine properties of structures on a substrate. Background Art

[0004] A lithographic apparatus is a machine configured to apply a desired pattern to a substrate. For example, a lithographic apparatus can be used in the manufacture of integrated circuits (ICs). A lithographic apparatus can project a pattern (often referred to as a "design layout" or "design") from a patterning device (e.g., a mask) onto a layer of radiation-sensitive material (resist) disposed on a substrate (e.g., a wafer).

[0005] To project a pattern onto a substrate, a lithographic apparatus can use electromagnetic radiation. The wavelength of this radiation determines the minimum size of features that can be formed on the substrate. Typical wavelengths currently in use are 365 nm (i-line), 248 nm, 193 nm, and 13.5 nm. Lithographic apparatus using extreme ultraviolet (EUV) radiation (having a wavelength in the 4 nm to 20 nm range, such as 6.7 nm or 13.5 nm) can be used to form smaller features on a substrate than lithographic apparatus using radiation having, for example, a 193 nm wavelength.

[0006] Low-k1 lithography can be used to process features smaller than the classical resolution limit of the lithographic apparatus. In such processes, the resolution equation can be expressed as CD = k1 × λ / NA, where λ is the wavelength of the radiation employed, NA is the numerical aperture of the projection optics in the lithographic apparatus, CD is the critical dimension (usually the minimum feature size printed, but in this case, the half-pitch), and k1 is the empirical resolution factor. Generally, the smaller k1, the more difficult it becomes to reproduce a pattern on the substrate that resembles the shape and dimensions planned by the circuit designer to achieve specific electrical functionality and performance. To overcome these difficulties, complex fine-tuning steps can be applied to the lithographic projection apparatus and / or design layout. These steps include, for example, but are not limited to, optimization of the NA, customized illumination schemes, the use of phase-shifted patterning devices, various optimizations of the design layout (such as optical proximity correction (OPC, sometimes also referred to as "optical and process correction") in the design layout, or other methods generally defined as "resolution enhancement techniques" (RET). Alternatively, a tight control loop for controlling the stability of the lithographic apparatus can be used to improve pattern reproduction at low k1.

[0007] During photolithography, it is often desirable to measure the resulting structures, for example, for process control and verification. Various tools are known for making these measurements, including scanning electron microscopes or various forms of metrology equipment, such as scatterometers. A general term for such tools may be metrology equipment or inspection equipment.

[0008] Overlay metrology tools are known that can measure overlay in target structures with pitches down to about 10 nm if the spacing between overlapping layers (e.g., gratings formed in different layers) is of a similar order of magnitude. Metrology tools are also known that can measure overlay between overlapping layers that are further apart, but this is only done if the pitch of the target structure is also correspondingly larger. Measuring overlay in target structures with a relatively small pitch (e.g., about 10 nm) and relatively large spacing between the overlying layers (e.g., greater than 100 nm) is difficult. A further challenge is the increasing use of layers of materials that are opaque to visible light, such as metal or carbon layers, or chalcogenide materials used, for example, in 3D memory applications. Portions of the target structure that lie beneath such opaque layers may be inaccessible to many existing metrology techniques based on scatterometers.

[0009] A specific metrology technique, referred to herein as photoacoustic subsurface atomic force microscopy (passAFM), and associated metrology equipment is described in WO2021028174A1, which is incorporated herein by reference. This technique aims to address one or more of the issues highlighted in the previous paragraph.

[0010] In passAFM, an AFM cantilever is used as a very high-frequency ultrasonic transducer (e.g., at frequencies around 100 GHz). The transducer is actuated via an optical pump pulse on the cantilever. The generated acoustic wave enters the sample via the cantilever tip, and the reflected echo (e.g., displacement or reflectivity) is detected when it returns to the cantilever surface via an optical probe beam.

[0011] It would be desirable to improve methods of performing such photoacoustic subsurface atomic force microscopy. Summary of the Invention

[0012] Embodiments of the invention are disclosed in the claims and the detailed description.

[0013] In a first aspect of the present invention, there is provided a substrate comprising at least one target, the target comprising: a reference substructure in a top layer; and a buried substructure in a buried layer located below the top layer, wherein the buried substructure comprises a non-constant pitch in at least one direction, the non-constant pitch causing a reflected wave reflected from the substructure by measurement of the target to be at least partially focused toward a focal point.

[0014] According to a second aspect of the present invention, there is provided a method for measuring a parameter value of interest by measuring a target, the target comprising: a reference substructure in a top layer; and a buried substructure in a buried layer below the top layer, wherein the buried substructure comprises a non-constant pitch in at least one direction, the non-constant pitch causing a reflected wave reflected from the substructure due to measurement of the target to be at least partially focused toward a focal point; the method comprising: a) determining the position of the reference structure via atomic force microscopy; b) generating an acoustic wave at the position of the reference structure determined at step a), and directing the acoustic wave toward the buried substructure; c) measuring an intensity measurement value of the reflected wave that has been reflected from the buried substructure due to step b) at the position of the reference structure determined at step a); and d) determining the parameter value of interest based on the intensity measurement value.

[0015] The present invention further provides a measurement device operable to perform the method of the second aspect.

[0016] These and other aspects and advantages of the apparatus and methods disclosed herein will be understood by considering the following description of exemplary embodiments and the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings, in which:

[0018] - Figure 1 A schematic overview of a lithographic apparatus is depicted;

[0019] - Figure 2 A schematic overview of a lithography unit is depicted;

[0020] - Figure 3 Depicts a schematic representation of overall photolithography, showing the collaboration between three key technologies for optimizing semiconductor manufacturing;

[0021] - Figure 4 is a schematic side view of a metrology tool having a cantilever probe, an ultrasound generating system, and an ultrasound detection system;

[0022] - Figure 5 is a schematic side cross-sectional view of a probe element in a transmit mode;

[0023] - Figure 6 yes Figure 4 A schematic side cross-sectional view of a probe element in a receiving mode;

[0024] - FIG7 is a diagram of (a) a probe element performing measurement on a target according to a first embodiment and (b) a probe element performing measurement on a target according to a second embodiment;

[0025] - Figures 8(a) and 8(b) respectively include illustrations of a sub-target including a first bias and a sub-target including a second bias according to an embodiment;

[0026] - Figures 9(a) and 9(b) respectively comprise illustrations of a first reference substructure layer according to an embodiment and a second reference substructure layer according to an embodiment; and

[0027] 10( a ), 10 ( b ), and 10 ( c ) respectively include illustrations of a first zone plate substructure layer according to an embodiment, a second zone plate substructure layer according to an embodiment, and a third zone plate substructure layer according to an embodiment. DETAILED DESCRIPTION

[0028] In this document, the terms "radiation" and "beam" are used to cover all types of electromagnetic radiation, including ultraviolet radiation (e.g., having a wavelength of 365 nm, 248 nm, 193 nm, 157 nm, or 126 nm) and EUV radiation (extreme ultraviolet radiation, e.g., having a wavelength in the range of about 5 nm to 100 nm).

[0029] As used herein, the terms "reticle," "mask," or "patterning device" should be broadly interpreted to refer to a general patterning device that can be used to impart an incident radiation beam with a patterned cross-section that corresponds to the pattern to be produced in a target portion of the substrate. In this context, the term "light valve" may also be used. In addition to classical masks (transmissive or reflective, binary, phase-shifting, hybrid, etc.), examples of other such patterning devices include programmable mirror arrays and programmable LCD arrays.

[0030] Figure 1A lithographic apparatus LA is schematically depicted. The lithographic apparatus LA comprises an illumination system (also called illuminator) IL configured to condition a radiation beam B (e.g., UV radiation, DUV radiation, or EUV radiation), a mask support (e.g., mask table) MT configured to support a patterning device (e.g., mask) MA and connected to a first positioner PM configured to accurately position the patterning device MA according to predetermined parameters, a substrate support (e.g., wafer stage) WT configured to hold a substrate (e.g., a resist-coated wafer) W and connected to a second positioner PW configured to accurately position the substrate support according to predetermined parameters, and a projection system (e.g., a refractive projection lens system) PS configured to project a pattern imparted to the radiation beam B by the patterning device MA onto a target portion C (e.g., a portion comprising one or more dies) of the substrate W.

[0031] In operation, the illumination system IL receives a radiation beam from a radiation source SO, for example, via a beam delivery system BD. The illumination system IL may include various types of optical components for directing, shaping, and / or controlling the radiation, such as refractive, reflective, magnetic, electromagnetic, electrostatic, and / or other types of optical components, or any combination thereof. The illuminator IL may be used to condition the radiation beam B so as to have a desired spatial and angular intensity distribution in its cross-section at the plane of the patterning device MA.

[0032] The term "projection system" PS as used in the present invention should be interpreted broadly to cover various types of projection systems suitable for the exposure radiation used and / or for other factors such as the use of immersion liquid or the use of a vacuum, including refractive, reflective, catadioptric, anamorphic, magnetic, electromagnetic and / or electrostatic optical systems or any combination thereof. Any use of the term "projection lens" herein may be considered synonymous with the more general term "projection system" PS.

[0033] The lithographic apparatus LA may be of a type in which at least a portion of the substrate may be covered by an immersion liquid having a relatively high refractive index (e.g. water) so as to fill the space between the projection system PS and the substrate W - this is also known as immersion lithography. Further information on immersion techniques is given in US 6,952,253, which is incorporated herein by reference.

[0034] The lithographic apparatus LA may also be of a type having two or more substrate supports WT (also referred to as a "dual stage"). In such a "multi-stage" machine, the substrate supports WT may be used in parallel, and / or a step of preparing a substrate W for subsequent exposure may be performed on one of the substrate supports WT while another substrate W on another substrate support WT is being used to expose a pattern on the other substrate W.

[0035] In addition to the substrate support WT, the lithographic apparatus LA may also include a measurement platform. The measurement platform is arranged to hold sensors and / or cleaning devices. The sensors may be arranged to measure properties of the projection system PS or properties of the radiation beam B. The measurement platform may hold multiple sensors. The cleaning devices may be arranged to clean a portion of the lithographic apparatus, such as a portion of the projection system PS or a portion of the system that provides immersion liquid. The measurement platform may be movable beneath the projection system PS when the substrate support WT is away from the projection system PS.

[0036] In operation, the radiation beam B is incident on a patterning device (e.g. a mask) MA held on the mask support MT and is patterned by a pattern (design layout) present on the patterning device MA. Having traversed the mask MA, the radiation beam B passes through the projection system PS, which focuses the beam onto a target portion C of the substrate W. With the aid of the second positioner PW and the position measurement system IF, the substrate support WT can be accurately moved, for example in order to position different target portions C in the path of the radiation beam B in a focused and aligned position. Similarly, the first positioner PM and possibly another position sensor (in Figure 1 The patterning device MA may be accurately positioned relative to the path of the radiation beam B using mask alignment marks M1, M2 and substrate alignment marks P1, P2. Although the substrate alignment marks P1, P2 occupy dedicated target portions as shown, the marks may be located in spaces between target portions. When the substrate alignment marks P1, P2 are located between the target portions C, they are referred to as scribe lane alignment marks.

[0037] like Figure 2As shown in FIG, the lithography apparatus LA may form part of a lithocell LC, sometimes also referred to as a litho cell or litho cluster, which often also includes equipment for performing pre- and post-exposure processes on a substrate W. Conventionally, these include a spin coater SC for depositing a resist layer, a developer DE for developing the exposed resist, and, for example, a chill plate CH and bake plate BK for regulating the temperature of the substrate W (e.g., for regulating the solvent in the resist layer). A substrate handling device or robot RO picks up substrates W from input / output ports I / O1 and I / O2, moves them between the various process equipment, and delivers them to a loading station LB of the lithography apparatus LA. The equipment in the lithocell, often also collectively referred to as a coating and developing system, is typically under the control of a coating and developing system control unit TCU, which itself may be controlled by a supervisory control system SCS, which in turn controls the lithography apparatus LA, for example, via a litho control unit LACU.

[0038] In order to correctly and consistently expose substrates W exposed by lithographic apparatus LA, it is desirable to inspect the substrates to measure properties of the patterned structures, such as overlay errors between subsequent layers, line thickness, critical dimensions (CDs), etc. For this purpose, an inspection tool (not shown) may be included in lithography cell LC. If errors are detected, adjustments may be made to the exposure of subsequent substrates or other processing steps to be performed on substrate W, particularly if the inspection is performed before other substrates W from the same batch or lot are yet to be exposed or processed.

[0039] An inspection apparatus, which may also be referred to as a metrology apparatus, is used to determine properties of a substrate W, and in particular, to determine how properties vary between different substrates W, or how properties associated with different layers of the same substrate W vary from layer to layer. The inspection apparatus is alternatively configured to identify defects on the substrate W, and may, for example, be part of the lithography cell LC, integrated into the lithography apparatus LA, or even be a separate device. The inspection apparatus may measure properties on a latent image (the image in the resist layer after exposure), a semi-latent image (the image in the resist layer after a post-exposure bake step (PEB), a developed resist image (in which either the exposed or unexposed portions of the resist have been removed), or even an etched image (after a pattern transfer step such as etching).

[0040] Typically, the patterning process in the lithographic apparatus LA is one of the most critical steps in processing, which requires a high accuracy in the sizing and placement of the structures on the substrate W. In order to ensure this high accuracy, three systems can be combined into a so-called "holistic" control environment, such as Figure 3One of these systems is the lithography apparatus LA, which is (physically) connected to a metrology tool MET (a second system) and to a computer system CL (a third system). The key to this "holistic" environment is optimizing the collaboration between these three systems to enhance the overall process window and provide a tight control loop to ensure that the patterning performed by the lithography apparatus LA remains within the process window. The process window defines a range of process parameters (e.g., dose, focus, overlay) within which a specific manufacturing process produces a defined outcome (e.g., a functional semiconductor device)—typically, the process parameters of the lithography or patterning process are allowed to vary within this defined outcome.

[0041] The computer system CL can use (part of) the design layout to be patterned to predict which resolution enhancement technology to use and perform computational lithography simulations and calculations to determine which mask layout and lithographic equipment settings achieve the maximum overall process window for the patterning process (in Figure 3 Typically, the resolution enhancement technique is arranged to match the patterning possibilities of the lithographic apparatus LA. The computer system CL may also be used (e.g. using input from the metrology tool MET) to detect where within the process window the lithographic apparatus LA is currently operating, to predict whether defects due to, for example, suboptimal processing may be present (e.g., in the process window). Figure 3 is depicted by an arrow pointing to “0” in the second scale SC2).

[0042] The metrology tool MET may provide input to the computer system CL to enable accurate simulations and predictions, and may provide feedback to the lithographic apparatus LA to identify, for example, possible drifts in the calibration state of the lithographic apparatus LA (e.g., Figure 3 ) depicted by multiple arrows in the third scale SC3.

[0043] During photolithography, it is desirable to frequently take measurements of the resulting structures, for example for process control and verification. Various tools are known for taking these measurements, including scanning electron microscopes or various forms of metrology equipment, such as scatterometers.

[0044] Figure 4is a schematic diagram of a metrology apparatus described in WO2021028174A1. The apparatus includes a cantilever probe 12 configured to provide high spatial resolution information about features present beneath the outer surface of an entity to be investigated with minimal or no risk of damage to the entity. The cantilever probe 12 is provided as part of a measurement system 25. In some embodiments, examples of which are given below, the entity to be investigated is a target structure 19 formed on a substrate W using a photolithographic process. In such embodiments, the measurement system 25 may be referred to as a metrology tool.

[0045] In an embodiment, the cantilever probe 12 includes a cantilever 14 and a probe element 16. The probe element 16 extends from the cantilever 14 toward a target structure 19 on a substrate W (at Figure 4 In an embodiment, the cantilever 14 and the probe element 16 are configured (e.g., via their material properties and dimensions) to perform the function of a cantilever in a standard atomic force microscope. In an embodiment, either or both of the cantilever 14 and the probe element 16 are formed from silicon.

[0046] In an embodiment, the measurement system 25 is configured to generate ultrasonic waves in the cantilever probe 12. The ultrasonic waves propagate through the probe element 16 and into the target structure 19. The ultrasonic waves are reflected from the target structure 19 back into the probe element 16 or into an additional probe element 32 (described below) extending from the cantilever 14.

[0047] In an embodiment, the photoacoustic effect is used to generate ultrasound waves in the cantilever probe 12. In some embodiments of this type, the generation of ultrasound waves is performed by directing a laser beam onto the cantilever probe 12. Figure 4 In the example shown, the laser beam is directed onto the cantilever probe 12 by a first laser unit 26. The first laser unit 26 may be considered to form all or part of the ultrasound generating system.

[0048] The properties of the laser beam provided by the first laser unit 26 are not particularly limited, so long as the desired ultrasonic waves are generated. For example, the laser beam may comprise a femtosecond laser. In one embodiment, laser pulses of between 10 fs and 900 fs, or between 10 fs and 500 fs, between 100 fs and 500 fs, between 100 fs and 300 fs, or approximately 200 fs may be used. The laser pulses may have a pulse energy of, for example, between 1 nJ and 10 nJ (e.g., approximately 6 nJ). As a specific example, the pulses may have a peak power of 30 kW. The pulse repetition rate may be between 10 MHz and 100 MHz (e.g., approximately 50 MHz), and the average power may be between 100 mW and 1 W (e.g., approximately 300 mW).

[0049] In an embodiment, Figure 4 As illustrated in FIG, an ultrasound generating layer 18 is disposed on the cantilever 14. A laser beam from a first laser unit 26 is directed onto the ultrasound generating layer 18 to generate ultrasound waves in the ultrasound generating layer 18. In this type of embodiment, the combination of the first laser unit 26 and the ultrasound generating layer 18 can be considered to form all or part of the ultrasound generating system. In embodiments, the ultrasound generating layer 18 is configured to provide a higher per-unit-area absorbance relative to the laser beam from the first laser unit 26 than the per-unit-area absorbance of the cantilever 14 in the absence of the ultrasound generating layer 18. It is also desirable that the photoacoustic conversion efficiency associated with the absorption be high and stable. The ultrasound generating layer 18 desirably has a high per-unit-area absorbance at the frequency of the laser beam from the first laser unit 20 and a heat diffusion rate on the order of the laser pulse duration (e.g., on the order of femtoseconds) to achieve high photoacoustic conversion efficiency. In embodiments, the ultrasound generating layer 18 can include a metallic material such as aluminum, gold, or titanium. Alternatively or additionally, the ultrasound generating layer 18 can be configured to include a highly absorptive carbon-based material such as amorphous carbon. The ultrasound generating layer 18 may comprise a single layer having a uniform composition over the thickness of the single layer. Alternatively, the ultrasound generating layer 18 may comprise a composite layer having a plurality of individual layers. In an embodiment, at least a subset of the individual layers have different compositions relative to each other. It is also desirable to arrange for efficient transmission of the generated ultrasound waves, for example by avoiding excessive reflection within the ultrasound generating layer 18 and / or at the interface between the ultrasound generating layer 18 and the cantilever 14. This can be achieved by reducing the magnitude of the acoustic impedance mismatch at the interface. In an embodiment, an impedance matching layer is provided between the ultrasound generating layer 18 and the cantilever 14. The acoustic impedance of the impedance matching layer is between the acoustic impedance of the ultrasound generating layer 18 and the acoustic impedance of the cantilever 14.

[0050] In an embodiment, the composition and dimensions of the ultrasound generating layer 18 are selected so that at least a portion of the ultrasound waves generated in the ultrasound generating layer 18 have a frequency greater than 15 GHz, optionally greater than 50 GHz, and optionally greater than 100 GHz. Providing ultrasound waves in the range of 15 GHz to 50 GHz provides submicron resolution of spatial features within the target structure 19. Providing ultrasound waves having a frequency greater than 100 GHz (e.g., in the range of 100 GHz to 200 GHz) provides nanometer resolution of spatial features within the target structure 19. Providing ultrasound waves in an intermediate range of 50 GHz to 100 GHz provides intermediate resolution of spatial features.

[0051] The thickness of the ultrasound generating layer 18 can affect the frequency of the generated ultrasound. When the thickness of the ultrasound generating layer 18 is comparable to or smaller than the skin depth (relative to the laser beam from the first laser unit 26), the thickness t of the ultrasound generating layer 18 and the frequency f of the ultrasound wave can be related by f = ν / 2t, where ν is the speed of sound in the ultrasound generating layer 18. If the thickness t is greater than the skin depth, the skin depth acts as a bottleneck. The skin depth is determined by the complex refractive index of the ultrasound generating layer 18.

[0052] Based on the above, forming the ultrasound generating layer 18 from a uniform aluminum layer having a thickness of, for example, 30 nm or a uniform amorphous carbon layer having a thickness of 85 nm or less will be suitable for generating ultrasound waves with frequencies greater than 100 GHz. As explained above, the required thickness depends on the speed of sound in the ultrasound generating layer 18. With a higher speed of sound, higher-frequency ultrasound waves can be generated from the same thickness of material. However, increasing the speed of sound may also increase reflection losses within the ultrasound generating layer 18 and / or at the boundary (if any) between the ultrasound generating layer 18 and the cantilever 14. The thickness of the ultrasound generating layer 18 will typically be less than 500 nm, optionally less than 250 nm, optionally less than 100 nm, and optionally less than 50 nm.

[0053] In some embodiments, the shape of the ultrasound generating layer 18 is configured to alter the properties (e.g., frequency) of the generated ultrasound and / or enhance conversion efficiency. For example, the ultrasound generating layer 18 may include one or more patterns having features with length scales smaller than the wavelength of the laser beam from the first laser unit 26. In some embodiments, the ultrasound generating layer 18 may include one or more rings of material, optionally closed rings, or optionally concentric circles. Alternatively, the ultrasound generating layer 18 may be arranged in a checkerboard pattern. The detailed dimensions and / or shape of any ultrasound generating layer 18 configured in this manner can be derived from an analysis of the vibration modes of the ultrasound generating layer 18.

[0054] In an embodiment, an ultrasonic detection system is provided that detects reflected ultrasonic waves reflected from a target structure 19. In some embodiments, detecting the reflected ultrasonic waves includes detecting a change in the optical reflectivity of the cantilever probe 12. Figure 4 In the example of FIG. 1 , the ultrasonic detection system includes a second laser unit 20 and a photodetector 22. The second laser unit 20 directs a laser beam onto the cantilever probe 12. In the illustrated embodiment, the laser beam is directed onto the ultrasound generating layer 18. The laser beam is reflected off the cantilever probe 12 (e.g., from the ultrasound generating layer 18) and detected by the photodetector 22. A data processing system 24 is provided for determining information about the target structure 19 from the detected reflected ultrasound waves.

[0055] In an embodiment, signal acquisition is performed in a pulse-echo imaging mode that switches between a transmit mode and a receive mode. In the transmit mode, the first laser unit 26 generates ultrasonic waves in the cantilever probe 12. The generated ultrasonic waves are transmitted to the target structure 19 through contact between the cantilever probe 12 and the target structure 19 (e.g., via the probe element 16 of the cantilever probe 12). In the receive mode, the second laser unit 20 detects the reflectivity of the cantilever probe 12 (e.g., by directing a laser beam onto the ultrasound generating layer 18, where it is reflected and detected by the photodetector 22). This approach makes it easy to distinguish between light reflected from the first laser unit 26 and light reflected from the second laser unit 20, as the first and second laser units 26 and 22 operate at different times. However, it is also possible to transmit and receive ultrasonic waves continuously, using other techniques to distinguish between reflections from the first laser unit 26 and reflections from the second laser unit 20. For example, the data processing system 24 may be configured to utilize a lock-in amplifier or similar technique to exploit the frequency and / or phase difference between the laser beams from the first laser unit 26 and the second laser unit 20. Figure 4 In the embodiment shown in FIG, the first laser unit 26 and the second laser unit 20 are separate devices. This is advantageous because the characteristics required for the laser beam of the first laser unit 26 (e.g., high power) are generally different from the characteristics required for the laser beam of the second laser unit 20 (e.g., low power). However, in other embodiments, the first laser unit 26 and the second laser unit 20 may be provided by a single unit that generates laser light for both generating ultrasonic waves and detecting changes in optical reflectivity that contain information about the ultrasonic waves reflected back from the target structure 19.

[0056] In an embodiment, the ultrasonic detection system (eg, the second laser unit 20 and the photodetector 22) is further configured to measure the deflection of the cantilever probe 12. This can be achieved, for example, by monitoring the change in the position of the reflected radiation spot on the photodetector 22.

[0057] The above embodiments provide several advantages compared to alternative approaches of generating ultrasound waves directly within the target structure 19 (eg by actuators on the back side of the target structure 19 opposite the AFM cantilever).

[0058] Generating ultrasonic waves directly in the target structure 19 may result in errors caused by material-related factors of the target structure 19. In addition, the highest achievable acoustic frequency is limited by the material properties of the target structure 19. Furthermore, the target structure 19 may be damaged by the process of generating the ultrasonic waves, which effectively limits the maximum power that can be used. Generating ultrasonic waves in the cantilever 12 makes it possible to avoid these problems, thereby providing improved accuracy, improved spatial resolution, lower risk of damage to the target structure 19 through higher ultrasonic frequencies, and / or higher input power without excessive risk of damage.

[0059] Figure 5 The probe element 16 is depicted operating in a transmit mode, wherein generated ultrasonic waves 28 enter the probe element 16 from the cantilever 14 (not shown) and propagate downwardly through the probe element 16 . Figure 6 Depicts Figure 5 Probe element 16 is shown operating in receive mode, where reflected ultrasonic waves propagate upward through probe element 16 and exit probe element 16 (arrow 30) into cantilever 14 (not shown). Probe element 16 is an example of a probe element that is tapered or conical, having a cross-sectional area that decreases toward target structure 19 (i.e., downward). In this particular example, the taper or taper extends throughout the vertical length of probe element 16. The cross-sectional shape is not particularly limited but, for example, can be approximately circular, such that the tapered or tapered portion of probe element 16 is conical. The tapered or tapered form serves to focus ultrasonic waves 28 toward target structure 19. However, the tapered or tapered form can also serve to defocus reflected ultrasonic waves, making their detection more challenging. In an embodiment, probe element 16 is formed from alternating layers of materials having a high acoustic refractive index and a low acoustic refractive index. The high acoustic refractive index material is, for example, silicon. The low acoustic refractive index material can be, for example, air or PMMA. When formed from alternating layers of materials having high and low acoustic refractive indices, the acoustic properties of element 16 may be further controlled or improved by adjusting the pitch of the alternating layers or the fill ratio or thickness of each layer.

[0060] Further embodiments are disclosed and described in WO2021028174A1, many of which are applicable to the concepts disclosed herein; for example providing corresponding separate probe elements for directing ultrasound waves into a target structure and receiving reflected ultrasound waves.

[0061] The focusing provided by the tapered or conical form of the probe element 16 causes the propagation of ultrasound waves in the target structure 19 to be similar to that from a point source. The three-dimensional radiation from a point source results in an intensity that is proportional to r 2 where r is the launch radius. Therefore, the intensity reaching the bottom of the target structure 19 is reduced by a factor t 2, where t is the thickness of the target structure 19. Therefore, without countermeasures, the reflection intensity reaching the top of the target structure 19 will be reduced by a factor t compared to the transmission intensity of the ultrasound at the tip of the probe element 16. 4 .

[0062] To address this issue, it is recommended to configure the target being measured so that ultrasonic waves reflected from the target are better directed toward a probe element (e.g., probe element 16 or, if separate transmit and receive probe elements are provided), a separate receive probe element. For example, the target can be configured to focus these reflected waves toward the probe element. This can be achieved by configuring the buried substructure of the target (or its multiple sub-targets) as a zone plate substructure (or Fresnel lens substructure).

[0063] Zone plates or Fresnel lenses are well-known concepts that can be used to focus waves, such as electromagnetic waves and / or acoustic waves. While the spacing or pitch of structures in a zone plate substructure can be determined according to the equation below, wave focusing can be achieved by providing substructures with a non-constant or varying pitch (in one, two, or more directions in the substrate plane). Therefore, the term "zone plate" as described herein should be understood to mean any substructure having a non-constant pitch in at least one direction, the varying pitch having the effect that reflected waves reflected from the substructure are at least partially focused toward a focal point (e.g., at a receiver or receiving probe element (which can be the same physical probe as the transmitting probe element or a different probe element)). Note that the focal point in this context can be interpreted as the point at which the image imaged by the zone plate structure is in focus, as distinct from the point at a distance from the lens defined by the focal length.

[0064] In an embodiment, the proposed target may comprise a top substructure or a (e.g., single) reference substructure. The reference substructure may comprise, for example, a single line or feature substructure, such as a "positive line" or positive feature (i.e., a line formed on an immediately underlying layer (in the resist)), or a "negative line" or negative feature (i.e., a line formed by a groove or trench within a layer formed on an immediately underlying layer (in the resist)). However, other shapes or arrangements of the top substructure are possible, including conventional gratings.

[0065] The basic concept of performing measurements using this target involves first determining the position of a reference or top substructure. This can be accomplished using conventional atomic force microscopy (AFM) measurements, for example, with the probe in contact with the reference substructure. PassAFM techniques can then be used to measure a zone plate substructure or a buried substructure (e.g., within a buried layer), where the received signal is stronger than with prior art methods due to the focusing effect of the zone plate substructure. This measurement of the zone plate substructure can include determining the intensity of the received signal, where the received signal is at peak intensity when the probe is centered on the zone plate substructure (i.e., when the probe is at or closest to the focus of the zone plate). The strength of the received signal (e.g., a strength metric such as intensity) thus indicates the position of the zone plate substructure relative to the probe. This means that the position of the zone plate substructure relative to the (conventionally measured) position of the reference substructure can be determined; such corresponding positions of one substructure relative to another include overlap.

[0066] Figure 7 schematically illustrates a cross-section of such a target according to two described examples. Figure 7(a) is a cross-section of a stack comprising a reference substructure 700 in a top layer, one or more intermediate layers 710, and a zone plate substructure 720 in a buried layer. In this example, reference substructure 700 comprises a single reference feature or line feature, more specifically, a negative line (i.e., a groove or trench feature in a resist layer) as already described. Zone plate substructure 720 comprises a zone plate arrangement, i.e., a substructure comprising varying pitches that provides a focusing effect on reflected waves from zone plate substructure 720 toward probe element 730 (which can be used for both transmission and reception). Figure 7(b) shows the same arrangement, but with reference subtarget 740 being a positive line (i.e., a feature formed from photoresist or a single line of resist).

[0067] Methods for determining the overlap from such a target as disclosed will be described. These methods are generally similar to the methods disclosed in the aforementioned WO2021028174A1. The information received from the signal after reflection from the target may include information about the overlap between the reference substructure and the zone plate substructure. The overlap represents the degree of mismatch between the reference substructure and the zone plate substructure. As mentioned in the introductory part of the specification, it is difficult or impossible to measure the overlap in targets with a relatively small pitch (e.g., about 10 nm) and a relatively large spacing between overlapping layers (e.g., greater than 100 nm) and / or in the presence of optically opaque material layers using existing techniques.

[0068] In some embodiments, overlay information can be obtained by detecting reflected ultrasonic waves at a measurement position of the cantilever relative to the target. In some embodiments, the measurement position is selected by using the cantilever probe in standard atomic force microscopy mode to detect the position of a reference substructure (e.g., a positive line, a negative line, or other reference feature in the reference substructure). For example, in some embodiments, the profile of the reference substructure is obtained by measuring the deflection of the cantilever caused by the interaction (e.g., contact) between the probe element and the reference substructure, and information about the zone plate substructure (e.g., overlay relative to the reference substructure or another reference) is obtained from the detected reflected ultrasonic waves.

[0069] The reflected ultrasonic wave is detected at the measurement position. At the measurement position, the probe element is aligned with the peak of the reference substructure. The alignment of the probe element with the reference substructure is determined by measuring the deflection of the cantilever caused by the interaction between the probe element and the reference substructure (e.g., contact mode, tapping mode, non-contact mode, etc.) (e.g., using standard atomic force microscopy mode).

[0070] To convert the measurement signal into an overlay value, an intentionally applied overlay bias can be used to determine the overlay. As mentioned above, the intensity of the reflected ultrasound measured at any position of the probe element aligned with a peak in the reference substructure varies as a function of the overlay according to a cosine function.

[0071] Overlap is typically a relatively small value, which means that the variation of intensity with overlap is nonlinear and relatively weak (i.e., near the peak of the cosine (or sine) function). In an embodiment, the variation of signal amplitude (e.g., intensity) with overlap is improved by using a target that includes multiple sub-targets, where one or more of the sub-targets includes an intentional overlap bias applied to them.

[0072] FIG8 illustrates an embodiment in which the target includes a first sub-target ( FIG8( a) ) and a second sub-target ( FIG8( b) ). In this scenario, the overlap biases −b, +b may comprise intentional displacements of a reference point 860 (e.g., the center) of a reference substructure 800 relative to a reference point 850 (e.g., the center) of a central feature 870 of a zone plate substructure 820. The first sub-target and the second sub-target may be positioned closely adjacent to each other on the substrate, for example, directly adjacent to each other. The overlap bias of the first sub-target differs from the overlap bias of the second sub-target. In the specific example shown, these biases are equal in magnitude and opposite in direction, but this is not required. This difference shifts the cosine variation mentioned above and improves the intensity variation of the overlap. For example, by carefully selecting the bias amplitude, the overlap-related cosine dependence of the measured signal amplitude (e.g., intensity) can be converted to a sine dependence, thereby providing a linear and relatively steep overlap-related variation in intensity.

[0073] In such an embodiment, the intensity difference (or measured signal amplitude difference) ,in are overlapping and is a proportionality constant that can be pre-derived using calibration measurements. For example, the constant can be determined using a set of calibration targets with a set of programmed bias values ​​(e.g., ranging from positive bias values ​​to negative bias values). Once the value is learned through calibration , only two sub-targets (e.g., with a first offset of +b and a second offset of -b) are needed to determine overlap. Therefore, overlap can be determined based on the difference between the intensity of the reflected ultrasonic wave detected from the zone plate sub-target of the first sub-target when the probe element is aligned with the reference substructure of the first sub-target, and the intensity of the reflected ultrasonic wave detected from the zone plate sub-target of the second sub-target when the probe element is aligned with the reference substructure of the second sub-target.

[0074] In some embodiments, the need for calibration measurements can be reduced or eliminated by using additional sub-targets (e.g., in a manner similar to that described in WO2021028174A1). In one such embodiment, four such sub-targets can be used, for example, a first sub-target, a second sub-target, a third sub-target, and a fourth sub-target. The first, second, third, and fourth sub-targets can each include two sub-structures, with an intentional overlap bias applied between the two sub-structures, as described above. The overlap biases for the first, second, third, and fourth sub-targets are all different. The overlap bias for the first sub-target can be -a+b. The overlap bias for the second sub-target can be +a+b. The overlap bias for the third sub-target can be -ab. The overlap bias for the fourth sub-target can be +ab, where a and b are constants. Overlap can be determined based on a combination of the measured signal amplitudes or intensities from each of these sub-targets (e.g., the intensity of the detected reflected ultrasound waves from each corresponding sub-target when the probe element is aligned with the reference sub-structure of each corresponding sub-target). More specifically, it can be understood that:

[0075]

[0076] And therefore, the overlap Can be identified as:

[0077]

[0078] Where I1, I2, I3, and I4 are the measured intensities of the first sub-goal, the second sub-goal, the third sub-goal, and the fourth sub-goal, respectively.

[0079] The zone plate substructure may comprise any suitable form, including a linear or one-dimensional zone plate structure, a two-dimensional (e.g., rectangular) zone plate structure, or a radial zone plate structure. The spacing of the elements of the zone plate structure may comprise any (non-constant pitch) spacing that provides a focusing effect.

[0080] The function of the metasurface structure 720 is not only to focus the waves generally on the tip of the AFM cantilever, but also to do so effectively for waves generated by point / local sources on the target surface. Thus, if the focal length of the Fresnel lens is f, then , where s0 = s i , and thus the distance d between the cantilever tip and the metasurface structure can be approximated as , where F is the focal length of the zone plate structure.

[0081] The focal length F of the zone plate substructure is related to the wavelength λ of the acoustic wave, the distance d from the source to the zone plate, and the pitch of the structure in the lens. The mathematical relationship between these can be expressed as follows:

[0082]

[0083] where r n is the zone plate substructure position (distance from the center substructure) or radius, where n is an integer (e.g., such that the first feature is r1 from the center, the next feature is r2, etc.). Based on this, in the embodiment, and assuming , since the goal is to focus the wave back to the source position (the tip of the cantilever) in reflection mode, the wave zone plate substructure feature position or radius within the proposed target can be determined according to the above equation.

[0084] In general, the zone plate size involves a trade-off between depth of focus, minimum printable CD (e.g., 50 nm, but this can be smaller using patterning techniques), and total target size (which can be fixed). As a specific example, a radial zone plate with a radius of approximately 5 μm and a minimum CD of 50 nm can achieve a depth of focus of 4 μm. For thick stacks such as 3D NAND, the depth of focus can be approximately 10 μm, resulting in a minimum CD of approximately 125 nm.

[0085] FIG9 is a schematic diagram, viewed from above, of two alternative reference substructure layers for a target arrangement comprising two offset subtargets +x, −x in a first direction and two offset subtargets −y, +y in a second direction. In this embodiment, the reference substructures are all of negative linear type. FIG9( b) shows a reference substructure layer for the same target arrangement, i.e., comprising two offset subtargets 920 in a first direction and two offset subtargets 930 in a second direction, but wherein the reference substructures are all of positive linear type. In embodiments using four offsets as already described, each of these target arrangements can include four subtargets per direction.

[0086] FIG10 shows an alternative zone plate substructure layer for a target arrangement including two offset subtargets +x, -x in a first direction and two offset subtargets -y, +y in a second direction. FIG10( a ) shows a linear zone plate substructure, FIG10( b ) shows a rectangular zone plate substructure, and FIG10( c ) shows a circular zone plate substructure. In an embodiment using four offsets as already described, each of these target arrangements can include four subtargets per direction.

[0087] It will be appreciated that the overlapping target arrangement may include any combination of the reference substructure layer depicted in FIG. 9 (or other reference substructure layers) and the zone plate substructure layer depicted in FIG. 10 (or other zone plate substructure layers).

[0088] Further embodiment clauses according to the present invention are described in the following numbered clauses:

[0089] 1. A substrate comprising at least one target, the target comprising: a reference substructure in a top layer; and a buried substructure in a buried layer located below the top layer, wherein the buried substructure comprises a non-constant pitch in at least one direction, the non-constant pitch causing a reflected wave reflected from the substructure due to measurement of the target to be at least partially focused toward a focal point.

[0090] 2. The substrate of clause 1 , wherein the reference substructure comprises a single reference feature.

[0091] 3. The substrate of clause 2, wherein the single reference feature comprises a single line feature.

[0092] 4. The substrate of clause 2 or 3, wherein the single reference feature comprises a feature formed of photoresist.

[0093] 5. The substrate of clause 2 or 3, wherein the single reference feature comprises a groove feature formed in a photoresist layer.

[0094] 6. A substrate according to any preceding clause, wherein the buried substructure comprises a zone plate substructure.

[0095] 7. The substrate of clause 6, wherein the zone plate substructure comprises a linear zone plate substructure.

[0096] 8. The substrate of clause 6, wherein the zone plate substructure comprises a rectangular zone plate substructure.

[0097] 9. The substrate of clause 6, wherein the zone plate substructure comprises a circular zone plate substructure.

[0098] 10. A substrate according to any of the preceding items, wherein the target includes a plurality of sub-targets, each sub-target including a corresponding reference substructure and a corresponding buried substructure; wherein the plurality of sub-targets includes at least a first group of sub-targets, the first group of sub-targets including at least a sub-target including a first bias and a sub-target including a second bias, each of the biases including a position bias between the reference substructure and the buried substructure.

[0099] 11. The substrate of clause 10, wherein the first set of sub-targets comprises at least a sub-target comprising a first bias, a sub-target comprising a second bias, a sub-target comprising a third bias, and a sub-target comprising a fourth bias.

[0100] 12. The substrate of clause 11, wherein the first bias is -a+b, the second bias is +a+b, the third bias is -ab, and the fourth bias is +ab, where a and b are constants.

[0101] 13. A substrate according to item 10, 11 or 12, wherein the plurality of sub-targets include the first group of sub-targets aligned with a first direction of the substrate plane and a corresponding second group of sub-targets aligned with a second direction of the substrate plane, the first direction being perpendicular to the second direction.

[0102] 14. A method of measuring a parameter value of interest by measuring a target, the target comprising: a reference substructure in a top layer; and a buried substructure in a buried layer below the top layer, wherein the buried substructure comprises a non-constant pitch in at least one direction, the non-constant pitch causing reflected waves reflected from the substructure due to measurement of the target to be at least partially focused toward a focal point; the method comprising:

[0103] a) determining the position of the reference structure by atomic force microscopy;

[0104] b) generating an acoustic wave at the location of the reference structure determined in step a) and directing the acoustic wave towards the buried substructure;

[0105] c) measuring a measure of the intensity of the reflected wave which has been reflected from the buried substructure as a result of step b) at the position of the reference structure determined at step a); and

[0106] d) determining a parameter value of interest based on the intensity measure value.

[0107] 15. The method of clause 14, wherein the strength measure of the reflected wave comprises an intensity value of the reflected wave.

[0108] 16. A method according to clause 14 or 15, wherein the parameter value of interest is an overlap value.

[0109] 17. A method according to any of clauses 14 to 16, wherein the target comprises a plurality of sub-targets, each sub-target comprising a corresponding reference substructure and a corresponding buried substructure; wherein the plurality of sub-targets comprises at least a first group of sub-targets, the first group of sub-targets comprising at least a sub-target comprising a first offset and a sub-target comprising a second offset, each of the offsets comprising a positional offset between the reference substructure and the buried substructure; and the method further comprises:

[0110] Perform steps a) to c) for each sub-goal to obtain a corresponding strength measurement value for each sub-goal; and

[0111] One or more parameter values ​​of interest are determined based on the strength metric value.

[0112] 18. The method of clause 17, wherein the method comprises:

[0113] obtaining a calibrated proportionality constant relating the parameter of interest to the intensity metric; and

[0114] The one or more parameter values ​​of interest are determined based on the intensity metric value and the calibrated constant of proportionality.

[0115] 19. The method of clause 17, wherein the first set of sub-goals comprises at least a sub-goal including a first bias, a sub-goal including a second bias, a sub-goal including a third bias, and a sub-goal including a fourth bias; and

[0116] Determining one or more parameter values ​​of interest based on the intensity metric value includes determining the one or more parameter values ​​of interest without requiring a calibrated constant of proportionality.

[0117] 20. The method of clause 19, wherein the first bias is -a+b, the second bias is +a+b, the third bias is -ab, and the fourth bias is +ab, where a and b are constants.

[0118] 21. A method according to item 18, 19 or 20, wherein the multiple sub-targets include a first group of sub-targets aligned with a first direction of a method plane and a corresponding second group of sub-targets aligned with a second direction of the method plane, the first direction being perpendicular to the second direction; and the method includes determining a first parameter value of interest from the first group of sub-targets and determining a second parameter value of interest from the second group of sub-targets.

[0119] 22. A method according to any of clauses 14 to 21, wherein the reference substructure comprises a single reference feature.

[0120] 23. A method according to clause 22, wherein the single reference feature comprises a single line feature.

[0121] 24. The method of clause 22 or 23, wherein the single reference feature comprises a feature formed from photoresist.

[0122] 25. The method of clause 22 or 23, wherein the single reference feature comprises a groove feature formed in a photoresist layer.

[0123] 26. A method according to any of clauses 14 to 25, wherein the buried substructure comprises a zone plate substructure.

[0124] 27. The method of clause 26, wherein the zone plate substructure comprises a linear zone plate substructure.

[0125] 28. The method of clause 26, wherein the zone plate substructure comprises a rectangular zone plate substructure.

[0126] 29. The method of clause 26, wherein the zone plate substructure comprises a circular zone plate substructure.

[0127] 30. A method according to any of clauses 14 to 29, wherein measuring an intensity measure of the reflected wave at step c) comprises measuring an intensity measure of the reflected wave at the focal point.

[0128] 31. The method of any of clauses 14 to 30, wherein generating the acoustic wave at step b) comprises generating the acoustic wave at the focus.

[0129] 32. A metrology device operable to perform the method according to any one of claims 17 to 31.

[0130] 33. The measurement device of clause 32, comprising a photoacoustic subsurface atomic force microscopy device.

[0131] 34. A mask set for manufacturing the substrate according to any one of claims 1 to 16, the mask set comprising a first mask for forming the buried layer and a second mask for forming the top layer, so as to form the target on the substrate.

[0132] Although specific reference may be made herein to the use of lithographic apparatus in IC manufacturing, it should be understood that the lithographic apparatus described herein may have other applications. Possible other applications include the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, flat panel displays, liquid crystal displays (LCDs), thin film magnetic heads, and the like.

[0133] Although specific reference is made herein to embodiments of the present invention in the context of detection or metrology equipment, embodiments of the present invention may be used in other equipment. Embodiments of the present invention may form part of mask inspection equipment, lithography equipment, or any equipment that processes objects such as wafers (or other substrates) or masks (or other patterning devices). The term "metrology equipment" may also refer to inspection equipment or inspection systems. For example, inspection equipment including embodiments of the present invention may be used to detect defects in a substrate or defects in a structure on a substrate. In such embodiments, the characteristic of interest of a structure on a substrate may relate to a defect in the structure, the absence of a particular portion of the structure, or the presence of an unwanted structure on the substrate.

[0134] Although specific reference is made to "measurement equipment / tools / systems" or "inspection equipment / tools / systems," these terms may refer to the same or similar types of tools, equipment, or systems. For example, an inspection or measurement equipment including embodiments of the present invention may be used to determine characteristics of structures on a substrate or on a wafer. For example, an inspection or measurement equipment including embodiments of the present invention may be used to detect defects in a substrate or defects in a structure on a substrate or on a wafer. In such embodiments, the characteristic of interest in a structure on a substrate may relate to a defect in the structure, the absence of a particular portion of the structure, or the presence of an unwanted structure on the substrate or on the wafer.

[0135] Although specific reference has been made above to the use of embodiments of the invention in the context of optical lithography, it will be appreciated that the invention is not limited to optical lithography and may be used in other applications, such as imprint lithography, where the context permits.

[0136] While the targets or target structures (more generally, structures on a substrate) described above are metrology target structures specifically designed and formed for the purpose of measurement, in other embodiments, the property of interest may be measured on one or more structures that are functional components of a device formed on a substrate. Many devices have regular grating-like structures. As used herein, the terms "structure," "target grating," and "target structure" do not require that the structure be specifically provided for the measurement being performed.

[0137] Although specific embodiments of the present invention have been described above, it will be understood that the present invention may be practiced in ways other than those described. The above description is intended to be illustrative and not restrictive. Therefore, those skilled in the art will appreciate that modifications may be made to the present invention as described without departing from the scope of the claims set forth below.

Claims

1. A substrate comprising at least one target, wherein the target comprises: Reference substructures in the top level; and a buried substructure in a buried layer below the top layer, wherein the buried substructure comprises a non-constant pitch in at least one direction, the non-constant pitch causing reflected waves reflected from the substructure due to measurement of the target to be at least partially focused toward a focal point.

2. The substrate according to claim 1, wherein The buried substructure includes a zone plate substructure.

3. The substrate according to claim 2, wherein The zone plate substructure includes a linear zone plate substructure.

4. The substrate according to claim 2, wherein The zone plate substructure includes a rectangular zone plate substructure.

5. The substrate according to claim 2, wherein The zone plate substructure includes a circular zone plate substructure.

6. A substrate according to any preceding claim, wherein The target includes multiple sub-targets, each sub-target includes the corresponding reference substructure and the corresponding buried substructure; wherein the multiple sub-targets include at least a first group of sub-targets, the first group of sub-targets includes at least a sub-target including a first offset and a sub-target including a second offset, each of the offsets including a position offset between the reference substructure and the buried substructure.

7. A method for measuring a parameter value of interest by measuring a target, the target comprising: Reference substructures in the top level; and a buried substructure in a buried layer below the top layer, wherein the buried substructure includes a non-constant pitch in at least one direction, the non-constant pitch causing reflected waves reflected from the substructure due to measurement of the target to be focused at least partially toward a focal point; The method comprises: a) determining the position of the reference structure via atomic force microscopy; b) generating an acoustic wave at the location of the reference structure determined in step a) and directing the acoustic wave towards the buried substructure; c) measuring an intensity measure of a reflected wave which has been reflected from the buried substructure as a result of step b) at the position of the reference structure determined at step a); and d) determining a parameter value of interest based on the intensity measure value.

8. The method according to claim 7, wherein: The intensity measure value of the reflected wave includes an intensity value of the reflected wave.

9. The method according to claim 7 or 8, wherein The parameter value of interest is the overlap value.

10. The method according to any one of claims 7 to 9, wherein: The target includes a plurality of sub-targets, each sub-target including a corresponding reference substructure and a corresponding buried substructure; wherein the plurality of sub-targets includes at least a first group of sub-targets, the first group of sub-targets includes at least a sub-target including a first offset and a sub-target including a second offset, each of the offsets including a position offset between the reference substructure and the buried substructure; And the method further includes: Perform steps a) to c) for each sub-goal to obtain a corresponding strength measurement value for each sub-goal; and One or more parameter values ​​of interest are determined based on the strength metric value.

11. The method according to any one of claims 7 to 10, wherein: The measuring of the intensity metric value of the reflected wave at step c) includes measuring the intensity metric value of the reflected wave at the focus.

12. The method according to any one of claims 7 to 11, wherein: The generating of the acoustic wave at step b) comprises generating the acoustic wave at the focus.

13. A metrology device operable to perform the method according to any one of claims 7 to 12.

14. The measurement device according to claim 13, comprising a photoacoustic subsurface atomic force microscopy device. 15 . A mask set for manufacturing the substrate according to claim 1 , the mask set comprising a first mask for forming the buried layer and a second mask for forming the top layer, so as to form the target on the substrate.

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