Sensor module, illuminator, metrology device and associated metrology method

Abstract

A sensor module for a metrology device is disclosed. The sensor module comprises an illumination device for illuminating a structure on a substrate, the illumination device comprising at least a first set of illuminators and a second set of illuminators, where the first set of illuminators comprises one or more illuminators, each illuminator being operable to illuminate the structure with a first illumination comprising a first optical characteristic, and the second set of illuminators being operable to illuminate the structure with a second illumination comprising a second optical characteristic. And wherein the second set of illuminators comprises one or more illuminators, each illuminator operable to illuminate the structure with a second illumination comprising a second optical characteristic different from the first optical characteristic; an optical system operable to capture scattered radiation generated by the illuminated structure scattered by the structure; and a detector operable to detect the scattered radiation.

Description

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims priority to European patent application 22188074.3 filed on August 1, 2022, the entire contents of which are incorporated herein by reference. Technical Field

[0003] The present invention relates to metrology, such as digital holographic microscopy, and 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 that is configured to apply a desired pattern to a substrate. A lithographic apparatus may be used, for example, in the manufacture of integrated circuits (ICs). For example, a lithographic apparatus may project a pattern (also often referred to as a "design layout" or "design") at a patterning device (e.g., a mask) onto a layer of radiation-sensitive material (resist) disposed on a substrate (e.g., a wafer).

[0005] To project a pattern on 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 a substrate. Typical wavelengths currently used 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 4-20 nm, such as 6.7 nm or 13.5 nm, can be used to form smaller features on a substrate compared to a lithographic apparatus using radiation with a wavelength of, for example, 193 nm.

[0006] Low-k 1 Photolithography can be used to process features whose size is smaller than the classical resolution limit of the photolithographic apparatus. In this type of process, the resolution formula can be expressed as CD = k 1 × λ / NA, where λ is the wavelength of the radiation used, NA is the numerical aperture of the projection optics in the lithographic apparatus, CD is the “critical dimension” (generally the smallest feature size printed, but in this case the half pitch), and k 1 is the empirical resolution factor. In general, k 1The smaller the NA, the more difficult it is to replicate a pattern on a substrate that is similar to the shape and size designed by the circuit designer to obtain specific electrical functionality and performance. To overcome these difficulties, complex fine-tuning steps can be applied to the lithography projection apparatus and / or the design layout. These include, for example, but are not limited to, optimization of the NA, customized illumination schemes, use of phase-shift patterning equipment, various optimizations of the design layout, such as optical proximity correction (OPC, sometimes also called "optical and process correction") in the design layout, or other methods generally defined as "resolution enhancement technology" (RET). Alternatively, a tight control loop for controlling the stability of the lithography apparatus can be used to improve low-k 1 The following pattern is reproduced.

[0007] During the manufacturing process, it is necessary to inspect the manufactured structure and / or measure the characteristics of the manufactured structure. Suitable inspection and metrology devices are known in the art. One known metrology device is a scatterometer, such as a dark field scatterometer.

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

[0009] The above-mentioned metrology equipment, more generally, may have the problem of limited illumination and detection wavelength range. In particular, it may be desirable to operate the metrology equipment in a wavelength range of, for example, 400nm to 1600nm. However, designing and / or implementing suitable illumination optics for this bandwidth may be difficult.

[0010] For example, in the context of digital microscopy, microscopes that can operate over this bandwidth have been addressed by implementing reflective optical components. However, these reflective optical components are difficult to manufacture and align. This problem has also been addressed by implementing a greater number of lens elements. As a result, component costs and device complexity are undesirably increased. In addition, the problem has been addressed by implementing a small number of lens elements, such as less than three lens elements. However, the use of a small number of lens elements means that it is more difficult to structurally generate an illumination spot of a suitable size over the entire wavelength range, and it is not possible for the illuminator to generate an illumination spot with a flat intensity profile.

[0011] Furthermore, the optical fiber connecting the fixed world to the sensing module may be optimized for specific optical properties, such as wavelength range, polarization, or spot size, but may not be suitable for operation under optical properties other than those specified.

[0012] It is desirable to provide an illumination device for a measurement device that is suitable for use over a wide wavelength range. Summary of the invention

[0013] In a first aspect of the present invention, a sensor module for a measurement device is provided, the sensor module comprising: an illumination device for illuminating a structure on a substrate, the illumination device comprising at least a first group of illuminators and a second group of illuminators, wherein the first group of illuminators comprises one or more illuminators, each illuminator being operable to illuminate the structure with a first illumination comprising a first optical characteristic, and wherein the second group of illuminators comprises one or more illuminators, each illuminator being operable to illuminate the structure with a second illumination comprising a second optical characteristic different from the first optical characteristic; an optical system being operable to capture scattered radiation generated by the illuminated structure and scattered by the structure; and a detector being operable to detect the scattered radiation.

[0014] In a second aspect of the present invention, an illumination device for a measurement device is provided; the illumination device comprises: a first group of illuminators, the first group of illuminators comprising one or more illuminators, each illuminator being operable to emit a first illumination comprising a first optical characteristic; a second group of illuminators, the second group of illuminators comprising one or more illuminators, each illuminator being operable to emit a second illumination comprising a second optical characteristic different from the first optical characteristic; and an illuminator holding device, the illuminator holding device accommodating the first group of illuminators and the second group of illuminators. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic diagrams in which:

[0016] - Figure 1 A schematic overview of a lithography apparatus is depicted;

[0017] - Figure 2 A schematic overview of a lithography cell is depicted;

[0018] - Figure 3 Depicted is a schematic representation of overall lithography, showing the collaboration between the three key technologies to optimize semiconductor manufacturing;

[0019] - 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;

[0020] - Figure 5 depicts a schematic overview of a level sensor arrangement according to an embodiment of the invention, which level sensor arrangement may include a dark-field digital holographic microscope;

[0021] - Figure 6depicts a schematic overview of an alignment sensor arrangement according to an embodiment of the present invention, which may include a dark-field digital holographic microscope;

[0022] - Figure 7 schematically depicts an example of a diffraction-based dark-field metrology device operating in a parallel acquisition scheme;

[0023] - Figure 8 Different examples of diffraction-based dark-field metrology equipment operating in a sequential acquisition scheme are schematically depicted;

[0024] - Fig. 9 An example of a dark-field digital holographic microscope operating in a sequential acquisition scheme is schematically depicted;

[0025] - Fig.10 Schematic depiction of a dark-field digital holographic microscope (df-DHM) operating in a parallel acquisition scheme;

[0026] - Fig.11A schematically depicts a dark-field digital holographic microscope (df-DHM) operating in a sequential acquisition scheme according to one embodiment;

[0027] - Fig. 11B Schematically depicts a Fig.11A The geometric NA space of the df-DHM shown;

[0028] - Fig. 11C schematically depicts a schematic diagram of an example arrangement of an illumination device;

[0029] - Fig. 12A schematically depicts a dark-field digital holographic microscope (df-DHM) operating with a parallel acquisition scheme according to one embodiment;

[0030] - Fig. 12B Schematically depicts a Fig. 12A The geometric NA space of the df-DHM shown in ; and

[0031] - Fig.13 Depicted is a block diagram of a computer system used to control a dark-field digital holographic microscope. DETAILED DESCRIPTION

[0032] In this document, the terms "radiation" and "beam" are used to cover all types of electromagnetic radiation, including UV radiation (e.g., having a wavelength of 365nm, 248nm, 193nm, 157nm or 126nm) and EUV (extreme ultraviolet radiation, e.g., having a wavelength of about 5-100nm).

[0033] The terms "reticle", "mask" or "patterning device" as used herein may be broadly interpreted as a general patterning device that can be used to impart a patterned cross-section to an incident radiation beam, corresponding to the pattern to be created in a target portion of the substrate. The term "light valve" may also be used in this context. In addition to conventional masks (transmissive or reflective, binary, phase-shifting, hybrid, etc.), other examples of such patterning devices include programmable mirror arrays and programmable LCD arrays.

[0034] Figure 1 A lithographic apparatus LA is schematically depicted. The lithographic apparatus LA comprises an illumination system (also referred to as an illuminator) IL configured to condition a radiation beam B (e.g., UV radiation, DUV radiation, or EUV radiation); a mask support (e.g., a mask table) MT constructed to support a patterning device (e.g., a 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., a wafer table) WT constructed to hold a substrate (e.g., a wafer coated with a resist) 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., a refractive projection lens system) PS configured to project a pattern imparted to the radiation beam B by the patterning device MA onto a target portion C (e.g., comprising one or more dies) of the substrate W.

[0035] In operation, the illumination system IL receives a radiation beam from a radiation source SO, for example via a beam delivery system BD. The illumination system IL may include various types of optical components, 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 so that it has a desired spatial and angular intensity distribution in its cross-section in the plane of the patterning device MA.

[0036] 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, applicable to the exposure radiation used, and / or to other factors, such as the use of 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.

[0037] 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 (e.g. water) to fill the space between the projection system PS and the substrate W, also known as immersion lithography. More information on immersion techniques is given in US6952253, which is incorporated herein by reference.

[0038] The lithographic apparatus LA may also be of a type having two or more substrate supports WT (also referred to as "dual stage"). In such "multi-stage" machines, the substrate supports WT may be used in parallel, and / or a step of preparing a substrate W for subsequent exposure may be performed on a substrate W on one substrate support WT while another substrate W on another substrate support WT is being used to expose a pattern on another substrate W.

[0039] In addition to the substrate support WT, the lithographic apparatus LA may further comprise a measuring table. The measuring table is arranged to hold sensors and / or cleaning equipment. The sensors may be arranged to measure properties of the projection system PS or properties of the radiation beam B. The measuring table may hold a plurality of sensors. The cleaning equipment may be arranged to clean parts of the lithographic apparatus, such as parts of the projection system PS or parts of a system for providing immersion liquid. The measuring table may be moved under the projection system PS when the substrate support WT is away from the projection system PS.

[0040] In operation, a radiation beam B is incident on a patterning device, for example a mask MA held on a mask support MT, and is patterned by a pattern (design layout) present on the patterning device MA. After passing through the mask MA, the radiation beam B passes through a projection system PS which focuses the radiation beam onto a target portion C of the substrate W. With the aid of a second positioner PW and a position measurement system IF, the substrate support WT can be precisely moved, for example in order to position different target portions C in the path of the radiation beam B in a focused and aligned position. Similarly, the first positioner PM and possibly a further position sensor (in Figure 1The patterning device MA (not explicitly shown) 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 of M2 and the substrate alignment marks P1 of P2. Although the substrate alignment marks P1, P2 are shown occupying dedicated target portions, they can be located in the space between target portions. When located between target portions C, the substrate alignment marks of P2, P1 are referred to as scribe line alignment marks.

[0041] like Figure 2 As shown, the lithography apparatus LA may form part of a lithography unit LC, sometimes also referred to as a lithography cell or (lithography) group, which typically also includes devices for performing pre-exposure and post-exposure processing on a substrate W. Conventionally, these devices include a spin coater SC for depositing a resist layer, a developer DE for developing the exposed resist, a cooling plate CH and a baking plate BK, for example for regulating the temperature of the substrate W, for example 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 them between different processing devices, and delivers substrates W to a loading station LB of the lithography apparatus LA. The devices in the lithography cell are also generally referred to as a track, typically under the control of 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.

[0042] In order for the substrate W exposed by the lithography apparatus LA to be correctly and consistently exposed, the substrate needs to be inspected to measure characteristics of the patterned structure, such as overlay errors between subsequent layers, line thickness, critical dimensions (CD), etc. To this end, an inspection tool (not shown) may be included in the lithography cell LC. If an error is detected, for example, adjustments may be made to the exposure of subsequent substrates or to other processing steps to be performed on the substrate W, particularly if the inspection is performed before other substrates W of the same batch are still to be exposed or processed.

[0043] The detection device, which may also be referred to as a metrology device, is used to determine properties of the substrate W, and in particular how properties vary from one substrate to another W, or how properties associated with different layers of the same substrate W vary from layer to layer. Alternatively, the inspection device may be configured to identify defects on the substrate W, and may for example be part of the lithography cell LC, or may be integrated into the lithography apparatus LA, or may even be a stand-alone device. The inspection device may measure properties on a latent image (the image in the resist layer after exposure), on a semi-latent image (the image in the resist layer after a post-exposure bake step PEB), on a developed resist image (wherein exposed or unexposed parts of the resist have been removed), or even on an etched image (after a pattern transfer step such as etching).

[0044] Typically, the patterning process in the lithography apparatus LA is one of the most critical steps in the processing process, which requires high precision in the size and position of the structures on the substrate W. To ensure this high precision, the three systems can be combined in a so-called "holistic" control environment, such as Figure 3 As shown. One of these systems is a lithography apparatus LA, which is (virtually) connected to a metrology tool MT (a second system) and a computer system CL (a third system). The key to this type of "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 a range of process parameters (e.g. dose, focus, overlay) within which a particular manufacturing process produces a defined result (e.g. a functional semiconductor device) - typically within these parameter ranges, variations in process parameters in the lithography process or patterning process are allowed.

[0045] The computer system CL can use (parts of) the design layout to be patterned to predict the resolution enhancement technology to be used, and perform computational lithography simulations and calculations to determine which mask layout and lithography apparatus settings can achieve the maximum overall process window ( Figure 3 Typically, the resolution enhancement technique is arranged to match the patterning possibilities of the lithographic apparatus LA. The computer system CL may also be used 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 (e.g. due to suboptimal processing) Figure 3 ), as depicted by an arrow pointing to “0” in the second scale SC2.

[0046] 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 ( Figure 3 ) as depicted by multiple arrows in a third scale SC3.

[0047] In a lithographic process, it is frequently necessary to measure the formed structures, for example for process control and verification. The tool used to perform such measurements is typically 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. A scatterometer is a versatile instrument that measures parameters of a lithographic process by having a sensor in a pupil or conjugate plane of a scatterometer objective, the measurement being generally referred to as pupil-based measurement, or by having the sensor in an image plane or in a plane conjugate to the image plane, in which case the measurement is generally referred to as image- or field-based measurement. Such scatterometers and associated measurement techniques are also described in patent applications US20100328655, US2011102753A1, US20120044470A, US20110249244, US20110026032 or EP1628164A, the entire contents of which are incorporated herein by reference. The aforementioned scatterometers can measure gratings using light from soft x-rays and visible to near-infrared wavelengths.

[0048] In a first embodiment, the scatterometer MT is an angle-resolved scatterometer. In such scatterometers, 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 produced 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.

[0049] In a second embodiment, the scatterometer MT is a spectroscopic scatterometer MT. In this type of spectroscopic scatterometer MT, radiation emitted by a radiation source is directed onto a target and radiation reflected or scattered from the target is directed to a spectrometer detector which measures the spectrum of the specularly reflected radiation (i.e. measures the intensity as a function of wavelength). From this data, the structure or profile of the target that gave rise to the detected spectrum can be reconstructed, for example by rigorous coupled wave analysis and nonlinear regression or by comparison with a library of simulated spectra.

[0050] 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 in 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 can also provide polarized radiation. Various embodiments of existing ellipsometers are described in U.S. patent applications 11 / 451599, 11 / 708678, 12 / 256780, 12 / 486449, 12 / 920968, 12 / 922587, 13 / 000229, 13 / 033135, 13 / 533110 and 13 / 891410, the entire contents of which are incorporated herein by reference.

[0051] 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 the reflection spectrum and / or an asymmetry of the detection configuration, which asymmetry is related to the degree of overlap. The two (typically overlapping) grating structures may be applied in two different layers (not necessarily continuous layers) and may be formed substantially at the same position on the wafer. The scatterometer may have a symmetrical detection configuration, such as described in co-owned patent application EP1628164A, so that any asymmetry is clearly discernible. This provides a method for directly measuring grating misalignment. Further examples for measuring the overlay error between two layers comprising periodic structures may be found in PCT patent application publication number WO 2011 / 012624 or in US patent application US20160161863, the entire contents of which are incorporated herein by reference.

[0052] Other parameters of interest may be focal length and dose. As described in U.S. Patent Application US2011-0249244, focus and dose may be determined simultaneously by scatterometry (or by scanning electron microscopy), the entire contents of which are incorporated herein by reference. A single structure may be used that has a unique combination of critical dimension and sidewall angle measurements for each point in the focus energy matrix (FEM also referred to as the focus exposure matrix). If these unique combinations of critical dimensions and sidewall angles are available, focus and dose values ​​may be uniquely determined from these measurements.

[0053] The metrology target may be a collection of composite gratings, formed by a lithography process, mainly in resist, but also after, for example, an etching process. Typically, the pitch and line width of the structures in the gratings strongly depend on the measurement optics (particularly the NA of the optics) in order to be able to capture the diffraction orders from the metrology target. As previously described, the diffraction signal can be used to determine the offset between two layers (also known as "overlay"), or can be used to reconstruct at least one component of the original grating produced by the lithography process. Such reconstruction can be used to provide guidance on the quality of the lithography process and can be used to control at least one component of the lithography process. The target can have smaller subdivisions that are configured to simulate the size of functional components of the design layout in the target. Due to this subdivision, the target will behave more similar to the functional components of the design layout, making the overall process parameter measurement more similar to the functional components of the design layout. The target can be measured in an underfill mode or an overfill mode. In the underfill mode, the spot generated by the measurement beam is smaller than the entire target. In the overfill mode, the spot generated by the measurement beam is larger than the entire target. In this type of overfill mode, different targets can also be measured simultaneously and thus different process parameters can be determined simultaneously.

[0054] Figure 4 A measurement device, such as a scatterometer, is depicted. 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 (i.e. a measurement of the intensity as a function of wavelength) of the specularly reflected radiation. From this data, the structure or profile 8 which gave rise to the detected spectrum can be reconstructed by a processing unit PU, for example by rigorous coupled wave analysis and nonlinear regression or by comparison with e.g. Figure 3 In general, for the reconstruction, the general form of the structure is known and some parameters are assumed from the knowledge of the process by which the structure was made, leaving only a few parameters of the structure to be determined from the scatterometry data. Such scatterometers can be configured as normal-incidence scatterometers or oblique-incidence scatterometers.

[0055] The overall measurement quality of the lithography parameters obtained by measuring the measurement target is at least partially determined by the measurement method used to measure the lithography parameters. The term "substrate measurement recipe" may include one or more parameters of the measurement itself, one or more parameters of the one or more patterns measured, or both. For example, if the measurement used in the substrate measurement recipe is an optical measurement based on diffraction, the one or more parameters measured may 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 recipe may 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, the entire contents of which are incorporated herein by reference.

[0056] Another measurement tool used in integrated circuit manufacturing is a topography measurement system, a level sensor or a height sensor. Such tools can be integrated in a lithography apparatus to measure the topography of a substrate (or wafer). A topography map of the substrate, also known as a height map, can be generated from these measurements, which indicates the height of the substrate as a function of the position on the substrate. The height map can then be used to correct the position of the substrate in the process of transferring the pattern to the substrate so as to provide a spatial image of the patterning device at an appropriate focus position on the substrate. It should be understood that "height" herein refers to a dimension outside the plane of the substrate (also known as the Z axis). Typically, a level sensor or height sensor is measured at a fixed position (relative to its own optical system), and the relative movement between the substrate and the optical system of the level sensor or height sensor causes the height measurement to be performed at a position on the substrate.

[0057] Examples of level sensors or height sensors known in the prior art include Figure 5 As shown, the figure only illustrates the operating principle. In this example, the level sensor comprises an optical system, which includes a projection unit LSP and a detection unit LSD. The projection unit LSP comprises a radiation source LSO providing a radiation beam LSB, which is imparted by a projection grating PGR of the projection unit LSP. The radiation source LSO can be, for example, a narrowband or broadband light 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 may 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 may additionally or alternatively include UV and / or IR radiation and any wavelength range suitable for reflection from the substrate surface.

[0058] The projection grating PGR is a periodic grating, including a periodic structure, causing the radiation beam BE1 to have a periodically varying intensity. The radiation beam BE1 with a periodically varying intensity is directed to a measurement position MLO on a substrate W having an incident angle α between 0 and 90 degrees, typically between 70 and 80 degrees, relative to an axis (Z axis) perpendicular to the incident substrate surface. At the measurement position MLO, the patterned radiation beam BE1 is reflected by the substrate W (indicated by arrow BE2) and directed to a detection unit LSD.

[0059] 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 indicative of the received light, for example indicating the intensity of the received light, such as a photodetector, or representing the spatial distribution of the received intensity, such as a camera. The detector DET may comprise any combination of one or more detector types.

[0060] 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 (tilted) angle of incidence α.

[0061] The projection unit LSP and / or the detection unit LSD may comprise further optical elements, such as lenses and / or mirrors, along the path of the patterned radiation beam between the projection grating PGR and the detection grating DGR (not shown).

[0062] In one embodiment, the detection grating DGR may be omitted and the detector DET may be placed where the detection grating DGR is located. Such a configuration provides a more direct detection of the projection grating PGR image.

[0063] 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 an array of measurement areas MLO or spots covering a larger measurement range.

[0064] Various general types of height sensors are disclosed in, for example, US7265364 and US7646471, both of which are incorporated by reference. A height sensor using ultraviolet radiation instead of visible light or infrared radiation is disclosed in US2010233600A1, which is incorporated herein by reference. In WO2016102127A1, which is incorporated by reference, a compact height sensor is disclosed that uses a multi-element detector to detect and identify the position of a grating image without the need for a detection grating.

[0065] Another metrology tool used in integrated circuit manufacturing is the alignment sensor. Thus, a key aspect of lithographic apparatus performance is the ability to correctly and accurately place an applied pattern relative to features placed in a previous layer (either by the same apparatus or a different lithographic apparatus). To this end, the substrate is provided with one or more sets of marks or targets. Each mark is a structure whose position can later be measured using a position sensor, typically an optical position sensor. The position sensor may be referred to as an "alignment sensor" and the marks may be referred to as "alignment marks".

[0066] 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 currently used in a lithographic apparatus is based on a self-referencing interferometer described in US6961116. Various enhancements and modifications of position sensors have been developed, such as those disclosed in US2015261097A1. All of these published contents are incorporated herein by reference.

[0067] Figure 6 is a schematic block diagram of a known alignment sensor embodiment, such as described 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 a steering optical device onto a mark, such as a mark AM located on a substrate W, as an illumination spot SP. In this example, the steering optical device includes a spot reflector SM and an objective lens OL. The diameter of the illumination spot SP illuminating the mark AM can be slightly smaller than the width of the mark itself.

[0068] 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 "diffracted" is intended to include the zeroth order diffraction (which may be referred to as reflection) from the mark. A self-referencing interferometer SRI, such as the type disclosed in the aforementioned US6961116, interferes with the beam IB with itself, after which the beam is received by the photodetector PD. Additional optics (not shown) may be included to provide separate beams in the event that the radiation source RSO creates more than one wavelength. If desired, the photodetector may be a single element, or it may comprise a plurality of pixels. The photodetector may comprise an array of sensors.

[0069] Steering optics, in this example comprising a spot mirror SM, may also be used to block zeroth order radiation reflected from the mark so that the information-bearing beam IB comprises only higher order diffracted radiation from the mark AM (this is not necessary for the measurement, but improves the signal-to-noise ratio).

[0070] The intensity signal SI is provided to the processing unit PU. By a combination of optical processing in block SRI and computational processing in unit PU, the values ​​of the X and Y position on the substrate relative to the reference frame are output.

[0071] A single measurement of the type shown 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 this to identify which period of the sine wave contains the mark position. The same process at coarser and / or finer levels can be repeated at different wavelengths to improve accuracy and / or achieve robust detection of the mark, regardless of the material the mark is made of and the material on and / or below which the mark is placed. The wavelengths can be optically multiplexed and demultiplexed so as to be processed simultaneously, and / or they can be multiplexed by time or frequency division.

[0072] In this example, the alignment sensor and the light spot SP remain stationary while the substrate W moves. Therefore, the alignment sensor can be rigidly and accurately mounted on 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 controls this movement by being mounted on a substrate support and a substrate positioning system that controls 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 one embodiment, one or more (alignment) marks are provided on the substrate support. Measurement of the position of the mark provided on the substrate support allows calibration of the position of the substrate support determined by the position sensor (e.g., relative to the frame to which the alignment system is connected). Measurement of the position of the alignment mark provided on the substrate allows determination of the position of the substrate relative to the substrate support.

[0073] 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. Such measurements may be performed on production substrates and / or dedicated metrology targets. There are various techniques for measuring the microstructures formed in 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.

[0074] Examples of known scatterometers include angle-resolved scatterometers of the type described in US2006033921A1 and US2010201963A1. Such scatterometers use relatively large targets, such as a 40 μm×40 μm grating, and the measurement beam generates a spot that is smaller than the grating (i.e., the grating is not filled). In addition to measuring feature shapes by reconstruction, diffraction-based overlay can be measured using such devices, as described in published patent application US2006066855A1. Diffraction-based overlay measurements using diffraction-order dark-field imaging enable overlay measurements on smaller targets. Examples of dark-field imaging measurements 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 are described in published patent publications 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. Using composite grating targets, multiple gratings can be measured in one image. The contents of all these patent applications are also incorporated herein by reference.

[0075] Dark field microscopes, such as the metrology equipment mentioned above, more generally suffer from the problem of limited angular range for illuminating a target and / or detecting the diffracted light of a target, since 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.

[0076] In a diffraction-based dark field metrology apparatus, a radiation beam is directed onto a metrology target and one or more properties of the scattered radiation are measured to determine a property of interest of the target. The properties of the scattered radiation may include, for example, intensity at a single scattering angle (e.g., as a function of wavelength) or intensity at one or more wavelengths as a function of the scattering angle.

[0077] Target measurements in dark field metrology may include, for example, measuring the positive first diffraction order I +1 The first intensity and the negative first diffraction order I -1 The second intensity of the , 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 from such intensity asymmetry measurements, e.g. the target being 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 pairs of overlapping sub-gratings patterned in different layers of the semiconductor device. Thus, the asymmetry of the target will depend on the alignment of the two layers, and thus the overlap. Other targets may be formed with structures that vary the exposure to varying degrees, based on the focus setting used during exposure; the measurements of which enable the focus setting to be inferred back (again via the intensity asymmetry).

[0078] Figure 7 and Figure 8 Two examples of diffraction based dark field metrology devices are schematically illustrated. Note that for simplicity, the two figures show only some of the components sufficient to describe the working principle of the two devices.

[0079] like Figure 7 As shown, the 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. Because the device is configured for dark field imaging, the zeroth diffraction order may be blocked by the 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 st DF. At the pupil plane of objective OB, a first wedge WG1 can be used to redirect the diffracted radiation to follow a desired beam path. Finally, an imaging lens can be used to convert the diffraction orders (e.g., the positive first diffraction order +1 st DF) is focused onto the image sensor IS so that a first image IM1 is formed at a first position.

[0080] Similarly, a second radiation illumination beam IB2 may be incident obliquely on the same overlapping target OT of the substrate WA from the opposite side of the system. 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, for example a negative first diffraction order -1 st DF, can be collected by the objective lens OB and then redirected by the second wedge WG2. Negative first diffraction order -1 stDF may then be focused by the imaging lens IL onto the image sensor IS such that a second image IM2 is formed at a second location.

[0081] Figure 7 The example of is operated in a parallel acquisition scheme. An overlapping target is illuminated simultaneously by two illumination beams IB1, IB2. Accordingly, two spatially separated images IM1, IM2 of the overlapping target are acquired simultaneously. Such parallel acquisition schemes allow fast measurement speeds and therefore high throughput. However, the pupil plane of the objective OB must be shared by two diffraction orders, such as +1 st DF and -1 st DF. The result of dividing the pupil into mutually exclusive illumination and detection pupils is that the illumination NA and detection NA are reduced accordingly. Although there is some flexibility in the trade-off between illumination NA and detection NA, it is not possible to ultimately make both illumination NA and detection NA as large as is usually desired in a single pupil. This results in each corresponding illumination beam and +1 st DF and -1 st The limited angular range of the DF beam, which in turn restricts the range of permissible grating pitch sizes and / or illumination wavelengths, places stringent requirements on the design of such metrology systems.

[0082] Figure 8 Another exemplary dark field measurement device (or Figure 7 different operating modes of the device). The main difference is Figure 8 The metrology equipment of is operated in a sequential acquisition scheme. In a sequential acquisition scheme, the metrology target OT is illuminated by only one illumination beam from one direction at any moment, so that only one image of the target is formed and acquired at any point in time. Figure 8 At a first time t=T1, a first illumination beam IB1 can be turned on and directed obliquely from one side of the metrology device onto an overlay target OT of the substrate WA. After interacting with the grating of the overlay target, multiple diffraction orders can be generated. At least one non-zero diffraction order, for example a positive first diffraction order +1 st DF can be collected by the objective lens OB and then focused onto the image sensor IS by the imaging lens IL.

[0083] 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 obliquely onto the same overlapping target from the opposite side of the metrology device. At least one generated diffraction order, for example the negative first diffraction order -1 stDF, may be collected by objective lens OB and subsequently focused onto image sensor IS to form a second image IM2 of the overlapping object. Note that images IM1 and IM2 may be formed at a common location on the image sensor.

[0084] Using this time-multiplexed acquisition scheme, the full NA of objective lens OB can be used to detect the diffracted beam +1 st DF and -1 st DF. The absence of restrictions on the objective NA means that a wider range of relevant design parameters such as grating pitch size, illumination wavelength and illumination angle is allowed, and greater flexibility in system design can be obtained. However, the fact that multiple image acquisitions are required means that the measurement speed is reduced, so the system throughput is affected.

[0085] Furthermore, accurate determination of, for example, overlay error depends on accurate measurement of small relative intensity differences (or intensity asymmetry) between the two acquired images IM1, IM2. A typical relative intensity difference is 10 times the intensity of one of the acquired images (e.g., IM1 or IM2). -4 Order of magnitude. Such small intensity differences can be easily masked by any intensity and / or wavelength fluctuations of the illuminating radiation. Therefore, it is required that the illuminating beam remain stable during continuous image acquisition. This can be achieved by using a stable light source that provides the required intensity and wavelength stability. Alternatively, additional hardware and software, such as intensity / wavelength monitoring equipment and corresponding feedback control loops, should be incorporated into the measurement equipment so that intensity and / or wavelength fluctuations of the illuminating beam are actively monitored and well compensated. In some cases, an intensity monitoring device can be used to actively track the intensity of the illuminating beam. The signal generated from the intensity monitoring device can be used to (e.g., electronically) correct for intensity fluctuations of the illuminating beam. All of these solutions increase the complexity and cost of the overall system.

[0086] 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 and reference radiation coherent with the object radiation, wherein the object radiation is obtained by illuminating a three-dimensional (3D) object 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 about the illuminated object, for example, 3D shape information. Based on the captured hologram image, the image of the object can be digitally reconstructed by using a computer reconstruction algorithm. As Figure 7and Figure 8 As described in the example of , an important advantage of hologram-based metrology over intensity-based metrology is that hologram-based metrology allows obtaining both intensity and phase information of an object. With the additional phase information, features of an object can be determined more accurately.

[0087] International patent application WO2019197117A1 (incorporated herein by reference) discloses a method and a metrology device based on dark-field digital holographic microscopy (df-DHM) for determining features, such as overlaps, of structures fabricated on a substrate. For the purpose of description, the international patent application WO2019197117A1 Figure 3 exist Fig. 9 Repeat in. Fig. 9 The disclosed df-DHM, which is particularly suitable for lithography process metrology, is schematically illustrated.

[0088] and Figure 7 and Figure 8 Compared to the previous example shown, Fig. 9 The df-DHM in FIG. 1 further comprises a reference optical unit 16, 18 for providing two additional reference radiation beams 51, 52 (reference radiation). The two reference radiation 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 sequentially to form two interference patterns. Coherence control is provided by adjusting the relative optical path difference (OPD) between the two scattered reference beams in each beam pair. However, there is no coherence control between the two beam pairs.

[0089] Due to the use of a single light source and inadequate coherence control, all four radiation beams, i.e. the first portion 41 of the scattered radiation 31, the first reference radiation 51, the second portion 42 of the scattered radiation 32 and the second reference radiation 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 desired information containing patterns and undesired artifact contribution patterns will overlap 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 is impractical in such an arrangement because it is technically challenging and time consuming to completely separate the superimposed interference patterns.

[0090] and Figure 8 Similar to the example, Fig. 9 The use of a sequential acquisition scheme in the example of allows the full NA of the objective lens to be used for illumination and detection. However, due to the sequential acquisition, this system also suffers from the 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 achieved at the same time.

[0091] Fig.10 The 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 illumination radiation beams, namely a first illumination radiation beam 1010 and a second illumination radiation beam 1020. In one embodiment, the two illumination beams 1010, 1020 may illuminate the metrology target 1060 simultaneously.

[0092] In one embodiment, the first illumination beam 1010 may be incident on the measurement target 1060 at a first angle of incidence in a first direction 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 in a second direction 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 one 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 may be different; for example, at relative angles of 180 degrees apart.

[0093] 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 scattered beam including a positive first diffraction order +1 st The first scattered beam 1011 of DF; the second illumination beam 1020 may correspond to a first diffraction order including a negative first diffraction order -1 st 1070.

[0094] Both scattered beams 1011, 1021 may be collected by an objective lens 1070 and subsequently refocused onto an image sensor 1080. The objective lens 1070 may include a plurality of lenses, and / or the df-DHM 1000 may include a lens system having two or more lenses, for example, similar to Fig. 9 The objective lens and imaging lens of an exemplary df-DHM are arranged such that a pupil plane of the objective lens is defined between the two lenses and an image plane is defined 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, namely 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 divided 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.

[0095] In one embodiment, in order to separate two at least partially spatially overlapping interference patterns (e.g., in the spatial frequency domain), the first reference beam 1030 may have a first incident angle relative to the optical axis OA, and the second reference beam 1040 may have a second incident angle relative to the optical axis OA; the first incident angle and the second incident angle are different. Alternatively or additionally, the first reference beam 1030 may have a first azimuth angle relative to the optical axis OA, and the second reference beam 1040 may have a second azimuth angle relative to the optical axis OA; the first azimuth angle and the second azimuth angle are different.

[0096] In order to generate an interference pattern, the two beams of each scattered reference beam pair should be at least partially coherent with each other to a sufficient degree to form an 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 phase offsets may include effects due to the optical path length (OPD) from the measurement target 1060 to the image sensor 1080 and due to interactions 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 beam pair is incoherent with any beam in the other beam 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, such as the two interference patterns formed by the corresponding 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.

[0097] The above-mentioned measurement equipment can usually use broadband radiation to make measurements in different wavelength ranges. In some cases, the full broadband radiation (or a wide range thereof) will be used for the measurement. However, more typically, for each measurement, one or more single wavelengths or narrow wavelength bands will be filtered out or selected in some way so that the illumination is configurable for the measurement.

[0098] It is increasingly desirable that the selectable range is wider, for example, between 400 nm and 1600 nm or more. In the measurement process, the illumination device (which may include the last part of the illumination branch) can be very close to the structure to be measured. It is designed to focus the light on a spot of similar size (e.g. small) on the structure to be measured, regardless of the wavelength used. The illumination device or each of its illuminators may include a lens or a reflector, which focuses the illumination to a spot on the structure to be measured. It is increasingly difficult to design an illumination device that is able to work accurately and in the same way for all wavelengths over a sufficiently large wavelength range.

[0099] Therefore, it is recommended to use an illumination device comprising multiple illuminators (or groups of illuminators), each covering a corresponding smaller wavelength band. In this way, each illuminator can be optimized for a smaller wavelength range, significantly reducing the design complexity. Each group of illuminators can be connected to the fixed world optics (e.g., all optics outside the sensor head) using its own set of optical fibers. In this way, the only additional component can be a fiber switch, which can also reside in the fixed world (outside the sensor head).

[0100] Therefore, an illumination device for a measurement device is disclosed; the illumination device comprises: a first group of illuminators, the first group of illuminators comprises one or more illuminators, each illuminator is operable to emit a first illumination comprising a first optical characteristic; a second group of illuminators, the second group of illuminators comprises one or more illuminators, each illuminator is operable to emit a second illumination comprising a second optical characteristic different from the first optical characteristic.

[0101] The irradiation device may include an illuminator holding device, such as an illuminator ring that accommodates the first group of illuminators and the second group of illuminators. The central area within the illuminator ring can be configured to receive an optical system (e.g., an objective system) of the measurement device. The detection NA defined by the central detection area can be, for example, greater than 0.5, greater than 0.6, greater than 0.7, greater than 0.8, or in the region of 0.85.

[0102] The first optical characteristic comprises at least a first wavelength range and / or a first polarization, and the second optical characteristic comprises at least a second wavelength range and / or a second polarization. In one embodiment, the first optical characteristic comprises at least a first illumination spot size and / or a spot shape; and the second optical characteristic comprises at least a second illumination spot size and / or a spot shape.

[0103] The first wavelength range may include, for example, a lower limit of 200nm to 600nm, 300nm to 600nm, 300nm to 500nm, or about 400nm. The first wavelength range may include, for example, an upper limit of 700nm to 1300nm, 800nm ​​to 1200nm, 900nm to 1100nm, or about 1000nm. The second wavelength range may include, for example, a lower limit of 700nm to 1300nm, 800nm ​​to 1200nm, 900nm to 1100nm, or about 1000nm. The second wavelength range may include, for example, an upper limit of 1300nm to 1900nm, 1400nm to 1800nm, 1500nm to 1700nm, or about 1600nm. The first wavelength range may partially overlap with the second wavelength range, or may not overlap with the second wavelength range.

[0104] Each of the first group of illuminators and the second group of illuminators may include only one illuminator.

[0105] The first set of illuminators and the second set of illuminators may each include at least pairs of illuminators, each pair of illuminators emitting illumination from pairs of different directions. The pairs of different directions may include substantially perpendicular directions (e.g., for measuring an x-target and a y-target simultaneously). The pairs of different directions may include substantially opposite directions (e.g., for simultaneously capturing two complementary diffraction orders, such as +1 and -1 diffraction orders).

[0106] The first set of illuminators and the second set of illuminators may each include two pairs of illuminators (eg, 90 degrees apart). This allows for simultaneous capture of two complementary diffraction orders, such as +1 and -1 diffraction orders, from the x-target and y-target.

[0107] The illuminator ring may be rotated about a center point of the illuminator ring, the illuminator ring being configured to rotate the first group of illuminators and the second group of illuminators about the center point; for example, to switch the illuminators of the different groups to a position that allows measurement of a specific wavelength or wavelength range. Conversely, the illuminator ring may be fixed with the illuminator groups, and thus fixed in position relative to the rest of the sensor module. In this case, the substrate may be rotated appropriately when illuminating with the first group of illuminators or illuminating with the second group of illuminators. In another embodiment, the illuminator ring may be fixed and the substrate does not rotate. In such examples, the angular spacing between the groups should be small enough (e.g., so that the illuminators corresponding to each group are very close) in which case the illumination beams (for at least one group of illuminators) will not be perfectly aligned relative to the target, so they will also illuminate the grating at a slightly tilted angle. Due to the finite NA of the illumination optics, the misalignment may be in the order of 7 degrees, causing some crosstalk. In this case, it is suggested that this crosstalk can be suppressed by additional signal processing. For example, in such embodiments, the angular spacing between the groups may be less than 15 degrees, less than 10 degrees, less than 8 degrees, less than 6 degrees, or less than 4 degrees.

[0108] The illumination apparatus may include one or more additional groups of illuminators, each group of illuminators being operable to illuminate the structure with illumination including corresponding additional optical properties (e.g., the entire wavelength range may be divided into two or more sub-ranges, each sub-range having a dedicated group of illuminators).

[0109] The illumination device may constitute part of a sensor module or a sensor head. The sensor module may comprise an illumination device, an optical system (e.g. comprising at least one objective lens) operable to capture scattered radiation generated by the illuminated structure and scattered by the structure; and a detector operable to detect the scattered radiation.

[0110] To make the sensor module as compact and light as possible, it may be kept physically separate 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 illuminator box may form part). In this case, the sensor module may comprise at least the objective or lens arrangement and the detector; e.g. a CCD / camera (and very little else, e.g. only auxiliary wiring etc.); e.g. Fig.10The objective lens 1070 and the detector 1080. The illuminator box may include, for example, the arrangement described in EP3964892A1 and / or WO2021121733A1. These two documents are incorporated herein by reference.

[0111] The following description will be made in the context of holography and digital holographic microscopy. In particular, the following description will be made in the context of dark-field digital holographic microscopy. However, it should be understood that the concepts disclosed herein are not limited thereto, but can be applied to any suitable metrology device, including a scatterometer, an alignment sensor, a level sensor, a bright-field DHM, or any other such device.

[0112] Fig.11A A dark-field digital holographic microscope (df-DHM) 1300 is depicted according to an embodiment of the present disclosure, which is suitable for determining features of interest of a structure 1308 (e.g., a blanket layer) fabricated on a substrate W. The df-DHM 1300 in this example operates according to a sequential acquisition scheme, similar to Fig. 9 df-DHM.

[0113] In one embodiment, the df-DHM 1300 includes a sensor module (not shown) including an illumination device for illuminating a structure 1308. The illumination device includes at least a first set of illuminators and a second set of illuminators. The first set of illuminators includes one or more illuminators, each illuminator operable to illuminate the structure 1308 with a first illumination 1323 including a first optical characteristic. The second set of illuminators includes one or more illuminators, each illuminator operable to illuminate the structure 1308 with a second illumination 1324 including a second optical characteristic different from the first optical characteristic.

[0114] In this example, the first optical characteristic includes at least a first wavelength range; and the second optical characteristic includes at least a second wavelength range. Additional optical characteristics can be envisioned. In one embodiment, the first optical characteristic includes at least a first polarization; and the second optical characteristic includes at least a second polarization. In one embodiment, the first optical characteristic includes at least a first illumination spot size; and the second optical characteristic includes at least a second illumination spot size. In one embodiment, the first optical characteristic includes at least a first illumination spot shape; and the second optical characteristic includes at least a second illumination spot shape.

[0115] In this example, the first group of illuminators includes a first illuminator 1321; the second group of illuminators includes a second illuminator 1322. The first illuminator 1321 can be operated to illuminate the structure 1308 at a first wavelength range. The second illuminator 1322 can be operated to illuminate the structure 1308 at a second wavelength range. The wavelength ranges can be defined as above. The illuminator group can include more than two groups of illuminators, each group of illuminators can be operated to illuminate the structure 1308 at a corresponding different wavelength range.

[0116] In one embodiment, the first illumination 1321 can be incident on the structure 1308 at a first angle of incidence in a first direction relative to the optical axis OA. The second illumination 1322 can be incident on the structure 1308 at a second angle of incidence in a second direction relative to the optical axis OA. The first angle of incidence of the first illumination 1321 and the second angle of incidence of the second illumination 1322 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.

[0117] Irradiation of structure 1308 by first illumination 1323 may generate first scattered radiation 1331 that is scattered by structure 1308, the first scattered radiation being produced by structure 1308 illuminated by first illumination 1323. Irradiation of structure 1308 by second illumination 1324 may generate second scattered radiation (not shown) that is scattered by structure 1308, the second scattered radiation being produced by structure 1308 illuminated by second illumination 1324.

[0118] The sensor module also includes an optical system 1302 that is operable to capture radiation generated by the illuminated structure 1308 and scattered by the structure 1308. In particular, the optical system is operable to capture first scattered radiation 1331 generated by the structure 1308 illuminated by the first illumination 1323 and second scattered radiation (not shown) generated by the structure 1308 illuminated by the second illumination 1324. In one embodiment, the optical system 1302 includes an objective lens 1309. In one embodiment, the optical system 1302 includes a plano-convex lens. Plano-convex lenses are iso-spherical and have relatively high aberrations. In one embodiment, the optical system 1302 includes an aspheric lens or a double aspheric lens. Plano-aspheric surfaces are non-iso-spherical and have relatively low aberrations. In one embodiment, the optical system 1302 includes a reflector optic. In one embodiment, the optical system 1302 defines a detection numerical aperture (NA). The detection NA defined by the central detection area can be, for example, greater than 0.5, greater than 0.6, greater than 0.7, greater than 0.8, or in the region of 0.85.

[0119] The sensor module also includes a detector 1306 operable to detect scattered radiation.

[0120] The sensor module further includes an illuminator ring (not shown) that accommodates the first illuminator 1321 and the second illuminator 1331. In one embodiment, a central region of the illuminator ring is configured to receive the optical system 1302. In one embodiment, the illuminator ring is rotatable around the optical axis of the optical system 1302. In one embodiment, the illuminator ring is configured to rotate the first group of illuminators and the second group of illuminators around the optical axis.

[0121] The optical system 1302 is configured to direct a portion 1341 of the first scattered radiation 1331 and / or a portion of the second scattered radiation from the structure 1308 to the detector 1306. Although substantially similar principles apply to directing a portion of the second scattered radiation to the detector 1306, the following discussion is about the portion 1341 of the first scattered radiation. Thus, the portion 1341 of the first scattered radiation 1331 is the portion of the first scattered radiation 1331 that reaches the detector 1306. Other portions of the first scattered radiation 1341 do not reach the detector 1306. In one embodiment, the portion 1341 of the first scattered radiation 1331 that reaches the detector 1306 excludes the specular reflection component of the first scattered radiation 1331. This can be achieved by arranging the polar incidence angle of the first illumination 1323 to be large enough to ensure that the specular reflection falls outside the detection NA of the optical system 1302, which will occur at the same polar incidence angle as the illumination. The detection NA of the optical system 1302 may be the detection NA of the objective 1309. Thus, the detector 1306 may be arranged for dark field measurements. In one embodiment, portion 1341 of the first scattered radiation 1331 is at least primarily composed of one or more non-zero order diffraction components scattered from structure 1308 (i.e., more than half or entirely composed of one or more non-zero order diffraction components), for example only a +1 order diffraction component or one or more +1, +2, +3 or higher positive non-zero order diffraction components.

[0122] In some embodiments, the sensor module is configured to illuminate the structure 1308 simultaneously with the first illumination 1323 from the first illuminator 1321 and the second illumination 1324 from the second illuminator 1322. In some embodiments, the sensor module is configured to illuminate the structure 1308 separately or individually with the first illumination 1323 from the first illuminator 1321 and the second illumination 1324 from the second illuminator 1322. In one embodiment, the sensor module is configured to switch between illuminating the structure 1308 with the first illumination 1323 from the first illuminator 1321 and illuminating the structure 1308 with the second illumination 1324 from the second illuminator 1322 by rotation of the substrate W. In one embodiment, the sensor module is configured to switch between illuminating the structure 1308 with the first illumination 1323 from the first illuminator 1321 and illuminating the structure 1308 with the second illumination 1324 from the second illuminator 1322 by rotation of the sensor module. In one embodiment, the sensor module is configured to switch between illuminating the structure 1308 with a first illumination 1323 from the first illuminator 1321 and illuminating the structure 1308 with a second illumination 1324 from the second illuminator 1322 by rotating the illuminator ring. In one embodiment, the sensor module is configured to switch between illuminating the structure 1308 with a first illumination 1323 from the first illuminator 1321 and illuminating the structure 1308 with a second illumination 1324 from the second illuminator 1322 by a combination of rotation of the illuminator ring, rotation of the substrate W, and / or rotation of the sensor module.

[0123] Fig. 11B Schematically depicts a Fig.11A 13. The geometric NA space of the df-DHM 1300 shown. The NA space may include a peripheral illumination region 1350 surrounding a central detection region 1360 (i.e., detection NA) including the optical system 1302. The first illuminator 1321 and the second illuminator 1322 are arranged within the peripheral illumination region 1350 and / or provide illumination within the peripheral illumination region 1350. The peripheral illumination region 1350 may be an NA region between the maximum NA of the system and the detection NA. For a non-immersion setting, the maximum NA of the system may be 1.

[0124] The first illumination 1323 may be incident on the structure 1308 from a first direction in the reference frame of the structure 1308. The second illumination 1324 may be incident on the structure 1308 from a second direction in the reference frame of the structure 1308. The first direction may be different from the second direction. For example, the first illumination 1323 may be incident on the structure 1306 at a first azimuth corresponding to the first direction, where 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 second illumination 1324 may be incident on the structure 1306 at a second azimuth corresponding to the second direction. The first azimuth of the first illumination 1323 and the second azimuth of the second illumination 1324 may be different; for example, 45 degrees apart. It will be appreciated that the first direction may be the same as the second direction, such that the first illumination 1323 and the second illumination 1324 originate from the same direction in the reference frame of the structure 1308.

[0125] like Fig. 11B As shown, the first illuminator 1321 is arranged to provide illumination within the peripheral illumination area 1350 from a first direction, and the second illuminator 1322 is arranged to provide illumination within the peripheral illumination area 1350 from a second direction. In this embodiment, the first illuminator 1321 is arranged to provide illumination at a first azimuth angle, and the second illuminator 1322 is arranged to provide illumination at a second azimuth angle, wherein the azimuth interval between the first azimuth angle and the second azimuth angle is 45 degrees. It should be understood that any suitable azimuth difference can be used. In the case where the illuminator ring is fixed, the azimuth difference can be small enough to reduce the crosstalk effect between the first illuminator 1321 and the second illuminator 1322.

[0126] In one embodiment, df-DHM 1300 is further operable to provide respective first reference illumination 1351 corresponding to first illumination 1323 and second reference illumination (not shown) corresponding to second illumination 1324. In one embodiment, first reference illumination 1351 and second reference illumination comprise plane waves or spherical waves.

[0127] In one embodiment, first reference illumination 1351 is provided by first illuminator 1321 and second reference illumination is provided by second illuminator. First illuminator 1321 provides first reference illumination 1351 onto detector 1306 while providing portion 1341 of first scattered radiation 1331. Similarly, second illuminator 1322 provides second reference illumination onto detector 1306 while being a portion of second scattered radiation. Notably, in embodiments where first illuminator 1321 and second illuminator 1322 are provided to structure 1308 individually or separately, the second reference illumination can be directed onto detector 1306 at a different time than first reference illumination 1351.

[0128] Detector 1306 is operable to detect a first interference pattern resulting from interference of a portion 1341 of first scattered radiation 1331 that reaches detector 1306 and a first reference illumination 1351. Portions 1341 of first scattered radiation 1331 that reaches sensor 1308 are at least sufficiently coherent with the first reference illumination 1351 at sensor 1308 to form a first interference pattern, and the first interference pattern is detectable by detector 1306. The first interference pattern is recorded by detector 1306. Similarly, the detector is operable to detect a second interference pattern resulting from interference of a portion of second scattered radiation that reaches detector 1306 and the second reference illumination.

[0129] Fig. 11C A schematic diagram depicts an example arrangement of illuminators, such as a first illuminator 1321 and a first reference illumination 1351 for providing a first illumination 1323. It should be appreciated that a substantially similar arrangement may be used for a second illuminator 1322 and a second reference illumination.

[0130] 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 1306). In one embodiment, the radiation source 1310 of the first illuminator 1321 has a wavelength range of 400nm to 1000nm. In one embodiment, the radiation source (not shown) of the second first illuminator 1322 has a wavelength range of 1000nm to 1600nm.

[0131] In embodiments where the first illumination 1323 and the second illumination 1324 originate from the same direction in the reference frame of the structure 1308, a single illuminator may be used and the radiation source may be configured to generate radiation of a controllable wavelength. In this embodiment, the radiation source may include a filtering unit for generating radiation of a controllable wavelength from radiation having a broadband spectral distribution.

[0132] The radiation beam is split by the beam splitter 1312 to provide a first illumination 1323 and a first reference illumination 1351. In the example shown, a first component of the split radiation beam representing the first reference illumination 1351 passes through a delay element 1314 and a reference optical unit 1316. The reference optical unit 1316 directs the first reference illumination 1351 onto the detector 1306. A second component of the split radiation beam representing the first illumination 1323 passes through an illumination optical unit 1320. The illumination optical unit 1320 directs the first illumination 1323 onto the structure 1308. The optical path length between the point at which the radiation beam is split by the beam splitter 1312 and the detector 1306 can be adjusted by the delay element 1314. The delay element 1314 can 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 WO2019 / 197117, for example, including a mirror. In this example, the delay element 1314 is disposed in the optical path between the beam splitter 1312 and the reference optical unit 1316, but the delay element 1314 may alternatively or additionally be disposed in the optical path between the beam splitter 1312 and the illumination optical unit 1320. In some embodiments, the path difference is set equal for each of the two optical paths between the beam splitter 1312 and the detector 1306 (i.e., via the structure 1308 and not via the structure 1308). This condition may be met by tuning the phase element 1314 and / or by precise alignment of the optical elements between the beam splitter 1312 and the detector 1306.

[0133] At a subsequent time after illuminating the structure 1308 with the first illumination 1323, the structure 1308 may be illuminated with an additional first illumination (not shown). The additional first illumination may be incident on the structure 1308 from a different direction than the first illuminator 1321 in the reference frame of the structure 1308. For example, the first illumination 1323 may be incident on the structure 1306 at a first azimuth corresponding to the first direction, and the additional first illumination may be incident on the structure 1306 at a third azimuth corresponding to the third direction. The first direction and the third direction may be substantially opposite directions. For example, the first azimuth may be angularly separated from the third azimuth by 180 degrees. The difference in direction may be implemented by changing the illumination direction in the reference frame of the optical system 1302, by rotating the substrate W (e.g., 180 degrees), or by a combination of changing the illumination direction in the reference frame of the optical system 1302 and rotating the substrate W. The illumination direction in the reference frame of the optical system 1302 may be changed by, for example, rotating a rotatable ring.

[0134] Similarly, at a subsequent time after illuminating structure 1308 with second illumination 1324, structure 1308 may be illuminated with an additional second illumination (not shown). The additional second illumination may be incident on structure 1308 from a different direction than the second illumination in the reference frame of structure 1308. For example, the second illumination may be incident on structure 1306 at a second azimuth corresponding to the second direction, and the additional second illumination may be incident on structure 1306 at a fourth azimuth corresponding to the fourth direction. The second direction and the fourth direction may be substantially opposite directions. For example, the second azimuth may be angularly separated from the fourth azimuth by 180 degrees. The difference in direction may be implemented by changing the illumination direction in the reference frame of optical system 1302, by rotating substrate W (e.g., 180 degrees), or by a combination of changing the illumination direction in the reference frame of optical system 1302 and rotating substrate W. The illumination direction in the reference frame of optical system 1302 may be changed by, for example, rotating a rotatable ring.

[0135] An additional first interference pattern is formed by interference between a portion of the additional first scattered radiation of the additional first illumination reaching the detector 1306 and the additional first reference illumination. A portion of the additional first scattered radiation reaching the sensor 1308 is at least sufficiently coherent with the additional first reference illumination at the sensor 1308 to form the additional first interference pattern and to make the additional first interference pattern detectable by the detector 1306. The detector 1306 records the additional first interference pattern. Similarly, an additional second interference pattern is formed by interference between a portion of the additional second scattered radiation reaching the detector 1306 and the additional second reference illumination.

[0136] One or more characteristics of the structure 1308 may be determined by a processing unit 1370 of the df-DHM 1300. The processing unit 1370 may use the first interference pattern and the additional first interference pattern recorded by the detector 1306 to determine the characteristic of the structure 1308 based on the first optical characteristic. The processing unit 1370 may also use the second interference pattern and the additional second interference pattern recorded by the detector 1306 to determine the characteristic of the structure 1308 based on the second optical characteristic.

[0137] In one embodiment, processing unit 1370 is coupled to detector 1306 to receive a signal including information about the interference pattern recorded by detector 1306. In one embodiment, processing unit 1370 corrects aberrations of optical system 1302.

[0138] Note that the foregoing embodiments may also be generalized to a df-DHM having a sensor module, wherein the first set of illuminators and the second set of illuminators each include multiple pairs of illuminators operable to illuminate the structure from different directions. The first set of illuminators and the second set of illuminators may each include at least pairs of illuminators, each pair of illuminators operable to emit illumination from pairs of different directions. The pairs of different directions may include substantially perpendicular directions (e.g., for measuring an x-target and a y-target simultaneously). The pairs of different directions may include substantially opposite directions (e.g., for simultaneously capturing two complementary diffraction orders, such as +1 and -1 diffraction orders). The first set of illuminators and the second set of illuminators may each include two pairs of illuminators (e.g., 90 degrees apart). This allows for simultaneous capture of two complementary diffraction orders, such as +1 and -1 diffraction orders, from the x-target and the y-target.

[0139] Fig. 12A A dark-field digital holographic microscope (df-DHM) 1400 is depicted, according to an embodiment of the present invention, which is suitable for determining features of interest of a structure 1408 (e.g., a blanket layer) fabricated on a substrate W. The df-DHM 1400 in this example operates according to a parallel acquisition scheme, similar to Fig.10 df-DHM.

[0140] In one embodiment, the df-DHM 1400 includes a sensor module (not shown) including an illumination device for illuminating a structure 1408. The illumination device includes at least a first set of illuminators and a second set of illuminators. The first set of illuminators includes one or more illuminators, each illuminator operable to illuminate the structure 1408 with a first illumination including a first optical characteristic. The second set of illuminators includes one or more illuminators, each illuminator operable to illuminate the structure 1408 with a second illumination including a second optical characteristic different from the first optical characteristic.

[0141] In some embodiments, the illumination apparatus further comprises one or more additional groups of illuminators, each group of illuminators being operable to illuminate the structure 1408 with illumination comprising corresponding additional optical properties. In this example, the one or more additional groups of illuminators comprise a third group of illuminators and a fourth group of illuminators. The third group of illuminators comprises one or more illuminators, each illuminator being operable to illuminate the structure 1408 with third illumination comprising third optical properties. The fourth group of illuminators comprises one or more illuminators, each illuminator being operable to illuminate the structure 1408 with fourth illumination comprising third optical properties.

[0142] In some embodiments, each group of illuminators each includes at least a pair of illuminators, each pair of illuminators being operable to illuminate structure 1408 from a pair of different directions. Each illuminator in a group of illuminators is configured to illuminate structure 1408 simultaneously. This may be similar to Fig.10 In other embodiments, each group of illuminators may include a group (e.g., a pair) of x-illuminators and a group (e.g., a pair) of y-illuminators. It should be understood that the multiple groups of illuminators may include any suitable number of illuminator groups.

[0143] Each group of illuminators includes different optical properties. In this embodiment, each group of illuminators includes a different wavelength range. For example, the first group of illuminators includes a first wavelength range of 400nm to 700nm, the second group of illuminators includes a second wavelength range of 700nm to 1000nm, the third group of illuminators includes a third wavelength range of 1000nm to 1300nm, and the fourth group of illuminators includes a fourth wavelength range of 1300nm to 1600nm. In some embodiments, the first wavelength range may partially overlap with the second wavelength range. It should be understood that each group of illuminators may include any suitable wavelength range. Additional optical properties can be envisioned. In one embodiment, the first optical property includes at least a first polarization; the second optical property includes at least a second polarization; the third optical property includes at least a third polarization; and the fourth optical property includes at least a fourth polarization. In one embodiment, the first optical property includes at least a first illumination spot size; the second optical property includes at least a second illumination spot size, the third optical property includes at least a third illumination spot size, and the fourth optical property includes at least a fourth illumination spot size.

[0144] In this embodiment, the first group of illuminators includes a first illuminator 1410A and an additional first illuminator 1420A. The first illuminator 1410A and the additional first illuminator 1420A can be operated to illuminate the structure 1408 at a first wavelength range. The second illuminator and the additional second illuminator can be operated to illuminate the structure 1408 at a second wavelength range. The second group of illuminators includes a second illuminator (not shown) and an additional second illuminator (not shown). The second illuminator and the additional second illuminator can be operated to illuminate the structure 1408 at the second wavelength range. The third group of illuminators includes a third illuminator (not shown) and an additional third illuminator (not shown). The third illuminator and the additional third illuminator can be operated to illuminate the structure 1408 at a third wavelength range. The fourth group of illuminators includes a fourth illuminator (not shown) and an additional fourth illuminator (not shown). The fourth illuminator and the additional fourth illuminator can be operated to illuminate the structure 1408 at a fourth wavelength range.

[0145] Illumination of structure 1408 may cause radiation to be scattered from structure 1408. Depending on the structure of structure 1408, the scattered radiation may include reflected radiation, diffracted radiation, or transmitted radiation. In this embodiment, structure 1408 may be an overlapping target based on diffraction; and each illuminator may correspond to a scattered beam including at least one non-zero diffraction order. Each scattered beam carries information of the illuminated structure 1408. For example, first illuminator 1410A may correspond to a positive first diffraction order +1 st The first scattered radiation 1411A of DF, and the additional first illuminator 1420B may correspond to the first diffraction order including the negative first diffraction order -1 st The second illuminator 1410B may correspond to the additional first scattered radiation 1421A of DF. st DF, and the additional second illuminator 1420B may correspond to a third scattered beam including the negative first diffraction order -1 st The third illuminator 1410C may correspond to a fourth scattered beam of DF (not shown). st The fifth scattered beam of DF (not shown), and the additional third illuminator 14020C can correspond to the fifth scattered beam including the negative first diffraction order -1 st The fourth illuminator 1410D may correspond to a sixth scattered beam of DF (not shown). st The seventh scattered beam of DF (not shown), and the additional fourth illuminator 1420D may correspond to a seventh scattered beam including the negative first diffraction order -1 st An eighth scattered beam of DF (not shown).

[0146] The sensor module also includes an optical system 1402 that is operable to capture radiation scattered by the structure 1408 generated by the illuminated structure 1408. For example, the optical system is operable to capture first scattered radiation 1411A generated by the structure 1408 illuminated by the first illumination 1410A, and additional first scattered radiation 1421A generated by the structure 1408 illuminated by the additional first illumination 1420A. In one embodiment, the optical system 1402 includes an objective lens 1409. In one embodiment, the optical system 1402 includes a plano-convex lens. The plano-convex lens is iso-spherical and has relatively high aberrations. In one embodiment, the optical system 1402 includes an aspheric lens or a double aspheric lens. The plano-aspheric surface is non-iso-spherical and has relatively low aberrations. In one embodiment, the optical system 1402 includes a reflector optical device. In one embodiment, the optical system 1402 defines a detection numerical aperture (NA). The detection NA defined by the central detection area may be, for example, greater than 0.5, greater than 0.6, greater than 0.7, greater than 0.8, or in the region of 0.85. Note that in some embodiments, one or more optical elements, such as lens combinations, may be used to achieve the same optical effect of objective 1409.

[0147] The sensor module also includes a detector 1306 operable to detect scattered radiation.

[0148] The sensor module further includes an illuminator ring (not shown) that accommodates the first group of illuminators, the second group of illuminators, the third group of illuminators, and the fourth group of illuminators. In one embodiment, a central region of the illuminator ring is configured to receive the optical system 1402. In one embodiment, the illuminator ring is rotatable around the optical axis of the optical system 1402. In one embodiment, the illuminator ring is configured to rotate the first group of illuminators, the second group of illuminators, the third group of illuminators, and the fourth group of illuminators around the optical axis.

[0149] The scattered radiation from each set of illuminators may be collected by objective lens 1409 and subsequently refocused onto detector 1406. Objective lens 1409 may include multiple lenses, and / or df-DHM 1400 may include a lens system having two or more lenses, e.g., similar to Fig. 9 An objective lens and an imaging lens of an exemplary df-DHM, thereby defining a pupil plane of the objective lens between the two lenses and an image plane at the focus of the imaging lens.

[0150] In some embodiments, the sensor module is configured to illuminate the structure 1308 simultaneously with illumination from each group of illuminators. In some embodiments, the sensor module is configured to illuminate the structure 1308 with each group of illuminators individually or one by one. In some embodiments, the sensor module is configured to switch between illumination from each group of illuminators by rotation of the substrate W. In one embodiment, the sensor module is configured to switch between illumination from each group of illuminators by rotation of the sensor module. In one embodiment, the sensor module is configured to switch between illumination from each group of illuminators by rotation of an illuminator ring. In one embodiment, the sensor module is configured to switch between illumination from each group of illuminators by a combination of rotation of the illuminator ring, rotation of the substrate W, and / or rotation of the sensor module.

[0151] The incident angle of the illumination refers to the angle between the optical axis (dashed line) or z-axis of the microscope in the xz plane and the incident illumination or its projection in the xz plane. The first illuminator 1410A may provide first illumination incident on the structure 1408 at a first angle of incidence in a first direction relative to the optical axis OA. The additional first illuminator 1420A may provide additional first illumination incident on the structure 1408 at a second angle of incidence in a second direction relative to the optical axis OA. The second illuminator may provide second illumination incident on the structure 1408 at a third angle of incidence in a third direction relative to the optical axis OA. The additional second illuminator may provide additional second illumination incident on the structure 1408 at a fourth angle of incidence in a fourth direction relative to the optical axis OA. The third illuminator may provide third illumination incident on the structure 1408 at a fifth angle of incidence in a fifth direction relative to the optical axis OA. The additional third illuminator may provide additional third illumination incident on the structure 1408 at a sixth angle of incidence in a sixth direction relative to the optical axis OA. The fourth illuminator can provide a fourth illumination incident on the structure 1408 at a seventh angle of incidence in a seventh direction relative to the optical axis OA. An additional fourth illuminator can provide an additional fourth illumination incident on the structure 1408 at an eighth angle of incidence in an eighth direction relative to the optical axis OA. The angle of incidence of each illuminator can be substantially the same. The angle of incidence of each illuminator 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.

[0152] Fig. 12B Schematically depicts a Fig. 12A 14. The NA space may include a peripheral illumination region 1450 surrounding a central detection region 1460 (i.e., detection NA) including the optical system 1402. Illuminators are arranged within the peripheral illumination region 1450 and / or provide illumination within the peripheral illumination region 1450.

[0153] like Fig. 12B As shown, each illuminator is arranged at a corresponding position within the peripheral area 1450, or provides illumination at a corresponding position within the peripheral area 1450. The second illuminator is labeled 1410B, the additional second illuminator is labeled 1420B, the third illuminator is labeled 1410C, the additional third illuminator is labeled 1420C, the fourth illuminator is labeled 1410D, and the additional fourth illuminator is labeled 1420D. In this embodiment, the azimuth angle of each illuminator is 45 degrees to the adjacent illuminator. In some embodiments, the illuminators are evenly distributed at an angle, but this is not necessary. It should be understood that any suitable azimuth spacing can be used.

[0154] 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 illuminator 1410A may provide first illumination incident on the structure 1408 at a first azimuth corresponding to the first direction. The additional first illuminator 1420A may provide additional first illumination incident on the structure 1408 at a second azimuth corresponding to the second direction. The first azimuth and the second azimuth may be different; for example, relative angles separated by 180 degrees. The second illuminator may provide second illumination incident on the structure 1408 at a third azimuth corresponding to the third direction. The additional second illuminator may provide additional second illumination incident on the structure 1408 at a fourth azimuth corresponding to the fourth direction. The third azimuth and the fourth azimuth may be different; for example, relative angles separated by 180 degrees. The third illuminator may provide third illumination incident on the structure 1408 at a fifth azimuth corresponding to the fifth direction. The additional third illuminator may provide additional third illumination incident on the structure 1408 at a sixth azimuth corresponding to the sixth direction. The fifth azimuth angle and the sixth azimuth angle can be different; for example, relative angles separated by 180 degrees. A fourth illuminator can provide fourth illumination incident on structure 1408 at a seventh azimuth angle corresponding to a seventh direction. An additional fourth illuminator can provide additional fourth illumination incident on structure 1408 at an eighth azimuth angle corresponding to an eighth direction. The seventh azimuth angle and the eighth azimuth angle can be different; for example, relative angles separated by 180 degrees.

[0155] In one embodiment, the azimuth angles of each illuminator are uniformly distributed around the xy plane. For example, the first azimuth angle and the third azimuth angle may be different, for example, 45 degrees apart. The third azimuth angle and the fifth azimuth angle may be different, for example, 45 degrees apart. The fifth azimuth angle and the fifth seventh azimuth angle may be different, for example, 45 degrees apart. The second azimuth angle and the fourth azimuth angle may be different, for example, 45 degrees apart. The fourth azimuth angle and the sixth azimuth angle may be different, for example, 45 degrees apart. The sixth azimuth angle and the eighth azimuth angle may be different, for example, 45 degrees apart. It will be understood that the azimuth angles do not need to be uniformly distributed, and there may be an uneven distribution of azimuth angles.

[0156] In an example embodiment of the first set of illuminators illuminating the configuration 1408, a portion 1412A of the first scattered radiation 1411A and a portion 1422A of the additional first scattered radiation 1421A are incident on the detector 1406 at a common location simultaneously. At the same time, the df-DWM may also be operable to provide two reference radiation beams, namely a first reference illumination 1430A and an additional first reference illumination 1440A, incident on the same location of the detector 1406. The four beams may be divided into two pairs of scattered radiation and reference radiation. For example, the first scattered reference illumination pair may include a portion 1412A of the first scattered radiation 1411A and the first reference illumination 1430. Similarly, a portion 1422A of the additional first scattered reference illumination pair may include the additional first scattered radiation 1421A and the additional first reference illumination 1420A. The two scattered reference illumination pairs may then form two interference patterns (holographic images) that at least partially overlap in the spatial domain.

[0157] It should be understood that each group of irradiation devices may have a corresponding set of reference irradiation pieces. For example, the second scattered reference irradiation pair may include a portion of the second scattered beam and a second reference irradiation (not shown). Similarly, the additional first scattered reference irradiation pair portion 1422A may include additional first scattered radiation 1421A and additional second reference irradiation (not shown). In some embodiments, the additional second reference irradiation is the same as the first reference irradiation, so that one or more groups of irradiators utilize the same reference irradiation.

[0158] In one embodiment, the first illumination, additional first illumination, first reference illumination 1430A and additional first reference illumination 1440A may be provided by an illumination branch including the first illuminator 1410A. Similarly, the second illumination device may provide the second illumination, additional second illumination, second reference illumination and additional second reference illumination. The third illumination device may provide the third illumination, additional third illumination, third reference illumination and additional third reference illumination. The fourth illumination device may provide the fourth illumination, additional fourth illumination, fourth reference illumination and additional fourth illumination. In one embodiment, the illumination device may be substantially similar to the illumination device of FIG. 11 or FIG. 12 of WO2021 / 121733.

[0159] One or more characteristics of the structure 1408 may be determined by a processing unit 1490 of the df-DHM 1400. For example, when the first illumination device 1410A is used, the processing unit 1490 determines the characteristics of the structure 1408 using the first interference pattern and the additional first interference pattern recorded by the detector 1406. In one embodiment, the processing unit 1490 is coupled to the detector 1406 to receive a signal including information about the first interference pattern and the additional first interference pattern recorded by the sensor 1490. In one embodiment, the processing unit 1490 corrects for aberrations of the objective lens 1409 of the df-DHM 1400. In one embodiment, the measurement of the first interference pattern and the additional first interference pattern is performed simultaneously (in parallel) in time using the radiation, and the processing unit 1490 is configured to determine the characteristics of the structure 1408 according to the first optical characteristic using the simultaneous (in parallel) measurements in time. The determination may be repeated for each additional illumination device such that the characteristics of the structure are determined according to the second optical characteristic, the third optical characteristic, and / or the fourth optical characteristic.

[0160] Note that the foregoing embodiments can also be generalized to df-DHMs having multiple sets of illuminator pairs and reference radiation beams. In some embodiments, the df-DHM has a sensor module wherein each set of illuminators includes two pairs of illuminators operable to illuminate the structure from four different directions. In some embodiments, the four different directions are substantially 90 degrees apart.

[0161] In some embodiments, the illuminator may provide multiple pairs of illumination and reference illumination. The illuminator may also provide sufficient coherence control between radiation illuminations so that only desired interference patterns are formed on the detector.

[0162] In one embodiment, the processing unit 1490 may be a computer system. The computer system may be equipped with an image reconstruction algorithm that performs 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 based on the results.

[0163] Fig.13 1500 is a block diagram of 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, 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 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.

[0164] The computer system 1500 may be coupled to a display 1512, such as a cathode ray tube (CRT) or a flat panel or touch screen display, via the bus 1502 for displaying information to a computer user. An input device 1514, including alphanumeric 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, trackball, or cursor direction keys, for communicating direction information and command selections to the processor 1504, and for controlling cursor movement on the display 1512. The input device typically has two degrees of freedom in 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 may also be used as an input device.

[0165] One or more methods described herein may be implemented by computer system 1500 in response to processor 1504 executing one or more sequences of one or more instructions contained in main memory 1506. These instructions may be read into main memory 1506 from another computer-readable medium, such as storage device 1510. Execution of the sequence 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 used to execute the sequence of instructions contained in main memory 1506. In alternative embodiments, hard-wired 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.

[0166] As used herein, the term "computer-readable medium" refers to any medium that participates in providing instructions to processor 1504 for execution. Such media can take a variety of 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 comprise 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, diskettes, hard disks, magnetic tapes, any other magnetic media, CD-ROMs, DVDs, any other optical media, punch cards, paper tapes, any other physical media with hole patterns, RAM, PROMs and EPROMs, FLASH-EPROMs, any other memory chips or cartridges, carrier waves described below, or any other medium that can be read by a computer.

[0167] Various forms of computer readable media may be used to transfer 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 may 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 may receive the data on the telephone line and convert the data into an infrared signal using an infrared transmitter. An infrared detector coupled to the bus 1502 may receive the data carried in the infrared signal and place the data on the bus 1502. The bus 1502 transfers 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 being executed by the processor 1504.

[0168] The computer system 1500 also preferably includes a communication interface 1518 coupled to the bus 1502. The communication interface 1518 provides a two-way data communication coupled 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 to 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 to 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.

[0169] 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 are exemplary forms of carrier waves that transport the information, and these signals carry the digital data to and from the computer system 1500.

[0170] The computer system 1500 can send messages and receive data, including program code, through the network(s), network link 1520, and communication interface 1518. In the example of the Internet, server 1430 can send the requested code for an application program through Internet 1528, ISP 1526, local network 1522, and communication interface 1518. For example, one such downloaded application program can provide one or more of the techniques described herein. The received code can be executed by processor 1504 as it is received, and / or stored in storage device 1510 or other non-volatile memory for later execution. In this way, computer system 1500 can obtain application code in the form of a carrier wave.

[0171] Additional embodiments are disclosed in the subsequent list of numbered clauses:

[0172] 1. A sensor module for a measuring device, the sensor module comprising:

[0173] an illumination apparatus for illuminating a structure on a substrate, the illumination apparatus comprising at least a first set of illuminators and a second set of illuminators, wherein the first set of illuminators comprises one or more illuminators, each illuminator being operable to illuminate the structure with a first illumination comprising a first optical characteristic, and wherein the second set of illuminators comprises one or more illuminators, each illuminator being operable to illuminate the structure with a second illumination comprising a second optical characteristic different from the first optical characteristic;

[0174] an optical system operable to capture scattered radiation generated by the illuminated structure and scattered by the structure; and

[0175] A detector is operable to detect the scattered radiation.

[0176] 2. A sensor module as defined in clause 1, wherein the first optical characteristic comprises at least a first wavelength range and the second optical characteristic comprises at least a second wavelength range.

[0177] 3. A sensor module as defined in clause 1 or 2, wherein the first optical property comprises at least the first polarization and the second optical property comprises at least the second polarization.

[0178] 4. An illumination device as defined in any preceding clause, wherein the first optical characteristic comprises at least a first spot size and the second optical characteristic comprises at least a second spot size.

[0179] 5. An illumination device as defined in any preceding clause, wherein the first optical characteristic comprises at least a first spot shape and the second optical characteristic comprises at least a second spot shape.

[0180] 6. A sensor module as defined in any preceding clause, wherein:

[0181] The first and second groups of illuminators are arranged and / or provide illumination in a peripheral illumination region surrounding a central detection region including the optical system.

[0182] 7. A sensor module as defined in any preceding clause, wherein the detection numerical aperture defined by the central detection area is greater than 0.7.

[0183] 8. A sensor module as defined in any preceding clause, wherein the detection numerical aperture defined by the central detection area is greater than 0.8.

[0184] 9. A sensor module as defined in any preceding clause, wherein the illumination device comprises one or more sets of additional illuminators, each set of illuminators being operable to illuminate the structure with illumination comprising respective additional optical characteristics.

[0185] 10. A sensor module as defined in any preceding clause, wherein each of the first set of illuminators and the second set of illuminators each comprises a single illuminator.

[0186] 11. A sensor module as defined in any one of clauses 1 to 9, wherein the first set of illuminators and the second set of illuminators each comprise at least pairs of illuminators, each pair of illuminators being operable to illuminate the structure from pairs of different directions.

[0187] 12. A sensor module as defined in clause 11, wherein the pair of different directions comprises substantially perpendicular directions.

[0188] 13. A sensor module as defined in clause 11, wherein the pair of different directions comprises substantially opposite directions.

[0189] 14. A sensor module as defined in any one of clauses 1 to 9, wherein the first set of illuminators and the second set of illuminators each comprise two pairs of illuminators operable to illuminate the structure from four different directions.

[0190] 15. A sensor module as defined in clause 14, wherein the four different directions are substantially 90 degrees apart.

[0191] 16. A sensor module as defined in any preceding clause, the sensor module being configured to simultaneously illuminate the structure with first illumination from a first set of illuminators and second illumination from a second set of illuminators.

[0192] 17. A sensor module as defined in clause 16, wherein the first set of illuminators and the second set of illuminators comprise an azimuthal spacing sufficient to minimize crosstalk between the first illuminators and the second illuminators.

[0193] 18. A sensor module as defined in any of clauses 1 to 15, the sensor module being configured to illuminate the structure with first illumination from a first set of illuminators and second illumination from a second set of illuminators, respectively.

[0194] 19. A sensor module as defined in clause 18, configured to switch between illuminating the structure with a first illumination from a first set of illuminators and illuminating the structure with a second illumination from a second set of illuminators by rotation of the substrate and / or the sensor module.

[0195] 20. A sensor module as defined in any of the preceding clauses, the sensor module further comprising an illuminator holding device which accommodates the first group of illuminators and the second group of illuminators, the illuminator holding device being an illuminator ring rotatable around the optical axis of the optical system, the illuminator ring being configured to rotate the first group of illuminators and the second group of illuminators around the optical axis.

[0196] 21. A sensor module as defined in any of clauses 1 to 20, wherein each group of illuminators comprises a respective different optical element, such that the optical element is identical within each group of illuminators but different between different groups of illuminators.

[0197] 22. A sensor module as defined in clause 21, wherein the optical elements are optimized for their respective optical properties.

[0198] 23. A digital holographic microscope configured to determine a characteristic of interest of a structure, comprising a sensor module according to any preceding clause.

[0199] 24. A digital holographic microscope as defined in clause 23, further operable to provide respective first reference illumination and second reference illumination, the first reference illumination having first optical properties corresponding to the first illumination and the second reference illumination having second optical properties corresponding to the second illumination.

[0200] 25. A digital holographic microscope as defined in clause 23 or 24, wherein the detector is operable to detect: a first interference pattern generated by interference of first scattered radiation and a first reference illumination, the first scattered radiation being generated by structure scattering the first illumination; and a second interference pattern generated by interference of second scattered radiation and a second reference illumination, the second scattered radiation being generated by scattering of the second illumination.

[0201] 26. A digital holographic microscope as defined in any one of clauses 23 to 25, comprising a dark-field digital holographic microscope.

[0202] 27. A scatterometer configured to determine a property of interest of a structure, comprising the sensor module of any one of clauses 1 to 22.

[0203] 28. An alignment sensor comprising the sensor module of any one of clauses 1 to 22.

[0204] 29. A level sensor comprising the sensor module of any one of clauses 1 to 22.

[0205] 30. An irradiation device for a measuring device; the irradiation device comprising:

[0206] A first set of illuminators, the first set of illuminators comprising one or more illuminators, each illuminator being operable to emit first illumination comprising a first optical characteristic

[0207] a second set of illuminators, the second set of illuminators comprising one or more illuminators, each illuminator operable to emit second illumination comprising a second optical characteristic different from the first optical characteristic; and

[0208] An illuminator holding device accommodates the first group of illuminators and the second group of illuminators.

[0209] 31. An illumination device as defined in clause 30, wherein the first optical characteristic comprises at least a first wavelength range and the second optical characteristic comprises at least a second wavelength range.

[0210] 32. An illumination device as defined in clause 30 or 31, wherein the first optical property comprises at least a first polarization and the second optical property comprises at least a second polarization.

[0211] 33. An illumination device as defined in any of clauses 30 to 32, wherein the first optical characteristic comprises at least a first spot size and the second optical characteristic comprises at least a second spot size.

[0212] 34. An illumination device as defined in any of clauses 30 to 33, wherein the first optical characteristic comprises at least a first spot shape and the second optical characteristic comprises at least a second spot shape.

[0213] 35. An irradiation apparatus as defined in any one of clauses 30 to 34, wherein:

[0214] The central area within the illuminator ring is configured to receive the optical system of the metrology device.

[0215] 36. An irradiation apparatus as defined in any one of clauses 30 to 35, wherein the illuminator holding device is an illuminator ring rotatable about a center point of the illuminator ring, the illuminator ring being configured to rotate the first group of illuminators and the second group of illuminators about the center point.

[0216] 37. An irradiation apparatus as defined in any one of clauses 30 to 36, wherein each of the first group of illuminators and the second group of illuminators each comprises a single illuminator.

[0217] 38. An illumination apparatus as defined in any one of clauses 30 to 36, wherein the first group of illuminators and the second group of illuminators each comprise at least pairs of illuminators, each pair of illuminators being operable to emit illumination from pairs of different directions.

[0218] 39. An irradiation apparatus as defined in clause 38, wherein the pair of different directions comprises substantially perpendicular directions.

[0219] 40. An irradiation apparatus as defined in clause 38, wherein the pair of different directions comprises substantially opposite directions.

[0220] 41. An illumination apparatus as defined in any one of clauses 30 to 36, wherein the first group of illuminators and the second group of illuminators each comprise two pairs of illuminators operable to emit illumination from four different directions.

[0221] 42. An irradiation apparatus as defined in clause 41, wherein the four different directions are substantially 90 degrees apart.

[0222] 43. An illumination apparatus as defined in any one of clauses 30 to 42, wherein the illumination apparatus comprises one or more sets of additional illuminators, each set of illuminators being operable to illuminate the structure with illumination comprising respective additional optical properties.

[0223] 44. An illumination apparatus as defined in any of clauses 30 to 43, wherein the first group of illuminators and the second group of illuminators comprise an azimuthal spacing sufficient to minimize crosstalk between the first illumination beam and the second illumination beam.

[0224] 45. An illumination apparatus as defined in any one of clauses 30 to 44, wherein each group of illuminators comprises a respective different optical element, such that the optical element is the same within each group of illuminators but different between different groups of illuminators.

[0225] 46. ​​An illumination device as defined in clause 45, wherein the optical elements are optimized for their respective optical properties.

[0226] Although the present invention may specifically mention the use of lithography apparatus in integrated circuit manufacturing, it should be understood that the lithography apparatus described herein may have other applications. Other possible applications include the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memory, flat panel displays, liquid crystal displays (LCDs), thin film magnetic heads, etc.

[0227] Although in this article, specific reference may be made to embodiments of the present invention in the context of a lithography apparatus, embodiments of the present invention may be used in other apparatuses. Embodiments of the present invention may form a component of a mask inspection apparatus, a metrology apparatus, or any apparatus that measures or processes, such as a wafer (or other substrate) or a mask (or other patterning device). These apparatuses are generally referred to as lithography tools. Such lithography tools may use vacuum conditions or ambient (non-vacuum) conditions.

[0228] Although specific reference may have been made above to the use of embodiments of the present invention in the context of optical lithography, it will be appreciated that the present invention is not limited to optical lithography and may be used in other applications, such as imprint lithography, where circumstances permit.

[0229] Although specific reference is made to a "measurement device / tool / system" or an "inspection device / tool / system," these terms may refer to the same or similar types of tools, devices, or systems. For example, an inspection or measurement device including embodiments of the present invention may be used to determine characteristics of structures on a substrate or wafer. For example, an inspection device or measurement device 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 relate to a defect in the structure, the absence of a particular component of the structure, or the presence of an unwanted structure on the substrate or wafer.

[0230] Although specific embodiments of the present invention have been described above, it should be understood that the present invention may be practiced in ways other than those described above. The above description is intended to be illustrative rather than limiting. 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 sensor module for a measuring device, the sensor module comprising: an illumination apparatus for illuminating a structure on a substrate, the illumination apparatus comprising at least a first set of illuminators and a second set of illuminators, wherein the first set of illuminators comprises one or more illuminators, each illuminator being operable to illuminate the structure with a first illumination comprising a first optical characteristic, and wherein the second set of illuminators comprises one or more illuminators, each illuminator being operable to illuminate the structure with a second illumination comprising a second optical characteristic, the second optical characteristic being different from the first optical characteristic; an optical system operable to capture scattered radiation generated by the illuminated structure and scattered by the structure; as well as A detector is operable to detect the scattered radiation.

2. The sensor module according to claim 1, wherein at least one of the following is present: - the first optical property comprises at least a first wavelength range, and the second optical property comprises at least a second wavelength range, - said first optical property comprises at least a first polarization and said second optical property comprises at least a second polarization, - the first optical characteristic comprises at least a first spot size, and the second optical characteristic comprises at least a second spot size, and - the first optical property comprises at least a first spot shape, and the second optical property comprises at least a second spot shape.

3. A sensor module according to any preceding claim, wherein: The first group of illuminators and the second group of illuminators are arranged and / or provide illumination within a peripheral illumination area surrounding a central detection area including the optical system.

4. The sensor module according to any one of the preceding claims, wherein a detection numerical aperture defined by the central detection area is greater than 0.

7.

5. A sensor module according to any one of the preceding claims, wherein the illumination device comprises one or more sets of additional illuminators, each set of illuminators being operable to illuminate the structure with illumination comprising corresponding additional optical properties.

6. The sensor module of any one of the preceding claims, wherein each of the first and second groups of illuminators each comprises a single illuminator.

7. A sensor module according to any one of claims 1 to 5, wherein the first and second groups of illuminators each comprise at least pairs of illuminators, each pair of illuminators being operable to illuminate the structure from a pair of different directions.

8. The sensor module according to claim 7, wherein at least one of the following is present: - the pair of different directions comprises substantially perpendicular directions; and - said pair of different directions comprises substantially opposite directions.

9. The sensor module of any one of claims 1 to 5, wherein the first and second groups of illuminators each comprise two pairs of illuminators operable to illuminate the structure from four different directions.

10. The sensor module of claim 9, wherein the four different directions are substantially 90 degrees apart.

11. The sensor module of any preceding claim, configured to simultaneously illuminate the structure with first illumination from the first set of illuminators and second illumination from the second set of illuminators.

12. The sensor module of claim 16, wherein the first and second groups of illuminators include azimuthal spacers sufficient to minimize crosstalk between the first and second illuminators.

13. The sensor module according to any one of the preceding claims, further comprising an illuminator holding device for accommodating the first group of illuminators and the second group of illuminators, the illuminator holding device being an illuminator ring capable of rotating around the optical axis of the optical system, the illuminator ring being configured to rotate the first group of illuminators and the second group of illuminators around the optical axis.

14. A digital holographic microscope configured to determine a characteristic of interest of a structure, comprising a sensor module as claimed in any preceding claim.

15. A scatterometer configured to determine a property of interest of a structure, comprising a sensor module as claimed in any one of claims 1 to 13.

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