Dark-Field Digital Holographic Microscope and Associated Measurement Methods

Abstract

The present invention discloses a dark-field digital holographic microscope configured to determine a property of interest of a structure. The dark-field digital holographic microscope includes: an illumination device configured to provide at least: a first pair of beams including a first illumination radiation beam (1010) and a first reference radiation beam (1030); and a second pair of beams including a second illumination radiation beam (1020) and a second reference radiation beam (1040); and one or more optical elements (1070) operable to capture first scattered radiation scattered by the structure and capture second scattered radiation scattered by the structure, the first scattered radiation and the second scattered radiation being generated by the first illumination beam and the second illumination beam, respectively. The beams of the first pair of beams are mutually coherent, and the beams of the second pair of beams are mutually coherent. The illumination device is configured to impose incoherence (ADI) between the first pair of beams and the second pair of beams.

Description

[0001] Cross - reference to related applications

[0002] This application claims priority to European Application No. 19216970.4 filed on December 17, 2019 and European Application No. 20167524.6 filed on April 1, 2020, and the entire contents of these applications are incorporated herein by reference. Technical field

[0003] The present invention relates to dark - field digital holographic microscopy and in particular to high - speed dark - field digital holographic microscopy, and to metrology applications in the manufacture of integrated circuits. Background art

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

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

[0006] Low k 1 Lithography can be used to process features smaller than the typical resolution limit of a lithographic apparatus. In such a process, the resolution 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” (usually the smallest feature size printed, but in this case the half - pitch) and k 1 is an empirical resolution factor. Typically, k 1The smaller it is, the more difficult it is to reproduce on a substrate a pattern that is similar in shape and size to that planned by a circuit designer in order to achieve a specific electrical functionality and performance. To overcome these difficulties, complex fine-tuning steps can be applied to the lithographic projection apparatus and / or the design layout. These steps include, for example but not limited to, optimization of the NA, custom illumination schemes, use of a phase-shifting patterning device, various optimizations of the design layout such as optical proximity correction (OPC, sometimes also referred to as "optical and process correction") in the design layout, or other methods generally defined as "resolution enhancement techniques" (RET). Alternatively, a strict control loop for controlling the stability of the lithographic apparatus can be used to improve the reproduction of the pattern at low k 1 underneath.

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

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

[0009] Dark-field microscopes such as the above-described metrology devices and, more generally, have a problem regarding an angular range with restricted illumination and detection, due to the requirement to share the total angular range (corresponding to the region within the angular resolution pupil) between the illumination path and the detection path. This situation limits the effective numerical aperture (NA) in illumination and / or detection. The problem of an increased effective NA in both illumination and detection has been solved by implementing an ordered acquisition scheme. As a result, the measurement speed is undesirably low.

[0010] There is a desire to provide a dark-field microscope having an increased effective NA of at least the detection optics in order to improve the resolution by capturing diffracted light over a larger range of diffraction angles. SUMMARY OF THE INVENTION

[0011] In a first aspect of the present invention, there is provided a dark-field digital holographic microscope configured to determine a property of interest of a structure. The dark-field digital holographic microscope comprises: an illumination device configured to provide at least: a first pair of beams, the first pair of beams comprising a first illumination radiation beam and a first reference radiation beam; and a second pair of beams, the second pair of beams comprising a second illumination radiation beam and a second reference radiation beam; and an imaging branch operable to detect at least first scattered radiation scattered by the structure and to detect second scattered radiation scattered by the structure, the first scattered radiation being generated by irradiating the structure with the first illumination radiation beam, the second scattered radiation being generated by irradiating the structure with the second illumination radiation beam, the imaging branch having a detection NA greater than 0.1; wherein the illumination device is configured such that: the first illumination radiation beam and the first reference radiation beam are at least partially temporally coherent and at least partially spatially coherent; the second illumination radiation beam and the second reference radiation beam are at least partially temporally coherent and at least partially spatially coherent; and the illumination device is configured to impose spatial incoherence and / or temporal incoherence between the first pair of beams and the second pair of beams.

[0012] In a second aspect of the present invention, there is provided a method of determining a property of interest of a target formed on a substrate by a lithography process. The method comprises: irradiating the target with a first illumination radiation beam and capturing the resulting first scattered radiation scattered from the target; irradiating the target with a second illumination radiation beam and capturing the resulting second scattered radiation scattered from the target; imposing spatial incoherence and / or temporal incoherence between a first pair of beams comprising the first illumination beam and a reference beam and a second pair of beams comprising the second illumination beam and a second reference beam such that: the beams of the first pair of beams are at least partially spatially coherent and at least partially temporally coherent, the beams of the second pair of beams are at least partially spatially coherent and at least partially temporally coherent, and any beam of the first pair of beams is spatially incoherent and / or temporally incoherent with respect to any beam of the second pair of beams; and simultaneously generating a first interference pattern resulting from the interference of the first scattered radiation with the first reference radiation beam and a second interference pattern resulting from the interference of the second scattered radiation with the second reference beam. BRIEF DESCRIPTION OF THE DRAWINGS

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

[0014] Figure 1 A schematic diagram of a lithography apparatus is depicted;

[0015] Figure 2 A schematic diagram of a lithography cell is depicted;

[0016] Figure 3 Schematic representation depicting overall lithography, which represents the collaboration between three key technologies for optimizing semiconductor manufacturing;

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

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

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

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

[0021] Figure 8 Schematically depicts a different example of a diffraction-based dark-field metrology device operating in a sequential acquisition scheme;

[0022] Figure 9 Schematically depicts an example of a dark-field digital holographic microscope operating in a sequential acquisition scheme;

[0023] Figure 10 Schematically depicts a dark-field digital holographic microscope (df-DHM) operating in a parallel acquisition scheme according to an embodiment;

[0024] Figure 11 Schematically depicts an illumination device capable of providing multiple radiation beams according to an embodiment;

[0025] Figure 12 Schematically depicts an illumination device capable of providing multiple radiation beams according to different embodiments;

[0026] Figure 13 Depicts a Fourier transform image in the spatial frequency domain;

[0027] Figure 14 Depicts a flowchart of a method for determining the amplitude and phase of a complex field according to another different embodiment; and

[0028] Figure 15 Depicts a block diagram of a computer system for controlling a dark-field digital holographic microscope. Detailed Description

[0029] In this document, the terms "radiation" and "beam" are used to encompass all types of electromagnetic radiation, including ultraviolet radiation (e.g., having a wavelength of 365 nm, 248 nm, 193 nm, 157 nm, or 126 nm) and extreme ultraviolet radiation (EUV, e.g., having a wavelength in the range of about 5 nm to 100 nm).

[0030] As used herein, the term "reticle", "mask", or "patterning device" can be broadly interpreted to mean a general patterning device that can be used to endow an incoming radiation beam with a patterned cross-section, the patterned cross-section corresponding to a pattern to be created in a target portion of a substrate. The term "light valve" can also be used in this context. Examples of such other patterning devices include programmable mirror arrays and programmable LCD arrays, in addition to classical masks (transmission or reflection, binary, phase-shifting, hybrid, etc.).

[0031] Figure 1 A lithographic apparatus LA is schematically depicted. The lithographic apparatus LA includes: an illumination system (also referred to as an illuminator) IL, which is configured to condition a radiation beam B (e.g., UV radiation, DUV radiation, or EUV radiation); a reticle support (e.g., a reticle stage) MT, which is configured to support a patterning device (e.g., a reticle) MA and is 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 stage) WT, which is configured to hold a substrate (e.g., a wafer coated with resist) W and is 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, which is configured to project the pattern imparted to the radiation beam B by the patterning device MA onto a target portion C (e.g., including one or more dies) of the substrate W.

[0032] In operation, the illumination system IL receives the radiation beam from a radiation source SO, for example via a beam delivery system BD. The illumination system IL can include various types of optical components for guiding, shaping i.e. conditioning, and / or controlling the radiation, such as refractive, reflective, magnetic, electromagnetic, electrostatic, and / or other types of optical components, or any combination thereof. The illuminator IL can be used to condition the radiation beam B to have a desired spatial and angular intensity distribution in the plane of the patterning device MA in its cross-section.

[0033] The term "projection system" PS as used herein should be construed broadly to encompass various types of projection systems suitable for the exposure radiation used or for other factors such as the use of an immersion liquid or the use of a vacuum, including refractive, reflective, catadioptric, anamorphic, magnetic, electromagnetic, and electrostatic optical systems, or any combination thereof. Any use of the term "projection lens" herein can be considered synonymous with the more general term "projection system" PS.

[0034] The lithographic apparatus LA can be of a type in which at least a portion of the substrate is covered by a liquid having a relatively high refractive index (e.g., water) in order to fill the space between the projection system PS and the substrate W - this is also known as immersion lithography. More information on immersion techniques is given in US6952253, which is incorporated herein by reference.

[0035] The lithographic apparatus LA can also be of a type having two or more substrate supports WT (also known as "dual platforms"). In such a "multi-platform" machine, the substrate supports WT can be used in parallel, and / or steps for preparing a substrate W for a subsequent exposure of the substrate W located on one of the substrate supports WT can be carried out while another substrate W on another substrate support WT is used for exposing a pattern on the other substrate W.

[0036] In addition to the substrate support WT, the lithographic apparatus LA can also include a measurement platform. The measurement platform is arranged to hold sensors and / or cleaning devices. The sensors can be arranged to measure properties of the projection system PS or of the radiation beam B. The measurement platform can hold a plurality of sensors. The cleaning device can be arranged to clean parts of the lithographic apparatus, such as parts of the projection system PS or parts of a system providing the immersion liquid. The measurement platform can move under the projection system PS when the substrate support WT is moved away from the projection system PS.

[0037] In operation, the radiation beam B is incident on a patterning device (e.g., a mask) MA held on a mask support MT and is patterned by the pattern (design layout) present on the patterning device MA. After passing through the mask MA, the radiation beam B passes through the projection system PS, which focuses the beam on a target portion C of the substrate W. By means of a second positioner PW and a position measurement system IF, the substrate support WT can be accurately moved, for example, in order to position different target portions C in the path of the radiation beam B at a focus and alignment position. Similarly, a first positioner PM and possibly another position sensor (which is not shown Figure 1(which is depicted explicitly in ) can be used to accurately position the patterning device MA relative to the path of the radiation beam B. Mask alignment marks M1, M2 and substrate alignment marks P1, P2 can be used to align the patterning device MA with the substrate W. Although the substrate alignment marks P1, P2 as illustrated occupy dedicated target portions, the substrate alignment marks can be located in the spaces between the target portions. When the substrate alignment marks P1, P2 are located between the target portions C they are referred to as scribe alignment marks.

[0038] As Figure 2 shown, the lithographic apparatus LA can form part of a lithographic cell LC (sometimes also referred to as a lithocell or (lithographic) cluster), which often also includes equipment for performing pre-exposure processes and post-exposure processes on the substrate W. Conventionally, these include a spin coater SC for depositing a resist layer, a developer DE for developing the exposed resist, a chill plate CH and a bake plate BK for regulating the temperature of the substrate W (for example for regulating the solvent in the resist layer). A substrate transfer device or robot RO picks up the substrate W from the input / output ports I / O1, I / O2, moves the substrate W between the different process equipment and transfers the substrate W to the feed table LB of the lithographic apparatus LA. The devices in the lithographic cell, which are generally collectively referred to as the track or coat develop system, are typically under the control of a track control unit or coat develop system control unit TCU, which itself can be controlled by a supervisory control system SCS, which can also control the lithographic apparatus LA via, for example, a lithography control unit LACU.

[0039] In order to correctly and consistently expose the substrate W exposed by the lithographic apparatus LA, it is desirable to inspect the substrate to measure the properties of the patterned structures, such as overlay errors between subsequent layers, line thickness, critical dimension (CD), etc. For this purpose, an inspection tool (not shown) can be included in the lithographic cell LC. If an error is detected, the exposure of subsequent substrates or other processing steps to be performed on the substrate W to be patterned can be adjusted, especially if the inspection is carried out before the other substrates W in the same lot or batch are still to be exposed or processed.

[0040] An inspection device, which can also be referred to as a metrology device, is used to determine the properties of a substrate W, and in particular how the properties of different substrates W vary or how the properties associated with different layers of the same substrate W vary between different layers. The inspection device is alternatively configured to identify defects on the substrate W and can be, for example, part of a lithography cell LC, or can be integrated into a lithography apparatus LA, or can even be a separate device. The inspection device can measure one or more properties regarding a latent image (the image in the resist layer after exposure), or regarding a semi-latent image (the image in the resist layer after a post-exposure bake step PEB), or regarding a developed resist image (where the exposed or unexposed portions of the resist have been removed), or even regarding an etched image (after a pattern transfer step such as etching).

[0041] Generally, the patterning process in a lithography apparatus LA is one of the most important steps in the process, and this step requires a high degree of accuracy in the sizing and placement of structures on the substrate W. To ensure this high degree of accuracy, three systems can be combined in the so-called "integrated" control environment schematically depicted in Figure 3 These systems, one of which is the lithography apparatus LA, which is (virtually) connected to a metrology tool MT (the second system) and to a computer system CL (the third system). The key to this "integrated" environment is to optimize the collaboration between these three systems to enhance the overall process window and provide a tight control loop to ensure that the patterning performed by the lithography apparatus LA remains within the process window. The process window defines the range of process parameters (such as dose, focus, overlay) within which a particular manufacturing process produces a defined result (e.g., a functional semiconductor device) - typically within this defined range, variations in the process parameters during the lithography process or patterning process are allowed.

[0042] The computer system CL can use (a portion of) the design layout to be patterned to predict which resolution enhancement techniques to use and perform computational lithography simulations and calculations to determine which mask layout and lithography apparatus settings achieve the maximum overall process window for the patterning process (depicted by the double arrow in the first scale SC1 in Figure 3 ). Generally, the resolution enhancement techniques are arranged to match the patterning capabilities of the lithography apparatus LA. The computer system CL can also be used to detect where within the process window the lithography apparatus LA is currently operating (e.g., using input from the metrology tool MT) to predict whether there might be defects due to, for example, sub-optimal processing (depicted by the arrow pointing to "0" in the second scale SC2 in Figure 3 ).

[0043] A metrology tool MT can provide an input to a computer system CL to enable accurate simulation and prediction, and can provide feedback to a lithographic apparatus LA to identify possible drifts, for example, in the calibration state of the lithographic apparatus LA (depicted by a plurality of arrows in the third scale SC3 in Figure 3 .

[0044] During the lithographic process, it is desirable to frequently measure the structures being produced, for example, for process control and verification. Tools used to make such measurements are commonly referred to as metrology tools MT. Different types of metrology tools MT for making these measurements are known, including scanning electron microscopes or various forms of scatterometer metrology tools MT. A scatterometer is a versatile instrument that allows the measurement of parameters of the lithographic process by having a sensor in the pupil or in a plane conjugate to the pupil of the objective of the scatterometer (the measurement is commonly referred to as pupil-based measurement), or by having a sensor in the image plane or in a plane conjugate to the image plane, in which case the measurement is commonly referred to as image- or field-based measurement. These scatterometers and related measurement techniques are further described in patent applications US20100328655, US2011102753A1, US20120044470A, US20110249244, US20110026032, or EP1,628,164A, which are incorporated herein by reference in their entirety. The aforementioned scatterometers can use light from the soft x-ray and visible light to near-IR wavelength ranges to measure gratings.

[0045] In a first embodiment, the scatterometer MT is an angular-resolved scatterometer. In such a scatterometer, a reconstruction method can be applied to the measured signal to reconstruct or calculate the properties of the grating. This reconstruction can be caused, for example, by simulating the interaction of the scattered radiation with a mathematical model of the target structure and comparing the simulated results with those measured. The parameters of the mathematical model are adjusted until the simulated interaction produces a diffraction pattern similar to the diffraction pattern observed from the real target.

[0046] In a second embodiment, the scatterometer MT is a spectroscopic scatterometer MT. In such a spectroscopic scatterometer MT, radiation emitted by a radiation source is directed onto a target and the reflected or scattered radiation from the target is directed onto a spectrometer detector that measures the spectrum of the specularly reflected radiation (i.e., a measurement of the intensity as a function of wavelength). From this data, the structure or profile of the target that produced the detected spectrum can be reconstructed, for example, by rigorous coupled-wave analysis and non-linear regression or by comparison with a simulated spectral library.

[0047] In the third embodiment, the scatterometer MT is an ellipsometric scatterometer. An ellipsometric scatterometer allows determination of parameters of a lithography process by measuring scattered radiation for each polarization state. Such metrology equipment emits polarized light (such as linearly polarized light, circularly polarized light, or elliptically polarized light) by using, for example, a suitable polarization filter in the illumination section of the metrology equipment. Sources suitable for the metrology equipment can also provide polarized radiation. Various embodiments of existing ellipsometric scatterometers are described in U.S. Patent Applications 11 / 451,599, 11 / 708,678, 12 / 256,780, 12 / 486,449, 12 / 920,968, 12 / 922,587, 13 / 000,229, 13 / 033,135, 13 / 533,110, and 13 / 891,410, which are incorporated herein by reference in their entirety.

[0048] Figure 4 depicts a metrology device, such as a scatterometer. The metrology device includes a broadband (white light) radiation projector 2 that projects radiation onto a substrate W. The reflected or scattered radiation is transmitted to a spectrometer detector 4, which measures the spectrum 6 of the specularly reflected radiation (i.e., a measurement of intensity as a function of wavelength). From this data, the structure or profile 8 that produced the detected spectrum can be reconstructed by a processing unit PU (e.g., by rigorous coupled-wave analysis and non-linear regression, or by comparison with a simulated spectral library as shown at the bottom of Figure 3 ). Generally, for reconstruction, the general form of the structure is known, and some parameters are assumed based on knowledge of the process used to fabricate the structure, leaving only a few parameters of the structure to be determined from the scatterometry measurement data. Such a scatterometer can be configured as a normal incidence scatterometer or an oblique incidence scatterometer.

[0049] The overall measurement quality of lithography parameters obtained via measurement of a metrology target is at least partially determined by the measurement selection scheme used to measure this lithography parameter. The term "substrate measurement selection scheme" can include one or more parameters of the measurement itself, one or more parameters of one or more patterns being measured, or both. For example, if the measurement used in the substrate measurement selection scheme is a diffraction-based optical measurement, one or more of the measured parameters can include the wavelength of the radiation, the polarization of the radiation, the angle of incidence of the radiation relative to the substrate, the orientation of the radiation relative to the pattern on the substrate, etc. One of the criteria for selecting a measurement selection scheme can be, for example, the sensitivity of one of the measurement parameters to process variations. More examples are described in U.S. Patent Application US2016-0161863, which is incorporated herein by reference in its entirety, and published U.S. Patent Application US 2016 / 0370717A1.

[0050] Another type of metrology tool used in IC manufacturing is a topography measurement system, a leveling sensor, or a height sensor. Such a tool can be integrated into the lithographic apparatus for measuring the topography of the top surface of a substrate (or wafer). A topographic map (also known as a height map) of the substrate can be generated based on these measurements indicating the height of the substrate as a function of its position on the substrate. This height map can then be used to correct the position of the substrate during transfer of the pattern onto the substrate, in order to provide a spatial image of the patterning device in the correct focus position on the substrate. It will be understood that "height" in this context refers to a dimension that is substantially out-of-plane with respect to the substrate (also known as the Z-axis). Typically, the leveling or height sensor performs measurements at a fixed location (relative to its own optical system), and relative movement between the substrate and the optical system of the leveling or height sensor causes height measurements at multiple locations across the entire substrate.

[0051] An example of a leveling or height sensor LS known in the art is schematically shown in Figure 5 which only illustrates the operating principle. In this example, the leveling sensor includes an optical system, which includes a projection unit LSP and a detection unit LSD. The projection unit LSP includes a radiation source LSO that provides a radiation beam LSB that is imparted with 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 light source (such as a polarized or unpolarized laser beam). The radiation source LSO can include multiple radiation sources having different colors, or wavelength ranges, such as multiple LEDs. The radiation source LSO of the leveling sensor LS is not limited to visible light radiation, but can additionally or alternatively cover UV and / or IR radiation and any wavelength range suitable for reflection from the surface of the substrate.

[0052] The projection grating PGR is a periodic grating including a periodic structure that generates a radiation beam BE1 having a periodically varying intensity. The radiation beam BE1 having a periodically varying intensity is directed towards a measurement site MLO on the substrate W, and the radiation beam BE1 has an angle of incidence ANG with respect to an axis (Z-axis) perpendicular to the incident substrate bottom surface that is between 0 degrees and 90 degrees (typically between 70 degrees and 80 degrees). At the measurement site MLO, the patterned radiation beam BE1 is reflected by the substrate W (indicated by the arrow BE2) and directed towards the detection unit LSD.

[0053] To determine the height level at the measurement site MLO, the level sensor further includes a detection system, which includes a detection grating DGR, a detector DET, and a processing unit (not shown) for processing the output signal of the detector DET. The detection grating DGR can be the same as the projection grating PGR. The detector DET generates a detector output signal, which indicates the received light, for example, indicates the intensity of the received light (such as a light detector), or represents the spatial distribution of the received intensity (such as a camera). The detector DET can include any combination of one or more detector types.

[0054] By means of triangulation techniques, the height level at the measurement site MLO can be determined. The detected height level is generally related to the signal intensity measured by the detector DET, and the signal intensity has a periodicity that particularly depends on the (tilted) incident angle ANG and the design of the projection grating PGR.

[0055] The projection unit LSP and / or the detection unit LSD can also include other optical elements (not shown) along the path of the patterned radiation beam between the projection grating PGR and the detection grating DGR, such as lenses and / or mirrors.

[0056] In an embodiment, the detection grating DGR can be omitted, and the detector DET can be placed at the position where the detection grating DGR is located. This configuration provides a more direct detection of the image of the projection grating PGR.

[0057] To effectively cover the surface of the substrate W, the level sensor LS can be configured to project an array of measurement beams BE1 onto the surface of the substrate W, thereby generating an array of measurement regions MLO or spots that cover a larger measurement range.

[0058] Various height sensors of a general type are disclosed, for example, in US7265364 and US7646471, which are incorporated by reference. A height sensor that uses UV radiation instead of visible or infrared radiation is disclosed in US2010233600A1, which is incorporated by reference. In WO2016102127A1, which is incorporated by reference, a compact height sensor that uses a multi-element detector to detect and identify the position of a grating image without a detection grating is described.

[0059] Another type of metrology tool used in IC manufacturing is an alignment sensor. Thus, a key aspect of the performance of the lithographic apparatus is the ability to place the applied pattern properly and accurately relative to features disposed in a previous layer (by the same or a different lithographic apparatus). For this purpose, the substrate is provided with one or more sets of marks or targets. Each mark is a structure whose position can be measured later 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".

[0060] The lithographic apparatus may include one or more (e.g., multiple) alignment sensors, and the position of alignment marks disposed on the substrate can be accurately measured by the one or more alignment sensors. The alignment (or position) sensor may use optical phenomena such as diffraction and interference to obtain position information based on the alignment marks formed on the substrate. An example of an alignment sensor used in current lithographic apparatuses is based on a self-referencing interferometer as described in US6961116. Various enhancements and modifications of the position sensor have been developed, such as those disclosed in US2015261097A1. All of these publications are incorporated herein by reference.

[0061] Figure 6 is a schematic block diagram of an embodiment of a known alignment sensor AS as described in and incorporated by reference in US6961116. A radiation source RSO provides a radiation beam RB having one or more wavelengths, which is steered by steering optics onto a mark, such as a mark AM located on a substrate W, as an illumination spot SP. In this example, the steering optics includes a spot mirror SM and an objective lens OL. The diameter of the illumination spot SP illuminating the mark AM may be slightly smaller than the width of the mark itself.

[0062] The radiation diffracted by the alignment mark AM (in this example via the objective lens OL) is collimated into an information-bearing beam IB. The term "diffraction" is intended to include zero-order diffraction from the mark (which may be referred to as reflection). A self-referencing interferometer SRI of the type disclosed, for example, in US6961116 mentioned above interferes with the beam IB itself, after which the beam is received by a photodetector PD. Additional optics (not shown) may be included to provide multiple separate beams in the case where the radiation source RSO produces more than one wavelength. The photodetector may be a single element, or it may include a number of pixels as required. The photodetector may include a sensor array.

[0063] In this example, the steering optical device including the spot mirror SM can also be used to block the zero-order radiation reflected from the marker, such that the information-carrying beam IB only includes higher-order diffracted radiation from the marker AM (this is not necessary for the measurement, but improves the signal-to-noise ratio).

[0064] The intensity signal SI is supplied to the processing unit PU. By a combination of optical processing in the block SRI and computational processing in the unit PU, values of the X position and the Y position on the substrate relative to a reference frame are output.

[0065] A single measurement of the type illustrated only fixes the position of the marker within a certain range corresponding to a certain spacing from the marker. A coarser measurement technique is used in combination with such a measurement to identify which period of the sine wave is the period containing the marked position. The same process carried out at a coarser and / or finer level can be repeated at different wavelengths for improving accuracy and / or for robust detection of the marker, i.e., robust detection, independent of the material from which the marker is made and the material on which and / or under which the marker is provided. The wavelengths can be optically multiplexed and demultiplexed to process the wavelengths simultaneously, and / or the wavelengths can be multiplexed by time division or frequency division.

[0066] In this example, the alignment sensor and the spot SP remain stationary while the substrate W moves. Thus, the alignment sensor can be mounted to the reference frame in a rigid and accurate manner while effectively scanning the marker AM in a direction opposite to the direction of movement of the substrate W. In such a movement, the substrate W is controlled by mounting the substrate W on a substrate support and controlling the movement of the substrate support by a substrate positioning system. A substrate support position sensor (e.g., an interferometer) measures the position of the substrate support (not shown). In an embodiment, one or more (alignment) markers are provided on the substrate support. Measuring the position of the marker provided on the substrate support allows calibration of the position of the substrate support as determined by the position sensor (e.g., relative to the frame to which the alignment system is connected). Measuring the position of the alignment marker provided on the substrate allows determination of the position of the substrate relative to the substrate support.

[0067] To monitor the lithography process, parameters of the patterned substrate are measured. The parameters can include, for example, overlay errors between successive layers formed in or on the patterned substrate. Such measurements can be performed on product substrates and / or on dedicated metrology targets. There are various techniques for making measurements of the microstructures formed during the lithography process, including using scanning electron microscopes and various specialized tools. A specialized inspection tool in a fast and non-invasive form is a scatterometer, in which a radiation beam is directed onto a target on the surface of the substrate, and the properties of the scattered or reflected beam are measured.

[0068] Examples of known scatterometers include angular-resolved scatterometers of the type described in US2006033921A1 and US2010201963A1. The targets used by such scatterometers are relatively large (e.g., 40 μm by 40 μm) gratings, and the measurement beam produces a spot smaller than the grating (i.e., the grating is underfilled). In addition to measuring feature shape by reconstruction, such devices can also be used to measure diffraction-based overlay, as described in published patent application US2006066855A1. Diffraction-based overlay metrology using dark-field imaging of diffraction orders enables overlay measurements of smaller targets. Examples of dark-field imaging metrology can be found in international patent applications WO 2009 / 078708 and WO 2009 / 106279, the entire disclosures of which are hereby incorporated by reference. Further developments of the techniques have been 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. Composite grating targets can be used to measure multiple gratings in one image. The contents of all these applications are also incorporated by reference into this text.

[0069] Dark-field microscopes such as the metrology devices described above and more generally have the problem of a limited angular range for the detection of light diffracted by the target and / or for the illumination of the target, because it can be required to share the total angular range (corresponding to the region within the angular resolution pupil) between the illumination path and the detection path. This situation limits the effective NA in illumination and detection.

[0070] 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 can include, for example, the intensity at a single scattering angle (e.g., according to wavelength), or the intensity at one or more wavelengths according to the scattering angle.

[0071] Measurements of the target in dark-field metrology can include, for example, measuring a first intensity I of the first diffraction order +1 and a second intensity (I of the -1 diffraction order -1 ), and calculating an intensity asymmetry (A = I +1 - I -1 ), which indicates the asymmetry on the target. The metrology target can include one or more grating structures, and a parameter of interest from the one or more grating structures can be inferred from such intensity asymmetry measurements. For example, the target is designed such that the asymmetry in the target varies with the parameter of interest. For example, in overlay metrology, the target can include at least one composite grating formed by at least a pair of overlapping sub-gratings, and the pair of overlapping sub-gratings are patterned in different layers of a semiconductor device. The asymmetry of the target will thus depend on the alignment of the two layers and thus on the overlay. Other targets can be formed with structures that are exposed to different degrees based on the focus settings used during exposure; measurements thereof enable the focus settings to be inferred inversely (again via intensity asymmetry).

[0072] Figure 7 and Figure 8 Schematically illustrate two examples of diffraction-based dark-field metrology apparatuses. Note that for simplicity, the two figures only show some of the components in the apparatuses that are sufficient for the purpose of describing the working principles of the two apparatuses.

[0073] As Figure 7 illustrated, a first illumination radiation beam IB1 can be incident obliquely from one side of the apparatus onto an overlay target on a substrate WA. The grating-based overlay target can diffract the first illumination beam into a number of diffraction orders. Since the apparatus is configured for dark-field imaging, the zero diffraction order can be blocked by an optical component or configured to fall completely outside the numerical aperture of the objective lens OB. At least one non-zero diffraction order, such as the positive first diffraction order (+1 st DF), can be collected by the objective lens OB. At the pupil plane of the objective lens OB, a first wedge WG1 can be used to redirect the diffracted radiation to follow the desired beam path. Finally, an imaging lens can be used to focus the diffraction order (e.g., the positive first diffraction order (+1 st DF)) onto an image sensor IS such that a first image IM1 is formed at a first location.

[0074] Similarly, a second illumination radiation beam IB2 can be obliquely incident from the opposite side of the system onto the same overlapping target OT of the substrate WA. The incident angle of the second illumination beam IB2 can be the same as the incident angle of the first illumination beam IB1. At least one non-zero diffraction order, such as the negative first diffraction order (-1 st DF), can be collected by the objective lens OB and then redirected by the second wedge WG2. The negative first diffraction order (-1 st DF) can 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.

[0075] Operating in a parallel acquisition scheme Figure 7 example. The overlapping target is simultaneously illuminated by both illumination beams IB1 and IB2. Correspondingly, two spatially separated images IM1 and IM2 of the overlapping target are simultaneously acquired. Such a parallel acquisition scheme allows for a fast measurement speed and thus allows for high throughput. However, two diffraction orders, such as the positive first diffraction order (+1 st DF) and the negative first diffraction order (-1 st DF), must share the pupil plane of the objective lens OB. The result of partitioning the pupil into mutually exclusive illumination and detection pupils is a consequent reduction in both the illumination NA and the detection NA. While there is some flexibility in the trade-off between the illumination NA and the detection NA, it is ultimately not possible to have both a large illumination NA and a large detection NA within a single pupil as is typically desired. This situation results in a limited angular range for each corresponding illumination beam and for the positive first diffraction order (+1 st DF) and the negative first diffraction order (-1 st DF) beams, which in turn limits the range of allowable grating pitch sizes and / or illumination wavelengths and thus imposes strict requirements for designing such metrology systems.

[0076] Figure 8 Schematically illustrates another exemplary dark-field metrology device (or Figure 7 different operating modes of the device). The main difference is that Figure 8 the metrology device operates in a continuous acquisition scheme. In the continuous acquisition scheme, only one illumination beam illuminates the metrology target OT from one direction at any given time, and thus only one image of the target is formed and acquired at any point in time. Refer to Figure 8, at the first time instance t = T1, the first illumination beam IB1 can be turned on and obliquely guided from one side of the measuring device onto the overlapping target OT of the substrate WA. After interaction with the grating of the overlapping target, multiple diffraction orders can be generated. At least one of the non-zero diffraction orders, such as the positive first diffraction order (+1 st DF), can be collected by the objective lens OB and then focused by the imaging lens IL onto the image sensor IS.

[0077] After the first image IM1 of the overlapping grating is acquired, at the 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 radiation beam IB2 can be directly obliquely incident onto the same overlapping target from the opposite side of the measuring device. At least one of the generated diffraction orders, such as the negative first diffraction order (-1 st DF), can be collected by the objective lens OB and then focused onto the image sensor IS to form the second image IM2 of the overlapping target. It should be noted that both images IM1 and IM2 can be formed at a common position on the image sensor.

[0078] With this time-division multiplexing acquisition scheme, the full NA of the objective lens OB can be used to detect the diffraction beams, namely the positive first diffraction order (+1 st DF) and the negative first diffraction order (-1 st DF). The unrestricted 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 and thus the system throughput is affected.

[0079] Furthermore, the accurate determination of, for example, the overlay error depends on the accurate measurement of the minimum relative intensity difference (or intensity asymmetry) between the two acquired images IM1 and IM2. A typical relative intensity difference is about 10 of the intensity of one of the acquired images (such as IM1 or IM2). -4Magnitude. This small intensity difference can easily be dwarfed by any intensity and / or wavelength fluctuations of the illuminating radiation. Therefore, an illumination beam is required to remain stable during consecutive image acquisitions. This can be achieved by using a stable light source that provides the desired intensity and wavelength stability. Alternatively, additional hardware and software, such as intensity / wavelength monitoring devices and corresponding feedback control loops, should be incorporated into the metrology device such that intensity and / or wavelength fluctuations of the illumination beam are actively monitored and well compensated. In some cases, an intensity monitoring device can be used to actively track the intensity of the illumination beam. The signal generated from the intensity monitoring device can be used to (e.g., electronically) correct the intensity fluctuations of the illumination beam. All these solutions add complexity and cost to the overall system.

[0080] Some or all of the foregoing problems can be solved by using digital holographic microscopy (particularly dark-field digital holographic microscopy). Digital holographic microscopy is an imaging technique that combines holography and microscopy. Different from other microscopy methods that record the projection image of an object, digital holographic microscopy records a hologram formed by the interference between object radiation obtained by illuminating a three-dimensional (3D) object and reference radiation that is coherent with the object radiation. An image can be captured using, for example, a charge-coupled device (CCD) or a complementary metal-oxide-semiconductor (CMOS). Since the object radiation is the radiation scattered from the object, the wavefront of the object radiation is modulated or shaped by the object. The scattered radiation can include reflected radiation, diffracted radiation, or transmitted radiation. Therefore, the wavefront of the object radiation carries information of the irradiated object, such as 3D shape information. Based on the captured image of the hologram, the image of the object can be numerically reconstructed by using a computer reconstruction algorithm. An important advantage of hologram-based metrology over intensity-based metrology is as described in Figure 7 and Figure 8 that hologram-based metrology allows obtaining both intensity information and phase information of an object. With the additional phase information, the characteristics of the object can be determined with better accuracy.

[0081] International patent application WO2019197117A1, which is incorporated herein by reference, discloses methods and metrology devices for determining the characteristics (e.g., overlay) of structures fabricated on a substrate based on dark-field digital holographic microscopy (df-DHM). For the purpose of description, Figure 9 of international patent application WO2019197117A1 is reproduced in Figure 3 . Figure 9 Schematically illustrates the disclosed df-DHM that is specifically adapted for lithography process metrology.

[0082] As shown in Figure 7 andFigure 8 compared with the foregoing example in Figure 9 the df-DHM in Figure 9 also includes reference optical units 16, 18 for providing two additional reference radiation beams 51, 52 (the reference radiation). Such two reference radiation beams 51, 52 are respectively paired with two corresponding portions 41, 42 of the scattered radiation beams 31, 32 (the object radiation). Two pairs of scattered reference beams are used to continuously form two interference patterns. Coherence control is provided by adjusting the relative optical path-length difference (OPD) between the two scattered reference beams within each pair of beams. However, coherence control is not available between the two pairs of beams.

[0083] Due to the use of a single light source and insufficient coherence control, all four radiation beams, namely, 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 reach the same position of the sensor 6 simultaneously, i.e., operated in a parallel acquisition scheme, multiple patterns including the desired information-containing pattern and the undesired artifact-causing pattern will overlap with each other. An undesired interference pattern can be formed by, for example, the interference between the portion 41 of the first scattered radiation 31 and the portion 42 of the second scattered radiation 32. Since it is technically challenging and time-consuming to completely separate the superimposed interference patterns, parallel acquisition is impractical for such an arrangement.

[0084] Similar to Figure 8 the example in Figure 9 using a continuous acquisition scheme in the example of Figure 9 allows the full NA of the objective lens to be used for both illumination and detection. However, the system suffers from the same problem of low measurement speed due to continuous acquisition. Therefore, it is desirable to have a df-DHM that can perform parallel acquisition so that high measurement speed and high design flexibility can be obtained simultaneously.

[0085] Figure 10 Schematically illustrates the imaging branch of a dark-field digital holographic microscope (df-DHM) 1000 according to an embodiment. The dark-field digital holographic microscope (df-DHM) includes an imaging branch and an illumination branch. In such an embodiment, a measurement target 1060 including a structure located 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 an embodiment, such two illumination beams, namely, the first illumination radiation beam 1010 and the second illumination radiation beam 1020, can simultaneously illuminate the measurement target 1060.

[0086] In an embodiment, the first illumination radiation beam 1010 may be incident on the measurement target 1060 at a first incident angle in a first direction with respect to the optical axis OA. The second illumination radiation beam 1020 may be incident on the measurement target 1060 at a second incident angle in a second direction with respect to the optical axis OA. The first incident angle of the first illumination radiation beam 1010 and the second incident angle of the second illumination radiation beam 1020 may be substantially the same. The incident angle 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, or in the range of 10 degrees to 90 degrees. The illumination of the measurement target 1060 may cause radiation to be scattered from the target. In an embodiment, the first illumination radiation beam 1010 may be incident on the measurement target 1060 at a first azimuth angle corresponding to the first direction. The second illumination radiation 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 radiation beam 1010 and the second azimuth angle of the second illumination radiation beam 1020 may be different; for example, opposite angles spaced 180 degrees apart.

[0087] Depending on the structure of the measurement target 1060, the scattered radiation may include reflected radiation, diffracted radiation, or transmitted radiation. In such an embodiment, the measurement target may be a diffraction-based overlay target; and each illumination beam may correspond to a scattered beam including at least one non-zero diffraction order. Each scattered beam carries information about the illuminated measurement target. For example, the first illumination radiation beam 1010 may correspond to a first scattered beam 1011 including the positive first diffraction order (+1 st DF); the second illumination radiation beam 1020 may correspond to a second scattered beam 1021 including the negative first diffraction order (-1 st DF). The zero diffraction order and other unwanted diffraction orders may either be blocked by a beam blocking element (not shown) or be configured to fall entirely outside the NA of the objective 1070. Thus, df-DHM may be operated in a dark field mode. It should be noted that in some embodiments, one or more optical elements (such as a lens combination) may be used to achieve the same optical effect of the objective 1070.

[0088] Due to the small size of the measurement target 1060, the imaging branch may have a net positive magnification (e.g., greater than 10× (i.e., greater than 10 times), greater than 20× (i.e., greater than 20 times), or greater than or equal to 30× (i.e., greater than or equal to 30 times)).

[0089] The scattered beams, namely the first scattered beam 1011 and the second scattered beam 1021, can be collected by the objective lens 1070 and then refocused onto the image sensor 1080. It should be noted that the objective lens 1070 of the imaging branch can be an imaging objective lens only used in the detection path (as shown) and not for illumination (e.g., focusing illumination onto the measurement target 1060). Thus, the illumination does not necessarily pass through the same objective lens as the scattered light. In other embodiments, an objective lens can be shared between the imaging branch and the illumination branch to collect the scattered radiation and focus the illumination onto the measurement target 1060.

[0090] It is desirable to collect as much scattered / diffracted light as possible, and thus a high-NA detection path or imaging branch is desirable. For example, in this regard, a high NA can be greater than 0.1, greater than 0.2, greater than 0.3, or greater than 0.4. In other embodiments, a high NA can refer to an NA of 0.8 or greater.

[0091] The objective lens 1070 can include multiple lenses, and / or the df-DHM 1000 can include a lens system having two or more lenses (e.g., imaging lenses and objective lenses similar to the exemplary df-DHG Figure 9 ), thereby defining a pupil plane between two lenses of the objective lens and an image plane located at the focal point of the imaging lens. In such an embodiment, a portion 1012 of the first scattered beam 1011 and a portion 1022 of the second scattered beam 1021 are simultaneously incident at a common position on the image sensor 1080. At the same time, two reference radiation beams, namely the first reference beam 1030 and the second reference beam 1040, are incident on the same position of the image sensor 1080. These four beams can be grouped into two pairs of scattered radiation and reference radiation. For example, the first scattered-reference beam pair can include the portion 1012 of the first scattered beam 1011 and the first reference beam 1030. Similarly, the portion 1022 of the second scattered-reference beam pair can include the second scattered beam 1021 and the second reference beam 1040. These two scattered-reference beam pairs can then form two interference patterns (holographic images), which at least partially overlap in the spatial domain.

[0092] In an 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 can have a first incident angle with respect to the optical axis OA, and the second reference beam 1040 can have a second incident angle with respect to the optical axis OA; the first incident angle and the second incident angle are different. Alternatively or additionally, the first reference beam 1030 can have a first azimuth angle with respect to the optical axis OA, and the second reference beam 1040 can have a second azimuth angle with respect to the optical axis OA; the first azimuth angle and the second azimuth angle are different.

[0093] To generate an interference pattern, the two beams of each scattered reference beam pair should be at least partially coherent with each other to an extent sufficient to form an interference pattern. It should be noted that each scattered radiation beam may have a phase shift relative to its corresponding illumination radiation. For example, at the image plane of the image sensor 1080, such a phase shift may include contributions due to the optical path length (OPD) from the measurement target 1060 to the image sensor 1080, i.e., the optical path, and through the interaction with the measurement target. As described above, it is necessary to control the coherence between the first scattered reference beam pair and the second scattered reference beam pair such that each beam of one pair is non-coherent with any beam of the other pair. In other words, interference should only occur between the beams within the same beam pair and be suppressed between different beam pairs. In this way, only the desired interference patterns (e.g., the two interference patterns formed by the corresponding scattered reference beam pairs) are formed on the image sensor 1080 in a superimposed manner, thus avoiding the problem of separating or removing undesired interference patterns.

[0094] More particularly, the coherence between the beams within the same beam pair should be both temporal and spatial coherence. The mutual coherence function between these beams depends on space and time. A typical approximation is to factorize such a function into a spatial component and a temporal component. However, when the beams are traveling at some angles (e.g., in an off-axis system), this approximation is no longer ideal. There should be sufficient coherence such that the object beam and the reference beam interfere on the camera. For the simplification of temporal and spatial coherence, this situation means that on the camera, each point has the same optical path length, i.e., the same optical path, from the source (e.g., within the "temporal coherence" length). For limited spatial coherence, each point of the reference beam should also be mapped to the corresponding point of the object beam (this situation may mean that the object and reference arms should get the image of the beam splitter on the camera). In the case of using single-mode fibers, the spatial coherence can be very large.

[0095] In addition to being spatially and temporally coherent with the reference beam (of the same beam pair), each object beam can be smooth (e.g., uniformly illuminated) over the entire region of the overlapping target.

[0096] In an embodiment, two illumination beams, i.e., the first illumination radiation beam 1010 and the second illumination radiation beam 1020, and two reference beams, i.e., the first reference beam 1030 and the second reference beam 1040, for the Figure 10 df-DHM can be provided by an illumination branch including an illumination device. Figure 11 Schematically illustrates an illumination device according to an embodiment. As Figure 11As shown, the light source 1110 can emit a primary radiation beam 1111 that is at least partially coherent. The primary radiation beam 1111 can include wavelengths ranging from soft x-rays to visible to near-IR. The primary radiation beam 1111 can be split into two beams, namely a first radiation beam 1112 and a second radiation beam 1114, by a first beam splitter 1120. In this embodiment, the first beam splitter 1120 can have a 50 / 50 splitting ratio, and thus the first radiation beam 1112 and the second radiation beam 1114 can have substantially the same power level. Subsequently, the two beams 1112, 1114 can follow two different beam paths respectively.

[0097] In one of the beam paths of the two beams 1112, 1114 (the second beam path in the example shown here, although this is largely arbitrary), the second radiation beam 1114 (or the first beam 1112) can be subject to a delay. In the example shown here, the delay is implemented via an incoherent delay arrangement such as an adjustable optical delay line AD1 that includes a prism 1132. The adjustable optical delay line AD1, or more generally the delay, can be used to control the OPD (or coherence) between the beam in the first beam path and the beam in the second beam path. This operation can be performed to ensure that these beams are not coherent, so that the first beam pair does not interfere with the second beam pair. Instead of such a delay line, a "hard" path difference may be intentionally introduced between the beam paths of the two beams 1112, 1114.

[0098] In an embodiment, the adjustable optical delay line AD1 can be operable such that the time delay between the two paths can be maintained as short as possible in order to have similar intensity fluctuations in the two beams while still imposing incoherence.

[0099] In a first beam path, a first radiation beam 1112 can enter a second beam splitter 1122, which can split the first radiation beam 1112 into two additional beams, namely a first illumination radiation beam 1010 and a first reference beam 1030. Depending on the splitting ratio of the second beam splitter 1122, the first illumination radiation beam 1010 and the first reference beam 1030 can have different powers. The splitting ratio of the second beam splitter 1122 can be 90 / 10, 80 / 20, 70 / 30, 60 / 40, or 50 / 50. In such an embodiment, the power of the first illumination radiation beam 1010 can be higher than the power of the first reference beam 1030. Each of the two beams, such as the first illumination radiation beam 1010 and the first reference beam 1030, can then be reflected by a reflection element 1140 into an optical delay line. Each optical delay line, whether fixed or adjustable, can include a reflective optical element to retro-reflect the incident radiation. The reflective optical element can be a right-angle prism 1130 or 1131. In some embodiments, the reflective optical element can be a pair of mirrors. In such an embodiment, the first illumination radiation beam 1010 can pass through a fixed optical delay line including prism 1131, while the first reference beam 1030 can pass through an adjustable optical delay line AD2 including prism 1130. In different embodiments, the first illumination radiation beam 1010 can pass through an adjustable optical delay line including prism 1130, while the first reference beam 1030 can pass through a fixed optical delay line including prism 1131. In either of these two scenarios, the relative OPD between the two beams, namely the first illumination radiation beam 1010 and the first reference beam 1030, can be adjustable. The two beams, namely the first illumination radiation beam 1010 and the first reference beam 1030, form a first pair of output beams. It should be noted that this is only one example of an adjustable path length arrangement for enabling adjustment of the OPD between the beams, namely the first illumination radiation beam 1010 and the first reference beam 1030. Any other suitable arrangement for achieving this scenario can be used alternatively.

[0100] The second radiation beam 1114 can be split into two beams by a third beam splitter 1124, such as a second illumination radiation beam 1020 and a second reference beam 1040. Depending on the splitting ratio of the third beam splitter 1124, these two beams can have different powers. In this embodiment, the splitting ratio of the third beam splitter 1124 can be the same as the splitting ratio of the second beam splitter 1122, such that the first illumination radiation beam 1010 and the second illumination radiation beam 1020 can have approximately the same power level, and the first reference beam 1030 and the second reference beam 1040 can have approximately the same power level. In this embodiment, the power of the second illumination radiation beam 1020 can be higher than the power of the second reference beam 1040. The two beams, namely the second illumination radiation beam 1020 and the second reference beam 1040, can then be reflected into two optical delay lines by reflection elements 1142 respectively, one optical delay line being fixed and the other optical delay line being adjustable. In this embodiment, the second illumination radiation beam 1020 can pass through a fixed optical delay line including a prism 1133, while the second reference beam 1040 can pass through an optical delay line AD3 including a prism 1134. In different embodiments, the second illumination radiation beam 1020 can pass through an adjustable optical delay line including 1134, while the second reference beam 1040 can pass through a fixed optical delay line including 1133. In either of these two scenarios, the relative OPD between the two beams, namely the second illumination radiation beam 1020 and the second reference beam 1040, can be adjustable. The two beams, namely the second illumination radiation beam 1020 and 1040, form a second pair of output beams. It should be noted that this is only one example of an adjustable path length arrangement that can be used to enable adjustment of the optical path length between the beams, namely the second illumination radiation beam 1020 and the second reference beam 1040. Any other suitable arrangement for achieving this situation can be alternatively used.

[0101] After leaving their respective optical delay lines, the four radiation beams, namely the first illumination radiation beam 1010, the first reference beam 1030, the second illumination radiation beam 1020, and the second reference beam 1040 can leave the illumination device 1100, and can be in the df-DHM (e.g. Figure 10In the corresponding bundle of the df-DHM), they are used as the illumination beam and the reference beam. In an embodiment, before leaving the illumination device 1100, some or all of these four beams can be respectively transmitted through additional optical elements (such as beam shaping elements, optical steering mirrors, optical polarization elements, and optical power control elements), so that the beam parameters, propagation directions, polarization states, and / or optical powers of each beam can be independently controlled. The beam parameters can include beam shape, beam diameter, and beam divergence. In an embodiment, the two beams of any one of the first pair of output beams and the second pair of output beams can have different power levels. One beam of the first pair of output beams can have approximately the same power level as one beam of the second pair of output beams.

[0102] From Figure 11 The propagation direction of the radiation beam exiting / leaving the illuminator determines the incident angle and azimuth angle of the beam relative to Figure 10 the optical axis OA of the df-DHM. On Figure 10 the top, the directions of the Cartesian reference coordinate system are shown. The incident angle of the beam refers to the angle between the optical axis (dashed line) or z-axis of the microscope and the incident beam or its projection in the x-z plane. The azimuth angle of the beam refers to the angle between the x-axis and the incident beam or its projection in the x-y plane.

[0103] Correspondingly, the optical delay line AD2 can be used to adjust the relative OPD between the first illumination radiation beam 1010 and the first reference beam 1030, while the third optical delay line AD3 can be used to adjust the relative OPD between the second illumination radiation beam 1020 and the second reference beam 1040. As long as the phase delay caused by the relative OPD is sufficient to cover the phase shift between each illumination radiation beam and its associated scattered radiation beam, the coherence between each scattered beam radiation (such as the first scattered beam 1011 or the second scattered beam 1021) and its paired reference beam radiation (such as the first reference beam 1030 or the second reference beam 1040) can be independently controlled or optimized. In addition, the tunable optical delay line AD1 can be used to intentionally add a sufficient phase delay between two pairs of illumination reference beams or two pairs of scattered reference beams (for example, the first scattered reference beam pair including part 1012 of the first scattered beam 1011 and the first reference beam 1030; the second scattered reference beam pair including part 1022 of the second scattered beam 1021 and the second reference beam 1040), so that any beam of one beam pair is incoherent with any beam of the other beam pair. In this way, only two required interference patterns are respectively formed by the two pairs of scattered reference beams on the image sensor 1080.

[0104] Figure 12 Schematically illustrate the illumination device 1200 according to different embodiments. Figure 12 The illumination device of Figure 11The irradiation device. The main difference is that, in Figure 12 the embodiment of Figure 11 , the three beam splitters 1220, 1222, 1224 can have different splitting ratios compared to their corresponding counterparts in the embodiment of Figure 11 . The result of using different splitting ratios can be that some or all of the four radiation beams output from the device, namely the first irradiation radiation beam 1010, the second irradiation radiation beam 1020, the first reference beam 1030, and the second reference beam 1040, can have different powers compared to their corresponding positional counterparts in the embodiment of

[0105] . Thus, Figure 11 shows an arrangement in which the first output branch (via the beam splitter 1122) includes a first pair of beam branches (e.g., the +1 diffraction order irradiation and the +1 reference beam), and the second output branch (via the beam splitter 1124) includes a second pair of beam branches (e.g., the -1 diffraction order irradiation and the -1 reference beam). In contrast, in Figure 12 , the first output branch is the irradiation branch (e.g., the +1 diffraction order and the 1st diffraction order irradiation beams), and the second output branch is the reference branch (e.g., the +1 diffraction order and the 1st diffraction order reference beams).

[0106] Regardless of the different beam combinations, the three adjustable optical delay lines AD1', AD2', AD3' can provide sufficient coherence (or OPD) control for all four beams such that only the desired interference pattern is formed on Figure 10 the image sensor 1080. In an embodiment, the delay line AD3' of the reference arm can be significantly longer (tens of mm) compared to the delay line AD2' of the irradiation arm.

[0107] In Figure 11In the arrangement, therefore, the adjustable optical delay line AD1 can implement an incoherent delay arrangement that is operable to impose a delay on one of the first or second branches relative to the other of the first or second branches; and the adjustable optical delay lines AD2, AD3 implement a coherent matching device for coherent matching of the beams within each pair of beams. In contrast, in Figure 12 the arrangement, the coherent matching device and the incoherent delay arrangement can be implemented together via a common optimization between the adjustable optical delay line AD1 and the adjustable optical delay lines AD2, AD3. It should be noted that in the latter case, if prism 1131 instead of prism 1130 is adjustable, or if prism 1133 instead of prism 1134 is adjustable, the optimization will be different.

[0108] The characteristics of the structure of the measurement target 1060 are determined by the processing unit 1090 of the measurement device. The processing unit 1090 uses the first interference pattern and the second interference pattern recorded by the image sensor 1080 to determine the characteristics of the structure of the measurement target 1060. In an embodiment, the processing unit 1090 is coupled to the image sensor 1080 to receive signals that include information about the first interference pattern and the second interference pattern recorded by the sensor 1080. In an embodiment, the processing unit 1090 corrects the aberration of the objective lens 1070 of the df-DHM 1000. In an embodiment, the measurements of the first interference pattern and the second interference pattern are performed simultaneously in time (in parallel) using radiation, and the processing unit 1090 is configured to use the measurements simultaneously in time (in parallel) to determine the characteristics of the structure of the measurement target 1060 on the substrate 1050.

[0109] In an embodiment, the processing unit 1090 uses the first interference pattern to calculate the complex field (where "complex" means that both amplitude information and phase information are present) of the radiation associated with the portion 1012 of the first scattered beam 1011 at the sensor 1080. Similarly, the processing unit 1090 uses the second interference pattern to calculate the complex field at the sensor 1080 associated with the portion 1022 of the second scattered beam 1021. This calculation of the complex radiation field based on the interference pattern formed by interfering the reference radiation with the radiation scattered from the object is collectively referred to as holography. Further details on how to perform such calculations in the context of metrology for lithography can be found, for example, in US2016 / 0061750A1, which is hereby incorporated by reference.

[0110] If the optical characteristics of the df-DHM 1000 are known, it is possible to back-propagate each of the calculated complex fields in a mathematical and computational manner to obtain the corresponding complex fields of the first scattered beam 1011 and the second scattered beam 1021 at the measurement target 1060.

[0111] It is known that the composite field provides additional information for determining the characteristics of the measurement target 1060 on the substrate 1050 relative to an alternative mode in which both phase and amplitude information are unavailable. For example, in European Patent Application EP18158745.2 filed on February 27, 2018, it has been disclosed how to use the phase information of scattered radiation to determine the overlay error between different layers on the substrate (an example of the characteristics to be determined of the structure). European Patent Application EP18158745.2 is hereby incorporated by reference.

[0112] In an embodiment, the characteristics of the structure of the measurement target 1060 are determined by comparing a first interference pattern with a second interference pattern. In an embodiment, the characteristics of the structure are determined based on the difference between the first interference pattern and the second interference pattern. The difference between the first interference pattern and the second interference pattern may, for example, contain information about the asymmetry in the structure of the measurement target 1060. Obtaining information about the asymmetry of the structure of the measurement target 1060 may provide information about overlay. In an embodiment, the phase information obtained from the calculated composite field is used to obtain overlay information, as described in EP18158745.2 filed on February 27, 2018. Overlay describes the unwanted misalignment between different patterns in the measurement target 1060 (such as patterns formed at different times, formed using different processes, and / or formed in different layers). In other embodiments, the characteristics being determined of the structure of the measurement target 1060 may include an error indicating an error in the focus of the radiation used in the lithography process to fabricate the structure of the measurement target 1060. In yet other embodiments, the characteristics being determined of the structure of the measurement target 1060 may include an error indicating an error in the radiation dose of the radiation used in the lithography process to fabricate the structure of the measurement target 1060.

[0113] It is important to minimize the contribution to the difference between the first interference pattern and the second interference pattern that does not originate from the structure of the measurement target 1060 (such as the aforementioned unwanted interference pattern). By using three adjustable optical delay lines AD1, AD2, AD3, those contributions are effectively suppressed by actually applying sufficient coherent control to four radiation beams (i.e., the first scattered beam 1011, the first reference beam 1030, the second scattered beam 1021, the second reference beam 1040).

[0114] To accurately calculate two composite fields, two interference patterns should be completely separated from background stray light and / or residual zero diffraction order. In addition, to extract target information from each interference pattern, two overlapping interference patterns should also be separated. Spatial frequency multiplexing can also be used to achieve complete separation of multiple overlapping interference patterns. Such a method is described in detail in U.S. Patent Application US20180011022A1, which is incorporated herein by reference.

[0115] Using spatial frequency multiplexing, the processing unit 1090 subjects the recorded image including multiple overlapping interference patterns to a two-dimensional (2D) Fourier transform to obtain a Fourier transform image. The transverse axis and the longitudinal axis of the obtained Fourier transform image respectively correspond to two axes in the spatial frequency coordinate system (fx, fy), namely fx and fy. In the obtained Fourier transform image, there are multiple spatial spectra, and each of the spectra corresponds to a part of the recorded image.

[0116] Figure 13 An exemplary 2D Fourier image in the spatial frequency domain is illustrated, which is obtained by subjecting the recorded image including two overlapping interference patterns to a 2D Fourier transform. As Figure 13 shown, the 2D Fourier image includes five spatial spectra: a basic spatial spectrum 1301 including a zero-order Fourier component, a first high-order spatial spectrum 1311 corresponding to the first interference pattern formed using the portion 1012 of the first scattered beam 1011, a second high-order spatial spectrum 1312 corresponding to the second interference pattern formed using the portion 1022 of the second scattered beam 1021, a first conjugate spatial spectrum 1321 conjugate to the first high-order spatial spectrum 1311, and a second conjugate spatial spectrum 1322 conjugate to the second high-order spatial spectrum 1312.

[0117] The center of the basic spectrum is the origin O of the spatial frequency coordinates. The position of the basic spectrum is fixed. However, the positions of the higher-order spectra and their conjugate spectra can be adjusted relative to the basic spectrum, for example, by changing the angle of incidence and / or the azimuth angle of each reference beam. The radial distance between the center of each higher-order spatial spectrum and the center of the basic spatial spectrum involves the angle between the optical axes of the scattered beams, i.e., the portions 1012 or 1022 of the first scattered beam 1011 or the second scattered beam 1021, and the optical axes of the first reference beam 1030 or the second reference beam 1040. The larger this angle, the farther away (relative to the basic spatial spectrum) the higher-order spatial spectrum will be. Thus, by providing a sufficiently large angle between the axis of a portion of the scattered beam and the axis of the reference beam, the first higher-order spatial spectrum 1311 or the second higher-order spatial spectrum 1312 can be completely separated from the basic spatial spectrum 1301. However, the angle of the reference beam cannot be arbitrarily high, because an increase in the angle between the optical axes of the scattered beams, i.e., the portions 1012 or 1022 of the first scattered beam 1011 or the second scattered beam 1021, and the optical axes of the first reference beam 1030 or the second reference beam 1040 results in a reduction in the fringe pitch of the hologram fringes. Ultimately, this angle is limited by the pixel pitch of the image sensor 1080. The fringes in the hologram (or interference pattern) must be adequately sampled by the sensor pixels. The maximum frequency in the hologram must satisfy the Nyquist criterion for sampling.

[0118] In addition, the azimuth angle of each reference beam affects the circumferential position of the spatial spectrum relative to the origin O. The circumferential position of the higher-order spatial spectrum is represented by the angle between the higher-order spatial spectrum and the spatial frequency axis fx. For example, the circumferential position of the first higher-order spatial spectrum is represented by the angle 1331. Thus, the first higher-order spatial spectrum 1311 and the second higher-order spatial spectrum 1312 can be completely separated from each other by ensuring that the difference between the azimuth angles of the two reference beams is sufficiently large.

[0119] Once separated, the processing unit 1090 extracts each higher-order spatial spectrum from the Fourier image and subsequently subjects the extracted higher-order spatial spectra to an inverse Fourier transform. It should be noted that since the two reference radiation beams are directly provided by the illumination device 1100, information about the reference radiation (such as the intensity distribution) at the image sensor 1080 can be determined either by calculation or by measurement. Based on the result of the inverse Fourier transform and the information about the reference radiation, the composite field of the pair of scattered radiation can be obtained, the details of which are described below. Ultimately, the composite field of the two scattered radiation beams is used to determine the characteristics of the structure of the measurement target 1060 and / or to correct the optical aberrations of the objective 1070 of the df-DHM 1000.

[0120] Continue to refer to Figure 13, existing methods only use the high-order spatial spectrum or sidebands (SBs) in the 2D Fourier image 1300, i.e., the first high-order spatial spectrum 1311 or the second high-order spatial spectrum 1312, to determine the amplitude and phase of the complex field of the scattered beam, i.e., the first scattered beam 1011 or the second scattered beam 1021, for the portions 1012 or 1022 thereof. The information contained in the base spatial spectrum or central band (CB) 1301 is completely discarded during the determination process. Thus, existing methods are vulnerable to noise limitations, such as low signal-to-noise ratio, which results in a reduction in throughput. According to different aspects of embodiments of the present invention, a method is provided for improving existing methods by providing a better and more accurate way to determine the amplitude and phase of the complex field of scattered radiation at an image sensor. This is achieved by considering the information contained in both the CB 1301 and the SB, i.e., the first high-order spatial spectrum 1311 or the second high-order spatial spectrum 1312.

[0121] It should be noted that another term for the Fourier image of Figure 13 is often referred to as the Fourier representation of a hologram. Four images of Figure 13 are obtained by transforming the hologram into its Fourier representation or Fourier spectrum in the spatial frequency domain via a (2D) Fourier transform.

[0122] Figure 14 FIG. shows a flowchart of a method for determining the amplitude and phase of a complex field according to an embodiment (e.g., the method may be performed by the processing unit 1090 or otherwise). Referring to Figure 14 , at step 1401, a hologram (or interference pattern) may be generated after irradiating an object or target, and subsequently, the hologram (or interference pattern) is transformed into its Fourier representation or Fourier spectrum in the spatial frequency domain via a Fourier transform. This Fourier representation has the advantageous property that the CB and the corresponding SB are spatially separated when the tilt angle of the reference wave used in the hologram is large enough to consider the spatial frequency content, i.e., the spatial frequency components, of the SB. It should be further noted that in Figure 13In this case, the SBs appear in pairs, where one pair includes an SB, namely the first high-order spatial spectrum 1311 and the first conjugate spatial spectrum 1321, and the other pair includes an SB, namely the second high-order spatial spectrum 1312 and the second conjugate spatial spectrum 1322. For each pair of SBs, the two SBs carry the same information because the two SBs are complex conjugates of each other, such that it is sufficient to select one SB from each pair of SBs. At step 1402, the CB in the Fourier representation can be selected, and the CB can subsequently be used to calculate the corresponding component in the image plane via the Fourier transform of the selected CB. At step 1403, one or more individual SBs can be selected in the Fourier representation, and each of the selected SBs can be used to calculate the corresponding component in the image plane via the inverse Fourier transform of the selected SB; at step 1404, the amplitude and / or phase of the composite field can be determined based on the calculated CB and SB components in the image plane. Details of the implementation of the method are described below.

[0123] It should be noted that Figure 14 The embodiments of are only non-limiting examples, and other embodiments may include more or fewer steps determined by specific requirements. For example, some embodiments may additionally include a step of irradiating an object or target, and this step can be used as the first step; some other embodiments may combine steps 1402 and 1403 into a single step, such that the inverse Fourier transform of the CB and the inverse Fourier transform of the SB can be performed in parallel, rather than sequentially in time.

[0124] In the case where a single interference pattern is sufficient, one of the illumination-reference beam pairs provided by the illumination device 1100 or 1200 can be used to irradiate the target 1060. Subsequently, at step 1401, the scattered radiation from the object or target is combined with the reference radiation provided from the illumination-reference beam pair to form the required single interference pattern. Such a single interference pattern can be transformed into a 2D Fourier representation in the spatial frequency domain via the Fourier transform. In such a case, the 2D Fourier representation (not shown) may include one CB and a pair of mutually conjugate SBs. The mutual tilt angle of the corresponding beams of the scattered reference beam pair can be arranged such that the resulting CB and SB do not overlap in the spatial frequency domain. Then, at step 1402, the CB in the Fourier representation can be selected and the CB can be used to calculate its corresponding component in the image plane via the inverse Fourier transform (i.e., as described below ). After that, at step 1403, one of the mutually conjugate SBs in the Fourier representation can be selected, and the SB can be used to calculate its corresponding component in the image plane via the inverse Fourier transform (i.e., as described below ). Finally, at step 1404, the calculated information (i.e., as described below and to determine the amplitude and phase of the complex field of scattered radiation from an object or target. Step 1404 is further explained by the following mathematical description.

[0125] In the following mathematical description, the inverse Fourier transform is applied separately to CB and SB, and is indicated by the real-valued function and the complex-valued function to indicate the corresponding components in the image plane, where are the 2D coordinates in the image plane. It should be noted that CB contains both the autocorrelation of the scattered beam and the autocorrelation of the reference beam. The power of the reference beam is given by:

[0126] . [1]

[0127] The complex-valued field in the image plane is indicated by , equal to the sample field (the field scattered from the sample / target) convolved with the point spread function of the imaging optics (e.g., objective lens 1070) represented by , i.e.:

[0128] . [2]

[0129] The complex-valued field in the image plane can be expressed in terms of the amplitude and the phase as:

[0130] . [3]

[0131] The hologram or interference pattern is modeled as:

[0132] , [4]

[0133] where indicates the wave vector of the reference wave, and s indicates the amplitude of the reference wave

[0134] , i.e., .

[0135] The least squares function for the estimation of the amplitude and phase of the complex-valued field can be defined as:

[0136] , [5]

[0137] The above least-squares function can be conveniently rewritten based on Parseval's theorem and the fact that CB and SB are well separated in the Fourier representation. After combining the corresponding contributions from CB and the two conjugate SBs that can be separated from CB, the above equation can be expressed more explicitly as:

[0138] , [6]

[0139] where and indicate the CB and SB components in the image plane derived from the experimentally measured hologram and the modeled CB and SB components, respectively, i.e.,

[0140] and are expressed as:

[0141] , [7]

[0142] and

[0143] . [8]

[0144] For simplicity, and will be referred to as the experimentally measured CB and SB components.

[0145] Via the derivative of the corresponding , i.e., Equation [6], the parameter fitting for the amplitude and the phase is obtained:

[0146] , [9]

[0147] .

[10]

[0148] The latter derivative of , i.e., Equation

[10] yields (for a specific value of

[0149] ,

[11]

[0150] From the above equation, the phase of the composite field can be estimated as:

[0151] .

[12]

[0152] Since the phase can be measured only from the sidebands, the estimated value of the phase does not depend on the amplitude or factor s. However, for the optimal signal-to-noise ratio in the sidebands, the value of s can be chosen as .

[0153] the previous derivative of, i.e., equation [9] yields (for any value of the position in the plane ):

[0154] .

[13]

[0155] The above relationship can be simplified to (neglecting the explicit dependency):

[0156] .

[14]

[0157] In this relationship, the first term involves CB, and the second term involves SB. For the following line of reasoning, the resolution of the amplitude for each of these two terms can be obtained separately, and this situation gives rise to the corresponding estimate for the amplitude at CB:

[0158] ,

[15]

[0159] and at SB:

[0160] ,

[16]

[0161] where ^ indicates the estimate.

[0162] Using these two expressions, namely equations

[15] and

[16] , the equation

[14] resulting from the derivative of with respect to A can be simplified to:

[0163] .

[17]

[0164] The true resolution (positive and real-valued) of the amplitude A at any image site R can be readily solved from the above cubic equation using standard mathematical methods

[17] . Some properties of interest for the true resolution can be further derived. Based on the fact that and , the two schemes can be considered as any of the following terms:

[0165] and ,

[18]

[0166] or vice versa:

[0167] and .

[19]

[0168] Differently expressed, this also implies any of the following:

[0169] ,

[20]

[0170] or vice versa:

[0171] .

[21]

[0172] In cases where two (overlapping) interference patterns are required (such as the operations described in the embodiments regarding Figure 10 ), the illumination-reference beam pair provided by the illumination device 1100 or 1200 can be used to illuminate the target 1060 for both. As described above, the two (overlapping) interference patterns can be formed by two pairs of scattered reference beams respectively. Therefore, the processing unit will need to determine the amplitude and phase of two composite fields, each composite field corresponding to a scattered beam. In some embodiments, the processing unit 1090 can also adopt the above four steps (i.e., Figure 14 steps 1401 to 1404 of Figure 13 ), for example, the example Fourier representation of Figure 13 to complete the determination process. However, at this time, the 2D Fourier representation can include the superposition of two corresponding CBs on top of each other, and two pairs of mutually conjugate and well-separated SBs (or a total of four SBs), such as Figure 13 's example Fourier representation. Each pair of mutually conjugate SBs contains the information of one of the two interference patterns. For example, the first pair of mutually conjugate SBs contains the information of the first interference pattern formed by the first pair of scattered reference beams; while the second pair of mutually conjugate SBs contains the information about the second interference pattern formed by the second pair of scattered reference beams. The two pairs of scattered reference beams can be arranged such that the resulting CBs and SBs do not overlap or intersect in the spatial frequency domain.

[0173] After the step 1401 of performing a Fourier transform on the captured image of the interference pattern to the spatial frequency domain, at step 1402, a central region including two overlapping corresponding CBs in the Fourier representation is selected, and the central region is used to calculate its corresponding component, i.e., in the image plane via an inverse Fourier transform . It should be noted that for ease of labeling, here represents the superposition CB of two corresponding pairs of scattered reference beams. The same notation will be used for the modeled version of this component. Subsequently, at step 1403, one SB from each pair of SBs in the Fourier representation is selected, and the SB is used to calculate its corresponding component in the image plane via an inverse Fourier transform. Therefore, two image plane components with exponents "1" and "2" respectively and , where these indices refer to two different interference patterns (or composite fields). Finally, at step 1404, based on the calculated information, such as , and , the amplitudes and phases of the two composite fields can be determined. Step 1404 is further explained by the following mathematical description, which is an extension of the aforementioned mathematical description (equations [1] to

[21] ) for a single hologram or interference pattern in this case.

[0174] In the following mathematical description content, the inverse Fourier transform is applied to each of the CBs of Figure 13 (such as 1301) and two selected SBs in the Fourier image (such as Figure 13 's first higher-order spatial spectrum 1311 and second higher-order spatial spectrum 1312) to obtain the corresponding components in the image plane. Such image plane components are represented by the real-valued function and the complex-valued functions and , where are the 2D coordinates in the image plane, and the indices "1" and "2" refer to two different interference patterns (or composite fields). Without loss of generality, assuming the same splitting ratio s is used for two separate holograms, the multiplexed hologram is modeled as an incoherent superposition of two separate holograms or interference patterns (where the indices "1" and "2" refer to two separate holograms):

[0175] .

[22]

[0176] The Fourier transform of the above multiplexed hologram contains a single CB in the Fourier representation (which results from two separate CBs and two separate SBs of two corresponding scattered reference beam pairs, and the two separate CBs and two separate SBs are modeled as follows:

[0177] ,

[23]

[0178] and

[0179] ,

[24]

[0180] .

[25]

[0181] The least squares function to be minimized for the multiplexed hologram is:

[0182] .

[26]

[0183] For the phases and The optimization of with respect to each phase function or is the same for the case of non - multiplexed (single) holograms, since the phase is only detectable in the corresponding SBs (and not in the CBs where the two corresponding scattered reference beams overlap). This means that the phase with respect to individual holograms will lead to i.e., the same resolution (since their side - bands are separated in the Fourier space of the multiplexed hologram):

[0184] ,

[27]

[0185] and

[0186] .

[28]

[0187] Similarly, for the optimization of the amplitude and it involves taking the derivative of with respect to each amplitude function or , and generating the following two equations in and :

[0188] ,

[29]

[0189] .

[30]

[0190] Applying the relations and , the above two equations

[29] and

[30] can be rewritten as:

[0191] ,

[31]

[0192] .

[32]

[0193] The above system of equations i.e., the simultaneous equations can be solved via various strategies. Without loss of generality, a particular strategy is described here as an example of an implementation. By dividing the two equations

[31] and

[32] , the ratio of the two amplitudes is obtained:

[0194] .

[33]

[0195] Applying equation

[33] to equation

[31] to eliminate , an equation with the only unknown parameter can be obtained (the explicit R - dependence is omitted):

[0196] ,

[34]

[0197] where ε is a small positive value that avoids noise amplification in regions of low power in the SB signal. The (positive and real-valued) amplitude of the first hologram included in the multiplexed hologram at a specific image location R can be readily solved from the cubic equation

[34] for the true solution. The amplitude of the second hologram included in the multiplexed hologram can then be derived from the ratio relationship, i.e., equation

[33] value.

[0198] It should be noted that the foregoing embodiments can also be further generalized or extended for df-DHM with multiple pairs of illumination and reference radiation beams. In some embodiments, each of the multiple illumination radiation beams can include different azimuth angles and / or different incident angles. Similarly, each of the multiple reference beams can include different azimuth angles and / or different incident angles. For example, in an embodiment, in addition to the two illumination radiation beams mainly located in the x-z plane, i.e., the first illumination radiation beam 1010 and the second illumination radiation beam 1020, two other illumination radiation beams (not shown) mainly located in the y-z plane can be used to illuminate the target 1060. Two additional reference radiation beams can also be used to pair with the two additional illumination radiation beams respectively. Such a situation may result in four at least partially spatially overlapping interference patterns, and each of the interference patterns can correspond to a pair of illumination and reference radiation beams. By appropriately configuring the azimuth angle and / or incident angle of each additional reference radiation beam, the four at least partially spatially overlapping interference patterns can be separable in the spatial frequency domain. In this way, more information about the structure of the target (e.g., y-axis asymmetry in the structure of the target) can be obtained.

[0199] In some embodiments, the illumination device can provide multiple pairs of illumination radiation beams and reference radiation beams. The illumination device can also provide sufficient coherence control between the radiation beams such that only the desired interference patterns will be formed on the image sensor. The multiple pairs of illumination radiation beams and reference radiation beams may result in the formation of multiple mutually incoherent, spatially overlapping interference patterns. Therefore, the multiplexed hologram can be modeled as an incoherent superposition of multiple individual holograms or interference patterns. Equation

[22] for the case of two overlapping holograms can then be further extended to include the amplitude and phase functions of all individual holograms. Therefore, when n holograms are formed, the hologram index, i.e., the exponent, should be extended from "1, 2" to "1, 2, 3 …… and n". It should be noted that as Figure 14 illustrated, the determination process should equally apply to any number of overlapping holograms, such as more than two overlapping holograms.

[0200] It should also be noted that different embodiments of methods for determining the amplitude and phase of one or more complex fields (e.g., Figure 14 embodiments) can be used in combination with Figures 10 to 12 embodiments or used independently of Figures 10 to 12 embodiments. When used independently, i.e., independently of each other, other types of df-DHM can be used to generate holograms or interference patterns.

[0201] In an embodiment, the processing unit 1090 can be a computer system. The computer system can be equipped with an image reconstruction algorithm for performing all of the foregoing tasks, including performing Fourier transforms, extracting each individual high-order spatial spectrum, performing inverse Fourier transforms, calculating complex fields, and determining the characteristics of the structure based on the results.

[0202] Figure 15 is a block diagram of a computer system 1500 that can assist in implementing the methods and processes disclosed in the present invention. The computer system 1500 includes a bus 1502 or other communication mechanism for communicating information, and a processor 1504 (or processors 1504 and 1505) coupled to the bus 1502 for processing information. The computer system 1500 also includes a main memory 1506 coupled to the bus 1502 for storing information and instructions to be executed by the processor 1504, such as random access memory (RAM) or other dynamic storage. The main memory 1506 can also be used to store transient variables or other intermediate information during the execution of instructions to be executed by the processor 1504. The computer system 1500 further includes a read-only memory (ROM) 1508 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.

[0203] The computer system 1500 can be coupled by a bus 1502 to a display 1512 for displaying information to a computer user, such as a cathode ray tube (CRT), a flat panel display, or a touch panel display. An input device 1514 including alphanumeric keys and other keys is coupled to the bus 1502 for communicating information and command selections to the processor 1504. Another type of user input device is a cursor control 1516 for communicating direction information and command selections to the processor 1504 and for controlling cursor movement on the display 1512, such as a mouse, a trackball, or cursor direction keys. Such input devices typically have two degrees of freedom in two axes (a first axis (e.g., x) and a second axis (e.g., y)), and such devices allow the device to specify a position in a plane. A touch panel (screen) display can also be used as an input device.

[0204] One or more of the methods described herein can be performed by the computer system 1500 in response to execution by the processor 1504 of one or more sequences of one or more instructions contained in the main memory 1506. These instructions can be read into the main memory 1506 from another computer-readable medium, such as the storage device 1510. Execution of the instruction sequences contained in the main memory 1506 causes the processor 1504 to perform the process steps described herein. One or more processors in a multiprocessing arrangement can also be used to execute the instruction sequences contained in the main memory 1506. In alternative embodiments, hardwired circuitry can be used in place of or in combination with software instructions. Thus, the description herein is not limited to any specific combination of hardware circuitry and software.

[0205] As used herein, the term "computer-readable medium" refers to any medium that participates in providing instructions to the processor 1504 for execution. Such a medium can take many forms, including but not limited to non-volatile media, volatile media, and transmission media. Non-volatile media includes, for example, optical or magnetic disks, such as the storage device 1510. Volatile media includes volatile memory, such as the main memory 1506. Transmission media includes coaxial cables, copper wire, and fiber optics, including the wires that make up the bus 1502. Transmission media can also take the form of acoustic or light waves, such as acoustic or light waves generated during radio frequency (RF) and infrared (IR) data communications. Common forms of computer-readable media include, for example, floppy disks, flexible disks, hard disks, magnetic tape, any other magnetic medium, CD-ROM, DVD, any other optical medium, punch cards, paper tape, any other physical medium with patterns of holes, RAM, PROM, and EPROM, FLASH-EPROM, any other memory chip or cartridge, a carrier wave as described below, or any other medium from which a computer can read.

[0206] Various forms of computer-readable media may be involved in carrying one or more sequences of one or more instructions to the processor 1504 for execution. For example, initially the instructions may be carried on a magnetic disk of a remote computer. The remote computer may load the instructions into its volatile memory and send the instructions using a modem via a telephone line. 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 carries the data to the main memory 1506, from which the processor 1504 retrieves and executes the instructions. The instructions received by the main memory 1506 may optionally be stored on the storage device 1510 before or after being executed by the processor 1504.

[0207] 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 coupling to a network link 1520 that is connected to a local area network 1522. For example, the communication interface 1518 may 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 may be a local area network (LAN) card to provide a data communication connection to a compatible LAN. A wireless link may also be implemented. In any such implementation, the communication interface 1518 sends and receives electrical, electromagnetic, or optical signals that carry digital data streams representing various types of information.

[0208] The network link 1520 typically provides data communication to other data devices via one or more networks. For example, the network link 1520 may provide a connection from the local area network 1522 to a main computer 1524 or to data equipment operated by an Internet service provider (ISP) 1526. The ISP 1526 in turn provides data communication services via the global packet data communication network (now commonly referred to as the “Internet” 1528). Both the local area network 1522 and the Internet 1528 use electrical, electromagnetic, or optical signals that carry digital data streams. The signals via the various networks and the signals on the network link 1520 and via the communication interface 1518 (which carry digital data to and from the computer system 1500) are exemplary forms of carriers that convey information.

[0209] The computer system 1500 can send messages and receive data, including program code, via a network, network link 1520, and communication interface 1518. In an Internet example, the server 1530 may transmit the requested code for an application via the Internet 1528, ISP 1526, local area network 1522, and communication interface 1518. For example, one such downloaded application may provide one or more of the techniques described herein. The received code may be executed by the processor 1504 when it is received, and / or stored in the storage device 1510 or other non-volatile memory for later execution. In this way, the computer system 1500 can obtain application code in the form of a carrier wave.

[0210] Additional embodiments are disclosed in the following numbered list of aspects:

[0211] 1. A dark-field digital holographic microscope configured to determine a property of interest of a structure, the dark-field digital holographic microscope comprising:

[0212] An illumination device configured to provide at least: a first beam pair including a first illumination radiation beam and a first reference radiation beam; and a second beam pair including a second illumination radiation beam and a second reference radiation beam; and

[0213] An imaging branch operable to at least detect a first scattered radiation scattered by the structure and detect a second scattered radiation scattered by the structure, the first scattered radiation being generated by the structure being irradiated by the first illumination radiation beam, the second scattered radiation being generated by the structure being irradiated by the second illumination radiation beam, the imaging branch having a detection NA greater than 0.1 and optionally greater than 0.8;

[0214] Wherein the illumination device is configured such that:

[0215] The first illumination radiation beam and the first reference radiation beam are at least partially temporally coherent and at least partially spatially coherent;

[0216] The second illumination radiation beam and the second reference radiation beam are at least partially temporally coherent and at least partially spatially coherent; and

[0217] The illumination device is configured to impose temporal incoherence and / or spatial incoherence between the first beam pair and the second beam pair.

[0218] 2. The dark-field digital holographic microscope according to aspect 1, wherein the illumination device is operable to: direct the first illumination radiation beam to illuminate the structure from a first direction; and direct the second illumination radiation beam to illuminate the structure from a second direction, the second direction being different from the first direction.

[0219] 3. The dark-field digital holographic microscope according to aspect 1 or 2, wherein the imaging branch includes a sensor, and the dark-field digital holographic microscope is operable to simultaneously capture an interference image on the sensor, the interference image including a first interference pattern generated by the interference of the first scattered radiation and the first reference beam, and a second interference pattern generated by the interference of the second scattered radiation and the second reference beam.

[0220] 4. The dark-field digital holographic microscope according to aspect 3, the dark-field digital holographic microscope being operable such that the first interference pattern and the second interference pattern at least partially overlap spatially on the sensor.

[0221] 5. The dark-field digital holographic microscope according to aspect 3 or 4, the dark-field digital holographic microscope being configured such that the first reference radiation beam and the second reference radiation beam are arranged to be incident at respective different azimuth angles with respect to the optical axis of the dark-field digital holographic microscope.

[0222] 6. The dark-field digital holographic microscope according to aspect 5, wherein the azimuth angle of the first reference radiation beam and the azimuth angle of the second reference radiation beam are configured to include a large enough difference such that the two interference patterns are separable in the spatial frequency domain.

[0223] 7. The dark-field digital holographic microscope according to any one of aspects 3 to 6, the dark-field digital holographic microscope being configured such that the first reference radiation beam and the second reference radiation beam are arranged to be incident at respective different incident angles with respect to the optical axis of the dark-field digital holographic microscope.

[0224] 8. The dark-field digital holographic microscope according to any one of aspects 3 to 7, including a processor operable to:

[0225] transform the interference image of the first interference pattern and the second interference pattern into a Fourier representation, wherein the Fourier representation includes a central frequency band and at least a pair of side frequency bands; and

[0226] determine the amplitude of the composite field of at least one of the first scattered radiation and the second scattered radiation based on at least one of the central frequency band and at least one of the side frequency bands of the at least a pair of side frequency bands.

[0227] 9. The dark-field digital holographic microscope according to aspect 8, wherein the at least one pair of side frequency bands includes:

[0228] A first pair of conjugate side frequency bands, the first pair of conjugate side frequency bands including first information related to the first interference pattern of the scattered reference beam pair, and

[0229] A second pair of conjugate side frequency bands, the second pair of conjugate side frequency bands including second information related to the second interference pattern of the scattered reference beam pair.

[0230] 10. The dark-field digital holographic microscope according to aspect 9, wherein the processor is operable to:

[0231] Use the central frequency band to calculate a first component in the interference images of the first interference pattern and the second interference pattern via an inverse Fourier transform;

[0232] Use the first side frequency band to calculate a second component in the interference images of the first interference pattern and the second interference pattern via an inverse Fourier transform;

[0233] Use the second side frequency band to calculate a third component in the interference images of the first interference pattern and the second interference pattern via an inverse Fourier transform; and

[0234] Determine a first amplitude and a first phase of a first composite field of the first scattered radiation and determine a second amplitude and a second phase of a second composite field of the second scattered radiation based on the first component, the second component, and the third component of the interference image.

[0235] 11. The dark-field digital holographic microscope according to aspect 10, wherein the processor is configured such that the determination of the first amplitude and the first phase of the composite field of the first scattered radiation and the determination of the second amplitude and the second phase of the composite field of the second scattered radiation further include:

[0236] Obtain a modeled image of the first interference pattern and the second interference pattern, the modeled image including a modeled first component, a modeled second component, and a modeled third component;

[0237] Define a performance function that describes the match between the interference image and the modeled image; and

[0238] Optimize (e.g., minimize) the performance function to obtain values for one or more of the following terms: the first phase, the second phase, the first amplitude, and the second amplitude.

[0239] 12. The dark-field digital holographic microscope according to aspect 11, wherein the processor is configured such that the optimization of the performance function further includes:

[0240] fitting the value of the first phase by obtaining the derivative of the performance function with respect to the first phase;

[0241] fitting the value of the second phase by obtaining the derivative of the performance function with respect to the second phase;

[0242] fitting the value of the first amplitude by obtaining the derivative of the performance function with respect to the first amplitude; and

[0243] fitting the value of the second amplitude by obtaining the derivative of the performance function with respect to the second amplitude.

[0244] 13. The dark-field digital holographic microscope according to any one of the preceding aspects, wherein the illumination device further includes a coherence matching device, the coherence matching device being operable to: delay one of the first illumination beam and the first reference beam relative to the other of the first illumination beam and the first reference beam in an adjustable manner so as to maintain the pair of first beams as at least partially coherent; and delay one of the second illumination beam and the second reference beam relative to the other of the second illumination beam and the second reference beam in an adjustable manner so as to maintain the pair of second beams as at least partially coherent.

[0245] 14. The dark-field digital holographic microscope according to aspect 13, wherein the illumination device includes a time delay device, the time delay device being configured to impose incoherence between the first pair of beams and the second pair of beams by being operable to delay one of the first pair of beams and the second pair of beams relative to the other of the first pair of beams and the second pair of beams.

[0246] 15. The dark-field digital holographic microscope according to aspect 14, wherein the time delay device includes an adjustable time delay device, the adjustable time delay device being operable to delay one of the first pair of beams and the second pair of beams relative to the other of the first pair of beams and the second pair of beams in an adjustable manner to impose the incoherence.

[0247] 16. The dark-field digital holographic microscope according to aspect 14 or 15, wherein the illumination device includes a first branch operable to provide the first pair of beams and a second branch operable to provide the second pair of beams, and wherein:

[0248] The time delay device includes at least a delay line that is operable to impose a delay on one of the first or second branches relative to the other of the first or second branches; and

[0249] The coherence matching device includes: a first coherence matching device in the first branch, the first coherence matching device being operable to delay at least one of the first reference beam and the first illumination beam in an adjustable manner; and a second coherence matching device in the second branch, the second coherence matching device being operable to delay at least one of the second reference beam and the second illumination beam in an adjustable manner.

[0250] 17. The dark-field digital holographic microscope according to aspect 14 or 15, wherein the illumination device includes: a first branch that is operable to provide the first illumination beam and the second illumination beam; and a second branch that is operable to provide the first reference beam and the second reference beam; wherein:

[0251] The coherence matching device and the time delay device are implemented by a common optimization of at least: a first adjustable delay line that is operable to impose an adjustable delay on one of the first or second branches relative to the other of the first or second branches; a second adjustable delay line in the first branch that is operable to impose an adjustable delay on at least one of the first illumination beam and the second illumination beam; and a third adjustable delay line in the second branch that is operable to impose an adjustable delay on at least one of the first reference beam and the second reference beam.

[0252] 18. The dark-field digital holographic microscope according to any one of the preceding aspects, wherein the first illumination radiation beam is configured to illuminate the structure at a first incident angle; the second illumination radiation beam is configured to illuminate the structure at a second incident angle different from the first incident angle.

[0253] 19. The dark-field digital holographic microscope according to any one of the preceding aspects, wherein the illumination device includes a single radiation source, and the illumination device is configured to generate the first pair of beams and the second pair of beams from the single radiation source.

[0254] 20. The dark-field digital holographic microscope according to aspect 19, wherein the single light source is configured to emit at least partially coherent radiation.

[0255] 21. The dark-field digital holographic microscope according to any of the foregoing aspects, wherein the illumination device is configured to generate the first reference beam and the second reference beam respectively at a first power level, and to generate the first illumination beam and the second illumination beam respectively at a second power level, the second power level being greater than the first power level.

[0256] 22. The dark-field digital holographic microscope according to any of the foregoing aspects, further comprising: one or more optical elements operable to capture first scattered radiation scattered by the structure, the first scattered radiation being generated by irradiating the structure with the first irradiation radiation beam; and to capture second scattered radiation scattered by the structure, the second scattered radiation being generated by irradiating the structure with the second irradiation radiation beam.

[0257] 23. The dark-field digital holographic microscope according to any of the foregoing aspects, wherein the imaging branch further comprises an objective lens operable to capture at least the first scattered radiation and the second scattered radiation.

[0258] 24. The dark-field digital holographic microscope according to any of the foregoing aspects, wherein the imaging branch has a net positive magnification.

[0259] 25. The dark-field digital holographic microscope according to any of the foregoing aspects, wherein the illumination device is configured such that each of the first illumination beam and the second illumination beam has a smooth profile to substantially uniformly irradiate the structure.

[0260] 26. A method of determining a characteristic of interest of a target formed on a substrate by a lithography process, the method comprising:

[0261] irradiating the target with a first irradiation radiation beam and capturing the resulting first scattered radiation scattered from the target;

[0262] irradiating the target with a second irradiation radiation beam and capturing the resulting second scattered radiation scattered from the target;

[0263] imposing spatial incoherence and / or temporal incoherence between a first beam pair including the first illumination beam and the first reference beam and a second beam pair including the second illumination beam and the second reference beam such that:

[0264] the beams of the first beam pair are at least partially spatially coherent and at least partially temporally coherent,

[0265] the beams of the second beam pair are at least partially spatially coherent and at least partially temporally coherent, and

[0266] Any beam of the first pair is spatially incoherent and / or temporally incoherent with respect to any beam of the second pair; and

[0267] Simultaneously generating a first interference pattern resulting from the interference of the first scattered radiation with a first reference radiation beam, and a second interference pattern resulting from the interference of the second scattered radiation with a second reference beam.

[0268] 27. The method according to aspect 26, further comprising:

[0269] Directing the first irradiation radiation beam to irradiate the target at a first incident angle; and directing the second irradiation radiation beam to irradiate the target at a second incident angle, the first incident angle being different from the second incident angle.

[0270] 28. The method according to any one of aspects 26 or 27, further comprising:

[0271] Directing the first irradiation radiation beam to irradiate the target at a first azimuth angle; and directing the second irradiation radiation beam to irradiate the target at a second azimuth angle, the first azimuth angle being different from the second azimuth angle.

[0272] 29. The method according to any one of aspects 26 to 28, further comprising:

[0273] Directing the first reference radiation beam and the second reference radiation beam to be incident at respective different azimuth angles with respect to the optical axis of the dark-field digital holographic microscope.

[0274] 30. The method according to aspect 29, wherein the azimuth angle of the first reference radiation beam and the azimuth angle of the second reference radiation beam form a difference that is large enough such that the two interference patterns are separable in the spatial frequency domain.

[0275] 31. The method according to any one of aspects 29 or 30, further comprising:

[0276] Directing the first reference radiation beam and the second reference radiation beam to be incident at respective different incident angles with respect to the optical axis of the dark-field digital holographic microscope.

[0277] 32. The method according to any one of aspects 26 to 31, further comprising:

[0278] Delaying one of the first illumination beam and the first reference beam relative to the other of the first illumination beam and the first reference beam in an adjustable manner so as to maintain the pair of first beams as at least partially coherent; and delaying one of the second illumination beam and the second reference beam relative to the other of the second illumination beam and the second reference beam in an adjustable manner so as to maintain the pair of second beams as at least partially coherent.

[0279] 33. The method according to aspect 32, further comprising:

[0280] Delaying one of the pair of first beams and the pair of second beams relative to the other of the pair of first beams and the pair of second beams in an adjustable manner so as to impose incoherence between the pair of first beams and the pair of second beams.

[0281] 34. The method according to any one of aspects 26 to 33, further comprising:

[0282] Generating the pair of first beams and the pair of second beams from a common radiation source, the pair of first beams including the first illumination radiation beam and the first reference radiation beam, and the pair of second beams including the second illumination radiation beam and the second reference radiation beam.

[0283] 35. The method according to any one of aspects 26 to 34, further comprising:

[0284] Setting the first reference beam and the second reference beam to a first power level and setting the first illumination beam and the second illumination beam to a second power level, the second power level being greater than the first power level.

[0285] 36. The method according to any one of aspects 26 to 35, comprising maintaining the time delay between the pair of first beams and the pair of second beams as short as possible when imposing the incoherence between the pair of first beams and the pair of second beams.

[0286] 37. The method according to any one of aspects 26 to 36, further comprising:

[0287] Imaging the first interference pattern and the second interference pattern such that the first interference pattern and the second interference pattern at least partially spatially overlap to obtain an interference image.

[0288] 38. The method according to aspect 37, comprising the further steps of:

[0289] Transforming the interference image of the first interference pattern and the second interference pattern into a Fourier representation, wherein the Fourier representation includes a central frequency band and at least a pair of side frequency bands; and

[0290] Determine at least the amplitude of the composite field of at least one of the first scattered radiation and the second scattered radiation based on the central frequency band and at least one of the at least one pair of side frequency bands.

[0291] 39. The method according to aspect 38, wherein the at least one pair of side frequency bands includes:

[0292] A first pair of conjugate side frequency bands, the first pair of conjugate side frequency bands including first information related to the first interference pattern, and

[0293] A second pair of conjugate side frequency bands, the second pair of conjugate side frequency bands including second information related to the second interference pattern.

[0294] 40. The method according to aspect 39, wherein the determining step further includes:

[0295] Using the central frequency band to calculate a first component in the interference images of the first interference pattern and the second interference pattern via an inverse Fourier transform;

[0296] Using the first side frequency band to calculate a second component in the interference images of the first interference pattern and the second interference pattern via an inverse Fourier transform;

[0297] Using the second side frequency band to calculate a third component in the images of the first interference pattern and the second interference pattern via an inverse Fourier transform; and

[0298] Determining a first amplitude and a first phase of the first composite field of the first scattered radiation and determining a second amplitude and a second phase of the second composite field of the second scattered radiation based on the first component, the second component, and the third component of the interference image.

[0299] 41. The method according to aspect 40, wherein each of the first pair of conjugate side frequency bands and the second pair of conjugate side frequency bands is separable from the central frequency band and any other side frequency bands.

[0300] 42. The method according to aspect 40 or 41, wherein the determining of the first amplitude and the first phase of the composite field of the first scattered radiation and the determining of the second amplitude and the second phase of the composite field of the second scattered radiation further include:

[0301] Obtaining a modeled image of the first interference pattern and the second interference pattern, the modeled image including a modeled first component, a modeled second component, and a modeled third component;

[0302] Define a performance function that describes the match between the interference image and the modeled image; and

[0303] Optimize (e.g., minimize) the performance function to obtain values for one or more of: the first phase, the second phase, the first amplitude, and the second amplitude.

[0304] 43. The method according to aspect 42, wherein the optimization of the performance function further comprises:

[0305] Fitting the value of the first phase by obtaining the derivative of the performance function with respect to the first phase;

[0306] Fitting the value of the second phase by obtaining the derivative of the performance function with respect to the second phase;

[0307] Fitting the value of the first amplitude by obtaining the derivative of the performance function with respect to the first amplitude;

[0308] Fitting the value of the second amplitude by obtaining the derivative of the performance function with respect to the second amplitude.

[0309] 44. A metrology device for determining a quantity of an attribute of interest of a structure on a substrate, the metrology device comprising a dark-field digital holographic microscope according to any one of aspects 1 to 22; or aspects 52 to 56.

[0310] 45. An inspection device for inspecting a structure on a substrate, the inspection device comprising a dark-field digital holographic microscope according to any one of aspects 1 to 25; or aspects 52 to 56.

[0311] 46. A method for determining at least the amplitude of a complex field that describes a structure, the method comprising:

[0312] Irradiating the structure with a first irradiation radiation beam and capturing the resulting first scattered radiation that has been scattered from the structure;

[0313] Irradiating the structure with a second irradiation radiation beam and capturing the resulting second scattered radiation that has been scattered from the structure;

[0314] Imaging a first interference pattern generated by the interference of the first scattered radiation with a first reference radiation beam and a second interference pattern generated by the interference of the second scattered radiation with a second reference beam, such that the first interference pattern and the second interference pattern at least partially spatially overlap to obtain an interference image;

[0315] Transform the interference images of the first interference pattern and the second interference pattern into a Fourier representation, where the Fourier representation includes a central frequency band and at least one pair of side frequency bands; and

[0316] Determine at least the amplitude of the composite field of at least one of the first scattered radiation and the second scattered radiation based on the central frequency band and the at least one pair of side frequency bands.

[0317] 47. The method according to aspect 46, wherein the at least one pair of side frequency bands includes:

[0318] A first pair of conjugate side frequency bands, the first pair of conjugate side frequency bands including first information related to the first interference pattern, and

[0319] A second pair of conjugate side frequency bands, the second pair of conjugate side frequency bands including second information related to the second interference pattern.

[0320] 48. The method according to aspect 47, wherein the determining step further includes:

[0321] Using the central frequency band to calculate a first component in the interference images of the first interference pattern and the second interference pattern via an inverse Fourier transform;

[0322] Using the first side frequency band to calculate a second component in the interference images of the first interference pattern and the second interference pattern via an inverse Fourier transform;

[0323] Using the second side frequency band to calculate a third component in the interference images of the first interference pattern and the second interference pattern via an inverse Fourier transform; and

[0324] Determine a first amplitude and a first phase of a first composite field of the first scattered radiation and a second amplitude and a second phase of a second composite field of the second scattered radiation based on the first component, the second component, and the third component of the interference images.

[0325] 49. The method according to aspect 48, wherein each of the first pair of conjugate side frequency bands and the second pair of conjugate side frequency bands is separable from the central frequency band and any other side frequency bands.

[0326] 50. The method according to aspect 48 or 49, wherein the determination of the first amplitude and the first phase of the composite field of the first scattered radiation and the determination of the second amplitude and the second phase of the composite field of the second scattered radiation further include:

[0327] Obtain modeled images of the first interference pattern and the second interference pattern, the modeled images including a modeled first component, a modeled second component, and a modeled third component;

[0328] Define a performance function that describes the match between the interference image and the modeled image; and

[0329] Optimize (e.g., minimize) the performance function to obtain values for one or more of: the first phase, the second phase, the first amplitude, and the second amplitude.

[0330] 51. The method according to aspect 50, wherein the optimization of the performance function further comprises:

[0331] Fitting the value of the first phase by obtaining the derivative of the performance function with respect to the first phase;

[0332] Fitting the value of the second phase by obtaining the derivative of the performance function with respect to the second phase;

[0333] Fitting the value of the first amplitude by obtaining the derivative of the performance function with respect to the first amplitude;

[0334] Fitting the value of the second amplitude by obtaining the derivative of the performance function with respect to the second amplitude.

[0335] 52. A dark-field digital holographic microscope configured to determine a property of interest of a structure, the dark-field digital holographic microscope comprising:

[0336] Irradiation means configured to (e.g., simultaneously) provide at least: a first beam pair including a first irradiation radiation beam and a first reference radiation beam; and a second beam pair including a second irradiation radiation beam and a second reference radiation beam, such that the dark-field digital holographic microscope can operate to (e.g., simultaneously) capture first scattered radiation scattered by the structure due to the structure being irradiated by the first irradiation radiation beam; and capture second scattered radiation scattered by the structure due to the structure being irradiated by the second irradiation radiation beam;

[0337] A sensor operable to simultaneously capture an interference image including a first interference pattern resulting from the interference of the first scattered radiation with the first reference beam and a second interference pattern resulting from the interference of the second scattered radiation with the second reference beam; and

[0338] A processor operable to:

[0339] Transform the interference images of the first interference pattern and the second interference pattern into a Fourier representation, where the Fourier representation includes a central frequency band and at least one pair of side frequency bands; and

[0340] Determine at least the amplitude of the composite field of at least one of the first scattered radiation and the second scattered radiation based on the central frequency band and the at least one pair of side frequency bands.

[0341] 53. The dark-field digital holographic microscope according to aspect 52, wherein the at least one pair of side frequency bands includes:

[0342] A first pair of conjugate side frequency bands, the first pair of conjugate side frequency bands including first information related to the first interference pattern, and

[0343] A second pair of conjugate side frequency bands, the second pair of conjugate side frequency bands including second information related to the second interference pattern.

[0344] 54. The dark-field digital holographic microscope according to aspect 53, wherein the processor is operable to:

[0345] Use the central frequency band to calculate a first component in the interference images of the first interference pattern and the second interference pattern via an inverse Fourier transform;

[0346] Use the first side frequency band to calculate a second component in the interference images of the first interference pattern and the second interference pattern via an inverse Fourier transform;

[0347] Use the second side frequency band to calculate a third component in the interference images of the first interference pattern and the second interference pattern via an inverse Fourier transform; and

[0348] Determine a first amplitude and a first phase of a first composite field of the first scattered radiation and a second amplitude and a second phase of a second composite field of the second scattered radiation based on the first component, the second component, and the third component of the interference images.

[0349] 55. The dark-field digital holographic microscope according to aspect 54, wherein the processor is configured such that the determination of the first amplitude and the first phase of the composite field of the first scattered radiation and the determination of the second amplitude and the second phase of the composite field of the second scattered radiation further include:

[0350] Obtain modeled images of the first interference pattern and the second interference pattern, the modeled images including a modeled first component, a modeled second component, and a modeled third component;

[0351] Define a performance function that describes the match between the interference image and the modeled image; and

[0352] Optimize (e.g., minimize) the performance function to obtain values for one or more of: the first phase, the second phase, the first amplitude, and the second amplitude.

[0353] 56. The dark-field digital holographic microscope according to aspect 55, wherein the processor is configured such that the optimization of the performance function further includes:

[0354] Fitting the value of the first phase by obtaining the derivative of the performance function with respect to the first phase;

[0355] Fitting the value of the second phase by obtaining the derivative of the performance function with respect to the second phase;

[0356] Fitting the value of the first amplitude by obtaining the derivative of the performance function with respect to the first amplitude; and

[0357] Fitting the value of the second amplitude by obtaining the derivative of the performance function with respect to the second amplitude.

[0358] Although specific reference may be made herein to the use of a lithographic apparatus in IC manufacture, it should be understood that the lithographic apparatus described herein may have other applications. Possible other applications include the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, flat panel displays, liquid crystal displays (LCDs), thin film magnetic heads, and the like.

[0359] Although embodiments of the present invention may be specifically referred to herein in the context of a lithographic apparatus, embodiments of the present invention may be used in other devices. Embodiments of the present invention may form part of a mask inspection apparatus, a metrology apparatus, or any apparatus for measuring or processing an object such as a wafer (or other substrate) or a mask (or other patterning device). These devices may generally be referred to as lithographic tools. Such lithographic tools may use vacuum conditions or ambient (non-vacuum) conditions.

[0360] Although the use of embodiments of the present invention may be specifically referred to above in the context of optical lithography, it should be understood that the present invention is not limited to optical lithography and may be used in other applications (e.g., imprint lithography) where the context permits.

[0361] Although specific embodiments of the present invention have been described above, it should be understood that the present invention may be practiced in other ways different from those described. The above description is intended to be illustrative, not restrictive. Thus, those skilled in the art will appreciate that the present invention as described may be modified without departing from the scope of the claims set forth below.

Claims

1. A dark-field digital holographic microscope configured to determine a property of interest of a structure, the dark-field digital holographic microscope comprises: an illumination device configured to provide at least: a first beam pair including a first illumination radiation beam and a first reference radiation beam; and a second beam pair including a second illumination radiation beam and a second reference radiation beam; and an imaging branch operable to at least detect a first scattered radiation scattered by the structure and detect a second scattered radiation scattered by the structure, the first scattered radiation being generated by the structure irradiated by the first illumination radiation beam, the second scattered radiation being generated by the structure irradiated by the second illumination radiation beam, the imaging branch having a detection NA greater than 0.1; wherein the illumination device is configured such that: the first illumination radiation beam and the first reference radiation beam are at least partially temporally coherent and at least partially spatially coherent; the second illumination radiation beam and the second reference radiation beam are at least partially temporally coherent and at least partially spatially coherent; and the illumination device is configured to impose temporal incoherence and / or spatial incoherence between the first beam pair and the second beam pair by introducing a delay between the first beam pair and the second beam pair, the delay being implemented via an incoherent delay arrangement including an adjustable optical delay line.

2. The dark-field digital holographic microscope according to claim 1, wherein the illumination device is operable to: direct the first illumination radiation beam to irradiate the structure from a first direction; and direct the second illumination radiation beam to irradiate the structure from a second direction different from the first direction.

3. The dark-field digital holographic microscope according to claim 1 or 2, wherein the imaging branch includes a sensor, and the dark-field digital holographic microscope is operable to simultaneously capture an interference image on the sensor, the interference image including a first interference pattern generated by the interference of the first scattered radiation and the first reference radiation beam, and a second interference pattern generated by the interference of the second scattered radiation and the second reference radiation beam.

4. The dark-field digital holographic microscope according to claim 3, the dark-field digital holographic microscope being operable such that the first interference pattern and the second interference pattern at least partially overlap spatially on the sensor.

5. The dark-field digital holographic microscope according to claim 3, the dark-field digital holographic microscope being configured such that the first reference radiation beam and the second reference radiation beam are arranged to be incident at respective different azimuth angles with respect to the optical axis of the dark-field digital holographic microscope.

6. The dark-field digital holographic microscope according to claim 5, wherein the azimuth angle of the first reference radiation beam and the azimuth angle of the second reference radiation beam are configured to include a large enough difference such that the two interference patterns are separable in the spatial frequency domain.

7. The dark-field digital holographic microscope according to claim 3, wherein the dark-field digital holographic microscope is configured such that the first reference radiation beam and the second reference radiation beam are arranged to be incident at respective different incident angles with respect to the optical axis of the dark-field digital holographic microscope.

8. The dark-field digital holographic microscope according to claim 3, comprising a processor operable to perform the following operations: transform the interference images of the first interference pattern and the second interference pattern into a Fourier representation, wherein the Fourier representation includes a central frequency band and at least one pair of side frequency bands; and determine the amplitude of the composite field of at least one of the first scattered radiation and the second scattered radiation based on at least one of the central frequency band and at least one of the side frequency bands of the at least one pair of side frequency bands.

9. The dark-field digital holographic microscope according to claim 8, wherein the at least one pair of side frequency bands comprises: a first pair of conjugate side frequency bands, the first pair of conjugate side frequency bands including first information related to the first interference pattern of the pair of scattered reference beams, and a second pair of conjugate side frequency bands, the second pair of conjugate side frequency bands including second information related to the second interference pattern of the pair of scattered reference beams.

10. The dark-field digital holographic microscope according to claim 9, wherein the processor is operable to: use the central frequency band to calculate a first component in the interference images of the first interference pattern and the second interference pattern via an inverse Fourier transform; use the first pair of conjugate side frequency bands to calculate a second component in the interference images of the first interference pattern and the second interference pattern via an inverse Fourier transform; use the second pair of conjugate side frequency bands to calculate a third component in the interference images of the first interference pattern and the second interference pattern via an inverse Fourier transform; and determine a first amplitude and a first phase of a first composite field of the first scattered radiation and a second amplitude and a second phase of a second composite field of the second scattered radiation based on the first component, the second component, and the third component of the interference images.

11. The dark-field digital holographic microscope according to claim 10, wherein the processor is configured such that the determination of the first amplitude and the first phase of the composite field of the first scattered radiation and the determination of the second amplitude and the second phase of the composite field of the second scattered radiation further comprises: obtaining a modeled image of the first interference pattern and the second interference pattern, the modeled image including a modeled first component, a modeled second component, and a modeled third component; defining a performance function that describes the match between the interference images and the modeled image; and optimizing the performance function to obtain values for one or more of the following terms: the first phase, the second phase, the first amplitude, and the second amplitude, the optimization including minimizing the performance function.

12. The dark-field digital holographic microscope according to claim 11, wherein the processor is configured such that the optimization of the performance function further comprises: Fitting the value of the first phase by obtaining the derivative of the performance function with respect to the first phase; Fitting the value of the second phase by obtaining the derivative of the performance function with respect to the second phase; Fitting the value of the first amplitude by obtaining the derivative of the performance function with respect to the first amplitude; and Fitting the value of the second amplitude by obtaining the derivative of the performance function with respect to the second amplitude.

13. The dark-field digital holographic microscope according to claim 1, wherein the illumination device further comprises a coherence matching device, the coherence matching device being operable to: delay one of the first illumination beam and the first reference beam relative to the other of the first illumination beam and the first reference beam in an adjustable manner so as to maintain the beams of the first beam pair as at least partially coherent; and delay one of the second illumination beam and the second reference beam relative to the other of the second illumination beam and the second reference beam in an adjustable manner so as to maintain the beams of the second beam pair as at least partially coherent.

14. The dark-field digital holographic microscope according to claim 13, wherein the illumination device comprises a time delay device configured to impose incoherence between the first beam pair and the second beam pair by being operable to delay one of the first beam pair and the second beam pair relative to the other of the first beam pair and the second beam pair.

15. The dark-field digital holographic microscope according to claim 14, wherein the time delay device comprises an adjustable time delay device operable to delay one of the first beam pair and the second beam pair relative to the other of the first beam pair and the second beam pair in an adjustable manner to impose the incoherence.

16. The dark-field digital holographic microscope according to claim 14 or 15, wherein the illumination device comprises a first branch operable to provide the first beam pair and a second branch operable to provide the second beam pair, and wherein: the time delay device at least comprises a delay line operable to impose a delay on one of the first branch or the second branch relative to the other of the first branch or the second branch; and the coherence matching device comprises: a first coherence matching device in the first branch, the first coherence matching device being operable to delay at least one of the first reference beam and the first illumination beam in an adjustable manner; and a second coherence matching device in the second branch, the second coherence matching device being operable to delay at least one of the second reference beam and the second illumination beam in an adjustable manner.

17. The dark-field digital holographic microscope according to claim 14 or 15, wherein the illumination device comprises: a first branch operable to provide the first illumination beam and the second illumination beam; and a second branch, the second branch being operative to provide the first reference beam and the second reference beam; wherein: the coherent matching device and the time delay device are implemented via a common optimization of at least: a first adjustable delay line operative to impose an adjustable delay on one of the first branch or the second branch relative to the other of the first branch or the second branch; a second adjustable delay line in the first branch operative to impose an adjustable delay on at least one of the first illumination beam and the second illumination beam; and a third adjustable delay line in the second branch operative to impose an adjustable delay on at least one of the first reference beam and the second reference beam.

18. The dark field digital holographic microscope according to claim 1, wherein the first illumination radiation beam is configured to illuminate the structure at a first angle of incidence; the second illumination radiation beam is configured to illuminate the structure at a second angle of incidence different from the first angle of incidence.

19. The dark field digital holographic microscope according to claim 1, wherein the illumination device includes a single radiation source, the illumination device being configured to generate the first pair of beams and the second pair of beams from the single radiation source.

20. The dark field digital holographic microscope according to claim 19, wherein the single radiation source is configured to emit at least partially coherent radiation.

21. The dark field digital holographic microscope according to claim 1, wherein the illumination device is configured to generate the first reference beam and the second reference beam respectively at a first power level, and to generate the first illumination beam and the second illumination beam respectively at a second power level greater than the first power level.

22. The dark field digital holographic microscope according to claim 1, further comprising: one or more optical elements operative to capture a first scattered radiation scattered by the structure, the first scattered radiation being generated by irradiating the structure with the first illumination radiation beam; and to capture a second scattered radiation scattered by the structure, the second scattered radiation being generated by irradiating the structure with the second illumination radiation beam.

23. The dark field digital holographic microscope according to claim 1, wherein the imaging branch further includes an objective lens operative to capture at least the first scattered radiation and the second scattered radiation.

24. The dark field digital holographic microscope according to claim 1, wherein the imaging branch includes a net positive magnification.

25. The dark field digital holographic microscope according to claim 1, wherein the illumination device is configured such that each of the first illumination beam and the second illumination beam includes a smooth profile to substantially uniformly illuminate the structure.

26. A method of determining a characteristic of interest of a target formed on a substrate by a lithography process using a dark field digital holographic microscope, the method comprising: Irradiate the target with a first irradiation radiation beam and capture the resulting first scattered radiation that has been scattered from the target; Irradiate the target with a second irradiation radiation beam and capture the resulting second scattered radiation that has been scattered from the target; By introducing a delay between a first beam pair including the first irradiation radiation beam and a first reference radiation beam and a second beam pair including the second irradiation radiation beam and a second reference radiation beam, spatial incoherence and / or temporal incoherence is imposed between the first beam pair and the second beam pair, the delay being implemented via an incoherent delay arrangement including an adjustable optical delay line such that: The beams of the first beam pair are at least partially spatially coherent and at least partially temporally coherent, The beams of the second beam pair are at least partially spatially coherent and at least partially temporally coherent, and Any beam of the first beam pair is spatially incoherent and / or temporally incoherent with respect to any beam of the second beam pair; And Simultaneously generate a first interference pattern resulting from the interference of the first scattered radiation and the first reference radiation beam, and a second interference pattern resulting from the interference of the second scattered radiation and the second reference beam.

27. The method according to claim 26, further comprising: Directing the first irradiation radiation beam to irradiate the target at a first incident angle; and directing the second irradiation radiation beam to irradiate the target at a second incident angle, the first incident angle being different from the second incident angle.

28. The method according to any one of claims 26 or 27, further comprising: Directing the first irradiation radiation beam to irradiate the target at a first azimuth angle; and directing the second irradiation radiation beam to irradiate the target at a second azimuth angle, the first azimuth angle being different from the second azimuth angle.

29. The method according to claim 26, further comprising: Directing the first reference radiation beam and the second reference radiation beam to be incident at respective different azimuth angles with respect to the optical axis of the dark-field digital holographic microscope.

30. The method according to claim 29, wherein the azimuth angle of the first reference radiation beam and the azimuth angle of the second reference radiation beam have a difference that is large enough such that the two interference patterns are separable in the spatial frequency domain.

31. The method according to claim 29, further comprising: Directing the first reference radiation beam and the second reference radiation beam to be incident at respective different incident angles with respect to the optical axis of the dark-field digital holographic microscope.

32. The method according to claim 26, further comprising: Delaying one of the first irradiation beam and the first reference beam relative to the other of the first irradiation beam and the first reference beam in an adjustable manner so as to maintain the beams of the first beam pair as at least partially coherent; and delaying one of the second irradiation beam and the second reference beam relative to the other of the second irradiation beam and the second reference beam in an adjustable manner so as to maintain the beams of the second beam pair as at least partially coherent.

33. The method according to claim 32, further comprising: Delay one of the first pair of beams and the second pair of beams relative to the other of the first pair of beams and the second pair of beams in an adjustable manner so as to impose incoherence between the first pair of beams and the second pair of beams.

34. The method according to claim 26, further comprising: Generating the first pair of beams and the second pair of beams from a common radiation source, the first pair of beams including the first irradiation radiation beam and the first reference radiation beam, and the second pair of beams including the second irradiation radiation beam and the second reference radiation beam.

35. The method according to claim 26, further comprising: Setting the first reference beam and the second reference beam to a first power level and setting the first irradiation beam and the second irradiation beam to a second power level, the second power level being greater than the first power level.

36. The method according to claim 26, comprising maintaining the time delay between the first pair of beams and the second pair of beams as short as possible when imposing the incoherence between the first pair of beams and the second pair of beams.

37. The method according to claim 26, further comprising: Imaging the first interference pattern and the second interference pattern such that the first interference pattern and the second interference pattern at least partially spatially overlap to obtain an interference image.

38. The method according to claim 37, comprising the further steps of: Transforming the interference image of the first interference pattern and the second interference pattern into a Fourier representation, wherein the Fourier representation includes a central frequency band and at least one pair of side frequency bands; and Determining the amplitude of the composite field of at least one of the first scattered radiation and the second scattered radiation based on at least one of the central frequency band and at least one of the at least one pair of side frequency bands.

39. The method according to claim 38, wherein the at least one pair of side frequency bands comprises: A first pair of conjugate side frequency bands, the first pair of conjugate side frequency bands including first information related to the first interference pattern, and A second pair of conjugate side frequency bands, the second pair of conjugate side frequency bands including second information related to the second interference pattern.

40. The method according to claim 39, wherein the determining step further comprises: Using the central frequency band to calculate a first component in the interference image of the first interference pattern and the second interference pattern via an inverse Fourier transform; Using the first pair of conjugate side frequency bands to calculate a second component in the interference image of the first interference pattern and the second interference pattern via an inverse Fourier transform; Using the second pair of conjugate side frequency bands to calculate a third component in the image of the first interference pattern and the second interference pattern via an inverse Fourier transform; and Determining a first amplitude and a first phase of a first composite field of the first scattered radiation and a second amplitude and a second phase of a second composite field of the second scattered radiation based on the first component, the second component, and the third component of the interference image.

41. The method according to claim 40, wherein each of the first pair of conjugate sidebands and the second pair of conjugate sidebands is separable from the central band and any other sidebands.

42. The method according to claim 40 or 41, wherein the determination of the first amplitude and the first phase of the composite field of the first scattered radiation and the determination of the second amplitude and the second phase of the composite field of the second scattered radiation further comprise: obtaining a modeled image of the first interference pattern and the second interference pattern, the modeled image including a modeled first component, a modeled second component, and a modeled third component; defining a performance function that describes the match between the interference image and the modeled image; and optimizing the performance function to obtain values for one or more of the following: the first phase, the second phase, the first amplitude, and the second amplitude, the optimization including minimizing the performance function.

43. The method according to claim 42, wherein the optimization of the performance function further comprises: fitting the value of the first phase by obtaining the derivative of the performance function with respect to the first phase; fitting the value of the second phase by obtaining the derivative of the performance function with respect to the second phase; fitting the value of the first amplitude by obtaining the derivative of the performance function with respect to the first amplitude; fitting the value of the second amplitude by obtaining the derivative of the performance function with respect to the second amplitude.

44. A metrology apparatus for determining a quantity of interest of a structure on a substrate, the metrology apparatus comprising a dark-field digital holographic microscope according to any one of claims 1 to 22.

45. An inspection apparatus for inspecting a structure on a substrate, the inspection apparatus comprising a dark-field digital holographic microscope according to any one of claims 1 to 25.

Images