Method and metrology tool and cantilever probe for determining information about a target structure

Abstract

The present disclosure relates to determining information related to a target structure formed on a substrate using a lithography process. In one arrangement, a cantilever probe having a cantilever and a probe element is provided. The probe element extends from the cantilever towards the target structure. Ultrasonic waves are generated in the cantilever probe. The ultrasonic waves propagate through the probe element into the target structure and are reflected back from the target structure into the probe element or into an additional probe element extending from the cantilever. The reflected ultrasonic waves are detected and used to determine information related to the target structure.

Description

Technical Field

[0001] The present invention relates to determining information related to structures formed on a substrate using a lithography process, in particular information obtained using a cantilever probe and ultrasound. Background Art

[0002] A lithographic apparatus is a machine configured to apply a desired pattern onto a substrate. For example, a lithographic apparatus can be used in the manufacture of integrated circuits (ICs). A lithographic apparatus can, for example, project a pattern of a patterning device (e.g., a mask), also often referred to as a "design layout" or "design", onto a layer of radiation-sensitive material (resist) disposed on a substrate (e.g., a wafer).

[0003] As semiconductor manufacturing processes have continued to progress, over the decades, the dimensions of circuit elements have become smaller and the amount of functional elements (such as transistors) per device has increased, following a trend commonly known as "Moore's Law". To keep up with Moore's Law, the semiconductor industry is continuously seeking technologies that can produce smaller features. To project a pattern onto a substrate, a lithographic apparatus can use electromagnetic radiation. The wavelength of this radiation determines the minimum size of the features patterned on the substrate. Typical wavelengths currently in use are 365 nm (i-line), 248 nm, 193 nm, and 13.5 nm. A lithographic apparatus using extreme ultraviolet (EUV) radiation (having a wavelength in the range of 4 - 20 nm, e.g., 6.7 nm or 13.5 nm) can be used to form smaller features on a substrate compared to a lithographic apparatus using radiation having, for example, a wavelength of 193 nm.

[0004] Metrology tools are used in many aspects of the IC manufacturing process and include scatterometer-based tools for inspecting / measuring products that have been exposed and / or etched in process control; for example, for measuring overlay.

[0005] Metrology tools are known that can measure the overlay in target structures having a pitch as low as about 10 nm when the separation between overlay layers (e.g., gratings formed in different layers) has a similar size order of magnitude. It is also known that metrology tools can measure the overlay between more widely spaced overlay layers, but only if the pitch of the target structure is correspondingly larger. It is difficult to measure the overlay in target structures having a relatively small pitch (e.g., about 10 nm) and a relatively large separation between overlay layers (e.g., greater than 100 nm).

[0006] Another challenge is the increased use of material layers 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 located beneath such opaque layers may be inaccessible to many existing scatterometer-based metrology techniques. SUMMARY OF THE INVENTION

[0007] The object of the present invention is to provide alternative or improved measurement techniques, for example for increasing the range of situations in which measurements can be effectively performed.

[0008] According to one aspect, there is provided a method for determining information related to a target structure formed on a substrate using a lithography process, comprising: providing a cantilever probe comprising a cantilever and a probe element, the probe element extending from the cantilever towards the target structure; generating ultrasonic waves in the cantilever probe, the ultrasonic waves propagating through the probe element into the target structure and reflecting back from the target structure into the probe element or into another probe element extending from the cantilever; and detecting the reflected ultrasonic waves and determining information related to the target structure based on the detected reflected ultrasonic waves.

[0009] According to one aspect, there is provided a metrology tool for determining information related to a target structure formed on a substrate using a lithography process, the metrology tool comprising: a cantilever probe having a cantilever and a probe element, the probe element being configured to extend from the cantilever towards the target structure; an ultrasonic wave generating system configured to generate ultrasonic waves in the cantilever probe such that the ultrasonic waves propagate through the probe element into the target structure and reflect back from the target structure into the probe element or into another probe element extending from the cantilever; and an ultrasonic wave detecting system configured to detect the reflected ultrasonic waves.

[0010] According to one aspect, there is provided a cantilever probe for determining information related to a target structure formed on a substrate using a lithography process, the cantilever probe comprising: a cantilever; a probe element configured to extend from the cantilever towards the target structure; and another probe element configured to extend from the cantilever towards the target structure, wherein: at least a portion of the probe element tapers to have a cross-sectional area decreasing towards the target structure; and at least a portion of the other probe element tapers to have a cross-sectional area increasing towards the target structure.

[0011] According to one aspect, there is provided a cantilever probe for determining information related to a target structure formed on a substrate using a lithography process, the cantilever probe comprising: a cantilever; and a probe element configured to extend from the cantilever towards the target structure, wherein: the probe element comprises a first portion and a second portion, the first portion and the second portion being formed of different materials; the first portion comprises an outer sheath region and the second portion comprises a central region located inside the outer sheath region, the central region and the outer sheath region being configured to act as waveguides to enable ultrasonic waves to propagate from the target structure through the probe element towards the cantilever.

[0012] According to one aspect, there is provided a cantilever probe for determining information related to a target structure formed on a substrate using a lithography process, the cantilever probe comprising: a cantilever; and a probe element configured to extend from the cantilever towards the target structure, wherein: the probe element comprises a longitudinally proximal portion and a longitudinally distal portion; the longitudinally proximal portion is connected to the cantilever and extends from the cantilever to the longitudinally distal portion; and the longitudinally distal portion is configured to extend from the longitudinally proximal portion towards the target structure, wherein: the longitudinally distal portion tapers to have a decreasing cross-sectional area towards the target structure, and the longitudinally proximal portion comprises at least a portion that does not taper or comprises at least a portion that tapers to have a decreasing cross-sectional area towards the cantilever. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying schematic drawings, in which like reference numerals represent like elements, and in which:

[0014] Figure 1 A schematic representation depicting overall lithography, showing the collaboration between three techniques for optimizing semiconductor manufacturing;

[0015] Figure 2 A schematic overview depicting a scatterometer device used as a metrology tool;

[0016] Figure 3 is a schematic side view of a metrology tool having a cantilever probe, an ultrasonic generation system, and an ultrasonic detection system;

[0017] Figure 4 is a schematic side cross-sectional view of the probe element in a transmission mode;

[0018] Figure 5 is in a reception mode Figure 4 of the probe element;

[0019] Figure 6is a schematic side cross-sectional view of a probe element and an additional probe element;

[0020] Figure 7 is a schematic side cross-sectional view of a probe element including a first part and a second part made of different materials;

[0021] Figure 8 is a schematic side cross-sectional view of an exemplary probe element having a longitudinal proximal portion and a longitudinal distal portion, wherein the longitudinal proximal portion tapers both towards and away from the cantilever;

[0022] Figure 9 is a schematic side cross-sectional view of an exemplary probe element having a longitudinal proximal portion and a longitudinal distal portion, wherein the longitudinal proximal portion tapers towards the cantilever;

[0023] Figure 10 is a schematic side cross-sectional view of an exemplary probe element having a longitudinal proximal portion and a longitudinal distal portion, wherein the longitudinal proximal portion has no taper, but there is a cross-sectional discontinuity between the longitudinal proximal portion and the longitudinal distal portion;

[0024] Figure 11 is a schematic side cross-sectional view of an exemplary probe element having a longitudinal proximal portion and a longitudinal distal portion, wherein the longitudinal proximal portion has no taper and there is no cross-sectional discontinuity between the longitudinal proximal portion and the longitudinal distal portion;

[0025] Figure 12 is a schematic side cross-sectional view depicting a probe element aligned with the peak of a first sub-structure;

[0026] Figure 13 depicts the variation of the measured intensity I as a function of the overlap ov when the probe element is aligned as shown in Figure 12 ;

[0027] Figure 14 is a schematic side cross-sectional view of a target structure including sub-structures with different pitches, and the resulting measurement for determining the Moore intensity pattern of the overlap;

[0028] Figure 15 is a schematic side cross-sectional view of a target structure including four sub-targets with different intentionally applied biases;

[0029] Figure 16 is a schematic side cross-sectional view of a target structure, wherein a first sub-structure is laterally displaced relative to a second sub-structure such that at least a portion of the second sub-structure does not overlap with any of the first sub-structures when viewed in a plane perpendicular to the substrate;

[0030] Figure 17 Depicting a lithographic system;

[0031] Figure 18 Is a schematic perspective view of a cantilever probe including a plurality of cantilevers;

[0032] Figure 19 Depicting ultrasonic waves generated in a cantilever probe of the type depicted in Figure 18 by irradiating the cantilever with structured illumination formed by diffraction;

[0033] Figure 20 Depicting ultrasonic waves generated in a cantilever probe of the type depicted in Figure 18 by irradiating the cantilever with structured illumination formed by an array of individually controllable elements;

[0034] Figure 21 Depicting reflected ultrasonic waves detected in a cantilever probe of the type depicted in Figure 18 by irradiating the cantilever with structured illumination formed by an array of individually controllable elements;

[0035] Figure 22 Depicting the detection of reflected ultrasonic waves in a cantilever probe of the type depicted in Figure 18 using a probe radiation beam focused to a line focus intersecting a plurality of cantilevers of the cantilever probe. Detailed Description

[0036] The patterning process performed by the lithographic apparatus LA during a lithography process requires high accuracy in the dimensions and the arrangement of structures on a substrate. To ensure such high accuracy, three systems can be combined into a so-called "integrated" control environment, as Figure 1 schematically depicted. In the example shown, one of these systems is the lithographic apparatus LA, which is (virtually) connected to a metrology tool MT (second system) and to a computer system CL (third system). The goal of such an "integrated" environment is to optimize the cooperation between these three systems to increase the overall process window and to provide a well-defined control loop that ensures that the patterning performed by the lithographic 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 yields a defined result (e.g., a functional semiconductor device) - typically the process parameters in the lithography process or patterning process are allowed to vary within the defined result.

[0037] The computer system CL may use the (portion of the) design layout to be patterned to predict which resolution enhancement techniques to use and perform computational lithography simulations and calculations to determine which mask layout and lithography apparatus settings achieve the maximum overall process window for the patterning process (depicted by the double arrow in Figure 1 ). Typically, the resolution enhancement techniques are arranged to match the patterning capabilities of the lithography apparatus LA. The computer system CL may also be used to (e.g., using input from the metrology tool MT) detect where within the process window the lithography apparatus LA is currently operating to predict whether there may be defects due to, for example, sub-optimal processing (depicted by the arrow pointing to "0" in Figure 1 ).

[0038] The metrology tool MT may provide input to the computer system CL to enable accurate simulations and predictions, and may provide feedback to the lithography apparatus LA to identify possible drifts in, for example, the calibration state of the lithography apparatus LA (depicted by the multiple arrows in Figure 1 ).

[0039] During the lithography process, it is desirable to frequently measure the structures produced, for example, for process control and verification. Tools used to make such measurements are commonly referred to as metrology tools MT. Different types of metrology tools MT for making such measurements are known, including scanning electron microscopes, atomic force microscopes, and various forms of scatterometer metrology tools MT. A scatterometer is a general-purpose instrument that allows the measurement of parameters of the lithography process by placing a sensor in the pupil or in a plane conjugate to the pupil of the objective of the scatterometer, and such measurement is typically referred to as pupil-based measurement; or by placing a sensor in the image plane or in a plane conjugate to the image plane, in which case the measurement is typically referred to as image- or field-based measurement. Such scatterometers and associated measurement techniques are further described in patent applications US20100328655, US2011102753A1, US20120044470A, US20110249244, US20110026032, or EP1,628,164A, which are incorporated herein by reference in their entirety. The aforementioned scatterometers may use light from the soft x-ray and visible to near-IR wavelength ranges to measure gratings.

[0040] In one embodiment, the scatterometer MT is an angular resolution scatterometer. In such a scatterometer, a reconstruction method can be applied to the measured signal to reconstruct or calculate the properties of the grating. Such reconstruction can be caused, for example, by simulating the interaction of the scattered radiation with a mathematical model of the target arrangement and comparing the simulated results with the measured results. The parameters of the mathematical model are adjusted until the simulated interaction produces a diffraction pattern similar to the diffraction pattern observed from the real target.

[0041] In another embodiment, the scatterometer MT is a spectroscopic scatterometer MT. In such a spectroscopic scatterometer MT, the radiation emitted by the radiation source is directed onto the target and the reflected or scattered radiation from the target is directed onto a spectrometer detector that measures the spectrum of the specularly reflected radiation (i.e., the measurement of the intensity as a function of wavelength). From such data, the structure or profile of the target that produced the detected spectrum can be reconstructed, for example, by rigorous coupled-wave analysis and non-linear regression or by comparison with a simulated spectral library.

[0042] In another embodiment, the scatterometer MT is an ellipsometric scatterometer. The ellipsometer allows the determination of the parameters of the lithography process by measuring the scattered radiation for each polarization state. Such a metrology device emits polarized light (such as linear, circular or elliptical) by using, for example, a suitable polarization filter in the illumination section of the metrology device. Sources suitable for the metrology device can also provide polarized radiation. Various embodiments of existing ellipsometers are described in U.S. Patent Applications 11 / 451,599, 11 / 708,678, 12 / 256,780, 12 / 486,449, 12 / 920,968, 12 / 922,587, 13 / 000,229, 13 / 033,135, 13 / 533,110 and 13 / 891,410, which are hereby incorporated by reference in their entirety.

[0043] In Figure 2 is depicted a metrology device, such as a scatterometer. It includes a broadband (white light) radiation projector 2 that projects radiation onto a substrate W. The reflected or scattered radiation is transmitted to a spectrometer detector 4 that measures the spectrum 6 of the specularly reflected radiation (i.e., the measurement of the intensity as a function of wavelength). From such data, the structure or profile 8 that produced the detected spectrum can be reconstructed by a processing unit PU, for example, by rigorous coupled-wave analysis and non-linear regression, or by comparison with as Figure 2The reconstruction is performed by comparing with a simulated spectral library shown at the bottom. Generally, for the reconstruction, the general form of the structure is known, and some parameters are assumed based on the knowledge of the process used to fabricate the structure, leaving only a few parameters of the structure to be determined based on the scattering measurement data. Such a scatterometer can be configured as a normal incidence scatterometer or an oblique incidence scatterometer.

[0044] In an embodiment, a cantilever probe 12 is provided, and an example of the cantilever probe 12 is depicted in Figure 3 The cantilever probe 12 is configured to provide high spatial resolution information related to features currently located beneath the outer surface of the entity to be studied in a manner that minimizes or poses 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 studied is a target structure 19 formed on a substrate W. In such an embodiment, 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 towards the target structure 19 on the substrate W (generally downward in Figure 3 ). In an embodiment, the cantilever 14 and the probe element 16 (e.g., by their material properties and dimensions) are configured to be capable of performing the cantilever function in a standard atomic force microscope. In an embodiment, either or both of the cantilever 14 and the probe element 16 are formed of 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 back from the target structure 19 into the probe element 16 or are reflected back into an additional probe element 32 extending from the cantilever 14 (described below).

[0047] In an embodiment, the ultrasonic waves are generated in the cantilever probe 12 using the photoacoustic effect. In some embodiments of this type, the generation of the ultrasonic waves is performed by directing a laser beam onto the cantilever probe 12. In Figure 3 the example of, the laser beam is directed into the cantilever probe 12 by a first laser unit 26. The first laser unit 26 may be considered to form all or part of an ultrasonic wave generation system.

[0048] The nature of the laser beam provided by the first laser unit 26 is not particularly limited as long as the desired ultrasonic waves are generated. For example, the laser beam may include femtosecond lasers. In one embodiment, a laser pulse of 200 fs with a pulse energy of 6 nJ is used, which generates a peak power of 30 kW. The repetition rate of such pulses is approximately 50 MHz and the average power is approximately 300 nW.

[0049] In an embodiment, as Figure 3 illustrated by way of example in, an ultrasonic wave generating layer 18 is provided on the cantilever 14. The laser beam from the first laser unit 26 is directed onto the ultrasonic wave generating layer 18 to generate ultrasonic waves in the ultrasonic wave generating layer 18. In this type of embodiment, the combination of the first laser unit 26 and the ultrasonic wave generating layer 18 can be considered to form all or part of an ultrasonic wave generating system. In an embodiment, the ultrasonic wave generating layer 18 is configured to provide a higher per unit area absorption rate relative to the laser beam from the first laser unit 26 than the cantilever 14 would provide per unit area in the absence of the ultrasonic wave generating layer 18. It is also desirable that the photoacoustic conversion efficiency associated with the absorption is high and stable. Desirably, the ultrasonic wave generating layer 18 has a high per unit area absorption rate at the frequency of the laser beam from the first laser unit 26 and has a thermal diffusion speed of the same order of magnitude as the laser pulse duration (e.g., femtosecond order of magnitude) to obtain a high photoacoustic conversion efficiency. In an embodiment, the ultrasonic wave generating layer 18 may include a metallic material such as aluminum, gold, or titanium. Alternatively or additionally, the ultrasonic wave generating layer 18 may be arranged to include a material based on highly absorbent carbon such as amorphous carbon. The ultrasonic wave generating layer 18 may include a single layer having a uniform composition throughout the thickness of the single layer. Alternatively, the ultrasonic wave generating layer 18 may include a composite layer having a plurality of individual layers. In an embodiment, at least a subset of the individual layers has a different composition relative to each other. It is also desirable to arrange for an effective transmission of the generated ultrasonic waves, for example by avoiding excessive reflections within the ultrasonic wave generating layer 18 and / or at the interface between the ultrasonic wave 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 ultrasonic wave generating layer 18 and the cantilever 14. The acoustic impedance of the impedance matching layer is between the acoustic impedance of the ultrasonic wave generating layer 18 and the acoustic impedance of the cantilever 14.

[0050] In an embodiment, the composition and dimensions of the ultrasonic generation layer 18 are selected such that at least a portion of the ultrasonic waves generated in the ultrasonic generation layer 18 have a frequency higher than 15 GHz, optionally higher than 50 GHz, and optionally higher than 100 GHz. Providing ultrasonic waves in the range of 15 GHz to 50 GHz provides spatial features with sub-micron resolution within the target structure 19. Providing ultrasonic waves with a frequency higher than 100 GHz (e.g., in the range of 100 GHz to 200 GHz) provides spatial features with nano-resolution within the target structure 19. Providing ultrasonic waves in the intermediate range of 50 GHz to 100 GHz provides spatial features with intermediate resolution.

[0051] The thickness of the ultrasonic generation layer 18 may affect the frequency of the generated ultrasonic waves. When the thickness of the ultrasonic generation layer 18 is comparable to or less than the penetration depth (associated with the laser beam from the first laser unit 26), the thickness of the ultrasonic generation layer 18 and the frequency of the ultrasonic waves can be related according to (when is the speed of sound in the ultrasonic generation layer 18). If the thickness is greater than the penetration depth, then the penetration depth becomes the bottleneck. The penetration depth is determined by the complex refractive index of the ultrasonic generation layer 18.

[0052] Based on the above, forming the ultrasonic generation layer 18 from a homogeneous aluminum layer with a thickness of 30 nm, or a homogeneous amorphous carbon layer with a thickness of 85 nm or less, will be suitable for generating ultrasonic waves with a frequency higher than 100 GHz. As explained above, the required thickness depends on the speed of sound in the ultrasonic generation layer 18. In the case of a higher speed of sound, higher frequency ultrasonic waves can be generated from the same thickness of material. However, increasing the speed of sound may also increase the reflection losses within the ultrasonic generation layer 18 and / or at the boundary (if any) between the ultrasonic generation layer 18 and the cantilever 14. The thickness of the ultrasonic generation 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 ultrasonic generation layer 18 is configured to modify the properties (e.g., frequency) of the generated ultrasonic waves and / or improve the conversion efficiency. For example, the ultrasonic generation layer 18 may include one or more patterns having a length scale smaller than the wavelength of the laser beam from the first laser unit 26. In some embodiments, the ultrasonic generation layer 18 may include one or more material rings, optionally closed rings, optionally concentric circles. Alternatively, the ultrasonic generation layer 18 may be arranged in a checkerboard pattern. The detailed dimensions and / or shape of any of the ultrasonic generation layers 18 configured in this way can be derived from the analysis of the vibration modes of the ultrasonic generation layer 18.

[0054] In an embodiment, an ultrasonic detection system is provided that detects reflected ultrasonic waves reflected back from the target structure 19. In some embodiments, the detection of the reflected ultrasonic waves includes detecting a change in the optical reflectivity of the cantilever probe 12. In Figure 3 an example, 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 ultrasonic generation layer 18. The laser beam is reflected off the cantilever probe 12 (e.g., off the ultrasonic generation layer 18) and detected by the photodetector 22. A data processing system 24 is provided for determining information related to the target structure 19 from the detected reflected ultrasonic 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, a first laser unit 26 generates ultrasonic waves in the cantilever probe 12. The generated ultrasonic waves are transmitted into the target structure 19 through the 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 ultrasonic wave generation layer 18, which is reflected and detected by the photodetector 22). This method allows for easy differentiation between the light reflected from the first laser unit 26 to the photodetector 22 and the light reflected from the second laser unit 20 to the photodetector 22, because the first laser unit 26 and the second laser unit 20 can operate at different times. However, it is also possible to continuously transmit and receive the ultrasonic waves and use other techniques to distinguish between the reflections originating from the first laser unit 26 and the reflections originating from the second laser unit 20. For example, the data processing system 24 can be configured to use a lock-in amplifier or a similar technique to utilize the frequency and / or the phase difference between the laser beams from the first laser unit 26 and the second laser unit 20. In Figure 3 the embodiment shown, 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 can be provided by a single unit that is used to generate the ultrasonic waves and to detect a change in the optical reflectivity that contains information related to the ultrasonic waves reflected back from the target structure 19.

[0056] In an embodiment, the ultrasonic detection system (e.g., the second laser unit 20 and the photodetector 22) is also 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 of the photodetector 22.

[0057] The above embodiments provide several advantages compared to alternative methods where, for example, ultrasonic waves are directly generated within the target structure 19 by an actuator on the dorsal side of the target structure 19 opposite the atomic force microscope cantilever. Directly generating the ultrasonic waves within the target structure 19 may result in errors caused by material-related factors of the target structure 19. Additionally, the highest achievable acoustic frequency is limited by the material properties of the target structure 19. Further, the target structure 19 may be damaged by the process of generating the ultrasonic waves, which effectively limits the highest power that can be used. Generating the ultrasonic waves within the cantilever probe 12 makes it possible to avoid these problems, thereby providing improved accuracy, improved spatial resolution, a lower risk of damage to the target structure 19, and / or a higher input power without an excessive risk of damage through a higher ultrasonic wave frequency.

[0058] Now with regard to Figures 4 to 11 the optional configuration of the probe element 16 and / or additional probe elements 32 will be discussed.

[0059] Figure 4 The probe element 16 is depicted operating in the transmit mode, where the generated ultrasonic wave 28 enters the probe element 16 from the cantilever 14 (not shown) and propagates downward through the probe element 16. Figure 5 Depicted Figure 4 is the probe element 16 operating in the receive mode, where the reflected ultrasonic wave propagates upward through the probe element 16 and exits (arrow 30) the probe element 16 into the cantilever 14 (not shown). The probe element 16 is an example of a probe element that is tapered to have a decreasing cross-sectional area toward the target structure 19 (i.e., downward). In this particular example, the taper is provided over the entire vertical length of the probe element 16. The cross-sectional shape is not particularly limited, but may approximate a circle, such that the tapered portion of the probe element 16 is conical. The tapered form serves to focus the ultrasonic wave 28 toward the target structure 19. However, the tapered form may also serve to defocus the reflected ultrasonic wave, making it more challenging to detect the reflected ultrasonic wave. In an embodiment, the probe element 16 is formed of alternating material layers having high and low acoustic refractive indices. The high acoustic refractive index material is, for example, silicon. The low acoustic refractive index material may be, for example, air or PMMA. The acoustic properties of the element 16 when formed of alternating material layers having higher and lower acoustic refractive indices can also be controlled or improved by adjusting the pitch or fill ratio of the alternating layers or the thickness of each layer.

[0060] The focusing provided by the tapered form of the probe element 16 causes the ultrasonic waves to propagate in the target structure 19 in a manner similar to propagation from a point source. Three-dimensional radiation from a point source results in an intensity that varies with decreases in an inversely proportional manner, where is the emission radius. Thus, the intensity reaching the bottom of the target structure 19 is reduced times, where is the thickness of the target structure 19. Thus, in the absence of countermeasures, the reflected intensity reaching the top of the target structure 19 will be reduced compared to the intensity of the ultrasonic waves at the tip of the probe element 16 times. Examples of methods for improving the detection efficiency are described below with respect to Figures 6 - 11 The method is based on separating the emission and reception functions of the probe element 16 and reducing defocusing or even magnifying the ultrasonic waves in the reception channel.

[0061] In an embodiment ( Figure 6 an example of which is depicted in Figure 6 ), the cantilever probe 12 includes two discrete probe elements: the probe element 16 and an additional probe element 32. The probe element 16 and the additional probe element 32 are configured such that the reflected ultrasonic waves are primarily detected by the additional probe element 32. In this type of embodiment, the ultrasonic waves generated in the cantilever probe 12 can propagate towards the target structure 19 primarily through the probe element 16. Thus,

[0062] In an embodiment, at least a portion of the probe element 16 tapers to have a decreasing cross-sectional area towards the target structure 19. In Figure 6 an example of Figure 6 all of the probe element 16 is tapered. The tapering of the probe element 16 focuses the ultrasonic waves onto the target structure 19. In an embodiment, at least a portion of the additional probe element 32 tapers to have an increasing cross-sectional area towards the target structure 19. In

[0063] Figures 7 to 11depicts an exemplary embodiment in which the reflected ultrasonic waves are mainly detected by the probe element 16. In an embodiment of this type, the additional probe element 32 may be omitted.

[0064] In an embodiment, as Figure 7 depicted, the ultrasonic waves propagating towards the target structure 19 mainly travel through the first portion 33 of the probe element 16 and the reflected ultrasonic waves propagating away from the target structure 19 mainly travel through the second portion 34 of the probe element 16. The first portion 33 is different from the second portion 34. In an embodiment, the first portion 33 and the second portion 34 have different geometries. In an embodiment, the first portion 33 and the second portion 34 are formed of different materials.

[0065] In the particular example shown, the first portion 33 includes an outer sheath region and the second portion 34 includes a central region located inside the outer sheath region (e.g., radially surrounded by the outer sheath region with respect to the average propagation direction of the ultrasonic waves). The central region and the outer sheath region act as waveguides to enable the reflected ultrasonic waves to propagate backward from the target structure 19 through the probe element 16 towards the cantilever 14. The waveguide behavior causes the reflected ultrasonic waves to mainly propagate through the central region of the probe element 16. In an embodiment, the acoustic impedance of the material forming the first portion 33 is higher than the acoustic impedance of the material forming the second portion 34. Complex structures such as Figure 7 shown can be relatively easily manufactured (e.g., using lithography) because at least a portion of the probe element 16 can be formed of silicon.

[0066] In an embodiment, at least a portion of the first portion 33 (e.g., the outer sheath region) tapers to have a cross-sectional area that decreases towards the target structure 19, as described above with respect to Figure 6 and the tapering focuses the ultrasonic waves onto the target structure 19. In an embodiment, at least a portion of the second portion 34 (e.g., the central region) tapers to have a cross-sectional area that decreases towards the cantilever 14, as described above with respect to Figure 6 and the tapering focuses the reflected ultrasonic waves towards the ultrasonic detection system, thereby improving the signal-to-noise ratio.

[0067] As Figures 8 to 10As illustrated, in some embodiments, the probe element 16 includes a longitudinal proximal portion 37 and a longitudinal distal portion 38. The longitudinal proximal portion 37 is connected to the cantilever 14 (not shown) and extends from the cantilever 14 to the longitudinal distal portion 38. The longitudinal distal portion 38 extends from the longitudinal proximal portion 37 toward the target structure 19 (not shown). The longitudinal distal portion 38 tapers to have a decreasing cross-sectional area toward the target structure 19. The tapering focuses the ultrasonic waves onto the target structure 19. The longitudinal proximal portion 37 includes at least a portion that tapers to have a decreasing cross-sectional area toward the cantilever 14, as Figure 8 and Figure 9 illustrated. In the example of Figure 8 , the longitudinal proximal portion 37 includes a portion 37A that tapers to have a decreasing cross-sectional area toward the cantilever 14 (not shown), and a portion 37B that tapers to have a decreasing cross-sectional area toward the target structure 19. In the example of Figure 9 , the entirety of the longitudinal proximal portion 37 tapers to have a decreasing cross-sectional area toward the cantilever 14. The tapering focuses the reflected ultrasonic waves toward the ultrasonic detection system.

[0068] In an embodiment, the longitudinal proximal portion 37 includes at least a non-tapering portion, as Figure 10 and Figure 11 illustrated. In Figure 10 and Figure 11 , the longitudinal proximal portion 37 has a constant cross-sectional area from the longitudinal distal portion 38 to the cantilever 14 (not shown). In Figure 10 , a discontinuity in the cross-sectional area exists at the interface between the longitudinal proximal portion 37 and the longitudinal distal portion 38. In Figure 11 , the cross-sectional area transitions continuously across the interface between the longitudinal proximal portion 37 and the longitudinal distal portion 38. Compared with the tapering probe element 16 of the type depicted in Figure 4 and Figure 5 , the lack of tapering facilitates manufacturing while reducing the defocusing of the reflected ultrasonic waves toward the ultrasonic detection system. Providing a longitudinal distal portion that tapers toward the target structure 19 provides a sharp tip that allows for the effective use of the cantilever probe 12 in a standard atomic force microscopy mode. Thus, a favorable balance of properties is achieved, allowing for the effective performance of both standard atomic force microscopy and ultrasonic-based measurements with the same cantilever 14 described above.

[0069] Embodiments are now described where the cantilever probe 12 is specifically used to determine information related to a target structure 19 formed on a substrate using a lithography process. In this context, as Figure 12Schematically depicted, the target structure 19 may include a first sub-structure 15 and a second sub-structure 17. The first sub-structure 15 overlaps with the second sub-structure 17. Information related to the target structure 19 may include information related to the overlap between the first sub-structure 15 and the second sub-structure 17 (marked as Figure 12 in ). The overlap represents the degree of misalignment between the first sub-structure 15 and the second sub-structure 17. As mentioned in the introduction section of the specification, it is difficult or impossible to measure the overlap in a target structure 19 having a relatively small pitch (e.g., about 10 nm) and a relatively large separation between the overlapping layers (e.g., about 100 nm) using the prior art, and / or it is difficult or impossible to measure the overlap in the target structure 19 in the presence of an optically opaque material layer using the prior art. The ultrasonic-based measurement described above can be used to address these challenges. Example methods are described below.

[0070] In some embodiments, information related to the overlap is obtained by detecting reflected ultrasonic waves at multiple positions of the cantilever probe 12 relative to the target structure 19. In an embodiment, the multiple positions are selected to detect the position of a reference feature in the first sub-structure 15 by using the cantilever probe in a standard atomic force microscopy mode. For example, in some embodiments, the profile of the first sub-structure 15 is obtained by measuring the deflection of the cantilever 14 caused by the interaction (e.g., contact) between the probe element 16 and the first sub-structure 15 at multiple positions of the probe element 16 relative to the first sub-structure 15, and by measuring information related to the second sub-structure 17 (e.g., overlap relative to the first sub-structure 15 or another reference).

[0071] In an embodiment, the first sub-structure 15 and the second sub-structure 17 each include a plurality of repeating elements (e.g., grating lines or groups of grating lines), as Figure 14 depicted. Reflected ultrasonic waves are detected at each position in a set of positions of the target structure 19 relative to the cantilever probe 12. Each position of the target structure 19 relative to the cantilever probe 12 is a position at which the probe element 16 is aligned with a different one of the peaks of the repeating elements of the first sub-structure 15, as Figure 12 depicted. The alignment of the probe element 16 with each repeating element is determined by measuring the deflection of the cantilever 14 caused by the interaction (e.g., contact) between the probe element 16 and the first sub-structure 15 (using the standard atomic force microscopy mode).

[0072] When the probe element 16 is aligned with the peak of the first sub-structure 15, it has been found that the intensity of the reflected ultrasonic wave varies as a determinable function of the relative position of the peak of the first sub-structure 15 and the corresponding underlying peak of the second sub-structure 17, this variation in turn depending on the overlap . The intensity with respect to the overlap can be expressed as and is schematically depicted in Figure 13 . The intensity is maximum when the peak of the first sub-structure 15 aligned with the probe element 16 is precisely aligned with the corresponding peak in the second sub-structure 17

[0073] The intensity of the reflected ultrasonic wave is related to the overlap , but the overlap can be obtained separately based on these measurements. This problem can be solved using the Moiré effect, as described below

[0074] If the first sub-structure 15 includes a grating having a pitch , the signal representing the intensity of the reflected ultrasonic wave measured at each of a plurality of positions where the probe element 16 is perfectly aligned with the peaks (lines) of the grating is expected to vary in the following manner

[0075]

[0076] where is the spatial coordinate of the probe element 16 on the first sub-structure 15 in the scan direction 42, and is the relative phase with respect to the starting point of the measurement. In an embodiment, as depicted in Figure 14 , the first sub-structure 15 is now arranged to have a different pitch from the second sub-structure 17. In Figure 14 , the pitch of the first sub-structure 15 is labeled and the pitch of the second sub-structure 17 is labeled . This difference in pitch gives rise to a signal that represents the measured intensity of the ultrasonic wave reflected from the target structure 19, including a beat frequency term related to the variation of the position of the probe element 16 relative to the target structure 19. The beat frequency term has a frequency defined by the difference between the pitches of the first sub-structure 15 and the second sub-structure 17, and a phase defined by the overlap . The phase of the beat frequency term can thus be detected and used to determine the overlap 。

[0077] The signal depends on the relative position of the first sub-structure 15 and the second sub-structure 17 in a manner similar to that described above, but due to the pitch difference between the first sub-structure 15 and the second sub-structure 17, the signal can be expressed as the sum of two cosine functions as follows: 。

[0078]

[0079] which produces a beat frequency:

[0080]

[0081] The phase term caused by the overlapping term is expressed as 。 The second cosine term in

[0082] In Figure 14 values are obtained at a plurality of points 40 corresponding to the alignment of different peaks among the plurality of peaks of the probe element 16 and the first sub-structure 15. Finally, the cosine function can be fitted through the points 40 (indicated by the dashed curve). Then, can be determined according to the phase of the fitted cosine function to obtain the overlap between the first sub-structure 15 and the second sub-structure 17 。Since the period of the beat frequency part of the signal can be made relatively large, the position offset corresponding to the overlap can be made relatively large, thereby providing high sensitivity. 。

[0083] The above method allows obtaining the overlap with high sensitivity and without measuring the reflected ultrasonic waves at a large number of positions. The technique can be applied using only the measurements obtained at a plurality of positions corresponding to the alignment between the probe element 16 and different peaks of the first sub-structure 15.

[0084] Now discuss an alternative embodiment where an intentionally applied overlap bias is used to obtain the overlap 。As mentioned above, the intensity of the reflected ultrasonic wave measured at any position where the peak of the probe element 16 is aligned with the first sub-structure 15 varies as a function of the overlap according to a cosine function:

[0085]

[0086] However, the overlap is typically a relatively small value, which means the intensity The variation with is non - linear and relatively weak. In an embodiment, a target structure 19 including a plurality of sub - targets is used to improve the intensity with variation, the plurality of sub - targets including one or more sub - targets having an overlapping bias intentionally applied to them. In some embodiments of this type, the target structure 19 includes a first sub - target and a second sub - target. The first sub - target and the second sub - target may be positioned very close to each other on the substrate W, such as adjacent to each other. The first sub - target and the second sub - target each include two sub - structures with an intentionally applied overlapping bias therebetween. The sub - structures of each sub - target may overlap with each other in the same manner as the first sub - structure 15 and the second sub - structure 17 (e.g., as described above with respect to Figure 12 and Figure 14 ). The overlapping bias of the first sub - target is different from the overlapping bias of the second sub - target. This difference shifts the cosine variation mentioned above and improves the with variation.

[0087] In some embodiments, the overlapping bias of the first sub - target is equal and opposite to the overlapping bias of the second sub - target. In an embodiment, the two sub - structures of the first sub - target have the same pitch as the two sub - structures of the second sub - target , the overlapping bias of is applied to the first sub - target and with respect to the cosine correlation is converted into a sine correlation, thereby providing a linear and relatively sharp variation of for smaller with variation. This is explained below.

[0088] In the case of applying the corresponding and biases, the respective intensities and of the reflected ultrasonic waves from the first sub - target and the second sub - target can be written as follows:

[0089]

[0090]

[0091] From this, it can be seen that

[0092]

[0093] And for smaller , the intensity difference is approximately given by:

[0094]

[0095] where .

[0096] The value of can be pre-derived using calibration measurements such that can be obtained based on the measurement of . For example, a set of calibration targets with a set of programmed bias values (e.g., ranging from -p to +p) can be used to determine the constant value. Once the value of is known through calibration, only two sub-targets (e.g., with an offset of and ) are needed to determine the overlap . Thus, the overlap can be determined based on the difference between the following intensities: the intensity of the detected reflected ultrasonic wave from the first sub-target when the probe element is aligned with the peak of the repeating element of the sub-structure of the first sub-target closest to the probe element (e.g., the uppermost sub-structure in the first sub-target); and the intensity of the detected reflected ultrasonic wave from the second sub-target when the probe element is aligned with the peak of the repeating element of the sub-structure of the second sub-target closest to the probe element (e.g., the uppermost sub-structure in the second sub-target).

[0097] In some embodiments, the need for calibration measurements is reduced or eliminated by using additional sub-targets. In one class of such embodiments ( Figure 15 schematically depicted in ), the target structure 19 includes a first sub-target 51, a second sub-target 52, a third sub-target 53, and a fourth sub-target 54. The first sub-target 51, the second sub-target 52, the third sub-target 53, and the fourth sub-target 54 each include two sub-structures with an intentionally applied overlap bias therebetween. The overlap biases of the first sub-target 51, the second sub-target 52, the third sub-target 53, and the fourth sub-target 54 are all different from each other. The two sub-structures of each of the first sub-target 51, the second sub-target 52, the third sub-target 53, and the fourth sub-target 54 have the same pitch . The overlap bias of the first sub-target 51 is . The overlap bias of the second sub-target 52 is . The overlap bias of the third sub-target 53 is . The overlap bias of the fourth sub-target 54 is . . and are constants. The overlap can be determined according to the combination of , , , and , where , , ,, and are defined as follows. is the intensity of the detected reflected ultrasonic wave from the first sub-target when the probe element is aligned with the peak of the repeating element of the sub-structure closest to the probe element of the first sub-target. is the intensity of the detected reflected ultrasonic wave from the second sub-target when the probe element is aligned with the peak of the repeating element of the sub-structure closest to the probe element of the second sub-target. is the intensity of the detected reflected ultrasonic wave from the third sub-target when the probe element is aligned with the peak of the repeating element of the sub-structure closest to the probe element of the third sub-target. is the intensity of the detected reflected ultrasonic wave from the fourth sub-target when the probe element is aligned with the peak of the repeating element of the sub-structure closest to the probe element of the fourth sub-target.

[0098] In an embodiment, , which results in the following relationship being true, as depicted in Figure 15 :

[0099]

[0100]

[0101] Then, the overlap can be obtained according to the following formula:

[0102]

[0103] As an example, for a target structure with a pitch of 60 nm, using a value of 5 nm would mean that the sub-targets have offsets of -10 nm, 20 nm, -20 nm, and 10 nm, respectively. Each sub-target can have multiple peaks / lines (e.g., as in Figure 15 , each sub-target has 3 peaks / lines). Within any given sub-target, the intensity I detected for each peak / line should be the same, but measuring multiple peaks improves the signal-to-noise ratio.

[0104] Figure 16Depicts an alternative method of setting the target structure 19, in which the first sub-structure 15 is (intentionally) laterally displaced relative to the second sub-structure 17 such that when viewed perpendicular to the plane of the substrate W, at least a portion of the second sub-structure 17 does not overlap with any of the first sub-structures 15. Then, the profile 35 of the first sub-structure 15 can be obtained by measuring the deflection of the cantilever 14 caused by the interaction (e.g., contact) between the probe element 16 and the first sub-structure 15 for a plurality of positions of the probe element 16 relative to the first sub-structure 15. The profile 47 of the second sub-structure 17 is obtained by detecting the reflected ultrasonic waves for a plurality of positions of the probe element 16 relative to the second sub-structure 17 within the portion of the second sub-structure 17 that does not overlap with any of the first sub-structures 15. Thus, the lateral displacement of the sub-structures allows the cantilever probe 12 to measure each sub-structure separately without any interference between the two sub-structures. Standard atomic force microscopy methods can be used to measure the uppermost first sub-structure 15, while the obscured second sub-structure 17 can be measured via the reflected ultrasonic waves, and the reflection process is not disturbed by any overlapping sub-structures that are not part of the second sub-structure 17. Since the magnitude of the intentionally applied lateral displacement is known, the overlap can be determined from the measured profiles 35 and 47 (as Figure 16 schematically indicated above).

[0105] In some embodiments, as Figure 18 illustrated in, the cantilever probe 12 includes a plurality of cantilevers 14 (labeled 14A - 14F in Figure 18 ). Each cantilever 14 can be configured and / or arranged to operate in any of the ways described above with respect to Figures 3 - 17 . Each cantilever 14 includes a respective probe element 16 (e.g., such that for each cantilever 14 under consideration, there is at least one probe element 16). Each probe element 16 can take any of the forms described above with respect to Figures 3 - 17 . Each probe element 16 is configured to extend from the cantilever 14 (the probe element 16 is associated with the cantilever 14) towards the target structure 19 ( Figure 18 not shown in). As shown in the upper right inset of Figure 18 , in the example of Figure 18 , the probe element 16 points upwards along the orientation of the figure. Thus, the target structure 19 will be located above the cantilever probe 12 along the orientation of this figure. In Figures 19 - 22In [the figure], the probe element 16 faces downward, so the target structure 19 will be located below the cantilever probe 12 along the orientation of these figures. In the illustrated example, the cantilever 14 is disposed on the support structure 60. The protruding tip region of the cantilever 14 protrudes on the edge of the support structure 60. In such an embodiment, the probe element 16 is attached to the protruding tip portion.

[0106] In some embodiments having a plurality of cantilevers 14, the generation of ultrasonic waves in the cantilever probe 12 (which can be performed using a suitably adapted version of the ultrasonic generation system discussed above) causes the ultrasonic waves to propagate through each corresponding probe element 16 into the target structure 19. In Figure 18 the example of [the figure], the ultrasonic waves can thus propagate through different probe elements 16 extending from two or more of the cantilevers 14 in the cantilever 14 (optionally, through the probe elements 16 on each of the cantilevers 14 in the cantilever 14). In an embodiment, the detection of the reflected ultrasonic waves (which can be performed using a suitably adapted version of the ultrasonic detection system discussed above) includes detecting the reflected ultrasonic waves from each of the corresponding probe elements 16.

[0107] Performing the generation and detection of ultrasonic waves through a plurality of probe elements 16 enables multiple measurements to be performed in parallel, thereby improving the overall measurement speed. In an embodiment of the cantilever probe 12 fabricated by etching a silicon wafer or the like, fabricating a cantilever probe 12 having a plurality of cantilevers 14 is not much more difficult than fabricating a cantilever probe 12 having a single cantilever 14. The etching pattern can be easily adjusted and the remaining processes generally will not require significant adaptation.

[0108] As described earlier, ultrasonic waves can be generated in the cantilever probe 12 using the photoacoustic effect. In this context, the radiation used to generate the ultrasonic waves can be referred to as pump radiation. The pump radiation can include a laser beam 70. The laser beam 70 can be provided by a first laser unit 26, as described above with respect to Figure 3 [the figure].

[0109] In some embodiments, as Figure 19 and Figure 20 illustrated in [the figure], the laser beam 70 is optically processed to provide structured illumination that matches the spatial distribution of the plurality of cantilevers 14. For example, the structured illumination can be characterized by a pitch that is at least approximately equal to the pitch of the cantilevers 14. In some embodiments, the optical processing of the laser beam 70 can include splitting the laser beam 70 into a plurality of sub-beams. Each sub-beam is then directed (e.g., focused) onto a different corresponding one of the plurality of cantilevers 14.

[0110] In an embodiment, asFigure 19 is schematically depicted, and the optical processing (e.g., splitting) of the laser beam 70 for providing the structured illumination is performed by diffraction. In the example shown, the laser beam 70 is directed through a diffraction grating 66 to produce diffracted radiation 72. The diffracted radiation 72 is focused by an optical device 64 to form a fringe pattern 62 on the cantilever 14. The fringe pattern 62 corresponds to the structured illumination. The diffraction grating 66 and the optical device can be configured such that the pitch of the fringe pattern 62 matches the pitch of the cantilever 14. The cantilever 14 can thus receive the respective maximum amount of radiation from the fringe pattern. Thereby, multiple cantilevers 14 can be pumped or excited simultaneously in an efficient and simple manner. The power from the laser beam 70 can be limited to mainly or exclusively fall on the cantilevers 14, thereby allowing the best utilization of the available laser power. In addition, stray light that may cause unwanted photoacoustic effects in and / or around the target structure 19 can be minimized or eliminated.

[0111] In another embodiment, as Figure 20 is schematically depicted, the optical processing (e.g., splitting) of the laser beam 70 is performed using an array 68 of individually controllable elements. For example, the array 68 can include an array of individually controllable micro - mirrors or microlenses. The use of the array 68 of individually controllable elements enhances flexibility. For example, the time for which each cantilever 14 is exposed to radiation (and thus, the generation of ultrasonic waves in the probe element 16 of each cantilever 14) can be flexibly controlled. In some embodiments, the array 68 of individually controllable elements can be used to simultaneously irradiate all the cantilevers 14 (in a manner similar to the Figure 19 arrangement where diffraction is used to split the laser beam 70). In other embodiments, the array 68 of individually controllable elements is configured to irradiate different cantilevers 14 at different times. For example, the array 68 of individually controllable elements can be controlled to irradiate individual cantilevers 14 one by one in sequence. Alternatively or additionally, the selective activation of the individually controllable elements can be used to implement signal processing techniques such as synthetic aperture imaging or coded excitation. Such signal processing techniques can be used to improve resolution and / or imaging depth performance.

[0112] In some embodiments, as Figure 21 and Figure 22 illustrated, the detection of the reflected ultrasonic waves includes directing a probe beam 80 of radiation onto the cantilever 14 and detecting the radiation reflected from the cantilever 14. For example, the probe beam 80 can be provided by a second laser unit 20, as described above with respect to Figure 3As described. The detected reflected radiation can be used to measure the change in reflectivity of the cantilever 14. The change in reflectivity provides information related to the reflected ultrasonic wave.

[0113] In an embodiment, as Figure 21 schematically depicted, the probe beam 80 is optically processed (e.g., split) to provide structured illumination that matches the spatial distribution of the plurality of cantilevers 14. In an embodiment, the probe beam 80 is divided into a plurality of sub-beams 84. Each sub-beam 84 is then directed to a different corresponding cantilever among the plurality of cantilevers 14. In the example shown, an array 82 of individually controllable elements is used to perform the optical processing of the probe beam 80. For example, the array 82 may include an array of individually controllable micro-mirrors or micro-lenses. The radiation reflected from each of the cantilevers 14 can be detected in parallel to increase the readout speed. In Figure 21 the example, discrete detectors 86 (e.g., photodiode sensors) are provided for detecting the radiation reflected from each corresponding cantilever 14 (such that six detectors 86 are provided in the example shown). Signal lines 88 allow the measurement results to be sent to the data processing system in parallel to determine information related to the target structure 19 based on the detected reflected ultrasonic waves.

[0114] In another embodiment, as Figure 22 depicted, the probe beam 80 is focused into a line focus. For example, a cylindrical lens can be used to produce the line focus. The line focus is arranged to intersect the plurality of cantilevers 14. The intersection of the line focus with the plurality of cantilevers 14 allows the focused radiation to be provided to the plurality of cantilevers 14 simultaneously in a convenient and controllable manner. The output radiation 90 can be captured and analyzed in various ways. In one embodiment, interferometry is used, for example, as described in "Parallel atomic force microscopy with optical interferometric detection" by T. Sulchek et al., Appl. Phys. Lett., Vol. 78, No. 12, pp. 1787 - 1789, March 2001.

[0115] In an embodiment, a metrology system 25 according to any of the embodiments described above is provided as a component of a lithography system 50, as Figure 17 schematically depicted. The lithography system 50 includes a lithography apparatus LA and the metrology system 25, and the lithography apparatus LA is configured to define a pattern for forming a target structure 19 on a substrate W.

[0116] Additional embodiments according to the present invention are described in the numbered aspects below:

[0117] 1. A method for determining information related to a target structure formed on a substrate using a lithography process, comprising:

[0118] Providing a cantilever probe comprising a cantilever and a probe element, the probe element extending from the cantilever towards the target structure;

[0119] Generating ultrasonic waves in the cantilever probe, the ultrasonic waves propagating through the probe element into the target structure and reflecting back from the target structure into the probe element or into another probe element extending from the cantilever; and

[0120] Detecting the reflected ultrasonic waves and determining information related to the target structure based on the detected reflected ultrasonic waves.

[0121] 2. The method according to aspect 1, wherein the ultrasonic waves are generated in the cantilever probe using the photoacoustic effect.

[0122] 3. The method according to aspect 2, wherein the generation of the ultrasonic waves is performed by directing a laser beam onto the cantilever probe.

[0123] 4. The method according to aspect 3, wherein an ultrasonic wave generating layer is provided on the cantilever and the laser beam is directed onto the ultrasonic wave generating layer to generate ultrasonic waves in the ultrasonic wave generating layer.

[0124] 5. The method according to aspect 4, wherein the ultrasonic wave generating layer comprises a metallic material.

[0125] 6. The method according to aspect 4 or 5, wherein the composition and size of the ultrasonic wave generating layer are selected such that at least a portion of the ultrasonic waves generated in the ultrasonic wave generating layer have a frequency higher than 15 GHz.

[0126] 7. The method according to any one of aspects 4 - 6, wherein the thickness of the ultrasonic wave generating layer is less than 500 nm.

[0127] 8. The method according to any one of the preceding aspects, wherein the detection of the reflected ultrasonic waves comprises detecting a change in the optical reflectivity of the cantilever probe.

[0128] 9. The method according to any one of the preceding aspects, wherein the reflected ultrasonic waves are detected at a plurality of positions of the cantilever probe relative to the target structure.

[0129] 10. The method according to any one of the preceding aspects, wherein the target structure comprises a first sub - structure superposed with a second sub - structure.

[0130] 11. The method according to aspect 10, wherein:

[0131] The profile of the first sub-structure is obtained by measuring the deflection of the cantilever caused by the interaction between the probe element and the first sub-structure at a plurality of positions of the probe element relative to the first sub-structure; and

[0132] Information related to the second sub-structure is obtained from the detected reflected ultrasonic waves.

[0133] 12. The method according to aspect 10 or 11, wherein the information related to the target structure includes information related to the overlap between the first sub-structure and the second sub-structure, and the overlap represents the degree of misalignment between the first sub-structure and the second sub-structure.

[0134] 13. The method according to aspect 12, wherein:

[0135] Each of the first sub-structure and the second sub-structure includes a plurality of repeating elements; and

[0136] The reflected ultrasonic waves are detected at each position in a set of positions of the target structure relative to the cantilever probe, and each position of the target structure relative to the cantilever probe is a position where the probe element is peak-aligned with a different one of the repeating elements of the first sub-structure.

[0137] 14. The method according to aspect 13, wherein the alignment of the probe element with each repeating element is determined by measuring the deflection of the cantilever caused by the interaction between the probe element and the first sub-structure.

[0138] 15. The method according to aspect 13 or 14, wherein:

[0139] The first sub-structure has a pitch different from that of the second sub-structure, such that the variation of the intensity of the ultrasonic waves reflected from the target structure with the position of the probe element relative to the target structure includes a beat frequency term, and the beat frequency term has a frequency defined by the difference between the pitches of the first sub-structure and the second sub-structure and a phase defined by the overlap; and

[0140] The phase of the beat frequency term is detected and used to determine the overlap.

[0141] 16. The method according to any one of aspects 12 - 14, wherein:

[0142] The target structure includes a first sub-target and a second sub-target;

[0143] The first sub-goal and the second sub-goal each include two sub-structures with an intentionally applied overlapping bias therebetween; and

[0144] The overlapping bias of the first sub-goal is different from the overlapping bias of the second sub-goal.

[0145] 17. The method according to aspect 16, wherein the overlapping bias of the first sub-goal is equal to and opposite to the overlapping bias of the second sub-goal.

[0146] 18. The method according to aspect 17, wherein:

[0147] The two sub-structures of the first sub-goal have the same pitch as the two sub-structures of the second sub-goal ;

[0148] The overlapping bias of the first sub-goal is ; and

[0149] The overlapping bias of the second sub-goal is .

[0150] 19. The method according to any one of aspects 16-18, wherein the overlap is determined based on the difference between the following intensities:

[0151] The intensity of the detected reflected ultrasonic wave from the first sub-goal when the probe element is aligned with the peak of the repeating element of the sub-structure of the first sub-goal closest to the probe element; and

[0152] The intensity of the detected reflected ultrasonic wave from the second sub-goal when the probe element is aligned with the peak of the repeating element of the sub-structure of the second sub-goal closest to the probe element.

[0153] 20. The method according to aspect 16, wherein:

[0154] The target structure further includes a third sub-goal and a fourth sub-goal;

[0155] The third sub-goal and the fourth sub-goal each include two sub-structures with an intentionally applied overlapping bias therebetween; and

[0156] The overlapping biases of the first sub-goal, the second sub-goal, the third sub-goal, and the fourth sub-goal are all different from each other.

[0157] 21. The method according to aspect 20, wherein:

[0158] Each of the two sub - structures of the first sub - target, the second sub - target, the third sub - target, and the fourth sub - target has the same pitch ;

[0159] The overlap offset of the first sub - target is ;

[0160] The overlap offset of the second sub - target is ;

[0161] The overlap offset of the third sub - target is ;

[0162] The overlap offset of the fourth sub - target is ;

[0163] and are constants; and

[0164] According to , , and to determine the overlap, where:

[0165] is the intensity of the detected reflected ultrasonic wave from the first sub - target when the probe element is aligned with the peak of the repeating element of the sub - structure of the first sub - target closest to the probe element;

[0166] is the intensity of the detected reflected ultrasonic wave from the second sub - target when the probe element is aligned with the peak of the repeating element of the sub - structure of the second sub - target closest to the probe element;

[0167] is the intensity of the detected reflected ultrasonic wave from the third sub - target when the probe element is aligned with the peak of the repeating element of the sub - structure of the third sub - target closest to the probe element; and

[0168] is the intensity of the detected reflected ultrasonic wave from the fourth sub - target when the probe element is aligned with the peak of the repeating element of the sub - structure of the fourth sub - target closest to the probe element.

[0169] 22. The method according to aspect 21, wherein:

[0170] ; and

[0171] The overlap is determined by . ​

[0172] 23. The method according to aspect 12, wherein:

[0173] The first sub-structure is laterally displaced relative to the second sub-structure such that at least a portion of the second sub-structure does not overlap with any of the first sub-structures when viewed in a plane perpendicular to the substrate;

[0174] The profile of the first sub-structure is obtained by measuring the deflection of the cantilever caused by the interaction between the probe element and the first sub-structure at a plurality of positions of the probe element relative to the first sub-structure; and

[0175] The profile of the second sub-structure is obtained by detecting the reflected ultrasonic waves at a plurality of positions of the probe element relative to the second sub-structure within a portion of the second sub-structure that does not overlap with any of the first sub-structures.

[0176] 24. The method according to any of the preceding aspects, wherein:

[0177] The cantilever probe includes both the probe element and the additional probe element; and

[0178] The reflected ultrasonic waves are mainly detected by the additional probe element.

[0179] 25. The method according to aspect 24, wherein the probe element is configured to direct the ultrasonic waves into the target structure at a position different from the position where the additional probe element receives the reflected ultrasonic waves.

[0180] 26. The method according to aspect 24 or 25, wherein:

[0181] At least a portion of the probe element tapers to have a decreasing cross-sectional area towards the target structure; and

[0182] At least a portion of the additional probe element tapers to have an increasing cross-sectional area towards the target structure.

[0183] 27. The method according to any one of aspects 1-23, wherein the reflected ultrasonic waves are mainly detected by the probe element.

[0184] 28. The method according to aspect 27, wherein:

[0185] The ultrasonic waves propagating towards the target structure mainly travel through a first portion of the probe element; and

[0186] The reflected ultrasonic waves propagating away from the target structure mainly travel through a second portion of the probe element different from the first portion.

[0187] 29. The method according to aspect 28, wherein the first part and the second part are formed of different materials.

[0188] 30. The method according to aspect 29, wherein the first part includes an outer sheath region and the second part includes a central region located inside the outer sheath region, and the central region and the outer sheath region are configured to act as a waveguide to cause the reflected ultrasonic wave to propagate backward from the target structure through the probe element toward the cantilever, such that the reflected ultrasonic wave mainly propagates through the central region of the probe element.

[0189] 31. The method according to any one of aspects 28 - 30, wherein at least a part of the first part tapers to have a cross-sectional area decreasing toward the target structure.

[0190] 32. The method according to any one of aspects 28 - 31, wherein at least a part of the second part tapers to have a cross-sectional area decreasing toward the cantilever.

[0191] 33. The method according to any one of the foregoing aspects, wherein:

[0192] the probe element includes a longitudinally proximal part and a longitudinally distal part;

[0193] the longitudinally proximal part is connected to the cantilever and extends from the cantilever to the longitudinally distal part; and

[0194] the longitudinally distal part extends from the longitudinally proximal part toward the target structure, wherein:

[0195] the longitudinally distal part tapers to have a cross-sectional area decreasing toward the target structure, and the longitudinally proximal part includes at least a part that does not taper or includes at least a part that tapers to have a cross-sectional area decreasing toward the cantilever.

[0196] 34. The method according to aspect 33, wherein the longitudinally proximal part includes a part that tapers to have a cross-sectional area decreasing toward the cantilever and a part that tapers to have a cross-sectional area decreasing toward the target part.

[0197] 35. The method according to aspect 33 or 34, wherein the longitudinally proximal part has a constant cross-sectional area from the longitudinally distal part to the cantilever.

[0198] 36. The method according to any one of aspects 1 - 35, wherein:

[0199] The cantilever probe includes a plurality of cantilevers, each cantilever having a respective probe element extending from the cantilever towards the target structure;

[0200] Generation of ultrasonic waves in the cantilever probe causes the ultrasonic waves to propagate through each respective probe element into the target structure; and

[0201] Detection of the reflected ultrasonic waves includes detecting the reflected ultrasonic waves from each respective probe element.

[0202] 37. The method according to aspect 36, wherein:

[0203] Generation of the ultrasonic waves is performed using a photoacoustic effect driven by a laser beam; and

[0204] The laser beam is optically processed to provide structured illumination that matches the spatial distribution of the plurality of cantilevers.

[0205] 38. The method according to aspect 37, wherein the optical processing is performed by diffraction.

[0206] 39. The method according to aspect 37, wherein the optical processing is performed using an array of individually controllable elements.

[0207] 40. The method according to any one of aspects 36 - 39, wherein detection of the reflected ultrasonic waves includes directing a probing beam of radiation onto the cantilever and detecting the radiation reflected from the cantilever.

[0208] 41. The method according to aspect 40, wherein the probing beam is optically processed to provide structured illumination that matches the cantilevers of the plurality of cantilevers.

[0209] 42. The method according to aspect 41, wherein the optical processing of the probing beam is performed using an array of individually controllable elements.

[0210] 43. The method according to aspect 40, wherein the probing beam is focused into a line focus that intersects the plurality of cantilevers.

[0211] 44. The method according to any one of aspects 40 - 43, wherein the radiation reflected from each of the cantilevers is detected in parallel.

[0212] 45. A metrology tool for determining information related to a target structure formed on a substrate using a lithography process, the metrology tool comprising:

[0213] A cantilever probe having a cantilever and a probe element, the probe element being configured to extend from the cantilever towards the target structure;

[0214] An ultrasonic generation system configured to generate ultrasonic waves in the cantilever probe such that the ultrasonic waves propagate through the probe element into the target structure and reflect back from the target structure into the probe element or into an additional probe element extending from the cantilever; and

[0215] An ultrasonic detection system configured to detect the reflected ultrasonic waves.

[0216] 46. The metrology tool according to aspect 45, wherein:

[0217] The cantilever probe includes a plurality of cantilevers, each cantilever having a respective probe element configured to extend from the cantilever toward the target structure;

[0218] The ultrasonic generation system is configured to cause ultrasonic waves to propagate through each respective probe element into the target structure; and

[0219] The ultrasonic detection system is configured to detect the reflected ultrasonic waves from each respective probe element.

[0220] 47. A cantilever probe for determining information related to a target structure formed on a substrate using a lithography process, the cantilever probe comprising:

[0221] A cantilever;

[0222] A probe element configured to extend from the cantilever toward the target structure; and

[0223] An additional probe element configured to extend from the cantilever toward the target structure, wherein:

[0224] At least a portion of the probe element tapers to have a decreasing cross-sectional area toward the target structure; and

[0225] At least a portion of the additional probe element tapers to have an increasing cross-sectional area toward the target structure.

[0226] 48. A cantilever probe for determining information related to a target structure formed on a substrate using a lithography process, the cantilever probe comprising:

[0227] A cantilever; and

[0228] A probe element configured to extend from the cantilever toward the target structure, wherein:

[0229] The probe element includes a first part and a second part, and the first part and the second part are formed of different materials;

[0230] The first part includes an outer sheath region and the second part includes a central region located inside the outer sheath region, and the central region and the outer sheath region are configured to act as waveguides to enable ultrasonic waves to propagate from the target structure through the probe element towards the cantilever.

[0231] 49. The probe according to aspect 48, wherein at least a part of the outer sheath region tapers to have a cross-sectional area decreasing towards the target structure.

[0232] 50. The probe according to aspect 48 or 49, wherein at least a part of the second part tapers to have a cross-sectional area decreasing towards the cantilever.

[0233] 51. A cantilever probe for determining information related to a target structure formed on a substrate using a lithography process, the cantilever probe comprising:

[0234] A cantilever; and

[0235] A probe element configured to extend from the cantilever towards the target structure, wherein:

[0236] The probe element includes a longitudinally proximal part and a longitudinally distal part;

[0237] The longitudinally proximal part is connected to the cantilever and extends from the cantilever to the longitudinally distal part; and

[0238] The longitudinally distal part is configured to extend from the longitudinally proximal part towards the target structure, wherein:

[0239] The longitudinally distal part tapers to have a cross-sectional area decreasing towards the target structure, and the longitudinally proximal part includes at least a part that does not taper or includes at least a part that tapers to have a cross-sectional area decreasing towards the cantilever.

[0240] 52. A lithography system, comprising:

[0241] A lithography apparatus configured to define a pattern for forming a target structure on a substrate; and

[0242] A metrology tool according to aspect 45.

Claims

1. A method for determining information related to a target structure formed on a substrate using a lithography process, wherein the target structure includes a first sub-structure and a second sub-structure, and the first sub-structure overlaps with the second sub-structure, the method comprises: providing a cantilever probe, the cantilever probe including a cantilever and a probe element, the probe element extending from the cantilever towards the target structure; generating ultrasonic waves in the cantilever probe, the ultrasonic waves propagating through the probe element into the target structure and reflecting back from the target structure into the probe element or into another probe element extending from the cantilever; and detecting the reflected ultrasonic waves and determining information related to the target structure based on the detected reflected ultrasonic waves, wherein the information related to the target structure includes information related to the overlap between the first sub-structure and the second sub-structure, the overlap indicating the degree of misalignment between the first sub-structure and the second sub-structure; wherein the information related to the overlap is determined based on the intensity of the detected reflected ultrasonic waves and a function representing the variation of the intensity with respect to the overlap; wherein the generation of the ultrasonic waves is performed by directing a laser beam onto the cantilever probe.

2. The method according to claim 1, wherein the ultrasonic waves are generated in the cantilever probe using the photoacoustic effect.

3. The method according to claim 1, wherein an ultrasonic wave generating layer is provided on the cantilever, and the laser beam is directed onto the ultrasonic wave generating layer to generate ultrasonic waves in the ultrasonic wave generating layer.

4. The method according to claim 3, wherein the ultrasonic wave generating layer includes a metallic material.

5. The method according to claim 3 or 4, wherein the composition and size of the ultrasonic wave generating layer are selected such that at least a part of the ultrasonic waves generated in the ultrasonic wave generating layer have a frequency higher than 15 GHz.

6. The method according to claim 3 or 4, wherein the thickness of the ultrasonic wave generating layer is less than 500 nm.

7. The method according to any one of claims 1 to 4, wherein the detection of the reflected ultrasonic waves includes detecting a change in the optical reflectivity of the cantilever probe.

8. The method according to any one of claims 1 to 4, wherein the reflected ultrasonic waves are detected at a plurality of positions of the cantilever probe relative to the target structure.

9. The method according to claim 1, wherein: the profile of the first sub-structure is obtained by measuring the deflection of the cantilever caused by the interaction between the probe element and the first sub-structure at a plurality of positions of the probe element relative to the first sub-structure; and information related to the second sub-structure is obtained from the detected reflected ultrasonic waves.

10. The method according to claim 9, wherein: each of the first sub-structure and the second sub-structure includes a plurality of repeating elements; and Detect the reflected ultrasonic waves at each position in a set of positions of the target structure relative to the cantilever probe, where each position of the target structure relative to the cantilever probe is a position where the probe element is peak-aligned with a different one of the repeating elements in the repeating elements of the first sub-structure.

11. The method according to claim 10, wherein the alignment of the probe element with each repeating element is determined by measuring the deflection of the cantilever caused by the interaction between the probe element and the first sub-structure.

12. The method according to claim 10 or 11, wherein: The first sub-structure has a different pitch from the second sub-structure, such that the variation of the intensity of the ultrasonic waves reflected from the target structure with the position of the probe element relative to the target structure includes a beat frequency term, the beat frequency term having a frequency defined by the difference between the pitches of the first sub-structure and the second sub-structure and a phase defined by the overlap; and The phase of the beat frequency term is detected and used to determine the overlap.

13. The method according to any one of claims 9 to 11, wherein: The target structure includes a first sub-target and a second sub-target; The first sub-target and the second sub-target each include two sub-structures, with an intentionally applied overlap bias therebetween; and The overlap bias of the first sub-target is different from the overlap bias of the second sub-target.

14. The method according to claim 13, wherein the overlap bias of the first sub-target is equal to the overlap bias of the second sub-target and opposite to the overlap bias of the second sub-target.

15. The method according to claim 14, wherein: The two sub-structures of the first sub-goal have the same pitch as the two sub-structures of the second sub-goal ; The overlapping bias of the first sub-goal is ; and The overlapping bias of the second sub-goal is .

16. The method according to claim 13, wherein the overlap is determined based on the difference between the following intensities: The intensity of the detected reflected ultrasonic waves from the first sub-target when the probe element is peak-aligned with the repeating element of the sub-structure of the first sub-target closest to the probe element; and The intensity of the detected reflected ultrasonic waves from the second sub-target when the probe element is peak-aligned with the repeating element of the sub-structure of the second sub-target closest to the probe element.

17. The method according to claim 13, wherein: The target structure further includes a third sub-target and a fourth sub-target; The third sub-target and the fourth sub-target each include two sub-structures, with an intentionally applied overlap bias therebetween; and The overlap biases of the first sub-target, the second sub-target, the third sub-target, and the fourth sub-target are all different from each other.

18. The method according to claim 17, wherein: The two substructures of each of the first sub-goal, the second sub-goal, the third sub-goal, and the fourth sub-goal have the same pitch ; The overlapping bias of the first sub-goal is ; The overlapping bias of the second sub-goal is ; The overlapping bias of the third sub-goal is ; The overlapping bias of the fourth sub-goal is ; and is a constant; and According to , , and to determine the overlap, where: is the intensity of the detected reflected ultrasonic wave from the first sub-target when the probe element is aligned with the peak of the repeating element of the sub-structure of the first sub-target closest to the probe element; is the intensity of the detected reflected ultrasonic wave from the second sub-target when the probe element is aligned with the peak of the repeating element of the sub-structure of the second sub-target that is closest to the probe element; is the intensity of the detected reflected ultrasonic wave from the third sub-target when the probe element is aligned with the peak of the repeating element of the sub-structure of the third sub-target closest to the probe element; and is the intensity of the detected reflected ultrasonic wave from the fourth sub-target when the probe element is aligned with the peak of the repeating element of the sub-structure of the fourth sub-target closest to the probe element.

19. The method according to claim 18, wherein: ; and By to determine the overlap .

20. The method according to claim 9, wherein: The first sub-structure is laterally displaced relative to the second sub-structure such that at least a portion of the second sub-structure does not overlap with any of the first sub-structures when viewed in a plane perpendicular to the substrate. The profile of the first sub-structure is obtained by measuring the deflection of the cantilever caused by the interaction between the probe element and the first sub-structure at a plurality of positions of the probe element relative to the first sub-structure; and the profile of the second sub-structure is obtained by detecting the reflected ultrasonic waves at a plurality of positions of the probe element relative to the second sub-structure within a portion of the second sub-structure that does not overlap with any of the first sub-structures.

21. The method according to any one of claims 1 to 4, wherein: the cantilever probe includes both the probe element and the additional probe element; and the reflected ultrasonic waves are mainly detected by the additional probe element.

22. The method according to claim 21, wherein the probe element is configured to direct the ultrasonic waves into the target structure at a position different from the position where the additional probe element receives the reflected ultrasonic waves.

23. The method according to claim 21, wherein: at least a portion of the probe element tapers to have a cross-sectional area that decreases towards the target structure; and at least a portion of the additional probe element tapers to have a cross-sectional area that increases towards the target structure.

24. The method according to any one of claims 1 to 4, wherein the reflected ultrasonic waves are mainly detected by the probe element.

25. The method according to claim 24, wherein: the ultrasonic waves propagating towards the target structure mainly travel through a first portion of the probe element; and the reflected ultrasonic waves propagating away from the target structure mainly travel through a second portion of the probe element that is different from the first portion.

26. The method according to claim 25, wherein the first portion and the second portion are formed of different materials.

27. The method according to claim 26, wherein the first portion includes an outer sheath region and the second portion includes a central region located inside the outer sheath region, and the central region and the outer sheath region are configured to act as waveguides to enable the reflected ultrasonic waves to propagate backward from the target structure through the probe element towards the cantilever, such that the reflected ultrasonic waves mainly propagate through the central region of the probe element.

28. The method according to claim 25, wherein at least a portion of the first portion tapers to have a cross-sectional area that decreases towards the target structure.

29. The method according to claim 25, wherein at least a portion of the second portion tapers to have a cross-sectional area that decreases towards the cantilever.

30. The method according to any one of claims 1 to 4, wherein: the probe element includes a longitudinally proximal portion and a longitudinally distal portion; the longitudinally proximal portion is connected to the cantilever and extends from the cantilever to the longitudinally distal portion; and the longitudinally distal portion extends from the longitudinally proximal portion towards the target structure, wherein: The longitudinal distal portion tapers to have a cross-sectional area that decreases toward the target structure, and the longitudinal proximal portion includes at least a part that does not taper or includes at least a part that tapers to have a cross-sectional area that decreases toward the cantilever.

31. The method according to claim 30, wherein the longitudinal proximal portion includes a part that tapers to have a cross-sectional area that decreases toward the cantilever and a part that tapers to have a cross-sectional area that decreases toward the target portion.

32. The method according to claim 30, wherein the longitudinal proximal portion has a constant cross-sectional area from the longitudinal distal portion to the cantilever.

33. The method according to any one of claims 1 to 4, wherein: the cantilever probe includes a plurality of cantilevers, each cantilever having a respective probe element extending from the cantilever toward the target structure; generation of ultrasonic waves in the cantilever probe causes the ultrasonic waves to propagate through each respective probe element into the target structure; and detection of the reflected ultrasonic waves includes detecting the reflected ultrasonic waves from each respective probe element.

34. The method according to claim 33, wherein: generation of the ultrasonic waves is performed using a photoacoustic effect driven by a laser beam; and the laser beam is optically processed to provide structured illumination that matches the spatial distribution of the plurality of cantilevers.

35. The method according to claim 34, wherein the optical processing is performed by diffraction.

36. The method according to claim 34, wherein the optical processing is performed using an array of individually controllable elements.

37. The method according to claim 33, wherein detection of the reflected ultrasonic waves includes directing a probing beam of radiation onto the cantilever and detecting the radiation reflected from the cantilever.

38. The method according to claim 37, wherein the probing beam is optically processed to provide structured illumination that matches the cantilevers of the plurality of cantilevers.

39. The method according to claim 38, wherein the optical processing of the probing beam is performed using an array of individually controllable elements.

40. The method according to claim 37, wherein the probing beam is focused into a line focus that intersects the plurality of cantilevers.

41. The method according to claim 37, wherein the radiation reflected from each of the cantilevers is detected in parallel.

42. A metrology tool for determining information related to a target structure formed on a substrate using a lithography process, wherein the target structure includes a first sub-structure and a second sub-structure, the first sub-structure overlapping the second sub-structure, the metrology tool comprises: a cantilever probe having a cantilever and a probe element, the probe element being configured to extend from the cantilever toward the target structure; An ultrasonic generation system, the ultrasonic generation system being configured to generate ultrasonic waves in the cantilever probe by directing a laser beam onto the cantilever probe, such that the ultrasonic waves propagate through the probe element into the target structure and reflect back from the target structure into the probe element or into an additional probe element extending from the cantilever; and An ultrasonic detection system, the ultrasonic detection system being configured to detect the reflected ultrasonic waves; wherein the information related to the target structure includes information related to the overlap between the first sub-structure and the second sub-structure, the overlap indicating the degree of misalignment between the first sub-structure and the second sub-structure; wherein the information related to the overlap is determined based on the intensity of the detected reflected ultrasonic waves and a function representing the variation of the intensity with respect to the overlap.

43. The metrology tool according to claim 42, wherein: The cantilever probe includes a plurality of cantilevers, each cantilever having a respective probe element configured to extend from the cantilever towards the target structure; The ultrasonic generation system is configured to cause ultrasonic waves to propagate through each respective probe element into the target structure; and The ultrasonic detection system is configured to detect the reflected ultrasonic waves from each respective probe element.

44. A cantilever probe for use in a metrology tool for determining information related to a target structure formed on a substrate using a lithography process according to any one of claims 1 - 41, the cantilever probe comprising: A cantilever; A probe element configured to extend from the cantilever towards the target structure; and An additional probe element configured to extend from the cantilever towards the target structure, wherein: At least a portion of the probe element tapers to have a cross-sectional area that decreases towards the target structure; and At least a portion of the additional probe element tapers to have a cross-sectional area that increases towards the target structure.

45. A cantilever probe for use in a metrology tool for determining information related to a target structure formed on a substrate using a lithography process according to any one of claims 1 - 41, the cantilever probe comprising: A cantilever; and A probe element configured to extend from the cantilever towards the target structure, wherein: The probe element includes a first portion and a second portion formed of different materials; The first portion includes an outer sheath region and the second portion includes a central region located inside the outer sheath region, the central region and the outer sheath region being configured to act as a waveguide to cause ultrasonic waves to propagate from the target structure through the probe element towards the cantilever.

46. The probe according to claim 45, wherein at least a portion of the outer sheath region tapers to have a cross-sectional area that decreases towards the target structure.

47. The probe according to claim 45 or 46, wherein at least a portion of the second portion tapers to have a cross-sectional area that decreases towards the cantilever.

48. A cantilever probe for a metrology tool for determining information related to a target structure formed on a substrate using a lithography process according to the method of any one of claims 1-41, the cantilever probe comprising: a cantilever; and a probe element configured to extend from the cantilever towards the target structure, wherein: the probe element includes a longitudinal proximal portion and a longitudinal distal portion; the longitudinal proximal portion is connected to the cantilever and extends from the cantilever to the longitudinal distal portion; and the longitudinal distal portion is configured to extend from the longitudinal proximal portion towards the target structure, wherein: the longitudinal distal portion tapers to have a cross-sectional area that decreases towards the target structure, and the longitudinal proximal portion includes at least a portion that does not taper or includes at least a portion that tapers to have a cross-sectional area that decreases towards the cantilever.

49. A lithography system, comprising: a lithography apparatus configured to define a pattern for forming a target structure on a substrate; and the metrology tool according to claim 42.

Images