Holographic metrology device and method

Abstract

A method of determining a parameter of interest of a structure including at least one first feature oriented along a first axis of a structure coordinate system and at least one second feature oriented along a second axis of the structure coordinate system. The method includes illuminating the first feature and the second feature with a first illumination from a first direction inclined with respect to the first and second axes to generate first scattered radiation from the first feature and second scattered radiation from the second feature; detecting a first interference pattern formed by interference between a portion of the first scattered radiation and a first reference illumination; detecting a second interference pattern formed by interference between a portion of the second scattered radiation and the first reference illumination; and determining the parameter of interest of the structure using the first interference pattern and the second interference pattern.

Description

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims priority to EP application 22193868.1, filed on September 5, 2022, and incorporated herein by reference in its entirety. Technical Field

[0003] The present invention relates to metrology such as digital holographic microscopy, in particular high-speed dark-field digital holographic microscopy, and to metrology applications in integrated circuit manufacturing. Background Art

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

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

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

[0007] During the manufacturing process it is necessary to inspect the manufactured structure and / or measure the properties of the manufactured structure. Suitable inspection and metrology equipment is known in the art. One of the known metrology equipment is a scatterometer and for example a dark field scatterometer.

[0008] Patent application publication US2016 / 0161864A1, patent application publication US2010 / 0328655A1, and patent application publication US2006 / 0066855A1 discuss embodiments of lithographic apparatus and embodiments of scatterometers. The cited documents are incorporated herein by reference.

[0009] More generally, the metrology arrangements mentioned above may have the problem of requiring at least four illuminators to measure the overlay of a structure or requiring more than one measurement to determine the overlay.

[0010] It is desirable to provide an illumination device for a metrology device that is suitable for use over a wide wavelength range. Summary of the invention

[0011] In a first aspect of the present invention, a method for determining a parameter of interest of a structure on a substrate is provided, the structure comprising at least one first feature oriented along a first axis of a structural coordinate system and at least one second feature oriented along a second axis of the structural coordinate system, the method comprising: illuminating the first feature and the second feature with a first illumination from a first direction inclined to the first axis and the second axis to generate a first scattered radiation from the first feature and a second scattered radiation from the second feature, detecting a first interference pattern formed by interference between a portion of the first scattered radiation and a first reference illumination; detecting a second interference pattern formed by interference between a portion of the second scattered radiation and the first reference illumination; and determining the parameter of interest of the structure using the first interference pattern and the second interference pattern.

[0012] In a second aspect of the present invention, a holographic metrology device is provided, comprising: a sensor module; a substrate support for supporting a substrate comprising a structure, the structure comprising at least one first feature oriented along a first axis of a structural coordinate system and at least one second feature oriented along a second axis of the structural coordinate system; and a reference illumination device for generating a first reference illumination; wherein the sensor module comprises: an illumination device comprising at least one illuminator, the at least one illuminator being operable to illuminate the structure with a first illumination from a first direction inclined to the first axis and the second axis to generate a first scattered radiation from the first feature and a second scattered radiation from the second feature; an optical system being operable to capture the first scattered radiation and the second scattered radiation; and a detector being operable to detect a first interference pattern formed by interference between a portion of the first scattered radiation and the first reference illumination and a second interference pattern formed by interference between a portion of the second scattered radiation and the first reference illumination. BRIEF DESCRIPTION OF THE DRAWINGS

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

[0014] Figure 1 depicts a schematic overview of a lithography apparatus;

[0015] Figure 2 A schematic overview of a lithography unit is depicted;

[0016] Figure 3 Depicted is a schematic representation of overall lithography showing the collaboration between three key technologies to optimize semiconductor manufacturing;

[0017] Figure 4 depicts a schematic overview of a scatterometry apparatus used as a metrology device, which may include a dark-field digital holographic microscope according to an embodiment of the present invention;

[0018] Figure 5 depicts a schematic overview of a level sensor device according to an embodiment of the present invention, which may include a dark-field digital holographic microscope;

[0019] Figure 6 depicts a schematic overview of an alignment sensor apparatus according to an embodiment of the present invention, which may include a dark-field digital holographic microscope;

[0020] Figure 7 schematically depicts an example of a diffraction-based dark-field metrology apparatus operating in a parallel acquisition scheme;

[0021] Figure 8 schematically depicting different examples of diffraction-based dark-field metrology devices operating in a sequential acquisition scheme;

[0022] Fig. 9 An example of a dark-field digital holographic microscope operating in a sequential acquisition scheme is schematically depicted;

[0023] Fig.10 Schematic depiction of a dark-field digital holographic microscope (df-DHM) operating in a parallel acquisition scheme;

[0024] Fig.11A Schematic depiction of a dark-field digital holographic microscope (df-DHM) operating in a parallel acquisition scheme;

[0025] Fig. 11B Depicted is the objective lens relative to df-DHM Fig.11A The k-space of df-DHM is shown;

[0026] Fig. 11C Describes the use Fig.11Adf-DHM, an image spectrum in the spatial frequency domain of the diffraction orders generated after illumination of the measurement target;

[0027] Fig.11D Describes the use Fig.11A df-DHM, a surrogate image spectrum in the spatial frequency domain of the diffraction orders generated after illumination of the measurement target;

[0028] Fig. 12A schematically depicts a dark-field digital holographic microscope (df-DHM) according to an embodiment;

[0029] Fig. 12B depicting a structure including two first features oriented along a first axis of a structural coordinate system and two second features oriented along a second axis of the structural coordinate system;

[0030] Fig. 12C Depicted Fig. 12A The relative position of each illuminator in df-DHM is visualized in k-space;

[0031] Fig.12D depicts a schematic representation of an example arrangement of illuminators; and

[0032] Fig.13 Depicted is a block diagram of a computer system for controlling a dark-field digital holographic microscope. DETAILED DESCRIPTION

[0033] In this document, the terms "radiation" and "beam" are used to cover all types of electromagnetic radiation, including ultraviolet radiation (e.g., wavelengths of 365, 248, 193, 157 or 126 nm) and EUV (extreme ultraviolet radiation, e.g., wavelengths in the range of about 5 to 100 nm).

[0034] The terms "reticle", "mask" or "patterning device" as used herein may be broadly interpreted as referring to a general patterning device that can be used to impart a patterned cross-section to an incoming radiation beam, corresponding to the pattern to be created in a target portion of a substrate. In this context, the term "light valve" may also be used. In addition to classical masks (transmissive or reflective, binary, phase-shifting, hybrid, etc.), other examples of such patterning devices include programmable mirror arrays and programmable LCD arrays.

[0035] Figure 1A lithographic apparatus LA is schematically depicted. The lithographic apparatus LA comprises an illumination system (also referred to as illuminator) IL configured to condition a radiation beam B (e.g. UV radiation, DUV radiation or EUV radiation), a mask support (e.g. mask table) MT configured to support a patterning device (e.g. mask) MA and connected to a first positioner PM (configured to accurately position the patterning device MA according to certain parameters), a substrate support (e.g. wafer stage) WT configured to hold a substrate (e.g. resist coated wafer) W and connected to a second positioner PW (configured to accurately position the substrate support according to certain parameters), and a projection system (e.g. refractive projection lens system) PS configured to project a pattern imparted to the radiation beam B by the patterning device MA onto a target portion C (e.g. comprising one or more dies) of the substrate W.

[0036] In operation, the illumination system IL receives a radiation beam from a radiation source SO (e.g., via a beam delivery system BD). The illumination system IL may include various types of optical components, such as refractive, reflective, magnetic, electromagnetic, electrostatic and / or other types of optical components, or any combination thereof, for directing, shaping and / or controlling the radiation. The illuminator IL may be used to condition the radiation beam B to have a desired spatial and angular intensity distribution in its cross-section at the plane of the patterning device MA.

[0037] The term "projection system" PS as used herein should be broadly interpreted as covering various types of projection systems, including refractive, reflective, catadioptric, anamorphic, magnetic, electromagnetic and / or electrostatic optical systems or any combination thereof, as appropriate with respect to the exposure radiation used and / or other factors such as the use of an immersion liquid or the use of a vacuum. Any use of the term "projection lens" herein may be considered synonymous with the more general term "projection system" PS.

[0038] The lithographic apparatus LA may be of a type in which at least a portion of the substrate may be covered by a liquid having a relatively high refractive index, such as water, to fill the space between the projection system PS and the substrate W, which is also known as immersion lithography. More information on immersion technology is given in US6952253, which is incorporated herein by reference.

[0039] The lithographic apparatus LA may also be of a type having two or more substrate supports WT (also referred to as a "dual stage"). In such a "multi-stage" machine, the substrate supports WT may be used in parallel, and / or steps for preparing a substrate W for subsequent exposure may be performed on a substrate W located on one of the substrate supports WT, while another substrate W on another substrate support WT is used to expose a pattern on another substrate W.

[0040] In addition to the substrate support WT, the lithographic apparatus LA may comprise a measurement platform. The measurement platform is arranged to hold sensors and / or cleaning means. The sensors may be arranged to measure properties of the projection system PS or properties of the radiation beam B. The measurement platform may hold a plurality of sensors. The cleaning means may be arranged to clean a part of the lithographic apparatus, e.g. a part of the projection system PS or a part of a system for providing immersion liquid. The measurement platform may be moved under the projection system PS when the substrate support WT is away from the projection system PS.

[0041] In operation, a radiation beam B is incident on a patterning device (e.g. a mask) MA and is patterned by a pattern (design layout) present on the patterning device MA, which is held on a mask support MT. After traversing the mask MA, the radiation beam B passes through a projection system PS which focuses the beam onto a target portion C of a substrate W. With the aid of a second positioner PW and a position measurement system IF, the substrate support WT can be accurately moved, for example in order to position different target portions C in the path of the radiation beam B at focused and aligned positions. Similarly, the first positioner PM and possibly another position sensor (not shown) are also provided. Figure 1 The patterning device MA (depicted explicitly in FIG. 1 ) can be used to accurately position the patterning device MA relative to the path of the radiation beam B. The patterning device MA and the substrate W can be aligned using the mask alignment marks M1, M2 and the substrate alignment marks P1, P2. Although the illustrated substrate alignment marks P1, P2 occupy dedicated target portions, they can be located in the space between target portions. When the substrate alignment marks P1, P2 are located between target portions C, these are referred to as scribe line alignment marks.

[0042] like Figure 2 As shown, the lithography apparatus LA may form part of a lithography cell LC, which is sometimes also referred to as a lithography cell or (lithography) cluster, which typically also includes equipment for performing pre- and post-exposure processes on a substrate W. Conventionally, these include a spin coater SC for depositing a resist layer, a developer DE for developing the exposed resist, a chill plate CH and a bake plate BK, for example, for regulating the temperature of the substrate W (e.g. for regulating the solvent in the resist layer). A substrate handler or robot RO picks up substrates W from input / output ports I / O1, I / O2, moves these substrates W between different process equipment, and delivers substrates W to a feed station LB of the lithography apparatus LA. The devices in the lithography cell (often also collectively referred to as tracks) are typically controlled by a track control unit TCU, which itself may be controlled by a supervisory control system SCS, which may also control the lithography apparatus LA, for example via a lithography control unit LACU.

[0043] In order for substrates W exposed by lithographic apparatus LA to be correctly and consistently exposed, it is desirable to inspect the substrates to measure properties of the patterned structures, such as overlay errors between subsequent layers, line thickness, critical dimensions (CD), etc. For this purpose, an inspection tool (not shown) may be included in lithography cell LC. If errors are detected, for example, adjustments may be made to the exposure of subsequent substrates or other processing steps to be performed on substrate W, particularly if the inspection is done before other substrates W of the same batch or lot are still to be exposed or processed.

[0044] An inspection apparatus (which may also be referred to as a metrology apparatus) is used to determine properties of a substrate W, in particular how properties vary from one substrate W to another, or how properties associated with different layers of the same substrate W vary from layer to layer. The inspection apparatus may alternatively be configured to identify defects on the substrate W, and may for example be part of a lithography cell LC, or may be integrated into a lithography apparatus LA, or may even be a stand-alone device. The inspection apparatus may measure properties on a latent image (the image in the resist layer after exposure) or a semi-latent image (the image in the resist layer after a post-exposure bake step PEB), or a developed resist image (where exposed or unexposed parts of the resist have been removed) or even an etched image (after a pattern transfer step such as etching).

[0045] Typically, the patterning process in the lithography apparatus LA is one of the most critical steps in the process, which requires high accuracy in the dimensioning and placement of structures on the substrate W. In order to ensure this high accuracy, the three systems can be combined into a so-called "holistic" control loop environment, such as Figure 3 Schematically depicted in . One of these systems is the lithography apparatus LA, which is (in practice) connected to a metrology tool MT (a second system) and a computer system CL (a third system). The key to this "holistic" environment is to optimize the cooperation between these three systems to enhance the overall process window, and to provide a tight control loop to ensure that the patterning performed by the lithography apparatus LA remains within the process window. The process window defines the range of process parameters (e.g. dose, focus, overlay) within which a specific manufacturing process will produce a defined result (e.g. a functional semiconductor device), and typically within which process parameters of a lithography process or patterning process are allowed to vary.

[0046] The computer system CL may use (a portion of) the design layout to be patterned to predict the resolution enhancement technique to be used, and perform computational lithography simulations and calculations to determine which mask layouts and lithography equipment settings achieve the maximum overall process window for the patterning process (in Figure 3Typically, the resolution enhancement technique is arranged to match the patterning possibilities of the lithographic apparatus LA. The computer system CL may also be used to detect where within the process window the lithographic apparatus LA is currently operating (e.g. using input from a metrology tool MT) to predict whether defects may be present due to, for example, suboptimal processing (e.g., Figure 3 ) is depicted by an arrow pointing to “0” in the second scale SC2.

[0047] The metrology tool MT may provide input to the computer system CL to enable accurate simulations and predictions, and may provide feedback to the lithographic apparatus LA to identify possible drifts, e.g. in the calibration state of the lithographic apparatus LA (in Figure 3 ) as depicted by multiple arrows in a third scale SC3.

[0048] During the lithographic process, it is desirable to frequently measure the structures created, for example for process control and verification. The tool that performs such measurements is generally referred to as a metrology tool MT. Different types of metrology tools MT for performing such measurements are known, including scanning electron microscopes or various forms of scatterometer metrology tools MT. Scatterometers are versatile instruments that allow parameters of the lithographic process to be measured by placing a sensor in the pupil of the scatterometer objective or in a plane conjugate to the pupil (these measurements are generally referred to as pupil-based measurements), or by placing the sensor in the image plane or in a plane conjugate to the image plane (in this case, these measurements are generally referred to as image-based or field-based measurements). 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 scatterometer described above can measure the grating using light from soft x-rays and visible to the near IR wavelength range.

[0049] In a first embodiment, the scatterometer MT is an angle-resolved scatterometer. In such a scatterometer, reconstruction methods may be applied to the measurement signals to reconstruct or calculate the properties of the grating. For example, such a reconstruction may be performed by simulating the interaction of the scattered radiation with a mathematical model of the target structure and comparing the simulation results with the measurement results. The parameters of the mathematical model are adjusted until the simulated interaction produces a diffraction pattern similar to that observed from a real target.

[0050] In a second embodiment, the scatterometer MT is a spectroscopic scatterometer MT. In such a spectroscopic scatterometer MT, radiation emitted by a radiation source is directed onto a target and reflected or scattered radiation from the target is directed to a spectrometer detector which measures the spectrum of the specularly reflected radiation (i.e. a measurement of the intensity as a function of wavelength). From this data the structure or profile of the target which produced the detected spectrum can be reconstructed, for example by rigorous coupled wave analysis and non-linear regression or by comparison with a library of simulated spectra.

[0051] In a third embodiment, the scatterometer MT is an ellipsometer. An ellipsometer allows the parameters of a lithography process to be determined by measuring the scattered radiation for each polarization state. Such a measurement device emits polarized light (such as linear, circular or elliptical) by using, for example, an appropriate polarization filter in the illumination section of the measurement device. A source suitable for the measurement device may 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 incorporated herein by reference in their entirety.

[0052] In one embodiment of the scatterometer MT, the scatterometer MT is adapted to measure the overlap of two misaligned gratings or periodic structures by measuring a reflection spectrum and / or an asymmetry in the detection configuration, the asymmetry being related to the degree of the overlap. The two (usually overlapping) grating structures may be applied in two different layers (not necessarily consecutive layers) and may be formed at substantially the same position on the wafer. The scatterometer may have a symmetrical detection configuration such as described in co-owned patent application EP1,628,164A, so that any asymmetry is clearly distinguishable. This provides a direct way of measuring misalignment in the gratings. Other examples for measuring the overlay error between two layers containing a periodic structure when measuring the target through the asymmetry of the periodic structure may be found in PCT patent application publication number WO 2011 / 012624 or in US patent application US 20160161863, which are incorporated herein by reference in their entirety.

[0053] Other parameters of interest may be focus and dose. Focus and dose may be determined simultaneously by a scatterometer (or alternatively by a scanning electron microscope), as described in U.S. Patent Application US2011-0249244, which is incorporated herein by reference in its entirety. A single structure may be used that has a unique combination of critical dimension and sidewall angle measurements for each point in a focus energy matrix (FEM, also called a focus exposure matrix). If these unique combinations of critical dimension and sidewall angle are available, then focus and dose values ​​may be uniquely determined from these measurements.

[0054] The metrology target may be the entirety of a composite grating, which is formed by a lithography process, mainly in a resist, but may also be formed after, for example, an etching process. Typically, the pitch and line width of the structures in the grating are largely determined by the measurement optics (particularly the NA of the optics) in order to be able to capture the diffraction orders from the metrology target. As indicated earlier, the diffraction signal may be used to determine the drift between two layers (also called 'overlay'), or may be used to reconstruct at least part of the original grating produced by the lithography process. This reconstruction may be used to provide guidance on the quality of the lithography process, and may be used to control at least part of the lithography process. The target may have smaller sub-segments, which are configured to mimic the size of a functional part of the design layout in the target. Due to the sub-segments, the behavior of the target will be more similar to the functional part of the design layout, so that the overall process parameter measurement better resembles the functional part of the design layout. The target may be measured in an underfill mode or in an overfill mode. In the underfill mode, the measurement beam generates a smaller spot than the overall target. In the overfill mode, the measurement beam generates a larger spot than the overall target. In this overfill mode, it is also possible to measure different targets simultaneously and thus determine different processing parameters simultaneously.

[0055] Figure 4 A measurement device such as a scatterometer is depicted in FIG. It comprises a broadband (white light) radiation projector 2 which projects radiation onto a substrate W. The reflected or scattered radiation is passed to a spectrometer detector 4 which measures the spectrum 6 of the specularly reflected radiation (i.e., measures the intensity as a function of wavelength). From this data, for example, by rigorous coupled wave analysis and nonlinear regression or by comparing Figure 3 The structure or profile 8 that produces the detected spectrum can be reconstructed by the processing unit PU by comparing it to the library of simulated spectra shown at the bottom. Typically, for the reconstruction, the general form of the structure is known and some parameters are assumed by knowledge of the process by which the structure was manufactured, with only a few parameters of the structure to be determined by scatterometry data. Such a scatterometer can be configured as a normal incidence scatterometer or an oblique incidence scatterometer.

[0056] The overall measurement quality of the lithographic parameters measured via the measurement target is at least partially determined by the measurement option used to measure the lithographic parameters. The term "substrate measurement option" can include one or more parameters of the measurement itself, one or more parameters of the one or more patterns being measured, or both. For example, if the measurement used in the substrate measurement option is a diffraction-based optical measurement, the one or more parameters measured can include the wavelength of the radiation, the polarization of the radiation, the angle of incidence of the radiation relative to the substrate, the orientation of the radiation relative to the pattern on the substrate, etc. One of the criteria for selecting the measurement option can be, for example, the sensitivity of one of the measurement parameters to process variations. More examples are described in U.S. patent application US2016-0161863 and published U.S. patent application US 2016 / 0370717A1, which are incorporated herein by reference in their entirety.

[0057] Another type of metrology tool used in IC manufacturing is a topography measurement system, a level sensor or a height sensor. Such a tool can be integrated in a lithography device to measure the topography of a substrate (or wafer). A topography map (also referred to as a height map) of a substrate can be generated from these measurements, which indicates the height of the substrate according to the position on the substrate. The height map can then be used to correct the position of the substrate during the transfer of the pattern onto the substrate so as to provide a spatial image of the pattern forming device in a properly focused position on the substrate. It is to be understood that in this context, "height" refers to a dimension (also referred to as the Z axis) that is clearly out of plane for the substrate. Typically, a level or height sensor performs measurements at a fixed position (relative to its own optical system), and the relative movement between the substrate and the optical system of the level or height sensor results in height measurements at positions across the substrate.

[0058] Figure 5 An example of a level or height sensor LS known in the art is schematically shown in FIG. Figure 5 Only the operating principle is illustrated. In this example, the level sensor comprises an optical system comprising a projection unit LSP and a detection unit LSD. The projection unit LSP comprises a radiation source LSO, which provides a radiation beam LSB imparted by a projection grating PGR of the projection unit LSP. The radiation source LSO can be, for example, a narrowband or broadband radiation source (such as a supercontinuum light source), polarized or unpolarized, pulsed or continuous, such as a polarized or unpolarized laser beam. The radiation source LSO can include a plurality of radiation sources with different colors or wavelength ranges, such as a plurality of LEDs. The radiation source LSO of the level sensor LS is not limited to visible radiation, but can additionally or alternatively cover UV and / or IR radiation and any wavelength range suitable for reflection from the substrate surface.

[0059] The projection grating PGR is a periodic grating including a periodic structure, which causes the radiation beam BE1 to have a periodically varying intensity. The radiation beam BE1 with periodic intensity variation is directed toward a measurement position MLO on the substrate W, and the radiation beam BE1 has an incident angle ANG relative to the axis (Z axis) perpendicular to the surface of the substrate, and the incident angle ANG is between 0 and 90 degrees, typically between 70 and 80 degrees. At the measurement position MLO, the patterned radiation beam BE1 is reflected by the substrate W (indicated by arrow BE2) and directed toward the detection unit LSD.

[0060] In order to determine the height level at the measuring position MLO, the level sensor further comprises a detection system comprising a detection grating DGR, a detector DET and a processing unit (not shown) for processing an output signal of the detector DET. The detection grating DGR may be identical to the projection grating PGR. The detector DET generates a detector output signal which is indicative of the received light (e.g. indicating the intensity of the received light) (such as a photodetector) or which represents a spatial distribution of the received intensity (such as a camera). The detector DET may comprise any combination of one or more detector types.

[0061] By means of triangulation techniques the height level at the measurement position MLO can be determined.The detected height level is typically related to the signal intensity measured by the detector DET, which has a periodicity that depends inter alia on the design of the projection grating PGR and the (oblique) angle of incidence ANG.

[0062] The projection unit LSP and / or the detection unit LSD may comprise further optical elements such as lenses and / or mirrors along the path (not shown) of the patterned radiation beam between the projection grating PGR and the detection grating DGR.

[0063] In an embodiment, the detection grating DGR may be omitted and the detector DET may be placed where the detection grating DGR is located. This configuration provides a more direct detection of the image of the projection grating PGR.

[0064] In order to effectively cover the surface of the substrate W, the level sensor LS may be configured to project an array of measurement beams BE1 onto the surface of the substrate W, thereby generating a measurement area MLO or an array of light spots covering a larger measurement range.

[0065] For example, various general types of height sensors are disclosed in US7265364 and US7646471, both of which are incorporated herein by reference. Height sensors that use UV radiation rather than visible or infrared radiation are disclosed in US2010233600A1, which is incorporated herein by reference. In WO2016102127A1 (incorporated herein by reference), a compact height sensor is described that uses a multi-element detector to detect and identify the position of a grating image without the need for a detection grating.

[0066] Another type of metrology tool used in IC manufacturing is the alignment sensor. Therefore, a key aspect of the performance of a lithographic apparatus is the ability to correctly and accurately place the applied pattern (either by the same apparatus or a different lithographic apparatus) relative to the features laid down in the previous layer. For this purpose, the substrate is provided with one or more sets of marks or targets. Each mark is a structure whose position can be measured at a later time using a position sensor (usually an optical position sensor). The position sensor may be referred to as an "alignment sensor" and the mark may be referred to as an "alignment mark".

[0067] The lithographic apparatus may include one or more (e.g., multiple) alignment sensors, by which the positions of alignment marks provided on the substrate can be accurately measured. The alignment (or position) sensor may use optical phenomena, such as diffraction and interference, to obtain position information from alignment marks formed on the substrate. An example of an alignment sensor used in a current lithographic apparatus is based on a self-referencing interferometer described in US6961116. For example, as disclosed in US2015261097A1, various enhancements and modifications of the position sensor have been developed. All of these disclosed contents are incorporated herein by reference.

[0068] Figure 6 is a schematic block diagram of an embodiment of a known alignment sensor AS, such as described, for example, in US6961116 (which is incorporated herein by reference). A radiation source RSO provides a radiation beam RB of one or more wavelengths, which is steered by steering optics onto a mark, such as a mark AM located on a substrate W, as an illumination spot SP. In this example, the steering optics include a spot reflector SM and an objective lens OL. The diameter of the illumination spot SP, with which the mark AM is illuminated, may be slightly smaller than the width of the mark itself.

[0069] Radiation diffracted by the alignment mark AM is collimated (in this example, via the objective lens OL) into an information carrying beam IB. The term "diffraction" is intended to include zero-order diffraction from the mark (which may be referred to as reflection). A self-referencing interferometer SRI (for example of the type disclosed in US6961116 mentioned above) causes the beam IB to interfere with itself, after which the beam is received by the photodetector PD. In the event that more than one wavelength is created by the radiation source RSO, additional optics (not shown) may be included to provide separate beams. If desired, the photodetector may be a single element, or it may comprise several pixels. The photodetector may comprise an array of sensors.

[0070] The steering optics comprising in this example a spot mirror SM can also be used to block zero-order radiation reflected from the mark so that the information-carrying beam IB comprises only high-order diffracted radiation from the mark AM (which is not essential for the measurement but improves the signal-to-noise ratio).

[0071] The intensity signal SI is supplied to the processing unit PU. By a combination of optical processing in block SRI and computational processing in unit PU, values ​​of the X position and Y position on the substrate relative to the reference frame are output.

[0072] A single measurement of the type illustrated only fixes the position of the mark within a specific range corresponding to one pitch of the mark. Coarse measurement techniques are used in conjunction with such measurements to identify which period of the sine wave is the period that contains the marked position. The same process at coarser or finer levels may be repeated at different wavelengths to increase accuracy and / or robustly detect the mark, regardless of the material the mark is made of and the material on and / or below the location where the mark is provided. The wavelengths may be optically multiplexed and demultiplexed so as to be processed simultaneously, and / or they may be multiplexed by time or frequency division.

[0073] In this example, the alignment sensor and the light spot SP remain stationary while the substrate W is moving. The alignment sensor can therefore be fixedly and accurately mounted to the reference frame while effectively scanning the mark AM in a direction opposite to the direction of movement of the substrate W. The substrate W is controlled in this movement by being mounted on a substrate support and a substrate positioning system (controlling the movement of the substrate support). A substrate support position sensor (e.g., an interferometer) measures the position of a substrate support (not shown). In an embodiment, one or more (alignment) marks are provided on the substrate support. Measuring the position of the mark provided on the substrate support allows the position of the substrate support determined by the position sensor to be calibrated (e.g., relative to a frame to which the alignment system is connected). Measuring the position of the alignment mark provided on the substrate allows the position of the substrate relative to the substrate support to be determined.

[0074] To monitor the lithography process, parameters of the patterned substrate are measured. The parameters may include, for example, overlay errors between successive layers formed in or on the patterned substrate. The measurements may be performed on production substrates and / or dedicated metrology targets. There are various techniques for measuring the microstructures formed during the lithography process, including the use of scanning electron microscopes and various specialized tools. A rapid and non-invasive form of specialized inspection tool is a scatterometer, in which a radiation beam is directed onto a target on the substrate surface and the properties of the scattered or reflected beam are measured.

[0075] Examples of known scatterometers include angle-resolved scatterometers of the type described in US2006033921A1 and US2010201963A1. The target used by such scatterometers is a relatively large (e.g. 40μm x 40μm) grating, and the measurement beam generates a spot that is smaller than the grating (i.e., the grating is underfilled). In addition to measuring feature shapes by reconstruction, diffraction-based overlay can also be measured using such equipment, as described in published patent application US2006066855A1. Diffraction-based overlay metrology using dark field imaging of diffraction orders enables overlay measurements on smaller targets. Examples of dark field imaging metrology can be found in international patent applications WO 2009 / 078708 and WO 2009 / 106279, which are incorporated herein by reference in their entirety. Further developments of this technology have been described in published patent disclosures US20110027704A, US20110043791A, US2011102753A1, US20120044470A, US20120123581A, US20130258310A, US20130271740A and WO2013178422A1. These targets can be smaller than the illumination spot and can be surrounded by product structures on the wafer. Multiple gratings can be measured in one image using a composite grating target. The contents of all these applications are also incorporated herein by reference.

[0076] More generally, dark field microscopes (such as the metrology devices mentioned above) have the problem of limited angular range for illuminating the target and / or detecting light diffracted by the target, because the total angular range (corresponding to the area within the angularly resolved pupil) may need to be shared between the illumination path and the detection path. This limits the effective NA in both illumination and detection.

[0077] In a diffraction-based dark field metrology setup, a radiation beam is directed onto a metrology target and one or more properties of the scattered radiation are measured to determine properties of interest of the target. The properties of the scattered radiation may include, for example, the intensity at a single scattering angle (e.g., as a function of wavelength) or the intensity at one or more wavelengths (as a function of the scattering angle).

[0078] Target measurements in dark field metrology may include, for example, measuring the first diffraction order I +1 The first intensity and the negative first diffraction order (I -1 ) and calculate the intensity asymmetry (A=I +1 -I -1 ), which is indicative of an asymmetry in the target. The metrology target may comprise one or more grating structures from which a parameter of interest may be inferred via such intensity asymmetry measurements, e.g. the target may be designed such that the asymmetry in the target varies with the parameter of interest. For example, in overlay metrology, the target may comprise at least one composite grating formed from at least one pair of overlapping sub-gratings, which are patterned in different layers of a semiconductor device. The asymmetry of the target will therefore depend on the alignment of the two layers, and hence on the overlay. Other targets may be formed by exposing structures that vary to varying degrees based on the focus setting used during exposure; measurement of which enables the focus setting to be inferred (again via the intensity asymmetry).

[0079] Figure 7 and Figure 8 Two examples of diffraction-based dark-field metrology devices are schematically illustrated. Note that for simplicity, the two figures only show some components sufficient to describe the working principle of the two devices.

[0080] like Figure 7 As illustrated, a first radiation illumination beam IB1 may be incident obliquely from one side of the device onto an overlapping target of the substrate WA. The grating-based overlapping target may diffract the first illumination beam into multiple diffraction orders. Since the device is configured for dark field imaging, the zeroth diffraction order may be blocked by optical components or configured to fall completely outside the numerical aperture of the objective lens OB. The objective lens OB may collect at least one non-zero diffraction order, such as a positive first diffraction order +1 DF. At the pupil plane of the objective lens OB, a first wedge WG1 may be used to redirect the diffracted radiation to follow a desired beam path. Finally, an imaging lens may be used to focus the diffraction order (e.g., the positive first diffraction order +1 DF) onto the image sensor IS so that a first image IM1 is formed at a first position.

[0081] Similarly, a second radiation illumination beam IB2 may be incident obliquely from the opposite side of the system onto the same overlapping target OT of the substrate WA. The angle of incidence of the second illumination beam IB2 may be the same as the angle of incidence of the first illumination beam IB1. At least one non-zero diffraction order (e.g., a negative first diffraction order -1 DF) may be collected by the objective lens OB and subsequently redirected by the second wedge WG2. The negative first diffraction order -1 DF may then be focused by the imaging lens IL onto the image sensor IS so that a second image IM2 is formed at a second position.

[0082] Figure 7An example of a method for measuring a measurement system in a parallel acquisition scheme is shown. An overlapping target is illuminated by two radiation illumination beams IB1, IB2 simultaneously. Accordingly, two spatially separated images IM1, IM2 of the overlapping target are acquired simultaneously. This parallel acquisition scheme allows a fast measurement speed, thereby achieving a high throughput. However, the pupil plane of the objective lens OB must be shared by two diffraction orders, such as +1 DF and -1 DF. As a result of dividing the pupil into mutually exclusive illumination and detection pupils, both the illumination NA and the detection NA are reduced accordingly. Although there is some flexibility in the trade-off between the illumination NA and the detection NA, it is ultimately impossible to make the illumination NA and the detection NA as large as generally desired within a single pupil. This results in a limited angular range for each corresponding illumination beam and the positive first DF and negative first DF beams, which in turn limits the range of allowable grating pitch sizes and / or illumination wavelengths, and therefore places strict requirements on the design of such a measurement system.

[0083] Figure 8 Another exemplary dark field measurement device (or Figure 7 The main difference is that Figure 8 The metrology device is operated in a sequential acquisition scheme. In the sequential acquisition scheme, at any time instance, the metrology target OT is illuminated by only one radiation illumination beam from one direction, so that only one image of the target is formed and acquired at any point in time. Figure 8 , at a first time instance t=T1, a first illumination beam IB1 can be turned on and directed obliquely from one side of the metrology device onto an overlapping target OT of a substrate WA. After interacting with the grating of the overlapping target, multiple diffraction orders can be generated. At least one of the non-zero diffraction orders (e.g., a positive first diffraction order +1 DF) can be collected by an objective lens OB and subsequently focused by an imaging lens IL onto an image sensor IS.

[0084] After acquiring the first image IM1 of the overlapping grating, at a second time instance t=T2, the first illumination beam IB1 is turned off and the second illumination beam IB2 is turned on. The second illumination beam IB2 may be directed tilted from the opposite side of the metrology device onto the same overlapping target. At least one of the generated diffraction orders (e.g., the negative first diffraction order -1 DF) may be collected by the objective lens OB and subsequently focused onto the image sensor IS to form a second image IM2 of the overlapping target. Note that the images IM1 and IM2 may be formed at a common location on the image sensor.

[0085] With this time-multiplexed acquisition scheme, the full NA of the objective lens OB can be used to detect the diffracted beams +1 DF and -1 DF. The lack of limitation in the target NA means that a wider range of relevant design parameters, such as grating pitch size, illumination wavelength and illumination angle, is allowed, and greater system design flexibility can be obtained. However, the fact that multiple image acquisitions are required means that the measurement speed is reduced, thus affecting the system throughput.

[0086] In addition, accurate determination of, for example, overlay error relies on accurate measurement of small relative intensity differences (or intensity asymmetry) between the two acquired images IM1, IM2. Typical relative intensity differences are about 10% of the intensity of one of the acquired images (e.g., IM1 or IM2). -4 . Such small intensity differences may easily be masked by any intensity and / or wavelength fluctuations of the illuminating radiation. Therefore, it is required that the illumination beam remains stable during continuous image acquisition. This can be achieved by using a stable light source that provides the desired intensity and wavelength stability. Alternatively, additional hardware and software (such as, for example, an intensity / wavelength monitoring device and a corresponding feedback control loop) should be incorporated into the measurement device so that intensity and / or wavelength fluctuations of the illumination beam are actively monitored and well compensated. In some cases, an intensity monitoring device can be used to actively track the intensity of the illumination beam. The signal generated from the intensity monitoring device can be used to correct (e.g. electronically) for intensity fluctuations of the illumination beam. All of these solutions increase the complexity and cost of the overall system.

[0087] Some or all of the above problems can be solved by using digital holographic microscopy, especially dark-field digital holographic microscopy. Digital holographic microscopy is an imaging technique that combines holography with microscopy. Unlike other microscopy methods that record projected images of objects, digital holographic microscopy records a hologram formed by the interference between object radiation obtained by irradiating a three-dimensional (3D) object with object radiation and a reference radiation coherent with the object radiation. The image can be captured using, for example, a charge-coupled device (CCD) or a complementary metal oxide semiconductor (CMOS). Since the object radiation is radiation scattered from the object, the wavefront of the object radiation is modulated or shaped by the object. The scattered radiation may include reflected radiation, diffracted radiation, or transmitted radiation. Therefore, the wavefront of the object radiation carries information of the irradiated object, such as 3D shape information. Based on the captured hologram image, the image of the object can be numerically reconstructed by using a computer reconstruction algorithm. As Figure 7 and 8 As described by the example of , an important advantage of hologram-based metrology compared to intensity-based metrology is that hologram-based metrology allows obtaining both intensity and phase information of an object. With the additional phase information, the properties of the object can be determined with better accuracy.

[0088] International patent application WO2019197117A1, incorporated herein by reference, discloses a method and a metrology device based on dark-field digital holographic microscopy (df-DHM) to determine the characteristics of structures fabricated on a substrate, such as overlay. Fig. 9 The international patent application WO2019197117A1 is reproduced in Figure 3 . Fig. 9 The disclosed df-DHM specifically adapted for lithography process metrology is schematically illustrated.

[0089] and Figure 7 and 8 Compared to the previous example shown, Fig. 9 The df-DHM in further comprises a reference optical unit 16, 18 for providing two additional reference illumination beams 51, 52 (reference illumination). The two reference illumination beams 51, 52 are paired with two corresponding parts 41, 42 of the scattered radiation beams 31, 32 (object radiation), respectively. The two scattered reference beam pairs are used in sequence to form two interference patterns. Coherence control is provided by adjusting the relative optical path difference (OPD) between the two scattered reference beams within each beam pair. However, no coherence control between the two beam pairs is available.

[0090] Due to the use of a single light source and insufficient coherence control, all four radiation beams, i.e., the first portion 41 of scattered radiation 31, the first reference illumination 51, the second portion 42 of scattered radiation 32, and the second reference illumination 52, are mutually coherent. If these four mutually coherent radiation beams are allowed to arrive at the same location of the sensor 6 simultaneously, i.e., operated in a parallel acquisition scheme, multiple interference patterns (including patterns and undesired artifact contribution patterns) comprising the desired information will overlap with each other. Undesired interference patterns may be formed, for example, by interference between portion 41 of the first scattered radiation 31 and portion 42 of the second scattered radiation 32. Parallel acquisition of such an arrangement is impractical, as it is technically challenging and time consuming to completely separate the superimposed interference patterns.

[0091] and Figure 8 Similar to the example, Fig. 9 The use of a sequential acquisition scheme in the example allows the full NA of the objective lens to be used for illumination and detection. However, due to the sequential acquisition, this system suffers from the same problem of low measurement speed. Therefore, it is desirable to have a df-DHM that can perform parallel acquisition so that high measurement speed and high design flexibility can be obtained simultaneously.

[0092] Fig.10An imaging branch of a dark field digital holographic microscope (df-DHM) 1000 is schematically illustrated. The dark field digital holographic microscope (df-DHM) comprises an imaging branch and an illumination branch. In this embodiment, a metrology target 1060 comprising a structure on a substrate 1050 is illuminated by two radiation illumination beams, namely a first radiation illumination beam 1010 and a second radiation illumination beam 1020. In an embodiment, the two illumination beams 1010, 1020 may illuminate the metrology target 1060 simultaneously.

[0093] In an embodiment, the first illumination beam 1010 may be incident on the measurement target 1060 at a first angle of incidence along a first direction of incidence relative to the optical axis OA. The second illumination beam 1020 may be incident on the measurement target 1060 at a second angle of incidence along a second direction of incidence relative to the optical axis OA. The first angle of incidence of the first illumination beam 1010 and the second angle of incidence of the second illumination beam 1020 may be substantially the same. The angle of incidence of each illumination beam may be, for example, in the range of 70 degrees to 90 degrees, in the range of 50 degrees to 90 degrees, in the range of 30 degrees to 90 degrees, in the range of 10 degrees to 90 degrees. The illumination of the measurement target 1060 may cause radiation to be scattered from the target. In an embodiment, the first illumination beam 1010 may be incident on the measurement target 1060 at a first azimuth angle corresponding to the first direction. The second illumination beam 1020 may be incident on the measurement target 1060 at a second azimuth angle corresponding to the second direction. The first azimuth angle of the first illumination beam 1010 and the second azimuth angle of the second illumination beam 1020 can be different; for example, at relative angles of 180 degrees apart.

[0094] Depending on the structure of the measurement target 1060, the scattered radiation may include reflected radiation, diffracted radiation, or transmitted radiation. In this embodiment, the measurement target may be an overlapping target based on diffraction; and each illumination beam may correspond to a scattered beam including at least one non-zero diffraction order. Each scattered beam carries information of the illuminated measurement target. For example, the first illumination beam 1010 may correspond to a first scattered beam 1011 including a positive first diffraction order +1 DF; the second illumination beam 1020 may correspond to a second scattered beam 1021 including a negative first diffraction order -1 DF. The zeroth diffraction order and other undesirable diffraction orders may be blocked by a beam blocking element (not shown) or configured to fall completely outside the NA of the objective lens 1070. Therefore, the df-DHM may be operated in a dark field mode. Note that in some embodiments, one or more optical elements (e.g., a lens combination) may be used to achieve the same optical effect of the objective lens 1070.

[0095] Both scattered beams 1011, 1021 may be collected by objective lens 1070 and subsequently refocused onto image sensor 1080. Objective lens 1070 may include multiple lenses, and / or df-DHM 1000 may include a lens system having two or more lenses, such as an objective lens and an imaging lens, similar to Fig. 9 An exemplary df-DHm of , thereby defining a pupil plane of the objective lens between the two lenses and an image plane at the focus of the imaging lens. In this embodiment, a portion 1012 of the first scattered beam 1011 and a portion 1022 of the second scattered beam 1021 are simultaneously incident on a common position of the image sensor 1080. At the same time, two reference radiation beams (i.e., a first reference beam 1030 and a second reference beam 1040) are incident on the same position of the image sensor 1080. The four beams can be grouped into two pairs of scattered radiation and reference radiation. For example, a first scattered reference beam pair can include a portion 1012 of the first scattered beam 1011 and a first reference beam 1030. Similarly, a portion 1022 of the second scattered reference beam pair can include a second scattered beam 1021 and a second reference beam 1040. The two scattered reference beam pairs can then form two interference patterns (holographic images) that at least partially overlap in the spatial domain.

[0096] In an embodiment, to separate two at least partially spatially overlapping interference patterns (e.g., in the spatial frequency domain), first reference beam 1030 may have a first angle of incidence relative to optical axis OA, and second reference beam 1040 may have a second angle of incidence relative to optical axis OA; the first angle of incidence and the second angle of incidence are different. Alternatively or additionally, first reference beam 1030 may have a first azimuth angle relative to optical axis OA, and second reference beam 1040 may have a second azimuth angle relative to optical axis OA; the first azimuth angle and the second azimuth angle are different.

[0097] In order to generate the interference pattern, the two beams in each scattered reference beam pair should be at least partially coherent with each other to a sufficient degree to form the interference pattern. Note that each scattered radiation beam may have a phase offset relative to its corresponding illumination radiation. For example, at the image plane of the image sensor 1080, such a phase offset may include contributions due to the optical path length (OPD) from the measurement target 1060 to the image sensor 1080 and the interaction with the measurement target. As described above, it is necessary to control the coherence between the first scattered reference beam pair and the second scattered reference beam pair so that each beam in one pair is incoherent with any beam in the other pair. In other words, interference should only occur between beams within the same beam pair and be suppressed between different beam pairs. In this way, only the desired interference pattern (e.g., two interference patterns formed by the relatively scattered reference beam pairs) is formed on the image sensor 1080 in a superimposed manner, thereby avoiding the problem of separating or removing undesired interference patterns.

[0098] The metrology device described above can provide an overlay measurement corresponding to a feature or a displacement between two layers on top of each other along a particular axis. For example, Fig.10 A dark field digital holographic microscope (df-DHM) 1000 includes a first illumination beam incident on a measurement target 1060 at a first azimuth angle corresponding to a first direction. The df-DHM 1000 also includes a second illumination beam 1020 incident on the measurement target 1060 at a second azimuth angle corresponding to a second direction. The first azimuth angle and the second azimuth angle can be different, for example, relative angles separated by 180 degrees.

[0099] A metrology device such as df-DHM 1000 may be suitable for obtaining overlapping measurements of features along an axis along which an illumination beam is located. For example, an illumination beam may be aligned along an x-axis and may then provide corresponding scattered beams corresponding to positive and negative diffraction orders. In particular, a first illumination beam 1010 may provide a scattered beam including at least a positive first diffraction order +1 DF, while a second illumination beam may provide a scattered beam including at least a negative first diffraction order -1 DF. Thus, in this setup, the df-DHM may provide overlapping x-components. Similarly, an illumination beam may be aligned along a y-axis and may then provide corresponding scattered beams corresponding to positive and negative diffraction orders. In particular, a first illumination beam 1010 may provide a scattered beam including at least a positive first diffraction order +1 DF, while a second illumination beam may provide a scattered beam including at least a negative first diffraction order -1 DF. Thus, in this setup, the df-DHM may provide overlapping y-components.

[0100] A limitation of a measurement device such as the df-DHM 1000 is that the overlay measurement may only include an overlay component along a certain axis, such as the x-component OVx discussed above. To obtain other overlay components, such as the y-component OVy, additional measurements are required. For example, to Fig.10 The df-DHM 1000 simultaneously acquires the overlapping x-component OVx and y-component OVy, which may require rotating the illumination beam relative to the wafer. This can be achieved, for example, by rotation of the wafer, rotation of the illumination beam about the optical axis, or a combination thereof. A disadvantage of rotating the illumination beam relative to the wafer may be that the throughput of the metrology device may be reduced due to the time spent rotating the illumination beam relative to the wafer.

[0101] Alternatively, means for acquiring overlapping x-components OVx and y-components OVy may include utilizing an additional set of illumination beams. Fig.11A Schematically illustrates a Fig.10 The dark-field digital holographic microscope (df-DHM) 1100 of the df-DHM 1000 includes a first illuminator group and a second illuminator group. Fig. 11BThe relative k-vector of an illumination beam provided by an illuminator in k-space relative to the objective lens 1170 of the df-DHM 1100 is schematically illustrated.

[0102] df-DHM 1100 can be used to measure the overlapping x-component OVx and y-component OVy of structure 1160, which includes an x ​​feature (or a first feature) oriented along the x-axis (or a first axis of the structure coordinate system) relative to the coordinate system of structure 1160 and a y feature (or a second feature) oriented along the y-axis (or a second axis of the structure coordinate system). Structure 1160 may include a substructure group. The substructure group may include 2 layers, each layer including at least one x-oriented feature and at least one y-oriented feature. For example, each layer may include features including at least one x-oriented feature and at least one y-oriented feature. In some embodiments, each layer includes 2 x-oriented features and 2 y-oriented features. It is to be understood that each layer may include any number of x-oriented features and y-oriented features. These layers may be stacked on top of each other. Although it has been described that each feature is oriented along a particular axis, it is to be understood that each feature may be oriented at least partially along an axis.

[0103] In particular, if Fig.11A As shown, the first illuminator group may include a first x-illuminator 1110 and a second x-illuminator 1120. The first x-illuminator 1110 may provide a first x-irradiation beam 1111, which is arranged to provide a first x-scattered beam 1112 having at least a positive first x-diffraction order +1 x-DF. The second x-illuminator 1120 may provide a second x-irradiation beam 1121, which is arranged to provide a second x-scattered beam 1122 having at least a negative first x-diffraction order -1 x-DF. The second illuminator group may include a first y-illuminator 1130 ( Fig.11A ) and a second y-illuminator 1140 ( Fig.11A 1 y-illuminator 1130 may provide a first y illumination beam (not shown) arranged to provide a first y scattered beam (not shown) having at least a positive first y diffraction order +1 y-DF. A second y illuminator 1140 may provide a second y illumination beam (not shown) arranged to provide a second y scattered beam (not shown) having at least a negative first y diffraction order -1 y-DF.

[0104] Each illumination beam can be incident on the structure 1160 of the wafer 1150 at a corresponding angle of incidence relative to the optical axis OA. The angle of incidence of each illumination beam can be substantially the same. The angle of incidence of each illumination beam can be, for example, in the range of 70 degrees to 90 degrees, in the range of 50 degrees to 90 degrees, in the range of 30 degrees to 90 degrees, and in the range of 10 degrees to 90 degrees.

[0105] Each illumination beam can be incident on the structure 1160 at a corresponding azimuth angle, and the relative position of each illuminator with respect to the objective lens 1170 is as shown in FIG. Fig. 11B As shown. Fig. 11B The relative position of each illuminator with respect to the objective lens 1170 in pupil space or k-space (k x , k y ) plane). Objective lens 1170 defines Fig. 11B The numerical aperture marked as 1171 in . Radiation outside this area defined by the numerical aperture 1171 will not be captured by the objective lens 1170 and therefore cannot contribute to the interference pattern captured by the image sensor 1180. Radiation within this area can be captured by the objective lens 1170 and therefore can contribute to the interference pattern recorded by the image sensor 1180. The azimuth of the beam refers to the angle between the x-axis and the incident beam or its projection in the xy plane. The first x-irradiator 1110 can provide a first x-irradiation beam 1111 at a first azimuth angle of 0 degrees. The second x-irradiator 1120 can provide a second x-irradiation beam 1121 at a second azimuth angle of 180 degrees. The first y-irradiator 1130 can provide a first y-irradiation beam at a third azimuth angle of 90 degrees. The second y-irradiator 1140 can provide a second y-irradiation beam at a fourth azimuth angle of 270 degrees.

[0106] Each scattered beam 1011, 1021 can be collected by an objective lens 1170 and subsequently refocused onto an image sensor 1180. The objective lens 1170 can include a plurality of lenses, and / or the df-DHM 1100 can include a lens system having two or more lenses, such as an objective lens and an imaging lens, similar to Fig. 9 An exemplary df-DHM is thus defined, defining a pupil plane of the objective lens between the two lenses and an image plane at the focus of the imaging lens.

[0107] like Fig.11AAs shown, a portion 1113 of the first x scattered beam 1112 and a portion 1123 of the second x scattered beam 1122 are simultaneously incident on the image sensor 1180 at a common location. At the same time, two x reference radiation beams (i.e., a first x reference beam 1114 and a second x reference beam 1124) are incident on the same location of the image sensor 1180. The first x reference beam 1114 can be provided by a first reference illuminator (not shown), and the second x reference beam can be provided by a second reference illuminator (not shown). These four beams can be grouped into two pairs of x scattered radiation and x reference illumination. For example, the first scattered reference beam pair can include a portion 1113 of the first x scattered beam 1112 and a first x reference beam 1114. Similarly, the second scattered reference beam pair can include a portion 1123 of the second x scattered beam 1122 and a second x reference beam 1124. The two scattered reference beam pairs can then form two interference patterns (holographic images), which can provide imaging information for the x features of the structure 1160.

[0108] Similarly, when the df-DHM 1100 utilizes a first y illumination beam and a second y illumination beam, two y reference radiation beams (i.e., a first y reference beam (not shown) and a second y reference beam (not shown)) can be simultaneously incident on the same position of the image sensor 1180 as the first y scattered beam and the second y scattered beam, respectively. The four beams can also be grouped into two pairs of y scattered radiation and y reference illumination. For example, the first scattered reference beam pair can include a portion of the first y scattered beam and the first y reference beam. Similarly, the second scattered reference beam pair can include a portion of the second y scattered beam and the second y reference beam. The two scattered reference beam pairs can then form two interference patterns (holographic images), which can provide imaging information for the y features of the structure 1160.

[0109] In order to be able to acquire all the information required to determine one overlap component (e.g., the x overlap component) in parallel, the reference illuminators can be arranged so that each reference beam interferes only with the corresponding portion of the scattered beam in its scattered reference beam pair. Using the x overlap component as an example, in order to separate two at least partially spatially overlapping interference patterns (e.g., in the spatial frequency domain), the first x reference beam 1114 can have a first angle of incidence relative to the optical axis OA, and the second x reference beam 1124 can have a second angle of incidence relative to the optical axis OA; the first angle of incidence and the second angle of incidence are different. Alternatively or in addition, the first x reference beam 1114 can have a first azimuth angle relative to the optical axis OA, and the second x reference beam 1124 can have a second azimuth angle relative to the optical axis OA; the first azimuth angle and the second azimuth angle are different. In this way, each reference beam can interfere with light from only one illuminator, and the orientation of the interference fringes on the image sensor 1180 can be changed by selecting the angle of incidence and / or azimuth angle of each reference beam. Fig. 11CThe image spectrum in the spatial frequency domain is depicted. The image spectrum includes a zero-order spatial frequency domain, which includes a basic spatial spectrum or baseband BB, which includes a zero-order Fourier component and a positive first x diffraction order +1 st The two (identical) higher-order spatial spectra or sidebands +SB1 and +SB2 and the negative first x diffraction order -1 st For example, by changing the angle of incidence and / or the azimuth of each x-reference beam, the position of the higher-order spatial spectra can be adjusted relative to the base spectrum.

[0110] The df-DHM 1100 may illuminate the structure 1160 using a first illuminator set and a reference illuminator so as to provide overlapping x-components. Subsequently, the df-DHM may illuminate the measurement target using a second illuminator set and a reference illuminator so as to provide overlapping y-components. Furthermore, if only the first illuminator set is used, then the image sensor 1180 will only image features that scatter light in the x-direction, and if only the second illuminator set is used, then the image sensor 1180 will only image features that scatter light in the y-direction. A disadvantage of the df-DHM 1100 is that four illuminators are required to provide full overlapping measurements in both directions in the substrate / target plane. Illuminators are relatively complex and expensive parts. Therefore, this requirement for four illuminators may increase the complexity and cost of the df-DHM 1100.

[0111] Alternatively, the df-DHM may use the first illuminator set and reference illuminator and the second illuminator set and reference illuminator to illuminate the structure 1160 simultaneously, providing overlapping x-components and y-components. While this may double the throughput of the df-DHM, this is at the expense of higher demands on the image sensor 1180.

[0112] Fig.11D An image spectrum in the spatial frequency domain of diffraction orders generated after illuminating the structure 1160 using a first illuminator set and a reference illuminator and a second illuminator set and a reference illuminator is depicted. The image spectrum includes a zero-order spatial frequency domain, which includes a basic spatial spectrum or baseband BB, which includes a zero-order Fourier component (depicted as a central circle) and a positive first x diffraction order +1 st Two (identical) higher-order spatial spectra or sidebands +SB1 and +SB2 of the x-DF and the negative first x-diffraction order -1 st The image spectrum also includes the positive first y diffraction order +1 st Two (identical) higher-order spatial spectra or sidebands +SB3 and +SB4 of the y-DF and the negative first y-diffraction order -1st Two (identical) higher order spatial spectra or sidebands -SB3 and -SB4 of y-DF. For example, by changing the angle of incidence and / or the azimuth of each x-reference beam or y-reference beam, the position of the higher order spatial spectra can be adjusted relative to the base spectrum.

[0113] It is increasingly desirable to use fewer illuminators and use less measurement time to obtain overlap measurements including an x ​​overlap component and a y overlap component. As discussed above, current components for obtaining overlaps including an x ​​overlap component and a y overlap component may require the use of four illuminators. In particular, a pair of x illuminators and a pair of y illuminators. In a setup such as this, the illumination beam from the x illuminator incident on the y-directed feature of the overlapping target may be diffracted, but will not be received by the objective of the optical system. Similarly, the illumination beam from the y illuminator incident on the x-directed feature of the overlapping target may be diffracted, but will not be received by the objective of the optical system. In this way, a large portion of the overlap information may be lost. In particular, half of the potential overlap information may be lost because the objective of the optical system does not receive the diffracted light.

[0114] Therefore, it is proposed to use an illumination device comprising at least one illuminator (or illuminator group), each illuminator / illuminator group being arranged to provide an illumination beam incident on at least one x-directed feature (first feature) and at least one y-directed feature (second feature) of the structure, so that both the x scattered radiation and the y scattered radiation are captured by the objective lens. In this way, the overlap comprising the x overlap component and the y overlap component can be acquired using fewer illuminators and using less measurement time.

[0115] Thus, an illumination arrangement of a metrology device for illuminating a structure on a substrate, the structure comprising at least one first feature oriented along a first axis of a structure coordinate system and at least one second feature oriented along a second axis of the structure coordinate system is disclosed.

[0116] The illumination device may include at least one illuminator (or group of illuminators), each illuminator / group of illuminators being operable to illuminate the structure with a first illumination from a first direction oblique to the first and second axes to generate a first scattered radiation from the first feature and a second scattered radiation from the second feature; and being operable to illuminate the structure with a second illumination from a second direction substantially opposite to the first direction to generate a third scattered radiation from the first feature and a fourth scattered radiation from the second feature. Thus, the illumination device may be operable to provide illumination incident on at least one x-directed feature (the first feature) and at least one y-directed feature (the second feature) of the structure such that both the x scattered radiation and the y scattered radiation are captured by the objective lens.

[0117] The first direction and the second direction may differ by an azimuth angle, wherein the azimuth angle of a beam refers to the angle between the x-axis and the incident beam or its projection in the xy plane, the xy plane being the plane of the structure. In particular, a first azimuth angle of a first illumination may differ from a second azimuth angle of a second illumination in a reference frame of the structure.

[0118] The first direction may be substantially opposite to the second direction, so the first azimuth may be substantially opposite to the second azimuth. In this context, substantially opposite may mean any of the following. In some embodiments, the difference between the first azimuth and the second azimuth is 180 degrees. In some embodiments, the difference between the first azimuth and the second azimuth may have a value within the range of 180 ± 1 degrees. In some embodiments, the difference between the first azimuth and the second azimuth may have a value within the range of 180 ± 2 degrees. In some embodiments, the difference between the first azimuth and the second azimuth may have a value within the range of 180 ± 3 degrees. In some embodiments, the difference between the first azimuth and the second azimuth may have a value within the range of 180 ± 5 degrees. In some embodiments, the difference between the first azimuth and the second azimuth may have a value within the range of 180 ± 10 degrees. In some embodiments, the difference between the first azimuth and the second azimuth may have a value within the range of 180 ± 15 degrees. In some embodiments, the difference between the first azimuth and the second azimuth may have a value within the range of 180 ± 20 degrees. In some embodiments, the difference between the first azimuth angle and the second azimuth angle may have a value within the range of 180±25 degrees. In some embodiments, the difference between the first azimuth angle and the second azimuth angle may have a value within the range of 180±30 degrees.

[0119] The first direction and the second direction and therefore the first azimuth and the second azimuth may be tilted to the x and y axes, respectively. In this context, the tilt angle may refer to any angle that is not aligned with one of the first axis or the second axis. In particular, in this context, tilt may mean any of the following. The first azimuth may be 45 degrees. The value of the first azimuth may be in the range of 45 ± 1 degrees. The value of the first azimuth may be in the range of 45 ± 2 degrees. The value of the first azimuth may be in the range of 45 ± 3 degrees. The value of the first azimuth may be in the range of 45 ± 5 degrees. The value of the first azimuth may be in the range of 45 ± 10 degrees. The value of the first azimuth may be in the range of 45 ± 15 degrees. The value of the first azimuth may be in the range of 45 ± 20 degrees. The value of the first azimuth may be in the range of 45 ± 25 degrees. The value of the first azimuth may be in the range of 45 ± 30 degrees. The first azimuth may be 135 degrees. The value of the first azimuth may be in the range of 135 ± 1 degrees. The value of the first azimuth may be in the range of 135 ± 2 degrees. The value of the first azimuth angle may be in the range of 135±3 degrees. The value of the first azimuth angle may be in the range of 135±5 degrees. The value of the first azimuth angle may be in the range of 135±10 degrees. The value of the first azimuth angle may be in the range of 135±15 degrees. The value of the first azimuth angle may be in the range of 135±20 degrees. The value of the first azimuth angle may be in the range of 135±25 degrees. The value of the first azimuth angle may be in the range of 135±30 degrees. The first azimuth angle may be 225 degrees. The value of the first azimuth angle may be in the range of 225±1 degree. The value of the first azimuth angle may be in the range of 225±2 degrees. The value of the first azimuth angle may be in the range of 225±3 degrees. The value of the first azimuth angle may be in the range of 225±5 degrees. The value of the first azimuth angle may be in the range of 225±10 degrees. The value of the first azimuth angle may be in the range of 225±15 degrees. The value of the first azimuth angle may be in the range of 225±20 degrees. The value of the first azimuth angle may be in the range of 225±25 degrees. The value of the first azimuth angle may be in the range of 225±30 degrees. The first azimuth angle may be 315 degrees. The value of the first azimuth angle may be in the range of 315±1 degree. The value of the first azimuth angle may be in the range of 315±2 degrees. The value of the first azimuth angle may be in the range of 315±3 degrees. The value of the first azimuth angle may be in the range of 315±5 degrees. The value of the first azimuth angle may be in the range of 315±10 degrees. The value of the first azimuth angle may be in the range of 315±15 degrees. The value of the first azimuth angle may be in the range of 315±20 degrees. The value of the first azimuth angle may be in the range of 315±25 degrees. The value of the first azimuth angle may be in the range of 315±30 degrees.

[0120] In the case where performance along a particular axis is more important than performance along another axis, the first azimuth or the second azimuth may include a deviation from any of the above azimuths. For example, if performance along the x-axis is more important than performance along the y-axis, the first azimuth may be, for example, 20 degrees.

[0121] In some embodiments, the first illumination is provided by a first illuminator and the second illumination is provided by a second illuminator. In such an embodiment, the first illumination and the second illumination may illuminate the structure simultaneously. The first illumination and the second illumination may be provided by mutual illumination devices, such as the illumination device 1100 of FIG. 11 of WO 2021 / 121733. Alternatively, the first illumination and the second illumination may be provided by respective illumination devices, respectively. The illumination device may include an illuminator holding device, such as an illuminator ring accommodating the first illuminator and the second illuminator. The central area within the illumination ring may be configured to receive an optical system (e.g., an objective system) of a measurement device. For example, the detection NA defined by the central detection area may be greater than 0.5, greater than 0.6, greater than 0.7, greater than 0.8, or within the region of 0.85.

[0122] In some embodiments, the illumination device comprises a first illuminator operable to provide a first illumination and a second illuminator operable to provide a second illumination. In some embodiments, the first illuminator and the second illuminator are operable to provide the first illumination and the second illumination simultaneously.

[0123] In some embodiments, the illumination device comprises a single illuminator operable to provide the first illumination and the second illumination. In some embodiments, the illumination device is operable to provide the first illumination and the second illumination separately or individually. In some embodiments, one or both of the structure and / or the illumination device is rotatable to rotate at least one illuminator a difference between the first orientation and the second orientation.

[0124] The illuminator ring may be rotated about a center point of the illuminator ring, the illuminator ring being configured to rotate the illumination device about the center point to switch between the first illumination and the second illumination. Conversely, the illuminator ring may be fixed with a single illuminator and thus fixed in position relative to the rest of the sensor module. In this case, the substrate (and structure) may be rotated appropriately to switch between the first illumination and the second illumination.

[0125] The illumination device may be part of a sensor module or a sensor head of a measurement device. Therefore, a sensor module for a measurement device is also disclosed. The sensor module may include an illumination device; an optical system (e.g., including at least an objective lens) operable to capture the first scattered radiation, the second scattered radiation, the third scattered radiation, and the fourth scattered radiation; and a detector operable to detect a first interference pattern formed by interference between a portion of the first scattered radiation and a first reference illumination, a second interference pattern formed by interference between a portion of the second scattered radiation and the second reference illumination, a third interference pattern formed by interference between a portion of the third scattered radiation and the first reference illumination, and a fourth interference pattern formed by interference between a portion of the fourth scattered radiation and the second reference illumination.

[0126] In order to make the sensor module as compact and light as possible, it may be physically separated from most of the optics, which may be placed, for example, in a beam preparation module elsewhere in the machine (e.g. separate from the fixed world of which the irradiator box may form a part). In this context, the sensor module may comprise at least an objective or lens arrangement and a detector; e.g. a CCD / camera (and little else, e.g. only auxiliary wiring etc.); e.g. Fig.10 The objective lens 1070 and the detector 1080. The illuminator box may include, for example, the arrangements described in EP3964892A1 and / or WO2021121733A1. These documents are incorporated herein by reference.

[0127] The sensor module may be part of a holographic metrology device configured to determine a parameter of interest of a structure. Therefore, a holographic metrology device is also disclosed, which may include a sensor module; a substrate support for supporting a substrate including a structure, the structure including at least one first feature oriented along a first axis of a structure coordinate system and at least one second feature oriented along a second axis of the structure coordinate system; and a reference illumination device for generating a first reference illumination and a second reference illumination.

[0128] The reference illumination arrangement may include a first reference illuminator operable to provide a first reference illumination; and a second reference illuminator operable to provide a second reference illumination. In some embodiments, the reference illumination arrangement includes a single reference illuminator operable to provide the first reference illumination and the second reference illumination. In some embodiments, the first reference illumination is at least partially coherent with the first illumination, and the second reference illumination is at least partially coherent with the second illumination.

[0129] The first reference illumination may be provided by the same illumination device as the first illumination. The second reference illumination may be provided by the same illumination device as the second illumination. The first reference illumination and the second reference illumination and the first illumination and the second illumination may be provided by the same illumination device.

[0130] The detector of the holographic measurement device can be operable to detect: a first interference pattern, which is produced by interference of first scattered radiation, the first scattered radiation is produced by scattering of the first illumination by the x-oriented features and the y-oriented features and the first reference illumination; and a second interference pattern, which is produced by interference of second scattered radiation, the second scattered radiation is produced by scattering of the second illumination by the x-oriented features and the y-oriented features and the second reference illumination.

[0131] Also disclosed is a digital holographic microscope configured to determine a parameter of interest of a structure, the structure comprising at least one first feature oriented along a first axis of a structural coordinate system and at least one second feature oriented along a second axis of the structural coordinate system, the digital holographic microscope comprising a holographic metrology device. The digital holographic microscope may comprise a dark-field digital holographic microscope.

[0132] Also disclosed is a method for determining a parameter of interest of a structure on a substrate, the structure comprising at least one first feature oriented along a first axis of a structural coordinate system and at least one second feature oriented along a second axis of the structural coordinate system, the method comprising: illuminating the first feature and the second feature with a first illumination from a first direction inclined to the first axis and the second axis to generate a first scattered radiation from the first feature and a second scattered radiation from the second feature; illuminating the first feature and the second feature with a second illumination from a second direction substantially opposite to the first direction to generate a third scattered radiation from the first feature and a fourth scattered radiation from the second feature; detecting a first interference pattern formed by interference between a portion of the first scattered radiation and a first reference illumination; detecting a second interference pattern formed by interference between a portion of the second scattered radiation and the second reference illumination; detecting a third interference pattern formed by interference between a portion of the third scattered radiation and the first reference illumination; detecting a fourth interference pattern formed by interference between a portion of the fourth scattered radiation and the second reference illumination; and determining the parameter of interest of the structure using the first interference pattern, the second interference pattern, the third interference pattern, and the fourth interference pattern.

[0133] It may be necessary to control the coherence between each scattered reference beam pair so that each beam in a pair is incoherent with any beam in the other pair. In other words, interference should only occur between beams within the same beam pair and be suppressed between different beam pairs. In this way, only the desired interference pattern (e.g., four interference patterns formed by relatively scattered reference beam pairs) is formed on the detector in a superimposed manner, thereby avoiding the problem of separating or removing undesired interference patterns.

[0134] The following description will be described in the context of holography and digital holographic microscopy. In particular, the following description will be described in the context of dark-field digital holographic microscopy. However, it should be understood that the concepts disclosed herein are not limited thereto and may be applied to any suitable metrology equipment, including scatterometers, alignment sensors, level sensors, bright-field DHMs, or any other such devices.

[0135] Fig. 12A A dark field digital holographic microscope (df-DHM) 1200 suitable for determining a parameter of interest of a structure 1260 (e.g., an overlay) fabricated on a substrate 1250 according to an embodiment of the present disclosure is depicted. The structure 1260 includes at least one first feature (e.g., an x-directed feature) oriented along a first axis of a structure coordinate system and at least one second feature (e.g., a y-directed feature) oriented along a second axis of the structure coordinate system. The df-DHM 1200 in this example operates according to a parallel acquisition scheme, similar to Fig.11A df-DHM.

[0136] Fig. 12B A structure 1260 is depicted that includes two first features oriented along a first axis of a structure coordinate system and two second features oriented along a second axis of the structure coordinate system. In particular, structure 1260 includes a first x-oriented feature 1261, a second x-oriented feature 1262, a first y-oriented feature 1263, and a second y-oriented feature. The first x-oriented feature 1261 and the second x-oriented feature 1262 are oriented along the x-axis of the structure coordinate system. The first y-oriented feature 1263 and the second y-oriented feature 1264 are oriented along the y-axis of the structure coordinate system. Therefore, in this example, the second axis of the structure coordinate system is perpendicular to the first axis of the structure coordinate system.

[0137] In an embodiment, the df-DHM 1200 includes a substrate support (not shown) for supporting a substrate 1250 including a structure 1260 .

[0138] In an embodiment, df-DHM 1200 includes a sensor module.

[0139] In an embodiment, the sensor module comprises an illumination arrangement comprising at least one illuminator operable to illuminate the structure 1260 with a first illumination from a first direction oblique to the first and second axes to generate a first scattered radiation from the first feature and a second scattered radiation from the second feature. The illumination arrangement may also be operable to illuminate the structure 1260 with a second illumination from a second direction substantially opposite to the first direction to generate a third scattered radiation from the first feature and a fourth scattered radiation from the second feature.

[0140] The illumination device of this embodiment comprises a first illuminator 1210 and a second illuminator 1220. The first illuminator is operable to illuminate the structure 1260 with a first illumination 1211 from a first direction. The second illuminator 1220 is operable to illuminate the structure 1260 with a second illumination 1221 from a second direction.

[0141] In an embodiment, first illumination 1211 can be incident on structure 1260 in a first direction at a first azimuth, where the azimuth of a beam refers to the angle between the x-axis and the incident beam or its projection in an xy plane, which is the plane of structure 1260. Second illumination can be incident on structure 1260 in a second direction at a second azimuth relative to the xy plane of structure 1260.

[0142] Fig. 12C The relative position of each illuminator relative to the structure side of the objective pupil space of the objective lens 1270 in k space (k x , k y ) image representation in the plane. Fig. 12C The center ring 1271 of is the objective pupil space 1271 of the objective 1270. The first direction is represented by a first k vector 1211 indicating a first azimuth angle, which in this example is 45 degrees. The second direction is represented by a second k vector 1221 describing a second azimuth angle, which in this example is 225 degrees. Therefore, in this example, the angular difference between the first azimuth angle and the second azimuth angle is 180 degrees.

[0143] In an embodiment, the first illumination 1211 can be incident on the structure 1260 at a first angle of incidence along a first OA direction relative to the optical axis OA. The second illumination 1221 can be incident on the structure 1260 at a second angle of incidence along a second OA direction relative to the optical axis OA. The first angle of incidence of the first illumination 1211 and the second angle of incidence of the second illumination 1221 can be substantially the same. The angle of incidence of each illumination beam can be, for example, in the range of 70 degrees to 90 degrees, in the range of 50 degrees to 90 degrees, in the range of 30 degrees to 90 degrees, in the range of 10 degrees to 90 degrees.

[0144] Irradiation of structure 1260 by first illumination 1211 may generate first scattered radiation 1212 scattered by structure 1260, the first scattered radiation 1212 resulting from illumination of structure 1260 by first illumination 1211. Irradiation of structure 1260 by second illumination 1221 may generate second scattered radiation 1222 scattered by structure 1260, the second scattered radiation 1222 resulting from illumination of structure 1260 by second illumination 1221.

[0145] First scattered radiation 1212 includes first scattered radiation from the first feature and second scattered radiation from the second feature. In particular, first scattered radiation 1212 includes positive first x diffraction order +1 x-DF from the x-oriented feature and positive first y diffraction order +1 y-DF from the y-oriented feature. Second scattered radiation 1222 includes third scattered radiation from the first feature and fourth scattered radiation from the fourth feature. In particular, second scattered radiation includes negative first x diffraction order -1 y-DF from the x-oriented feature. st x-DF and the negative first y diffraction order -1 from the y-directed feature st y-DF.

[0146] In an embodiment, the sensor module further comprises an optical system 1202 operable to capture radiation scattered by the structure 1260, which is generated by the structure 1260 being illuminated. In particular, the optical system 1202 is operable to capture a first scattered radiation 1212 generated by the structure 1260 being illuminated by the first illumination 1211 and a second scattered radiation 1222 generated by the structure 1260 being illuminated by the second illumination 1221.

[0147] In an embodiment, the optical system 1202 includes an objective lens 1270. In an embodiment, the optical system 1202 includes a plano-convex lens. A plano-convex lens is equiplanar and has relatively high aberrations. In an embodiment, the optical system 1202 includes a planoasphere or a double aspheric lens. A planoasphere is non-equiplanar and has relatively low aberrations. In an embodiment, the optical system 1202 includes a reflector optical device. In an embodiment, the optical system 1202 defines a detection numerical aperture (NA). For example, the detection NA defined by the central detection area can be greater than 0.5, greater than 0.6, greater than 0.7, greater than 0.8, or in the region of 0.85.

[0148] The sensor module also includes a detector 1280 operable to detect scattered radiation.

[0149] Optical system 1202 is configured to direct portion 1213 of first scattered radiation 1212 toward detector 1280 . Optical system 1202 is also configured to direct portion 1223 of second scattered radiation 1222 toward detector 1280 .

[0150] The following is discussed with respect to portion 1213 of the first scattered radiation 1212, although substantially similar principles apply to directing portion 1223 of the second scattered radiation 1222 to the detector 1280. Portion 1213 of the first scattered radiation 1212 is the portion of the first scattered radiation 1212 that reaches the detector 1280. Other portions of the first scattered radiation 1341 do not reach the detector 1280. In an embodiment, portion 1213 of the first scattered radiation 1212 that reaches the detector 1280 does not include a specular reflection component of the first scattered radiation 1212. This may be achieved by arranging the polar incidence angle of the first illumination 1211 to be large enough to ensure that specular reflections that would occur at the same polar incidence angle as the illumination fall outside the detection NA of the optical system 1202. The detection NA of the optical system 1202 may be the detection NA of the objective 1270. Thus, the detector 1280 may be arranged for dark field measurements. In an embodiment, a portion 1213 of the first scattered radiation 1212 consists at least primarily of one or more non-zero order diffraction components scattered from the structure 1260 (i.e., more than half or all), such as only a +1 order diffraction component or one or more of positive non-zero order diffraction components of +1, +2, +3 or higher order. In particular, the first scattered radiation 1212 includes a positive first x diffraction order +1 diffracted by the x-directed features and a positive first y diffraction order +1 diffracted by the y-directed features. The second scattered radiation 1222 includes a negative first x diffraction order -1 diffracted by the x-directed features and a negative first y diffraction order -1 diffracted by the y-directed features.

[0151] In some embodiments, the sensor module is configured to illuminate the structure 1260 with the first illumination 1211 from the first illuminator 1210 and the second illumination 1221 from the second illuminator 1220 simultaneously. In some embodiments, the sensor module is configured to illuminate the structure 1260 with the first illumination 1211 from the first illuminator 1210 and the second illumination 1221 from the second illuminator 1220 separately or individually. In an embodiment, the sensor module is configured to switch between illuminating the structure 1260 with the first illumination 1211 from the first illuminator 1210 and illuminating the structure 1260 with the second illumination 1221 from the second illuminator 1220 by rotation of the substrate W. In an embodiment, the sensor module is configured to switch between illuminating the structure 1260 with the first illumination 1211 from the first illuminator 1210 and illuminating the structure 1260 with the second illumination 1221 from the second illuminator 1220 by rotation of the sensor module. In an embodiment, the sensor module is configured to switch between illuminating the structure 1260 with a first illumination 1211 from the first illuminator 1210 and illuminating the structure 1260 with a second illumination 1221 from the second illuminator 1220 by a rotation of the illuminator ring. In an embodiment, the sensor module is configured to switch between illuminating the structure 1260 with a first illumination 1211 from the first illuminator 1210 and illuminating the structure 1260 with a second illumination 1221 from the second illuminator 1220 by a combination of a rotation of the illuminator ring, a rotation of the substrate W and / or a rotation of the sensor module.

[0152] In an embodiment, df-DHM 1200 is further operable to provide respective first reference illumination 1214 corresponding to first illumination 1211 and second reference illumination 1224 corresponding to second illumination 1221. In an embodiment, first reference illumination 1214 and second reference illumination 1224 comprise plane waves or spherical waves.

[0153] In an embodiment, first reference illumination 1214 is provided by first illuminator 1210 and second reference illumination 1224 is provided by second illuminator 1220. First illuminator 1210 provides first reference illumination 1214 onto detector 1280 simultaneously with portion 1213 of first scattered radiation 1212. Similarly, second illuminator 1220 provides second reference illumination 1224 onto detector 1280 simultaneously with portion 1224 of second scattered radiation 1222. Notably, in embodiments where first illuminator 1210 and second illuminator 1220 are provided to structure 1260 individually or separately, second reference illumination 1224 may be directed onto detector 1280 at a different time than first reference illumination 1214.

[0154] The detector 1280 may be operable to detect a portion of the first scattered radiation (ie, the positive first x diffraction order +1 st x-DF) and the first reference radiation 1214; a first interference pattern formed by the interference between a portion of the second scattered radiation (ie, the positive first y diffraction order +1 st y-DF) and the second reference illumination 1224; a second interference pattern formed by the interference between a portion of the third scattered radiation (ie, the negative first x diffraction order -1 st x-DF) and the first reference illumination 1214; and a portion of the fourth scattered radiation (ie, the negative first y diffraction order -1 st 1280 and a fourth interference pattern formed by interference between the first reference illumination 1214 and the first scattered radiation and the third scattered radiation. The first reference illumination 1214 is at least sufficiently coherent with the first scattered radiation and the third scattered radiation so that the first interference pattern and the third interference pattern can be detected by the detector 1280. The second reference illumination 1224 is at least sufficiently coherent with the second scattered radiation and the fourth scattered radiation so that the second interference pattern and the fourth interference pattern can be detected by the detector 1280.

[0155] Fig.12D Depicted is a schematic representation of an example arrangement of illuminators, such as a first illuminator 1210 for providing a first illumination 1211 and a first reference illumination 1214. It is appreciated that a substantially similar arrangement may be used for a second illuminator 1220 to provide a second illumination 1221 and a second reference illumination 1224.

[0156] The radiation source 1310 provides a radiation beam to the beam splitter 1312. The radiation source 1310 generates a radiation beam of electromagnetic radiation that is temporally and spatially coherent, or temporally and spatially partially coherent, or temporally coherent and spatially partially incoherent (but coherent enough to interfere at the detector 1280). In an embodiment, the radiation source 1310 of the first illuminator 1210 has a wavelength range of 400 nm to 1000 nm. In an embodiment, the radiation source (not shown) of the second first illuminator 1220 has a wavelength range of 1000 nm to 1600 nm.

[0157] The radiation beam is split by a beam splitter 1312 to provide a first illumination 1211 and a first reference illumination 1214. In the example shown, a first portion of the split radiation beam represents the first reference illumination 1214, which passes through a delay element 1314 and a reference optical unit 1316. The reference optical unit 1316 directs the first reference illumination 1214 onto a detector 1280. A second portion of the split radiation beam represents the first illumination 1211, which passes through an illumination optical unit 1320. The illumination optical unit 1320 directs the first illumination 1211 onto the structure 1260. The optical path length between the point at which the radiation beam is split by the beam splitter 1312 and the detector 1280 may be adjusted by the delay element 1314. The delay element 1314 may include any suitable arrangement for introducing a phase delay, for example by controllably increasing the path length of the radiation passing through the delay element 1314. The delay element 1314 can be implemented based on the principles discussed with reference to FIG. 12 of WO 2019 / 197117, for example involving a reflector. In the present example, the delay element 1314 is provided in the optical path between the beam splitter 1312 and the reference optical unit 1316, but the delay element 1314 can alternatively or additionally be provided in the optical path between the beam splitter 1312 and the illumination optical unit 1320. In some embodiments, for each of the two optical paths between the beam splitter 1312 and the detector 1280 (i.e., via the structure 1260 rather than via the structure 1260), the path difference is set to be equal. This condition can be met by tuning the phase element 1314 and / or by accurate alignment of the optical elements between the beam splitter 1312 and the detector 1280.

[0158] One or more parameters of interest of the structure 1260 may be determined by a processor 1290 of the df-DHM 1200. The processor 1290 may determine the parameter of interest of the structure 1260 using the first interference pattern, the second interference pattern, the third interference pattern, and the fourth interference pattern recorded by the detector 1280. In particular, the processor 1290 may determine the parameter of interest in a direction defined by the first axis (i.e., the x-axis) through the first interference pattern and the third interference pattern; and determine the parameter of interest in a direction defined by the second axis (i.e., the y-axis) through the second interference pattern and the fourth interference pattern. The parameter of interest may be an overlap of the structure 1260.

[0159] In an embodiment, processor 1290 is coupled to detector 1280 to receive a signal comprising information about the interference pattern recorded by detector 1280. In an embodiment, processor 1290 corrects aberrations of optical system 1202.

[0160] In an embodiment, the processing unit 1290 may be a computer system. The computer system may be equipped with an image reconstruction algorithm for performing all of the above tasks, including performing a Fourier transform, extracting each individual high-order spatial spectrum, performing an inverse Fourier transform, calculating the complex field, and determining the characteristics of the structure 1260 based on the results.

[0161] Fig.13 1 is a block diagram illustrating a computer system 1500 that can assist in implementing the methods and processes disclosed herein. The computer system 1500 includes a bus 1502 or other communication mechanism for communicating information and a processor 1504 (or multiple processors 1504 and 1505) coupled to the bus 1502 for processing information. The computer system 1500 also includes a main memory 1506, such as a random access memory (RAM) or other dynamic storage device, which is coupled to the bus 1502 for storing information and instructions to be executed by the processor 1504. The main memory 1506 can also be used to store temporary variables or other intermediate information during the execution of instructions to be executed by the processor 1504. The computer system 1500 also includes a read-only memory (ROM) 1408 or other static storage device, which is coupled to the bus 1502 for storing static information and instructions for the processor 1504. A storage device 1510, such as a magnetic disk or optical disk, is provided and coupled to the bus 1502 for storing information and instructions.

[0162] The computer system 1500 can be coupled to a display 1512, such as a cathode ray tube (CRT) or a flat panel or touch panel display, via the bus 1502 for displaying information to a computer user. An input device 1514 including alphanumeric keys and other keys is coupled to the bus 1502 for communicating information and command selections to the processor 1504. Another type of user input device is a cursor control 1516, such as a mouse, a trackball, or cursor direction keys, for communicating direction information and command selections to the processor 1504 and controlling cursor movement on the display 1512. The input device typically has two degrees of freedom on two axes, a first axis (e.g., x) and a second axis (e.g., y), which allows the device to specify a position in a plane. A touch panel (screen) display can also be used as an input device.

[0163] In response to processor 1504 executing one or more sequences of one or more instructions contained in main memory 1506, one or more methods described herein may be performed by computer system 1500. Such instructions may be read into main memory 1506 from another computer-readable medium such as storage device 1510. Execution of the sequences of instructions contained in main memory 1506 causes processor 1504 to perform the process steps described herein. One or more processors in a multi-processing arrangement may also be employed to execute the sequences of instructions contained in main memory 1506. In alternative embodiments, hardwired circuitry may be used in place of or in combination with software instructions. Therefore, the description herein is not limited to any specific combination of hardware circuitry and software.

[0164] The term "computer-readable medium" as used herein refers to any medium that participates in providing instructions to the processor 1504 for execution. Such media can take many forms, including but not limited to non-volatile media, volatile media, and transmission media. Non-volatile media include, for example, optical or magnetic disks, such as storage device 1510. Volatile media include dynamic memory, such as main memory 1506. Transmission media include coaxial cables, copper wires, and optical fibers, including the wires that make up bus 1502. Transmission media can also take the form of sound waves or light waves, such as those generated during radio frequency (RF) and infrared (IR) data communications. Common forms of computer-readable media include, for example, floppy disks, foldable disks, hard disks, magnetic tape, any other magnetic media, CD-ROMs, DVDs, any other optical media, punch cards, paper tape, any other physical media with a pattern of holes, RAM, PROMs and EPROMs, flash EPROMs, any other memory chips or memory cartridges, carrier waves described below, or any other media from which a computer can read.

[0165] Various forms of computer readable media may be involved in carrying one or more sequences of one or more instructions to the processor 1504 for execution. For example, the instructions may initially be carried on a disk of a remote computer. The remote computer can load the instructions into its dynamic memory and send the instructions over a telephone line using a modem. A modem local to the computer system 1500 can receive the data on the telephone line and use an infrared transmitter to convert the data into an infrared signal. An infrared detector coupled to the bus 1502 can receive the data carried in the infrared signal and place the data on the bus 1502. The bus 1502 carries the data to the main memory 1506, from which the processor 1504 retrieves and executes the instructions. The instructions received by the main memory 1506 may optionally be stored on the storage device 1510 before or after execution by the processor 1504.

[0166] Preferably, the computer system 1500 also includes a communication interface 1518 coupled to the bus 1502. The communication interface 1518 provides a bidirectional data communication coupling to a network link 1520, which is connected to a local network 1522. For example, the communication interface 1518 can be an integrated services digital network (ISDN) card or a modem to provide a data communication connection with a corresponding type of telephone line. As another example, the communication interface 1518 can be a local area network (LAN) card to provide a data communication connection with a compatible LAN. A wireless link can also be implemented. In any such implementation, the communication interface 1518 sends and receives electrical signals, electromagnetic signals, or optical signals that carry digital data streams representing various types of information.

[0167] The network link 1520 typically provides data communication to other data devices through one or more networks. For example, the network link 1520 may provide a connection to a host computer 1524 or data equipment operated by an Internet Service Provider (ISP) 1526 through a local network 1522. The ISP 1526, in turn, provides data communication services through a global packet data communication network, now commonly referred to as the "Internet" 1428. Both the local network 1522 and the Internet 1528 use electrical, electromagnetic, or optical signals that carry digital data streams. The signals through the various networks and the signals on the network link 1520 and through the communication interface 1518, which carry the digital data to and from the computer system 1500, are exemplary forms of carrier waves that transport the information.

[0168] The computer system 1500 can send messages and receive data, including program code, through (multiple) networks, network links 1520, and communication interfaces 1518. In the Internet example, the server 1430 can send a request code for an application program through the Internet 1528, ISP 1526, local network 1522, and communication interface 1518. For example, one such download application can provide one or more technologies described herein. The received code can be executed by the processor 1504 when it is received, and / or stored in the storage device 1510 or other non-volatile storage device for later execution. In this way, the computer system 1500 can obtain the application code in the form of a carrier wave.

[0169] Other embodiments are provided in the following numbered item list:

[0170] 1. A method of determining a parameter of interest of a structure on a substrate, the structure comprising at least one first feature oriented along a first axis of a structure coordinate system and at least one second feature oriented along a second axis of the structure coordinate system, the method comprising:

[0171] illuminating the first feature and the second feature with a first illumination from a first direction oblique to the first and second axes to generate first scattered radiation from the first feature and second scattered radiation from the second feature,

[0172] detecting a first interference pattern formed by interference between a portion of the first scattered radiation and a first reference illumination;

[0173] detecting a second interference pattern formed by interference between a portion of the second scattered radiation and the first reference illumination; and

[0174] A parameter of interest of the structure is determined using the first interference pattern and the second interference pattern.

[0175] 2. The method according to clause 1, further comprising:

[0176] illuminating the first feature and the second feature with a second illumination from a second direction substantially opposite the first direction to generate third scattered radiation from the first feature and fourth scattered radiation from the second feature;

[0177] detecting a third interference pattern formed by interference between a portion of the third scattered radiation and the second reference illumination;

[0178] detecting a fourth interference pattern formed by interference between a portion of the fourth scattered radiation and the second reference illumination; and

[0179] A parameter of interest of the structure is determined using the first interference pattern, the second interference pattern, the third interference pattern, and the fourth interference pattern.

[0180] 3. A method according to item 2, wherein the azimuth angle between the first direction and the second direction is in the range of 180±30 degrees.

[0181] 4. A method according to item 2, wherein the azimuth angle between the first direction and the second direction is within the range of 180±5 degrees.

[0182] 5. A method according to item 2, wherein the azimuth angle between the first direction and the second direction is in the range of 180±1 degrees.

[0183] 6. A method according to any preceding clause, wherein the first direction is defined by an azimuth angle within a range of 45±30 degrees, an azimuth angle within a range of 135±30 degrees, an azimuth angle within a range of 225±30 degrees, or an azimuth angle within a range of 315±30 degrees.

[0184] 7. A method according to any one of clauses 1 to 5, wherein the first direction is defined by an azimuth angle within a range of 45±5 degrees, an azimuth angle within a range of 135±5 degrees, an azimuth angle within a range of 225±5 degrees or an azimuth angle within a range of 315±5 degrees.

[0185] 8. A method according to any one of clauses 1 to 5, wherein the first direction is defined by an azimuth angle within a range of 45±1 degrees, an azimuth angle within a range of 135±1 degrees, an azimuth angle within a range of 225±1 degrees or an azimuth angle within a range of 315±1 degrees.

[0186] 9. A method according to any of clauses 2 to 8, wherein the first illumination is provided by a first illuminator and the second illumination is provided by a second illuminator.

[0187] 10. The method of clause 9, wherein the steps of illuminating the first feature and the second feature with the first illumination and illuminating the first feature and the second feature with the second illumination are performed simultaneously.

[0188] 11. A method according to any one of clauses 2 to 8, wherein the first illumination and the second illumination are both provided by a single illuminator.

[0189] 12. The method of clause 11, further comprising, between the step of illuminating the first feature and the second feature with the first illumination and the step of illuminating the first feature and the second feature with the second illumination:

[0190] One or both of the structure and / or the single illuminator are rotated to rotate the structure relative to the single illuminator by a difference between the first direction and the second direction.

[0191] 13. A method according to any of clauses 2 to 12, wherein the first reference illumination is provided by a first reference illuminator and the second reference illumination is provided by a second reference illuminator.

[0192] 14. A method according to any one of clauses 2 to 12, wherein the first reference illumination and the second reference illumination are from a single reference illuminator.

[0193] 15. A method according to any preceding clause, wherein the first reference illumination is at least partially coherent with the first illumination.

[0194] 16. A method according to any of clauses 2 to 15, wherein the second reference illumination is at least partially coherent with the second illumination.

[0195] 17. A method according to any of the preceding clauses, comprising:

[0196] determining the parameter of interest in a direction defined by the first axis from the first interference pattern; and

[0197] The parameter of interest is determined by the second interference pattern in a direction defined by the second axis.

[0198] 18. A method according to any one of clauses 2 to 17, comprising:

[0199] determining the parameter of interest in a direction defined by the first axis from the first interference pattern and the third interference pattern; and

[0200] The parameter of interest is determined in a direction defined by the second axis by the second interference pattern and the fourth interference pattern.

[0201] 19. A method according to any preceding clause, wherein:

[0202] The first scattered radiation comprises a first diffraction order and the second scattered radiation comprises a second diffraction order.

[0203] 20. A method according to any one of clauses 2 to 19, wherein:

[0204] The third scattered radiation comprises a third diffraction order and the fourth scattered radiation comprises a fourth diffraction order.

[0205] 21. A method according to any preceding clause, wherein the parameter of interest is overlap.

[0206] 22. A method according to any preceding clause, wherein the first axis is perpendicular to the second axis.

[0207] 23. A holographic measurement device, comprising:

[0208] Sensor module;

[0209] a substrate support for supporting a substrate comprising a structure comprising at least one first feature oriented along a first axis of a structure coordinate system and at least one second feature oriented along a second axis of the structure coordinate system; and

[0210] A reference irradiation device, used to generate a first reference irradiation;

[0211] The sensor modules include:

[0212] an illumination arrangement comprising at least one illuminator operable to illuminate the structure with a first illumination from a first direction oblique to the first and second axes to generate first scattered radiation from a first feature and second scattered radiation from a second feature;

[0213] an optical system operable to capture the first and second scattered radiation; and

[0214] A detector is operable to detect a first interference pattern formed by interference between a portion of the first scattered radiation and the first reference illumination and a second interference pattern formed by interference between a portion of the second scattered radiation and the first reference illumination.

[0215] 24. A holographic measurement device according to clause 23, wherein:

[0216] The reference illumination device is further operable to generate a second reference illumination;

[0217] the illumination arrangement being further operable to illuminate the structure with a second illumination from a second direction substantially opposite to the first direction to generate third scattered radiation from the first features and fourth scattered radiation from the second features;

[0218] The optical system is further operable to capture the third scattered radiation and the fourth scattered radiation; and

[0219] The detector is further operable to detect a third interference pattern formed by interference between a portion of the third scattered radiation and the second reference illumination and a fourth interference pattern formed by interference between a portion of the fourth scattered radiation and the second reference illumination.

[0220] 25. A holographic measurement device according to clause 24, wherein the first direction and the second direction differ by an azimuth angle in the range of 180±30 degrees.

[0221] 26. A holographic measurement device according to clause 24, wherein the first direction and the second direction differ by an azimuth angle in the range of 180±5 degrees.

[0222] 27. A holographic measurement device according to clause 24, wherein the first direction and the second direction differ in azimuth angle within the range of 180±1 degrees.

[0223] 28. A holographic measurement device according to any one of clauses 23 to 27, wherein the first direction is defined by an azimuth angle within a range of 45±30 degrees, an azimuth angle within a range of 135±30 degrees, an azimuth angle within a range of 225±30 degrees or an azimuth angle within a range of 315±30 degrees.

[0224] 29. A holographic measurement device according to any one of clauses 23 to 27, wherein the first direction is defined by an azimuth angle within a range of 45±5 degrees, an azimuth angle within a range of 135±5 degrees, an azimuth angle within a range of 225±5 degrees or an azimuth angle within a range of 315±5 degrees.

[0225] 30. A holographic measurement device according to any one of clauses 23 to 27, wherein the first direction is defined by an azimuth angle within a range of 45±1 degrees, an azimuth angle within a range of 135±1 degrees, an azimuth angle within a range of 225±1 degrees or an azimuth angle within a range of 315±1 degrees.

[0226] 31. A holographic metrology apparatus according to any one of clauses 24 to 30, wherein the illumination means comprises:

[0227] a first illuminator operable to provide a first illumination, and

[0228] A second illuminator is operable to provide a second illumination.

[0229] 32. A holographic metrology apparatus according to clause 31, wherein the first illuminator and the second illuminator are operable to provide the first illumination and the second illumination simultaneously.

[0230] 33. A holographic measurement device according to any one of clauses 24 to 30, comprising:

[0231] A single illuminator is operable to provide a first illumination and a second illumination.

[0232] 34. A holographic metrology apparatus according to any of clauses 23 to 33, wherein one or both of the structure and / or the illumination device are rotatable to rotate at least one illuminator by a difference between a first orientation and a second orientation.

[0233] 35. A holographic metrology apparatus according to any one of clauses 24 to 34, wherein the reference illumination device comprises:

[0234] a first reference illuminator operable to provide a first reference illumination; and

[0235] A second reference illuminator is operable to provide a second reference illumination.

[0236] 36. Holographic metrology apparatus according to any of clauses 24 to 34, wherein the reference illumination arrangement comprises a single reference illuminator operable to provide the first reference illumination and the second reference illumination.

[0237] 37. A holographic metrology apparatus according to any of clauses 23 to 36, wherein the first reference illumination is at least partially coherent with the first illumination.

[0238] 38. A holographic metrology apparatus according to any of clauses 2 to 37, wherein the second reference illumination is at least partially coherent with the second illumination.

[0239] 39. A holographic metrology apparatus according to any of clauses 23 to 28, further comprising a processor operable to determine a parameter of interest of the structure using the first interference pattern and the second interference pattern.

[0240] 40. A holographic metrology apparatus according to clause 39, wherein the processor is further operable to:

[0241] determining the parameter of interest in a direction defined by the first axis from the first interference pattern; and

[0242] The parameter of interest is determined by the second interference pattern in a direction defined by the second axis.

[0243] 41. A holographic metrology apparatus according to any of clauses 24 to 38, further comprising a processor operable to determine a parameter of interest of the structure using the first interference pattern, the second interference pattern, the third interference pattern and the fourth interference pattern.

[0244] 42. A holographic metrology apparatus according to clause 41, wherein the processor is further operable to:

[0245] determining the parameter of interest in a direction defined by the first axis from the first interference pattern and the third interference pattern; and

[0246] The parameter of interest is determined in a direction defined by the second axis by the second interference pattern and the fourth interference pattern.

[0247] 43. A holographic metrology apparatus according to any of clauses 39 to 42, wherein the parameter of interest is overlap.

[0248] 44. A holographic measurement device according to any one of clauses 28 to 43, wherein the first axis is perpendicular to the second axis.

[0249] 45. A digital holographic microscope configured to determine a parameter of interest of a structure comprising at least one first feature oriented along a first axis of a structural coordinate system and at least one second feature oriented along a second axis of the structural coordinate system, the digital holographic microscope comprising a holographic measurement device of any one of items 23 to 44.

[0250] 46. ​​A digital holographic microscope according to any of item 45, including a dark-field digital holographic microscope.

[0251] 47. A scatterometer comprising the sensor module of any one of clauses 23 to 44.

[0252] 48. An alignment sensor comprising the sensor module of any one of items 23 to 44.

[0253] 49. A level sensor comprising the sensor module of any one of clauses 23 to 44.

[0254] Although specific reference may be made herein to the use of lithographic apparatus in the manufacture of ICs, it should be understood that the lithographic apparatus described herein may have other applications. Possible other applications include the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, flat panel displays, liquid crystal displays (LCDs), thin film magnetic heads, and the like.

[0255] Although embodiments of the present invention may be specifically referenced herein in the context of lithographic equipment, embodiments of the present invention may be used in other equipment. Embodiments of the present invention may form part of a mask inspection equipment, a metrology equipment, or any equipment that measures or processes objects such as a wafer (or other substrate) or a mask (or other pattern forming device). These equipment may generally be referred to as lithographic tools. Such lithographic tools may use vacuum conditions or ambient (non-vacuum) conditions.

[0256] Although specific reference may have been made above to the use of embodiments of the invention in the context of optical lithography, it will be appreciated that the invention is not limited to optical lithography and may be used in other applications, such as imprint lithography, where the context permits.

[0257] Although specific reference is made to "measurement equipment / tool / system" or "inspection equipment / tool / system", these terms may refer to the same or similar types of tools, equipment, or systems. For example, an inspection or measurement equipment including embodiments of the present invention may be used to determine characteristics of structures on a substrate or wafer. For example, an inspection equipment or measurement equipment including embodiments of the present invention may be used to detect defects in a substrate or defects in a structure on a substrate or wafer. In such embodiments, the characteristic of interest of a structure on a substrate may be related to a defect in the structure, the absence of a particular portion of the structure, or the presence of an unwanted structure on the substrate or wafer.

[0258] Although specific embodiments of the present invention have been described above, it is to be understood that the present invention may be practiced in a manner other than that described. The above description is intended to be illustrative, not restrictive. Therefore, it will be apparent to those skilled in the art that modifications may be made to the described invention without departing from the scope of the claims set forth below.

Claims

1. A holographic measurement device, comprising: Sensor module; a substrate support for supporting a substrate comprising a structure comprising at least one first feature oriented along a first axis of a structure coordinate system and at least one second feature oriented along a second axis of the structure coordinate system; as well as A reference irradiation device, used to generate a first reference irradiation; The sensor module comprises: an illumination arrangement comprising at least one illuminator operable to illuminate the structure with a first illumination from a first direction oblique to the first and second axes to generate first scattered radiation from the first features and second scattered radiation from the second features; an optical system operable to capture the first and second scattered radiation; and A detector is operable to detect a first interference pattern formed by interference between a portion of the first scattered radiation and a first reference illumination and a second interference pattern formed by interference between a portion of the second scattered radiation and the first reference illumination.

2. The holographic measurement device according to claim 1, wherein: The reference illumination device is further operable to generate a second reference illumination; the illumination arrangement being further operable to illuminate the structure with a second illumination from a second direction substantially opposite to the first direction to generate third scattered radiation from the first feature and fourth scattered radiation from the second feature; The optical system is further operable to capture the third scattered radiation and the fourth scattered radiation; and The detector is further operable to detect a third interference pattern formed by interference between a portion of the third scattered radiation and the second reference illumination and a fourth interference pattern formed by interference between a portion of the fourth scattered radiation and the second reference illumination. 3 . The holographic measurement device according to claim 2 , wherein the azimuth angle between the first direction and the second direction is within a range of 180±30 degrees.

4. The holographic measurement device according to any one of claims 1 to 3, wherein the first direction is defined by an azimuth angle within a range of 45±30 degrees, an azimuth angle within a range of 135±30 degrees, an azimuth angle within a range of 225±30 degrees, or an azimuth angle within a range of 315±30 degrees.

5. The holographic measurement device according to any one of claims 2 to 4, wherein the illumination device comprises: a first illuminator operable to provide said first illumination, and A second illuminator is operable to provide the second illumination.

6. A holographic metrology apparatus according to claim 5, wherein the first and second illuminators are operable to provide the first and second illumination simultaneously.

7. The holographic measurement device according to any one of claims 2 to 4, comprising: A single illuminator is operable to provide said first illumination and said second illumination.

8. The holographic metrology apparatus according to any one of claims 1 to 7, wherein one or both of the structure and / or the illumination device are rotatable to rotate the at least one illuminator by the difference between the first direction and the second direction.

9. The holographic metrology apparatus according to any one of claims 2 to 8, wherein the reference illumination device comprises: a first reference illuminator operable to provide said first reference illumination; as well as A second reference illuminator is operable to provide the second reference illumination.

10. A holographic metrology apparatus according to any one of claims 2 to 8, wherein the reference illumination arrangement comprises a single reference illuminator operable to provide the first reference illumination and the second reference illumination.

11. The holographic metrology apparatus according to any one of claims 1 to 10, wherein at least one of the following is present: The first reference illumination is at least partially coherent with the first illumination, and The second reference illumination is at least partially coherent with the second illumination.

12. A digital holographic microscope configured to determine a parameter of interest of a structure, wherein the structure comprises at least one first feature oriented along a first axis of a structural coordinate system and at least one second feature oriented along a second axis of the structural coordinate system, wherein the digital holographic microscope comprises the holographic measurement device of any one of claims 1 to 11.

13. The digital holographic microscope according to any one of claims 12, comprising a dark-field digital holographic microscope.

14. A method of determining a parameter of interest of a structure on a substrate, the structure comprising at least one first feature oriented along a first axis of a structure coordinate system and at least one second feature oriented along a second axis of the structure coordinate system, the method comprising: illuminating the first feature and the second feature with a first illumination from a first direction oblique to the first axis and the second axis to generate first scattered radiation from the first feature and second scattered radiation from the second feature, detecting a first interference pattern formed by interference between a portion of the first scattered radiation and a first reference illumination; detecting a second interference pattern formed by interference between a portion of the second scattered radiation and the first reference illumination; as well as The parameter of interest of the structure is determined using the first interference pattern and the second interference pattern.

15. A scatterometer comprising the holographic metrology device according to any one of claims 1 to 11.

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